Studies of Cyclobuta[1,2:3,4]dicyclopentene
F IGURE 1. Cyclobuta[1,2:3,4]dicyclopentene (1), 1,6-didehydro[10]annulene (2), and tricyclic, 10π structures of formula C10H6.
isomerization of 2 to 1,5-didehydronaphthalene (4) occurs
at -51 °C, t1/2 ∼25 min, eq 1]. While extensively studied
To further explore the question of whether 1 should
be characterized as an aromatic molecule and to estimate
its heat of formation, we undertook theoretical studies
using the Gaussian 98 suite of programs.9,10 In addition
to structure 1, we investigated its isomer, 1,6-didehydro-
[10]annulene (2), as well as other related structures (vide
infra). Molecular geometries were optimized with Becke’s
three-parameter hybrid density-functional/Hartree-
Fock method11 using the correlation functional of Lee,
Yang, and Parr12 and the 6-31G(d,p) basis set (B3LYP/
6-31G(d,p)). Frequency calculations were also conducted
at this level of theory. In each case, the absence of
imaginary frequencies confirmed that the structure we
calculated was a local minimum.
Although our input geometry for 1 had C2ν symmetry
and a carbon skeleton with alternant C-C single and
double bonds, the calculated structure was found to be
planar with nonalternant bonds and D2h symmetry,
consistent with a delocalized 10π-electron system (Figure
2). The calculated structure of 2 was also planar with
nonalternant bonds and D2h symmetry. The latter cal-
culation is consistent with low-temperature solution
NMR experiments, which provided evidence for a dia-
magnetic ring current in 2 and which led to the conclu-
sion that 2 has “static or time-averaged D2h symmetry”
at -90 °C.3 Our calculations show that 1 is 4.7 kcal/mol
higher in energy than 2, which is perhaps not surprising
given the undoubtedly greater strain energy of 1 versus
2.
theoretically, neither 1 nor any substituted structure
containing the tricyclic 10π skeleton of 1 has been
synthesized. Interesting and unresolved issues concern-
ing the aromaticity of 1 (vide infra), its reactivity, and
its thermal stability led us to undertake both experimen-
tal and theoretical studies in an effort to access and
characterize structure 1; these studies ultimately leave
open the question of whether 1 is capable of existing as
an independent chemical entity.
Th eor etica l Stu d ies
Although cyclobuta[1,2:3,4]dicyclopentene (1) has not
been prepared in the laboratory, it has been the subject
of several theoretical studies. Both Hu¨ckel5 and semiem-
pirical molecular orbital methods6 were employed in
studies seeking to determine if 1 is properly categorized
as an aromatic molecule. Unfortunately, little consensus
has emerged from the prior theoretical studies of 1. While
some authors have concluded that 1 is an antiaromatic5
or nonaromatic molecule,6a,c-e another, using the SINDO
approximation method, categorized 1 as a “moderately
aromatic” compound.6b Given the limitations of the HMO
method in predicting the aromaticity of hydrocarbons (in
part, because it ignores electrostatic effects)7 and the
difficulties associated with parametrization in semiem-
pirical methods,8 it is perhaps not surprising that no clear
picture of the aromatic properties of 1 has arisen from
theoretical studies that have employed these methods.
To provide an estimate of the resonance stabilization
energy (RSE) of 1, we calculated the enthalpic changes
(9) Frisch, M. J .; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J . R.; Zakrzewski, V. G.; Montgomery, J . A.,
J r.; Stratman, R. E.; Burant, J . C.; Dapprich, S.; Millam, J . M.; Daniels,
A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J .; Barone, V.;
Cossi, M.; Cammi, R.; Mennuci, B.; Pomelli, C.; Adamo, C.; Clifford,
S.; Ochterski, J .; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma,
K.; Malik, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J . B.;
Cioslowski, J .; Ortiz, J . V.; Baboul, G.; Stefanov, B. B.; Liu, G.;
Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.;
Fox, D. J .; Keith, T. A.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara,
A.; Gonzales, C.; Challacombe, M.; Gill, P. M. W.; J ohnson, B.; Chen,
W.; Wong, M. W.; Andres, J . L.; Head-Gordon, M.; Replogle, E. S.;
Pople, J . A. GAUSSIAN 98; Gaussian, Inc.: Pittsburgh, PA, 1998.
(10) For an alternative calculation method for the determination of
aromaticity (NICS), see: Schleyer, P. v. R.; Manoharan, M.; Wang,
Z.-X.; Kiran, B.; J iao, H.; Puchta, R.; Hommes, N. J . R. v. E. Org. Lett.
2001, 3, 2465.
(5) (a) Zhou, Z.; Parr, R. G. J . Am. Chem. Soc. 1989, 111, 7371. (b)
Gastmans, J . P.; Ferraz, M. H. M. Tetrahedron 1977, 33, 2205. (c) Hess,
B. A., J r.; Schaad, L. J . J . Org. Chem. 1971, 36, 3418.
(6) (a) Glidewell, C.; Lloyd, D. Tetrahedron 1984, 40, 4455. (b) J ug,
K. J . Org. Chem. 1983, 48, 1344. (c) Toyota, A. Bull. Chem. Soc. J pn.
1975, 48, 1152. (d) Nakajima, T. Pure Appl. Chem. 1971, 28, 219. (e)
Dewar, M. J . S.; Trinajastic, N. Tetrahedron Lett. 1967, 8, 3121.
(7) Borden, W. T. Modern Molecular Orbital Theory for Organic
Chemists; Prentice-Hall: Englewood Cliffs, NJ , 1975; Chapter 5.
(8) Anh, N. T.; Frison, G.; Solladie-Cavallo, A.; Metzner, P. Tetra-
hedron 1998, 54, 12841.
(11) Becke, A. D. J . Chem. Phys. 1993, 98, 5648.
(12) Lee, C.; Yang, G.; Parr, R. G.; Phys. Rev. B 1988, 37, 785.
J . Org. Chem, Vol. 69, No. 7, 2004 2517