Influence of Lewis acid strength on hydride transfer to unsaturated substrates

Hydride transfer promoted by the coordination of a substrate molecule to a Lewis acid is a critical step in many catalytic transformations. This computational study investigates the nature of the interaction between a polar substrate molecule and a Lewis acid by examining the influence of Lewis acid strength on the ability to reduce (transfer a hydride to) the coordinated substrate molecule. To investigate this interaction, the coordination of 10 probe substrates to seven Lewis acids was analyzed. Coordination of the probe substrate molecules to a Lewis acid resulted in a more favorable reduction of the substrate molecule by 20-70 kcal mol^-1. Further examination of the coordination of the substrate molecules to Lewis acids of varying Lewis acid strengths resulted in a direct linear correlation between the ability of the Lewis acid-substrate adduct to accept a hydride and the Lewis acid strength. The linear correlations also revealed that between 44 and 70% of the Lewis acidity of the Lewis acids translated to the Lewis acid-substrate adducts. From the results obtained in this study, the minimum Lewis acid strength needed to activate the substrates for the reduction with [BH₄]^- and the implications of employing a Lewis acid to promote the reduction of an unsaturated polar substrate in catalytic reactions are also described.