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ball milling for the quantitative and specific solvent

b Faculty 5, Department of Chemistry, University of Oldenburg, 26188 Edewecht, Germany

Ball milling is used as a facile, efficient, cheap, and environmentally friendly procedure for the solvent-free three-component reaction of aromatic aldehydes with malononitrile and dimedone, or 1,3-cyclohexanedione, resorcinol, and α- and β-naphthol. The reactions proceed quantitatively at room temperature by milling stoichiometric mixtures of the reagents in the presence of 10 mol% Na2CO3. This method offers the advantages of short reaction times, low cost, quantitative yields and simple work-up with no need of any organic solvent. It is thus enviro-economic without producing dangerous wastes. The orientation selectivity with β-naphthol and resorcinol is discussed. Existing structural misattributions are pointed out and clarified by 1H NMR spin coupling analysis

The versatile synthesis of 2-amino-3-cyano-4-aryl-4H-chromenes is presently one of the most studied vehicles for the presentation of new catalysts with mostly weak acids and bases, even though uncatalyzed reactions of aldehydes with malononitrile and 1,3-diketones provide quantitative wasteless yields in stoichiometric solventless melt-reactions at still moderate temperatures (125–150 °C). Clearly, the uncatalyzed syntheses provide the best possible scale-up possibilities,1 but also enviro-economic catalysis by milling with soda at room temperature is of interest for reactions with less reactive reagents.2 The general importance of chromene derivatives or amino-4H-chromenes has been amply reported in ref. 3 and in the numerous papers cited therein, but we focus here on the importance of 2-amino-3-cyano-4-aryl-4H-chromenes. These compounds have been identified as having the capacity to prevent various types of human diseases.4,5 These include antiallergic,6 antitumor7–9 and antibacterial activities.10 Most recent interest has focused on these compounds with additional substituents for anti-microbial activities in pharmaceutical applications and biological studies.11,12 Furthermore, the 2-amino-4-aryl-3-cyano-substituted 4H-chromenes are being used as building blocks for the synthesis of human drugs for many kinds of neurodegenerative and other serious diseases.13,14 For example, most 4H-chromene derivatives with particular substituents show antibacterial activity even against highly resistant Staphylococcus aureus and others.15 Several of these compounds are thus already commercially available. Importantly, also both the 2-amino and the 3-cyano substituents have been variously converted with standard transformations to create a multitude of compounds with useful applications. But unfortunately there is some confusion as to the correct chemical structure of these very important commercial compounds, because the necessary corrections of unduly claimed orientation selectivities in some of the multiple component reactions (MCRs) were never performed, and the reasons for the selectivities were not discussed.

ball milling for the quantitative and specific solvent

The synthesis by Knoevenagel condensation + Michael addition of the highly substituted 4H-chromenes has been varied by using many methods in recent years in an attempt to improve ease and efficiency by catalysis. The two-step approach by cyclization of preformed arylidenemalononitriles with β-dicarbonyl compounds is accelerated by the presence of an organic base such as piperidine,16 morpholine, pyridine,17 and triethylamine.18,19 Furthermore, despite our report of an early quantitative wasteless procedure for stoichiometric melt reactions without catalyst,1 further two-step procedures have been tried to decrease the temperature by use of a catalyst at the expense of lower yield and the need for chromatographic workup.20–37 This is also true for the later tried MCRs for the preparation of 4H-chromenes. The catalytic processes for the one-pot synthesis of 2-amino-4-aryl-3-cyano-substituted 4H-chromenes have been recently complemented by the application of potassium phthalimido-N-hydroxylate in water as another base though by use of 10% excess of the reagent malononitrile,3 which however creates additional wastes when recycling the catalyst. Other approaches utilized for example hexadecyltrimethylammonium bromide (HTMAB),20 sodium stearate,21 4-dodecylbenzenesulfonic acid (DBSA),20 KF–Al2O3,22 the rare earth perfluorooctanoate (Re(PFO)3),23 (S)-proline,24 (NH4)2HPO4,25 NMe4OH,26 MgO,27,28 tetrabutylammonium fluoride (TBAF),29 CeCl3·7H2O,30 ZnO-beta-zeolite,31 1,4-diazabicyclo[2.2.2]octane (DABCO),32 silica gel-supported polyphosphoric acid (PPA–SiO2),33γ-Fe2O3,34 Caro's acid silica gel,35 and amino-functionalized ionic liquids.36 Some of these methods applied organic solvent such as ethanol, dimethylformamide (DMF) or acetic acid,24,37 others water with propylpiperazine-N-sulfonic acid,38 or hydrocalcite,39 or 1-deoxy-1-methylaminosorbitol.40 Also, heating by microwave41 or ultrasonic irradiation42 was applied. However, most of these procedures have drawbacks of producing dangerous wastes or providing less than quantitative yield of pure product. These are as yet only overcome upon stoichiometric heating of the components at 150 °C and 100–130 °C without catalyst.1 Some catalysis protocols still require elevated temperatures, often hazardous organic solvents, and organic bases. Furthermore, the catalysts are often expensive or not commercially available, and tedious workup procedures are the rule with many wastes, because of only moderate product yields, requiring chromatographic workup and recrystallization. Thus, quantitative yield of the target products at room temperature omitting wasteful workup is highly desired. We therefore applied the usefulness of the solventless (solvent-free when choosing aqueous workup) stoichiometric MCR technique with the easily removable environmentally safe catalyst sodium carbonate in a ball-mill,2,43–47 and keep in mind the possibility that ball milling is scalable to hundreds of grams,48–52 which contrasts with recent reviews on milling in organic synthesis with complaints of lack of scale-up developments.53 But the existence of the reviews48–52 on such scale-up (further applications in ref. 48–52) has been pointed out.54 We have already reported that combining MCRs and ball milling allows for the synthesis of a wide variety of interesting organic building blocks.43–52,55,56 We also reported that sodium carbonate can be a useful basic catalyst in kneading milling.44–47 This technique benefits from both the economical and the synthetic point of view by not only omitting the unnecessary use of organic solvents, but also enhancing the rate of many organic reactions.

