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(mu-OH)(P- P)]2[BF4]2 (1a-d), where P-P = R2PCH2CH2PR2 (R = Me, dmpe, 1a; Et, depe, 1b; i- Pr, dippe, 1c; Cy, dcype, 1d), and [Pt(P-P)(H2O)2][BF4]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 degrees C in a chlorinated solvent/H2O 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 Delta(nu) over bar = (nu) over bar (C=N)(coord) - (nu) over bar (C=N)(free) of the isocyanide moiety in complexes of the type [PtCl(CN-2,6-(Me)2C6H3)(P- P)][BF4] (2a-e). The values of Delta(nu) over bar 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.