Electrophilic reactions conjugated substitution

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. 

Complementary to the conjugate substitution reaction in which the nucleophile is transferred directly from the tetraalkyl ferrate to the allylic ligand, preformed low-valent Fe complexes can form reactive allyl-iron complexes via an SN2 -type mechanism (path C, Equations (7.8) and (7.9), Scheme 7.16], These complexes react with incoming nucleophiles and electrophiles in a substitution reaction. Depending on the nature ofthe iron complex employed in the reaction, either o- or Jt-allyl complexes are generated. 

Conjugate substitution electrophilic alkenes bearing leaving groups can promote substitution reactions at C=C related to those at C=0 Nucleophilic aromatic substitution electron-poor aromatic rings that allow substitution reactions with nucleophiles rather than the usual electrophiles... 

The chemistry of all three heterocyclic ring systems contains some Su prises. Pyrrole, for example, is both an amine and a conjugated diene, its chemical properties are not consistent with either of these structural features. Unlike most other amines, pyrrole is not basic (Section 24.4) unlike most other conjugated dienes, pyrrole undergoes electrophilic substitution rather than addition reactions. The same is true of furan and thiophene Both react with electrophiles to give substitution products. 

An example of a quantitative SMR study correlating electronic properties and catalytic parameters is provided by the glutathione conjugation of para-substituted l-chloro-2-nitro-benzene derivatives (183). The values of log/j2 (second order rate constant of the nonenzy-matic reaction) and log (enzymatic reaction catalyzed by various glutathione transferase preparations) were correlated with the Hammett resonance cr value of the substrates, a measure of their electrophilicity. Regression equations with positive slopes and values in the range 0.88-0.98 were obtained. These results quantitate the influence of substrate electrophilicity on nucleophilic substitutions mediated by glutathione, be they enzymatic or nonenzymatic. 

Conjugate substitution electrophilic alkenes bearing leaving groups can promote substitution reactions at C=C related to those at C=0... 

In this next section we are going to consider reactions related to conjugate substitutions but in which the double bond is part of an aromatic ring. We spent some considerable time in Chapter 21 explaining that aromatic rings are nucleophilic electrophiles attack them, and typical aromatic reactivity is to undergo electrophilic substitution. 

As indicated in Scheme 1, Pd-catalyzed conjugate substitution can be achieved by the reaction of /3-metallo-a,/3-unsaturated carbonyl compounds with organic electrophiles. 

When acrylates or related compounds are employed as electrophiles in conjugate addition reactions, a conveniently substituted C-H pro-nucleophile has to be employed in order to generate a new stereogenic center. For this reason, most of the methodologies reported in this context involve the use of 2-sub-stituted 1,3-dicarbonyl compounds or related derivatives as Michael donors. 

The equilibrium between a-silyl alkoxides and silojq carbanions can be shifted toward the carbanion side by introduction of a conjugating group into either or both the acylsilane and the nucleophile. In 1980, Reich et al. reported that treatment of all l-substituted acylsilanes with vinyllithium followed by a variety of electrophiles affords a-substituted enol silyl ethers 22 via a siloxy allyllithium intermediate 21 fScheme 8.1Similar reactions using phenyllithium give products in which electrophilic quenching occurred at the benzylic position. 

A powerful application of such chiral oxazolines is found in addition reactions to substituted naphthalenes (Scheme 3.14) [71-73]. Chiral oxazoline 82 undergoes conjugate addition reactions with carbanions and other nucleophiles to furnish a stabilized carbanion intermediate 83 [71]. This could be trapped with a variety of electrophiles to provide products such as 84 with 98 2 diastereoselectivity. Following auxiliary removal, the sequence provides... 

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