Library

X
feed icon rss

Your email was sent successfully. Check your inbox.

An error occurred while sending the email. Please try again.

Proceed reservation?

Export
  • 1
    Electronic Resource
    Electronic Resource
    A Stopped-Flow Kinetic Study of the Reaction of trans-1-(Dimethylamino)-1,3-butadiene with Dimethyl Dicyanofumarate (1992)
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 125 (1992), S. 1665-1667 
    Details
    ISSN: 0009-2940
    Keywords: Zwitterion formation and decay, kinetics of ; Cycloadditions ; Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: trans-1-(Dimethylamino)-1,3-butadiene and dimethyl dicyanofumarate react in acetonitrile and dichloromethane at -40°C with a second-order rate constant of 107 to 108 l mol-1 s-1. This was determined by following the decay of the UV absorption of diene and dienophile with a stopped-flow spectrofluorimeter. After disapperance of the reagents a new absorption (λmax = 400 nm, lgε = 2.514) is recorded which decays by a first-order process. The analysis of the kinetic data (Ea = 14.5 ± 0.1 kcal mol-1, lgA = 14.71 ± 0.05, ΔH# = 14.1 ± 0.1 kcal mol-1, δS# = 7.2 ± 0.2 cal mol-1 K-1, ΔG# = 11.95 ± 0.01 kcal mol-1) in combination with stereochemical studies on the cycloaddition of the E/Z isomeric dienophiles dimethyl dicyanofumarate and dicyanomaleate leads to the interpretation that the new species is a zwitterion.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cber.19921250723
    Library Location Call Number Volume/Issue/Year Availability
    BibTip Others were also interested in ...
    Paper (German National Licenses)
    Fulltext
  • 2
    Electronic Resource
    Electronic Resource
    trans-1-(Dimethylamino)-1,3-butadiene - Concerted and Stepwise (4 + 2) Cycloadditions (1992)
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 125 (1992), S. 1647-1656 
    Details
    ISSN: 0009-2940
    Keywords: Cycloadditions ; Diels-Alder reactions ; Kinetics ; Zwitterionic intermediates ; Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: trans-1-(Dimethylamino)-1,3-butadiene is allowed to react with methyl acrylate, acrylonitrile, maleo- and fumaronitrile, dimethyl maleate and fumarate, as well as with dimethyl dicyanomaleate and dicyanofumarate. In all cases except the two last ones where only a single isomer is obtained mixtures (endo/exo) of cycloadducts are formed. Structures are determined by a combination of one- and two-dimensional NMR spectroscopy. The E/Z isomeric maleo- and fumaronitrile add stereospecifically to the diene. Cycloaddition of dimethyl maleate and fumarate leads to the same mixture of isomers in almost the same ratio. The reaction of dimethyl dicyanomaleate and dimethyl dicyanofumarate yields a cycloadduct which seems to be the thermodynamically most stable one. The results are interpreted in such a manner that a concerted cycloaddition takes place with maleo- and fumaronitrile, while stepwise reactions, presumably proceeding via zwitterions, are observed with dimethyl maleate, fumarate, the strongly electrophilic dimethyl dicyanomaleate and dicyanofumarate. With increasing capability of the dienophile to stabilize a negative charge the mechanism changes from a concerted to a stepwise reaction.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cber.19921250721
    Library Location Call Number Volume/Issue/Year Availability
    BibTip Others were also interested in ...
    Paper (German National Licenses)
    Fulltext
  • 3
    Electronic Resource
    Electronic Resource
    Reactions of 1,1-bis(dimethylamino)-1,3-butadiene with olefins - (4 + 2) cycloadditions and zwitterion formation (1992)
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 125 (1992), S. 1657-1664 
    Details
    ISSN: 0009-2940
    Keywords: (4 + 2) Cycloaddition ; Zwitterion formation ; Ketene aminal ; Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 1,1-Bis(dimethylamino)-1,3-butadiene (1) as a strong donor diene (E1/2 = 0.03 V vs. SCE, 1st IPv = 6.94 eV) is treated with acrylonitrile, dimethyl dicyanofumarate, and tetracyanoethylene. Cycloaddition with acrylonitrile is slow and requires elevated temperatures at which elimination of dimethylamine with formation of 1-cyano-2-(dimethylamino)-1,3-cyclohexadiene occurs. The reaction of 1 with TCNE at -40°C in acetonitrile is very fast and leads to the zwitterion 5. At T ≥ -20°C, 5 eliminates hydrogen cyanide to give a highly colored (λmax = 489 nm, lgε = 4.716) merocyanine. A zwitterion 9 generated from 1 and dimethyl dicyanofumarate can be isolated in crystalline form. The structural analysis shows that the zwitterions are produced by attack of the olefin at the antiperiplanar conformation of 1. Both zwitterions which can be trapped as crystalline picrates cannot be converted to cycloadducts. These results suggest that cycloaddition is observed if zwitterion formation becomes energetically unfavorable.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cber.19921250722
