Page 313 - Mechanism and Theory in Organic Chemistry

consistent with it being a set of equilibrating bicyclobutonium ions.g0 The
spectrum of the cation derived from 1-methylcyclobutyl chloride is best explained
if the cyclobutyl cation is in rapid equilibrium with the corresponding cyclo-
propylcarbinyl cation and the equilibrium favors the former, as shown in
Equation 6.35.91

(6.35)

The nmr spectrum of the 4,4-dimethylcyclopropylcarbinyl cation in super-
acid is only in agreement with the bisected structure of the ion (78). In structure
78 the two methyl groups are not equivalent--one is cis to the cyclopropyl ring

78
and the other is trans. Indeed, the nmr spectrum shows two different peaks due
to methyl groups separated by 0.54 ppm, which do not coalesce up to - 30°C,
at which temperature ring opening occurs.92
Molecular orbital calculations suggest that for the parent ion and for
methylated ions, 78 and 79, the bisected cyclopropylcarbinyl structure is the
structure of lowest energy.93 They also predict that the cyclopropylcarbinyl-
H
'V-CH~

79
cyclopropylcarbinyl conversions, which we know must occur from label scram-
bling experiments (Equation 6.32) and other rearrangements, have as a transition
state a puckered cyclobutyl cation.94 Figure 6.12 shows the proposed reaction
coordinate diagram for the parent system and the gem-dimethyl system. The most
stable cation in the latter system, in agreement with Olah's work, is that which is
stabilized by having the gem-dimethyl group on the same carbon as the positive
charge.

The Norbornyl Cationg5
In discussing the cyclopropylcarbinyl cation before the norbornyl cation we have,
chronologically, put the cart before the horse. The first experimentally docu-
mented example of anchimeric assistance by a CLC a bond was announced by
Winstein and Trifan in 1949.96 These workers studied the solvolysis of exo- and

See note 89, but see also W. J. Hehre and P. C. Hiberty, J. Amer. Chem. Soc., 96, 302 (1974).
91 M. Saunders and J. Rosenfeld, J. Amer. Chem. Soc., 92, 2548 (1970).
92 (a) See note 34, p. 280; (b) C. U. Pittman, Jr., and G. A. Olah, J. Amer. Chem. Soc., 87, 2998
(1965).
O3 (a) See note 58(e), p. 288; (b) W. J. Hehre and P. C. Hiberty, J. Amer. Chem. Soc., 96, 302 (1974).
O4 See note 93(b).
O5 See note 59(c), p. 288.
Be S. Winstein and D. S. Trifan, J. Amer. Chem. Soc., 71, 2953 (1949).