Additive Effects of Amines on Asymmetric Hydrogenation of Quinoxalines Catalyzed by Chiral Iridium Complexes
| Autor: | Antonio Togni, Kazushi Mashima, Rino Schwenk, Yusuke Kita, Takashi Ohshima, Atsuhiro Iimuro, Takuto Nagano |
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| Rok vydání: | 2012 |
| Předmět: |
Reaction mechanism
Molecular Structure Diphenylphosphine Stereochemistry Organic Chemistry Asymmetric hydrogenation Enantioselective synthesis chemistry.chemical_element Disproportionation General Chemistry Iridium Medicinal chemistry Catalysis chemistry.chemical_compound chemistry Catalytic cycle Quinoxalines Organometallic Compounds Hydrogenation Amines |
| Zdroj: | Chemistry - A European Journal. 18:11578-11592 |
| ISSN: | 0947-6539 |
| DOI: | 10.1002/chem.201201366 |
| Popis: | The additive effects of amines were realized in the asymmetric hydrogenation of 2-phenylquinoxaline, and its derivatives, catalyzed by chiral cationic dinuclear triply halide-bridged iridium complexes [{Ir(H)[diphosphine]}(2)(μ-X)(3)]X (diphosphine = (S)-2,2'-bis(diphenylphosphino)-1,1'-binaphthyl [(S)-BINAP], (S)-5,5'-bis(diphenylphosphino)-4,4'-bi-1,3-benzodioxole [(S)-SEGPHOS], (S)-5,5'-bis(diphenylphosphino)-2,2,2',2'-tetrafluoro-4,4'-bi-1,3-benzodioxole [(S)-DIFLUORPHOS]; X = Cl, Br, I) to produce the corresponding 2-aryl-1,2,3,4-tetrahydroquinoxalines. The additive effects of amines were investigated by solution dynamics studies of iridium complexes in the presence of N-methyl-p-anisidine (MPA), which was determined to be the best amine additive for achievement of a high enantioselectivity of (S)-2-phenyl-1,2,3,4-tetrahydroquinoxaline, and by labeling experiments, which revealed a plausible mechanism comprised of two cycles. One catalytic cycle was less active and less enantioselective; it involved the substrate-coordinated mononuclear complex [IrHCl(2)(2-phenylquinoxaline){(S)-BINAP}], which afforded half-reduced product 3-phenyl-1,2-dihydroquinoxaline. A poorly enantioselective disproportionation of this half-reduced product afforded (S)-2-phenyl-1,2,3,4-tetrahydroquinoxaline. The other cycle involved a more active hydride-amide catalyst, derived from amine-coordinated mononuclear complex [IrCl(2)H(MPA){(S)-BINAP}], which functioned to reduce 2-phenylquinoxaline to (S)-2-phenyl-1,2,3,4-tetrahydroquinoxaline with high enantioselectivity. Based on the proposed mechanism, an Ir(I)-JOSIPHOS (JOSIPHOS = (R)-1-[(S(p))-2-(dicyclohexylphosphino)ferrocenylethyl]diphenylphosphine) catalyst in the presence of amine additive resulted in the highest enantioselectivity for the asymmetric hydrogenation of 2-phenylquinoxaline. Interestingly, the reaction rate and enantioselectivity were gradually increased during the reaction by a positive-feedback effect from the product amines. |
| Databáze: | OpenAIRE |
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