Tungsten oxide, amine oxidation

Alkenyloxacarbenes, with tungsten carbonyls, 5, 675 Alkenyl oxiranes, for C-N bonds via amination, 10, 704 (Alkenyloxy)hydrosilanes, hydrosilylation-oxidation, 10, 832 Alkenylphosphonium iridium(III) complexes, preparation,... 

Although molybdenum and tungsten enzymes carry the name of a single substrate, they are often not as selective as this nomenclature suggests. Many of the enzymes process more than one substrate, both in vivo and in vitro. Several enzymes can function as both oxidases and reductases, for example, xanthine oxidases not only oxidize purines but can deoxygenate amine N-oxides [82]. There are also sets of enzymes that catalyze the same reaction but in opposite directions. These enzymes include aldehyde and formate oxidases/carboxylic acid reductase [31,75] and nitrate reductase/nitrite oxidase [83-87]. These complementary enzymes have considerable sequence homology, and the direction of the preferred catalytic reaction depends on the electrochemical reduction potentials of the redox partners that have evolved to couple the reactions to cellular redox systems and metabolic requirements. 

It is worthwhile to comment on the catalytic species. As opposed to oxometal species, which convert amines to imines, hydroperoxymetal complexes (MOOH) convert amines to nitrones. Thus the oxidation of amines is a convenient way of distinguishing the active species. The reactivity of oxometal versus peroxometal species is illustrated in Fig. 22. In practice, tungsten is the catalyst of choice to convert amines to nitrones [130]. 

Fe2(CO)9, and Fe3(CO)12, respectively. Similar disproportionations occurred with Ni(CO)4 and Co2(CO)8 which gave anionic species such as [Ni2(CO)6]2, [Ni3(CO)8]2, [Co(CO)4] , etc., upon treatment with ammonia or other amines. In contrast to the carbonyls of iron, nickel and cobalt, those of chromium, molybdenum and tungsten reacted with pyridine and 1,2-ethylenediamine to afford substitution products of the general composition M(CO)6 (py) (n = 1, 2, and 3) and M(CO)4(en) with the metal remaining in the oxidation state zero [25], Mainly as the result of this work, Hieber became convinced that the metal carbonyls should be regarded as true coordination compounds, and the coordinated CO should not be considered a radical but a monodentate ligand like NH3, pyridine, etc. He held this view despite the criticism by several of his contemporaries [3, 19] and was very pleased to see that in most textbooks published after 1940 this view had been accepted. 

It is possible to speed up aliphatic tertiary amine oxidation by adding tungstate or molybdate catalysts.334 However, for oxidation of aromatic and particularly heterocyclic tertiary nitrogen, a stronger system than hydrogen peroxide alone is required. iV-Oxidation of heterocycles is of pivotal importance in industrial chemical synthesis.335 Catalysed systems have been applied and these are dominated by metal peroxo systems based on molybdenum or tungsten. For example, quinoxaline and pyrazine may be oxidized to mono- or... 

The anti addition of amines to the double bond of cationic (alkene)(cyclopentadienyl)di-carbonyliron complexes and to the analogous molybdenum and tungsten complexes has been reported31 33. The adducts underwent carbonyl insertion-cyclization to give chelate complexes, which were then oxidized to /8-lactams. For example, from the Fp complexes of ( )- and (Z)-2-butene the corresponding /8-lactams were obtained diastereoselectively in 10-15% yield by the direct oxidation of the benzylamine adducts with chlorine at low temperature33. The stereochemistry was determined by H-NMR spectroscopy. 

When the Schiff base tungsten(0) compound mcr-[W(CO)(L)] (L = Ph2PC6H4CHNC6H4PPh2 Scheme 17) was exposed to HC1 gas, it converted to m-[WnCl2(CO)2(L )] (L = Ph2PC6H4CH2NHC6H4PPh2) as the final product. The 7r-iminium intermediate, i.e., the protonated complex mer-[W(CO)3(LH)]BF4, and the final product were both characterized structurally. In this reaction, the metal center has undergone a two-electron oxidation (W° to W11) and the original imine bond has been reduced to a secondary amine, as indicated by X-ray analysis and spectroscopic measurements.197... 

Tungsten catalysis. A primary amine with the group CH2NH2 on oxidation with 35% hydrogen peroxide in the presence of a catalytic amount of sodium tungstate (Na2W04 2H20, supplied by Fisher) is oxidized via the hydroxylamine to the... 

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