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Mechanism in Polyoxometalate-Catalyzed Homogeneous Hydrocarbon Oxo Transfer Oxidation. The [Co4(H2O)2P2W18O68]10-/p-Cyano-N,N-dimethylaniline N-Oxide Selective Catalytic Epoxidation System

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journal contribution
posted on 22.05.1996 by Xuan Zhang, Ken Sasaki, Craig L. Hill
The Co-substituted heteropolyanions [Co4P2W18O68]10- and [CoPW11O39]5- catalyze the highly selective epoxidation of disubstituted alkenes and stilbenes by p-cyano-N,N-dimethylaniline N-oxide (CDMANO). Terminal alkenes are not readily epoxidized. The following d-electron transition metal-substituted (TMSP) complexes are less selective and two orders of magnitude less reactive than the Co complexes:  [MnIIIPW11O39]4-, [MnIIPW11O39]5-, [FeIIIPW11O39]4-, and [NiIIPW11O39]5-. The system (TBA)8H2[Co4P2W18O68] (TBA = n-Bu4N+) (TBA1)/CDMANO/alkene/CH3CN solvent is homogeneous throughout. The values for K1 (constant for 1:1 association of the following ligands with 1 under the catalytic conditions in these studies = acetonitrile solution, 25 or 50 °C) are 275 ± 13 (N-methylimidazole), 4.3 ± 0.1 (pyridine), 59 ± 3 (4-picoline N-oxide), 22 ± 1 (4-cyanopyridine N-oxide), and 57 ± 5 (N-methylmorpholine N-oxide, MMNO, a model for CDMANO). Comparisons of the electronic absorption spectra of 1 under catalytic turnover and several other conditions indicate formation of the 1:1 CDMANO adduct, a result also consistent with thermodynamic binding and kinetic data. Chromatographic separation and spectral (UV−visible, NMR) evidence indicate that the brown color in the epoxidation reactions evident after many turnovers results from condensed heterocyclic structures from oxidation of the principal product derived from CDMANO during catalysis, p-cyano-N,N-dimethylaniline (CDMA). Evaluation of the kinetics of cyclohexene epoxidation by CDMANO over a wide range of conditions affords the following empirical rate law:  +{d[epoxide]/dt}initial = k‘[cyclohexene]i[CDMANO]i[1]total)/(k‘‘[CDMANO]i + k‘‘‘[cyclohexene]i + k‘‘‘‘[cyclohexene]i[CDMANO]i + k‘‘‘‘‘). This is inconsistent with several common catalytic oxygenation mechanisms but consistent with a three-step mechanism:  an initial pre-equilibrium association of 1 and CDMANO; loss of CDMA and formation of a reactive high-valent cobalt intermediate; and then transfer of oxygen from the intermediate to alkene.