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1.
Werner Fuss Wolfram E Schmid Sergei A Trushin Paul S Billone William J Leigh 《Chemphyschem》2007,8(4):592-598
Photochemical pericyclic reactions are believed to proceed via a so-called pericyclic minimum on the lowest excited potential surface (S(1)), which is common to both the forward and backward reactions. Such a common intermediate has never been directly detected. The photointerconversion of 1,3-butadiene and cyclobutene is the prevailing prototype for such reactions, yet only diene ring closure proceeds with the stereospecificity that the Woodward-Hoffmann rules predict. This contrast seems to exclude a common intermediate. Using ultrafast spectroscopy, we show that the excited states of two cyclobutene/diene isomeric pairs are linked by not one, but by two common minima, p* and ct*. Starting from the diene side (cyclohepta-1,3-diene and cycloocta-1,3-diene), electrocyclic ring closure passes via the pericyclic minimum p*, whereas ct* is mainly responsible for cis-trans isomerization. Starting from the corresponding cyclobutenes (bicyclo[3.2.0]heptene-6 and bicyclo[4.2.0]octene-7), the forbidden isomer is formed from ct*. The path branches at the first (S(2)/S(1)) conical intersection towards p* and ct*. The fact that the energetically unfavorable ct* path can compete is ascribed to a dynamic effect: the momentum in C=C twist direction, acquired--such as in other olefins--in the Franck-Condon region of the cyclobutenes. 相似文献
2.
Polypropylene (PP) has become an indispensable material in our daily lives. Annual worldwide production of PP is now more than 30000000 tons and is predicted to grow at an annual rate of about 6% during the first decade of the 21st century. Commercial production of PP began in 1957 with the use of TiCl(3) catalysts established by Ziegler and Natta. However, the low activities and low stereospecificities of the catalysts resulted in large amounts of catalyst residue and atactic PP in the product, necessitating steps for their removal in commercial production. As a means of finding appropriate catalysts, we developed MgCl(2)-supported TiCl(4) catalysts, which basic concept was introduction of organic compounds onto the inorganic crystal catalyst surface. This addition led to remarkable enhancements in stereospecificity with extremely high activity. Use of the new catalysts enlarged and simplified the PP production process by eliminating the steps previously required for removal of catalyst residue and atactic PP. In addition, it greatly improved the properties of the PP, enabling a much wider range of PP applications by replacing metal and engineering plastics with the highly stereoregular PP. Therefore, these catalysts helped the rapid establishment of the current PP industry and now play a major role in production. The latest MgCl(2)-supported TiCl(4) catalyst is providing precise control of the isotactic PP structure. Future expectations for this type of catalyst are to acquire a single-site nature and to contribute to the creation of a new class of hybrid materials. 相似文献
3.
自然界广泛存在铁屎米酮(Canthinone)骨架(1)的生物碱。在夹竹桃科Pleiocarpa talbo-tii W.中,就发现有光学活性的具有六氢铁屎米酮骨架的生物碱存在。另据报道,3-位上有取代基的六氢铁屎米酮化台物具有抗缺氧活性、扩张血管活性和毒性作用。对这类生 相似文献
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5.
Jonas Scharfbier Benjamin M. Gross Martin Oestreich 《Angewandte Chemie (International ed. in English)》2020,59(4):1577-1580
A method for the synthesis of benzylsilanes starting from the corresponding ammonium triflates is reported. Silyl boronic esters are employed as silicon pronucleophiles, and the reaction is catalyzed by copper(I) salts. Enantioenriched benzylic ammonium salts react stereospecifically through an SN2‐type displacement of the ammonium group to afford α‐chiral silanes with inversion of the configuration. A cyclopropyl‐substituted substrate does not undergo ring opening, thus suggesting an ionic reaction mechanism with no benzyl radical intermediate. 相似文献
6.
