During nitroxide-mediated polymerization (NMP) in the presence of a nitroxide R2(R1)NO*, the reversible formation of N-alkoxyamines [P-ON(R1)R2] reduces significantly the concentration of polymer radicals (P*) and their involvement in termination reactions. The control of the livingness and polydispersity of the resulting polymer depends strongly on the magnitude of the bond dissociation energy (BDE) of the C-ON(R1)R2 bond. In this study, theoretical BDEs of a large series of model N-alkoxyamines are calculated with the PM3 method. In order to provide a predictive tool, correlations between the calculated BDEs and the cleavage temperature (Tc), and the dissociation rate constant (k(d)), of the N-alkoxyamines are established. The homolytic cleavage of the N-OC bond is also investigated at the B3P86/6-311++G(d,p)//B3LYP/6-31G(d), level. Furthermore, a natural bond orbital analysis is carried out for some N-alkoxyamines with a O-C-ON(R1)R2 fragment, and the strengthening of their C-ON(R1)R2 bond is interpreted in terms of stabilizing anomeric interactions. 相似文献
During nitroxide-mediated polymerization (NMP) in the presence of a nitroxide R2(R1)NO*, the reversible formation of N-alkoxyamines [P-ON(R1)R2] reduces significantly the concentration of polymer radicals (P*) and their involvement in termination reactions. The control of the livingness and polydispersity of the resulting polymer depends strongly on the magnitude of the bond dissociation energy (BDE) of the C-ON(R1)R2 bond. In this study, theoretical BDEs of a large series of model N-alkoxyamines are calculated with the PM3 method. In order to provide a predictive tool, correlations between the calculated BDEs and the cleavage temperature (T(c)), and the dissociation rate constant (k(d)), of the N-alkoxyamines are established. The homolytic cleavage of the N-OC bond is also investigated at the B3P86/6-311++G(d,p)//B3LYP/6-31G(d), level. Furthermore, a natural bond orbital analysis is carried out for some N-alkoxyamines with a O-C-ON(R1)R2 fragment, and the strengthening of their C-ON(R1)R2 bond is interpreted in terms of stabilizing anomeric interactions. 相似文献
Since Gomberg's discovery of radicals as chemical entities, the interest around them has increased through the years. Nowadays, radical chemistry is used in the synthesis of 75% of all polymers, inevitably establishing a close relationship with Solid-Phase Organic Synthesis. More recently, the interest of organic chemists has shifted towards the application of usual "in-solution" radical chemistry to the solid-phase, ranging from the use of supported reagents for radical reactions, to the development of methodologies for the synthesis of small molecules or potential libraries. The aim of this review is to put in perspective radical chemistry, moving it away from its origin as a synthetic means for solid supports, to becoming a useful tool for the synthesis of small molecules. 相似文献
Free radical addition reactions in the presence of cobaloximes and related compounds have been modelled. Several mechanisms are presented and similarities with the “persistent radical effect” noted by Daikh and Finke are discussed. Cobaloximes and salophen
derivatives are widely used in organic synthesis to build carbon-carbon bonds, whereas in polymer synthesis they are used as catalytic chain transfer agents in the production of oligomers. This work shows that these reactions are closely related and also demonstrates the influence that an external radical source has on the overall reaction kinetics. 相似文献
The first total synthesis of chaetocin (1), a potent histone methyltransferase inhibitor, is described in detail. Key reactions were radical bromination for α-oxidation of the diketopiperazine ring, and reductive radical coupling for construction of the dimeric core structure. Stereoselective construction of the disulfide bridges was achieved via substitution reaction with H2S. The total synthesis of 1 was accomplished in nine steps starting from known d-amino acid derivatives. Total synthesis of non-natural ent-chaetocin (ent-1) was also achieved via the established synthetic route, starting from l-amino acid derivatives. 相似文献
