Medium Effects on the Rates of Stereomutation of a Pair of Diastereomeric Cyclopropanones. Ground State Stabilization in Nucleophilic Solvents Induces Deviation from Solvent Polarity Controlled Behavior

The synthesis of the two stereoisomers of
spiro(bicyclo[2.2.1]heptane-2,1‘-cyclopropan)-2‘-one,
3a and
4a, from diazomethane and the ketene
2-carbonylylbicyclo[2.2.1]heptane in ether at 195 K yields
a ∼1.6 to 1 ratio.
At 245 K, the ratio changes in a first-order manner, with an
observed rate constant of 1.7 × 10-4
s-1, to an equilibrium
ratio of 0.8 to 1. The temperature dependence of the
interconversion of 3a and 4a (GC method) and that
of their
dideuterio derivatives 3b and 4b (NMR method)
have been determined and yield activation parameters
Ea = 16.3
± 1.4 kcal/mol and log A = 10.4 ± 1.4 (A in
s-1) (GC method) and
Ea = 15.3 ± 1.4 and log A = 9.6 ± 1.4
(NMR
method). The free energies of activation at 239 K have been
determined in four solvents:  dichloromethane (16.1
kcal/mol), acetone (17.7), hexane (17.9), and ether (19.1). The
solvent dependence does not correlate well with
commonly used measures of solvent polarity, and the reaction is
unexpectedly slow in acetone and ether. This
deceleration is explained in terms of nucleophilic association of these
solvents with the carbonyl groups in the
cyclopropanones, leading to a ground state stabilization.