Doubly hydrogen-bridged 1,2-diphenylenediboranes derived from 9-chloro-9-borafluorene and ligand exchange reactions

Cyclic 1,2-diphenylenediboranes containing a doubly hydrogen-bridged structure, including 1,2-(2,2^′biphenylylene)diborane(I) and 1,2-(2,2^′biphenylylene)-1,2-diethyldiborane (II), are conveniently prepared by treating 9-chloro-9-borafluorene with NaBH₄ and Na(Et)₃BH, respectively. The reaction mechanism involves an initial Cl-H exchange to form 9-borafluorene containing a reactive 5-member ring diarylborane moiety, which subsequently engages in a facile ring expansion with the in situ formed B-H containing residue (BH₃ or HBEt₂) to result in cyclic 1,2-diphenylenediboranes compounds. The doubly hydrogen-bridged structure shows good thermal stability up to 50 °C. Upon thermal cleavage at higher temperature, all free B-H groups become very reactive involving hydroboration with α-olefin. The complexization study also reveals that this intradiborane moiety forms a 1:2 complex with a strong base, such as pyridine.