Pyrolysis of sulphoxides

On pyrolysis, sulphoxides with a (3-hydrogen undergo syn elimination to give alkenes (Scheme 6.24). The pyrolysis of sulphoxides takes place at about 80 °C. Like other concerted pericyclic reactions, this reaction is also highly stereoselective. For example, the anff-sulphoxide (1) on pyrolysis gives mainly frawi-methylstilbine (2), whereas the corresponding jyn-sulphoxide (3) gives predominantly cw-methylstilbine (4). 

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. 

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