The stereochemistry of
base-catalyzed H/D exchange on 13
beta2.gif" BORDER=0 ALIGN="middle">-su
bstituted ethyl
butanoates in ethanol-
dhas
been studied in order to analyze the steric and electronic factorswhich control the diastereoselectivity ofelectrophilic attack on enolate anions. Electrophilic deuterationof the enolate anion also determines the stereoselectivityof 1,4-conjugate addition of ethanol-
d to
,
beta2.gif" BORDER=0 ALIGN="middle">-unsaturated esters. Experimental conditions were selectedwhichrigorously exclude the effects of ion pairing and aggregation. Theresearch showed that stereoelectronic factorsgenerally produce higher stereoselection than steric effects do.Electronegative heteroatom su
bstituents at C-3produceda 10:1 ratio of the2
R*,3
R*/2
R*,3
S*2-deuterio
butanoates. In the most sta
ble transition states forelectrophilic attack,these electronegative su
bstituents occupy an antiperiplanar position tothe forming C-D
bond. Only with a
beta2.gif" BORDER=0 ALIGN="middle">-
tert-
butyl su
bstituent did steric effects produce high stereoselection, andit fell off rapidly with a decrease in car
bon
branching. Protonation of acyclic
beta2.gif" BORDER=0 ALIGN="middle">-ethoxy aldehyde and ketoneenolates follows the same diastereoselectivity patternas the
beta2.gif" BORDER=0 ALIGN="middle">-ethoxy ester enolate,
but protonation of the cyanocar
banionfrom a
beta2.gif" BORDER=0 ALIGN="middle">-ethoxy nitrile gives much lowerstereoselection.