Side reactions hydrogenolysis

The side reaction of hydrogenolysis of the methyl-ruthenium intermediate to methane also may become predominant when the carbonyl insertion-methyl migration step of the process (Scheme 1) proceeds at a low rate. To reduce this drawback some Lewis acid promoters (i.e. metal alkali cations, classical Lewis acids such as AII3, SbCl etc.)... 

Hydrogen and a catalyst.2 0 The most common catalysts are platinum and ruthenium, but homogeneous catalysts have also been used.281 Before the discovery of the metal hydrides this was one of the most common ways of effecting this reduction, but it suffers from the fact that C=C, CssC, C=N and C=N bonds are more susceptible to attack than C=0 bonds.282 For aromatic aldehydes and ketones, reduction to the hydrocarbon (9-37) is a side reaction, stemming from hydrogenolysis of the alcohol initially produced (0-78). 

Since the hydrogenolysis of cyclohexane and cyclohexane derivatives is less probable than the thermodynamically favored dehydrogenation to form aromatic compounds, most studies address hydrogenolysis only in connection with aromati-zation as an unwanted side reaction. An interesting observation by Somorjai showed, however, that hydrogenolysis of cyclohexane to form n-hexane becomes competitive with aromatization on Pt single crystals with increasing kink density.302 On a Pt surface where approximately 30% of the atoms in the steps are in kink positions, benzene and n-hexane are formed in 1 1 ratio. 

The hydrogenolysis of the carbinol C—O bond and the perhydrogenation of the aryl groups may be side-reactions. In most cases, hydrogenolysis is undesirable, but selective hydrogenolysis349 may be the aim of certain synthetic processes. Practical... 

Hydrogenation of anilines usually affords a saturated amine as the major product, but several side reactions, including hydrogenolysis, reductive hydrolysis, and reductive coupling, may accompany and even dominate the reduction. Hydrogenolysis is important only in certain activated molecules (10). 

Carnahan et al. obtained good yields of alcohols and glycols by hydrogenation of lower mono- and dicarboxylic acids over ruthenium dioxide or Ru-C at 135-225°C and 34-69 MPa H2 (eqs. 10.1 and 10.2).8 In general, the optimum temperature was about 150°C. The chief side reaction was hydrogenolysis of the alcohols, as exemplified in the formation of ethanol from oxalic acid and of butanol and propanol from succinic acid (see eq. 10.2). Platinum and palladium catalysts were ineffective under similar or even more severe conditions. 

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