aldol condensation experimental procedure

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Organic chemistry

Course: organic chemistry   >   unit 12.

• Aldol reaction

Aldol condensation

• Mixed (crossed) aldol condensation
• Mixed (crossed) aldol condensation using a lithium enolate
• Retro-aldol and retrosynthesis
• Intramolecular aldol condensation

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Video transcript

Organic Chemistry Laboratory II Dibenzalacetone by the Aldol Condensation Experimental Procedure

Figure 1:   Reaction Scheme for Preparation of Dibenzalacetone

The first step of the reaction involves generation of the enolate of acetone, using sodium hydroxide as base.  The enolate then reacts with the carbonyl carbon of the benzaldehyde in a nucleophilic acyl addition.  A second enolate of acetone is generated which then reacts with another molecule of benzaldehyde.  The resulting dibenzalacetone contains a ketone and two alkene functional groups, that are conjugated.

The dibenzalacetone product will be characterized by melting point and TLC analysis, and the percent yield will be determinied.  The overall purpose of this experiment is to illustrate the mixed aldol condensation as a way to prepare a,b -unsaturated carbonyl compounds.

Solvent-Free Aldol Condensation Reactions: Synthesis of Chalcone Derivatives Supplementary Material

Supplementary information for Comprehensive Organic Chemistry Experiments for the Laboratory Classroom © The Royal Society of Chemistry 2017

Experiment Notes Instructor Notes 1 Experimental Tips 2 Optional Modifications 3

Figures Experimental photos 4 1H NMR 5 IR spectra 7 Thin-Layer Chromatography (TLC) Analysis 10

Instructor Notes

This inquiry-based laboratory features one of the most powerful carbon-carbon bond forming reactions in organic chemistry while highlighting several green and sustainable principles. At the beginning of the experiment, each student obtains an “unknown” benzaldehyde derivative (4-chlorobenzaldehyde, 4-bromobenzaldehyde, or 3-bromobenzaldehyde) to use in the solvent-free aldol condensation. The procedure involves grinding acetophenone with one equivalent of sodium hydroxide and benzaldehyde derivative for ten minutes using a mortar and pestle. Each chalcone is then isolated by suction filtration after washing with water. Although the crude chalcone is often found to have sufficient purity for product characterization, recrystallization is performed with 95% ethanol to remove trace impurities. Once students isolate their product, they use a variety of techniques ( melting point determination, thin-layer chromatography (TLC), NMR and IR spectroscopy) in order to determine the identity of their chalcone. This simple experiment is ideal for intermediate organic chemistry students as it is easily and consistently executed in high yield, incorporates problem solving and highlights green chemistry principles as the synthesis minimizes waste production (no reaction solvent) and proceeds with high atom economy.

This experiment was designed for CHM 249 (Organic Chemistry) at the Department of Chemistry at the University of Toronto. The undergraduate program in the Department of

Chemistry includes a first year twelve week introductory course in organic chemistry. In second year, degree students continue with CHM 249 (enrollment of approximately 60-80 students), which takes place over a twelve week semester. This specialist course introduces students to numerous organic transformations with an emphasis on mechanistic theory and synthetic applications. The course also incorporates eleven weeks of laboratory work (4.5 h each week) which provides students with valuable hands-on experience.

This experiment was developed with two undergraduate students who tested the reproducibility of the experiment prior to its introduction into CHM 249 where it was performed by an additional 129 students over two years. After implementing this new laboratory, positive feedback was received from students who enjoyed learning about solvent-free reactions and carrying out the grinding technique. The results can be found in Scheme SM 4.2.2.1.1.

Scheme SM 4.2.2.1.1 – Chalcone synthesis results

Experimental Tips

As the experiment requires students to use 5 mmol of benzaldehyde derivatives of varying molecular weight, it is recommended that samples are provided in vials to maintain benzaldehyde anonymity. It is recommended that the vials be labelled

“unknown A”, “unknown B”, and “unknown C” corresponding to 4-chlorobenzaldehyde, 4-bromobenzaldehyde, and 3-bromobenzaldehyde, respectively.  At room temperature, 4-chlorobenzaldehyde and 4-bromobenzaldehyde are both white solids while 3-bromobenzaldehyde is a colorless liquid (mp 20 °C). It is recommended that all sample vials are kept in a chemical refrigerator prior to the experiment and stored on ice during the experiment in order to maintain benzaldehyde anonymity, as all derivatives are white solids at reduced temperatures.  Although the melting points of 4-bromochalcone (117-119 °C) is similar to 4- chlorochalcone (113-117 °C), students rarely confuse the two since the products are of sufficient purity to obtain accurate melting point data.

Optional Modifications

This experiment was designed for a 2-3 hour laboratory session where students perform one chalcone synthesis and a variety of characterization techniques. However, there are several procedural modifications that can be employed to cater to your unique teaching laboratory:

Experiment Simplification:  Instructors may choose to disclose the identity of the benzaldehyde derivatives  TLC analysis can be a time consuming technique for students with limited experience, and as such, can be omitted from the experiment.  Students can analyze the purity and structure of their unique chalcone by collecting IR and 1H NMR spectra independently, but these methods may be time consuming and costly. Alternatively, the spectra (one or both) can be provided directly to students for post-laboratory analysis. For example, students may collect an IR spectrum of their chalcone product while the 1H NMR can be provided in the laboratory manual.

Experiment Extension:  Students may perform two or three chalcone syntheses if time permits. Alternatively, students can work in groups of three where each student performs a unique chalcone synthesis. After each student isolates pure product, they can share the material with each partner for independent analysis. This method allows each student to gain experience with one chalcone synthesis but enhances the inquiry-based learning component of the experiment as they are required to characterize all three chalcones .

Experimental Photos

Figure SM 4.2.2.1.1 – Reaction mixture after grinding for 10 minutes.

Figure SM 4.2.2.1.2 – Collection of the crude solid by suction filtration on a Büchner funnel.

Figure SM 4.2.2.1.3 – Pure chalcone products after recrystallization (all pale yellow crystals).

1H NMR Spectra

1 Figure SM 4.2.2.1.4: H NMR spectrum of trans-4-chlorochalcone (CDCl3, 400 MHz)

1 Figure SM 4.2.2.1.5: H NMR spectrum of trans-4-bromochalcone (CDCl3, 400 MHz)

1 Figure SM 4.2.2.1.6: H NMR spectrum of trans-3-bromochalcone (CDCl3, 400 MHz)

Figure SM 4.2.2.1.7: IR spectrum of trans-4-chlorochalcone (CHCl3 solution)

Figure SM 4.2.2.1.8: IR spectrum of trans-4-bromochalcone (CHCl3 solution)

Figure SM 4.2.2.1.9: IR spectrum of trans-3-bromochalcone (CHCl3 solution)

Thin-Layer Chromatography (TLC) Analysis

Figure SM 4.2.2.1.10: Chalcone TLC analysis. Each chalcone was run alongside acetophenone using a 3:1 hexanes/ethyl acetate solvent system.

Synthesis of (E)-chalcones [(E)-1,3-diarylprop-2-en-1-ones]

Supplementary Material

In this work, which is planned for 1 session or 2 sessions 3 hours each, students (individually or in groups of two) will synthesize (E)-chalcone derivatives by the reaction of acetophenones with benzaldehydes . This experimental work illustrates the aldol condensation reaction. The desired product is obtained directly by filtration or can be used to illustrate chromatographic techniques (in a second session). The amount of solvent ( methanol or THF) necessary should be adjusted because with some substitution pattern after the base addition a precipitate is produced; in these cases more solvent should be added to allow a proper stirring. The benzaldehydes should be added with the reaction container immersed in an ice bath. The (E)-chalcone derivatives show different colours depending of the substituent present in the 1 aromatic rings (R= H, white, R= OH yellow, if R =NO2 derivatives are more orange). This experiment was performed by nearly 500 students and the yields and melting points are an average of the students’ results.

