Radical contraction of 1,3,2-dioxaphosphepanes to 1,3,2-dioxaphosphorinanes: a kinetic and (17)O NMR spectroscopic study

Two diastereomeric 5-bromo-4-phenyl-2-phenoxy-2-oxo-1,3,2-dioxophosphepanes have been synthesized and used to study the contraction of 4-phenyl-2-phenoxy-2-oxo-1,3,2-dioxophosphorinan-5-yl radicals. Kinetics were determined by competition methods and demonstrate Arrhenius parameters typical of rearrangements of this kind. Isotopic labeling reveals that all rearrangements are formally of the 1,2-type with retention of configuration at phosphorus. Analysis of the stereochemistry of the rearrangements, however, reveals the two diastereomers to take different paths with respect to the geometry of the presumed alkene radical cation intermediate.