Ultrasound-assisted synthesis of RO- and RS-substituted (hetero)acenes via oxo- and thio-Friedel-Crafts/Bradsher reactions

• Published in: Ultrasonics sonochemistry 2019-11, Vol.58, p.104640-104640, Article 104640
• Main Authors: Bałczewski, Piotr, Kowalska, Emilia, Skalik, Joanna, Koprowski, Marek, Owsianik, Krzysztof, Różycka-Sokołowska, Ewa
• Format: Article
• Language: English
• Subjects: Acenes; Cyclization; Deacetalization; Electrophilic aromatic substitution; Oxo- and thio-Friedel-Crafts/Bradsher reactions; Ultrasounds
• ISSN: 1350-4177; 1873-2828
• DOI: 10.1016/j.ultsonch.2019.104640

[Display omitted] •Use of ultrasounds in the oxo- and thio-Friedel-Crafts/Bradsher reactions.•The first SEAr sonocyclisation leading to RO- and RS-substituted acenes.•Over 7500-fold reduction in reaction time for the oxo-variant.•On average 2-fold reduction in reaction time for the thio-variant.•Sono-deacetalization of acetalaryl(aryl)methyl ethers without subsequent cyclization. Two heteroatom-variants of the Friedel-Crafts/Bradsher cyclization of o-acetalaryl(aryl)methyl ethers and o-dithioacetalaryl(aryl)methyl thioethers, have been realized with the ultrasound assistance. The environmentally friendly “oxo-variant” (Oxo-F-C/B), proceeding in a medium containing mineral acid and a large amount of water (HClaq/CH3CN) led to a very efficient formation of RO-substituted (hetero)acenes in less than 5 min. In the “thio-variant” (Thio-F-C/B), o-dithioacetalaryl(aryl)methyl thioethers underwent ultrasound-assisted cyclization in nonaqueous medium (FeCl3/KI/EtOH) in less than 25 min., in lower yields than in the “oxygen variant” to give RS-substituted (hetero)acenes. The RO-(hetero)acenes cyclized at 25–60 °C in aqueous media but did not cyclize in organic solvents while the RS-(hetero)acenes required higher temperatures 55–60 °C and cyclized in organic solvents but did not react in aqueous media. The acceleration of the ultrasound-assisted reactions compared to the reactions carried out under silent conditions exceeded 7500 times in the most effective example of the oxo-variant and on average 2 times for the thio-variant. The plausible reaction mechanisms under ultrasound and silent conditions have been proposed. The ultrasonic mechanism involves disturbing of solvation layers and formation of the reactive (“naked”) carbocations upon operation of the shock wave produced by the bubble collapse. The o-acetalaryl(aryl)methyl ethers underwent a selective ultrasound-assisted deacetalization to give o-formylaryl(aryl)methyl ethers, without subsequent cyclization under the acidic reaction conditions.