New hydrogen donors for amine-free photoinitiating systems in dental materials.

OBJECTIVES: The two-component Camphorquinone (CQ)/aromatic amine system is well-established and clearly corresponds to the reference system used in all photopolymerizable dental adhesives and composites. However, this CQ/amine system still suffers from the presence of aromatic amines that can be referenced as toxic. Therefore, the aim of this work is to develop amine-free photoinitiating systems (PISs) for the polymerization of a representative dental methacrylate resins upon blue light irradiation. The proposed strategy is based on the in-silico design (by molecular modelling) of new hydrogen donors (amine-free) bearing a copolymerizable moiety (methacrylate functionality) to ensure their low migration/leaching properties from the synthesized polymer. The new proposed PISs are compared to the well-established CQ/amine system for the polymerization of different methacrylate blends upon exposure to a commercial blue dental LED centered at 477 nm. METHODS: Molecular orbitals calculations are used to design new hydrogen donors exhibiting low C-H bond dissociation energies. Based on this in-silico design, the syntheses of new co-initiators are reported here for the first time. Real-time FTIR experiments are used to monitor the photopolymerization profiles. Color indexes measurements were also carried out to investigate the bleaching properties of the new proposed systems. RESULTS: Three new co-initiators are proposed as alternatives to aromatic amines in dental materials in combination with camphorquinone. The performances of the new proposed amine-free PISs for the photopolymerization of thick (1.4 mm) samples of methacrylate upon exposure to a blue dental LED under air are excellent. Similar or better polymerization performances are obtained with the new proposed amine-free systems compared to those reached with the CQ/amine reference. Excellent bleaching properties are also found. The involved chemical mechanisms are investigated through molecular orbitals calculations.

A New Phosphine for Efficient Free Radical Polymerization under Air.

A new phosphine is proposed as efficient coinitiator for camphorquinone (CQ)-based photoinitiating systems for the free radical polymerization of (meth)acrylates. Remarkably, this new co-initiator can exhibit two functionalities: a phosphine moiety to overcome oxygen inhibition and an iodonium salt moiety as counter cation to initiate the polymerization process. Excellent polymerization performances in the presence of CQ for the free radical polymerization of methacrylates under blue light are observed, and amine-free systems can be easily developed from the proposed structure. The photopolymerization of composites is also investigated in the presence of the new phosphine (without iodonium counter cation) and very interesting depth of cure can be obtained from the new developed photoinitiating system after only 20 s of irradiation with a blue light-emitting diode.

Palladium nanoparticles embedded in mesoporous carbons as efficient, green and reusable catalysts for mild hydrogenations of nitroarenes.

The reduction of nitroarenes is the most efficient route for the preparation of aromatic primary amines. These reductions are generally performed in the presence of heterogeneous transition metal catalysts, which are rather efficient but long and tedious to prepare. In addition, they contain very expensive metals that are in most cases difficult to reuse. Therefore, the development of efficient, easily accessible and reusable Pd catalysts obtained rapidly from safe and non-toxic starting materials was implemented in this report. Two bottom-up synthesis methods were used, the first consisted in the impregnation of a micro/mesoporous carbon support with a Pd salt solution, followed by thermal reduction (at 300, 450 or 600 °C) while the second involved a direct synthesis based on the co-assembly and pyrolysis (600 °C) of a mixture of a phenolic precursor, glyoxal, a surfactant and a Pd salt. The obtained composites possess Pd nanoparticles (NPs) of tunable sizes (ranging from 1-2 to 7.0 nm) and homogeneously distributed in the carbon framework (pores/walls). It turned out that they were successfully used for mild and environment-friendly hydrogenations of nitroarenes at room temperature under H2 (1 atm) in EtOH in the presence of only 5 mequiv. of supported Pd. The determinations of the optimal characteristics of the catalysts constituted a second objective of this study. It was found that the activity of the catalysts was strongly dependent on the Pd NPs sizes, i.e., catalysts bearing small Pd NPs (1.2 nm obtained at 300 °C and 3.4 nm obtained at 450 °C) exhibited an excellent activity, while those containing larger Pd NPs (6.4 nm and 7.0 nm obtained at 600 °C, either by indirect or direct methods) were not active. Moreover, the possibility to reuse the catalysts was shown to be dependent on the surface chemistry of the Pd NPs: the smallest Pd NPs are prone to oxidation by air and their surface was gradually covered by a PdO shell decreasing their activity during reuse. A good compromise between intrinsic catalytic activity (i.e. during first use) and possibility of reuse was found in the catalyst made by impregnation followed by reduction at 450 °C since the hydrogenation could be performed in only 2 h in EtOH or even in water. The catalyst was quantitatively recovered after reaction by filtration, used at least 7 times with no loss of efficiency. Advantageously, almost Pd-free primary aromatic amines were obtained since the Pd leaching was very low (<0.1% of the introduced amount). Compared to numerous reports from the literature, the catalysts described here were both easily accessible from eco-friendly precursors and very active for hydrogenations under mild and "green" reaction conditions.

