DIASTEREOSELECTIVITY OF ENOLATE ANION PROTONATION - H D EXCHANGE OF BETA-SUBSTITUTED ETHYL BUTANOATES IN ETHANOL-D/

The stereochemistry of base-catalyzed H/D exchange on 13 beta-substitu ted ethyl butanoates in ethanol-d has been studied in order to analyze the steric and electronic factors which control the diastereoselectiv ity of electrophilic attack on enolate anions. Electrophilic deuterati on of the enolate anion also determines the stereoselectivity of 1,4-c onjugate addition of ethanol-d to alpha,beta-unsaturated esters. Exper imental conditions were selected which rigorously exclude the effects of ion pairing and aggregation. The research showed that stereoelectro nic factors generally produce higher stereoselection than steric effec ts 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 subs tituents occupy an antiperiplanar position to the forming C-D bond. On ly with a beta-tertbutyl substituent did steric effects produce high s tereoselection, and it fell off rapidly with a decrease in carbon bran ching. Protonation of acyclic beta-ethoxy aldehyde and ketone enolates follows the same diastereoselectivity pattern as the beta-ethoxy este r enolate, but protonation of the cyanocarbanion from a beta-ethoxy ni trile gives much lower stereoselection.