The CF₃ Group as a Synthon of the CF⁺ Unit in Palladium Chemistry

The homoleptic trifluoromethyl-palladium(II) complex [Pd(CF₃)₄]^2- (1) is shown to be highly active towards amines. Thus, when treated with primary amines RNH₂, it readily undergoes aminolysis of one of the CF₃ ligands affording the isocyanide complexes [(CF₃)₃Pd(CNR)]^- (R=aryl). In this process the original CF₃ group undergoes total defluorination. Interestingly, the reaction of 1 with secondary amines R₂NH proceeds with loss of just two F-substituents, whereby the Fischer-type fluoroaminocarbene complexes [(CF₃)₃Pd(CFNR₂)]^- are formed (R=Et, Ph). The reaction of 1 with diamines affords different [(CF₃)₃Pd(NHC)]^- complexes containing sterically non-demanding NHC ligands. Representative examples of various topologies are reported based on the common imidazolidin-2-ylidene or benzimidazolin-2-ylidene rings as well as the expanded-ring perimidin-2-ylidene. This metal-tailored synthetic route, where a CF₃ group acts as a pre-carbenic unit, is unprecedented in the vast NHC-chemistry. It takes place under very mild conditions and is envisaged to be extensible to other non-isolable NHC ligands. The key difluorocarbene intermediate [(CF₃)₃Pd(CF₂)]^- is experimentally detected.