The Mechanistic Diversity of the Thermal and Photochemical Decomposition of Bis(phenylphosphonoyl)Peroxides: Concerted Polar, Homolytic, and Electron-Transfer Processes On the Reactivity of (Phenylphosphonoyl)oxyl Radicals

The thermal and photochemical decomposition of the first bis(phenylphosphonoyl)peroxides, dioxybis[(n‐butoxy)phenyl‐phosphane oxide] (5), and dioxybis[(phenoxy)phenylphosphane oxide] (6) has been studied in various solvents by 1H‐, 13C‐, and 31P‐NMR spectroscopy, laser flash photolysis (LFP), and ESR spin‐trapping experiments. Kinetic studies reveal at 20°C a ca. 270 times slower thermal decay for 5 than for 6, which primarily results from a lower A factor rather than differences in the activation energies. The thermal decay of 5 occurs predominantly by a novel, presumably concerted polar rearrangement with formation of a thermally unstable, mixed phosphonoyl‐phosphoryl anhydride. Photolysis of 5 induces homolytical cleavage of the peroxy bond with release of [(n‐butoxy)phenylphosphonoyl]oxyl radicals 7. Radical 7 is characterized by a broad, transient UV/Vis absorption spectrum in the 400 to >700 nm range (λmax ca. 580 nm), as has been demonstrated by 248‐nm LFP of 5 in acetonitrile solution. The short lifetime of this absorption indicates an extremely high reactivity (in hydrogen ion and addition) of this electrophilic radical. The thermal and photochemical decomposition of peroxide 6 leads to a virtually identical product distribution, suggesting O–O bond cleavage to be the major initial reaction under both conditions. LFP at 248 and 308 nm of a solution of 6 in acetonitrile initially produces a weak, broad absorption at ca. 500 nm and stronger bands at 280 and 400 nm. The highly transient 500‐nm absorption is assigned to the [(phenoxy)phenylphosphonoyl]oxyl radical 8, the other bands are attributed to the phosphonoyloxy‐substituted benzene radical cation 8Z. The formation of this species can be explained in terms of electron transfer in the first‐formed oxyl radical 8 and/or the intact peroxide 6, followed by cleavage of the peroxy bond. The decay of 8Z is accompanied by the build‐up of the absorption spectrum of a 1,4‐dioxy‐substituted biphenyl radical cation.