Dehydrogenation of secondary alcohols

The oxidative dehydrogenation of secondary alcohols to ketones on iridium at 130°C has been measured by Le Nhu Thanh and Kraus (i-Zi), and the rates have been correlated by the Taft equation [series 112, four reactants of the structure R CH(OH)CH3, slope 4.7]. 

While alcohols and acids are considered to be primary products (see Figure 4) ketones are probably formed in secondary reactions which only occur at higher temperatures 2). In Table II it can be seen that as the temperature increases, ketone production increases at the expense of alcohols up to a point after which both decrease (due to hydrogenation to hydrocarbons which under FT conditions are thermodynamically more stable (2), It has been suggested (2) that ketones result from the direct reaction between alcohols and surface carbon atoms and/or from the dehydrogenation of secondary alcohols (eg under high temperature FT conditions acetone and isopropyl alcohol are in thermodynamic equilibrium (2). 

SOLVENT-FREE DEHYDROGENATION OF SECONDARY ALCOHOLS IN THE ABSENCE OF HYDROGEN ACCEPTORS USING ROBINSON S CATALYST... 

The procedure presented for the dehydrogenation of secondary alcohols is very easy to reproduce and high atom utilization achieved presents a major benefit over hydrogen-transfer oxidations. Results for the dehydrogenation of various substrates using Ru(OCOCF3)2(CO)(PPh3)2 as a catalyst are presented in Table 5.2. 

The preparation of ketones by dehydrogenation of secondary alcohols over zinc and copper catalysts and the decarboxylation condensation of acids over manganous oxide or thoria have been adequately covered by standard reference books on catalysis. However, the more complete but equally serviceable catalytic syntheses involving either an aldol or a Tischenko ester type of condensation have been virtually ignored. 

Ethoxyacetaldehyde, an aldehyde ether, is readily prepared in 35% yield from Cellosolve by the vapor-phase dehydrogenation technique. Similar techniques are employed for the catalytic dehydrogenation of secondary alcohols (method 181). 

Raney nickel, Ni(R), effects the dehydrogenation of secondary alcohols to ketones in excellent yields at moderate temperatures in the presence [927] or absence [927, 928] of hydrogen acceptors. 

Dehydrogenation of secondary alcohols to ketones is carried out analogously to the aldehyde syntheses described above the same oxidants and procedures are used. The reactions are in general smoother, since the ketones formed are appreciably more stable than the aldehydes towards an excess of oxidant. 

A mild method, much favored in the steroid series, for preparation of ketones is the Oppenauer dehydrogenation of secondary alcohols under catalysis by metal alkoxides (see page 322) ... 

Reactions of oxidoreduction are an example of chiral inversion that takes place by the intermediacy of two opposing metabolic processes. The alcohol/ketone equilibrium mediated by alcohol dehydrogenase enzyme is an abundant reaction. The dehydrogenation of secondary alcohols to a ketone proceeds with substrate stereoselectivity in oxidation, while the hydrogenation of the ketone metabolite is product selective to one face of the carbonyl group. The consequence of the metabolism of the secondary alcohols may involve chiral inversion of this center, which can result in an altered proportion of the two enantiomers or epimers. 

On the large scale, oxidation (dehydrogenation) of secondary alcohols to ketones is usually achieved using air and a catalyst - copper oxide is an established one. Aldehydes are often prepared by hydroformylation of olefins rather than from the corresponding alcohol. However, on the smaller scale, a large and diverse number of carbonyl compounds are... 

Nickel is a weU-recognized promoter of hydrogen-transfer reactions and hence it is expected that it can be appHed in the reactions under discussion. In accord, acceptor-free dehydrogenation of secondary alcohols by Ni NPs supported on alumina has been reported. Low-coordinated Ni° sites and metal/support interfaces are beheved to play significant roles in the catalytic cycle. 

Kon K, Hakim Siddiki SMA, Shimizu K. Size- and support-dependent Pt nanocluster catalysis for oxidant-free dehydrogenation of alcohols. J Catal. 2013 304 63-71. Shimizu K, Kon K, Shimura K, Hakim SSMA. Acceptor-firee dehydrogenation of secondary alcohols by heterogeneous cooperative catalysis between Ni nanoparticles and acid—base sites of alumina supports. J Catal. 2013 300 242-250. 

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