In the first step of this radical reaction, also known as the initiation step,
AIBN undergoes homolysis with the loss of N2 to generate two eqivalents
of the isobutryonitrile radical.
Then the isobutyronitrile radical abstracts a hydrogen atom from PhSH to
generate PhS. The thiophenyl radical is the chain carrier that starts each
reaction and is regenerated at the end of each reaction.
The next step, called propagation, occurs when the PhS radical adds to the alkene
of the vinylcyclopropane, which leads to the ring breaking open to generate a
radical on the carbon bearing the COOMe group. This radical then attacks the
alkene of the cyclohexenone piece to generate a new radical. This radical then
attacks the double bond between that was formed in the initial addition/ring-opening step to form
a new bond and place a radical on the carbon one removed from the top cyclopentane. This
radical then undergoes loss of PhS to re-form the double bond in the product and
generate PhS.
