Curved Hammett Plot Research Articles

Direct Aroylation of Olefins through a Cobalt/Photoredox-Catalyzed Decarboxylative and Dehydrogenative Coupling with α-Oxo Acids.

A photoredox/cobalt dual catalytic procedure has been developed that allows benzoylation of olefins. Here the photoredox catalyst effects the decarboxylation of α-ketoacids to form benzoyl radicals. After addition of this radical to styrenes, the cobalt catalyst abstracts a H-atom. Hydrogen evolution from the putative cobalt hydride intermediate allows a Heck-like aroylation without the need for a stoichiometric oxidant. Mechanistic studies reveal that electronically different styrenes lead to a curved Hammett plot, thus suggesting a change in product-determining step in the catalytic mechanism.

Correlating the para-Substituent Effects on Ru(II)-Polypyridine Photophysical Properties and on the Corresponding Hybrid P450 BM3 Enzymes Photocatalytic Activity

Ru(II)-diimine complexes covalently attached near the heme active site of P450 BM3 enzymes have been used to rapidly inject electrons and drive selective C-H functionalization upon visible light irradiation. Herein, we have generated a series of hybrid P450 BM3 enzymes containing a photosensitizer of general formula [Ru(4,4'-X2bpy)2(PhenA)]2+ where X = Cl, H, tBu, Me OPhe, OMe, or NMe2, bpy = 2,2'-bipyridine, and PhenA = 5-acetamido-1,10-phenanthroline. We then probed the effect of electron-withdrawing and -donating groups at the para position of the 4,4'-X2bpy ligands on the corresponding hybrid enzymes photocatalytic activity. A 3-fold improvement in initial reaction rate was noted when varying the substituent from Cl to tBu, however, the reaction rates decrease thereafter with the more electron donating groups. In order to rationalize those effects, we investigated the variation of the substituent on the photophysical properties of the corresponding [Ru(4,4'-X2bpy)2(bpy)]2+ model complexes. Several linear correlations were established between the E(III/II) potential, the MLCT emission, and absorption energies as well as the logarithm of the luminescence quenching rate vs the summative Brown-Okamoto parameter (Σσp+). Moreover, a downward curved Hammett plot is observed with the hybrid enzyme initial reaction rate revealing mechanistic details about the overall light-driven enzymatic process.

Solvent hydrogen bonding and structural effects on nucleophilic substitution reactions. Part-7 : Reaction of benzenesulphonyl chloride with substituted benzylamines in acetonitrile/dimethylsulfoxide mixtures

Substitution reactions of eleven para- and meta-substituted benzylamines with benzenesulphonyl chloride in different mole fractions of dimethylsulfoxide (DMSO) in acetonitrile (AcN) have been investigated conductometrically. The second order rate constants found to increase with an increase in the mole fraction of DMSO up to 0.5 mole fraction and beyond that it remained nearly constant. Multiple correlation analysis of the rate data via Kamlet-Taft's solvatochromic parameters revealed that the solvent H-bonding and dipolarity/polarizability plays a dominant role in governing the reactivity and also solvent-solvent interaction influenced the preferential solvation and consequently the rate of the reaction. A curved Hammett plot was obtained and a mechanism was proposed to account for this nonlinear behaviour observed.

Reactions of Alkyl Radicals with Substituted Toluenes and the Effect of Substituents on Dissociation Energies of Benzyl CH Bonds

AbstractReactions of isopropyl and of undecyl radicals with meta‐ and para‐substituted toluenes are reported. The results demonstrate that the reactivities of toluenes are due to both benzyl‐H abstraction and addition of the alkyl radicals to the aromatic ring. Relative reactivities yield curved Hammett plots, consistent with kinetic data reported by Dütsch and Fischer. Abstractions and ring additions occur with comparable rates, but opposite Hammett slopes. Addition is favored by electron‐withdrawing and abstraction by electron‐donating substituents. The effects of substituents on the dissociation energies of benzyl CH bonds are shown to be the major factor influencing reaction rates for benzyl‐H abstraction by alkyl radicals.

Reinterpretation of curved Hammett plots in reaction of nucleophiles with aryl benzoates: change in rate-determining step or mechanism versus ground-state stabilization.

