Cubane, cuneane, and their carboxylates: A calorimetric, crystallographic, calculational, and conceptual coinvestigation

Article abstract of DOI:10.1021/jo050471+

This study is a multinational, multidisciplinary contribution to the thermochemistry of dimethyl1,4-cubanedicarboxylate and the corresponding isomeric, cuneane derivative and provides both structural and thermochemical information regarding the rearrangement of dimethyl 1,4-cubanedicarboxylate to dimethyl 2,6-cuneanedicarboxylate. The enthalpies of formation in the condensed phase at T = 298.15 K of dimethyl 1,4-cubanedicarboxylate (dimethyl pentacyclo-[4.2.0.0.0.0]octane-1,4-dicarboxylate) and dimethyl 2,6-cuneanedicarboxylate (dimethyl pentacyclo-[3.3.0.0.0.0]octane-2,6- dicarboxylate) have been determined by combustion calorimetry, Δ_fH°_m(cr)/kJ·mol^-1 = -232.62 ± 5.84 and -413.02 ± 5.16, respectively. The enthalpies of sublimation have been evaluated by combining vaporization enthalpies evaluated by correlation-gas chromatography and fusion enthalpies measured by differential scanning calorimetry and adjusted to T = 298.15 K, Δ_cr ^gH_m(298.15 K)/kJ·mol^-1 = 117.2 ± 3.9 and 106.8 ± 3.0, respectively. Combination of these two enthalpies resulted in Δ_fH°_m(g., 298.15 K)/kJ·mol^-1 of-115.4 ± 7.0 for dimethyl 1,4-cubanedicarboxylate and -306.2 ± 6.0 for dimethyl 2,6-cuneanedicarboxylate. These measurements, accompanied by quantum chemical calculations, resulted in values of Δ_fH_m(g, 298.15 K) = 613.0 ± 9.5 kJ·mol^-1 for cubane and 436.4 ± 8.8 kJ·mol^-1 for cuneane. From these enthalpies of formation, strain enthalpies of 681.0 ± 9.8 and 504.4 ± 9.1 kJ·mol^-1 were calculated for cubane and cuneane by means of isodesmic reactions, respectively. Crystals of dimethyl 2,6-cuneanedicarboxylate are disordered; the substitution pattern and structure have been confirmed by determination of the X-ray crystal structure of the corresponding diacid.

Full text of DOI:10.1021/jo050471+

Cubane, Cuneane, and Their Carboxylates: A Calorimetric,
Crystallographic, Calculational, and Conceptual Coinvestigation
Maria Victoria Roux,*,†,‡ Juan Z. Da´valos,† Pilar Jime´nez,† Rafael Notario,†,§ Obis Castan˜o,|
,#
,X
James S. Chickos,*, William Hanshaw, Hui Zhao, Nigam Rath, Joel F. Liebman,∇,$
Behzad S. Farivar,∇ and A. Bashir-Hashemi2,)
Instituto de Quı´mica Fı´sica “Rocasolano”, CSIC, Serrano 119, 28006 Madrid, Spain, Departamento de
Quı´mica Fı´sica, Universidad de Alcala´ de Henares, 28871 Alcala´ de Henares, Spain, Department of
Chemistry & Biochemistry, University of Missouri St. Louis, 8001 Natural Bridge Road,
St Louis, Missouri 63121-4499, Department of Chemistry & Biochemistry, University of Maryland,
Baltimore County, Baltimore, Maryland 21250, and ERC at AFRL/PRSP, 10 East Saturn Boulevard,
Edwards AFB, California 93524
jsc@umsl.edu
Received March 9, 2005
This study is a multinational, multidisciplinary contribution to the thermochemistry of dimethyl1,4-
cubanedicarboxylate and the corresponding isomeric, cuneane derivative and provides both
structural and thermochemical information regarding the rearrangement of dimethyl 1,4-
cubanedicarboxylate to dimethyl 2,6-cuneanedicarboxylate. The enthalpies of formation in the
condensed phase at T ) 298.15 K of dimethyl 1,4-cubanedicarboxylate (dimethyl pentacyclo-
[4.2.0.0.2,50.3,804,7]octane-1,4-dicarboxylate) and dimethyl 2,6-cuneanedicarboxylate (dimethyl pentacyclo-
[3.3.0.0.2,40.3,706,8]octane-2,6-dicarboxylate) have been determined by combustion calorimetry,
∆fH°m(cr)/kJ‚mol-1 ) -232.62 ( 5.84 and -413.02 ( 5.16, respectively. The enthalpies of sublimation
have been evaluated by combining vaporization enthalpies evaluated by correlation-gas chroma-
tography and fusion enthalpies measured by differential scanning calorimetry and adjusted to T )
298.15 K, ∆crgHm(298.15 K)/kJ‚mol-1 ) 117.2 ( 3.9 and 106.8 ( 3.0, respectively. Combination of
these two enthalpies resulted in ∆fH°m(g., 298.15 K)/kJ‚mol-1 of -115.4 ( 7.0 for dimethyl 1,4-
cubanedicarboxylate and -306.2 ( 6.0 for dimethyl 2,6-cuneanedicarboxylate. These measurements,
accompanied by quantum chemical calculations, resulted in values of ∆fHm(g, 298.15 K) ) 613.0 (
9.5 kJ‚mol-1 for cubane and 436.4 ( 8.8 kJ‚mol-1 for cuneane. From these enthalpies of formation,
strain enthalpies of 681.0 ( 9.8 and 504.4 ( 9.1 kJ‚mol-1 were calculated for cubane and cuneane
by means of isodesmic reactions, respectively. Crystals of dimethyl 2,6-cuneanedicarboxylate are
disordered; the substitution pattern and structure have been confirmed by determination of the
X-ray crystal structure of the corresponding diacid.
Introduction
befits the symmetry and accompanying esthetics, large
strain energy and eight tertiary carbons all capable of
possible functionalization, the chemistry of this seemingly
simple eight-carbon hydrocarbon (C8H8) and its deriva-
tives has blossomed.3 Soon after the first synthesis of
cubane, there were measurements of the enthalpies of
combustion and of sublimation.4 The values of both
measurements have been questioned.5-7 Cuneane is a
valence isomer of cubane of considerably lower symmetry.
That cuneanes arise from cubanes by Ag+-catalyzed
rearrangement suggests that the cuneanes are more
(enthalpically) stable than the corresponding cubanes.
This is the 40th anniversary of the first successful
synthesis of the polycyclic hydrocarbon cubane.1,2 As
† Instituto de Qu´ımica F´ısica “Rocasolano”.
‡ To whom all correspondence about combustion calorimetry is to
be addressed.
§ To whom all correspondence about quantum chemical calculations
is to be addressed.
| Universidad de Alcala´ de Henares.
University of Missouri St. Louis.
# To whom all correspondence about phase-transition enthalpies is
to be addressed.
X To whom all correspondence about crystallography is to be
addressed.
∇ University of Maryland, Baltimore County.
