Page 748 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

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SECTION 8.2
The Annulenes

Z,E,E,Z,E,Z,E Z,E,Z,E,Z,Z,E

The spectrum also reveals a significant diamagnetic (aromatic) ring current. The
internal hydrogens [C(3), C(6), C(10), C(13)] are very far upfield ( =−0 61ppm). 36
The interconversion of the two forms involves a configurational change from E to
Z of at least one double bond, and the E for this process is about 10 kcal/mol. The
a
crystal structure for [14]annulene shows the Z,E,E,Z,E,Z,E-form to be present in the
solid. 82 The bond lengths around the ring range from 1.35 to 1.41 Å, but do not show
the alternating pattern of short and long bonds expected for a localized polyene. There
is some distortion from planarity, especially at carbon atoms 3, 6, 10, and 13, which
is caused by nonbonded repulsions between the internal hydrogens. MP2/6-31G* and
B3LYP/6-31G* calculations find the delocalized structure as the only minimum. 83
A 14-electron system can be generated in circumstances in which the steric
problem associated with the internal hydrogens of [14]annulene can be avoided. This
can be achieved in 10b,10c-dihydropyrene systems, in which the annulene ring is built
around a saturated core.

R R

84
Several derivatives of this ring system have been synthesized. The properties of these
compounds indicate that the conjugated system has aromatic character. They exhibit
NMR shifts characteristic of a diamagnetic ring current. Typical aromatic substitution
reactions can be carried out. 85 An X-ray crystal structure (R = C H shows that the
2 5
bond lengths are in the aromatic range (1.39–1.40 Å), and there is no strong alternation
around the ring. 86 The peripheral atoms are not precisely planar, but the maximum
deviation from the average plane is only 0.23 Å. The dimethyl derivative is essentially
planar with bond lengths between 1.38 and 1.40 Å.
Another family of 14 -electron systems is derived from structure 3. 87

syn-3 anti-3

82
C. C. Chiang and I. C. Paul, J. Am. Chem. Soc., 94, 4741 (1972).
83 C. H. Choi, M. Kertesz, and A. Karpfen, J. Am. Chem. Soc., 119, 11994 (1997).
84
(a) R. H. Mitchell and V. Boekelheide, J. Am. Chem. Soc., 96, 1547 (1974); (b) V. Boekelheide and
T. A. Hylton, J. Am. Chem. Soc., 92, 3669 (1970); (c) H. Blaschke, C. E. Ramey, I. Calder, and
V. Boekelheide, J. Am. Chem. Soc., 92, 3675 (1970); (d) V. Boekelheide and J. B. Phillips, J. Am. Chem.
Soc., 89, 1695 (1967); (e) R. H. M. Mitchell, V. S. Iyer, N. Khalifa, R. Mahadevan, S. Venugopalan,
S. A. Weerawarna, and P. Zhou, J. Am. Chem. Soc., 117, 1514 (1995).
85 J. B. Phillips, R. J. Molyneux, E. Sturm, and V. Boekelheide, J. Am. Chem. Soc., 89, 1704 (1967).
86 A. W. Hanson, Acta Crystallogr., 23, 476 (1967).
87
E. Vogel, Pure Appl. Chem., 28, 355 (1971).