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

Abstract

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 abstraction 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.