Hydrogen peroxide vanadyl complexes

The desire to convert benzene directly to phenol with 30% hydrogen peroxide was mentioned in Chap. 4. A polymer-supported salicylimine vanadyl complex (1 mol%) was used to catalyze this reaction. Phenol was obtained in 100% yield at 30% conversion.217 There was no leaching of the metal. The catalyst was recycled ten times after which it started to break up. Oxidation of ligands is often a problem with oxidation catalysts. Inorganic supports not subject to such oxidation need to be tried to extend the life of such catalytic agents. 

Three general classes of vanadium-containing compounds are of interest for their utility as insulin-mimetic agents (1) inorganic vanadium salts, both anionic (vanadates [V04] ) and cationic (vanadyl VO +) (2) complexes resulting from combination of vanadium(V) and hydrogen peroxide(s) (mono- and di-peroxovanadates, [V0(02 )(H2O)2(L-0f ( =0,1,2) and [V0(02)2 (L-Oj ( = 1,2, 3, and L = e.g., bipyridil, oxalate, phenanthroline, picoli-nate), and (3) chelated vanadium(IV) complexes (Scheme 5.8) [160], Vanadyl,... 

The oxidized electron transfer mediator (ETMox). namely the peroxo complexes of methyltrioxorhenium (MTO) and vanadyl acetylacetonate [VO(acac)2] and flavin hydroperoxide, generated from its reduced form (Figure 1.1) and H2O2, recycles the N-methylmorpholine (NMM) to N-methylmorpholine N-oxide (NMO), which in turn reoxidizes the Os(VI) to Os(VIII). While the use of hydrogen peroxide as oxidant without any electron transfer mediators is inefficient and nonselective, various alkenes were oxidized to diols in good to excellent yields employing this mild triple catalytic system (Scheme 1.2). 

The flavin is an efficient electron-transfer mediator, but rather unstable. Several transition metal complexes, for instance vanadyl acetylacetonate, can also activate hydrogen peroxide and are capable of replacing the flavin in the dihydroxylation reaction [18]. 

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