Tautomerism 1,1-enediamine-amidine

Although the enediamine form is predominant, the amidine tautomers exist in some the imidazole and benzimidazole forms in the crystals, whereas in solution, tautomeric equilibrium between 13 and 13 has been observed33 (equation 1). The equilibrium is affected by the substituent X, and electron-withdrawing chlorine of 13 favors the... 

Except for 12 and 13 b-d, the isolation of the amidine tautomer has been reported only recently. Spectral data show that alkylated 1,1-enediamines such as 16 tautomerize predominantly to 4,5-dihydroimidazole derivatives 16 3 6 (equation 4). More interestingly, the adducts of enediamines and diethyl diazenedicarboxylate exist exclusively as the enediamine tautomer when R2 = H (17) and as the amidine tautomer when R2 = Et (17 )37 (equation 5). Since 17 and 17 differ only in an ethyl group, it appears that the steric effect plays an important role in determining the tautomeric equilibrium. 

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