1,3‐Imidazole‐Based Mesoionic Carbenes and Anionic Dicarbenes: Pushing the Limit of Classical N‐Heterocyclic Carbenes

Abstract

Classical N‐heterocyclic carbenes (NHCs) featuring the carbene center at the C2‐position of 1,3‐imidazole framework (i.e. C2‐carbenes) are well acknowledged as very versatile neutral ligands in molecular as well as in materials sciences. The efficiency and success of NHCs in diverse areas is essentially attributed to their persuasive stereoelectronics, in particular the potent σ‐donor property. The NHCs with the carbene center at the unusual C4 (or C5) position, the so‐called abnormal NHCs (aNHCs) or mesoionic carbenes (iMICs), are however superior σ‐donors than C2‐carbenes. Hence, iMICs have substantial potential in sustainable synthesis and catalysis. The main obstacle in this direction is rather demanding synthetic accessibility of iMICs. The aim of this review article is to highlight recent advances, particularly by the author's research group, in accessing stable iMICs, quantifying their properties, and exploring their applications in synthesis and catalysis. In addition, the synthetic viability and use of vicinal C4,C5‐anionic dicarbenes (ADCs), also based on an 1,3‐imidazole framework, are presented. As will be apparent on following pages, iMICs and ADCs hold potentials in pushing the limit of classical NHCs by enabling access to conceptually new main‐group heterocycles, radicals, molecular catalysts, ligands sets, and more.

Divergent Reactivity of a Cyclic Bis‐Hydridostannylene: A Masked Sn(I) Diradicaloid

“…Very recently, we reported an open‐shell singlet tin(I) diradicaloid 1 and other main‐group heterocycles [13] based on an anionic dicarbene (ADC=PhC{N(Dipp)C} 2 ; Dipp=2,6‐ i Pr 2 C 6 H 3 ) [14] as a green crystalline solid [15] . The calculated diradical character for 1 amounts to 37% [15] .…”

Steigerung der π‐Akzeptoreigenschaft mesoionischer Carbene durch Carbonylierung mit Kohlenmonoxid

“…Wie erwartet, reagieren iMICs ( VII ) mit schwachen π‐Akzeptoreigenschaften nicht mit CO [19] . VII (R = H) können jedoch weiter deprotoniert werden, um anionische Dicarbene (ADCs, VIII ) zu erhalten, die als leistungsstarke monoanionische Vier‐Eletronen‐Donatoren für den Zugang zu einer Vielzahl von Hauptgruppen‐Heterozyklen fungieren [5a,20] . Daher war es naheliegend, dass VIII , die sowohl elektrophile als auch nukleophile Funktionalitäten aufweisen, wie Organometallverbindungen [21] in der Lage sein sollten, CO durch kooperative Effekte zu aktivieren.…”

Boosting the π‐Acceptor Property of Mesoionic Carbenes by Carbonylation with Carbon Monoxide

“…Expectedly, iMICs ( VII ) with poor π‐acceptor properties do not react with CO [19] . However, VII (R = H) can be further deprotonated to afford anionic dicarbenes (ADCs, VIII ), which serve as powerful monoanionic four‐electron donor frameworks in accessing a variety of main‐group heterocycles [5a,20] . We prompted to reason that VIII featuring both electrophilic and nucleophilic functionalities, like polar organometallic compounds, [21] should be able to activate CO by cooperative effects.…”