Structures and conformations of heteroatom-bridged calixarenes

The structures and conformational energies of twelve MeN-, O-, S-, MeP-, CO-bridged homocalix[4] arenes and two kinds of O-bridged alternate hybrid-calix[4] arenes have been calculated at the B3LYP/6-31G* level of theory. The 1,3-alternate or twisted-1,3-alternate conformations are found to be the lowest energy structures in all cases except for MeP-bridged calix[4] pyridine and calix[4] benzene, for which the twist-pinched cone and partial cone are the most stable conformations, respectively. The conformational energy differences calculated between the lowest energy and the next conformation are on the order of 2.0-3.0 kcal/mol, but smaller for the S- and MeP-bridged compounds. Analysis of the structures and relative energies shows that the phenyl hydrogens have electrostatic attractions with the lone pairs of the heteroatoms and steric repulsions with the methyl groups on bridgehead groups. Conversely, the lone electron pair in the pyridyl compounds engage in a repulsive electrostatic interaction. The interactions of 1,3-aromatic rings play a minor but important role in the relative stability sequence. This detailed understanding of the factors governing the conformational space of hetero calixarences can be used to design conformationally biased analogs of these interesting compounds. Copyright (C) 2011 John Wiley & Sons, Ltd. Supporting information may be found in the online version of this paper.