Acetylenes and noble-metal compounds. Part 14. Insertion reactions of 6-chloro-2,2,4,5,7,7-hexamethylocta-3,5-dien-3-ylpalladium compounds; the PdCl2-catalysed carbonylation of acetylenes to cyclopentadienones

Abstract

Carbonylation of the kinetically favoured σ-butadienyl complex [Pd(CButCMeCMeCButCl)(bipy)Cl](2; bipy = 2,2′-bipyridyl) gives an insoluble complex (6) which is suggested to be [Pd{η3-[graphic omitted]O]Cl}n]. Complex (6) on heating or on treatment with trimethyl phosphite gives 3,4-dimethyl-2,5-di-t-butylcyclopentadienone (4) which could also be obtained catalytically by carbonylation of ButC2Me in the presence of [Pd(NCPh)2Cl2]. Complex (6) reacts with pyridine (py) to give the exocyclic η3-allylic complexes [Pd2{η3-CH2[graphic omitted]Me}2Cl2] and [Pd{η3-CH2[graphic omitted]Me}(py)Cl], which with HCl generates (4). The acetylenes PhC2Ph and PhC2But are also carbonylated catalytically in the presence of [Pd(NCPh)2Cl2] to tetraphenylcyclopentadienone and 3,4-diphenyl-2,5-di-t-butylcyclopentadienone respectively. A new mechanism for the formation of cyclopentadienones viaσ-butadienylpalladium complexes is proposed. The σ,π-butadienyl complex [Pd3(η3-CButCMeCMeCButCl)2Cl4] reacts with mono-t-butyl-acetylene to give 1,3-dimethyl-2,4,6-tri-t-butylbenzene, a result which agrees with previous proposals for the mechanism of acetylene cyclotrimerisation reactions. The bipy ligand in complex (2) and in its thermodynamically favoured isomer (3) is labile and can be transferred to MCl2 to give [M(bipy)Cl2](M = Pd or Pt) and [Pd2(CButCMeCMeCButCl)2Cl2] which rearranges to the cyclobutadiene complex [Pd2(C4But2Me2)2Cl4].