1.2- Diazepin-4-ones, tautomerism

A well-studied case of tautomerism is that of l,2-diazepin-4-ones tautomers 47a and 47b were characterized by NMR, the former being the most stable (85JOC2141). 3,5-Dihydro-4//-2,3-benzodiazepin-4-ones exist as 48a and not as 4-hydroxy tautomers 48b (74JHC401). 

Wentrup s paper [98JCS(P2)2247] contains one of the very few contributions to the tautomerism of functionalized 1,3-diazepines. l,3-Dihydro-l,3-diazepin-2-ones (49a) exist as such and not as hydroxy tautomers 49b ( H and NMR in DMSO solution and IR in the solid state). 

Annular tautomerism between one or more NH-forms and various CH forms of 1,2-diazepines is possible with the favored species dependent upon the nature and position of substituents (69ACS3125, 70T739). Interestingly, unlike monocyclic azepines and tautomeric 1,2-diazepines, 5H- 1,2-diazepines (47) exist preferentially in their bicyclic diazanorcaradiene form (48) (72JA2770). 

Iminopropadienones are highly reactive species that have been used as three-carbon fragments in the synthesis of 1,4-diazepines <2002JOC2619>. Aryl- and neopentyl-substituted iminopropadienone derivatives are stable at 25 °C and the latter, 99, reacted with iV,iV -dimethyl-l,2-diaminoethane to afford a 55% yield of the l,4-diazepin-5-one 100, a compound that partially tautomerized to the enamide 101 upon Kugelrohr distillation. 

Some pyridodiazepin-4-ones exist as different valence bond tautomers, and may even isomerize to five-membered ring-containing products. Pyridine-3,4-diamine condensed with ethyl acetoacetate in toluene to give a mixture of tautomeric diazepin-4-ones, but in xylene, the product was an imidazopyridine which was also obtained by dry fusion of the diazepin-4-one [2327],... 

Substituents, such as OH, SH, and NHj, can also participate in tautomeric equilibria, for example compare (13) with (14)-(16). The tautomerism of l,2-diazepin-4-ones has been thoroughly investigated by Moore et al. and the two keto tautomers (11) and (12) found to predominate in an equilibrium ratio of ca. 8 1, respectively (Equation (1)) <68JA1369, 73JOC2939>. 5 f-2,3-Benzo-diazepin-l-ones and benzodiazepine-1-thiones have also been established to exist in their oxo and thiono tautomeric forms <70BSF2237>. 

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