Electron transfer and singlet oxygen mechanisms in the photooxygenation of dibutyl sulfide and thioanisole in MeCN sensitized by N-methylquinolinium tetrafluoborate and 9,10-dicyanoanthracene. The probable involvement of a thiadioxirane intermediate in electron transfer photooxygenations

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 photooxygenations lead to different product distributions, makes it unlikely that, as currently believed, the two processes involve the same intermediate, i.e., a nucleophilic persulfoxide. Furthermore, the cooxidation of Ph2SO observed in the DCA-induced photooxygenation of Bu2S was drastically reduced when the reaction was performed in the presence of 0.5 M biphenyl as a cosensitizer, that is, under conditions where an (indirect) ET mechanism should operate. This observation confirms that a persulfoxide is formed in singlet oxygen but not in ET photosulfoxidations. The latter conclusion was further supported by the observation that also the intermediate formed in the reaction of thianthrene radical cation with KO2, a reaction which mimics step d (Scheme 2) in the ET mechanism of photooxygenation, is an electrophilic species, being able to oxidize Ph2S but not Ph2SO. It is thus proposed that the intermediate involved in ET sulfoxidations is a thiadioxirane, whose properties (it is an electrophilic species) seem more in line with the observed chemistry. Theoretical calculations concerning the reaction of a sulfide radical cation with O2-* provide a rationale for this proposal.     

Photosensitized oxidation of alkyl phenyl sulfoxides. C-S bond cleavage in alkyl phenyl sulfoxide radical cations

The 3-cyano-N-methylquinolinium perchlorate (3-CN-NMQ(+)ClO4(-))-photosensitized oxidation of phenyl alkyl sulfoxides (PhSOCR1R2R3, 1, R1 = R2 = H, R3 = Ph; 2, R1 = H, R2 = Me, R3 = Ph; 3, R1 = R2 = Ph, R3 = H; 4, R1 = R2 = Me, R3 = Ph; 5, R1 = R2 = R3 = Me) has been investigated by steady-state irradiation and nanosecond laser flash photolysis (LFP) under nitrogen in MeCN. Steady-state photolysis showed the formation of products deriving from the heterolytic C-S bond cleavage in the sulfoxide radical cations (alcohols, R1R2R3COH, and acetamides, R1R2R3CNHCOCH3) accompanied by sulfur-containing products (phenyl benzenethiosulfinate, diphenyl disulfide, and phenyl benzenethiosulfonate). By laser irradiation, the formation of 3-CN-NMQ(*) (lambda(max) = 390 nm) and sulfoxide radical cations 1(*+) , 2(*+), and 5(*+) (lambda(max) = 550 nm) was observed within the laser pulse. The radical cations decayed by first-order kinetics with a process attributable to the heterolytic C-S bond cleavage leading to the sulfinyl radical and an alkyl carbocation. The radical cations 3(*+) and 4(*+) fragment too rapidly, decaying within the laser pulse. The absorption band of the cation Ph2CH(+) (lambda(max) = 440 nm) was observed with 3 while the absorption bands of 3-CN-NMQ(*) and PhSO(*) (lambda(max) = 460 nm) were observed just after the laser pulse in the LFP experiment with 4. No competitive beta-C-H bond cleavage has been observed in the radical cations from 1-3. The C-S bond cleavage rates were measured for 1(*+), 2(*+), and 5(*+). For 3(*+) and 4(*+), only a lower limit (ca. >3 x 10(7) s(-1)) could be given. Quantum yields (Phi) and fragmentation first-order rate constants (k) appear to depend on the structure of the alkyl group and on the bond dissociation free energy (BDFE) of the C-S bond of the radical cations determined by a thermochemical cycle using the C-S BDEs for the neutral sulfoxides 1-5 obtained by DFT calculations. Namely, Phi and k increase as the C-S BDFE becomes more negative, that is in the order 1 < 5 < 2 < 3, 4, which is also the stability order of the alkyl carbocations formed in the cleavage. An estimate of the difference in the C-S bond cleavage rate between sulfoxide and sulfide radical cations was possible by comparing the fragmentation rate of 5(*+) (1.4 x 10(6) s(-1)) with the upper limit (10(4) s(-1)) given for tert-butyl phenyl sulfide radical cation (Baciocchi, E.; Del Giacco, T.; Gerini, M. F.; Lanzalunga, O. Org. Lett. 2006, 8, 641-644). It turns out that sulfoxide radical cations undergo C-S bond breaking at a rate at least 2 orders of magnitude faster than that of corresponding sulfide radical cations.     

