Coluccini et al.
MM calculations confirm that the structures of the more
and of the less stable rotamers correspond to those
determined by NMR in solution. The syn-anti intercon-
version barriers of rotamers of 1 and of 2 were experi-
mentally determined by line shape analysis of the
temperature-dependent NMR spectra, and the corre-
sponding pathways are believed to occur via a cogwheel
mechanism, identified as the one-ring-flip process. In the
case of the hydrocarbon 2 it was observed that at lower
temperatures another motion (dubbed correlated torsion)
takes place with the same mechanism, and a value for
this second barrier was experimentally determined.
Exp er im en ta l Section
Ma t er ia ls. 1,4-Bis(m esit oyl)-2,3,5,6-t et r a m et h ylb en -
zen e (1). 1,4-Dibromodurene (7 mmol in 40 mL of dry THF)
was first reacted with n-BuLi at -78 °C (7 mmol, solution 1.6
M in hexane), then with mesitylaldehyde (10 mmol in 5 mL of
dry THF). After the temperature was raised, the mixture was
treated with NH4Cl, extracted with ether, and dried (Na2SO4)
to give the 1-bromo-4-methylhydroxymesityl-2,3,5,6-tetram-
ethylbenzene. The intermediate was treated again with n-BuLi
at -78 °C and then with mesitylaldehyde to give, after workup,
1,4-bis(methylhydroxymesityl)-2,3,5,6-tetramethylbenzene. The
diol (4 mmol in 40 mL of CH2Cl2) was then treated with PCC
(10 mmol). After 1 h, Et2O was added to the mixture and the
solution was filtered on decalite and concentrated at reduced
pressure. The crude product was purified by use of silica gel
chromatography (petroleum ether/Et2O 30/1) to give 1 (2.5
mmol, overall yield over 1,4 dibromodurene 35%). Single
crystals suitable for X-ray diffraction were obtained by slow
evaporation from absolute ethanol or chloroform.
1H NMR (CDCl3, 400 MHz) δ 2.03 (12H, s), 2.19 (12H, s),
2.29 (6H, s), 6.87 (4H, s). 13C NMR (CDCl3, 100.6 MHz) δ 17.63
(CH3), 21.71 (CH3), 22.24 (CH3), 130.56 (CH), 132.19 (Cq),
137.23 (Cq), 138.2 (Cq), 141.13 (Cq), 144.22 (Cq), 202.6 (CO).
Anal. Calcd for C30H34O2: C, 83.79; H, 9.23. Found: C, 83.84;
H, 9.15. Mp 184-186°C (from ethanol).
1,4-Bis(m esitilvin yl)-2,3,5,6-tetr a m eth ylben zen e (2). A
solution of 1 (2.5 mmol in 15 mL of dry hexane) was treated
with MeLi (8 mmol in Et2O) and heated to reflux for 3 h.24
The mixture was subsequently quenched with water, extracted
F IGURE 6. Single-crystal X-ray structure of 2 showing the
propeller-like shape of the anti Ci rotamer (Au type).
crystal corresponds to that of the less abundant rotamer
observed at the equilibrium in solution.
The X-ray structural determination, combined with the
knowledge of the syn-to-anti interconversion barrier (13.1
kcal mol-1), offers the opportunity of testing in an
independent way the assignment of the syn structure to
the more abundant rotamer in solution. From the mea-
sured barrier, in fact, the half-lifetimes of the rotamers
are estimated to be close to 5 min at about -85 °C. Thus
the very same crystal of 2 used for X-ray diffraction
(which contains only the rotamer anti) was dissolved in
CD2Cl2 at this temperature21 and the sample transferred
as quickly as possible into the probe of the NMR
spectrometer, cooled to -90 °C. In this way the system
should not have sufficient time to reach a complete
equilibrium and a proportion of the anti rotamer, greater
than that expected at the equilibrium, would conse-
quently be observed.
Indeed we measured in this experiment an initial 3.5:1
ratio, and observed a decreasing of the proportion of the
minor rotamer with time, until a 5.2:1 ratio22 was
obtained when the equilibrium conditions were finally
achieved. This allowed us to identify the minor rotamer
as that having the same structure (anti) observed in the
single crystal by X-ray crystallography, a result in
agreement with the previous assignment.
with Et2O, dried (Na2SO4), and purified on
a silica gel
chromatography column (petroleum ether/Et2O 10/1) to obtain
1,4-bis(1-hydroxyethylmesityl)-2,3,5,6-tetramethylbenzene. The
intermediate diol (0.35 mmol in 20 mL of CHCl3) was then
treated with P2O5 (5 mmol) for 15 h and the solution was
filtered and concentrated at reduced pressure. Crystallization
from acetone yielded 2 as a white solid. Single crystals suitable
for X-ray diffraction were obtained by slow evaporation from
acetone.
Con clu sion s
1H NMR (CDCl3, 400 MHz) δ 2.10 (12H, s), 2.22 (6H, s),
5.50 (2H, d, J ) 4.0 Hz), 5.60 (2H, d, J ) 4.0 Hz), 6.81 (4H, s).
13C NMR (CDCl3, 100.6 MHz) δ 18.95 (CH3), 20.83 (CH3), 22.0
(CH3), 125.37 (CH2), 129.61 (CH), 132.78 (Cq), 135.83 (Cq),
In both compounds 1 and 2 we have been able to
observe, by X-ray diffraction, the crystal structure of the
less stable of the two rotamers (syn and anti, respectively)
detected in solution by low-temperature NMR spectros-
copy. Although other examples of this type have been
reported,23 such a feature is not that usually observed.
136.33 (Cq), 141.28 (Cq), 144.91 (Cq). Anal. Calcd for C32H38
:
C, 89.80; H, 10.20. Found: C, 89.82; H, 10.18. Mp 190-192
°C.
NMR Mea su r em en ts. The assignment of the 13C spectra
was carried out by DEPT and 2D heteronuclear correlation
(gHMBC sequence). The samples for the low-temperature
measurements were prepared by connecting the NMR tubes
containing the compound and some deuterated solvent for
(21) Methylene chloride was selected because, even at temperatures
in the range -80 to - 85 °C, it is capable of dissolving compound 2
quite rapidly and is still fluid enough as to yield sufficiently resolved
spectra for distinguishing the signals of the syn from those of the anti
form. At lower temperatures the time required to dissolve the
compound was too long, thus allowing the equilibrium to be established
before the sample could be transferred into the NMR probe.
(22) The conformer ratio for the equilibrium in CD2Cl2 increases
from 2.9:1 to 5.2:1 by lowering of the temperature from -60 to -90
°C. The more stable species increases its proportion not only according
to the Boltzamann equation but also because of the larger value of
the dielectric constant at lower temperatures.19
(23) Taha, M.; Marks, V.; Gottlieb, H. E.; Biali, S. E. J . Org. Chem.
2000, 65, 8621. Marks, V.; Gottlieb, H. E.; Melman, A.; Byk, G.; Cohen,
S.; Biali, S. E. J . Org. Chem. 2001, 66, 6711. Marks, V.; Nahmany,
M.; Gottlieb, H. E.; Biali, S. E. J . Org. Chem. 2002, 67, 7898.
(24) Roberts, R. M.; El-Khawaga, A. M.; Roengsumram, S. J . Org.
Chem. 1984, 49, 1380.
7272 J . Org. Chem., Vol. 68, No. 19, 2003