Chloroform and deuterated chloroform are cancer suspect agents and mutagens.

Transcription

1 JCE # LAB DCUMETATI Chemicals and CAS egistry umbers Acetone [ ] Isobutyraldehyde [ ] L-proline [ ] D-proline [ ] Diethyl ether [ ] Magnesium sulfate [ ] Chloroform [ ] Deuterated chloroform [ ] Benzaldehyde [ ] Acetophenone [ ] Safety and Hazards Acetone is flammable. Isobutyraldehyde is flammable and an irritant. Diethyl ether is flammable. Chloroform and deuterated chloroform are cancer suspect agents and mutagens. Benzaldehyde is an irritant. Acetophenone is an irritant. 1

2 Instructor otes All reagents are used as received from the manufacturer. When L-proline is the catalyst, the enantiomer is the major product of the condensation. A mixture consisting of a 96% excess of the enantiomer has a specific rotation of (in CHCl 3, c = 0.6) according to reference 2 of the Student Handout. Isobutyraldehyde is one of the few branched aliphatic aldehydes that affords an appreciable yield of crossed-condensation product. If thin layer chromatography is performed on the product, iodine staining is necessary to visualize the compound. A 3:1 hexanes/ethyl acetate mixture is an appropriate starting eluent. The product yields that students obtain typically range from 50-65%. Typical specific rotations that students obtain are (in CHCl 3, c 1.5). A typical enantiomeric excess is approximately 70% as determined by polarimetry. This ee is significantly lower than the 96% reported in reference 2 of the Student Handout. However, the reaction conditions in the literature procedure are different than those employed here, and the literature ee was determined by chiral HPLC. For the isobutyraldehyde adduct, 1 H M (60 MHz, CDCl 3 ) δ: 3.83 (q, 1 H, J = 5.9 Hz), 2.56 (d, 2 H, J = 6.1 Hz), 2.19 (s, 3 H), 1.63 (septet, 1 H, J = 6.3 Hz), 0.92 (d, 6 H, J = 6.4 Hz). 13 C M (15 MHz, CDCl 3 ) δ: 208.5, 70.7, 45.9, 31.8, 28.7, 16.7, The H hydrogen is not apparent in the spectrum. According to a research article (Kandasamy, S.; otz, W.; Bui, T.; Barbas III, C.F. J. Am. Chem. Soc. 2001, 123, ), the presence of water in the reaction mixture leads to a loss of enantioselectivity and decreases the rate of product formation. The 2

3 rationale for the loss of enantioselectivity is that the water disrupts hydrogen bonding in the transition state. In our experience, water prevents the condensation reaction from occurring. When benzaldehyde is used in place of isobutyraldehyde, the product mixture contains not only condensation product but also unreacted benzaldehyde and dehydrated adduct. The enantiomeric excess is also lower. If thin layer chromatography is performed on this product mixture, short-wave ultraviolet light is sufficient for visualization. A 3:1 hexanes/ethyl acetate mixture is an appropriate starting eluent. If students wish to substitute a different ketone, such as acetophenone, for acetone in the procedure, they should be warned to consider how easy removal of the excess from the crude product will be. elatively dilute solutions (<2 g/100 ml of chloroform) give us optimal polarimetry data. 3

4 Student Handout ALDL CDESATI eferences: 1. Doxsee, K.; Hutchison, J.E. Green rganic Chemistry: Strategies, Tools, and Laboratory Experiments Pacific Grove, CA: Brooks/Cole, List, B.; Lerner,.A.; Barbas III, C.F. J. Am. Chem. Soc. 2000, 122, Gröger, H.; Wilken, J. Angew. Chem. Int. Ed. 2001, 40, Cauble, D.F.; Krische, M.J. Chemtracts rg. Chem. 2002, 15, othenberg, G.; Downie, A.P.; aston, C.L.; Scott, J.L. J. Am. Chem. Soc. 2001, 123, Compounds that contain the carbonyl group display a surprisingly diverse range of reaction chemistry, and the discovery and study of such reactions form the heart of much modern synthetic organic chemistry. ften when a nucleophile forms a bond with a carbonyl carbon, the mechanism proceeds in such a way that a molecule of water is released. Because of this formation of water, such reactions were first known as condensation reactions. As research provided greater insight into mechanistic pathways, the condensation label was eventually applied to virtually any nucleophilic acyl addition or substitution in which two compounds combine or are "condensed" into a single molecule. These reactions are valuable because they result in more elaborate carbon skeletons, and they frequently result in the generation of a new chirality center. 4

