A kinetic template effect in arylphosphonium salt formation.

This thesis describes studies of a "kinetic template effect" which assists the formation of arylphosphonium salts from aryl halides and tertiary phosphines in the presence of a transition metal halide catalyst in refluxing ethanol. The "kinetic template effect" arises from the presence in the aryl halide of a limited range of orthosubstituents capable of intramolecular coordination with the metal at a critical stage of the reaction. In Chapter One, the "kinetic template effect" is compared with the better known "thermodynamic template effect". Earlier work on related "kinetic template effects" in the formation of aryl-phosphorus bonds is reviewed. The evidence for the possible involvement of aryl-metal intermediates in such reactions is also discussed. Chapter Two is concerned with the design and synthesis of potential template molecules. A model is proposed for the features necessary in the template substituent in terms of the nature and position of the donor atom or group essential for the replacement of the ortho-halogen under mild conditions. Chapter Three describes a kinetic study of the nickel (II) catalysed reactions of ortho-haloaryl Schiff's base and ortho-haloarylazo-dyestuff templates with tertiary phosphines. A rate law is deduced which indicates a first order dependence in each reactant, i.e. rate a [template] [phosphine] [catalyst], an overall third order expression. Rate studies also indicate that the nature of the orthohalogen is important, the order of replacement being I>Br>Cl. The effects of substituents remote from the ortho-haloaryl template are also considered. A mechanistic scheme consistent with the rate data is proposed.The X-ray crystal structures of two arylphosphonium salts derived from template aryl halides are discussed in Chapter Four, providing unequivocal proof that the position of replacement of halogen in substrates bearing more than one replaceable halogen, in different positions, is ortho with respect to the template donor group.