Cationic [2,6-Bis(2‘-oxazolinyl)phenyl]palladium(II) Complexes:  Catalysts for the Asymmetric Michael Reaction

Reaction of 1,3-dicyanobenzene with β-amino alcohols ( S )-H 2 NCHRCH 2 OH (R = i Pr, i Bu, t Bu, CH 2 Cy, CH 2 Ph) and ( R )-H 2 NCHPhCH 2 OH gave new 1,3-bis(2‘-oxazolinyl)benzenes (30−51%). These, together with 1,3-bis(4‘,4‘-dimethyl-2‘-oxazolinyl)benzene, were treated with LDA/TMEDA followed by the addition of PdBr 2 (1,5-COD) to give [2,6-bis(2‘-oxazolinyl)phenyl]palladium(II) bromide complexes (21−41%). In two cases no complexes were obtained (R = Ph, CH 2 Ph) due to ring opening of the oxazolines by LDA/TMEDA. Treatment of the palladium complexes with AgBF 4, AgOTf, or AgSbF 6 in wet CH 2 Cl 2 provided a series of cationic [2,6-bis(2‘-oxazolinyl)phenyl]palladium complexes (28−87%) containing water coordinated to palladium, as established by an X-ray crystal structure analysis of ( S, S )-[2,6-bis(4‘-isopropyl-2‘-oxazolinyl)phenyl]aquopalladium(II) trifluoromethanesulfonate. All of the cationic complexes proved to be efficient catalysts for the Michael reaction between α-cyanocarboxylates and methyl vinyl ketone and between acrylonitrile and activated Michael donors. Selectivities of up to 34% ee were obtained for the formation of ( R )-ethyl 2-cyano-2-methyl-5-oxohexanoate.