Thermal Racemization of Substituted Indolinobenzospiropyrans:  Evidence of Competing Polar and Nonpolar Mechanisms

A series of 6-substituted indolinobenzospiropyrans were resolved by chiral stationary phase HPLC and rate constants krac for their thermal racemization were measured by circular dichroism spectropolarimetry at 60 °C in three different solvents: cyclohexane, 90:10 hexanes/2-propanol, and acetonitrile. Results show that the spiropyrans undergo thermal racemization most rapidly in acetonitrile, with krac values ranging from 9.3 × 10-5 to >5.0 × 10-3 s-1, and least rapidly in cyclohexane, with krac values ranging from 6.8 × 10-6 to 4.6 × 10-4 s-1. V-shaped plots of log krac vs Hammett σp constants in 90:10 hexanes/2-propanol and acetonitrile suggest that thermal racemization of the 6-substituted spiropyrans proceed via two competing mechanisms: a polar mechanism involving heterolytic C(sp3)−O bond cleavage with anchimeric assistance from the indoline nitrogen and a nonpolar electrocyclic ring opening mechanism with no anchimeric assistance from the indoline nitrogen. The outcome of this competition appears to be strongly influenced by solvent polarity: plots of log krac vs σp and σ- in cyclohexane show a near-linear correlation with negative slope, which is consistent with the nonpolar mechanism. However, an increase in solvent polarity results in a shift toward a linear correlation with positive slope, which is consistent with the polar mechanism.