5' 5 R1 4' 6' 6 4 AB1' 3' 1 3 2' 2 R O Figure SM 4.2.2.2.1 - (E)-chalcone structure

Table of Results: Reaction Yield and Melting Point of some (E)-chalcones

Entry Substituent R Substituent R1 Yield of the crude (%) Melting point (°C) 1* H H 65-75 56-57 2 OH H 75-85 87-89

3 OH OCH3 70-85 89-90

4 OH NO2 70-85 153-154 *The unsubstituted derivative is presented to help students and/or instructors in the NMR analysis

Typically the TLC Rf of acetophenone and chalcone using mixtures of hexane/ethyl acetate are very different. If the eluent used is 80-20% (hexane/ethyl acetate) chalcone will have an Rf of approximately

0.5 (see Fig. SMX.2.). 2’-Hydroxychalcone spot is usually yellow and can be seen by human eye whereas acetophenone can be seen using an UV lamp and TLC plates of Silica gel 60 F254.

Figure SM 4.2.2.2.2 – Typical TLC plate after elution

The most important aspects in the 1H NMR analyses are the presence of AB spin systems due to the vinylic protons H- and H-. It is also interesting to highlight the coupling constant value (J ~ 16 Hz) characteristic of an (E)-configuration. The presence of a 2’-hydroxyl group is essential to the intramolecular cyclization of 2’-hydroxychalcones and gives a very distinct singlet in the 1H NMR spectra (δ 12-13 ppm); its presence in the chalcone A ring also gives a very interesting inequivalence of the aromatic protons that allow students to study important aspects such as multiplicity and long range coupling constants and also the shielding effect of the hydroxy group. Finally we also present examples of the presence, in the B ring, of electron withdrawing (nitro) and electron donating (OCH3) groups, which allow studying the effects on chemical shifts. In the 13C NMR spectra the signal due to the carbonyl group carbon (C-1) is also characteristic of these compounds (δ 190-195 ppm). The deshielding effect of the carbonyl group in the proton and carbon resonances of the A and B rings and also in the β–position of the α,β-unsaturated system can also be explored in the 1H and 13C NMR spectra of these compounds. In the following figures are given, as examples, the 1H and 13C NMR spectra of (E)-chalcone, (E)-2’-hydroxychalcone, (E)-2’-hydroxy-4-methoxychalcone and (E)-2’- hydroxy-4-nitrochalcone.

1 Figure SM 4.2.2.2.3 - H NMR spectrum (300 MHz, CDCl3) of the (E)-chalcone.

13 Figure SM 4.2.2.2.4 - C NMR spectrum (75 MHz, CDCl3) of the (E)-chalcone

1 Figure SM 4.2.2.2.5 - H NMR spectrum (300 MHz, CDCl3) of the (E)-2’-hydroxychalcone.

13 Figure SM 4.2.2.2.6 - C NMR spectrum (75 MHz, CDCl3) of the (E)-2’-hydroxychalcone.

1 Figure SM 4.2.2.2.7 - H NMR spectrum (300 MHz, CDCl3) of the (E)-2’-hydroxy-4-methoxychalcone.

13 Figure SM 4.2.2.2.8 - C NMR spectrum (75 MHz, CDCl3) of the (E)-2’-hydroxy-4-methoxychalcone.

1 Figure SM 4.2.2.2.9 - H NMR spectrum (300 MHz, CDCl3) of the (E)-2’-hydroxy-4-nitrochalcone.

13 Figure SM 4.2.2.2.10 - C NMR spectrum (75 MHz, CDCl3) of the (E)-2’-hydroxy-4-nitrochalcone.

Experimental procedure for microwave synthesis:

1. In a fume hood add to a two-necked flask, equipped with a magnetic stirring bar, fiber-optic temperature control and reflux condenser (Figure SM 4.2.2.2.11) was charged with 0.2 mL of the desired acetophenone and 10 mL of dried THF. 2. Put the round bottom flask in an ice bath placed on a stirring plate and slowly add 2 equiv. of sodium hydride. 3. Put the two-necked flask in Ethos SYNTH microwave (Milestone Inc.) (Figure SM 4.2.2.2.11) and irradiate at constant power of 400 W for 15 minutes. 4. After this period pour the reaction mixture into a vessel containing ice (~ 20 g) and add hydrochloric acid to adjust pH to ~ 2. 5. The formed solid is filtered using a vacuum filtration apparatus.

Figure SM 4.2.2.2.11 - Reaction setup apparatus.

A free-solvent approach for chalcone synthesis via aldol reaction

Experimental notes: …………………………………………………………………..……………..1 Table of yield, m.p. and IR spectrum of recrystallized chalcone………………….………2 Figures: Photos of the experiment……………………………………………………………………..2 1H, 13C NMR and EIMS spectra……………………………………………………………...3 Proposed mechanism for chalcone synthesis 5

This experiment aims the synthesis of the chalcone (E)-1-(4'-chlorophenyl)-3-phenyl-2-propen-1-one, prepared by a Claisen-Schmidt aldol condensation between an aldehyde lacking an α-hydrogen (benzaldehyde) and a ketone (4´-chloroacetophenone) in the presence of solid NaOH. Therefore, 0.5 mL (5.0 mmol., 1 eq.) of benzaldehyde, 0.65 mL (5.0 mmol., 1 eq.) of 4´- chloroacetophenone and a pellet of NaOH (weighing approximately 0.2 g, 5.0 mmol., 1 eq.) were added into a porcelain mortar (8 cm diameter). After few seconds of grinding with the aid of a pestle, the reaction mixture became a yellow paste. The mixture was grinded for a period of 20-30 minutes until totally solidified when a beige solid powder became (Figure SM 4.2.2.3.1). At the end of grinding, 10 mL of distilled water was added to the mortar (to facilitate the product isolation by filtration) and further well mixed with the product so obtained using the pestle and a spatula to remove the solid from mortar´s wall. The resulting solid was collected by vacuum filtration on a Büchner funnel. The mortar and pestle were washed with more 10 mL of distilled water, in order to remove remained solid therein. This solid was added to the same filter and all of product on the filter was washed with distilled water (10 mL). The solid so obtained was recrystallized from ethanol, collected by vacuum filtration on a Büchner funnel and the crystals so collected were allowed to dry in an oven. Once dried, the product was weighed and their melting point determined. The structure of the obtained compound was elucidated by m.p., IR (KBr), EIMS and NMR spectroscopy (solutions were made in DMSO-d6 and spectra obtained using a 600 MHz instrument). Table SM 4.2.2.3.1 describes the average values of yield, m.p. and IR spectrum of pure chalcone. The 1H NMR, 13C NMR and EIMS spectra of the recrystallized chalcone are shown in Figures SM 4.2.2.3:2 to XM X.5. The doublets of the alkene hydrogens, typical of chalcones, arise at 7.78 and 7.95 ppm. Students can concluded that chalcone is trans by measuring the large coupling constant (15,6 Hz) of the alkene hydrogens. The reproducibility of this experiment was assessed by its repetitive execution, namely by 2nd year Biochemistry students from University of Trás-os-Montes e Alto Douro, in each year.

Table SM 4.2.2.3.1 – Yield, m.p. and IR spectrum of recrystallized chalcone.

Yield (%) 90-95 m.p. (ºC) 97-98 -1 IR (KBr, max, cm 1662, 1603, 1483, 1444, 1335, 1210, 1092, 1025, 1008, 975, 825, 757, 683

Photos of the experiment

a b c Figure SM 4.2.2.3.1 – Reaction mixture: a) paste at the beginning of the grinding; b) using the spatula to remove the solid from mortar´s wall; c) beige solid obtained after complete grinding.

1H, 13C NMR and EIMS spectra

1 Figure SM 4.2.2.3.2 – H NMR spectrum (600 MHz, DMSO-d6) of recrystallized chalcone.

Figure SM 4.2.2.3.3 – 1H NMR spectrum detail showing the chalcone peaks

13 Figure SM 4.2.2.3.4 – C NMR spectrum (600 MHz, DMSO-d6) of recrystallized chalcone.

Figure SM 4.2.2.3.5– DEPT 135 NMR spectrum (600 MHz, DMSO-d6) of recrystallized chalcone

Figure SM 4.2.2.3.6 – EIMS (EI-TOF) spectrum of recrystallized chalcone.

