PREPARATION AND EVALUATION OF CHIRAL HIGH-PERFORMANCE LIQUID-CHROMATOGRAPHIC STATIONARY PHASES OF MIXED CHARACTER (PI-DONOR AND PI-ACCEPTOR) FOR THE RESOLUTION OF RACEMIC COMPOUNDS

Up to now, most stationary phases have had either pi-acceptor or pi-donor characteristics. In this study, the behaviour of stationary phases having mixed characteristics (pi-donor and pi-acceptor) was investigated and compared with that of silicas bonded either with the same pi-acceptor group or with the same pi-donor group. The chiral selectors, (S)-1-(alpha-naphthyl)ethylamine associated with (S)-phenylalanine and (S)-N-(3,5-dinitrobenzoyl)phenylalanine, were bound to a gamma-aminopropylsilanized silica gel. Stationary phases possessing mixed character were obtained either by bonding the two chiral selectors on the same silica or by mixing two chiral silicas bonded with only one of two chiral selectors. The selectivity of these chiral stationary phases possessing pi-donor and pi-acceptor characters are similar; the alpha-values are intermediate between those shown by the two chiral stationary phases with only pi-donor or pi-acceptor character. The results obtained in the resolution of a series of racemic compounds with either pi-donor or pi-acceptor character show that the chiral recognition mechanism is probably more complex than the conventional face-to-face pi-interactions usually described.