Roux et al.
data integration. Analysis of the integrated data did not show
any decay. Final cell constants were determined by a global
refinement of xyz centroids of 4034 reflections (θ < 25.97°).
Collected data were corrected for systematic errors using
SADABS20 based on the Laue symmetry using equivalent
reflections. The integration process yielded 11,712 reflections
of which 1592 were independent reflections.
calculations predict the 1,3-dicarbomethoxy isomer to be
approximately 16 kJ‚mol-1 less stable than the corre-
sponding 2,6- isomer. Only the top pathway produces
dimethyl 1,3-cuneanedicarboxylate. On the basis of prod-
uct analysis (40/900 mg), about 5% of the reaction
proceeds through this formal pathway. The formation of
the 1,3-isomer suggests that the rearrangement is not
governed solely by thermodynamics or by the principle
of least motion.
Crystal data and intensity data collection parameters are
provided in the Supporting Information. Structure solution and
refinement were carried out using the SHELXTL software
package.21 The structure was solved by direct methods and
refined successfully in the space group C2/c. Full matrix least-
Experimental Section
2
squares refinement was carried out by minimizing Σw(Fo
-
Dimethyl 2,6-Cuneanedicarboxylate. A mixture of di-
methyl 1,4-cubanedicarboxylate [1.0 g, 1H NMR(CDCl3); δ 4.25
(s, 6H), 3.72 (s, 6H, -OMe) ppm] and silver perbromate (1.0
g) in 50 mL of dry toluene was heated at 100 °C for 12 h. The
progress of the reaction was followed by 1H NMR. The mixture
was concentrated under vacuum and chromatographed on
silica gel using methylene chloride/hexane (1/1) as eluent.
Approximately 860 mg of pure dimethyl 2,6-cuneanedicar-
boxylate was obtained (recrystallized from methylene chloride/
hexane) [1H NMR (CDCl3) δ 3.62 (s, 6H, -OMe), 3.10(m, 6H)
ppm; mp 118-119 °C, lit19 mp 116-117 °C)] along with
dimethyl 1,3-cuneanedicarboxylate (∼40 mg: 1H NMR (CDCl3)
δ 3.76 (s, 3H, -OMe), 3.73 (s, 3H, -OMe), 3.23 (m, 4H), 2.56
(m, 2H) ppm; mp 124-125 °C, lit.19 mp 122-123 °C. The 2,6-
cuneane diester was analyzed both by gas chromatography and
by DSC to determine purity. Both analyses did not indicate
the presence of any other isomer.
2,6-Cuneanedicarboxylic Acid. Dimethyl 2,6-cuneanedi-
carboxylate (32 mg, 0.15 mmol) was stirred in 400 µL of 1 N
NaOH and a few microliters of ethanol overnight at room
temperature. The next day, the solution was acidified with 6
N HCl and extracted several times with 10 mL portions of
ether. The ether layers were combined and allowed to evapo-
rate. A white crystalline residue remained (20 mg) which was
dissolved in hot ethanol and allowed to stand. The crystals
isolated, mp >250 dec; νmax broad absorption from 3300 to
2500, 1659.5, 1432, 886, 734.4 cm-1 (IR spectrum obtained
using an ATR accessory), were analyzed by X-ray crystal-
lography.
X-ray Crystal Structure: Dimethyl 2,6-Cuneanedicar-
boxylate. The crystal structure of dimethyl 2,6-cuneanedi-
carboxylate was disordered. Attempts at solving the crystal
structure provided unrealistic bond distances due to disorder.
The structure was solved both in C2/c and in a lower symmetry
space groups Cc to see if a better disorder model can be
obtained. The Cc structure is presented as Figure 1 since this
model identifies the relative position of the two carbomethoxy
groups and eliminates the possibility of a rearrangement
occurring during base-catalyzed hydrolysis of the diester.
Additional structural information can be found in the Sup-
porting Information.
2,6-Cuneanedicarboxylic Acid. Colorless crystals were
grown by slow evaporation from an ethanol solution at ambient
temperature. A crystal with dimensions 0.10 × 0.07 × 0.04
mm3 was mounted on a glass fiber in a random orientation.
