Zwitterionic structures

The sydnones may be represented by structures (123a-d), of which the zwitterionic structure (123a) most clearly implies an aromatic sextet. The diamagnetic susceptibility exaltation for Af-phenylsydnone of ll.Ox 10 cm moP is comparable with the corresponding value for pyrrole (10.2x10 ). 3-p-Bromophenylsydnone (123 R = H, R = p-bromophenyl) is essentially planar however, the O—N bond and 0(1)—C(5) bond lengths are not very different from normal single bond distances. 

Consider 1,3-pentadiene and 1,4-pentadiene. Which, if either, would benefit from the type of resonance described above Draw appropriate resonance contributors for this isomer. Indicate the likely importance of different zwitterionic structures which you might draw. Compare the energies of 1,3-pentadiene and 1,4-pentadiene. Which one is more stable ... 

As mentioned in Section II,B, solutions of y9-hydroxypyridines in the nonpolar solvents chloroform and carbon tetrachloride show sharp infrared absorption bands near 3600 cm indicating that they exist in the hydroxy form. Infrared spectral data also led Mason to conclude that -hydroxypyridines probably exist largely as such in the solid state and exhibit O— 0 hydrogen bonding, a conclusion which is contrary to an earlier proposal favoring a zwitterion structure. 

Mercaptoquinolines of this type exist in the zwitterion or the zwitterion-extended quinone form to a greater extent than do the analogous hydroxy compounds ° (see Table V and Section II,R), and the color of 8-mercaptoquinoline has been attributed to the zwitterion structure.The concentration of the zwitterion decreases as the dielectric constant of the solvent decreases in the order H2O > MeOH > EtOH > Bu OH as indicated by the change in the molecular extinction coefficient. ... 

The ultraviolet spectra of the pyridinecarboxylic acids (334) were initially interpreted assuming that the proportion of the zwitterion structure 335 was not appreciable,and the early pK work was inconclusive. However, Jaffe s calculations based on the Hammett equation indicated that about 95% of nicotinic and isonicotinic acids existed in the zwitterion form, and ultraviolet spectral data showed that the actual percentages of picolinic, nicotinic, and isonieotinie acids existing in the zwitterion form in aqueous solution are 94, 91, and 96%, respectively.This was later confirmed by Stephenson and Sponer, who further demonstrated that the proportion of the zwitterion form decreases in solvents of low dielectric constant, becoming very low in ethanol. Dipole moment data indicate that isonicotinic acid exists as such in dioxane, and 6-hydroxypyridine-3-carboxyiic acid has been shown to exist in form 336 u.sing pK data. ... 

For N-hydroxy-1,2,4-triazole, the zwitterionic structure 128 was suggested [89AX(C)782,96CHEC-II(4)127]. Whereas N -substituted 3-hydroxy-1,2,5-triazoles prefer the hydroxy form 129, 2-hydroxy-1,3,4-triazoles exist as 0X0 forms 130 (Scheme 49) [76AHC(S1), pp. 380,383,388,389],... 

Pontevedrine (369) is a red compound from Papaveraceae (71TL3093). The UV-VIS absorption maxima Imax in ethanol were found at 245 (4.59), 312 (4.28), 325 (4.39), and 470 (4.01) nm. On addition of acid or alkali no changes were observable. A zwitterionic structure was proposed on the basis of the NMR data (Scheme 110). 

Zwitterion structures (206 207 etc.) as well as the mesoionic structure 208 have been proposed for these compounds.The exact significance of structure 208 and the term mesoionic is unclear in... 

The zwitterionic structure 221a was assigned to 2-nitroaminothia-zole since its infrared spectrum contained a broad band at 3200 cm ... 

Diazoalkanes and related compounds are not suitable guests for the types of hosts discussed above. Very weak complexation was found with diazodicyanoimidazole (2.53 Sheppard et al., 1979) in which the mesomeric zwitterionic structure with a formal diazonio group (see Secs. 2.6 and 6.2) is dominant. However, no complexation was found for another compound with a formal diazonio group, the ben-zothiazol-azidinium salt 2.50 (Szele and Zollinger, 1982). 

Thus, in the first case, if the amine is o-aminobenzoic acid, an explosive diazonium salt with a zwitterion structure is obtained ... 

The electrostatic interactions in the complexes 19 were obviously sufficient to favor the zwitterionic structure in a manner that formation of the usual o-QM was suspended, so that all reactions typical of o-QMs in their quinoid form (such as [4 + 2]-cycloadditions) were suppressed or at least slowed down. Decomposition of the complex of a-tocopherol was immediate by fast heating to 40 °C or above. This caused disintegration of the complex 19, immediate rotation of the methylene group into the ring plane, and thus formation of the o-QM, which then showed the classical chemistry of such compounds. 