The ongoing hype with multiple repetitions of the MCR syntheses in question, providing high melting-point products, bears the risk that the characterization of the products rests solely on melting point comparison rather than on interpretation of 1H NMR coupling schemes that must fit with the claimed orientation specificities in these reactions. Unfortunately, there are never-resolved discrepancies in the claimed molecular structures when for example β-naphthol or resorcinol was the reagent for Michael additions followed by cyclization, and one must not invoke melting points of published unassigned or merely claimed molecular structures without considering a safe structure elucidation.

effect of al(oh) particle size on microstructures and

a Research Laboratory of Green Organic Synthesis and Polymers, Department of Chemistry, Iran University of Science and Technology, 16846 Tehran, Iran E-mail: [email protected]

b Faculty 5, Department of Chemistry, University of Oldenburg, 26188 Edewecht, Germany

Ball milling is used as a facile, efficient, cheap, and environmentally friendly procedure for the solvent-free three-component reaction of aromatic aldehydes with malononitrile and dimedone, or 1,3-cyclohexanedione, resorcinol, and α- and β-naphthol. The reactions proceed quantitatively at room temperature by milling stoichiometric mixtures of the reagents in the presence of 10 mol% Na2CO3. This method offers the advantages of short reaction times, low cost, quantitative yields and simple work-up with no need of any organic solvent. It is thus enviro-economic without producing dangerous wastes. The orientation selectivity with β-naphthol and resorcinol is discussed. Existing structural misattributions are pointed out and clarified by 1H NMR spin coupling analysis

The versatile synthesis of 2-amino-3-cyano-4-aryl-4H-chromenes is presently one of the most studied vehicles for the presentation of new catalysts with mostly weak acids and bases, even though uncatalyzed reactions of aldehydes with malononitrile and 1,3-diketones provide quantitative wasteless yields in stoichiometric solventless melt-reactions at still moderate temperatures (125–150 °C). Clearly, the uncatalyzed syntheses provide the best possible scale-up possibilities,1 but also enviro-economic catalysis by milling with soda at room temperature is of interest for reactions with less reactive reagents.2 The general importance of chromene derivatives or amino-4H-chromenes has been amply reported in ref. 3 and in the numerous papers cited therein, but we focus here on the importance of 2-amino-3-cyano-4-aryl-4H-chromenes. These compounds have been identified as having the capacity to prevent various types of human diseases.4,5 These include antiallergic,6 antitumor7–9 and antibacterial activities.10 Most recent interest has focused on these compounds with additional substituents for anti-microbial activities in pharmaceutical applications and biological studies.11,12 Furthermore, the 2-amino-4-aryl-3-cyano-substituted 4H-chromenes are being used as building blocks for the synthesis of human drugs for many kinds of neurodegenerative and other serious diseases.13,14 For example, most 4H-chromene derivatives with particular substituents show antibacterial activity even against highly resistant Staphylococcus aureus and others.15 Several of these compounds are thus already commercially available. Importantly, also both the 2-amino and the 3-cyano substituents have been variously converted with standard transformations to create a multitude of compounds with useful applications. But unfortunately there is some confusion as to the correct chemical structure of these very important commercial compounds, because the necessary corrections of unduly claimed orientation selectivities in some of the multiple component reactions (MCRs) were never performed, and the reasons for the selectivities were not discussed.

