Pyrazolonderivate. IV [9]. Zur Tautomerie und Stereochemie von 4‐Aminomethylen‐Δ²‐pyrazolinonen‐(5)

Abstract

Pyrazolone Derivatives. IV. Tautomerism and Stereochemistry of 4‐Aminomethylene‐Δ2‐pyrazolinones‐(5) Tautomerism and stereochemistry of compounds of type 1 were investigated by 1H‐n.m.r.‐ and i.r.‐spectroscopy. Dynamic 1H‐n.m.r. study with 15N‐labelled compounds shows, that 1 exist in solution as 4‐aminomethylene‐Δ2‐pyrazolinones‐(5) with strong intramolecular hydrogen bonds. Compared to 4‐unsubstituted Δ2‐pyrazolinones‐(5) polar solvents do not change the tautomeric equilibrium of 1. The N‐aryl‐substituted compounds have a higher NH‐acidity than N‐alkyl‐substituted derivatives. In crystalline state the N‐alkyl‐substituted compounds 1g, h exist in the E‐configuration as 4‐aminomethylene‐Δ2‐pyrazolinones‐(5) with intermolecular hydrogen bond. At higher temperatures or in solution the Z‐isomer is formed spontaneously.