Complex tauto- and rotamerism of 2-(R-phenyl)-1,2,3,4-tetrahydroquinazolines

• Published in: Journal of physical organic chemistry Vol. 18; no. 8; pp. 737 - 742
• Main Authors: Gawinecki, Ryszard, Kolehmainen, Erkki, Kuczek, Agnieszka, Pihlaja, Kalevi, Ośmiałowski, Borys
• Format: Journal Article
• Language: English
• Published: Chichester, UK John Wiley & Sons, Ltd 01.08.2005
• Subjects: conjugation; hydrogen bond; multinuclear magnetic resonance; ring-chain tautomerism; rotamers; Schiff bases; temperature effect
• ISSN: 0894-3230; 1099-1395
• DOI: 10.1002/poc.927

Detailed NMR spectral analysis of CDCl3 solutions of 2‐(R‐phenyl)‐1,2,3,4‐tetrahydroquinazolines reveals three or four tautomeric forms. Apart from 2‐[(benzylideneamino)methyl]aniline, the other chain tautomeric forms are present only in minor quantities. In general, electron‐donating substituents increase the contribution of all chain forms. Lowering the temperature of the CDCl3 solution of 2‐(R‐phenyl)‐1,2,3,4‐tetrahydroquinazolines decreases the content of the 2‐[(benzylideneamino)methyl]aniline form. At the same time, the amount of the ring form increases. Opening of the tetrahydropyrimidine ring in 2‐(R‐phenyl)‐1,2,3,4‐tetrahydroquinazolines was found to be an endothermic process especially for less electron‐donating substituents. Copyright © 2005 John Wiley & Sons, Ltd. Tautomeric equilibrium between 2‐(R‐phenyl)‐1,2,3,4‐tetrahydroquinazolines, 2‐[(benzylideneamino)methyl] anilines [C1(major)] and 2‐(benzylideneamino)benzyl‐amines [C2(minor)] takes place in chloroform solution.