1,721,092 research outputs found
Catalytic Stereoselective SN1-Type Reactions Promoted by Chiral Phosphoric Acids as Brønsted Acid Catalysts
No full textIn recent years, the field of organocatalysis has extensively explored the use of carbenium ions for practical C−C bond forming reactions. In order to control the formation of new stereocenters, the generation of carbenium ions was investigated by exploring asymmetric Brønsted acid catalysis. Oxocarbenium ions, iminium ions, and other carbenium ions stabilized by heteroatoms could also be generated by Brønsted acid catalysis, through the departure of suitable leaving groups. The aforementioned reactivity has recently been exploited leading to remarkable achievements. The application of chiral Brønsted acids in the generation of heteroatom stabilized carbenium ions has recently been investigated, with the demonstration of important results. Although diverse architectural arrays of Brønsted acid catalysts were conceived, BINOL-derived phosphoric acids have played a dominant role in this area of research. Their low pKa values combined with the possibility to alter their architecture, and their ability to assemble ordered transition states was exploited in different SN1-type reactions allowing the formation of a variety of products with high enantiomeric excesses. In addition, the generation of highly stabilized chiral counter anions allowed the exploitation of counterion asymmetric catalysis with various nucleophiles
Teach new tricks for an old dog: Cheap and abundant iron complexes for new catalytic transformations
No full textThe development of iron complexes as catalysts in organic transformations is attractive for a number of reasons. Iron is one of the most abundant metals in the earth's crust, it is cheaper than the rare metals, and its toxicity is low. As the cost of rare and precious metals will increase in the Future, the development of new synthetic strategies based on iron are critical. In this brief summary, we discuss two innovative iron-catalysed C-C bond forming reactions through radicals, generated by redox chemistry, or by photocatalysis. These works shine light on the possibility to promote radical transformations by iron complexes that can be advantageously used as starting point to develop new methodologies for the formation of C-C bonds with a cheap and environmental friendly approach
From QCA (Quantum Cellular Automata) to Organocatalytic Reactions with Stabilized Carbenium Ions
No full textWhat do quantum cellular automata (QCA), "on water" reactions, and SN1-type organocatalytic transformations have in common? The link between these distant arguments is the practical access to useful intermediates and key products through the use of stabilized carbenium ions. Over 10 years, starting with a carbenium ion bearing a ferrocenyl group, to the 1,3-benzodithiolylium carbenium ion, our group has exploited the use of these intermediates in useful and practical synthetic transformations. In particular, we have applied the use of carbenium ions to stereoselective organocatalytic alkylation reactions, showing a possible solution for the "holy grail of organocatalysis". Examples of the use of these quite stabilized intermediates are now also considered in organometallic chemistry. On the other hand, the stable carbenium ions are also applied to tailored molecules adapted to quantum cellular automata, a new possible paradigm for computation. Carbenium ions are not a problem, they can be a/the solution
Design of BODIPY dyes as triplet photosensitizers: electronic properties tailored for solar energy conversion, photoredox catalysis and photodynamic therapy
No full textBODIPYs are renowned fluorescent dyes with strong and tunable absorption in the visible region, high thermal and photo-stability and exceptional fluorescence quantum yields. Transition metal complexes are the most commonly used triplet photosensitisers, but, recently, the use of organic dyes has emerged as a viable and more sustainable alternative. By proper design, BODIPY dyes have been turned from highly fluorescent labels into efficient triplet photosensitizers with strong absorption in the visible region (from green to orange). In this perspective, we report three design strategies: (i) halogenation of the dye skeleton, (ii) donor-acceptor dyads and (iii) BODIPY dimers. We compare pros and cons of these approaches in terms of optical and electrochemical properties and synthetic viability. The potential applications of these systems span from energy conversion to medicine and key examples are presented
Synergistic Stereoselective Organocatalysis with Indium(III) Salts
