Division of Organic Chemistry, Group of Functional Materials Synthesis, Centre of Molecular and Macromolecular Studies, Polish Academy of Sciences, Sienkiewicza 112, 90-363 Łódź, Poland; Faculty of Mathematics and Natural Sciences, Institute of Chemistry, Health and Food Sciences, Department of Material & Structural Chemistry, Jan Długosz University in Częstochowa, Armii Krajowej 13/15, 42-201 Częstochowa, Poland.
Division of Organic Chemistry, Group of Functional Materials Synthesis, Centre of Molecular and Macromolecular Studies, Polish Academy of Sciences, Sienkiewicza 112, 90-363 Łódź, Poland.
Ultrason Sonochem. 2019 Nov;58:104640. doi: 10.1016/j.ultsonch.2019.104640. Epub 2019 Jun 14.
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 (HCl/CHCN) 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 (FeCl/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.
两种芳基（芳基）甲基-O-缩醛和-O-二硫缩醛芳基（芳基）甲基硫醚的 Friedel-Crafts/Bradsher 环化的杂原子变体，在超声辅助下已经实现。环保的“氧化变体”（Oxo-F-C/B），在含有矿物酸和大量水的介质（HCl/CHCN）中进行，导致 RO 取代（杂）薁在不到 5 分钟的时间内以非常有效的方式形成。在“硫变体”（Thio-F-C/B）中，O-二硫缩醛芳基（芳基）甲基硫醚在非水介质（FeCl/KI/EtOH）中，在不到 25 分钟的时间内进行超声辅助环化，收率低于“氧变体”，得到 RS 取代（杂）薁。RO-（杂）薁在 25-60°C 的水介质中环化，但在有机溶剂中不环化，而 RS-（杂）薁需要更高的温度 55-60°C，并在有机溶剂中环化，但在水介质中不反应。与在无声条件下进行的反应相比，超声辅助反应的加速超过了 Oxo-F-C/B 最有效实例中的 7500 倍，平均为 Thio-F-C/B 的 2 倍。在超声和无声条件下提出了合理的反应机制。超声机制涉及干扰溶剂化层，并在由气泡坍塌产生的冲击波作用下形成反应性（“裸露”）碳正离子。O-缩醛芳基（芳基）甲基醚在酸性反应条件下，选择性地进行超声辅助脱缩醛，得到 O-甲酰基芳基（芳基）甲基醚，而不随后进行环化。
