Kinetics and mechanism of the addition of triphenylphosphoniocyclopentadienide to tetrachloro-p-benzoquinone

Abstract

The kinetics and mechanism of the reaction of triphenylphosphoniocyclopentadienide 1 with tetrachloro-p-benzoquinone (chloranil 2a) to yield 2,3,5-trichloro-6-(3-triphenylphosphoranylidene-cyclopenta-1,4-dienyl)-p-benzoquinone 3 in dichloromethane solution are reported. The rate limiting step was found to be electrophilic attack of tetrachloro-p-benzoquinone on the aromatic cyclopentadiene ring of the ylide. A thermochemical study of the system showed a small enthalpy and a large negative entropy of activation in agreement with the existence of a highly polar betaine as ah intermediate in the reaction. The proposed mechanism involves bimolecular transfer of a proton to a molecule of ylide. No evidence was found either for the formation of a π-complex or for acid catalysis of the reaction.