Page 466 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

The transition structure involves a three-center, two-electron bond and corresponds 447
to a symmetrically bridged structure. As indicated above, the bridged structure may
actually be an intermediate in some cases. The migration process can be concerted with SECTION 4.4
the formation of the carbocation; that is, the migration can begin before the bond to the Structure and Reactions
of Carbocation
leaving group at the adjacent carbon atom is completely broken. The phenonium ion Intermediates
case discussed in Section 4.3 is one example. The ease of migration is also influenced
by strain. In general, a shift that will reduce strain is favored.

4.4.5. Bridged (Nonclassical) Carbocations

In the discussion of carbocation rearrangements, we encountered examples of
bridged ions that require expansion of bonding concepts beyond the two-center, two-
electron bonds that suffice for most stable organic molecules. These bridged carboca-
tions, involve delocalization of electrons and formation of three-center, two-electron
bonds, and are sometimes called nonclassical ions. The recognition of the import-
ance of bridged structures largely originated with a specific structure, the norbornyl
cation, and the issue of whether its structure is classical or bridged. 133 The special
properties of this intermediate were recognized on the basis of studies by Saul Winstein
and his collaborators. The behavior of norbornyl systems in solvolytic displacement
reactions was suggestive of neighboring-group participation by a saturated carbon-
carbon bond. Evidence for both enhanced rate and unusual stereoselectivity was
developed from the study of acetolysis of exo-2-norbornyl sulfonates. The acetolyses
of both exo-2-norbornyl brosylate and endo-2-norbornyl brosylate produce exclusively
exo-2-norbornyl brosylate. The exo-brosylate is more reactive than the endo isomer by
a factor of 350. 134 Furthermore, enantiomerically pure exo-brosylate gives completely
racemic exo-acetate, and the endo-brosylate gives acetate that is at least 93% racemic.
These results suggest the involvement of an achiral species. Since the secondary
norbornyl cation is chiral, it cannot account for the racemization.

AcOH AcOH
OBs O CCH 3 O CCH 3
2
2
KOAc KOAc
OBs
exo endo
Both acetolyses were considered to proceed by way of a rate-determining
formation of a carbocation. The rate of ionization of the endo-brosylate was considered
normal, since its reactivity was comparable to that of cyclohexyl brosylate. Winstein

133 H. C. Brown, The Nonclassical Ion Problem, Plenum Press, New York, 1977; H. C. Brown, Tetrahedron,
32, 179 (1976); P. D. Bartlett, Nonclassical Ions, W. A. Benjamin, New York, 1965; S. Winstein, in
Carbonium Ions, Vol. III, G. A. Olah and P. v. R. Schleyer, eds., Wiley-Interscience, New York, 1972,
Chap. 22; G. D. Sargent, ibid., Chap. 24; C. A. Grob, Angew. Chem. Int. Ed. Engl., 21, 87 (1982);
G. M. Kramer and C. G. Scouten, Adv. Carbocation Chem., 1, 93 (1989).
134
S. Winstein and D. S. Trifan, J. Am. Chem. Soc., 71, 2953 (1949); 74, 1147, 1154 (1952); S. Winstein,
E. Clippinger, R. Howe, and E. Vogelfanger, J. Am. Chem. Soc., 87, 376 (1965).