Sigma orbitals migration

We shall distinguish two different sigmatropic changes, which we represent schematically in Fig. 20, namely the migration of a group via a sigma orbital or by a pi orbital. 

The electrophilic nature of sulfuryl chloride is validated in a study on the chlorination of anthracene derivatives the observed substituent effects conform well with molecular orbital modeling859. Chlorination of ethyl 3,4,5-trialkylpyrrole-2-carboxylates results in a substantial amount of / -side-chain chlorinated products in addition to the expected a-side-chain chlorination860. This has been ascribed to the electrophilic nature of the reagent861 and the mechanism proposed involves an allylic migration of the halogen from the a-sigma adduct to the adjacent / -side chain. 

In this reaction a hydrogen that is sigma bonded to the carbon that is adjacent to the pi bond (C-l) migrates to the end carbon of the pi bond (C-3). As this occurs, the pi bond migrates from C-2-C-3 to C-l-C-2. A picture of the orbitals involved in this reaction may help make the process clearer ... 

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