Excursion to the World of Heptacyclic Compounds Made of Azulenes and Acetylenedicarboxylates

Azulenes and acetylenedicarboxylates react under acid catalysis (Brønsted or Lewis) and form (2aRS,8aSR)‐2a,8a‐dihydrocyclopenta[cd]azulene‐1,2‐dicarboxylates as intermediate products, which then dimerize by central bond‐formation between C(2a¹) and C(2′a¹) and various peripheral C,C′‐atoms of the dihydroazulene fragments, depending on the substituents present. The reactions are often accompanied by the formation of side‐products, such as 2‐(azulen‐1‐yl)fumarates and ‐maleates and others caused by H‐shifts of the primary intermediates. H‐Shifts between the two tetrahydrocyclopenta[cd]azulene parts of the heptacyclic structures were also found.     

Synthesis and photophysical properties of azuleno[1′,2′:4,5]pyrrolo[2,1-b]quinazoline-6,14-diones: Azulene analogs of tryptanthrin

Azulene analog of tryptanthrin, azuleno[1′,2′:4,5]pyrrolo[2,1-b]quinazoline-6,14-dione, was successfully prepared by the condensation reaction of azuleno[2,1-b]pyrrole-2,3-dione with isatoic anhydride in the presence of sodium hydride or diisopropylethylamine (DIPEA). Its 2-halo derivatives were also obtained in high yields by the condensation reaction with 5-haloisatoic anhydrides in the presence of DIPEA. Reactivity toward electrophilic reagents was revealed by halogenation with N-bromosuccinimide (NBS) or N-iodosuccinimide (NIS) to afford 12-halo derivatives in high yields. Among the halo derivatives, 2-iodo and 12-iodo derivatives were reactive enough to afford phenylethynyl derivatives under Pd-catalyzed Sonogashira cross-coupling conditions. Within the phenylethynyl derivatives, only 12-phenylethynyl derivative was transformed into its 1,1,4,4-tetracyanobutadiene (TCBD) derivative by the reaction with TCNE. Amphoteric redox properties of the novel azulene analogs of tryptanthrin were characterized by spectroscopic and voltammetric analyses.     

Synthesis of N,N,N′,N′-tetrasubstituted 1,3-bis(4-aminophenyl)azulenes and their application to a hole-injecting material in organic electroluminescent devices

After a preliminary search of the reaction conditions for the Suzuki-Miyaura cross-coupling of haloazulenes with arylboronic acids, the title compounds were synthesized either by the direct coupling reaction between 1,3-dihaloazulene and the corresponding N,N-disubstituted 4-aminophenylboronic acids or by a two-step sequence involving the cross-coupling with 4-bromophenylboronic acid and subsequent Pd-catalyzed amination. Application of the title diamines to a hole-injecting material in organic electroluminescent devices was carried out to provide their prominent characteristics as a novel durable, non-cyanine and non-polyamine substance without color fade. The diamine derivatives, extended by an ethynyl unit between the azulenyl core and the 4-aminophenyl moiety, were also synthesized and found, unfortunately, unsuitable for vacuum deposition in preparing a multilayer composite.     

Report on an Unusual Cascade Reaction between Azulenes and 2,3‐Dichloro‐5,6‐dicyano‐1,4‐benzoquinone (=4,5‐Dichloro‐3,6‐dioxocyclohexa‐1,4‐diene‐1,2‐dicarbonitrile; DDQ)

The oxidation of 1‐(3,8‐dimethylazulen‐1‐yl)alkan‐1‐ones 1 with 2,3‐dichloro‐5,6‐dicyano‐1,4‐benzoquinone (=4,5‐dichloro‐3,6‐dioxocyclohexa‐1,4‐diene‐1,2‐dicarbonitrile; DDQ) in acetone/H₂O mixtures at room temperature does not only lead to the corresponding azulene‐1‐carboxaldehydes 2 but also, in small amounts, to three further products (Tables 1 and 2). The structures of the additional products 3 – 5 were solved spectroscopically, and that of 3a also by an X‐ray crystal‐structure analysis (Fig. 1). It is demonstrated that the bis(azulenylmethyl)‐substituted DDQ derivatives 5 yield on methanolysis or hydrolysis precursors, which in a cascade of reactions rearrange under loss of HCl into the pentacyclic compounds 3 (Schemes 4 and 7). The found 1,1′‐[carbonylbis(8‐methylazulene‐3,1‐diyl)]bis[ethanones] 4 are the result of further oxidation of the azulene‐1‐carboxaldehydes 2 to the corresponding azulene‐1‐carboxylic acids (Schemes 9 and 10).     

Out-of-plane deformation of the azulene ring in crystal structures of simply substituted azulene derivatives

The crystal structures of 1,3-bis(4-bromophenyl)- and 1,3-di(2-thienyl)azulenes (5 and 6) were elucidated by X-ray analysis. Two aryl groups connect to the azulenyl core with dihedral angles of 34.9-41.6° and the two aryl planes of the groups slant against the azulene ring toward different ways in their crystal structures. It was also found that the azulene rings of 5 and 6 showed a slight out-of-plane deformation in the way that the hydrogen atoms at the 4- and 8-positions are apart from the neighboring aryl ortho-hydrogen atoms to fill in the vacant space made by the slanting aryl planes.     

