Electron Paramagnetic Resonance and Computational Studies of Radicals Derived from Boron-Substituted N-Heterocyclic Carbene Boranes

Abstract

Fifteen second-generation NHC-ligated boranes with aryl and alkyl substituents on boron were prepared, and their radical chemistry was explored by electron paramagnetic resonance (EPR) spectroscopy and calculations. Hydrogen atom abstraction from NHC-BH(2)Ar groups produced boryl radicals akin to diphenylmethyl with spin extensively delocalized across the NHC, BH, and aryl units. All of the NHC-B·HAr radicals studied abstracted Br-atoms from alkyl bromides. Radicals with bulky N,N'-dipp substituents underwent dimerization about 2 orders of magnitude more slowly than first-generation NHC-ligated trihydroborates. The evidence favored head-to-head coupling yielding ligated diboranes. The first ligated diboranyl radical, with a structure intermediate between that of ligated diboranes and diborenes, was spectroscopically characterized during photolysis of di-t-butyl peroxide with N,N'-di-t-butyl-imidazol-2-ylidene phenylborane. The reactive site of B-alkyl-substituted NHC-boranes switched from the boron center to the alkyl substituent for both linear and branched alkyl groups. The β-borylalkyl radicals obtained from N,N'-dipp-substituted boranes underwent exothermic β-scissions with production of dipp-Imd-BH(2)· radicals and alkenes. The reverse additions of NHC-boryl radicals to alkenes are probably endothermic for alkyl-substituted alkenes, but exothermic for conjugated alkenes (addition of an NHC-boryl radical to 1,1-diphenylethene was observed). A cyclopropylboryl radical was observed, but, unlike other α-cyclopropyl-substituted radicals, this showed no propensity for ring-opening.

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

“…[110] Unsere Gruppe charakterisierte Strukturen und Reaktionen von substituierten Borylradikalen NHC-BH(C)R mit R = Aryl, Alkyl und BH(R)-NHC (d. h. ein Diboranylradikal). [111] Bertrand und Mitarbeiter isolierten und charakterisierten ein durch zwei CAACLiganden stabilisiertes Borylen. [112] Wang und Robinson verçffentlichten einen Überblick über ihre Komplexe zwischen NHC-Liganden und reaktiven Verbindungen verschiedener Hauptgruppenelemente einschließlich Bor.…”

Influence of Electronic Effects on the Reactivity of Triazolylidene‐Boryl Radicals: Consequences for the use of N‐Heterocyclic Carbene Boranes in Organic and Polymer Synthesis

“…[5] We now report how polar effects on the NHC affect the radical reactivity of NHC-boranes, and what this means for their structural optimization as radical mediators and Type II photopolymerization co-initiators. For that purpose, the library of triazolylidene NHCs developed by the Rovis group was an attractive target for investigation, as one can easily prepare NHCs with strongly electron-withdrawing to strongly donating substituents.…”

Isolierung eines neutralen, Bor‐haltigen Radikals, stabilisiert durch ein cyclisches (Alkyl)(amino)carben

“…Vor allem Lacôte, Curran et al haben NHC-stabilisierte Borylradikale (A) intensiv untersucht, jedoch wurden diese wegen ihrer Kurzlebigkeit nur spektroskopisch beobachtet. [4] In ähnlicher Weise entwickelten LalevØe et al eine Reihe Pyridin-stabilisierter Borylradikale (B), die von Zipse et al auf Basis von Rechnungen vorhergesagt wurden.…”

“…[4] Somit erschienen uns Aryl(halogen)borane als geeignete Kandidaten für die Stabilisierung eines Borylradikals unter Vermeidung einer Dimerisierung durch Radikalhomokupplung. Die CAAC-stabilisierten Thienyl-(1) oder Duryl-substituierten Borane (2; Duryl = 2,3,5,6-Tetramethylphenyl) wurden in Ausbeuten von 77 bzw.…”