oxides and nitrones in the presence of Mg(II) cation as catalyst were evaluated. The presence of acetonitrile as co-solvent was found to be fundamental for the Lewis acid catalysis on solid-phase. The regio- and stereochemical outcome of nitrile oxide cycloadditions is influenced by nearly stoichiometric quantities of the cation, whilst catalytic amounts of Mg(II) influence both the reactivity and the
Evans' chiralauxiliary was grafted onto both Merrifield and Wang resins and, after functionalisation, they were used as chiral dipolarophiles in a 1,3-dipolarcycloaddition involving both a nitrileoxide and a nitrone. The cycloadducts were cleaved and analysed by chiral HPLC: the recovered supported chiral oxazolidinone was functionalised and reused in further cycloadditions. The stereochemical results
The highly chemo- and enantioselective organocatalytic three-component reaction between N-arylhydroxylamines, aldehydes and α,β-unsaturated aldehydes is presented; the reaction gives access to isoxazolidines in high yields with >25:1 dr and 91–99% ee.
A new method has been demonstrated to solve the aggregation issue of metalcomplex catalysts; Steric protection of the metal center of catalysts was effective in the cases of DBFOX/Ph-transition metalcomplexes by structural modification of chiral ligand. The new ligands have been successfully applied to the nitrone cycloadditions to a variety of α,β-unsaturated aldehydes. Excellent enantioselectivities
Solvent Enhancement of Reaction Selectivity: A Unique Property of Cationic Chiral Dirhodium Carboxamidates
作者:Xiaochen Wang、Carolin Weigl、Michael P. Doyle
DOI:10.1021/ja202676a
日期:2011.6.22
1,3-Dipolar cycloadditionreactions of nitrones with α,β-unsaturated aldehydes catalyzed by a cationic chiral dirhodium(II,III) carboxamidate with (R)-menthyl (S)-2-oxopyrrolidine-5-carboxylate ligands in toluene increase reaction rates, give optimum regioselectivities, and enhance stereoselectivities compared to the same reactions performed in traditionally used halocarbon solvents. Rate and enantioselectivity