MOLECULAR-ORBITAL STUDY OF THE MECHANISM OF PLATINUM(0)-CATALYZED ALKENE AND ALKYNE DIBORATION REACTIONS

A theoretical study has been carried out for the mechanism of Pt(0)-ca talyzed alkyne and alkene diboration reactions with the B3LYP density functional method. Two different paths are studied, path A where the f irst step is B-B oxidative addition and path B where the first step is alkyne/alkene coordination. Though the coordination energy of acetyle ne and ethylene to Pt(PH3)(2) is larger than the energy gain of oxidat ive addition of (OH)(2)B-B(OH)2 to Pt(PH3)(2), the trend reverses as t he size of substituents on alkynes, alkenes, and (OH)(2)B-B(OH)(2) inc reases, and for large alkynes path A is expected to be favored over pa th B. Path A has been shown to proceed via the following steps: (a) co ordination of (OH)(2)B-B(OH)(2) to Pt(PH3)(2), (b) oxidative addition of the B-B bond to Pt, (c) dissociation of one phosphine ligand, (d) c oordination of alkyne/alkene to form a pi-complex, (e) migratory inser tion of alkyne/alkene into a Pt-B bond, (f) migration of the CHxCHxB(O H)(2) (x = 1 or 2) group to become cis to B(OH)(2), (g) recoordination of phosphine, and (h) elimination of (OH)(2)BCHx-CHxB(OH)(2) product. The rate-determining step is found to be phosphine dissociation step c, in agreement with the experiment. The observed difference between a lkyne and alkene diboration reactions originates from the difference i n energetics in the step e and has been explained in terms of lower de formation energy and larger B-C sigma bond energy for alkyne than for alkene in (B(OH)(2))(PH3)Pt-CHxCHx-B(OH)(2). The experimental stereose lectivity has been explained in terms of rigidity of the C-C pi bond.