Stereoselective synthesis of rhodium(I) 4-(dialkylamino)triazol-5-ylidene complexes

The 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(N-HC)(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.