From the reactivity of N-heterocyclic carbenes to new chemistry in ionic liquids

Abstract

N-Heterocyclic carbenes have numerous applications in synthetic chemistry. We detail the reactivity and chemistry of these molecules including investigations into their reactions with small reagents, their use for the preparation of polarised azines and their potential application as NLO materials. The chemistry of imidazolium salts, which are related to NHCs by the addition of a proton, is also discussed. New chemistry for ionic liquids is also revealed.

Reactivity of Electrogenerated Imidazole-2-Ylidenes in Ionic Liquids: Synthetic Implications

“…Imidazole-2-ylidene is a singlet carbene, quite stable as it is stabilized by the two π-donor nitrogen atoms (Figure 1), so it behaves as nucleophile or base, rather than a diradical (9). These carbenes can be generated also by electrochemical deprotonation, in an oneelectron reduction with production of molecular hydrogen (Scheme 2) (8,10). Imidazole-2-ylidene can be considered a neutral electrogenerated base (11), regenerating its probase, the imidazolium cation, when acting as a base.…”

“…The reactivity of the corresponding N-heterocyclic carbene (NHC), an imidazole-2-ylidene, has been studied after the pioneering work of Arduengo, in 1991 (7). NHCs are among the most basic neutral compounds known, with pKa values ranging from 22 to 24 in DMSO (8), and they have many applications in organic synthesis, mainly as ligands in transition metal complexes. Imidazole-2-ylidene is a singlet carbene, quite stable as it is stabilized by the two π-donor nitrogen atoms (Figure 1), so it behaves as nucleophile or base, rather than a diradical (9).…”

Investigation of the Role of Electrogenerated N-Heterocyclic Carbene in the Staudinger Synthesis in Ionic Liquid

“…Electrochemistry can also be useful in the generation of NHCs from the corresponding IL. In fact, the monoelectronic cathodic reduction of an imidazolium cation leads to the formation of the corresponding carbene and molecular hydrogen (Scheme 2) [19,[24][25][26].…”

“…N-Heterocyclic carbenes have been frequently used in organic chemistry as ligands and, more recently, as organocatalysts [18][19][20][21] in many reactions, such as the annulation of enals and sulfonylimines, [10,11] the Aza-Morita-Baylis-Hillman reaction of cyclic enones and N-tosylimines, [12] the benzoin condensation, [9,13] the Stetter reaction, [9,13] the Mannich Reaction [15] and the Staudinger reaction [16,17]. These stabilized singlet carbenes behave as nucleophilic organic catalysts, due to the presence of p-donor heteroatoms adjacent to the divalent carbon atom [22].…”

Ionic liquids: a versatile medium for palladium-catalyzed reactions

“…ILs have made significant progress in recent years in the catalytic processes. [14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30] They are liquid salts in which the ions are poorly coordinated. ILs are not simply salts dissolved in liquid but they usually exist as liquid below 100 °C, or even at room temperature (room temperature ionic liquids; RTILs).…”