H(+), CH(3)(+), and R(3)Si(+) carborane reagents: when triflates fail.
H(+), CH(3)(+), and R(3)Si(+) carborane reagents: when triflates fail.
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DOI：
10.1021/ar900159e
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发表时间：
2010-01-19
影响因子：
18.3
通讯作者：
Reed CA
中科院分区：
文献类型：
--
作者：
Reed CA
几十年来，三氟甲酸、三氟甲酯和三烷基三氟硅烷试剂分别作为H+、CH3+和R3Si+的清洁亲电源，在合成化学中起到了很好的作用。然而，一些弱碱性底物对这些试剂不起反应。此外，三氟阴离子可能对亲电活化的底物表现出不想要的亲核性。在这项研究中，我们描述了用碳硼烷试剂取代三氟化钠类亲电试剂的方法。使用CHB11R5X6型碳硼烷阴离子−(R=H，Me，X；X=Br，Cl)-一类显著惰性、弱亲核阴离子的成员-而不是三氟化钠，显著增加了这些试剂的亲电性，并关闭了随后的阴离子亲核化学。因此，H(碳硼烷)酸可以清洁地质子化苯、磷苯、C60等，而三氟酸则不能。类似地，CH3(碳硼烷)试剂可以甲基化对沸腾的纯净三氟化甲酯具有惰性的底物，包括苯、磷苯、磷腈和形成双正离子乙烷类似物Me6N22+的五甲基肼离子。甲基碳硼烷在从简单烷烃中提取氢化物以生成可分离的碳正离子盐(如叔丁基阳离子)方面也令人惊讶地有效。三烷基硅基碳硼烷试剂R3Si(Carborane)从底物中提取卤化物，产生具有前所未有的反应活性的阳离子。例如，从氟里昂中提取氟化物以形成碳正离子，从IrCl(CO)(PPh3)2中提取氯以形成配位不饱和Ir阳离子，该阳离子在室温下与氯苯进行氧化加成，以及环-N3P3Cl6的硅基化反应产生用于磷腈聚合的催化剂，该催化剂在室温下起作用。尽管目前碳硼烷试剂的价格太高而不能广泛使用，但作为制备活性阳离子和催化剂的专用试剂，它仍然引起了人们的极大兴趣。
For decades, triflic acid, methyl triflate and trialkylsilyl triflate reagents have served synthetic chemistry well as clean, strong electrophilic sources of H+, CH3+ and R3Si+ respectively. However, a number of weakly basic substrates are unreactive towards these reagents. In addition, triflate anion can express undesired nucleophilicity towards electrophilically activated substrates. In this Account, we describe methods that replace triflate-based electrophilic reagents with carborane reagents. Using carborane anions of type CHB11R5X6− (R = H, Me, X; X = Br, Cl) – members of a class of notably inert, weakly nucleophilic anions – instead of triflate significantly increases the electrophilicity of these reagents and shuts down subsequent nucleophilic chemistry of the anion. Thus, H(carborane) acids cleanly protonate benzene, phosphabenzene, C60 etc. while triflic acid does not. Similarly, CH3(carborane) reagents can methylate substrates that are inert to boiling neat methyl triflate, including benzene, phosphabenzenes, phosphazenes, and the pentamethylhydrazinium ion which forms the dipositive ethane analogue, Me6N22+. Methyl carboranes are also surprisingly effective in abstracting hydride from simple alkanes to give isolable carbocation salts e.g. t-butyl cation. Trialkylsilyl carborane reagents, R3Si(carborane), abstract halides from substrates to produce cations of unprecedented reactivity. For example, fluoride is extracted from freons to form carbocations, chloride is extracted from IrCl(CO)(PPh3)2 to form a coordinatively-unsaturated iridium cation that undergoes oxidative addition with chlorobenzene at room temperature, and silylation of cyclo-N3P3Cl6 produces a catalyst for the polymerization of phosphazenes that functions at room temperature. Although currently too expensive for widespread use, carborane reagents are nevertheless of considerable interest as specialty reagents for making reactive cations and catalysts.
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