ZrOCl2·8H2O on montmorillonite K10 accelerated conjugate addition of amines to α,β-unsaturated alkenes under solvent-free conditions

doi:10.1016/j.tet.2005.10.006

Tetrahedron

Volume 62, Issue 4, 23 January 2006, Pages 672-677

https://doi.org/10.1016/j.tet.2005.10.006 Get rights and content

Abstract

At room temperature, ZrOCl₂·8H₂O on montmorillonite K10 efficiently catalyzes conjugate addition of amines to a variety of conjugated alkenes such as α,β-unsaturated carbonyl compounds, carboxylic esters, nitriles and amides under solvent-free conditions. The catalyst can be recycled for subsequent reactions without any appreciable loss of efficiency.

Graphical Abstract

Introduction

The development of new synthetic methods leading to β-amino ketones or their derivatives has attracted much attention in organic synthesis. These β-amino ketones are attractive targets for chemical synthesis because of their prevalence and wide utility. Among the methods for generating β-amino ketones, the Mannich reaction is a classic method for the preparation of these derivatives.¹ However, due to the drastic reaction conditions and the long reaction times, the classic Mannich reaction presents serious disadvantages.¹ The approach based on conjugate addition of amines to α,β-unsaturated carbonyl compounds is one of the most simple and effective alternative methods for preparing β-amino ketones.²

The Michael reaction, which was discovered many years ago, has been used as one of the most useful methods for effecting carbon–carbon bond formation and later has also been efficiently manipulated for carbon-sulfur and carbon-nitrogen bond forming processes.² This reaction is usually carried out under acid or base catalysis.³ However, to avoid side reactions, occasionally encountered in the presence of a strong acid or a base, a number of alternative procedures have been developed in the past few years and in particular, various Lewis acid-induced reactions have been reported.⁴ Unfortunately, many of these procedures often require a large excess of reagents, long reaction time and drastic reaction conditions in acetonitrile or 1,2-dichloroethane which are toxic. In some cases, a stoichiometric amount of Lewis acid such as AlCl₃, TiCl₄ or SnCl₄ are required.⁴

The use of zirconium(IV) salts as an efficient Lewis acid for various transformations, such as electrophilic amination of activated arenes,⁵ transthioacetylization of acetals,⁶ deoxygenation of heterocyclic-N-oxides,⁷ reduction of nitro compounds,⁸ conversion of carbonyl compounds to 1,3-oxathiolanes,⁹ selective oxidation of primary and benzylic alcohols,¹⁰ synthesis of nitriles from O-arylaldoximes,¹¹ chemoselective N-nitrosation of secondary amines,¹² Michael reaction of 1,3-dicarbonyls and enones,¹³ opening of epoxide rings by amines,¹⁴ reactions of indole, 1-methylindole, and pyrrole with α,β-unsaturated ketone¹⁵ and Friedel-Crafts reactions has been well documented in the literature.

Synthetic chemists have been using highly dispersed mineral solids with extensive specific areas, for a considerable time. Many organic reactions have been devised in which the reagents are deposited on various inorganic solid supports. These reagents have advantages over the conventional homogeneous solution techniques: easy set-up and work-up, mild experimental conditions, high yields and/or selectivity.¹⁶ Montmorillonite K10 had a great impact in organic synthesis and has offered major breakthroughs for the fine chemicals manufacturing industries.¹⁷

As part of our research on chemical transformations,¹⁸ in this paper we report a simple and environmentally benign methodology for the conjugate addition of amines to α,β-unsaturated esters, nitriles, amides and ketones under neutral, solvent-free conditions and at room temperature using ZrOCl₂·8H₂O on montmorillonite K10 as catalyst.

Section snippets

Results and discussion

Different types of amines (primary and secondary) were subjected to conjugate addition reactions on the clay-supported zirconium oxychloride. The overall reaction is as shown in Scheme 1.

We started to study the conjugate addition catalyzed by ZrOCl₂ supported on the montmorillonite K10 by examining the conditions required for the reaction involving piperidine and methylacrylate to afford adduct 1a. A summary of the results obtained is provided in Table 1. Entries 1–7 show the effect of various

Conclusion

In conclusion, we have developed a new simple method for accelerating the conjugate reaction of amines with α,β-unsaturated olefins by using the ZrOCl₂·8H₂O/montmorillonite K10. Due to the stability of the catalyst under the reaction conditions, it is possible to recover it by simple filtration and use it again after reactivating it by heating in vacuum at 80 °C. The present procedure provides an efficient and general methodology for the preparation of β-amino esters, nitriles, amides and

General

NMR spectra were recorded on a Bruker ACF 500. IR spectra were measured using a Perkin Elmer 781 spectrometer. Column chromatography was performed on silica gel, Merck grade 60. CH₂Cl₂ was distilled before use. ZrOCl₂·8H₂O and other chemicals were purchased from Fluka or Merck.

Procedure for preparation of catalyst

ZrOCl₂·8H₂O (3.35 g) was added to deionized water (20 mL) and the mixture was stirred for 5 min until complete dissolution of the zirconium oxychloride. Montmorillonite K10 (6.65 g) was then added. The resulting suspension

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ZrOCl2·8H2O on montmorillonite K10 accelerated conjugate addition of amines to α,β-unsaturated alkenes under solvent-free conditions

Abstract

Introduction

Section snippets

Results and discussion

Conclusion

General

Procedure for preparation of catalyst

Tetrahedron Lett.

J. Am. Chem. Soc.

Chem. Lett.

Synlett

Asian J. Chem.

Tetrahedron Lett.

Angew. Chemi. Int. Ed. Engl.

Synthesis

Tetrahedron

Tetrahedron

Angew. Chem., Int. Ed. Engl.

Conjugated Addition Reactions in Organic Synthesis

Synlett

J. Org. Chem.

Tetrahedron

Synlett

Tetrahedron

Synth. Commun.

Tetrahedron Lett.

Tetrahedron Lett.

Synthesis

Tetrahedron: Asymmetry

J. Chem. Soc., Perkin Trans. 1

Tetrahedron

J. Org. Chem.

Synlett

Synlett

ZrOCl₂·8H₂O on montmorillonite K10 accelerated conjugate addition of amines to α,β-unsaturated alkenes under solvent-free conditions