Heterogeneous catalytic isotopic exchange of benzylic compounds in solution

Isotopic exchange reactions of bibenzyl and benzylic derivatives with deuterium (D2) and tritium (T2) gas, catalyzed by Pd/C, have been performed in solution. Catalyst pre-washed with the reacting gas showed improved H/D or H/T exchange. In aprotic solvents such as dioxane, ethyl acetate, and cyclohexane, 3.1 to 3.5 D atoms were exchanged under standard conditions in 1 h, while in benzene 1 D atom was exchanged. D atoms adsorbed on the catalyst surface were rapidly replaced by H from methanol, diluting the D2 gas phase. Compounds containing O or N atoms enhanced the rate of exchange, while S atoms inhibited the catalyst activity. The H/D exchange obeyed pseudo-first order kinetics when D2 gas was used in large excess. When T2 gas replaced D2, a substantial decrease in rate of exchange was observed, due to the slower dissociative chemisorption of T2 on the catalyst surface. The results indicated that this exchange process occurred through two simultaneous mechanisms. The breaking of the C-H bond appeared to be the rate-determining step. 22 refs., 1 fig., 5 tabs