Nickel(0) Catalysts in Organic Synthesis

(A) Transfer hydrogenation of imines to the corresponding amines was efficiently catalyzed by a Ni(0)-NHC (N-heterocyclic carbene) complex. Using this catalyst, a variety of aldimines and ketimines were reduced in good to excellent yields under mild conditions. (B) Buchwald and Zhang reported a Ni(0)-catalyzed process for the transformation of enynes to bicyclic cyclopentenones. This method is tolerant of a variety of functional groups including esters, ketones, nitriles, ethers, and amines. It is also more effective with more highly substituted olefins than is the corresponding titanium procedure. (C) The Ni(0)-catalyzed cross-coupling of vinyl triflates with alkyl Grignard reagents have been achieved in good to excellent yield by Busacca and coworkers. The effects of triflate substitution, solvent and especially ligands have been investigated. (D) Louie and co-workers used Ni(0)-imidazolylidene complexes to catalyze the coupling reactions of various symmetrical diynes with CO2 under mild conditions to produce pyrones. They later reported the Ni(0)-catalyzed cycloaddition of CO2 with various asymmetrical diynes to afford regioisomeric mixtures of pyrones. (E) The cross-coupling reaction of aryl mesylates or chloroarenes with arylboronic acids proceeded efficiently in the presence of a Ni(0) catalyst and K3PO4, and various biaryls were synthesized in high yields. (F) Dehalogenation of aryl halides was efficiently performed in refluxing THF using a catalytic combination composed of Ni(0)-NHC-b-hydrogencontaining alkoxide. A Ni(0)-IMes·HCl-(i-Pr)ONa combination proved to be efficient for the dehalogenation of aryl chlorides, bromides, and iodides in short reaction times and under mild conditions. (G) Some stable Ni(0) catalysts, such as Ni[P(OPh)3]4 and Ni[P(OEt)3]4, were employed to efficiently promote the Heck reactions of aryl and vinyl halides with olefins and alkynes. (H) The selective synthesis of N-aryl- or N,Nc-diarylpiperazines and trimethylene (bis)piperidines from the corresponding diamines and aryl chlorides was achieved using a catalyst combination of Ni(0) associated to 2,2c-bipyridine. The Ni(0)/2,2c-bipyridine catalyst is also effective for the sequential arylation of piperazine. (I) A Ni(0) catalyst was used for the direct arylation of malononitrile, a bdifunctionalized compound, from halogenated aromatic substrates. The catalytic system is quite simple: Ni(PPh3)3, generated in situ from NiBr2(PPh3)2, PPh3 and zinc. Good yields and excellent selectivities have been obtained in a-arylmalononitriles from iodobenzene, but also from bromo- or chloro-aromatic substrates.