The joint selectivity challenges presented by the asymmetric one-carbon ring expansion of aziridines into azetidines requires a potential catalyst not only to select for the [1,2]-Stevens rearrangement in preference to cheletropic extrusion of olefins, but also to exert enantiocontrol over potential radical intermediates. Nature utilizes ring-size manipulation in the biosynthesis of natural products, with common strategies for biocatalytic one-carbon ring expansion including oxidative ring expansions 17 and carbocation rearrangements. Furthermore, enzymes derived from cytochrome P450BM3, such as cytochromes P411, and other hemoproteins have emerged as powerful catalysts for carbene transfer reactions, and formation of strained rings such as cyclopropanes and cyclopropenes with excellent stereoselectivities has been reported. The most common reactions of enzymatic iron-carbenoid intermediates are additions across π-systems or X-H bond insertions: biocatalytic C-N bond insertion through Stevens rearrangements of any kind have yet to be reported. We envisioned that a carbene transfer enzyme could potentially achieve the requisite chemo-and stereoselection necessary to perform this challenging reaction (Figure ).