Homogeneous Decatungstate-Catalyzed Photooxygenation of Tetrasubstituted Alkenes: A Deuterium Kinetic Isotope Effect Study

Mo2C as Pre-Catalyst for the C-H Allylic Oxygenation of Alkenes and Terpenoids in the Presence of H2O2

In this study, commercially available molybdenum carbide (Mo2C) was used, in the presence of H2O2, as an efficient pre-catalyst for the selective C-H allylic oxygenation of several unsaturated molecules into the corresponding allylic alcohols. Under these basic conditions, an air-stable, molybdenum-based polyoxometalate cluster (Mo-POM) was formed in situ, leading to the generation of singlet oxygen (1O2), which is responsible for the oxygenation reactions. X-ray diffraction, SEM/EDX and HRMS analyses support the formation mainly of the Mo6O192− cluster. Following the proposed procedure, a series of cycloalkenes, styrenes, terpenoids and methyl oleate were successfully transformed into hydroperoxides. After subsequent reduction, the corresponding allylic alcohols were produced with good yields and in lab-scale quantities. A mechanistic study excluded a hydrogen atom transfer pathway and supported the twix-selective oxygenation of cycloalkenes on the more sterically hindered side via the 1O2 generation.

Direct Photocatalyzed Hydrogen Atom Transfer (HAT) for Aliphatic C-H Bonds Elaboration

Direct photocatalyzed hydrogen atom transfer (d-HAT) can be considered a method of choice for the elaboration of aliphatic C–H bonds. In this manifold, a photocatalyst (PC_HAT) exploits the energy of a photon to trigger the homolytic cleavage of such bonds in organic compounds. Selective C–H bond elaboration may be achieved by a judicious choice of the hydrogen abstractor (key parameters are the electronic character and the molecular structure), as well as reaction additives. Different are the classes of PCs_HAT available, including aromatic ketones, xanthene dyes (Eosin Y), polyoxometalates, uranyl salts, a metal-oxo porphyrin and a tris(amino)cyclopropenium radical dication. The processes (mainly C–C bond formation) are in most cases carried out under mild conditions with the help of visible light. The aim of this review is to offer a comprehensive survey of the synthetic applications of photocatalyzed d-HAT.

Highly Stereoselective Synthesis of Tetrasubstituted Acyclic All-Carbon Olefins via Enol Tosylation and Suzuki-Miyaura Coupling

A highly stereocontrolled synthesis of tetrasubstituted acyclic all-carbon olefins has been developed via a stereoselective enolization and tosylate formation, followed by a palladium-catalyzed Suzuki-Miyaura cross-coupling of the tosylates and pinacol boronic esters in the presence of a Pd(OAc)2/RuPhos catalytic system. Both the enol tosylation and Suzuki-Miyaura coupling reactions tolerate an array of electronically and sterically diverse substituents and generate high yield and stereoselectivity of the olefin products. Judicious choice of substrate and coupling partner provides access to either the E- or Z-olefin with excellent yield and stereochemical fidelity. Olefin isomerization was observed during the Suzuki-Miyaura coupling. However, under the optimized cross-coupling reaction conditions, the isomerization was suppressed to <5% in most cases. Mechanistic probes indicate that the olefin isomerization occurs via an intermediate, possibly a zwitterionic palladium carbenoid species.

C–H bond activation with Ge-oxyl complex generated by photoinduced-electron-transfer of di(hydroxo)porphyrin GeIV complex

Visible-light irradiation of MeOH solution containing di(hydroxo)tetraphenylporphyrin atogermanium(IV) complex ( tppGe ( OH )2; 1a), cumene, and Fe 3+ ion ( Fe ( NO 3)3) as an electron acceptor gave cumyl alcohol as an oxidative product along with Fe 2+ ion as a reductive product. The quantum yield (Φox) and turn over frequency (TOF) for the formation of cumyl alcohol was 0.033 and 111.1 h-1, respectively. The addition of KOH aqueous solution (1 mM) into reaction solution led to an increase of Φox to 0.047. The free energy change (ΔG) with electron transfer from excited triplet state (31a*) to Fe 3+ was estimated as a large negative value (-1.37 eV). Furthermore, in 1a-photosensitized oxidation of MeOH in the presence of K 2 PtCl 6 as an electron acceptor, formaldehyde ( HCHO ) was formed in Φox = 0.034 and TOF = 120.0 h-1. The isotope effect for the formation of HCHO (Φox(H)/Φox(D) = 5.04) was observed when MeOH -d4 was employed as an substrate. Both formation of cumyl alcohol and formaldehyde was not observed at all in the case of photosensitized reactions by tppGe ( OMe )2 (1b) having two axial methoxo ligands. These findings indicate the photosensitized reaction was initiated by photoinduced electron transfer from 31a* to Fe 3+ or K 2 PtCl 6 to generate porphyrin radical cation (1a+•), which underwent a proton dissociation of hydroxo axial ligand to give tpp ( OH ) Ge - O • ( Ge -oxyl complex) as key intermediate. The Ge -oxyl complex oxidized substrates through a hydrogen-atom abstraction. It is strongly suggested that 1a can act as a good sensitizer for being able to activate C – H bond of organic compounds.

Heteropolytungstic acids incorporated in an ordered mesoporous zirconia framework as efficient oxidation catalysts

Ordered mesoporous composite materials consisting of nanocrystalline ZrO₂ and 12-phosphotungstic and 12-silicotungstic acids were synthesized and shown to be superior catalysts than individual components.

Efficient degradation of methabenzthiazuron photoinduced by decatungstate anion in water: kinetics and mechanistic studies

This study concerns the elimination of methabenzthiazuron (MBTU) photocatalysed by sodium decatungstate salts W10O32(4-)·(DTA) in aqueous solution under irradiation at 365 nm. Ninety percentage of MBTU (10(-4) M) is mineralised in the presence of the photocatalyst (2×10(-4) M) after 7 d under exposure and the formation of nitrate, sulphate and ammonium confirmed this phenomenon. In aerated conditions, the photodegradation rate of MBTU clearly increased in the presence of DTA by a factor of 40 when compared to direct photolysis with ΦMBTU=2.5×10(-2) and t1/2 (MBTU)=1.4 h. Oxygen appeared essential since 2 times inhibition of MBTU disappearance and the photocatalytic cycle interrupt were observed in the absence of oxygen. The degradation mechanism has been elucidated through the photoproducts identification by LC-ESI-MS analysis. Two processes were implied in the degradation: electron transfer and H atom abstraction reactions both involving W10O32(4-∗) excited state species. In the primary steps of the degradation, the aromatic ring hydroxylation was observed by electron transfer leading to OH-MBTU isomers and H atom abstraction reaction gave benzthiazuron and a supposed demethylated product. Secondary oxidations permitted the hydroxylation of both products.