Of pseudoazulenes

There are early reviews discussing the chemistry and properties of pseudoazulenes.311312 Unsubstituted pseudoazulenes are rather unstable, but benzocon-densation and substituents stabilize the molecule, for the latter in agreement with the electronegativities of the heteroatoms. No Diels—Alder reactions occur between pseudoazulenes and dienes, a fact which is consistent with a certain degree of aromaticity. A pseudoazulene with a pyranic ring was formed from benzyl and cyclopentadiene with sodium methox-ide.313... 

Since that time many new compounds of the pseudoazulene type have been described but there are only a few reports giving a clear review of the total field.2,37 Moreover, there are a number of reviews about special pseudoazulenes38-40 in which the overall concept of pseudoazulenes received little notice. 

Table I, containing compounds 26-87, summarizes all pseudoazulenes known through July 1979, as well as the corresponding literature. Not included are hydroderivatives of known and hitherto unknown pseudo-azulene systems containing many oxo and thio compounds. Such pseudoazulenes are predominantly of the type produced by introduction of an NR group. Only one system (78) of pseudoazulenes containing a selenium group is known, whereas tellurium compounds are unknown because of difficulties...

Quaternary salts (90) from azalenes obtained from heterocyclic bases and phenacyl halides or derivatives of a-halo carbon acids also contain an acidic exocyclic CH2 group. Therefore, during the deprotonation, mixtures of pseudoazulenes 91 and ylides 98 can be formed, especially if Y = CH. °5 Quaternary salts (90) containing one pyrrole-type nitrogen (Y = N) de-protonate exclusively to pseudoazulenes166 (Scheme 6). Analogous behavior... 

The condensation reaction gives very high yields, but the preparation of the reactants (106) is often complicated. For the preparation of pseudoazulene systems 5 and 6 formed by exchange of two C—C bonds of the seven-membered ring in azulene (Scheme 1), five-membered dicarbonyl compounds of type 107 are used. Their reaction with alkyl or aryl hydrazines gives heterocyclic bases (108), which can be transformed to the pseudoazulene system (49) by alkylation and deprotonation (see Section III,A,l).136... 

In addition to the general methods of preparation already described, there are some methods that hitherto have not been extended to a large number of pseudoazulenes. 

Cycloaddition reactions can also lead to the formation of pseudoazulene lattices, for example, the reaction of acetylene dicarboxylates with diazo-cyclopentadienes cyclopenta[c]pyradizines (55).140 142... 

A 1,3-dipolar cycloaddition of indanoneenamines and nitrilimines produces the indeno[2,1 -cjpyrazole (52).135 The yields of such cyclization reactions are high (60-80%). Reactions of A-alkylpyridine-3-ylides with picryl chloride involving cyclization give a mixture of pseudoazulenes 35 and 36.103 An additional preparation of pyrindine 26 is from 1,2- and 1,4-oxides of azonine.55... 

Under the influence of air, oxygen, and light numerous pseudoazulenes decompose very quickly (see Section V,E). Solutions of pseudoazulenes in aprotic solvents (particularly n-hexane and benzene) can often be stored for long periods. Nearly all compounds decompose in protic solvents within a short time. 

The class of pseudoazulenes has attracted theoretical interest due to the analogy with hydrocarbon azulene.46,57,61,65 7,74.79-82.101.102.118,129.158, 161.189 Jhese calculations should also answer the question of whether the pseudoazulenes should be considered as heteroaromatic compounds. A large amount of work deals with the electronic spectra of the pseudoazulenes (see Section IV.E).54-57-60-62-64,67,74,96,117,118,170,191 Furthermore, there are calculations that are dedicated exclusively to questions of reactivity.61 126, 133 These results, a portion of which have already been summarized.2,38 allow the following general remarks. 

The different electron density distribution causes a considerable dipole moment in the ground state of pseudoazulenes (Section IV,D). Corresponding calculations have been carried out for various systems.54,60-62-63,1 17,170-171... 

Quantum chemical calculations also allow statements to be made about the reactivity of pseudoazulenes. The results have been generalized for simple systems in Scheme 15. 

Comprehensive material concerning the calculation of electronic spectra of pseudoazulenes deals with the question of whether these compounds also possess the interesting absorption properties of the azulenes. The calculations show that all unsubstituted pseudoazulenes should possess a long-wavelength electronic transition between 500 and 700 nra. corresponding to an S0 - S, transition with n,n character. For simple representatives of the [b]-series this long-wavelength transition is bathochromically shifted in the order of oxalenes to azalenes to thialenes. 

