C-O Hydrogenolysis

Basically, the cleavage of the C-O bond takes place through two mechanisms. The first mechanism (the hydrogenolytic cleavage) is a hydrogen-assisted bond-cleavage reaction, whereas the second mechanism is an ionic insertion of the metal into the C-O bond. The characteristic features of the two mechanisms are summarized in Table 4.1. 

During the insertion mechanism, the metal is inserted into the carbon-oxygen bond. The insertion is promoted by a strong metal—oxygen interaction. It is thought that unreduced metal ions may play an important role in the insertion mechanism (electrophilic catalysis). The type of the catalyst, the method of preparation, and the additives can influence the concentration and stability of these ions. 

Electrophilic catalysis may play an important role in the case of the similar benzylic carbon, too. For an O-benzyl system, it was found in a 1997 experiment that palladium oxide is a much more effective catalyst than palladium metal when the catalyst has been prereduced with chemical reducing agents. This finding shows very clearly that the electrophilic character of the unreduced metal ions plays an important role in the hydrogenolysis of the benzyl C—O bonds. Additional support for this mechanism is the fact that a small amount of butylamine can inhibit the hydrogenolysis of the benzyl C—O bond. 

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Another interesting point is the very small difference in the 5neld of the hydrogenolysis product diphenylmethane, over all the catalysts (Table 1). This is contrary to the results on C-C bond hydrogenolysis over bimetallic and Ti02 supported catalysts (1). This indicates that the sites for C-O hydrogenolysis are probably different from C-C bond hydrogenolysis. 

Complete C-O Hydrogenolysis of Sugars and Sugar Alcohols to Alkanes Using... 

Nakagawa Y, Mori K, Chen K, Amada Y, Tamura M, Tomishige K (2013) C-O hydrogenolysis of Re-modified Ir catalyst in alkane solvent. Appl Catal A 468 418-425... 

FIGURE 9.12 Volcano plot of the C-O hydrogenolysis activities for MMo/alumina relative to CoMo/alumina [93]... 

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