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 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)­CNtBu, Ph2CCH2, 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).