Asymmetric catalysis, 132 -: Metal-catalyzed enantioselective α-ketol rearrangements

被引:0
作者
Brunner, H [1 ]
Stöhr, F [1 ]
机构
[1] Univ Regensburg, Inst Anorgan Chem, D-93040 Regensburg, Germany
关键词
rearrangements; isomerizations; ketones; asymmetric catalysis;
D O I
暂无
中图分类号
O62 [有机化学];
学科分类号
070303 ; 081704 ;
摘要
Promoted by catalytic amounts of transition-metal complexes, the tertiary alpha-hydroxy ketones 1, 3, 5/6 undergo alpha-ketol rearrangements to afford equilibrium mixtures of isomers with a reorganization of the carbon skeleton. The range of metal complexes catalyzing the isomerizations is large; the best results were obtained with the catalyst systems NiCl2/TMEDA, Ni(acac)(2), and Ni(acac)(2),TMEDA (TMEDA N, N, N', N'-tetramethyl-1, 2-diaminoethane). The catalytic rearrangements were performed at 130 degrees C in the absence of solvent, with a Ni-II/ligand/substrate ratio of 1:2:100. The equilibrium composition of the model system 1/2 is 12.5:87:5. The conversion of the achiral substrates 1 and 3 into the chiral products 2 and 4 can be used for kinetic resolution However, the reverse reactions 2 --> 1 and 4 --> 3 in the equilibrations narrow the window for asymmetric induction with enantioselective catalysts of the metal component/optically active ligand type. In system 1, the highest enantiomeric excess was achieved with the catalyst systems NiCl2/pybox [18.9% (S)-2] and Ni(acac)(2)/pybox [19.3% (R)-2] {pybox = 2,6-bis[(S)-4-isopropyl(oxazolin-2'-yl)]pyridine}. The alpha-ketol rearrangement of 3 with the Ni(acac)(2)/pybox catalyst resulted in a maximum enantiomeric excess of 37.1% (S)-4.
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页码:2777 / 2786
页数:10
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