1′,3′,3′-Trimethyl-6-nitrospiro[2H-1-benzopyran-2,2′-indoline]: its thermal enantiomerization and the equilibration with its merocyanine

The enantiomers of the title compound, the important photochromic material (RS)-1b, have been enriched semipreparatively by liquid chromatography. As a consequence, we were able to determine the barrier of the thermal interconversion (R)-1b(S)-1b via time-dependent polarimetry, amounting to ΔG^≠=85.9 kJ/mol at 22.0°C in d⁶-DMSO (Table 2). The thermal equilibration of the corresponding merocyanine 2b was monitored in d⁶-DMSO by time-dependent ¹H NMR, resulting in ΔG₁^≠=102.8 and ΔG₂^≠=92.0 kJ/mol at 22°C (Table 1). This means that, starting from (RS)-1b, the opened isomer 2b is attained by a slow reaction (ΔG₁^≠=102.8 kJ/mol, Fig. 4). Therefore, the merocyanine 2b cannot be identified with the intermediate required for the fast process of C(sp³)–O bond cleavage (ΔG^≠=85.9 kJ/mol) upon the above enantiomerization of (RS)-1b. Apparently, these two thermal isomerizations (Fig. 4) are independent of each other. The structure of the unknown intermediate of the interconversion (R)-1b(S)-1b must therefore differ from the known one of merocyanine 2b.

Table 1. Equilibration between spiro compounds (RS)-1 and merocyanines 2 at 22°C, measured by time-dependent UV absorptions[3] for (RS)-1a2a and by time-dependent ¹H NMR intensities for the other compounds

Full-size table (8K)

Article Outline

1. Introduction

2. Equilibration of the merocyanine 2b with the spiro compound (RS)-1b

3. Preparative separation and characterization of the enantiomers of the spiro compound (RS)-1b

4. Enantiomerization of the spiro compounds (R)- and (S)-1b

5. Discussion of the two different isomerizations investigated

6. Experimental

6.1. General methods

6.2. (±)-6-Nitro-1′,3′,3′-trimethylspiro[2H-1-benzopyran-2,2′-indoline] 1b[43]

6.3. (+)₄₃₆-6-Nitro-1′,3′,3′-trimethylspiro[2H-1-benzopyran-2,2′-indoline] 1b

6.4. (−)₄₃₆-6-Nitro-1′,3′,3′-trimethylspiro[2H-1-benzopyran-2,2′-indoline] 1b

6.5. 4-Nitro-2-[(E)-2′-(1′′,3′′,3′′-trimethyl-3H′′′-2′′-indoliumyl)-1′-ethenyl]-1-phenolate 2b[19]

Acknowledgements

References

1. Introduction

Many derivatives of 1′,3′,3′-trimethylspiro[2H-1-benzopyran-2,2′-indoline] 1a (Scheme 1) are of interest because of their photochromism.[2] The parent molecule 1a can be transformed photochemically into the merocyanine 2a which isomerizes thermally with a very high rate back to 1a.[3] Therefore, unsubstituted 1a has no practical value with respect to photochromism. This situation changes upon the introduction of a nitro group into the 6-position: the title compound 1b has probably been cited in the literature most often among all photochromic materials. The corresponding merocyanine 2b is obtained by irradiation and reverts to the equilibrium mixture (Scheme 1) consisting predominantly of the spiro compound 1b. The rate of isomerization of 2b is much lower than that of the 2a→1a reversal.[3, 4, 5, 6, 7 and 8] Although analogs have now been found which are more stable to light than 1b, the latter has been significant for the development of practical applications of photochromism and continues to be significant for basic research,[2, 9 and 10] e.g. with respect to 1b chemically bonded to another molecule. A further nitro group in the 8-position again changes the properties: only a very small amount of the spiro compound 1c appears in the thermal equilibrium[11 and 12] ( Scheme 1) in dipolar aprotic solvents, which means that the observed photochromism is a reversible one with limited applicability.