Synthesis of Complex Boron–Nitrogen Heterocycles Comprising Borylated Triazenes and Tetrazenes Under Mild Conditions

The reactions of organic azides with diaryl­(dihalo)­diboranes(4) were explored, resulting in the observation of a number of surprising reactivity patterns. The reaction of phenyl azide with 1,2-diaryl-1,2-dihalodiboranes(4) resulted in the formation of five-membered rings comprising diboryl-triazenes with retention of the boron–boron bond, while the reaction of the peculiar 1,1-di­(9-anthryl)-2,2-difluorodiborane(4) with phenyl azide yielded a six-membered ring bearing a diboryl-triazene, whereby the B–B bond was ruptured by the insertion of an arylnitrene-like reactive intermediate. Both types of heterocycles feature unprecedented connectivity patterns and are very rare examples of boryl triazenes beyond the more common 1,2,3-triazolatoboranes. They are also the product of a unique type of aryl migration from a boron center to the phenyl azide γ-nitrogen center. Lastly, the substitution of 1,2-diaryl-1,2-dihalodiboranes(4) with azide groups, using trimethylsilyl azide as the transfer reagent, yielded boryl-tetrazaboroles and diboryldiazadiboretidines (as side-products), invoking the intermediacy of the first N-boryl-substituted iminoboranes, which are BN isosteres of monoborylated alkynes. The synthetic results are complemented with mechanistic proposals derived from quantum-chemical calculations.本研究探索了有机叠氮化物(organic azides)与二芳基(二卤)乙硼烷(4)之间的反应，观察到多种出乎意料的反应模式。叠氮苯(phenyl azide)与1,2-二芳基-1,2-二卤代乙硼烷(4)反应后，生成了含二硼基三氮烯的五元环，且硼-硼键得以保留；而特殊的1,1-二(9-蒽基)-2,2-二氟乙硼烷(4)与叠氮苯反应则得到了含二硼基三氮烯的六元环，此时B-B键因类芳基氮宾活性中间体(arylnitrene-like reactive intermediate)的插入而发生断裂。这两类杂环均具有前所未有的连接方式，是相较于更常见的1,2,3-三唑基硼烷(1,2,3-triazolatoboranes)而言极为罕见的二硼基三氮烯实例。它们同时还是芳基从硼中心迁移至叠氮苯γ-氮中心这一独特反应类型的产物。最后，以叠氮三甲基硅烷(trimethylsilyl azide)作为转移试剂，将1,2-二芳基-1,2-二卤代乙硼烷(4)中的卤素替换为叠氮基团，得到了二硼基四氮硼杂环戊烷(boryl-tetrazaboroles)与二硼基二氮杂二硼环丁烷(diboryldiazadiboretidines，作为副产物)，该过程涉及首例N-硼基取代的亚胺硼烷(iminoboranes)中间体——其为单硼基取代炔烃的BN等排体。本研究通过量子化学计算(quantum-chemical calculations)得到的机理推测，对上述合成结果进行了补充。