O,S-Acetals in a New Modification of oxo-Friedel–Crafts–Bradsher Cyclization—Synthesis of Fluorescent (Hetero)acenes and Mechanistic Considerations

Abstract

This paper presents the use of O,S-acetals in a new modification of the oxo-Friedel–Crafts–Bradsher cyclization. In this reaction, under mild reaction conditions (25 °C), three- and four-ring fused RO-acenes (major) and/or HO(CH2)2S-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 HO(CH2)2S-group, Hammett’s constants). Further increasing the reaction temperature, HCl concentration or prolonging reaction time, surprisingly, yielded a 2:1 mixture of cis and trans dimeric isomers, as the only products of this cyclization. The DFT calculations confirmed a greater stability of the cis isomer compared to the trans isomer. The formation of unexpected dimeric products and HO(CH2)2S-acenes sheds light on the mechanism of oxo-Friedel–Crafts–Bradsher cyclization, involving competitive O/S atom protonation in strained O,S-acetals and in strain-free side groups of intermediate species.

The First Example of the Friedel–Crafts Cyclization Leading to (10-Hydroxy-9,10-dihydroanthr-9-yl)phosphonium Salts without the Expected Bradsher Dehydration

“…In case of bulky triphenylphosphine, this ratio was even 12.3:1 (trans-13Cl:cis-13Cl), while in case of less-hindered diphenylmethylphosphine and dimethylphenylphosphine, it was almost 1.2:1 (trans-14MsO:cis-14MsO and trans-16BF 4 :cis-16BF 4 ). The intermediates of this reaction were most probably acyclic phosphonium salts 5A-10A, as in other hetero-F-C-B reactions [14,17], but they were not detected by the 31 P{ 1 H} NMR spectroscopy and were not isolated due to their rapid cyclization under an acidic conditions. The structure of trans-13Cl was further confirmed by a chloride anion exchange reaction with KPF 6 , leading to trans-13PF 6 (Scheme 2).…”

“…Earlier, we elaborated a new approach to mono-hetero (Z = OR, SR)-substituted acenes IV via the hetero-Friedel-Crafts-Bradsher (F-C-B) cyclization of ortho-O,O-acetals, S,S-dithioacetals and O,S-thioacetals II obtained from diarylmethanols I [14][15][16][17]. In this reaction, a new benzene ring, fused to two other (hetero)aromatic moieties, ArI and ArII, is formed to give acenes IV (Scheme 1, path a).…”