Article pubs.acs.org/IECR
Alkoxycarbonylation of Industrially Relevant Anilines Using Zn4O(O2CCH3)6 as Catalyst Elisenda Reixach,† Robert M. Haak,† Stefan Wershofen,‡ and Anton Vidal-Ferran*,†,§ †
Institute of Chemical Research of Catalonia (ICIQ), Avinguda Països Catalans, 16, E-43007 Tarragona, Spain Bayer MaterialScience, D-47812 Krefeld, Germany § Catalan Institution for Research and Advances Studies (ICREA), Passeig Lluís Companys 23, E-08010 Barcelona, Spain ‡
S Supporting Information *
ABSTRACT: An efficient procedure is presented for the alkoxycarbonylation of industrially important amines using environmentally friendly alkyl carbonates as reagents and Zn4O(OAc)6 as catalyst. Aromatic amines in particular (thirteen examples) consistently give high yields (up to 98%), regardless of the electronic properties of the substituents or the ring substitution pattern.
1. INTRODUCTION Industrial companies are continuously working on improving their process technologies. For example, in the preparation of isocyanates (key precursors of polyurethanes), researchers have sought alternatives to the traditionally used reagent phosgene.1 So far, there are no alternative technologies available to manufacture major aromatic isocyanates economically on an industrial scale. Carbamates, which are established intermediates for the preparation of fine chemicals, can be transformed into isocyanates by thermal decomposition,2 and therefore, they have become an interesting alternative for the synthesis of polyurethanes.1,3 A widely studied method to prepare carbamates is the catalyzed alkoxycarbonylation of aliphatic and aromatic amines with organic carbonates (Scheme 1).4
1). This is especially important for the carbamates derived from aromatic diamines TDA or MDA, which are widely used industrially as polyurethane precursors. Previously, we reported on the use of Zn(OAc)2·2H2O as catalyst for the alkoxycarbonylation of industrially relevant aromatic diamines as precursors of polyurethanes.5e,15 In this transformation, yields as high as 98% for the methoxycarbonylation of MDA and 95% for the methoxycarbonylation of TDA were reached. As part of our ongoing studies on zinccatalyzed alkoxycarbonylations,5e we considered using Zn4O(OAc)6 as a catalyst. Although this cluster was first prepared in 1926,16 its catalytic activity was only recently examined. Ohshima et al. described the use of the fluorinated analogue, Zn4O(O2CCF3)6, as a catalyst for the synthesis of oxazolines via tandem condensation−cyclodehydration of β-amino alcohols with esters,17 as well as transesterifications,18 acylations,19 cycloadditions,20 and cyclic carbonate synthesis,21 with good to excellent results in terms of yield and selectivity. Although Zn4O(OAc)6 was included in some of their catalyst screenings (refs 17 and 21), it was not explored in detail in any of these procedures. Furthermore, neither this cluster nor any of its derivatives had been studied in the alkoxycarbonylation of aliphatic or aromatic amines. To improve upon the results we obtained earlier5e and to expand the scope of the reaction, we explored the use of Zn4O(OAc)6 for alkoxycarbonylation in detail. Herein, we report on Zn4O(OAc)6 as catalyst in the alkoxycarbonylation of various aromatic amines, a number of them of considerable industrial interest.
Scheme 1. Alkoxycarbonylation of Amines
Various catalysts have been explored for this transformation, such as zinc,5 sodium6 or lead compounds,7 Yb(OTf)3,8 titanium9 or zirconium10 tetraalkoxides, CeO2-supported gold,11 scandium12 or bismuth13 derivatives, and several organic and inorganic bases.14 Although a number of these procedures show excellent results, some of their limitations may include low selectivity, high catalyst loadings, the need for a cocatalyst, or the use of toxic metals. Consequently, given the economic impact of the products of this amply studied reaction, the search for new, better-performing catalysts remains active. The main challenge is to develop a procedure that gives high carbamate conversions and at the same time furnishes the target products with very high alkoxycarbonylation selectivities (i.e., N-alkylation of the starting amines does not take place; Scheme © 2012 American Chemical Society
2. EXPERIMENTAL SECTION 2.1. General Remarks. All starting materials were commercially available and were used without further Received: Revised: Accepted: Published: 16165
May 19, 2012 October 30, 2012 November 23, 2012 November 23, 2012 dx.doi.org/10.1021/ie301315k | Ind. Eng. Chem. Res. 2012, 51, 16165−16170
Industrial & Engineering Chemistry Research
Article
