Complexes of Borane and N-Heterocyclic Carbenes: A New Class of Radical Hydrogen Atom Donor

Abstract

Calculations suggest that complexes of borane with N-heterocyclic carbenes (NHC) have B-H bond dissocation energies more then 20 kcal/mol less than free borane, diborane, borane-THF, and related complexes. Values are in the range of popular radical hydrogen atom donors like tin hydrides (70-80 kcal/mol). The resulting prediction that NHC borane complexes could be used as radical hydrogen atom donors was verified by radical deoxygenations of xanthates by using either AIBN or triethylborane as initiator.

Olefin Hydroborations with Diamidocarbene–BH3 Adducts at Room Temperature

“…Over time or upon heating the reaction mixture to 55 • C, the two multiplets converted to a single multiplet resonance at δ 3.67 ppm (see Supplementary Materials, Figure S17). Similar results were reported by Curran and co-workers, who attributed the phenomena to "chain walking" of a carbene-BH 3 adduct [19][20][21][22][23][24][25][26]. Additionally, when cis-or trans-3-hexene was introduced to 1a, compound 10 was obtained as the sole product, as indicated by the presence of a single multiplet at δ 3.67 ppm (C 6 D 6 ).…”

“…In the absence of any external initiators, the NHC-BH3 adducts typically do not react with olefins. However, as shown in Scheme 1, the NHC-BH3 adducts do facilitate the hydroboration of a wide range of organic molecules at elevated temperatures and/or upon the addition of a radical initiator [19][20][21][22][23][24][25][26][27]. Scheme 1.…”

Die Verknüpfung von Boran mit N‐heterocyclischen Carbenen: effektive Wasserstoffdonoren für Radikalreaktionen

“…[4] Ein Beitrag einer Arbeitsgruppe um Fensterbank, Lacôte, Malacria und Curran (im Folgenden als FLMC abgekürzt) hat diesem Forschungsfeld kürzlich eine neue Richtung gegeben. [5] Als das Ergebnis der Vereinigung von Organoboranchemie und N-heterocyclischen Carbenen (NHCs) erhielten sie NHC-Boran-Komplexe, die eine neue vielversprechende Klasse von Wasserstoffdonoren für radikalische Reaktionen bilden. Die Bindungsdissoziationsenergie (BDE) von Boran (BH 3 ) ist mit 445.5 kJ mol À1 für einen Wasserstoffdonor viel zu hoch, sie kann jedoch durch Bindung an Lewis-Basen verringert werden.…”

Metal‐Free Dihydrogen Oxidation by a Borenium Cation: A Combined Electrochemical/Frustrated Lewis Pair Approach

“…[6] In addition, the bond-dissociation energy of the BÀH bond is weakened by the coordination of a carbene ligand. [8,9] As we demonstrate herein, the combination of these properties improves the chemical stability of borenium Lewis acid catalysts toward unwanted side reactions (e.g. reaction with the solvent) and further decrease the voltage required to oxidize H 2 at a carbon electrode by 910 mV (equivalent to 175.6 kJ mol À1 ).…”