Department of Chemistry, University of Minnesota - Twin Cities, 207 Pleasant St SE, Minneapolis, Minnesota 55455, USA.
Nat Chem. 2016 Jan;8(1):63-8. doi: 10.1038/nchem.2386. Epub 2015 Nov 2.
Pyrroles are structurally important heterocycles. However, the synthesis of polysubstituted pyrroles is often challenging. Here, we report a multicomponent, Ti-catalysed formal [2+2+1] reaction of alkynes and diazenes for the oxidative synthesis of penta- and trisubstituted pyrroles: a nitrenoid analogue to classical Pauson-Khand-type syntheses of cyclopentenones. Given the scarcity of early transition-metal redox catalysis, preliminary mechanistic studies are presented. Initial stoichiometric and kinetic studies indicate that the mechanism of this reaction proceeds through a formally Ti(II)/Ti(IV) redox catalytic cycle, in which an azatitanacyclobutene intermediate, resulting from [2+2] alkyne + Ti imido coupling, undergoes a second alkyne insertion followed by reductive elimination to yield pyrrole and a Ti(II) species. The key component for catalytic turnover is the reoxidation of the Ti(II) species to a Ti(IV) imido via the disproportionation of an η(2)-diazene-Ti(II) complex.
吡咯是结构上重要的杂环。然而，多取代吡咯的合成通常具有挑战性。在这里，我们报告了炔烃和重氮化合物的 Ti 催化多组分、形式 [2+2+1] 反应，用于氧化合成五元和三元取代的吡咯：氮宾类似物到经典的 Pauson-Khand 型环戊烯酮合成。鉴于早期过渡金属氧化还原催化的稀缺性，提出了初步的机理研究。初步的计量和动力学研究表明，该反应的机理通过一个形式上的 Ti(II)/Ti(IV)氧化还原催化循环进行，其中来自 [2+2] 炔烃+Ti 亚氨基偶联的氮杂钛环丁烯中间体经历第二次炔烃插入，然后进行还原消除，生成吡咯和 Ti(II)物种。催化周转的关键组分是通过η(2)-重氮-Ti(II)配合物的歧化将 Ti(II)物种氧化再生成 Ti(IV)亚氨基。
