4,5-Disubstituted N,N'-di-tert-alkyl imidazolium salts: new synthesis and structural features.
4,5-Disubstituted N,N'-di-tert-alkyl imidazolium salts: new synthesis and structural features.
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4,5-二取代的N,N-二叔烷基咪唑鎓盐：新合成和结构特征。
DOI：
10.1002/chem.201002805
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发表时间：
2011
期刊：
影响因子：
--
通讯作者：
I. Lyapkalo
中科院分区：
文献类型：
--
作者：
Anastasia A. Grishina;S. M. Polyakova;R. Kunetskiy;I. Císařová;I. Lyapkalo
大约二十年前，稳定的N-杂环卡宾（NHC）的里程碑式发现引发了人们的极大兴趣，并导致了有机和有机金属合成的许多突破。NHC是一种多用途的高效刘易斯碱催化剂，有助于扩大有机催化的范围和适用性。此外，NHC被认为是过渡金属基催化的有效辅助配体。由N，N ′-二取代咪唑鎓盐或咪唑-2-硫酮衍生的不饱和咪唑-2-亚基构成了最大的一类稳定的NHC。通常的做法是将大体积的取代基（最常见的是2，6-二取代的芳基或叔基团）连接到咪唑-2-亚基的氮中心，以获得配体和络合物的更好的动力学稳定性。叔取代基倾向于在C2亲核中心附近产生比正交取向的芳基取代基更大的空间效应（图1）。最近关于N，N '-二取代咪唑-2-亚基与[Ni（CO）4] [5]的相互作用以及大体积咪唑-2-亚基刘易斯碱-刘易斯酸对活化H2的研究清楚地证明了叔取代基（II型，R'= H）比2，6-二烷基苯基（I型）具有上级空间效应。用叔烷基取代基修饰证明对于膦配体的效率至关重要，例如PACHTUNGTRENNUNG（CMe 3）2 Me，[7]联芳基-2-基-二叔丁基膦或PACHTUNGTRENNUNG（CMe 3）3[9]在具有挑战性的Pd催化的偶联反应中。叔N，N '-二取代的咪唑-2-亚基被认为比甚至最基本的和空间要求高的膦如PACHTUNGTRENNUNG（CMe 3）3更具给电子性和更大体积。[10]大量的过渡金属配合物与咪唑-2-亚基配体已被制备和表征，钯配合物已被发现是非常有效的芳基氯的催化活化。然而，尽管具有两个或三个叔取代基的膦配体的种类令人印象深刻，但合成化学家可以使用N，N '-二叔烷基咪唑-2-亚基的阵列似乎非常狭窄。基本上，迄今为止，该亚家族中只有两种配体，N，N '-二叔丁基和N，N'-二金刚烷-1-基-咪唑-2-亚基，已被广泛用于有机和有机金属合成，它们在4和5位都是未取代的（II，R '= H，参见图1）。这是一个相当大的缺点，因为不允许调节配体的立体电子性质，并且未取代的位置容易发生副反应。14]因此，合成4或4，5-（二）取代的N，N ′-二叔烷基咪唑-2-亚基配体的模块化和有效的方法是必然需要的。4位和5位的取代不仅可以保护配体和催化配合物免受不希望的副反应的影响，而且它们的电子性质的变化可以直接预测[a] A。A. Grishina，S. M.波利亚科瓦河A.库涅茨基博士M.捷克共和国科学院Lyapkalo有机化学和生物化学研究所，v. v. i.，Flemingovo nam. 2 16610布拉格6（捷克共和国）传真：（+420）220183578电子邮件：polyakova@uochb.cas.cz [B] I.查尔斯大学无机化学系Hlavova 2030，12840布拉格2（捷克共和国）[†]逝世，2010年9月10日。本文的支持信息可在http://dx.doi.org/10.1002/chem201002805下找到。图1.咪唑-2-亚基的N，N '-二芳基和叔取代基的空间效应的示意图比较。
The landmark discovery of stable N-heterocyclic carbenes (NHCs) approximately two decades ago has sparked immense interest and led to numerous breakthroughs in organic and organometallic synthesis. NHCs are versatile and efficient Lewis base catalysts, instrumental in extending the scope and applicability of organocatalysis. Moreover, NHCs are recognised as potent ancillary ligands for transition-metal-based catalysis. The unsaturated imidazol-2-ylidenes, derived from N,N’disubstituted imidazolium salts or imidazole-2-thiones, constitute one of the largest group of stable NHCs. It is a general practice to attach bulky substituents, most commonly 2,6-disubstituted aryl or tertiary groups, to the nitrogen centres of imidazol-2-ylidenes to attain better kinetic stabilisation of the ligands and complexes. Tertiary substituents tend to engender greater spatial effects in the vicinity of the C2 nucleophilic centre than orthogonally orientated aryl substituents (Figure 1). Recent studies on the