Diastereoselectivity of Enolate Anion Protonation. H/D Exchange of β-Substituted Ethyl Butanoates in Ethanol-d

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Abstract

The stereochemistry of base-catalyzed H/D exchange on 13 β-substituted ethyl butanoates in ethanol-d has been studied in order to analyze the steric and electronic factors which control the diastereoselectivity of electrophilic attack on enolate anions. Electrophilic deuteration of the enolate anion also determines the stereoselectivity of 1,4-conjugate addition of ethanol-d to α,β-unsaturated esters. Experimental conditions were selected which rigorously exclude the effects of ion pairing and aggregation. The research showed that stereoelectronic factors generally produce higher stereoselection than steric effects do. Electronegative heteroatom substituents at C-3 produced a 10:1 ratio of the 2R*,3R*/2R*,3S* 2-deuteriobutanoates. In the most stable transition states for electrophilic attack, these electronegative substituents occupy an antiperiplanar position to the forming C−D bond. Only with a β-tert-butyl substituent did steric effects produce high stereoselection, and it fell off rapidly with a decrease in carbon branching. Protonation of acyclic β-ethoxy aldehyde and ketone enolates follows the same diastereoselectivity pattern as the β-ethoxy ester enolate, but protonation of the cyanocarbanion from a β-ethoxy nitrile gives much lower stereoselection.

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