Page 468 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

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+ SECTION 4.4
nonclassical
bridged ion Structure and Reactions
of Carbocation
Intermediates
A
classical classical
ion ion
B
+
+

C

Fig. 4.13. Contrasting energy profiles for stable and
unstable bridged norbornyl cation. (A) Bridged ion
is a transition structure for rearrangement between
classical structures. (B) Bridged ion is an intermediate
in rearrangement of one classical structure to the other.
(C) Bridged ion is the most stable structure.

The two alternative descriptions of the norbornyl cation were tested very exten-
sively. In essence, the question that is raised has to do with the relative energy of
the bridged structure. Is it lower in energy than the classical ion and therefore an
intermediate to which the classical ion would collapse or is it a transition structure
(or higher-energy intermediate) in the rapid isomerization of two classical structures?
Figure 4.13 illustrates the energy profiles corresponding to the various possibilities.
When the techniques for direct observation of carbocations became available,
the norbornyl cation was subjected to study by those methods. The norbornyl cation
was generated in SbF -SO -SOF and the temperature dependence of the proton
5
2
2
13
magnetic resonance spectrum was examined. 136 Subsequently, the C NMR spectrum
was studied and the proton spectrum was determined at higher field strength. These
studies excluded rapidly equilibrating classical ions as a description of the norbornyl
cation under stable ion conditions. 137 The resonances observed in the 13 C spectrum
were assigned. None of the signals appear near the position where the C(2) carbon
of the classical secondary 2-propyl cation is found. Instead, the resonances for the
norbornyl cation appear at relatively high field and are consistent with the bridged-ion
structure. 138 Other NMR techniques were also applied to the problem and confirmed
the conclusion that the spectra observed under stable ion conditions could not be the
result of averaged spectra of two rapidly equilibrating ions. 139 It was also determined
136 P. v. R. Schleyer, W. E. Watts, R. C. Fort, Jr., M. B. Comisarow, and G. A. Olah, J. Am. Chem. Soc.,
86, 5679 (1964); M. Saunders, P. v. R. Schleyer, and G. A. Olah, J. Am. Chem. Soc., 86, 5680 (1964).
137
G. A. Olah, G. K. SuryaPrakash, M. Arvanaghi, and F. A. L. Anet, J. Am. Chem. Soc., 104, 7105
(1982).
138 G. A. Olah, G. Liang, G. D. Mateescu, and J. L. Riemenschneider, J. Am. Chem. Soc., 95, 8698 (1973);
G. A. Olah, Acc. Chem. Res., 9, 41 (1976).
139
C. S. Yannoni, V. Macho, and P. C. Myhre,J. Am. Chem. Soc., 104, 7105 (1982); M. Saunders and
M. R. Kates, J. Am. Chem. Soc., 102, 6867 (1980); M. Saunders and M. R. Kates, J. Am. Chem. Soc.,
105, 3571 (1983).