5 When a new chirality center forms during the course of a reaction, the resulting product might consist of a 50%-50% (or racemic) mixture of enantiomers, or the amount of one enantiomer might exceed the amount of the other enantiomer. In the latter situation, it is useful to measure this enantiomeric excess (ee). If the specific rotation of one of the enantiomers is known, polarimetry can be used to determine ee. [You will use this technique for determining ee in this project.] The technique can be unreliable, however, because rotation values vary with solvent and temperature, and uncertainty in the concentration contributes to uncertainty in the specific rotation value. The most reliable data correlate the optical rotation with an independent analytical determination of ee. Sometimes certain hydrogen atoms in the enantiomers (e.g., a hydrogen bonded at the chirality center) are magnetically and chemically inequivalent and can be distinguished in the 1 H M spectrum. Integration of these signals can also allow determination of ee. Integration, however, can carry noticeable experimental error because different relaxation times can lead to different peak intensities. A pulse delay is necessary to account for any differences in relaxation time. Another way to find ee is to separate the enantiomers from each other and measure the individual amounts. Chemists used to routinely accomplish this task by treating the enantiomeric mixture with a chiral reagent, thereby forming a diastereomeric mixture. Because diastereomers have different chemical and physical properties, these mixtures could be resolved by one of several means. For example, one diastereomer might crystallize while the other remained in solution. nce separated, the diastereomers were treated with a reagent to remove the added group so the original enantiomers could be recovered. A second method for separating enantiomers is to treat the mixture with an achiral reagent followed by an enzyme that selectively 5

6 catalyzes the reverse reaction to re-form one of the enantiomers. nce separated, the compound that does not react with the enzyme can be treated to re-form the other enantiomer. The preferred technique for determining ee is chiral HPLC (refer to the background information on chromatography). This method permits the separation of the enantiomers without any prior chemical treatment of the mixture. Chiral HPLC requires a column packed with chiral material. The two enantiomers interact differently with this chiral stationary phase and, therefore, have different retention times. Integration of the resolved peaks allows the determination of ee. ne important condensation reaction is the aldol condensation, which usually involves the reaction between two molecules of aldehydes or ketones in the presence of base to form β- hydroxycarbonyl compounds (Scheme 1). (The name aldol is a contraction of aldehyde and alcohol, which are the two functional groups in the simplest condensation product.) These compounds are readily dehydrated, sometimes under the initial reaction conditions, to form α,βunsaturated carbonyl compounds. The reaction requires that at least one of the starting aldehydes or ketones have α-hydrogens. Crossed aldol reactions involve two different aldehydes or ketones. The Claisen-Schmidt modification of the aldol condensation involves the reaction between a ketone with α-hydrogens and an aldehyde. C + CH C CH 2 C CH C CH 2 H + C CH H C CH 2 Scheme 1. Mechanism of Base-Promoted Aldol Condensation The catalytic asymmetric aldol condensation is a fundamental C-C bond forming reaction in biology, too. In contrast to laboratory methods that require a base to form an enolate, biological systems use enzymes, class I and class II aldolases, to catalyze the direct aldolization 6

