Defining the optimal inductive and steric requirements for a cross-coupling catalyst using the energetic span model

This paper interrogates the efficiency of a model cross-coupling reaction catalyzed by palladium complexes, using a quantum mechanical/molecular mechanical (QM/MM) methodology that permits ‘independent’ treatments of the inductive and steric effects of the phosphine ligands of the palladium catalysts. To test the efficiency of the catalyst we calculate turnover frequencies (TOFs), based on ‘the energetic-span model’ which considers the TOF determining transition state (TDTS) and TOF determining intermediate (TDI) as the reaction-rate controlling factors. Four different TDTS and TDI species are considered: the square planar diphosphine palladium complex, the anionic monophosphine, the T-shaped monophosphine, and the anionic phosphine-free species. Two different requirements are found to typify the most efficient catalysts: either bulky ligands that hinder the stabilization of diphosphines, or medium size ligands with high electron withdrawing power. Both conditions reduce the energetic span of the cycle by destabilizing the TDI more than the TDTS, thereby leading to enhanced TOFs. The approach used here can serve for future theoretical or experimental designs of new palladium catalysis.

The energetic span model for kinetic assessment of catalytic cycles identifies the best phosphine ligand for a theoretically computed cross-coupling reaction.Figure optionsDownload high-quality image (73 K)Download as PowerPoint slide