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(CBu^tCMeCMeCBu^tCl)(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 Bu^tC₂Me 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 PhC₂Bu^t 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-CBu^tCMeCMeCBu^tCl)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(CBu^tCMeCMeCBu^tCl)₂Cl₂] which rearranges to the cyclobutadiene complex [Pd₂(C₄Bu^t₂Me₂)₂Cl₄].