Thermal Elimination Reactions of Xanthates, N-Oxides, Sulfoxides, and Selenoxides

Thermal )6-elimination reactions of acetates, benzoates, xanthates, sulfoxides, selenoxides, and N-oxides are also group transfer reactions. All these elimination reactions are yn-stereospecific and proceed through a cyclic six membered—or five membered—ring transition state of 6e process by intramolecular transfer of hydrogen atom, where all the participating orbitals have suprafacial interactions. These reactions are fundamentally retro-group transfer reactions. 

Pyrolysis of xanthate esters known as Chugaev reaction requires temperatures of 150-250 °C. For example, the pyrolysis of xanthates 4 and 5 gives more substituted olefins (Saytzeff products) as major products. 

Pyrolysis of N-oxides known as Cope elimination takes place at lower temperatures (100-150 °C). The pyrolysis of sulphoxides and selenoxides takes place easily below 100 °C because of weaker C-S and C-Se bonds. As for example, ery/firo-N-oxide 6 and eryT/iro-sulfoxide 7 on pyrolysis give cis and trans olefins as major product, respectively. 

Similarly, yn-elimination of PhSeOH from selenoxide 8 and selenoxides derived from selenides 9-11 in presence of H2O2 occurs below rt to produce enones in high yields. 

Suggest a mechanism of the following reactions More than one step may be involved. 

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