Stereoselective Cross-Coupling of Grignard Reagents and Conjugated Dienylbromides using Iron Salts with Magnesium Alkoxides

A convenient procedure allowing iron-catalyzed cross-coupling of alkyl or aryl Grignard reagents and conjugated dienyl bromides is reported, relying on the use of cheap and non-toxic magnesium alkoxides as sole additives. An excellent stereoselectivity is observed in the alkyl-dienyl series. This sequence has been applied to the synthesis of the sex pheromone of codling moth, illustrating its applicability for obtaining targets of industrial interest. Preliminary mechanistic studies carried out on the aryl-dienyl cross-coupling suggest that in situ generated ate homoleptic organoiron(II) species act as catalytically relevant intermediates. A modified preparative method for the realization of THF solutions of dienyl bromides as ready-to-use coupling partners is also discussed, circumventing the thermal instability of those derivatives.

Automated summary

A convenient procedure for iron-catalyzed cross-coupling of alkyl or aryl Grignard reagents and conjugated dienyl bromides has been reported. The use of cheap and non-toxic magnesium alkoxides as sole additives led to excellent stereoselectivity in the alkyl-dienyl series. Preliminary mechanistic studies suggest in situ generated ate homoleptic organoiron(II) species act as catalytically relevant intermediates.