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

Prey, Sven, Herok, Christoph, Fantuzzi, Felipe, Bolte, Michael, Lerner, Hans‐Wolfram, Engels, Bernd, Wagner, Matthias (2023) Multifaceted behavior of a doubly reduced arylborane in B–H-bond activation and hydroboration catalysis. Chemical Science, 14 (4). pp. 849-860. ISSN 2041-6520. (doi:10.1039/D2SC05518J) (KAR id:98799)

Abstract

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 DBAMe2, 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]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]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 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.

Item Type: Article
DOI/Identification number: 10.1039/D2SC05518J
Subjects: Q Science > QD Chemistry
Divisions: Divisions > Division of Natural Sciences > Chemistry and Forensics
Funders: University of Kent (https://ror.org/00xkeyj56)
University of Würzburg (https://ror.org/00fbnyb24)
Goethe University Frankfurt (https://ror.org/04cvxnb49)
Depositing User: Felipe Fantuzzi
Date Deposited: 06 Dec 2022 12:59 UTC
Last Modified: 24 Feb 2023 13:56 UTC
Resource URI: https://kar.kent.ac.uk/id/eprint/98799 (The current URI for this page, for reference purposes)

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