Cleavage reactions hydrogenolysis

Under alkaline conditions isolating lignin degradation products which are essentially of a phenyl.ethyl rather than a phenylpropyl nature is structurally important and requires a lignin structure by which the 7-carbon may be removed as a result of a 0-7 carbon-carbon cleavage reaction, either by direct alkaline hydrolysis or alkali-catalyzed hydrogenolysis. 

In the discussion of the fully conjugated systems, it was pointed out that the N—O bond is very easily cleaved by hydrogenolysis. Therefore fused isoxazolidine systems find use in chiral syntheses. Examples of cleavage reactions are demonstrated with LAH for the iV-methylpiperidine derivative (69) (78TL4647), hydrogenolysis over Raney nickel for the isoquinoline (70) (69CB736) and metal in acid solution for the piperidine (71) (78TL2753). 

In general, reductive elimination involving an organyl and a hydride is more facile than that between two organyls indeed, hydrogenolysis of metal alkyls represents one of the most important cleavage reactions, which is often incorporated within catalytic cycles (Figures 4.22, 6.14). 

M-C interaction in the product as well as reactant state results in weaker interaction between reacting hydrocarbon fragments, so that barriers for the reaction in both directions increase. This in line with the observed lower rate of hydrocarbon hydrogenolysis observed for Pt as compared with Ni. On Ni the rate of C-H activation is lowered more than that for the CH -CHy bond cleavage reaction. Because of the stronger metal-carbon bond of Pt than Ni, the rate of methane formation by recombination of adsorbed hydrogen with adsorbed CH3 will be lower on the former. 

Under H2, the rhodium(I)-catalyzed C-C bond cleavage reaction of cyclobutanone 73 afforded ring-opened alcohol 74 (Scheme 3.42). Based on the state-of-the-art knowledge of cyclobutanone/cyclobutanol chemistry, we may assume that hydrogenolysis proceeds through ring opening of rhodium cyclobutanolates by P-carbon elimination. 

Hydrogenolysis is analogous to hydrolysis and ammonolysis, which involve the cleavage of a bond induced by the action of water and ammonia, respectively. Chemical bonds that are broken by hydrogenolysis reactions include carbon—carbon, carbon—oxygen, carbon—sulfur, and carbon—nitrogen bonds. An example of hydrogenolysis is the hydrodealkylation of toluene to form benzene and methane ... 

The structure of 82 was established by alkaline ring cleavage to benzilic acid amide and by hydrogenolysis to (C6H5)2CH—CONH— COCfiHs. These reactions also served to eliminate 83 as the structure of the 169° compound. The other possible isomeric structure, (C6H5)2C(CN)0C0C6H5, which could have formed after 0-acylation, was ruled out by its independent synthesis from bromodiphenyl-acetonitrile and silver benzoate. 

Another group of reactions with the predominant cleavage of the ring comprises catalytic hydrogenation of isoxazole derivatives and has been investigated only recently. The most commonly used catalyst has been Raney nickel,but use has sometimes been made of platinum catalysts. Hydrogenolysis of the 0—N bond (172—>173) occurs in isoxazole, its homologs,and their functional derivatives, for example, isoxazole carboxylic acids- and 5-aminoisoxazoles. ... 

Under the conditions used for the reductive cleavage of the O—N bond in isoxazoles, a similar reaction readily occurs with isoxa-zolines, e.g. on treatment with organomagnesium compounds and on catalytic hydrogenation.- Hydrogenolysis of the O—N bond (180 181 182 183) was used to elucidate the structure of isoxa-zolones from -ketoesters. -... 

Catalytic hydrogenolysis is the cleavage of a molecule into fragments by hydrogen in the presence of a catalyst It is a useful and frequent synthetic reaction. This chapter is organized around the type of bond being cleaved. 

Hydrogenolysis (Section 26.7) Cleavage of a bond by reaction with hydrogen. Benzylic ethers and esters, for instance, are cleaved by hydrogenolysis. 

The values of the adsorption coefficient of hydrogen for both reactions were practically identical (1.9 and 2.1 atm-1). Here, the selectivity of the branched reactions depends on the partial pressure of methylcyclopentane. This difference may be accounted for by assuming that either the cleavage of the C—C bond of methylcyclopentane in the (3-position and in the 7-position with respect to the methyl group does not take place on the same sites of the surface of platinum (or on the sites of the same activity), or that the mechanism of hydrogenolysis is more complex than that ex-... 

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