An Air-Stable Organoboron Compound, Dithienooxadiborepine: Preparation and Functionalization.

Abstract

π-Conjugated organoboron molecules, which are easy to prepare, stable against moisture, and easy to functionalize, are scarce. Here, we report a one-pot synthesis of an air-stable organoboron compound, dithienooxadiborepine 1 in 17% yield on a 600 mg scale without separation or handling an air-sensitive intermediate. Dithienooxadiborepine 1 showed excellent stability under ambient conditions, allowing conventional column chromatography purification. Functionalization of 1 was realized via direct bromination using NBS and further Stille coupling reactions, giving access to longer π-conjugated molecules 5A and 5B. Single-crystal structures of compounds 4, 5A, and 5B not only unambiguously verified the chemical identity of dithienooxadiborepine 1 but also revealed that both the seven-member oxadiborepine ring and the 5-7-5 fused dithienooxadiborepine ring system are planar. UV-vis absorption and fluorescence emission measurements of 5A and 5B showed bathochromic shifted absorption and emission relative to 1, evidencing good π-conjugation. Cyclic voltammograms of 5A and 5B displayed two reduction peaks corresponding to two electron-accepting events at two boron atoms. These results proved dithienooxadiborepine 1 a potent π-conjugating building block for electron-accepting materials.