On the possible isomerisation pathway for triarylmethanes Ar3CX, including propeller crowns

The crystal structures of two propeller crown ethers are presented. The properties of the seven propeller crown structures examined to date are compared critically with geometric information retrieved from the Cambridge Crystallographic Database for other Ar3CX systems. Following the method of Bye, Schweizer, and Dunitz (BSD) each conformation was regarded as a sample point defined by the torsion angles of its three aryl groups, and possible features of the potential energy surface for the interconversion of the propeller isomers were considered graphically. Low-energy stereoisomerisation paths were apparent; the two-ring and one-ring flip transition states of earlier hypotheses did not lie on these paths.