$ To whom all correspondence about strain energies is to be
addressed.
(1) Eaton, P. E.; Cole, T. W. J. Am. Chem. Soc. 1964, 86, 962.
(2) For almost contemporaneous alternative approaches to the
synthesis of cubane, see: (a) Barborak, J. C.; Watts, L.; Pettit, R. J. J.
Am. Chem. Soc. 1966, 88, 1328. (b) Chin, C. G.; Cuts, W. H.;
Masamune, S. J. Chem. Soc. Chem. Commun. 1966, 880.
2 ERC at AFRL/PRSP.
) To whom all correspondence about organic syntheses is to be
addressed.
10.1021/jo050471+ CCC: $30.25 © 2005 American Chemical Society
Published on Web 06/04/2005
J. Org. Chem. 2005, 70, 5461-5470
5461

Roux et al.
This study is a combined experimental and theoretical
contribution to the thermochemistry of dimethyl 1,4-
cubanedicarboxylate and the corresponding cuneane
derivative and provides structural and thermochemical
information regarding the rearrangement of dimethyl
1,4-cubanedicarboxylate to dimethyl 2,6-cuneanedicar-
boxylate. The thermochemical data is also augmented by
theoretical calculations on the relative stabilities of the
two title compounds as well as on the parent compounds
and other related derivatives.
The enthalpies of formation in the condensed phase
at T ) 298.15 K of dimethyl 1,4-cubanedicarboxylate
(dimethyl pentacyclo[4.2.0.0.2,50.3,804,7]octane-1,4-dicar-
boxylate) and dimethyl 2,6-cuneanedicarboxylate (di-
methyl pentacyclo[3.3.0.0.2,40.3,706,8]octane-2,6-dicarbox-
ylate) have been determined by combustion calorimetry.
The enthalpies of sublimation have been evaluated by
combining vaporization enthalpies evaluated by correla-
tion gas chromatography and fusion enthalpies measured
by differential scanning calorimetry. Values of ∆fH°m(g,
298.15 K) were calculated from these measurements. The
structure of dimethyl pentacyclo[3.3.0.0.2,40.3,706,8]octane-
2,6-dicarboxylate obtained from the rearrangement of
dimethyl 1,4-cubanedicarboxylate has been determined
by an X-ray crystal structure of the corresponding diacid.
Some details on the X-ray-determined, crystallographi-
cally disordered structure of dimethyl 2,6-cuneanedicar-
boxylate are also given.
FIGURE 1. Projection view of dimethyl 2,6-cuneanedicar-
boxylate.
FIGURE 2. Projection view of 2,6-cuneanedicarboxylic acid
with 25% thermal ellipsoids.
Results
A. X-ray Crystallography. The structure of dimethyl
2,6-cuneanedicarboxylate and 2,6-cuneanedicarboxylic
acid as solved by X-ray crystallography is illustrated in
Figures 1 and 2. Table 1 contains a comparison of some
bond distances between 2,6-cuneanedicarboxylic acid and
octamethylcuneane,8 the only other compound in the
Cambridge Data Base with this ring structure. The latter
contains two crystallographically unique molecules in the
asymmetric unit cell. The bond lengths in Table 1 are
displayed by imposing the C2 molecular axis of symmetry
present in 2,6-cuneanedicarboxylic acid on both molecules
while maintaining the numbering system shown in
TABLE 1. Comparison of Bond Lengths between
2,6-Cuneanedicarboxylic Acid and Octamethylcuneane
bond lengths (Å)
2,6-cuneane-
dicarboxylic acid
octamethyl-
cuneanea
bond
C(1)-C(2); C(1)#1-C(2)#1
C(1)-C(4); C(1)#1-C(4)#1
C(1)-C(3); C(1)#1-C(3)#1
C(2)-C(3); C(2)#1-C(3)#1
C(2)-C(2)#1
C(3)-C(4)#1; C(4)-C(3)#1
C(4)-C(4)#1
1.518(4)
1.524(4)
1.528(4)
1.458(5)
1.492(6)
1.524(4)
1.585(6)
1.520(2)
1.551(3)
1.535(7)
1.520(2)
1.532(1)
1.551(3)
1.572(2)
a Octamethylcuneane contains two crystallographically unique
molecules in the asymmetric unit; values cited are average values.8
(3) For some recent reviews, see: (a) Griffin, G. W.; Marchand, A.
P. Chem. Rev. 1989, 89, 997. (b) Eaton, P. E. Angew. Chem., Int. Ed.
Engl. 1992, 31, 1421. (c) Higuchi, H.; Ueda, I. In Carbocyclic Cage
Compounds; Chemistry and Applications; Osawa, E. J., Yonemitsu,
O., Eds.; VCH: New York, 1992; p 217. (d) Bashir-Hashemi, A.; Iyer,
S. Alster, J.; Slagg, N. Chem. Ind. 1995, 14, 551. (e) Tsanaktsidis, J.
Advances in Strain in Organic Chemistry; Halton, B., Ed.; JAI Press
Inc.: Greenwich, CT, 1997; Vol. 6, p 67. (f) Bashir-Hashemi, A.
Carbocyclic and Heterocyclic Cage Compounds and Their Building
Blocks, Advances in Strained and Interesting Organic Molecules; Laali,
K., Ed.; JAI Press Inc.: Stamford, CT, 1999; p 1. (g) Bashir-Hashemi
A.; Higuchi, H. In The Chemistry of Cyclobutanes; Rappoport, Z.,
Liebman, J. F., Eds.; Wiley: Chichester, 2005; p 873. (h) Quintanilla,
E.; Da´valos, J. Z.; Abboud, J. L. M.; Alkorta, I. In The Chemistry of
Cyclobutanes; Rappoport, Z., Liebman, J. F., Eds.; Wiley: Chichester,
2005; p 177.
(4) Kybett, B. D.; S. Carroll, S.; Natalis, P.; Bonnell, D. W.;
Margrave, J. L.; Franklin, J. L. J. Am. Chem. Soc. 1966, 88, 626.
(5) Diky, V. V.; Frenkel, M.; Karpushenkava, L. S. Thermochim. Acta
2003, 408, 115.
(6) Bashir-Hashemi, A.; Chickos, J. S.; Hanshaw, W.; Zhao, H.;
Farivar, B. S.; Liebman, J. F. Thermochim. Acta 2004, 424, 91.
(7) Kirklin, D. R.; Churney, K. L.; Domalski, E. S. J. Chem.
Thermodyn. 1989, 21, 1105.
(8) Imgartinger, H.; Strack, S.; Gleiter, R.; Brand, S.; Acta Crystal-
logr. C Cryst. Struct. Commun. 1997, 53, 1145.
Figure 1. The bond lengths for the octamethylcuneane
are averages of the distances as reported for the two
molecules.8 With the exception of the C4-C4#1 bond
length, all others appear shorter in the diacid. The
eclipsed methyl interactions that are present in octa-
methylcuneane are likely to contribute to the longer bond
lengths observed.
