Methyl cation affinities of commonly used organocatalysts.

Methyl cation affinities (MCAs) and proton affinities (PAs) of a variety of N- and P-based organocatalysts have been calculated at the MP2(FC)/6-31+G(2d,p)//B98/6-31G(d) level of theory. Correlations between MCA and PA values have been used to identify factors leading to the potentially poor predictive value of PA data for organocatalytic activity. One of the relevant factors concerns steric effects between organocatalysts and the reactant electrophiles, which are not well-modeled by reaction with a proton. A second important factor concerns systematic differences in bond strengths between second- and third-row elements. This latter point makes MCA values much better descriptors of the catalytic activity of phosphanes than PA or pKa data.