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The reversibly formed C?N bond plays a very important role in dynamic covalent chemistry and the C?N/C?N exchange of components between different imine constituents to create dynamic covalent libraries has been extensively used. To facilitate diversity generation, we have investigated an organocatalyzed approach, using L ‐proline as catalyst, to accelerate the formation of dynamic libraries of [n×n] imine components. The organocatalysis methodology has also been extended, under somewhat modified conditions, to reversible C?C/C?N exchange processes between Knoevenagel derivatives of barbituric acid and imines, allowing for the generation of increased diversity. 相似文献
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A mechanistic model is presented for the base‐catalyzed intramolecular cyclization of polycyclic unsaturated alcohols of type A to ethers D (Scheme 1). The alkoxide anion B is formed first in a fast acid‐base equilibrium. For the subsequent reaction to D , a carbanion‐like transition state C is proposed. This mechanism is in full agreement with our results regarding the influence of substituents on the regioselectivity and the rate of cyclization. We studied the effect of alkyl substituents in allylic position (alkylated endocylic olefinic alcohols 1 – 3 ) and, especially, at the exocyclic double bond ( 12 – 15 ). The fastest cyclization (krel=1) is 12 → 16 , which proceeds via a primary carbanion‐like transition state ( E : R1=R2=H). The corresponding processes 13 → 17 and 14 → 17 are characterized by a less‐stable secondary carbanion‐like transtition state ( E : R1=Me, R2=H, or vice versa) and are slower by a factor of 104. The slowest reaction (krel ca. 10−6) is the cyclization 15 → 18 via a tertiary carbanion‐like transition state ( E : R1=R2=Me). 相似文献
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Organocatalyzed and Uncatalyzed CC/CC and CC/CN Exchange Processes between Knoevenagel and Imine Compounds in Dynamic Covalent Chemistry 下载免费PDF全文
Molecular diversity generation through reversible component exchange has acquired great importance in the last decade with the development of dynamic covalent chemistry. We explore here the recombination of components linked by C?C and C?N bonds through reversible double‐bond formation, and cleavage in C?C/C?C and C?C/C?N exchange processes. The reversibility of the Knoevenagel reaction has been explored, and C?C/C?C C/C exchanges have been achieved among different benzylidenes, under organocatalysis by secondary amines such as L ‐proline. The substituents of these benzylidenes were shown to play a very important role in the kinetics of the exchange reactions. L ‐Proline is also used to catalyze the reversible C?C/C?C exchange between Knoevenagel derivatives of barbituric acid and malononitrile. Finally, the interconversion between Knoevenagel derivatives of dimethylbarbituric acid and imines (C?C/C?N exchange) has been studied and was found to occur rapidly in the absence of catalyst. The results of this study pave the way for the extension of dynamic combinatorial chemistry based on C?C/C?C and C?C/C?N exchange systems. 相似文献
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Grzegorz Mlosto Katarzyna Urbaniak Mihaela Gulea Serge Masson Anthony Linden Heinz Heimgartner 《Helvetica chimica acta》2005,88(10):2582-2592
The reaction of the methyl (dialkoxyphosphinyl)‐dithioformates (= methyl dialkoxyphosphinecarbodithioate 1‐oxides) 10 with CH2N2 at − 65° in THF yielded cycloadducts which eliminated N2 between − 40 and − 35° to give the corresponding phosphonodithioformate S‐methanides ( =methylenesulfonium (dialkoxyoxidophosphino)(methylthio)methylides) 11 (Scheme 3). These reactive 1,3‐dipoles were intercepted by aromatic thioketones to yield 1,3‐dithiolanes. Whereas the reaction with thiobenzophenone ( 12b ) led to the sterically more congested isomers 15 regioselectively, a mixture of both regioisomers was obtained with 9H‐fluorene‐9‐thione ( 12a ). Trapping of 11 with phosphono‐ and sulfonodithioformates led exclusively to the sterically less hindered 1,3‐dithiolanes 16 and 18 , respectively (Scheme 4). In addition, reactive CC dipolarophiles such as ethenetetracarbonitrile, maleic anhydride, and N‐phenylmaleimide as well as the NN dipolarophile dimethyl diazenedicarboxylate were shown to be efficient interceptors of 11 (Scheme 5). 相似文献
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Issa Yavari Ramin Pashazadeh Reza Hosseinpour Ehsan Ghanbari 《Helvetica chimica acta》2013,96(12):2191-2195
An efficient synthesis of alkyl 5‐(alkylimino)‐3‐aryl(alkyl)‐2‐(dicyanomethylene)‐4‐hydroxythiazolidine‐4‐carboxylates, containing polarized C?C bonds, via reaction of malononitrile, arylisothiocyanates, and the Nef‐isocyanide adducts, under basic conditions, is described. 相似文献
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Short Access to Belt Compounds with Spatially Close CC Bonds and Their Transannular Reactions 下载免费PDF全文
Prof. Dr. Pelayo Camps Dr. Tània Gómez Ane Otermin Dr. Mercè Font‐Bardia Dr. Carolina Estarellas Prof. Dr. Francisco Javier Luque 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(40):14036-14046
