Dehydrogenative Oxidation of Alcohols

Moseley, K. and Maitlis, P.M. (1969) Journal of The American Chemical Society, 91, 5970. 

and Richards, R.L (1982) Comprehensive Organometallic Chemistry, Vol. 5, Iridium (eds G. Wilkinson, F.G.A. Stone and E.W. Abel), Pergamon, 

6 Buchanan, ).M., Stryker, J.M. and Bergman, R.G. (1986) Journal of the American Chemical Society, 108, 

1) Other than hydrogen transfer reactions, catalytic applications of Cp lr complexes for the deuteration of organic molecules [81-84], asymmetric Diels-Alder reactions [85, 86], 

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Lattice silver also can perform a dehydrogenative oxidation of alcohols with O2. For example, fert-butyl alcohol can be oxidized to isobutylene oxide on an O2 covered Ag(l 10) surface at elevated temperatures (85). However, other oxidation products also were produced. Experiments using 02 labeling revealed that the oxygen in the product is from the original alcohol and they believe the hydrogen atom from the methyl C—H bond is directly transferred to either O2 or another molecule of tcrf-butyl alcohol. Lattice silver is still widely used in industry and further studies hold promise for other industrially suitable methods (Fig. 14). 

Oxoreductases include enzymes such as dehydrogenases, oxidases and peroxidases which catalyse transformations such as oxidation of alcohols to carbonyls and dehydrogenation of functionalized alkanes to alkenes. 

The enzyme isocitrate dehydrogenase is one of the enzymes of the Krebs or citric acid cycle, a major feature in carbohydrate metabolism (see Section 15.3). This enzyme has two functions, the major one being the dehydrogenation (oxidation) of the secondary alcohol group in isocitric acid to a ketone, forming oxalosuccinic acid. This requires the cofactor NAD+ (see Section 11.2). For convenience, we are showing non-ionized acids here, e.g. isocitric acid, rather than anions, e.g. isocitrate. 

Oxidation of Cu with O2 is markedly slower than the analogous oxidation by TEMPO. The existence of a copper-centered dehydrogenation step was supported by investigating kinetic isotope effects and Hammett correlation studies using different substituted benzyhc alcohols. The /1-hydrogen abstraction was postulated to occur in a concerted mechanism with an -coordinated TEMPO radical bgand (Scheme 4). As such, this TEMPO-mediated copper-catalyzed oxidation of alcohols bears resemblance... 

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). 

Anodic dehydrogenations, e.g., oxidations of alcohols to ketones, have been treated in Sect. 8.1 and formation of olefins by anodic elimination of C02 and H+ from carboxylic acids was covered in Sect. 9.1. Therefore this section is only concerned with anodic bisdecarboxylations of v/odicarboxylic acids to olefins. This method gives usually good results when its chemical equivalent, the lead tetraacetate decarboxylation, fails. Combination of bisdecarboxylation with the Diels-Alder reaction or [2.2] -photosensitized cycloadditions provides useful synthetic sequences, since in this way the equivalent of acetylene can be introduced in cycloadditions. 

The silver-loaded titania photocatalysts possess high activity in the photocatalytic decomposition of ozone [38], photoreduction of various thiols [39], photodestruction of 1,4-dichlorobenzene [40], dehydrogenation and oxidation of alcohols (e.g. of 2-propanole [41,42]), decoloration of textile waste water [43], photokilling of bacteria [44], and others. For additional detailed information on this topic, one can use a number of review articles and books [e.g. 45-48]. 

The aerobic oxidation of alcohols catalysed by low-valent late-transition-metal ions, particularly those of group VIII elements, involves an oxidative dehydrogenation mechanism. In the catalytic cycle (Fig. 5) ruthenium can form a hydridometal species by /1-hydride elimination from an alkoxymetal intermediate, which is reoxidized by dioxygen, presumably via insertion of 02 into the M-H bond with formation of H202. Alternatively, an alkoxymetal species can decompose to a proton and the reduced form of the catalyst (Fig. 5), either directly or via the intermediacy of a hydridometal intermediate. These reactions are promoted by bases as cocatalysts, which presumably facilitate the formation of an alkoxymetal intermediate and/or /1-hydride elimination. 

Other ruthenium-based catalysts for the aerobic oxidation of alcohols have been described where it is not clear if they involve oxidative dehydrogenation by low-valent ruthenium, to give hydridoruthenium intermediates, or by high-valent oxoruthenium. Masutani et al. [107] described (nitrosyl)Ru(salen) complexes, which can be activated by illumination to release the NO ligand. These complexes demonstrated selectivity for oxidation of the alcoholic group versus epoxidation, which was regarded as evidence for the intermediacy of Ru-oxo moieties. Their excellent alcohol coordination properties led to a good enantiomer differentation in the aerobic oxidation of racemic secondary alcohols (Fig. 19) and to a selective oxidation of primary alcohols in the presence of secondary alcohols [108]. 

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. 

The most important applications of peroxyacetic acid are the epoxi-dation [250, 251, 252, 254, 257, 258] and anti hydroxylation of double bonds [241, 252, the Dakin reaction of aldehydes [259, the Baeyer-Villiger reaction of ketones [148, 254, 258, 260, 261, 262] the oxidation of primary amines to nitroso [iJi] or nitrocompounds [253], of tertiary amines to amine oxides [i58, 263], of sulfides to sulfoxides and sulfones [264, 265], and of iodo compounds to iodoso or iodoxy compounds [266, 267] the degradation of alkynes [268] and diketones [269, 270, 271] to carboxylic acids and the oxidative opening of aromatic rings to aromatic dicarboxylic acids [256, 272, 271, 272,273, 274]. Occasionally, peroxyacetic acid is used for the dehydrogenation [275] and oxidation of aromatic compounds to quinones [249], of alcohols to ketones [276], of aldehyde acetals to carboxylic acids [277], and of lactams to imides [225,255]. The last two reactions are carried out in the presence of manganese salts. The oxidation of alcohols to ketones is catalyzed by chromium trioxide, and the role of peroxyacetic acid is to reoxidize the trivalent chromium [276]. 

The domain of oxidations with silver oxide includes the conversion of aldehydes into acids [63, 206, 362, 365, 366, 367 and of hydroxy aromatic compounds into quinones [171, 368, 369]. Less frequently, silver oxide is used for the oxidation of aldehyde and ketone hydrazones to diazo compounds [370, 371], of hydrazo compounds to azo compounds [372], and of hydroxylamines to nitroso compounds [373] or nitroxyls [374] and for the dehydrogenation of CH-NH bonds to -C=N- [375]. Similar results with silver carbonate are obtained in oxidations of alcohols to ketones [376] or acids [377] and of hydroxylamines to nitroso compounds [378]. 

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