Pyrene-tagged carbohydrate-based mixed P/S ligand: spacer effect on the Rh(i)-catalyzed hydrogenation of methyl α-acetamidocinnamate.

Abstract

The post-functionalization of a chiral catalyst offers the advantage of providing it with additional physical characteristics that, together with its enantioselective capacity, increase its overall synthetic value. Taking advantage of the modularity and polyfunctionality of carbohydrate-derived ligands, herein we report the synthesis of two mixed P/S catalysts functionalized with a pyrene group through the 6 position of the sugar by carbon chains of different lengths. Using the hydrogenation of methyl (Z)-α-acetamidocinnamate as the model reaction has shown that the proximity of the pyrenyl group to the catalytic center is detrimental to the activity and enantioselectivity of the hydrogenation process, the most efficient catalyst being the complex derived from pyrenebutyric acid 12. The study of the supramolecular π-π interaction of the most active complex 12 with SWCNTs by UV-Vis spectroscopy shows, that in ethyl acetate complex 12 is totally adsorbed onto the SWCNT surface, while in methylene chloride there is an equilibrium between the adsorbed and the free form of the complex, allowing the use of complex 12 and SWCNTs in a catch and release process. Interestingly, it has been determined that the nanocatalyst 12/SWCNT is more enantioselective than complex 12 alone, affording (S)-N-acetylphenyl alanine 16 in quantitative yield and 96% ee.