Rational design of nucleophilic amine sites via computational probing of steric and electronic effects

Abstract

Accessibility of the nucleophilic site in organocatalysts is essential to ensure adequate catalytic activity. Gas-phase trimethylborane (TMB) Lewis basicity and Brønsted proton basicity of several amine based organocatalysts have been calculated using the CBS-QB3 model chemistry. This TMB basicity scale can, as opposed to the proton basicity scale, account for steric effects encountered in the initial nucleophilic attack of the nitrogen free electron pair on a substrate. Since such a step is the first one in several amine catalyzed reactions, severe steric hindrance of the nucleophilic center would render the catalyst ineffective. Comparing the TMB basicity and proton basicity with the experimentally observed catalytic activity of both homogeneous and heterogeneously supported amine sites found in literature for the aldol reaction of acetone with 4-nitrobenzaldehyde showed that, due to the inclusion of these steric effects, the TMB basicity scale is a much better predictor of catalytic activity than the proton basicity. According to this computational Lewis basicity scale, potential steric hindrance in alternative nitrogen containing active sites was probed. This resulted in 3-propylpyrrolidine being proposed among the most promising monofunctional amine groups and 1-(methylamino)propan-2-ol among the most promising bifunctional amine-hydroxyl groups for heterogeneous aldol reaction catalysts.