Allylic halogen, hydrogenolysis

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. 

Allylic and benzylic heteroatom substituents such as -OR, -SR, and halogens undergo concomitant hydrogenolysis during Birch reduction. However, benzylic -OH groups are converted to alkoxides, and the resultant electron-rich -CHjO moiety resists further reduction. 

Certain types of functionality can be removed and replaced by hydrogen under catalytic hydrogenation conditions. This is called hydrogenolysis. For example, aromatic halogen substituents are usually removed by reduction over metal catalysts. Aliphatic halides are less reactive but hydrogenolysis is promoted by base. Carbon oxygen bonds at benzyl and allyl positions are cleaved by catalytic hydrogenation. ... 

---