9-Borafluorenes: Synthesis, Properties, and Reactivity.

This review covers all aspects of 9-borafluorene chemistry, from the first attempted synthesis in 1960 to the present. This class of molecules consists of a tricyclic system featuring a central antiaromatic BC4 ring with two fused arene groups. The synthetic routes to all 9-borafluorenes and their adducts are presented. The Lewis acidity and photophysical properties outlined demonstrate potential utility as sensors and in electronic materials. The reactivity of borafluorenes reveals their prospects as reagents for the synthesis of other boron-containing molecules. The appealing traits of 9-borafluorenes have stimulated investigations into analogues that bear different aromatic groups fused to the central BC4 ring. Finally, we offer our views on the challenges and future of borafluorene chemistry.

Ligation of Boratabenzene and 9-Borataphenanthrene to Coinage Metals.

The reactions of boratabenzene and borataphenanthrene anions with group 11 Ph3PMCl reagents furnished η2 coordination complexes, with the exception of the copper boratabenzene species that adopted an η6 mode. The binding of arene ligands to copper in an η6 manner is rare, and altering the ancillary ligand on copper to an N-heterocyclic carbene switched the binding of the boratabenzene to η2, indicating that such ligands are capable of vacating coordination sites. The η2 coordination complexes bind side-on, akin to olefins, via a borataalkene unit, although with the carbon atom much more proximal to the metal center than boron.

The 9-Borataphenanthrene Anion.

The 9-borataphenanthrene anion is easily accessed by deprotonation of a 9,10-dihydro-9-boraphenanthrene and its diverse reactivity is investigated. Alkylation occurs at the carbon atom adjacent to boron, and room temperature hydroboration occurs across the B=C bond. The π-manifold of the central BC5 ring coordinates to chromium in an η6 fashion while only the B=C unit binds η2 to gold, indicating versatility of the 9-borataphenanthrene anion as a ligand. Supporting calculations rationalize the reactivity and aromaticity is corroborated by nucleus-independent chemical shift (NICS) indices.

Assessing the donor ability of boratabenzene and 9-borataphenanthrene anions through metal complexes with carbonyl ligands.

A series of anionic group 6 tricarbonyl and neutral rhodium dicarbonyl complexes featuring a boratabenzene (L1, with a phenyl on boron, a trimethylsilyl group on the adjacent carbon and methyl groups on the other carbons) and a borataphenanthrene ligand (L2, with a phenyl group on boron and a trimethylsilyl group on the adjacent carbon) are prepared. The donor ability of the boracyclic ligands is evaluated experimentally and theoretically by the stretching frequencies of the CO ancillary ligands. Overall, the donor ability of the ligands falls into the following trend: L1 > cyclopentadienyl > L2 > mesitylene.

Successive carbene insertion into 9-phenyl-9-borafluorene.

The reactions of 9-phenyl-9-borafluorene with trimethylsilyldiazomethane in a 1 : 1 and 1 : 2 stoichiometry furnished the corresponding BC5 and BC6 heterocycles via the formal insertion of one and two carbene units.