Conformational changes of 2,11-dithia[3.3]metacyclophane. A new look using VT NMR and calculation

The conformational changes of 2,11-dithia[3.3]metacyclophane are reexamined utilizing VT NMR data and calculations (ab initio, semiempirical, density functional, and molecular mechanics) to show that the syn to syn' inversion that occurs with exchange of the benzylic hydrogens proceeds most easily through bridge wobbles of the anti isomers and that the critical barrier is the conversion of the syn - chair - chair isomer to the anti-chair-chair isomer, barrier (found) = 9.3-9.6 kcal/mol, barrier (calc) = 10.3 kcal/mol, such that when this conversion is slow on the NMR time scale, the benzylic hydrogens no longer exchange, and the syn-chair-chair isomer becomes frozen. The syn-boat-chair isomer, however, can continue to invert to the anti-boat-chair isomer until lower temperatures, and thus, the benzylic hydrogens continue to exchange for this isomer. Thus, while bridge wobbles of the syn isomers have the largest barriers, bridge wobbles of the anti isomers have the smallest barriers, and so the barriers of the syn-to-anti conversions play a much greater role than previously determined.