Boroles from alumoles: accessing boroles with alkyl-substituted backbones via transtrielation

The alumole (CpAlC4Et4)-Al-3t (Cp-3t = 1,2,4-tris(tert-butyl)cyclopentadienyl) is reported to be capable of transferring its butadiene moiety to aryl(dihalo)boranes to generate boroles through aluminum-boron exchange. The products feature a rare alkyl-substituted backbone, which, as shown in other examples, often leads to dimerization due to insufficient steric protection of the antiaromatic borole ring. Sterically crowded aryl groups bound to the boron atom are shown to prevent dimerization, allowing access to the first monomeric derivatives of this type. Results from UV-vis spectroscopy, electrochemistry, and DFT calculations reveal that the alkyl substituents cause remarkable modifications in the optical and electronic properties of the boroles compared to their perarylated counterparts.

Automated summary

The rare alkyl-substituted boroles were synthesized through aluminum-boron exchange reaction. Sterically crowded aryl groups prevent dimerization and allow access to monomeric derivatives. The alkyl substituents significantly modify the optical and electronic properties compared to the perarylated boroles.