The Photooxygenation of Benzyl, Heteroarylmethyl, and Allyl Sulfides

Aerobic Photocatalysis: Oxidation of Sulfides to Sulfoxides

Sulfoxides constitute one of the most important functional groups in organic chemistry found in numerous pharmaceuticals and natural products. Sulfoxides are usually obtained from the oxidation of the corresponding sulfides. Among various oxidants, oxygen or air are considered the greenest and most sustainable reagent. Photochemistry and photocatalysis is increasingly applied in new, as well as traditional, yet demanding, reaction, like the aerobic oxidation of sulfides to sulfoxides, since photocatalysis has provided the means to access them in mild and effective ways. In this review, we will summarize the photochemical protocols that have been developed for the oxidation of sulfides to sulfoxides, employing air or oxygen as the oxidant. The aim of this review is to present: i) a historical overview, ii) the key mechanistic studies and proposed mechanisms and iii) categorize the different catalytic systems in literature.

Photooxidation of thiosaccharides mediated by sensitizers in aerobic and environmentally friendly conditions

A series of β-d-glucopyranosyl derivates have been synthesized and evaluated in photooxidation reactions promoted by visible light and mediated by organic dyes under aerobic conditions. Among the different photocatalysts employed, tetra-O-acetyl riboflavin afforded chemoselectively the respective sulfoxides, without over-oxidation to sulfones, in good to excellent yields and short reaction times. This new methodology for the preparation of synthetically useful glycosyl sulfoxides constitutes a catalytic, efficient, economical, and environmentally friendly oxidation process not reported so far for carbohydrates.

Photochemistry of triphenylamine (TPA) in homogeneous solution and the role of transient N-phenyl-4a,4b-dihydrocarbazole. A steady-state and time-resolved investigation

Irradiation of triphenylamine with a laser pulse (355 nm) provided intermediate N-phenyl-4a,4b-tetrahydrocarbazole (DHC0) whose reactivity depends on the reaction media and atmospheres used.

Visible-light photocatalytic aerobic oxidation of sulfides to sulfoxides with a perylene diimide photocatalyst

Photosensitized oxygenation has been recognised as a modern method of incorporating oxygen into a substrate, as it offers environmentally benign alternatives to several conventional synthetic procedures. A metal-free aerobic selective sulfoxidation photosensitized by a perylene diimide photocatalyst has been developed. The reaction utilizes visible light as the driving force and molecular oxygen as the oxidant. The advantages of the developed method include high efficiency and selectivity, extremely simple operation and work-up procedure, mild reaction conditions, and practical application in late-stage functionalization.

Simple low cost porphyrinic photosensitizers for large scale chemoselective oxidation of sulfides to sulfoxides under green conditions: targeted protonation of porphyrins

Large scale chemoselective photooxidation of sulfides to sulfoxides in the presence of the diacids ofmeso-tetra(phenyl)porphyrin with different acids is reported.

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.

Photooxidation of Dibenzothiophene and 4,6-Dimethyldibenzothiophene Sensitized by N-Methylquinolinium Tetrafluoborate: Mechanism and Intermediates Investigation

Photooxidation of dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (DMDBT) sensitized by N-methylquinolinium tetrafluoborate (NMQ(+)BF4-) has been investigated in O2-saturated acetonitrile solutions. Nearly 100% oxidation of DBT and DMDBT was observed, and the oxidized products are predominantly composed of sulfoxides and sulfones, which are formed via photoinduced electron transfer (ET). Such ET processes were studied with fluorescence quenching of NMQ+, time-resolved transient absorption measurement, and ESR experiments. The fluorescence of NMQ+ is efficiently quenched by DBT and DMDBT via diffusion-controlled processes, with bimolecular quenching constants of 1.6 x 10(10) M(-1) s(-1) for DBT and 2.3 x 10(10) M(-1) s(-1) for DMDBT. The electron-transfer nature of the quenching is evidenced by the transient absorption measurement of the neutral radical NMQ*, which is formed by electron transfer from the substrates (DBT or DMDBT) to the excited singlet state of NMQ+. The ESR spectra of the superoxide radical anion (O2*-) trapped by 5,5-dimethyl-1-pyrroline N-oxide (DMPO) in the photooxygenation of DBT and DMDBT as well as their sulfoxides manifest that O2 traps an electron from NMQ* to form O2*-. The fact that the formation of sulfoxides and sulfones is greatly suppressed in the presence of benzoquinone (BQ), an efficient electron trap for NMQ* and O2*-, further indicates an ET process in the photooxidation of DBT and DMDBT. As inferred from the control experiments, the role of singlet oxygen (1O2) in the photooxidation is negligible. The intermediates responsible for the formation of sulfoxides and sulfones have been examined in detail.