Page 327 - Mechanism and Theory in Organic Chemistry

H H H
H2 14,H
.-.
H3&\:c-H . . - H~C l)l--E< H
A
.a
+
C
411
H3H HHH
Hydride shifts of higher order than 1,3 occur if (1) the migration origin and
the migration terminus are close together and (2) the geometry of the system
allows overlap of the orbitals of hydrogen and of the migration terminus. Be-
cause of the proximity requirement such shifts are rare in acyclic systemslZ5 in
nucleophilic solvents,lZ6 but in reactions of medium ring (7-12 carbon atoms)

I
0
HCOOH HCOOH*
A

la5 For a summary of these, see J. L. Fry and G. J. Karabatsos, in Olah and Schleyer, Eds., Carbonium
Ions, Vol. 11, pp. 555-557.
lZe Saunders and Stofko have observed 1,3- 1,4-, and 1,5-intramolecular shifts from tertiary to
tertiary center in superacid and have calculated the activation barriers to be 8.5, 12-13, and 6-7
kcal mole-l, respectively. (M. Saunders and J. J. Stofko, Jr., J. Amer. Chem. Soc., 95, 252 (1973)).