Steric Effects in the Baeyer−Villiger Oxidation of Ketones Catalyzed by Platinum(II) Lewis Acid Complexes with Coordinated Electron-Donor Alkyl Diphosphines

The synthesis and characterization of new hydroxo-bridged platinum(II) complexes of the type [Pt(μ-OH)(P−P)] 2 [BF 4 ] 2 ( 1a − d ), where P−P = R 2 PCH 2 CH 2 PR 2 (R = Me, dmpe, 1a; Et, depe, 1b; i -Pr, dippe, 1c; Cy, dcype, 1d ), and [Pt(P−P)(H 2 O) 2 ][BF 4 ] 2 ) (where R= t -Bu, dtbpe, 1e ) are reported. These complexes were tested in the Baeyer−Villiger oxidation of 2-methylcyclohexanone, 2-methylcyclopentanone, and cyclobutanone with 35% hydrogen peroxide as oxidant. The reactions were performed at 20 and 70 °C in a chlorinated solvent/H 2 O two-phase system. Within the above-reported series of complexes, 1e gave the highest catalytic activity and productivity in the oxidation of cyclic ketones. The Lewis acidity of 1a − e was investigated through the determination of the wavenumber shift Δν̄ = ν̄(C⋮N) coord − ν̄(C⋮N) free of the isocyanide moiety in complexes of the type [PtCl(CN-2,6-(Me) 2 C 6 H 3 )(P−P)][BF 4 ] (P−P = 2a − e ). The values of Δν̄ showed that compounds 2a − e have comparable Lewis acidity, thus indicating that the difference in the catalytic activity observed for 1a − e must be largely ascribed to steric requirements imparted by the alkyl diphosphine ligand.