B. Thermochemistry. The standard enthalpies of
combustion and formation of dimethyl 1,4-cubane-dicar-
boxylate and dimethyl 2,6-cuneanedicarboxylate in the
condensed phase at T ) 298.15 K are given in the last
two columns of Table 2. The energy of combustion of
dimethyl 1,4-cubanedicarboxylate was previously deter-
mined by Kirklin et al.7 and by Avdonin et al.9 Compari-
son of the results obtained by these authors with the ones
(9) Avdonin, V. V.; Kirpichev, E. I.; Rubtsov, Y. I.; Romanova, L.
E.; Ivanova, M. E.; Eremenko, L. T. Russ. Chem. Bull. 1996, 45, 2342.
5462 J. Org. Chem., Vol. 70, No. 14, 2005

Cubane, Cuneane, and Their Carboxylates
TABLE 2. Standard Molar Energy, Enthalpy of Combustion, and Enthalpy of Formation at T ) 298.15 K and p0
)
101.325 kPa
∆U°m(cr)/
kJ‚mol-1
∆cH°m(cr)/
kJ‚mol-1
∆fH°m(cr)/
compound
kJ‚mol-1
dimethyl 1,4-cubanedicarboxylate
dimethyl 2,6-cuneanedicarboxylate
-6201.94 ( 5.63
-6021.55 ( 4.91
-6204.48 ( 5.63
-6024.08 ( 4.91
-232.62 ( 5.84
-413.02 ( 5.16
TABLE 3. Comparison of the Available Standard Energies and Enthalpies of Combustion and Formation of
1,4-Dimethyl Cubanedicarboxylate at T ) 298.15 K
purity/
no. of
amt
react
∆cu°(cr)/
J‚ g-1
∆cH°m(cr)/
kJ‚ mol-1
∆fH°m(cr)/
kJ‚mol-1
entry mol % char method expts
aux subst
ref
1
2
3
>99.7 DSC
SBCa
SBCa
7
nac
6
benzoic acid ma ≈ 0.131 g -28220.22 ( 1.95 -6218.09 ( 1.42 -218.99 ( 2.12 Kirklin7
99.9 CO2
no
m ≈ 0.180 g -28187.3
-6205.9
-231.2 ( 2.5
Avdonin9
99.9 DSC SMCb
benzoic acid m ≈ 0.035 g -28162.3( 9.3
-6204.48 ( 5.63 -232.62 ( 5.84 This work
a SBC: macro bomb combustion calorimeter. b SMC: semimicro bomb combustion calorimeter. c na: not available.
TABLE 4. Summary of Vaporization Enthalpies (Enthalpies in kJ‚mol-1
)
∆lgH°m (298.15K) (lit.)
mix 1
77.9
84.8
87.8 ( 2.0
mix 2
mix 3
63.8
78.1
85.1
mix 4
77.9
∆lgH°m (298.15 K) meana
diethyl malonate
dimethyl phthalate
diethyl phthalate
dimethyl cubanedicarboxylate
dimethyl cuneanedicarboxylate
dibutyl phthalate
65.03
77.1
85.6
64.7 ( 2.0
78.0 ( 2.2
84.8 ( 2.2
88.5 ( 2.2
89.7 ( 2.1
101.6 ( 2.2
78.0
84.7
89.2 ( 2.4
84.7
90.2 ( 2.0
89.2 ( 2.2
101.7
101.5
101.7
101.5
a The uncertainty represents 2 standard deviations in the mean uncertainty associated with the intercept of the equations derived
from the correlations.
TABLE 5. Fusion Enthalpies of Dimethyl
obtained in our work is given in Table 3. Avdonin et al.9
1,4-Cubanedicarboxylate and Dimethyl
2,6-Cuneanedicarboxylate (Enthalpies in kJ‚mol-1
)
report complete combustion without the use of an aux-
iliary substance. Kirklin et al.7 sandwiched layers of the
cubane diester with benzoic acid. Small quantities of
carbon deposits were observed on the crucible walls. We
experienced soot formation in the absence of an auxiliary
substance and in a few experiments using benzoic acid.
Complete combustion was generally observed when ben-
zoic acid was layered with the cubane diester as described
by Kirklin et al.7 and only the results of these experi-
ments are reported. The results obtained in this study,
however, are in best agreement with the work of Avdonin
et al.9 Combustion of dimethyl 2,6-cuneanedicarboxylate,
which was also conducted in the presence of the auxiliary,
benzoic acid, does not appear to have been studied
previously, nor has that of any other cuneane derivative.
No soot formation was observed in these experiments as
well.
run
m/mg
J/g
onset/°C
∆
cr(1)lH°m(Tfus
)
Tfus/K
Dimethyl 1,4-Cubanedicarboxylate
1a
2a
3a
4b
8.66 176.4
6.17 175.6
6.90 178.8
1.02
167.1
164.5
163.5
164.5
38.8
38.6
39.3
35.7
38.1 ( 1.6
41.0 ( 3.0
average
438.2 ( 1.5
437.8 ( 0.5
lit.c
Dimethyl 2,6-cuneanedicarboxylate
1a
6.47 104.6
5.89 107.6
5.54 111.6
1.10
119.4
119.3
119.2
23.0
2a
23.7
3a
24.5
4b
22.2
23.4 ( 1
average
392.7 ( 0.1
a St. Louis. b Madrid. c Reference 7.
Vaporization enthalpies were obtained by correlation
gas chromatography.6,10 The results of these studies are
summarized in Table 4. Also included in Table 4 is a
summary of how well the vaporization enthalpy of the
standards were reproduced. Since the retention times of
dimethyl 1,4-cubane and 2,6-cuneanedicarboxylates were
very similar, their vaporization enthalpies were evalu-
ated in separate experiments; the values of both are very
similar. As noted above, the literature values are aver-
ages of several determinations; details are available in
the Supporting Information.
dimethyl cubanedicarboxylate were also measured previ-
ously. The values measured compare within the experi-
mental uncertainty with those reported earlier by Kirklin
et al.7 No additional phase transitions were observed for
either diester between T ) 298 K and T ) Tfus. An
average value of 38.1 ( 1.6 kJ‚mol-1 was used in all
subsequent calculations. Although the fusion entropies
of these two valence isomers might be expected to be
similar in magnitude, the smaller fusion entropy of the
dimethyl cuneanedicarboxylate is consistent with the fact
that the crystal for this material is disordered and that
it exists as a racemic material. Furthermore, the low
temperature thermal behavior of this material was not
investigated.
The fusion enthalpies of both the cubane and cuneane
diesters are listed in Table 5. The fusion enthalpy of
(10) For a description of the technique see: Chickos, J. S.; Hosseini,
S.; Hesse, D. G. Thermochim. Acta 1995, 249, 41. Chickos, J. S.; Hesse,
D. G.; Hosseini, S.; Nichols, G.; Webb, P. Thermochim. Acta 1998, 313,
101.
The sublimation enthalpy of both diesters can be
obtained by combining the vaporization enthalpy avail-
able at T ) 298.15 K with the fusion enthalpy once
J. Org. Chem, Vol. 70, No. 14, 2005 5463

Roux et al.
