Abstract
The temperature dependences of (i) the broad-band decoupled 13C n.m.r. spectrum of 5,6,11,12,17,18-hexahydrotribenzo[a.e, i]cyclododecene (1) and (ii) the 1H n.m.r. spectra of its 2,3,8.9,14,15- (2) and 1.4,7,10,13.16- (3) hexamethyl derivatives have been interpreted in terms of ring inversion between enantiomeric C2 conformations. Conformational analysis on these molecules has been carried out with the aid of strain energy calculations on selected conformations of the parent hydrocarbon (1) and its 1.4.7.1 0,13.16-hexamethyl derivative (3). Useful correlations between calculated and experimental thermodynamic parameters were found. The temperature dependences of the 1H n.m.r. spectra of the 2.3.8.9- (1 5) and 1,4,7,10- (1 6) tetramethyl derivatives of 5,6,11,12-tetrahydrodibenzo[a, e]cyclo-octene (14) have been interpreted in terms of interconversion of chair- and boat-like conformations. Strain energy calculations on selected conformations of the 1.4.7.10-tetramethyl derivative (14) have led to useful correlations between calculated and experimental thermodynamic parameters.
| Original language | English (US) |
|---|---|
| Pages (from-to) | 1398-1414 |
| Number of pages | 17 |
| Journal | Journal of the Chemical Society, Perkin Transactions 1 |
| DOIs | |
| State | Published - 1978 |
Funding
10 and 11 and refers in turn to the 4a,5-, 5,6-, 6,6a-, 10a.l1-, 11,12-, and 12,12a-bonds. The descriptors TS1 and TS1* refer to degener- ate conformations defined by coplanarity of atoms 11, 12, 12a, 4a, 5, and 6. Further degenerate analogues are the TS2 and TS2* conformations (see refs. 10 and 11)defined by coplanarity of atoms 5, 6, 6a, 10a, 11, and 12 in compound (15) in the form of (i) the four homotopic aromatic protons and (ii) the four homotopic aromatic (i) +20 "C and (ii) -70 "C indicate (see Table 5) that the signals for the aromatic protons are the most suitable for line-shape analysis (see Figure 2) employing the two-site exchange program (see method I in the Experimental section) over the temperature range -52 to -71 "C. The results of this analysis (Table 6) show that the C (37; R1= H, R2= Me) and MEB (38; R1= H, R2 = Me) conformations (see Figure 14) of (15) are populated to the extent of C : MEB N 60 : 40 in CDC13-- CS, (2 : 1) at -71 "C and that the free energy of activ- ation is 10.5kcal mol-l for C -+ Boat interconversion in close agreement with the values of 10.2 (refs. 10-12) and 10.0 (ref. 13) reported for tetrahydrodibenzocyclo-octene (14). This correspondence in conformational behaviour between (14) and (15), together with strain energy calculations discussed previously for (14), suggest that (i) the 2,3,8,9-tetramethyl derivative (15) is a suitable model for tetrahydrodibenzocyclo-octene (14) and (ii) C ---t Boat interconversion in the 2,3,8,9-tetramethyl derivative (15) involves a transition-state conformation (see Figure 15) of the TS1 (39; R1= H, R2 = Me) type. We thank Dr. B. F. Taylor for his collaboration in record- ing the 13C n.ni.r. spectra. We gratefully acknowledge financial support (to J. S. S.) from the State Scholarship Foundation of the Government of Greece and the award of an S.R.C. Research Studentship (to D. J. B.). [7/1692 Received, 26th Sefitember, 19771
ASJC Scopus subject areas
- General Chemistry