Multifaceted behavior of a doubly reduced arylborane in B–H-bond activation and hydroboration catalysis

Alkali-metal salts of 9,10-dimethyl-9,10-dihydro-9,10-diboraanthrancene (M(2)[DBA-Me(2)]; M(+) = Li(+), Na(+), K(+)) activate the H–B bond of pinacolborane (HBpin) in THF already at room temperature. For M(+) = Na(+), K(+), the addition products M(2)[4] are formed, which contain one new H–B and one new B–Bpin bond; for M(+) = Li(+), the H(−) ion is instantaneously transferred from the DBA-Me(2) unit to another equivalent of HBpin to afford Li[5]. Although Li[5] might commonly be considered a [Bpin](−) adduct of neutral DBA-Me(2), it donates a [Bpin](+) cation to Li[SiPh(3)], generating the silyl borane Ph(3)Si–Bpin; Li(2)[DBA-Me(2)] with an aromatic central B(2)C(4) ring acts as the leaving group. Furthermore, Li(2)[DBA-Me(2)] catalyzes the hydroboration of various unsaturated substrates with HBpin in THF. Quantum-chemical calculations complemented by in situ NMR spectroscopy revealed two different mechanistic scenarios that are governed by the steric demand of the substrate used: in the case of the bulky Ph(H)C[double bond, length as m-dash]NtBu, the reaction requires elevated temperatures of 100 °C, starts with H–Bpin activation which subsequently generates Li[BH(4)], so that the mechanism eventually turns into “hidden borohydride catalysis”. Ph(H)C[double bond, length as m-dash]NPh, Ph(2)C[double bond, length as m-dash]O, Ph(2)C[double bond, length as m-dash]CH(2), and iPrN[double bond, length as m-dash]C[double bond, length as m-dash]NiPr undergo hydroboration already at room temperature. Here, the active hydroboration catalyst is the [4 + 2] cycloadduct between the respective substrate and Li(2)[DBA-Me(2)]: in the key step, attack of HBpin on the bridging unit opens the bicyclo[2.2.2]octadiene scaffold and gives the activated HBpin adduct of the Lewis-basic moiety that was previously coordinated to the DBA-B atom.