Vinylamine tautomerism

Dihydroazoles can exist in at least three forms (cf. Section 4.01.1.3), which in the absence of substituents are tautomeric with each other. The forms in which there is no hydrogen on at least one ring nitrogen normally predominate because imines are generally more stable than vinylamines in aliphatic chemistry. Thus for dihydropyrazoles the stability order is A" (hydrazone) (288) > A (azo) (289) >A (enehydrazine) (290). 

Diphenylcyclopropene thione also reacts with Schiff bases 379 via the tautomeric vinylamine form to give betaines 380, which tautomerize to the bicyclic thio amides 3 /247 ... 

Likewise, Schiff bases of type 379 react with triafulvenes again (see p. 75) via the tautomeric vinylamine form to give products of C—C-insertion 253. In the presence of a COOR group at triafulvene C4, the cross-conjugated system 536 undergoes facile cyclization to 2-pyridones 537 by loss of alcohol. 

In Table 33 are given the values of AH for the tautomeric change calculated from the differences in bond energies only. It is gratifying to find that these values correspond with the generally accepted opinions that, in simple cases, (1) the ketones and aldehydes are stable with respect to the vinyl alcohols, (2) the amides are stable with respect to the imidols, (3) the aldimines are stable with respect to the vinylamines, (4) the oximes are stable with respect to nitroso compounds, (5) diazohydroxy compounds are stable with respect to nitrosamines, (6) hydrazones are stable with respect to azo com-... 

Valence tautomerization 17, 801 Vanadate s. Titanium vanadate Vanadium-tin 16, 414 — pentoxide 16, 300 Verley s. Meerwein Vinyl acetate as additive 18, 806 Vinylamines s. 1,2,2-Trichloro-vinylamines... 

---