Quantum chemical calculations and nuclear magnetic resonance measurements on benzyl-type carbanions. Part 2. Influence of countercations and interacting unsaturated systems

Abstract

Quantum chemical calculations, n.m.r. and spectrophotometric measurements are carried out to study the influence of countercations and interacting unsaturated systems on the structural and electronic properties of benzyl-type carbanions. The calculated geometry of benzyl-Li compares favourably with X-ray data on a related structure. ¹³C, ¹H n.m.r. shifts and ¹J_CH coupling constants of benzyl, o- and p-CH₃O-benzyl-Li, -Na, and -K compounds show a fair overall agreement with the ab initio-calculated charge distributions and structural parameters for the terminal members of the Li-, Na-, and K- series, the Li compound and the free carbanion, modelling the K salt. Both theory and experiment indicate that, when passing from the anion to the alkali-metal compound, an important destruction of the resonance saturation, present in the CH₃O derivatives, occurs due to the presence of the countercation, the reduction being more important with decreasing cation radius. The n.m.r. data for α-alkyl-substituted compounds suggest that steric factors make the position of the cation in the C_α region less favourable, the effect being more pronounced for larger cation radius. The resonance saturation effect in the above mentioned systems may also be influenced by intermolecular effects, e.g. it may change during a chemical reaction. Ab initio calculations on the interaction energy between the benzyl-type carbanions and unsaturated systems showing increasing delocalization possibilities for incoming negative charge indicate that the larger this delocalization possibility (ethene < butadiene < styrene), the more important the destruction of resonance saturation. Along this series the parallel conformation of the CH₃O group in the p-CH₃O compounds gradually becomes less disfavoured. The calculated effect is however not strong enough yet in order to show full agreement with the observed increase in the k(–) value for the addition reaction to 1,1-diphenylethene when passing from polystyryl to poly-p-methoxystyryl carbanions. Larger basis sets and extensive geometry optimization should be carried out in order to settle this problem.