Thermal Fragmentation of Trihaloethyl and Hexafluoro-2-propyl (α-Hydroxyiminobenzyl)phosphonates. Solvent Effects and the Trapping of Metaphosphate

Article abstract of DOI:10.1021/jo00079a029

Reactions of 2,2,2-trihaloethyl or 1,1,1,3,3,3-hexafluoro-2-propyl benzoylphosphonate anions 7 or 10 with hydroxylamine gave corresponding (α-hydroxyiminobenzyl)phosphonates, 3 or 11, respectively, as predominantly (E)-isomers.Refluxing (E)-3a or (E)-3b in EtOH or 2-PrOH caused them to fragment to benzonitrile and mixed phosphodiesters: ethyl trihaloethyl phosphate (12) or 2-propyl trihaloethyl phosphate (13), respectively. (E)-3a did not undergo any fragmentation in boiling water or MeOH.Refluxing (E)-3a in aprotic solvents led to the formation of benzonitrile andP,P'-bis(2,2,2-trifluoroethyl)pyrophosphate (4a).The rate of fragmentation increased with solvent polarity.Similar behavior was exhibited by 11.The fragmentation of anions 3 and 11 is interpreted in terms of a dissociative mechanism leading to the formation of metaphosphate in the first step.The lack of reactivity in water and methanol is rationalized by assuming stabilization of the starting material by H-bond formation, while the rate enhancement by polar solvent is attributed to stabilization of the transition state and solvation of the departing OH(1-) by the solvent.The metaphosphate formed in the thermal fragmentation of 3a in MeCN was trapped by styrene oxide, as evidenced by the isolation and identification of 2-oxo-2-(2',2',2'-trifluoroethyl)-4-phenyl-1,3,2-dioxaphospholane as a mixture of diastereoisomers 18 and 19.