Harnessing carbene polarity: Unified catalytic access to donor, neutral, and acceptor carbenes

, halide, aryl, heteroaryl). This Fe-catalyzed method harnesses α-Cl radicals and couples an exceptionally wide array of carbenes in (2+1) cyclopropanations and σ-bond insertions. This mild, robust, and electronically tunable synthetic method facilitated the development of a better classification system for catalytic metal carbenes (validated by both kinetic and thermodynamic quantification), as well as a carbene "click-like" reaction and aqueous adaptation for chemical biology applications.