Carbon-13 chemical shifts as a probe for conformational and electronic effects in alkyl vinyl ethers

Abstract

The principle of additivity of substituent chemical shifts (SCS) is applied to the carbon-13 chemical shifts of β carbons (δβ) in a number of 1,1-disubstituted ethylenes and propenes, which includes some α-substituted alkyl vinyl ethers. An additivity relationship is observed for 34 compounds that indicates an independent polarization mechanism across the double bond for each geminal substituent. Positive deviations from additivity are observed for compounds bearing bulky substituents. The magnitude of deviations is taken as an indication of the severity of steric interactions among substituents.Reductions in efficiency of transmission of substituent effects in ring substituted α-phenyl alkyl vinyl ethers, relative to para-substituted styrenes, are interpreted as the result of a variable dihedral angle between the ring and vinyl group. An explanation of the relationship between conformation and transmission of substituent effects is presented as an alternative to existing views. An angular dependence of π-polarization and resonance effects is consistent with the chemical shift data.