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The hydrosilylation reaction, describing the addition of Si-H bonds to unsaturated bonds, is performed in the presence of catalysts, usually highly active platinum catalysts. This work focuses on the study of a photoinduced hydrosilylation by the use of benzophenone which promotes the addition reaction of olefin on different hydrosilanes. The reactivity of silanes towards addition onto the double bond during hydrosilylation appears to depend on their structure. It was observed that the consumption of Si-H and C[double bond, length as m-dash]C functional groups increases with the irradiation time, and reaches a maximum of approx. 51% in the case of diphenylsilane. The hydrosilylation products are determined with 1 H NMR, HSQC, DEPT, COSY and 13 C NMR. The main product corresponds to the single adduct of the silyl radical onto the double bond. Substitution of the Si-H bond by two or three phenyls groups (triphenylsilane, diphenysilane) enhances the yield of the reaction, although diphenylsilane was found to be more efficient than triphenylsilane because of its lower steric hindrance. The ketyl radical formed after hydrogen abstraction by the triplet state of benzophenone likely forms benzopinacol, a reaction which reduces the overall yield of the hydrosilylation reaction. All these experiments are in line with DFT calculations of the Gibbs free energy of the reactions involved. This sheds new light on the photoinduced hydrosilylation process and opens the way to more active combinations of photoinitiator/silane/vinylsilane systems., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Although N-heterocyclic carbenes (NHCs) have brought profound changes in catalytic organic synthesis, their generation generally requires an inert atmosphere and harsh conditions. To overcome these limitations, an air-stable NHC photogenerator has been developed involving two mild components: 1,3-bis(mesityl)imidazolium tetraphenylborate (IMesH + BPh 4 - ) and electronically excited isopropylthioxanthone (ITX). In this study, the photochemical mechanism is investigated via the accurate identification of the transient species and photoproducts. Electron transfer reaction between the excited triplet state of ITX and BPh 4 - is demonstrated as being the primary photochemical step. Nanosecond laser spectroscopy shows an efficient quenching and the formation of the expected ITX radical anion. The oxidized borane species is not observed, suggesting that this short-lived species could dissociate very rapidly to give the phenyl radical - successfully identified using electron paramagnetic resonance - and triphenylborane. As regards the final photoproducts, 1 H and 13 C NMR spectroscopies support the formation of the targeted NHC, 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes), suggesting the occurrence of a subsequent proton transfer reaction between ITX radical anion and imidazolium cation (IMesH + ). Gas chromatography-mass spectrometry reveals three other products: biphenyl, isopropylthioxanthene and ITX. Their formation can be reconciled with a 2-step mechanism of photoinduced electron/proton transfer reactions. 11 B NMR spectroscopy demonstrates that the main organoboron photoproduct is diphenylborinic acid formed by oxidation of BPh 3 . Due to its Lewis acidity, Ph 2 BOH can react with IMes to yield an NHC-boron adduct.

Base catalyst and oxidant are usually necessary to promote the polymerization of poly(disulfide) oligomers through oxidative coupling of the terminal SH groups into S-S bonds. In this study, we prove that self-polymerization of bifunctional (disulfide) oligomer films can take place in a matter of minutes under UVC irradiation (254 nm, 10.5 mW cm -2 ). The resulting insoluble polymer is characterized using solid-state NMR, 1 H T 2 NMR relaxation measurements, thermal analysis, and Fourier-transform infrared spectroscopy and proves to have similar composition as a model poly(disulfide) prepared under oxidative conditions, but distinct physical properties. These differences are explained by a change in polymer architecture due to a higher ratio of cyclization relative to linear polymerization. Homolytic photocleavage of internal S-S bonds creates thiyl groups close to each other, driving an increased kinetic feasibility for the cyclization reaction by radical coupling. The subsequent formation of mechanically interlocked macrocycles (polycatenane network) is proposed to account for film properties analogous to those of a cross-linked polymer., Competing Interests: The authors declare no competing financial interest.

Direct incorporation of molecular oxygen into small organic molecules has attracted much attention for the development of new environmentally friendly oxidation processes. In line with this approach, bioinspired systems mimicking enzyme activities are of particular interest since they may perform catalysis in aqueous media. Demonstrated herein is the incorporation of a natural flavin cofactor (FMN) into the specific microenvironment of a water-soluble polymer which allows the efficient reduction of the FMN by NADH in aqueous solution. Once reduced, this artificial flavoenzyme can then activate molecular dioxygen under aerobic conditions and result in the Baeyer-Villiger reaction at room temperature in water., (© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

A modular total synthesis of mycolactone A/B, the exotoxin produced by Mycobacterium ulcerans, has been achieved through the orchestration of several Pd-catalyzed key steps. While this route leads to a mixture of the natural product and its C12 epimer (4 : 1 ratio), this was inconsequential from the biological activity standpoint. Compared to the previously reported routes, this synthetic blueprint allows the late-stage modification of the toxin, as exemplified by the preparation of [22,22,22- 2 H 3 ]-mycolactone A/B.

