Supramolecular selectivity in solid phase catalytic hydrogenation of phenylalkynyl and azobenzene derivatives

Abstract

Solid phase catalytic hydrogenation of phenylalkynyl and of azobenzene derivatives was studied with accordance to a mechanism proposing a supramolecular selective hydrogenation of assemblies of the unsaturated substrates on a metallic catalyst matrix. The kinetics of the solid phase reaction is shown to be affected by a combination of three factors: intermolecular distances, intact of alignment of molecules and the rigidity of the molecular stacks. A stepwise stereospecific hydrogenation of diphenylacetylene (DPA) via the cis-isomer intermediate was found, while a one-step hydrogenation to saturated bibenzyl derivative took place in the case of diphenylethynylbenzene (PEB). Solid phase hydrogenation of azobenzene derivatives resulted in a fast reaction giving aniline derivatives as the final product. The hydrogenation kinetics of substituted azobenzene with hydroxy- or amino-group could be significantly increased as a result of better alignment, or drastically decreased due to the increase in the rigidity of the molecular assembly. A secondary small hydrogen–deuterium isotopic effect kh/kd=1.45 was measured, indicating a weakening of the carbon–hydrogen bond as the rate determining step of the reacting molecule. The presence of traces of impurities such as water or peroxides, during the process of the construction of the molecular stacks on the metallic catalyst, results in a major reduction of the hydrogenation rate as an outcome of interference in the self-assembly process.