9-Borafluorene

9-borafluorenes are a class of boron-containing heterocycles consisting of a tricyclic system with a central BC4 ring with two fused arene groups.

[1] 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.

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.

In addition to alkynes, phosphaalkynes, and carbenes, insertion reactions into 9-borafluorenes have been shown with other functional groups such as azides[17] and carbonyls.

DFT calculations led the authors to describe the borafluorene complex as an L2 ligand with significant nickel backbonding into the empty boron p orbital.

[22] 9-borafluorenes can participate in ring-opening reactions to form oligomers, which often contain three-center two-electron bonds, in order to fulfill the octet on the boron atom.

For example, it has been reported that a 1:1 mixture of 9-bromo-9-borafluorene and triethylsilane at room temperature for several weeks led to a pentameric species as a result of a ring-opening reaction.

[8] However, 9-borafluorenes with substituents possessing an additional Lewis basic functional group, such as 8-hydroxyquinoline, show higher quantum yield due to increased rigidity of the molecule.

[12] A similar phenomenon was observed with BODIPY and aza-BODIPY coordinating to the boron center, where the HOMO-LUMO gaps of each π system were relatively unchanged, but increased rigidity led to improved quantum yield.

π donor groups such as tBuO and iPr2N were found to blue-shift the absorption peak attributed to raising the energy of the LUMO, while electron acceptor groups such as MesF (2,4,6-(tris(trifluoromethyl))phenyl) were found to red-shift the absorption by lowering the energy of the LUMO.

The sensing was attributed to the binding of fluoride ion increasing the HOMO-LUMO gap by changing the pπ-π* conjugation.

It was observed that a solution of the sensor in a weakly coordinating solvent was red at room temperature but became colorless upon cooling.

Thermolysis of dialkyl or diaryl-2-biphenylboranes to yield 9-borafluorene. (R = Et, n Pr, i Bu, Ph)
Formation of a reactive borylene intermediate by treatment of 2,6-Mes 2 C 6 H 3 BCl 2 (Mes = 2,4,6-trimethylphenyl) with lithium metal, followed by C–C σ bond insertion to yield the 9-borafluorene.
Synthetic route to 9-borafluorenes using transmetalation reactions. Successful synthesis of 9-borafluorene derivatives have been performed with M=Mg, Hg, SnR 2 , SiMe 2 .
Lewis base (LB) adduct to 9-borafluorene (R = Cl, Br, OTf, Ph).
Intramolecular Lewis acid-base adduct of a 9-borafluorene.
Reduction of a 9-borafluorene by lithium metal (Ar = 4- t Bu-C 6 H 4 ) [ 13 ] .
Insertion of diphenylacetylene into 9-chloro-9-borafluorene. The same reaction has been performed for the bromine and triflate derivatives. [ 15 ]
Azide, carbene, carbonyl, and phosphaalkyne insertion reactions for 9-phenyl-9-borafluorene. (Ad = adamantyl)
Synthesis of metal-boryl 9-borafluorene complex with Mn(I) by reaction of 9-chloro-9-borafluorene and NaMn(CO) 4 (PPh 3 ).
Reaction of 9-phenyl-9-borafluorene with the AlCp* tetramer to yield the η 1 9-borafluorene complex, with boron acting as the acceptor.
Successful synthesis of an η 5 9-borafluorene-nickel complex.
Oligomerization reactions of 9-H-9-borafluorene following its synthesis by treatment of 9-bromo-9-borafluorene by triethylsilane. The pentamer converts to the dimer under heating.
Adducts of 9-borafluorene which all fluoresce at λ=435 nm.
Frontier molecular orbitals of Mes F -9-borafluorene and i Pr 2 N-9-borafluorene calculated at a B3LYP/6-31+G(d) level.
Fluoride sensor whose emission maximum is ~140 nm blue shifted upon fluoride binding. (Ar = 4-(Ph 2 N)-C 6 H 4 , 2-thienyl, 2-bithienyl)
NHC-stabilized 9-borafluorenium cation that displays thermochromism.