Elsevier

Tetrahedron

Volume 66, Issue 34, 21 August 2010, Pages 6858-6863
Tetrahedron

Cs2.5H0.5PW12O40 catalyzed diastereoselective synthesis of β-amino ketones via three component Mannich-type reaction in water

https://doi.org/10.1016/j.tet.2010.06.052Get rights and content

Abstract

A series of insoluble salts of Keggin heteropoly compounds were prepared and used as catalysts for the Mannich-type reaction of benzaldehyde, aniline, and cyclohexanone in water. Among them, Cs2.5H0.5PW12O40 showed excellent catalytic activity. Effects of surfactant, catalyst loading and temperature were studied to introduce the best reaction condition. The optimized reaction conditions were extended to Mannich reaction of various aldehydes, ketones, and amines in water. This rapid procedure afforded structurally divers β-amino ketones with major anti diastereoselectivity. Additionally, four new compounds were reported. The catalyst was recovered and reused for subsequent runs.

Introduction

One of the important goals in organic chemistry is the development of new methodologies for the synthesis of functionalized biologically active compounds with structural diversity from simple and readily available precursor molecules.

β-Amino carbonyl compounds are important intermediates for various pharmaceutical and natural product syntheses including β-amino alcohols, β-amino acids, and lactams.1, 2, 3 Among the various methods for the synthesis of these compounds, Mannich-type reaction constitutes a method of choice in many cases due to its versatility.4

Numerous versions of the Mannich reaction have been developed to overcome drawbacks associated with the classical Mannich reaction. One of the efficient routes is to conduct a one-pot three-component reaction of an aldehyde, an amine and a ketone (Scheme 1) with the assistance of ultrasound irradiation5 or using Lewis acids,6, 7, 8 Lewis base,9 Brönsted acids,10, 11, 12, 13, 14 organocatalyst15 or praline16 to catalyze the reaction.

Despite the merits of these methodologies, problems recycling and reusing of the catalysts, harsh reaction conditions, toxicity or difficulty in product separation remain concerns. Therefore, the search for new readily available green catalysts is still being actively pursued. In this context, we focused on the development of reusable heteropoly acid (HPA) catalyst to perform this transformation under milder reaction conditions and to enhance the generality of the reaction.

Section snippets

Results and discussion

The pKa of HPAs is lower than that of the conventional solid acid catalysts.17 Therefore, they are generally active catalysts, which allow efficient operation under mild conditions. Owing to inherent advantages of HPAs, heterogeneous acid catalysis by these compounds has the potential of a great economic reward and green benefits. In order to achieve this goal, one way is to exchange their protons with large alkali metal cations such as K+, Cs+, or [(n-Bu)4N]+ to form insoluble salts.18

Taking

Conclusion

Cs2.5PW as heterogeneous catalyst showed effective catalytic activity in the synthesis of β-amino ketones via three component Mannich reaction in water. The good yields, mild conditions, low catalyst loading and convenient operations as well as no formation of by-products, such as aldol or deamination products made it believable that this protocol would play important role in the synthesis of natural products.

General

H3PMo12O40, [(n-C4H9)4N]Br, [(t-C4H9)4N]Br, Cs2CO3, TiCl4, SnCl4, KCl, HOAc, H3PO4, sodium molybdate and sodium tungstate were obtained from Merck and SnCl2 from Fluka were used as supplied. H3PW12O40, Co(OAc)2·4H2O and potassium persulfate purchased from Aldrich. CPB, CTAB, Triton X-100, SOS, SDeS, and SDS were obtained from Aldrich and used as received without further purification. The organic materials were commercially available and purchased from Fluka, Merck, and Aldrich chemical

Acknowledgements

The authors thank the Razi University Research Council and Kermanshah Oil Refining Company for support of this work.

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