Synthesis, characterisation and application of pyridine-modified chitosan derivatives for the first non-racemic Cu-catalysed Henry reaction

Abstract

The preparation, characterisation and application of two pyridine-modified chitosan derivatives (C1 and C2) containing Cu(OAc)2 adsorbed as catalysts for the conversion of benzaldehyde into 2-nitro-1-phenylethanol are described. Quantitative solid-state 13C multiple-contact cross-polarization, magic-angle-spinning, nuclear magnetic resonance (MC-CP MAS NMR) measurements confirmed the successful grafting of 2-pyridinecarboxaldehyde and 6-methylpyridine-2-carboxaldehyde to the chitosan backbone and indicated that 47(±2)% of the NH2 groups were grafted for both C1 and C2. The use of C1-Cu(OAc)2 as a catalyst in the nitroaldol reaction led to 96(±1)% conversion and 19(±4)% enantiomeric excess (ee), while the use of C2-Cu(OAc)2 as a catalyst also promoted the nitroaldol reaction, affording almost quantitatively the expected 2-nitro-1-phenylethanol (98(±1)%) with 14.5(±1.5)% ee.