RESUMEN
Oxidative addition of the tridentate N-heterocyclic carbene (NHC) diphosphine ligand precursor ([PCP]H)PF(6) (1) {[PCP] = o-(i)Pr(2)PC(6)H(4)(NC(3)H(4)N)o-C(6)H(4)P(i)Pr(2)} to Ni(COD)(2) results in the formation of the nickel(II) hydride complex ([PCP]NiH)PF(6) (2). This hydride undergoes a rapid reaction with ethylene to generate a nickel(0) complex in which an ethyl group has been transferred to the carbene carbon of the original NHC-diphosphine ligand. If the first intermediate is the anticipated square-planar nickel(II) ethyl species, then the formation of the product would require a process that involves a trans C-C coupling of the NHC carbon and a presumed Ni-ethyl intermediate. Deuterium-labeling studies provide evidence for migratory insertion of the added ethylene into the Ni-H bond rather than into the Ni-carbene linkage; this is based on the observed deuterium scrambling, which requires reversible beta-elimination, alkene rotation, and hydride readdition. However, density functional theory studies suggest that a key intermediate is an agostic ethyl species that has the Ni-C bond cis to the NHC unit. A possible transition state containing two cis-disposed carbon moieties was also identified. Such a process represents a new pathway for catalyst deactivation involving NHC-based metal complexes.
RESUMEN
A series of molecules capable of releasing of the strong organometallic Lewis acid B(C6F5)3 upon exposure to 254 nm light have been developed. These photo Lewis acid generators (PhLAGs) can now serve as photoinitiators for several important B(C6F5)3-catalyzed reactions. Herein is described the synthesis of the triphenylsulfonium and diphenyliodonium salts of carbamato- and hydridoborates, their establishment as PhLAGs, and studies aimed at defining the mechanism of borane release. Factors affecting these photolytic reactions and the application of this concept to photoinduced hydrosilylation reactions and construction of siloxane scaffolds are also discussed.