Mechanism of the insertion reactions of alkynes with phosphanickelacycles

Article abstract of DOI:10.1021/om00012a023

The mechanism of the insertion reaction of substituted alkynes (PhCCPh (X), PhCCCOOEt (Y) MeOOCCCCOOMe (Z)) into the Ni-C bond of trans-[{Ni(2-C₆H₄CH₂PPh ₂)(μ-Cl)}₂] and trans-[NiCl(C-P)P′] (C-P = 2-C₆H₄CH₂PPh₂, P′ = PBz₃ (1a), PMe₂Ph (1b); C-P = 2-C₆H₄-CH₂PEt₂, P′ = PBz₃ (1′a)) complexes has been investigated by means of ³¹P NMR and UV-vis spectroscopy; in all cases insertion of only one molecule of alkyne has been found. The regioselectivity of the reaction is very well defined; only the regioisomer arising from a 1,2-cis addition on the Ni-C bond is obtained. The kinetics of the reactions have been studied via UV-vis spectroscopy. Results agree with the formation of a highly ordered transition state still containing the monodentate phosphine P′ in its coordination sphere, the effect of the metalated phosphine being less important (for example ΔH? = 43, 60, 40 kJ mol^-1 and ΔS? = -118, -67, -125 J Kr^-1 mol^-1 for the 1a + Y, 1b + Y, and 1′a + Y systems, respectively). A mechanism proceeding via the formation of a pentacoordinate intermediate aggregate, not detectable by NMR or visible spectroscopy, under the conditions of the study, is proposed. Only for the weaker π-acceptor alkyne PhCCPh is the observed rate constant found to be dependent on the alkyne concentration, indicating that for the other two alkynes studied the formation of the above-mentioned aggregate is complete under the kinetic conditions used (> 10-fold excess of alkyne).