Synthesis of palladium(II) cyclometalated complexes of 2-beta-disubstituted ethenyl-8-quinolinols and steric interactions of adjacent substituents onchelate ring stability

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 triden tate. Cyclometalation of the ligands with Na-2[PdCl4] in the presence of py ridine in MeOH took place at 25 degrees C to give (ca. 60%) the neutral che late complex possessing a Pd(II)-C sigma bond. When the pyridine was exchan ged with triphenylphosphine, steric interactions arose through three substi tuents bonded to alpha-C, beta-C of chelate ring and PPh3. As revealed from X-ray analysis his (benzyl ester) groups in Pd complex, the mutual repulsi on by adjacent groups causes the distinction between disubstituents. The re solution of the diastereomer was thus accomplished by the column chromatogr aphy. These indicate that Pd-C bond in cyclometalate ring is stable even un der the inherent structural molecular strain.