Conformational analysis of [3.3.3](1,3,5)cyclophane systems

Abstract

2,2,11,11,20,20-Hexadeuterio[3.3.3](1,3,5)cyclophane 1-d6 and its precursor, [3.3.3](1,3,5)cyclophane-2,11,20-trione 2, were synthesized and their conformational behavior was examined by a variable temperature 1H NMR method; 1-d6 (ca. 1% CD2Cl2 solution) exists in a mixture of the conformer A (C3h) and the conformer B (Cs) in a 27:73 ratio at −70 °C with the energy barriers for the trimethylene bridge inversion processes [Formula: see text] being 12.4 kcal/mol (Tc = −7 °C, 270 MHz). Conformer B is more stable than A by 0.4 kcal/mol based on the populations of both conformers. In contrast to the conformational behavior of 1, inversion of the tricarbon bridges of 2 is not frozen even at −90 °C because of the increased flexibility of these tricarbon bridges. Keywords: conformational analysis, cyclophane, tri-n-butyltin deuteride.