Synthesis of coumarin derivatives by palladium complex catalyzed intramolecular Heck reaction: Preparation of a 1,2-cyclobutadiene-substituted CpCoCb diphosphine chelated palladium complex

A cobalt-sandwich diphosphine chelated palladium complex 2, [(η5-cyclopentadienyl)(η4-1,2-diphenyl-3,4-bis(diphenylphosphino-κP)cyclobutadiene)cobalt(I)]palladium(II)dichloride, was prepared from the reaction of a cyclobutadiene-substituted CpCoCb diphosphine 1, (η5-cyclopentadienyl) (η4-1,2-diphenyl-3,4-bis-diphenylphosphinocyclobutadiene)cobalt(I), with one molar equivalent of Pd(COD)Cl2. Both compounds were characterized by spectroscopic means as well as single-crystal X-ray diffraction methods. Application of purified 2 as the catalytic precursor in intramolecular Heck cyclization reaction on 4-hydroxycoumarin resulted in the formation of several derivatives with intensity enhanced fluorescence property.