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Expanding the Scope of Asymmetric Electrophilic Atom-Transfer Reactions: Titanium- and Ruthenium-Catalyzed Hydroxylation of β-Ketoesters
Patrick Y. Toullec, Cristina Bonaccorsi, Antonio Mezzetti, Antonio Togni and Barry M. Trost
Proceedings of the National Academy of Sciences of the United States of America
Vol. 101, No. 16 (Apr. 20, 2004), pp. 5810-5814
Published by: National Academy of Sciences
Stable URL: http://www.jstor.org/stable/3371918
Page Count: 5
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The enantioselective formation of a quaternary stereogenic center coinciding with a hydroxylation process is a very rare reaction from a homogeneous catalysis point of view. Indeed, to our knowledge, no asymmetric transition-metal-catalyzed direct hydroxylation has been reported before. We describe here our initial study concerning the enantioselective α-hydroxylation of various β-ketoesters catalyzed by Lewis-acidic complexes. Specifically, it was found that the Ti complex [ TiCl2((R,R)-1- Np- TADDOLato)( MeCN)2] affords the hydroxylated products in high yield and enantioselectivities up to 94% enantiomeric excess when using 2-(phenylsulphonyl)-3-(4-nitrophenyl)oxaziridine as the oxidizing agent. Chiral enantiopure compounds of the latter type have been used previously in stoi-chiometric asymmetric hydroxylation reactions. We also show that, in a complementary approach with H2 O2 as the oxidant, the Ru(II) complex [ RuCl( OE2)((S,S)- PNNP)] PF6 catalyzes the same type of transformation in a case of substrates showing a very substantial extent of enolization under reaction conditions; being, however, unreactive toward only weakly enolized β-ketoesters.
Proceedings of the National Academy of Sciences of the United States of America © 2004 National Academy of Sciences