Proposed mechanism for chalcone synthesis

H H H H H H O OH O C O C H O O O C C H C C H C C C H H H O H

Cl Cl Cl Cl

H H H OH O C O C C C C H H

Scheme SM 4.2.2.3.1 - Proposed mechanism for chalcone synthesis

Preparation of dibenzylideneacetone Supplementary Material

Experimental notes Background topics……………………………………………………………………………………..1 Experimental details………..………………………………………………………………………….1 Figures Photos of the experiment…………………………………………………………………….……….3 IR and 1H-NMR spectra…………………………………………………….…………………………5

Experimental notes Background topics This experiment aims to illustrate the reactivity of carbonyl compounds at both carbonyl and α carbon atoms. An insight at tautomeric equilibrium (formation of enol/enolates) and nucleophilic addition to the carbonyl group should be emphasized. The Claisen-Schmidt reaction, a cross aldol condensation, is particularly adequate to illustrate these important features of the carbonyl Chemistry. The experiment is appropriate to medium level students, who are encouraged to rationalise the mechanism of the reaction and some experimental details through the answers to a set of additional questions. It was realized by more than 150 students of the Faculty of Sciences and Technology, Universidade Nova de Lisboa (in classes of 22 students/ class, 11 groups of two), who accepted very well the work and enjoyed its execution. Two main aspects are very important and should be understood and explained by the students: (1) the occurrence of a spontaneous dehydration reaction in a basic medium (E1cB mechanism), after the aldol formation by addition of the ketone enolate to the aldehyde carbonyl, and (b) the reason of the yellow colour of the final product (extended conjugation of the π system and absorption in the Visible). Suggest the students to write the reaction mechanism: formation of the ketone enolate, nucleophilic addition to the C=O bond of the aldehyde, dehydration of the aldol. Ask about the molar ratio ketone/aldehyde (1:2 favours double condensation; ketone in excess favours mono-condensation). IUPAC name for dibenzylideneacetone is 1,5-diphenylpenta-1,4-dien-3-one.

Experimental details Experimentally the work is very simple, with low difficulty and hazard levels. Ethanol is the only organic solvent used, at room temperature for the synthesis and at boiling point to the recrystallization. About 80-90 mL of ethanol will be needed for recrystallization. An interesting suggestion can be made to the students: to keep a small amount of the crude product (before purification by recrystallization) and to dry it. Then, when measuring the melting point of a Supplementary information for Comprehensive Organic Chemistry Experiments for the Laboratory Classroom © The Royal Society of Chemistry 2017 sample of the purified product, also determine the melting point of a sample of the crude product. Both experimental values should be compared and discussed. Ask the students about the reasons of (eventual) differences, also including the value reported in the literature. Some experimental results obtained by the students in the laboratory are presented in Table 4.2.2.4.1.

Table 4.2.2.4.1. Typical experimental results obtained in the Laboratory Yield of crude product Not significant. The crude product contains a large amount of water and some unreacted benzaldehyde, even after air drying. Melting point of crude product 94-101ºC Yield of recrystallized product (average) 4,1 g (55%); lower: c.a 40%; higher: c.a 70% Melting point of purified product 104-106ºC

The preparation of samples for spectral analysis is also very important. Students should be familiarized with the technique of preparing a solid transparent disk for IR spectroscopy, by using a small amount of a dried sample of the compound and KBr and the adequate material. For 1H-NMR spectroscopy test the solubility of dibenzylideneacetone in acetone , chloroform or carbon tetrachloride (or other solvents). After making the choice (in terms of availability, cost, toxicity…) use the chosen solvent in the corresponding fully deuterated form. CDCl3 is a good choice.

Figures Photos of the experiment

Figure 4.2.2.4.1. Chemicals Figure 4.2.2.4.2. Set up of experimental apparatus

Figure 4.2.2.4.3. Starting the product formation Figure 4.2.2.4.4. End of reaction Supplementary information for Comprehensive Organic Chemistry Experiments for the Laboratory Classroom © The Royal Society of Chemistry 2017

Figure 4.2.2.4.5. Recovery of crude product Figure 4.2.2.4.6. Air drying of crude product

Figure 4.2.2.4.7. Hot filtration after recrystallization Figure 4.2.2.4.8. Starting crystal formation

Figure 4.2.2.4.9. Collecting crystals by suction filtration Figure 4.2.2.4. 10. Crystals of purified product

Figure 4.2.2.4.11. IR spectrum of dibenzylideneacetone (in KBr disk)

1 Figure 4.2.2.4.12. H-NMR spectrum of dibenzylideneacetone (in CDCl3)

L-proline Catalyzed Aldol Reaction of 4-Nitrobenzaldehyde with

Acetone Supplementary Material

Figures Figure 1 - HPLC analysis of racemic product catalyzed by pyrrolidine……... 2 Figure 2 - HPLC analysis of chiral product catalyzed by L-proline………… 2 Figure 3 - 1H NMR spectrum of (4R)-(4-Nitrophenyl)-4-hydroxy-2-butanone………. 3

The main purpose of this experiment is to illustrate the utility of L-proline as the catalysis in the preparation of chiral compounds. The experimental simplicity of this kind of reactions lies in mild reaction conditions without the requirement of pre-formation or isolation of unstable enamines, or preactivation of carbonyl compounds. It also can draw the student’s attention and interest to the application of commercially available, non-toxic, natural amino acid as catalysts in asymmetric synthesis and the origin of chirality in nature. In order to accurately determining the yield and enantioselectivity of reaction product，running flash column chromatography is necessary in the product purification process. Based on the TLC analysis, the size of column, the quantity of silica gel and the polarity of eluent are the three carefully-decided crucial factors for the fast and economical chromatography isolation. Students need to have learned advanced organic chemistry, in which the concepts of chirality, asymmetric synthesis and enamine formation were taught. The students should have acquired the basic knowledge of instrumental analysis and be skilled enough in order to perform the reaction at a micro-scale and handle the work-up process as here planned.

Additional notes on the preparation of(4R)-(4-Nitrophenyl)-4-hydroxy-2-butanone: This procedure was successfully implemented at the described scale. The product yields obtained by the students are about 53~74%, and the values of ee are about 62~69%.

Using ethyl acetate/petroleum ether = 1/3 as the eluting solvent, the Rf value of the product is about 0.2-0.3. We recommend the column with the diameter about 20 mm and the length about 200 mm. The column will fill to about 100 mm with the amount of silica being used. The ee of product was determined by chiral HPLC analysis (Daicel Chiralpak AS-H, isopropanol/hexane = 50/50, 1.0 mL/min, λ = 254 nm, tR (major) = 7.4 min, tR (minor) = 8.7 min.). Authentic racemic standard for the HPLC analysis was synthesized using pyrrolidine as a catalyst.

# Time Area Height Width Symmetry Area% 1 7.4 8125.5 569.2 0.213 0.635 49.259 2 8.729 8370 472.5 0.2747 0.617 50.741

Figure 4.2.2.5.1‐ HPLC analysis of racemic product catalyzed by pyrrolidine.

# Time Area Height Width Symmetry Area% 1 7.361 9488.8 703.7 0.2093 0.624 84.319 2 8.749 1764.7 112.7 0.2447 0.811 15.681

Figure 4.2.2.5.2‐ HPLC analysis of chiral product catalyzed by L-proline.

1 Figure 4.2.2.5.3 - H NMR spectrum (400MHz, CDCl3) of (4R)-(4-Nitrophenyl)-4-hydroxy-2-butanone.

Preparation of a β-nitrostyrene derivative by the Henry reaction: comparison of a conventional and a microwave- assisted method

In the experiment proposed herein the advantages of using microwave-assisted organic synthesis methods (MAOs) in the preparation of β-nitrostyrene derivatives are explored. Comparing with the conventional synthesis of β-nitrostyrene derivatives, it requires shorter heating times and relative smaller amounts of the nitroalkane reactant. The use of more resource-efficient, greener and economical synthetic routes is undeniably a trend in modern organic chemistry laboratories. Additionally, the use of MAOs has been reported to increase product yields and ease of work-up and purification compared to conventional “heating and stirr” methods.1, 2

Students’ laboratorial sessions The experiments were designed for chemistry-based curricula, namely medicinal chemistry courses, and are best suited for students that have organic chemistry (theoretical and practical) foundations. This same experiment was tested in a first year master chemistry course including over 25 students. The suggested experiments were easy to perform and produced clear and obvious results which directed the students in a more applied and real organic chemistry lesson. The average success of students in the integrated experience was quite satisfactory (>90%). Students obtained adequate amount of compound to complete the hands-on experience of structural characterisation. No extraordinary difficulties in the work-up step and acquisition of NMR spectra have been detected. However, in the interpretation of NMR data, some drawbacks have been noticed; students were advised to use molecular model kits to assist in the visualization process. Furthermore, a theoretical class for data discussion is strongly recommended. A lab report was written according standard guidelines.

