Synthesis and characterization of Palladium (II) p-AllylComplexes with Chiral Phosphinocarbene Ligands. Kinetics and Mechenism of allylic Amination.

New palladium η3-allyl complexes with chelating phosphinocarbene ligands (P-C) have been preparedand fully characterized. The adopted synthetic strategy is based on the reaction between the appropriate silver precursors and the labile [Pd(η3-allyl)OCOCF3]2. These complexes react with amines in the presenceof dimethyl fumarate, yielding the new palladium(0) complexes [Pd(η3-allyl)(P-C)] and allylamine. Under pseudo-first-order conditions, the amination rates obey the simple law kobs ) k2[NHR2], and this observation seems to rule out the possibility of a simultaneous amine attack at the central atom with displacement of the bidentate ancillary ligand (or of one of its donor groups). The k2 values increase with increasing basicity of the amine, with decreasing steric hindrance at the allyl fragment, and with increasing bulkiness of the P-C heterocyclic nitrogen substituent. Moreover a remarkable decrease of the amination rate is observed for the phosphinocarbene complexes as compared to the “isostructural” phoshinopyrazole (P-N) derivatives, in agreement with the strong σ-donating and weak π-accepting character of the carbene ligands. The new posphinocarbene ligands are tested in the palladium-catalyzed amination of 1,3-diphenylallylethyl carbonate, and the resulting low enantioselectivity may support the view of the comparable trans influence of carbene and phosphine ligands, which renders the corresponding trans allyl carbons electronically equivalent.