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

Abstract

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.

Boron-Doped Polycyclic π-Electron Systems with an Antiaromatic Borole Substructure That Forms Photoresponsive B–P Lewis Adducts

“…Namely, the B–P bond in 7a · PT is cleaved in the excited state to afford base-free 7a * in the excited state and not PT * (Figure c). While such photodissociation behavior has been reported for the Lewis adducts of some boron-doped PAHs perpendicularly coordinated by pyridine, , this is the first example of the photoresponsive behavior of heteroatom-doped polycyclic π-systems connected through dative bonds. It should be noted that in borole-embedded 7a photodissociation scarcely proceeded when pyridine was employed as the Lewis base (Figure S8) because the binding constant of 7a with pyridine is 5 orders of magnitude higher than that of 7a · PT .…”

“…A variety of boron-doped PAHs, including PAHs with a non-hexagonal boracycle, have been developed to date and used not only as optoelectronic materials but also as organocatalysts. , Among these, fully conjugated polycyclic π-electron systems such as 1 , 2 , and 3 , in which three aryl groups at the boron center are held in a planar fashion, have gained much attention (Figure b). Owing to their structural constraints, this class of molecules is stable in the presence of air and water despite the absence of steric protection .…”

Proton to hydride umpolung at a phosphonium center via electron relay: a new strategy for main-group based water reduction

“…Main group compounds have shown a growing potential for transition metal-like reactivities, and thus offering an economical alternative to metal-based chemical transformations. [1][2][3][4][5][6][7][8] One fundamentally important process related to sustainable energy production is the direct generation of high-energy dihydrogen molecules by water splitting (water reduction). 9,10 Currently, besides direct electrolysis, [9][10][11][12][13][14][15] artificial water reduction strategies usually require a reduced form of (s-or d-block) metal as a reactant or catalyst, where heterolytic O-H bond cleavage during the water splitting occurs through oxidative addition over one or two metal centres (Chart 1A).…”

Boron-Based Lewis Acid Catalysis: Challenges and Perspectives

“…Using earth-abundant, less toxic, firstrow transition metals, chemists are re-examining well known strategies to make them more environmentally-friendly [29][30][31]. Boron-based compounds are widely employed as main group catalysts: boronic acids prominently for the activation of hydroxy functional groups [32], doubly B-doped (hetero)arenes as activators of p-block molecules [33], and boranes capable of activating a large number of substrates via a variety of transformations (Figure 2). While boronic acids and doubly B-doped (hetero)arenes are extensively employed in organic synthesis, boranes catalysis is a less developed field.…”