Anomalous reactivity of radical cations produced by photosensitized oxidation of 4-methoxybenzyl alcohol derivatives: role of the sensitizer

Steady-state and nanosecond laser flash photolysis measurements of 4-methoxybenzyl alcohol (1a), 4-methoxy-alpha-methylbenzyl alcohol (1b), 4,4'-dimethoxydiphenylmethanol (1c) and 4-methoxy-alpha,alpha'-dimethylbenzyl alcohol (1d) were carried out in air-equilibrated CH(2)Cl(2) and CH(3)CN solutions, in the presence of 9,10-dicyanoanthracene (DCA) and N-methylquinolinium tetrafluoroborate (NMQ(+)BF(4)(-)) as sensitizers. In particular, steady-state irradiation with DCA produced carbonyl compounds and, with NMQ(+)BF(4)(-), carbonyl compounds, ethers (substrates 1a-c ) and styrene (substrate 1d ) while time-resolved investigations gave evidence of charged species produced upon irradiation. The effect of solvent polarity on the reactivity was investigated; in the case of DCA, the reactivity increased with the solvent polarity, while the opposite was obtained when NMQ(+)BF(4)(-) was used. Quantum mechanical calculations at semiempirical (INDO/1-CI) and DFT (B3LYP/6-311G(d)) levels were used to support transient assignments and to obtain the charge and spin density distributions, respectively. The different photooxidation mechanisms operative with the neutral and charged sensitizer were rationalized in terms of the reactivity of free and complexed radical cations, respectively.     

Visible photooxidation of dibenzothiophenes sensitized by 2-(4-methoxyphenyl)-4, 6-diphenylpyrylium: an electron transfer mechanism without involvement of superoxide

We report here on a new electron-transfer mechanism for visible-light photooxidation of sulfides in which no superoxide ion is involved. Visible-light irradiation of 2-(4-methoxyphenyl)-4, 6-diphenylpyrylium tetrafluoroborate (MOPDPP(+)BF(4)(-)) in an O(2)-saturated acetonitrile solution containing dibenzothiophene (DBT) results in nearly 100% conversion to oxygenated products, DBT sulfoxide and sulfone. The photooxidation of DBT is initiated by a photoinduced electron-transfer process, where the excited MOPDPP(+) traps an electron from the ground-state DBT to form MOPDPP(*) and DBT radical cation. Such a mechanism is consistent with the studies of laser flash photolysis, electron spin resonance, and fluorescence quenching of the irradiated system. The photogenerated DBT radical cation undergoes a coupling reaction with O(2) to produce the intermediate responsible for the formation of the oxygenated products. The presence of O(2) has no effect on the decay kinetics of the transient absorption of MOPDPP(*), indicating that no redox reaction occurs between MOPDPP(*) and O(2), and thus no superoxide ion (O(2)(*-)) is formed. Moreover, the ESR signal of MOPDPP(*) was significantly enhanced in the presence of O(2), consistent with the assumption that the photogenerated DBT radical cation couples with O(2) to form the oxygen-adduct, which is subject to further reactions (Scheme 3) leading to the final oxygenated products. Similar results have been obtained when using 10-methylacridine hexafluorophosphate (AcrH(+)PF(6)(-), which has similar reduction potential in the ground state as MOPDPP(+)) as the sensitizer. This finding provides a possibility for the photooxidation of sulfides with dioxygen utilizing visible light (solar energy) and is also of significance in clarification of the reaction mechanism.     

