Resolution of optical isomers by high-performance liquid chromatography, using coated and bonded chiral charge-transfer complexing agents as stationary phases

Ten racemic helicenes ([5] to [14]) and two double helicenes {diphenanthro[4,3-a; 3′,4′-o]picene (I) and 8,20-dibromodiphenanthro[4,3-a; 4′,3′-j]chrysene (II)} were resolved using high-performance liquid chromatography. R(−)- and S(+)-2-(2,4,5,7-tetranito-9-fluorenylideneaminooxy)propionic acid (TAPA) and three R(−)-homologues derived from butyric (TABA), isovaleric (TAIVA) and hexanoic (TAHA) acids were used as chiral charge-transfer (C.T.) complex-forming stationary phases, in situ coated on silica microparticles. The bulkiness of the group at the chiral centre of the C.T. acceptor is critical for the ease of resolution of the C.T. donors. The [6]-[14]-helicenes and the double helicenes I and II were completely resolved on R(−)-TAPA. On the other hand, the optical isomers of [5]-helicene could be separated only on R(−)-TABA and baseline resolution required ten recycling steps. R(−)-TAPA was also incorporated as a bonded solid phase and as a salt. The resolution factors, r, are given, a mechanism of resolution is proposed and the scope of the method and its significance are discussed.