Imidazolin-2-ones tautomerism

Imidazolidin-2-one, l-(5-nitro-2-thiazolyl)-pharmacological activity, 6, 328 Imidazolidin-4-one, l-aryl-3-phenyl-2-thioxo- C NM S, 355 Imidazolidinones C NMR, 5, 355 Imidazolidin-2-ones nucleophilic displacement, 5, 428 polymers, 1, 279-280 reactivity, 5, 376 synthesis, 5, 466, 471 Imidazolidin-4-ones synthesis, 5, 468 Imidazoline, 2-alkyl-synthesis, 5, 463 Imidazoline, 2-amino-applications, 5, 498 Imidazoline, 2-aryl-synthesis, 5, 463 Imidazoline, 2-methyl-synthesis, 5, 487 Imidazoline, 2-nitroamino-synthesis, 5, 471 2-Imidazoline, 2-arylamino-tautomerism, 5, 368 2-Imidazoline, 1-benzyl-methylation, 5, 425 2-Imidazoline, 1,2-diaryl-synthesis, 5, 463... 

Retro-Hantzsch, mechanism of, 84, 85, 102 Rhodanine, alkylation of, 419 ambident reactivity of. 419 reaction with aryldiazonium salts, 419 reaction with halogenothiazoles, 79 Rice cultures, 135, 136, 137 Ring-chain tautomerism, 113 Ring opening, of A-2-thiazoline-5-one, 433 Ring transformation, 5 amino-A-4-thiazo-line-2-thione to 4-mercapto-imidazoline-2-thione, 399 5,5-diphenyl-2,4-thiazolidinedithione, to 4,5-diphenyl-A-4-thiazoline-2-thione, 37 3... 

Methylimidazolin-4-ones can exist in three possible forms (38-40 X = 0) (Scheme 16). In the solid state, l,2-dimethyl-5-phenylimidazolin-4-one exists in the OH form (39 X = 0), but insolubility precluded studies in solution. Compounds stated to be 1-acyl-imidazolin-4-ones have been formulated as the 4-oxo forms (analogous to 38 X = O) on IR evidence. Where possible, it seems that a non-conjugated carbonyl form (e.g. 34, 42 X = O) is preferred to the conjugated version (e.g. 37,41 X = O), but although substituents may have a marked effect, the nature of the solvent is not of particular importance in determining the position of tautomeric equilibrium for these compounds. 

The best known of the potential mercaptoimidazoles are the imidazoline-2- and benz-imidazoline-2-thiones, which resemble imidazolin-2-ones in that the tautomeric form (53 X = S) is the preferred form. The crystal structure and the HNMR spectrum of 1,3-dimethyl-3H-imidazoline-2-thione have been interpreted as showing partial double bond character in the N—C—N system, but no aromaticity (70CC56). However, the preference for a betaine structure (56) rather than (57) or (58) should be accepted with caution since it is really only a resonance structure similar to others which undoubtedly contribute to the overall structures of oxo-, thio- and amino-imidazoles. Measurement of the piSTa values for a series of imidazoline-2-thiones substituted variously on C-4, C-5, N-1 and N-3 by hydrogen, phenyl or methyl shows that all of the values are similar. Approximate Kr values calculated show that these compounds exist even more in the thione forms (53, X = S 58) than do the corresponding thiazoline-2-thiones and oxazoline-2-thiones. The UV spectra in aqueous solution support thione structures, as do dipole moment and X-ray studies (76AHC(S1)280, p. 400). 

The formation of imidazoquinoxalines apparently proceeds through the tautomeric form D formed at the first stage of the reaction of the bases of Schiff C in which nucleophilic attack of atom N4 on the imine atom of carbon occurs with the closure of the imidazoline cycle the aromatization of which when exposed to the DMSO or oxygen of the air leads to the imidazo[l,5-fl]quinoxalin-4-one derivatives. 

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