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

The reaction path for Sm(III)-catalyzed alkene hydroboration reaction by ca techolborane has been investigated using ab initio MO methods. The stationa ry 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 pi-complex. Then, ethylene insertion into the Sm-H bond takes place leading to stable Cp2SmC2H5 after passing through th e barrier of 4.2 kcal/mol. In the following step the model borane adds to C p2SmC2H5 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 ste p, the dissociation of the hydroborated product, C2H5B(OH)(2) takes place w ith a large endothermicity of 40.4 kcal/mol. Because of the small activatio n energies, the rate determining step may be the last step. This reaction m echanism is different from that for Rh(I)-catalyzed hydroboration. The effe cts of the Sm atom on this energy profile is discussed. In addition, the bo nding features of all the stationary structures on the reaction path are ob tained from topological analysis of the corresponding electron density dist ributions. (C) 1999 Elsevier Science B.V. All rights reserved.