Controllable and Diverse Migrations of Tetracoordinate Borons: Beyond 1,2-Shift

Conspectus

The chemistry of tetracoordinate borons has long been dominated by the classic 1,2-metalate shift, a powerful yet inherently predictable rearrangement where the migrating aptitude is dictated by the electronic bias of the substituents. Breaking free from this paradigm to achieve controllable and diverse migration─beyond 1,2-shift─remains a formidable challenge. In this Account, we describe our systematic efforts to unlock three previously inaccessible dimensions of tetracoordinate boron reactivity: ionic remote migrations, radical remote migrations, and programmable sequential multiple migrations on a single boron center. Across these three manifolds, the feasibility of migration is governed by three common factors: migration distance is constrained by the size of the cyclic transition state (rings of 5–7 members are viable); the aptitude of a migrating group depends on its ability to stabilize developing charge in ionic pathways or radical character in radical pathways; and conformational preorganization is essential for ionic remote migrations, while radical variants are more tolerant due to the extended lifetime of the radical intermediate. Distilled from systematic mechanistic studies, these principles establish a predictive framework for the design of new migrations.

Our journey began with the realization that the conventional 1,2-shift is not the only option. By carefully designing the tetracoordinate boron frameworks and the external stimulus, we can direct a substituent to migrate over a longer distance. In the ionic manifold, we developed remote 1,n-metalate shifts (n ≥ 3) in which heteroatoms (hydroxyl, bromine, hydride) and carbon substituents migrate over 1,3-, 1,4-, and even 1,7-distances, enabling stereospecific ketoxime synthesis, enantioselective construction of axially chiral alkenes, skeletal editing of N-heterocycles, a Passerini-type multicomponent reaction, enolate-driven C(sp2)–C(sp3) cross-coupling, and asymmetric indole reduction─all with a level of control unattainable by classical methods. Second, we turned to a completely orthogonal activation mode: radical-induced remote migrations. Under visible-light photoredox conditions, selective cleavage of a C(sp3)–B bond in tetracoordinate boron species generates an alkyl radical that undergoes unprecedented long-distance migration (1,4-, 1,5-, and the first 1,6-radical shift), with the incorporation of both radical and boron moiety into the products with complete atom economy─solving a long-standing problem in photoredox deboronations of alkyl tetracoordinate borons with boron species discarded as waste. Third, we explored the untapped potential of sequential multiple migrations on a single tetracoordinate boron atom. By judicious selection of inducing reagents─identical or distinct electrophiles, bifunctional reagents, or transition-metal catalysts─two or even three successive 1,2- and 1,3-migrations can be programmed in a controlled order, transforming simple boronates into complex, densely functionalized architectures (e.g., tetrasubstituted alkenyl halides, benzothiophenes, polysubstituted alkenes, deuterated allylboronates, 1,1-diarylalkanes, and diarylmethylamines) in a single operation.

Collectively, these discoveries demonstrate that tetracoordinate borons are not merely reactive intermediates confined to the classic 1,2-shift, but rather represent a versatile migration platform whose reactivity─distance, mechanism, and sequence─can be rationally controlled. Mechanistic insights from DFT calculations and kinetic studies rationalize the observed migratory aptitudes and offer a predictive framework for designing future migrations. Our work opens a new chapter in organoboron chemistry, where controllable and diverse migration becomes a synthetic reality.