Substituent Rate Effects in Sigmatropic Homodienyl [1,5]-Hydrogen Migrations

1990 ◽  
Vol 43 (8) ◽  
pp. 1357 ◽  
Author(s):  
GG Pegg ◽  
GV Meehan
Keyword(s):  
Transition State ◽  
Charge Separation ◽  
Substituent Effect ◽  
Substituent Effects ◽  
Aryl Group ◽  
Rate Enhancement ◽  
Polar Substituent ◽  
Rate Effects ◽  
Electronic Perturbation

The small but systematic polar substituent effect on the rates of [1,5]- homodienyl hydrogen migrations previously observed for the series of alkenylcyclopropane substrates (1) has been further explored through two additional substrate series (4) and (6) bearing the aryl probe at C4 and at the migration origin C1 respectively. However, negligible substituent effects were apparent at these positions. It now seems that the substituent effect originally observed at C5 reflects electronic perturbation by the substituent, rather than an inherent charge separation in the transition state for homodienyl rearrangement. The aryl group at C1 of the series (6) does, however, afford a substantial rate enhancement relative to the parent cis-1-methyl-2-vinylcyclopropane. In addition, exclusive formation of (1E,4Z)-1-arylhexa-1,4-diene rearrangement products is observed. These kinetic and stereochemical observations are interpreted in terms of a stereoelectronically favoured equatorial conformation for the aryl group in the transition state for rearrangement.

scholarly journals Substituent effects in the decarboxylation reactions of coordinated arylcarboxylates in dinuclear copper complexes, [(napy)Cu2(O2CC6H4X)]+

2017 ◽  
Vol 23 (6) ◽  
pp. 351-358 ◽  
Author(s):  
Qiuyan Jin ◽  
Jiaye Li ◽  
Alireza Ariafard ◽  
Allan J Canty ◽  
Richard AJ O’Hair
Keyword(s):  
Density Functional Theory ◽  
Transition State ◽  
Density Functional ◽  
Copper Complexes ◽  
Substituent Effects ◽  
Density Functional Theory Calculations ◽  
Molecular Formula ◽  
Binuclear Complexes ◽  
Aryl Group ◽  
Functional Theory

A combination of gas-phase ion trap mass spectrometry experiments and density functional theory (DFT) calculations have been used to examine the role of substituents on the decarboxylation of 25 different coordinated aromatic carboxylates in binuclear complexes, [(napy)Cu2(O2CC6H4X)]+, where napy is the ligand 1,8-naphthyridine (molecular formula, C8H6N2) and X = H and the ortho ( o), meta ( m) and para ( p) isomers of F, Br, CN, NO2, CF3, OAc, Me and MeO. Two competing unimolecular reaction pathways were found: decarboxylation to give the organometallic cation [(napy)Cu2(C6H4X)]+ or loss of the neutral copper benzoate to yield [(napy)Cu]+. The substituents on the aryl group influence the branching ratios of these product channels, but decarboxylation is always the dominant pathway. Density functional theory calculations reveal that decarboxylation proceeds via two transition states. The first enables a change in the coordination mode of the coordinated benzoate in [(napy)Cu2(O2CC6H4X)]+ from the thermodynamically favoured O, O-bridged form to the O-bound form, which is the reactive conformation for the second transition state which involves extrusion of CO2 with concomitant formation of the CO2 coordinated organometallic cation, [(napy)Cu2(C6H4X)(CO2)]+, which then loses CO2 in the final step to yield [(napy)Cu2(C6H4X)]+. In all cases the barrier is highest for the second transition state. The o-substituted benzoates show a lower activation energy than the m-substituted ones, while the p-substituted ones have the highest energy, which is consistent with the experimentally determined normalised collision energy required to induce fragmentation of [(napy)Cu2(O2CC6H4X)]+.

