Boron–Boron Single Bonds Mimicking Transition Metals

This review article deals with a special transition‐metal‐like reactivity of ambiphilic (electrophilic and nucleophilic) diborane molecules with a BB single bond, namely the ability to coordinate a substrate and reduce it in their coordination sphere with the electrons from the BB bond. Specially designed diboranes are synthesized for this purpose, being Lewis‐acidic at boron and at the same time providing a filled BB bond orbital of high energy, from which electrons could be transferred to the substrate. A fine tuning of the substituents at the boron is required to balance the ambiphilic (electrophilic and nucleophilic) properties of the diborane. Moreover, diborane stability is an important issue for larger‐scale applications, since strongly electrophilic diboranes are generally air sensitive. The discussion in the last sections focuses on diboranes with guanidinate substituents. These diboranes are air‐stable and easy‐to‐handle sources of ambiphilic cationic diboranes and are efficiently synthesized by BB dehydrocoupling reactions. They are particularly suitable for processes in which substrate coordination and electron transfer proceed in two separate steps. The development of a transition‐metal‐like reactivity with stable, easy‐to‐handle diboranes paves the way to widen the applications of diboranes for further applications in synthetic chemistry.