Interpretation Of Substituent Effects Research Articles

Experimental and Computational Studies on the Acetate-Assisted C-H Activation of N-Aryl Imidazolium Salts at Rhodium and Iridium: A Chloride Additive Changes the Selectivity of C-H Activation.

Combined experimental and computational mechanistic studies of the reactions of unsymmetrical, para-substituted N-aryl imidazolium salts, L2-R1,R2, at [MCl2Cp*]2 (M = Rh, Ir) in the presence of NaOAc are reported. These proceed via intermediate N-heterocyclic carbene complexes that then allow an internal competition between two differently substituted aryl rings toward C-H activation to be monitored. At 348 K in dichloroethane C-H activation of the aryl with the more electron-withdrawing substituents is generally favored. DFT calculations show similar barriers for proton transfer and dissociative HOAc/Cl- ligand substitution, with proton transfer favoring electron-donating substituents, and ligand substitution favoring electron-withdrawing substituents. Microkinetic simulations reproduce the experimental preference implying that the ligand substitution step dominates selectivity. For several substrates, notably L2-F,OMe and L2-F,H, running the C-H activation reactions at 298 K in the presence of added [Et4N]Cl reverses the selectivity. The greater availability of chloride in solution makes an alternative dissociative interchange ligand substitution mechanism accessible, leaving proton transfer as selectivity determining and so favoring electron-donating substituents. Our results highlight the potential importance of the ligand substitution step in the interpretation of substituent effects and demonstrate how a simple additive, [Et4N]Cl, can have a dramatic effect on selectivity by changing the mechanism of ligand substitution.

Interpretation of substituent effects on 13C and 15N NMR chemical shifts in 6-substituted purines

A range of purine derivatives modified at position 6 of the basic purine skeleton exhibit a variety of biological activities. Several derivatives are used or tested nowadays for pharmacological treatments. The present work aims to analyze the effects of substituents on the electron distribution in the purine core as reflected by NMR chemical shifts. We collected a comprehensive set of experimental NMR data for a variety of 6-substituted purines (-NH(2), -NHMe, -NMe(2), -OMe, -Me, -CCH, and -CN) and determined the molecular and crystal structures of three derivatives (-NHMe, -CCH, and -CN) by X-ray diffraction. The density-functional methods calibrated in our recent study (Phys. Chem. Chem. Phys., 2010, 12, 5126) have been employed to enable understanding of the substituent-induced changes in the NMR chemical shifts of the atoms in the purine skeleton. Analyses of the nuclear shielding using localized molecular orbitals (LMOs), specifically the natural LMOs (NLMOs) and Pipek-Mezey LMOs, were used to break down the values of the isotropic (13)C and (15)N NMR chemical shifts and the chemical shift tensors into the contributions of the individual LMOs. The experimental and calculated trends in the chemical shift of the N-3 atom correlate nicely with the Hammett constants (σ(para)) and the calculated natural charges on N-3, whereas the contributions of the LMOs to the N-1 and C-6 chemical shifts are found to be more complex.

Analysis of Substituent Effects in Naphthalene Skeleton

The dissociation constants of 45 substituted 1- and 2-naphthoic acids in six organic solvents (methanol, ethanol, acetonitrile, dimethylformamide, dimethyl sulfoxide, pyridine) have been used to evaluate the substituent effects in the naphthalene skeleton. The dissociation constants data have been treated traditionally by using Dewar-Grisdale equation and the Taft DSP equation, and the alternative interpretation of substituent effects (AISE) method. Quantitative comparison of substituent effects and their transmission from different positions, combination of inductive and resonance effects from various positions, and effects of solvents are discussed. Best results are predominantly provided by the AISE. Substituent effects are stronger in the case of substituents affecting from ring of the naphthalene skeleton bearing carboxylic group than from ring not bearing carboxylic group, and there are no substantial differences between the individual positions of this ring. The contribution of resonance effects at 4α and 8β positions distinctly changes with the medium. Special quality of substituent H was found in comparison with other substituents presumably due their bulkier character. The principal component analysis (PCA) has been also applied to treat the mean dissociation constants using the single substituent property approach. The dissociation together with reactivity data for other naphthalene derivatives have been tested for comprehensive evaluation using PCA. There was found a large similarity of naphthalene reactivity data tested from the point of view of substituent effects.

