Thiolate push

Only irreversible removal of the dithiol or of the solvolysis products can push the equilibrium to the right. Methods of choice are transacetalization to a highly reactive carbonyl derivative, alkylation to sulfide, oxidation of the thiol and formation of metal thiolates, for which mercury(II) salts are frequently used. 

Figure 17 A schematic view of the involvement of axial hgands and distal residues in the push-puU mechanism for 0-0 bond cleavage of an iron-boimd peroxide in thiolate-ligated (left) and histidine-hgated (right) systems such as cyUochrome P450 and horseradish peroxidase, respectively ...

The mechanism of the acid-catalyzed leaving group departure is probably concerted, with transition states 94 for the and 95 for the step, respectively. There is an increase in and with increasing basicity of the thiolate ion. This indicates the operation of an electronic push by the RS group, presumably because of developing resonance (96) in the transition state. From the slopes of plots of... 

Figure 2.6. Calculated (UB3LYP/LACV3P ) push effect of the thiolate ligand (a) on the reduction energy 2 3, and (b) on the reduction of 1 -r 1 .

The thiolate ligand was implicated as a crucial factor in the 0-0 bond cleavage process through its push effect that leads to Cpd I (ref [54]). Ogliaro et al. have addressed this issue by comparing the proton affinity of 6 to a reference complex without a thiolate ligand. The thiolate ligand was found to increase the proton affinity of ferric peroxide by 81 kcal mol . At the same time, the protonated reference complex devoid of thiolate loses water spontaneously as well. Thus, the push effect of the thiolate does not concern... 

Ogliaro, E, S.P. De Visser, and S. Shaik (2002). The push effect of the thiolate ligand in cytochrome P450 A theoretical gauging. J. Inorg. Biochem. 91, 554-567. 

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