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Trends in Photochemistry & Photobiology   Volumes    Volume 4 
Aerobic photolysis of saturated C-H bond with decatungstate anion, mechanism and products
Charles Giannotti, Christoph Richter
Pages: 43 - 54
Number of pages: 12
Trends in Photochemistry & Photobiology
Volume 4 

Copyright © 1997 Research Trends. All rights reserved

The aerobic photocatalysed oxidation of alkanes in the presence of decatungstates Q4W10O32Q = Na, Me4N, Pr4N, Bu4N, Hex4N leads to hydroperoxides as primary products. In the case of adamantane after reduction, a mixture of adamantanol-1, adamantanol-2 and adamantanone is obtained. The quantum yield of the Ad oxidation is 0.11. Nitrile solvents and organic counter-ions of the catalyst compete in this photooxidation with alkane substrate. The influence of this competition on the quantum yields, rate and selectivity for the Ad oxidation is quantitatively evaluated, a similar study with catalyst such as [Xn+Wl2040]8-n where X = Si, Co2+, Co3+is also reported. In such case the suggested mechanism envisages the hydroperoxides formation in the solvent cage followed by their photodecomposition to alcohols and ketone with λ>260 nm light. Generated during light excitation the ligand to metal charge transfer exited state of decatunsgtate anion W10O324- relaxes for t < 30 ps to a transient X which persists for 5 ns. In the nanosecond time domain it reacts with all components of the photocatalytic system (substrate, organic counter-ion and solvent) to give one-electron reduced decatungstate species HW10O324- and an organic radical R.. The latter is quenched by dioxygen at a near diffusion controlled rate, forming a peroxy radical ROO. which reoxidizes HW10O324-. Thus closing the catalytic cycle. Irradiation of deaerated acetonitrile solution of nitroxide radicals and Q4W10O32 produces a gradual decrease in the ESR signal of these radicals. The reduced blue species HW10O324- and the nitrosonium salt are found. We have been able to oxidize in a selective manner with a relatively good yield the 1,8 cineol which is difficult to achieve by ordinary chemical methods. Using solar light (from Plataforma Solar of Almeria) we have performed the photooxidation of adamantane in a more large scale (5 1) and the results obtained correlate well with those we have reported in a smaller scale.
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