REGIOSELECTIVITY IN HYDROCARBON ACTIVATION ON ZEOLITE-Y - STEREOCHEMISTRY OF H-D EXCHANGE

The H-D exchange between alkanes and zeolites normally proceeds with t he formation of adsorbed carbenium ions and olefins as intermediates. Cycloalkanes were used to study the regioselectivity and stereochemist ry of the elimination/protonation reactions on zeolites to establish t he role of the framework in proton transfer reactions. A preferential exchange in the secondary equatorial position adjacent to the tertiary center, relative to the axial one was observed. This result is in agr eement with recent theoretical calculations of elimination/protonation reactions occurring on the zeolite surface, which point out that an e clipsed conformation between the eliminating proton and the cationic c enter is necessary for the reaction to take place. Therefore, in cyclo alkanes, the equatorial hydrogens can be better accommodated in the ec lipsed orientation, This result is in opposition to what is observed f or elimination/protonation reactions of cycloalkanes in solution, wher e the axial protons are easier eliminated, With t-butyl-cyclohexane an almost exclusive exchange at the methyl position was observed, This r esult cannot be explained by the present accepted mechanism involving olefins and carbenium ions. (C) 1997 Academic Press.