Selective Double Carbomagnesiation of Internal Alkynes Catalyzed by Iron-N-Heterocyclic Carbene Complexes: A Convenient Method to Highly Substituted 1,3-Dienyl Magnesium Reagents

Controlled
multicarbometalation of alkynes has been envisaged as
an efficient synthetic method for dienyl and polyenyl metal reagents,
but an effective catalyst enabling the transformation has remained
elusive. Herein, we report that an iron­(II)-N-heterocyclic carbene
(NHC) complex (IEt2Me2)2FeCl2 (IEt2Me2 = 1,3-diethyl-4,5-dimethylimidazol-2-ylidene)
can serve as a precatalyst for the double carbometalation of internal
unsymmetrical alkynes with alkyl Grignard reagents, producing highly
substituted 1,3-dienyl magnesium reagents with high regio- and stereoselectivity.
Mechanistic studies suggest the involvement of low-coordinate organoiron­(II)-NHC
species as the in-cycle intermediates. The strong σ-donating
nature of IEt2Me2 and its appropriate steric
property are thought the key factors endowing the iron-NHC catalyst
fine performance.

控制多碳金属化反应被构想为合成二烯基和聚烯基金属试剂的高效合成方法，然而，一种能够实现这一转化的有效催化剂一直难以寻觅。本研究中，我们报道了一种铁(II)-N-杂环卡宾(NHC)配合物(IEt2Me2)2FeCl2（IEt2Me2 = 1,3-二乙基-4,5-二甲基咪唑-2-基）可作为内不对称炔烃与烷基格氏试剂双重碳金属化反应的预催化剂，从而生成高度取代的1,3-二烯基镁试剂，具有高区域选择性和立体选择性。机理研究表明，低配位有机铁(II)-NHC物种作为循环中的中间体。IEt2Me2的强σ供电子性质及其适宜的立体性质被认为是赋予铁-NHC催化剂优异性能的关键因素。