Chiral recognition in jet-cooled complexes of (1R,2S)-(+)-cis-1-amino-2-indanol and methyl lactate: on the importance of the CH...pi interaction

Complexation between (1R,2S)-(+)-cis-1-amino-2-indanol (AI) and the two enantiomers of methyl lactate has been studied by means of laser-induced fluorescence, resonance-enhanced two-photon ionisation, and IR-UV double resonance spectroscopy, in the region of 3 microm. Two isomeric complexes have been spectroscopically characterised for each diastereoisomer. Comparison with ab initio calculations shows that the most stable form is an insertion structure, common to the two diastereoisomers, in which the OH group of methyl lactate inserts into the intramolecular bond of AI. This structure shows almost no chiral discrimination. A secondary structure has been observed, which is specific to each enantiomer. It involves a main hydrogen bond from the OH group of methyl lactate to AI together with weaker hydrogen bonds, which depend on chirality. The enantioselectivity in the hydrogen bond topology is due to a weak stabilizing CH...pi interaction, involving the CH located on the asymmetric carbon of methyl lactate, which can be obtained for one of the enantiomers only.