Effect of meta-substituents of phenolic antioxidants—proposal of secondary substituent effect

Phenolic antioxidants were studied extensively in the 1960s. During that time, the action mechanism might have seemed to be clarified completely. Based on our recent study, however, there remain some unclear points on the interpretation of substituent effects. In this work, o-methoxyphenols having m-substituent have been investigated in detail to make any m-substituent effect clear. It was found that the m-substituent of an m-substituted o-methoxyphenol may behave as if it were on the p-position. This fact is explained by the suggestion that the m-substituent affects its p-substituent, which then influences its o-position by resonance. That is, an m-substituent on the phenol can affect the antioxidant activity, if the phenol also has an o-substituent. Such a substituent effect is proposed as a “secondary substituent effect”.

Chemometric Analysis of Substituent Effects. IX. Alternative Interpretation of Substituent Effects (AISE) - Orthogonal Model

Continuing the previous communications, the present paper suggests an orthogonal model of alternative interpretation of substituent effects (AISE). Following this approach, substituents are classified into three classes. Those of class II are defined as substituents whose atom bound to the basic skeleton bears a character of nucleophile, i.e. it is able of an intramolecular nucleophilic interaction with the reaction centre. The class III includes substituents whose atom bound to the basic skeleton is electrophilic in nature and can correspondingly interact with the reaction centre too. The class I includes the substituents possessing none of the above-mentioned characteristics. Within the model suggested the substituent effects can be described by a family of three lines corresponding to the substituent classes described with a single substituent constant at the coordinate axis. The validity of the model suggested has successfully been tested by comparison with other correlation equations using 318 reaction series taken from literature. It has been found that the value of the point of intersection at the coordinate axis (the isosubstituent constant) is smaller in processes with electron deficit and grater in those with electron excess in the reaction centre. The slopes of the individual lines in the family of lines (the reaction constants) decrease in the order II > I > III in most cases, the only exception being the processes with direct conjugation between a negatively charged reaction centre and the substituent and some gas phase processes.

Chemometric Analysis of Substituent Effects. X. Unification and Reparametrization of Substituent Constants in Hammett-Type Equations by Means of Alternative Interpretation of Substituent Effects (AISE)

The paper deals with evaluation of validity of substituent scales σm, σp, σp0, σp+, and σp- in Hammett-type equations. Possible reasons of unreliability of these scales and conceivable ways of verification of individual substituent constants are discussed. On the basis of Alternative Interpretation of Substituent Effects (AISE) and four additional conditions, a regression nonlinear model has been suggested which includes the dependence of all above-mentioned substituent scales upon a single interpreting variable - the σi substituent constant. This model has been applied to the 32 most frequent substituents from each scale, i.e. altogether 160 substituent constants taken from literature. The optimization of 17 unknown regression parameters gave the residual standard deviation s = 0.180 and, after excluding 12 remote values, s = 0.061. The analysis of residua proved their standard and statistical appropriateness of the model. The agreement between literature values and predicted values of substituent constants was the best with the σm scale; on the other hand, the largest differences were encountered with the σp+ scale. The found differences above a current reliability limit of substituent constants were due first to unreliability of some literature data and next to solvent effects and (in some cases) specific properties of substituents.

Chemometric Analysis of Substituent Effects. VIII. Alternative Interpretation of Substituent Effects (AISE)

