Phthaloylcobalt complexes in synthesis. Ligand modifications leading to a practical naphthoquinone synthesis

Article abstract of DOI:10.1021/om00137a004

Modification of the ligands of trigonal-bipyramidal (phthaloyl)Co(PPh₃)₂Cl has led to an understanding of how this complex reacts with alkynes to give naphthoquinones when activated with AgBF₄. Through X-ray crystal structure determinations and temperature-dependent ¹H NMR spectra, it has been determined that reaction with alkynes requires conversion of (phthaloyl)Co(PPh₃)₂Cl into a six-coordinate complex with a dissociable ligand above or below the plane defined by the phthaloylcobalt ring. Treatment of (phthaloyl)Co(PPh₃)₂Cl with dimethylglyoxime in pyridine effects such a transformation in high yield, and the resulting air-stable complex (phthaloyl)Co(dimethylglyoxime)(pyridine)Cl reacts with alkynes at 80°C to produce excellent yields of naphthoquinones. A simple synthesis of menaquinones has been realized by using this method. In addition, SnCl₄ has been shown to catalyze the quinone forming reaction at room temperature.