Page 297 - Mechanism and Theory in Organic Chemistry

H
YzH5 I /P
CH~*CH~OH ,,,,+ HNO, > CH,-C-C
1
NH2 C2H5 'H
30% inversion
14
(Comparison of Equations 6.23 and 6.13 shows how the stereochemistry at C,
may depend on the lifetime of the carbocation.)
The stereochemistry at the migration origin cannot always be studied
because, as in the pinacol and semipinacol rearrangements and as in Wagner-
Meerwein shifts where migration is followed by loss of a proton, the migration
origin often becomes trigonal in the product. When it is tetrahedral in the product
its stereochemistry varies and is not yet fully understood. If the nucleophile
attacks before the migrating group has fully departed, then it must come in from
the back side and give inversion at the migration origin. We saw a dramatic
example of this in Cram's work on the solvolysis of 3-phenyl-2-butyl tosylate,
in which solvent attacks the phenonium ion directly and the migration origin,
C,, is almost entirely inverted. If the migrating group has completely departed
from the migration origin before the nucleophile attacks and there is time for
rotation about the C,-C, bond, then racemization should result. (If there is not
time for free rotation, attack from one side might be less hindered than attack
from the other.) Racemization at C, occurs, for example, in the deamination of
(S)-1-amino-2-cyclohexylbutane (30), which affords, among other products,
racemic 2-cyclohexylbutan-2-01 (31).43

S R,S
30 3 1
A number of cases have been found in which 1,2-hydride shifts occur with
retention at C,. For example, (S)-6-(aminomethy1)-2-methyloctane (32) is
deaminated in aqueous perchloric acid to give 2,6-dimethyloctan-6-01 in 35 per-
cent yield. This product is formed with 41 percent retention at C,. No fully
satisfactory explanation for the retention has been proposed.44

Migratory Aptitudes
The relative ease with which alkyl and aryl groups migrate is called their
migratory aptitude. Unfortunately, migratory aptitudes are not absolute quantities;
values determined in one reaction under one set of conditions may differ enor-

43 W. Kirmse and W. Gruber, Chem. Ber., 104, 1789 (1971).
44 W. Kirmse and H. Arold, Chem. Ber., 104, 1800 (1971).