Pseudo-cyclic structures of mono- and di-azaderivatives of malondialdehydes. Synthesis and conformational disentanglement by computational analyses

• Published in: Organic & biomolecular chemistry 2014-11, Vol.12 (44), p.8997-9010
• Main Authors: Romero-Fernández, María P, Ávalos, Martín, Babiano, Reyes, Cintas, Pedro, Jiménez, José L, Light, Mark E, Palacios, Juan C
• Format: Article
• Language: English
• ISSN: 1477-0520; 1477-0539
• DOI: 10.1039/c4ob01507j

Mono- and diaza-derivatives of malondialdehydes, namely 3-alkyl(aryl)amino-2-arylacroleins and 1,5-dialkyl(aryl)-3-arylvinamidines are open-chain systems in which extended electron delocalization and pseudoaromaticity can be envisaged. A set of diversely functionalized compounds has been synthesized and characterized by spectroscopic data and X-ray diffractometry. Quantum-chemical calculations were performed for all possible neutral tautomers and conformers in the gas phase and compared to those in polar solvents (CHCl3, DMSO, and EtOH) at the M06-2X/6-311++G(d,p) level. Tautomeric equilibria and conformational preferences can be rationalized in terms of structural factors, which can be roughly estimated as summation or subtractions of intramolecular interactions. As expected, a key role is played by intramolecular hydrogen bonds whose strength varies from the gas phase to polar ethanol. This issue also delves into the concept of resonance-assisted H-bond, where the donor and acceptor atoms are connected by a π-conjugated system. The most stable conformers (structures a and c) possess a high degree of pseudoaromaticity as inferred from HOMA indexes and other delocalization parameters.