Proton transfer reactions of N -aryl triazolium salts: unusual ortho -substituent effects
Proton transfer reactions of N -aryl triazolium salts: unusual ortho -substituent effects
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N-芳基三唑盐的质子转移反应：不寻常的邻位取代基效应
DOI：
10.1002/poc.3399
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发表时间：
2014
影响因子：
1.8
通讯作者：
Tucker D
中科院分区：
文献类型：
--
作者：
Tucker D
先前对三唑基N-杂环卡宾的三唑离子共轭酸的C(3)-氢/氘交换反应的研究揭示了在酸性条件下N1-质子化为二价盐的机制变化。有趣的是，数据表明，相对于邻位具有氢或甲基取代基而不是氟的其他 N-芳基取代基，在存在 N-五氟苯基取代基的情况下，pKaN1 有所增加。为了探讨 N-五氟苯基取代基是否存在明显的供体效应，该效应不同于该基团更常见的吸电子效应，我们研究了四种三唑鎓盐在 N-芳环的 2-位和/或 6-位上与杂原子或杂原子取代基的类似氘交换反应。这些包括带有 N-2,4,6-三溴苯基 11、N-2,6-二氯苯基 12、N-2-吡啶基 13 和 N-2-嘧啶基 14 取代基的三唑盐。发现 11、12 和 14 的 logkex-pD 曲线在较低 pD 下显示出与先前研究的 N-五氟苯基三唑盐相似的趋势，因此支持 pKaN1 存在明显的供体效应。令人惊讶的是，N-吡啶盐 13 的 logkex-pD 曲线独特地显示出较低 pD 下的酸催化作用。我们在此提出，该数据最好通过调用 N-(2-吡啶基)取代基的分子内一般碱作用与三唑鎓环上的 N1-质子化来解释。最后，氘氧离子催化的 C(3)-H/D 交换的二阶速率常数（kDO，M−1s−1）可从 pD >1.5 的数据获得，用于提供四种三唑盐 11、12、13、14 的 C(3)-碳酸 pKaC3 值的估计。 © 2014 作者。 《物理有机化学杂志》由 John Wiley & Sons Ltd 出版。
Previous studies of the C(3)‐hydrogen/deuterium exchange reactions of the triazolium ion conjugate acids of triazolyl N‐heterocyclic carbenes revealed a change of mechanism under acidic conditions with N1‐protonation to a dicationic salt. Interestingly, the data suggested anincreasein pKaN1in the presence of aN‐pentafluorophenyl substituent relative to otherN‐aryl substituents with hydrogens or methyl substituents rather than fluorines at theortho‐positions. To probe the presence of an apparent donor effect of aN‐pentafluorophenyl substituent, which differs from the more common electron withdrawing effect of this group, we have studied the analogous deuterium exchange reactions of four triazolium salts with heteroatoms or heteroatom substituents in the 2‐position and/or 6‐position of theN‐aryl ring. These include triazolium salts withN‐2,4,6‐tribromophenyl11,N‐2,6‐dichlorophenyl12,N‐2‐pyridyl13andN‐2‐pyrimidinyl14substituents. The logkex– pDprofiles for11,12and14were found to show similar trends at lower pDs as for the previously studiedN‐pentafluorophenyl triazolium salt, hence supporting the presence an apparent donor effect on pKaN1. Surprisingly, the logkex– pDprofile forN‐pyridyl salt13uniquely showed acid catalysis at lower pDs. We propose herein that this data is best explained by invoking an intramolecular general base role for theN‐(2‐pyridyl) substituent in conjunction with N1‐protonation on the triazolium ring. Finally, the second order rate constants for deuteroxide ion catalysed C(3)‐H/D exchange (kDO, M−1s−1), which could be obtained from data at pDs >1.5, were used to provide estimates of C(3)‐carbon acid pKaC3values for the four triazolium salts11,12,13,14. © 2014 The Authors. Journal of Physical Organic Chemistry published by John Wiley & Sons Ltd.