Notes: - Students must read the details of the experiments and prepare a procedure flow chart to be followed in the class; - Students must previously explore microwave-assisted synthesis features; - Students must check the identity and purity of each product by TLC, NMR (1H and 13C) and melting point determination; Supplementary information for Comprehensive Organic Chemistry Experiments for the Laboratory Classroom © The Royal Society of Chemistry 2017

- Students must be familiar with β-nitrostyrene derivatives structural features; - Students must review NMR spectroscopic concepts.

Additional notes on the preparation of β-nitrostyrene derivatives

In the so called conventional method (Figure SM 4.2.2.6.1a), the first 6 hours of reaction must be performed prior to the actual lab session by the instructor or the student, since the required heating time is about 7 hours or even more. The average yields fluctuate from 90 to 95%, depending on the reaction time and experience of the operator. In the work-up step the progress of the reaction can be monitored by TLC, using petroleum ether/ diethyl ether 50:50 (v/v), as eluent; a small difference between the Rf of the aldehyde and the corresponding β-nitrostyrene derivative is observed, being the β-nitrostyrene spot easily observed due to its strong yellow colour (Rf nitrostyrene > Rf benzaldehyde). Before the extraction step it is important to evaporate (ca. half of the initial volume) some of the nitroalkane to prevent distribution of nitrostyrene between the two layers (organic and aqueous).

Figure SM 4.2.2.6.1. Henry’s reaction setup: a) Conventional method and b)

Microwave-assisted apparatus.

In microwave-assisted method (Figure SM 4.2.2.6.1b) the reaction time may vary according to the apparatus used, mainly due to fluctuations on temperature/pressure control. This experiment was performed and fully optimised using a Biotage Initiator 2.5 Microwave oven. TLC was used to monitor the progress of the reaction. At various time Supplementary information for Comprehensive Organic Chemistry Experiments for the Laboratory Classroom © The Royal Society of Chemistry 2017 intervals during the reaction, mixture samples were taken and subjected to TLC analysis. The average yields also fluctuate from 90 to 95%, depending on reaction conditions and operator skills.

Although the yields obtained for the two methods are very similar, the students will have the opportunity to observe the simplification of the technique as well the significant reduction of preparation time of a simple organic compound , recurring to microwave-assisted organic synthesis approach. The melting point evaluation could be compared with the literature values to assess compound purity.3

Herein is proposed the study of a specific β-nitrostyrene derivative (4-hydroxy-3- methoxybenzaldehyde), however it was already tested the same method, with equally satisfactory results, for the preparation of other β-nitrostyrene and β-methyl-β- nitrostyrene derivatives, starting for a wide variety of substituted benzaldehydes.

β-nitrostyrene derivatives preparation: the chemistry behind and beyond

The synthetic route proposed in this work involves a Henry reaction type condensation between the carbonyl compound (benzaldehyde) and the nitroalkane in basic conditions. The basic catalyst deprotonates the nitroalkane in the α-Carbon to the strong electron withdrawing group (NO2) and the resulting resonance-stabilised anionic intermediate then attacks the carbonyl compound. This step yields a β-nitroalcohol, which, after heat-promoted dehydration, forms the intended β-nitrostyrene derivative.

The proposed mechanism for the Henry reaction is depicted in Figure SM 4.2.2.6.2. Supplementary information for Comprehensive Organic Chemistry Experiments for the Laboratory Classroom © The Royal Society of Chemistry 2017

Figure SM 4.2.2.6.2. Proposed mechanism for the first step of the Henry reaction to

obtain the β-nitroalcohol.

The intermediate is difficult to isolate, especially when aromatic aldehydes are used.4

The nitroaldolic condensation is a classic synthetic reaction that implies the formation of a carbon-carbon bond. This reaction occurs easily in the presence of basic catalysts of organic or inorganic nature, as primary and tertiary amines, alkaline metal hydroxides, carbonates and alkoxides, among others.4, 5 Ammmonium acetate and buffer solution of ammonium acetate in acetic acid are the most common catalysts used in the Henry reaction setup6. Unwanted parallel pathways can occur along with the aldolic condensation, as the Cannizzaro reaction, or polymerization can occur simultaneously to the formation of the nitroalkenes.7 These parallel reactions can be minimized, or even avoided, with the use of the proper catalyst.8

The Henry reaction proceeds in a non-aqueous environment mainly because nitrostyrenes are highly insoluble in water. The final compounds are usually very coloured, from light yellow to intense orange, due to the existence of a pronounced Supplementary information for Comprehensive Organic Chemistry Experiments for the Laboratory Classroom © The Royal Society of Chemistry 2017 electronic delocalization through the double bond and the aromatic ring (highly conjugated system). The β-nitrostyrene derivatives show a predominance of the E isomer (trans) as could be inferred by the analysis of the coupling constants of the vinylic protons (Figure SM 4.2.2.6.3) in the 1H NMR spectrum.9 The aromatic substitution pattern of the compounds could also be concluded by the NMR analysis.

The 1H, 13C and DEPT spectra are depicted in Figures SM 4.2.2.6.3, SM 4.2.2.6.4, SM

4.2.2.6.5, and SM 4.2.2.6.6.

Structural analysis

Figuure SM 4.2.2.6.3. 1H NMR spectrum (400 MHz, DMSO) of 4-hydroxy-3-methoxy-β- nitrostyrene.

Figuure SM 4.2.2.6.4. Expansion of 1H NMR spectrum (400 MHz, DMSO) of 4-hydroxy- 3-methoxy-β-nitrostyrene.

Figuure SM 4.2.2.6.5. 13C NMR spectrum (100 MHz, DMSO) of 4-hydroxy-3-methoxy-β- nitrostyrene. Supplementary information for Comprehensive Organic Chemistry Experiments for the Laboratory Classroom © The Royal Society of Chemistry 2017

Figuure SM 4.2.2.6.6. DEPT-135 spectrum (100 MHz, DMSO) of 4-hydroxy-3-methoxy- β-nitrostyrene.

1. C. O. Kappe, D. Dallinger and S. S. Murphree, in Practical Microwave Synthesis for Organic Chemists, Wiley-VCH Verlag GmbH & Co. KGaA, 2009, pp. 291- 299. 2. G. Yan, A. J. Borah and L. Wang, Org. Biomol. Chem., 2014, 12, 6049-6058. 3. N. Milhazes, R. Calheiros, M. P. M. Marques, J. Garrido, M. N. D. S. Cordeiro, C. Rodrigues, S. Quinteira, C. Novais, L. Peixe and F. Borges, Bioorg. Med. Chem., 2006, 14, 4078-4088. 4. R. Ballini and G. Bosica, J. Org. Chem., 1997, 62, 425-427. 5. C. D. Wang and S. Wang, Synth. Commun., 2002, 32, 3481-3486. 6. C. B. Gairaud and G. R. Lappin, J. Org. Chem., 1953, 18, 1-3. 7. G. Sartori, F. Bigi, R. Maggi, R. Sartorio, D. J. Macquarrie, M. Lenarda, L. Storaro, S. Coluccia and G. Martra, J. Catal., 2004, 222, 410-418. 8. A. G. M. Barrett and G. G. Graboski, Chem. Rev., 1986, 86, 751-762. 9. R. E. Clavijo, R. Araya-Maturana, B. K. Cassels and B. Weiss-López, Spectrochim. Acta A: Molec. Spectrosc., 1994, 50, 2105-2115.

Additional Reading

J. E. McMurry, Organic Chemistry, Brooks Cole, Belmount, 8th ed., 2011.

D. L. Pavia, G. M. Lampman, G. S. Kriz, J. A. Vyvyan, Introduction to Spectroscopy, Belmount, 8th ed., 2008.

A. Ault,Techniques and Experiments for Organic Chemistry, University Science Books, Sausalito, 6th ed., 2008.

Synthesis of Aurone derivatives through acid-catalysed

aldol condensation

Purpose of the experiment……………………………………………………………………………………1 General Notes / Troubleshooting Information…………………………………………...……………….3 Photos of the experiment………………………………………………………………………………..……3 1H and 13C NMR spectra………………………………………………………………………………………5

Aurones are minor tricyclic flavonoids , structurally isomers of flavones, which have been studied not only for their physiological properties and effects on Nature, but also for their therapeutic potential. From the synthetic approaches towards aurones , the aldol condensation between benzofuran-3(2H)- one and benzaldehydes is one of the most popular due to its compatibility with a wide range of substituents.

