Hybrid Inorganic-Organic Cross-Metathesis between Diborenes and Acetylene

The ruthenium-catalyzed cross-metathesis of alkenes and alkynes, which splits the alkene C=C double bond and couples one-half to each carbon of the alkyne C C triple bond, is one of the most efficient tools for the synthesis of 1,3-dienes, with wide-ranging applications, including pharmaceutical and polymer chemistry. In contrast, inorganic main-group metathesis reactions are restricted to a handful of examples of heavier p-block multiple bonds (P=P, Ge=Ge, and E=E, E Ge, Sn, Pb). We now report the first examples of thermally induced, transition-metal-free cross-metathesis between an organic alkyne and inorganic cyclic alkyl(amino)carbene (CAAC)-stabilized B=B double bonds, which yield fully planar, p-delocalized 1,8-diaza-3,6-diboraoctatetraenes. Density functional theory studies show that these compounds have an open-shell singlet biradical ground state with a thermally accessible closed-shell state. In-depth computational mechanistic analyses show that they are formed via a biradical cycloaddition-cycloreversion mechanism. Finally, unlike their organic counterparts, these B,N-analogues of octatetraene can undergo two-electron chemical reduction to form diamagnetic dianions.

Automated summary

This study reports the first examples of thermally induced, transition-metal-free cross-metathesis between an organic alkyne and inorganic B=B double bonds, resulting in the formation of novel compounds with unique properties. These compounds form via a biradical mechanism and possess distinctive reducibility.