The influence of electron delocalization upon the stability and structure of potential N-heterocyclic carbene precursors with 1,3-diaryl-imidazolidine-4,5-dione skeletons

Targeting N-heterocyclic carbenes (NHCs) with increased π-acceptor character featuring N-fluorophenyl substituents, the molecular 2-chloro-1,3-bis(fluorophenyl)imidazolidine-4,5-diones (1a–c) were isolated from the condensation of the corresponding formamidine with oxalyl chloride. These formal adducts of NHCs with hydrogen chloride demonstrated reactivity akin to that of alkyl halides: 1,3,1′,3′-tetrakis(2,6-dimethylphenyl)-[2,2′]diimidazolidinyl-4,5,4′,5′-tetraone (2b) was formed via the reductive coupling of 1b, while 1,3-bis(2,6-diisopropylphenyl)-4,5-dioxoimidazolidin-2-yl acetate (3c) was formed as the result of a metathesis reaction with mercury(II) acetate. Chloride abstraction resulted in the formation of imidazolium-4,5-dione salts (4a–c) that decomposed rapidly, except in the case of the kinetically-stabilized 1,3-bis(2,6-diisopropylphenyl)imidazolium-4,5-dione hexafluorophosphate 4c. All imidazolium-4,5-dione hexafluorophosphate salts decomposed to neutral 2-fluoro-1,3-bis(aryl)imidazolidine-4,5-diones (5a–c) via fluoride abstraction. 2-Methoxy-1,3-di(aryl)imidazolidine-4,5-diones (6a–c) were also prepared and they failed to undergo thermolysis and yield the free NHCs. Computational analyses revealed that the instability of NHCs with an oxalamide skeleton, as well as that of imidazolium-4,5-diones, results from a π-framework which extends over both carbonyl moieties and gives rise to a very low energy LUMO, rendering the compounds in question highly electrophilic.