Electrophilic addition index

CONTENTS Preface, Brian Halton. Matrix-Isolation of Strained Three-Membered Ring Systems, Wolfram Sander and Andreas Kirschfield. Synthetic Utility of Cyclobutanones, Harold W. Moore and Benjamin R. Xerxa. Electrophilic Additions to Bicyclo [1.1.0] butanes, Manfred Christl. From Spiro-pentanes to Linear and Angular Oligo- and Polytriangulanes, Armin de Meijere and Sergei Kozhushkov. [N] Staffanes, Piotr Kaszynski and Josef Michl. Index. ... 

A new structure-activity relationship, Xjj =yS+l, where y is a negative constant, S is the total steric effect, and 4 is the total inductive effect, correlated strongly with available measurements of ozonolysis. New rate coefficients were measured for ozonolysis of a number of unsaturated heteroatomic compounds and it has been emphasized that the inductive effect rather than the steric effect is important in predicting their reactivity %, the inductive effect index, was compared with the Taft a constant and rates of reaction of hydroxyl radical with a given species it correlated strongly in both cases (which should be unaffected by steric factors) suggesting a universal response by olefinic species towards electrophilic addition. ... 

In addition to the above prescriptions, many other quantities such as solution phase ionization potentials (IPs) [15], nuclear magnetic resonance (NMR) chemical shifts and IR absorption frequencies [16-18], charge decompositions [19], lowest unoccupied molecular orbital (LUMO) energies [20-23], IPs [24], redox potentials [25], high-performance liquid chromatography (HPLC) [26], solid-state syntheses [27], Ke values [28], isoelectrophilic windows [29], and the harmonic oscillator models of the aromaticity (HOMA) index [30], have been proposed in the literature to understand the electrophilic and nucleophilic characteristics of chemical systems. 

An attempt has been made to analyse whether the electrophilicity index is a reliable descriptor of the kinetic behaviour. Relative experimental rates of Friedel-Crafts benzylation, acetylation, and benzoylation reactions were found to correlate well with the corresponding calculated electrophilicity values. In the case of chlorination of various substituted ethylenes and nitration of toluene and chlorobenzene, the correlation was generally poor but somewhat better in the case of the experimental and the calculated activation energies for selected Markovnikov and anti-Markovnikov addition reactions. Reaction electrophilicity, local electrophilicity, and activation hardness were used together to provide a transparent picture of reaction rates and also the orientation of aromatic electrophilic substitution reactions. Ambiguity in the definition of the electrophilicity was highlighted.15... 

Density functional theory (DFT) provides an efficient method to include correlation energy in electronic structure calculations, namely the Kohn-Sham method 1 in addition, it constitutes a solid support to reactivity models.2 DFT framework has been used to formalize empirical reactivity descriptors, such as electronegativity,3 hardness4 and electrophilicity index.5 The frontier orbital theory was generalized by the introduction of Fukui function,6 and new reactivity parameters have also been proposed.7,8 Moreover, relationships between those parameters have been found, and general methods to relate new quantities exist.9... 

From the relation between Fukui function and local softness, electrophilic and nucleophilic local softnesses can be computed. Donor and acceptor sites can also be identified by large values of both types of local softnesses in addition, it can be used to compare sites of different molecules and to identity which one is softer or harder. The elec-trophilicity index can also be extended to a local context,21 and a comparison of the electrophilicity of sites in different species can be made. 

The global electrophilicity index to, which measures the stabilization in energy when the system acquires an additional electronic charge AN from the environment, has been given the following simple expression 21... 

A quantitative relationship between Hammett substituent constant (a) for substituted ethylene and the global electrophilicity index has been found.122 Therefore, it is not surprising to find a good correlation between In k for the addition of HOCH2CII2S to substituted a-nitrostilbenes reported by Bernasconi et a/.116 and the global electrophilicity index co, as shown in Figure 4. 

Figure 4 Plot of ln( ) vs the electrophilicity index a> for the reaction of the addition of HOCH2CH2S to substituted a-nitrostilbenes. Rate coefficients k from reference 116...

In this chapter, we have reviewed the usefulness of the global and local electrophilicity indexes to quantitatively account for the reactivity and selectivity patterns observed in a large series of classical organic reactions. The global electrophilicity index, w, categorizes within an unique absolute scale the propensity of the electron acceptors to acquire additional electronic charge from the environment. This classification allowed an impressive number of systems in DA reactions to be rationalized in terms of their reaction mechanisms in polar and nonpolar processes. The global electrophilicity scale provides a simple way to assess the more or less polar character of a process on the... 

The electrophilicity index also accounts for the electrophilic activation/deactivation effects promoted by EW and electron-releasing substituents even beyond the case of cycloaddition processes. These effects are assessed as responses at the active site of the molecules. The empirical Hammett-like relationships found between the global and local electrophilicity indexes and the reaction rate coefficients correctly account for the substrate selectivity in Friedel-Crafts reactions, the reactivity of carbenium ions, the hydrolysis of esters, the reactivity at the carbon-carbon double bonds in conjugated Michael additions, the philicity pattern of carbenes and the superelectrophilicity of nitronium, oxonium and carboxonium ions. This last application is a very promising area of application. The enhanced electrophilicity pattern in these series results from... 

Beyond the present applications of the electrophilicity index in organic chemistry, there are some others that are being considered in our group. For instance, based on the encouraging results that follow from the comparison between the electrophilicity index and the reaction rate coefficients, it is expected that quantitative comparisons between the electrophilcity index and the Hammett substituent constants may open the possibility of having new theoretically predicted a values that have not been yet experimentally determined. Consider for instance the case of multiple substitutions where the additivity rules may not apply. 

Using their CNDO results Helland and Skancke calculated indices of reactivity (frontier electron density, FED, for electrophilic substitution frontier orbital density, FOD, for nucleophilic substitution frontier radical density, FRD, for radical substitution) for the thienopyridines. It was indicated that the FED index has its highest value for C-3 in the [2,3-]- and 3,2- -fused systems and for C-2 in the [3,4-1-fused isomers. As far as the former group is concerned, the predictions are in agreement with experimental observations (see Section IV,A.). Little experimental evidence is available for the [3,4-]-fused systems, but it seems highly probable that they would have a considerable tendency to undergo addition reactions at the 1,3-positions, since the product would contain a normal rather than a quinoid pyridine ring [Eq. (23)]. 

Schoeller and Brinker applied single electron perturbation theory to carbene/ alkene additions and elaborated a construct that was almost indentical to our FMO analysis. [74] Schoeller also focused on estimating LUMO(carbene)/ HOMO(alkene) and LUMO(alkene)/HOMO(carbene) differential energies, and derived a selectivity index , S, which roughly paralleled and predicted the electrophilicity of the dihalocarbenes and the nucelophilicity of (e.g.), C(NH2)2, C(0H)2, and C(SMe)2. [74]... 

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