Regioselective Alkyne Cyclotrimerization with an In Situ-Generated [Fe(II)H(salen)]·Bpin Catalyst

A mild, efficient, and regiospecific catalytic cyclotrimerization of alkynes to form 1,2,4-substituted arenes has been discovered. From a cheap and air-stable [Fe­(salen)]2-μ-oxo complex and readily available pinacol borane (HBpin), a monomeric [FeH­(salen)]·Bpin species formed in situ acts as the active catalyst. This species is shown to feature a hemilabile salen ligand stabilized via interactions with the boron entity. The formation, identity, and reaction mechanism of the active species are supported by complementary kinetic, spectroscopic, and computational data. The active catalyst undergoes hydrometallation of a coordinated alkyne to form a vinyl iron species, stepwise additions of two more alkynes across the Fe–C bond to form a pendant triene, which upon ring-closure forms the arene product. The catalytic cycle is closed by substitution of the product with the alkyne substrate. With the active [FeH­(salen)]·Bpin catalyst, atom-efficient, intermolecular trimerization is shown with high regioselectivity for a diverse range of substrate substitution patterns and presence of functional groups.

本研究发现了一种温和、高效且具有区域专一性的炔烃催化环三聚反应，可生成1,2,4-取代芳烃(1,2,4-substituted arenes)。以廉价且空气稳定的[Fe(salen)]₂-μ-氧络合物([Fe(salen)]₂-μ-oxo complex)与易得的频哪醇硼烷(HBpin)为原料，原位生成的单核[FeH(salen)]·Bpin物种即为活性催化剂。该物种的特征在于存在通过硼实体相互作用稳定的半稳定salen配体(hemilabile salen ligand)。活性物种的生成、结构身份及反应机理得到了动力学、光谱学与计算数据的互补佐证。活性催化剂先对配位炔烃进行氢金属化(hydrometallation)，生成乙烯基铁物种(vinyl iron species)；随后在Fe-C键上逐步加成另外两分子炔烃，形成悬挂式三烯(pendant triene)，该中间体经环闭合反应生成芳烃产物。催化循环通过产物与炔烃底物的取代反应完成。采用该[FeH(salen)]·Bpin活性催化剂，可实现原子经济性的分子间三聚反应，对多种取代模式的底物及含官能团的底物均表现出优异的区域选择性。