Conformational behavior of dithia[n.3.3](1,3,5)cyclophanes and dithia[n.3.3](1,2,6)cyclophanes

Abstract

The conformational behavior of a series of crown-fused dithia[n.3.3](1,2,6)cyclophanes ( 126- CPs ) and dithia[n.3.3](1,3,5)cyclophanes ( 135- CPs ) was investigated by variable-temperature 1H and 13C NMR spectroscopy, X-ray crystallography and density functional theory (DFT) calculations. Single crystal X-ray structure analysis showed that two thia-bridges in 126- CPs adopted a pseudochair– pseudochair ( cc) conformation and the cyclophane decks underwent a ring-tilting motion in the case of [10.3.3](1,2,6)cyclophane ( 1a ). In contrast, the thia-bridges in 135- CPs took both cc and pseudoboat– pseudochair ( bc) conformations, and the ring-tilting process was also found in [10.3.3](1,3,5)cyclophane ( 2a ). Variable temperature 1H NMR study revealed that there was no wobbling-motion for two thia-bridges in 126- CPs while thia-bridges in 135- CPs experienced a wobbling-process with a conformational barrier of 9.21 and 8.80 kcal mol −1, respectively, for 2a and [13.3.3](1,3,5)cyclophane ( 2b ). DFT calculations for the two cyclophanes series revealed that 126- CPs preferred a cc conformation which was consistent with the experimental observation; similarly, 135- CPs took a preferential cc conformation, agreeing with 2a having a predominant cc conformer ( cc: bc ratio=70:30), but not 2b having a predominant bc conformer ( cc: bc ratio=15:85) in the solid state.