Page 454 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

Up to this point, we have considered only carbocations in which the cationic 435
2
carbons are sp hybridized and planar. When this hybridization cannot be achieved,
carbocations are of higher energy. In a classic experiment, Bartlett and Knox demon- SECTION 4.4
strated that the tertiary chloride 1-chloroapocamphane was inert to nucleophilic substi- Structure and Reactions
of Carbocation
tution. 92 Starting material was recovered unchanged even after refluxing for 48 h in Intermediates
ethanolic silver nitrate. The unreactivity of this compound is attributed to the structure
2
of the bicyclic system, which prevents rehybridization to a planar sp carbon. Back-
side nucleophilic solvent participation is also precluded because of the bridgehead
location of the C−Cl bond.
CH 3 CH 3

Cl
The apocamphyl structure is particularly rigid, and bridgehead carbocations
become accessible in more flexible structures. The relative solvolysis rates of
the bridgehead bromides 1-bromoadamantane, 1-bromobicyclo[2.2.2]octane, and
1-bromobicyclo[2.2.1]heptane illustrate this trend. The relative rates for solvolysis in
80% ethanol at 25 C are shown. 93

Br Br Br
1 10 –3 10 –10
The relative reactivity of tertiary bridgehead systems toward solvolysis is well correlated
withtheincreaseinstrainthatresultsfromconversionoftheringstructuretoacarbocation,
as calculated by molecular mechanics. 94 This result implies that the increased energy
associated with a nonplanar carbocation is proportional to the strain energy present in the
ground state reactant. The solvolysis rates also correlate with bridgehead cation stability
measured by gas phase hydride affinity and MP2/6-311G MO calculations. 95
∗∗
Alkenyl carbocations in which the cationic carbon is sp hybridized are about
2
15 kcal higher in energy than similar cations in which the cationic center is sp (see
Figure 3.18). 96 This is because of the higher electronegativity of the orbital with
increasing s-character. The intermediacy of substituted vinyl cations in solvolysis
reactions has been demonstrated, but direct observation has not been possible for
97
simple vinyl cations. Most examples of solvolytic generation of vinyl cations involve
very reactive leaving groups, especially trifluoromethanesulfonate (triflates). Typical
products include allenes, alkynes, and vinyl esters. 98

92 P. D. Bartlett and L. H. Knox, J. Am. Chem. Soc., 61, 3184 (1939).
93
For a review of bridgehead carbocations see R. C. Fort, Jr., in Carbonium Ions, Vol. IV, G. A. Olah
and P. v. R. Schleyer, eds., Wiley-Interscience, New York, 1973, Chap. 32.
94 T. W. Bentley and K. Roberts, J. Org. Chem., 50, 5852 (1985); R. C. Bingham and P. v. R. Schleyer, J.
Am. Chem. Soc., 93, 3189 (1971); P. Müller and J. Mareda, Helv. Chim. Acta, 70, 1017 (1987); P. Müller,
J. Mareda, and D. Milin, J. Phys. Org. Chem., 8, 507 (1995).
95
E. W. Della and W. K. Janowski, J. Org. Chem., 60, 7756 (1995); J. L. M. Abboud, O. Castano, E. W.
Della, M. Herreros, P. Muller, R. Notario, and J.-C. Rossier, J. Am. Chem. Soc., 119, 2262 (1997).
96
V. D. Nefedov, E. N.Sinotova, and V. P. Lebedev, Russ. Chem. Rev., 61, 283 (1992).
97 H.-U. Siehl and M. Hanack, J. Am. Chem. Soc., 102, 2686 (1980).
98
For reviews of vinyl cations, see Z. Rappoport in Reactive Intermediates, R. A. Abramovitch, ed., Vol.
3, Plenum Press, New York, 1983; Dicoordinated Carbocations, Z. Rappoport and P. J. Stang, eds.,
John Wiley & Sons, New York, 1997.