Dehydrogenation of Alcohols to Aldehydes or Ketones

The synthesis of aldehydes or ketones by dehydrogenation of alcohols is one chemical route of many possible alternatives. The hydroformylation of olefins is the most utilized synthetic route, although dehydrogenation has found a place in the production of fragiunce aldehydes. From the Ullmann Encyclopedia [40] it is apparent that dehydrogenation becomes predominant when the carbon number is Cg or more for straight-chain aldehydes. The unsaturated Cio aldehydes from the transformation of essential oils are a second important field where dehydrogenation is often employed (Table 1). 

Dehydrogenation is normally performed at high temperatures and low pressures, preferably with hydrogen as carrier gas. This is acceptable as long as the chemical stability of the molecules tolerates such severe conditions. When this is not so, different alternatives must be considered. Such alternatives are reactions in liquid phase at the appropriate temperature and pressure by use of (i) solvent, (ii) a purging inert gas (iii) hydrogen acceptors or (iv) even low surface-area catalysts. The conditions are a function of the type of reaction and must be adjusted in consequence. 

Farrauto, C. H. Bartholomew. Fundamentals of Industrial Catalytic Processes, Chapman and Hall, New York, 1997, p. 630. 

Knozinger, J. Weitkamp (eds.). Handbook of Heterogeneous Catalysis, Vol. 5, Wiley-VCH, Weinheim, 1996, p. 2159. 

Satterfield, Heterogeneous Catalysis in Industrial Practice, McGraw-Hill. New York. 1991, p. 554. 

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Platonov and co-workers have made a significant contribution to the study of alcohol dehydrogenation by means of rhenium catalysts (310). Rhenium disulfide was found to be efficient in promoting dehydrogenation of alcohols to aldehydes or ketones (acetone), and in the dehydrogenation of cyclohexanol to phenol and a small amount of cyclohexanone (308),... 

Dimethyl sulfide and chlorine or, better still, dimethyl sulfide and N-chlorosuccinimide, form a system capable of the selective dehydrogenation of alcohols to aldehydes or ketones. The intermediates, such as (013)28 C1 Cr, react with bases according to the scheme in equation 25 [720],... 

Complexes of these metals catalyze the dehydrogenation of alcohols to aldehydes or ketones, producing gaseous or transferring hydrogen... 

Dehydrogenation of alcohols to aldehyde or ketone allows subsequent bond construction steps which would not be possible for the parent alcohols. Hence, a variety of iridium, rhodium or ruthenium phosphine, pincer and related complexes, that are efficient catalysts for the dehydrogenation of alcohols, can potentially be appHed for the related hydrogen-transfer reactions, thus leading to new added-value compounds. The hydrogen atoms transfer to a sacrificial hydrogen acceptor, such as a carbonyl compound or an olefin which is reduced to the corresponding alcohol or alkane. 

Oxidation or Dehydrogenation of Alcohols to Aldehydes and Ketones C,0-Dihydro-elimination... 

Benzeneseleninic anhydride, C5HjSe(0)0(0)SeC5Hs, which is prepared in situ from diphenyldiselenide and tert-hniyX hydroperoxide, is used for the oxidation of alcohols to aldehydes or ketones [525]. This reagent is a suitable dehydrogenating agent for the introduction of double bonds a to carbonyl groups [526] and the regeneration of ketones from their oximes, semicarbazones, and phenylhydrazones [527]. 

Acetone, cyclohexanone, benzophenone, cinnamaldehyde, and other carbonyl compounds are hydrogen acceptors in the Oppenauer oxidation of alcohols to carbonyl compounds. The reaction is catalyzed by Raney nickel [961], aluminum alkoxides [962], tris(isopropoxide), or tris(tert-bu-toxide) as bases soluble in organic solvents [963, 964]. These dehydrogenations of alcohols to aldehydes and ketones require refluxing or distillations and have given way to dimethyl sulfoxide oxidations, which take place at room temperature. 

