The Oxidation of Amines and Alcohols

Oxidative dehydrogenation reactions of alcohols and amines are widespread in enzymatic biochemistry, and are of potential importance with regard to the operation of fuel cells based on simple alcohols such as methanol. The nature of products, and their rates of formation, may vary depending on the reaction conditions, and a role of metal ions has been recognized. The oxidation of amines may lead to a variety of products (nitriles, nitro species, etc.) although dehydrogenated diimine products are obtained quantitatively when the oxidation of the amine occurs via coordination to metal centers. A review is available on the mechanisms of oxidative dehydrogenations of coordinated amines and alcohols (93). 

A large amount of work has been devoted to N-binding macrocyclic complexes of Ni, Cu and Fe, which yield imine ligand products. Bidentate amine ligands, mainly ethylenediamine (en), have been used with Ru and Os complexes. The oxidation of coordinated ethylenediamine and related ligands stops at the diimine stage and does not continue to the dinitrile. The a,a -diimine entity -N=C-C = N- formed in the four-electron oxidation is particularly stable (93). 

Given the characterization of mononuclear [Fe(CN)5(en)]3 (98), as well as of other bound diamines (99), their ability toward reactive dehydrogenation could be expected, probably in a more effective way if dinuclear complex formation is allowed for. Some precedent exists for the autoxidation of bound 4-aminomethylpyridine to 

4-cyanopyridine in [Fe(CN)5]3 (100). The preparation of these putative dinuclear complexes, bridged by unsaturated diimine ligands could be an interesting issue, reinforced by the possibility of preparing new mixed-valence species, and stimulated by the renaissance experienced in this field, related to the diverse electronic delocalization patterns (9,10). 

Given the characterization of mononuclear [Fe(CN)5(en)] (98), as well as of other bound diamines (99), their ability toward reactive dehydrogenation could be expected, probably in a more effective way if dinuclear complex formation is allowed for. Some precedent exists for the autoxidation of bound 4-aminomethylpyridine to 4-cyanopyridine in [Fe(CN)5] (100). The preparation of these putative dinuclear complexes, bridged by unsaturated diimine ligands could be an interesting issue, reinforced by the possibility of preparing new mixed-valence species, and stimulated by the renaissance experienced in this field, related to the diverse electronic delocalization patterns (9,10). 

Disproportionation reactions of free NH2OH have earlier been reported to produce N2, N2O, and NH3 (104). Given that these processes are very slow, it has been proposed that they should have originated from metal ion impurities, suggesting that previous coordination of NH2OH is a prerequisite to disproportionation (105). Unfortimately, the redox properties of coordinated 

The NH2OH disproportionation reaction catalyzed by NP (75) was considered in a detailed kinetic study, performed spectrophotometri-cally and gas volumetrically 108). The found products were N2, N2O, and NH3, and a mechanistic interpretation identified a fast route, probably effected by radicals derived from a bridged hydroxylamine dinuclear complex. The slow route was associated with the intermediacy of a nitroside complex. 

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The Oxidation of Amines and Alcohols The Disproportionation of Hydroxylamine Miscellaneous Reactions... 

Commercial RuC13 xH20 is widely used as a catalyst (see references 3 and 2320 for details of earlier studies). Examples include the oxidation of amines and alcohols,2321 the production of glycol esters from synthesis gas,2322 and the generation of 1,3-diol derivatives from reaction of dienes and alkenes with aldehydes and carboxylic adds.2323... 

Fleishmann, M., Korinek, K. and Pletcher, D. (1972) The kinetics and mechanism of the oxidation of amines and alcohols at oxide covered nickel, silver, copper and cobalt electrodes. Journal of the Chemical Society, Perkin Transactions 2, 31,1396. 

The incorporation of Ti into various framework zeolite structures has been a very active research area, particularly during the last 6 years, because it leads to potentially useful catalysts in the oxidation of various organic substrates with diluted hydrogen peroxide [1-7]. The zeolite structures, where Ti incorporation has been achieved are ZSM-5 (TS-1) [1], ZSM-11 (TS-2) [2] ZSM-48 [3] and beta [4]. Recently, mesoporous titanium silicates Ti-MCM-41 and Ti-HMS have also been reported [5]. TS-1 and TS-2 were found to be highly active and selective catalysts in various oxidation reactions [6,7]. All other Ti-modified zeolites and molecular sieves had limited but interesting catalytic activities. For example, Ti-ZSM-48 was found to be inactive in the hydroxylation of phenol [8]. Ti-MCM-41 and Ti-HMS catalyzed the oxidation of very bulky substrates like 2,6-di-tert-butylphenol, norbomylene and a-terpineol [5], but they were found to be inactive in the oxidation of alkanes [9a], primary amines [9b] and the ammoximation of carbonyl compounds [9a]. As for Ti-P, it was found to be active in the epoxidation of alkenes and the oxidation of alkanes and alcohols [10], even though the conversion of alkanes was very low. Davis et al. [11,12] also reported that Ti-P had limited oxidation and epoxidation activities. In a recent investigation, we found that Ti-P had a turnover number in the oxidation of propyl amine equal to one third that of TS-1 and TS-2 [9b]. As seen, often the difference in catalytic behaviors is not attributable to Ti sites accessibility. 

