Stable cyclic (alkyl) (amino) carbenes as rigid or flexible, bulky, electron-rich ligands for transition-metal catalysts:: A quaternary carbon atom makes the difference
Stable cyclic (alkyl) (amino) carbenes as rigid or flexible, bulky, electron-rich ligands for transition-metal catalysts:: A quaternary carbon atom makes the difference
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DOI：
10.1002/anie.200501841
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发表时间：
2005-01-01
影响因子：
16.6
通讯作者：
Bertrand, G
中科院分区：
文献类型：
--
作者：
Lavallo, V;Canac, Y;Bertrand, G
实现特定转化的催化剂的可用性对工业界和学术界都至关重要。多年来，均相催化的成功在很大程度上归功于各种配体框架的发展，这些配体框架已被用于调整各种含金属体系的行为。配体设计的进步不仅使已知过程在范围、温和性和催化剂负载方面得到了改进，而且还发现了新的选择性反应。钯催化的偶联反应是一个很好的例子，它被广泛应用于从合成有机化学到材料科学的各个领域对于这些催化过程来说，它们代表了合成化学家可用的一些最强大和最通用的工具，由于使用了体积大、富含电子的磷化氢A和环二氨基碳烯（n-杂环碳烯（NHCs）） B，最近取得了重大进展（图1）这些配体稳定了活性的催化物质，加速了重要的催化步骤，即氧化加成、金属转化和还原消除。另一方面，过大的位阻会给大体积反应物的耦合带来一些缺点为了克服这个问题，Glorius和同事利用环己烷的构象灵活性成功地开发了具有“柔性立体体”的配体。本文报道了稳定的环（烷基）-（氨基）碳烯（CAACs） C的合成（图1）。用强σ给体烷基取代NHCs B的一个电负性氨基取代基，使得CAAC配体C比a和B更富电子。此外，由于在碳中心的α位置上存在一个四元碳原子，碳C具有与配体a和B显著不同的立体环境（图1），并放大了柔性立体体效应。研究表明，羰基化合物的α-芳基化反应中，碳烯C特有的电子和位阻性质使得caac -钯配合物的合成具有很高的效率。
The availability of catalysts to perform specific transformations is critical for both industry and academia. Over the years, the success of homogeneous catalysis can be attributed largely to the development of a diverse range of ligand frameworks that have been used to tune the behavior of a variety of metal-containing systems. Advances in ligand design have allowed not only for improvements of known processes in terms of scope, mildness, and catalyst loadings, but also for the discovery of new selective reactions. A good illustration is given by palladium-catalyzed coupling reactions, which are applied to a wide area of endeavors ranging from synthetic organic chemistry to materials science.[1] For these catalytic processes, which represent some of the most powerful and versatile tools available for synthetic chemists, major advances have recently been reported thanks to the use of bulky, electron-rich, phosphines A and cyclic diaminocarbenes (N-heterocyclic carbenes (NHCs)) B (Figure 1).[2] These ligands stabilize the active catalytic species, and accelerate the important catalytic steps, namely oxidative addition, transmetallation, and reductive elimination. On the other hand, excessive steric hindrance can present some drawbacks for the coupling of bulky reactants.[3] To overcome this problem Glorius and co-workers have successfully developed ligands with “flexible steric bulk” using the conformational flexibility of cyclohexane.[4]Herein we report the synthesis of stable cyclic (alkyl)-(amino) carbenes (CAACs) C (Figure 1). The replacement of one of the electronegative amino substituents of NHCs B by a strong σ-donor alkyl group makes the CAAC ligands C even more electron-rich than A and B. Moreover, owing to the presence of a quaternary carbon atom in a position α to the carbene center, carbenes C feature steric environments that differentiate them dramatically from both ligands A and B (Figure 1), and amplifies flexible steric bulk effect. We show that the peculiar electronic and steric properties of carbenes C allow for the synthesis of CAAC-palladium complexes that are highly efficient for the catalytic α-arylation of carbonyl compounds.