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

Article abstract of DOI:10.1021/jacs.9b01998

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 M₂[DBA] are readily accessible from the respective neutral DBAs and Li metal, Na metal, or KC₈. In the first step, the ambiphilic M₂[DBA] activate H₂ in a concerted, metal-like fashion. The rates of H₂ 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 H₂-addition rate to a higher extent than more weakly coordinating Na⁺/K⁺ ions. For the hydrogenation of unsaturated compounds, we identified Li₂[4] (Me substituents at boron) as the best performing catalyst; its substrate scope encompasses Ph(H)CNtBu, Ph₂CCH₂, and anthracene. The conversion of E-Cl to E-H bonds (E = C, Si, Ge, P) was best achieved by using Na₂[4]. The latter protocol provides facile access also to Me₂Si(H)Cl, a most important silicone building block. Whereas the H₂-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).