The reduction of the nitro group represents a powerful and widely used transformation that allows to introducing an amino group in the molecule. New synthetic strategies for complex functionalized molecular architectures are deeply needed, including highly efficient and selective nitro reduction methods, tolerant of a diverse array of functional moieties and protecting groups. Since chiral amino groups are ubiquitous in a variety of bioactive molecules such as alkaloids, natural products, drugs, and medical agents, the development of reliable catalytic methodologies for the nitro group reduction is attracting an increasing interest also in the preparation of enantiomerically pure amines. In this context, the modern reduction methods should be chemoselective and respectful of the stereochemical integrity of the stereogenic elements of the molecule. The review will offer an overview of the different possible methodologies available for this fundamental transformation, with special attention on the most recent contributions in the field, especially in the last ten years: hydrogenations, metal dissolving and hydride transfer reductions, catalytic transfer hydrogenations, and metal-free reductions. The main advantages or limitations for the proposed methods will be briefly discussed, highlighting in some cases the most important features of the presented reduction methodologies from an industrial point of view. ■ CONTENTSAuthor Information 444 Corresponding Authors 444 Notes 444 Acknowledgments 444 References 444
A simple procedure to convert protein data bank files (.pdb) into a stereolithography file (.stl) using VMD software (Virtual Molecular Dynamic) is reported. This tutorial allows generating, with a very simple protocol, threedimensional customized structures that can be printed by a low-cost 3D-printer, and used for teaching chemical education topics. With the use of the free licensed and multiplatform software, colored input geometries can be obtained by a simple-click modification procedure in order to generate .obj and .mtl files. An easy protocol to create personal .pdb files for 3D-printing technology is also reported.
3D-printed flow reactors were designed, fabricated from different materials (PLA, HIPS, nylon), and used for a catalytic stereoselective Henry reaction. The use of readily prepared and tunable 3D-printed reactors enabled the rapid screening of devices with different sizes, shapes, and channel dimensions, aimed at the identification of the best-performing reactor setup. The optimized process afforded the products in high yields, moderate diastereoselectivity, and up to 90 % ee. The method was applied to the continuous-flow synthesis of biologically active chiral 1,2-amino alcohols (norephedrine, metaraminol, and methoxamine) through a two-step sequence combining the nitroaldol reaction with a hydrogenation. To highlight potential industrial applications of this method, a multistep continuous synthesis of norephedrine has been realized. The product was isolated without any intermediate purifications or solvent switches.
Catalyzing C À Cbond-forming reactions with earthabundant metals under mild conditions is at the heart of sustainable synthesis.T he cyclotrimerization of alkynes is av aluable atom-efficient reaction in organic synthesis that is enabled by several metal catalysts,i ncluding iron. This study reports an effective iron-catalyzedc yclotrimerization for the regioselective synthesis of 1,2,4-substituted arenes (1 mol % catalyst, toluene,2 08 8C, 5min). Ad ual activation mechanism (substrate deprotonation, reductive elimination) renders the simple Fe II precatalyst highly active in the absence of any reductant.Metal-catalyzed cyclotrimerization reactions of alkynes constitute akey approach for the construction of substituted aromatic compounds and have been developed to great maturity since their first discovery in the mid-19th century and the early studies of Reppe in 1948.[1] Today,s everal transition metals display good catalytic activity in this highly atom-efficient, redox-neutral, complexity-generating reaction.[2] Thes earch for sustainable base-metal approaches has recently prompted the development of iron-catalyzed processes.[3] High activities were especially observed with lowvalent iron catalysts,w hich can be generated from iron(0) precursors or by reactions of Fe II complexes with suitable reductants.T he first report utilized iron carbonyl catalysts at high temperatures.[4] Recently,v arious combinations of iron complexes,ligands,and zinc have been shown to exhibit high catalytic activity and wide applicability.[5] Alternatively,elaborate iron complexes in low oxidation states (+ 1, 0) have also been applied.[6] Most catalyst developments were fueled by the notion that al ow-valent iron species would engage in coordinative activation of the softly Lewis basic alkyne, facilitate the iron-centered oxidative cyclization step,a nd be regenerated upon reductive elimination. However,t hese methods require the preparation and handling of highly reactive iron complexes or the presence of suitable reductants.T he conceptual relation between reductants and bases led us to postulate amechanistically distinct approach to the design of effective iron(II) precatalysts in which as imple internal base would mimic the role of an external reductant. With one equivalent of at erminal alkyne,adual activation mechanism of the inactive FeX 2 involving sequential alkyne deprotonation and reductive alkyne elimination would result in an overall reduction in the absence of an actual reductant (Scheme 1). We surmised that FeX 2 complexes constitute most simple precatalysts with inexpensive,e asily accessible ligands Xt hat are bulky (facile dissociation), strongly basic (alkyne deprotonation), and lipophilic (solubility in organic solvents).With these framework conditions,weinvestigated various ferrous salts and identified iron(II) bis(1,1,1,3,3,3-hexamethyldisilazan-2-ide) (Fe(hmds) 2 )a sa na ctive catalyst. Documented herein are the benefits of this simple catalytic system, which presents tangible advances over the cu...
Cyclopentadienone-based iron complexes were used for the first time to successfully catalyze the diastereoselective hydrogenation of enantiopure imines. Chiral amines, including valuable biologically active products, were obtained often as enantiomerically pure compounds. Computational studies helped to elucidate the chemical and stereochemical aspects of the iron-catalyzed reaction.
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