Page 319 - Mechanism and Theory in Organic Chemistry

The exo compound actually solvolyzes 5.7 times faster than the endo compound.
Since there is no obvious route for a bond participation here, it appears that there
must be an increase in nonbonded strain in the transition state of 87 of RT ln 5.7
or - 1 kcal m~le-~.~O~

As Sargent has pointed out, experiments and examination of molecular
models both indicate that this system should offer more extreme steric hindrance
to endo ionization than the norbornyl system does. One kcal mole-l, then, is
an upper limit for the increase in nonbonded interaction experienced by the
leaving group in going from the ground to the transition state in endo-2-nor-
bornyl tosylate.lo7 But 1 kcal mole-I cannot be responsible for the exolendo
ionization rate ratio of 1550 reported by Winstein and Trifan, a ratio since
corroborated by hundreds of other studies.
More recently Nordlander and co-workers adopted a different approach to
determine whether endo ionization of 2-norbornyl derivatives is hindered.lo8
They solvolyzed exo- and endo-2-noxbornyl tosylate in the strongly ionizing
but very weakly nucleophilic solvent, trifluoroacetic acid, and found the ex01
endo rate ratio to be 1120. They then compared the rates of solvolysis of endo-2-
norbornyl tosylate and of 2-adamantyl tosylate in two solvents-trifluoroacetic
acid and much more nucleophilic acetic acid. Using an analysis suggested by
Schleyer,log they reasoned that if ionization of endo-2-norbornyl tosylate is
sterically hindered, its rate should show a large dependence on the nucleophili-
city of the solvent. But 2-adamantyl tosylate cannot solvolyze with solvent assis-
tance (see Section 5.4, p. 243). Thus the ratio of the rate of solvolysis of endo-2-
norbornyl tosylate to the rate of solvolysis of 2-adamantyl tosylate should be much
larger in acetic acid than in trifluoroacetic acid. In fact, the ratio is 30 times
greater in acetic acid than in trifluoroacetic acid. If the rates of solvolyses of
trans-2-methylcyclopentyl tosylate (88) and 2-adamantyl tosylate are compared
in the same two solvents, the ratio of rates is 31 times greater in acetic acid.
Thus endo-2-norbornyl tosylate seems to be acting normally-that is, like other
secondary tosylates with a branch in the /3 position.

loe H. C. Brown, I. Rothberg, P. v. R. Schleyer, M. M. Donaldson, and J. J. Harper, Proc. Nut.
Acad. Sci. U.S., 56, 1653 (1967).
lo' See note 59(c), p. 288.
lo8 J. E. Nordlander, R. R. Gruetzmacher, W. J. Kelly, and S. P. Jindal, J. Amn. Chem. Soc., 96, 181
(1974).
\ ,
log P. V. R. Schleyer, J. L. Fry, L. K. M. Lam, and C. J. Lancelot, J. Amn. Chem. Soc., 92, 2542
(1970).