TABLE 6. Sublimation Enthalpies of Dimethyl 1,4-Cubanedicarboxylate and Dimethyl 2,6-Cuneanedicarboxylate
(Enthalpies in kJ‚mol-1
)
∆
crlH°m(Tfus
)
Tfus/K ∆Cp∆Ta/kJ‚mol-1
∆
crlH°m(298 K)b ∆lgH°m(298 K)
∆
gH°m(298 K)
cr
dimethyl 1,4-cubanedicarboxylate
dimethyl 2,6-cuneanedicarboxylate
38.1 ( 1.6
23.4 ( 1.0
438.2
392.7
-9.4 ( 2.8
-6.3 ( 1.9
28.7 ( 3.2
17.1 ( 2.1
88.5 ( 2.2
89.7 ( 2.1
117.2 ( 3.9
106.8 ( 3.0
a Heat capacities of the crystal and liquid phase were estimated. Dimethyl 1,4-cubanedicarboxylate, dimethyl 2,6-cuneanedicarboxylate:
Cp(cr) ) 236.2; Cp(l) ) 355.8 J‚mol-1.K-1, respectively; b Uncertainty ((2σ) assumed to be 0.3 of the magnitude of the temperature
adjustment. at T ) 298.15 K with the condensed phase enthalpy of formation of dimethyl 1,4-cubane-dicarboxylate and dimethyl 2,6-
cuneanedicarboxylate results in the enthalpies of formation in the gas phase given in Table 7.
TABLE 7. Standard Molar Enthalpies of Sublimation
∆
gHm and Formation ∆fH°m of the Solid and ∆fH°m of
cr
the Gas at T ) 298.15 K and p0) 101.325 KPa for
Dimethyl 1,4-cubanedicarboxylate and Dimethyl
2,6-cuneanedicarboxylate; Enthalpies in KJ‚Mol-1
compound
∆fH°m(cr)
∆
gHm
∆fH°m(g)
cr
dimethyl 1,4-cubane-
dicarboxylate
-232.6 ( 5.8 117.2 ( 3.9 -115.4 ( 7.0
dimethyl 2,6-cuneane- -413.0 (5.2 106.8 ( 3.0 -306.2 ( 6.0
FIGURE 3. Numbering of the positions in cubane and
cuneane rings.
dicarboxylate
TABLE 8. Comparison of the Calculated Relative
Enthalpies of Formation of Dimethyl
adjusted to from T ) Tfus to T ) 298.15 K. A protocol for
11
doing so using eq 1 has been described previously.
Benzenedicarboxylates with Experiment (Enthalpies in
kJ‚mol-1
)
∆cr Hm(298.15 K)/kJ‚mol-1 ) ∆cr Hm(Tfus/K)+
l
l
compound
∆fH°m(g)expt ∆∆fH°m(g)expt ∆∆fH°m(g)calcd
[0.15Cp(cr) - 0.26Cp(l) - 9.83][Tfus - 298.15]/1000
dimethyl 1,2-benzene- -606.1 ( 2.7a
dicarboxylate
21.9
18.2
(1)
-606.2 ( 2.6b
21.2
-1.2
dimethyl 1,3-benzene- -629.2 ( 2.0a
dicarboxylate
1.8
Molar heat capacities of the crystal (Cp(cr)) and liquid
phase (Cp(l)) were estimated; the following values were
used for both dimethyl 1,4-cubane-dicarboxylate and
dimethyl 2,6-cuneanedicarboxylate: Cp(cr) ) 236.2; Cp(l)
) 355.8 J‚mol-1.K-1. The results of the temperature
adjustment to the fusion enthalpies are given in Table
6. Combining the sublimation enthalpies at T ) 298.15
K with the condensed-phase enthalpy of formation of
dimethyl 1,4-cubanedicarboxylate and dimethyl 2,6-cu-
neanedicarboxylate results in the enthalpies of formation
in the gas phase given in Table 7.
C. Theoretical Calculations. The results from the
theoretical calculation for cubane, methyl cubanecar-
boxylate, the three isomers of dimethyl cubanedicarboxy-
late, cuneane, the three methyl cuneanecarboxylates, the
10 dimethyl cuneanedicarboxylates, and the reference
compounds used in the calculations are available in the
Supporting Information. Numbering of the positions in
cubane and cuneane rings is shown in Figure 3. It is
worth noting that isomers generally have very similar
values of zero-point vibrational energies and thermal
corrections, when comparing cubane with cuneane, meth-
yl cubanecarboxylate with the methyl cuneanecarboxy-
lates, and the dimethyl cubanedicarboxylates with the
dimethyl cuneanedicarboxylates.
-627.1 ( 1.1b
0.3
0.0
dimethyl 1,4-benzene- -628.0 ( 1.0a
dicarboxylate
0.0
-627.4 ( 1.1b
0.0
a Value taken from ref 40a. b Value taken from ref 40b.
TABLE 9. Energies at 0 K and Enthalpies at 298 K for
Cubane and Cuneane Obtained at Different Gaussian-n
Levelsa
compound
cubane
level
E0
H298
∆fH°m(g)
G2(MP2)
G2
-308.84760a -308.84201a
599.5
595.5
607.1
608.7
439.1
435.3
445.0
445.3
-308.85324a -308.84765a
G3//B3LYP -309.21009 -309.20450
G3
-309.21826 -309.21259
-308.90883 -308.90311
-308.91438 -308.90866
cuneane
G2(MP2)
G2
G3//B3LYP -309.27245 -309.26673
G3 -309.28016 -309.27435
a All values in hartrees (1 hartree ) 2625.5 kJ‚mol-1). a In last
column, enthalpies of formation, in kJ‚mol-1, calculated using bond
separation reactions. a Value taken from ref 41.
in the case of the parent compounds, cubane and cu-
neane, is possible to carry out such type of calculations,
to obtain more reliable enthalpies of formation, as it is
discussed in the next Section. Calculations have been
performed at different Gaussian-n levels. The results
obtained are collected in Table 9.
Choosing dimethyl 1,4-benzenedicarboxylate as the
reference compound, Table 8 compares the relative
enthalpies of formation of dimethyl benzenedicarboxy-
lates in gas phase at T ) 298.15 as calculated by theory
and measured experimentally.
The size of the methyl and dimethyl carboxylates
studied in this work does not permit calculations at very
high levels, because of their computational cost. However,
Discussion
One of the more interesting and perhaps elusive
properties of cubane and its derivatives, is its strain
energy, i.e., the enthalpy difference between the enthalpy
of formation of the cubane and that of an unstrained
reference material. We have chosen to use as our refer-
(11) Chickos, J. S.; Hanshaw. W. J. Chem. Eng. Data 2004, 49, 77.
