Synthesis of Palladium (II) Cyclometalated Complexes of 2-β-Disubstituted
Ethenyl-8-quinolinols and Steric Interactions of Adjacent Substituents on Chelate Ring Stability

Shinichi OHFUCHI*, Chitoshi KITAMURA, Yoshihiko MAEKAWA††,
George R. NEWKOME††† and Akio YONEDA

Department of Applied Chemistry, Kobe City College of Technology; 8-3, Gakuenhigashi-machi, Nishi-ku,Kobe-shi 651-2194 Japan
Department of Applied Chemistry,Faculty of Engineering, Himeji Institute of Technology;
2167, Shosha, Himeji-shi 671-2201 Japan
†† Nagase-CIBA Ltd.; 236, Nakai, Tatsuno-machi, Tatsuno-shi 679-4124 Japan
††† Department of Chemistry, University of South Florida, Tampa, FL 33620, U.S.A

2-Ethenyl-8-quinolinols containing bulky substituents in the 2-position of the ethenyl group have been prepared. From the NMR and X-ray analysis data, O, N and 2-C atoms of the ligand are expected to be in-plane of the tridentate. Cyclometalation of the ligands with Na2[PdCl4] in the presence of pyridine in MeOH took place at 25 °C to give (ca. 60%) the neutral chelate complex possessing a Pd (II)-C σ bond. When the pyridine was exchanged with triphenylphosphine, steric interactions arose through three substituents bonded to α-C, β-C of chelate ring and PPh3. As revealed from X-ray analysis bis (benzyl ester) groups in Pd complex, the mutual repulsion by adjacent groups causes the distinction between disubstituents. The resolution of the diastereomer was thus accomplished by the column chromatography. These indicate that Pd-C bond in cyclometalate ring is stable even under the inherent structural molecular strain.

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