Stabilization zwitterion

The formation of nitrocyclobutanes (44) is attributed to the contribution of the structure B" to the reactivity of stabilized zwitterionic intermediate B. The contribution of C-alkylation decreases due to steric hindrance caused by the presence of substituents at the a-position of the initial nitroalkene (42). 

Scheme 3.45, Eq. 2) through the most stabilized zwitterion to give only one stereoisomer of target 5-methylene-substituted nitronate (35 h). 

Addition of PhPdl to the allene triggers cyclopropyl ring opening to generate a cr-palladium species, which readily leads to a 1,3,5-triene through /3-elimination. From the observed diastereoselectivities, the reaction seemingly proceeds stepwise via the well-stabilized zwitterionic intermediate. 

Of wider interest are mesomerically-stabilized zwitterions of the general formula (39) with R and R other than H. The charge on the nucleus arises without the intervention... 

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). 

Electrostatic Strong interaction between ions for charges and q, separated by r 2, and solvent dielectric constant, water has large g stabilizes zwitterion formation... 

Several reaction mechanisms were proposed. One is based on a concept that Lewis acids, like BF3, coordinate directly with oxygen of the acetal. This results in ring opening that is induced to form a resonance-stabilized zwitterion ... 

Semi-empirical calculations of the molecular orbitals reveal that both the ground and the excited states of squaraines exhibit donor-acceptor-donor (D-A-D) intramolecular charge transfer [67]. This class of compounds possesses a resonance-stabilized zwitterionic structure. Typical squaraines have a four-membered central electron-deficient ring and two-electron donor groups. As monomers in solution, these compounds strongly absorb at wavelengths above 600 nm with high molar absorptions (e > 10 1 mol cm ) and intense fluorescent emission with a small Stokes shift moreover, squaraines are photostable [66, 67]. 

The first transformation begins with addition of the thiamine conjugate base to the a-keto carbon of pyruvate. The adduct thus formed decarboxylates readily, generating CO2 and a resonance-stabilized zwitterion. This product is an acetyl anion equivalent, which participates in nucleophilic attack on a sulfur atom of a substance called lipoamide. 

Such an outcome is plausible only if a concerted rather than a zwitterionic mechanism takes place when Dess-Martin periodinane is employed (Scheme 8.2). With IBX, such a clearcut scenario is not observable since as a highly polar reagent commonly DMSO as solvent was used, which stabilizes zwitterionic intermediates as well. Under the influence of Lewis acids, the mechanistic pathway most probably involves an open-chain intermediate and the product distribution mirrors the thermodynamic stability of the stereoisomeric products. 

---