Proton and carbon-13 nuclear magnetic resonance studies of the conformational dynamic properties of seven-membered rings. 2,4-Benzodioxepin and its derivatives

Abstract

Blanchette, A locally written computer program, NLLSQ, based on the Marquardt algo-with the values predicted on the basis of the curve shown in Figure 3, e.g., r i t h~n ,~~ was used. We thank Dr. Eric Enwall for kindly obtaining for us the computer-drawn plot shown in Figure 3. CH values for torsion angles, $6, of 60 and 180' are predicted to have (42) The observed values of J14wcH measured in 15N-enriched amino acids opposite signs "due to a substituent effect involving the lone pair electrons measured approximately at their isoelectric points agree remarkably well on the nitrogen and the *-electrons of the carbonyl groups".z8b = 3.6 and flAh& = 1.3 H z .~~ Abstract: The conformational and dynamic properties of 1,3-dioxa-5,6-benzocycloheptene (5,2,4-benzodioxepin) and three of its 2,2-disubstituted and two of its 2-monosubstituted derivatives have been investigated by IH and I3C DNMR methods.Analysis of the spectra of low temperatures (below coalescence) indicates that the most stable seven-membered ring conformations detected for solutions in CHFzCl are C (79%) and T B (21%) for 5, TB for 6, 7, and 8 (the disubstituted derivatives), C for the 2-methyl-derivative (9), and T B for the 2-methoxy derivative (10). Thus the polar nature of the single methoxy substituent causes a change in the ring conformation as a consequence of the anomeric effect. Free-energy barriers were determined for the chair inversion of 5 (8.0 kcal/mol) and for the twist-boat pseudorotation of 6,7,8, and 10 (10.0,9.0,9.9, and 6.7 kcal/ mol, respectively). The substituent effects on both the conformational and dynamic properties of the seven-membered rings are discussed and explained

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

“…Information on the geometry of the major form is derived in part from the proton-proton coupling constants reported in Table 1, where it is seen that all couplings are similar to those for the chair forms of 8 , 8a, 8b, and 8d. In addition, the I3C parameters offer an equally valuable conformational probe because of the sensitivity of 6C.5 to ring conformation as was shown earlier (8,15) for 2 and 3, and in this work for 8d.…”

“…At the outset it is useful to recall that the TB form of 2 is stabilized by the generalized anomeric effect (8 'For a solution in dimethyl ether. The quantity of the second of the two conformers is given in brackets whenever an equilibrium exists.…”

“…Whereas, for the analogous six-membered orthoformate 2-methoxy-1,3-dioxane, both axial and equatorial chair conformations (i.e. C, and C, respectively) are observed (lo), the 2-methoxy derivative of 2 was found to exist only in the TB form (8). Presumably, repulsive steric interactions destabilize the C, form while electronic factors destabilize the C, form and stabilize the TB form.…”

“…Previous studies (7) could not clearly distinguish experimentally between these forms because they interconvert easily as a result of the low energy barrier for the pseudorotation motion involved, although calculations suggest that 5 is more stable than 4 (7). In contrast, the dioxepins 2 and 3 provided an opportunity (8,9) to investigate experimentally the stabilizing effect associated with the g i g t acetal arrangement in a ring system because the only TB form possible for these molecules has the appropriate bond disposition as shown by 6 . On the other hand, the features of 5 can be found in a benzocycloheptene derivative only if a change in the position of the oxygen atoms takes place as in 7, which represents the TB form of 1,3-benzodioxepin (8).…”

“…The various signals at -137OC are labelled in accordance with conformational assignments made in the next section. Finally, a free energy of activation of 8.4 kcal/mol is calculated at -77OC (the coalescence temperature T,) from the splitting of the H-2 signal using the two-site approximation and a transmission coefficient of one-half (8).…”