Stereochemistry of 1,2-elimination and proton-transfer reactions: toward a unified understanding.

Many mechanistic and stereochemical studies have focused on the breaking of the C-H bond through base-catalyzed elimination reactions. When we began our research, however, chemists knew almost nothing about the stereospecificity of addition-elimination reactions involving conjugated acyclic carbonyl compounds, even though the carbonyl group is a pivotal functional group in organic chemistry. Over the last 25 years, we have studied the addition-elimination reactions of ß-substituted acyclic esters, thioesters, and ketones in order to reach a comprehensive understanding of how electronic effects influence their stereochemistry. This Account brings together our understanding of the stereochemistry of 1,2-elimination and proton-transfer reactions, describing how each study has built upon previous work and contributed to our understanding of this field. When we began, chemists thought that anti stereospecificity in base-catalyzed 1,2-elimination reactions occurred via concerted E2 mechanisms, which provide a smooth path for anti elimination. Unexpectedly, we discovered that some E1cBirrev reactions produce the same anti stereospecificity as E2 reactions even though they proceed through diffusionally equilibrated, "free" enolate-anion intermediates. This result calls into question the conventional wisdom that anti stereochemistry must result from a concerted mechanism. While carrying out our research, we developed insights ranging from the role of historical contingency in the evolution of hydratase-dehydratase enzymes to the influence of buffers on the stereochemistry of H/D exchange in D2O. Negative hyperconjugation is the most important concept for understanding our results. This idea provides a unifying view for the largely anti stereochemistry in E1cBirrev elimination reactions and a basis for understanding the stereoelectronic influence of electron-withdrawing ß-substituents on proton-transfer reactions.

Does activation of the anti proton, rather than concertedness, determine the stereochemistry of base-catalyzed 1,2-elimination reactions? Anti stereospecificity in E1cB eliminations of ß-3-trifluoromethylphenoxy esters, thioesters, and ketones.

As part of a comprehensive investigation on the stereochemical aspects of base-catalyzed 1,2-elimination reactions, we have studied a set of acyclic carbonyl substrates that react by an irreversible E1cB mechanism with largely anti stereospecificity. (2)H NMR data show that these reactions using KOH in EtOH/H(2)O under non-ion-pairing conditions produce a minimum of 85-89% anti elimination on stereospecifically labeled tert-butyl (2R*,3R*)- and (2R*,3S*)-3-(3-trifluoromethylphenoxy)-2,3-(2)H(2)-butanoate, S-tert-butyl (2R*,3R*)- and (2R*,3S*)-3-(3-trifluoromethylphenoxy)-2,3-(2)H(2)-butanethioate, and the related ketones, (4R*,5R*)- and (4R*,5S*)-5-(3-trifluoromethylphenoxy)-4,5-(2)H(2)-3-hexanone. With both diastereomers of each substrate available, the KIEs can be calculated and the innate stereoselectivities determined. The elimination reactions of the ß-3-trifluoromethylphenoxy substrates occur by E1cB mechanisms with diffusionally equilibrated enolate-anion intermediates. Thus, it is clear that anti elimination does not depend solely upon concerted E2 mechanisms. Negative hyperconjugation provides a satisfactory explanation for the anti stereospecificity exhibited by our carbonyl substrates, where the leaving group activates the anti proton, leading to the enolate intermediate. The activation of the anti proton by negative hyperconjugation may also play a role in the concerted pathways of E2 mechanisms. We have also measured the rates of the hydroxide-catalyzed elimination reactions of butanoate, thiobutanoate, and ketone substrates in EtOH/H(2)O, with ß-tosyloxy, acetoxy, and 3-trifluoromethylphenoxy nucleofuges.

Effect of buffer general acid-base catalysis on the stereoselectivity of ester and thioester H/D exchange in D2O.

As part of a comprehensive investigation on the stereochemistry of base-catalyzed 1,2-elimination and H/D exchange reactions of carbonyl compounds, we have found that the stereoselectivity of H/D exchange of 3-hydroxybutyryl N-acetylcysteamine (3) in D(2)O is strongly influenced by the presence of buffers. This buffer effect is also operative with a simple acyclic ester, ethyl 3-methoxybutanoate (7). Buffers whose general-acid components are cyclic tertiary ammonium ions are particularly effective in changing the stereoselectivity. (2)H NMR analysis showed that without buffer, H/D exchange of 3 produces 81-82% of the 2R*, 3R* diastereomer of 2-deuterio 3 (the anti product). In the presence of 0.33 M 3-quinuclidinone buffer, only 44% of the 2R*, 3R* diastereomer was formed. With ester 7, the stereoselectivity went from 93-94% in DO(-)/D(2)O to 60% in the presence of buffer. Phosphate buffer, as well as others, also showed substantial effects. The results are put into the context of what is known about the mechanism of H/D exchange of esters and thioesters, and the relevance of the buffer effect on the mechanism of the enoyl-CoA hydratase reaction is discussed. It is likely that hydrogen bonding in the enolate-buffer acid encounter complex is an important stereochemical determinant in producing a greater amount of the 2R*, 3S* diastereomer (the syn product). Studies that involve the protonation of enolate anions in D(2)O need to include the buffer general acid in any understanding of the stereoselectivity.