Sulfinates and sulfonates as high performance co-initiators in CQ based systems: Towards aromatic amine-free systems for dental restorative materials.

OBJECTIVE: The aim of our study is to develop amine-free photoinitiating systems (PISs) for the polymerization of representative dental methacrylate resins under blue light irradiation. PISs based on camphorquinone (CQ)/sulfinate and CQ/sulfonate, eventually in combination with an iodonium salt, are proposed and compared to the well-established CQ/amine system. The polymerization performances of thick (1.4 mm) samples of different methacrylate blends upon exposure to a commercial blue LED centered at 477 nm under air are described. Finally, the performances of the new developed PISs are evaluated for dental composites application. METHODS: FTIR is used to monitor the photopolymerization profiles. ESR spectroscopy and electrochemical experiments are used to identify the radicals generated. Mechanical properties measurements and color stability measurements are carried out to determine the key properties of the dental composites prepared. RESULTS AND SIGNIFICIANCE: The performances of the new proposed PISs for the photopolymerization of thick (1.4 mm) samples of methacrylate upon exposure to a blue dental LED under air are excellent. Similar or better performances and bleaching properties are obtained with the new proposed amine-free systems compared to those reached with the CQ/amine reference system. Dental composites with excellent mechanical properties and exceptional color stability are obtained. The involved chemical mechanisms for the initiation step were also established.

1-Aryl-2-(triisopropylsilyl)ethane-1,2-diones: Toward a New Class of Visible Type I Photoinitiators for Free Radical Polymerization of Methacrylates.

1-Aryl-2-(triisopropylsilyl)ethane-1,2-diones (SEDs) are proposed here as a new class of visible Type I photoinitiators (PIs) for free radical polymerization under air upon exposure to blue (@455 nm) and green (@520 nm) LEDs. Remarkably, these new systems present good polymerization performances and excellent bleaching properties compared to camphorquinone-based systems, and transparent polymers are obtained upon visible light irradiation. Real-time Fourier transform infrared spectroscopy is used to monitor the polymerization profiles. Molecular orbital calculations are also carried out for a better understanding of the structure/reactivity relationship of the photoinitiators.

Aryliodonium Ylides as Novel and Efficient Additives for Radical Chemistry: Example in Camphorquinone (CQ)/Amine Based Photoinitiating Systems.

Diaryliodonium salts are well-established compounds in free radical chemistry and are already used as photoinitiators (free radical or cationic polymerization), but the presence of counter anions is a strong drawback. Indeed, a counter anion is always required (e.g., SbF6-) leading to potential toxicity issues or release of HF. In the present paper, counter anion-free and fluoride-free aryliodonium salts are proposed, that is, aryliodonium ylides (AY) are studied here as new and efficient additives for radical chemistry and an example is provided for the camphorquinone (CQ)/amine based photoinitiating systems (PISs) for the polymerization of thick (1.4 mm) and thin (20-13 µm) methacrylates under air and blue light irradiation. The newly proposed PISs, for example, CQ/amine/AY, presented excellent polymerization performances and good bleaching properties were obtained after polymerization. Real-time Fourier transform infrared spectroscopy (RT-FTIR) was used to monitor the photopolymerization profiles. The chemical mechanisms involved were investigated using electron spin resonance (ESR).

Reusable magnetic Pd x Co y nanoalloys confined in mesoporous carbons for green Suzuki-Miyaura reactions.

