Nickel-Catalyzed Reductive Conjugate Addition to Enones via Allylnickel Intermediates

An alternative method to copper-catalyzed conjugate addition followed by enolate silylation for the synthesis of β-disubstituted silyl enol ether products (R1(R2)­HCCHC­(OSiR4 3)­R3) is presented. This method uses haloarenes instead of nucleophilic aryl reagents. Nickel ligated to either neocuproine or bipyridine couples an α,β-unsaturated ketone or aldehyde (R2HCCHC­(O)­R3) with an organic halide (R1–X) in the presence of a trialkylchlorosilane reagent (Cl–SiR4 3). Reactions are assembled on the benchtop and tolerate a variety of functional groups (aldehyde, ketone, nitrile, sulfone, pentafluorosulfur, and N-aryltrifluoroacetamide), electron-rich iodoarenes, and electron-poor haloarenes. Mechanistic studies have confirmed the first example of a catalytic reductive conjugate addition of organic halides that proceeds via an allylnickel intermediate. Selectivity is attributed to (1) rapid, selective reaction of LNi0 with chlorotriethylsilane and enone in the presence of other organic electrophiles, and (2) minimization of enone dimerization by ligand steric effects.