Hydrogenolysis of Carbon-Oxygen Bonds

Ionic hydrogenation is the noncatalytic addition of hydrogen to carbon-carbon or carbon-heteroatom double bonds, and the hydrogenolysis of carbon-oxygen or carbon-halogen single bonds. Thus, the conversion of aldehydes and ketones to the corresponding hydrocarbon is one of its many manifesta-... 

The hydrogenolysis of allyl-oxygen bonds can be avoided by the use of Rh or Ru catalysts, and modified Pt catalysts have been effective. Epoxides also survive conditions used to hydrogenate carbon-carbon double and triple bonds. 

A second example exploits the fact that the mixed hydride reagent is capable of hydrogenolysis of certain carbon-oxygen bonds. Thus, treatment of cyclohexanone ketal (Chapter 7, Section IX) with lithium aluminum hydride-aluminum chloride results in the rupture of a C-O bond to give the oxyethanol derivative. 

Hydrogenolysis, without ring reduction, of the carbon-oxygen bond in phenols cannot be depended on, but by conversion of the phenol to a better leaving group, such as is formed by interaction of the phenol with 2-chlorobenzoxazole, l-phenyl-5-chlorotetrazole, phenylisocyanate,... 

Hydrogenation of phthalic anhydride over copper chromite afforded 82.5% yield of the lactone, phthalide, and 9.8% of o-toluic acid resulting from hydrogenolysis of a carbon-oxygen bond [1015]. Homogeneous hydrogenation of a,a-dimethylsuccinic anhydride over tris(triphenylphos-phine)rhodium chloride gave 65% of a,a-dimethyl- and 7% of )S,)S-dimethyl-butyrolactone [1016]. 

On hydrogenation, phenols may undergo saturation to the cyclo-hexanol, hydrogenolysis of the carbon-oxygen bond, or partial hydrogenation to the cyclohexanone. 

In the dry Pauson-Khand reaction system oxygen is necessary in order to prevent hydrogenolysis of the allylic carbon-oxygen bond in the product. This process is presumably effected by hydridocobalt... 

Aromatic ketones represent a rather special case in dissolving metal reductions. Under many conditions pinacol formation is the predominent reaction path (see Volume 3, Chapter 2.6). Also, the reduction potentials of aromatic carbonyl compounds are approximately 1 V less negative than their aliphatic counterparts. The reductions of aromatic ketones by metals in ammonia are further complicated by the fact that hydrogenolysis of the carbon-oxygen bond can take place (Chapter 1.13, this volume) and Birch reduction may intervene (Chapter 3.4, this volume). 

A potentially useful chemoselective dissolving metal reagent for the reduction of aromatic ketones in the presence of other functional groups is the combination Zn-DMSO and aqueous potassium hydroxide. In three examples (benzophenone, fluorenone and 4-benzoylpyridine), the yields of secondary alcohols were over 90%. Two other ketones (xanthone and thioxanthone) gave mixtures of alcohol and the hydrocarbon obtained by hydrogenolysis of a carbon-oxygen bond. ... 

Hydrogenolysis of the carbon-oxygen bond in alcohols, enols and phenols is relatively difficult, regardless of type. 

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