Ortho-Palladation of (Z)-2-Aryl-4-Arylidene-5(4H)-Oxazolones. Structure and Functionalization

Ortho-palladated complexes of (Z)-2-aryl-4-arylidene-5(4H)-oxazolones have been prepared through oxidative addition. The reaction of (Z)-2-phenyl-4-(2-bromobenzylidene)-5(4H)-oxazolone (4) with Pd₂(dba)₃·CHCl₃ gives the six-membered cyclopalladated dinuclear complex [Pd(μ-Br)(o-C₆H₄CHCNC(O)OCPh)]₂ (7). The reaction of 7 with PPh₃ gives dinuclear 9, which incorporates one phosphine per Pd atom through cleavage of the Pd−N bond, and preserves the bromide bridging system. However, reaction with PPh₂Me gives mononuclear 8, which incorporates two phosphines as a results of the cleavage of the μ-Br system and N displacement. In contrast, the reaction of 7 with pyridine gives complex 12 due to simple cleavage of the Br bridge, leaving the N-bonding intact. Therefore, three different reaction pathways have been characterized. The reactivity of the Pd−C bond in 7 has also been examined, and functionalized oxazolones can be obtained. The reaction of 7 with PhI(OAc)₂ in acetic acid gives the starting oxazolone C₆H₄-2-Br-CHCNC(O)OCPh (4), through the presumed oxidation of the Pd center and C−Br bond formation by reductive coupling. In contrast, the reaction of the acetate dimer 14 with PhI(OAc)₂ in acetic acid gives C₆H₄-2-OAc-CHCNC(O)OCPh (20) through C−O coupling. When treatment of 7 with PhI(OAc)₂ is performed in MeOH or EtOH, the oxazolones C₆H₄-2-OR-CHCNC(O)OCPh (R = Me (18), Et (19)) are obtained. The reaction of 7 with CO in alcohols ROH gives cleanly the oxazolones C₆H₄-2-CO₂R-CHCNC(O)OCPh (R = Me (21), ⁱPr (22)) through CO migratory insertion into the Pd−C bond and further nucleophilic attack of the RO⁻ fragment.