9-borafluorenes are a class of boron-containing heterocycles consisting of a tricyclic system with a central BC4 ring with two fused arene groups. 9-borafluorenes can be thought of as a borole with two fused arene rings, or as a trigonal planar boron atom with an empty p orbital bridging two biphenyl rings. However, 9-borafluorenes are generally less reactive than boroles due to less antiaromatic character and Lewis acidity. Containing highly conjugated π systems, 9-borafluorenes possess interesting photophysical properties. In addition, 9-borafluorenes are good Lewis acids. This combination of properties enables potential uses such as in light-emitting materials, solar cells, and sensors for some molecules.
Synthesis
The earliest successful synthesis of a 9-borafluorene was reported in 1963 by Köster and Benedikt, who performed thermolysis of dialkyl- or diaryl-2-biphenylboranes to release an alkane and yield the 9-borafluorene. Treatment of the resulting 9-alkyl or 9-arylborafluorene with boron trichloride yields the 9-chloroborafluorene, which can be functionalized to a variety of derivatives.
A particularly interesting synthesis of 9-borafluorenes was reported by Grigsby and Power. 2,6-Mes2C6H3BX2 (X = Cl, Br) was treated with lithium metal in diethyl ether to yield the reactive, highly electron-deficient borylene intermediate, which is able to insert into the strong C–C σ bond to form a 9-borafluorene.
A highly useful synthetic route to 9-borafluorenes is transmetalation reactions that utilize a heteroatom in the 9-position. 9-mercurafluorenes, 9-silafluorenes, and 9-stannafluorenes have been utilized in syntheses with generally good yield. While earlier synthetic methods often suffered from some substituents on the biphenyl framework leading to poor yield and selectivity, transmetalation methods generally tolerate substitution, giving rise to greater variety.
Functionalization
9-halo-9-borafluorenes are by far the most common precursors to functionalization of 9-borafluorenes at the boron center. The main strategies to accomplish late-stage functionalization at the boron center are metal halide elimination reactions using organometallic reagents, trialkylsilyl halide elimination, and hydrogen halide elimination using a base and either an amine or alcohol.
Reactivity
Lewis acid-base adducts
9-borafluorenes are highly Lewis acidic at the boron center and readily form Lewis acid-base adducts to satisfy the octet for the boron atom. In these adducts, the boron center is no longer trigonal planar and no longer has its empty p orbital that participates in conjugation in the π system in 9-borafluorene. Adducts involving Lewis bases such as pyridines, phosphines, ethers, carbenes, and nitriles have been described. The reactions involve simple reaction of the 9-borafluorene with the Lewis base at room temperature or low temperature in moderate to high yield. In addition, 9-borafluorenes whose boron is substituted with a group that also contains a Lewis base, such as 8-hydroxyquinioline, can form intramolecular adducts via the substituent's Lewis base donating to the boron atom to form a boron spirocenter.
Reduction
The 4π antiaromatic BC4 ring can undergo two-electron reduction to form a 6π aromatic system to form a dianion.
This can be accomplished by reduction by lithium metal. X-ray diffraction studies show that upon reduction, the B–C bond shortens from 1.64 Å to 1.54 Å owing to the gain of aromaticity. One-electron reduction of 9-bromo-9-borafluorene NHC and CAAC adducts has also yielded isolable neutral 9-borafluorene radicals.
B–C bond insertion
The endocyclic B–C bond in 9-borafluorenes is often susceptible to insertion by a variety of reagents.
Alkynes and phosphaalkynes have been shown to insert into the B–C bond to yield a 7-membered ring system. Mechanistic calculations revealed that the reaction between diphenylacetylene and 9-chloro-9-borafluorene occurs first by coordination of the alkyne π bond to the boron center, followed by insertion into the B–C bond in a concerted step with a single transition state to yield the BC6 system. In contrast, mechanistic calculations indicate that the reaction of 1-adamantyl-phosphaalkyne and 9-phenyl-9-borafluorene occurs via a concerted transition state.
Carbenes have also been shown to perform insertion. Bartholeme, Bluer, and Martin reacted 9-phenyl-9-borafluorene with CH(TMS)=N2 which generates a carbene via loss of N2. Insertion generated a BC5 system, which could then undergo a subsequent insertion with another equivalent of the carbene to yield a symmetric species with a BC6 system.