Thermolysis of 5,5-dimethyl-4-aryl-Δ1-1,2,4-triazolin-3-ones in solution. Products, kinetics, substituent effects, and solvent effects

1981 ◽  
Vol 59 (1) ◽  
pp. 100-105 ◽  
Author(s):  
Lubomira M. Cabelkova-Taguchi ◽  
John Warkentin
Keyword(s):  
Transition State ◽  
Solvent Effects ◽  
Charge Separation ◽  
Substituent Effects ◽  
Activation Parameters ◽  
Oxidative Cyclization ◽  
First Order ◽  
Temperature Range ◽  
Aryl Amine ◽  
First Order Kinetics

A series of 5,5-dimethyl-4-aryl-Δ1-1,2,4-triazolin-3-ones (Ar = C6H5, p-C6H4CH3, p-C6H4OCH3, p-C6H4Cl, and p-C6H4Br) were prepared from the corresponding 4-arylsemicarbazones of acetone by oxidative cyclization on alumina. The triazolinones decompose in solution to N2, CO, and isopropylidene aryl amine, with first order kinetics, in the temperature range 148–200 °C. Average activation parameters are ΔH≠ = 35 kcal mol−1 and ΔS≠ = 8 cal K−1 mol−1. Substituent effects are correlated through σ-constants but the thermolyses are relatively insensitive to substituents, with ρ = −0.17 at 172.5 °C. Solvent effects indicate a transition state that is less polar than the ground state.It is tentatively concluded that the triazolinone fragmentation, like the analogous thermolysis of a Δ3-1,3,4-oxadiazolin-2-one, may be a fully-concerted but nonsynchronous process with a transition state involving little, if any, charge separation. Other mechanisms, except for those involving highly polar (e.g. zwitterionic) transition states, have not been ruled out.

Effects of Bromine Substituents on Rates of Acetate-promoted Enolization of Ketones

1972 ◽  
Vol 50 (19) ◽  
pp. 3242-3247 ◽  
Author(s):  
R. A. Cox ◽  
J. Warkentin
Keyword(s):  
Transition State ◽  
Carbonyl Group ◽  
Substituent Effects ◽  
Steric Effects ◽  
Ionization Constants ◽  
Polar Substituent ◽  
Methylene Group ◽  
Better Than

Effects of bromine substituents on rates of acetate-catalyzed bromination of ketones are reported. Approximate polar contributions to the rates were obtained, based on the approximation that the steric effects of bromine and methyl are identical. Such polar substituent effects are quite large at the α′-position of an α-bromo ketone, leading to the conclusion that the carbonyl group is much better than the methylene group in transmitting polar effects.Rate enhancements caused by α-bromine substituents are largely entropic in origin and rates correlate well with ionization constants of α-bromo acids. These results are interpreted as indicative of an enolate-like transition state for acetate-catalyzed enolization.

Substituent Effects on the Ionization Equilibria of 9-Aryl-9-fluorenols and 10-Aryl-10-hydroxyanthracen-9-ones

1975 ◽  
Vol 53 (11) ◽  
pp. 1635-1641 ◽  
Author(s):  
Trevor W. Toone ◽  
Edward Lee-Ruff ◽  
Alan C. Hopkinson
Keyword(s):  
Transition State ◽  
Steric Hindrance ◽  
Major Part ◽  
Positive Charge ◽  
Substituent Effects ◽  
Hammett Equation ◽  
Aryl Group ◽  
Apparent Contradiction ◽  
Ionization Equilibria

The title compounds were prepared and their basicities measured. In both systems the major part of the molecule is held planar, while an aryl group is free to rotate. This may interact with the positive charge but steric hindrance to the ortho hydrogens may be expected to oppose planarity. In these systems no evidence was found for a reduction of the normal σ+ value due to failure to achieve planarity. A formula is presented which gives the approximate degree of resemblance (α) of the transition state to the products and, in the case of the ionization of triarylmethanols, α is found to be linear with pKR+. It is suggested that this could resolve the apparent contradiction between the Hammett equation and the Hammond postulate.