A kinetic study is reported for the reaction of the anionic nucleophiles OH-, CN-, and N 3 - with aryl benzoates containing substituents on the benzoyl as well as the aryloxy moiety, in 80 mol % H2O - 20 mol % dimethyl sulfoxide at 25.0 degrees C. Hammett log k vs sigma plots for these systems are consistently nonlinear. However, a possible traditional explanation in terms of a mechanism involving a tetrahedral intermediate with curvature resulting from a change in rate-determining step is considered but rejected. The proposed explanation involves ground-state stabilization through resonance interaction between the benzoyl substituent and the electrophilic carbonyl center in the two-stage mechanism. Accordingly, the data are nicely accommodated on the basis of the Yukawa-Tsuno equation, which gives linear plots for all three nuceophiles. Literature reports of the mechanism of acyl transfer processes are reconsidered in this light.

Ketene-forming eliminations from aryl phenylacetates promoted by R2NH in MeCN. Origin of the curved Hammett plot

Elimination reactions of YC6H4CH2C(O)OC6H3-2-X-4-NO2 promoted by R2NH in MeCN have been studied kinetically. The reactions are second-order and exhibit Bronsted β = 0.52–0.87, |βlg| = 0.37–0.65, and ρH = 1.9–4.1. The β and ρH values decrease as the leaving group becomes less basic and the |βlg| values decrease with a stronger base. The results are interpreted in terms of an E1cb-like transition state with extensive Cβ–H bond cleavage, significant negative charge development at the β-carbon, and limited Cα–OAr bond cleavage. For all reactions, the Hammett plots are curvilinear and show downward curvature. The curvature is more pronounced as the leaving group becomes poorer. The results have been attributed to the highly carbanionic transition state, which is moved towards the reactant on the horizontal reaction coordinate by a stronger electron-withdrawing substituent.

.pi.-Plus Contributions in .DELTA.E and .DELTA.C Dual-Parameter Correlations. Curved Hammett Plots and 19F Shifts

Structure-reactivity analysis with the dual-parameter ΔE and ΔC substi- tuent constants illustrates deviations that can arise with conjugating substituents 4-X in a para position. Complications arise when the inte- raction of a 4-X substituent with the reaction site (rates or equili- bria) or with a spectroscopic measurement (NMR shift) gives rise to ad- ditional π-contributions thereby leading to a deviation in a single- parameter Hammett linear free energy plot. The deviations for the 4-X groups are referred to in the ΔE and ΔC correlations as π-plus effects. Their presence in systems that give curved Hammett plots for rates, equilibria, and 19 F chemical shifts is illustrated

Nucleophilic substitution at sulphonyl sulphur. Part 4. Hydrolysis of substituted thiophenesulphonyl halides in water–acetone mixtures

Hydrolysis of substituted thiophene-2-sulphonyl bromides and chlorides have been studied in water and in water–acetone mixtures at 25°C. Curved Hammett plots with an upward concavity are obtained in water and in water-acetone with up to 40% acetone. For solvents richer in acetone (>80%) the Hammett plots become straight lines with positive slopes for both chlorides and bromides. The result are rationalized in terms of different timing between nucleophile–sulphur bond making and sulphur–halogen bond-breaking on changing substituent, nucleofuge, and solvent. The application of the More O'Ferrall potential energy surface model is consistent with an SN2-type mechanism which can shift towards an SN1 or SAN process depending on different factors. On the other hand, the VBCM model, recently proposed by Pross, makes predictions which are in conflict with the experimental results. A comparison between some solvolysis reactions of benzoyl chlorides and thiophene-2-sulphonyl chlorides indicate that there is close analogy between these two electrophilic centres.

Nucleophilic substitution at sulphonyl sulphur part 3. Hydrolysis of substituted thiophene‐2‐sulphonyl chlorides catalysed by silver nitrate and silver nitrite salts

AbstractHydrolysis reactions of substituted thiophene‐2‐sulphonyl chlorides (5‐methyl, 5‐H, 5‐chloro, 5‐nitro) catalysed by silver nitrate and silver nitrite salts have been studied in water at 25°. Salt effects by potassium nitrate, sodium perchlorate and mercuric bromide have also been investigated. For the catalysis effected with silver nitrate, the pseudo‐first order rate constants depend on the first power of silver ion concentration, while nitrate anion is not involved in the transition state. The corresponding curved Hammett plot suggests a transition state with a partially developed sulphonylium character. With silver nitrite, the rate dependence on both silver and nitrite ion concentrations would indicate that silver is involved in the transition state as well as nitrite. Also, in this case the curved Hammett plot obtained suggests a partial positive charge on the sulphur atom in the transition state. A comparison with uncatalysed hydrolysis reactions would support a previous interpretation that thiophene‐2‐sulphonyl chlorides hydrolyze by an SN2 type mechanism which can shift toward an SN1 or an SAN process depending on the ring substituent.