This experiment aims at the preparation of a small library of aurone derivatives (1a-c) through a simple acid-catalysed aldol condensation (more specifically, a Claisen-Schmidt condensation), starting from cheap and readily available starting materials. Additionally, the product is quickly precipitated from the reaction medium and the pure product is easily isolated by filtration, avoiding the use of expensive purification techniques and equipment. Since no organic solvent is used either in the reaction or purification step, this synthetic methodology is attractive from a sustainable point of view and emphasis can be given to this aspect, as well as, to the therapeutic potential of the synthetized compounds.

The experiment was designed to be performed in a 100 mg scale of benzofuran-3(2H)-one (2) and using three different benzaldehydes (3a-c, Scheme SM 4.2.2.7.1).1 It encompasses a 15-45 minute reaction (depending on the benzaldehyde used), elementary techniques such as precipitation, filtration, and product characterization by melting point and NMR. The procedure is operationally simple and can be performed by any undergraduate student following organic chemistry courses where the concepts of nucleophilic addition to the carbonyl group and NMR spectroscopy are discussed.

Scheme SM 4.2.2.7.1

The reproducibility of the experiment was assessed by its repetitive execution namely by:

1) six undergraduate students attending the laboratory classes of the Organic Chemistry I course (first year of the Integrated Master of Pharmaceutical Sciences Degree, FFUL); and 2) three graduate students (i.e. one Master student and two PhD candidates) with different backgrounds (Biochemistry, Pharmaceutical Sciences, Chemistry) and different levels of experience in organic chemistry lab work.

Table SM 4.2.2.7.1 summarizes the conditions used and results obtained using the three different benzaldehydes, with yields in the range of: 45–59% (1a), 50–73% (1b) and 62–95% (1c).

Table SM 4.2.2.7.1 - Experiments conducted in a round bottom flask starting from benzofuran-3(2H)-one and different benzaldehydes at r.t.

Entry R Reaction time Product Isolated Yield Melting Point (min) (%) (ºC) 1a H 30 1a, yellow solid 45-51 110-113 2b H 30 1a, yellow solid 59 113-114 a 3 NMe2 40 1b, orange solid 50-62 174-176 c 4 NMe2 40 1b, orange solid 69-73 178-180 a 5 NO2 15 1c, yellow solid 80-85 N.D. d 6 NO2 15 1c, yellow solid 62 216-220 b 7 NO2 15 1c, yellow solid 95 222-225 a Experiments performed individually by two undergraduate students; b experiments performed by PhD candidate 1; c two runs of the same experiment performed by PhD candidate 2; d experiment performed by a M.Sc. student. 2

General Notes / Troubleshooting Information:

 The aspect of the reaction mixture varies depending on the benzaldehyde used. If R = H, the

mixture maintains the yellow colour during the course of the reaction; if R = NMe2 the colour

changes from yellow to dark orange; and if R = NO2 a precipitate is formed during the reaction (Figure SM 4.2.2.7.1).

 Since the reaction solvent is acetic acid it is recommended the following procedure to monitor the progression of the reaction. TLC preparation: at 15-min time intervals, use a Pasteur pipette to

transfer a drop of the reaction mixture to an eppendorf with 0.5 mL distilled H2O. Add approximately an equal volume of EtOAc and shake the solution. After phase separation spot a TLC plate with the

organic phase and the limiting starting material (Figure SM 4.2.2.7.2). Compound 1a: Rf = 0.69 2 (Hex/EtOAc, 7:3) ; compound 1b and 1c: Rf = 0.33 and 0.40, respectively (Hex/EtOAc, 8:2).

 In order to avoid loss, product precipitation and wash should be done with cold distilled water. Since the product is filtered-off from an aqueous solution it is advice to use a porcelain Büchner funnel and filter paper Whatman No.1.

Photos of the experiment:

Figure SM 4.2.2.7.1 – Illustrative photos of the reaction with different benzaldehydes (R = NMe2 and NO2) at t = 0 min.

Figure SM 4.2.2.7.2 – Illustrative photos of the reaction with different benzaldehydes (R = NMe2 and NO2) at completion and subsequent product isolation.

Figure SM 4.2.2.7.3 – TLC preparation for compound 1b and visualization at 254 nm, 366 nm, and without UV light, respectively.

1H and 13C NMR spectra:

The stereochemistry of the carbon-carbon double bond in aurones 1a-c was assigned as having the (Z)-configuration based on the 1H and 13C chemical shifts for the exocyclic double bond. The 1H NMR data obtained reveal that the chemical shifts for the β-hydrogen atoms for aurones 1a-c range from 6.80 to 6.95 ppm, which is consistent with reported values for both (Z)-aurones (ca. 6.70 ppm) and (E)-aurones (ca. 7.10 ppm), making the attribution ambiguous. However, the 13C NMR spectra were much more conclusive, revealing that the chemical shift for the exocyclic carbon range from 109 to 115 ppm, in line with the values reported for (Z)-aurones, the thermodynamically more stable isomer. In contrast, the 13C chemical shifts for the exocyclic carbon in (E)-aurones are usually observed at significantly higher frequency (ca.120-130 ppm).3 The 13C NMR spectrum for compound 1a is reported in reference 4.

1 Figure SM 4.2.2.7.4 – H spectrum of compound 1a (400 MHz, CDCl3) and peak assignment.

1 Figure SM 4.2.2.7.5 – H spectrum of compound 1b (300 MHz, CDCl3) and peak assignment.

13 Figure SM 4.2.2.7.6 – C NMR spectrum of compound 1b (300 MHz, CDCl3) and peak assignment.

1 Figure SM 4.2.2.7.7 – H NMR spectrum of compound 1c (300 MHz, CDCl3) and peak assignment.

13 Figure SM 4.2.2.7.8 – C NMR spectrum of compound 1c (300 MHz, CDCl3) and peak assignment.

1 The execution of the experiment at a larger scale or using other substituted benzaldehydes might require increased reaction times. 2 The Rf between the starting material and compound 1a is similar but they spot differently at 366 nm. 3 A. Pelter, R. S. Ward and T. I. Gray, J. Chem. Soc.-Perkin Trans. 1, 1976, 2475. 4 M. P. Carrasco, A. S. Newton, L. Goncalves, A. Gois, M. Machado, J. Gut, F. Nogueira, T. Hanscheid, R. C. Guedes, D. dos Santos, P. J. Rosenthal and R. Moreira, Eur. J. Med. Chem., 2014, 80, 523.

Synthesis of pyrazole heterocycles

Thomas A. Logothetis

Experiment notes

The TLC monitoring for both reactions involves a mini-work-up, i.e. the partitioning of an aliquot of the reaction mixture between water and diethyl ether, and utilising the organic layer for TLC analysis.

Reaction 1: Synthesis of 2-(4-chlorobenzylidene)quinuclidin-3-one

This reaction affords the aldol condensation product in a high yield only if the following parameters are taken into consideration by a skilled student; lower yields should be expected if the specifications are not meticulously adhered to. In our year 2 laboratory class this reaction is also used to discuss in depth aspects that normally do not feature high in lectures, i.e. surface area and particles sizes’ influ- ence on reaction kinetics; TLC monitoring and visualisation techniques and their physicochemical ba- sis; and different techniques of heating and temperature controlling.

The sodium hydroxide used in this reaction should have a reasonably large surface area, i.e. a small particle size: Pellets or flakes will not work well whereas small pearls or freshly ground NaOH work well. However, the weighing needs to be reasonably fast due to the hygroscopic nature of the base.

This reaction demands precise temperature control, best via internal measurement, although careful monitoring of the external heating works sufficiently well. Ideally, the temperature is kept in the given range (externally: 60-65 °C) to achieve a reasonably fast reaction, but not higher than 65 °C as this will lead to a diminished yield. This would be apparent in a darkening reaction mixture, due to side- Supplementary information for Comprehensive Organic Chemistry Experiments for the Laboratory Classroom © The Royal Society of Chemistry 2017 product formation. Reactions starting from 4-chloro- or 4-bromobenzaldehyde are particularly sensitive to excess temperatures, whereas 4-fluoro- and 4-methylbenzaldehyde are slightly more tolerant.

