text: A general double group transfer reaction which is synchronous can be represented as an interaction between a component with p π electrons and a component with q π electrons as shown.
  Then the selection rules are the same as for the generalized cycloaddition reactions.  That is
  * For supra/supra or antara/antara double group transfers, if p + q = 4n + 2 it is thermally allowed, and if p + q = 4n it is photochemically allowed
  * For supra/antara double group transfers, if p + q = 4n it is thermally allowed, and if p + q = 4n + 2 it is photochemically allowed
  This is summarized in the following table:
  The case of q = 0 corresponds to the thermal elimination of the "transferred" R groups.  There is evidence that the pyrolytic eliminations of dihydrogen and ethane from 1,4-cyclohexadiene and 3,3,6,6-tetramethyl-1,4-cyclohexadiene, respectively, represent examples of this type of pericyclic process.
  The ene reaction is often classified as a type of group transfer process, even though it does not involve the transfer of two σ-bonded groups.  Rather, only one σ-bond is transferred while a second σ-bond is formed from a broken π-bond. As an all suprafacial process involving 6 electrons, it is symmetry-allowed under thermal conditions.  The Woodward-Hoffmann symbol for the ene reaction is [2 + 2 + 2] (see below).