OXIDATIVE ADDITION OF CATECHOLBORANE TO IRX(CO)(DPPE) COMPLEXES

The oxidative addition of catecholborane (1,3,2-benzodioxaborole) to the Ir(I) cis-phosphine complexes IrX(CO)(dppe) (X = Br, I, H, BO2C6H4; dppe = 1,2-bis(diphenylphosphino)ethane) has been found to proceed stereoselectively under kinetic control. Of the four complexes that can be formed by cis oxidative addition of the B-H bond to IrBr(CO)(dppe), the one having hydride trans to phosphorus and boron trans to bromide is formed in >99% yield while addition to IrI(CO)(dppe) occurs similarly but with only 90% stereoselectivity. Isomerization of the initially formed diastereomers to thermodynamically less stable diastereomers which display hydride trans to halide and boron trans to phosphorus occurs after 1 day. The mechanism for the isomerism for X = Br has been determined to proceed via reductive elimination/oxidative addition processes. The oxidative addition reactions of catecholborane to [IrH(CO)(dppe)] and [Ir(BO2C6H4)(CO)(dppe)], generated from IrH3(CO)(dppe) and IrH2(BO2C6H4)(CO)(dppe), respectively, also occur in a cis fashion with 99% stereoselectivity yielding Ir(III) products with hydride trans to CO and boron trans to phosphorus. The observed stereoselectivity for catecholborane addition to IrX(CO)(dppe) (X = Br, I) is reversed with respect to addition to IrX(CO)(dppe) (X = H, BO2C6H4) and is accounted for by electronic factors involving the pi-basicity of the halide ligands.