Comparison of imine to olefin insertion reactions: Generation of five- and six-membered lactams via a nickel-mediated co, olefin, co, imine insertion cascade

Cationic nickel complexes of the form L₂Ni(CH ₃)(RN=C(H)R)⁺ (L₂ = bidentate nitrogen donor) have been probed for their ability to mediate olefin and imine migratory insertion reactions. While these complexes do not undergo alkene insertion into the nickel-methyl bond, the use of olefin-tethered imines CH₂= CH(CH₂)_nN=C(R)aryl (n = 1-3; R = H, methyl, phenyl) and the addition of CO initiates competitive olefin and imine insertion into the in situ generated nickel-acyl complex. The insertion selectivity shows clear steric and electronic influences and suggests that strong imine coordination can allow it to compete with the more rapid insertion of alkenes. This reactivity has been employed to develop a CO/alkene/CO/imine insertion cascade to synthesize five- and six-membered-ring lactams.