Bashir-Hashemi, A.; Chickos, J. S.; Hanshaw, W.; Zhao, H.; Farivar,
B. S.; Liebman, J. F. Thermochim. Acta 2004, 424, 91.
5464 J. Org. Chem., Vol. 70, No. 14, 2005

Cubane, Cuneane, and Their Carboxylates
TABLE 10. Experimental and Calculated Enthalpies of
a
Formation (kJ‚mol-1
)
compound
∆fH°m(g)expt
∆fH°m(g)calcd
∆strainHm(g)
methane
-74.4 ( 0.4b
-83.8 ( 0.3b
ethane
propane
isobutane
cyclopropane
cyclobutane
cyclopentane
benzene
-104.7 ( 0.5b
-134.2 ( 0.6b
53.3 ( 0.5b
27.7 ( 1.1b
-76.4 ( 0.7b
82.9 ( 0.5c
methyl benzoate
adamantane
methyl cyclopropane-
carboxylate
methyl cyclobutane-
carboxylate
-276.1 ( 4.0d
-134.6 ( 2.2b
-309.8 ( 0.7f
0.0e
FIGURE 4. Homodesmic reaction for the calculation of the
enthalpy of formation of cubane and cuneane using ∆fH°m(g)
of adamantane and methyl 1-adamantanecarboxylate as refer-
ence.
-327.7 ( 1.3f
methyl 1-adamantane- -495.4 ( 2.7g
0.0e
carboxylate
dimethyl 1,4-cubane-
-115.4 ( 7.0h,i
-100.1 ( 4.4k
j
j
674.2 ( 9.8h
ence material for an unstrained quaternary carbon atom
bearing a carbomethoxy group (C-CO2CH3), the differ-
ence in the experimental enthalpies of formation of
methyl 1-adamantanecarboxylate and adamantane, Fig-
ure 4. Also included in Figure 4 is the homodesmic
reaction relating dimethyl cuneanedicarboxylate to cu-
neane. The theoretical calculations indicate that both
reactions are not exactly thermoneutral, resulting in
enthalpies of reaction 6.8 and 21.0 kJ‚mol-1, in the case
of cubane and cuneane, respectively. An uncertainty of
(4 kJ‚mol-1 in the calculated enthalpies of reaction has
been assumed. Combining these results with the experi-
mental enthalpies of formation given in Table 10 results
in enthalpies of formation, ∆fH°m(g, 298.15 K) ) 613.0 (
9.5 and 436.4 ( 8.8 kJ‚mol-1, for cubane and cuneane,
respectively. As has been indicated above, we have
carried out calculations on cubane and cuneane at
different high Gn levels. Applying the bond separation
reaction method12,13 and using the following bond separa-
tion reaction
dicarboxylate
678.5h
595.8k
dimethyl 2,6-cuneane- -306.2 ( 6.0h,i
483.4 ( 8.8h
dicarboxylate
484.7h
cubane
622.2 ( 3.7l 613.0 ( 9.5h,m 681.0 ( 9.8h,°
602.7 ( 7.3h,n 670.7 ( 9.4h,n
685.3h,p
cuneane
436.4 ( 8.8h,m 504.4 ( 9.1h,o
441.2 ( 7.3h,n 509.2 ( 9.4h,n
505.7h,p
bicyclobutane
217.1 ( 0.8b
276.0q
266.0r
260.7k
255.1s
methyl 1-bicyclobutane- -164.6 ( 0.7b
carboxylate
222.3k
a Uncertainties represent (2σ; values in bold are recommended.
b Value taken from ref 17. c Value taken from ref 42. d Value taken
from ref 43. e Used as reference, taken to be strainless. This is,
admittedly, somewhat contentious. However, what is more im-
portant in the current context is the assumption that these species
have the same strain energy. f Value taken from ref 18. g Value
taken from ref 44. h This work. See text. i Experimental value from
Table 7. j See Table 11. k Value taken from ref 7. l Value taken
from ref 4. m Calculated from the homodesmic reaction of Figure
4. n Results from Gaussian-n calculations. o Calculated from the
isodesmic reaction of Figure 5. p Results from MP2(FULL)/6-
31G(d) level. q Calculated from ∆fH°m(g)expt of bicyclobutane + 5
ethane f 2 propane + 2 isobutane. r Calculated using Benson’s
strain free group values (ref 45). s Calculated using the strain
energy of bicyclobutane and the exothermicity of the following
reaction: bicyclobutane + methyl 1-adamantanecarboxylate f
methyl 1-bicyclobutanecarboxylate + adamantane.
C8H8 (g) + 16CH4 (g) f 12C2H6 (g)
(2)
enthalpies of formation, ∆fH°m(g, 298.15 K) ) 602.7 (
7.3 and 441.2 ( 7.3 kJ‚mol-1 are calculated for cubane
and cuneane, respectively, as average of the values
calculated at the different Gaussian-n levels (see Table
9). Values of 597.514 and 610.915 kJ‚mol-1 have been
previously calculated for cubane at the G2(MP2,SVP) and
G3(MP2) levels. A value of 460 kJ‚mol-1 had been
estimated for cuneane using group additivity.16
Using the isodesmic reactions and the stoichiometry
of Figure 5, strain enthalpies of 681.0 ( 9.8 and 504.4 (
9.1 kJ‚mol-1 are calculated for cubane and cuneane,
respectively, using their enthalpies of formation derived
from reactions in Figure 4, whereas strain enthalpies of
670.7 ( 9.4 and 509.2 ( 9.4 kJ‚mol-1 are calculated using
their enthalpies of formation derived from Gaussian-n
(12) Raghavachari, K.; Stefanov, B. B.; Curtiss, L. A. J. Chem. Phys.
1997, 106, 6764.
FIGURE 5. Isodesmic reaction for the calculation of the strain
enthalpy of cubane and cuneane; enthalpies in kJ‚mol-1using
∆fH°m(g) of ethane and isobutane as reference.
(13) Hehre, W. J.; Radom. L.; Schleyer, P. v. R.; Pople, J. A. Ab Initio
Molecular Orbital Theory; Wiley: New York, 1986.
(14) Wang, H.; Law, C. K. J. Phys. Chem. B 1997, 101, 3400.
(15) Rogers, D. W. J. Mol. Struct. 2000, 556, 207. In his paper, the
value calculated, 146 kcal‚mol-1 (Table 3), was incorrectly transcribed
to section 4.2 as 148.7 kcal‚mol-1 (personal communication with
Professor Donald W. Rogers).
calculations. The corresponding values obtained directly
from the theoretical calculations, at the MP2(FULL)/6-
31G(d) level, are 685.3 and 505.7 kJ‚mol-1, respectively.
Similar estimations of the strain energy of the dicar-
(16) Hassenru¨ck, K.; Martin, H.-D.; Walsh, R. Chem. Ber. 1988, 121,
369.
J. Org. Chem, Vol. 70, No. 14, 2005 5465

Roux et al.
methyl cuneanecarboxylate +
benzene f cuneane + methyl benzoate (4)
dimethyl cubanedicarboxylate +
benzene f cubane +
dimethyl benzenedicarboxylate (5)
dimethyl cuneanedicarboxylate +
benzene f cuneane +
dimethyl benzenedicarboxylate (6)
FIGURE 6. Isodesmic reaction for the calculation of the strain
enthalpy of dimethyl 1,4-cubanedicarboxylate and dimethyl
2,6-cuneanedicarboxylate.
