Hydrogen and amine activation by a frustrated Lewis pair of a bulky n-heterocyclic carbene and B(C6F5)3
Hydrogen and amine activation by a frustrated Lewis pair of a bulky n-heterocyclic carbene and B(C6F5)3
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DOI：
10.1002/anie.200802596
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发表时间：
2008-01-01
影响因子：
16.6
通讯作者：
Stephan, Douglas W.
中科院分区：
文献类型：
--
作者：
Chase, Preston A.;Stephan, Douglas W.
N-杂环卡宾（NHC），[1]一类稳定且可分离的卡宾，[2]是一种研究充分的配体，通常可以取代有机金属配合物中的膦，在催化中有广泛的用途。[3，4] NHC本身也被用作有机催化剂[5 - 7]，并且越来越多的报道表明各种稳定的卡宾具有独特和出乎意料的反应性。例如，Bertrand及其同事最近的研究表明，某些（氨基）-（烷基）卡宾直接与二氢和氨反应，[8]模拟非金属氧化加成过程（方案1a）。小的主族物种，如P4 [9，10]也与卡宾反应。我们小组目前的工作涉及"受抑刘易斯对"（FLP）的反应性，[11，12]这是一种刘易斯酸和碱的组合，其中空间位阻阻止或限制加合物的形成。这些未淬灭的位点可以具有独特的反应性。例如，大体积膦和硼烷的混合物直接与二氢[13，14]和烯烃反应（方案1b）。[15此外，化合物（2，4，6-Me3C6H2）2P（C6F4）B（C6F5）2是能够与二氢可逆反应的主族体系的第一个实例（方案1c）。[17]这些物种的氢活化能力已被利用在亚胺，腈和氮丙啶的无金属催化还原。[18然而，迄今为止，已知在FLP型化学中具有反应性的化合物的范围相对较小。[20 - 22]在本文中，我们提出了大体积NHC，其在FLP化学中是与刘易斯酸三（五氟苯基）硼烷[23 - 25]的可行碱性伴侣，显示出与二氢和胺的键活化反应性。在后一种情况下，NHC还催化与三（五氟苯基）硼烷的伯和仲烷基胺加合物的消除反应，以直接且干净地产生各种氨基硼烷。在我们发现基于卡宾的FLP的最初努力中，进行了NHC 1，3-双（2，6-二异丙基苯基）-1，3-咪唑-2-亚基（IDipp）与B（C6F5）3的反应。所得产物（1）的11B NMR谱显示出在Δ 15处的单共振。6ppm，这表明四配位硼中心，表明形成了式（IDipp）B（C6F5）3的简单刘易斯加合物（方案2）。化合物1的~1H、~（19）F和~（13）C {~1H} NMR谱都很复杂，这是由于这种大体积加合物的旋转受限所致。此外，通过X射线晶体结构测定证实了该制剂（图1）。Power和菲利普斯描述了唯一报道的NHC/B（C6F5）3加合物，其含有1，3，4，5-四甲基-1，3-咪唑-2-亚基。后一种物种的BdC（卡宾）键长为1.6407（18），[26]与1中发现的1.663（5）的BdC距离相比。IDipp中的芳烃取代基上的iPr基团所呈现的更大的空间需求大概是较长的Bis-C键的原因。值得注意的是，刘易斯酸-碱加合物1证明对二氢或烯烃不反应，并且在溶液中长时间不稳定，分解得到产物的混合物。
N-Heterocyclic carbenes (NHCs),[1] one class of stable and isolable carbenes,[2] are a well-studied ligand that can often replace phosphines in organometallic complexes, which have found extensive use in catalysis.[3, 4] NHCs are also utilized as organocatalysts in their own right,[5–7] and there are growing number of reports of unique and unexpected reactivity of a variety of stable carbenes. For example, recent studies from Bertrand and co-workers showed that certain (amino)-(alkyl) carbenes react directly with dihydrogen and ammonia,[8] mimicking metalloid oxidative addition processes (Scheme 1a). Small main-group species, such as P4[9, 10] also react with carbenes. Current work from our group involves the reactivity of “frustrated Lewis pairs”(FLPs),[11, 12] a combination of Lewis acids and bases in which steric encumbrance prevents or limits adduct formation. These unquenched sites can have unique reactivity. For example, mixtures of bulky phosphines and boranes react directly with dihydrogen [13, 14] and olefins (Scheme 1b).[15, 16] Furthermore, the compound (2, 4, 6-Me3C6H2) 2P (C6F4) B (C6F5) 2 is the first example of a main group system that can reversibly react with dihydrogen (Scheme 1c).[17] The hydrogen-activating ability of these species has been exploited in metal-free catalytic reduction of imines, nitriles, and aziridines.[18, 19] However, the range of compounds known to be reactive in FLP-type chemistry is relatively small to date.[20–22] Herein, we present bulky NHCs which are viable basic partners with the Lewis acid tris (pentafluorophenyl) borane [23–25] in FLP chemistry, showing bond activation reactivity with dihydrogen and amines. In the later case, the NHC also catalyzes an elimination reaction with primary and secondary alkyl amines adducts of tris (pentafluorophenyl) borane to directly and cleanly generate a variety of aminoboranes. In our initial efforts to uncover carbene-based FLPs, the reaction of the NHC 1, 3-bis (2, 6-diisopropylphenyl)-1, 3-imidazol-2-ylidene (IDipp) with B (C6F5) 3 was performed. The 11B NMR spectrum of the resulting product (1) shows a single resonance at À15. 6ppm, which is indicative of a fourcoordinate boron center, suggesting the formation of the simple Lewis adduct of formulation (IDipp) B (C6F5) 3 (Scheme 2). The 1H, 19F and 13C {1H} NMR spectra of 1 are all quite complex, which is attributable to restricted rotation in this bulky adduct. In addition this formulation was confirmed by an X-ray crystal structure determination (Figure 1). Power and Phillips described the only reported NHC/B (C6F5) 3 adduct, which contains 1, 3, 4, 5-tetramethyl-1, 3-imidazol-2-ylidene. This latter species had a BÀ C (carbene) bond length of 1.6407 (18),[26] which compares with the BÀC distance of 1.663 (5) found in 1. The greater steric demands presented by the iPr groups on the arene substituents in IDipp presumably account for the longer BÀC bond. It is noteworthy that Lewis acid–base adduct 1 proved unreactive towards dihydrogen or olefins Scheme 2), and is not stable for extended periods in solution, decomposing to give a mixture of products.