Study of the Reactions of Substituted Allenes with Tris(triphenylphosphine)nickel(0). An Analysis of the Factors Affecting the Regio- and Stereochemistry of ?-Complex Formation and Coupling To Form Nickelacyclopentane Complexes

Article abstract of DOI:10.1021/ja00297a022

The reactions of ethylallene (ETA), tert-butylallene (TBA), phenylallene (PHA), cyanoallene (CYA), and methxoyallene (MEA) with tris(triphenylphosphine)nickel(0) <(TPP)3Ni> have been investigated and compared with the reaction of dimethylallene (DMA) with (TPP)3Ni reported earlier.At low temperatures (< -70 degC) ETA, TBA, and DMA reversibly form mono- and bis-allene ?-complexes which can be detected by NMR. ?-Complex formation involves predominantly the least substituted double bond.DMA displaces ETA and TBA, while MEA in turn displaces DMA from the ?-complexes.PHA and CYA undergo immediate coupling at low temperatures (< -70 degC), and the relative stabilities and structures of their ?-complexes could not be determined.The relative stabilities and structures of the ?-complexes are discussed in terms of steric and electronic effects.The cis bis-?-complexes undergo coupling to form nickelacyclopentane ?-complexes: the relative reactivity sequence being CYA > PHA > MEA >> DMA > ETA > TBA.Evidence for a competitive electron-transfer process with CYA is presented.The structures of the intermediate substituted nickelacyclopentane complexes are inferred from the structures of the hydrocarbon products formed in reductive-elimination and hydrogen-migration, reductive-elimination reactions and from the structures of ketonic products formed on reaction with carbon monoxide.Specific ?-?, ?-?, and ?-? modes of coupling in syn and anti conformations of the bis-?-complexes have been identified.The regio- and stereochemistry of formation of the nickelacyclopentane ?-complexes are discussed in terms of steric and electronic (orbital energy and AO coefficients) effects in the rate-determining transition state for ?-complex formation.