Degenerate tautomeric equilibria

Apart from the IH and 2H tautomers 2a and 2b, isomeric forms in which the migrating proton takes up its position on one of the carbon atoms in the ring are formally possible. For the case of unsubstituted pyrazole 2, R = r2 = r3 = tjjg tautomeric equilibrium should be represented by the scheme involving 4H 2e and a pair of degenerate 5H species 2c and 2d (Scheme 2). 

Configuration of A. /V-disubstituted amidines is thus regularly E, reversed rather than in amidines with a free NH2 group which are Z95. Derivatives with only one substituent at the amino group are in the middle and their conformation may be sensitive to small steric effects. In 24 a tautomeric equilibrium would be degenerate, hence the problem is simplified. In the case of IV,-diaryl derivatives, three forms are possible (24a-c) the fourth would be very improbable for steric reasons. From dipole moments an equilibrium... 

Very interesting tautomeric properties are inherent in polycyclic systems that contain annulated phenol and quinone rings. The simplest model for these compounds is napht-hazarin 32 which can exist, both in solution and in the solid state, as a fast equilibrium mixture of several tautomers (32a-32c) where forms 32a and 32b (i.e. a degenerate tautomeric pair of identical 1,4-diones) predominate (equation 12). 

The prototropic degenerated tautomerization of a benzotriazolyl-imidazoUdinone derivative 35 (Scheme 5.26) is frozen at room temperature [70]. At higher temperatures, the equilibrium gets fast on the NMR timescale and both the rate constant kj and the free energy of activation AG = 19.3 kcalmol" could be determined by dynamic H NMR spectroscopy. 

Annular prototropie tautomerism of 1,2,3-triazole (v-triazole) and its C-substituted derivatives involves the equilibrium of three possible isomers 24a-24c. In the ease of the parent eompound (R = H), 24a and 24c are degenerate isomers (Seheme 11). 

N-NMR spectroscopy provides a useful tool in tautomeric studies. Thus, the N-NMR spectrum of lb (R = R = Me) shows a broad average signal for the two equivalent nitrogen atoms of the A B degenerate equilibrium when drops of acid are added to accelerate the rate (86MRC444). 

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