Ionization intramolecular

Like the un-ionized hydroxyl group, an alkoxy group is a weak nucleophile. Nevertheless, it can operate as a neighboring nucleophile. For example, solvolysis of the isomeric p-bromobenzenesulfonate esters 6 and 7 leads to identical prxKluct nuxtures, suggesting the involvement of a common intermediate. This can be explained by involvement of the cyclic oxonium icai which would result from intramolecular participation. ... 

Intramolecular cycloadditions of substrates with a cleavable tether have also been realized. Thus esters (37a-37d) provided the structurally interesting tricyclic lactones (38-43). It is interesting to note that the cyclododecenyl system (w = 7) proceeded at room temperature whereas all others required refluxing dioxane. In each case, the stereoselectivity with respect to the tether was excellent. As expected, the cyclohexenyl (n=l) and cycloheptenyl (n = 2) gave the syn adducts (38) and (39) almost exclusively. On the other hand, the cyclooctenyl (n = 3) and cyclododecenyl (n = 7) systems favored the anti adducts (41) and (42) instead. The formation of the endocyclic isomer (39, n=l) in the cyclohexenyl case can be explained by the isomerization of the initial adduct (44), which can not cyclize due to ring-strain, to the other 7t-allyl-Pd intermediate (45) which then ring-closes to (39) (Scheme 2.13) [20]. While the yields may not be spectacular, it is still remarkable that these reactions proceeded as well as they did since the substrates do contain another allylic ester moiety which is known to undergo ionization in the presence of the same palladium catalyst. 

The authors conclude from these results that these (Z)/(E) rearrangements proceed through the ionization-rearrangement pathway. They assume that the ratios quoted arise solely from the difference in polarity of methanol and ethanol. For an intramolecular rearrangement (kY) one would not expect to find significantly different rates for these two solvents. 

The ionization of (E)-diazo methyl ethers is catalyzed by the general acid mechanism, as shown by Broxton and Stray (1980, 1982) using acetic acid and six other aliphatic and aromatic carboxylic acids. The observation of general acid catalysis is evidence that proton transfer occurs in the rate-determining part of the reaction (Scheme 6-5). The Bronsted a value is 0.32, which indicates that in the transition state the proton is still closer to the carboxylic acid than to the oxygen atom of the methanol to be formed. If the benzene ring of the diazo ether (Ar in Scheme 6-5) contains a carboxy group in the 2-position, intramolecular acid catalysis is observed (Broxton and McLeish, 1983). 

Mass spectrometry is a useful tool to detect the existence of reactive iron-imido intermediates. In intramolecular aromatic aminations, Que and coworkers used electrospray ionization mass spectrometry to show the presence of a molecular ion at m/z 590.3 and 621.2, which could be attributed to the formation of [(6-(o-TsN-C6H4)-TPA)Fe ]+ and [(6-(o-TsN-C6H4)-TPA)Fe° OMe)]+. With the isoto-... 

The form and shape of a molecule (i.e. its steric and geometric features) derive directly from the molecular genotype , but they cannot be observed without a probe. Furthermore, they vary with the conformational, ionization and tautomeric state of the compound. Thus, the computed molecular volume can vary by around 10% as a function of conformation. The same is true of the molecular surface area, whereas the key (i.e. pharmacophoric) intramolecular distances can vary much more. 

Like the stereoelectronic features that generate them, the MIFs are highly sen-sihve to the conformahonal and ionization state of the molecule. However, they in turn have a marked intramolecular influence on the conformahonal and ionization equilibria of the compound. It is the agency of the MIFs that closes the circle of influences from molecular states to stereoelectronic features to MIFs (Fig. 1.3). 

The difference between the log P of a given compound in its neutral form (log P ) and its fully ionized form (log P ) has been termed dialog P ) and contains series-specific information, and expresses the influence of ionization on the intermolecular forces and intramolecular interactions of a solute [44, 51, 52]. 

Of particular interest when considering ionizable compounds is the difference of lipophilicity between the neutral species and one of its ionic forms, because ionization dramatically alters intramolecular interactions (such as electronic conjugation, internal ionic and hydrogen bonds, polarity, hydrophilic folding, and shielding). In a given solvent system, diff (log is approximately constant for compounds with similar chemical... 

Saponification of Ionized Esters by Intramolecular Hydrogen Transfer Cleavage of the Alkyl/Oxygen Bond... 

Entropic contributions to the acceleration of first-order reactions by microwaves should be negligible (AS = = 0). When ionization (SN1 or E,) or intramolecular addition (cyclizations) processes are involved a microwave effect could be viewed as resulting from a polarity increase from GS to TS, because of the development of dipolar intermediates (Scheme 3.5). 

For most of the molecules, the electron flow is from D to A, which is supported by the anti-Aviram-Ratner mechanism of Fig. 11a. However, there may be yet another possibility, shown in Fig. 21 implicit in the analyses of Figs. 9-11 has been the Aviram and Ratner assumption [79] that auto-ionization is a less efficient competing process. If the electric field induces intramolecular ionization first by sufficiently altering the orbital energies, then the direction of electron flow may occur in the anti-AR direction (Fig. 21) ... 

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