Zwitterion stability

One possible mechanism is electrophilic attack of the complexed carbene carbon atom at the terminal carbon of the diene. The resulting zwitterionic intermediate can now eliminate the metallic group (CO)5M directly, or, alternatively, the metallic group can migrate to yield a new, more stable zwitterion (stabilization of the allyl cation by the heteroatom X). 

Both oxetanes were formed with exclusively the exo-phenyl configuration. The regio- and diastereoselectivity observed are in accord with the assumption of a PET process involving the oxidation of the ascorbic acid derivatives and the formation of the carbonyl radical anions. In these special instances 1,4-biradical and 1,4-zwitterion stabilization result in similar product regiochemistry. The relative configuration of the products favors the assumption of a PET-process. 

Bush, M.F. Forbes, M.W. Jockusch, R.A. Oomens, J. Polfer, N.C. Saykally, R.J. WUhams, E.R. Infrared spectroscopy of cationized lysine and epsUon-N-methyllysine in the gas phase Effects of alkah-metal ion size and proton afhnity on zwitterion stability. J. Phys. Chem. A 2007, 111, 7753-7760. 

Bush ME, Forbes MW, Jockusch RA, Oomens J, Polfer NC, Saykally RJ, Williams ER (2007) Infrared spectroscopy of cationized lysine and epsilon-N-methyllysine in the gas phase effects of alkali-metal ion size and proton affinity on zwitterion stability. J Phys Chem A 111 7753-7760... 

Bush MF, Oomens J, Williams ER (2009) Proton affinity and zwitterion stability new results from infrared spectroscopy and theory of cationized lysine and analogues in the gas phase. J Phys Chem A 113 431 38... 

Dunbar RC, Steill J, Polfer NC, Oomens J (2009) Peptide length, steric effects and ion solvation govern zwitterion stabilization in barium-chelated di- and tiipeptides. J Phys Chem B 113 10552-10554... 

O Brien JT, Prell JS, Berden G, Oomens J, Williams ER (2010) Effects of anions on the zwitterion stability of Glu, His and Arg investigated by IRMPD spectroscopy and theory. Int J Mass Spectrom 297 116-123... 

Estephan, Z.G., Jaber, J.A., and Schlenoff, JJl. (2010) Zwitterion-stabilized silica nanopartides toward nonstick nano. Langmuir, 26,16884-16889. 

When the nitrogen atom is substituted by a nitrophenacyl group, OH attack gives the betainic zwitterion (Scheme 13). which is soluble in organic solvents (32). The stability of the C-betainic or ylid structure has been explained as an effect of resonance of the negative charge in the molecule (33, 34). 

The initial reaction between a ketene and an enamine is apparently a 1,2 cycloaddition to form an aminocyclobutanone adduct (58) (68-76a). This reaction probably occurs by way of an ionic zwitterion intermediate (75). The thermal stability of this adduct depends upon the nature of substituents Rj, R2, R3, and R. The enolic forms of 58 can exist only if Rj and/or R4 are hydrogens. If the enamine involved in the reaction is an aldehydic enamine with no 3 hydrogens and the ketene involved is di-substituted (i.e., R, R2, R3, and R4 are not hydrogens), then the cyclo-butanone adduct is thermally stable. For example, the reaction of dimethyl-ketene (61) with N,N-dimethylaminoisobutene (10) in isopropyl acetate... 

Many of the properties oj -hydroxypyridines are typical of phenols. It was long assumed that they existed exclusively in the hydroxy form, and early physical measurements seemed to confirm this. For example, the ultraviolet spectrum of a methanolic solution of 3-hydroxypyridine is very similar to that of the 3-methoxy analog, and the value of the dipole moment of 3-hydroxypyridine obtained in dioxane indicates little, if any, zwitterion formation. However, it has now become clear that the hydroxy form is greatly predominant only in solvents of low dielectric constant. Comparison of the pK values of 3-hydroxypyridine with those of the alternative methylated forms indicated that the two tautomeric forms are of comparable stability in aqueous solution (Table II), and this was confirmed using ultraviolet spectroscopy. The ratios calculated from the ultraviolet spectral data are in good agreement with those de-... 

The type of extended quinonoid form (cf. 133), if any, which can stabilize the zwitterion form is shown. 

According toB3-LYP/6-31G calculations, the triplet state of 88 is -5.1 kcal/ mol lower in energy than the zwitterionic singlet state. The order is reversed for the pyridine-bridged 89. Likewise, for l,l, 2,2, 3,3 -tetrathiadiazafulvalene, the state was found to be more stable than the state [99JA6657]. However, one should keep in mind that B3-LYP calculations often lead to an exaggerated stabilization of triplet states. 

Together with a shift of the proton from the a-carbon to the alkoxide oxygen, the tertiary amine is eliminated from the addition product to yield the unsaturated product 3. Early examples of the Baylis-Hillman reaction posed the problem of low conversions and slow reaction kinetics, which could not be improved with the use of simple tertiary amines. The search for catalytically active substances led to more properly adjusted, often highly specific compounds, with shorter reaction times." Suitable catalysts are, for example, the nucleophilic, sterically less hindered bases diazabicyclo[2.2.2]octane (DABCO) 6, quinuclidin-3-one 7 and quinuclidin-3-ol (3-QDL) 8. The latter compound can stabilize the zwitterionic intermediate through hydrogen bonding. ... 

The formation of a single complex species rather than the stepwise production of such species will clearly simplify complexometric titrations and facilitate the detection of end points. Schwarzenbach2 realised that the acetate ion is able to form acetato complexes of low stability with nearly all polyvalent cations, and that if this property could be reinforced by the chelate effect, then much stronger complexes would be formed by most metal cations. He found that the aminopolycarboxylic acids are excellent complexing agents the most important of these is 1,2-diaminoethanetetra-aceticacid (ethylenediaminetetra-acetic acid). The formula (I) is preferred to (II), since it has been shown from measurements of the dissociation constants that two hydrogen atoms are probably held in the form of zwitterions. The values of pK are respectively pK, = 2.0, pK2 = 2.7,... 

The primary ozone-olefin addition product splits into a molozonide zwitterion (3). The zwitterion (3) then stabilizes by splitting into a carbonyl compd (4) and another zwitterion (5). 

More recently, Stepanov et al. (1989) investigated the acid-base properties of the zwitterion 3.22 which is obtained in the diazotization of 5-amino-3-nitro-l,2,4-triazole. Under alkaline conditions the (Z)-diazoate dianion 3.23 is formed. It can be isomerized thermally to give the (E)-diazoate dianion 3.24. If the solution of this compound is acidified, the primary addition of a proton takes place at the anionic ring nitrogen yielding 3.25, and subsequently the hydrogen-bond-stabilized (Z)-iso-mer (3.26). Further acidification gives the nitrosoamine (3.27). 

The diazonio group of one zwitterion is stabilized by intermolecular interactions with the carboxylato oxygens of two neighbouring zwitterions. The same type of coordination is observed in crystals of benzene diazonium chloride, tribromide, and tetrafluoroborate (Andresen and Romming, 1962 Romming, 1963 Cygler et al., 1982). 

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