Substituent effects on the are remarkable. Electron-withdrawing groups at the 5 -position, e.g., 5 -nitro-substitution (indoline component), and donor substituent at the 8-position (benzothiopyran component) in 44 leads to a longer wavelength shift. As the polarity of the solvent increases, the max of the colored form of spiroindolinobenzothiopyran results in hypsochromic shift. This can be interpreted as the existence of a polar structural component of the colored form in the ground state. Kinetic study has suggested that the zwitterionic structure largely contributes to the colored form of 6-nitrospiroindolinobenzothiopyran, as well as spiropy-rans.97 Based on H-NMR and X-ray analysis,98 99 the existence of an... 

The crystal and molecular structures of 6,6-dimethyl-3-methylthio-6,7-dihydro[l,2,4]triazino[l,6-c]quinazolin-5-ium-l-olate 461, obtained by condensation of 455 with acetone, confirmed its zwitterionic structure (75CSC295). Analogues of 461 were also prepared for thermolysis studies (74T3997). 

The electronic nature of the NSN fragment was studied, using both ab initio and DFT methods, for a series of 1,2,5-thiadiazoles and compared to the established zwitterionic structure of naphtha[l,8-rz/][l,2,6]thiadiazine 6 (Figure 1). 

Addition of ammonia as a model nucleophile to nitrile oxides was studied by a semiempirical MNDO method, for fulminic acid and acetonitrile oxide (121). The reaction is exothermic and proceeds in two steps. The first (and rate-determining) step is the formation of a zwitterionic structure as intermediate. The second step, which involves transfer of a proton, is very fast and leads to the formation of Z-amidoximes in accordance with experimental data. Similar results were... 

An unusual zwitterionic structure 38 formed by reaction of 1,4-di(tert-butyl) butadiyne with two equivalents of di(tert-butyl)aluminum hydride was investigated... 

The differences in the elution patterns for the two sorbents have been explained98 by invoking the zwitterionic structures for the antibiotics under the basic pH conditions employed for strata-X-C... 

Dihydroflavopereirine (3) has been isolated from Strychnos usambarenis Gilg (12). The zwitterionic structure of the alkaloid has been proved by spectroscopic investigation and by direct comparison of the alkaloid with a synthetic 5,6-dihydroflavopereirine (3) sample. 

Nitriles attached to [l,2,4]triazolo[l,5- ]pyridones were also converted to hydrazide adducts or 1,3,4-triazoles <2004CPA260>. Sulfur in zwitterionic structure 106 was readily alkylated with ethyl iodide (Equation 14), affording the salt 107 <2003JCM755>. 

Figure 4.1 The zwitterionic structure of two ot-amino acids, and the condensation reaction to form a dipeptide, linked by a peptide bond.

Exocyclic Double Bonds C=W Groups Betaine, and Zwitterionic Structures... 

By chance, the existence of the borane complex 330 of 329 was discovered. The liberation of 330 occurred with the best efficiency with sodium bis(trimethylsilyl)-amide from the borane complex 327 of 326. When styrene or furan was used as the solvent, three diastereomeric [2 + 2]-cycloadducts 328 and [4 + 2]-cycloadducts 331, respectively, were obtained in 30and 20% yield (Scheme 6.70) [156]. With no lone pair on the nitrogen atom, 330 cannot be polarized towards a zwitterionic structure, which is why its allene subunit, apart from the inductive effect of the nitrogen atom, resembles that of 1,2-cydohexadiene (6) and hence undergoes cycloaddition with activated alkenes. It is noted that the carbacephalosporin derivative 323 (Scheme 6.69) also does not have a lone pair on the nitrogen atom next to the allene system because of the amide resonance. 

An unusual tautomerism occurs in the system 56. It, too, is found in two crystal forms. One has a melting point of 108 to 110°C, is insoluble in petroleum ether, and is assigned the zwitterionic structure 56a. The second melts at 77 to 78°C, is soluble in hot petroleum ether, and is assigned structure 56b (118). 

The Diels-Alder reaction is the best known and most widely used pericyclic reaction. Two limiting mechanisms are possible (see Fig. 10.11) and have been vigorously debated. In the first, the addition takes place in concerted fashion with two equivalent new bonds forming in the transition state (bottom center, Fig. 10.11), while for the second reaction path the addition occurs stepwise (top row, Fig. 10.11). The stepwise path involves the formation of a single bond between the diene (butadiene in our example) and the dienophile (ethylene) and (most likely) a diradical intermediate, although zwitterion structures have also been proposed. In the last step, ring closure results with the formation of a second new carbon carbon bond. Either step may be rate determining. 

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