No full textThe compatibility of indium(III) Lewis acids with water and amines allows their employment in synergistic and cooperative catalysis. Stereoselective organocatalytic S(N)1-type reactions, in which carbenium ions are generated, are promoted by the presence of indium salts. The peculiar properties of indium salts can be exploited in organocatalysis for the design of water-compatible, benign, green processes. The development of such indium(III)-promoted organocatalytic procedures is the focus of our recent research, a summary of which is presented in this article.The compatibility of indium(III) Lewis acids with water and amines allows their employment in synergistic and cooperative catalysis. Stereoselective organocatalytic SN1-type reactions, in which carbenium ions are generated, are promoted by the presence of indium salts. The peculiar properties of indium salts can be exploited in organocatalysis for the design of water-compatible, benign, green processes. The development of such indium(III)-promoted organocatalytic procedures is the focus of our recent research, a summary of which is presented in this article. © Georg Thieme Verlag Stuttgart.New York
Stereoselective Organocatalytic Addition of Nucleophiles to Isoquinolinium and 3,4-dihydroisoquinolinium Ions: A Simple Approach for the Synthesis of Isoquinoline Alkaloids
No full textThe frequent occurrence of chiral 1-substituted-1,2,3,4-tetrahydroisoquinoline ring systems in a large number of alkaloids, possessing a broad spectrum of biological and pharmaceutical properties, has prompted out a considerable interest in their synthesis. Economical and valuable stereoselective processes based on organocatalytic transformations represent a new avenue for approaching isoquinoline alkaloids with efficiency and creativity
Iron-Promoted Radical Reactions: Current Status and Perspectives
No full textCatalysis based on precious metals has reached an incredible level of sophistication and has found widespread use and application, both in academia and in industry. However, owing to the environmental impact, cost, and low abundance, catalysis based on inexpensive, abundant, and environmentally benign first-row transition metals has become a valuable alternative to traditional reactions promoted by their less-abundant counterparts. From economic and ecological perspectives, iron salts and iron metal complexes are highly promising candidates to replace costly and toxic metals. In recent years, iron complexes have been used to promote the formation of radicals. Photoredox chemistry is a valuable method for the generation of radical species under mild conditions, but it often relies on rare-earth-metal complexes. The employment of iron complexes for the generation of radicals is attractive for the development of sustainable, simple, and effective procedures. In this Focus Review, we highlight and summarize recent radical reactions promoted by iron complexes and their application in organic reactions
Hydrogenation of Calix[4]pyrrole: From the Formation to the Synthesis of Calix[4]pyrrolidine
Full text linkCalix[4]pyrrolidine, is a topological analogue of the chromophore of the pigments of life. An efficient two-step procedure, starting with the condensation between acetone and pyrrole followed by a heterogenous catalytic hydrogenation, has been developed. Many unsuspected aspects demonstrating the influence of the topology on the physicochemical and chemical properties emerged. The simple, symmetric structure showed unique attributes
Stereoselective SN1-Type Reaction of Enols and Enolates
No full textStereoselective alkylation of enolates represents a valuable and important procedure for accessing carbon-carbon-bond frameworks in natural and nonnatural product synthesis. Usually, activated electrophilic partners that react through an SN2 mechanism are employed. To overcome the limitations due to reduced reactivity and steric hindrance, SN1-type reactions can be considered a valid and practical alternative. Accessible enolates can be used in stereoselective (diastereo- or enantioselective) reactions with electrophilic carbenium ions, either used as stable reagents or generated in situ from suitable precursors. The results achieved in this active field are summarized in this review. 1 Introduction 2 Alcohols in SN1-Type Reactions with Enolates 2.1 Enantioselective Reactions with Metal Complexes 2.2 Organocatalytic Enantioselective Reactions 3 Alcohols and Alcohol Derivatives in SN1-Type Reactions with Enolates: Enantioselective Reactions with Metal Enolates 4 Isolated Carbenium Ions in SN1-Type Reactions with Enolates: Enantioselective Reactions with Metal Enolates 5 Miscellaneous 6 Conclusion
Octa-1,7-diene-4,5-diamine Derivatives: Useful Intermediates for the Stereoselective Synthesis of Nitrogen Heterocycles and Ligands for Asymmetric Catalysis
No full textThe octa-1,7-diene-4,5-diamine skeleton is usually prepared by the addition of allylic organometallic reagents to glyoxal diimines. The relative and absolute configurations of the C4 and C5 stereocenters are dependent on the nature of the (chiral) nitrogen substituents. These compounds are useful intermediates for the construction of NHC carbene complexes and nitrogen heterocycles, such as 2,2′-bipyrrolidines, 2,5-diazabicyclo[2.2.1]heptanes, amino- and hydroxy-substituted 7-azanorbornane, and diamino-substituted cyclohexanols and cyclohexanediols
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