The Thermal Decomposition of N-Arylcarbamates of t-Alcohols. 2. Effects of Solvent, Temperature, and Substituents

1972 ◽  
Vol 50 (21) ◽  
pp. 3478-3487 ◽  
Author(s):  
S. J. Ashcroft ◽  
Melanie P. Thorne
Keyword(s):  
Diphenyl Ether ◽  
Substituent Effects ◽  
Reaction Rates ◽  
Specific Rate ◽  
Cyclic Transition ◽  
Tertiary Alcohols ◽  
Carbonium Ion ◽  
Tertiary Carbon ◽  
Cyclic Transition State ◽  
Influence Of Solvent

The influence of solvent, ring substituents, and tertiary carbon atom substituents on the decomposition at 423–473 K of some N-arylcarbamates of tertiary alcohols (p-XC6H4NHCO2C(CH3)2R, where X = CH3, H, Cl, NO2, R=C≡CH, C2H5, CH=CH2) has been investigated. In dodecane, diphenyl ether, acetophenone, nitrobenzene, and decanol, the reactions were first order yielding chiefly carbon dioxide, amine, and olefin. t-Pentyl carbamates yielded the olefins 2-methyl-1-butene and 2-methyl-2-butene in a 3:1 mixture. Solvent effects are reflected in variations in enthalpies and entropies of activation, but effects on the actual values of the specific rate constants are small. Substituent effects are also small, with changes in R from C≡CH to CH=CH2 producing only a tenfold rate increase. Variations in X yield Hammett σρ plots with slopes of about 0.50. These results are consistent with the charge separated cyclic transition state proposed before (1). In ethylene glycol, increased reaction rates and 1:1 yields of the olefins from the t-pentyl carbamates indicate a mechanism with greater carbonium ion character.

scholarly journals Confronting the Challenging Asymmetric Addition of Vinyl Arene Pronucleophiles into Ketones: Ligand-Controlled Regiodivergent Processes Through a Dearomatized Allyl-Cu Species

2021 ◽  
Author(s):  
Yuyang Dong ◽  
Alexander Schuppe ◽  
Binh Khanh Mai ◽  
Peng Liu ◽  
Stephen Buchwald
Keyword(s):  
Transition State ◽  
Functional Groups ◽  
Computational Studies ◽  
Ancillary Ligand ◽  
Reductive Coupling ◽  
Cyclic Transition ◽  
Tertiary Alcohols ◽  
Rapid Construction ◽  
Cyclic Transition State

The selective reductive coupling of vinyl arenes and ketones represents a versatile approach for the rapid construction of enantiomerically enriched tertiary alcohols. Herein, we demonstrate a CuH-catalyzed regiodivergent coupling of vinyl arenes and ketones, in which the selectivity is controlled by the ancillary ligand. This approach leverages an in situ generated benzyl- or dearomatized allyl-Cu intermediate, yielding either the dearomatized or exocyclic addition products, respectively. The method exhibits excellent regio-, diastereo- and enantioselectivity, and tolerates a range of common functional groups and heterocycles. Computational studies suggest that the regio- and enantioselectivity are controlled by the ancillary ligand, while the diastereoselectivity is enforced by steric interactions between the alkyl-Cu intermediate and ketone substrates in a six-membered cyclic transition state.

scholarly journals Confronting the Challenging Asymmetric Addition of Vinyl Arene Pronucleophiles into Ketones: Ligand-Controlled Regiodivergent Processes Through a Dearomatized Allyl-Cu Species

2021 ◽  
Author(s):  
Yuyang Dong ◽  
Alexander Schuppe ◽  
Binh Khanh Mai ◽  
Peng Liu ◽  
Stephen Buchwald
Keyword(s):  
Transition State ◽  
Functional Groups ◽  
Computational Studies ◽  
Ancillary Ligand ◽  
Reductive Coupling ◽  
Cyclic Transition ◽  
Tertiary Alcohols ◽  
Rapid Construction ◽  
Cyclic Transition State

The selective reductive coupling of vinyl arenes and ketones represents a versatile approach for the rapid construction of enantiomerically enriched tertiary alcohols. Herein, we demonstrate a CuH-catalyzed regiodivergent coupling of vinyl arenes and ketones, in which the selectivity is controlled by the ancillary ligand. This approach leverages an in situ generated benzyl- or dearomatized allyl-Cu intermediate, yielding either the dearomatized or exocyclic addition products, respectively. The method exhibits excellent regio-, diastereo- and enantioselectivity, and tolerates a range of common functional groups and heterocycles. Computational studies suggest that the regio- and enantioselectivity are controlled by the ancillary ligand, while the diastereoselectivity is enforced by steric interactions between the alkyl-Cu intermediate and ketone substrates in a six-membered cyclic transition state.