Alternative interpretation of substituent effects (AISE) starts from the presumption that a substituent only possesses a single property described by a single substituent constant. This property is transmitted to the reaction centre by three different ways depending on the interaction type in the triad reaction centre - basic skeleton - substituent. For interpretation it is substantial whether or not the substituent has p electrons at the atom adjacent to the basic skeleton. If it has none, the substituent belongs to class I and operates only by its basic effect described by the mentioned single substituent constant. Substituents of class II possess a free electron pair at the atom adjacent to the basic skeleton, and those of class III have a multiple bond between the first and the second atoms which is polarized in the direction from the basic skeleton. Substituent effects in class I are described by a substituent constant identical with σI constant. Substituents in classes II and III show additional effects proportional to the same constant. Hence, a separate treatment of substituent effects in the individual classes provides three straight lines intersecting in a common point. Mathematically, the description of substituent effects in this approach is expressed by a family of lines with a single explaining variable. The point of intersection, which is referred to as the iso-effect point, is not identical with the classic standard substituent - hydrogen - but is near to CN substituent. The approach given has the advantage of adopting a single substituent constant whose scale can be adjusted relatively precisely. Its drawback (like in the case of the correlation equations derived from the principle of separation of substituent effects) lies in a more extensive set of substituents needed for a correlation. The AISE principle has been applied to 318 series of experimental data describing effects of 32 substituents in a large variety of chemical models (aliphatic, alicyclic, aromatic, heteroaromatic, with or without direct conjugation between reaction centre and substituent) in both chemical reactions and equilibria. A comparison with two other correlation relations with two and three substituent constants for interpretation of substituent effects based on the principle of separation of the individual substituent effects showed that the closeness of AISE based correlations is comparable with that of the correlation equations currently used. It was somewhat less successful in the models with direct conjugation between reaction centre and substituent but the AISE principle can be used even in these cases.

Kinetic analysis of a generalized catalytic selective oxidation reaction

A kinetic model is presented for a generalized catalytic selective oxidation reaction in which lattice oxygen is the proximate source of oxygen. The model takes into account the effects on the overall kinetics due to finite residence time of intermediates on the surface, diffusional rate of lattice oxygen, and overall stoichiometry of the reaction. It is shown that except under the condition when adsorption of the hydrocarbon reactant is the slow step, the rate expression derived from this model differs from that obtained with a model in which these effects are neglected. One important consequence of the model is that the rate of consumption of hydrocarbon can be changed if the selectivity of the reaction is changed, even when reoxidation of the catalyst is the rate-limiting step. In such a circumstance, the rate might show an order dependence in oxygen partial pressure, the apparent activation energy reflects that of reoxidation and the temperature dependence of the selectivity, and the apparent preexponential factor is also influenced by the stoichiometry of the reaction. The implications of these to the interpretation of substituent effects and deuterium kinetic isotope effects are discussed.

Polar radicals. 15. Interpretation of substituent effects on the mechanism of electrolytic reduction of the carbon-halogen bond in series of substituted benzyl halides

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTPolar radicals. 15. Interpretation of substituent effects on the mechanism of electrolytic reduction of the carbon-halogen bond in series of substituted benzyl halidesDennis D. Tanner, James A. Plambeck, Darwin W. Reed, and Thomas W. MojelskyCite this: J. Org. Chem. 1980, 45, 25, 5177–5183Publication Date (Print):December 1, 1980Publication History Published online1 May 2002Published inissue 1 December 1980https://doi.org/10.1021/jo01313a031RIGHTS & PERMISSIONSArticle Views220Altmetric-Citations30LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (894 KB) Get e-Alerts Get e-Alerts

Carbon-13 Chemical Shifts of the Carbonyl Group. II. A Correlation with CO Pi-Bond Polarity

The 13C chemical shift of the carbonyl group in molecules of the type XCOY is considered in terms of the theory for the local paramagnetic term σpAA for 13C shifts presented by Karplus and Pople. On the assumption of (i) an invariant, nonpolar sigma network, (ii) a constant average excitation energy, and (iii) neglect of neighbor anisotropy and intramolecular dispersion effects, a formula is derived which relates the chemical shifts to the polarity of the carbonyl pi bond. When the limitations imposed by Assumptions (i) and (iii) are minimized by an adjustment based on the 13C shifts of symmetrical disubstituted ethylenes, the resultant semiempirical theory predicts that the ``corrected'' carbonyl shifts should correlate with the CO pi-bond polarity, or substituent parameters to which the latter is related. The experimental data are found to correlate with Taft's reactivity parameter σI, in accord with the interpretation of substituent effects in terms of π-bond polarity in the carbonyl group.