Azulene stabilization

In contrast to the significant resonance stabilization of azulene, pentalene and heptalene are indicated to be destabilized relative to a reference polyene ... 

The thiepin 13 (X = S) was found to be inert toward catalytic hydrogenation. The authors ascribe the stability of 13 (X = S) to a contribution of the charge-separated structures 13a and 13 b to the ground state of 13 (X = S) 71). An X-ray structural determination showed that 13 (X = S) is nearly planar with a disordered crystal structure similar to that of azulene 73). 

Azulene (Ci0H8) has attracted the interest of many research groups over the years due to its unusual properties as well as its attractive blue color. The azulene system has the tendency to stabilize cations, as well as anions, owing to its remarkable polarizability (4). Indeed, the pvalue for the cyclopro-... 

We have reported the synthesis of a series of azulene-substituted methyl cations, i.e., tri(l-azulenyl)methyl, di(l-azulenyl)phenylmethyl, and (1-azul-enyl)diphenylmethyl cations (2a+, 3a+, and 4a+) (Figure 6) (6). In order to examine substituent effects on the azulene rings and to enhance their stabilities, a series of the cations (2b-d+, 3b-d, and 4b-d+) bearing tert-buty groups on each azulene ring were also synthesized (7). 

The dication 212+ composed of two methylium units connected to a p-phenylene spacer would be a candidate for new Wurster type violene-cyanine hybrid (Figure 12) (15). The reaction of four molar amounts of azulene 6b with terephthalaldehyde yielded the hydro precursor. Synthesis of the dication 212+ was accomplished by hydride abstraction with DDQ in almost quantitative yield. The dication 212+ was expected to show destabilization, but instead it exhibited high thermodynamic stability just like the corresponding monocation 3b+. 

Di(l-azulenyl)(6-azulenyl)methyl cation (24+) represented in Figure 17 exemplifies the cyanine-cyanine hybrid (20). Di(l-azulenyl)methylium unit in 24+ acts as a cyanine terminal group. The tropylium substructure stabilizes the cationic state (24+). Reduction of 24+ should afford the neutral radical 24, which is stabilized by capto-dative substitution effect, because 24 is substituted with azulenes in the donor and acceptor positions. The anionic state (24") is also stabilized by contribution of the cyclopentadienide substructure, which should exhibit the third color change in this system. 

Theoretische Uberlegungen von Brown sagen fiir die bisher nicht dargestellten nichtbenzoiden Kohlenwasserstoffe Pentalen und Heptalen starke Farbigkeit voraus. Pentalen sollte stabiler als Azulen, Heptalen dagegen ziemlich unbestandig sein. 

Wittig reactions with pyrrole-2-aldehyde led to the esters (79) which were cyclisized to 3a-azaazulen-4-ones (80).104,105 4-Methylene-3a-aza-azulenes (81) have been obtained from 80 with stabilized phos-phoranes.36 Reaction of dimethyl acetylenedicarboxylate with 81 could not be achieved. A similar cycloaddition was successful in the synthesis of cycl[3,3,3]azines (2) (Section V). 

Exercise 21-10 Suggest reasons why (a) the stabilization energy of biphenylene is less than twice that of benzene, and (b) the heat of combustion of naphthalene is less than that of azulene. 

The essential requirement for an exact determination of the physicochemical properties of a compound is its stability under the conditions of measurement. Thus, pseudoazulenes are difficult to measure because they are rather unstable in comparison to most azulenes. The simplest unsubstituted representatives of some systems are so thermally unstable that it is nearly impossible to isolate them. Phenyl substitutents have a stabilizing effect, so polyaryl-substituted compounds can be kept at room temperature for some months without decomposing noticeably. An especially strong stabilizing effect is given by the picryl group.106... 

Azulenes are stable to acids or bases. Most pseudoazulenes, however, decompose in acidic or alkaline media. Some compounds dissolve in strong acids without decomposition (see Section V,C), on dilution with water or aqueous bases, however, only polymeric products can be isolated.86 Even in these cases phenyl substituents have a stabilizing effect after heating for 5-10 min in concentrated or semiconcentrated acids, polyaryl-substituted pyrindines (26) for example, can be recovered nearly undecomposed, whereas the parent structure is totally decomposed.51... 

For these reasons the reactivity of pseudoazulenes is higher than that of the carbocyclic analogs, the azulenes. Thus, a generally lower stability toward electrophilic, nucleophilic, and radical reagents results. This explains why the general stability of some pseudoazulenes is so low (see Section IV,A). 

The only satisfactory approach to this problem at present involves the use of perturbations methods. Attempts to calculate resonance energies indirectly, by comparing calculated total energies of a compound with that of a single classical structure for it, have proved ineffective. Thus calculations of this kind have failed to account for the special stability of monocyclic compounds containing 4n + 2 electrons (Hiickel s rule), and they predict that all the compounds XVII-XXIII should be aromatic whereas the only one that is aromatic is azulene (XXII). 

Many aromatic compounds have considerable resonance stabilization but do not possess a benzene nucleus, or in the case of a fused polycyclic system, the molecular skeleton contains at least one ring that is not a benzene ring. The cyclopentadienyl anion C5HJ, the cycloheptatrienyl cation C7H+, the aromatic annulenes (except for [6]annulene, which is benzene), azulene, biphenylene and acenaphthylene (see Fig. 14.2.2(b)) are common examples of non-benzenoid aromatic hydrocarbons. The cyclic oxocarbon dianions C Of (n = 3,4,5,6) constitute a class of non-benzenoid aromatic compounds stabilized by two delocalized n electrons. Further details are given in Section 20.4.4. 

There are several types of aromatic systems in addition to the ones described above. On example is azulene, a member of a class of nonalternant conjugated hydrocarbons.28 As a result of the different bridging pattern as compared to benzene, the n-energy levels are considerably shifted leading to light absorption in the visible region, and a purple color, as well as reduced n-electron stabilization. 

Stabilization by the introduction of bulky /er/-butyl groups into the azulene rings should be attributable to not only their steric and inductive electronic effects but also C-C hyperconjugative effects induced by the o-bond of the tert-butyl groups. The p/ R value of 14.3 for methyl cation 2b with six tert-butyl groups is the highest value ever reported for a methyl cation substituted with only hydrocarbon groups. 

Another probe of the local environment around the azulene solute molecule was the shift of the frequency of the S3 Sq absorption band, as a function of density. This solvachromic shift, or stabilization of the electronic states of the solute in the presence of the solute, was accurately described using the same radial distribution function used to reproduce the collisional deactivation rates. The applicability of the same radial distribution function to treat collisional deactivation and solvachromic frequency shifts suggests that both have a similar dependence on local density. 

An example is shown in Figure 2.15 where an even perturbation produces the orbitals of azulene and an odd perturbation (/ j) those of naphthalene from the perimeter orbitals of cyclodecapentaene. (Cf. Example 2.5.]) The energies of 0, and 0,. are not affected to the first order by the odd perturbation producing naphthalene, whereas 0s is stabilized and 0,- destabilized. Therefore, 0, becomes the HOMO and 0. the LUMO, and the HOMO-LUMO splitting AEhomo-lumo he same as for cyclodecapentaene. The HOMO- LUMO transition is referred to as L according to Platt. In azulene, on the other hand, 0, is destabilized and becomes the HOMO, whereas 0s- is stabilized and becomes the LUMO. The HOMO-LUMO splitting is markedly smaller than for cyclodecapentaene, and the HOMO-LUMO transition is of the Lb type. 

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