Two domino Diels–Alder adducts were obtained from 3,7‐bis(cyclopenta‐2,4‐dien‐1‐ylidene)‐cis‐bicyclo[3.3.0]octane and dimethyl acetylenedicarboxylate or N‐methylmaleimide under microwave irradiation. From the first adduct, a C20H24 diene with C2v symmetry was obtained by Zn/AcOH reduction, hydrolysis, oxidative decarboxylation, and selective hydrogenation. Photochemical [2+2] cycloaddition of this diene gave a thermally unstable cyclobutane derivative, which reverts to the diene. However, both the diene and the cyclobutane derivatives could be identified by X‐ray diffraction analysis upon irradiation of the diene crystal. New six‐membered rings are formed upon the transannular addition of bromine or iodine to the diene. The N‐type selectivity of the addition was examined by theoretical calculations, which revealed the distinct susceptibility of the doubly bonded carbon atoms to the bromine attack. 相似文献
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Metal Alkoxide Promoted Regio‐ and Stereoselective CO and CC Metathesis of Allenoates with Aldehydes 下载免费PDF全文
Minyan Wang Dr. Zhao Fang Prof. Dr. Chunling Fu Prof. Dr. Shengming Ma 《Angewandte Chemie (International ed. in English)》2014,53(12):3214-3217
The reaction of 2,3‐allenoates and aldehydes in the presence of an alkoxide affords alkyl 4,5‐diaryl‐3‐oxo‐2‐propylpent‐4(E)‐enoates and cis‐3,4‐diaryloxetanes through a formal C?O and C?C metathesis. A mechanism for this reaction has been proposed. 相似文献
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Highly Enantioselective Rhodium(I)‐Catalyzed Activation of Enantiotopic Cyclobutanone CC Bonds 下载免费PDF全文
Laetitia Souillart Dr. Evelyne Parker Prof. Dr. Nicolai Cramer 《Angewandte Chemie (International ed. in English)》2014,53(11):3001-3005
The selective functionalization of carbon–carbon σ bonds is a synthetic strategy that offers uncommon retrosynthetic disconnections. Despite progress in C? C activation and its great importance, the development of asymmetric reactions lags behind. Rhodium(I)‐catalyzed selective oxidative additions into enantiotopic C? C bonds in cyclobutanones are reported. Even operating at a reaction temperature of 130 °C, the process is characterized by outstanding enantioselectivity with the e.r. generally greater than 99.5:0.5. The intermediate rhodacycle is shown to react with a wide variety of tethered olefins to deliver complex bicyclic ketones in high yields. 相似文献
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Helmut Poleschner Reiner Radeglia Hinrich Meyer 《Magnetic resonance in chemistry : MRC》1984,22(8):480-485
This paper reports the 77Se NMR chemical shifts of 1,3-dithiole-, 1,3-thiaselenole- and 1,3-diselenole-2-ones, -thiones and -selones, of the corresponding saturated compounds 1,3-diselenolane-2-one, -thione and -selone, and the 1,3-thiaselenolium tetrafluoroborates, either unsubstituted or substituted by morpholino, ethylthio or ethylseleno groups in the 2-position. The 77Se chemical shift values of the ring selenium and the C?Se groups are compared with the 13C chemical shift values of neighbouring carbon atoms. The relationships between the 77Se chemical shifts of the C?Se groups and the wavelengths of their n→* absorption in the UV-visible spectrum are discussed with respect to the significance of the δE term in the contribution of the paramagnetic screening and the electron density distribution. 相似文献
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J. Kao 《Journal of computational chemistry》1988,9(8):905-923
Ab initio molecular orbital theory has been systematically employed to investigate the structures, energies, and conformations of 27 nitrogen-containing compounds. A force field has been developed, on the basis of available experimental and theoretical data, to permit extension of the molecular orbital-based molecular mechanics calculations to hetero-conjugated systems. Structures, heats of formation, and conformations of more than 65 molecules were studied by the developed force field and the results were compared with the available experimental data. 相似文献
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《Journal of mass spectrometry : JMS》2005,40(6):796-806
Dissociative electron ionization (70eV) of selenophene (C4H4Se) generates m/z 106 ions of composition [H2, C2, 80Se]+? and m/z 105 ions of [H, C2, 80Se]+. From tandem mass spectrometric experiments, Density Functional Theory (DFT) and ab initio calculations, it is concluded that these ions have the structure of selenoketene H2C?C?Se+? (1a+? )and selenoketyl HC?C?Se+ (2a+) ions respectively. The calculations predict that selenoketene ion 1a+? is separated by high energy barriers from its isomers selenirene (H e)+? 1b+?, ethyne selenol (HCCSeH)+? 1c+?, (CCHSeH)+? 1d+? and (CCSeH2)+? 1e+?. The selenoketyl ion 2a+ is separated by high barriers from its isomers (CCHSe)+ 2b+, and (CCSeH)+ 2c+. Neutralization‐reionization mass spectra (NRMS) of these structurally characterized ions confirmed that the corresponding neutral analogues, selenoketene H2CCSe 1a and selenoketyl radical HCCSe 2a? are stable in the rarefied gas phase. The relative, dissociation, and isomerization energies for selenoketene and selenoketyl ions and neutrals studied at B3LYP/6–31G(d,p) and G2/G2(MP2) levels are used to support and interpret the experimental results. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献