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 (PR 3 = PEt 3 ( 2a ), PPhBz 2 ( 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⋮CCO 2 Me into 2b forms mainly ( 4b ), whereas CF 3 C⋮CCO 2 Et gives exclusively the alternative regioisomer ( 10b ). On the basis of these observations, a model based on frontier orbital interactions is proposed. The alkynylphosphines RC⋮CPPh 2 insert regiospecifically into the Ni−C bonds of 2a, b to give initially complexes that contain three-membered phosphanickelacycles formed by coordination of the PPh 2 group to the adjacent metal. These species are unstable and subsequently dimerize (R = CO 2 Me) 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.