C₂-Symmetric Copper(II) Complexes as Chiral Lewis Acids. Catalytic Enantioselective Carbonyl−Ene Reactions with Glyoxylate and Pyruvate Esters

The C 2 -symmetric complexes [Cu( S,S )- tert -butylbis(oxazolinyl)](SbF 6 ) 2 ( 2a ), its bis(aquo) counterpart [Cu( S,S )- tert -butylbis(oxazolinyl)(H 2 O) 2 ](SbF 6 ) 2 ( 4 ), and [Cu( S,S )-phenylbis(oxazolinyl)](OTf) 2 ( 1c ) have been shown to catalyze the enantioselective ene reaction between glyoxylate esters and various olefins. Monosubstituted, disubstituted, and trisubstituted olefins each react with ethyl glyoxylate in the presence of 0.2−10 mol % of catalysts 1, 2, and 4 to afford the ene products in good yield and enantioselectivity. Complex 2a has also been shown to catalyze the addition of 1,1-disubstituted olefins to methyl pyruvate in good yields (76−95%) and excellent enantioselectivies (≥98% ee). The synthetic utility of the glyoxylate−ene reaction was demonstrated by conversion of the resulting α-hydroxy ester into the corresponding methyl ester, free acid, Weinreb amide, and α-azido ester, all with no detectable racemization.