A Boratafulvene

• Published in: Angewandte Chemie International Edition 2021-09, Vol.60 (36), p.20055-20060
• Main Authors: Heitkemper, Tobias, Naß, Leonard, Sindlinger, Christian P.
• Format: Article
• Language: English
• Subjects: Aromaticity; borane; borata fulvene; boroles; Boron; C-H acidity; Electronic structure; main group chemistry; Reagents; Substitution reactions
• ISSN: 1433-7851; 1521-3773
• DOI: 10.1002/anie.202107968

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 pKa value of the exocyclic B−CH3 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 Me3SnCl 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. The first boratafulvene was accessed by deprotonation of a B‐methyl borole. The boratafulvene is a new C‐nucleophilic synthon for substituted boroles and reacts with carbonyls in a bora‐Wittig‐type reaction transferring the methylene group.