Rhodium catalyzed hydroformylation of olefins

DFT and CCSD(T) methods were used to examine 61 different rhodium catalysts for the hydroformylation of ethylene. The carbon monoxide (CO) stretching frequency was a key electronic parameter to understand the pi-accepting nature of the ligand. Normally, pi-accepting ligands lead to increased CO stretching frequencies and a reduction in CO dissociation energy. There was no relationship between CO dissociation energy and CO stretching frequency. However, a clear relationship exists between the ethylene insertion barrier (from the rhodium dicarbonyl hydride resting state) and the CO stretching frequency as stronger pi-accepting ligands systematically led to a reduction in the barrier. Due to the multistep nature of the rate-limiting step, the overall barrier can be divided into the CO/ethylene equilibrium and an intrinsic ethylene insertion barrier and both are systematically reduced as the pi-accepting nature of the ligand is increased. A comparison of the carbonylation transition state (TS) to the ethylene insertion TS allowed us to understand reversibility of olefin insertion. While the ethylene insertion TS systematically decreases with increasing CO stretching frequency, the carbonylation TS is relatively flat. The lines cross at 2156 cm(-1) implying a change in the rate-limiting step in this region given a standard set of process conditions. (c) 2018 Wiley Periodicals, Inc.