Backbone-controlled LUMO energy induces intramolecular C–H activation in ortho-bis-9-borafluorene-substituted phenyl and o-carboranyl compounds leading to novel 9,10-diboraanthracene derivatives

The choice of backbone linker for two ortho-bis-(9-borafluorene)s has a great influence on the LUMO located at the boron centers and, therefore, the reactivity of the respective compounds. Herein, we report the room temperature rearrangement of 1,2-bis-(9-borafluorenyl)-ortho-carborane, C2B10H10-1,2-[B(C12H8)]2 ([2a]) featuring o-carborane as the inorganic three-dimensional backbone and the synthesis of 1,2-bis-(9-borafluorenyl)benzene, C6H4-1,2-[B(C12H8)]2 (2b), its phenylene analog. DFT calculations on the transition state for the rearrangement support an intramolecular C–H bond activation process via an SEAr-like mechanism in [2a], and predicted that the same rearrangement would take place in 2b, but at elevated temperatures, which indeed proved to be the case. The rearrangement gives access to 3a and 3b as dibora-benzo[a]fluoroanthene isomers, a form of diboron polycyclic aromatic hydrocarbon (PAH) that had yet to be explored. The isolated compounds 2b, 3a, and 3b were fully characterized by NMR, HRMS, cyclic voltammetry (CV), single-crystal X-ray diffraction analysis, and photophysical measurements, supported by DFT and TD-DFT calculations.


Additional reactions
A clean reaction was observed for the synthesis of 5 when the reaction was carried out in THF-d8 as a one pot synthesis with an excess of 9-Br-9-borafluorene, with the formation of a single new species by NMR spectroscopy (see below). As the ring-opened species arises from the deuterated THF solvent, the alkyl chain was not observed in the 1 H and 13 C NMR spectrum.
Compound 5 was not detected by HRMS (LIFDI) as it is an anion.
When this reaction was repeated in non-deuterated THF, removal of all volatiles in vacuo after the formation of 9-(4-bromobutoxy)-9-borafluorene led to the formation of some impurities.
Continuing the reaction in THF-d8 resulted in a mixture of compounds in the 1 H and 11 B NMR spectra.

S25
When 9-(Me2S)-9-Br-9-borafluorene was reacted with 1,2-Li2-1,2-C2B10H10·(Et2O)2 in C6D6, solubility was low and the reaction led to a complex mixture, as observed by NMR spectroscopy and HRMS. When the same reaction was carried out in Me2S, reaction between the solvent and 1,2-Li2-1,2-C2B10H10·(Et2O)2 formed a complex mixture from which a crystal of 1-MeS-2-(Me2S-9-borafluorene)-1,2-C2B10H10 was isolated and characterized by single-crystal X-ray diffraction. To confirm the reaction between the solvent and dilithium salt, as similar reactions are known for n-butyllithium, 25  Single-crystal X-ray diffraction Table S1. Single-crystal X-ray diffraction data and structure refinements of 2b, 3a, 3b, 3a·THF, 5, 9-(Me2S)-9-Br-9-borafluorene, 1-MeS-2-(Me2S-9-borafluorene)-1,2-C2B10H10, and 9-(4bromobutoxy)-9-borafluorene.    Figure S1. Solid state molecular structure of 2b from single-crystal X-ray diffraction at 100 K. Atomic displacement ellipsoids are drawn at the 50% probability level and hydrogen atoms are omitted for clarity. Figure S2. Solid state molecular structure of 3a from single-crystal X-ray diffraction at 100 K. Atomic displacement ellipsoids are drawn at the 50% probability level and hydrogen atoms are omitted for clarity. Figure S3. Solid state molecular structure of 3b from single-crystal X-ray diffraction at 100 K. Atomic displacement ellipsoids are drawn at the 50% probability level and hydrogen atoms and solvent molecules are omitted for clarity. Only one of four symmetry-independent molecules is shown. Figure S4. Solid state molecular structure of 3a·THF from single-crystal X-ray diffraction at 100 K. Atomic displacement ellipsoids are drawn at the 50% probability level and solvent molecules and hydrogen atoms are omitted for clarity. Figure S5. Solid state molecular structure of 5 from single-crystal X-ray diffraction at 100 K, on the left with the lithium counterion and on the right from a different angle. Atomic displacement ellipsoids are drawn at the 50% probability level, and hydrogen atoms and the minor occupied components of disordered THF and alkyl groups are omitted for clarity. Only one of two symmetry-independent anions and cations are shown. Figure S6. Solid state molecular structure of the dimethyl sulfide adduct of 9-Br-9-borafluorene, 9-(Me2S)-9-Br-9-borafluorene, from single-crystal X-ray diffraction at 100 K. Atomic displacement ellipsoids are drawn at the 50% probability level and hydrogen atoms are omitted for clarity. Figure S7. Solid state molecular structure of an isolated product 1-MeS-2-(Me2S-9borafluorene)-1,2-C2B10H10 from the reaction in SMe2 described above from single-crystal Xray diffraction at 100 K. Atomic displacement ellipsoids are drawn at the 50% probability level and hydrogen atoms are omitted for clarity. Figure S8. Solid state molecular structure of 9-(4-bromobutoxy)-9-borafluorene from singlecrystal X-ray diffraction at 100 K. Atomic displacement ellipsoids are drawn at the 50% probability level and hydrogen atoms are omitted for clarity.