Nickel catalysts Negishi cross-coupling reactions

The cross-coupling of organozinc compounds has been accomplished using a number of metal enolate complexes, namely Rh(acac)(H2C=CH2)2 (6) , Co(acac)3 (30) °, Ni(acac)2 (47) , Li(acac) (107) and Cr(acac)3 (108) . For instance, complex 47 proved to be an extremely effective catalyst system for the Negishi cross-coupling of arylzinc halides (109) with aryl (110), heteroaryl and alkenyl halides, triflates and nonaflates to give the corresponding biaryl compounds (111) (equation 30). The solvent played an important role in these reactions and optimal conditions were found with 8 1 mixtures of THF and A-ethylpyrrolidinone (nep). Only 0.05 mol% of the nickel complex... 

Secondary alkyl hahdes are a much more challenging class of electrophiles for cross-coupling reactions [138]. The use of nickel catalysts allows couphng of these substrates with organoboranes in the presence of a 1,2-diatnine hgand (Scheme 1.9) [139]. Similar results have been obtained in the Negishi couphngs with nickel catalysts [140]. 

Unsymmetrical biaryh and diarylmethanes. Negishi et al have used a nickel-(0) complex or a palladium(0) complex as catalyst for the cross-coupling of aryl-or benzylzinc halides with aryl halides at room temperature to form biaryls or diarylmethanes. The zinc derivatives are prepared by reaction of aryl- or benzyl-lithium with zinc chloride or bromide. 

Alkenyl-alkenyl cross-coupling. Baba and Negishi have prepared a catalyst from this Pd(II) complex and 2 equiv. of diisobutylaluminum hydride that promotes this coupling reaction. Tetrakis(triphenylphosphine)palladium(0) is inactive, as is material prepared in situ from palladium chloride, triphenylphos-phine and HAKr-CtHg) . A nickel catalyst prepared from Ni(acac)2, PfCnHsja, and diisobutylaluminum hydride is somewhat less efficient. The coupling Involves (E)-alkenylalanes (4, 158, 159) and alkenyl halides. The products are (E,E)- and (E,Z)-dienes. 

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