Cu(ii)-mediated tautomerization for the pyrazole-nitrile coupling reaction†

IF 3.5 3区 化学 Q2 CHEMISTRY, INORGANIC & NUCLEAR
Chang-Chih Hsieh, Chia-Wei Chen, Ming-Hsi Chiang and Yih-Chern Horng
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引用次数: 0

Abstract

This study investigates the mechanism of prototropic tautomerization in metal-bound asymmetric pyrazole (R-PzH) ligands during Cu(II)-mediated PzH-MeCN coupling reactions. Intrinsic prototropic tautomerization of metal-bound ligands has not been previously documented. Various new bis-pyrazolylamidino Cu(II) complexes, [Cu(R-Pz(HNC(Me)))2(ClO4)2], from the coupling reaction, and tetrakis pyrazole Cu(II) complexes, [Cu(R-PzH)4(ClO4)2], with symmetric and asymmetric C-monosubstituted R-PzH ligands were synthesized and characterized. Kinetic UV-vis studies revealed slower coupling rates with asymmetric R-PzH ligands, further reduced with increasing substituent size due to steric hindrance impeding tautomerization. Structural analysis showed preferential binding of asymmetric 3(5)-R-PzH to Cu(II) ions at the C5 position of PzH, requiring a tautomeric shift to the C3 position for the subsequent coupling reaction. DFT calculations confirmed the greater stability of the C5-tautomeric [Cu(R-PzH)4(ClO4)2] complexes over their C3 counterparts, explaining the rate discrepancies. A mechanism involving counteranion-mediated proton transfer during tautomerization is proposed to account for coupling product formation.

Abstract Image

铜(II)介导的吡唑-腈偶联反应的互变异构化
本文研究了Cu(II)介导的PzH-MeCN偶联反应中金属结合的不对称吡唑(R-PzH)配体的原向异构化机制。金属结合配体的固有原变性以前没有文献记载。通过偶联反应合成了多种新型双吡唑胺基Cu(II)配合物[Cu(R-Pz(HN=C(Me))2(ClO4)2]和对称和不对称C-单取代R-PzH配体的四(PzH) Cu(II)配合物[Cu(R-PzH)4(ClO4)2],并对其进行了表征。动力学UV-vis研究表明,与不对称R-PzH配体的偶联速率较慢,随着取代基尺寸的增加,由于位阻阻碍了互变异构化,偶联速率进一步降低。结构分析表明,不对称的3(5)-R-PzH在PzH的C5位置优先结合Cu(II)离子,需要在随后的偶联反应中互变异构移位到C3位置。DFT计算证实了c5 -互变异构体[Cu(R-PzH)4(ClO4)2]复合物比它们的C3对应物更稳定,解释了速率差异。在互变异构过程中,提出了一种涉及反阴离子介导的质子转移的机制来解释偶联产物的形成。
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来源期刊
Dalton Transactions
Dalton Transactions 化学-无机化学与核化学
CiteScore
6.60
自引率
7.50%
发文量
1832
审稿时长
1.5 months
期刊介绍: Dalton Transactions is a journal for all areas of inorganic chemistry, which encompasses the organometallic, bioinorganic and materials chemistry of the elements, with applications including synthesis, catalysis, energy conversion/storage, electrical devices and medicine. Dalton Transactions welcomes high-quality, original submissions in all of these areas and more, where the advancement of knowledge in inorganic chemistry is significant.
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