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 E_a = 16.3 ± 1.4 kcal/mol and log A = 10.4 ± 1.4 (A in s^-1) (GC method) and E_a = 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.