Catalytic Hydrogenolysis of n, n-Dihalobicyclo [n-3. 1. 0] alkanes

In order to investigate the effect of halogens in the regioselective cleavage of cyclopropane rings, difluoro, dichloro and dibromo derivatives of n, n-dihalobicyclo [n-3. 1. O]alkanes (n=6, 7 and 9) were hydrogenated with palladium-charcoal (Pd-C) or Raney nickel catalyst in methanol solution at room temperature under the atmospheric pressure. The effect of amines as additives was also investigated. When the difluoro derivatives were hydrogenated over Pd-C, the compounds with n=6, 7 and 9 were all cleaved exclusively at the condensed bonds to give the corresponding ring-enlarged products, whereas only the compound with n=6 was hydrogenolyzed by the use of Raney nickel to give the ring-enlarged products. The addition of amines retarded the hydrogenolysis and the retarding effect of amines increased in order of the adsorption strength of amines to the catalysts. The hydrogenation reactivity of dichloro derivatives in the Pd-C-catalyzed hydrogenation was relatively low and only the compound with n=6 was ring-enlarged. On the other hand, Raney nickel-catalyzed hydrogenation of dichloro derivatives, n=6, 7 and 9, proceeded smoothly in the presence of amines and th e C-Cl bond was cleaved to give monochlorocyclopropanes, accompanied by small amounts of methylcycloalkanes, respectively. Dibromo derivatives, n=7 and 9, were hydrogenolyzed at the C-Br bond over both Pd-C and Raney nickel in the presence of amines to give monobromocyclopropanes, accompanied by small amounts of methylcycloalkanes. These results may be explained by the differences in the contribution degree of the two factors: the strain of the cyclopropane ring caused by the substitution of halogens and the strength of the adsorption of halogens to the catalysts.