Kinetic Studies of [4n+2]π-Thermal Cyclodimerization of 1-(3-Pyridazinyl)-3-oxidopyridinium Betaines

1992 ◽  
Vol 57 (9) ◽  
pp. 1951-1959 ◽  
Author(s):  
Madlene L. Iskander ◽  
Samia A. El-Abbady ◽  
Alyaa A. Shalaby ◽  
Ahmed H. Moustafa
Keyword(s):  
Energy Gap ◽  
Activation Parameters ◽  
Kinetic Studies ◽  
Cyclic Transition ◽  
Concerted Mechanism ◽  
First Order ◽  
Thermodynamic Activation Parameters ◽  
Cyclic Transition State ◽  
Complete Dissociation ◽  
Homo Lumo

The reactivity of the base induced cyclodimerization of 1-(6-arylpyridazin-3-yl)-3-oxidopyridinium chlorides in a pericyclic process have been investigated kinetically at λ 380 nm. The reaction was found to be second order with respect to the liberated betaine and zero order with respect to the base. On the other hand dedimerization (monomer formation) was found to be first order. It was shown that dimerization is favoured at low temperature, whereas dedimerization process is favoured at relatively high temperature (ca 70 °C). Solvent effects on the reaction rate have been found to follow the order ethanol > chloroform ≈ 1,2-dichloroethane. Complete dissociation was accomplished only in 1,2-dichloroethane at ca 70 °C. The thermodynamic activation parameters have been calculated by a standard method. Thus, ∆G# has been found to be independent on substituents and solvents. The high negative values of ∆S# supports the cyclic transition state which is in favour with the concerted mechanism. MO calculations using SCF-PPP approximation method indicated low HOMO-LUMO energy gap of the investigated betaines.

Reactions of the radical cation derived from 2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS·+) with amino acids. Kinetics and mechanism

2000 ◽  
Vol 78 (8) ◽  
pp. 1052-1059 ◽  
Author(s):  
C Aliaga ◽  
E A Lissi
Keyword(s):  
Amino Acids ◽  
Radical Cation ◽  
Sulfonic Acid ◽  
Biological Fluids ◽  
Reaction Rates ◽  
Kinetics And Mechanism ◽  
Specific Rate ◽  
Wide Range ◽  
Initial Ph ◽  
Secondary Reactions

Stable free radicals derived from 2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS·+) have been extensively employed to monitor the antioxidant capacity of biological fluids and beverages. Besides reacting with typical antioxidants (such as phenols or thiols) these radicals react with a variety of hydrogen or electron donors. The present work reports on the kinetics and mechanism of these radical reactions with several amino acids. Reaction rates notably increase when the pH of the media increases and, when measured under similar conditions, follows the ordercysteine > > tryptophan > tyrosine > histidine > cystineThe kinetics of the process is interpreted in terms of a mechanism comprising an initial pH dependent reversible step, followed by secondary reactions of the substrate derived radical with itself or with another ABTS·+; this simple three-step mechanism leads to very complex kinetic expressions. The specific rate constants of several of the elementary steps were determined by working under a wide range of substrate, radical, and ABTS concentrations. The values obtained for the initial interaction between the ABTS derived radical and the substrate range from 0.5 M–1 s–1 to 1.9 × 106 M–1 s–1 for histidine and cysteine, respectively.Key words: ABTS radical cation, 2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonic acid), amino acids, kinetics.

Divergent Reactivity of Stannane and Silane in the Trifluoromethylation of PdII: Cyclic Transition State versus Difluorocarbene Release

2018 ◽  
Vol 130 (46) ◽  
pp. 15301-15305 ◽  
Author(s):  
Maoping Pu ◽  
Italo A. Sanhueza ◽  
Erdem Senol ◽  
Franziska Schoenebeck
Keyword(s):  
Transition State ◽  
Cyclic Transition ◽  
Cyclic Transition State

scholarly journals Asymmetric Synthesis of Tertiary α -Hydroxyketones by Enantioselective Decarboxylative Chlorination and Subsequent Nucleophilic Substitution

Molecules ◽  
2020 ◽  
Vol 25 (17) ◽  
pp. 3902
Author(s):  
Mei Kee Kam ◽  
Akira Sugiyama ◽  
Ryouta Kawanishi ◽  
Kazutaka Shibatomi
Keyword(s):  
Asymmetric Synthesis ◽  
Nucleophilic Substitution ◽  
Primary Amine ◽  
Tetrabutylammonium Hydroxide ◽  
Tertiary Alcohols ◽  
Tertiary Carbon ◽  
Chiral Tertiary Alcohols

Chiral tertiary α-hydroxyketones were synthesized with high enantiopurity by asymmetric decarboxylative chlorination and subsequent nucleophilic substitution. We recently reported the asymmetric decarboxylative chlorination of β-ketocarboxylic acids in the presence of a chiral primary amine catalyst to obtain α-chloroketones with high enantiopurity. Here, we found that nucleophilic substitution of the resulting α-chloroketones with tetrabutylammonium hydroxide yielded the corresponding α-hydroxyketones without loss of enantiopurity. The reaction proceeded smoothly even at a tertiary carbon. The proposed method would be useful for the preparation of chiral tertiary alcohols.

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