The role of conjugative interactions in acidic and basic character of five membered aromatic heterocyclics

Conformational Properties of Oxazole-Amino Acids: Effect of the Intramolecular N–H···N Hydrogen Bond

“…82 It was shown that proton affinity for the nitrogen atom is substantially higher in comparison with the oxygen atom in the oxazole ring. 83 All of these data support our findings that N−H•••N hydrogen bond in the conformation β2 is much stronger than N−H•••O hydrogen bond in the conformation C5.…”

“…The relatively strong hydrogen bond ability of nitrogen in comparison with oxygen is explained on the basis of its basicity as well as weak nonparticipation in the conjugative interaction of ring cloud . It was shown that proton affinity for the nitrogen atom is substantially higher in comparison with the oxygen atom in the oxazole ring . All of these data support our findings that N–H···N hydrogen bond in the conformation β2 is much stronger than N–H···O hydrogen bond in the conformation C5.…”

Deprotonation of Isoxazole: A Photoelectron Imaging Study

“…10 Computational work by Kaur et al confirmed that the C5 site in isoxazole is more acidic than C4, while deprotonation at C3 led to the breaking of the O−N bond, making the C3 comparison more challenging. 11 In a study of 1H-1,2,3-triazole, a 5-member heterocyclic aromatic with three nitrogen atoms, Ichino et al found that deprotonation at various carbon positions initiated fragmentation or N−N bond fission. 12 In the present study, we explore the deprotonation of isoxazole in its gas-phase reaction with O − and the various anion states that result.…”

“…Cole et al attempted to selectively deprotonate isoxazole at C5 via collisional decarboxylation of isoxazole-5-carboxylic acid, but found that upon collision, isoxazole lost HCN rather than the carboxy group . Computational work by Kaur et al confirmed that the C5 site in isoxazole is more acidic than C4, while deprotonation at C3 led to the breaking of the O–N bond, making the C3 comparison more challenging . In a study of 1H-1,2,3-triazole, a 5-member heterocyclic aromatic with three nitrogen atoms, Ichino et al found that deprotonation at various carbon positions initiated fragmentation or N–N bond fission …”

Polymorphism in N-(5-methylisoxazol-3-yl)malonamide: understanding the supramolecular structure and the crystallization mechanism

“…Its insertion into molecules can make it capable of molecular recognition or even self-recognition. Isoxazole rings present the most basic site nitrogen atom, whose lone pair of electrons is freely available for bonding to a proton; 18 in contrast, the isoxazole-oxygen atom is highly electronegative, and its electron density is less available for interactions.…”