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.