Steady-state and laser flash photolysis study of the carbon-carbon bond fragmentation reactions of 2-arylsulfanyl alcohol radical cations

The N-methylquinolinium tetrafluoroborate (NMQ(+))-sensitized photolysis of the erythro-1,2-diphenyl-2-arylsulfanylethanols 1-3 (1, aryl = phenyl; 2, aryl = 4-methylphenyl; 3, aryl = 3-chlorophenyl) has been investigated in MeCN, under laser flash and steady-state photolysis. Under laser irradiation, the formation of sulfide radical cations of 1-3, in the monomeric (lambda(max) = 520-540 nm) and dimeric form (lambda(max) = 720-->800 nm), was observed within the laser pulse. The radical cations decayed by first-order kinetics, and under nitrogen, the formation of ArSCH(*)Ph (lambda(max) = 350-360 nm) was clearly observed. This indicates that the decay of the radical cation is due to a fragmentation process involving the heterolytic C-C bond cleavage, a conclusion fully confirmed by steady-state photolysis experiments (formation of benzaldehyde and the dimer of the alpha-arylsulfanyl carbon radical). Whereas the fragmentation rate decreases as the C-C bond dissociation energy (BDE) increases, no rate change was observed by the replacement of OH by OD in the sulfide radical cation (k(OH)/k(OD) = 1). This suggests a transition state structure with partial C-C bond cleavage where the main effect of the OH group is the stabilization of the transition state by hydrogen bonding with the solvent. The fragmentation rate of 2-hydroxy sulfanyl radical cations turned out to be significantly slower than that of nitrogen analogues of comparable reduction potential, probably due to a more efficient overlap between the SOMO in the heteroatom and the C-C bond sigma-orbital in the second case. The fragmentation rates of 1(+*)-3(+*) were found to increase by addition of a pyridine, and plots of k(base) against base strength were linear, allowing calculation of the beta Bronsted values, which were found to increase as the reduction potential of the radical cation decreases, beta = 0.21 (3(+*)), 0.34 (1(+*)), and 0.48 (2(+*)). The reactions of 1(+*) exhibit a deuterium kinetic isotope effect with values that increase as the base strength increases: k(OH)/k(OD) = 1.3 (pyridine), 1.9 (4-ethylpyridine), and 2.3 (4-methoxypyridine). This finding and the observation that with the above three bases the rate decreases in the order 3(+*) > 1(+*) > 2(+*), i.e., as the C-C BDE increases, suggest that C-C and O-H bond cleavages are concerted but not synchronous, with the role of OH bond breaking increasing as the base becomes stronger (variable transition state). It is probable that, with the much stronger base, 4-(dimethylamino)pyridine, a change to a stepwise mechanism may occur where the slow step is the formation of a radical zwitterion that then rapidly fragmentates to products.     

Properties of excited radical cations of substituted oligothiophenes

Excited-state properties of radical cations of substituted oligothiophenes ( nT (*+), n denotes the number of thiophene rings, n = 3, 4, 5) in solution were investigated by using various laser flash photolysis techniques including two-color two-laser flash photolysis. nT (*+) generated by photoinduced electron transfer to p-chloranil or resonant two-photon ionization (RTPI) by using the first 355-nm ns laser irradiation was selectively excited with the second picosecond laser (532 nm). Bleaching of the absorption of nT (*+) together with growth of a new absorption was observed during the second laser irradiation, indicating the generation of nT (*+) in the excited state ( nT (*+)*). The D 1 state lifetime was estimated to be 34 +/- 4, 24 +/- 2, and 18 +/- 1 ps for 3T (*+), 4T (*+), and 5T (*+), respectively. In the presence of hole acceptor (Q), bleaching of nT (*+) and growth of Q (*+) were observed upon selective excitation of nT (*+) during the nanosecond-nanosecond two-color two-laser flash photolysis, indicating the hole transfer from nT (*+)(D 1) to Q. Recovery of nT (*+) was also observed together with decay of Q (*+) because of regeneration of nT (*+) by hole transfer from Q (*+) to nT at the diffusion-limiting rate. It was suggested that the hole transfer rate ( k HT) from nT (*+)(D 1) to Q depended on the free-energy change for hole transfer (-Delta G = 1.41-0.46 eV). The estimated k HT faster than the diffusion-limiting rate can be explained by the contribution of the static quenching for the excited species in the presence of high concentration of Q (0.1-1.0 M).     

