Isomerizations involving thermal sigmatropic migrations

Thermal sigmatropic shifts involve the uncatalyzed intramolecular migration of a o- bond, flanked by one or more 7r-electron systems, to a new position within the molecule, viz. 

Most examples of this class of reaction involve hydrogen shifts, but reactions are known in which alkyl, aryl, and other groups undergo sigmatropic migrations.  

In dienyl 1,5-shifts, = 0 in homodienyl shifts, n There is one example of the reverse of reaction (43) in which n = 3. Table 10 lists activation parameters for 1,5-hydrogen shifts which have been studied at several temperatures. 

Only c/5-1,3-dienes can attain this geometry, and only cis dienes isomerize by this mechanism. If R and R differ in size, the most stable transition state is that in which the bulkier group is oriented equatorially. This accounts for the observation that isomerization of m-1,3-hexadiene gives predominantly cw,/rflAi -2,4-hexadiene.  

The transition-state geometry in homodienyl 1,5-hydrogen shifts is probably non-planar, viz. 

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