排序方式: 共有30条查询结果,搜索用时 187 毫秒
21.
《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2017,129(15):4242-4246
Manganese(IV)‐oxo complexes are often invoked as intermediates in Mn‐catalyzed C−H bond activation reactions. While many synthetic MnIV‐oxo species are mild oxidants, other members of this class can attack strong C−H bonds. The basis for these reactivity differences is not well understood. Here we describe a series of MnIV‐oxo complexes with N5 pentadentate ligands that modulate the equatorial ligand field of the MnIV center, as assessed by electronic absorption, electron paramagnetic resonance, and Mn K‐edge X‐ray absorption methods. Kinetic experiments show dramatic rate variations in hydrogen‐atom and oxygen‐atom transfer reactions, with faster rates corresponding to weaker equatorial ligand fields. For these MnIV‐oxo complexes, the rate enhancements are correlated with both 1) the energy of a low‐lying 4E excited state, which has been postulated to be involved in a two‐state reactivity model, and 2) the MnIII/IV reduction potentials. 相似文献
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《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2017,129(28):8299-8302
A concise, protecting‐group‐free total synthesis of the unusual brominated sesquiterpene aplydactone is described. Our synthesis features a [2+2] photocycloaddition, a Wolff ring contraction, an unusual remote C−H functionalization to establish the highly strained tetracyclic core, and a hydrogen‐atom transfer (HAT) reaction to access the bromine‐containing stereocenter. A finely tuned conformation of the α‐diazoketone precursor is the key for the success of the late‐stage transannular C−H insertion to deliver a bridged six‐membered ring and a quaternary stereocenter (C6) between two quaternary carbon atoms (C1 and C7). 相似文献
23.
Braddock A. Sandoval Sarah I. Kurtoic Megan M. Chung Kyle F. Biegasiewicz Todd K. Hyster 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2019,131(26):8806-8810
Flavin‐dependent ene‐reductases (EREDs) are known to stereoselectively reduce activated alkenes, but are inactive toward carbonyls. Demonstrated here is that in the presence of photoredox catalysts, these enzymes will reduce aromatic ketones. Mechanistic experiments suggest this reaction proceeds through ketyl radical formation, a reaction pathway that is distinct from the native hydride‐transfer mechanism. Furthermore, this reactivity is accessible without modification of either the enzyme or cofactors, allowing both native and non‐natural mechanisms to occur simultaneously. Based on control experiments, we hypothesize that binding to the enzyme active site attenuates the reduction potential of the substrate, enabling single‐electron reduction. This reactivity highlights opportunities to access new catalytic manifolds by merging photoredox catalysis with biocatalysis. 相似文献
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Daniel Delony Markus Kinauer Martin Diefenbach Serhiy Demeshko Christian Würtele Max C. Holthausen Sven Schneider 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2019,131(32):11087-11090
A terminal iridium oxo complex with an open‐shell (S=1) ground state was isolated upon hydrogen atom transfer (HAT) from the respective iridium(II) hydroxide. Electronic structure examinations support large spin delocalization to the oxygen atom. Selected oxo transfer reactions indicate the ambiphilic reactivity of the iridium oxo moiety. Calorimetric and computational examinations of the HAT revealed a bond dissociation free energy for the IrO?H bond that is sufficient for hydrogen atom abstraction towards C?H bonds and small contributions from entropy and spin–orbit coupling to the HAT thermochemistry. 相似文献
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27.
《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2017,129(46):14576-14580
The sluggish oxidants [FeIV(O)(TMC)(CH3CN)]2+ (TMC=1,4,8,11‐tetramethyl‐1,4,8,11‐tetraazacyclotetradecane) and [FeIV(O)(TMCN‐d12)(OTf)]+ (TMCN‐d12=1,4,7,11‐tetra(methyl‐d3)‐1,4,7,11‐tetraazacyclotetradecane) are transformed into the highly reactive oxidant [FeIV(O)(TMCO)(OTf)]+ ( 1 ; TMCO=4,8,12‐trimethyl‐1‐oxa‐4,8,12‐triazacyclotetradecane) upon replacement of an NMe donor in the TMC and TMCN ligands by an O atom. A rate enhancement of five to six orders of magnitude in both H atom and O atom transfer reactions was observed upon oxygen incorporation into the macrocyclic ligand. This finding was explained in terms of the higher electrophilicity of the iron center and the higher availability of the more reactive S=2 state in 1 . This rationalizes nature's preference for using O‐rich ligand environments for the hydroxylation of strong C−H bonds in enzymatic reactions. 相似文献
28.
《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2017,129(52):16848-16852
Although there has been significant progress in the development of transition‐metal‐catalyzed hydrosilylations of alkenes over the past several decades, metal‐free hydrosilylation is still rare and highly desirable. Herein, we report a convenient visible‐light‐driven metal‐free hydrosilylation of both electron‐deficient and electron‐rich alkenes that proceeds through selective hydrogen atom transfer for Si−H activation. The synergistic combination of the organophotoredox catalyst 4CzIPN with quinuclidin‐3‐yl acetate enabled the hydrosilylation of electron‐deficient alkenes by selective Si−H activation while the hydrosilylation of electron‐rich alkenes was achieved by merging photoredox and polarity‐reversal catalysis. 相似文献
29.
Daniel E. Diaz David A. Quist Austin E. Herzog Andrew W. Schaefer Ioannis Kipouros Mayukh Bhadra Edward I. Solomon Kenneth D. Karlin 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2019,131(49):17736-17740
The dioxygen reactivity of a series of TMPA‐based copper(I) complexes (TMPA=tris(2‐pyridylmethyl)amine), with and without secondary‐coordination‐sphere hydrogen‐bonding moieties, was studied at ?135 °C in 2‐methyltetrahydrofuran (MeTHF). Kinetic stabilization of the H‐bonded [( TMPA)CuII(O2.?)]+ cupric superoxide species was achieved, and they were characterized by resonance Raman (rR) spectroscopy. The structures and physical properties of [( TMPA)CuII(N3?)]+ azido analogues were compared, and the O2.? reactivity of ligand–CuI complexes when an H‐bonding moiety is replaced by a methyl group was contrasted. A drastic enhancement in the reactivity of the cupric superoxide towards phenolic substrates as well as oxidation of substrates possessing moderate C?H bond‐dissociation energies is observed, correlating with the number and strength of the H‐bonding groups. 相似文献
30.
Yulong Kuang Kai Wang Xiangcheng Shi Xiaoqiang Huang Eric Meggers Jie Wu 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2019,131(47):17015-17019
Enantioenriched 1,4‐dicarbonyl compounds are versatile synthons in natural product and pharmaceutical drug synthesis. We herein report a mild pathway for the efficient enantioselective synthesis of these compounds directly from aldehydes through synergistic cooperation between a neutral eosin Y hydrogen atom transfer photocatalyst and a chiral rhodium Lewis acid catalyst. This method is distinguished by its operational simplicity, abundant feedstocks, atom economy, and ability to generate products in high yields (up to 99 %) and high enantioselectivity (up to 99 % ee). 相似文献