Conformations of 1,3,5-triaryl-1,3,5-triazacyclohexanes: comparison of the o-, m-, and p-fluorophenyl compounds

Abstract

Crystallographic studies of 1,3,5-tris-(o-, m-, and p-fluorophenyl)-1,3,5-triazacyclohexane disclose examples of diaxial–equatorial chair conformations for 1,3,5-triaryl-1,3,5-triazacyclohexanes. The repulsion between axial aryl groups is relieved by the N–C(aryl) axial bonds being displaced outwards from ideal tetrahedral positions by 17–21°. The orientation of the equatorial aryl ring about the N–C(aryl) bond depends on the position of the F substituent. In the p-F compound the angle between the equatorial aromatic ring and the symmetry plane of the triazacyclohexane ring is 90° and there is maximum N lone-pair–π-orbital overlap. With increasing steric effects in the m- and o-F compounds the analogous angles are 54.7 and 22.5° and the small N lone-pair–π-orbital overlap in the o-F case is associated with that N atom being more pyramidal than the other N atoms in the o-, m-, and p-fluoro compounds.