Page 283 - Mechanism and Theory in Organic Chemistry

leads to the rearranged product phenyl 1-phenylethyl ket~ne.~ Apparently the
mechanism is that shown in Equation 6.5. A 1,2-shift ___-__---- converts one vigl - cation-to - -.-
another. hut the rearranged one is stabilized by conjugation with a benzene ring.
-
OTf 4
%
5 4-c=c\CH,
20% H,O (6.5)
CH,

Under the long-lived conditions of carbocations in superacid (Section 5.3,
p. 235), 1,2-shifts interconverting ions of like stability also occur and are very
rapid. For example, at - 180°C the five methyl groups of 2,3,3-trimethylbutyl
cation have only one peak in the nmr. This observation implies that the methyl
shift in Equation 6.6 occurs at the rate of 75 x lo3 sec-I with an activation barrier
of < 5 kcal mole
CH, H,C
+* 1 CH, - I +
CH3-C-C-CH3 CH3-C-C-CH, (6.6)
I I I I
H,C CH, H,C CH,
Occasionally rearrangements from more stable to less stable carbocations
occur, but only if (1) the energy difference between them is not too large or (2)
the carbocation that rearranges has no other possible rapid reactions open to it.9
For example, in superacid medium, in the temperature range 0-40°C, the proton
nmr spectrum of isopropyl cation indicates that the two types of protons are
exchanging rapidly. The activation energy for the process was found to be
16 kcal mole-l. In addition to other processes, the equilibrium shown in Equation
6.7 apparently occurs.1° In the superacid medium, no Lewis base is available
H

(6.7)
H H H H H H H
either to add to the carbocation or to accept a proton from it in an elimination
reaction, and in the absence of such competing reactions there is ample time for
the endothermic 1,2-hydride shift to take place.
1,2-Shifts have stereochemical as well as energetic requirements. &order-
for such rearrangements to occur. the C-Z (Z_=_-group) - bond at the
--
migation ongin =st Ii<Lgrr~the plane described by the vacant p
--
-
orbital on the adjacent carbon-and the Cp.--C4 bond as in Figure 6.1-that is, the
-./ - - cxlidmust be 0". For example,
dihedral angle between Z and the empty$-
-
M. A. Imhoff, R. H. Summerville, P. v. R. Schleyer, A. G. Martinez, M. Hanack, T. E. Dueber,
and P. J. Stang, J. Amer. Chem. Soc., 92, 3802 (1970).
G. A. Olah and J. Lukas, J. Amer. Chem. Soc., 89, 4739 (1967).
For references to a number of such rearrangements, see (a) note 3(b) and (b) M. Saunders, P.
Vogel, E. L. Hagen, and J. Rosenfeld, Accts. Chem. Res., 6, 53 (1973).
lo See note 9(b).