Six- and seven-membered ring carbenes: Rational synthesis of amidinium salts, generation of carbenes, synthesis of Ag(I) and Cu(I) complexes

Reactions of neat 1,3- and 1,4-dibromides with N,N′-diarylformamidines in the presence of diisopropylethylamine (DIPEA) afford corresponding amidinium salts in high yields (>80%). Six- and seven-membered ring amidinium salts bearing bulky Mes (2,4,6-Me3C6H2) and Dipp (2,6-iPr2C6H3) aryl groups were prepared using this method. Free six-membered ring carbene 6-Dipp was generated from amidinium salt using LiHMDS as a base. NHC–Ag(I) complexes were obtained by the reactions of amidinium salts with Ag2O. NHC complexes of Pd and Rh are not accessible by deprotonation of amidinium salts, nor by transmetallation of Ag(I) complexes. However NHC–Cu(I) complexes were obtained by transmetallation of NHC–Ag(I). Thus, transmetallation of six- and seven-membered NHC–Ag(I) complexes was documented for the first time.

Graphical abstractA new efficient method of synthesis of six- and seven-membered ring amidinium bromides bearing bulky Mes (mesityl) and Dipp (2,6-(i-Pr)-Ph) aryl groups was developed. Deprotonation of these salts with Ag2O leads to (NHC)AgBr (NHC – N-heterocyclic carbene) complexes. These complexes undergo further transmetallation when treated with CuBr to give (NHC)CuBr.Figure optionsDownload full-size imageDownload as PowerPoint slide