Sigmatropic rearrangements classification

As outlined previously (Section 3,2), reactions here considered as cyclo-eliminations are those in which n a-bonds plus m tt-bonds are transformed into (n — I) (j-bonds plus (m + 1) Tr-bonds. Most of the reactions in this category have elsewhere in the literature been defined as cyclo-additions and sigmatropic rearrangements. It is felt that the particular classification adopted here is more rigorous and less confusing. 

The largest body of data on the MT of various migrants have been obtain l at present for the six-fold degenerate rearrangement of l-R" -l,2,3,4,5,6-hexamethyl-benzenonium ions (5) which proceeds by consecutive 1,2-shifts of R (1,2- or 1,6-sigmatropic shifts by the Woodward-Hoffmann classification... 

Cyclobutenyl cations, e.g., pentamethylcyclobutenyl (74) 257.258.260) j.g j own to display no tendency to rearrangement by a 1,2-methyl shift (forbidden sigmatropic shift of order [1.4] according to the Woodward and Hoffmann classification ). 

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