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.