Series II:
bomethoxy derivatives, Figure 6, results in strain en-
thalpies of 674.2 ( 9.0 and 483.4 ( 8.8 kJ‚mol-1 for
dimethyl 1,4-cubanedicarboxylate and dimethyl 2,6-cu-
neanedicarboxylate, respectively. The corresponding val-
ues obtained directly from the theoretical calculations,
at the MP2(FULL)/6-31G(d) level, are 678.5 and 484.7
kJ‚mol-1, respectively.
methyl cubanecarboxylate +
adamantane f cubane +
methyl 1-adamantanecarboxylate (7)
methyl cuneanecarboxylate +
adamantane f cuneane +
methyl 1-adamantanecarboxylate (8)
Kybett et al.4 report a strain energy of 657 kJ‚mol-1
for cubane. Their difficulty in obtaining complete com-
bustion has already been noted, and the sublimation
enthalpy reported for cubane has also been shown to be
in serious error.5,6 Kirklin et al.7 using the sublimation
enthalpy data of cubane were fortuitously able to esti-
mate the sublimation enthalpy of dimethyl cubanedicar-
boxylate as 118.9 kJ‚mol-1; this number is within the
experimental uncertainty of the value obtained in this
study. Their enthalpy of formation, ∆fH°m(g, 298.15 K),
of cubane, 665.3 kJ‚mol-1, was derived from data on the
dicarbomethoxy derivative (Table 7, entry 1) using cy-
clobutane and bicyclobutane and their corresponding
carbomethoxy derivatives as reference materials in ho-
modesmic reactions similar to those shown in Figure 6.
A strain enthalpy of 674.7 kJ‚mol-1 was calculated using
the enthalpy of formation of gaseous cubane and a strain
free group increment for a tertiary sp3 carbon, (C-(C)3-
(H) ) -1.17 kJ‚mol-1).7 Despite obvious differences in
the numerical values used, the strain energies calculated
are identical to the results of this work.
dimethyl cubanedicarboxylate +
2 adamantane f cubane +
2 methyl 1-adamantanecarboxylate (9)
dimethyl cuneanedicarboxylate +
2 adamantane f cuneane +
2 methyl 1-adamantanecarboxylate (10)
Depending of the values taken for the enthalpies of
formation of cubane and cuneane, different values of the
enthalpies of formation for all the methyl and dimethyl
cubane- and cuneanecarboxylates are obtained. These
values are collected in Table 11. As can be observed from
the table, the calculated values for dimethyl 1,4-cu-
banedicarboxylate and dimethyl 2,6-cuneanedicarboxy-
late are in very good agreement with those values
experimentally determined in this work.
The rearrangement of cubanes to the more stable
cuneanes confirmed by the thermochemical measure-
ments is also consistent with theory. An isomerization
enthalpy calculated by theory at the MP2(FULL)/6-31G-
(d) level as -179.6 kJ‚mol-1 compares to a value of
-161.5 kJ‚mol-1 obtained from Gaussian-n calculations.
For the isomerization of dimethyl 1,4-cubanedicarboxy-
late to dimethyl 2,6-cuneanedicarboxylate, a theoretical
isomerization enthalpy of -193.9 kJ‚mol-1 can be com-
A decrease in strain energy is often observed in
going from the parent compound to a substituted deriva-
tive. Using similar homodesmic and isodesmic reac-
tions for methyl 1-bicyclobutanecarboxylate and bicy-
clobutane (Figures 4 and 5, Table 10), Kirklin et al.7
calculated a difference in strain enthalpy of 38.4 kJ‚mol-1
per carbomethoxy substituent. In this work, the
total decrease in strain for cubane and cuneane going
to the corresponding 1,4- and 2,6-dicarbomethoxy
pared to the experimental value of -190.8 ( 9.2 kJ‚mol-1
.
Are these enthalpy of formation and isomerization
values plausible? Although cubane is unequivocally a
pentacyclic compound, it is still appealing to describe it
as composed of six rings or faces, namely six cyclobu-
tanes. Likewise, cuneane is describable as six ring or
faces but in this case, two apiece of cyclopropane, cy-
clobutane, and cyclopentane. Accordingly, the isomeriza-
tion of cubane to cuneane corresponds to the transfor-
mation of four cyclobutanes into two cyclopropanes and
two cyclopentanes. Because cubane and cuneane are
composed of the same number and types of groups (C(C)3-
(H), tertiary sp3), the isomerization enthalpy corresponds
directly to the change in strain energy of the two
polycycles. Because cyclopropane, cyclobutane and cyclo-
pentane all have the same groups (C(C)2(H)2, secondary
derivatives is calculated as 6.8 and 21.0 kJ‚mol-1
,
respectively. This is considerably less than previous
estimates.7
Theoretical values of the enthalpies of formation for
all the methyl and dimethyl cubane- and cuneanecar-
boxylates studied in this work can be obtained using two
sets of isodesmic reactions with benzene/methyl benzene-
carboxylates (series I reactions, 3-6) or adamantane/
methyl 1-adamantanecarboxylate (series II reactions,
7-10) as references.
Series I:
methyl cubanecarboxylate + benzene f cubane +
methyl benzoate (3)
5466 J. Org. Chem., Vol. 70, No. 14, 2005

Cubane, Cuneane, and Their Carboxylates
TABLE 11. Calculated Relative and Absolute Enthalpies of Formation for All the Cubanes and Cuneanes Studied in
This Work (Enthalpies in kJ‚mol-1
)
∆fH°m(g)calc from series I reactions
∆fH°m(g)calc from series II reactions
a
∆∆fH°m(g)calcd
b
c
b
c
cubane
179.6
0.0
613.0
436.4
602.7
441.2
613.0
436.4
602.7
441.2
cuneane
methyl cubanecarboxylate
187.0
11.5
0.0
250.9
240.6
248.0
237.7
methyl 1-cuneanecarboxylate
methyl 2-cuneanecarboxylate
methyl 3-cuneanecarboxylate
dimethyl 1,2-cubanedicarboxylate
dimethyl 1,3-cubanedicarboxylate
dimethyl 1,4-cubanedicarboxylate
dimethyl 1,2-cuneanedicarboxylate
dimethyl 1,3-cuneanedicarboxylate
dimethyl 1,4-cuneanedicarboxylate
dimethyl 1,5-cuneanedicarboxylate
dimethyl 2,3-cuneanedicarboxylate
dimethyl 2,4-cuneanedicarboxylate
dimethyl 2,6-cuneanedicarboxylate
dimethyl 2,7-cuneanedicarboxylate
dimethyl 2,8-cuneanedicarboxylate
dimethyl 3,7-cuneanedicarboxylate
78.4
83.2
75.6
80.4
66.9
71.7
64.1
68.9
2.8
69.7
74.5
66.9
71.7
191.4
195.6
193.9
10.0
16.4
10.7
20.6
15.2
8.3
-102.5
-104.9
-103.7
-280.8
-281.1
-286.8
-270.3
-275.6
-282.6
-294.5
-294.7
-297.5
-261.4
-112.8
-115.2
-114.0
-276.0
-276.3
-282.0
-265.5
-270.8
-277.8
-289.7
-289.9
-292.7
-256.6
-117.9
-113.7
-115.4
-296.2
-289.9
-295.6
-285.7
-291.1
-298.0
-306.3
-303.4
-306.2
-276.8
-128.2
-124.0
-125.7
-291.4
-285.1
-290.8
-280.9
-286.3
-293.2
-301.5
-298.6
-301.4
-272.0
0.0
2.8
0.0
29.5
a MP2(FULL)/6-31G(d) level. b Values calculated using for cubane and cuneane their enthalpies of formation obtained using reactions
in Figure 4. c Values calculated using for cubane and cuneane using enthalpies of formation obtained from theoretical calculations at
several Gn levels. See text.
