Page 470 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

The result is a structure that is 7.5 kcal/mol above the complex. The complex 451
1
13
structure also gives better agreement with the experimental C and H NMR chemical
shifts than the corner-protonated structure. The preference for the complex structure SECTION 4.4
persists in computations using a continuum solvent model. The picture that emerges Structure and Reactions
of Carbocation
from this analysis is of a primarily electrostatic attraction between a C(1)−C(2) double Intermediates
bond and a carbocation-like C(6). 148 Such a structure would be poised for nucleophilic
attack anti to C(6), as is observed to occur.
Let us now return to the question of solvolysis and how it relates to the structure
under stable ion conditions. To relate the structural data to solvolysis conditions, the
primary issues that must be considered are the extent of solvent participation and the
nature of solvation of the cationic intermediate. The extent of solvent participation has
been probed by comparison of solvolysis characteristics in TFA with acetic acid. The
exo-endo reactivity ratio in TFA is 1120, compared to 280 in acetic acid. While the
endo isomer shows solvent sensitivity typical of normal secondary tosylates, the exo
isomer reveals a reduced sensitivity. This result indicates that the TS for solvolysis of
the exo isomer possesses a greater degree of charge dispersal, which is consistent with
formation of a bridged structure. This fact, along with the rate enhancement of the exo
isomer, indicates that the participation commences prior to ionization, and leads to
the conclusion that bridging is a characteristic of the solvolysis TS, as well as of the
stable ion structure. 149
Another line of evidence indicating that bridging is important in solvolysis comes
from substituent effects for groups placed at C(4), C(5), C(6), and C(7) in the norbornyl
system. The solvolysis rate is most strongly affected by C(6) substituents and the exo
isomer is more sensitive to these substituents than the endo isomer. This implies that
the TS for solvolysis is especially sensitive to C(6) substituents, as would be expected
if the C(1)−C(6) bond participates in solvolysis. 150
Computation of the TS using ionization of the protonated exo and endo alcohols
as a model has been done using B3LYP/6-311+G calculations. 151 The results confirm
∗
that participation occurs during the ionization process and is greater for the exo than the
endo system. However, the stabilization resulting from the participation is considerably
less than the full stabilization energy of the bridged carbocation. A difference of
3.7 kcal/mol is calculated between the exo and endo TSs. Figure 4.14 illustrates the
relative energy relationships.
Many other cations besides the norbornyl cation have bridged structures. 152
Scheme 4.5 shows some examples that have been characterized by structural studies
or by evidence derived from solvolysis reactions. To assist in interpretation of the
bridged structures, the bond representing the bridging electron pair is darkened in a
corresponding classical structure. Not surprisingly, the borderline between classical
and bridged structures is blurred. There are two fundamental factors that prevent
an absolute division: (1) The energies of the two (or more) possible structures may

148 N. H. Werstiuk, H. M. Muchall, and S. Noury, J. Phys. Chem. A, 104, 11601 (2000).
149
J. E. Nordlander, R. R. Gruetzmacher, W. J. Kelly, and S. P. Jindal, J. Am. Chem. Soc., 96, 181 (1974).
150 F. Fuso, C. A. Grob, P. Sawlewicz, and G. W. Yao, Helv. Chim. Acta, 69, 2098 (1986); P. Flury and
C. A. Grob, Helv. Chim. Acta, 66, 1971 (1983).
151 P. R. Schreiner, P. v. R. Schleyer, and H. F. Schaefer, III, J. Org. Chem., 62, 4216 (1997).
152
V. A. Barkhash, Top. Current Chem., 115–117, 1 (1984); G. A. Olah and G. K. SuryaPrakash, Chem.
Brit., 19, 916 (1983).