| Summary: | The rates of the methoxide ion induced protiodeiodination of a number of polychloroiodoarenes in dimethyl sulphoxide-methanol (9:1, v/v) have been measured at 323·2K. Chlorine substituents activate all positions in the order, o-Cl > m-Cl > p-Cl, although the more fully substituted polychloroiodoarenes show much weaker substituent effects. The true reagent effecting the reactions appears to be the dimsyl anion, and the rates of reaction in some cases reach and exceed that expected of an encounter-controlled process. This may account for the major decrease in efficiency of further activating substituents. The extent to which concomitant methoxydehalogenation occurs has been checked partly by product analysis and partly by comparison with the rates of methoxydechlorination of some allied polychlorobenzenes. Methoxydehalogenation is an expected mode of reaction in a number of cases. Only in studies of some non-ortho-substituted compounds is extensive methoxydehalogenation observed. The presence of an ortho-chlorine substituent promotes the protiodeiodination reaction to the exclusion of methoxydehalogenation. The addition of fluorene to the reaction medium to provide a second and competing carbanion causes the formation of V9,9 -bifluorenylidene whose presence suggests the intermediacy of 9,9'-iodofluorene. This and a number of other observations suggest that the mechanism of the reduction involves the loss of iodine as I<sup>delta+</sup> towards a suitable nucleophile and that, despite the similarity of reaction conditions, the S<sub>RN</sub>1 mechanism is not operating in these systems.
|
|---|
