Electrophiles summary

Section 12 1 On reaction with electrophilic reagents compounds that contain a ben zene ring undergo electrophilic aromatic substitution Table 12 1 m Section 12 1 and Table 12 3 m this summary give examples... 

A Summary of Substituent Effects in Aromatic Substitution A summary of the activating and directing effects of substituents in electrophilic aromatic substitution is shown in Table 16.2. 

Substituent effect, additivity of, 570 electrophilic aromatic substitution and, 560-563 summary of. 569 Substitution reaction, 138 Substrate (enzyme), 1041 Succinic acid, structure of, 753 Sucralose, structure of. 1006 sweetness of, 1005 Sucrose, molecular model of. 999 specific rotation of, 296 structure of, 999 sweetness of, 1005 Sugar, complex, 974 d, 980 L, 980... 

We will give a brief summary of the general principles of diffusion and preassociation in electrophilic substitutions before discussing the values in Table 3-1 (for more comprehensive reviews see North, 1964 and Ridd, 1978). A precursor A (HN02 in... 

These reactions in weakly alkaline solutions are faster than the heterolytic (Dn + AN)-like hydroxy-de-diazoniation, which, for most diazonium ions, (depending on their electrophilicity), is dominant below pH 2-4. As shown by Ishino et al. (1976), an increase in rate, corresponding to the occurence of other mechanisms in addition to the heterolytic hydroxy-de-diazoniation, is observable at pH 3.7-7.0. The increase is dependent on the substituent in the specifically substituted benzenediazo-nium ion. The slope d(log )/d(pH) was found to be in the range 0.22-1.09 (see summary of the work of Ishino et al. by Zollinger, 1983, p. 624). 

In summary then, the kinetics and related data are most consistent with protonated acetyl nitrate as the reagent in this medium. It is unfortunate that there is doubt as to the nature of the electrophile, as this medium combines high reactivity with good solvent properties, which has made it popular for studying substituent effects in nitration. Some relative reactivities (mostly obtained under competition conditions) are given in Table 20. 

A summary of electrophilic additions to triple bonds and allenes involving a vinyl cation is given in Table IV. 

Summary of Electrophilic Additions to Acetylenes and Allenes Involving Vinyl Cations... 

For a summary and detailed discussion of the stereochemistry of electrophilic additions to alkenes and alkynes see R. C. Fahey, in Topics in Stereochemistry, Vol. 3,... 

In summary, then, the orientation of electrophilic thallation can be controlled by an appropriate manipulation of reaction conditions. Under conditions of kinetic control, ortho substitution results when chelation of the electrophilic reagent (TTFA in the studies described above) with the directing substituent permits intramolecular delivery of the electrophile, and para substitution results when such capabilities are absent this latter result is an expression of the very large steric requirements of the bulky thallium electrophile. Under conditions of thermodynamic control, however, meta substitution is observed. 

In summary, the same factors that operate in the electrophile, namely steric, chelation, and polar effects, govern facial selectivity for enolates. The choice of the Lewis acid can determine if the enolate reacts via a chelate. The final outcome depends upon the relative importance of these factors within the particular TS. 

In summary, the apparent redox modulation of lycopene certainly affects two important redox sensitive transcription factors at higher concentrations of lycopene. However, electrophilic lycopene oxidation products cannot be ruled out as the major activators and the activation may be due to specific molecular interactions. 

In summary, transifion-metal-catalyzed alkene-polymerization reactions highlight the metal-induced electrophilic activation of C—C n bonds to form carbo-cation-like alkene complexes. Considerations involving substituent pi-donor or pi-acceptor strength (i.e., tendency toward carbocation formation) will be useful in similarly rationalizing polymerization reactions (4.105) for more general alkenes. 