sp3) we may equate the enthalpy of the reaction
4Cyclo-(CH2)4f 2Cyclo-(CH2)3 + 2Cyclo-(CH2)5 (11)
with the desired strain energy change. From the enthal-
pies of formation of the three cycloalkanes taken from
ref 17, we find an exothermicity or strain lessening of
-157.0 ( 2.5 kJ‚mol-1, a value encouragingly close to
the above exothermicity of the cubane to cuneane rear-
rangement. Consider now the rearrangement of the
cubane and cuneane dicarboxylate esters. This process
differs from that of the parent hydrocarbons by trans-
forming two esters on four-membered rings into two on
three-membered rings. Since the two substituents that
compose dimethyl cubane and cuneane dicarboxylate are
identical, we may estimate the difference of the rear-
rangement enthalpy of the esters and hydrocarbons to
be twice the exothermicity of the reaction
FIGURE 7. Three distinct least motion pathways for the
rearrangement of dimethyl 1,4-cubanedicarboxylate involving
Cyclo-(CH2)3 +
methyl cyclobutanecarboxylate f Cyclo-(CH2)4 +
methyl cyclopropanecarboxylate (12)
inversion of configuration of both migrating CHCR (R ) H, R
) -CO2CH3) centers.
spectively. This difference is 14.2 kJ‚mol-1, corroborating
theory with our thermochemical experiments and models.
How to consider the processes by which the cubanes
isomerize into the corresponding cuneanes? Figure 7
illustrates three formal pathways by which dimethyl 1,4-
cubanedicarboxylate can rearrange to a mixture of di-
methyl 1,3- and dimethyl 2,6-cuneanedicarboxylates.8
The migrating bonds in Figure 7 are shown in bold and
the rearrangement requires inversion of configuration of
both migrating CHCR (R ) H, or -CO2CH3) centers. The
role of the metal ion is ignored; therefore these pathways
should not be considered in a mechanistic context. That
other isomeric dicarbomethoxy isomers are not formed
even though the theoretical calculations (Table 11)
predict some of them to be as stable as the 1,3 and 2,6
isomers, suggests that the rearrangement is not more
extensive than illustrated in this figure. The theoretical
Accepting the enthalpies of formation of the parent
cycloalkanes from ref 17 and that of the methyl carboxyl-
ate esters from ref 18, we find a reaction exothermicity
of -7.7 ( 1.9 kJ‚mol-1. The strain energy difference
between dimethyl 1,4-cubanedicarboxylate and dimethyl
2,6-cuneanedicarboxylate is expected to be 15.4 kJ‚mol-1
higher than that of the difference between cubane and
cuneane. We had earlier derived (in this paper) theoreti-
cal values of 6.8 and 21.0 kJ‚mol-1 for the total decrease
in strain for cubane and cuneane going to the corre-
sponding 1,4- and 2,6 dicarbomethoxy derivatives, re-
(17) Pedley, J. B. Thermochemical Data and Structures of Organic
Compounds; TRC Data Series: College Station, TX, 1994; Vol. 1.
(18) Verevkin, S. P.; Ku¨mmerlin, M.; Beckhaus, H.-D.; Galli, C.;
Ru¨chardt, C. Eur. J. Org. Chem. 1998, 579.
J. Org. Chem, Vol. 70, No. 14, 2005 5467

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

Cubane, Cuneane, and Their Carboxylates
empirical formula and massic energy of combustion of the
cotton-thread fuse used in the experiments with this micro-
bomb, C1.000H1.740O0.871 and -(17410 ( 37) J‚g-1, respectively,
were determined in the Madrid laboratory. All samples were
weighed on an ultra-micro balance and corrections of apparent
mass to mass were made.
For the correction of apparent mass to mass, conversion of
the energy of the actual bomb process to that of the isothermal
process, and corrections to the standard state, we have used
the values of density F, massic heat capacity cp, and (δV/δT)p
and (δV/δT)p (see Table S9, Supporting Information). The
density and heat capacity at T ) 298.15 K for dimethyl 2,6-
cuneanedicarboxylate were determined in this work by X-ray
and DSC, respectively.
Standard-state corrections were made according to Hubbard
et al.24 The atomic weights of the elements used are those
recommended by IUPAC in 1999.25 The results of the combus-
tion experiments are given in Table 2 (and Table S10,
Supporting Information). The symbols in these tables have the
same meaning as those in ref 26 and the experimental values
have been derived, similarly. The massic energy of combustion
of the compound is referenced to the final temperature of the
experiments, T ) 298.15 K. The uncertainties in the standard
molar energy and enthalpy of combustion are twice the final
overall standard deviation of the mean and were estimated
as outlined by Olofsson.27 The standard molar energy and
enthalpy of combustion reference to the following combustion
reaction:
used. For each compound, a fresh sample was used for each
run. After calibration, several runs with high-purity benzoic
acid and indium were performed under the same conditions
as the experimental measurements; results for accuracy in
temperatures and enthalpy determinations were 0.2 and 1.6%,
respectively. Details are given in ref 32.
