Vinylic halogens, hydrogenolysis

A very active elemental rhodium is obtained by reduction of rhodium chloride with sodium borohydride [27]. Supported rhodium catalysts, usually 5% on carbon or alumina, are especially suited for hydrogenation of aromatic systems [iTj. A mixture of rhodium oxide and platinum oxide was also used for this purpose and proved better than platinum oxide alone [i5, 39]. Unsaturated halides containing vinylic halogens are reduced at the double bond without hydrogenolysis of the halogen [40]. 

Water does not dissolve many organic compounds but it can be used as a solvent, especially in hydrogenations of acids and their salts. It may have some deleterious effects for example it was found to enhance hydrogenolysis of vinylic halogens [66],... 

The conspicuously high speed of hydrogenolysis of allylic and vinylic halogens as compared with those removed further from the multiple bonds or halogens in alkanes implies that the multiple bond participates in a multicenter transition state [63]. Such a mechanism would account even for the surprisingly easy hydrogenolysis of allylic and vinylic fluorine [66, 528, 529, 530]. 

A generalization derived from many studies is that the tendency toward hydrogenolysis of vinylic and allylic oxygen, nitrogen, and halogen increases in the order ruthenium rhodium palladium < platinum < irridium. In vinylic systems especially, platinum favors hydrogenolysis more than palladium in allylic systems, the trend is not so clear but appears to be the reverse (26). Iridium is placed in this sequence on the basis of limited data. 

HYDROGENOLYSIS OF THE CARBON-HALOGEN BOND IN VINYL HALIDES... 

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