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.

1H-1,2,3-Triazol-5-ylidenes as Catalytic Organic Single-Electron Reductants

“…Meanwhile, carbene A Ph was converted into its conjugate acid C Ph via a two-step mechanism. First a single-electron transfer (SET) leads to the radical cation B Ph , followed by a H atom transfer (HAT) likely from tetrahydrofuran (BDE = 92 kcal·mol –1 ) as stated by previous reports on carbene radical cations. − Note that even under harsher conditions (excess A Ph and higher temperature), the second reduction of 1a was still not observed. Similar results have been reported by Thorley et al for the stochiometric reduction of quinones by lithium diisopropylamide (LDA, Scheme , blue pathway). , Indeed, calculations showed that the first reduction of the quinone is exothermic by 23.76 kcal·mol –1 , while the second reduction is endothermic by 14.32 kcal·mol –1 .…”

“…Regardless of this issue, the question remains whether a stable singlet carbene could act as an organic reducing agent. Along this line, several groups − have explored the oxidation of NHCs and cyclic (alkyl)­(amino)­carbenes (CAACs) but have never been able to isolate the corresponding radical cation. In all cases, this species immediately abstracts a hydrogen atom from the solvent, which generates the carbene conjugate acid (Scheme a).…”

Aldiminium Cations as Countercations to Discrete Main Group Fluoroanions

“…They are usually prepared by ion exchange from the corresponding chloride derivatives using KBF 4 [8]. Recently, [ Me CAACH][BF 4 ] was prepared by Jana's group from CAAC: carbene and NO [BF 4 ] [9]. The Me CAAC: carbene, which acted as a one-electron reducing agent, was chemically oxidized with NO[BF 4 ] to form a transient radical cation, which was subsequently converted to [ Me CAACH][BF 4 ] by hydrogen abstraction [9].…”

“…Recently, [ Me CAACH][BF 4 ] was prepared by Jana's group from CAAC: carbene and NO [BF 4 ] [9]. The Me CAAC: carbene, which acted as a one-electron reducing agent, was chemically oxidized with NO[BF 4 ] to form a transient radical cation, which was subsequently converted to [ Me CAACH][BF 4 ] by hydrogen abstraction [9]. The same method led to the formation of [ Me CAACH][SbF 6 ] when NO[SbF 6 ] was used [9].…”

A crystalline doubly oxidized carbene

“…More recently, attention has shifted to the oxidation of stable singlet carbenes since they possess a more electron-rich carbon center 16 . Several groups have systematically explored the oxidation of N-heterocyclic carbenes (NHCs) and cyclic (alkyl)(amino)carbenes (CAACs) with various single-electron oxidants 17,18,19,20 . In all cases the reaction afforded the conjugate acid of carbenes (Fig.…”