We report herein Pd x Co y nanoalloys confined in mesoporous carbons (Pd x -Co y @MC) prepared by an eco-friendly one-pot approach consisting in the co-assembly of readily available and non-toxic carbon precursors (phloroglucinol, glyoxal) with a porogen template (pluronic F-127) and metallic salts (H2PdCl4 and Co(NO3)2·6H2O) followed by thermal annealing. Three Pd x Co y @MC materials with different alloy compositions were prepared (C1: x/y = 90/10; C2: x/y = 75/25; C3 and C4: x/y = 50/50). The nanoalloys were uniformly distributed in the carbon framework and the particle sizes depended on the alloy composition. These composites were then used for Suzuki-Miyaura reactions using either H2O or a 1 : 1 H2O/EtOH mixture as solvent. The Pd50Co50@MC catalyst C3 proved to be the most efficient catalyst (in terms of efficiency and magnetic recovery) affording the coupling products in good to excellent yields. After reaction, C3 was recovered quantitatively by simple magnetic separation and reused up to six times without loss of efficiency. The amount of palladium lost in the reaction mixture after magnetic separation was very low (ca. 0.1 % wt of the amount initially used).

In Situ Generated Ruthenium-Arene Catalyst for Photoactivated Ring-Opening Metathesis Polymerization through Photolatent N-Heterocyclic Carbene Ligand.

1,3-Bis(mesityl)imidazolium tetraphenylborate (IMesH+ BPh4- ) can be synthesized in one step by anion metathesis between the corresponding imidazolium chloride and sodium tetraphenylborate. In the presence of 2-isopropylthioxanthone (sensitizer), an IMes N-heterocyclic carbene (NHC) ligand can be photogenerated under irradiation at 365 nm through coupled electron/proton transfer reactions. By combining this tandem NHC photogenerator system with metathesis inactive [RuCl2 (p-cymene)]2 precatalyst, the highly active RuCl2 (p-cymene)(IMes) complex can be formed in situ, enabling a complete ring-opening metathesis polymerization (ROMP) of norbornene in the matter of minutes at room temperature. To the best of our knowledge, this is the first example of a photogenerated NHC. Its exploitation in photoROMP has resulted in a simplified process compared to current photocatalysts, because only stable commercial or easily synthesized reagents are required.

Structural investigation of cyclo-dioxo maleimide cross-linkers for acid and serum stability.

The biochemical characteristics of hetero-bifunctional cross-linkers used in bioconjugates are of essential importance to the desired features of the final adduct (i.e. antibody-drug conjugates). These include stability in biological media, chemical and biological reactivities, cleavability under defined conditions, and solubility. In our previous work, we introduced a new amino-to-thiol linker, maleimidomethyl dioxane (MD), as an alternative to classical maleimide conjugation, with increased hydrophilicity and serum stability due to succinimidyl ring-opening. In this work, we investigate the generality of linkers containing a dioxo-ring with regard to their ability to self-hydrolyze and their surprising stability at a low pH. We synthesized four FRET probes which allowed us to address the stability of the dioxo-ring and to study the maleimide ring-opening and the thiol-exchange processes by means of detecting and measuring the generation of fluorescence. It was found that the ring expansion (from a 5- to a 6-membered ring) improved the stability of the probes in aqueous media, and the increase of the chain length between the dioxo-ring and the succinimide ring (from methylene to ethylene) decreased the rate of succinimidyl ring-opening.

An efficient photochemical route to Pd nanoparticles; application to the one-step synthesis of Pd@polymer nanocomposite films.

We have developed a facile, efficient, low cost and 'green' photochemical approach to preparing surfactant-free Pd nanoparticles and Pd-immobilized@acrylate photo-polymer films at room temperature, under air and without any additional treatment. The reaction system only includes a photo-initiator used as a generator of free radicals and a Pd(II) salt. In ethanol solution, the photochemical reduction of Pd(II) to Pd(0) generates very small metal particles with a narrow size distribution (2-4 nm). Furthermore, we have shown that the formation of Pd nanoparticles from a Pd(II) salt can be reversible thus allowing easy handling and safe storage with the possibility of generating the nanoparticles just before use. In the presence of an acrylate bifunctional monomer, Pd@polymer film was obtained through a 'one-pot, one-step' process resulting from a simultaneous photo-reduction of Pd(II) and photo-polymerization of acrylate units. The simultaneous generation of a 3D polymer network and of metal particles leads to a homogeneous distribution of Pd nanoparticles in the photo-polymer matrix with an average diameter of approximately 3.7 ± 1.1 nm. Such as-prepared Pd@polymer films were found to efficiently catalyze the Mizoroki-Heck reaction in the presence of only 0.9 mequiv. of supported palladium. The major interest of this arrangement is its recoverability and reusability, which makes it very attractive both from a practical and economical viewpoint. Finally, it is worth noting that this innovation offers a great advantage over concurrent methods in that it is simply generated within minutes, it is highly stable, and there is sharp monodispersity in the size of the Pd nanoparticles that can be stored for months without alteration of their physico-chemical properties and catalytic activity.

