A04 • Catalysis with Reduced Arylboranes

Owing to their vacant p_z orbitals, trigonal-planar boron atoms embedded in polycyclic aromatic hydrocarbons (PAHs) impart unique electronic properties to the resulting B_n-PAHs. Particularly, their ability to promote one- or two-electron reductions is significant. As an incentive for this project, we have demonstrated that the dianion salts of doubly reduced 9,10-dihydro-9,10-diboraanthracenes (R₂-DBAs; R: B-bonded substituents) represent a promising class of transition-metal-free catalysts.
These catalysts readily mediate the addition of element–element σ- and π-bonds across their two boron centers. Among the activated substrates are molecules containing H‒H, H‒CCR, H‒Bpin, and F‒Ar^F bonds, as well as R₂C=CR₂, RC≡CR, and OC=O functionalities. In the context of catalysis, [R₂-DBA]^2‒ -mediated hydrogenation, hydride-transfer, and CO₂-disproportionation reactions have been successfully carried out. Our planned research will cover the following areas: (1) development of chemically more robust R₂-DBAs by embedding the boron atoms into rigid planar scaffolds (‘stabilization by structural constraint’).
We will evaluate how this enforced rigidity affects the required planar-to-bowl conformational change of the boron centers upon tetracoordination, potentially shifting the reaction mechanism from oxidative addition or nucleophilic substitution to single-electron transfer. These questions will be addressed using selected fluoroarenes (F‒Ar^F) as substrates. (2) Activation of diboranes(4) by [R₂-DBA]^2‒ and utilization of the activation products as B₂(OR)₄ and [B(OR)₂]⁻-transfer reagents. This subproject is closely linked to (3), the hydroborative conversion of the abundant greenhouse gas CO₂ into C₁ building blocks for organic synthesis. (4) Transition from chemical reducing agents to electrochemical methods for the (re)generation of the [R₂-DBA]^2‒ dianions from neutral R₂-DBAs during catalytic cycles. Collaboration between experimentalists and theoreticians will be essential for elucidating catalytic mechanisms ‒ particularly single-electron transfer vs. concerted closedshell pathways ‒ designing new catalysts and reactions and clarifying the influence of the countercation.