Photooxygenations of PhSMe and Bu2S sensitized by N-methylquinolinium (NMQ+) and 9,10-dicyanoanthracene (DCA) in O2-saturated MeCN have been investigated by laser and steady-state photolysis. Laser photolysis experiments showed that excited NMQ+ promotes the efficient formation of sulfide radical cations with both substrates either in the presence or in absence of a cosensitizer (toluene). In contrast, excited DCA promotes the formation of radical ions with PhSMe, but not with Bu2S. To observe radical ions with the latter substrate, the presence of a cosensitizer (biphenyl) was necessary. With Bu2S, only the dimeric form of the radical cation, (Bu2S)2+*, was observed, while the absorptions of both PhSMe+* and (PhSMe)2+* were present in the PhSMe time-resolved spectra. The decay of the radical cations followed second-order kinetics, which in the presence of O2, was attributed to the reaction of the radical cation (presumably in the monomeric form) with O2-* generated in the reaction between NMQ* or DCA-* and O2. The fluorescence quenching of both NMQ+ and DCA was also investigated, and it was found that the fluorescence of the two sensitizers is efficiently quenched by both sulfides (rates controlled by diffusion) as well by O2 (kq = 5.9 x 10(9) M(-1) s(-1) with NMQ+ and 6.8 x 10(9) M(-1) s(-1) with DCA). It was also found that quenching of 1NMQ* by O2 led to the production of 1O2 in significant yield (PhiDelta = 0.86 in O2-saturated solutions) as already observed for 1DCA*. The steady-state photolysis experiments showed that the NMQ+- and DCA-sensitized photooxygenation of PhSMe afford exclusively the corresponding sulfoxide. A different situation holds for Bu2S: with NMQ+, the formation of Bu2SO was accompanied by that of small amounts of Bu2S2; with DCA, the formation of Bu2SO2 was also observed. It was conclusively shown that with both sensitizers, the photooxygenations of PhSMe occur by an electron transfer (ET) mechanism, as no sulfoxidation was observed in the presence of benzoquinone (BQ), which is a trap for O2-*, NMQ*, and DCA-*. BQ also suppressed the NMQ+-sensitized photooxygenation of Bu2S, but not that sensitized by DCA, indicating that the former is an ET process, whereas the second proceeds via singlet oxygen. In agreement with the latter conclusion, it was also found that the relative rate of the DCA-induced photooxygenation of Bu2S decreases by increasing the initial concentration of the substrate and is slowed by DABCO (an efficient singlet oxygen quencher). To shed light on the actual role of a persulfoxide intermediate also in ET photooxygenations, experiments in the presence of Ph2SO (a trap for the persulfoxide) were carried out. Cooxidation of Ph2SO to form Ph2SO2 was, however, observed only in the DCA-induced photooxygenation of Bu2S, in line with the singlet oxygen mechanism suggested for this reaction. No detectable amounts of Ph2SO2 were formed in the ET photooxygenations of PhSMe with both DCA and NMQ+ and of Bu2S with NMQ+. This finding, coupled with the observation that 1O2 and ET ph...

A laser flash photolysis study of the spectral properties and beta-scission reactions of a series of ring-substituted cumyloxyl radicals has been carried out. All cumyloxyl radicals display a broad absorption band in the visible region of the spectrum, which decays on the microsecond time scale, leading to a strong increase in absorption in the UV region of the spectrum, which is attributed to the corresponding acetophenone formed after beta-scission of the cumyloxyl radicals. The position of the visible absorption band is red-shifted by the presence of electron-donating ring substituents, while a blue-shift is observed in the presence of electron-withdrawing ring substituents, suggesting that + R ring substituents promote charge separation in the excited cumyloxyl radical through stabilization of the partial positive charge on the aromatic ring of an incipient radical zwitterion. Along this line, an excellent Hammett-type correlation between the experimentally measured energies at the visible absorption maxima of the cumyloxyl radicals and sigma(+) substituent constants is obtained. A red-shift is also observed on going from MeCN to MeCN/H(2)O for all cumyloxyl radicals, pointing toward a specific effect of water. The ring substitution does not influence to a significant extent the rate constants for beta-scission of the cumyloxyl radicals, which varies between 7.1 x 10(5) and 1.1 x 10(6) s(-1), a result that suggests that cumyloxyl radical beta-scission is not governed by the stability of the resulting acetophenone. Finally, k(beta) increases on going from MeCN to the more polar MeCN/H(2)O 1:1 for all cumyloxyl radicals, an observation that reflects the increased stabilization of the transition state for beta-scission through increased solvation of the incipient acetophenone product.

The mesolytic cleavage of a beta-C-X bond (ArCR(2)-X(*+) --> ArCR(2)(*/+) + X(+/*)) is one of the most important reactions of alkylaromatic radical cations. In this Account, our group's results concerning some fundamental aspects of this process (cleavage mode, structural and stereoelectronic effects, competitive breaking of different beta-bonds, nucleophilic assistance, possible stereochemistry, carbon vs oxygen acidity in arylalkanol radical cations) are presented and critically discussed for reactions where X = H, CR(3), SR, and SiR(3). Several examples illustrating how this information was exploited as a tool to detect electron-transfer mechanisms in chemical and enzymatic oxidations are also reported.

The radical cations of thioanisole (1), p-methylthioanisole (2), and the benzyl phenyl sulfides 3-5 have been produced by pulse radiolysis in aqueous solutions, using SO 4 •-or Tl 2+ as oxidizing species. The radical cations 1 •+ -5 •+ exhibit very similar UV spectra, with strong absorptions between 300-350 and 500-600 nm. In contrast to aliphatic thioether radical cations, 1 •+ -5 •+ do not undergo dimerization (via formation of a three-electron bond with the parent thioethers). In the absence of bases, 1 •+ is a long-lived species with a lifetime >30 ms, whereas 3 •+ , 4 •+ , and 5 •+ decay rapidly by both C-S bond and C-H bond cleavage with k C-H ) 1.3 × 10 3 s -1 and k C-S ) 1.3 × 10 3 s -1 for 3 •+ and k C-H ) 0.95 × 10 3 s -1 and k C-S ) 2.65 × 10 3 s -1 for 4 •+ . In the presence of OH -or HPO 4 2-, also 1 •+ undergoes a deprotonation process, with a rate larger than those of the benzyl phenyl sulfide radical cations. For example, the rate constant for the OH --induced deprotonation is 3.4 × 10 7 M -1 s -1 for 1 •+ and 9.5 × 10 6 M -1 s -1 for 3 •+ . Thioanisole radical cation 1 •+ was also produced by reduction of thioanisole sulfoxide. Under these conditions, it was possible to study the reaction of 1 •+ with a number of nucleophiles or electron donors. It was found that 1 •+ reacts with I -, N 3 -, PhS -, PhSH, Br -, and SCN -by an electron transfer mechanism, producing the oxidized form of the nucleophile. This reaction is diffusion controlled with the first four nucleophiles, which are more easily oxidized than 1 (E°< 1.45 V); slower reactions were observed with SCN -(E°) 1.62 V) and with Br -(E°) 1.96 V). NO 3 -(E°) 2.3V) is unreactive (k < 10 6 M -1 s -1 ).