Page 299 - Mechanism and Theory in Organic Chemistry

The migratory aptitudes obtained in this way were p-anisyl, 880; p-tolyl, 40;
phenyl, 1; p-chlorophenyl, 0.47 Depovere points out that the larger values for
the migratory aptitudes ofp-anisyl and p-tolyl here as compared to Bachmann's
data are due to the twofold role of the electron-donating groups in 37: They
migrate better from the /? carbon and they facilitate ionization at the a carbon.
We have already mentioned that migratory aptitudes are dependent on the
reaction and on the conditions under which the reaction is carried out. An
example of the latter type of variation is that in the pinacol rearrangement of
triphenylethylene glycol, the phenyl/hydrogen migration ratio may vary by a
factor of 180 (from 7.33 to 0.41) when the catalyst is changed from concentrated
sulfuric acid to HC1 in water/di~xane.~~
A striking example of the former type of variation can be gained from a
comparison of the migratory aptitudes in the pinacol rearrangement (see above)
with those in a deamination reaction. For example, the semipinacolrearrangement
of 38 gives the following migratory aptitudes : p-anisyl, 1.5 ; p-tolyl, 1.3 ; phenyl, 1 ;
p-chlorophenyl, 0.9.49

Generally, deamination reactions show a lower selectivity than pinacol and
Wagner-Meerwein rearrangements. This has been explained in two ways. First,
the "hot" carbocation theory says that carbocations formed from deamination
are of extremely high energy and therefore lacking in dis~rimination.~~ Huisgen,
on the other hand, has proposed that in deamination, because the energy of
ionization is very low, the absolute differences in the activation energies for the
possible subsequent reactions are small. This follows if one assumes a constant
ratio between the activation energies of the various steps on the reaction path.51
(See also Section 5.2, p. 226.)
A novel method of measuring migratory aptitudes has been published by
Shubin and co-worker~.~~ They studied the temperature at which the two methyls
of 39 became equivalent in superacid solution in the nmr and found the following
results for various substituents X: H, 70°C; CH,, - 100°C; C1, - 55°C; F, - 70°C;
CF,, 0°C.

47 P. Depovere and R. Devis, Bull. Soc. Chim. France, 479 (1969).
48 C. J. Collins, J. Amer. Chem. Soc., 77, 5517 (1955).
D. Y. Curtin and M. C. Crew, J. Amer. Chem. Soc., 76, 3719 (1954).
D. Semenow, C.-H. Shih, and W. G. Young, J. Amer. Chem. Soc., 80, 5472 (1958).
R. Huisgen and C. Riichardt, Justus Liebigs Ann. Chem., 601, I (1956).
62 V. G. Shubin, D. V. Korchagina, G. I. Borodkin, B. G. Derendjaev, and V. A. Koptyug, J.
Chem. Soc., D, 696 (1970).