Cationic [2,6-bis(2′-oxazolinyl)phenyl]palladium(II) complexes:: Catalysts for the asymmetric Michael reaction

Reaction of 1,3-dicyanobenzene with beta-amino alcohols (S)-H2NCHRCH2OH (R = Pr-i, Bu-i, Bu-t, CH2Cy, CH2Ph) and (R)-H2NCHPhCH2OH gave new 1,3-)bis(2'-oxazolinyl)benzenes (30-51%). These, together with 1,3-bis(4',4'-dimethyl-2'-oxazolinyl)ben were treated with LDA/TMEDA followed by the addition of PdBr2(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, CH2Ph) due to ring opening of the oxazolines by LDA/TMEDA. Treatment of the palladium complexes with AgBF4, AgOTf, or AgSbF6 in wet CH2Cl2 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 alpha-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.