Information about absorption spectra of pseudoazulenes can be found in nearly every paper mentioned in Table I. In most cases the spectral data provide a comparison with the corresponding azulene. For unstable pseudoazulenes electronic spectra are also used to demonstrate the structure of the product of synthesis.53,83 104 113,116 123 184 188 192 Systematic investigations on absorption spectra of pseudoazulenes have been made 63 66 68 69 92 96 117,1 is... 

Systematic investigations on infrared spectra of pseudoazulenes have not been published. Data can be found only in the experimental sections of papers. Investigations on 2-phenyl-2H-pyrindine (29b),85 cyclopenta[c]thio-pyran (31a),85 4-methyl-4//-cyclopenta[6]quinoline (39a),11J-114 5-methyl-5H-cyclopenta[c]quinoline (42a),123 and oxalenes 27, 33, 40, 44231 show that the absorption frequencies fall into three main regions 600-1250 cm" 1 (in-plane and out-of-plane bonding vibrations of CH), 1300-1620 cm"1 (aromatic C—C bonds), and 2980-3120 cm" 1 (C H stretching vibrations). 

Detailed studies of the mass spectra of pseudoazulenes have not been reported data are only described in the experimental sections of papers, usually without any discussion. More detailed data have been reported for pseudoazulene systems 35 and 36,105 39,113,114 42,123 65,163 80,195 81,197 82,198 and 83.198 Mass ion data have been reported for 2655,56 and 56.143... 

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). 

Reactions in which pseudoazulenes formally react as dienes are unknown. Experiments with the Diels-Alder reaction of pseudoazulenes 26, 27,39, and 40 have not been successful. With dimethyl acetylene dicarboxylate in cold benzene or methylene chloride these pseudoazulenes react by addition (Eq. 8).236... 

Systematic investigations of oxidation and reduction reactions of pseudoazulenes are not available. Many pseudoazulenes are sensitive to air but their reaction behavior with other oxidizing reagents is unknown. The air sensitivity of many pseudoazulenes is inconsistent with the aromaticity of these compounds. 

Very different results are obtained by hydrogenation of pseudoazulenes. Chemical and catalytic reduction gives a variety of products. The six-mem bered ring, however, is often perhydrogenated these reactions are discussed in Section V,E. 

Several authors have reported that they did not succeed in isolating a number of pseudoazulenes when solutions in dilute mineral or glacial acetic acids were diluted with water or other bases.51-71,77-86 It was only possible to recover polymer-like material. In the subsequent decomposition the nucleophile attacks the conjugated acid of the pseudoazulenes at a position adjacent to the heteroatom, probably resulting in a fission. The hydrogen-deuterium exchange catalyzed by acids has been reported for systems 29,86 31,86 and 49.135... 

A considerable number of electrophilic substitution reactions are known for pseudoazulenes. They are listed in Table VI. Electrophilic substitution reactions of pseudoazulenes take place at C-l and C-3 of the five-membered... 

Another characteristic of electrophilic reactions of pseudoazulenes is the application of numerous cations as the electrophile, for example, diazonium salts and Vilsmeier- Haack s reagent (see Table VI), tropylium ion,135 triphenylmethyl cation,"4 pyrylium ion,119 and dithiolium ion.166 Very stable cations are formed (e.g., 120) addition of base releases the substituted pseudoazulene (see example in Eq. 10). Generally reactions of this type are thermodynamically favored (see also Section IV,B). The site of substitution... 

Halide atoms at the five-membered ring of pseudoazulenes can be substituted by different nucleophiles. The reaction of 5,7-dichlorocyclopenta-[cjthiapyrane (31b) with silver nitrite in ethanol at reflux gave a chloronitro derivative (31c) in 46°0 yield.86 Loss of halide from 31b formed positive... 

Closer investigations of pseudoazulenes 26, 27, 39, and 4()50,214 revealed that irradiation of these systems in the presence of 302 produced singlet oxygen. It was possible to demonstrate the formation of 02 by quenching experiments with DABCO and 2,5-dimethylfuran. In a thermal reaction 02 adds to the pseudoazulenes, whereby dioxetane derivatives (126) are... 

Azulene is one of the most interesting molecules from the photophysical point of view. A picosecond measurement of the vibrational energy decay in matrix isolated polyatomic molecules shows at 4K that molecular modes in a polyatomic matrix does not affect decay in the 2 vibrational manifold . The S2 Sq fluorescence of pseudoazulenes has been studies in Shpolski... 

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