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. Synthesis of dienic scaffolds by Fe‐mediated coupling between Grignard reagents and dienyl bromides is reported. This methodology involves the use of magnesium alkoxides as additives, and can be applied to targets of industrial interest, such as insect pheromones. Mechanistic studies suggest that the active species is an organoiron(II) complex, showing that no reduction of the precursor to low oxidation states is required.