Mechanistic Studies on Metaphosphate Generation from Fragmentation of 2,3-Oxaphosphabicyclooctene Derivatives

Article abstract of DOI:10.1021/ja00018a045

The rates of fragmentation of 3-R-6-methyl-N-phenyl-2,3-oxaphosphabicyclo<2.2.2>oct-5-ene-8,9-dicarboximide- 3-oxide, where R = EtN or EtO, and of the 3-sulfide, where R - EtO, were determined in various solvents and at various temperatures.All reactions were first-order, as they were also when alcohols were present as trapping agents for the released metaphosphoric acid derivatives.From the small solvent effects on rates and activation parameteers, it was concluded that the thermal fragmentations occurred by a retrocycloaddition process.This was supported by the similarity in rates for the three substrates, which differ considerably in polarity at phosphorus.The metaphosphates showed selectivity in their reactivity to alcohol mixtures; rates were faster with more nucleophilic alcohols (ethanol vs 2,2,2-trifluoroethanol) and less crowded alcohols (ethanol vs tert-butyl alcohol).A secondary reaction of the released metaphosphate with two of the substrates was detected by kinetics measurements; first-order rate constants were larger when alcohol was not present to remove metaphosphate.Quantum yields in photochemical fragmentations of the substrates were unchanged when alcohols were present; metaphosphates generated in this fashion showed selectivity effects toward alcohols as seen in the thermal fragmentation.No evidence was found for radical intermediates in either process.