An Improved Catalyst System for the Iron‐Catalyzed Intermolecular Ring‐Expansion Reactions of Epoxides

A considerable improvement is reported in the iron-catalyzed ring-expansion reactions of epoxides generating tetrahydrofuran derivatives by formal insertion of an alkene. Optimization of the catalyst system revealed that a preformed [Fe(salen)] complex minimizes the formation of polymerization side-products so that increased yields of intermolecular reactions were obtained. However, more importantly, the scope of the reaction could also be enlarged considerably. The iron-catalyzed ring-expansion reaction can now be applied to some styrene oxide derivatives, acting as radical donors, as well as to a wide variety of acceptor-substituted acyclic alkenes and cyclic dienes that act as radical acceptors. The use of unsymmetrical radical acceptors led to interesting questions concerning the regiochemistry of the reactions. The conservation of the stereochemistry of the starting materials in the products was investigated through a study of the reactions of E- and Z-configured acceptor-substituted double bonds. The reactions of fumaric and maleic esters were performed and the ratios of diastereomeric and regioisomeric products were determined.