Anions aromatic stabilization

MO calculations, 7, 364 Oxetene, 2-isopropylidene-polysubstituted rearrangement, 7, 377 Oxetene, tetramethyl-structure, 7, 366 Oxetenes, 7, 363-402 decomposition, 7, 375 metabolism, 1, 243 molecular dimensions, 7, 366 thermal stability, 7, 370 Oxetenyl anions aromaticity, 7, 371 Oxetenyl cations aromaticity, 7, 371 Oxichlororaphine occurrence, 3, 196 Oxichromic developers in colour photography, 1, 378-379 Oxidation... 

The pKa values for 3-phenyl- and 6-methyl-3-phenyl-2//-thiopyran-S,S-dioxides have been discussed in connection with the aromatic stabilization energies of their corresponding anions (91JOC4218). 

The chemistry and properties of heteronins 221 (Scheme 82) have been reviewed.264-266 These compounds are thermally stable and possess delocalized planar molecules with strong diamagnetic ring currents.2673 Schleyer and co-workers calculated by ab initio and density functional methods aromatic stabilization energies as well as other properties of heteronins, and found that only 221 anions with X = N and P are aromatic and planar.2676... 

According to a semiempirical study by Malar, the different polyphospholide anions have 86—101% aromaticities of that of the cyclopentadienide anion. Chesnut and Quin reported on the basis of GIAO NMR calculations using a triple-valence quality basis set that the phospholide anion s aromaticity is 63% that of the cyclopentadienide anion. The aromatic stabilization energy (ASE) obtained by Schley-er et al. from eq 2 (X = P ) was 90% that of the cyclopentadienide anion. 

All of these oxocarbon anions are aromatic according to simple molecular orbital calculations. The aromatic stabilization of the anion is apparently responsible for the fact that squaric acid (HjC O,) is about as strong as sulfuric acid.104 There is a considerable and growing body of knowledge of the chemistry of these systems, but most of it is probably more appropriate to a discussion of organic chemistry.105... 

Dimerization of the sodium naphthalenide radical anion would result in a loss of aromatic stabilization, but this is not true for 1, which can form a C-C bond and a resonance-stabilized bis-phenylmethyl dianion, 2 (Section 26-4C). 

The two free hydroxy groups are First protected with acetic anhydride. In a second step the acetyl group is reductively cleaved by a Birch reduction with lithium in liquid ammonia.19 Lithium dissolves in the ammonia with the formation of solvated electrons. Stepwise electron transfer to the aromatic species (a SET process) leads first to a radical anion, which stabilizes itself as benzylic radical 38 with loss of the oxygen substituent. A second SET process generates a benzylic anion, which is neutralized with ammonium chloride acting as a proton source (see Chapter 12). 

A particularly strong type of resonance stabilization is found for those compounds which form an aromatic ring upon removal of a proton. The enhanced aromatic stability of the conjugate base translates into a large increase in acidity of the acid. Whereas the doubly ally lie proton of 1,4-pentadiene is predicted to have a pKa % 40 due to resonance stabilization of the anion, the doubly allylic proton in cyclopentadiene has a pKa = 16 because the resulting anion produces an aromatic jt system. 

The cyclopentadiene anion is stabilized by five equivalent resonance structures. The anion is an aromatic anion by virtue of it being a six-jr-electron system. The indenyl anion is stabilized by a total of seven resonance contributors. However, they are nonequivalent and all but one require that the aromatic cloud of the benzene ring is disrupted. Thus, while the negative charge is well delocalized, the resonance stabilization is less than that of the cyclopentadiene system. Thus the proton is not as easily removed, making indene a weaker acid. 

Fig. 6 Reaction energy profile for reactions 34a/34b (A), 35a/35b (B), and 36a/36b (C). (A) and (B) Aromatic stabilization of the transition state is greater than that of benzene or cyclopentadienyl anion, respectively. (C) Anti-aromatic destabilization (positive ASE) of the transition state is less than that of cyclobutadiene the high barrier results from the additional contribution by angular and torsional strain at the transition state.

Reaction between 2 -hydroxyacetophenone and benzaldehyde (Claisen-Schmidt condensation) in the absence of a solvent at 423 K giving 2 -hydroxy chalcones and flavanones has been successfully performed with MgO as a solid base catalyst/581 A conversion of 40 % after 1 h with 67 % selectivity to chalcone was achieved. The influence of the solvent and the effects of a substituent on the aromatic ring were investigated by Amiridis et a//59,6"1 The reaction was carried out on MgO at 433 K. Dimethyl sulfoxide (DMSO) showed a strong promoting effect on the reaction, which was attributed to the ability of this dipolar aprotic solvent to weakly solvate anions and stabilize cations so that both become available for reaction. In this case, a conversion of 2-hydroxyacetophenone of 47 % with a selectivity to flavanone of 78 % was achieved after 30 min in a batch reactor. Further investigations1611 showed that DMSO significantly increases the rate of the subsequent isomerization of the 2 -hydroxychalcone intermediate to flavanone. 

Anti-aromaticity was predicted by the Hiickel approach for conjugated cyclic planar structures with 4n 7i electrons due to the presence of two electrons in antibonding orbitals, such as in the cydopropenyl anion, cydobutadiene, and the cydopentadienyl cation (n = 1), and in the cydoheptatrienyl anion and cydooctatetraene (n = 2). It has been argued that a simple definition of an anti-aromatic molecule is one for which the 1H NMR shifts reveal a paramagnetic ring current, but the subject is controversial. The power of the Hiickel theory indeed resides not only in the aromatic stabilization of cydic 4n + 2 electron systems, but also in the destabilization of those with An electrons [22, 27, 42]. 

We have seen that aromatic stabilization leads to unusually stable hydrocarbon anions such as the cyclopentadienyl anion. Dianions of hydrocarbons are rare and are usually... 

Huckel s rule is very useful and it helps us to predict and understand the aromatic stability of numerous other systems. Cyclopentadiene, for example, has two double bonds that are conjugated but the whole ring is not conjugated since there is a methylene group in the ring. However, this compound is relatively easy to deprotonate (see next chapter, p. 196) to give a very stable anion in which all the bond lengths are the same. How many... 

The intermediate anion is stabilized by electronegative nitrogen and by delocalization round the ring. These reactions have some similarity to nucleophilic aromatic substitution (Chapter 23) but are more similar to carbonyl reactions. The intermediate anion is a tetrahedral intermediate that loses the best leaving group to regenerate the stable aromatic system. Nucleophiles such as amines or thiolate anions work well in these reactions. 

Thiin dioxides form anions 478 that appear to have some aromatic stability. The anions react with aldehydes (Scheme 69). 

The stability of anionic systems is governed by several factors (a) carbon hybridization (b) effective overall n-conjugation (c) inductive effects (d) aromatic stabilization and (e) environmental factors, e.g., ion-solvation equilibrium. 

The pyrrole anion, C4H4N -, is a 6 n electron species that has the same electronic structure as the cyclopentadienyl anion. Both of these anions possess the aromatic stability of 6 n electron systems. 

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