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)(η2-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 (ea-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.