Ab initio mechanistic investigation of samarium(III)-catalyzed olefin hydroboration reaction

Abstract

The reaction path for Sm(III)-catalyzed alkene hydroboration reaction by catecholborane has been investigated using ab initio MO methods. The stationary structures on the model reaction path considering ethylene as alkene, Cp 2SmH as an active catalyst, and HB(OH) 2 as model borane were obtained at the RHF and MP2 levels, and the MP4SDQ energy calculations were carried out at the MP2 structures. In the reaction, initially ethylene coordinates to the active catalyst to form a π-complex. Then, ethylene insertion into the Sm–H bond takes place leading to stable Cp 2SmC 2H 5 after passing through the barrier of 4.2 kcal/mol. In the following step the model borane adds to Cp 2SmC 2H 5 to form a borane complex which thereafter passes through the small barrier of 1.1 kcal/mol giving rise to a product complex. In the final step, the dissociation of the hydroborated product, C 2H 5B(OH) 2 takes place with a large endothermicity of 40.4 kcal/mol. Because of the small activation energies, the rate determining step may be the last step. This reaction mechanism is different from that for Rh(I)-catalyzed hydroboration. The effects of the Sm atom on this energy profile is discussed. In addition, the bonding features of all the stationary structures on the reaction path are obtained from topological analysis of the corresponding electron density distributions.