C₁-Symmetric Aminosulfoximines as Ligands in Copper-Catalyzed Carbonyl-Ene Reactions

Abstract

Highly modular C₁-symmetric aminosulfoximines were prepared and applied as chiral ligands in copper-catalyzed enantio­selective carbonyl-ene reactions. The optimized system catalyzed the conversion of pyruvates and 1,1-disubstituted olefins yielding the corresponding hydroxy esters with high enantiomeric excesses (up to 91% ee) in moderate yields.

Key words

asymmetric catalysis - α-hydroxy esters - carbonyl-ene reactions - copper - sulfoximines

References

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18

A pronounced effect of ortho-alkoxy groups was observed in hetero-Diels-Alder reactions catalyzed by copper complexes bearing N-quinolyl sulfoximines. For details, see ref. 11.

19

Experimental Procedure.
In a dried Schlenk-flask under an argon atmosphere, CuCl₂ (6.7 mg, 0.05 mmol) was suspended in dry CH₂Cl₂ (2 mL) and then treated with AgClO₄ (20.7 mg, 0.1 mmol). The mixture was stirred for 60 min at r.t. under exclusion of light and subsequently, aminosulfoximine 1a or 1g (0.05 mmol) was added. Stirring of the resulting blue suspension was continued for another 30 min followed by the addition of the enophile (0.5 mmol) and the olefin (5-20 equiv). After stirring for 48 h at r.t., the mixture was diluted with Et₂O (50 mL) and filtered through a plug of silica gel. The solvent was removed under reduced pressure, and the corresponding product was purified by column chromatography (pentane-EtOAc, 15:1 for 9a and 9c, 10:1 for 9b) and obtained as colorless oil.

20

Unfortunately, 9b was obtained as a mixture with enophile 7b, which was inseparable by column chromatography. Use of (chiral) HPLC, however, allowed the determination of the enantiomeric ratio of 9b (80% ee). The ‘yield’ of 9b was then estimated from the NMR spectra (44%).