Aryl sulfoxide radical cations. Generation, spectral properties, and theoretical calculations

Aromatic sulfoxide radical cations have been generated by pulse radiolysis and laser flash photolysis techniques. In water (pulse radiolysis) the radical cations showed an intense absorption band in the UV region (ca. 300 nm) and a broad less intense band in the visible region (from 500 to 1000 nm) whose position depends on the nature of the ring substituent. At very low pulse energy, the radical cations decayed by first-order kinetics, the decay rate increasing as the pH increases. It is suggested that the decay involves a nucleophilic attack of H(2)O or OH(-) (in basic solutions) to the positively charged sulfur atom to give the radical ArSO(OH)CH(3)(*). By sensitized [N-methylquinolinium tetrafluoborate (NMQ(+))] laser flash photolysis (LFP) the aromatic sulfoxide radical cations were generated in acetonitrile. In these experiments, however, only the band of the radical cation in the visible region could be observed, the UV band being covered by the UV absorption of NMQ(+). The lambda(max) values of the bands in the visible region resulted almost identical to those observed in water for the same radical cations. In the LFP experiments the sulfoxide radical cations decayed by second-order kinetics at a diffusion-controlled rate, and the decay is attributed to the back electron transfer between the radical cation and NMQ(*). DFT calculations were also carried out for a number of 4-X ring substituted (X = H, Me, Br, OMe, CN) aromatic sulfoxide radical cations (and their neutral parents). In all radical cations, the conformation with the S-O bond almost coplanar with the aromatic ring is the only one corresponding to the energy minimum. The maximum of energy corresponds to the conformation where the S-O bond is perpendicular to the aromatic ring. The rotational energy barriers are not very high, ranging from 3.9 to 6.9 kcal/mol. In all radical cations, the major fraction of charge and spin density is localized on the SOMe group. However, a substantial delocalization of charge and spin on the ring (almost 50% for the 4-methoxy derivative and around 30% for the other radical cations) is also observed. This suggests some conjugative interaction between the MeSO group and the aromatic system that may become very significant when a strong electron donating substituent like the MeO group is present. The ionization energies (IE) of the 4-X ring substituted neutral aromatic sulfoxides were also calculated, which were found to satisfactorily correlate with the experimental E(p) potentials measured by cyclic voltammetry.     

Mechanistic borderline between one-step hydrogen transfer and sequential transfers of electron and proton in reactions of NADH analogues with triplet excited states of tetrazines and Ru(bpy)(3)2+

Efficient energy transfer from Ru(bpy)(3)(2+) (bpy = 2,2'-bipyridine, denotes the excited state) to 3,6-disubstituted tetrazines [R(2)Tz: R = Ph (Ph(2)Tz), 2-chlorophenyl [(ClPh)(2)Tz], 2-pyridyl (Py(2)Tz)] occurs to yield the triplet excited states of tetrazines ((3)R(2)Tz(*)), which have longer lifetimes and higher oxidizing ability as compared with those of Ru(bpy)(3)(2+). The dynamics of hydrogen-transfer reactions from NADH (dihydronicotinamide adenine dinucleotide) analogues has been examined in detail using (3)R(2)Tz(*) by laser flash photolysis measurements. Whether formal hydrogen transfer from NADH analogues to (3)R(2)Tz(*) proceeds via a one-step process or sequential electron and proton transfer processes is changed by a subtle difference in the electron donor ability and the deprotonation reactivity of the radical cations of NADH analogues as well as the electron-acceptor ability of (3)R(2)Tz(*) and the protonation reactivity of R(2)Tz(*)(-). In the case of (3)Ph(2)Tz(*), which is a weaker electron acceptor than the other tetrazine derivatives [(ClPh)(2)Tz; Py(2)Tz], direct one-step hydrogen transfer occurs from 10-methyl-9,10-dihydroacridine (AcrH(2)) to (3)Ph(2)Tz(*) without formation of the radical cation (AcrH(2)(*)(+)). The rate constant of the direct hydrogen transfer from AcrH(2) to (3)Ph(2)Tz(*) is larger than that expected from the Gibbs energy relation for the rate constants of electron transfer from various electron donors to (3)Ph(2)Tz(*), exhibiting the primary deuterium kinetic isotope effect. On the other hand, hydrogen transfer from 9-isopropyl-10-methyl-9,10-dihydroacridine (AcrHPr(i)) and 1-benzyl-1,4-dihydronicotinamide (BNAH) to (3)R(2)Tz(*) occurs via sequential electron and proton transfer processes, when both the radical cations and deprotonated radicals of NADH analogues are detected by the laser flash photolysis measurements.     

