Mechanism in polyoxometalate-catalyzed homogeneous hydrocarbon oxo transfer oxidation. The [Co-4(H2O)(2)P2W18O68](10)-/p-cyano-N,N-dimethylaniline N-oxide selective catalytic epoxidation system

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: [(MnPW11O39)-P-III](4-), [(MnPW11O39)-P-II](5-), [(FePW11O39)-P-III](4-), and [(NiPW11O39)-P-II](5-). The system (TBA)(8)H-2[Co4P2W18O68] (TBA = n-Bu(4)N(+)) (TBA1)/CDMANO/alkene/CH3CN solvent is homogeneous throughout. The values for K-1 (constant for 1:1 association of the following ligands with 1 under the catalytic conditions in these studies = acetonitrile solution, 25 or 50 degrees 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) + ktriple prime[cyclohexene](i) + ktriple prime[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.