Kinetic Study of Hydrolysis of Benzoates. Part XXVI. Variation of the Substituent Effect with Solvent in Alkaline Hydrolysis of Substituted Alkyl Benzoates

2006 ◽  
Vol 71 (11-12) ◽  
pp. 1557-1570 ◽  
Author(s):  
Vilve Nummert ◽  
Mare Piirsalu ◽  
Ilmar A. Koppel
Keyword(s):  
Kinetic Study ◽  
Alkaline Hydrolysis ◽  
Rate Constants ◽  
Substituent Effect ◽  
Substituent Effects ◽  
Alkyl Substituent ◽  
Solvent Parameters ◽  
Substituent Constants ◽  
Hydrolysis Of ◽  
Main Factor

The second-order rate constants k2 (dm3 mol-1 s-1) for the alkaline hydrolysis of substituted alkyl benzoates C6H5CO2R have been measured spectrophotometrically in aqueous 0.5 M Bu4NBr at 50 and 25 °C (R = CH3, CH2Cl, CH2CN, CH2C≡CH, CH2C6H5, CH2CH2Cl, CH2CH2OCH3, CH2CH3) and in aqueous 5.3 M NaClO4 at 25 °C (R = CH3, CH2Cl, CH2CN, CH2C≡CH). The dependence of the alkyl substituent effects on different solvent parameters was studied using the following equations:      ∆ log k = c0 + c1σI + c2EsB + c3∆E + c4∆Y + c5∆P + c6∆EσI + c7∆YσI + c8∆PσI     ∆ log k = c0 + c1σ* + c2EsB + c3∆E + c4∆Y + c5∆P + c6∆Eσ* + c7∆Yσ* + c8∆Pσ* .  ∆ log k = log kR - log kCH3. σI and σ* are the Taft inductive and polar substituent constants. E, Y and P are the solvent electrophilicity, polarity and polarizability parameters, respectively. In the data treatment ∆E = ES - EH2O , ∆Y = YS - YH2O , ∆P = PS - PH2O were used. The solvent electrophilicity, E, was found to be the main factor responsible for changes in alkyl substituent effects with medium. When σI constants were used, variation of the polar term of alkyl substituents with the solvent electrophilicity E was found to be similar to that observed earlier for meta and para substituents, but twice less when σ* constants were used. The steric term for alkyl substituents was approximately independent of the solvent parameters.

Nucleophilic cleavage of diaryl disulphides

1973 ◽  
Vol 26 (1) ◽  
pp. 121 ◽  
Author(s):  
DAR Happer ◽  
JW Mitchell ◽  
GJ Wright
Keyword(s):  
Transition State ◽  
Reaction Rate ◽  
Substituent Effects ◽  
Butyl Alcohol ◽  
Sulphur Atom ◽  
Cyanide Ion ◽  
Benzene Rings ◽  
Nucleophilic Cleavage ◽  
Rapid Equilibrium ◽  
Electron Withdrawal

The rates of cleavage of 14 symmetrically substituted diaryl disulphides by cyanide ion have been measured in 60% aqueous t-butyl alcohol at pH 9.2. A plot of log k against σ� shows that while the reaction rate is accelerated by inductive electron withdrawal from the benzene rings, substituents capable of conjugative interaction are not correlated by their σ� parameters. +R substituents cause reaction to occur much faster than predicted on the basis of their σ� values, while -R substituents react more slowly than predicted. Measurement of rates of cleavage of three series of unsymmetrically substituted disulphides by cyanide or hydroxide shows that these unusual substituent effects arise from substituents in the thiocyanate-forming aryl ring. This behaviour is explained in terms of a change in the electronic behaviour of the thio- cyanate-forming sulphur atom from -I, + R in the disulphide to -I,-R in the rate-determining transition state for the reaction. The study does not show whether the cleavage involves an SN2 process or rapid equilibrium formation of a pentacovalent intermediate.

Polar substituent effects on13C NMR spectra in ethylcis-2-substituted cyclopropane carboxylates

1982 ◽  
Vol 20 (3) ◽  
pp. 159-161 ◽  
Author(s):  
Yoshiaki Kusuyama ◽  
Satoru Inoshita ◽  
Ginjiro Okada ◽  
Minoru Yanagi ◽  
Kenjiro Tokami ◽  
...  
Keyword(s):  
Nmr Spectra ◽  
Substituent Effects ◽  
Polar Substituent
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