Conformational dynamic investigation of 1,5-benzodioxepin and its dimethyl derivative by ¹H and ¹³C nmr

Abstract

The conformational and dynamic properties of 1,5-dioxa-6,7-benzocycloheptene (2; 1,5-benzodioxepin) and its dimethyl derivative (7) have been investigated by dynamic 1H and 13C nmr methods. Analysis of the spectra at low temperatures (below coalescence) indicates that the most stable seven-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) as well as for the twist-boat pseudorotation of 7 (6.8 kcal/mol). The conformational properties of these seven-membered heterocycles are discussed and compared to those known for 2,4-benzodioxepins.

Conformational consequence of stereoelectronic effects in seven-membered cyclic acetals: nuclear magnetic resonance investigation of 1,3-benzodioxepin and its 2-substituted derivatives

“…were done by calculation of spectra at low temperature using a slightly modified version of the DNMR-2 program on a CDC Cyber 173 computer (24). The simulations were done by a visual comparison of the calculated and experimental spectra.…”

“…Calculations of AS* and AH* were not expected to be sufficiently reliable because of the uncertainty in temperature (t3"), uncertainties in T2 arising from possible small longrange coupling, as well as the fact that the conformational populations and chemical shifts were kept constant through the whole temperature interval. The complete treatment was done as reported previously (24).…”

“…The substitution of C-5 by an oxygen atom in 5-oxabenzocycloheptene (9) lowers the energy barrier to 9.5 kcal/mol (13). Furthermore, the presence of two oxygen atoms in the ring reduces the value to 7.9 kcal/mol for 2 and to 6.5 kcal/mol for 1,5-dioxa-6,7-benzocycloheptene (14) (8,24). The free energy barrier determined for 8 (8.4 kcal/mol) is therefore higher than that of both 2 and 14, whereas an intermediate value might have been expected if the effect of each oxygen atom were independent and additive.…”