Origin of enantioselective reduction of quaternary copper D,L amino acid complexes under vibrational activation conditions

Phenylglycine (Phg), a non natural amino acid was used as chiral selector for the distinction of L/D amino acids through enantioselective Cu-II reduction. The investigation of the chiral effects was performed based on the dissociation of copper quaternary complexes involving phenylglycine: [Cu-II,(Phg,AA(1),AA(2)-H)](+). The main cleavage was the competitive formal loss of Phg and [Phg-H](+) yielding the [Cu-II,(AA(1),AA(2)-H)](+) and [Cu-I,(AA(1),AA(2))](+) ions respectively. The later involving a stepwise process with CO2 loss and a concomitant Cu-II reduction. The relative extent of this competitive process appeared to be strongly enantioselective. This led us to explore the origin of the observed enantiomeric reduction from dissociations of copper AAs complexes. With this aim, smaller [Cu-II,(Phg,AA-H)](+) ternary complexes have been investigated and MSn together with double resonance-IRMPD experiments have been conducted. The existence of zwitterions was considered in order explain the role of gas phase acidity and proton affinity on the enantioselective reduction. (C) 2011 Elsevier B.V. All rights reserved.