Reactions of iron carbenes with α,ß-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,ß-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.