Methyl Cation Affinities of Organocatalysts

Significance

Many organocatalysts possess a Lewis basic reactive site and many organocatalytic transformations involve nucleophilic attack at carbon. Therefore, methyl cation affinities (MCAs), in this context defined as the reaction enthalpies of the transformation shown in equation 1 (see scheme), are much better descriptors of catalytic activity than the respective proton affinities (PAs, see equation 2). The paper in hand provides a set of computed MCA and PA values for a broad variety of N- and P-based organocatalysts. The study revealed two factors leading to deviations from the generally good qualitative correlation of MCA and PA values: Firstly, steric effects, which are larger for methyl cations than for protons and secondly, systematic differences in bond strengths between second row (N) and third row (P) elements.