Page 771 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

754 The bond at the ring fusion is quite long. A molecular mechanics calculation on this
molecule, which includes an SCF-MO treatment of the planar conjugated system,
CHAPTER 8
concluded that the molecule is slightly destabilized (4 kcal/mol) relative to a polyene
Aromaticity reference. 191 HF/STO-3G calculations found a small stabilization. 192 An experimental
determination of H also suggests a small (3.7 kcal/mol) stabilization. 193
H 2
The tricyclic fused systems known as s-indacene and as-indacene have also
been of interest in assessing the range of aromaticity. s-Indacene derivatives have
bond lengths varying from 1.39 to 1.44 Å in the crystal structure. 194 MO and DFT
calculations vary in predicting the relative energy of localized and delocalized struc-
tures. 195 B3LYP/6-31G* calculations place the structures within 0.1 kcal/mol of one
another. 196 An aromatic stabilization of 10.8 kcal/mol is calculated based on the
following isodesmic reaction, which is much less than for anthracene. 197 The NICS
values for both rings are positive and indicate a paramagnetic ring current.

+ 2

ASE = 10.8 kcal/mol

The possibility of extra stabilization in systems that have conjugated components
exocyclic to the ring has also been examined. Such substituents complete conjugated
rings but are not part of the cyclic system. Some representative structures are shown
in Scheme 8.5.
Cyclopropenes and cyclopentadienes with exocyclic double bonds provide the
possibility of dipolar resonance structures that suggest aromatic character in the cyclic
structure.

+ – +
CH 2 CH 2 – CH 2
CH 2
triafulvene fulvene

For methylenecyclopropene, a microwave structure determination has established bond
lengths that show the strong alternation anticipated for a localized structure. 198 The
molecule does have a significant (1.90 D) dipole moment, implying a contribution from
the dipolar resonance structure. The net stabilization calculated at the MP/6-31G* level
is small and comparable to the stabilization of 1,3-butadiene. The molecular geometry

191
N. L. Allinger and Y. H. Yuh, Pure Appl. Chem., 55, 191 (1983).
192 D. Cremer, T. Schmidt, and C. W. Bock, J. Org. Chem., 50, 2684 (1985).
193
W. Roth, H. -W. Lennartz, E.Vogel, M. Leiendecker, and M. Oda, Chem. Ber., 119, 837 (1986).
194 K. Hafner, B. Stowasser, H. -P. Krimmer, S. Fischer, M. C. Böhm, and H. J. Lindner, Angew. Chem.
Intl. Ed. Engl., 25, 6201 (1986); J. D. Dunitz, C. Kruger, H. Imgartinger, E. F. Maverick, Y. Wang,
and M. Nixdorf, Angew. Chem. Intl. Ed. Engl., 27, 387 (1988).
195
C. Gellini, G. Cardini, P. R. Salvi, G. Marconi, and K. Hafner, J. Phys. Chem., 97, 1286 (1993);
R. H. Hertwig, M. C. Holthausen, W. Koch, and Z. B. Maksic, Angew. Chem. Intl. Ed. Engl., 33, 1192
(1994); R. H. Hertwidg, M. C. Holthausen, and W. Koch, Intl. J. Quantum Chem., 54, 147 (1995).
196
M. Nendel, B. Goldfuss, B. Beno, K. N. Houk, K. Hafner, and H.-J. Lindner, Pure Appl. Chem., 71,
221 (1999).
197 M. Nendel, B. Goldfuss, K. N. Houk, and K. Hafner, Theochem, 461-2, 23 (1999).
198
T. D. Norden, S. W. Staley, W. H. Taylor, and M. D. Harmony, J. Am. Chem. Soc., 108, 7912 (1986).