Stability and Unimolecular Reactivity of Palladate(II) Complexes [L n PdR 3 ] − (L=Phosphine, R=Organyl, n =0 and 1)

The reduction of Pd II precatalysts to catalytically active Pd 0 species is a key step in many palladium‐mediated cross‐coupling reactions. Besides phosphines, the stoichiometrically used organometallic reagents can afford this reduction, but do so in a poorly understood way. To elucidate the mechanism of this reaction, we have treated solutions of Pd(OAc) 2 and a phosphine ligand L in tetrahydrofuran with RMgCl (R=Ph, Bn, Bu) as well as other organometallic reagents. Analysis of these model systems by electrospray‐ ionization mass spectrometry found palladate(II) complexes [L n PdR 3 ] − ( n =0 and 1), thus pointing to the occurrence of transmetallation reactions. Upon gas‐phase fragmentation, the [L n PdR 3 ] − anions preferentially underwent a reductive elimination to yield Pd 0 species. The sequence of the transmetallation and reductive elimination, thus, constitutes a feasible mechanism for the reduction of the Pd(OAc) 2 precatalyst. Other species of interest observed include the Pd IV complex [PdBn 5 ] − , which did not fragment via a reductive elimination but lost BnH instead.