interaction of N,N’-disubstituted imidazol-2-ylidenes with [Ni(CO)4] [5] as well as on activation of H2 by bulky imidazol-2-ylidene Lewis base–Lewis acid pairs clearly attest to the superior steric effect of tertiary substituents (type II, R’=H) than 2,6-dialkylphenyl (type I). Decoration with tert-alkyl substituents proved essential for the efficiency of phosphane ligands, for example, PACHTUNGTRENNUNG(CMe3)2Me,[7] biaryl-2-yl-di-tert-butylphosphanes or PACHTUNGTRENNUNG(CMe3)3[9] in challenging Pd-catalysed coupling reactions. Tertiary N,N’-disubstituted imidazol-2-ylidenes are regarded to be more electron donating and more bulky than even the most basic and sterically demanding phosphanes, such as PACHTUNGTRENNUNG(CMe3)3.[10] A multitude of transition-metal complexes with the imidazol-2-ylidene ligands have been prepared and characterised and palladium complexes have been found to be highly efficient in catalytic activation of aryl chlorides. However, whereas an impressive variety of phosphane ligands with two or three tertiary substituents are at the disposal of synthetic chemists, the array of N,N’-di-tert-alkyl imidazol-2-ylidenes appears to be exceedingly narrow. Essentially, only two ligands of this subfamily, N,N’-di-tertbutyland N,N’-diadamantan-1-yl-imidazol-2-ylidenes, have been extensively employed in organic and organometallic syntheses so far, both being unsubstituted at positions 4 and 5 (II, R’= H, see Figure 1). This is a considerable shortcoming because no tuning of stereoelectronic properties of the ligands is permitted, and the unsubstituted positions are vulnerable to side reactions. 14] Thus, a modular and efficient method for synthesis of 4or 4,5-(di)substituted N,N’-di-tert-alkyl imidazol-2-ylidene ligands is certainly in demand. Not only would the substitutions at positions 4 and 5 protect the ligands and the catalytic complexes from undesirable side reactions, but the variation of their electronic properties could directly and predict[a] A. A. Grishina, Dr. S. M. Polyakova, R. A. Kunetskiy, Dr. I. M. Lyapkalo Institute of Organic Chemistry and Biochemistry Academy of Sciences of the Czech Republic, v. v. i. , Flemingovo nam. 2 16610 Prague 6 (Czech Republic) Fax: (+420) 220183578 E-mail : polyakova@uochb.cas.cz [b] Dr. I. C sařov Department of Inorganic Chemistry, Charles University Hlavova 2030, 12840 Prague 2 (Czech Republic) [†] Deceased, September 10, 2010. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem201002805. Figure 1. Schematic comparison of steric effects of N,N’-di-aryl and -tertiary substituents of imidazol-2-ylidenes.
影响因子：
16.6
作者：
Burgos, Carlos H.;Barder, Timothy E.;Buchwald, Stephen L.
通讯作者：
Buchwald, Stephen L.
影响因子：
15
作者：
Kirchhoff, JH;Netherton, MR;Fu, GC
通讯作者：
Fu, GC