Abstract
The effect of para-substituents on the geminal coupling constants in benzylic methylene groups has been studied as a function of the conformation at the benzene—CH2 bond. The slopes of Hammett plots of Jgemvs. σ are large (−1.6 to −1.9) when the geometry permits a hyperconjugative interaction between the benzene ring and the methylene group but negligible (+ 0.2) when this interaction is geometrically restricted. The effect of a change in electronegativity of the group attached to the methylene carbon has also been investigated. The change in the slope, ρ, from −1.7 to −1.6 on protonation of the nitrogen attached to the methylene group in a series of isoindolines shows this effect to be extremely small. Solvent effects on ρ are also very small. The potential of this J − σ relation for conformational analysis of benzyl groups is discussed in comparison with existing methods.
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