Synthesis and characterization of novel five-membered palladacycles

3-(2-Chloroquinolin-3-yl)-1,5-bis(3,4,5-trimethoxy-phenyl)-pentane-2,4-dione derivatives 3a–b were conveniently synthesized in excellent yields (82% each) by tandem Knoevenagel condensation reactions of 2-chloro-3-carbaldehyde-quinoline 1a–b with 3,4,5-trimethoxy acetophenone, followed by a base catalyzed Michael addition, such as DBU (1,8-diazabicyclo[5,4,0]undec-7-ene) with or without solvent. The reactions of 3a–b with Pd(dba)2 in the presence of PPh3 (1:2) in degassed acetone provided the dinuclear palladium complexes {Pd(C,N-2-C9H4N–CH–[–CH2CO(3,4,5-(OMe-)3–C6H2-]2–3-R-6)Cl(PPh3)}2 [(R = H (4a), R = OMe (4b)] in moderate yields (38% and 43%), which in turn reacted with an excess of isonitrile XyNC (Xy = 2,6-Me2C6H3) to give the corresponding palladacycles 5a–b in moderate yields (45% and 43%). The palladacycles 5a–b were also obtained in similar yields (32% and 33%) via a one-pot oxidative addition reaction of 3a-b with isonitrile XyNC:Pd(dba)2 (4:1). The products were characterized by satisfactory elemental analysis and spectral studies (IR, 1H, and 31P NMR). The crystal structure of 5a was determined by X-ray crystallography diffraction studies.

Graphical abstractNovel palladacycles have been synthesized via a one-pot oxidative addition reaction of chloro-quinoline derivatives with Pd(dba)2 and 2,6-dimethylphenyl isocyanide XyNC (Xy = 2,6-Me2C6H3) or via the insertion reaction of isocyanide XyNC into a Pd–C bond of novel dinuclear palladium complexes. The sequence of reactions led eventually to compounds that could potentially possess pharmacological properties. The solid state structure of the selected complex 5a shows interesting features.Figure optionsDownload full-size imageDownload as PowerPoint slide