Page 293 - Mechanism and Theory in Organic Chemistry

constant for solvolysis of 16 (X = H) is only 18.016.08 times faster than the
solvolytic rate constant-a rate enhancement of about three. This corresponds to
59 percent anchimeric assistance.
Primary /3-aryl tosylates have also been shown to undergo solvolysis by two
distinct pathways-aryl- and solvent-assisted.32 Tertiary P-aryl tosylates, how-
ever, ionize to a stable carbocation and seem to require no assistance in isomeriza-
ti~n.~~
The unsubstituted phenonium ion, as well as other phenonium ions substi-
tuted with electron-donating groups, have been recently observed as stable ions
in superacid medium.34 That the structure is actually 18 and not an unsym-
metrically bridged ion (19) nor a nonclassical ion (20) (see Section 6.2) in which
there are three-center bonds was shown by the nmr evidence. The ring carbon
that is bonded to the aliphatic carbons was established by 13C shifts to be tetra-
hedral in nature; and 13C and proton chemical shifts in the ring were similar to
those of cations shown to have Structure 21.

The central carbon of the migrating group and the carbons of the migration
terminus and of the migration origin all undergo bonding changes during a
Wagner-Meerwein shift, and the stereochemistry at each may change.
The orbital picture we have previously formulated (Figure 6.2) predicts
that the stereochemistry of the migrating group will be retained during the migra-
tion since, in this picture, the migrating group uses the same lobe of the same
orbital to bond to both migration origin and migration terminus. Predominant
retention is in fact observed, but some racemization may occur. For example,
in the semipinacol rearrangement36 of (3s) - 1 -amino-2,3-dimethyl-2-pentanol
(22), the product 23 that arises from migration of the S-butyl group accounts for
33 percent of the product. The chirality of the S-butyl group in 23 is only 86-88
percent retained.37 Kirmse and co-workers have proposed that the pathway for

3a J. M. Harris, F. L. Schadt, P. v. R. Schleyer, and C. J. Lancelot, J. Amer. Chem. Soc., 91, 7508
(1969).
33 H. C. Brown and C. J. Kim, J. Amer. Chem. Soc., 90, 2082 (1968).
34 (a) G. A. Olah and R. D. Porter, J. Amer. Chem. Soc., 93, 6877 (1971) and references therein;
(b) G. A. Olah, Angew. Chem. Znt. Ed., 12, 173 (1973).
35 D. J. Cram, in Steric Effects in Organic Chemistry, M. S. Newman, Ed., Wiley, New York, 1956.
36 The semipinacol rearrangement is the rearrangement that ensues when a 8-amino alcohol is
deaminated as in the following equation. See Problem 6.4.
R' R R' R R '
1 I HNO I I + I
R'-C-C-NH, R'-C-C-N=N d R'-C-C-R + N, + H+
I I I I II I
HO R HO R 0 R
37 W. Kirmse and W. Gruber, Chem. Ber., 106, 1365 (1973); W. Kirmse, W. Gruber, and J. Knist,
Chem. Ber., 106, 1376 (1973).