R-X Bonds at Saturated Carbons

Bases are added frequently as promoters in catalytic dehalogenations to neutralize the liberated halogen acid that may inhibit the action of catalyst.196 Denton et al. studied the effects of added potassium acetate to the rates of hydrogenolysis of various halogen compounds with a Pd-C as catalyst in methanol (Table 13.8).196 

Heptyl bromide was not hydrogenolyzed whatsoever, even in the presence of potassium acetate, while phenethyl bromide and 3-phenylpropyl bromide were hydo-genolyzed at considerable rates with addition of the base. Ethyl 3-bromopropionate, which resisted hydrogenolysis in neutral alcohol, as mentioned above, was hydrogenolyzed in the presence of the base, although at a slow rate. It is seen that the bomine located at the a-position is markedly activated by electron-withdrawing benzoyl and ethoxycarbonyl groups. 

TABLE 13.8 Effects of Added Potassium Acetate on the Rates of Hydrogenolysis of Halogen Compounds 1  

Dehalogenations at saturated carbons are seen quite commonly with cyclic compounds of the type RCH2X. Equations 13.105,197 13.106,198 and 13.107199 are examples of such dechlorination, debromination, and deiodination, respectively, using palladium or platinum as catalyst. Equations 13.108200 and 13.109201 are those using Raney Ni. 

Raney Ni has also been used for removal of a secondary bromine, and has been applied for the synthesis of deoxy cyclitols with addition of Amberlite IR-4B anion-exchange resin to bind the liberated acid202 205 (eq. 13.110204). 

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