7 of two unmodified carbonyl compounds. Whereas class II aldolases use a zinc co-factor, class I aldolases take advantage of an enamine mechanism that involves the ε-amino group of an activesite lysine residue (Scheme 2). This amino group forms an imine with the ketone of dihydroxyacetone-3-phosphate (DHAP). The imine tautomerizes to an enamine, which adds to the aldehyde of glyceraldehyde-3-phosphate (G3P) as the imine re-forms. Finally, the imine undergoes hydrolysis to liberate the β-hydroxy adduct. H P P 3 H H Class I aldolase 2-3 P DHAP H Lys(aldolase) Lys(aldolase) 3 P H 2-2- H H P 3 2- Lys(aldolase) 3 P H (imine) tautomerization H G3P P P H (enamine) H Scheme 2. Catalytic cycle of direct aldol condensation with a class I aldolase If simple organic molecules can act like these enzymes, such processes would allow the cost-effective manufacture of chiral building blocks on an industrial scale. These organic catalysts should not only function like an enzyme, but should also show easy availability in 7

8 either enantiomeric form at comparably low prices. The molecules should have low molecular weight, be easily separated from the product, and be easily recovered after work-up without racemization. ecently, chemists have found that L-proline, a naturally-occurring amino acid, can function as a small-molecule asymmetric class I aldolase mimic. The proposed enamine mechanism of the proline-catalyzed aldol condensation involves an iminium ion intermediate rather than an imine (Scheme 3). H H C 2 H H C 2 H C 2 H tautomerization H 2 C 2 H C 2 H H H H H Scheme 3. Proposed catalytic cycle of direct aldol condensation with L-proline 8

9 During the addition of the enamine to the aldehyde, the rigid pyrrolidine ring of the enamine and an intermolecular hydrogen bond between the carboxylate group of the enamine and the aldehyde oxygen both contribute to the enantioselection. If the amino group of the proline is nucleophilic enough to form an enamine, why is it not basic enough to simply remove an α-proton from the acetone and catalyze the aldol condensation that way? ne reason is that proline also contains a carboxylic acid. Protons of carboxylic acids are more acidic than α-protons of ketones. If the amino group were going to act as a base, it would deprotonate the carboxylic acid. The carboxylic acid also activates the ketone carbonyl, so nucleophilic addition is more favorable. In general, an enamine mechanism is more likely when the catalyst contains two or more functional groups that can act in concert (e.g., one as an acid/electrophile, the other as a base/nucleophile). Because isobutyraldehyde has an α-hydrogen, one might expect that it could combine with proline to form an enamine that would condense with another molecule of isobutyraldehyde or with acetone, but no such reaction products are observed. ne explanation is that the proline is statistically more likely to encounter acetone molecules because the acetone is used in large excess. Another explanation is that even if an enamine were to form from isobutyraldehyde, the two methyl groups would sterically hinder the addition of the enamine to another carbonyl compound. The product of the aldol condensation between acetone and isobutyraldehyde also contains α-hydrogens, yet it does not undergo a second condensation. If the product were to form an enamine with proline, steric effects would make that enamine less reactive than the enamine from acetone. Therefore, any remaining isobutyraldehyde is more likely to react with the acetone-derived enamine. 9

10 This experiment illustrates several of the principles of green chemistry. The reaction proceeds at ambient temperature and pressure and requires neither an inert atmosphere nor purification and drying of the reagents. The reagents require no modification. The promoter is used in catalytic rather than stoichiometric amount. Moreover, the catalyst is theoretically obtainable from renewable sources, biodegradable, and readily available in both enantiomeric forms. The water-solubility of the catalyst allows for easy removal from the reaction mixture. Furthermore, when only one enantiomeric product is desired, an enantioselective process generally offers a higher atom economy. Condensation reactions, by following an addition mechanism, also have an inherently higher atom economy. However, this experiment also illustrates some of the trade-offs involved in green chemistry. The most glaring example is that the acetone is used in large excess so as to function as both reagent and solvent and to suppress side reactions, such as dehydration of the adduct or self-condensation of the isobutyraldehyde. This large excess leads to poor overall atom economy and effective mass yield. In addition, the acetone is volatile and flammable. Another volatile and flammable organic solvent, ether, is used for extracting the product from the reaction mixture. The separated aqueous layer contains not only the proline catalyst but also acl and possibly some unreacted acetone and isobutyraldehyde. Finally, the reaction proceeds slowly. Product does not begin to appear for about 24 hours, and at least 48 hours are necessary to obtain an appreciable yield. For a process to be recognized as green, it ultimately needs to be viable in an industrial setting. The time limitation alone calls into question the viability of this process. Procedure Because isobutyraldehyde is an irritant and some aldol products can trigger allergic reactions in individuals, you are strongly encouraged to wear gloves when performing these 10