Formation of carbon-sulfur and carbon-selenium bonds by palladium-catalyzed decarboxylative cross-couplings of hindered 2,6-dialkoxybenzoic acids.

A simple route to diaryl sulfides using a decarboxylative palladium-catalyzed reaction between electron-rich 2,6-dialkoxybenzoic acid derivatives and diaryl disulfides is reported. This coupling proceeds efficiently in the presence of Pd(CF(3)CO(2))(2) and Ag(2)CO(3) in a 65:1 mixture of 1,4-dioxane and tetramethylene sulfoxide (TMSO). We present also the first formation of a carbon-selenium bond via a palladium-catalyzed decarboxylative cross-coupling.

Reusable polystyrene-supported Pd catalyst for Mizoroki-Heck reactions with extremely low amounts of supported Pd.

In this communication, we report that by running Mizoroki-Heck reactions of various aryl iodides in toluene-DMF, only extremely low amounts of supported Pd (0.04 mequiv.) are required to obtain almost quantitative yields. The catalyst was reused successfully up to three times.

Biaryl synthesis via decarboxylative Pd-catalyzed reactions of arenecarboxylic acids and diaryliodonium triflates.

A novel simple and efficient synthesis of biaryls via a Pd-catalyzed decarboxylative cross-coupling reaction of arenecarboxylic acids and diaryliodonium triflates is described. The PdCl2/DPEphos catalytic system in the presence of Ag2CO3 in DMSO was found to be the most efficient. Various biaryls, including sterically hindered biaryls, were synthesized with yields ranging from 37 to 85%.

Highly efficient and reusable supported pd catalysts for Suzuki-Miyaura reactions of aryl chlorides.

Syntheses of air- and moisture-stable heterogeneous (tert-butylarylphosphino)polystyrene-supported Pd catalysts and their use for versatile Suzuki-Miyaura reactions of aryl chlorides and arylboronic acids under non-anhydrous conditions are reported. The catalysts are easily recovered by filtration. They can be used many times (more than seven) without showing any loss of activity, and the amount of Pd leached is extremely low (<0.1%).

Synthesis of biaryls via decarboxylative Pd-catalyzed cross-coupling reaction.

[reaction: see text] A simple and efficient route to biaryls via Pd-catalyzed decarboxylative cross-couplings of arene carboxylic acids and aryl iodides is reported. The PdCl2/AsPh3 catalytic system in the presence of Ag2CO3 in DMSO was found to be particularly efficient to perform this transformation. This reaction can be extended to the synthesis of various biaryls, including sterically hindered biaryls, with yields ranging from 58% to 90%.

Facile introduction of SH group on aromatic substrates via electrophilic substitution reactions.

Herein, we describe a mild and efficient two-step procedure to introduce a thiol group on aromatic substrates. First, reaction with an activated sulfoxide leads to an arylsulfonium salt intermediate. Then, two successive beta-elimination-based dealkylation reactions afford the desired arylthiols in good to excellent yields.

Reinvestigation of the noncatalyzed coupling of aryllithium with haloarene: a novel aromatic nucleophilic substitution pathway.

Noncatalyzed coupling reactions of aryllithiums and haloarenes proceed not only through the well-known aryne route but also, in some cases, through a novel addition-elimination pathway. Indeed, ortho-chloro- and ortho-bromomethoxyarenes lead selectively to the corresponding ortho-biaryls through a chelation-driven aromatic nucleophilic substitution pathway. Contrary to common belief, such noncatalyzed coupling reactions often proceed with high regioselectivity and high yield. These results underline the potency of such simple reactions and open up a straightforward access to a wide range of biaryl structures; this also appears particularly useful for large-scale and biaryl building-block syntheses, as only cheap and readily available substrates are involved.