Free-energy surfaces for liquid-phase reactions and their use to study the border between concerted and nonconcerted alpha,beta-elimination reactions of esters and thioesters.

Distinguishing between the concerted second-order mechanism for beta-eliminations and nonconcerted mechanisms with discrete carbanion intermediates is very difficult experimentally, but the ability of quantum chemistry to find stationary points of the free-energy surface in liquid-phase solutions, even for complex reagents, provides a new tool for elucidating such mechanisms. Here we use liquid-phase density functional theory calculations to find transition states and intermediates on the free-energy surfaces of four base-initiated alpha,beta-eliminations of acetoxy and mesyloxy esters and their analogous thioesters. The geometries, free energies, and charge distributions of these structures support a stepwise irreversible first-order elimination from a conjugate base (E1cB(I)) mechanism with acetoxy ester 3, acetoxy thioester 4, and mesyloxy thioester 6. However, mesyloxy ester 5, which has an excellent nucleofuge and a less-acidic proton, follows a concerted but asynchronous E2 mechanism with an E1cB-like transition state. The anti transition state is more favorable than the syn one, even for the poorer nucleofuge and more-acidic thioesters. The article includes a general scheme for describing liquid-phase reactions in terms of free-energy surfaces.

Stereochemistry of 1,2-elimination reactions at the E2-E1cB interface--tert-butyl 3-tosyloxybutanoate and its thioester.

Experimental data on the stereoselectivity of base-catalyzed 1,2-elimination reactions that produce conjugated carbonyl compounds are scarce in spite of the importance of these reactions in organic and biochemistry. As part of a comprehensive study in this area, we have synthesized stereospecifically-deuterated beta-tosyloxybutanoate esters and thioesters and studied the stereoselectivity of their elimination reactions under non-ion pairing conditions. With the availability of both the (2R*,3R*) and (2R*,3S*) diastereomers the innate stereoselectivity could be determined unambiguously. (1)H and (2)H NMR data show that these substrates produce 5-6% syn elimination, the usual amount for acyclic substrates undergoing E2 reactions. Contrary to earlier suggestions, activation by a carbonyl group has virtually no influence upon the stereoselectivity. Elimination of the (2R*,3R*) diastereomer of the beta-tosyloxyester and thioester produces 21-25% of the (Z)-alkene, much more than observed with a poorer beta-nucleofuge. A relatively large amount of (Z)-alkene product seems to be a good marker for an E2 pathway, in which the transition state is E1cB-like, rather than an E1cB(irrev) mechanism. Syn KIE values were higher than those for anti elimination for the esters as well as the thioesters. Experimental challenges to the synthesis of stereospecifically-deuterated beta-tosyloxyesters are discussed.

Novel syn intramolecular pathway in base-catalyzed 1,2-elimination reactions of beta-acetoxy esters.

As part of a comprehensive investigation of electronic effects on the stereochemistry of base-catalyzed 1,2-elimination reactions, we observed a new syn intramolecular pathway in the elimination of acetic acid from beta-acetoxy esters and thioesters. 1H and 2H NMR investigation of reactions using stereospecifically labeled tert-butyl (2R*,3R*)-3-acetoxy-2,3-2H2-butanoate (1) and its (2R*,3S*) diastereomer (2) shows that 23 +/- 2% syn elimination occurs. The elimination reactions were catalyzed with KOH or (CH3)4NOH in ethanol/water under rigorously non-ion-pairing conditions. By contrast, the more sterically hindered beta-trimethylacetoxy ester produces only 6 +/- 1% syn elimination. These data strongly support an intramolecular (Ei) syn path for elimination of acetic acid, most likely through the oxyanion produced by nucleophilic attack at the carbonyl carbon of the beta-acetoxy group. The analogous thioesters, S-tert-butyl (2R*,3R*)-3-acetoxy-2,3-2H2-butanethioate (3) and its (2R*,3S*) diastereomer (4), showed 18 +/- 2% syn elimination, whereas the beta-trimethylacetoxy substrate gave 5 +/- 1% syn elimination. The more acidic thioester substrates do not produce an increased amount of syn stereoselectivity even though their elimination reactions are at the E1cb interface.