11 experiments. Acetone, isobutyraldehyde, and diethyl ether are flammable, so open flames should be avoided. Conduct operations in a fume hood whenever possible. Aldol Procedure To a 25-mL one-neck round-bottom flask equipped with a magnetic stir bar and a stopper or rubber septum, add 1.0 ml (0.79 g, mol) of isobutyraldehyde, 14 ml (11 g, 0.19 mol) of acetone, and 0.23 g ( mol) of L-proline. Magnetically stir the reaction mixture at room temperature in a fume hood for one week. Dilute the mixture with 50 ml of saturated aqueous acl solution and transfer to a separatory funnel. Extract the mixture with diethyl ether (1 x 15 ml). Dry the separated organic layer over MgS 4, remove the drying agent by gravity filtration into a clean, dry, weighed 50-mL one-neck round-bottom flask, and remove the solvent by rotary evaporation. Analyze your compound by I, 1 H M, 13 C M, and polarimetry. If your product is a solid, measure a melting point. Assess the purity of your product. If necessary, purify the product. nce you have isolated pure product, calculate your percent yield, atom economy, efficiency, effective mass yield, and cost for the experiment. Estimation of Enantiomeric Excess A literature reference for the specific rotation of the enantiopure aldol adduct is not available. n the other hand, there are two literature reports of specific rotations for mixtures of which the enantiomer is the major component: Specific otation Enantiomeric Excess eference % List, B.; Lerner,.A.; Barbas III, C.F. J. Am. Chem. Soc. 2000, 122, % amachandran, P.V.; Xu, W.-c.; Brown, H.C. Tetrahedron Lett. 1996, 37,

12 We also know that a racemic mixture (ee = 0%) will show no rotation. Use all three data points to generate a graph of ee vs. specific rotation with a linear fit. Use the equation of the straight line to solve for the ee that corresponds with the average specific rotation of your product mixture. ound your result to the nearest whole percent. Additional Work Perform at least one of the following tasks: 1. epeat the procedure using a smaller amount of catalyst and analyze the outcome. 2. epeat the procedure using a smaller amount of acetone and analyze the outcome. 3. epeat the procedure using D-proline as the catalyst and analyze the outcome. 4. epeat your procedure using a different aldehyde (e.g., benzaldehyde) and analyze the outcome. 5. epeat your procedure using a different ketone (e.g., acetophenone) and analyze the outcome. 6. epeat your procedure using one equivalent of acetone and 10 ml of water as the solvent and analyze the outcome. 7. epeat the procedure, but maintain the reaction mixture at reflux temperature for 2 h instead of room temperature for one week. Analyze the outcome. 8. xidize your product to a diketone (with instructor approval of the procedure). 9. educe your product to a diol (with instructor approval of the procedure). 10. ther (with approval of the instructor). Clean-up: All reaction products should be placed into properly labeled vials and submitted to the instructor. All aqueous solutions and spent drying agent can be flushed down the drain with copious amounts of water. All organic solutions should be placed into the appropriate solvent 12

13 waste container. Melting point capillaries, Pasteur pipets, tlc plates, and tlc capillaries should be placed into the broken glass receptacle. Filter paper can be placed into the regular trash. Clean your glassware, and wash your hands. Questions Answer the following questions in your lab notebook: 1. Was the original reaction enantioselective? If so, which enantiomer was major? 2. What other possible condensation products might have formed? 3. What was green about the initial procedure? 4. What was not green about the initial procedure? 5. How might the initial procedure be made greener? 6. What was the outcome of your second reaction? 7. What was green about your second procedure? 8. What was not green about your second procedure? 9. How might your second procedure be made greener? 13