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[SiPh3], generating the silyl borane Ph3Si-Bpin; Li-2[DBA-Me-2] with an aromatic central B2C4 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 = NtBu, the reaction requires elevated temperatures of 100 degrees C, starts with H-Bpin activation which subsequently generates Li[BH4], so that the mechanism eventually turns into hidden borohydride catalysis. Ph(H)C=NPh, Ph2C=O, Ph2C=CH2, and iPrN=C=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.

Automated summary

Alkali-metal salts of 9,10-dimethyl-9,10-dihydro-9,10-diboraanthrancene can activate the H-B bond of pinacolborane at room temperature, leading to the formation of new bonding products. Depending on the alkali metal used, the products can contain new H-B and B-Bpin bonds. The Li+ salt results in the transfer of the H- ion to another pinacolborane molecule to form Li[5]. This Li[5] compound donates a [Bpin](+) cation to Li[SiPh3] to generate Ph3Si-Bpin, while Li-2[DBA-Me-2] acts as a leaving group. Moreover, Li-2[DBA-Me-2] can catalyze the hydroboration of various unsaturated substrates in THF, with different reaction mechanisms observed depending on the steric demands of the substrate.