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Closed Access item Synthesis, Structure, and Applications of N-Dialkylamino-N‘-alkylimidazol-2-ylidenes as a New Type of NHC Ligands

Authors:Ros, Abel
Monge, David
Alcarazo, Manuel
Álvarez, Eleuterio
Lassaletta, José M.
Fernández, Rosario
Keywords:Alkylation, Allylic, Carbene complexes, Crystal structure, Molecular structure, Cyclization, Transition metal, Catalysts
Issue Date:18-Nov-2006
Publisher:American Chemical Society
Citation:Organometallics 25(26): 6039-6046 (2006)
Abstract: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.
Description:8 páginas, 3 figuras, 1 tabla, esquemas.
Publisher version (URL):http://dx.doi.org/10.1021/om060788f
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