Electrophilic Additions to Multiple Bonds

In spite of these uncertainties, however, the utility of the reactivity-selectivity principle has been illustrated for a number of diverse areas of mechanistic interest. Such applications are being extended to other areas as well. For example, Olah has recently studied the mechanism of electrophilic addition to multiple bonds using selectivity data and concluded that the transition states of the bromine addition to alkenes are of a 7r-complex nature (Olah and Hockswender, 1974). Finally the large number of reactivity-selectivity relationships which have been discovered offer considerable experimental support for the various expressions and formulations of the Hammond postulate whose profound effect on modem mechanistic chemistry is now beyond question. 

The spectrum of mechanisms governing the reactions of electrophilic addition to multiple bonds is quite broad. The solvent, the polarity of electrophilic agent, the type and conformation of substituents at a multiple bond and its polarity all have a substantial effect on kinetics and stereochemistry of the reactions in question [1-3]. The most general and important features of various mechanisms are summarized in the following scheme ... 

The reactions of carbenes, which are apparently unique in displaying electrophilic character in strongly basic solutions, include substitution, addition to multiple bonds, and co-ordination with lone pairs of electrons to form unstable ylides. This last reaction is of obvious relevance to a consideration of the reactions of heterocyclic compounds with carbenes and will be summarized. 

In Part 2 of this book, we shall be directly concerned with organic reactions and their mechanisms. The reactions have been classified into 10 chapters, based primarily on reaction type substitutions, additions to multiple bonds, eliminations, rearrangements, and oxidation-reduction reactions. Five chapters are devoted to substitutions these are classified on the basis of mechanism as well as substrate. Chapters 10 and 13 include nucleophilic substitutions at aliphatic and aromatic substrates, respectively, Chapters 12 and 11 deal with electrophilic substitutions at aliphatic and aromatic substrates, respectively. All free-radical substitutions are discussed in Chapter 14. Additions to multiple bonds are classified not according to mechanism, but according to the type of multiple bond. Additions to carbon-carbon multiple bonds are dealt with in Chapter 15 additions to other multiple bonds in Chapter 16. One chapter is devoted to each of the three remaining reaction types Chapter 17, eliminations Chapter 18, rearrangements Chapter 19, oxidation-reduction reactions. This last chapter covers only those oxidation-reduction reactions that could not be conveniently treated in any of the other categories (except for oxidative eliminations). 

All of the above mentioned examples of vinyl cation intermediates have involved electrophilic additions to triple bonds or allenes or participation in solvolyses of such multiple bonds. In a sense, these reactions derive from analogies in normal... 

The equimolar mixture of bromine trifluoride and bromine can be employed for the bro-mofluorination of fluorine-free alkcncs such as (Z)- and( )-l,2-dichloroethene, methyl acrylate and its a-substituted derivatives.1 The reactions are carried out in l,l,2-trichoro-l,2,2-tri-fluoroethane (CFC-113) using a 25-35 % excess of bromine trifluoridc/ bromine. The direction of the bromine monofluoride" addition to multiple bonds in the esters, much as in other electrophilic reactions, depends on the electronic nature of the substituent R. 

Despite these complications, the enormous and unique synthetic potential of radical reactions is also obvious. Radicals are highly reactive species and their addition to multiple bonds occurs easily, even with crowded substrates, under mild and essentially neutral conditions. Radical reactions are not generally sensitive to the influence of polar effects and tolerate the presence of functional groups otherwise incompatible with electrophilic and/or nucleophilic reagents. 

Carbenes, in general, are very reactive electrophilic species. Their activity depends to some extent on the method and conditions of preparation, on the nature of the substituent groups and also on the presence or absence of metals or metallic salts. Carbenes undergo a variety of reactions, including insertion into C—H bonds, addition to multiple bonds and skeletal rearrangements. The simplest carbene, methylene itself, attacks primary, secondary and tertiary C—H bonds indiscriminantly. However, alkyl carbenes can undergo selective intramolecular C—H, N—H or O—H insertion to provide useful synthetic transformations. In general, no intermolecular reactions are observed when intramolecular insertion is possible. 

Org. Chem. 1978, 43, 4207-4215 a related azepine formation through addition of N-unsubstituted azidirines to electrophilic carbon-carbon multiple bond systems such as acrylonitrile followed by aza-[3,3]-Claisen rearrangement was reported by Hassner (c) A. Hassner, R. D Costa,... 

Electrophilic Additions to Carbon-Carbon Multiple Bonds... 

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