Page 311 - Mechanism and Theory in Organic Chemistry

starting material.85 Indeed, Figure 6.10 presents the results of CNDO calcula-
tions on the barrier to rotation of the carbinyl It appears that a 30"
rotation from the symmetrical structure (a = 0°, see 74) leads to only a small
decrease in stabilization.
H*H

Is the cyclopropylcarbinyl system also the first-formed ion in solvolysis of
cyclobutyl derivatives? The evidence is conflicting. Majerski, Borcid, and
Sunko studied the reactions shown in Equations 6.33 and 6.34 and found that
when the starting material is the cyclopropylcarbinyl methanesulfonate, the label
scrambling is less complete in the cyclopropylcarbinol than in the cyclobutanol;
similarly, cyclobutyl methanesulfonate gives less label scrambling in the cyclo-
butanol than in the cyclopropylcarbinol.87 (In Equation 6.33, the numbers
show the distribution of the CD, group. In Equation 6.34, they show the distri-
bution of the CH, group.)

yield too low
for analysis

74.. {voH (6.34)
a
OH
24%
+
H

O
-
-
&
.
52% 24% yield too low
for analysis
This would make it appear that cyclopropyl and cyclobutyl derivatives each
solvolyze to give ions that are similar in structure to the starting material. Solvent
capture may occur at this stage. If it does not, the first-formed ion rearranges.
On the other hand, there is now a good deal of evidence that the solvolysis
of most cyclobutyl derivatives does lead directly to the cyclopropylcarbinyl
cation. For example, orbital symmetry considerations (Section 1 1.3) indicate that
the conversion of cyclobutyl cations into cyclopropylcarbinyl cations should
occur by disrotatory ring opening as shown in Figure 6.1 1 ; but any steric factors
that would hinder such a process decelerate most cyclobutyl solvolyses. Thus
See note 84(b).
Be K. B. Wiberg and J. G. Pfeiffer, J. Amr. Chcm. Soc., 92, 553 (1970).
a7 See note 84(a), p. 297.