Disclosing Cyclic(Alkyl)(Amino)Carbenes as One-Electron Reductants: Synthesis of Acyclic(Amino)(Aryl)Carbene-Based Kekulé Diradicaloids

Abstract

Herein, we disclose cyclic(alkyl)(amino)carbenes (CAACs) to be one-electron reductants under the formation of a transient radical cation as indicated by EPR spectroscopy. The disclosed CAAC reducing reactivity was used to synthesize acyclic(amino)(aryl)carbene-based Thiele and Chichibabin hydrocarbons, a new class of Kekulé diradicaloids. The results demonstrate CAACs to be potent organic reductants. Notably, the acyclic(amino)(aryl)carbene-based Chichibabin's hydrocarbon shows an appreciable population of the triplet state at room temperature, as evidenced by both variable-temperature NMR and EPR spectroscopy.

Keywords: EPR spectroscopy; carbene; electrochemistry; radical cation; reductant.

Scheme 1

One‐electron oxidation of N‐heterocyclic carbene I (top) and cyclic(alkyl)(amino)carbene V (bottom) (R=monoanionic organic substituent and the counter anions are omitted for clarity).

Figure 1

a) Cyclic voltammogram of 1 in THF (0.1 M Bu₄NPF₆) measured at a scan rate of 20 mV s⁻¹. Inset: Differential pulse voltammograms (DPV) of 1 in THF at a scan rate of 20 mV s⁻¹ with 0.2 M Bu₄NPF₆). b) Experimental X‐band EPR spectrum of a solution of 2 in toluene generated in situ at 200 K. The isotropic signal has a g value of 2.0043 and a ¹⁴N hyperfine coupling of a(N)=16.7 MHz (5.9 G).

Scheme 2

Oxidation of 1 (Dip=2,6‐iPr₂C₆H₃).

Scheme 3

Reactions of 1 with 4, 5, and 6.

Figure 2

a) Variable‐temperature ¹H NMR spectra of 9 (selected region) in [D₈]THF. b) Variable‐temperature X‐band EPR spectra of 9 in THF between 255 and 300 K. C) plots of double‐integral intensity (A) vs. T. The red line shows the fit to the Bleaney–Bowers equation (experimental parameters: microwave frequency=9.38 GHz, microwave power=0.2 mW, modulation amplitude=0.5 G, modulation frequency=100 kHz). d) Molecular structure of 9 with thermal ellipsoids at the 50 % probability level. All H atoms are omitted for clarity. Selected bond lengths [Å] and angles [°]: N1−C1 1.389(2), C1−C8 1.479(2), C1−C2 1.400(2), C2−C3 1.431(2), C3−C4 1.359(2), C4−C5 1.425(2), C5−C5’ 1.419(21), N1−C1−C8 114.44(14).

Figure 3

Energy scanning of the rotation of the C−C bond between the two phenyl rings of 9 for the broken‐symmetry singlet (orange) and the triplet (blue) states at the UBP86‐D3/Def2SVP level of theory.

Figure 4

Cyclic voltammograms of 4, 5, and 6 in CH₃CN/0.1 M Bu₄NPF₆ measured at a GC working electrode.