S-1 Supporting Information For Synthesis and Characterization of B-Heterocyclic π-Radical and Its Reactivity as a Boryl Radical Yoshitaka Aramaki, Hideki Omiya, Makoto Yamashita,* Koji Nakabayashi, Shin-ichi Ohkoshi,* Kyoko Nozaki* General All manipulations were carried out with Schlenk technique under argon purified by passing through a hot column packed with BASF catalyst R3-11 or in the argon-filled glove box (Miwa MFG) unless otherwise noted. Most of reagents were purchased from Tokyo Chemical Industry Co., Ltd. (TCI), Aldrich Chemical, Co. Ltd. and Kanto Kagaku Co., Ltd. Toluene, THF, and hexane were purified by passing through a solvent purification system. C 6 D 6 was distilled from Na/benzophenone solution before use. BF 3 OEt 2 was distilled under reduced pressure in the presence of CaH 2 before use. KH was washed with hexane and dried under reduced pressure in the glove box before use. Low-temperature reaction at -35 °C was performed with SW-M01 small stirrer of Nissin Laboratory Instruments in freezer of glove box. The 1 H, 11 B 19 F and 13 C{ 1 H} NMR spectra were recorded on a JEOL JNM-ECP 500 or JEOL JNM-ECS400 MHz spectrometers with residual protiated solvent for 1 H, deuterated solvent for 13 C{ 1 H}, an external BF 3 ·OEt 2 for 11 B, and an external C 6 F 6 for 19 F used as reference. Elemental analyses were performed by the Microanalytical Laboratory of the Department of Chemistry, Faculty of Science, the University of Tokyo. Melting points were measured on a MPA100 Optimelt Automated Melting Point System. ESR spectra were recorded on JEOL JES-FA200. UV/vis absorption chart was recorded on a SHIMADZU UV-3150 spectrophotometer. Cyclic voltammogram of 1 mM THF solution of 3 were recorded on ALS600D in an argon-filled glove box. Diimine 1 was prepared according to the literature reported method. S1 Synthesis of 2 To a 80 mL Schlenk flask placed with 1 (3.15 g, 6.26 mmol) and KH (502 mg, 12.5 mmol), 75 mL of THF was added and the resulting mixture was refluxed for 2.5 h under argon atmosphere. The solution was cooled to 0 °C with an ice bath and BF 3 OEt 2 (0.786 mL, 6.26 mmol) was added slowly by a syringe. After the removal of the ice bath, the mixture was stirred overnight at room temperature. Then, insoluble salt was filtered off through a pad of Celite ® and the volatiles were removed in vacuo. The residue was dissolved in CH 2 Cl 2 and the resulting suspension was passed through a pad of silica gel. After volatiles were removed from the filtrate under reduced pressure, the crude product was washed by hexane and gave pale yellow crystalline solid (1.37 g, 40%). 1 H NMR