Alkyne Insertions with Nickel-Allyl Complexes

Alkyne Insertions with Nickel-Allyl Complexes 

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Alkyne Insertions with Nickel-Allyl Complexes... 

Two catalytic cycles are proposed to explain the difference in selectivity. In both cases, catalytic cycle is initiated by the oxidative addition of an alkynylstannane to nickel(O) species, leading to the formation of alkynylnickel(ll) complex 77 (Scheme 24).92 Then, an allene is inserted into the nickel(ll) complex in a manner which avoids steric repulsion with the butyl group to afford the anti-ir-a y complex 80. The carbometallation of the terminal alkyne can take place at the non-substituted allylic carbon of the corresponding syn-Ti-a y complex 78. The stereoselectivity is determined by the relative rate of the two possible insertion modes which depend on the ligand used. A bidentate... 

The ease of interaction of Ni(0) complexes with organic substrates has been shown to depend upon both the ligands on nickel and the solvent. The presence of a,a-bipyridyl with the Ni(0) complex and the alkyne led to the isolation of a nickelacyclopropene, an observation in accord with the recently proposed metallocyclic pathway for the Ni(0)-cata-lyzed trimerization of alkynes. Allylic and benzylic ethers and epoxides have been observed to undergo oxidative insertion of Ni(0) into their C-0 bonds with solvent (TMEDA > THE > Et O > CeHe) and ligand (EtsP > PhsP a,a-bipy > COD) effects consistent with an electron-transfer attack by Ni(0). With such sulfur heterocycles as dibenzothiophene, phenoxathiin, phenothiazine, and thian-threne, a 1 1 admixture of (COD)2Ni with a,a-bipyridyl gave as the principal product the desulfurized, ring-contracted cyclic product. 

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