Free radical reactions mediated by transition metal complexes
The work described in this thesis is concerned with an investigation into the role and behaviour of odd-electron transition metal complexes in promoting organic reactions. The metathesis reaction between cyclohexane and tetrachloromethane in the presence of a series of transition metal carbonyl-containing complexes has been studied and shown to afford chlorocyclohexane and trichloromethane in good yield. A detailed kinetic study of such a reaction was undertaken using [Re[sub]2(CO)[sub]10] as the metal complex. This study indicated that the reaction followed a free-radical chain mechanism in which the metal complex acted solely as an initiator. A survey has been made of the relative efficiencies of other metal carbonyl complexes in initiating this reaction and these findings have been compared with the known behaviour and thermal stabilities of the complexes. The scope of the reaction has also been investigated using a number of different halogenating agents and various saturated and alkyl aromatic substrates. An investigation has also been made into the role of transition metal complexes in promoting the reaction between toluene and sulphuryl chloride. A study of the reaction in the presence of a series of transition metal carbonyl-containing complexes shows that the predominant reaction leads to formation of chlorotoluenes via an even-electron (ionic) mechanism. Conversely, when the same reaction is performed in the presence of the platinum and palladium complexes [M(PPh[sub]3)[sub]4] and [MCl[sub]2(PPh[sub]3)[sub]2] (M = Pt or Pd), the predominant product is benzyl chloride formed via an odd-electron (radical) mechanism. Under the reaction conditions the zerovalent complexes are rapidly oxidised to [MCl[sub]2(PPh[sub]3)[sub]2] and it is proposed that these species are primarily responsible for radical chain initiation. The [RuCl[sub]2(PPh[sub]3)[sub]3]-mediated reaction between toluene and sulphuryl chloride proves to be equally selective but promotes formation of chlorotoluenes. A study of the effect of sulphuryl chloride on the metal complexes utilised in the above reactions has shown that sulphuryl chloride is a very effective mild chlorinating agent for dinuclear or coordinatively unsaturated metal complexes. Thus the reaction between [M[sub]2(CO)[sub]10] (M = Mn or Re) and sulphuryl chloride affords almost quantitative yields of [M(CO)[sub]5Cl] (M= Mn or Re) in a considerably more convenient procedure than previously reported. Moreover, reaction with [Re[sub]2(CO)[sub]8(PPh[sub]3)[sub]2] produces both the cis- and trans-isomers of [ReCl(CO)[sub]4PPh[sub]3], the latter being hitherto unknown. Treatment of [Pt(PPh[sub]3)[sub]4] with sulphuryl chloride similarly affords cis- and trans-[PtCI[sub]2(PPh[sub]3)[sub]2]. The halogenation of toluene with di-chlorine in the presence of a number of transition metal complexes has also been studied. At elevated temperatures in the presence of [M(PPh[sub]3)[sub]4] and [MCl[sub]2(PPh[sub]3)[sub]2] ] (M = Pt or Pd) the reaction is essentially identical to the analogous reaction with sulphuryl chloride. At 0[sup]o C however, this free radical reaction is supressed and only chlorotoluenes were formed via an ionic mechanism involving higher oxidation state complexes of platinum and palladium.