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═N tBu, 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).

Synthesis and Ring Strain of a Benzoborirene-N-Heterocyclic Carbene Adduct.

The reduction of an N-heterocyclic carbene (1,3-diisopropyl-4,5-dimethylimidazolin-2-ylidene, IiPr Me 2 ) adduct of dichloro(ortho-bromophenyl)borane by tert-butyl lithium at low temperature yields the IiPr Me 2 adduct A of parent benzoborirene, a highly strained boron-containing bicyclic compound. A is unstable at room temperature and dimerizes at low temperature to the bis-IiPr Me 2 adduct of 9,10-dihydro-9,10-diboraanthracene, characterized by single-crystal X-ray crystallography.

Selective CO2 Splitting by Doubly Reduced Aryl Boranes to Give CO and [CO3 ]2.

Alkali metal salts M2 [1] (M=Li, Na) of doubly reduced 9,10-dimethyl-9,10-dihydro-9,10-diboraanthracene (1) instantaneously add the C=O bond of CO2 across their boron centers to furnish formal [4+2]-cycloadducts M2 [2]. If only 1 equiv of CO2 is supplied, these products are stable. In the presence of excess CO2 , however, C-O bond cleavage occurs and an O2- equivalent is transferred to CO2 to furnish CO and [CO3 ]2- . With M=Li, Li2 CO3 precipitates and the neutral 1 is liberated such that it can be reduced again to establish a catalytic cycle. With M=Na, [CO3 ]2- remains coordinated to both boron atoms in a bridging mode (Na2 [4]). A mechanistic scenario is proposed, based on isolated intermediates and model reactions.

Reversible Dihydrogen Activation by Reduced Aryl Boranes as Main-Group Ambiphiles.

A new approach to main-group H2 activation combining concepts of transition-metal and frustrated Lewis pair chemistry is reported. Ambiphilic, metal-like reactivity toward H2 can be conferred to 9,10-dihydro-9,10-diboraanthracene (DBA) acceptors by the injection of two electrons. The resulting [DBA]2- ions cleave the H-H bond with the formation of hydridoborates under moderate conditions (T=50-100 °C; p<1 atm). Depending on the boron-bonded substituents R, the addition is either reversible (R=C≡CtBu) or irreversible (R=H). The reaction rate is strongly influenced by the nature and the coordination behavior of the countercation (Li+ slower than K+ ). Quantum-chemical calculations support the experimental observations and suggest a concerted, homolytic addition of H2 across both boron atoms. As proven by the successful conversion of Me3 SiCl into Me3 SiH, the system Li2 [DBA]/H2 appears generally relevant for the hydrogenation of element-halide bonds.