Electrophile common types

A full description of how a reaction occurs is called its mechanism. There are two general kinds of mechanisms by which reactions take place radical mechanisms and polar mechanisms. Polar reactions, the more common type, occur because of an attractive interaction between a nucleophilic (electron-rich) site in one molecule and an electrophilic (electron-poor) site in another molecule. A bond is formed in a polar reaction when the nucleophile donates an electron pair to the electrophile. This movement of electrons is indicated by a curved arrow showing the direction of electron travel from the nucleophile to... 

Electrophilic addition of sulfenyl compounds at carbon-carbon double bonds, extensively studied and reviewed2,4 715 106, finds numerous synthetic applications owing to the regio- and stereoselectivity of the addition26. The most common types of agents for the electrophilic addition of sulfur to double and triple bonds are sulfenyl halides (RSX,... 

Electrophiles, the most common type of reactive intermediate, are molecules with an electron-deficient atom, so having a full or partial positive charge. 

Carbon-centered electrophiles are compounds or intermediates which are electron poor and thus capable of accepting electrons from electron donors. To be an electron acceptor, an electrophile must have an unfilled orbital on carbon available for overlap with a filled orbital of the donor. Unfilled atomic p orbitals or antibonding orbitals (both a and it ) are the most common types of acceptor orbitals. The most common carbon electrophiles fall into four major categories ... 

Skin cancer is the most common type of cancer. Damage to skin DNA from sunlight is the most common cause of skin cancer. This causes mutations that result in formation of cancer cells and that suppress the immune responses that normally prevent replication of such cells. The class of chemicals most commonly associated with causing skin cancer are the polycyclic aromatic hydrocarbons from sources such as coal tar. These can be metabolized to electrophilic substances that bind with DNA to initiate cancer (see Figures 7.3 and 8.3). Arsenic in drinking water has been established as a cause of precancerous lesions, called arsenical keratoses, and squamous cell carcinoma of skin. 

Aromatic compounds are generally stable and they do not react like alkenes. They prefer to undergo reactions in which the stable aromatic ring is retained. The most common type of reaction for aromatic rings is electrophilic substitution, but reduction is also possible. 

Nucleophiles are either negatively charged or neutral species with a pair of electrons in a high energy filled orbital that they can donate to electrophiles. The most common type of nucleophile has a nonbonding lone pair of electrons. Usually these are on a heteroatom such as O, N, S, or P. 

Conversion of a 1,2-diamine into a pyrazine ring is a common type of reaction (see Section 6.3.1.). In this dinucleophile -I- dielectrophile approach, heteroatoms function as nucleophiles and 1,2-dicarbonyl or related compounds, as the electrophilic component. The difference in relative reactivities at the termini of the individual components may influence the formation of a single product alternatively, the relative reactivities may be so similar that a mixture of isomers is formed. 

Common types of radicals that add to n bonds are those that can be generated from alkyl halides, mercaptans, thiophenols, thioacids, aldehydes, and ketones. Like the corresponding electrophilic additions to double bonds, many radical additions are either regiospecific or highly regioselective. 

This commoner type of reaction involves the attack of carbon or heteroatom nucleophiles onto carbonyl compounds, by direct or conjugate addition, and onto imines. Because we have just dealt with boranes we shall start with the reaction of allylic boron compounds with such electrophiles. You might strictly not call this nucleophilic attack. 

Substitution at halogen is another common type of reaction (3.66). Even when phosphorus enjoys a full outer octet of electrons, it may accept more and show electrophilic behaviour (3.67) and (3.68). 

The second set of examples involves the use of thionium ions as electrophiles in inter- and intramolecular processes to obtain a-substituted sulfides (see 24 25, Scheme 20.7T which is the most common type of Pummerer reaction. Applications of this classical Pummerer rearrangement are exemplified in the synthesis of trans-solamin, the synthesis of indolizidine alkaloids, and the synthesis of the CDE ring of erinacine E. The first exanple fScheme 20.10 uses Pummerer chemistry in the generation of a thionium ion, which reacts in an intermolecular tin-mediated ene reaction the second one fScheme 20.11 uses Pummerer chemistry to introduce a nitrogen-containing heterocycle by intramolecular addition to form the coniceine core and the third example fScheme 20.12 is an intramolecular silicon-induced Pummerer reaction with oxygenated nucleophiles applied to the synthesis of a precursor of erinacine. Details of these Pummerer-based strategies are discussed below. 

A common type of reaction with alkenes in which an electrophilic species adds to a ir bond. 

We study several common types of electrophiles, how each is generated, and the mechanism by which it replaces hydrogen on an aromatic ring. 

Most of the compounds in this class have been prepared from preexisting crown ether units. By far, the most common approach is to use a benzo-substituted crown and an electrophilic condensation polymerization. A patent issued to Takekoshi, Scotia and Webb (General Electric) in 1974 which covered the formation of glyoxal and chloral type copolymers with dibenzo-18-crown-6. The latter were prepared by stirring the crown with an equivalent of chloral in chloroform solution. Boron trifluoride was catalyst in this reaction. The polymer which resulted was obtained in about 95% yield. The reaction is illustrated in Eq. (6.22). 

---