N-Heterocyclic Carbene Boryl Radicals: A New Class of Boron-Centered Radical

Abstract

Reduction of xanthates by N-heterocyclic carbene boranes (NHC-boranes) has been suggested to occur by a radical chain mechanism involving heretofore unknown NHC-boryl radicals. In support of this suggestion, both the expected borane dithiocarbonate product and an unexpected borane xanthate product have now been isolated. These are the first NHC-boranes with boron-sulfur bonds, and their structures have been secured by spectroscopic and crystallographic means. The first rate constants for H-atom transfer from an NHC borane complex were determined by using the ring opening of a substituted cyclobutylcarbinyl radical as a clock reaction. The rate constant for reaction of the NHC-borane with a secondary alkyl radical at ambient temperature is 4 x 10(4) M(-1) s(-1), and the Arrhenius function displayed an entropic term (log A term) that was typical for a bimolecular reaction. The B-H bond dissociation energy of an NHC-borane complex has been estimated at 88 kcal/mol. The putative NHC-boryl radical in these transformations has been detected by EPR spectroscopy. Spectral analysis suggests that it is a pi-radical, analogous to the benzyl radical.

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.…”

Komplexe von N‐heterocyclischen Carbenen mit Boranen: Synthese und Reaktionen

“…Unser erster Vorschlag über ein NHC-Boryl-Radikal als Zwischenstufe bei der Xanthatreduktion stammt aus dem Jahr 2008. [56] Seitdem nehmen jedoch die Berichte über Bildung, Struktur und Reaktionen dieser Borylradikale [76] rasch zu. NHC-Boryl-Radikale wurden durch ESR-Spektroskopie (direkt [76,77] und mit "Spin-Trapping" [93] ) und UV/Vis-Spektroskopie bei der Laser-Blitzlichtphotolyse beobachtet.…”

“…[56] Seitdem nehmen jedoch die Berichte über Bildung, Struktur und Reaktionen dieser Borylradikale [76] rasch zu. NHC-Boryl-Radikale wurden durch ESR-Spektroskopie (direkt [76,77] und mit "Spin-Trapping" [93] ) und UV/Vis-Spektroskopie bei der Laser-Blitzlichtphotolyse beobachtet. [92,93] Mit DFT berechnete Strukturen und experimentelle Beobachtungen ergeben ein konsistentes Bild.…”

“…[56,75,93] Der Schätzwert von 88 kcal mol À1 für die B-H-BDE von dipp-Imd-BH 3 (2) stammt dagegen aus der Anwendung einer Evans-Polanyi-Beziehung auf Grundlage der gemessenen Geschwindigkeitskonstanten für die H-Abstraktion durch Kohlenstoffradikale. [76] Ungeachtet des exakten Werts sind diese B-H-Bindungen um einiges schwächer als die B-H-Bindungen von freien Boranen und Borankomplexen mit Ethern, Aminen und sogar Phosphinen. [74] Vergleichbar niedrige Werte haben nur noch andere p-Komplexe von Boran, zum Beispiel Pyridin-Boran.…”