New Phosphine Oxides as High Performance Near- UV Type I Photoinitiators of Radical Polymerization.

Carbazole structures are of high interest in photopolymerization due to their enhanced light absorption properties in the near-UV or even visible ranges. Therefore, type I photoinitiators combining the carbazole chromophore to the well-established phosphine-oxides were proposed and studied in this article. The aim of this article was to propose type I photoinitiators that can be more reactive than benchmark phosphine oxides, which are among the more reactive type I photoinitiators for a UV or near-UV light emitting diodes (LED) irradiation. Two molecules were synthesized and their UV-visible light absorption properties as well as the quantum yields of photolysis and photopolymerization performances were measured. Remarkably, the associated absorption was enhanced in the 350-410 nm range compared to benchmark phosphine oxides, and one compound was found to be more reactive in photopolymerization than the commercial photoinitiator TPO-L for an irradiation at 395 nm.

Stereodivergent hydrogermylations of α-trifluoromethylated alkynes and their applications in cross-coupling reactions.

The hydrometalation of alkynes with group 14 elements such as tin- or silyl hydrides is a classical transformation in organic synthesis. Strangely, among the group 14 elements, the use of germanium hydrides is rarely seen. Two efficient, stereodivergent, and broadly applicable routes to (Z)- and (E)-α-CF3-vinylgermanes by regio- and stereoselective hydrogermylation of α-trifluoromethylated alkynes under radical or transition-metal-catalyzed conditions are reported. Furthermore, we demonstrate that the resulting stereodefined fluorinated building blocks are remarkable cross-coupling partners, provided that the vinylgermane is appropriately tuned electronically, as demonstrated by the synthesis of trisubstituted (Z)- and (E)-α-trifluoromethylated alkenes.

On the synthesis, characterization and reactivity of N-heteroaryl-boryl radicals, a new radical class based on five-membered ring ligands.

The synthesis and physical characterization of a new class of N-heterocycle-boryl radicals is presented, based on five membered ring ligands with a N(sp(2) ) complexation site. These pyrazole-boranes and pyrazaboles exhibit a low bond dissociation energy (BDE; BH) and accordingly excellent hydrogen transfer properties. Most importantly, a high modulation of the BDE(BH) by the fine tuning of the N-heterocyclic ligand was obtained in this series and could be correlated with the spin density on the boron atom of the corresponding radical. The reactivity of the latter for small molecule chemistry has been studied through the determination of several reaction rate constants corresponding to addition to alkenes and alkynes, addition to O2 , oxidation by iodonium salts and halogen abstraction from alkyl halides. Two selected applications of N-heterocycle-boryl radicals are also proposed herein, for radical polymerization and for radical dehalogenation reactions.

Formation of N-heterocyclic carbene-boryl radicals through electrochemical and photochemical cleavage of the B-S bond in N-heterocyclic carbene-boryl sulfides.

The B-S bond in N-heterocyclic carbene (NHC)-boryl sulfides can be cleaved homolytically to NHC-boryl or NHC-thioboryl and thiyl radicals using light, either directly around 300 nm or with a sensitizer at a longer wavelength (>340 nm). In contrast, the electrochemical reductive cleavage of the B-S bond is difficult. This easy photolytic cleavage makes the NHC-boryl sulfides good type I photopolymerization initiators for the polymerization of acrylates under air.

Mechanistic and preparative studies of radical chain homolytic substitution reactions of N-heterocyclic carbene boranes and disulfides.

Reactions of 1,3-dimethylimidazol-2-ylidene-borane (diMe-Imd-BH3) and related NHC-boranes with diaryl and diheteroaryl disulfides provide diverse NHC-boryl monosulfides (diMe-Imd-BH2SAr) and NHC-boryl disulfides (diMe-Imd-BH(SAr)2). Heating in the dark with 1 equiv of disulfide favors monosulfide formation, while irradiation with 2 equiv disulfide favors disulfide formation. With heteroaryl disulfides, the NHC-borane in the primary NHC-boryl sulfide product migrates from sulfur to nitrogen to give new products with a thioamide substructure. Most substitution reactions are thought to proceed through radical chains in which homolytic substitution of a disulfide by an NHC-boryl radical is a key step. However, with electrophilic disulfides under dark conditions, a competing ionic path may also be possible.

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.

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%).