Hydrolysis of α-cyanobenzylideneanilines. Part 2. Kinetic studies in basic media

The base-catalysed hydrolysis of α-cyanobenzylideneanilines (I) was studied in aqueous dioxan (40 and 50% v/v) at 30°C. The rate of hydrolysis was first-order with respect to hydroxide ion. The plot of the logarithm of second-order rate constants for (I) with substituents on the C-phenyl ring against the σ value was linear (ρ 2.10), whereas that for (I) with substituents on the N-phenyl ring was concave. These results were compared with those for the reaction of (I) with ethoxide ion, where similar substituent effects were observed. These results were interpreted in terms of a mechanism involving tetrahedral addition intermediate, where the attack of hydroxide ion on the substrate is probably rate-determining. The curved Hammett plot cannot be ascribed to a change in mechanism, but seems to result from some other factors.

Studies on the decomposition of tetra-alkylammonium salts in solution. Part III: Interpretation of Hammett correlations

Hammett free energy correlations are reported for the decomposition reactions of tetra-alkylammonium halides in chloroform and acetone solution. All the salts contained a substituted benzyl group attached to the nitrogen atom and decomposed to yield a benzyl iodide and dimethylaniline as the only products.The curved Hammett plots for para-substituted benzyl compounds are compared to the linear plot found previously for a series of salts each containing a para-substituted phenyl group, and it is concluded that curved Hammett plots cannot be used as evidence for a change of mechanism, unless direct conjugative effects between substituent and reaction site are absent.

Substituent effect on lysozyme-catalysed hydrolysis of some beta-aryl di-N-acetylchitobiosides.

Measurements are reported on the kinetics of the lysozyme-catalysed hydrolysis of several beta-aryl di-N-acetylchitobiosides, some of which have been synthesized for the first time. The catalytic rate constants (k(cat.)) at 45 degrees yield a curved Hammett plot (concave up) and the plot of DeltaH(double dagger) versus DeltaS(double dagger) has a sharp break. Substrates with electron-withdrawing groups exhibit a kinetic deuterium isotope effect (k(H) (cat.)/k(D) (cat.)), whereas those with electron-donating groups show no such isotope effect. The results suggest the operation of different mechanisms for the two types of substrates.

Kinetics of the neutral and alkaline hydrolysis of aromatic sulphonyl chlorides in water

The hydrolyses of various substituted benzenesulphonyl chlorides (substituents: p-H,o-NO2,m-NO2,p-NO2,p-F,p-Me,p-OMe) in water in the pH range 3–11 have been studied. Rate coefficients and activation parameters have been determined for the neutral water (solvolysis) reaction and the alkaline hydrolysis reaction. The data are in agreement with an SN2 mechanism for both reactions. In solvolysis, bond-making and bond-breaking are comparable in the transition state, but in alkaline hydrolysis bond-formation predominates. The alkaline hydrolysis rates follow the Hammett equation with a ρ-value of +1·564. The solvolysis rates give a curved Hammett plot.

Kinetic isotope effects. Part 2.—Rates of abstraction of hydrogen and tritium from acetophenone and some para- and meta-substituted acetophenones in alkaline media

Measurements are reported on the kinetics of the base-catalyzed ionization of acetophenone and a series of para- and meta-substituted acetophenones, some of which cover a temperature range. The bromination and detritiation results at 25° yield curved Hammett plots and the magnitude of the isotope effect can be correlated with the pK of the corresponding benzoic acids (the higher values being associated with the weaker acids), and the rates of ionization. The activation energy data are in line with Wiberg's prediction that “in general, in any series of related reactions, the reaction with the lowest activation energy will have the lowest isotope effect”. The ratio of the Arrhenius factor (AT/AH) is in all case except one close to the theoretical limits set for a three-centre reaction and for para-methoxyacetophenone the value (14) is considerably higher. Curvature of the Arrhenius plot for this compound is also obtained.