Cycloreversion of azetidines via oxidative electron transfer. steady-state and time-resolved studies

Cycloreversion of cis- and trans-1,2,3-triphenylazetidine (c-2 and t-2) is achieved by electron transfer to (tris(4-bromophenyl)aminium radical cation (5 (*+)). Stepwise C-N and C-C bond cleavage of azetidine radical cations leads to cis- and trans-stilbene, together with N-benzylideneaniline, as final products. Mechanistic evidence is provided by quenching studies, using laser flash photolysis to generate 5 (*+) from its neutral precursor.     

Transients in the oxidative and H-atom-induced degradation of 1,3,5-trithiane. Time-resolved studies in aqueous solution

The (*)OH-induced oxidation of 1,3,5-trithiacyclohexane (1) in aqueous solution was studied by means of pulse radiolysis with optical and conductivity detection. This oxidation leads, via a short-lived (*)OH radical adduct (<1 micros), to the radical cation 1(*+) showing a broad absorption with lambda(max) equal to 610 nm. A defined pathway of the decay of 1(*+) is proton elimination. It occurs with k = (2.2 +/- 0.2) x 10(4) s(-1) and yields the cyclic C-centered radical 1(-H)(*). The latter radical decays via ring opening (beta-scission) with an estimated rate constant of about 10(5) s(-1). A distinct, immediate product (formed with the same rate constant) is characterized by a narrow absorption band with lambda(max) = 310 nm and is attributed to the presence of a dithioester function. The formation of the 310 nm absorption can be suppressed in the presence of oxygen, the rationale for this being a reaction of the C-centered cyclic radical 1(-H)(*) with O(2). The disappearance of the 310 nm band (with a rate constant of 900 s(-1)) is associated with the hydrolysis of the dithioester functionality. A further aspect of this study deals with the reaction of H(*) atoms with 1 which yields a strongly absorbing, three-electron-bonded 2sigma/1sigma* radical cation [1(S therefore S)-H](+) (lambda(max) = 400 nm). Its formation is based on an addition of H(*) to one of the sulfur atoms, followed by beta-scission, intramolecular sulfur-sulfur coupling (constituting a ring contraction), and further stabilization of the S therefore S bond thus formed by protonation. [1(S therefore S)-H](+) decays with a first-order rate constant of about 10(4) s(-1). Its formation can be suppressed by the addition of oxygen which scavenges the H(*) atoms prior to their reaction with 1. Complementary time-resolved conductivity experiments have provided information on the quantification of the 1(*+) radical cation yield, the cationic longer-lived follow-up species, extinction coefficients, and kinetics concerning deprotonation processes as well as further reaction steps after hydrolysis of the transient dithioesters. The results are also discussed in the light of previous photochemical studies.     

