In comparison to other Group 13 carbene analogs, boryl anions [BR2]- represent a rather new class of compounds. Given the electron excess of such anions and the vacant p orbital at the boron atom, boryl anions are highly reactive and thermodynamically unstable. The isolation of the first boryl anion by Yamashita and Nozaki in 2006 provided access to the synthesis of a plethora of new boron–element bonds. While the vast majority of boryl anions is synthesized via reduction of haloborane precursors, Bertrand and co-workers showed in 2013 that the usage of electron-withdrawing cyano substituents and a cyclic alkyl(amino)carbene (CAAC) ligand induce an umpolung of the B–H bond, thus enabling the deprotonation of the corresponding (CAAC)-stabilized (dicyano)hydroborane. Previous work from Braunschweig and co-workers has shown the deprotonation of a (CAAC)-supported cyano(dihydro)borane, demonstrating that only one cyano group is already sufficient to induce this B-H bond umpolung. In this work, we present the synthesis of the (CAAC)-stabilized (cyano)hydro boryl anions 1a and 1b as well as the boron- and nitrogen-centered reactivity towards organic electrophiles and haloboranes, respectively (Scheme 1). These reactions led not only to the fully unsymmetrically substituted cyano(hydro)organoboranes 2a,b, but also to the first examples of compounds with cumulenic B=C=N=B units, namely the 1,4-dibora-2-azabutatriens 3a,b.

Boron- versus nitrogen-centered nucleophilic earctivity of (Cyano)hydroboryl anions: Synthesis of cyano(hydro) organoboranes and 2-Aza-1,4-diborabutatrienes

ACS Spring 2022

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