Initially developed as an elastomer with an excellent record of barrier and chemical resistance properties, poly(disulfide) has experienced a revival linked to the dynamic nature of the S-S covalent bond. A novel photobase-catalyzed oxidative polymerization of multifunctional thiols to poly(disulfide) network is reported. Based solely on air oxidation, the single-step process is triggered by the photodecarboxylation of a xanthone acetic acid liberating a strong bicyclic guanidine base. Starting with a 1 μm thick film based on trithiol poly(ethylene oxide) oligomer, the UV-mediated oxidation of thiols to disulfides occurs in a matter of minutes both selectively, i.e., without overoxidation, and quantitatively as assessed by a range of spectroscopic techniques. Thiolate formation and film thickness determine the reaction rates and yield. Spatial control of the photopolymerization serves to generate robust micropatterns, while the reductive cleavage of S-S bridges allows the recycling of 40% of the initial thiol groups., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

A click chemistry synthetic strategy based on aza-Michael addition and radical photopolymerization is proposed to generate a polymeric network via three time-controlled steps. The selection of primary diamines and diacrylates allows two consecutive aza-Michael reactions to occur. This reaction sequence affords the unique opportunity to interpose a radical photopolymerization reaction, enhancing the cross-link density. Consequently, the second aza-Michael addition appears as a valuable postconsolidation step of the polymer network.

Historically, the synthesis of aqueous polymer dispersions has focused on radical chain-growth polymerization of low-cost acrylate or styrene emulsions. Herein, we demonstrate the potential of UV-initiated thiol-ene step-growth radical polymerization, departing from a nontransparent difunctional monomer miniemulsion based on ethylene glycol dithiol and diallyl adipate. Performed without solvent and at ambient conditions, the photopolymerization process is energy-effective, environmentally friendly, and ultrafast, leading to full monomer consumption in 2 s, upon irradiating a miniemulsion contained in a 1 mm thick quartz cell microreactor. The resultant linear poly(thioether ester) particles have an average diameter of 130 nm. After water evaporation, they yield a clear elastomeric film combining chemical resistance and high degree of crystallinity (55%).

We report on the synthesis and the photophysical properties of a trans-nitroalkoxystilbene dimer (DPNS). The fluorescence quantum yield (Φ(f)), the Stokes shift, and the quantum yield for the trans-to-cis photoisomerization (Φ(t→c)) are strongly dependent on the nature of the solvent. Upon increasing solvent polarity, Φ(f) increases together with the decrease of Φ(t→c). This solvent-induced reverse behavior mainly stems from the progressive stabilization of a highly polar twisted internal charge transfer state (TICT) at excited singlet level which opens a competing channel to photoisomerization. In the presence of hydroxylic substrates (i.e., alcohols or water), fluorescence of DPNS is strongly quenched due to a hydrogen bonding interaction at excited state. The efficiency of the process is clearly correlated to the H-bond donor ability of the quencher. In aqueous solution, the major formation of a 2:1 host-guest complex with β-cyclodextrins (β-CD) prevents the quenching by H(2)O and leads to a 50-fold increase of the fluorescence signal together with a strong band blue-shift with respect to that of the free chromophore. This latter effect was rationalized in terms of a severe reduction of the solvent-induced stabilization of the TICT state. As a consequence, the trans-to-cis photoisomerization reaction is reactivated and leads to a paradoxical 14-fold increase of Φ(t→c) even though DPNS is sequestered in β-CD cavities.

Photoacid-catalyzed sol-gel polymerization is now recognized as a powerful single-step synthetic approach to the synthesis of hybrid films, which can be distinguished from conventional sol-gel methods by higher reactivity and a solvent-free process. Despite its utility, the mechanism is not yet understood, in particular what chemical, physical, and photochemical parameters determine the precise sequence, kinetics, and advancement of this UV inorganic photopolymerization. Here, using mainly transmission real-time Fourier transformed infrared (RT-FTIR) spectroscopy, we characterize in situ the hydrolysis-condensation reactions of oligomeric silicon alkoxides and the formation of byproducts. Systematic review and assessment of numerous processing variables (relative humidity, film thickness, precursor structure, nature, and the concentration of photoacid generator) prove that the reaction kinetics are controlled by the two independent phenomena: the intrinsic chemical reaction rates and the water vapor permeation into the film.

[reactions: see text] The stereoselective outcome of Pd(II)- or Ag(I)-catalyzed intramolecular N-alkylation to afford 1,3-disubstituted 1,2,3,4-tetrahydroisoquinolines was examined. In the absence of additional substituents, Pd(II) allows a facile access to the cis isomers, while Ag(I) favors formation of the trans isomers. The same observation was made for the synthesis of 2,5-disubstituted pyrrolidines. Possible reasons for the observed stereoselectivities are discussed.