Studies of stereocontrol in two types of radical equivalents of Mannich addition reactions offer new insights for application to aminosugar synthesis. In the first method, haloacetal addition (Ueno-Stork reaction) is extended to dihydroxyhydrazones, leading to an adduct with the unexpected 3-epi-l-daunosamine configuration. A neighboring α-benzyloxy substituent causes a dramatic reversal of stereocontrol compared with hydrazones where this substituent is absent; vicinal dipole repulsion is proposed to account for the diastereoselectivity. In the second method, radical addition-cyclization with thiophenol and treatment with fluoride leads to diastereoselective group transfer from a silicon-tethered ethynyl group to the CN bond of hydrazones, affording anti-hydrazino alcohols with a trans-2-(phenylthio)vinyl substituent. The one-pot process occurs under neutral, tin-free radical conditions, and offers stereocontrol which is complementary to the haloacetal method. Synthetic utility of the radical Mannich concept is demonstrated in a brief asymmetric synthesis of N-trifluoroacetyl-l-daunosamine from achiral precursors. 相似文献
Allenes are valuable organic molecules that feature unique physical and chemical properties. They are not only often found in natural products, but also act as versatile building blocks for the access of complex molecular targets, such as natural products, pharmaceuticals, and functional materials. Therefore, many remarkable and elegant methodologies have been established for the synthesis of allenes. Recently, more and more methods for radical synthesis of allenes have been developed, clearly emphasizing the associated great synthetic values. In this perspective, we will discuss recent important advances in the synthesis of allenes via radical intermediates by categorizing them into different types of substrates as well as distinct catalytic systems. The mechanistic studies and synthetic challenges will be highlighted.Recent important advances in the synthesis of allenes via radical strategies are highlighted. 相似文献
Cyclopropylcarbinyl --> homoallyl and related rearrangements of radical ions (a) are frequently used as mechanistic "probes" to detect the occurrence of single electron transfer in chemical and biochemical processes, (b) provide the basis for mechanism-based drug design, and (c) are important tools in organic synthesis. Unfortunately, these rearrangements are poorly understood, especially with respect to the effect of substrate structure on reactivity. Frequently, researchers assume that the same factors which govern the reactivity of neutral free radicals also pertain to radical ions. The results reported herein demonstrate that in some cases structure-reactivity trends in radical ion rearrangements are very different from neutral radicals. For radical ions, delocalizations of both charge and spin are important factors governing their reactivity. 相似文献
The specifics of the radical polymerization of styrene and methyl methacrylate in the presence of ruthenium closo- and exo-nido-carborane complexes with phosphine and diphosphine ligands were investigated. It was shown that, depending on a coinitiator, the polymerization proceeds through the atom transfer radical mechanism or the reverse atom transfer radical mechanism to high conversions without gelation to yield macromolecules with a low polydispersity. The influence of the ligand environment, the oxidation state of ruthenium atoms in the carborane complexes, and the temperature conditions on the specific features of the polymer synthesis was established. 相似文献
The synthesis and "round trip radical cyclization" of 11-iodo-2,7,11-trimethyldodec-6-en-5-one are described. The round trip cyclization is a sequence of 5-exo, 6-endo, and 5-exo cyclizations in which the last radical cyclization occurs at the same carbon atom as the initial radical generation. The key second (6-endo) cyclization produces two stereoisomers, one of which cyclizes efficiently to isogymnomitrene ketone, while the other cyclizes inefficiently to gymnomitrene ketone. Efforts to influence the kinetic or thermodynamic outcome of the second cyclization were not successful, and the results are contrasted with a related cyclization of Jung and Rayle where thermodynamic control was readily established. 相似文献
The present study demonstrates the indirect electrocatalytic synthesis of isonicotinic acid using a Ni complex, [NiII(Me4-(NO2Bzo)2[14]tetraeneN4)], in an acetonitrile solution at room temperature. The complex was used as an excellent electrocatalyst for the reduction of carbon dioxide. The results indicate that the electrocatalytic reduction product of CO2 (CO2−) has a dual role in the electrosynthesis of isonicotinic acid. The dual activity of CO2− involved indirect electrocatalytic reduction of pyridine as well as its radical reaction with pyridine radical anion to form isonicotinic acid. Finally, EC′C′C mechanism was proposed for the synthesis of isonicotinic acid. In contrast, the reaction of pyridine with CO2 in the absence of the complex follows an EC′C mechanism, and the final product is 4,4′-bipyridine. 相似文献
