Elimination Reactions of (E)-2,4,6-TrinitrobenzaldehydeO-Benzoyloximes Promoted by R2NH/R2NH2+in 70 mol% MeCN (aq). Effects of the β-Aryl Group and Leaving Group on Nitrile-Forming Transition States

2016 ◽  
Vol 37 (6) ◽  
pp. 871-876 ◽  
Author(s):  
Sang Yong Pyun ◽  
Kyu Cheol Paik ◽  
Man So Han ◽  
Bong Rae Cho
Keyword(s):  
Transition States ◽  
Aryl Group ◽  
Leaving Group ◽  
Elimination Reactions

Mechanisms of aromatic nucleophilic substitution by the ambident thiocyanate ion

1973 ◽  
Vol 26 (2) ◽  
pp. 273 ◽  
Author(s):  
DE Giles ◽  
AJ Parker
Keyword(s):  
Nucleophilic Substitution ◽  
Transition States ◽  
Reactivity Ratio ◽  
Substitution Reactions ◽  
Aromatic Nucleophilic Substitution ◽  
Thiocyanate Ion ◽  
Sn2 Reactions ◽  
Leaving Group ◽  
Electrophilic Reactivity ◽  
Saturated Carbon Atom

Sulphur/nitrogen reactivity ratios in a series of aromatic nucleophilic substitution reactions of ambident thiocyanate ion have been determined. There are profound differences from the pattern found in SN2 reactions at a saturated carbon atom. Abnormal transition states, involving interactions between entering and leaving group, are likely in the bond-breaking step of the intermediate complex in reactions of thiocyanate ion with 1-fluoro-2,4-dinitrobenzene and with 2,4- dinitrophenyl 4-toluenesulphonate. The nitro-substituted aryl thiocyanates are shown to be tri-functional electrophiles, with reactive centres at aromatic carbon, at cyanide carbon, and at sulphur. Aryl 4-toluenesulphonates are bifunctional electrophiles with reactive centres at aryl carbon and sulphonyl sulphur. The site of attack by nucleophiles depends on the nature of the nucleophile. The sulphur/nitrogen reactivity ratio of ambident SCN-, and the electrophilic reactivity of tri- and bi-functional substrates, are in most instances consistent with the Hard and Soft Acids and Bases principle. Exceptions to the principle in some instances reveal differences between the SNAr and SN2 mechanisms, and in others indicate abnormal transition states.

Ambident Heterocyclic Reactivity: Alkylation of 4-Substituted and 2,4-Disubstituted Benzimidazoles

1994 ◽  
Vol 47 (8) ◽  
pp. 1523 ◽  
Author(s):  
MR Haque ◽  
M Rasmussen
Keyword(s):  
Hydrogen Bonding ◽  
Transition State ◽  
Electrostatic Field ◽  
Transition States ◽  
Alkyl Halides ◽  
Methyl Group ◽  
Site Selectivity ◽  
Leaving Group ◽  
Standard Set ◽  
Specific Association

The N1/N3-alkylation patterns of 4-amino-, 4-methyl- and 4-nitro-benzimidazole anions, and their 2-methyl analogues, with a standard set of primary alkyl halides (in dimethylformamide, 30°) have been determined and compared. The observed regioselectivities are dominated by proximal effects-electrostatic field, non-bonded steric and in some cases specific association (hydrogen bonding)-the interplay of which is critically dependent on the (variable) geometries of the SN2 transition states involved, in particular on the N---C distance of the developing N-alkyl bonds. The presence of a symmetrically placed 2-methyl group produces an enhanced N1/N3 site selectivity, very sensitive to the loose-tight nature of the transition state. Halide leaving group effects on butylation regioselectivities of 2-unsubstituted, 2-ethoxy-, 2-methyl- and 2-chloro-4-methylbenzimidazole anions, whilst small, are consistent with a Bell-Evans-Polanyi analysis of SN2 transition state variations, with the earlier transition states of CH3(CH2)3I leading to reduced regioselectivities.

The Formation of Alkenes by the Thermal Elimination Reaction of N-Methyl-4-alkoxypyridinium Iodides. II. An Investigation of the Mechanism

1972 ◽  
Vol 50 (8) ◽  
pp. 1188-1191
Author(s):  
George H. Schmid ◽  
Aaron W. Wolkoff
Keyword(s):  
Elimination Reaction ◽  
Product Composition ◽  
Thermal Elimination ◽  
Leaving Group ◽  
Elimination Reactions ◽  
Leaving Groups

A comparison of the products from elimination reactions of a number of compounds containing various leaving groups with those containing the N-methyl oxypyridinium leaving group suggests that the elimination is not occurring by means of a simple E1 mechanism. Changing the anion of the salt from iodide to methyl-sulphate and tetrafluoroborate affects the product composition indicating that the anion is taking part in the reaction. The mechanism of this reaction appears to be on the E1-E2 borderline.

The effect of the leaving group on orientation in E2 elimination reactions

1960 ◽  
Vol 1 (39) ◽  
pp. 1-3 ◽  
Author(s):  
William H. Saunders ◽  
Susan R. Fahrenholtz ◽  
John P. Lowe
Keyword(s):  
Leaving Group ◽  
Elimination Reactions

Elimination reactions. Leaving group tendencies in E2C, E2H and SN2 reactions

1968 ◽  
Vol 9 (50) ◽  
pp. 5183-5188 ◽  
Author(s):  
D.J. Lloyd ◽  
A.J. Parker
Keyword(s):  
Sn2 Reactions ◽  
Leaving Group ◽  
Elimination Reactions

Kinetics and mechanism of the aminolysis of ethyl aryl carbonates in acetonitrile

1998 ◽  
Vol 76 (6) ◽  
pp. 710-716 ◽  
Author(s):  
Han Joong Koh ◽  
Ji-Won Lee ◽  
Hai Whang Lee ◽  
Ikchoon Lee
Keyword(s):  
Transition States ◽  
Interaction Constant ◽  
Kinetics And Mechanism ◽  
Base Catalysis ◽  
Large Magnitude ◽  
Stepwise Mechanism ◽  
Leaving Group ◽  
Rate Limiting

The aminolysis reactions of ethyl aryl carbonates with benzylamines in acetonitrile at 25.0°C are investigated. The base-catalyzed path, k2, disappears when strong nucleophiles (X = p-CH3O and p-CH3) react with a substrate activated by a strong nucleofuge (Z = p-NO2). The large magnitude of rho x (-1.7 to -2.5), rho z (3.4 to 4.3), and rho xz (1.4) values, and relatively large kH/kD (1.6 to 1.8) found for the uncatalyzed path (k1) can be accounted for in terms of a stepwise mechanism with rate-limiting expulsion of the phenoxide leaving group. The catalyzed process (k2) is characterized by the much smaller magnitude of rho x (-1.0 to -1.7), rho z (0.4 to 0.7), and rho xz (0.2), the larger kH/kD (2.1 to 2.5) values, and the lower Δ Hdouble dagger values (1.8-1.9 kcal mol-1) than those of the uncatalyzed process (k1) with large negative Δ Sdouble dagger values (-65 to -67 cal K-1 mol-1). These results are consistent with four- and six-centered transition states for the two processes, k1 and k2, respectively.Key words: ethyl aryl carbonates, aminolysis mechanism, base catalysis, stepwise mechanism, cross-interaction constant.

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