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41.
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Nucleophilic vinylic substitutions of 4H-pyran-4-one and 2-methyl-4H-pyran-4-one with ammonia were calculated by the B3LYP method using the 6-31G(d,p) basis set. Bulk solvent effects of aqueous
solution were estimated by the polarized continuum and Poisson–Boltzmann self-consistent reaction field models using the 6-311+G(d,p)
basis set. In the gas phase different mechanisms were found for the two reaction systems calculated. The reaction of 4H-pyran-4-one proceeds through enol, whereas a feasible path for the less reactive 2-methyl-4H-pyran-4-one is the mechanism through a keto intermediate. Addition of ammonia in concert with proton transfer is the rate-determining
step ofthe reaction. The mechanism proceeding either by a bimolecular nucleophilic substitution (SN2) or by one involving a tetrahedral zwitterionic intermediate is shown to be unlikely in the gas phase or nonpolar solution.
The effects of bulk solvent not only consist in a reduction of the various activation barriers by about 25–40 kJ mol−1 but also in a change in the reaction mechanism.
Received 26 May 2002 / Accepted 26 July 2002 /
Published online: 14 February 2003 相似文献
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Polystyrene-supported 2-azidoethyl phenyl selenide and 3-azidopropyl phenyl selenide reagents have been developed and applied to the traceless solid-phase organic synthesis of 1-vinyl- and 1-allyl-1,2,3-triazoles by CuI-mediated azide–alkyne cycloadditions and subsequent cleavage from the polymer support through oxidation–elimination reaction with 30% hydrogen peroxide. The advantages of this method include straightforward operation, good yield and purity of the products, and good stability and lack of odor for the reagents. 相似文献
45.
Angelo Ranise Francesco LucchesiniMatteo Caviglia Silvana AlfeiAndrea Spallarossa Chiara Caneva 《Tetrahedron》2013
A simple one-pot procedure for the stereoselective synthesis of Knoevenagel-type indoles is described. The method is based on the in situ reaction of indole iminium salts (four of them are fully characterized) with acyclic symmetrical and unsymmetrical active methylene reagents in the presence of triethylamine. In general, the overall yields are moderate to good. Some of relevant reaction parameters and steric effects affecting stereoselectivity are discussed. 相似文献
46.
Exploring Regioselective Bond Cleavage and Cross‐Coupling Reactions using a Low‐Valent Nickel Complex 下载免费PDF全文
Addison N. Desnoyer Florian W. Friese Weiling Chiu Marcus W. Drover Dr. Brian O. Patrick Prof. Dr. Jennifer A. Love 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(12):4070-4077
Recently, esters have received much attention as transmetalation partners for cross‐coupling reactions. Herein, we report a systematic study of the reactivity of a series of esters and thioesters with [{(dtbpe)Ni}2(μ‐η2:η2‐C6H6)] (dtbpe=1,2‐bis(di‐tert‐butyl)phosphinoethane), which is a source of (dtbpe)nickel(0). Trifluoromethylthioesters were found to form η2‐carbonyl complexes. In contrast, acetylthioesters underwent rapid Cacyl?S bond cleavage followed by decarbonylation to generate methylnickel complexes. This decarbonylation could be pushed backwards by the addition of CO, allowing for regeneration of the thioester. Most of the thioester complexes were found to undergo stoichiometric cross‐coupling with phenylboronic acid to yield sulfides. While ethyl trifluoroacetate was also found to form an η2‐carbonyl complex, phenyl esters were found to predominantly undergo Caryl?O bond cleavage to yield arylnickel complexes. These could also undergo transmetalation to yield biaryls. Attempts to render the reactions catalytic were hindered by ligand scrambling to yield nickel bis(acetate) complexes, the formation of which was supported by independent syntheses. Finally, 2‐naphthyl acetate was also found to undergo clean Caryl?O bond cleavage, and although stoichiometric cross‐coupling with phenylboronic acid proceeded with good yield, catalytic turnover has so far proven elusive. 相似文献
47.
Shawna M. Liff William T. Pennington Dennis W. Smith Jr. 《Journal of polymer science. Part A, Polymer chemistry》2014,52(2):232-238
A novel class of semifluorinated perfluorocyclohexenyl (PFCH) aryl ether homo/copolymers was successfully synthesized with high yield through the step‐growth polymerization of commercially available bisphenols and decafluorocyclohexene in the presence of a triethylamine base. The synthesized polymers exhibit variable thermal properties depending on the functional spacer group (R). PFCH aryl ether copolymers with random and alternating architectures were also prepared from versatile bis‐perfluorocyclohexenyl aryl ether monomers. The PFCH polymers show high thermal stabilities with a 5% decomposition temperature ranging from 359 to 444 °C in air and nitrogen atmosphere. These semifluorinated PFCH aromatic ether polymers contain intact enchained PFCH olefin moieties, making further reactions such as crosslinking and application specific functionalization possible. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 232–238 相似文献
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Philip Eckert Prof. Michael G. Organ 《Chemistry (Weinheim an der Bergstrasse, Germany)》2022,28(35):e202200665
Bulky Pd−N-heterocyclic carbene (NHC) catalysts (e. g., N-(di-2,6-(3-pentyl)phenyl), IPent) have been shown to have significantly higher reactivity in a wide variety of cross-coupling applications (i. e., C−C, C−S, C−N) than less hindered variants (e. g., N-(di-2,6-(isopropyl)phenyl), IPr). Further, chlorinating the backbone of the NHC ring sees an even greater increase in reactivity. In the cross-coupling of (hetero)aryl electrophiles to secondary alkyl nucleophiles, making the N-aryl groups larger reduces the amount of β-hydride elimination leading to alkene byproducts and chlorinating the NHC core had an even greater effect, all but eliminating alkene formation. In the present study involving the cross-coupling of primary alkyl electrophiles and nucleophiles, a sharp and surprising reversal of all of the above trends was observed. Bulkier catalysts had generally slower rate of reaction and β-hydride elimination worsened leading to extensive amounts of alkene byproducts. 相似文献
50.