Unexpected steric effects of "remote" alkyl groups on the rate of conjugate additions to alkyl alpha,beta-ethylenic sulfones, sulfoxides, and esters.
Examination of conjugated ethylenic sulfones, sulfoxides, and esters in Michael-type addition reactions reveals, for the first time, that the size of the heteroatom-attached alkyl group affects the rate of conjugate addition. Molecular modeling strongly suggests that what are generally considered to be "remote" alkyl groups in -CbetaH=CalphaHS(O)n-alkyl systems and -CH2CbetaH=CalphaHCOO-alkyl systems are actually not remote from the beta-carbon atom of the Michael accepting unit. Molecular modeling clearly shows that the alkyl groups in these Michael acceptors shield the beta-carbons in the following order: Et<i-Pr<t-Bu. Competition experiments establish the relative rates of Michael additions to be in the following order: Et>i-Pr>t-Bu.
Usera
Aimee R
AR
Department of Chemistry, School of Arts and Sciences, The Johns Hopkins University, Baltimore, Maryland 21218, USA.
Posner
Gary H
GH
D000478:Alkylation; D004952:Esters / Q000737:chemistry*; D005030:Ethylenes / Q000737:chemistry*; D008958:Models, Molecular; D015394:Molecular Structure; D013450:Sulfones / Q000737:chemistry*; D013454:Sulfoxides / Q000737:chemistry*
D004952:Esters; D005030:Ethylenes; D013450:Sulfones; D013454:Sulfoxides; C036216:ethylene