Regioselectivities of the insertion reactions of unsymmetrical alkynes with the nickelacycles [NiBr(C₆H₄CH₂PPh₂-2)(L)] (L = tertiary phosphine)

The regioselectivities of the insertions of various alkynes into the Ni-Ph bond of the five-membered nickelacycles [NiBr(C₆H₄CH₂PPh₂)(PR₃)] (PR₃ = PEt₃ (2a), PPhBz₂ (2b)) have been determined by multinuclear NMR spectroscopic studies of the products. In some cases, the resulting seven-membered nickelacycles have also been structurally characterized by X-ray analyses. The results show that electronic factors are very important in controlling the regioselectivity of these insertion reactions. For example, insertion of MeC≡CCO₂Me into 2b forms mainly [NiBr{o-C(Me)=C(CO₂Me)C₆H₄CH₂ PPh₂}(PPhBz₂)] (4b), whereas CF₃C≡CCO₂Et gives exclusively the alternative regioisomer (10b). On the basis of these observations, a model based on frontier orbital interactions is proposed. The alkynylphosphines RC≡CPPh₂ insert regiospecifically into the Ni-C bonds of 2a,b to give initially complexes that contain three-membered phosphanickelacycles formed by coordination of the PPh₂ group to the adjacent metal. These species are unstable and subsequently dimerize (R = CO₂Me) or undergo rearrangement (R = Me, Ph) to give a bicyclic nickelacycle in which the metal is part of both a six- and a four-membered ring.