Perimidine, prototropic tautomerism

Valence and prototropic tautomeric reactions are among the most important mechanisms that govern transformations of a broad variety of photochromic organic systems.1,2 Until recently, no examples of photochromic compounds have been reported whose photochromic behavior was due to a combination of these two tautomeric reactions. Such a combination, which is characteristic of ring-chain tautomerism3 has been implemented in the photochromic and thermochromic rearrangements of a novel type of heterocyclic photochromes, derivatives of 2,3-dihydro-2-spiro-4 -(2, 6 -di-iert-butylcyclohexadien-2, 5 -one)perimidine la and its analogs.4 The occurrence of a proton transfer step is in accord with the fact that the AvV -dimethyl derivative of la exhibits no photochromic properties. 

The C spectrum (15.04 MHz) of perimidine (13 R = H) (Table 1) is consistent with a symmetrical structure, reflecting rapid prototropic tautomerism between the annular nitrogen atoms as found for H NMR. Tautomerism is excluded in 1-alkyl derivatives as shown for (13 R = Me) in Table 1. The relative C chemical shifts parallel those of the corresponding protons in the H NMR spectrum. The relative order of the chemical shifts in C NMR of 1,8-naphthalenediamine is the same as in perimidine. All carbocyclic peaks other than C9b exhibit a pronounced upfield shift, in accord with the 7r-excessive character of the carbocyclic rings in the perimidine system. On the other hand, C2... 

Finally, the Se NMR evidence of l-mesityl-l,3-dihydro-imidazole-2-selone was much more consistent with the selone 43a (5( Se) = 28ppm, Vsec = 231 Hz] than with the selenol tautomer 43b (Scheme 5.34) [83]. The free energy of activation of prototropic tautomerism of ethyl 2H-perimidine-2-carboxylate 44 (Scheme 5.35)... 

---