An Asymmetric SN2 Dynamic Kinetic Resolution

The SN2 reaction exhibits the classic Walden inversion, indicative of the stereospecific backside attack of the nucleophile on the stereogenic center. Observation of the inversion of the stereocenter provides evidence for an SN2-type displacement. However, this maxim is contingent on substitution proceeding on a discrete stereocenter. Here we report an SN2 reaction that leads to enantioenrichment of product despite starting from a racemic mixture of starting material. The enantioconvergent reaction proceeds through a dynamic Walden cycle, involving an equilibrating mixture of enantiomers, initiated by a chiral aminocatalyst and terminated by a stereoselective SN2 reaction at a tertiary carbon to provide a quaternary carbon stereocenter. A combination of computational, kinetic, and empirical studies elucidates the multifaceted role of the chiral organocatalyst to provide a model example of the Curtin–Hammett principle. These examples challenge the notion of enantioenriched products exclusively arising from predefined stereocenters when operating through an SN2 mechanism. Based on these principles, examples are included to highlight the generality of the mechanism. We anticipate the asymmetric SN2 dynamic kinetic resolution to be used for a variety of future reactions

Zinc Complexes of β

We report the synthesis of α-aminonitriles by one-pot coupling reaction of aldehyde, amines and trimethylsilyl cyanide (TMSCN) using β-ketoiminato zinc complexes as a pre-catalyst in very good yield under mild reaction condition. Homoleptic zinc complex [{κ2-(2,4,6-Me3C6H2NC(Me)=CHC(Me)=O)}2Zn] (1 a) was synthesized by the treatment of protic ligand [(2,4,6-Me3C6H2NHC(Me)=CHC(Me)=O)] (L1H) with potassium hydride and anhydrous zinc diiodide in 2 : 2 : 1 molar ratio in THF. Analogous reactions using [(2,6-iPr2C6H3NHC(Me)=CHC(Me)=O)] (L2H) and [(Ph2CHNHC(Me)=CHC(Me)=O)] (L3H) as protic ligands, dinuclear zinc complexes [{κ3-(2,6-iPr2C6H3NC(Me)=CHC(Me)=O)}ZnI]2 (1 b) and [Zn(Ph2CHNHC−(Me)=CHC(Me)=O)ZnI2] (1 c) were obtained in good yield. Molecular structures of ligands L1H, L3H, and zinc complexes 1 a, 1 b, and 1 c were established by single-crystal x-ray diffraction analysis. Dinuclear zinc complexes 1 b, and 1 c exhibited high transformation, greater selectivity and broad substrate scope for the synthesis of α-aminonitrile under mild condition. A most plausible mechanism for synthesis α-aminonitrile is proposed based on several controlled reactions

Iridium-Catalyzed Homogeneous Hydrogenation and Hydrosilylation of Carbon Dioxide

The knowledge of the potential of transition metal-based complexes as catalysts for the reduction of CO2 has grown significantly over the last few decades. This chapter focuses on the progress made during recent years in the field of homogeneous iridium-catalyzed reduction of CO2 by using hydrogen and/or silicon hydrides as reducing agents, comparing them with homogeneous catalysts based on other transition metals. The reported studies on iridium-catalyzed CO2 reduction processes show that an important point to keep in mind when designing a catalyst is the nature of the reducing agent (hydrogen, hydrosilanes, and/or hydrosiloxanes). Thus, iridium(III) half-sandwich complexes with 4,4′-dihydroxy-bipyridine (DHBP) or 4,7-dihydroxy-1,10-phenanthroline (DHPT) ligands, and iridium(III)-PNP pincer complexes have proven to be excellent catalysts for the hydrogenation of CO2 to formic acid. However, Ir(III)-NSiNMe (NSiN = fac-bis-(4-methylpyridine-2-yloxy)methylsilyl) and Ir(III)-NSiMe (NSiMe = 4-methylpyridine-2-yloxydimethylsilyl) species are not stable under hydrogen atmosphere but are effective catalysts for the reduction of CO2 with hydrosiloxanes to silylformate under solvent-free conditions and moderate CO2 pressures and temperatures. Moreover, while using iridium(III)-DHBP half-sandwich complexes, high CO2 and H2 pressures are required to achieve the catalytic CO2 hydrogenation to methanol; Ir-NSiMe species catalyze the reduction of CO2 to methoxysilane with hydrosiloxanes under low CO2 pressure.Peer reviewe

Selective Hydrogenation of Carbon Dioxide into Methanol

International audienceThis chapter is dedicated to methanol synthesis from carbon dioxide and hydrogen. Methanol, chemical formula CH3OH, is an important platform molecule which can be transformed into a large number of other chemicals, i.e., formaldehyde, acetic acid, dimethyl ether, methyl tert-butyl ether, and methyl methacrylate, as well as complex hydrocarbon mixtures, e.g., gasoline and diesel. Up to date, methanol is produced at industrial scale by steam reforming of natural gas, leading to high environmental impacts. The selective hydrogenation of carbon dioxide into methanol can be a good alternative since it is possible to capture carbon dioxide from industrial processes and to produce hydrogen from renewable energies, e.g., solar energy and wind energy.From a thermodynamic point of view, carbon dioxide hydrogenation is strongly influenced by the total pressure, temperature, and feeding composition. The use of a catalyst is also mandatory to control the kinetic and the selectivity into methanol. Among solid catalysts studied, copper-based catalysts have been found to be the best catalytic systems. Promoters like zinc oxide were usually used. Nickel-, palladium-, and silver-based catalysts also showed good catalytic performance compared to copper-based catalysts. Soluble catalysts have been intensively studied for this hydrogenation. Ru complexes appeared as the best homogeneous catalyst. Other metal-free homogeneous catalysts, e.g., N-heterocyclic carbenes, have been found to be active and selective in this reaction. Efforts have been made on the mechanistic study of the reaction in both the gas and liquid phases. Large industrial production has started in several countries showing the interest and the feasibility of the process