Organocatalyzed Beckmann rearrangement of cyclohexanone oxime by trifluoroacetic acid in aprotic solvent
Introduction
Nowadays, the production of ε-caprolactam is mainly based on the Beckmann rearrangement of cyclohexanone oxime employing oleum as homogeneous catalyst [1], [2]. In the industrial process problems due to product separation, hazardous working conditions, corrosion of the reactor and formation of large amounts of ammonium sulfate as by-product are encountered [1], [2].
In order to overcome these problems a large variety of solid acids were employed as catalysts in the Beckmann rearrangement of cyclohexanone oximes both in gas and liquid phase processes [3], [4], [5]. However, the fast catalyst deactivation, which is the main problem encountered with the heterogeneous systems, limits their practical application only to complex plants with continuum catalyst regeneration [6].
Recently, progresses on the Beckmann rearrangement are observed by using organic co-catalysts and promoters and these studies directly derive from the early studies of Beckmann and Kuhara [7], [8]. In their original works they respectively employed acetic anhydride and acetyl chloride as promoters for oximes rearrangement. In particular, Kuhara recognized the acetyl oxime as the active intermediate able to promote the reaction in the presence of hydrochloric acid [8]. Later, species like acetyl- and picril-oximes or oxime carbonate, tosylate, sulfonate etc. were extensively studied by many authors [8], [9], [10]. In general, reagents that allow the Beckmann rearrangement of ketoximes at relatively low temperatures convert the oximes to more reactive ether or ester intermediates [11]. These compounds, respect to the corresponding oximes, have a lower electronic density on the nitrogen atom and consequently a greater tendency to rearrange even without Brønsted acids [9], [12], [13], [14]. More recently, it has been observed that 2,4,6-trichlorotriazine (TCT) in N,N-dimethylformamide (DMF) is able to react with oximes giving in high yield the corresponding amides [15]. A catalytic process has been developed by using TCT in the presence of ZnCl2 as Lewis acid at 353–373 K in CH3CN. In such a system, however, substrates like cycloalkanone oximes do not react [16].
The need of using strong inorganic acids in the Beckmann rearrangement of cyclohexanone oxime is accepted as a common synthetic practice [17]. However, on considering the recent developments regarding the mechanism of the rearrangement, this kind of reaction does not necessarily need high protonation ability [18]. For instance, cyclohexanone oxime in aqueous solvent is fully protonated in the pH range 2–3 [19]. In the acid catalyzed Beckmann rearrangement the key points are the proton transfer from the nitrogen to the oxygen and the concerted extraction of the water molecule with the displacement of the carbon atom. This process is allowed by the formation of electron-poor nitrogen, which is the driving force for the rearrangement together with a strong solvent participation effect which consists in assisting the 1–2 shift by a proton-jump type mechanism, a particular solvent participation mechanism of the acid itself [18], [20].
The presence of a strongly electrophilic nitrogen may induce side reactions in the presence of nucleophiles such as water and/or the non protonated oxime [21], [22]. As a matter of fact, a strong acid is required to ensure the complete protonation of the oxime, thus avoiding side reactions due to the non protonated oxime [21], [22]. The solvent may play an important role on aiding the hydrogen transfer as well as the concerted water extraction with the consequent rearrangement [18]. This is the case of the Beckmann rearrangement of oximes in the presence of sulfamic acid [23]. Such a weak inorganic acid (pKa = 1.18) allows the rearrangement of several oximes in non aqueous solvents: in particular, with cyclohexanone oximes a yield of 40% in ε-caprolactam at 363 K after 6 h has been reported [23].
The rearrangement of the acetyl cyclohexanone oxime gives acetyl caprolactam, which readily reacts in the presence of cyclohexanone oxime to give ε-caprolactam and acetyl cyclohexanone oxime [7]. In a previous paper a catalytic cycle based on the rearrangement of the acetyl cyclohexanone oxime to acetyl caprolactam was attempted, but several side reactions limited its synthetic efficacy [21]. On the progress of these studies, the synthesis of trifluoroacetyl cyclohexanone was carried out by addition of trifluoroacetic anhydride to the oxime, but together with the trifluoracetylated product a clear yield in ε-caprolactam was observed. Starting from this evidence we gave some preliminary results on the rearrangement of cyclohexanone oxime promoted by CF3COOH in non aqueous solvent, where a practically quantitative conversion was obtained [21].
In the present paper we give new insight on the Beckmann rearrangement of cyclohexanone oxime catalyzed by CF3COOH in aprotic solvent: data relative to cyclohexanone oxime protonation equilibrium, interaction of ε-caprolactam with the acid, solvent effect on reaction kinetics and apparent activation energy are also given.
Section snippets
Materials
Reagents (cyclohexanone oxime, ε-caprolactam, trifluoroacetic acid), were purchased from Aldrich. The purity of the commercially available samples was checked by the usual methods (melting point, TLC, HPLC, GC and GC–MS) and further purifications were carried out when necessary. In particular, cyclohexanone oxime was crystallized from cyclohexane, dried in vacuo and stored under nitrogen at 248 K. Reaction solvents were HPLC grade products used without further purifications. Deuterated
Concentration profile: reagent, intermediates and products
A typical concentration profile of the Beckmann rearrangement in acetonitrile as a solvent is reported in Fig. 1. A qualitative analysis shows that the reaction follows an apparent zero order kinetics until conversion of 70–80%. In the other investigated solvents analogous reaction profiles are obtained, even though a noticeable difference on the initial reaction rate is observed (see Table 1 and Fig. 2). Except in benzonitrile as a solvent, where the reaction has a selectivity of 84% in
Conclusions
The synthetic interest of using CF3COOH as acid catalyst in the Beckmann rearrangement in aprotic solvent is due to several factors, such as the good activity and selectivity and the ease of the separation stage if compared with the common industrial processes. Respect to other organocatalyzed Beckmann rearrangements this reaction is a step forward because of the facile separation of the catalyst from the reaction mixture [15], [16], [23]. The reaction is also environmental friendly due to the
Acknowledgements
Financial support by Ca’ Foscari University of Venice is gratefully acknowledged (Ateneo fund 2007). A special thank to Mr. Claudio Tortato for the helpful discussions.
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