title: Efficient synthetic protocols for the preparation of common N-heterocyclic carbene precursors.
abstract: The one-pot condensation of glyoxal, two equivalents of cyclohexylamine, and paraformaldehyde in the presence of aqueous HBF4 provided a straightforward access to 1,3-dicyclohexylimidazolium tetrafluoroborate (ICy·HBF4). 1,3-Dibenzylimidazolium tetrafluoroborate (IBn·HBF4) was obtained along the same lines. To synthesize 1,3-diarylmidazolium salts, it was necessary to isolate the intermediate N,N'-diarylethylenediimines prior to their cyclization. Although this additional step required more time and reagents, it led to a much more efficient overall process. It also proved very convenient to carry out the synthesis of imidazolinium salts in parallel to their imidazolium counterparts via the reduction of the diimines into diammonium salts. The critical assembly of the C(2) precarbenic unit was best achieved with paraformaldehyde and chlorotrimethylsilane in the case of imidazolium derivatives, whereas the use of triethyl orthoformate under microwave irradiation was most appropriate for the fast and efficient synthesis of imidazolinium salts. This strategy was applied to the synthesis of six common N-heterocyclic carbene precursors, namely, 1,3-dimesitylimidazolium chloride (IMes·HCl), 1,3-dimesitylimidazolium tetrafluoroborate (IMes·HBF4), 1,3-dimesitylimidazolinium chloride (SIMes·HCl), 1,3-bis(2,6-diisopropylphenyl)imidazolium chloride (IDip·HCl or IPr·HCl), 1,3-bis(2,6-diisopropylphenyl)imidazolinium chloride (SIDip·HCl or SIPr·HCl), and 1,3-bis(2,6-bis(diphenylmethyl)-4-methylphenyl)imidazolium chloride (IDip*·HCl or IPr*·HCl).
authors.affiliation: Laboratory of Catalysis, Institut de Chimie (B6a), Allée du six Août 13, Quartier Agora, Université de Liège, 4000 Liège, Belgium.
authors.forename: Morgan
authors.initials: M
authors.lastname: Hans
authors.affiliation: Laboratory of Catalysis, Institut de Chimie (B6a), Allée du six Août 13, Quartier Agora, Université de Liège, 4000 Liège, Belgium; Faculty of Chemistry, Adam Mickiewicz University in Poznań, Umultowska 89b, 61-614 Poznań, Poland.
authors.forename: Jan
authors.initials: J
authors.lastname: Lorkowski
authors.affiliation: Laboratory of Catalysis, Institut de Chimie (B6a), Allée du six Août 13, Quartier Agora, Université de Liège, 4000 Liège, Belgium.
authors.forename: Albert
authors.initials: A
authors.lastname: Demonceau
authors.affiliation: Laboratory of Catalysis, Institut de Chimie (B6a), Allée du six Août 13, Quartier Agora, Université de Liège, 4000 Liège, Belgium.
authors.forename: Lionel
authors.initials: L
authors.lastname: Delaude
keywords: cyclization; experimental procedure; imidazolinium salt; imidazolium salt; microwave heating