Synthesis of Pd(II) and Pt(II) complexes possessing bicyclo[3.2.0]heptanyl phosphinite ligands: Identification of a novel Pd(II) precatalyst for 1,6-diene cycloisomerisation

The synthesis of monodentate and bidentate phosphinite ligands, possessing the unusual bicyclo[3.2.0]heptane framework, are reported. A convenient, tin-free synthesis of a key intermediate, namely 3-endo-6-endo-dihydroxybicyclo[3.2.0]heptane, is described. The air-sensitive phosphinite ligand 1 was either protected as the borane adduct, which is air-stable, or reacted directly with bis(acetonitrile)palladium(II)chloride to give the novel air and moisture stable palladium(II) complex 11. A platinum(II) relative 12 has also been synthesised by reaction of phosphinite 1 with bis(benzonitrile)platinum(II)chloride. Each complex has been thoroughly characterised and their molecular structures confirmed by X-ray diffraction studies. In catalytic applications, such as cross-coupling reactions of organometallic reagents with organohalides, an unexpectedly poor activity has been established for 11. For example, Suzuki–Miyaura cross-coupling of activated and deactivated aryl bromides with aryl boronic acids, in the presence of catalytic quantities of 11, proceed in low yield, accompanied by substantial homocoupling. Palladium agglomeration, to produce catalytically inactive Pd black, is rapid in these reactions, under both aqueous and non-aqueous conditions. The poor reactivity is proposed to arise through an unfavourable near tetrahedral ‘(PP′)Pd(0)’ geometry, which slows the oxidative addition step in the catalytic cycle with either activated or deactivated aryl halides. The steric bulk of the ligand and the associated large P–M–P′ bite angle, particularly at the palladium zero oxidation state, is proposed to account for the poor reactivity. However, we have established that cationic derivatives of 11 promote the cycloisomerisation of diallylmalonate in a regioselective fashion.