Preliminary examination and data collection was performed
using a single-crystal X-ray diffractometer using graphite
monochromated Mo KR radiation (λ ) 0.71073 Å) equipped
with a sealed tube X-ray source at T ) 150 K. Preliminary
unit cell constants were determined with a set of 45 narrow
frames (0.4° in $) scans. The data set collected consists of 3636
frames with a frame width of 0.3° in $ and counting time of
15 s/frame at a crystal to detector distance of 4.900 cm. The
double pass method of scanning was used to exclude any noise.
The collected frames were integrated using an orientation
matrix determined from the narrow frame scans. SMART and
SAINT software packages were used for data collection and
Fc2)2. The non-hydrogen atoms were refined anisotropically to
convergence. The hydrogen atoms were treated using ap-
propriate riding model (AFIX m3). The final residual values
were R(F) ) 4.5% for 1176 observed reflections [I > 2σ(I)] and
wR(F2) ) 11.6%; s ) 1.0 for all data. A projection view of the
molecule with non-hydrogen atoms represented by 25% prob-
ability ellipsoids, and showing the atom labeling is presented
in Figure 2. Additional structural information can be found
in the Supporting Information.
Complete listings of the atomic coordinates for the non-
hydrogen atoms and the geometrical parameters, positional
and isotropic displacement coefficients for hydrogen atoms,
anisotropic displacement coefficients for the non-hydrogen
atoms are deposited with the Cambridge Crystallographic
Data Center (CCDC 272994, dimethyl 2,6-cuneanedicarboxy-
late; CCDC 272995, 2,6-cuneanedicarboxylic acid).
Combustion Experiments. The energy of combustion of
dimethyl 1,4-cubanedicarboxylate and dimethyl 2,6-cuneanedi-
carboxylate were determined in Madrid in an isoperibolic static
micro-bomb calorimeter developed recently. A detailed descrip-
tion of the calorimetric system can be found in earlier
papers.22,23 The calorimetric temperatures were measured to
within (1 × 10-4 K by means of a 100 Ω platinum resistance
thermometer, using a calibrated resistance bridge interfaced
to a microcomputer programmed to calculate the adiabatic
temperature change. The energy of reaction was always
referenced to the final temperature of T ) 298.15 K. The
energy equivalent of the calorimeter ꢀ(calor) was determined
from the combustion of benzoic acid, NIST standard reference
sample 39j. From nine calibration experiments, ꢀ(calor) )
(2102.54 ( 0.56) J‚K-1, where the uncertainty quoted is the
standard deviation of the mean. To obtain only CO2 as the
oxidized carbon product of the combustion reactions of both
compounds (i.e., no CO), several methods and auxiliary
substances were used. The best results were obtaining when
benzoic acid NIST 39j was used as an aid in the combustion
reaction. The calorimetric standard, SRM 39j, has a certified
specific energy of combustion of -(26434 ( 3) J g-1. This
reduces to 26414.0 J g-1 at 298.15 K and standard-state
conditions. Three benzoic acid pellets and two pellets of the
compound studied were alternatively placed and weighed into
the platinum crucible. The method followed was the same
previously described by Kirklin, Churney and Domalski.7 To
verify complete combustion at the end of each experiment, the
total quantity of gas in the bomb was slowly released (0.2
cm3‚s-1) through Dra¨ger tubes. No traces of CO were detected
(sensitivity levels were approximately 1 × 10-6 mass fraction).
No traces of carbon residue were observed in any of the
runs. The massic energies of combustion of both compounds
were determined by burning the samples in oxygen, with 0.05
cm3 of water added to the bomb. The combustion bomb was
flushed and filled with oxygen, previously freed from combus-
tible impurities, at an initial pressure of 3.04 MPa. The
(20) Blessing, R. H. Acta Crystallogr. 1995, A51, 33.
(21) Sheldrick, G. M.;Bruker Analytical X-ray Division, Madison,
WI, 2001.
(22) Da´valos, J. Z.; Roux, M. V. Meas. Sci. Technol. 2000, 11, 1421.
(23) Roux, M. V.; Torres, L. A.; Da´valos, J. Z. J. Chem. Thermodyn.
2001, 33, 949.
(19) Cassar, L.; Eaton, P. E.; Halpern, J. J. Am. Chem. Soc. 1970,
92, 6366.
5468 J. Org. Chem., Vol. 70, No. 14, 2005