Construction of a Diverse Range of Boron Heterocycles via Ring Expansion of a Carboranyl-Substituted 9-Borafluorene.

Direct insertion of unsaturated substrates into a five-membered borole ring is a useful method to obtain valuable heterocycles containing one or more three-coordinate boron atoms. A highly Lewis acidic 9-o-carboranyl-9-borafluorene, in which the o-carboranyl substituent is connected via one of the cluster carbon atoms to the boron atom of the 9-borafluorene unit, was found to react with a vast array of unsaturated molecules, such as alkynes, aldehydes and various organic azides, to form larger boraheterocyclic products. The ring expansion reactions of the central borole ring proceed rapidly at room temperature, cementing the role of the o-carboranyl substituent in enhancing the insertion reactivity of 9-borafluorenes.

Alkene insertion reactivity of a -carboranyl-substituted 9-borafluorene.

The synthesis of 9-borafluorene with an electron-withdrawing -carboranyl substituent and its reactions with a series of alkenes are described. The -carboranyl substituent is bonded one of the cluster carbon atoms to the boron atom of the 9-borafluorene moiety. In all cases, the reactions afford partly saturated analogs of borepins ( 6,7-dihydroborepins) by unprecedented alkene insertion into the endocyclic B-C bond of the borole ring.