LINEAR FREE-ENERGY RELATIONSHIPS IN THE REDUCTION OF IMINES OVER A PD SEPIOLITE CATALYST

In this work we synthesized a series of imines of formula RC6H4CHNC6H5 and C6H5CHNC6H4R (R OH-, CH3O-, CH3-, HOOC-, CH3-CO, and CH3COO-) and subjected them to hydrogenation with Hz over a 3% Pd catalyst supported on sepiolite extracted from Vallecas (Madrid, Spain). From the relationships found between In A and Ea we inferred the occurrence of a compensating effect in the process. We also determined the influence of substituents on the reduction rate from the σ contributions reported by Hammett. By taking N-benzylideneaniline as a reference, we found the presence of electron-withdrawing substituents in either aromatic ring increased the rate of reduction of the >CNbond, while that of electron-releasing groups had the opposite effect. Substituents on the benzylidene ring were found to have a more marked effect than those on the phenylimine ring. A general multi-step reduction pathway similar to that reported by Horiuti-Polanyi (Trans. Faraday Soc. 30, 1164 (1934)) is proposed to account for the experimental results obtained. © 1992.