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)-catalyzed alkyne and alkene diboration reactions with the B3LYP density functional method. Two different paths are studied, path A where the first step is B-B oxidative addition and path B where the first step is alkyne/alkene coordination. Though the coordination energy of acetylene and ethylene to Pt(PH3)(2) is larger than the energy gain of oxidative addition of (OH)(2)B-B(OH)2 to Pt(PH3)(2), the trend reverses as the size of substituents on alkynes, alkenes, and (OH)(2)B-B(OH)(2) increases, and for large alkynes path A is expected to be favored over path B. Path A has been shown to proceed via the following steps: (a) coordination 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) coordination of alkyne/alkene to form a pi-complex, (e) migratory insertion of alkyne/alkene into a Pt-B bond, (f) migration of the CHxCHxB(OH)(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 alkyne and alkene diboration reactions originates from the difference in energetics in the step e and has been explained in terms of lower deformation 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 stereoselectivity has been explained in terms of rigidity of the C-C pi bond.