Zinc complexes formed by 2,2′-bipyridine and 1,10-phenanthroline moieties combined with 2-azanorbornane: Modular chiral catalysts for aldol reactions

Abstract

Chiral scaffolds of 2-azabicyclo[2.2.1]heptane and 2-azabicyclo[3.2.1]octane were used for the construction of new modular catalysts containing complexing moieties pyridine, 2,2′-bipyridine and 1,10-phenanthroline appended by an imine linkage. The coordination abilities of the new ligands towards Zn(ii) were investigated using NMR and UV spectroscopy. The plausible structures of the [ZnL2]2+ and [ZnLXn](2-n)+ complexes formed were established by comparison of the experimental and DFT-calculated NMR spectra. The catalytic application of the [ZnLXn](2-n)+-type complexes in the asymmetric aldol reaction of ketones with aromatic aldehydes produced an excess of the respective syn-aldols in up to >98% ee.