Abstract
Cyclopentyl methyl ether and 2-methyltetrahydrofuran, low impact ethereal solvents forming a positive azeotrope with water, were successfully employed as solvents in the synthesis of a variety of acetals carried out under Dean–Stark conditions in the presence of heterogeneous acidic catalysts. Under these conditions, ammonium salts, either as such or supported on SiO2, performed better or equally well than widely employed homogeneous and heterogeneous acidic catalysts such as p-toluenesulfonic acid, Amberlyst 15®, or Montmorillonite K10. Several examples highlight the advantage of tuning the relative acidities of ammonium salts by appropriately selecting the counterion. Within one of these examples, our protocol clearly outweighs the classic p-toluenesulfonic acid/toluene protocol in terms of chemoselectivity. Silica-supported catalysts were characterized by SEM, TEM, and FTIR spectroscopies, as well as by N2 physisorption. Such a characterization reveals an even distribution of ammonium salts on silica, thus confirming the formation of expected catalytic supports.
Highlights
The setup of widely employed reactions under economical, practical, and environmentally friendly conditions is a challenge of wide interest in contemporary organic chemistry
Starting from these premises and following our interest in the development of reaction procedures in environmentally friendly solvents [20,21,22,23], we investigated the acetalization of aldehydes and ketones in well-known low impact ethereal solvents forming positive azeotropes with H2 O (Table 1), i.e., cyclopentyl methyl ether (CPME) [24,25,26,27] and 2-methyltetrahydrofuran (2-MeTHF) [28,29,30,31]
Few reactions were run with a homogeneous acidic catalyst, i.e., para-toluenesulfonic acid (p-TSA), or with two widely employed, commercially available, heterogeneous acidic catalysts, i.e., Amberlyst 15® and Montmorillonite K10 (Mont K10)
Summary
The setup of widely employed reactions under economical, practical, and environmentally friendly conditions is a challenge of wide interest in contemporary organic chemistry. From this point of view, great attention should be dedicated to the appropriate choice of solvents as they represent one of the major contributions to the environmental impact of fine chemical synthesis [1,2,3,4,5,6]. Acetals are widely employed in multistep organic synthesis for the protection of carbonyl compounds towards basic and nucleophilic reagents, due to their general stability to a wide range of reagents and ease of removal.
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