Regioselectivities of the Insertion Reactions of Unsymmetrical Alkynes with the Nickelacycles [NiBr(C<sub>6</sub>H<sub>4</sub>CH<sub>2</sub>PPh<sub>2</sub>-2)(L)] (L = Tertiary Phosphine)
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The regioselectivities of the insertions of various alkynes into the Ni−Ph bond of the five-membered nickelacycles (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 (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.
