Surprising Formation of Highly Substituted Azulenes on Thermolysis of 4,5,6,7,8-Pentamethyl-2H-cyclohepta[b]furan-2-one and Heptalene Formation with the New Azulenes

Abstract

Dedicated to Edgar Heilbronner on the occasion of his 80th birthday Der Mensch kommt nicht um die Tatsache herum, dass die Sinne zwar als höchste Herren seine Erkenntnisfähigkeit bestimmen, ihm aber keine Sicherheit bieten, da sie in jeder Lage sich leicht täuschen lassen. Michel de Montaigne, Essais (Übers. Hans Stilett)Heating of 4,5,6,7,8-pentamethyl-2H-cyclohepta[b]furan-2-one (1a) in decalin at temperatures b 1708 leads to the development of a blue color, typical for azulenes. It belongs, indeed, to two formed azulenes, namely 4,5,6,7,8-pentamethyl-2-(2,3,4,5,6-pentamethylphenyl)azulene (4a) and 4,5,6,7,8-pentamethylazulene (5a) (cf. Scheme 2 and Table 1). As a third product, 4,5,6,7-tetramethyl-2-(2,3,4,5,6-pentamethylphenyl)-1H-indene (6a) is also found in the reaction mixture. Neither 4,6,furan-2-one (1c) exhibit, on heating, such reactivity. However, heating of mixtures 1a/1b or 1a/1c results in the formation of crossed azulenes, namely 4,6,8-trimethyl-2-(2,3,4,5,6-pentamethylphenyl)azulene (4ba) and 2-(2,3,4,5,6-pentamethylphenyl)azulene (4ca), respectively (cf. Scheme 3). The formation of small amounts of 4,6,8-trimethylazulene (5ba) and azulene (5ca), respectively, besides 1H-indene 6a is also observed. The observed product types speak for an [8 2]-cycloaddition reaction between two molecules of 1a or between 1b and 1c, respectively, with 1a, whereby 1a plays in the latter two cases the part of the two-atom component (cf.Figs. 5 ± 7 and Schemes 4 ± 6). Strain release, due to the five adjacent Me groups in 1a, in the [8 2]cycloaddition step seems to be the driving force for these transformations (cf . Table 3), which are further promoted by the consecutive loss of two molecules of CO 2 and concomitant formation of the 10p-electron system of the azulenes. The new azulenes react with dimethyl acetylenedicarboxylate (ADM) to form the corresponding dimethyl heptalene-4,5-dicarboxylates 20, 22, and 24 (cf. Scheme 7), which give thermally or photochemically the corresponding double-bond-shifted (DBS) isomers 20', 22', and 24', respectively. The five adjacent Me groups in 20/20' and 24/24' exert a certain buttressing effect, whereby their thermal DBS process is distinctly retarded in comparison to 22/22', which carry isolated Me groups at C(6), C(8), and C(10). This view is supported by X-ray crystal-structure analyses of 22 and 24 (cf. Fig. 8 and Table 5).

Helical Nanographenes Embedded with Contiguous Azulene Units

“…Although considerable efforts have been devoted to theoretically elucidate the nature of azulene-embedded graphenes and GNRs, − detailed experimental investigations are still limited, probably because preparation of such graphenes in a structurally uniform manner is not a straightforward process owing to the isomerization among five-, six-, and seven-membered rings. , The bottom-up chemical synthesis of nanocarbons bearing model structures of such graphenes can resolve this problem. − However, the bottom-up synthesis of nanocarbons with multiple azulene units is still limited because of the absence of reliable methods, unlike that of other nanocarbons composed only of hexagonal rings and isolated nonhexagonal ring(s). − Peña, Pascual, and co-workers and Müllen, Fasel, Feng, and co-workers , addressed this issue by conducting on-surface dehydrocyclization of rationally designed precursors to incorporate azulene units into nanocarbons. Mastalerz, Zhang, and Chi also achieved in-solution construction of highly fused azulene units via Scholl cyclization of ingeniously tailored precursors.…”

Helical Graphene Nanostructures Embedded with an Azulene Cluster

“…Compared to those of other nanocarbons composed only of hexagonal rings and isolated non-hexagonal ring(s), [29][30][31] the bottom-up chemical syntheses of nanocarbons with multiple azulene units are still limited, because of the absence of reliable methods, especially in-solution ones. [32][33][34] Peña, Pascual and co-workers, 35 and Müllen, Fasel, Feng and co-workers 36 elegantly addressed this situation by utilising on-surface dehydrocyclisation of rationally designed precursors to incorporate azulene units into nanocarbons. Mastalerz 37…”

Helical Graphene Nanostructures Embedded with an Azulene Cluster

“…Compared to those of other nanocarbons composed only of hexagonal rings and isolated non-hexagonal ring(s), [29][30][31] the bottom-up chemical syntheses of nanocarbons with multiple azulene units are still limited, because of the absence of reliable methods, especially in-solution ones. [32][33][34] Peña, Pascual and co-workers, 35 and Müllen, Fasel, Feng and co-workers 36 elegantly addressed this situation by utilising on-surface dehydrocyclisation of rationally designed precursors to incorporate azulene units into nanocarbons.…”