Chemistry of the 2,3‐oxaphosphabicyclo[2.2.2]octene ring system: Extrusion of metaphosphates

AbstractThe 2,3‐oxaphosphabicyclo[2.2.2]octene 3‐oxide (or sulfide) ring system is of considerable value because it easily fragments on being heated or irradiated (254 nm) to provide three‐coordinate phosphoryl species. The system is synthesized by O‐insertion with peracids into a CP bond of 7‐phosphanorbornene derivatives with a variety of P‐substituents. With rare exception, the insertion has been found to proceed with retention of the configuration at phosphorus, as established by X‐ray and NMR techniques. The thermal fragmentation that produces the metaphosphate derivatives EtOPO2, EtOP(S)O, and Et2NPO2 follows first‐order kinetics, and is independent of the concentration of a trapping agent for these species. Solvent effects and activation parameters join in defining a retrocycloaddition mechanism that ejects the free metaphosphate. The species PhPO2 can also be easily generated either thermally or photochemically. Metaphosphates have been found to attack ethereal oxygen in epoxides and oxetanes, and may undergo anchimeric participation with a properly placed methoxy group on the substituent used in the 2,3‐oxaphosphabicyclo[2.2.2]octene precursor.