Quantum chemical calculations and nuclear magnetic resonance measurements on benzyl-type carbanions. Part 1. The effect of an aromatic methoxy substituent (resonance saturation) and α-alkyl substitution

Abstract

Ab initio STO-3G quantum chemical calculations are reported for benzyl-type carbanions. The influence of aromatic ring substitution by a methoxy group (ortho or para) and of α-alkyl substitution on the electronic properties of the carbanions are investigated. Energetics (e.g. the energy difference between perpendicular and parallel methoxy group conformations, substituent interaction energies) and charge distributions (e.g. total and π-electronic charge excess on the aromatic ring carbons, negative charge excess on the methoxy group) reveal the importance of a resonance saturation effect in these systems due to the simultaneous presence of two π-electron-donating substituents in the aromatic nucleus, the effect being always larger in the case of para-substitution. Upon introduction of an α-alkyl substituent sp²-hybridization is still more favoured and a negative charge transfer to the alkyl group, which acts as an inductive acceptor, occurs. The charge distribution in the aromatic nucleus is hardly changed when compared with the α-unsubstituted case. The ¹³C and ¹H n.m.r. chemical shifts of the benzyl-potassium compounds generally correlate well with the calculated charge distribution in the anions. The non-additivity of the π-electron-donating CH₂^– and CH₃O substituents also shows up in the ¹³C shift data, the effect again being larger for para than for ortho substitution. When analysing the effect of α-alkyl substitution, a small polarizing effect due to the presence of the countercation should probably be taken into account.