Group 16 atoms, nucleophilic substitution proposed mechanism

A reasonable mechanism for the iodine oxidation of 5-Trt cysteine peptides is given in Scheme 6. 45 Reaction of iodine with the divalent sulfur atom leads to the iodosulfonium ion 5 which is then transformed to the sulfenyl iodide 6 and the trityl cation. Sulfenyl iodides are also postulated as intermediates in the iodine oxidation of thiols to disulfides. The disulfide bond is then formed by disproportionation of two sulfenyl iodides or by reaction between the electrophilic sulfur atom of R -S-I and the nucleophilic S-atom of a second R -S-Trt molecule. The proposed mechanism suggests that any sulfur substitution (i.e., thiol protecting group) capable of forming a stabilized species on cleavage, such as the trityl cation, can be oxidatively cleaved by iodine. 

In the another hypothesis, it has been assumed [37] that the rotation of the alkyl group R in the coordination sphere of platinum entails an appreciable intensification of the nucleophilic properties of the carbon atom, which should facilitate the electrophilic attack on the latter by the deuterium cation D. The alkyl group rotates synchronously with the approach of D and the agostic coordination between the C-H bond and platinum ion is formed. Since it is evident that several hydrogen atoms can be substituted by deuterium in the alkyl group before it is split off from platinum, the proposed mechanism explains satisfactorily the multiplicity of different types of H-D exchange in the presence of platinum(ll) complexes. 

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