Su Xiaojun, Baker J J, Martin Caleb D
Department of Chemistry and Biochemistry , Baylor University , One Bear Place #97348 , Waco , TX 76798 , USA . Email:
Chem Sci. 2019 Oct 29;11(1):126-131. doi: 10.1039/c9sc04053f. eCollection 2020 Jan 7.
Monomeric boroles have been gaining attention as reagents for the synthesis of heterocycles due to their ability to insert atoms into the BC ring in a single step. Although unique boron frameworks can be accessed this methodology, the products feature aryl substitution on the carbon centers as steric bulk is required to preclude borole dimerization. This work demonstrates that insertion chemistry is possible with Diels-Alder dimeric boroles and that such reactivity is not exclusive to monomeric boroles with bulky groups. With 1-phenyl-2,3,4,5-tetramethylborole dimer, the formal 1,1-insertion of a nitrene and sulfur generate the six-membered aromatic 1,2-azaborine and 1,2-thiaborine, respectively. The isolation of the 1,2-thiaborine enabled the synthesis of an η-chromium complex. Benzophenone and diphenylketene readily insert a CO unit to generate BOC seven-membered rings confirming dimeric boroles can serve as monomeric synthons in 1,2-insertion reactions. An epoxide did not furnish the anticipated eight-membered BOC ring, instead provided a bicyclic system with a BOC ring. The insertion chemistry was demonstrated with two other borole dimers featuring different substitution with diphenylketene as a substrate. This work elevates borole insertion chemistry to a new level to access products that do not require bulky substitution.
由于单体硼杂环戊二烯能够一步将原子插入BC环中，因此作为合成杂环的试剂受到了关注。尽管通过这种方法可以获得独特的硼骨架，但由于需要空间位阻来防止硼杂环戊二烯二聚，产物在碳中心上具有芳基取代。这项工作表明，狄尔斯-阿尔德二聚体硼杂环戊二烯也可以进行插入反应，而且这种反应性并非具有庞大基团的单体硼杂环戊二烯所独有。对于1-苯基-2,3,4,5-四甲基硼杂环戊二烯二聚体，氮烯和硫的形式上的1,1-插入分别生成了六元芳香族1,2-氮硼环和1,2-硫硼环。1,2-硫硼环的分离使得能够合成一种η-铬配合物。二苯甲酮和二苯基乙烯酮很容易插入一个CO单元以生成七元BOC环，这证实了二聚体硼杂环戊二烯可以在1,2-插入反应中作为单体合成子。一种环氧化物没有生成预期的八元BOC环，而是提供了一个带有BOC环的双环体系。以二苯基乙烯酮为底物，用另外两种具有不同取代基的硼杂环戊二烯二聚体证明了插入反应。这项工作将硼杂环戊二烯插入反应提升到了一个新的水平，以获得不需要庞大取代基的产物。
