Novel intramolecular cyclocarbonylations involving pi-alkene-hydridoiron intermediates: From [eta(5)-C5H5(CO)(2)Fe]-substituted (Z)-enals to alpha,beta-butenolides and gamma-butyrolactones

In order to broaden the application of [eta(5)-C5H5(CO)(2)Fe]-substituted e nals, reaction cascades were developed for the construction of five-membere d lactone skeletons, initiated by the regioselective reduction of the aldeh yde functionality with sodium borohydride or K-Selectride. Depending on the iron compound and the reagent employed, alpha,beta-butenolides or gamma-la ctones were obtained. The key steps in the reaction cascade for the formati on of alpha,beta-butenolides involve carbonylation and reductive eliminatio n. Labeling experiments, which were carried out to provide mechanistic deta ils of the subsequent gamma-lactone formation, are in agreement with a redu ction step which involves a pi-alkene hydridoiron intermediate. Proposed re action pathways are given.