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 (M₂[DBA-Me₂]; 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 M₂[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-Me₂ unit to another equivalent of HBpin to afford Li[5]. Although Li[5] might commonly be considered a [Bpin]^- adduct of neutral DBA-Me₂, it donates a [Bpin]⁺ cation to Li[SiPh₃], generating the silyl borane Ph₃Si-Bpin; Li₂[DBA-Me₂] with an aromatic central B₂C₄ ring acts as the leaving group. Furthermore, Li₂[DBA-Me₂] 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[BH₄], so that the mechanism eventually turns into "hidden borohydride catalysis". Ph(H)C[double bond, length as m-dash]NPh, Ph₂C[double bond, length as m-dash]O, Ph₂C[double bond, length as m-dash]CH₂, 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 Li₂[DBA-Me₂]: 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.