Sulphamic acid: An efficient, cost-effective and recyclable solid acid catalyst for three-component synthesis of homoallylic amines

Abstract

Homoallylic amines are synthesized by the three-component coupling reaction of aldehydes, anilines and allyltributylstannane using heterogeneous solid acid catalyst, sulphamic acid under solvent-free conditions in excellent yields. The catalyst can be recovered by simple filtration and can be recycled in subsequent reactions. The method is simple, cost-effective and environmentally benign.

Introduction

Homoallylic amines are valuable intermediates in the synthesis of β-amino acids, β-lactams, γ-lactams, aziridines, amines, HIV-protease inhibitors and biologically active nitrogen-containing natural products [1], [2]. Hence, the synthesis of homoallylic amines is an essential task in organic synthesis. In general, homoallylic amines are prepared either by addition of organometallic reagents to imines or by nucleophilic addition of allylsilane, allyltin, allylboron, allylgermane or allylgallium reagents to imines in the presence of acid catalysts [2](c), [3]. Accordingly several methods have been reported for the allylation of aldimines in the presence of Lewis acids, such as TiCl₄, Sc(OTf)₃, BF₃ · OEt₂, PdCl₂(PPh₃)₂ or PtCl₂(PPh₃)₂, Seletfluor^™, Bi(OTf)₃, and iodine [4]. However, most of the synthetic protocols reported so far suffer from high temperatures, prolonged reaction times, harsh reaction conditions, low yields of the products and the use of hazardous and often expensive acid catalysts. Furthermore, many of these catalysts are deactivated or sometimes decomposed by amines and water that exit during imine formation. And also, the main disadvantage of most of the existing methods is that the catalysts are destroyed in the work-up procedure and cannot be recovered or re-used. Therefore, the development of simple, convenient and environmentally benign approaches for the synthesis of homoallylic amines is still desirable.

In recent years, the use of solid acids as heterogeneous catalysts has received tremendous interest in different areas of organic synthesis [5]. Heterogeneous solid acids are advantageous over conventional homogeneous acid catalysts as they can be easily recovered from the reaction mixture by simple filtration and can be reused after activation or without activation, thereby making the process economically more viable. During the last few years, sulphamic acid (NH₂SO₃H, SA) has emerged as a substitute for conventional acidic catalysts. Sulphamic acid is a common organic acid with mild acidity, in volatility and incorrositivity, is insoluble in common organic solvents. It is a white crystalline solid [6a], with outstanding physical stability and is commercially available cheap chemical. It has already been determined that SA is composed not of the amino sulfonic acid, but rather of ⁺H₃N⁻SO₃ zwitterionic units by both X-ray and neutron diffraction techniques [6](b), [6](c). In recent years, SA has been used as an efficient heterogeneous acid catalyst for acid catalyzed reactions, such as acetalization [7a], esterification [7](b), [7](c), acetylation of alcohols and phenols [7d], nitrile formation [7e], tetrahydropyranylation of alcohols [7f], transesterification of β-ketoesters [7g]. Moreover, some important organic transformations, including Biginelli condensation [7h], Beckmann rearrangement [7i], inter- and intramolecular imino Diels–Alder reactions [7j] and very recently Pechmann condensation [7k], have also been carried out in the presence of SA. The usual catalytic feature and intrinsic zwitterionic property of SA is very different from the conventional acidic catalyst, which encouraged us to investigate further applications of SA as an acidic catalyst in the other carbon–carbon and carbon-heteroatom bond forming reactions. However, there are no reports in the use of SA for the synthesis of homoallylic amines. The use of SA as a recyclable catalyst makes the process convenient, economic and environmentally benign.