Page 469 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

450 that 3,2- and 6,2-hydride shifts were occurring under stable ion conditions. Activation
energies of 10.8 and 5.9 kcal/mol, respectively, were measured for these processes.
CHAPTER 4
Nucleophilic Substitution
2,6-shift 2,3-shift
+
+ E = 5.9 H + H E = 10.8
a
a
H 6 3 H
These results, which pertain to stable ion conditions, provide a strong case that
the most stable structure for the norbornyl cation is the symmetrically bridged ion.
How much stabilization does the bridging provide? An estimate based on molecular
mechanics calculations and a thermodynamic cycle suggests a stabilization of about
6 ± 1kcal/mol. 140 A gas phase value based on mass spectrometric measurements is
11 kcal/mol. 141 Gas phase hydride affinity and chloride affinity data also show the
∗
norbornyl cation to be especially stable. 142 MO calculations (MP2/6-31G ) give a
bridged structure that is 13.6 kcal more stable than the classical secondary structure and
13
predicts C chemical shifts and coupling in agreement with the experimental results. 143
The C(1)−C(6)=C(2)−C(6) distance is found to be 1.832 Å by an MBPT(2)/DZP
computation. 144 The difference in energy between the two structures is reduced only
slightly when calculations include the effect of solvation, indicating that the bridged
ion would be more stable than the classical ion, even in solution. 145 There is also good
agreement between calculated and observed infrared spectra. 146
Werstiuk and Muchall computed the structure at the B3LYP and QCISD levels.
The minimum energy structure was found to be a bridged ion with a C(1)−C(6)
distance of 1.892 Å and a C(1)−C(2) distance of 1.389 Å (B3LYP/6-311+G(2d,2p)).
They applied AIM concepts to a description of the structure. 147 This analysis resulted
in the description of the norbornyl cation as a complex, consistent with the relatively
long C(1)−C(6) and C(2)−C(6) and short C(1)−C(2) distances indicated above. The
bond critical points found by the AIM analysis show a T-configuration with the bond
from C(6) intersecting with the C(1)−C(2) critical point. There is no bond path directly
to C(1) or C(2). Carbon 6 is then best described as tetravalent, with the C(1)−C(2)
double bond as the fourth ligand. These computations also examined the effect of
bringing C(6) closer to C(1) and C(2) to form the more strongly bridged structure that
would be implied by a corner-protonated cyclopropane representation.

H H H H
+
+
π complex corner-protonated
cyclopropane
140 P. v. R. Schleyer and J. Chandrasekhar, J. Org. Chem., 46, 225 (1981).
141 M. C. Blanchette, J. L. Holmes, and F. P. Lossing, J. Am. Chem. Soc., 109, 1392 (1987).
142
R. B. Sharma, D. K. S. Sharma, K. Hiraoka, and P. Kebarle, J. Am. Chem. Soc., 107, 3747 (1985).
143 P. v. R. Schleyer and S. Sieber, Angew. Chem. Int. Ed. Engl., 32, 1606 (1993).
144
S. A. Perera and R. J. Bartlett, J. Am. Chem. Soc., 118, 7849 (1996).
145
P. R. Schreiner, D. L. Severance, W. L. Jorgensen, P. v. R. Schleyer, and H. F. Schaefer, III, J. Am.
Chem. Soc., 117, 2663 (1995).
146 W. Koch, B. Liu, D. J.DeFrees, D. E. Sunko, and H.Vancik, Angew. Chem. Int. Ed. Engl., 29, 183
(1990).
147
N. H. Werstiuk and H. M. Muchall, J. Phys. Chem. A, 104, 2054 (2000); N. H. Werstiuk and
H. M. Muchall, Theochem, 463, 225 (1999).