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.