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Stereoselective Synthesis of Rhodium(I) 4-(Dialkylamino)triazol-5-ylidene Complexes

AuthorsRos, Abel ; Alcarazo, Manuel; Iglesias-Sigüenza, Javier; Díez, Elena; Álvarez, Eleuterio ; Fernández, Rosario; Lassaletta, José M.
Asymmetric hydrosilylation
Issue Date27-Aug-2008
PublisherAmerican Chemical Society
CitationOrganometallics 27(18): 4555-4564 (2008)
AbstractThe synthesis of 1-alkyl-1,2,4-triazolium salts containing chiral 4-dialkylamino substituents was accomplished by selective alkylation at N-1 of the corresponding 4-(dialkylamino)-1,2,4-triazoles. The latter were synthesized by reaction of N,N-dimethylformamidazine dihydrochloride with N,N-dialkylhydrazines in pyridine. Alternatively, the synthesis of 4-[(2S,5S)-2,5-diphenylpyrrolidin-1-yl]-1-phenyl-1,2,4-triazolium perchlorate was performed by reaction of 3-phenyl-1,3,4-oxadiazolium perchlorate with (2S,5S)-1-amino-2,5-diphenylpyrrolidine. These triazolium salts were transformed into neutral [RhCl(NHC)(COD)] complexes (NHC = 4-(dialkylamino)-1-alkyl(phenyl)triazol-5-ylidene) by reaction with [RhCl(COD)]2 in the presence of triethylamine. The products, featuring restricted C(carbene)−Rh bond rotation, were formed with high levels of selectivity in all cases, the de’s being >98% for complexes bearing the (2S,5S)-2,5-diphenylpyrrolidino group. A cationic heterobidentate NHC/S complex was also synthesized in a highly stereoselective way by alkylation of 4-[(2S,5S)-2,5-diphenylpyrrolidin-1-yl]-1,2,4-triazole with (R)-1-bromo-2-(cyclohexylthio)-3-methylbutane, followed by treatment of the resulting triazolium salt with [RhCl(COD)]2/Et3N and ensuing bromide abstraction by AgSbF6. A preliminary evaluation of the neutral and cationic Rh complexes in the asymmetric hydrosilylation of acetophenone indicated good catalytic activity and moderate enantioselectivities for most complexes, reaching a promising 62% ee in the case of the cationic complex.
Description10 páginas, 6 figuras, 1 tabla, 8 esquemas.
Publisher version (URL)http://dx.doi.org/10.1021/om800533g
Appears in Collections:(IIQ) Artículos
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