Rate and equilibrium constants for the addition of triazolium salt derived N-heterocyclic carbenes to heteroaromatic aldehydes.
Rate and equilibrium constants for the addition of triazolium salt derived N-heterocyclic carbenes to heteroaromatic aldehydes.
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DOI：
10.1039/d2sc05704b
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发表时间：
2022-12-21
期刊：
影响因子：
8.4
通讯作者：
中科院分区：
文献类型：
--
作者：
杂芳族醛通常优先或专门用于一系列NHC催化的工艺中，这些工艺通过产生反应性二氨基烯醇或Breslow中间体（BI）进行，其独特反应性的原因目前尚不清楚。本文报道了N-芳基三唑鎓 NHC与杂芳醛反应的速率常数和平衡常数的测定，为NHC和杂芳醛结构对BI形成的影响提供了深入的了解。NHC催化剂和杂芳醛结构的变化显着影响所观察到的加合物形成，衰变为起始材料和向前反应为BI的动力学参数。特别地，用3-卤素（Br，F）和3-甲基取代的吡啶-2-甲醛衍生物两者观察到大的影响，其相对于苯甲醛衍生物基本上有利于四面体中间体的形成。关键的观察结果表明，空间位阻的增加导致三唑骨架中大的（2，6-二取代的）-N-Ar基团的k2和k−1降低，并且在吡啶-2-甲醛衍生物的3-位上有空间要求的醛取代基，但在6-位上没有。作为这项研究的一部分，20四面体加合物的N-芳基三唑衍生的NHC到杂芳醛加成后形成的分离和表征进行了描述。这些加合物是NHC催化的杂芳醛的Umpolung加成的关键中间体，并且是BI前体。杂原子和一个额外的取代基都邻位的杂芳族醛的邻位的组合效果的结果在异常大的增强四面体加合物形成的NHC加成的平衡常数。
Heteroaromatic aldehydes are often used preferentially or exclusively in a range of NHC-catalysed processes that proceed through the generation of a reactive diaminoenol or Breslow Intermediate (BI), with the reason for their unique reactivity currently underexplored. This manuscript reports measurement of rate and equilibrium constants for the reaction between N-aryl triazolium NHCs and heteroaromatic aldehydes, providing insight into the effect of the NHC and heteroaromatic aldehyde structure up to formation of the BI. Variation in NHC catalyst and heteroaromatic aldehyde structure markedly affect the observed kinetic parameters of adduct formation, decay to starting materials and onward reaction to BI. In particular, large effects are observed with both 3-halogen (Br, F) and 3-methyl substituted pyridine-2-carboxaldehyde derivatives which substantially favour formation of the tetrahedral intermediate relative to benzaldehyde derivatives. Key observations indicate that increased steric hindrance leads to a reduction in both k2 and k−1 for large (2,6-disubstituted)-N-Ar groups within the triazolium scaffold, and sterically demanding aldehyde substituents in the 3-position, but not in the 6-position of the pyridine-2-carboxaldehyde derivatives. As part of this study, the isolation and characterisation of twenty tetrahedral adducts formed upon addition of N-aryl triazolium derived NHCs into heteroaromatic aldehydes are described. These adducts are key intermediates in NHC-catalysed umpolung addition of heteroaromatic aldehydes and are BI precursors. The combined effect of the heteroatom and an additional substituent both ortho to the aldehydic position in heteroaromatic aldehydes result in exceptionally large enhancements in equilibrium constants for tetrahedral adduct formation on NHC addition.
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影响因子：
8.4
作者：
Ishii T;Nagao K;Ohmiya H
通讯作者：
Ohmiya H
影响因子：
1.8
作者：
BRESLOW, R;KIM, R
通讯作者：
KIM, R
影响因子：
3.6
作者：
Hsieh, Ming-Hsiu;Huang, Gou-Tao;Yu, Jen-Shiang K.
通讯作者：
Yu, Jen-Shiang K.
DOI：
10.1002/anie.201501840
发表时间：
2015-06-01
期刊：
Angewandte Chemie (International ed. in English)
影响因子：
--
作者：
Collett CJ;Massey RS;Taylor JE;Maguire OR;O'Donoghue AC;Smith AD
通讯作者：
Smith AD
影响因子：
4.9
作者：
Enders, Dieter;Grossmann, Andre;Raabe, Gerhard
通讯作者：
Raabe, Gerhard