Hybrid QM/MM study of propene insertion into the Rh-H bond of HRh(PPh3)2(CO)(η2-CH2=CHCH3):: the role of the olefin adduct in determining product selectivity

A hybrid QM/MM computational investigation of propene insertion into the Rh-H bond of HRh(PPh3)(2)(CO)(eta (2)- CH2=CHCH3) was performed to address the issue of kinetic (transition state) versus thermodynamic (ground state) determination of regioselectivity in hydroformylation catalysis. Two propene adduct isomers, one having an equatorial-axial arrangement of the two PPh3 co-ligands (ee-3) and the other having a bis-equatorial arrangement (ee-1), were predicted to be the most stable. The adduct ea-3 was predicted to be more stable than ee-1, by 1.0 kcal mol(-1). and based on the computed Boltzmann populations ea-3 is expected to be present in roughly a three-fold excess over ee-1. Based on the computed energy barriers leading to the corresponding linear and branched Rh-propyl products, ea-3 generates the linear insertion product almost exclusively, while ee-1 produces primarily the branched product. The 1.6 kcal mol(-1) difference in their respective activation energies translates into a fifteen-fold greater reactivity for ee-1 than ea-3. Hence, two separate reaction channels exist, one leading to the branched insertion product which is derived from the more active, minor propene adduct (ee-1). and one leading to the linear insertion product which originates from the less active, major propene adduct (ea-3). Thus the regioselectivity in hydroformylation catalysis may be rationalized in terms of ground state discrimination between the two most stable isomers of the propene adducts. (C) 2001 Elsevier Science B.V. All rights reserved.