SIPr·HCl

SIPr·HCl (1,3-bis(2,6-diisopropylphenyl)imidazolinium chloride) is a precursor to the free N-heterocyclic carbene (NHC) 1,3-bis(2,6-diisopropylphenyl)-2-imidazolidinylidene (SIPr). SIPr is a saturated NHC ligand. The salt is also used to form catalysts in situ for a variety of reactions, for example amination, Heck coupling, ring-opening metathesis polymerisation (ROMP) and hydrogenation. It has been used in a one-pot synthesis of N-heterocyclic carbene–allylpalladium complexes.

N-Arylation of indoles. In a study of palladium/imidazolium salt systems for amination reactions, less reactive indoles were converted to N-aryl indoles with SIPr·HCl as the supporting ligand. The palladium(II) acetate/SIPr·HCl/sodium hydroxide system was efficient for the N-arylation of diverse indoles with aryl bromides.

Suzuki–Miyaura coupling with aryldiazonium salts. In one study, SIPr·HCl with palladium(II) acetate, without added base, was used as the catalyst to cross-couple aryl, vinyl and alkyl boronic acids and boronates with aryldiazonium tetrafluoroborates. Diazonium ions formed in situ also took part in the coupling.

Mizoroki–Heck coupling with aryldiazonium salts. In a related study, the same combination of SIPr·HCl and palladium(II) acetate, without added base, gave Heck coupling products from olefins and aryldiazonium tetrafluoroborates. Diazonium ions formed in situ directly from anilines also coupled.

α-Arylation of esters. In one study, palladium(0) with an imidazolinium salt of SIPr as the carbene precursor mediated the coupling of aryl halides with ester enolates to give α-aryl esters. In the same study, the tert-butyl esters of naproxen and flurbiprofen were prepared from tert-butyl propionate and aryl bromides with Pd(dba)₂ and the same carbene precursor.

Iron-catalysed biaryl coupling. In one study, iron fluorides in combination with SIPr catalysed the cross-coupling of aryl chlorides with aryl Grignard reagents to give biaryls. The authors describe the metal fluoride/NHC combinations in this work as a route to unsymmetrical biaryls without palladium catalysts or phosphine ligands.

Chemoselective Suzuki–Miyaura coupling. In the palladium-catalysed Suzuki–Miyaura coupling of chloroaryl triflates, one study found that SIPr as ligand led to selective coupling at the chloride. With SIMes, the related carbene with 2,4,6-trimethylphenyl groups, the coupling was selective for the triflate.