Anodic oxidations of amines and alcohols can also be effected by reaction with the oxide films formed anodically at Ag, Cu, and Co electrodes (Table 7), as well as at Ni/ An H abstraction mechanism to the oxide film was proposed (cf. Ref. 40) rather than electron transfer to the anode surface. The latter would lead to organic ion reactions with proton transfer rather than free radical reactions which appear to be the indicated type of process in these anodic oxidations. 

Rate constants for a series of oxidations of amines and alcohols at anodically oxidized Ni, Ag, Cu, and Co electrodes were measured. The oxidations proceed by reaction with the oxide layer coupled with its electrochemical regeneration by an anodic reoxidation step. Reaction with solution-... 

The oxidation of primary and secondary alcohols in the presence of 1-naphthylamine, 2-naphthylamine, or phenyl-1-naphthylamine is characterized by the high values of the inhibition coefficient / > 10 [1-7], Alkylperoxyl, a-ketoperoxyl radicals, and (3-hydroxyperoxyl radicals, like the peroxyl radicals derived from tertiary alcohols, appeared to be incapable of reducing the aminyl radicals formed from aromatic amines. For example, when the oxidation of tert-butanol is inhibited by 1-naphthylamine, the coefficient /is equal to 2, which coincides with the value found in the inhibited oxidation of alkanes [3], However, the addition of hydrogen peroxide to the tert-butanol getting oxidized helps to perform the cyclic chain termination mechanism (1-naphthylamine as the inhibitor, T = 393 K, cumyl peroxide as initiator, p02 = 98 kPa [8]). This is due to the participation of the formed hydroperoxyl radical in the chain termination ... 

Cross double carbonylation of amines and alcohols Oxamates can be prepared by double carbonylation of amines and alcohols in the presence of (CH,CN)2PdCl2 as catalyst with 02 and Cul as oxidant and co-catalyst. This reaction is particularly efficient when applied to (3-amino alcohols. 

There is, though, a major difference in the way that amines and alcohols behave toward oxidizing agents. Amines generally show more complex behavior on oxidation because, as we shall see, nitrogen has a larger number of stable oxidation states than oxygen. 

It is difficult to anticipate the optimum activation time for the oxidation of a certain alcohol. Hindered alcohols are expected to require more than 15 min. On the other hand, a prolonged activation time, although not deleterious for the oxidation of many alcohols, whose corresponding alkox-ydisulfonium chlorides are stable, may promote side reactions, particularly in allylic, benzylic and propargylic alcohols. In such alcohols, it may be advisable to use a very short activation time at a very low temperature, followed by a prolonged reaction with an amine at low temperature. 

The chemoselectivity of the dioxirane oxyfunctionalization usually follows the reactivity sequence heteroatom (lone-pair electrons) oxidation > JT-bond epoxida-tion > C-H insertion, as expected of an electrophilic oxidant. Because of this chemoselectivity order, heteroatoms in a substrate will be selectively oxidized in the presence of C-H bonds and even C-C double bonds. In allylic alcohols, however, C-H oxidation of the allylic C-H bond to a,/ -unsaturated ketones may compete efficaciously with epoxidation, especially when steric factors hinder the dioxirane attack on the Jt bond. To circumvent the preferred heteroatom oxidation and thereby alter the chemoselectivity order in favor of the C-H insertion, tedious protection methodology must be used. For example, amines may be protected in the form of amides [46], ammonium salts [50], or BF3 complexes [51] however, much work must still be expended on the development of effective procedures which avoid the oxidation of heteroatoms and C-C multiple bonds. 

Fleischmann et al. [549] studied the electro-oxidation of a series of amines and alcohols at Cu, Co, and Ag anodes in conjunction with the previously described work for Ni anodes in base. In cyclic voltammetry experiments, conducted at low to moderate sweep rates, organic oxidation waves were observed superimposed on the peaks associated with the surface transitions, Ni(II) - Ni(III), Co(II) -> Co(III), Ag(I) - Ag(II), and Cu(II) - Cu(III). These observations are in accord with an electrogenerated higher oxide species chemically oxidizing the organic compound in a manner similar to eqns. (112) (114). For alcohol oxidation, the rate constants decreased in the order kCn > km > kAg > kCo. Fleischmann et al. [549] observed that the rate of anodic oxidations increases across the first row of the transition metals series. These authors observed that the products of their electrolysis experiments were essentially identical to those obtained in heterogeneous reactions with the corresponding bulk oxides. 

Glutamate, which now contains the nitrogen atom of the former amino acid, next undergoes an o. idative denmin-atjort to yield ammonJum ion and regerseraced u-ketc lutarate. The oxidation of the amine to an imine ii mechanistically similar to the oxidation of a secondary alcohol to a ketone and is carried out by NAD. The imine is then hydrolyzed in the usual way. 

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