Formation of carbon-hydrogen bonds

Industrial hydrogenation of multiple C—C bonds is nowadays effected to a preponderating extent by catalytic methods. Because of the many variations possible they have also become a regular part of preparative and analytical work in the laboratory. Nevertheless, particularly in chemical research, other reducing agents have established their place, and the more so because they supplement catalytic procedures in the most satisfactory manner and even exceed them in selectivity. 

Addition of hydrogen to C=C bonds and aromatic systems 1. Chemical reducing agents 

The most important processes for addition of hydrogen to the ethylenic bond and to aromatic and heterocyclic systems comprise reduction by nonnoble metals in suitable solvents. All other reducing agents, such as tin(n) chloride, hydrogen iodide, and even complex metal hydrides, are much less important for reduction of multiple C—C bonds. 

Although the reducing action of the first-named systems was earlier ascribed to so-called nascent hydrogen, it is now known, particularly through the work of Willstatter,3 Hiickel4 

However, Huckel6 returned to the nascent hydrogen hypothesis in order to explain the reduction of isolated aromatic nuclei by the system alkali-metal-liquid ammonia-alcohol.5 7 Since this system contains no metal surface to catalyse the recombination of H atoms (2H - H2), it may be the resulting concentration of free hydrogen atoms that triggers the primary process. 

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In the solid state, the equilibrium is in favor of the hydrido complex (III), and its crystal structure and that of the osmium(II) analog have been determined (38). Chatt also observed that, on heating the equilibrium mixture of (II) and (III), naphthalene was eliminated and the product Ru(dmpe)2 was also a tautomeric mixture. Here the tautomer-ism involves breaking and re-formation of carbon-hydrogen bonds in the methyl groups of the phosphine ligands (IV and V) ... 

Hydrogenolysis of hydrocarbons involves the rupture of carbon-carbon bonds and the formation of carbon-hydrogen bonds. The simplest hydrogenolysis reaction of a hydrocarbon is the conversion of ethane to methane... 

J. Halpem, Formation of Carbon-Hydrogen Bonds by Reductive Elimination , Acc. Chem. Res., 1982, 15, 332. 

The formation of carbon-hydrogen bonds from halogen-containing compounds has been carried out by a number of procedures reductive dehalogenation of halogeno-phenol with Raney alloys in alkaline D2O provides a route to deu-teriated phenols treatment of dichlorobenzene with Na or K in liquid ammonia ( solvated electrons ) gives benzene, and, in addition, aniline via benzyne. ... 

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