Nicolas Blanchard

Copper-Mediated Coupling Reactions and Their Applications in Natural Products and Designed Biomolecules Synthesis

6.4. Polyynes 3123 6.5. Using R-Hydroxy Stannanes 3124 6.6. Using the Hurtley Reaction 3124 6.7. Using a Methylenation Reaction 3125 7. Conclusions and Future Prospects 3125 8. Uncommon Abbreviations 3125 9. Acknowledgments 3125 10. Note Added in Proof 3125 11. References 3126 * Authorstowhomcorrespondenceshouldbeaddressed(evano@chimie.uvsq.fr, nicolas.blanchard@uha.fr). † Université de Versailles Saint Quentin en Yvelines. ‡ Université de Haute-Alsace. Chem. Rev. 2008, 108, 3054–3131 3054

Subtle Ligand Effects in Oxidative Photocatalysis with Iridium Complexes: Application to Photopolymerization

Iridium(III) complexes were designed and evaluated as efficient photoinitiators of polymerization reactions in combination with iodonium salts and silanes. Mechanistically, these reactions were shown to proceed through oxidative photoredox catalysis, generating aryl and silyl radicals under very soft irradiation conditions (blue LED, xenon lamp, and even sunlight). These radicals can initiate the free radical polymerization of acrylates or can be oxidized during the catalytic cycle to promote the ring-opening polymerization of epoxy monomers. Remarkably, both the (photo)chemical reactivity and the practical efficiency are dramatically affected by the ligands. In addition, the central role played by the oxidation ability of the excited state of the photocatalyst is discussed.

Efficient dual radical/cationic photoinitiator under visible light: a new concept

A new concept based on the photoredox catalysis is proposed for the design of a photoinitiating system able to efficiently generate, in a single catalytic cycle, both a radical and a cation as initiating species. This is exemplified here by the tris(1,10-phenanthroline)ruthenium(II) (Ru(phen)32+) complex in combination with a diphenyl iodonium salt and a silane. Interesting and promising photopolymerization profiles in free radical photopolymerization (FRP) and free radical promoted cationic photopolymerization (FRPCP) are obtained under air and upon xenon lamp exposure and even soft irradiation conditions (fluorescence bulb, sunlight). An acrylate/epoxide blend is also easily polymerized under air using a fluorescent bulb. The mechanisms are investigated by ESR and Laser Flash Photolysis experiments.

A Novel Photopolymerization Initiating System Based on an Iridium Complex Photocatalyst

The use of a photocatalyst (tris(2-phenylpyridine)iridium [Ir(ppy)(3)]) being able to generate both radicals and cations to initiate free radical polymerization and ring opening polymerization is presented. Remarkably, under soft irradiations (fluorescence bulb, sunlight), excellent cationic polymerization profiles and final conversions are obtained. The involved mechanisms are investigated by ESR experiments.

Daucus carota L. mediated bioreduction of prochiral ketones

Stereoselective reductions of ketones to secondary alcohols are of the utmost importance in organic synthesis. Very high selectivities are observed with traditional reducing agents, mainly based on boron or transition metals, complexed with chiral ligands. Bioreductions mediated by intact cells from cut plants, vegetables and fruits are attractive alternatives and could facilitate transition towards a more biobased economy. This emerging area highlights the recent results obtained in the aqueous bioreduction of prochiral ketones using carrot roots. The applications of this methodology to asymmetric protonation, dynamic kinetic resolution and the synthesis of biologically relevant targets are presented.

New thioxanthone and xanthone photoinitiators based on silyl radical chemistry

The ability of a new efficient photoinitiator (TX–Si), based on the well-known thioxanthone chromophore linked to a disilylacetylene moiety and exhibiting a red-shifted absorption, to initiate both free radical polymerization and cationic polymerization is checked. A comparison with the parent compound (isopropylthioxanthone) is provided. High rates of polymerization and high final conversions are obtained. The migration of TX–Si out of the polymer film is noticeably reduced (3-fold factor) compared to that observed with ITX. A similar modification of the xanthone chromophore (XT–Si) also leads to an efficient cleavable photoinitiator and opens a door for the design of a new class of potentially interesting compounds for UV curing applications. As characterized by ESR spin trapping, TX–Si and XT–Si generate silyl radicals under light irradiation evidencing a Type I photoinitiator character. The analysis of the excited state processes through laser flash photolysis and molecular orbital calculations allows explaining the observed photochemical behavior.

A Diverted Total Synthesis of Mycolactone Analogues: An Insight into Buruli Ulcer Toxins

Mycolactones are complex macrolides responsible for a severe necrotizing skin disease called Buruli ulcer. Deciphering their functional interactions is of fundamental importance for the understanding, and ultimately, the control of this devastating mycobacterial infection. We report herein a diverted total synthesis approach of mycolactones analogues and provide the first insights into their structure-activity relationship based on cytopathic assays on L929 fibroblasts. The lowest concentration inducing a cytopathic effect was determined for selected analogues, allowing a clear picture to emerge by comparison with the natural toxins.

New Boryl Radicals Derived from N‐Heteroaryl Boranes: Generation and Reactivity

Recently, boryl radicals have been the subject of revived interest. These structures were generated by hydrogen-abstraction reactions from the corresponding boranes (i.e., from amine or phosphine boranes). However, the classical issue remains their high B--H bond-dissociation energy (BDE), thereby preventing a very efficient hydrogen-abstraction process. In the present paper, new N-heteroaryl boranes that exhibiting low B--H BDE are presented; excellent hydrogen-transfer properties have been found. Both the generation and the reactivity of the associated boryl radicals have been investigated through their direct observation in laser flash photolysis. The boryl radical interactions with double bonds, oxygen, oxidizing agent, and alkyl halides have been studied. Some selected applications of N-heteroaryl boryl radicals as new polymerization-initiating structures are proposed to evidence their high intrinsic reactivity.

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.

Ruthenium-Catalyzed Nucleophilic Ring-Opening Reactions of a 3-Aza-2-oxabicyclo[2.2.1]hept-5-ene with Alcohols

Ruthenium-catalyzed nucleophilic ring-opening reactions of a 3-aza-2-oxabicyclo[2.2.1]hept-5-ene with alcohols were investigated. When a neutral ruthenium(II) catalyst, Cp*RuCl(COD), was used in MeOH, the trans-1,2-ring opened product was formed as the only regio- and stereoisomer. On the other hand, when a cationic ruthenium(II) catalyst, [CpRu(CH(3)CN)(3)]PF(6), was used in MeOH, the cis-1,2-ring opened product was formed exclusively. Moderate to excellent stereoselectivity (70:30 to 100:0) was observed with various alcohols.