Cyclopropanation of unactivated alkenes with non-stabilized iron carbenes

Cyclopropanes are ubiquitous in medicines, yet robust synthetic access to a wide range of sterically and electronically diverse analogs remains a challenge. To address the synthetic limitations of the most direct strategy, (2+1) cycloaddition, we sought to develop a variant that employs non-stabilized carbenes. We present herein an FeCl₂-catalyzed cyclopropanation that uniquely employs aliphatic (enolizable) aldehydes as carbene precursors. A remarkably broad range of alkenes may be coupled with these non-stabilized, alkyl carbenes. This extensive scope enables the synthesis of novel classes of cyclopropanes bearing alkyl, benzyl, allyl, halide, and heteroatom substituents, as well as spirocyclic and fused bicycles. Over 40 examples illustrate the broad generality, efficiency, selectivity, functional group tolerance, and practical utility of this approach. Mechanistic insights, gathered from stereochemical probes and competition experiments, are included to reveal the applicability of this non-stabilized carbene route for novel cyclopropane synthesis.