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

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.