Orthogonal assisted tandem reactions for the upgrading of bio-based aromatic alcohols using chitin derived mono and bimetallic catalysts

Abstract

The upgrading of a benzyl-type alcohols was explored via an orthogonal tandem sequence comprised of a first oxidative step producing the corresponding aldehydes, and a subsequent reductive amination to achieve both secondary and tertiary amines. To the scope, acetonitrile (ACN) was used as a solvent and a source/precursor of reactant amines, and different heterogeneous catalysts based on Rh and Mo, were designed as mono- and bi-metallic systems in the form of metal nanoparticles dispersed on a chitin-derived N-doped carbons. A parametric analysis carried out separately for the oxidation and the reductive amination allowed to choose the best performant catalyst for both the reactions of the tandem process. A one-pot two-step protocol was implemented accordingly: as an example, benzyl alcohol was quantitatively and selectively oxidised to benzaldehyde (>99%) which in turn, was converted to N-benzylethanamine (66%) or N-benzyl-N-ethylethanamine (60%) in the presence of [Rh(5%)-N/C–Mo(5%)]-N/C or [Rh(3%)-N/C–Mo(5%)]-N/C as catalysts, respectively. The tandem sequence proved successful also for other bio-based benzyl-type alcohols that afforded the corresponding secondary/tertiary amines in yields up to 53–93%. Overall, the study proved the viability of an innovative method aimed not only at process intensification for multistep synthesis, but also at the valorization of substrates (alcohols) and biopolymers (chitin) derived from biomass.