Crystalline Cyclic (Alkyl)(amino)carbene‐tetrafluoropyridyl Radical

Abstract

A stable cyclic (alkyl)(amino)carbene (CAAC) 1 inserts into the para-CF bond of pentafluoropyridine, and after fluoride abstraction, the iminium-pyridyl adduct [3](+) was isolated. A cyclic voltammetry study shows a reversible three-state redox system involving [3](+) , [3](⋅) , and [3](-) . The CAAC-pyridyl radical [3](⋅) , obtained by reduction of [3](+) with magnesium, has been spectroscopically and crystallographically characterized. In contrast to the lack of π communication between the CAAC and the pyridine units in cation [3](+) , the unpaired electron of [3](⋅) is delocalized over an extended π system involving both heterocycles.

Reactivity of NHC Alane Adducts towards N‐Heterocyclic Carbenes and Cyclic (Alkyl)(amino)carbenes: Ring Expansion, Ring Opening, and Al−H Bond Activation

“…[23] The most important NMR shifts of compounds 1-4 are summarized in Ta ble 1. The 1 HNMR spectra of the NHC·AlH 3 adducts 1-4 in solution reveal sharp, significantly shifted resonances for the NHC ligand, whereas the aluminum-bound hydrogen atoms appear as very broad peaks (range > 2.5 ppm) due to directb ondingt ot he 27 Al atom (I = 5/2) and the resulting splitting in combination with the quadrupole moment( q = 14.7 fm 2 )o ft he aluminum nucleus. However,t he aluminumbound hydrogen atoms of the adducts 1-4 can be detectedi n the 1 H{ 27 Al} NMR spectrum in the range 4.39-4.60 ppm.…”

“…The 1 HNMR spectra of the NHC·AlH 3 adducts 1-4 in solution reveal sharp, significantly shifted resonances for the NHC ligand, whereas the aluminum-bound hydrogen atoms appear as very broad peaks (range > 2.5 ppm) due to directb ondingt ot he 27 Al atom (I = 5/2) and the resulting splitting in combination with the quadrupole moment( q = 14.7 fm 2 )o ft he aluminum nucleus. However,t he aluminumbound hydrogen atoms of the adducts 1-4 can be detectedi n the 1 H{ 27 Al} NMR spectrum in the range 4.39-4.60 ppm. The 1 HNMR spectrum of adduct 3 shows, fore xample, ad oublet at 0.94 ppm for the methyl groups and as eptet at 5.25 ppm for the methine protons of the isopropyl substituents.…”

“…Compared with NHCs, cAACs have a smaller HOMO–LUMO gap and are thus simultaneously stronger electrophiles and nucleophiles . These features enable cAACs to activate small molecules such as CO, H 2 , P 4 , and other enthalpically strong single bonds, such as B−H, B−C, C−H, and C−F …”

Titanium Imido Complexes by Displacement of –SiMe3 and C–H Bond Activation in a TiIII Amido Complex, Promoted by a Cyclic (Alkyl)(Amino) Carbene (cAAC)

“…Thus, cAACs may accept an electron, intramolecularly from a low-valent center such as Si(0) 21 or Cl 2 Si 22 to assume diradicaloid character in the resulting bis(cAAC) adducts ( I and II in Scheme 3), or upon chemical reduction to allow isolation of stable ligand-centered carboxy 23 and pyridyl 24 radicals. From these perspectives, the zwitterionic complex 3 possesses two redox active sites, the cAAC ligand and the Ti(IV) metal center.…”

Stable Organic Radicals Derived from N‐Heterocyclic Carbenes

“…In 2015, Bertrand and co‐workers reported the synthesis of NHC–aryl radical 56 from cAAC 36 and pentafluoropyridine (Scheme ) . The C−F activation of pentafluoropyridine by using the cAAC results in 55 and subsequent fluoride abstraction and reduction generate cAAC–tetrafluoropyridyl radical 56 .…”