Reversible cyclobutane formation : the ring closure of σ,π-2-(penta-methylcyclopentadienyl)- 2-phenylethylpalladium compounds to (2-4-η-1,2,3,4,5- pentamethyl-6-endo- and -6-exo-phenylbicyclo[3.2.0]hept-2- enyl)palladium compounds

Abstract

The cyclisation of the dynamic σ,π-complex[{P[graphic omitted]e₅CHPhCH₂)Cl}₂](1α,β) is shown to lead to the bicyclo[3.2.0]hept-2-enylpalladium complexes, [Pd(γ-C₅[graphic omitted]H₂)Cl]₂(1 γ, phenyl endo to palladium) and [Pd(δ-C₆[graphic omitted]H₂)Cl]₂(1δ; phenyl exo). The exo-phenyl isomer is the most stable end product and the cyclisation to a cyclobutane (1αβ)→(1γ) is reversible. Solvate complexes of the α⇌β form [[graphic omitted]e₅CHPhCH₂)L₂][PF₆](L = MeCN or Me₂CO) were obtained but attempts to prepare the cyclo-octadiene (cod) or cycle-octatetraene (cot) derivatives led to the γ(phenyl endo) cyclised forms, [Pd(γ-C[graphic omitted]H₂(L₂)][PF₆(L₂= cod or cot), from which the pure halides [{Pd(γ-C[graphic omitted]H₂)X}₂](X = Cl, Br, or I) and the pentane-2,4-dionate were prepared. Analogous derivatives of the δ-series (phenyl exo) were also synthesised. The ¹H and ¹³C n.m.r. spectra have been measured; a novel form of isomerism about the halide bridge is described for (1 γ), and the Karplus relation of coupling constant to dihedral angle has been shown to be obeyed in a near-planar cyclobutane by a comparison of the hydrogen positions determined from the X-ray structure of [Pd(γ-C[graphic omitted]H₂)(acac)] and the cyclobutane J(H–H) values in a number of complexes.

The reversible cyclisation of a σ,π-complex to a cyclobutane derivative allows a reinterpretation of the cubane to cuneane isomerisation catalysed by [PdCl₂(PhCN)₂].