Phosphine Adducts of 1,2-Dibromo-1,2-dimesityldiborane(4): Between Bridging Halides and Rearrangement Processes
Phosphine Adducts of 1,2-Dibromo-1,2-dimesityldiborane(4): Between Bridging Halides and Rearrangement Processes
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DOI：
10.1002/anie.201201673
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发表时间：
2012-01-01
影响因子：
16.6
通讯作者：
Radacki, Krzysztof
中科院分区：
文献类型：
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作者：
Braunschweig, Holger;Damme, Alexander;Radacki, Krzysztof
现代有机化学和高价值化学品的生产强烈依赖于将多种不同功能纳入有机框架的可能性。在这方面，硼基在很久以前就已经建立，将通用的可及性与独特的化学多样性相结合。有价值的次级产物包括例如烯烃、醇和酮。同样重要的是它们作为Suzuki-Miyaura型偶联反应中的试剂的突出意义。已经开发了各种合成方法，例如作为盐消除、硼氢化或二硼化反应，以使得能够方便地引入硼基。特别地，二硼化在促进从市售的二硼烷（4）试剂，即B2cat 2（cat=儿茶酚合）和B2 pin 2（pin=频哪醇合）同时产生两个反应位点方面具有极大的兴趣。[1]这种方法的一个主要缺点是几乎在所有情况下都需要过渡金属催化，除了高活性的乙硼烷（4）衍生物。[2]这种二硼化反应性的第一个例子是在1954年发表的B2Cl 4，它在没有任何添加剂的情况下很容易添加到乙烯的双键上。[2a]然而，催化过程是迄今为止占主导地位的，因此目前的研究集中在替代的无金属系统。Hoveyda等人最近报道了由N-杂环卡宾（NHC）促进的α，β-不饱和酮的有效无金属β-硼化。[3]中立1：B2 pin 2的1 NHC加合物（1;方案1）被提出作为催化活性物质，尽管其确切结构当时仍然未知。然而，这些发现刺激了这一领域的研究，并且已经基于中性和阴离子sp2-sp3二硼化合物开发了用于有机底物硼化的其他无金属系统。[4]因此，现在可以获得关于有机催化剂的实际组成的更详细的信息，这清楚地强调了sp2-sp3二硼烷作为过渡金属[5]和有机催化的[4]硼基化反应中的中间体的高度相关性。Marder等人的精细光谱和理论研究最终证实了中性NHC加合物1在溶液和固态中的存在。[6]此外，Kleeberg埃塔尔.最近成功地分离了相关的阴离子物种B2 pin 2·Xc 3（2; Xc 3 = OMeCl 2，OtBuCl 2，4-tBu-C6 H4 OCl 2，Fc 3;方案1），其在有机亲电试剂的无金属硼基化中显示出高潜力。[7]考虑到这些基本的发展，似乎相当令人惊讶的是，乙硼烷（4）与刘易斯碱的简单1：1加合物已经被忽视了很长一段时间。因此，充分表征的sp2-sp3二硼化合物的数量仍然相对较少。最初，Marder和Norman研究了B2cat 2和B2（1，2-S2 C6 H4）对氮和磷供体的反应性，得到相应的单加合物和双加合物B2（1，2-E2 C6 H4）· L（3; E= O，S; L= 4-甲基吡啶，PMe 2 Ph，PEt 3;方案1）和B2（1，2-E2 C6 H4）· L2（4）。[8]后来，分子内刘易斯碱配位被建立用于频哪醇合（二异丙醇胺合）二硼（5; PDIPA-二硼;方案1），其被证明是在α，β-不饱和底物的铜催化的β硼化中高度有用的sp2-sp3二硼试剂。[9]与最近公开的衍生物1和2一起，很明显，少数已知的sp2-sp3乙硼烷仅衍生自氧代和硫基乙硼烷（4）前体。我们对反应性更强的卤素取代的二硼烷的配位化学产生了兴趣（4...
Modern organic chemistry and the production of high-value chemicals strongly rely on the possibility to incorporate a manifold of different functionalities into the organic framework. In this regard, the boryl group has been established a long time ago, combining versatile accessibility with a unique chemical diversity. Valuable secondary products include for example alkenes, alcohols, and ketones. Of equal importance is their outstanding significance as reagents in Suzuki–Miyaura-type coupling reactions. Various synthetic approaches, such as salt elimination, hydroboration, or diboration reactions, have been developed to enable a convenient introduction of the boryl group. In particular, the diboration has been of great interest in facilitating the simultaneous generation of two reactive sites from commercially available diborane (4) reagents, that is, B2cat2 (cat= catecholato) and B2pin2 (pin= pinacolato).[1] One major drawback of this approach is the requirement of transition-metal catalysis in almost all cases, except for highly reactive diborane (4) derivatives.[2] The first example of such diboration reactivity was published back in 1954 for B2Cl4, which readily adds to the double bond of ethylene in the absence of any additive.[2a] However, catalytic processes are by far predominant, for which reason current research focuses on alternative metal-free systems. Hoveyda et al. recently reported the efficient metal-free β-boration of α, β-unsaturated ketones promoted by an N-heterocyclic carbene (NHC).[3] A neutral 1: 1 NHC adduct of B2pin2 (1; Scheme 1) was proposed as the catalytically active species, even though its exact structure remained unknown at that time. Nevertheless, these findings stimulated research in this area and other metal-free systems for the borylation of organic substrates have been developed based on both neutral and anionic sp2–sp3 diboron compounds.[4] As a result, more detailed information on the actual composition of the organic catalyst is now available, which clearly emphasizes the high relevance of sp2–sp3 diboranes as intermediates in both transition-metal [5] and organocatalyzed [4] borylation reactions. Elegant spectroscopic and theoretical studies by Marder et al. eventually verified the existence of the neutral NHC adduct 1 both in solution and in the solid state.[6] Furthermore, Kleeberg etal. recently succeeded in the isolation of related anionic species B2pin2· XÀ(2; XÀ= OMeÀ, OtBuÀ, 4-tBu-C6H4OÀ, FÀ; Scheme 1), which showed a high potential in the metal-free borylation of organic electrophiles.[7]With these fundamental developments in mind, it appears rather surprising that simple 1: 1 adducts of diboranes (4) with Lewis bases have been neglected for a long time. Accordingly, the number of fully characterized sp2–sp3 diboron compounds is still comparatively small. Initially, Marder and Norman studied the reactivity of B2cat2 and B2 (1, 2-S2C6H4) towards nitrogen and phosphorus donors to afford the corresponding mono-and bis-adducts B2 (1, 2-E2C6H4)· L (3; E= O, S; L= 4-picoline, PMe2Ph, PEt3; Scheme1) and B2 (1, 2-E2C6H4)· L2 (4).[8] Later on, intramolecular Lewis base coordination was established for pinacolato (diisopropanolaminato) diboron (5; PDIPA-diboron; Scheme 1), which was shown to be a highly useful sp2–sp3 diboron reagent in the copper-catalyzed βboration of α, β-unsaturated substrates.[9] Together with the more recently published derivatives 1 and 2, it becomes evident that the few known sp2–sp3 diboranes are exclusively derived from oxo-and sulfurbased diborane (4) precursors. We became interested in the coordination chemistry of more reactive halide-substituted diboranes (4 …