Effects of substituents on aryl groups during the reaction of triarylphosphine radical cation and oxygen

In a previous report (S. Yasui, S. Tojo and T. Majima, J. Org. Chem., 2005, 70, 1276), we presented the results from the laser flash photolysis (LFP) and product analysis of the 9,10-dicyanoanthracene (DCA)-photosensitized oxidation of triarylphosphine (Ar(3)P) in acetonitrile under air, which showed that the photoreaction results in the oxidation of Ar(3)P to give the corresponding phosphine oxide (Ar(3)P=O) in a nearly quantitative yield, and that the reaction is initiated by the electron transfer (ET) from Ar(3)P to DCA in the singlet excited state ((1)DCA*), producing the triarylphosphine radical cation Ar(3)P (+). This radical cation decays through radical coupling with O(2) to afford the peroxy radical cation Ar(3)P(+)-O-O*, which we proposed to be the intermediate leading to the product Ar(3)P=O. We now examined this photoreaction in more detail using ten kinds of Ar(3)P with various electronic and steric characteristics. The decay rate of Ar(3)P*(+) measured by the LFP was only slightly affected by the substituents on the aryl groups of Ar(3)P. During the photolysis of trimesitylphosphine (Mes(3)P), the peroxy radical cation intermediate (Mes(3)P(+)-O-O*) had a lifetime long enough to be spectrophotometrically detected. The quantum yields of Ar(3)P=O increased with either electron-withdrawing or -releasing substituents on the aryl groups, suggesting that a radical center is developed on the phosphorus atom during the step when the quantum yield is determined. In addition, the o-methyl substituents in Ar(3)P decreased the quantum yield. These results clearly indicated that Ar(3)P(+)-O-O* undergoes radical attack upon the parent phosphine Ar(3)P that eventually produces the final product, Ar(3)P=O.     

Reaction of triarylphosphine radical cations generated from photoinduced electron transfer in the presence of oxygen

[reaction: see text] The 9,10-dicyanoanthracene (DCA)-sensitized photoreaction of triarylphosphines (1) was carried out in acetonitrile under aerobic conditions. Phosphine 1 was oxidized to the corresponding phosphine oxide with no appreciable side reactions. Product analysis and laser flash photolysis experiments suggest that the radical cation of 1 formed by the electron transfer from 1 to DCA in the singlet excited state ((1)DCA) reacts with O(2) to eventually afford the phosphine oxide.     

Spectroscopic and calorimetric studies on the mechanism of methylenecyclopropane rearrangements triggered by photoinduced electron transfer

2-(dideuteriomethylene)-1,1-bis(4-methoxyphenyl)cyclopropane (d(2)-1) undergoes degenerate rearrangement in both singlet- and triplet-sensitized electron-transfer photoreactions. Nanosecond time-resolved absorption spectroscopy on laser flash photolysis of the unlabeled 1 with 9,10-dicyanoanthracene, 1,2,4,5-tetracyanobenzene, or N-methylquinolinium tetrafluoroborate as an electron-accepting photosensitizer gives rise to two transients with lambda(max) at 500 and 350 nm assigned to the dianisyl-substituted largely twisted trimethylenemethane cation radical (6.+) and the corresponding diradical (6..), respectively. These intermediates are also detected, respectively, by steady state and nanosecond time-resolved EPR with chloranil or anthraquinone as a sensitizer. The degenerate rearrangement of d(2)-1 thus proceeds via these two different types of intermediates in a cation radical cleavage-diradical cyclization mechanism. Energetics based on nanosecond time-resolved photoacoustic calorimetry support this mechanism. A comparison of the reactivities and the spectroscopic results of 1, 1,1-bis(4-methoxyphenyl)-2-methylenespiro[2.2]pentane (2), and 1-cyclopropylidene-2,2-bis(4-methoxyphenyl)cyclopropane (3) suggest that the reversible methylenecyclopropane rearrangement between 2 and 3 proceeds via a similar mechanism.     

2,10－莰烷二醇衍生的钛络合物催化的不对称硫醚氧化及其机理

在2,10－莰烷二醇/钛催化的亚砜化反应中,枯烯过氧化氢(CHP)和叔丁基过氧化氢(TBHP)分别给出R构型和S构型亚砜。在动力学拆分过程中,用CHP作氧化剂导致亚砜的构型发生逆转,但是用TBHP则保留不变。基于这些结果和电喷雾质谱(ESI－MS)数据,亚砜化反应的机理推测为分子内亲核氧转移到络合的硫醚底物上。     

