Page 751 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

734 the pattern is short, short, long, rather than alternating. The NMR spectrum indicates
an aromatic ring current. 100 The chemical reactivity of the molecule also justifies its
CHAPTER 8 101
classification as aromatic.
Aromaticity
There are also examples of [18]annulene systems constructed around a saturated
central core, such as in compound 4. 102 In this compound, the internal protons are
at very high field (−6to −8ppm), whereas the external protons are far downfield
(∼ 9 5ppm).

H
H H
H
H H

4

The chemical shift data can be used as the basis for calculating the diamagnetic
ring current by comparing the value with the maximum ring current expected for a
completely delocalized system. By this criterion, the flexible [18]annulene maintains
only about half (0.56) of the maximum ring current, whereas the rigid ring in 4 gives
a value of 0.88, indicating more effective conjugation in this system.
The synthesis of annulenes has been carried forward to larger rings as well.
[20]Annulene, 103 [22]annulene, 104 and [24]annulene 105 have all been reported. The
NMR spectra of these compounds are consistent with regarding [22]annulene as
aromatic, whereas the [20] and [24] analogs are not. The dominant structure for
[24]annulene has a repeating Z,E,E-motif of double bonds. The internal hydrogens
1
are at lower field in the H-NMR spectrum than the external ones, which is consistent
with a paramagnetic ring current. There is another minor conformation that has the
same configuration of the double bonds. There is also evidence for a bond shift process
‡
that interchanges single and double bonds. This process occurs with a G of about
10 kcal/mol. 106 Although the properties of these molecules have not been studied as
completely as for the smaller systems, they are consistent with the predictions of the
Hückel rule.

100 Y. Gaoni, A. Melera, F. Sondheimer, and R. Wolovsky, Proc. Chem. Soc., 397 (1965).
101
I. C. Calder, P. J. Garratt, H. C. Longuet-Higgins, F. Sondheimer, and R. Wolovsky, J. Chem. Soc. C,
1041 (1967).
102 T. Otsubo, R. Gray, and V. Boekelheide, J. Am. Chem. Soc., 100, 2449 (1978).
103
B. W. Metcalf and F. Sondheimer, J. Am. Chem. Soc., 93, 6675 (1971).
104 R. M. McQuilkin, B. W. Metcalf, and F. Sondheimer, J. Chem. Soc., Chem. Commun., 338 (1971).
105 I. C. Calder and F. Sondheimer, J. Chem. Soc., Chem. Commun., 904 (1966).
106
J. F. M. Oth and Y. de J. de Zelicourt, Helv. Chim. Acta, 82, 435 (1999).