Electron withdrawing groups mechanism

The reactions of electrophilic alkenes (alkenes attached to electron-withdrawing groups) with enamines produce one or more of the following products simple alkylation (2), 1,2 cycloaddition (3), and 1,4 cycloaddition (4). Competition with C alkylation by N alkylation is inconsequential and therefore will be largely ignored (5,7). A stepwise ionic mechanism leading to these products necessarily involves the formation of a zwitterion intermediate (1) as the first step, which is then followed either by one of the... 

Ordinary vinylie substrates react very poorly if at all by these mechanisms, but substitution is greatly enhanced in substrates of the type ZCH=CHX, where Z is an electron-withdrawing group such as HCO, RCO, EtOOC, ArS02, NC, F, and so on, since these P groups stabilize the carbanion ... 

This mechanism does not apply to unsubstituted or N,N-disubstituted aryl carbamates, which hydrolyze by the normal mechanisms. Carboxylic esters substituted in the a position by an electron-withdrawing group (e.g., CN or COOEt) can also hydrolyze by a similar mechanism involving a ketene intermediate. These elimination-addition mechanisms usually are referred to as ElcB mechanisms, because that is the name given to the elimination portion of the mechanism (p. 1308). 

When carboxylate ions are decarboxylated, the mechanism is entirely different, being of the Sgl type. Evidence for this mechanism is that the reaction is first order and that electron-withdrawing groups, which would stabilize a carbanion, facilitate the reaction." ... 

Similar additions have been successfully carried out with carboxylic acids, anhydrides, acyl halides, carboxylic esters, nitriles, and other types of compounds. These reactions are not successful when the alkene contains electron-withdrawing groups such as halo or carbonyl groups. A free-radical initiator is required, usually peroxides or UV light. The mechanism is illustrated for aldehydes but is similar for the other compounds ... 

In the ElcB mechanism the question of orientation seldom arises because the mechanism is generally found only where there is an electron-withdrawing group in the P position, and that is where the double bond goes. 

The reaction proceeds with isolated double bonds and electron-rich alkynes. Electron-withdrawing groups in the acetylene moiety decelerated the reaction. A plausible mechanism implies the activation of the olefin by coordination of the metal triflate followed by nucleophilic attack of the acetylene or acetylide (Scheme 31). 

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