Quinones as Co-Catalysts and Models for the Surface of Active Carbon in the Phosphovanadomolybdate-Catalyzed Aerobic Oxidation of Benzylic and Allylic Alcohols: Synthetic, Kinetic, and Mechanistic Aspects
✍ Scribed by Ronny Neumann; Alexander M. Khenkin; Inga Vigdergauz
- Publisher
- John Wiley and Sons
- Year
- 2000
- Tongue
- English
- Weight
- 156 KB
- Volume
- 6
- Category
- Article
- ISSN
- 0947-6539
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✦ Synopsis
Quinones have been considered as reactive compounds present on the surface of active carbon. Thus, the co-catalytic use of quinones combined with the phosphovanadomolybdate polyoxometalate, PV2Mo10O40(5-), has been studied as an analogue of the known PV2Mo10O405-/C catalyst in oxidative dehydrogenation reactions. From the synthetic point of view both biphasic the quinone (org)-Na5PV2Mo10O40- (aq) and monophasic quinone (org)- 4Q5PV2Mo10O40-(org) [4Q = (nC4H9)4-N+] systems are effective for the selective oxidation of benzylic and allylic alcohols to their corresponding aldehydes. Kinetic measurements carried out on the model oxidative dehydrogenation of 4-methylbenzyl alcohol in the presence of p-chloranil, 4Q5PV2Mo10O40, and molecular oxygen showed that the reaction was non-elementary, although the 4-methylbenzyl alcohol oxydehydrogenation was the rate-determining step. ESR measurements showed the presence of the semiquinone of p-chloranil, probably as a complex with the polyoxometalate. This proposed complex was shown to be a more potent oxidant than p-chloranil. Thus, for the oxidation of 4-methoxytoluene the semiquinone complex was active, whereas p-chloranil alone was inactive. Beyond the importance of understanding quinone-phosphovanadomolybdate polyoxometalate-catalyzed reactions, insight gained from the formation of semiquinone active species can be applied for heterogeneous and aerobic oxidative transformations catalyzed by PV2Mo10O405- with carbon matrices as active supports.