Reactions of guaiazulene with methyl terephthalaldehydate and 2-hydroxy- and 4-hydroxybenzaldehydes in methanol in the presence of hexafluorophosphoric acid: comparative studies on molecular structures and spectroscopic, chemical and electrochemical properties of monocarbocations stabilized by 3-guaiazulenyl and phenyl groups

Reaction of guaiazulene (1) with methyl terephthalaldehydate (2) in methanol in the presence of hexafluorophosphoric acid at 25 °C for 2 h under aerobic conditions gives (3-guaiazulenyl)[4-(methoxycarbonyl)phenyl]methylium hexafluorophosphate (5) in 94% yield. Similarly, reactions of 1 with 2-hydroxybenzaldehyde (3) and 4-hydroxybenzaldehyde (4) under the same reaction conditions as 2 give (3-guaiazulenyl)(2-hydroxyphenyl)methylium hexafluorophosphate (6) and (3-guaiazulenyl)(4-hydroxyphenyl)methylium hexafluorophosphate (7) in 89 and 97% yields, respectively. Comparative studies on the molecular structures as well as the spectroscopic, chemical and electrochemical properties of the monocarbocation compounds 5-7 stabilized by 3-guaiazulenyl and 4-(methoxycarbonyl)phenyl (or 2-hydroxy- or 4-hydroxyphenyl) groups are reported.     

Direct aromatization of chlorohydroazulenones with triflic anhydride: access to chloroazulenyl triflates

A direct and efficient aromatization of chlorohydroazulenones has been achieved by using triflic anhydride and then lutidine or tropylium cation to afford selectively chloroazulenes and chloroazulenyl triflates, respectively.     

Synthesis and redox behavior of 1,2-dihydro-1-oxabenz[a]azulen-2-ones

Three new 1,2-dihydro-1-oxabenz[a]azulen-2-one derivatives, 1a (R¹=H, R²=Me), 1b (R¹=H, R²=Ph), and 1c (R¹=COOEt, R²=Me), have been synthesized by the reaction of 2-hydroxyazulene (2a) and its 1-ethoxycarbonyl derivative 2b with ethyl acetoacetate (3a) or ethyl benzoylacetate (3b) in the presence of aluminum chloride. To our knowledge, these are the first examples of this type of compound, although the yield of the products is low in some cases. Their electronic properties were studied in detail utilizing the analyses of 1,2-dihydro-1-oxabenz[a]azulen-2-one derivative 1a by the spectroscopic and voltammetric analyses. The analyses revealed that the fused α-pyrone system lowers both the HOMO and the LUMO energies, relative to those of parent azulene (10), but has much pronounced effect on the LUMO, consequently, leading to decrease in HOMO–LUMO gap, compared with those of 10. These results should be attracted to the development of amphoteric redox materials. Reactivity toward electrophilic reagents was also examined by bromination and Vilsmeier–Haack formylation reactions of 1a. To evaluate the scope of the reaction products we have examined Sonogashira cross-coupling reaction of the bromination products with trimethylsilylacetylene and conversion of the formylation product to dibromoolefin by the reaction with phosphorous ylide prepared with CBr₄ and Ph₃P. Effective extension of the π-electron system in the ethynyl products has been revealed by the spectroscopic analysis. These reaction products would be attracted to the application as a terminal group for electronic applications.     

From Blue Azulenes to Blue Heptalenes – New Strongly Polarized π‐Convertible Heptalenes

The 1,5,6,8,10‐pentamethylheptalene‐4‐carboxaldehyde ( 4b ) (together with its double‐bond‐shifted (DBS) isomer 4a ) and methyl 4‐formyl‐1,6,8,10‐tetramethylheptalene‐5‐carboxylate ( 15b ) were synthesized (Schemes 3 and 7, resp.). Aminoethenylation of 4a / 4b with N,N‐dimethylformamide dimethyl acetal (=1,1‐dimethoxy‐N,N‐dimethylmethanamine=DMFDMA) led in DMF to 1‐[(1E)‐2‐(dimethylamino)ethenyl]‐5,6,8,10‐tetramethylheptalene‐2‐carboxaldehyde ( 18a ; Scheme 9), whereas the stronger aminoethenylation agent N,N,N′,N′,N″,N″‐hexamethylmethanetriamine (=tris(dimethylamino)methane=TDMAM) gave an almost 1 : 1 mixture of 18a and 1‐[(1E)‐2‐(dimethylamino)ethenyl]‐5,6,8,10‐tetramethylheptalene‐4‐carboxaldehyde ( 20b ; Scheme 11). Carboxylate 15b delivered with DMFDMA on heating in DMF the expected aminoethenylation product 19b (Scheme 10). The aminoethenylated heptalenecarboxaldehydes were treated with malononitrile in CH₂Cl₂ in the presence of TiCl₄/pyridine to yield the corresponding malononitrile derivatives 23b, 24b , and 26a (Schemes 13 and 14). The photochemically induced DBS process of the heptalenecarboxaldehydes as ‘soft’ merocyanines and their malononitrile derivatives as ‘strong’ merocyanines of almost zwitterionic nature were studied in detail (Figs. 10–29) with the result that 1,4‐donor/acceptor substituted heptalenes are cleaner switchable than 1,2‐donor/acceptor‐substituted heptalenes.     

Efficient synthesis and applications of 2-substituted azulene derivatives: towards highly functionalized carbo- and heterocyclic molecules

An efficient synthesis of 2-substituted azulene derivatives (3-6) was accomplished from ethyl 2-oxo-2H-cyclohepta[b]furan-3-carboxylate 1 and its derivative 2, which in turn were prepared from readily available tropolone. Compounds 1 and 2 were utilized to construct densely functionalized benz[a]azulene and azulene-furan frameworks (16-25, 29-34, 37, 38).