A new class of C<sub>1</sub>-symmetric monosulfoximine ligands for enantioselective hetero Diels–Alder reactions
作者:Carsten Bolm、Marinella Verrucci、Oliver Simic、Pier Giorgio Cozzi、Gerhard Raabe、Hiroaki Okamura
DOI:10.1039/b309556h
日期:——
Quinoline-based C1-symmetricsulfoximines have been used as chiral ligands in copper-catalyzedasymmetric hetero Diels-Alder reactions leading to cycloadducts with up to 96% ee.
A group of chiral [(salen)Cr(III)] + BF 4 - complexes, with enhanced steric hindrance in 3,3'-positions of salicylidene moiety, has been synthesized and applied for the oxo-Diels-Alder reaction of alkyl glyoxylates with cyclohexa-1,3-diene. A readilyaccessiblecomplex that bears bulky adamanthyl substituents revealed its potential, leading to the cycloadducts with excellent selectivity (up to endolexo
Ethylene-Bridged Bissulfoximines in Copper-Catalyzed Enantioselective Hetero-Diels--Alder Reactions
作者:Carsten Bolm、Marinella Verrucci、Oliver Simic、Christian P. R. Hackenberger
DOI:10.1002/adsc.200505104
日期:2005.10
Ethylene-bridgedbissulfoximines were applied as chiral ligands in copper-catalyzedenantioselective hetero-Diels–Alder reactions. After optimization of the reaction conditions, products with up to 99% ee were obtained.
The stereoselective hetero Diels–Alderreaction between ethyl glyoxylate and cyclohexadiene catalyzed by [Cu(II)t-Bu-(box)](OTf)2 was investigated. The reaction was performed step-by-step and the geometry of the Cu(II) complexes formed in the course of the catalysis was analysed by EPR spectroscopy, advanced pulsed EPR methods (ENDOR, and HYSCORE) and DFT calculations. Our results show that one triflate
The zinc(II)-catalysed hetero-DielsâAlder reaction
of the conjugated dienes 2,3-dimethylbuta-1,3-diene and
cyclohexa-1,3-diene with ethyl glyoxylate has been studied in the
presence of different C2-symmetric bisoxazolines.
The zinc(II)-catalysed reaction of 2,3-dimethylbuta-1,3-diene
with ethyl glyoxylate gives both the hetero-DielsâAlder and ene
products, the former being the major product with an enantiomeric excess
up to 87%. The hetero-DielsâAlderâ¶ene ratio is relatively
independent of the catalytic system and the solvent and is in the range
1â¶0.5â1â¶0.8. For the zinc(II)-catalysed
reaction of cyclohexa-1,3-diene with ethyl glyoxylate, the
hetero-DielsâAlder product is formed in up to 84% isolated yield
and with an enantiomeric excess up to 65%. The enantiomeric excess for
this reaction is very dependent on the solvent, with MeNO2
generally lowering the enantiomeric excess compared with
CH2Cl2. Based on the absolute stereochemistry of
the hetero-DielsâAlder products and semi-empirical calculations on
different bisoxazolineâzinc(II)âethyl glyoxylate
intermediates the mechanism for the reaction is discussed.