Room-Temperature C-C Bond Cleavage of an Arene by a Metallacarborane
Room-Temperature C-C Bond Cleavage of an Arene by a Metallacarborane
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DOI：
10.1002/anie.201001555
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发表时间：
2010-01-01
影响因子：
16.6
通讯作者：
Welch, Alan J.
中科院分区：
文献类型：
--
作者：
Ellis, David;McKay, David;Welch, Alan J.
通过过渡金属物种活化和裂解C3 C键是当前强烈关注的领域，[1]并且，尽管它们仍然相对罕见，但已知许多提供C3 C单键断裂的系统。[2-4]相比之下，芳族C12 C键的裂解被认为是非常困难的。六碳芳香环可以在高温下在气相中裂解[5]，而在不太极端的条件下，这些环被酶裂解，这是全球碳循环的重要组成部分。[6]然而，关于非生物体系中芳环低温裂解反应的报道却很少。在已知的例子中，（仿生）氧化裂解是更常见的过程，但通常被认为是难以控制的，[7]而还原裂解则更为罕见。[8]然而，氧化和还原裂解反应通常涉及芳环的显著初始化学修饰。Sattler和Parkin最近描述了芳香杂环（喹喔啉）在908 ℃下的C12 C键断裂。[9]我们在此报道了在室温下由金属碳硼烷在没有对芳烃进行其他化学修饰的情况下对简单芳烃中的芳香族C12 C键进行前所未有的裂解。1，1 '-二的处理（邻碳硼烷），[10-11]（1'，2'-closo-C2B10H11）-2-closo-C2B10H11（图1），在萘的存在下，在THF中加入过量的Li，随后与[{Ru（对伞花烃）Cl 2} 2]（对伞花烃= 1-iPr，4-MeC 6 H4），得到暗红色金属碳硼烷1-（1 '，2'-closo-C2 B10 H11）-4-{C10 H14 Ru（p-cymene）}-4，1，6-closo-RuC 2B 10 H11作为唯一可分离的产物（约为1.5 g）。20%产率）。通过质谱、1H和11B {1H} NMR光谱以及最终通过单晶X射线衍射表征产物。[12]在1H光谱中，除了在大约d= 4.7和2.2 ppm处的宽CcageH共振外，还存在归属于不对称络合物中η6-对伞花烃配体的CH 3C 6 H4 CHMe 2质子的正常共振（d= 6.5和5.5 ppm之间的四个dd，具有约3 J和4J偶联。6 Hz和1.5 Hz）。然而，通常归属于CH 3C 6 H4 CHMe 2、CH 3C 6 H4 CHMe 2和CH 3C 6 H4 CHMe 2质子的信号都显示为加倍。此外，还有两个高频双峰共振（d= 9.6，9.4 ppm）和两个额外的共振（d= 4.5，4.1 ppm），它们表现为明显的三重峰。总的来说，这些数据表明产物中存在两种不同的C10 H14单元;一种是常规的η6-对伞花烃配体，但另一种似乎经历了主要的结构变化。11B {1H}光谱相对没有信息，在d= 6和1025 ppm之间有9个共振，包括d= 1010处的多重信号。6ppm，占10个硼原子。质谱证实分子式为C25 H50 B20 Ru 2（包络线以m/z 757为中心），这意味着双（碳硼烷）加上两个{RuC 10 H14}单元，但是，如从NMR光谱中明显的，该分子是不对称的，并且对伞花烃配体之一已经被显著改变。晶体学研究产生了图2所示的分子结构。图3显示了分子中心部分的另一个视图。
The activation and cleavage of CÀC bonds by transition-metal species is an area of intense current interest,[1] and, although they are still relatively rare, a number of systems that afford the breaking of CÀC single bonds are known.[2–4] In contrast, the cleavage of aromatic CÀC bonds is considered to be extraordinarily difficult. Six-carbon aromatic rings can be cleaved in the gas phase at high temperatures,[5] whereas under less extreme conditions such rings are cleaved by enzymes, an important part of the global carbon cycle.[6] However, there are very few reports of low-temperature cleavage reactions of aromatic rings in nonbiological systems. Of the examples that are known,(biomimetic) oxidative cleavage is the more common process but is generally regarded as difficult to control,[7] whereas reductive cleavage is much more rare.[8] However, both oxidative and reductive cleavage reactions typically involve significant initial chemical modification of the aromatic ring. Sattler and Parkin recently described the cleavage of a CÀC bond in an aromatic heterocycle (a quinoxaline) at 908C.[9] We report herein the unprecedented cleavage of an aromatic CÀC bond in a simple arene at room temperature by a metallacarborane without other chemical modification to the arene. Treatment of 1, 1’-bis (o-carborane),[10–11] 1-(1’, 2’-closo-C2B10H11)-2-closo-C2B10H11 (Figure 1), with an excess of Li in THF in the presence of naphthalene and subsequent reaction with [{Ru (p-cymene) Cl2} 2](p-cymene= 1-iPr, 4-MeC6H4), affords the dark red metallacarborane 1-(1’, 2’-closo-C2B10H11)-4-{C10H14Ru (p-cymene)}-4, 1, 6-closo-RuC2B10H11 as the only isolable product (in ca. 20% yield) after workup (involves TLC methods). The product was characterized by mass spectroscopy, 1H and 11B {1H} NMR spectroscopy, and ultimately by single-crystal X-ray diffraction.[12]In the 1H spectrum there are, in addition to broad CcageH resonances at approximately d= 4.7 and 2.2 ppm, the normal resonances assigned to the CH3C6H4CHMe2 protons of a η6-pcymene ligand in an asymmetric complex (four dd between d= 6.5 and 5.5 ppm with 3J and 4J couplings of ca. 6 Hz and 1.5 Hz, respectively). However, the signals normally assigned to the CH3C6H4CHMe2, CH3C6H4CHMe2, and CH3C6H4CHMe2 protons all appear doubled. In addition there are two high-frequency doublet resonances (d= 9.6, 9.4 ppm) and two additional resonances (d= 4.5, 4.1 ppm) which appear as apparent triplets. Collectively these data suggest that there are two different C10H14 units in the product; one is a regular η6-p-cymene ligand but the other appears to have been subjected to a major structural change. The 11B {1H} spectrum is relatively uninformative with nine resonances between d= 6 and À25 ppm, including a multiple signal at d= À10. 6 ppm that accounts for ten boron atoms. The mass spectrum confirms the molecular formula as C25H50B20Ru2 (envelope centered on m/z 757) which implies bis (carborane) plus two {RuC10H14} units, but, as is evident from the NMR spectra, the molecule is asymmetric and one of the p-cymene ligands has been substantially altered. A crystallographic study resulted in the molecular structure shown in Figure 2. Figure 3 shows an alternative view of the central part of the molecule.