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]N t Bu, 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 i PrN[double bond, length as m-dash]C[double bond, length as m-dash]N i Pr 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.