Azulenes preparation

Although a small sample of 2,4,6-trimethylpyrylium perchlorate may with care be recrystallized from acetic acid to give white crystals, m.p. 245-247° dec., it is recommended that this not be done with larger quantities. The 2,4,6-trimethylpyrylium perchlorate is of satisfactory purity for use in the 4,6,8-trimethyl-azulene preparation without further purification. 

By application of the analogy principle regarding azulene and pseudo-azulene, preparations for pseudoazulenes can be found. By analogy to... 

Substituted azulenes can be prepared in the same manner by the use of substituted cyclopentadienes or substituted pentamethinium salts. 

Azulyl sulphoxides 127 have also been prepared by a reaction involving a direct electrophilic substitution on the azulene ring by alkane- or arenesulphinyl chlorides186 (equation 69). Preparation of the methyl and phenyl sulphoxides of 4,6,8-tri-methylazulene and 4,6,8-tri-isopropylazulene by this method resulted in fair yields (57— 72%). However, the substitution on azulene itself gave only low yields of the corresponding sulphoxides. 

Azulene quinones [49b] are compounds related to the family of tropones and are considered to possess great biological and physiological potential. Several polycyclic compounds have been prepared by high pressure (3kbar, PhCl, 130°C, 15h) Diels-Alder reaction of 3-bromo-l,5-azulene quinone (137) and 3-bromo-l,7-azulene quinone (138) with several dienophiles. The cycloadditions were regioselective and afforded cycloadducts in reasonable to good yields (Scheme 5.20). 

Simple resonance theory predicts that pentalene (48), azulene (49), and heptalene (50) should be aromatic, although no nonionic canonical form can have a double bond at the ring junction. Molecular orbital calculations show that azulene should be stable but not the other two, and this is borne out by experiment. Heptalene has been prepared but reacts readily with oxygen, acids, and bromine, is easily hydrogenated, and polymerizes on standing. Analysis of its NMR spectrum shows that it is... 

Of the fundamental nonalternant hydrocarbons, only two prototypes were known about fifteen years ago azulene (XI, Fig. 5), the molecular structure of which was determined by Pfau and Plattner and fulvene (XIX) synthesized by Thiec and Wiemann. Early in the 1960 s many other interesting prototypes have come to be synthesized. Doering succeeded in synthesizing heptafulvene (XX) fulvalene (XXI) and heptafulvalene (XXIII). Prinzbach and Rosswog reported the synthesis of sesquifulvalene (XXII). Preparation of a condensed bicyclic nonalternant hydrocarbon, heptalene (VII), was reported by Dauben and Bertelli . On the other hand, its 5-membered analogue, pentalene (I), has remained, up to the present, unvanquished to many attempts made by synthetic chemists. Very recently, de Mayo and his associates have succeeded in synthesizing its closest derivative, 1-methylpentalene. It is added in this connection that dimethyl derivatives of condensed tricyclic nonaltemant hydrocarbons composed of 5- and 7-membered rings (XIV and XV), known as Hafner s hydrocarbons, were synthesized by Hafner and Schneider already in 1958. 

The Friedel-Crafts acylation at the 3-position of the azulene ring was possible due to the effect of the electron-withdrawing 1-methoxycarbonyl group. 183 has been prepared previously in an eight-step synthetic route in an unsatisfactory reaction yield156. 

This procedure illustrates a simple and general method for preparing azulenes. It is far more convenient and proceeds in much better yield than previous syntheses of azulenes involving... 

Azulene-substituted methyl cations were prepared as illustrated in Figure 7. The hydro derivatives (7a-d) became good precursors for the methyl cations. These derivatives were readily obtained by the reaction of azulenes 6a-d with 1-formyl compounds 5a-d under acidic conditions. The synthesis of the tri(l-... 

Figure 7. Preparation of the azulene-substituted methyl cation salts 2a-it PFf ...

The tri(azulenyl)methane derivative 24+ including a 6-azulenyl group was prepared by the reaction of azulene 6b with diethyl 6-formylazulene-l,3-dicarboxylate. Synthesis of 24+ was accomplished by hydride abstraction with DDQ. Cation 24+ was isolated as a hexafluorophosphate salt by treatment with aqueous HPF6. The new salt is a stable, deep-green colored crystals, that can be... 

Arylbenzotriazoles (797) are prepared via 2-nitro- and 2-amino-diphenylamines (Scheme 161). The 2-nitrodiphenylamines (796) are prepared from the appropriate aniline by reaction with 2-fluoronitrobenzene in the presence of KF <808215,85JCS(Pl)2725>. Azo-coupling of 2-amino-1-cyano-azulene (798) with p-tolyldiazonium chloride gives (799) (Scheme 162). Catalytic reduction of (799) quantitatively yields the diamino derivative (800), which on diazotization affords 9-cyano-azuleno[l,2-J]triazole (801) in 77% yield <85TL335>. 

Preparation of some azulenylmagnesium species was achieved by the halogen-magnesium exchange reactions of iodoazulenes with lithium tributylmagnesate at low temperatures (equations 29-33) . The reactions offer access to a variety of functionalized azulenes including azulenylphosphine, -stannane and -boronic ester. 

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... 

Hafner s synthesis of azulene,216 the cyclopentadienyl anion can also be used to introduce the five-membered fragment. This anion reacts with heterocyclic quaternary salts in a complex manner the pseudoazulene, however, is obtained in one step. The total yields are only about 10%, but it is possible to use starting materials that are easily available in large quantities. This variant was applied to prepare 2H-cyclopenta[thiadiazolium salts145 and cyclopenta[c]thiopyranes (31) using N-methyl thiazoliumbromide.90... 

A further illustrative use of a diazoketone, in this case as a source of a car-bene, is given in the preparation of azulene (Expt 6.16). 

The preparation of azulene (Expt 6.16) provides an interesting series of rearrangements.9 The key step in the synthesis is the intramolecular insertion reaction, carried out under conditions of high dilution, of the ketocarbene (22) into the 1,2-position of the benzene ring. The unstable norcaradiene (23) which is formed then ring-opens to the bicyclic trienone (24), which isomerises to the more stable cross-conjugated trienone (25) during isolation. Dehydration of the trienone with a mixture of phosphorus pentoxide and methanesulphonic acid yields azulene. 

These polymers, particularly poly(pyrrole), are most conveniently prepared from the parent molecule via electrolysis. So far, furan, pyrrole, thiophene, and various methylated derivatives have been polymerized by this procedure (10). The anodic polymerization apparently also works for relatively electron rich aromatic compounds such as aniline and azulene (11). 

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