Reactions of Iron Carbenes with α,β‐Unsaturated Esters by Using an Umpolung Approach: Mechanism and Applications

An Umpolung approach, in which a phosphorus ylide moiety was introduced to increase the electron density of the double bond, was developed to activate electron‐deficient alkenes for reaction with electrophilic iron carbenes. In tandem with the Wittig reaction, the reactions of α,β‐unsaturated esters with in situ generated Fe? carbene complexes delivered formal C? H insertion products through cyclopropanation/ring‐opening reactions. DFT calculations and cross‐experiments indicate that, in this process, the ring opening of the cyclopropylmethyl ylide intermediate is rapid and reversible and the subsequent proton transfer is the rate‐determining step. Further studies revealed that, based on the choice of the ylide and ester groups, as well as the base, the reaction could be steered towards either the ring‐opening pathway or to the production of vinyl cyclopropanes.