Chemoselective synthesis of β-amino acid derivatives by hydroamination of activated olefins using AlSBA-15 catalyst prepared by post-synthetic treatment

Abstract

β-Amino acid derivatives have a wide variety of applications viz. in the synthesis of peptide analogues, precursor for amino alcohols, optically active amino acids, lactums and diamines. Chemoselective anti-Markovnikov hydroamination reaction of activated olefins was effectively used to synthesize β-amino acid derivatives using AlSBA-15 and AlMCM-41 catalysts. These catalysts with different Si/Al ratios were synthesized by isomorphous substitution of aluminium into the framework of SBA-15, which induces the Brönsted and Lewis acid sites. The structural integrity of the catalysts was established by characterizing with XRD, N₂-sorption, TEM, NH₃-TPD, ²⁷Al MAS NMR and ²⁹Si MAS NMR techniques. Hydroamination of ethyl acrylate with aniline was used as a test reaction, which gave N-[2-(ethoxycarbonyl)ethyl]aniline with high selectivity. The performance of AlSBA-15 catalyst was also determined with different acrylates and amines to know the general applicability of the catalyst in hydroamination reactions.

Introduction

The catalytic addition of an N–H bond across a multiple bond (hydroamination), to give valuable nitrogen-containing molecules is of great interest to both academic and industrial researchers [1]. The hydroamination reaction proceeds with 100% atom economy which makes it one of the most desirable processes applicable in the area of natural products, pharmaceuticals, dyes, fine chemicals, polymers and surfactants [2]. A prime example for this type of reaction is the hydroamination of easily available olefins with amines. Regarding thermodynamics, the addition of amines to alkenes is approximately thermoneutral, whereas the corresponding addition to alkynes is slightly exothermic [3]. There are several reports on hydroamination of unactivated alkenes especially catalyzed by transition metal complexes in homogeneous and heterogeneous conditions [4], [5], [6], [7]. On the other hand, hydroamination of activated olefins (acrylate derivatives) is a Michael type addition reaction, which is a simplistic approach to synthesize amino acid derivatives. These amino acid derivatives have a wide variety of applications viz. in the synthesis of peptide analogues, precursor for amino alcohols, optically active amino acids, lactums and diamines [8], [9]. In earlier studies, Brönsted and Lewis acids such as H₂SO₄, HBF₄ and FeCl₃ have been used for this reaction [10], [11]. Recently hydroamination of acrylates with amines has been reported with copper and bismuth salts [12], [13], and complexes of Ni(II) and Pd(II) [14], [15]. However, homogeneous methods suffer from tedious work-up procedures, low catalyst recyclability and above all the environmental problems. Scope of heterogeneous organic transformations is growing due to their well-documented advantages over homogeneous catalytic systems. But surprisingly there are very few reports available on the use of heterogeneous catalysts for hydroamination of activated olefins. Zeolite beta and clays have been reported so far for this reaction [16], [17], [18], [19]. Zeolites cannot be used for larger substrates because of their smaller pore sizes, whereas clays have low thermal stability which affects the catalyst regeneration. Palladium complexes immobilized on silica and alumina surfaces have been reported for hydroamination of O-activated alkenes [20]. Mesoporous materials such as MCM-41 and SBA-15 have advantages over zeolites for their larger pore size and higher surface area [21]. The isomorphous substitution of aluminium into the mesoporous framework of MCM-41 and SBA-15 induces the Brönsted and Lewis acidity [22]. AlSBA-15 has a larger pore diameter, thicker pore walls and higher hydrothermal stability compared to MCM-41. Hence AlSBA-15 has been used for hydroamination of activated olefins and the results are compared with that of AlMCM-41.

To create Brönsted and Lewis acidic sites in SBA-15, much effort has been made on the incorporation of heteroatom, such as Al, in the framework of mesoporous silica by post-grafting or doping (one-pot synthesis). However, it is very difficult to introduce the metals directly into SBA-15 due to the easy dissociation of metal–O–Si bonds under strong acidic conditions. Only a few studies on the direct synthesis of Al-SBA-15 have been reported so far [23], [24], [25], [26], [27], [28]. Thus, the post-synthesis method for the alumination of mesoporous silicas, which are obtained by ion exchange, becomes a useful alternative. The researchers have demonstrated that Al can be effectively incorporated into siliceous MCM-41 and MCM-48 materials via various post-synthesis procedures [29], [30], [31]. The authors claimed that the materials produced via the post-synthesis method have structural integrity, acidity and catalytic activity similar to those of materials prepared by in situ method. However, as of present, very few post-synthesis alumination methods for SBA-15 have been reported [32], [33], [34], [35], [36]. In our earlier studies, we have successfully demonstrated montmorillonite clays and mesoporous materials as catalysts for intermolecular hydroamination of activated olefins and alkynes with amines [19], [37], [38], [39], [40]. The present study deals with the synthesis, characterization and the applications of AlSBA-15 in the synthesis of β-amino acid derivatives by the hydroamination of activated olefins with amines.