-Acetals in a New Modification of -Friedel-Crafts-Bradsher Cyclization-Synthesis of Fluorescent (Hetero)acenes and Mechanistic Considerations.

This paper presents the use of -acetals in a new modification of the -Friedel-Crafts-Bradsher cyclization. In this reaction, under mild reaction conditions (25 °C), three- and four-ring fused -acenes (major) and/or -acenes (minor) are formed, the latter products having never been observed before in this type of cyclization. In this way, two electronically different fluorophores could be obtained in a single cyclization reaction, one of them having strong electron donor properties (+M effect of alkoxy groups) and the other having donor-acceptor properties (+M and -I effects of the -group, Hammett's constants). Further increasing the reaction temperature, HCl concentration or prolonging reaction time, surprisingly, yielded a 2:1 mixture of and dimeric isomers, as the only products of this cyclization. The DFT calculations confirmed a greater stability of the isomer compared to the isomer. The formation of unexpected dimeric products and -acenes sheds light on the mechanism of -Friedel-Crafts-Bradsher cyclization, involving competitive O/S atom protonation in strained -acetals and in strain-free side groups of intermediate species.