Dual Role of Doubly Reduced Arylboranes as Dihydrogen- and Hydride-Transfer Catalysts.

Doubly reduced 9,10-dihydro-9,10-diboraanthracenes (DBAs) are introduced as catalysts for hydrogenation as well as hydride-transfer reactions. The required alkali metal salts M ₂ [DBA] are readily accessible from the respective neutral DBAs and Li metal, Na metal, or KC ₈ . In the first step, the ambiphilic M ₂ [DBA] activate H ₂ in a concerted, metal-like fashion. The rates of H ₂ activation strongly depend on the B-bonded substituents and the counter cations. Smaller substituents (e.g., H, Me) are superior to bulkier groups (e.g., Et, pTol), and a Mes substituent is even prohibitively large. Li ⁺ ions, which form persistent contact ion pairs with [DBA] ^2- , slow the H ₂ -addition rate to a higher extent than more weakly coordinating Na ⁺ /K ⁺ ions. For the hydrogenation of unsaturated compounds, we identified Li ₂ [4] (Me substituents at boron) as the best performing catalyst; its substrate scope encompasses Ph(H)C═N tBu, Ph ₂ C═CH ₂ , and anthracene. The conversion of E-Cl to E-H bonds (E = C, Si, Ge, P) was best achieved by using Na ₂ [4]. The latter protocol provides facile access also to Me ₂ Si(H)Cl, a most important silicone building block. Whereas the H ₂ -transfer reaction regenerates the dianion [4] ^2- and is thus immediately catalytic, the H ^- -transfer process releases the neutral 4, which has to be recharged by Na metal before it can enter the cycle again. To avoid Wurtz-type coupling of the substrate, the reduction of 4 must be performed in the absence of the element halide, which demands an alternating process management (similar to the industrial anthraquinone process).