    Library Location Call Number Volume/Issue/Year Availability
    BibTip Others were also interested in ...
    Paper (German National Licenses)
    Fulltext
  • 4
    Electronic Resource
    Electronic Resource
    Rearrangements of free radicals, XI. Sigmatropic and electrocyclic reactions of bicyclo[3.2.0]heptadienyl radicals, 3-Quadricyclanyl radicals, and 7-norbornadienyl radical (1985)
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 118 (1985), S. 3500-3512 
    Details
    ISSN: 0009-2940
    Keywords: Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: Umlagerungen von Freien Radikalen, XI. Sigmatrope und elektrocyclische Reaktionen von Bicyclo[3.2.0]heptadienyl-Radikalen, 3-Quadricyclanyl-Radikalen und des 7-Norbornadienyl-RadikalsDie Umlagerung matrixisolierter organischer Radikale mit Bicyclo[3.2.0]heptadienyl-Struktur (2, 5, 11a,b), 3-Quadricyclanyl-Gerüst (16, 26, 27) und von 7-Norbornadienyl-Radikal 15 wurde untersucht. Alle Systeme führen letztlich zu Radikalen mit Tropylium-Struktur. 16, 26 und 27 isomerisieren zunächst in Radikale mit Bicyclo[3.2.0]heptadienyl-Gerüst, bevor Tropylium-Radikale gebildet werden. 7-Norbornadienyl-Radikal 15 ist der instabilste Vertreter auf der C7-Hyperfläche. Sigmatrope 1,2-Vinylverschiebungen von Radikalen mit Bicyclo[3.2.0]heptadienyl-Gerüst wurden nicht beobachtet. Substituenten ändern die relativen Stabilitäten der Radikale nicht.
    Notes: The rearrangement of matrix-isolated organic radicals with bicyclo[3.2.0]heptadienyl structure (2, 5, 11a,b), 3-quadricyclanyl structure (16, 26, 27), and of 7-norbornadienyl radical 15 is studied. Final rearrangement products are radicals with tropylium structure. 16, 26, and 27 isomerize to radicals with bicyclo[3.2.0]heptadienyl skeleton before electrocyclic ring opening to tropylium radicals takes place. 7-Norbornadienyl radical 15 is the least stable radical on the C7-hypersurface. Sigmatropic 1,2-vinyl shifts in bicyclo[3.2.0]heptadienyl radicals could not be observed. Substituents do not influence the rearrangement behaviour.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cber.19851180907
    Library Location Call Number Volume/Issue/Year Availability
    BibTip Others were also interested in ...
    Paper (German National Licenses)
    Fulltext
  • 5
    Electronic Resource
    Electronic Resource
    Are Charge-Transfer Complexes Intermediates in Diels-Alder Reactions? A Case Study of the Reaction of 1,2-Dimethylenecyclopentane with Tetracyanoethylene (1991)
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 124 (1991), S. 2811-2817 
    Details
    ISSN: 0009-2940
    Keywords: Cycloaddition ; Diels-Alder reaction ; Charge-transfer complex ; Kinetics ; Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The Diels-Alder reaction of 1,2-dimethylenecyclopentane (1) and tetracyanoethylene (TCNE) is kinetically studied by conventional and stopped-flow UV/Vis absorption-spectroscopic techniques. A transient charge-transfer (CT) absorption band with λmmax=490 nm (half-life at 20°C ca. 3.4 × 10-2 s) is detected in dichloromethane solution. For the molar extinction coefficient a value of e490=310 M-1 cm-1 has been determined from measurements of the optical densitity at zero time as a function of excess diene concentration. With the aid of computer simulation the combined kinetic analysis of the disappearance of reactants and CT complex reveals that the data obtained from experiments at equimolar concentration and at a single temperature (20°C) can be interpreted equally well by both the model where the CT complex is an intermediate on the reaction path and where it is in a nonreactive side equilibrium with the reactants. Activation parameters have been determined for the disappearance of the starting compounds and of the CT complex. Contrary to what is expected for a CT complex being an intermediate in a concerted Diels-Alder reaction a more negative activation entropy for the decay of the CT complex than for the disappearance of the reactants is found. This suggests that the CT complex is unlikely to exhibit a transition-state-like structure as is required for a concerted cycloaddition. Experiments under pseudo-first-order conditions have led to an extension of the reaction scheme by a second-order reaction of the CT complex with the excess diene, the rate constant of which is virtually identical to that for the direct bimolecular cycloaddition. The conclusion is drawn for our case that there are more arguments against than arguments for the CT complex being a preorientation complex in the [4 + 2] cycloaddition.
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cber.19911241225
    Library Location Call Number Volume/Issue/Year Availability
    BibTip Others were also interested in ...
    Paper (German National Licenses)
    Fulltext
Close ⊗
This website uses cookies and the analysis tool Matomo. More information can be found here...