Synthesis, Crystal Structures, and Laser Flash Photolysis oftert-Butyl Aroylperbenzoates¹

Abstract

tert-Butyl aroylperbenzoates (1-7) were synthesized. Single-crystal structures for 2 and 5 show that the perester and benzophenone carbonyl groups are almost coplanar in each. Laser flash photolysis (LFP, lambdaex = 355 nm) of 1-5 in CCl4 produces the corresponding aroylphenyl radicals (9-13). The lifetimes of the para aroyl-substituted phenyl radicals (9-12) are similar (approximately 0.4 micros), but each is shorter lived than the meta aroyl-substituted phenyl radical (13). LFP of 2, 6, and 7 also produces different (tert-butyldioxycarbonylbenzoyl)benzyl radicals (8, 14, and 15, lambdamax approximately 320 nm). The lifetimes of each in CCl4 have been found to be approximately 17-18 micros. The effect of substituents on the quantum yield of decomposition of 1-7 and the lifetimes of 9-13 is discussed.

Ultrafast Pump−Probe Studies oftert-Butyl Aroylperbenzoates and Triplet Energy Interception by the O−O Bond

“…That the τ T values of BAPs are less than 1 ns whereas the τ T values of nonperester derivatives of benzophenone are in the microsecond regime indicates that the most likely process that is responsible for the decay of the triplet state is fission of the O-O bond to produce radicals. 4,5,12 To achieve this result, the electronic energy initially resident on the benzophenone moiety needs to become resident on the peroxy moiety as a result of intramolecular energy transfer (IET). Two questions that arise from the observations are the following: (i) What is the nature of the solvent effect?…”

“…[10][11][12] The aroylphenyl radicals (λ max ) 550 nm) have lifetimes of 0.4-0.6 µs in CCl 4 , and the absolute bimolecular rates of reactions of these radicals range from 7.56 × 10 7 to 1.68 × 10 9 M -1 s -1 (at 22 ( 2 °C). 4,12 BAPs are also known to induce photopolymerization of double-bond-containing monomers. 11,13 The perester bond of BAPs dissociates as a result of the intramolecular triplet energy dissipation.…”

“…10,11 The quantum yields of dissociation (φ diss ) of the O-O bond are near unity for many of the BAPs. [10][11][12] The aroylphenyl radicals (λ max ) 550 nm) have lifetimes of 0.4-0.6 µs in CCl 4 , and the absolute bimolecular rates of reactions of these radicals range from 7.56 × 10 7 to 1.68 × 10 9 M -1 s -1 (at 22 ( 2 °C). 4,12 BAPs are also known to induce photopolymerization of double-bond-containing monomers.…”

“…Products from steady-state photolysis of BAPs indicate that homolytic dissociation of the perester (O−O) bond leads to formation of the corresponding aroylbenzoyloxyl and aroylphenyl radicals. , The quantum yields of dissociation (φ diss ) of the O−O bond are near unity for many of the BAPs. − The aroylphenyl radicals (λ max = 550 nm) have lifetimes of 0.4−0.6 μs in CCl 4 , and the absolute bimolecular rates of reactions of these radicals range from 7.56 × 10 7 to 1.68 × 10 9 M -1 s -1 (at 22 ± 2 °C). , BAPs are also known to induce photopolymerization of double-bond-containing monomers. , …”

The Chemistry of Peresters

“…C‐C(O)‐O bond angles are often somewhat compressed (109–110°) for an sp 2 center as compared with 121° for esters . For an electron poor tert ‐butyl perbenzoate, the observed C‐C(=O)‐O angle is 110.2°; For the highly hindered tri‐ tert ‐butyl perbenzoate, the value can be as low as 109° …”