Isomerism/Tautomerism in Hydrazones Derived from Thiophenaldehydes and 2-Hydrazinoquinoline

The structures of 2-thiophenecarbaldehyde 2-quinolylhydrazone, C₁₄H₁₁N₃S, (1), 2-acetylthiophene 2-quinolylhydrazone, C₁₅H₁₃N₃S, (2), 2(1H)-quinolone 5-bromo-2-thienylmethylidenehydrazone, C₁₄H₁₀BrN₃S, (3), and 5-chloro-2-thiophenecarbaldehyde 2-quinolylhydrazone, C₁₄H₁₀ClN₃S, (4), show that (1), (2) and (4) exist as the antiE isomer while (3) is the tautomer of the syn(Z) isomer, being protonated on the quinoline nitrogen rather than on the exocyclic nitrogen. The photochemical isomerization/tautomerization of (3) to the antiE isomer is discussed.

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