Vanadium‐Catalysed Aerobic Cleavage of C−C Bonds in Substituted Cyclohexanones To Afford Carboxylic Acids: Two Model Complexes with Tetrahedral Geometry around Vanadium( V )

Two monosilanol/disilanol derivatives were synthesised by treatment of Ph3SiOH or Ph2Si(OH)2 with [nBu4N]VO3 (3) in ethanol solution. The mononuclear anionic dioxo vanadium(V) species [{Ph3SiO}2VO2]− (4) and the dinuclear complex anion [{Ph2SiO2VO2}2]2− (5) thus obtained are unusual and novel model compounds with ≡Si−O−V(O)2O linkages characterised by single‐crystal X‐ray diffraction. The catalytic activities of these tert‐butylammonium complexes, together with those of the simple vanadium derivatives 3 and 5, of the vanadyl precursors VOSO4·5H2O (1) and [VO(acac)2] (2), and also of “H6[PMo9V3O40]·aq”, were tested for aerobic C−C bond cleavage in 2‐hydroxycyclohexanone and 2‐methylcyclohexanone. Monomeric vanadium species show the same chemistry as the polyoxometalate but with lower yields of acids or keto acids, and so the redox and acid properties of HPA‐n (n = 3, 4) are better tuned. Mechanisms for these oxidation reaction, including electron transfer from a substrate coordinated to vanadium, are proposed. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)