1H dynamic NMR and X-ray crystal structure studies of conformational preferences in dibenzo[c,h][1,6]diazecines

Abstract

The conformational equilibria and preferences in dibenzo[c,h][1,6]diazecines have been investigated by ¹H DNMR measurements and X-ray crystallography. It has been found that restricted rotation about the exocyclic N–S bond in 6,13-ditosyl-6,7,13,14-tetrahydro-5H,12H-dibenzo[c,h][1,6]diazecine 2 becomes slow on the NMR timescale in the temperature range 200–300 K. The free energy of activation was determined to be 43.5 ± 0.5 kJ mol^–1 for this dynamic process. Steric hindrance at the transition state is deemed to be the most important contribution to the N,S barrier. From X-ray crystal structure analyses, it appears that the influence of the N-inversion on this barrier is negligible. The conformation in the solid state is also the preferred one in solution. The X-ray structure illustrates the spatial vicinity of the tosyl groups relative to the annelated benzene moieties of the 10-membered ring. Consistent with these results are high field shifts and corresponding splittings in the aromatic part of the ¹H NMR spectrum of compound 2.