Reversible Intramolecular Cycloaddition of Phosphaalkene to an Arene Ring

The phosphepinium cation 1 is deprotonated by base
generating a phosphaalkene that undergoes cycloaddition to the N-bound
aromatic ring affording the 2-phosphabicyclo[2.2.2]­octa-5,7-diene 2. The analogous deprotonation reaction of the less bulky
phosphepinium cation 3 affords a reversible equilibrium
between the phosphaalkene 4 and the corresponding cycloaddition
product 5. This latter observation represents the first
reversible cycloaddition of a main group multiply bonded species with
an arene ring. The bicyclic species 2 was also shown
to be oxidized or alkylated in reactions with S8 and MeOTf,
affording 6 and 7, respectively. This finding
and its implications for related cycloadditions of other main group
multiply bonded species are considered computationally.