Amino acid-functionalized multi-walled carbon nanotubes: A metal-free chiral catalyst for the asymmetric electroreduction of aromatic ketones

Abstract

We report new metal-free chiral electrodes for catalyzing the asymmetric reduction of aromatic ketones. Multi-walled carbon nanotubes (MWCNTs) were functionalized by chiral amino acids. The resulting amino acid-functionalized MWCNTs were fully characterized by surface analysis and spectroscopic tools and were used as a cathode for the electroreduction of 2,2,2-trifluoroacetophenone in MeCN/n-amyl alcohol (1:1)-TEAI (0.1 M). Electroreduction at l-arginine-MWCNTs and d-arginine-MWCNTs cathodes afforded, respectively, (R)- and (S)-α-(trifluoromethyl) benzyl alcohol in 60–61% yield with 43–44% enantiomeric excess (ee). The l-lysine-MWCNTs cathode gave a lower ee value of the R enantiomer (30%). The yields and ee values were a little bit lower for the electroreduction of acetophenone and other arylalkyl ketones. These amino acid-functionalized MWCNTs electrodes, in addition to showing good catalytic performance, are very stable and can be reused without loss of activity and selectivity.