Synthesis, Structure, and Applications of N-Dialkylamino-N‘-alkylimidazol-2-ylidenes as a New Type of NHC Ligands^§

Condensation of enantiopure trans-(2S,5S)-1-amino-2,5-diphenylpyrrolidine (7) with formylaminoacetaldehyde (8) followed by POCl₃-promoted cyclization of the resulting hydrazone 9 provides an easy, unprecedented entry to trans-1-[(2S,5S)-2,5-diphenylpyrrolidin-1-yl]-1H-imidazole (6). This key compound can be readily alkylated with simple or functionalized alkyl halides to afford a series of imidazolium salts 10 and 21a−c containing the (2S,5S)-2,5-diphenylpyrrolidino group as a common characteristic. Reaction of the N‘-isopropyl derivative 10 with Ag₂O followed by transmetalation of the resulting silver carbene 11 with [MCl(COD)]₂ (M = Rh, Ir) afforded the corresponding [MCl(COD)(NHC)] complexes 12 and 13. Reaction of 12 with CO yielded the expected [RhCl(CO)₂(NHC)] product 14. The comparative analysis of the CO stretching frequencies of 14 with literature data reveals that the presence of the N-dialkylamino group does not modify the excellent σ-donor ability of the imidazol-2-ylidene ligand. On the other hand, thioether-containing imidazolium salts 21a−c can be transformed into Pd complexes 23a−c with bidentate C/S ligands by transmetalation of the corresponding silver carbenes 22a−c with [Pd(allyl)(COD)]⁺SbF₆^-. A preliminary study reveals that these complexes are suitable catalysts in allylic substitution reactions.