The formation of a metallacycloheptadienyl intermediate in the reaction of palladacyclopentadienyl derivatives with tetracyanoethylene

The palladium(II) derivatives of the type 1 Pd(LL′)(C₄R₁R₂R₃R₄)] (LL′ = HNSPh:2-(phenylthiomethyl)-pyridine (A), BiPy: 2,2′-bipyridyl (B), DPPE: bis-diphenylphosphinoethane (C), NEOC: neocuproine (2,2′-dimethyl-o-phenanthroline) (D), R₁ = R₄ = COOMe, R₂, R₃ = C₁₀H₆ (a), R₁ = R₃ = C₆H₅, R₂ = R₄ = COOMe (b), R₁ = R₂ = R₃ = R₄ = COOMe (c)) react with the electron poor olefin tetracyanoethylene (TCNE) to yield under mild conditions the type 2 cycles C₆(CN)₄R₁R₂R₃R₄ and the corresponding palladium(0) olefin derivative Pd(η²]-TCNE)(LL′)]. The olefin insertion reactions are usually fast, but in the case of the reaction of complex 1Da with TCNE accumulation of an intermediate is observed. The low temperature NOESY spectrum allows the determination of the intermediate structure which can be described as a hepta-membered metallacycle species.