TLC monitoring indicates that the reaction takes circa 60 min of heating. The visualisation of the limit- ing reagent is difficult as it neither oxidises nor shows up under UV irradiation. While a stock solution of pure 3-quinuclidinone hydrochloride only produces barely visible whiteish spots at the baseline, the reaction mixture is alkaline and the 3-quinuclidinone travels up slightly from the baseline and can be oxidised with a permanganate dip and subsequent heating of the TLC plate, producing spots that are easier to notice.

The product easily crystallises and this is exploited in the isolation and purification step.

Reaction 2: Synthesis of 4-[5-(4-chlorophenyl)-1H-pyrazol-3-yl]piperidine

This reaction is straightforward and takes around 45-60 minutes of heating. Again the product forms a solid easily. The only precaution necessary is related to the handling of hydrazine hydrate, which should only be done in a well ventilated fume cupboard. It readily forms a hydrazone with acetone in an exothermic reaction, which is exploited in decontamination and cleaning procedures.

The TLC monitoring uses two eluents which differ strongly in polarity, which reflects the polarity differ- ences of the starting material and the product. Removing a sample from the reaction mixture is achieved by briefly lifting the reflux condenser and using a Pasteur pipette. Note: The reaction is not carried out under reflux conditions but some hydrazine hydrate will collect at the bottom of the reflux condenser. This should be rinsed with acetone during clean-up.

Optional features

The described sequence of two reactions can be complemented by two more steps, namely the Boc- protection of the secondary amine in the piperidine ring followed by a bromination of the pyrazole ring Supplementary information for Comprehensive Organic Chemistry Experiments for the Laboratory Classroom © The Royal Society of Chemistry 2017 as described in the chapter “Selective Boc-protection and bromination of pyrazoles” to afford a versa- tile scaffold.

Another feature of the synthesis of a pyrazole heterocycle is that the starting aldehyde can be easily modified to afford a potentially large library of products. Although this script only describes the synthe- sis starting from 4-chlorobenzaldehyde, three more para-substituted benzaldehydes have been tested and used in practicals (see tabulation of experimental results below) employing the same conditions.

Theoretically, ortho and meta-substituted as well as disubstituted benzaldehydes should be possible starting materials, although these options have not been tested.

Experimental results

The following tables summarise the results achieved with second year undergraduate students aver- aged over several years. Starting materials are para-substituted benzaldehydes, i.e. 4-chlorobenzal- dehyde, 4-bromobenzaldehyde, 4-fluorobenzaldehyde, and 4-methylbenzaldehyde (4-tolylaldehyde).

The colour of the crystalline products from reaction 1 is yellow (intensive yellow for the chloro and bromo derivatives), the powdery solids produced in reaction 2 are all pale yellow.

Typical melting points and yields for products of reaction 1 (aldol condensation):

Residue mp [°C] Recrystallisation solvent Yield [%]

R = Cl 112-114 ethanol 80-85

R = Br 119-121 ethanol 80-85

R = F 118-120 ethanol 75-80

R = Me 110-112 ethanol 70-75

Table SM 4.2.2.8.1: Data for 2‐(4‐chlorobenzylidene)quinuclidin‐3‐one

Typical melting points and yields for products of reaction 2 (pyrazole formation):

Residue mp [°C] Recrystallisation solvent(s) Yield [%]

R = Cl 174-175 ethanol 65-70

R = Br 184-186 ethanol 65-70

R = F 178-180 ethanol 60-65

R = Me 128-130 ethanol 60-65

Table SM 4.2.2.8.2: Data for 4‐[5‐(4‐chlorophenyl)‐1H‐pyrazol‐3‐yl]piperidine

The ethanol recrystallisation also works with a mixed solvent system using ethanol and water.

Photos of the products

Figure SM 4.2.2.8.1: Aldol condensation product (left) and pyrazole heterocycle (right)

Typical TLC plates for both reactions

Reaction 1 (aldol condensation):

The limiting reagent, 3-quinuclidinone hydrochloride (Q), is not visible under UV light. When using a Supplementary information for Comprehensive Organic Chemistry Experiments for the Laboratory Classroom © The Royal Society of Chemistry 2017 coloured stain like potassium permanganate it produces a faint white spot due to the lack of penetra- tion of the reagent into the spotted material. The aldehyde (SM) is clearly visible under UV irradiation and the spot can be oxidised. Sometimes the aldehyde is contaminated with a little of its correspond- ing acid, depending on quality and age of this starting material. If it is spotted from a concentrated so- lution, the acid spot becomes visible under UV light only. Depending on the concentration of the reac- tion mixture (R) in the solution from which it is spotted, the now deprotonated 3-quinuclidinone and

(oxidation only) and an intermediate can be observed.

Figure SM 4.2.2.8.2: Exemplary observations on TLC plates when monitoring reaction 1 (left) and reaction 2 (middle & right)

Reaction 2 (pyrazole formation):

Due to the strongly different polarity of the starting material (SM) and the product (P) it is advised to use two different eluents to follow the reaction (R). In the more polar eluent ethylene glycol sometimes becomes visible under oxidative staining.

Mechanistic details

The two mechanisms are briefly summarised below. The base used in the aldol condensation would first abstract the ammonium proton followed by a proton from the CH2 group next to the carbonyl; the bridgehead proton next to the carbonyl cannot be abstracted as the lone pair would occupy a sp3 or- bital lobe that cannot overlap with the carbonyl -system/orbital set (Bredt’s rule). The pKa values would be similar to ammonium, acetone and alkane (pKa ~9, ~20, ~40). Supplementary information for Comprehensive Organic Chemistry Experiments for the Laboratory Classroom © The Royal Society of Chemistry 2017

Step 1 is an aldol formation with subsequent elimination of water, in total a typical aldol condensation:

Scheme SM 4.2.2.8.1: Steps of reaction 1

The decontamination of hydrazine exploits its fast and exothermic reaction with acetone to form the corresponding hydrazone, which is less toxic and not a reported carcinogen . Discussion of this topic during a lab class can lead to the second mechanism, which consists of a Michael addition followed by an intramolecular hydrazone formation, and finally an aromatisation with opening of the bicyclic sys- tem to afford a piperidine and pyrazole ring.

Scheme SM 4.2.2.8.2: Steps of reaction 2

The spectra depicted here illustrate the chloro-derivatives only. Only spectra asked to obtain are ex- hibited here. Spectra not showing here have been found to be less useful in the context of this practi- cal. NMR spectra stemming from the fluoro-derivatives would exhibit additional coupling between 1H and 19F nuclei as well as between 13C and 19F nuclei. Mass spectra of the bromo-derivatives would present – like the described chlorine containing products – a typical isotope pattern. In infrared spectra the (C-X) can be seen depending on the resolution and spectral width of the spectrometer in the re- gion 1030 – 1100 cm-1. The (C-H) are visible between 2850 – 3000 cm-1 and the (C=O) absorbs around 1710 cm-1. Supplementary information for Comprehensive Organic Chemistry Experiments for the Laboratory Classroom © The Royal Society of Chemistry 2017

The mass spectrum below shows two peaks found for [M+H]+ with the 3:1 isotope pattern with 2 amu difference typical for Cl1 as evidenced by the detected mass (column 1) and relative abundance in %

Figure SM 4.2.2.8.3: ES+ (MeCN) of 2‐(4‐chlorobenzylidene)quinuclidin‐3‐one

Figure SM 4.2.2.8.4: IR (ATR) of 2‐(4‐chlorobenzylidene)quinuclidin‐3‐one Supplementary information for Comprehensive Organic Chemistry Experiments for the Laboratory Classroom © The Royal Society of Chemistry 2017

13 Figure SM 4.2.2.8.5: C NMR (100 MHz, CDCl3) of 2‐(4‐chlorobenzylidene)quinuclidin‐3‐one

13 Figure SM 4.2.2.8.6: C NMR (100 MHz, CDCl3) of 4‐[5‐(4‐chlorophenyl)‐1H‐pyrazol‐3‐yl]piperidine

1 H NMR (CDCl3, 400 MHz):  [ppm] = 7.67 (d, J = 8.3 Hz, 2H), 7.36 (d, J = 8.3 Hz, 2H), 6.35 (s, 1H), 3.19 (dt, J = 12.2, 3.1 Hz, 2H), 2.81 (tt, J = 12.1, 3.9 Hz, 1H), 2.74 (td, J = 12.2, 2.5 Hz, 2H), 1.97 (d, br, J = 12.5 Hz, 2H), 1.71 (qd, J = 12.7, 3.6 Hz, 2H).

