Synthesis of 2,6-Bis(2-oxazolinyl)phenylplatinum(II) NCN Pincer Complexes by Direct Cyclometalation. Catalysts for Carbon−Carbon Bond Formation

Abstract

1,3-Bis(4‘,4‘-dimethyl-2‘-oxazolinyl)benzene (5a) and 5-nitro-1,3-bis(4‘,4‘-dimethyl-2‘-oxazolinyl)benzene (5b) were heated in dry acetic acid with K2PtCl4 to give the corresponding 2,6-bis(4‘,4‘-dimethyl-2‘-oxazolinyl)phenylplatinum(II) chloride complexes 6a and 6b in 49 and 11% yield, respectively. The X-ray structure of 6b is reported. The main side product observed in the platination of 5a was identified as di(2-methyl-2-N-acetyl)propyl isophthalate. In contrast, use of Pd(OAc)2 with 5a in this protocol, followed by addition of LiBr, gave 2,6-bis(4‘,4‘-dimethyl-2‘-oxazolinyl)phenylpalladium(II) bromide in only 3% yield. Treatment of 6a with AgOTf and AgSbF6 in acetone gave quantitatively the corresponding cationic 2,6-bis(4‘,4‘-dimethyl-2‘-oxazolinyl)phenyl(aquo)platinum(II) complexes 15a and 15b. Similarly treatment of 6b with AgOTf in acetone gave 4-nitro-2,6-bis(4‘,4‘-dimethyl-2‘-oxazolinyl)phenyl(aquo)platinum(II) trifluoromethanesulfonate (15c) (72%). Complexes 15a−c, together with 2,6-bis(4‘,4‘-dimethyl-2‘-oxazolinyl)phenylaquopalladium(II) triflate (15d), were applied as catalysts for the Michael reaction between methyl vinyl ketone and ethyl cyanoacetate and the Diels−Alder reaction between acrylonitrile and cyclopentadiene. In both cases platinum complex 15a was found to be the most active, with the 4-nitro group of 15c resulting in decreased catalytic activity.