Andrei V. Afonin Dmitry V. Pavlov Igor A. Ushakov Natalia A. Keiko 《Magnetic resonance in chemistry : MRC》2012,50(7):502-510
In the 13C NMR spectra of methylglyoxal bisdimethylhydrazone, the 13C‐5 signal is shifted to higher frequencies, while the 13C‐6 signal is shifted to lower frequencies on going from the EE to ZE isomer following the trend found previously. Surprisingly, the 1H‐6 chemical shift and 1J(C‐6,H‐6) coupling constant are noticeably larger in the ZE isomer than in the EE isomer, although the configuration around the –CH═N– bond does not change. This paradox can be rationalized by the C–H?N intramolecular hydrogen bond in the ZE isomer, which is found from the quantum‐chemical calculations including Bader's quantum theory of atoms in molecules analysis. This hydrogen bond results in the increase of δ(1H‐6) and 1J(C‐6,H‐6) parameters. The effect of the C–H?N hydrogen bond on the 1H shielding and one‐bond 13C–1H coupling complicates the configurational assignment of the considered compound because of these spectral parameters. The 1H, 13C and 15N chemical shifts of the 2‐ and 8‐(CH3)2N groups attached to the –C(CH3)═N– and –CH═N– moieties, respectively, reveal pronounced difference. The ab initio calculations show that the 8‐(CH3)2N group conjugate effectively with the π‐framework, and the 2‐(CH3)2N group twisted out from the plane of the backbone and loses conjugation. As a result, the degree of charge transfer from the N‐2– and N‐8– nitrogen lone pairs to the π‐framework varies, which affects the 1H, 13C and 15N shieldings. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
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Insights into the naphthoquinone photoredox reactions have been gained from the reactions of a C-glycosyl naphthoquinone. The retentiveness comes from the balance of the lifetime of intermediary zwitterionic species relative to the conformational change to bring about the stereo-mutating C−C bond rotation. 相似文献
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We present a novel approach for empirical parametrization of the Karplus equation, based on differentiation not only according to the class of compound but also according to the type of spin-spin coupling constant. In the case of -piperidones, this approach allows us to satisfactorily predict all the vicinal proton-proton spin-spin coupling constants, in contrast to previously proposed modified Karplus equations.N. N. Semenov Institute of Chemical Physics, Russian Academy of Sciences, 117977 Moscow. Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 7, pp. 1566–1574, July, 1992. 相似文献
9.
The reaction of -iodo--nitrostilbene with morpholine or piperidine yields -nitro--morpholino- or -nitro--piperidinostilbene. The reaction is regioselective and stereospecific: only the iodine, and not the nitro group, is substituted; the product obtained has acis structure regardless of whether the initial substrate had acis- or atrans-configuration. The prereaction stage probably includes the formation of a charge-transfer complex between substrate and reagent.A. N. Nesmeyanov Institute of Organoelemental Compounds, Russian Academy of Sciences, 117813 Moscow. Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 5, pp. 1214–1216, May, 1992. 相似文献
10.
Stereospecific Nucleophilic Substitution with Arylboronic Acids as Nucleophiles in the Presence of a CONH Group
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Duanshuai Tian Dr. Chengxi Li Dr. Guoxian Gu Henian Peng Prof. Dr. Xumu Zhang Prof. Dr. Wenjun Tang 《Angewandte Chemie (International ed. in English)》2018,57(24):7176-7180
Stereospecific nucleophilic substitution was achieved for the first time with arylboronic acids as nucleophiles. This transition‐metal‐free coupling between chiral α‐aryl‐α‐mesylated acetamides and arylboronic acids provided access to a series of chiral α,α‐diaryl acetamides with excellent enantioselectivity and moderate to good yields. The CONH functionality proved to be crucial for bridging the reactants and promoting the reaction. Efficient syntheses of a cannabinoid CB1 receptor ligand, the antidepressant (S)‐diclofensine, and a key chiral building block of the inhibitor implitapide were successfully accomplished by using this method. 相似文献