Calabrese Camilla, Maris Assimo, Uriarte Iciar, Cocinero Emilio J, Melandri Sonia
Dipartimento di Chimica "G. Ciamician", Università degli Studi di Bologna, Via Selmi 2, 40126, Bologna, Italy.
Dpto. Química Física, Universidad del País Vasco (UPV/EHU), Apartado 644, 48080, Bilbao, Spain.
Chemistry. 2017 Mar 13;23(15):3595-3604. doi: 10.1002/chem.201604891. Epub 2016 Dec 29.
The effects of halogenation on the tautomeric and conformational equilibria of the model system 2-hydroxypyridine/2-pyridone have been investigated through chlorine substitution at positions 3, 4, 5, and 6. In the gas phase, the lactim syn-periplanar tautomer (OH ) was the predominant species for all compounds over the lactam form (C=O) and the less abundant anti-periplanar lactim (OH ). However, the population of the three species was shown to be dependent on the position of the chlorine substitution. Chlorination in position 5 or 6 strongly stabilizes the OH tautomer, whereas the C=O form has a significant population when the ring is chlorinated in positions 3 or 4. Overall, the OH form is the least favourable form, although the 3-substitution favours the population of this tautomer. In addition, the C=O tautomer is strongly stabilized in the solvent, which makes it the dominant form in some substituted species. This study has been performed by means of rotational spectroscopy in the gas phase and/or theoretical calculations in the isolated phase and in solution. Both the OH and C=O forms of 5-chloro-2-hydroxypyridine and the OH form of 6-chloro-2-hydroxypyridine were experimentally observed. All transitions displayed a complex nuclear hyperfine structure owing to the presence of the chlorine and nitrogen nuclei. For all species, a full quadrupolar hyperfine analysis has been performed. This has provided crucial information for the unambiguous identification of tautomers.
通过在2-羟基吡啶/2-吡啶酮模型体系的3、4、5和6位进行氯取代，研究了卤化对其互变异构和构象平衡的影响。在气相中，对于所有化合物，内酰胺烯醇式顺式共平面互变异构体（OH）比内酰胺形式（C=O）和含量较少的反式共平面内酰胺烯醇式（OH）占主导地位。然而，这三种异构体的比例显示取决于氯取代的位置。5位或6位氯化强烈稳定OH互变异构体，而当环在3位或4位氯化时，C=O形式有显著比例。总体而言，OH形式是最不利的形式，尽管3-取代有利于这种互变异构体的比例。此外，C=O互变异构体在溶剂中强烈稳定，这使其在一些取代物种中成为主要形式。本研究通过气相旋转光谱法和/或孤立相及溶液中的理论计算进行。实验观察到了5-氯-2-羟基吡啶的OH和C=O形式以及6-氯-2-羟基吡啶的OH形式。由于存在氯和氮原子核，所有跃迁都显示出复杂的核超精细结构。对于所有物种，都进行了完整的四极超精细分析。这为明确鉴定互变异构体提供了关键信息。
