A rigid cyclic (alkyl)(amino) carbene ligand leads to isolation of low-coordinate transition-metal complexes
A rigid cyclic (alkyl)(amino) carbene ligand leads to isolation of low-coordinate transition-metal complexes
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DOI：
10.1002/anie.200502566
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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
在过去的十年中，二氨基n-杂环碳烯（NHCs） A作为过渡金属催化剂的辅助配体和有机催化剂本身的应用已被证明是非常富有成效的NHCs的高效是由于其较强的σ-给体性质和高空间要求的结构。[1,2] NHCs不仅产生了改进的过渡金属催化剂，而且还导致了不寻常的低配位金属配合物[2]的分离，[2]通常在催化过程中起关键作用。最近，我们报道了稳定的环（烷基）（氨基）碳烯（CAACs） B[3]的合成，并证明了CAACs比NHCs更强的σ给体，并导致了酮和醛的α-芳基化的高效钯催化剂。在这里，我们报道了刚性CAAC配体的特殊空间和电子性质允许制备低配位金属配合物，迄今为止不能与任何其他配体分离。为了进一步评价刚性CAAC B1的电子性能，我们尝试制备了[RhCl (CO) 2 (B1)]配合物，因为这种类型的铑配合物的CO伸展频率被认为是衡量配体L的σ-给体能力的一个很好的指标，并且有大量的数据可供比较carbenb1与半等量的[{RhCl (cod) 2} 2]（cod= 1,5 -环二烯）反应生成配合物1，硅胶柱层析纯化后得到橙色结晶，收率79%（方案1）。单晶x射线衍射明确了1的结构按照经典程序，用过量的CO处理氯仿中的1溶液。令人惊讶的是，产物（95%产率）的13c NMR谱显示只有一个CO信号，具有较大的耦合常数（JRh， C= 134 Hz），而不是两个较小的信号（JRh， C= 33-82 Hz），正如[RhCl (CO) 2 (L)]配合物所观察到的那样众所周知，当大块的配体L存在时，[RhCl (CO) 2L]配合物很容易失去一个CO配体
In the last decade the use of diamino N-heterocyclic carbenes (NHCs) A as ancillary ligands for transition-metal catalysts and as organic catalysts on their own has proven very fruitful.[1] The efficiency of NHCs is attributed to their strong σ-donor properties and sterically demanding structure.[1, 2] Not only have NHCs yielded improved transition metal catalysts, but they have also lead to the isolation of unusual low-coordinate metal complexes,[2] which often play a key role in catalytic processes. Recently, we reported the synthesis of stable cyclic (alkyl)(amino) carbenes (CAACs) B [3] and demonstrated that CAACs can be even stronger σ donors than NHCs, and lead to highly efficient palladium catalysts for the α-arylation of ketones and aldehydes. Here we report that the peculiar steric and electronic properties of rigid CAAC ligands allow the preparation of low-coordinate metal complexes, hitherto not isolable with any other ligands.To further evaluate the electronic properties of rigid CAAC B1, we attempted to prepare the [RhCl (CO) 2 (B1)] complex, since the CO stretching frequencies of this type of rhodium complexes are recognized as an excellent measure of the σ-donor ability of the ligand L, and a large amount of data is available for comparison.[4] Reaction of carbene B1 with half an equivalent of [{RhCl (cod) 2} 2](cod= 1, 5-cyclooctadiene) afforded complex 1, which after purification by column chromatography on silica gel was obtained as orange crystals in 79% yield (Scheme 1). The structure of 1 was unambiguously determined by single-crystal X-ray diffraction.[5] Following the classical procedure, a solution of 1 in chloroform was treated with an excess of CO. Surprisingly, the 13 C NMR spectrum of the product (95% yield) showed only one CO signal, with a large coupling constant (JRh, C= 134 Hz), and not two signals with smaller Jvalues (JRh, C= 33–82 Hz), as observed for [RhCl (CO) 2 (L)] complexes.[4] It is known that when bulky ligands L are present [RhCl (CO) 2L] complexes can readily lose one CO ligand