Reactions of Cyanoboranes with a Palladium−PMe3 Complex: Mechanism for the Catalytic Cyanoboration of Alkynes

Reversible oxidative addition of the B−CN bond to palladium was observed in a reaction of a 13 C-labeled cyanoborane, having an N, N ‘-dimethyl- o -phenylenediamido ligand on the boron atom, with a palladium−PMe 3 complex, which was generated from (π-cyclopentadienyl)(π-allyl)palladium(II) with 2 equiv of PMe 3 . The same borylbis(trimethylphosphine)palladium(II) cyanide complex was formed in the reaction of borylbis(trimethylphosphine)palladium(II) chloride with trimethylsilyl cyanide. Reaction of the palladium−PMe 3 complex with a cyanoborane bearing a tethered carbon−carbon triple bond gave a (2-borylalkenyl)palladium(II) cyanide complex, whose structure was determined by a single-crystal X-ray analysis, via insertion of the C−C triple bond into the B−Pd bond of a transiently formed borylpalladium(II) cyanide complex. The (2-borylalkenyl)palladium(II) cyanide complex underwent reductive elimination at 60 °C. Significant retardation of the reductive elimination step by addition of 2 equiv of trimethylphosphine was observed.