A Boratafulvene

Structurally authenticated free B‐alkyl boroles are presented and electronic implications of alkyl substitution were assessed. Deprotonation of a boron‐bound exocyclic methyl group in a B‐methyl borole yields the first 5‐boratafulvene anion—an isomer to boratabenzene. Boratafulvene was structurally characterized and its electronic structure probed by DFT calculations. The p K a value of the exocyclic B−CH 3 in a set of boroles was computationally approximated and confirmed a pronounced acidic character caused by the boron atom embedded in an anti‐aromatic moiety. The non‐aromatic boratafulvene reacts as a C‐centered nucleophile with the mild electrophile Me 3 SnCl to give a stannylmethyl borole, regenerating the anti‐aromaticity. As nucleophilic synthons for boroles, boratafulvenes thus open an entirely new avenue for synthetic strategies toward this highly reactive class of heterocycles. Boratafulvene reacts as a methylene transfer reagent in a bora‐Wittig‐type reaction generating a borole oxide.