Page 757 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

740 cyclopentadienyl cation is also calculated to be antiaromatic by magnetic suscep-
tibility and chemical shift criteria. 127 Its pK has been estimated as –40, using
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CHAPTER 8 128
an electrochemical cycle. The heterolytic bond dissociation energy to form the
Aromaticity cation from cyclopentadiene is 258 kcal/mol, which is substantially more than for
formation of an allylic cation from cyclopentene but only slightly more than the
252 kcal/mol required for formation of an unstabilized secondary carbocation. 64 The
high energy of the cyclopentadienyl cation is also indicated by ionization studies
in solution. A rate retardation of 10 −14 relative to cyclopentyl analogs has been
estimated from solvolytic rate data. 129 Solvolysis of cyclopentadienyl halides assisted
by silver ion is extremely slow, even though the halide is doubly allylic. 130 When
cyclopentadienyl bromide and antimony pentafluoride react at −78 C, an EPR

spectrum is observed, which indicates that the cyclopentadienyl cation is a triplet. 131
Similar studies indicate that the pentaisopropyl 132 and pentachlorocyclopentadienyl
cation are also triplets, but the ground state of the pentaphenyl derivative is a
singlet.
The relative stability of the anions derived from cyclopropene and cyclopentadiene
by deprotonation is just the reverse of the situation for the cations. Cyclopentadiene
is one of the most acidic hydrocarbons known, with a pK of 16.0. 133 The pK’s
a
of triphenylcyclopropene and trimethylcyclopropene have been estimated as 50 and
62, respectively, using electrochemical cycles 134 (see Section 6.1). The unsubstituted
compound would be expected to fall somewhere between and thus must be about 40
powers of 10 less acidic than cyclopentadiene. MP2/6-311+G(2df,2pd) and B3LYP/6-
311+G(2df,2pd) calculations indicate a small destabilization of the cyclopropenyl
anion, relative to the cyclopropyl anion. 135 Thus the six -electron cyclopentadienide
anion is enormously stabilized relative to the four -electron cyclopropenide ion, in
agreement with the Hückel rule.
The Hückel rule predicts aromaticity for the six -electron cation derived
from cycloheptatriene by hydride abstraction and antiaromaticity for the planar eight
-electron anion that would be formed by deprotonation. The cation is indeed very
stable, with a pK + of +4.7. 136 Salts containing the cation can be isolated as a result
R
of a variety of preparative procedures. 137 On the other hand, the pK of cyclohep-
tatriene has been estimated at 36. 134 This value is similar to normal 1,4-dienes and
does not indicate strong destabilization. The seven-membered eight -electron anion
is probably nonplanar. This would be similar to the situation in the nonplanar eight
-electron hydrocarbon, cyclooctatetraene.

127
H. Jiao, P. v. R. Schleyer, Y. Mo, M. A. McAllister, and T. T. Tidwell, J. Am. Chem. Soc., 119, 7075
(1997).
128 R. Breslow and S. Mazur, J. Am. Chem. Soc., 95, 584 (1975).
129
A. D. Allen, M. Sumonja, and T. T. Tidwell, J. Am. Chem. Soc., 119, 2371 (1997).
130 R. Breslow and J. M. Hoffman, Jr., J. Am. Chem. Soc., 94, 2110 (1972).
131 M. Saunders, R. Berger, A. Jaffe, J. M. McBride, J. O’Neill, R. Breslow, J. M. Hoffman, Jr.,
C. Perchonock, E. Wasserman, R. S. Hutton, and V. J. Kuck, J. Am. Chem. Soc., 95, 3017 (1973).
132
H. Sitzmann, H. Bock, R. Boese, T. Dezember, Z. Havlas, W. Kaim, M. Moscherosch, and L. Zanathy,
J. Am. Chem. Soc., 115, 12003 (1993).
133 A. Streitwieser, Jr., and L. L. Nebenzahl, J. Am. Chem. Soc., 98, 2188 (1976).
134
R. Breslow and W. Chu, J. Am. Chem. Soc., 95, 411 (1973).
135 G. N. Merrill and S. R. Kass, J. Am. Chem. Soc., 119, 12322 (1997).
136 W. v. E. Doering and L. H. Knox, J. Am. Chem. Soc., 76, 3203 (1954).
137
T. Nozoe, Prog. Org. Chem., 5, 132 (1961); K. M. Harmon, in Carbonium Ions, Vol. IV, G. A. Olah
and P. v. R. Schleyer, eds., Wiley-Interscience, New York, 1973, Chap. 2.