effect of al(oh) particle size on microstructures and

The synthesis by Knoevenagel condensation + Michael addition of the highly substituted 4H-chromenes has been varied by using many methods in recent years in an attempt to improve ease and efficiency by catalysis. The two-step approach by cyclization of preformed arylidenemalononitriles with β-dicarbonyl compounds is accelerated by the presence of an organic base such as piperidine,16 morpholine, pyridine,17 and triethylamine.18,19 Furthermore, despite our report of an early quantitative wasteless procedure for stoichiometric melt reactions without catalyst,1 further two-step procedures have been tried to decrease the temperature by use of a catalyst at the expense of lower yield and the need for chromatographic workup.20–37 This is also true for the later tried MCRs for the preparation of 4H-chromenes. The catalytic processes for the one-pot synthesis of 2-amino-4-aryl-3-cyano-substituted 4H-chromenes have been recently complemented by the application of potassium phthalimido-N-hydroxylate in water as another base though by use of 10% excess of the reagent malononitrile,3 which however creates additional wastes when recycling the catalyst. Other approaches utilized for example hexadecyltrimethylammonium bromide (HTMAB),20 sodium stearate,21 4-dodecylbenzenesulfonic acid (DBSA),20 KF–Al2O3,22 the rare earth perfluorooctanoate (Re(PFO)3),23 (S)-proline,24 (NH4)2HPO4,25 NMe4OH,26 MgO,27,28 tetrabutylammonium fluoride (TBAF),29 CeCl3·7H2O,30 ZnO-beta-zeolite,31 1,4-diazabicyclo[2.2.2]octane (DABCO),32 silica gel-supported polyphosphoric acid (PPA–SiO2),33γ-Fe2O3,34 Caro's acid silica gel,35 and amino-functionalized ionic liquids.36 Some of these methods applied organic solvent such as ethanol, dimethylformamide (DMF) or acetic acid,24,37 others water with propylpiperazine-N-sulfonic acid,38 or hydrocalcite,39 or 1-deoxy-1-methylaminosorbitol.40 Also, heating by microwave41 or ultrasonic irradiation42 was applied. However, most of these procedures have drawbacks of producing dangerous wastes or providing less than quantitative yield of pure product. These are as yet only overcome upon stoichiometric heating of the components at 150 °C and 100–130 °C without catalyst.1 Some catalysis protocols still require elevated temperatures, often hazardous organic solvents, and organic bases. Furthermore, the catalysts are often expensive or not commercially available, and tedious workup procedures are the rule with many wastes, because of only moderate product yields, requiring chromatographic workup and recrystallization. Thus, quantitative yield of the target products at room temperature omitting wasteful workup is highly desired. We therefore applied the usefulness of the solventless (solvent-free when choosing aqueous workup) stoichiometric MCR technique with the easily removable environmentally safe catalyst sodium carbonate in a ball-mill,2,43–47 and keep in mind the possibility that ball milling is scalable to hundreds of grams,48–52 which contrasts with recent reviews on milling in organic synthesis with complaints of lack of scale-up developments.53 But the existence of the reviews48–52 on such scale-up (further applications in ref. 48–52) has been pointed out.54 We have already reported that combining MCRs and ball milling allows for the synthesis of a wide variety of interesting organic building blocks.43–52,55,56 We also reported that sodium carbonate can be a useful basic catalyst in kneading milling.44–47 This technique benefits from both the economical and the synthetic point of view by not only omitting the unnecessary use of organic solvents, but also enhancing the rate of many organic reactions.

The ongoing hype with multiple repetitions of the MCR syntheses in question, providing high melting-point products, bears the risk that the characterization of the products rests solely on melting point comparison rather than on interpretation of 1H NMR coupling schemes that must fit with the claimed orientation specificities in these reactions. Unfortunately, there are never-resolved discrepancies in the claimed molecular structures when for example β-naphthol or resorcinol was the reagent for Michael additions followed by cyclization, and one must not invoke melting points of published unassigned or merely claimed molecular structures without considering a safe structure elucidation.

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