The Kinetics and Mechanism of the Reaction of Onium Cyclopentadienylides with Tetrahalo-p-Benzoquinones

Abstract

The reaction of triphenylphosphonium cyclopentadienylide (1) with halogen-substituted p-benzoquinones (4) is shown to give a new class of dipolar (zwitterionic) dyes (5) containing phosphorus. The general structure of these molecules has been investigated by a combination of mass spectrometry and multinuclear (¹H, ^l3C and 3l^P) nmr using the specialist techniques of DEPT spectroscopy, homonuclear (COSY) and heteronuclear, 2-D nmr. In addition, stopped-flow (uv/vis) techniques have been used to study the kinetics of the reactions and hence demonsrate that the rate-limiting step is nucleophilic addition of the ylid to the quinone, followed by a rapid loss of halide ion. The mechanism follows the classical pattern associated with nucleophilic aromatic substitution in activated aryl halides.