Zwitterionic vinyl carbanion

Substituted imidazoles A/-R Im undergo a stereoselective, uncatalysed ring opening on reaction with ynones PhC=CCOR and water in MeCN at 45-60 C to produce (Z,Z)-propenylaminoethenylformamides R N(CH=0)CH=CH-NH-C(Ph)=COR. The reaction is believed to proceed via a zwitterionic vinyl carbanion, generated by the initial nucleophilic addition of imidazole to the C=C bond. ... 

However we rationalized that using silicon as a migrating group could result in a unique stabilization of the energy surface of olefin isomerization. This rationalization was based on silicon s well known ability to stabilize both a-carbanions and p-carbocations. Thus a hypothetical "dual-stabilized" zwitterion would be produced by a 90° twist of a vinyl silane, and a following 1,2-shift of Silicon would produce a singlet carbene possessed of the same hyperconjugative stabilization as in a P-silyl cation (Eq. 5). 

The success of the reaction depends on the formation of the zwitterionic 7t-allylpalladium complex, which is subsequently trapped with the electron-poor olefins to afford the desired vinylcyclopentane. This is achieved through a nucleophilic addition of Pd onto the vinyl group, which results in an opening of the cyclopropane ring, revealing the zwitterionic 7i-allylpalladium complex. The presence of the ester moieties stabilizes the carbanion, while the carbocation is stabilized by the 7t-allylpalladium complex. This is followed by a Michael addition of the electron poor olefins onto the carbanion to form the second intermediate, which rapidly... 

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See also in sourсe #XX -- [ ]

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