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

Abstract

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.

Quinone‐Catalyzed Selective Oxidation of Organic Molecules

“…Neumann and coworkers reported an aerobic oxidation of allylic and benzylic alcohols using 5 mol% o -chloranil with 1.5 mol% Na 5 PV 2 Mo 10 O 40 at 90 °C in H 2 O/decalin under 1 atm O 2 . [62] …”

Chinon‐katalysierte, selektive Oxidation organischer Moleküle

“…Neumann et al. haben eine aerobe Oxidation von Allyl‐ und Benzylalkoholen mit 5 Mol‐% o ‐Chloranil und 1.5 Mol‐% Na 5 PV 2 Mo 10 O 40 bei 90 °C in H 2 O/Decalin unter 0.1 MPa O 2 beschrieben 62…”

Enhancing the Reduction Potential of Quinones via Complex Formation

“…7 Similarly, quinone compounds have a wide range of application in synthetic chemistry, catalysis, and electrochemistry. [8][9][10][11][12][13][14] Active research continues to assess the usefulness of quinone in lithium-O2 batteries. [15][16][17] Quinones are very good oxidizing agents and can undergo one or two electron reduction, forming monoanion and dianion radical, respectively, depending on the conditions.…”