Table 1 summarizes the experimental results obtained in our laboratory on the kinetics of the normal dehydrogenation of hydrocarbons (hexahydro-aromatics to aromatics, the open chain compounds butylene to butadiene, and ethylbenzene to styrene), of amines to ketimines, and of alcohols to aldehydes or to ketones, respectively, in the presence of metallic or oxide catalysts. Equation (1) was found to apply in all cases. Ko and h are given by... 

Oxidations by oxygen and catalysts are used for the conversion of alkanes into alcohols, ketones, or acids [54]-, for the epoxidation of alkenes [43, for the formation of alkenyl hydroperoxides [22] for the conversion of terminal alkenes into methyl ketones [60, 65] for the coupling of terminal acetylenes [2, 59, 66] for the oxidation of aromatic compounds to quinones [3] or carboxylic acids [65] for the dehydrogenation of alcohols to aldehydes [4, 55, 56] or ketones [56, 57, 62, 70] for the conversion of alcohols [56, 69], aldehydes [5, 6, 63], and ketones [52, 67] into carboxylic acids and for the oxidation of primary amines to nitriles [64], of thiols to disulfides [9] or sulfonic acids [53], of sulfoxides to sulfones [70], and of alkyl dichloroboranes to alkyl hydroperoxides [57]. 

Four important coenzymes contain nucleotides as part of their structures. We have already mentioned coenzyme A (for its structure, see page 312), which contains ADP as part of its structure. It is a biological acyl-transfer agent and plays a key role in fat metabolism. Nicotinamide adenine dinucleotide (NAD) is a coenzyme that dehydrogenates alcohols to aldehydes or ketones, or the reverse process It reduces carbonyl groups to alcohols. It consists of two nucleotides linked by the 5 hydroxyl group of each ribose unit. 

Most examples of quinone dehydrogenations adjacoit to have been earned out on steroidal ketones and are essentially limited to readily enolizable species. Reactions on esters and amides (Table 8) are far less common and, because of their relatively low ease of enolization, require hanh conditions. Thus, unless stabilization of the intermediate carbonium ion is possible, - elevated temperatures and prolonged reaction times are required (Table 8), which increases the incidence of unwanted side reactions. Frequent by-products are those arising as a result of Diels-Alder reactions or Michael addition to the quinone." Allylic alcohols may be rapidly oxidized to aldehydes or ketones under these conditions and requite prior protection. 

Dehydrogenations, which involve the elimination of hydrogen Ifom organic molecules, lead to compounds containing double bonds, multiple bonds, or aromatic rings. For practical reasons, only the formation of carbon-carbon double bonds, of carbon-nitrogen double bonds in cyclic amines, and of aromatic rings (both carbocyclic and heterocyclic) will be discussed in this chapter. The conversion of alcohols into aldehydes and ketones and of amines into imines and nitriles will be discussed in the chapter Oxidations (Chapter 3). 

The acceptorless dehydrogenation can be used not only for simple transformation of alcohols to the corresponding ketones or aldehydes or for... 

For the dehydrogenation of CH—XH structures, for example, of alcohols to ketones, of aldehydes to carboxylic acids, or of amines to nitriles, there is a wealth of anodic reactions available, such as the nickel hydroxide electrode [126], indirect electrolysis [127, 128] (Chapter 15) with I , NO, thioanisole [129, 130], or RUO2/CP [131]. Likewise, selective chemical oxidations (Cr(VI), Mn02, MnOJ, DMSO/AC2O, Ag20/Celite , and 02/Pt) [94] are available for that purpose. The advantages of the electrochemical conversion are a lower price, an easier scale-up, and reduced problems of pollution. 

Hundreds of substances of many types have been tested as dehydration catalysts and found active. Lists can be found in the literature [69,76,85] and we need to name here only such catalysts which show high activity and selectivity. The latter parameter is more important because a number of solids, especially oxides, can catalyse both the dehydration and the dehydrogenation of alcohols. The formation of aldehydes or ketones is then a parallel reaction to the dehydration, and the ratio of the rates depends on the nature of the catalyst. Only few oxides are clean dehydration or dehydrogeneration catalysts, but the selectivity may be shifted to some extent in either direction by the method of catalyst preparation. 

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