Enolimine tautomer

Morino and Snell32 ascribed the 420-nm species to a protonated internal pyridoxal-P-lysine aldimine and the 337-nm species to either a dipolar ionic form of the aldimine or to a substituted aldamine (an adduct at C-4 ). However, the dipolar ionic form absorbs at about 360 nm in aspartate transaminase and in the model systems containing Schiff bases of A-methylated PLP.34 It was suggested that the 337-nm species may be the enolimine tautomer.35,36 ... 

Although there is a very fast (on the NMR time scale) double proton transfer in quinoline 123, the enaminone/enolimine tautomers 123a and 123b are the only ones present in the chloroform solution. Ab initio calculations of the relative stabilities of tautomers using the PCM solvation model produced results contradictory to the experimental data (03CEJ2710). 

Equation (9), for the azomethine derivatives in Equation (10) (R = H, f-Bu, Br, NO2) containing the aldimine fragment in an aromatic nucleus, the enolimine tautomer predominates (04RKZ30). 

The relative stabifization of the E or K tautomers in 2-4 depends mostly on the position of benzoannelation (Figure 10.3). In contrast to 2 and 3, imines of 3-hydroxy-2-naphthaldehyde 4 exist exclusively as the enolimine tautomers [50 - 54]. Loss of aromaticity in 2 and 3 is restricted to one ring of the naphthalene moiety, whereas in 4 both rings are involved, which leads to destabilization of the... 

Other hetarylazomethines containing several ligating tautomeric moieties may be illustrated by tautomeric equilibrium (14), in which o-tosylamino(hydroxy)aldimines containing 2-amino- (hydroxy-, mer-capto-, hydroseleno-) substituted benzazoles (X = NTs, O Y = NR R = H, Aik, Ar) participate (07JCC1493). When Y = NR, the enolimine 2-amino-tautomer is realized. For X = O, S, and Se, the 2-keto, thio-, and selenoazole tautomers are more expressed. 

Tautomeric Schiff bases of gossypol 37 form stable complexes with various cations [120-122]. Upon protonation of 37 the enolimine form E is realized, and in crown complexes with Li", Na, and K" " the ketoenamine tautomer K is formed (Figure 10.25). 

Aza-15-crown-5 containing 4-(phenyldiazenyl)naphthalen-l-ol 38 exists in the enolimine form E. Complexation with alkaline-earth metal cations reverses the equilibrium to tautomer K (Figure 10.26). This process is accompanied by strong bathochromic (60-100nm) and hyperchromic effects in the absorption spectra [123]. 

Reactions of 44 with alkali and alkaline-earth metal cations are accompanied by considerable displacement of tautomeric equilibrium toward enolimine complexes 45 K. The intensity of the absorption maximum at 480 nm (corresponding to the tautomer K) in the electronic spectra decreases by 25% for alkaline-earth and 15% for alkali metal cations with the subsequent increase of intensity of the 420 nm absorption band. Compound 44 showed high sensitivity for Cu and Co " " ions. Acetonitrile and dimethyl sulfoxide solutions of 44 exhibit strong fluorescence with maximum at 530 nm. Addition of Cu " and Co " " leads to the formation of chelates 46 demonstrating pronounced naked-eye CHEQ effect the fluorescence intensity decreased by a factor of 39 and 33, respectively, without any appreciable shift of the emission maximum (Figure 10.30). 

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