Chiral Bis(Phospholane) PCP Pincer Complexes: Synthesis, Structure, and Nickel-Catalyzed Asymmetric Phosphine Alkylation

Abstract

A known ligand precursor, prepared by an improved method from 1,3-bis(bromomethylbenzene), a base, and (R,R)-2,5-dimethylphospholane-borane, was deprotected with DABCO and used to prepare chiral m-xylene-based PCP pincer complexes. Reaction with Ni(DME)Br2 and NEt3 gave square planar Ni(DuPinPhos)(Br) (9), which was crystallographically characterized. Heating with [Ir(COE)2Cl]2 yielded the dinuclear hydride complex [Ir(DuPinPhos)(H)(Cl)]2(μ-C6H4(CH2-dimethylphospholanyl)2] (10), which contained both pincer chelates and a bridging bis(phospholane). Nickel complex 9 was a catalyst precursor for enantioselective alkylation of secondary phosphines with 2-bromomethylnaphthalene using the base NaOSiMe3. The dynamic behavior of a catalytic intermediate, the P-stereogenic terminal phosphido complex Ni(DuPinPhos)(PPhIs) (Is = 2,4,6-(i-Pr)3C6H2), was characterized by variable temperature NMR spectroscopy.