Imidazolium‐Oxazoline Salts in Ruthenium‐Catalyzed Allylic Substitution and Cross Metathesis of Formed Branched Isomers
作者:Hamed Ben Ammar、Bechir Ben Hassine、Cédric Fischmeister、Pierre H. Dixneuf、Christian Bruneau
DOI:10.1002/ejic.201000718
日期:2010.10
Imidazolium-oxazoline chlorides have been prepared from chloroacetonitrile and used to generate bidentate mixed NHC-oxazoline ligands for ruthenium-catalyzed substitution of cinnamylchloride by phenols. These ligands associated to [RuCp*(MeCN)3][PF6] promote allylic substitution reactions at room temperature with high regioselectivity in favour of the branched isomers giving terminal alkenes. These allylic ethers
Lithiated 2-(1-chloroethyl)-4,4-dimethyl-2-oxazoline 7 adds to electron-poor alkenyl heterocycles to afford substituted cyclopropanes in excellent yields. A route to chiral nonracemic heterosubstituted cyclopropanes, starting from optically active 2-chloromethyl-2-oxazolines, is highlighted as well.
Asymmetric induction afforded by chiral azolylidene N-heterocyclic carbenes (NHC) catalysts
作者:Ragnhild B. Strand、Trygve Helgerud、Tina Solvang、Amund Dolva、Christian A. Sperger、Anne Fiksdahl
DOI:10.1016/j.tetasy.2012.09.004
日期:2012.10
Screening studies of new chiral imidazolium and triazolium based NHC salts I-VIII as ligands in asymmetric organometallic catalysis and as organocatalysts showed that these catalysts efficiently promoted the reactions. Moderate enantioselectivities (55-57% ee) were obtained in the asymmetric Cu-NHC catalysed conjugate additions of diethylzinc to cyclohexenone, in accordance with most previous studies. The chiral induction afforded in the gold(I)-NHC catalysed cyclopropanation reactions was low (< 28% ee). However, these results represent the first reported chiral gold(I)-NHC catalysed olefin cyclopropanation. The NHC-organocatalysed asymmetric cross-annulation of cinnamaldehyde and trifluoroacetophenone gave lower enantioselectivity (< 50% ee) but higher yields of the gamma-lactone product relative to previous reports. The enantioselectivities obtained varied considerably, even within a group of structurally closely related NHCs. This study demonstrates the challenge of designing NHCs with a general ability to induce asymmetry in a broader range of reactions. (C) 2012 Elsevier Ltd. All rights reserved.
STEREOSELECTIVE SYNTHESIS OF ENDOTHELIN RECEPTOR ANTAGONISTS