Vaporization Enthalpies. The vaporization enthalpies of
dimethyl 1,4-cubane and 2,6-cuneanedicarboxylates were mea-
sured by correlation-gas chromatography. All gas chromato-
graphic standards were commercial samples and were used
without any further purification. The vaporization enthalpies
of the standards were obtained from the literature. Most of
the values of the standards reported in the literature have
been measured under different experimental conditions. The
available literature values are summarized in the Supporting
Information along with a summary of how each value was
chosen. Entries for each standard were adjusted from the mean
temperature of measurement to T ) 298.15 K using eq 14
where Cpl refers to the molar heat capacity of each liquid
standard which was estimated and Tm refers to the mean
temperature of measurement.33
∆lgHm(298.15 K)/J mol-1 ) ∆lgHm(Tm) + (10.58 +
0.26 Cpl)(Tm - 298.15) (14)
Each standard was analyzed by gas chromatography and
found to be at least 99 mol % pure. Correlation gas chroma-
tography experiments were performed on an instrument
equipped with a split/splitless capillary injection port and a
flame ionization detector run at a split ratio of approximately
100/1. Retention times were recorded to three significant
figures following the decimal point; they are provided in the
Supporting Information. The instrument was run isothermally
using either a 15 (mixtures 1, 3, and 4) or 30 m (mixture 4)
phenylmethyl silicone capillary column. Helium was used as
the carrier gas. At the temperatures of the experiments, the
retention time of the solvent, CH2Cl2, which increased with
increasing temperature, was used to determine the dead
volume of the column. Adjusted retention times, ta, were
calculated by subtracting the measured retention time of the
solvent from the retention time of each analyte as a function
of temperature, usually over a 30 K range. Column tempera-
tures were controlled by the gas chromatograph and were
monitored independently. Temperature was maintained con-
stant by the gas chromatograph to (0.1 K.
A plot of ln(1/ta) against 1/T(K) resulted in straight lines
characterized by the parameters available in the Supporting
Information. Correlation of the enthalpy of transfer from the
column to the gas phase at the mean temperature of measure-
ment, ∆slnvHm(Tm), against the vaporization enthalpies of the
standards, ∆lgHm (298.15 K), resulted in the correlation
equations (15-18). These equations were used to calculate the
vaporization enthalpy of the cubane and cuneane diesters.
C12H12O4(cr) + 13O2(g) ) 12CO2(g) + 6H2O(l) (13)
The standard molar energy of combustion and standard
molar enthalpies of combustion and formation of dimethyl 1,4-
cubanedicarboxylate and dimethyl 2,6-cuneanedicarboxylate
in solid phase were determined from the results reported in
Table S10, Supporting Information. The values for the stan-
dard molar enthalpies of formation of H2O(l) and CO2(g) at T
) 298.15 K are respectively: -(285.830 ( 0.042) and -(393.51
( 0.13) kJ‚mol-1; values were taken from CODATA.28
Thermochemical Analysis: Fusion Enthalpies. A dif-
ferential scanning calorimeter equipped with an intracooler
unit was used in Madrid and a standard DSC was used in St.
Louis to determine the purities, melting temperatures, heat
capacities and enthalpies of fusion of dimethyl 1,4-cubanedi-
carboxylate and dimethyl 2,6-cuneanedicarboxylate. The in-
strument’s temperature scale was calibrated by measuring the
melting temperature of the recommended high-purity refer-
ence materials: benzoic acid,29 tin, and indium.30,31 The power
scale was calibrated with high-purity indium (mass fraction:
>0.99999) as reference material.31 Thermograms of samples
hermetically sealed in aluminum pans were recorded in a
nitrogen atmosphere. All the pans were weighed before and
after the experiments in order to confirm that no product had
volatilized. The samples were weighed on microbalances with
a sensitivity of 1 × 10-6 g (Madrid) and 1 × 10-5 g (St. Louis),
respectively. For determination of the purities, temperatures
and enthalpies of transitions, a heating rate of 0.04 K s-1 was
∆lgHm(298 K) ) (1.3562 ( 0.078) ∆slnvHm(457 K) +
(11.68 ( 1.0)r2 ) 0.9967 (15)
(24) Hubbard, W. N.; Scott, D. W.; Waddington, G. In Experimental
Thermochemistry; Rossini, F. D., Ed.; Interscience: New York, 1956;
Chapter 5.
∆lgHm(298 K) ) (1.3406 ( 0.090) ∆slnvHm(457 K) +
(13.48 ( 1.2)r2 ) 0.9954 (16)
(25) IUPAC. Pure Appl. Chem. 2001, 73, 667.
(26) Westrum, E. F., Jr. Presentation of combustion calorimetric
data in the primary literature. In Combustion Calorimetry; Sunner,
S., Månsson, M., Eds.; Pergamon Press: Oxford, 1979; Chapter 7.
(27) Olofsson, G. Assignment of uncertainties. In Combustion Cal-
orimetry; Sunner, S., Månsson, M.: Eds.; Pergamon Press: Oxford,
1979; Chapter 6.
∆lgHm(298 K) ) (1.379 ( 0.084) ∆slnvHm(412 K) + (6.79 (
0.93)r2 ) 0.9963 (17)
(28) CODATA. Recommended key values for thermodynamics, 1975.
J. Chem. Thermodyn. 1976, 8, 603.
∆lgHm(298 K) ) (1.395 ( 0.088) ∆slnvHm(458 K) +
(7.794 ( 1.10)r2 ) 0.9960 (18)
(29) Andon, R. J. L.; Connett, J. E. Thermochim. Acta 1980, 42, 241.
(30) Indium and tin, standard materials, and melting points sup-
plied by Perkin-Elmer.
(31) Sabbah, R.; Xu-wu, A. Chickos, J. S.; Plana˜s Leitao, M. L.; Roux,
M. V.; Torres, L. A. Thermochim. Acta 1999, 331, 93.
Computational Details. Standard ab initio molecular
orbital calculations13 were performed with the Gaussian03
J. Org. Chem, Vol. 70, No. 14, 2005 5469

Roux et al.
series of programs.34 For all the species included in this study,
full geometry optimizations were carried out at the HF/6-31G-
(d) level. The corresponding harmonic vibrational frequencies
were evaluated at the same level of theory to confirm that the
optimized structures found correspond to minima of the
potential energy surface and to evaluate the corresponding
zero-point vibrational energies, ZPE, and the thermal correc-
tions at 298 K. ZPE values were scaled by the empirical factor
0.9135.35 All the minima found at the HF/6-31G(d) level were
again fully re-optimized at the MP2(FULL)/6-31G(d) level.
Electronic energies, zero-point vibrational energies, and ther-
mal correction to enthalpies for all the cubanes and cuneanes
studied in this work, and other compounds used as references,
are collected in Table S13, Supporting Information. In the case
of parent compounds, cubane and cuneane, and because of
their smaller size, we have carried out calculations at several
Gaussian-n levels: G2(MP2),36 G2,37 G3//B3LYP,38 and G3,39
the values being collected in Table 9.
of the structure and energetics of a pair of cubane and
cuneane isomers have been made. It is found that
dimethyl cuneane-2,6-dicarboxylate is thermodynami-
cally some 190 kJ‚mol-1 more stable than its synthetic
precursor, the more symmetric, dimethyl cubane-1,4-
dicarboxylate.
Acknowledgment. The support of the Spanish
MEC/DGI under Projects BQU2003-05827 and FIS2004-
02954-C03-01 and the Research Board of the University
of Missouri is gratefully acknowledged.
Supporting Information Available: Experimental de-
tails as well as X-ray crystallographic data (CIF), retention
times, details on the theoretical calculations, and references
to literature data used. This material is available free of charge
via the Internet at http://pubs.acs.org.
Summary
Through a combined calorimetric, crystallographic, and
computational effort, the first definitive determinations
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