Mechanism of the Photodissociation of 4-Diphenyl(trimethylsilyl)methyl-N,N-dimethylaniline

On irradiation in hexane (248- and 308-nm laser light) 4-diphenyl(trimethylsilyl)methyl-N,N-dimethylaniline, 2, undergoes photodissociation of the C-Si bond giving 4-N,N-dimethylamino-triphenylmethyl radical, 3(*) (lambda(max) at 343 and 403 nm), in very high quantum yield (Phi = 0.92). The intervention of the triplet state of 2 (lambda(max) at 515 nm) is clearly demonstrated through quenching experiments with 2,3-dimethylbuta-1,3-diene, styrene, and methyl methacrylate using nanosecond laser flash photolysis (LFP). The formation of 3(*) is further demonstrated using EPR spectroscopy. The detection of the S(1) state of 2 was achieved using 266-nm picosecond LFP, and its lifetime was found to be 1400 ps, in agreement with the fluorescence lifetime (tau(f) = 1500 ps, Phi(f) = 0.085). The S(1) state is converted almost exclusively to the T(1) state (Phi(T) = 0.92). In polar solvents such as MeCN, 2 undergoes (1) photoionization to its radical cation 2(*)(+), and (2) photodissociation of the C-Si bond, giving radical 3(*) as before in hexane. The formation of 2(*)(+) occurs through a two-photon process. Radical cation 2(*)(+) does not fragment further, as would be expected, to 3(*) via a nucleophile(MeCN)-assisted C-Si bond cleavage but regenerates the parent compound 2. Obviously, the bulkiness of the triphenylmethyl group prevents interaction of 2(*)(+) with the solvent (MeCN) and transfer to it of the electrofugal group Me(3)Si(+). The above results of the laser flash photolysis are supported by pulse radiolysis, fluorescence measurements, and product analysis.     

Chiral N-alkyl-2,4,6-triphenylpyridiniums as enantioselective triplet photosensitizers. laser flash photolysis and preparative studies

Three N-alkylpyridinium photosensitizers having chiral alkyl groups have been prepared by reacting 2,4,6-triphenylpyrylium tetrafluoroborate ion with (1R,2S)-(-)-norephedrine, (S)-(+)-2-(aminomethyl)pyrrolidine, and (R)-(-)-1-cyclohexylethylamine. Laser flash photolysis allows detection of the corresponding triplet excited states that are quenched by hydrogen atom donors and electron donors. Asymmetric quenching of the chiral triplet excited state was observed using enantiomerically pure 1,2-diamino cyclohexane as quencher. Low enantiomeric excess values (up to 7%) were measured for the photochemical cyclization of 5-methyl-4-hexenoic acid to its corresponding gamma-lactone using these chiral N-alkylpyridinium as photosensitizers.     

Kinetics, products, and mechanism of the reaction of diphenylcarbonyl oxide with sulfoxides

The kinetics of the reactions of diphenylcarbonyl oxide with dimethyl, di-n-hexyl, diphenyl, dibenzyl, andn-hexylbenzyl sulfoxides in acetonitrile was studied by flash photolysis at 295 K. The oxidation of sulfoxide affords the corresponding sulfone as the main reaction product, and diphenyl sulfide also forms in the case of Ph₂SO. Solvent effect on the reaction kinetics and the composition of the reaction products was studied. The reaction mechanism is discussed, which includes two parallel pathways: the nucleophilic attack of carbonyl oxide at the sulfur atom of sulfoxide and the formation of the cyclic sulfurane intermediatevia the electrophilic 1,3-cycloaddition of Ph₂COO at the S=O bond. The sulfurane undergoes fragmentationvia parallel channels to form sulfone or sulfide. Published inIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1504–1509, September, 2000.     

