An Investigation of Substituent Effects on Coupling Constants to Vinyl Protons in Styrene Derivatives

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Abstract
Experimental long-range phenyl proton–vinyl proton coupling constants in 4-substituted styrenes are substituent independent. This is also predicted by INDO–finite perturbation theory calculations of these coupling constants. Comparison with calculated and experimental long-range coupling constants for 4-substituted benzaldehydes suggests that the previously reported substituent dependence for the latter coupling constants arises from substituent-induced changes in molecular geometry.Geminal vinyl coupling constants in 4-substituted styrenes, α-methylstyrenes, and α-t-butylstyrenes are substituent dependent with substituent effects increasing as phenyl and vinyl groups are twisted out of planarity. These trends are reproduced by INDO–FPT calculations. It is concluded that the substituent effects are primarily transmitted through space.Both experimental and calculated vinyl 13C–1H coupling constants show strong stereospecific substituent effects. From the pattern of results (particularly the greater field dependence for JC(β)H(9) than JC β)H(8)) it is concluded that these coupling constants.also reflect through-space substituent effects. This is supported by calculations on model compounds with no intervening phenyl group.

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