1 Figure SM 4.2.2.8.7: H NMR (400 MHz, CDCl3) of 4‐[5‐(4‐chlorophenyl)‐1H‐pyrazol‐3‐yl]piperidine

The conformational analysis of the piperidine ring of the final product is based on analysing coupling

2,3 patterns and constants ( JHH) and relating the information to the torsion angle via the Karplus curve.

Data are listed together with the spectrum and from the large triplet coupling constant in the tt it can be derived that the CH in the piperidine ring has an equatorial substituent (the pyrazole residue), i.e that the hydrogen atom occupies the axial position. Supplementary information for Comprehensive Organic Chemistry Experiments for the Laboratory Classroom © The Royal Society of Chemistry 2017

1 Figure SM 4.2.2.8.8: Expansion of H NMR (400 MHz, CDCl3) of 4‐[5‐(4‐chlorophenyl)‐1H‐pyrazol‐3‐yl]piperidine Supplementary information for Comprehensive Organic Chemistry Experiments for the Laboratory Classroom © The Royal Society of Chemistry 2017

A green approach to 3-carbonylchromones Ana Bornadiego, Jesús Díaz, Ana G. Neo, Carlos F. Marcos*

Notes for the instructor

The experiment was designed for an intermediate level organic chemistry course. However, as the experimental protocols are simple and reliable, it could also be suitable for introductory courses. Protocol was comfortably performed in a three-hour session with groups of 8-10 students organized in teams of 2. However, as microwave irradiation times are very short, it can be easily adapted to larger groups only slightly increasing the length of the lab session, even if only one microwave reactor is available. Both reactions give reasonably pure products and can be used with any further purification processes. Student’s typical yields range from 70-90% (average 80%) for the first step, and 15-60% (average 32%) for the second step.

The experiment was performed as part of the teaching laboratory sessions corresponding to a course of Organic Chemistry included in the first year of the BSc in Biochemistry. The students can easily perform the experimental procedure, though they have some difficulties understanding the mechanisms involved in the reactions. This problem can be satisfactorily solved scheduling a pre-lab seminar and a pre-lab assignment in which emphasis on both mechanistic and environmental aspects of the reactions was made.

Liquid reagents were measured using micropipettes with disposable tips.

Anhydrous pyridine should be used to guarantee consistent high yields of the chromone . Optimally, pyridine can be distilled immediately before the addition. Recently distilled pyridine stored under nitrogen over KOH is also a good option.

Liquid-liquid extraction of the crude of the chromone synthesis can be performed using a separation funnel, as explained in the experiment description, or, alternatively, in the same reaction vial with the help of a Pasteur pipette. In the latter case, 4-6 washings with CuSO4 solution (c.a. 2 mL) are necessary. Supplementary information for Comprehensive Organic Chemistry Experiments for the Laboratory Classroom © The Royal Society of Chemistry 2017

Figure SM 4.2.2.9.1. Different phases of the experiment. (A) Reaction mixture of the enaminone synthesis after microwave irradiation. (B) Filtration of the enaminone after crystallisation in toluene . (C) Reaction mixture of the chromone synthesis before microwave irradiation.

Figure SM 4.2.2.9.2. Synthesis of the chromone: Liquid-liquid partition of the reaction crude with copper sulphate (A) and brine (B).

Mechanism of the reactions Supplementary information for Comprehensive Organic Chemistry Experiments for the Laboratory Classroom © The Royal Society of Chemistry 2017

Figure SM 4.2.2.9.3. Mechanism for the synthesis of the enaminone intermediate.

Figure SM 4.2.2.9.4. Mechanism for the synthesis of the chromone.

Spectroscopic data and spectra

(E)-3-(dimethylamino)-1-(2-hydroxyphenyl)prop-2-en-1-one.

Orange solid; mp: 133-137 ºC.

IR (cm-1): 2920, 1628, 1548.

1 H NMR (400 MHz, CDCl3) δ 13.94 (s, 1H, OH), 7.89 (d, J = 12.1 Hz, 1H), 7.70 (d, J = 8.0 Hz, 1H), 7.35 (t, J = 8.5 Hz, 1H), 6.94 (d, J = 8.3 Hz, 1H), 6.81 (t, J = 8.1 Hz, 1H), 5.79 (d, J = 12,1 Hz, 1H), 3.18 (s, 3H), 2.96 (s, 3H) ppm.

13 C NMR (126 MHz, CDCl3) δ 191.7 (C), 163.2 (C), 154.9 (CH), 134.1 (CH), 128.4 (CH), 120.5 (C),

118.4 (CH), 118.2 (CH), 90.3 (CH), 45.6 (CH3), 37.6 (CH3) ppm.

MS (CI) m/z (%) 220 (M+29, 79), 193 (M+, 92), 191 (100), 190 (35), 147 (90), 98 (100). Supplementary information for Comprehensive Organic Chemistry Experiments for the Laboratory Classroom © The Royal Society of Chemistry 2017

1 Figure SM 4.2.2.9.3. H NMR spectrum (500 MHz, CDCl3) of the enaminone.

13 Figure SM 4.2.2.9.4. C NMR spectrum (126 MHz, CDCl3) of the enaminone. Supplementary information for Comprehensive Organic Chemistry Experiments for the Laboratory Classroom © The Royal Society of Chemistry 2017

Methyl 2-oxo-2-(4-oxo-4H-chromen-3-yl)acetate

Bright brown solid; mp: 131-135 ºC.

IR (cm-1) 3063, 2955, 1731, 1694, 1649.

1 H NMR (500 MHz, CDCl3) δ 8.66 (s, 1H), 8.25 (d, J = 7.9 Hz, 1H), 7.77 (t, J = 8.5 Hz, 1H), 7.56 (d, J = 8.4 Hz, 1H), 7.52 (t, J = 7.5, 1H), 4.00 (s, 3H), ppm.

13 C NMR (126 MHz, CDCl3) δ 184.7 (C), 174.8 (C), 164.4 (C), 162.4 (CH), 156.3 (C), 135.2 (CH),

127.1 (CH) 126.5 (CH), 124.9 (C), 120.1 (C), 118.8 (CH), 53.2 (CH3) ppm.

MS (CI) m/z (%) 261 (M+29, 22), 234 (M+2, 85), 233 (M+1, 98), 232 (M+, 99), 217 (69), 121 (22).

1 Figure SM 4.2.2.9.5. H NMR spectrum (500 MHz, CDCl3) of the chromone. Supplementary information for Comprehensive Organic Chemistry Experiments for the Laboratory Classroom © The Royal Society of Chemistry 2017

13 Figure SM 4.2.2.9.6. C NMR spectrum (126 MHz, CDCl3) of the chromone. Supplementary information for Comprehensive Organic Chemistry Experiments for the Laboratory Classroom © The Royal Society of Chemistry 2017

Synthesis of indigo and dyeing process

The main purpose of this experiment is the preparation of indigo, a product that students are in contact daily. It also aims to draw the student’s attention to enolate anions, Aldol-type reactions and redox chemistry involved on dyeing process. This experiment was tested by students of intermediate organic chemistry because the reaction mechanism of indigo formation is complex1-4 (Baeyer-Drewson reaction). On the other hand, this demonstration is also appropriate for high school level students, due to its simple and fast procedure. When acetone, sodium hydroxide and 2-nitrobenzaldehyde are mixed, the reaction begins with an enolate ion attack to at the carbonyl group of 2-nitrobenzaldehyde to form a hydroxy ketone. The reaction continues in a series of condensations, tautomerizations and dehydratations to give indigo within a matter of seconds. Indigo is not soluble in water (vat dye), so the students have the opportunity of learn about dyeing processes involving oxidation and reduction reactions. An alternative method in two steps, but not tested in the classroom, can be performed starting from 2-nitrobenzaldehyde with addition of nitromethane and base catalyst, followed by reduction with sodium hydrosulfite5.

Additional notes on the preparation of indigo

More precipitated product is obtained if the filtration is made in the next day of the lab session. Student yields are usually 65-75%. The melting point found in the literature is 390-392ºC and sublimes without decomposition to form copper-red prisms6. This mark is too high to be recorded at classroom. The

NMR spectra performed in acetone and DMSO-d6, did not show any signals due to insolubility of this dye. Supplementary information for Comprehensive Organic Chemistry Experiments for the Laboratory Classroom © The Royal Society of Chemistry 2017

Figure SM 4.2.2.10.1 – Indigo after precipitation and vacuum filtration

Additional notes on dying of cotton7

If substantial amounts of blue-purple solid remain, the solution should be decanted before cotton immersion.