Intermolecular complexes between sulfide radical cations from β-hydroxy sulfides and phosphate

Triplet-sensitized oxidation of 2-(methylthio)ethanol (2-MTE) and photosensitized formaldehyde formation from 2-MTE were measured in aqueous solutions. The formaldehyde quantum yields were measured in steady-state experiments; whereas the time-resolved detection of dimeric sulfur radical cations (S∴S)⁺ was followed in nanosecond laser flash photolysis. 4-Carboxybenzophenone was the triplet sensitizer in both types of experiments. It was found that the lifetimes of the (S∴S)⁺ radicals increased in the presence of phosphate buffer, sodium perchlorate, or D₂O. There were corresponding decreases in the formaldehyde yields for these same experimental factors. The phosphate case was discussed in some detail in terms of a new intermolecular complex between the sulfide radical cations and the oxyanions.     

Identification of the 1,3,2,4-Dithiadiazolyl RCNSNS(*) Radical Dimers in Solution, Their Dimeric Concerted Photochemically Symmetry-Allowed Rearrangement to 1,2,3,5-Dithiadiazolyl RCNSSN(*) by the Net Exchange of Adjacent Cyclic Sulfur and Nitrogen Atoms, and the Photolysis of RCNSNS(*)

The 1,3,2,4-dithiadiazolyl RCNSNS(*) radicals undergo an unprecedented concerted rearrangement to the thermodynamically more stable 1,2,3,5-dithiadiazolyl RCNSSN(*) radicals by the net exchange of adjacent cyclic sulfur and nitrogen atoms. The UV-visible spectra of RCNSNS(*) (R = Ph, p-O(2)NC(6)H(4), 3,5-(O(2)N)(2)C(6)H(3), CF(3)) in solution show bands at 250 nm (strong) and 680 nm (very weak) attributable to monomer and two dimer bands at 376 and 480 nm, the positions of which are independent of the substituents, providing direct identification of the radical dimers in solution. The dimerization equilibrium constant (K(298) approximately 0.7 for R = Ph) at room temperature was derived from the enthalpy and entropy changes for the dimerization of PhCNSNS(*) (DeltaH(d) degrees = -19.0 kJ/mol, DeltaS degrees = -66.5 J/mol) estimated by a variable-temperature ESR spectroscopic study. In addition, RCNSNS(*) (R = Bu(t), Ph) undergo an apparent unimolecular photolysis to RCN and possibly SNS(*) (analogue of ONO(*)). The photochemical rearrangement and dissociation (for R = Ph and 3,5-(O(2)N)(2)C(6)H(3)) were shown to proceed by irradiation of the radical dimer (376 and 480 nm) and monomer (250 nm), respectively. Thus, the radical rearrangement reasonably occurs via a concerted dimeric pathway shown by molecular orbital calculations (CNDO) to be photochemically symmetry-allowed. In addition, we propose that the radical dissociation proceeds via a concerted unimolecular photochemically symmetry-allowed process.     

Electron-transfer and acid-base properties of a two-electron oxidized form of quaterpyrrole that acts as both an electron donor and an acceptor

Electron-transfer interconversion between the four-electron oxidized form of a quaterpyrrole (abbreviated as P4 for four pyrroles) and the two-electron oxidized form (P4H2) as well as between P4H2 and its fully reduced form (P4H4) bearing analogous substituents in the alpha- and beta-pyrrolic positions was studied by means of cyclic voltammetry and UV-visible spectroelectrochemistry combined with ESR and laser flash photolysis measurements. The two-electron oxidized form, P4H2, acts as both an electron donor and an electron acceptor. The radical cation (P4H2*+) and radical anion (P4H2*-) are both produced by photoinduced electron transfer from dimeric 1-benzyl-1,4-dihydronicotinamide to P4H2, whereas the cation radical form of the compound is also produced by electron-transfer oxidation of P4H2 with [Ru(bpy)3]3+. The ESR spectra of P4H2*+ and P4H2*- were recorded at low temperature and exhibit spin delocalization over all four pyrrole units. Thus, the two-electron oxidized form of the quaterpyrrole (P4H2) displays redox and electronic features analogous to those seen in the case of porphyrins and may be considered as a simple, open-chain model of this well-studied tetrapyrrolic macrocycle. The dynamics of deprotonation from P4H2*+ and disproportionation of P4H2 were examined by laser flash photolysis measurements of photoinduced electron-transfer oxidation and reduction of P4H2, respectively.