In summary, we have shown that stable cationic charge centers can significantly enhance the reactivities of adjacent electrophilic centers. Most of the studied systems involve reactive dicationic electrophiles. A number of the reactive dications have been directly observed by low temperature NMR. Along with their clear structural similarities to superelectrophiles, these dicationic systems are likewise capable of reacting with very weak nucleophiles. Utilization of these reactive intermediates has led to the development of several new synthetic methodologies, while studies of their reactivities have revealed interesting structure-activity relationships. Based on the results from our work and that of others, it seems likely that similar modes of activation will be discovered in biochemical systems (perhaps in biocatalytic roles) in the years to come. 

In summary, the Danheiser reactions are a family of [3 + 2] annulations that take place between allenylsilanes and diverse electrophiles. The products can be carbo-cycles or heterocycles. In all cases, the annulations proceed most efficiently when the allenylsilane has a non-hydrogen substituent on the carbon atom bearing the silicon. This is a consequence of the common mechanistic pathway that proceeds through a vinyl carbocation intermediate. 

In summary (Scheme 15), 2-lithiopiperidines and 2-lithiopyrrolidines appear to be very versatile nucleophiles for the elaboration of these heterocyclic systems, affording a variety of 2-substituted heterocycles in excellent yields. The stereoselectivity of the reaction is near 100% in the piperidine series with most carbonyl electrophiles (retention of configuration) and alkyl halides (inversion of configuration). In the pyrrolidine series, the selectivity is also near 100% with carbonyl electrophiles (retention), but less selective (inversion predominates) with alkyl halides (less problematic with Af-aUylpyrrolidines). 

This short summary has aimed to highlight a few of the more important aspects of the orientation of electrophilic substitution of pyridines and their benzo analogues. Strictly, reactions that involve metallation could be treated under this heading but they will be considered as involving a nucleophilic attack at a ring hydrogen (see Section 2.05.5). Electrophilic cyclizations of a substituent on to a pyridine will be mentioned briefly, but Chapter 2.06 should be consulted for those reactions. 

When substituted benzene undergoes electrophilic attack, groups already on the ring affect the reactivity of the benzene ring as weU as the orientation of the reachon. A summary of these effects of substituents on reachvity and orienta-hon of electrophihc substituhon of substituted benzene is presented below. 

Summary This paper has reviewed recent developments in the synthesis of telechelic and semitele-chelic polymers via anionic methods. The two anionic approaches (electrophilic termination and functional initiation) to the synthesis of these materials were discussed. The advantages of anionic methods (e.g.,... 

Figure 6.1. Summary of the most common electrophilic addition reactions of olefins. In each case, the olefin reacts as a Lewis base. All reactions are regioselective. The overall stereochemistry is (a) stereospecific anti (b) stereospecific syn (c) not stereospecific, in general.

It is well known that not all attempts to explain the reactivity of individual positions in electrophilic substitution reactions have been successful. There are three main lines along which attempts have been made to remove discrepancies between theory and experiment (for a summary, see ref. 147) (1) introduction into the HMO treatment of additional empirical parameters (inductive effect), (2) invoking the addition-elimination mechanism, and (3) invoking different reactivity of the protonated and unprotonated forms. 

In the above examples, the nucleophilic role of the metal complex only comes after the formation of a suitable complex as a consequence of the electron-withdrawing effect of the metal. Perhaps the most impressive series of examples of nucleophilic behaviour of complexes is demonstrated by the p-diketone metal complexes. Such complexes undergo many reactions typical of the electrophilic substitution reactions of aromatic compounds. As a result of the lability of these complexes towards acids, care is required when selecting reaction conditions. Despite this restriction, a wide variety of reactions has been shown to occur with numerous p-diketone complexes, especially of chromium(III), cobalt(III) and rhodium(III), but also in certain cases with complexes of beryllium(II), copper(II), iron(III), aluminum(III) and europium(III). Most work has been carried out by Collman and his coworkers and the results have been reviewed.4-29 A brief summary of results is relevant here and the essential reaction is shown in equation (13). It has been clearly demonstrated that reaction does not involve any dissociation, by bromination of the chromium(III) complex in the presence of radioactive acetylacetone. Furthermore, reactions of optically active... 

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