Oxidative dehydrogenation of amines

CuOTf-catalyzed synthesis of polysubstituted pyrroles from a-diazoketones, nitroalkenes, and amines was reported by Lu, Wang, and coworkers. The corresponding polysubstituted pyrroles could be obtained in moderate yields using air as the oxidant. This cascade process of the polysubstituted pyrrole formation involves an NH insertion of carbene, a copper-catalyzed oxidative dehydrogenation of amine, and a [3+2] cycloaddition of azomethine ylide [21] (Scheme 8.9). 

In 2010, Wang s group reported a copper-catalyzed three-component reaction of a-diazoketones 122 with amines 123 and nitroalkenes 124 under aerobic conditions (Scheme 5.86) [87], The cascade reaction follows a sequence of an N—H insertion, a copper-catalyzed oxidative dehydrogenation of amine, a [3+2] cycloaddition, and finally, an elimination of HNOj/dehydrogenation, affording a variety of polysubsti-tuted pyrroles 125. 

Oxidative dehydrogenations of many macrocyclic ligand complexes have now been documented. Typically, these reactions involve conversion of coordinated secondary amines to imine groups. 

A useful example is the oxidative dehydrogenation of primary amines to nitriles. The amine (2 mmol) is added dropwise or in small portions to a vigoronsly stirred solutuion of [RnO ] prepared as above (100 cm ) the reaction is complete when the dark orange colonr of [RuO ] reappears. The solution is extracted with dieth-ylether (3x25 cm ), dried over MgSO and the ether removed [549]. 

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). 

Scheme 70). The precise position of oxidative dehydrogenation can vary for the same iigand associated with different metals such as nickel(II) and iron(II).210 212 Ruthenium(II) and iron(II) are particularly effective for the oxidative dehydrogenation of 1,2-diamine ligands213 214 and even more simple amines.215 216... 

Na5PV2Moio04o supported on active carbon is active for oxidative dehydrogenation of benzylic alcohols and amines without overoxidation of benzalde-hyde and benzylamine in the liquid phase (357). The suppression of the overoxidation may be due to the lower oxidizing ability of Na5PV2Moio04o relative to its acid form. 

We have already seen that imines may be formed by the oxidative dehydrogenation of co-ordinated amines and that this is a commonly observed process, particularly in macrocyclic systems. Likely mechanisms for these dehydrogenations were suggested in Chapter 5, which emphasised the role of the variable oxidation state metal ions in the process. These reactions are quite general and many examples involving iron or ruthenium complexes have been studied in detail. 

HRP catalyzes the oxidative dehydrogenation of a wide range of electron-rich aromatic compounds. The result of this radical formation pathway is dimerization and subsequent oligomerization of the substrates [76-78]. Peroxidases have been used to catalyze polymerizations of phenols (e.g. p-cresol and guaiacol) and aromatic amines (e.g. aniline, and o-phenyldiamine) [79, 80]. N- and O-dealkylations are also useful electron transfer reactions catalyzed by peroxidases. These reactions are used in industrial wastewater treatment and may have synthetic applications [81]. 

In general, Os complexes are highly effective for the oxidative dehydrogenation of coordinated amine and alcohol... 

The heterobimetallic complexes [N(n-Bu)4] [Os(N)R2(/u.-0)2Cr02] catalyze the selective oxidation of alcohols with molecular oxygen. A mechanism in which alcohol coordinates to the osmium center and is oxidized by B-hydrogen elimination (see -Hydride Elimination) is consistent with the data. The hydroxide adduct of OSO4, [0s(0H)204], with ferric cyanide and other co-oxidants catalyzes the oxidative dehydrogenation of primary aromatic and aliphatic amines to nitriles, the oxidation of primary alcohols to carboxylic acids, and of secondary alcohols to ketones. Osmium derivatives such as OsCb catalyze the effective oxidation of saturated hydrocarbons in acetonitrile through a radical mechanism. ... 

This chapter highlights the ruthenium-catalyzed dehydrogenative oxidation and oxygenation reactions. Dehydrogenative oxidation is especially useful for the oxidation of alcohols, and a variety of products such as ketones, aldehydes, and esters can be obtained. Oxygenation with oxo-ruthenium species derived from ruthenium and peroxides or molecular oxygen has resulted in the discovery of new types of biomi-metic catalytic oxidation reactions of amines, amides, y3-lactams, alcohols, phenols, and even nonactivated hydrocarbons tmder extremely mild conditions. These catalytic oxidations are both practical and useful, and ruthenium-catalyzed oxidations will clearly provide a variety of futrue processes. 

Newmann R, Levin M (1991) Selective aerobic oxidative dehydrogenation of alcohols and amines catalyzed by a supported molybdenum-vanadium heteiopolyanion salt. J Oig Chem 56 5707-5710... 

Dehydrogenation of amines may be achieved by thermolysis of sulphinamides, although benzylic or allylic activation is required to control the regioselectivity of elimination (Scheme 1)/ A biomimetic oxidative deamination of primary... 

The use of primary amines as suitable substrates for the synthesis of 2-substituted benzimidazoles, benzoxazoles, and benzothiazoles with CuBr2 catalyst was developed by Zhou and coworkers (Scheme 8.54). A key step in this reaction is the formation of an imine intermediate via oxidative dehydrogenation of benzylamine with the copper complex and molecular oxygen [94]. 

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