Sigma orbitals 5.5 -sigmatropic

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. 

Hiickel-type systems (such as Hilcfcel pericyclic reactions and suprafacial sigmatropic shifts) obey the same rules as for sigma electron. The rationale for this observation is clear If the overlap between adjacent p-electron orbitals is positive along the reaction coordinate, only the peraiutational mechanism can... 

For examples of the use of these terms see CYCLOADDITION and SIGMATROPIC REARRANGEMENT. See also ANTI PI (jl) ORBITAL SIGMA (o) ORBITAL. 

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