Electrophilic addition conjugated systems

As conjugated systems with alternating TT-charges, the polymethine dyes are comparatively highly reactive compounds (3). Substitution rather than addition occurs to the equalized TT-bond. If the nucleophilic and electrophilic reactions are charge-controHed, reactants can attack regiospeciftcaHy. 

Addition to conjugated systems can also be accomplished by any of the other three mechanisms. In each case, there is competition between 1,2 and 1,4 addition. In the case of nucleophilic or free-radical attack, the intermediates are resonance hybrids and behave like the intermediate from electrophilic attack. Dienes can give 1,4 addition by a cyclic mechanism in this way ... 

For N-methyl arylamines, electrophilic N-sulfonyloxy esters appear to be strong candidates for the ultimate carcinogenic metabolites. However, additional studies are required as these conclusions are circumstantially based on their comparative reactivity with nucleophiles and on the failure of other metabolic conjugation systems to esterify N-hydroxy-N-methyl arylamines (9,187). 

Instead of a triflate, the electrophile on the glycosyl acceptor can be an a,(3-unsaturated carbonyl group. This is the case reported in Fig. 25, in which a stereoselective Michael addition of the 1-thiosugar 56 to the a,(3-conjugated system of levoglucosenone 57, generated after deprotection a couple of L-fucopyranosyl-4-thiodisaccharides 61 and 62 presenting inhibitory activity on a-L-fucosidase.54... 

In principle, pyrylium salts can be transformed to pyrans by the addiiion of an appropriate reagent to the 2-, 4-, or 6-position of the heterocycle. Although most transformations seem to be nucleophilic additions (because of the electrophilic character of the pyrylium substrates) some reactions, such as hydrogenation or one-electron reductions, are of a radical nature. Many nucleophilic additions to pyrylium salts proceeding via unstable pyran intermediates seem to be of general synthetic interest, especially in the fields of aromatic and other conjugated systems, l97 200 but are of little interest in pyran chemistry. 

Cyclic addition reactions of 16a with diynes or cyanamide are interpreted as being initiated by an electrophilic attack of the cyanamide or the terminal carbon atom of a C=C bond. Both result in ring expansion and the formation of 2,4-As,As-diphosphapyridine 39 <1998ZNB443> or l,3-A5,A5-diphosphabenzenes 40, respectively (Scheme 10). The diyne reaction is not limited to conjugated systems. 1,5-Hexadiyne and 1,7-octadiyne can be used as well <1997JOM(529)223>. 

Acid catalysts promote conjugate addition of alcohols to a, p unsaturated carbonyl compounds by protonating the carbonyl group and making the conjugated system more electrophilic. Methanol adds to this ketone exceptionally well, for example, in the presence of an acid catalyst known as Dowex 50 . This is an acidic resin—just about as acidic as sulfuric acid in fact, but completely insoluble, and therefore very easy to remove from the product at the end of the reaction by filtration. 

Benzene is aromatic because it has six electrons in a cyclic conjugated system. We know it is aromatic because it is exceptionally stable and it has a ring current and hence large chemical shifts in the proton NMR spectrum as well as a special chemistry involving substitution rather than addition with electrophiles. This chapter and the next are about the very large number of other aromatic systems in which one or more atoms in the benzene ring are replaced by heteroatoms such as N, O, and S. There are thousands of these systems with five- and six-membered rings, and we will examine just a few. 

The positions of the Cl—C, HO—C, and RO—C bonds allows them to conjugate with the Hg—C bond ( cr,cr-conjugation ). Hence, an electrophile (with the latter two substrates), or an agent which can bind Hg-+, or a reagent which combines both properties, such as HCl, will break the conjugated cr-bonds, create at the center of the molecule the new 77-bond, and eliminate olefin or acetylene. This is formally similar to the 1,4-addition occurring in 77,77-conjugated systems. 

Because of the weakness of hydrazoic acid as a potential electrophilic reagent one would expect electrophilic addition of hydrazoic acid only in systems in which a powerful electron-releasing group is conjugated with the double bond. An interesting example of such an... 

The theoretical study by Khan et al. [13] on the stereochemistry of nucleophilic addition of organometallics to unsaturated substrates can also be applied to conjugated systems. Of the three major factors affecting the orientation of the nucleophile to the substrate (see Chapter 16), the ability of the substrate to discriminate between the nucleophilic and electrophilic character of the Grignard reagent is the one most susceptible to influence by conjugation with an adjacent 7r-system. They concluded that the accessible electrophilic site (the metal) is the source of the stereochemical preference for the electron-rich olefin face. 

Electrophilic addition to simple alkenes takes place in such a way as to form the most stable intermediate carbonium ion. Addition to a,j3-unsaturatec arbonyl compounds, too, is consistent with this principle to see that this is so, however, we must look at the conjugated system as a whole. As in the case of conjugated dienes (Sec. 8.20), addition to an end of the conjugated system is preferr jd, since this yields (step 1) a resonance-stabilized carbonium ion. Addition to the carbonyl oxygen end would yield carbonium ion 1 addition to the j3-catbon end would yield carbonium ion II. 

The addition reactions take place at a carbon-carbon multiple bond, or carbon-hetero atom multiple bond. Because of this peculiarity, the addition reactions are not common as the first step in pyrolysis. The generation of double bonds during pyrolysis can, however, continue with addition reactions. The additions can be electrophilic, nucleophilic, involving free radicals, with a cyclic mechanism, or additions to conjugated systems such as Diels-Alder reaction. This type of reaction may explain, for example, the formation of benzene (or other aromatic hydrocarbons) following the radicalic elimination during the pyrolysis of alkanes. In these reactions, after the first step with the formation of unsaturated hydrocarbons, a Diels-Alder reaction may occur, followed by further hydrogen elimination ... 

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