ORGN 628

Unexpected steric effects of remote alkyl groups on the rate of conjugate additions to alkyl α,β-ethylenic sulfones and sulfoxides and to alkyl crotonates

Aimee R. Usera, arusera@jhu.edu and Gary H. Posner, ghp@jhu.edu. Department of Chemistry, The Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218-2682

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 and x-ray crystallography strongly suggest that what are generally considered to be “remote” alkyl groups in –CH=CHS(O)_n-Alkyl systems and –CH₂CH=CHCOO-Alkyl crotonate systems are actually not remote from the b-carbon atom of the Michael accepting unit. Molecular modeling clearly shows that the alkyl groups in these Michael acceptors shield the b-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.

Physical Organic Chemistry: Calculations, Mechanisms, Photochemistry, and High-Energy Species
8:00 AM-12:00 PM, Thursday, 30 March 2006 Georgia World Congress Center -- C303, Oral

Division of Organic Chemistry

The 231st ACS National Meeting, Atlanta, GA, March 26-30, 2006