Synthesis, characterization, and rearrangements of [(1-methylcyclobutyl)methyl]platinum(II) complexes. Very mild ring-strain-induced carbon-carbon activation

Article abstract of DOI:10.1021/om00090a004

Complexes trans-Pt(PMe₃)₂ClR, where R = (1-methylcyclobutyl)methyl (mcbm, 1), (adamantyl)methyl (adm, 8), 4-methylpentyl-1,1-d₂ (9), and 4-methyl-4-pentenyl (10), and also PtCl(dmpe)(mcbm) [dmpe = 1,2-bis(dimethylphosphmo)ethane] (2) have been prepared. Pyrolysis of 1 or 2 at 140°C yields 2-methyl-1,4-pentadiene (12) as the only organic product, and trans-HPt(PMe₃)₂Cl (13) is isolated in high yield in the case of 1. Added PMe₃ retards the pyrolysis of 1, leading to formation of [HPt(PMe₃)₃]Cl (14) and exhibiting kinetics consistent with initial PMe₃ dissociation. Decomposition of 8 requires heating at 240°C for hours. Pyrolysis of 9 at 140°C forms 13 and 4-methyl-1-pentene-1,1-d₂ with very little rearrangement of the deuterium label. Treatment of 1 with Ag⁺ in acetone at -80°C forms [trans-Pt(PMe₃)₂(mcbm)(acetone)]⁺ (15) that rearranges above -40°C to {cis-Pt(PMe₃)₂[1,4,5-n-(CH₂) ₃C(Me)=CH₂]}⁺ (16) and above -10°C to a mixture of [Pt(PMe₃)₂(2-4-n-2-methylpentenyl)]⁺ (17a) and [Pt(PMe₃)₂(1-3-n-2-methylpentenyl)]⁺ (17b), which is isolated as the PF₆^- salt. Reaction of 1 with Ag⁺ in CD₂Cl₂ at -80 °C leads within 30 min to direct formation of 16, representing an extremely mild C-C activation step. Reaction of 10 with Ag⁺ at -80°C followed by warming to -20°C also generates 16. In contrast, 8 and Ag⁺ from [Pt(PMe₃)₂(adm)(acetone)]⁺ which is isolable at ambient temperature. Mechanistic implications of these reactions are discussed.