Synthesis of triphenylene-fused phosphole oxides via C–H functionalizations

The synthesis of triphenylene-fused phosphole oxides has been achieved through two distinct C–H functionalization reactions as key steps. The phosphole ring was constructed by a three-component coupling of 3-(methoxymethoxy)phenylzinc chloride, an alkyne, and dichlorophenylphosphine, involving the regioselective C–H activation of the C2 position of the arylzinc intermediate via 1,4-cobalt migration. The resulting 7-hydroxybenzo[b]phosphole derivative was used for further π-extension through Suzuki–Miyaura couplings and a Scholl reaction, the latter closing the triphenylene ring. The absorption and emission spectra of the thus-synthesized compounds illustrated their nature as hybrids of triphenylene and benzo[b]phosphole.


Materials and methods
General. All reactions with air or moisture-sensitive compounds were performed using standard Schlenk techniques in oven-dried reaction vessels under a nitrogen or argon atmosphere.
Analytical thin-layer chromatography (TLC) was performed on Merck 60 F254 silica gel plates.
Flash column chromatography was performed using 40-63 μm silica gel (Si 60, Merck). 1 H, 13 C and 31 P nuclear magnetic resonance (NMR) spectra were recorded on JEOL ECA-400 (400 MHz) or Bruker AV-400 (400 MHz) and AV-300 (300 MHz) NMR spectrometers. 1 H and 13 C NMR spectra are reported in parts per million (ppm) downfield from an internal standard, tetramethylsilane (0 ppm) and CHCl3 (77.0 ppm), respectively. 31 P NMR spectra are referenced to an external reference (85% H3PO4, 0 ppm). High-resolution mass spectra (HRMS) were obtained with a Q-Tof Premier LC HR mass spectrometer. Melting points were determined using a capillary melting point apparatus and are uncorrected. UV-vis and fluorescence spectra were recorded on a Shimadzu UV-1800 spectrophotometer and a Shimadzu RF-5301PC spectrofluorophotometer, respectively.

Materials.
Unless otherwise noted, commercial reagents were purchased from Alfa Aesar, Sigma Aldrich, TCI and other commercial suppliers and used as received. Xantphos and CoCl2 (anhydrous, 97%) were purchased from Alfa Aesar and used as received. Tetrahydrofuran (THF) and 1,4-dioxane were distilled over Na/benzophenone. Dichloromethane (DCM) was distilled over calcium hydride. Anhydrous dimethylformamide (DMF) was purchased from Sigma-Aldrich. 3-(Methoxymethoxy)phenylmagnesium bromide was prepared from the corresponding 3-(methoxymethoxy)phenyl bromide and magnesium turnings in anhydrous THF and titrated before use.

Synthesis of triphenylene-fused phosphole oxides
The synthesis and characterization of 7-hydroxybenzo[b]phosphole oxide (3) and its triflate 4 are described in our previous work. 1

Suzuki-Miyaura coupling between 4 and bromoarylboronic acid
Typical procedure: 7-(2-Bromophenyl)-2,3-dibutyl-1-phenylphosphindole 1-oxide (6a). A 25 mL oven-dried Schlenk tube was charged with 4 (0.97 g, 2.0 mmol) and DMF (6 mL). The resulting solution was degassed through freeze-pump-thaw cycles (3 times). To the degassed solution, Pd(PPh3)4 (115.6 mg, 0.10 mmol), K3PO4 (0.64 g, 3.0 mmol), and 2bromophenylboronic acid (0.48 g, 2.4 mmol) were added sequentially. The resulting mixture was stirred at 85 °C for 60 h and then allowed to cool to room temperature. The reaction mixture was extracted with EtOAc (3 × 30 mL). The combined organic layer was dried over MgSO4 and concentrated under reduced pressure. The residue was purified by flash chromatography on silica gel (eluent: EtOAc/hexane 1:2) to afford the title compound as colorless oil (0.72 g, 73%). Signal broadening was observed in the 1 H and 13 C NMR spectra of this compound and compound 7b, presumably due to the slow rotation of the aryl-aryl bond. The presence of a minor peak in the corresponding 31 P NMR spectra suggests the existence of a mixture of equilibrating diastereomers. 1  sequentially. The resulting mixture was stirred at 100 °C for 24 h. The reaction mixture was allowed to cool to room temperature, diluted with water, and extracted with EtOAc (3 × 20 mL).
The combined organic layer was dried over MgSO4 and concentrated under reduced pressure.
The residue was purified by flash chromatography on silica gel (eluent: EtOAc/hexane 1:2) to afford the desired ortho-teraryl product.
Note: The 1 H NMR spectra of teraryl compounds 7a-c (CDCl3, 298 K) displayed extensive signal broadening, and most of the signals did not show resolved spin-spin coupling. Some of the 13 C NMR signals also showed substantial broadening, which made it difficult to match the number of the observed peaks to the number of the unique carbons. This is presumably due to the restricted, slow rotation of the aryl-aryl bonds compared to the time scale of the NMR measurement.