Many nickel-based catalysts have been reported for cross-coupling reactions of nonactivated alkyl halides. The mechanistic understanding of these reactions is still primitive.Here we report a mechanistic study of alkyl−alkyl Kumada coupling catalyzed by a preformed nickel(II) pincer complex ([(N 2 N)Ni−Cl]). The coupling proceeds through a radical process, involving two nickel centers for the oxidative addition of alkyl halide. The catalysis is second-order in Grignard reagent, first-order in catalyst, and zero-order in alkyl halide. A transient species, [(N 2 N)Ni-alkyl 2 ](alkyl 2 -MgCl), is identified as the key intermediate responsible for the activation of alkyl halide, the formation of which is the turnover-determining step of the catalysis.

The nickel­(II) hydride complex [(MeN2N)­Ni-H] (2) was synthesized by the reaction of [(MeN2N)­Ni-OMe] (6) with Ph2SiH2 and was characterized by NMR and IR spectroscopy as well as X-ray crystallography. 2 was unstable in solution, and it decomposed via two reaction pathways. The first pathway was intramolecular N–H reductive elimination to give MeN2NH and nickel particles. The second pathway was intermolecular, with H2, nickel particles, and a five-coordinate Ni­(II) complex [(MeN2N)2Ni] (8) as the products. 2 reacted with acetone and ethylene, forming [(MeN2N)­Ni-O i Pr] (9) and [(MeN2N)­Ni-Et] (10), respectively. 2 also reacted with alkyl halides, yielding nickel halide complexes and alkanes. The reduction of alkyl halides was rendered catalytically, using [(MeN2N)­Ni-Cl] (1) as catalyst, NaO i Pr or NaOMe as base, and Ph2SiH2 or Me­(EtO)2SiH as the hydride source. The catalysis appears to operate via a radical mechanism.

Isomerization and olefin exchange experiments show that β-H elimination is kinetically viable but thermodynamically unfavorable in [( Me NN 2 )Ni-alkyl] complexes. The intermediacy of Ni-hydride species was corroborated by a trapping experiment. The alkyl complex [( Me NN 2 )Ni-propyl ] catalyzes olefin isomerization.

The mechanism of alkyl-aryl Kumada coupling catalyzed by the nickel pincer complex Nickamine was studied. Experiments using radical-probe substrates and DFT calculations established a bimetallic oxidative addition mechanism. Kinetic measurements showed that transmetalation rather than oxidative addition was the turnover determining step. The transmetalation involved a bimetallic pathway.3

A method for the synthesis of the industrially relevant monomers adipic acid, 1,6-hexanediol (HDO), and 1,6-hexanediamine (HMD) via isomerizing hydroformylation of 1,3-butadiene is described. The aldehyde intermediates are protected in situ as acetals to avoid hydrogenation to pentanal. Adipic aldehyde diacetal is obtained in good yields, and the first examples for the conversion toward adipic acid, 1,6-hexanediol, and 1,6-hexanediamine are shown.