Scheme 2. Alkoxycarbonylation of Anilines: Starting Materials, Products, Intermediates, and Alkoxycarbonylating Agents
cm−1. Anal. Calcd for C12H18O13Zn4: C, 22.81; H, 2.87. Found: C, 22.66; H, 2.72. 2.3. Catalytic Experiments. General Procedure for Catalytic Experiments. All reactions, except the ones using nbutylamine, were performed in a stainless steel autoclave. In a typical experiment, the aromatic diamine (2−4 mmol), the corresponding organic carbonate (DMC or DEC), and the catalyst were introduced into a Teflon vessel placed inside a 25 mL autoclave. The atmosphere was purged with N2, then the autoclave was closed and heated to the desired temperature, after which it was allowed to stir at that temperature for the time indicated in Tables 1 and 2. The reaction mixture was stirred at ca. 800 rpm. Once the reaction was finished, the autoclave was removed from the heating mantle, initially allowed to cool for 30 min in the air, and was then cooled down further, in an ice bath, for 30 min. The yields and conversions of all the compounds derived from MDA, 2,4-TDA, or 2,6TDA were determined by HPLC using external standards. In all the cases where isolated yields are reported, the carbamates were isolated and purified by chromatography over silica gel using mixtures of hexane and ethyl acetate as eluent. The physical and spectroscopic data of the obtained products were in good agreement with the reported ones (see Supporting Information (SI) for details). Methoxycarbonylation of n-Butylamine. Methoxycarbonylation of n-butylamine was performed in Ace pressure tubes. In a typical experiment, the amine (2−4 mmol), the required amounts of DMC, and the catalyst were introduced into an Ace tube inside of a glovebox. The tube was heated to the desired temperature for the corresponding reaction time. Methyl butyl carbamate was isolated and purified by chromatography over
purification unless stated otherwise. When required, dry solvents were obtained using a solvent purification system (SPS) of Innovative Technology, Inc. DMC and DEC were stored over 4 Å molecular sieves, and their residual water content was measured on a Karl Fischer apparatus (< 10 ppm).22 NMR spectra were recorded on a Bruker Avance 400 Ultrashield spectrometer at ambient temperature (1H 400 MHz, 13C 100 MHz) except when stated otherwise, and chemical shifts are given in ppm relative to the residual solvent peak. Crude products of catalytic reactions were analyzed in an Agilent 1200 series HPLC system equipped with UV detector and quantified using external standards. Melting points were determined by DSC with a heating rate of 10 °C/min. 2.2. Preparation of Zn4O(OAc)6. Acetic acid (0.743 mL, 13 mmol) was dissolved in diethyl ether (30 mL). One half of this solution (15 mL) was added under argon over an ice− water cooled solution of diethyl zinc (6 mL, 6.5 mmol, 1.08 M in toluene) in diethyl ether (240 mL) at an addition rate of 15 mL/h. After that, the second half of the acetic acid solution was added while oxygen was bubbled through the reaction mixture. An addition rate of 15 mL/h and a temperature of 0 °C were maintained. During the second addition, a white solid precipitated. The suspension was filtered and the residue was dissolved in dichloromethane (20 mL) and filtered again. The resulting filtrate was concentrated under reduced pressure and dried at 50 °C under vacuum, yielding 850 mg (1.34 mmol, 83%) of a white solid identified as Zn4O(OAc)6. Mp = 253 °C (lit.24 252 °C). 1H NMR (500 MHz, DMSO-d6) δ 1.8 (CH3); 13 C{1H} NMR (500 MHz, DMSO-d6) δ 22.3 (CH3), 176.4 (CO). IR (neat): 1594, 1438, 1057, 1035, 666, 615, 550, 525 16166
dx.doi.org/10.1021/ie301315k | Ind. Eng. Chem. Res. 2012, 51, 16165−16170
Industrial & Engineering Chemistry Research
Article
Table 1. Alkoxycarbonylation of Aromatic Amines with Zn4O(OAc)6a entry
substrate
organic carbonate
catalyst loading (%)
molar ratio of carbonate to substrate
yield (%) 1
yield (%) 2
1 2 3d 4 5 6 7 8e 9f 10f 11 12g 13 14
aniline aniline 4,4′-MDA 4,4′-MDA 4,4′-MDA 4,4′-MDA 4,4′-MDA 4,4′-MDA 4,4′-MDA 4,4′-MDA 2,4-TDA 2,4-TDA 2,6-TDA (2,4:2,6)-TDA (80:20)
DMC DEC DMC DMC DMC DMC DMC DMC DEC DEC DMC DEC DMC DMC
0.25 0.50 0.25 0.25 0.25 0.25 0.125 0.0625 1.00 0.50 0.625 0.625 1.25 0.625
25:1 25:1 25:1 20:1 16:1 12.5:1 25:1 25:1 25:1 25:1 25:1 25:1 30:1 30:1
96b 91b 97 (93)b 95 95 74 95 94 97 94 98 (93)b 96 90 95h
n.i.c n.i. 1 1 1 1 1 2 n.d. n.d. 1 3 2 1
T = 180 °C for entries 1−10 and 190 °C for entries 11−14, reaction t = 120 min except in entries 8 (260 min) and 12 (300 min). bIsolated yield. n.i. = not isolated. dSide-products 4-(4-aminobenzyl)-N-methylaniline and 4,4′-methylenebis(N-methylaniline) were obtained in a combined yield of