A general and single step synthesis of polyhydroxydihydrochalcones from the readily available phenols and dihydrocinnamic acids using BF3·Et2O is described. The method allows the synthesis of a wide range of compounds with multiple phenolic hydroxyls and other substituents. These dihydrochalcones are converted into homoisoflavonoids by DMF/PCl5 and the methodology has been applied to the synthesis of naturally occurring phloretin and 5,7-dihydroxy-3-[(4-hydroxyphenyl)methyl]-4H-chromen-4-one. The antioxidant activity of dihydrochalcones and homoisoflavonoids was determined by superoxide free radical (NBT) and DPPH free radical scavenging methods. Polyhydroxydihydrochalcones 3c, 3f, 3g and homoisoflavonoids 4c, 4f, 4g displayed excellent antioxidant activity. 相似文献
We disclose herein the first example of merging photoredox catalysis and copper catalysis for radical 1,4-carbocyanations of 1,3-enynes. Alkyl N-hydroxyphthalimide esters are utilized as radical precursors, and the reported mild and redox-neutral protocol has broad substrate scope and remarkable functional group tolerance. This strategy allows for the synthesis of diverse multi-substituted allenes with high chemo- and regio-selectivities, also permitting late stage allenylation of natural products and drug molecules.An efficient synthesis of multi-substituted allenes by metallaphotoredox-catalyzed decarboxylative 1,4-carbocyanation of 1,3-enynes is described.相似文献
A facile entry to the synthesis of natural and unnatural substituted maleic anhydrides based on the Barton radical decarboxylation is described. The radicals, generated by the photolysis of N-hydroxy-2-thiopyridone esters derived from succinic and alkyl acids reacted, respectively, with electron deficient olefin phenyl maleimide by a consecutive two-step radical addition, afforded the corresponding disubstituted maleic anhydrides 1a-f. 相似文献
Dihydrophenazines and their analogues have been widely used as strong reducing photoredox catalysts in radical chemistry, such as organocatalyzed atom transfer radical polymerization (O-ATRP). However, when dihydrophenazines were employed as organic photoredox catalysts (OPCs) to mediate O-ATRP, the initiator efficiency was nonquantitative due to cross-coupling between dihydrophenazines and radical species. Here, a new kind of core modification for dihydrophenazines, phenoxazines and phenothiazines was developed through this cross-coupling process. Mechanistic studies suggested that the radical species would be more likely to couple with OPC'' radical cations rather than the ground-state OPC. Core modification of OPCs could stabilize the radical ions in an oxidative quenching catalytic cycle. Significantly, core modifications of OPCs could lower the energy of light required for photoexcitation. Compared with their noncore-modified counterparts, all the core-modified dihydrophenazines and phenoxazines exhibited efficient performance in controlling O-ATRP for the synthesis of poly(methyl methacrylate) with higher initiator efficiencies under the irradiation of simulated sunlight.Photomediated core modification of organic photocatalysts through a radical/radical cation cross-coupling process enables the ability to stabilize radical ions and prevent undesirable side reactions.相似文献
The use of a bisaminooxy compound as initiator for nitroxide‐mediated radical polymerization (NMRP) of styrene or n‐butyl acrylate allows the synthesis of α,ω‐nitroxide‐capped polymers. At high temperatures and with the addition of acetic anhydride, it was found that these polymers could be applied as macroinitiators in the free‐radical polymerization of methyl methacrylate. This enables the synthesis of block copolymers with only minor contents of homopolymer.
The structure of bis‐TIPNO, the bisaminooxy compound used as an initiator for the nitroxide‐mediated radical polymerization of styrene or n‐butyl acrylate. 相似文献
3,4-Dihydroquinolin-2-ones are of great importance in the areas of pharmaceuticals. However, the direct intramolecular radical cyclizations of the corresponding amide compounds favor 5-exo products 2. Reports on the radical cyclization reactions producing 3,4-dihydroquinolin-2-one derivatives are limited. Herein, an efficient tandem reaction combining radical and ionic processes was developed, which provides a practical synthetic strategy for the synthesis of substituted 3,4-dihydroquinolin-2-ones from simple and readily available precursors under neutral reaction conditions. 相似文献