1′,3′,3′-trimethyl-6, 8-dinitrospiro [2H-1-benzopyran-2,2′-indoline]:: Fast thermal enantiomerization and slow thermal equilibration with a ring-opened isomer

The transformation of the title compound (RS)-2 into the merocyanine 1 was studied by time-dependent H-1-NMR in DMF-D-7 at 22 degrees C. An equilibrium containing 1.9% (RS)-2 is established; the free enthalpy of activation for the (XS)-2 --> 1 process amounts to 101.3 kJ mol(-1). The interconversion of the enantiomers of the title compound was studied by temperature-dependent H-1-NMR in DMF-D-7. The barrier to ring opening (60.1 kJ mol(-1), 23 degrees C) was determined for the first time. Apparently (RS)-2 represents the 2H-1-benzopyran with the lowest Delta G not equal-value for this process which occurs as often as 155 times per second at 23 degrees C. The characterization and all H-1-NMR measurements of (RS)-2 were facilitated by its isolation as a solid. The two reactions, namely interconversion of enantiomers and transformation into a merocyanine, are found to be kinetically independent of each other. Their mechanisms are discussed on the basis of the scheme of free enthalpy changes.