Iron(III)-Catalyzed Enantioselective Synthesis of Allenes

Significance

Olefination of ketenes with ethyl diazoacetate using an Fe(III) catalyst and triphenyl­phosphine provides an efficient synthesis of 4-mono­substituted and 4,4-disubstituted allenic esters. Only 0.5 mol% of Fe(TCP)Cl is required and the reaction conditions are mild. The phosphine ligand can also be recovered and reused. The scope of the reaction is broad and encompasses a variety of substituents on the ketene, including H, Br, alkyl and aryl groups. A one-pot strategy has also been developed for unstable ketenes by synthesizing the ketene in situ. An asymmetric version is possible by using a chiral phosphine ligand, and affords chiral allenes in good yields and excellent enantioselectivities.