Basic Organic Reactions with Electrophilic Active Centers

BASIC ORGANIC REACTIONS WITH ELECTROPHILIC ACTIVE CENTERS 

AU four of the elementary reactions in a cationic polymerization involve electrophilic or cationic intermediates. Thus, initiation, propagation, transfer, and termination may be classified as either nucleophilic substitution, electrophilic addition, elimination, rearrangement, or possibly as a pericyclic reaction. Initiation occurs in alkene polymerizations by either addition of acid to the alkene, or by ionization of a covalent initiator followed by addition of the resulting carbocationic intermediate to an olefin s double bond. Although initiation is an electrophilic addition (AdE) reaction in 

The polymer chain grows by propagation with regeneration of the same type of acti ve species. Thus, propagation is an Ads reaction in carbo-cationic alkene polymerizations [Eq. (49)], and an Sn2 reaction in most ring-opening polymerizations, 

Alternatively, covalent propagation may proceed by a pericyclic reaction involving a multicenter rearrangement such as a group transfer polymerization no examples of these type of reactions have been reported so far. 

The active species are destroyed in termination. As discussed in Section A, this may occur by rearrangement of less stable carbenium ions to more stable and unreactive carbenium ions, by nucleophilic attack on carbenium or onium ions to form nonstrained onium ions, or by reaction of a nucleophile such as the counteranion with a carbenium or onium ion to form inactive covalent species. Carbenium ions also terminate by hydride abstraction from some other molecule in the system. 

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V. BASIC ORGANIC REACTIONS WITH ELECTROPHILIC ACTIVE CENTERS... 

From a theoretical point of view, the key issue has been the basic nature of the metalation step, where the R groups moves from a R -H bond to a M-R bond. C-H activation is very common in organic chemistry as it allows the formation of functionalized hydrocarbons. Different mechanisms had been proposed for this metalation step, including electrophilic aromatic substitution, a-bond metathesis, oxidative addition/reductiveelimination and Heck-like insertion. Theoretical studies have facilitated narrowing the mechanistic possibilities to two main options oxidative addition/reductive elimination and proton abstraction by a base. In the oxidative addition/reductive elimination process the metal is inserted in the C-H bond with formal increase in the oxidation state of the metal, and the hydride leaves the metal coordination sphere of the metal afterwards. In the proton abstraction mechanism, the metal does not interact directly with the proton, which is captured by a base, with simultaneous formal creation of a carbanion that binds to the metal center. The mechanism of the reaction will depend on the presence of a base able to abstract the proton and of the existence of an energetically accessible oxidation state for the metal. 

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