Figure SM 4.2.2.10.2 – Piece of cotton after dyeing process

IR spectra:

Students easily identify N-H stretching band between 3250 and 3400 cm-1 and the strong absorption C=O stretching band (1600-1640 cm-1). IR spectrum of indigo is also available in SDBS (nº 21446)8.

Figure SM 4.2.2.10.3 - IR (KBr) of indigo

1 J. Jack Li, Name reactions: A collection of detailed reaction mechanisms, Springer, 2nd ed., 2003, 14. 2 A. Mckillop, Advanced Problems in Organic reaction mechanisms, Pergamon, 1997,127. 3 V. Bianda, J. A. Constenla, R. Haubrichs, P. L. Zaffalon, Chemistry: Bulgarian Journal of Science Education, 2013, 22 (1), 52. 4 Kirk-Othmer, Encyclopedia of Chemical Technology, Wiley, 2nd ed., 1970, 11, 562-575. 5 L. F. Tietze, Th. Eicher, Reactions and Syntheses in the organic chemistry laboratory, University Science Books, Oxford University Press, 1989, 366-367. 6 Kirk-Othmer, Encyclopedia of Chemical Technology, Wiley, 2nd ed. 1970, 11, 565. 7 D. W. Boykin, J. Chem. Ed., 1998, 75 (6), 769. 8 http://sdbs.db.aist.go.jp/sdbs/cgi-bin/direct_frame_top.cgi

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23.8: The Aldol Reaction and Condensation of Ketones and Aldehydes

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A useful carbon-carbon bond-forming reaction known as the Aldol Reaction is yet another example of electrophilic substitution at the alpha carbon in enolate anions. The fundamental transformation in this reaction is a dimerization of an aldehyde (or ketone) to a beta-hydroxy aldehyde (or ketone) by alpha C–H addition of one reactant molecule to the carbonyl group of a second reactant molecule. Due to the carbanion like nature of enolates they can add to carbonyls in a similar manner as Grignard reagents. For this reaction to occur at least one of the reactants must have alpha hydrogens.

The aldol reactions for acetaldehyde and acetone are shown as examples.

Example: Aldol Reactions

Aldol Reaction Mechanism

Step 1: Enolate formation

Step 2: Nucleophilic reaction by the enolate

Step 3: Protonation

Aldol Condensation: the dehydration of aldol products to synthesize α, β unsaturated carbonyls (enones)

The products of aldol reactions often undergo a subsequent elimination of water, made up of an alpha-hydrogen and the beta-hydroxyl group. The product of this \(\beta\) -elimination reaction is an α,β-unsaturated aldehyde or ketone. Base-catalyzed elimination occurs with heating. The additional stability provided by the conjugated carbonyl system of the product makes some aldol reactions thermodynamically driven and mixtures of stereoisomers ( E & Z ) are obtained from some reactions. Reactions in which a larger molecule is formed from smaller components, with the elimination of a very small by-product such as water, are termed Condensations . Hence, the following examples are properly referred to as aldol condensations . Overall the general reaction involves a dehydration of an aldol product to form an alkene:

Going from reactants to products simply

Example: Aldol Condensation from an Aldol Reaction Product

Aldol Condensation Base Catalyzed Mechanism

1) Form enolate

2) Form enone

Aldol Condensation Acid Catalyzed Mechanism

Under acidic conditions an enol is formed and the hydroxy group is protonated. Water is expelled by either and E1 or E2 reaction.

When performing both reactions together always consider the aldol product first then convert to the enone. Note! The double bond always forms in conjugation with the carbonyl.

Example: Aldol Condensation Directly from the Ketones or Aldehydes

Aldol Reactions in Multiple Step Synthesis

Aldol reactions are excellent methods for the synthesis of many enones or beta hydroxy carbonyls. Because of this, being able to predict when an aldol reaction might be used in a synthesis in an important skill. This accomplished by mentally breaking apart the target molecule and then considering what the starting materials might be.

Fragments which are easily made by an aldol reaction

Steps to 'reverse' the aldol reaction (from the final aldol product towards identifying the starting compounds).

1) From an enone break the double bond and form two single bonds. Place and OH on the bond furthest from the carbonyl and an H on the bond closest to the carbonyl.

2) From the aldol product break the C-C bond between the alpha carbon and the carbon attached to the OH. Then turn the OH into a carbonyl and add an hydrogen to the other carbon.

Example: Determining the Reactant when given the Aldol Condensation Product

What reactant must be used to make the following molecule using an aldol condensation?

Mixed Aldol Reactions and Condensations

The previous examples of aldol reactions and condensations used a common reactant as both the enolic donor and the electrophilic acceptor. The product in such cases is always a dimer of the reactant carbonyl compound. Aldol condensations between different carbonyl reactants are called crossed or mixed reactions, and under certain conditions such crossed aldol condensations can be effective.

Example: Mixed Aldol Reaction (One Product)

The success of these mixed aldol reactions is due to two factors. First, aldehydes are more reactive acceptor electrophiles than ketones, and formaldehyde is more reactive than other aldehydes. Second, aldehydes lacking alpha-hydrogens can only function as acceptor reactants, and this reduces the number of possible products by half. Mixed aldols in which both reactants can serve as donors and acceptors generally give complex mixtures of both dimeric (homo) aldols and crossed aldols. Because of this most mixed aldol reactions are usually not performed unless one reactant has no alpha hydrogens.

The following abbreviated formulas illustrate the possible products in such a case, red letters representing the acceptor component and blue the donor. If all the reactions occurred at the same rate, equal quantities of the four products would be obtained. Separation and purification of the components of such a mixture would be difficult.

Example: Products of a Mixed Aldol Reaction

The aldol condensation of ketones with aryl aldehydes to form α,β-unsaturated derivatives is called the Claisen-Schmidt reaction.

Example: Claisen-Schmidt Reaction

Intramolecular aldol reaction

Molecules which contain two carbonyl functionalities have the possibility of forming a ring through an intramolecular aldol reaction. In most cases two sets of \(\alpha\) hydrogens need to be considered. As with most ring forming reaction five and six membered rings are preferred (less ring strain).

As with other aldol reaction the addition of heat causes an aldol condensation to occur.

12. Draw the bond-line structures for the products of the reactions below. Note: One of the reactions is a poorly designed aldol condensation producing four different products.

Contributors and Attributions

Dr. Dietmar Kennepohl FCIC (Professor of Chemistry, Athabasca University )

Prof. Steven Farmer ( Sonoma State University )

William Reusch, Professor Emeritus ( Michigan State U. ), Virtual Textbook of Organic Chemistry

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Aldol condensation: green perspectives

• Published: 15 January 2022
• Volume 19 , pages 2171–2190, ( 2022 )

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• Savita Bargujar 1 &
• Sonia Ratnani   ORCID: orcid.org/0000-0002-5725-3354 1  

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Aldol reaction is one of the most established reactions employed for the construction of new C–C bond with application in chemical synthesis and biochemical domains. Conventionally, aldol reaction involved the use of basic catalyst in hydroalcoholic medium or use of strong bases in toxic and flammable organic solvents. Apart from long reaction times, such conditions result in mixtures of ketols and α , β unsaturated ketones and side products from competitive side reactions along with aldol products. In recent years, considerable efforts and interest have been put for the development of catalytic methods in green context for this transformation. Though numerous variants of homogenous and heterogeneous catalysts have been examined for better results, there are environment concerns associated with catalytic aldol reaction. Increasing ecological awareness and environment-friendly reactions has prompted chemists to involve improved strategies such as micellar medium, microwave irradiation and ultrasonics as alternative routes for aldol condensation. This review summarizes and updates the research on these routes with a green perspective. Reactions performed under these methodologies with or without use of heterogeneous catalysis have been highlighted with reference to the principles of green chemistry. The protocols have been primarily complied from pedagogical journals with depiction of green component wherever possible.

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Bargujar, S., Ratnani, S. Aldol condensation: green perspectives. J IRAN CHEM SOC 19 , 2171–2190 (2022). https://doi.org/10.1007/s13738-021-02464-w

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