Modular Synthesis of Substituted [n]Helicenes (n = 5–7) Starting from Arylmethyl Sulfone and Arylene Dialdehyde: Aldol-Type Condensation, Photocyclization, and Desulfonylative Arylation

Bis(p-methoxyphenysulfonyl)-substituted helicenes were successfully synthesized via UV light irradiation of the corresponding (E,E)-bis(2-aryl-2-(p-methoxyphenysulfonyl)ethenyl)arylenes, which were prepared through aldol-type condensation of arylene dialdehydes with arylmethyl p-methoxypheny sulfones. By varying the combination of dialdehydes and arylmethyl p-methoxypheny sulfones, a series of helicene homologues, ranging from [5] to [7]helicenes, were obtained. The p-methoxyphenysulfonyl groups in these helicenes were efficiently replaced with Grignard reagents via Ni-catalyzed Kumada–Tamao–Corriu coupling, yielding the corresponding alkyl- and aryl-substituted derivatives. These dialkyl- and diaryl-substituted helicenes were further expanded into larger π-conjugated systems. The trimethylsilylmethyl-substituted derivative underwent sequential bromination, phosphonation, and a Wittig–Horner reaction with arylaldehydes, affording olefinic helicenes. Furthermore, FeCl₃-promoted oxidative annulation of the biphenyl-1-yl-substituted derivative resulted in the formation of a “two-blade propeller” closed [6]helicene, fused with dibenzo[g,p]chrysene arrays.