Enantioselective inclusion of methyl phenyl sulfoxides and benzyl methyl sulfoxides by (R)-phenylglycyl-(R)-phenylglycine and the crystal structures of the inclusion cavities

Crystalline (R)-phenylglycyl-(R)-phenylglycine [(R,R)-1] includes methyl phenyl sulfoxides (2 and 3) and benzyl methyl sulfoxides (4) with high enantioselectivity. The dipeptide exhibited different stereoselectivity depending on four structural isomers of methyl tolyl sulfoxide (C8H10OS): R for methyl 2-tolyl sulfoxide, S for methyl 3-tolyl sulfoxide, and racemic for methyl 4-tolyl sulfoxide. A structural isomer, benzyl methyl sulfoxide, was included in racemic form. Chlorophenyl methyl sulfoxides 3 (C7H7ClOS) with a similar volume showed the same enantioselectivity for their recognition. By single-crystal X-ray analyses of these inclusion compounds, it was elucidated that (R,R)-1 molecules self-assembled to form layer structures and included the sulfoxides between these layers and that the origin of the enantioselectivity based on chiral cavities was induced by conformation of the C-terminal phenyl group of the dipeptide. The relative position between the ammonio proton and the C-terminal phenyl group in one molecule of the dipeptide determined the stereochemistry of the methyl sulfinyl groups to be recognized. Various positional isomers of methyl xylyl sulfoxide having the formula of C9H12OS were subjected to the enantioselective inclusion by (R,R)-1 crystals and these results are also discussed.