Aj. Edwards et al., Regioselectivities of the insertion reactions of unsymmetrical alkynes with the nickelacycles [NiBr(C6H4CH2PPh2-2) (L)] (L = tertiary phosphine), ORGANOMETAL, 20(13), 2001, pp. 2864-2877
The regioselectivities of the insertions of various alkynes into the Ni-Ph
bond of the five membered nickelacycles [NiBr(C6H4CH2PPh2)(PR3)] (PR3 = PEt
3 (2a), PPhBz(2) (2b)) have been determined by multinuclear NMR spectroscop
ic studies of the products. In some cases, the resulting seven-membered nic
kelacycles 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 M
eC drop CCO2Me into 2b forms mainly [NiBr{o-C(Me) = C(CO2Me)C6H4CH2PPh2}(PP
hBz(2))] (4b), whereas CF3C drop CCO2Et gives exclusively the alternative r
egioisomer (10b). On the basis of these observations, a model based on fron
tier orbital interactions is proposed. The alkynylphosphines RC drop CPPh2
insert regiospecifically into the Ni-C bonds of 2a,b to give initially comp
lexes that contain three-membered phosphanickelacycles formed by coordinati
on 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.