Side-chain 13C nuclear magnetic resonance shifts in ring-substituted styrenes. The effect of β-substituents on β-carbon shifts

Abstract

A large number of compounds of the type XC₆H₄–CHCYZ have been prepared and the effect of varying X, Y, and Z on the ¹³C n.m.r. chemical shifts of the β-(terminal) carbon of the ethenyl group has been investigated. For Y and Z constant and X meta or para to the side-chain the dominant influence is the electronic effect of X. For meta-X, excellent correlations of the C_β shifts with Hammett σ^meta constants are obtained in all cases. For para-X, there is evidence that both the inductive and resonance effect of X influence the shift with the extent of the contribution of the latter being greatest in situations where resonance interaction between X and Y/Z is expected to be strongest. Attempts to investigate the degree of this interaction by means of Taft's Dual Substituent Parameter Equation were only partly successful, and showed that in our systems such analysis is better regarded as a qualitative guide to the extent of resonance interaction that a quantitative measure of it.