Electrophile nucleophile

Theoretical reactivity indices of heteroaromatic systems distinguish reactivity toward electrophilic, nucleophilic and homolytic reactions. 

Cyclocondensation reactions starting from two components are possible only when both of them have two reactive centers An initial electrophilic-nucleophilic interaction yielding a linear product is followed by a second electrophilic-nucleo-... 

This is the reverse of the first step in the SnI mechanism. As written here, this reaction is called cation-anion recombination, or an electrophile-nucleophile reaction. This type of reaction lacks the symmetry of a group transfer reaction, and we should therefore not expect Marcus theory to be applicable, as Ritchie et al. have emphasized. Nevertheless, the electrophile-nucleophile reaction possesses the simplifying feature that bond formation occurs in the absence of bond cleavage. 

Some electrophile-nucleophile reactions are guided more by orbital interactions than by electrostatics. The key interaction involves the donor orbital on the nucleophile, i.e., the highest-occupied molecular orbital (HOMO). Examine the HOMO of enamine, silyl enol ether, lithium enolate and enol. Which atom is most nucleophilic, i.e., which site would produce the best orbital overlap with an electrophile ... 

The azinones and their reaction characteristics are discussed in some detail in Section II, E. Because of their dual electrophilic-nucleophilic nature, the azinones may be bifunctional catalysts in their own formation (cf. discussion of autocatalysis below) or act as catalysts for the desired reaction from which they arise as byproducts. The uniquely effective catalysis of nucleophilic substitution of azines has been noted for 2-pyridone. 

What about the second reactant, HBr As a strong acid, HBr is a powerful proton (H+) donor and electrophile. Thus, the reaction between HBr and ethylene is a typical electrophile-nucleophile combination, characteristic of all polar reactions. 

A Grignard reaction begins with an acid-base complexation of Vfg2+ to the carbonyl oxygen atom of the aldehyde or ketone, thereby making the carbonyl group a better electrophile. Nucleophilic addition of R then produces a tetrahedral magnesium alkoxide intermediate, and protonation by addition of water... 

This chapter has taken the reader through a number of microwave-assisted methodologies to prepare and further functionalize 2-pyridone containing heterocycles. A survey of inter-, intramolecular-, and pericyclic reactions together with electrophilic, nucleophilic and transition metal mediated methodologies has been exemplified. Still, a number of methods remain to be advanced into microwave-assisted organic synthesis and we hope that the smorgasbord of reactions presented in this chapter will inspire to more successful research in this area. 

The Hammett equation has also been shown to apply to many physical measurements, including IR frequencies and NMR chemical shifts. The treatment is reasonably successful whether the substrates are attacked by electrophilic, nucleophilic, or free-radical reagents, the important thing being that the mechanism be the same within a given reaction series. 

In alternant hydrocarbons (p. 55), the reactivity at a given position is similar for electrophilic, nucleophilic, and free-radical substitution, because the same kind of resonance can be shown in all three types of intermediate (cf. 20,22, and 23). Attack at the position that will best delocalize a positive charge will also best delocalize a negative charge or an unpaired electron. Most results are in accord with these predictions. For example, naphthalene is attacked primarily at the 1 position by NOj, NHJ, and Ph, and always more readily than benzene. 

The thiophene ring can be elaborated using standard electrophilic, nucleophilic, and organometallic chemistry. A variety of methods have been developed to exploit the tendency for the thiophene ring (analogous to that of furan and pyrrole) to favor electrophilic substitution and metallation at its a-carbons. Substitution at the p-carbons is more challenging, but this problem can also be solved by utilizing relative reactivity differences. 

We mentioned before that we need to consider four factors when choosing whether a reaction will go by an SnI or Sn2 mechanism. These four factors are electrophile, nucleophile, leaving group, and solvent. We will go through each factor one at a time, and we will see that the difference between the two mechanisms is the key to understanding each of these four factors. Before we move on, it is very important that you understand the two mechanisms. For practice, try to draw them in the space below without looking back to see them again. 

Hydroxide is the nucleophile Water is the nucleophile Water is the nucleophile MeCl is the electrophile Nucleophile Base... 

In the present study the dimer (salen)CoAlX3 showed enhanced activity and enantioselectivity. The catalyst can be synthesized easily by readily commercially available precatalyst Co(salen) in both enantiomeric forms. Potentially, the catalyst may be used on an industrial scale and could be recycled. Currently we are looking for the applicability of the catalyst to asymmetric reaction of terminal and meso epoxides with other nucleophiles and related electrophile-nucleophile reactions. 

This electrophile/nucleophile dichotomy can be looked upon as a special case of the acid/base idea. The classical definition of acids and bases is that the former are proton donors, and the latter proton acceptors. This was made more general by Lewis, who defined acids as compounds prepared to accept electron pairs, and bases as substances that could provide such pairs. This would include a number of compounds not previously thought of as acids and bases, e.g. boron trifluoride (39),... 

Addition reactions, too, can be electrophilic, nucleophilic or radical in character, depending on the type of species that initiates the process. Addition to simple carbon-carbon double bonds is normally either electrophile-, or radical-, induced e.g. addition of HBr,... 

Correlation between electrophilicity-nucleophilicity and Lewis acidity-basicity ... 

The nitrogen atoms in ADC compounds are highly electrophilic. Nucleophilic attack on nitrogen is easy, and as with electrophilic acetylenes, such as dimethyl acetylenedicarboxylate, it seems likely that some cycloaddition reactions of ADC compounds with unsymmetrical substrates proceed via a stepwise mechanism. PTAD is a powerful electrophile, although TCNE is more reactive, and chlorosulfonyl isocyanate is more reactive still.58... 

Arenetellurenyl halides are dihalogen-like electrophiles nucleophiles coordinate with tellurium and generate in many cases linear Y-Te-X arrangements (Y=0, S, Se, Te nucleophile donor atom, X=C1, Br, I). Examples with Se- or Te-containing ligands are not so numerous.9,83... 

Of these, only the contact ion pair is the critical precursor to electrophile/ nucleophile interactions (see, for example, Hughes et al., 1933 Hughes and Ingold, 1935 Ingold, 1969 Cordes and Dunlap, 1969 Kessler and Feigel, 1982 Troughton et al., 1984 for the microscopic reverse, see Winstein et al., 1954 Winstein and Robinson, 1958 Shiner, 1970 Harris,... 

However, in contrast to such whimsical and playful description, some metaphors developed into serious definitions that became essential to rigorous chemical classification and explanation. Indeed, the electrophilic/nucleophilic language is an example from the development of chemical theory which is the subject of later chapters of this book. Let us consider three other examples of metaphor-tumed-convention that dominated eighteenth- and nineteenth-century chemistry. 

The discussion and classification of reagents is masterful in identifying Ingold s new nomenclature and principles with more widely known oxidation-reduction and acid-base theory. The 1953 lectures at Cornell University, published as Structure and Mechanism in Organic Chemistry, follow this same strategy, showing how old classification schemes overlap with each other and how apparent inconsistencies disappear as old schemes are incorporated into the new one. Nineteenth-century Berzelian electrochemical dualism, revived by Lapworth and Robinson in the cationic/anionic schema, disappears into the electrophilic/nucleophilic language. 

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