CONFORMATIONAL DYNAMIC INVESTIGATION OF 1,5-BENZODIOXEPIN AND ITS DIMETHYL DERIVATIVE BY H-1 AND C-13 NMR

The conformational and dynamic properties of 1,5-dioxa-6,7-benzocycloheptene (2; 1,5-benzodioxepine) and its dimethyl derivative (7) were investigated by dynamic 1H and 13C NMR methods. Analysis of the spectra at low temperatures (below coalescence) indicates that the most stable 7-membered ring conformations detected for solutions in CHF2Cl are a mixture of the chair (C, 80%) and the twist-boat (TB, 20%) for 2 and a mixture of TB (64%) and C (36%) for 7. Free energy barriers were determined for the chair inversion of 2 (6.5 kcal/mol) and 7 (7.5 kcal/mol) and for the TB pseudorotation of 7 (6.8 kcal/mol). The conformational properties of these 7-membered heterocycles are discussed and compared to those known for 2,4-benzodioxepines.