Thermal Racemization of Thiaheterohelicenes

Thermal racemizations of four thiaheterohelicenes (TH) with alternant thiophene and benzene rings were examined and compared with those of carbohelicenes (CH). The racemization rate of [5]TH was very fast at room temperature, and the rates of [7]TH and [9]TH were found to be 1/5 to 1/50 times slower than the corresponding CH having a similar internal angle. The ΔS values obtained by temperature-variable experiments showed a large difference between [7]TH (−103.9±4.8 JK−1 mol−1) and [9]TH (−66.3±4.0), suggesting different conformations in the transition state. CNDO calculations on the transitional conformations of [5]TH demonstrated that a geometry, such as that distorting the benzene rings, has a lower energy than a geometry such as that twisting the thiophene ring. Based upon these results, the conformations in the transition states for the THs are inferred.