Page 742 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

more stabilized compounds exhibit the greatest magnetic susceptibility. 41 The various 725
criteria all correlate, although there may be variation in the degree of correlation for
different types of compounds. 42 Aromaticity is thus best conceived of as a single SECTION 8.2
characteristic owing to structural factors that results in both stabilization and the The Annulenes
phenomena associated with electron mobility.

8.2. The Annulenes

The term annulene refers to completely conjugated monocyclic polyenes. 43 The
synthesis and study of annulenes has been extended well beyond the first several
members of the series, cyclobutadiene, benzene, and cyclooctatetraene, which were
described in the Introduction. The generality and limits of the Hückel rule can be
tested by considering the properties of the annulene series. In this section, we consider
the properties of the annulenes and related compounds in the context of aromaticity.

8.2.1. Cyclobutadiene

Although several derivatives of cyclobutadiene are known and are discussed
shortly, cyclobutadiene itself has been observed only as a matrix-isolated species, that
is trapped at very low temperature in a frozen inert gas. The first successful synthesis
of cyclobutadiene was achieved by release from a stable iron complex. 44 Various
trapping agents react with cyclobutadiene to give Diels-Alder adducts, indicating
that it is reactive as both a diene and a dienophile. 45 Dehalogenation of trans-3,
4-dibromocyclobutene gave the same reaction products. 46

dimerization
H H
CH 2 CHCO 2 CH 3

CO CH 3
2

In the absence of trapping agents, a characteristic dimer is produced, which is the
result of one cyclobutadiene molecule acting as a diene and the other as a dienophile
in a Diels-Alder reaction. This dimerization is an extremely fast reaction and limits
the lifetime of cyclobutadiene, except at extremely low temperatures.

41
P. v. R. Schleyer, P. K. Freeman, H. Jiao, and B. Goldfuss, Angew. Chem. Int. Ed. Engl., 34, 337
(1995); C. W. Bird, Tetrahedron, 52, 9945 (1996).
42 M. K. Cyranski, T. M. Krygowski, A. R. Katritzky, and P. v. R. Schleyer, J. Org. Chem., 67, 1333
(2002).
43
F. Sondheimer, Pure Appl. Chem., 28, 331 (1971); Acc. Chem. Res., 5, 81 (1972).
44 L. Watts, J. D. Fitzpatrick, and R. Pettit, J. Am. Chem. Soc., 87, 3253 (1965).
45 L. Watts, J. D. Fitzpatrick, and R. Pettit, J. Am. Chem. Soc., 88, 623 (1966); J. C. Barborak, L. Watts,
and R. Pettit, J. Am. Chem. Soc., 88, 1328 (1966); D. W. Whitman and B. K. Carpenter, J. Am. Chem.
Soc., 102, 4272 (1980).
46
E. K. G. Schmidt, L. Brener, and R. Pettit, J. Am. Chem. Soc., 92, 3240 (1970).