POLYOXOMETALATE SYSTEMS FOR THE CATALYTIC SELECTIVE PRODUCTION OF NONTHERMODYNAMIC ALKENES FROM ALKANES - NATURE OF EXCITED-STATE DEACTIVATION PROCESSES AND CONTROL OF SUBSEQUENT THERMAL-PROCESSES IN POLYOXOMETALATE PHOTOREDOX CHEMISTRY

The photooxidations of exemplary branched acyclic alkanes and cycloalkanes by a range of polyoxotungstates varying in charge density, ground-state redox potential, acidity, and other properties were examined in detail. The organic products generated in these reactions depend on the polyoxometalate used, and in particular on the ground-state redox potential of the complex. Under anaerobic conditions acyclic branched alkanes yield principally alkenes, while cycloalkanes yield principally alkenes and dimers. Alkyl methyl ketones, derived inpart from reaction with acetonitrile solvent, and isomerized alkanes are produced with some alkane substrates. Under aerobic conditions, autoxidation, initiated by radicals generated in the photoinduced redox chemistry, is observed. Under anaerobic conditions the polyoxotungstates with formal redox potentials more negative than-1.0 V vs Ag/AgNO3(CH3CN), such as W10O324-and W6O192-, photochemically dehydrogenate branched acyclic alkanes in high selectivity to-olefins and the least substituted alkenes, products heretofore undocumented in photooxidation reactions catalyzed by polyoxometalates. In contrast, the polyoxotungstates, regardless of structural family, with ground-state formal redox potentials less negative than-1.0 V vs Ag/AgNO3(CH3CN), such as α-PW12O403- and α-P2W18O626-, photodehydrogenate these alkanes in high selectivity to the thermodynamic or most substituted alkenes. The ratio of the most substituted alkenes to other alkenes is higher in the latter reactions than the ratio seen under conditions of acid equilibration. Several lines of evidence confirm the importance of alkyl radicals as intermediates and/or implicate hydrogen abstraction as the dominant mode of attack of the excited state of W10O324- on the alkane substrate: the ratio of alkenes to dimers at early reaction times, the relative rates of polymerization of the various alkenes generated in the reaction (a heretofore undocumented reaction of polyoxometalate excited states), the suppression of all alkene and other organic products generated under anaerobic conditions in favor of the most substituted (tertiary) hydroperoxide upon addition of O2 to the reaction, the primary kinetic isotope effect, Kcyciohexane-K12/Kcyciohexane-d12 of 2.5 for oxidation by the excited state of W|0O324', the relative reactivities of differentalkanes, the relative reactivities of tertiary vs primary C-H bonds, the regiochemistry of the alkyl fragments in the dimeric byproducts, and other data. The rate law established for photooxidation of alkanes by W10O324- (approximately first order in alkane, first order in light intensity, and variable order in W10O324-) coupled with the constancy of the ratios of olefinic to dimeric products over a wide range of both light intensity and W10O324- concentration and other data establish that the major factor determining the regiochemistry in production of the nonthermodynamic alkenes is bimolecular radical disproportionation while the major factor in determining the regiochemistry in production of the thermodynamic or more substituted alkenes is deprotonation of carbonium ions generated by oxidation of the intermediate radicals by ground-state polyoxotungstates. © 1990, American Chemical Society. All rights reserved.