Threat to the Throne: Can Two Cooperating Boron Atoms Rival Transition Metals in Chemical Bond Activation and Catalysis?

Certain electron‐rich 1,4‐diborabenzene derivatives efficiently activate single, double, and triple bonds and thereby increasingly compete with transition metals in homogeneous catalysis. This review compares the activation of three model substrates (H2, H2C=CH2, CO2) by (i) 9,10‐dihydro‐9,10‐diboraanthracene dianions, (ii) their neutral carbene‐stabilized congeners, (iii) 1,3,2,5‐diazadiborinines, and (iv) 1,4,2,5‐diazadiborinines. Distinct structure‐properties relationships become apparent, the most influential factors being (i) the steric demands of the B‐bonded substituents, (ii) the charges on the B‐doped (hetero)arenes, (iii) charge polarization as a result of additional N‐doping, and (iv) the energies and nodal structures of the frontier orbitals. The observed reactions are explained by a transition metal‐like activation mechanism. If the two boron atoms are chemically inequivalent, contributions of a B(+I)/B(+III) mixed‐valence state determine the observed regioselectivities when polar substrates are added. The lessons learned from the conversions of the model substrates are subsequently used to rationalize the behavior of the B2 heterocycles also toward more sophisticated substrate molecules. Finally, catalytic cycles based on H2‐ and H−‐transfers, hydroboration reactions, and CO2 reductions will be covered.