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 k(rac) for their thermal racemization were measured by circular dichroism spectropolarimetry at 60 degrees 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 k(rac) values ranging from 9.3 x 10(-5) to >5.0 x 10(-3) s(-1), and least rapidly in cyclohexane, with k(rac) values ranging from 6.8 x 10(-6) to 4.6 x 10(-4) s(-1). V-shaped plots of log k(rac) vs Hammett sigma(p) 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(sp(3))-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 k(rac) vs sigma(p) and sigma(-) 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.