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1 Available online at Chinese Chemical Letters 20 (2009) Nuclephilic ring opening of epoxides promoted by multi-site phase-transfer catalyst: An efficient and eco-friendly route to synthesis of b-hydroxy-thiocyanate Ali Reza Kiasat *, Roya Mirzajani, Haji Shalbaf, Tahereh Tabatabaei Chemistry Department, College of Science, Shahid Chamran University, Ahvaz , Iran Received 4 January 2009 Abstract A highly effective and mild protocol for ring opening of epoxides with NH 4 SCN in the presence of catalytic amount of a multisite phase-transfer catalyst, a,a 0,a 00 -N-hexakis(triethylammoniummethylene chloride)-melamine, is developed. A variety of b-hydroxy thiocyanates as important intermediates in agricultural and pharmaceutical chemistry were obtained in high yields with excellent regioselectivity and in short reaction times. # 2009 Ali Reza Kiasat. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. Keywords: Multi-site phase-transfer catalyst; b-hydroxythiocyanate; Thiocyanohydrin; Epoxide; Ring opening; Regioselectivity Phase-transfer catalysis (PTC) has been recognized as a convenient and highly useful synthetic tool in organic synthesis due to its operational simplicity, mild reaction conditions, safety considerations, suitability for large scale reactions, and environmental concerns. Because of these salient features, the research and the developments of novel PTC especially to carry out valuable organic reactions have received considerable industrial interest [1,2]. The paramount considerations in the selection of the catalyst are economy of scale and efficiency of the PTC, particularly on the explicit industrial scale preparation of organic compounds. In order to cater to these needs, multi-site phasetransfer catalysts (MPTC) have been developed. In general, MPTCs have more potential of offering greater PTC activity and to effect a particular synthetic transformation under mild reaction conditions [3,4]. Regioselective ring opening of epoxides with nucleophilic reagents is a useful tool for the preparation of several 1,2-disubstituted products and is an important reaction for synthetic organic and medicinal chemists [5]. Among the vast number of nucleophiles that have been employed in the ring opening, thiocyanate anion (which affords the corresponding thiirane) has received considerable attention [5]. The formation of thiiranes from oxiranes and thiocyanates has been explained by the intermediacy of the corresponding b-hydroxythiocyanate. However, b-hydroxythiocyanates have not been isolated due to rapid conversion into the corresponding thiiranes [6]. It has been reported that the presence of some hydroquinone or DDQ is required to stabilize the produced b-hydroxythiocyanates, and to inhibit its conversion to thiirane [7]. Although a few useful reagents reported for the ring opening of epoxide to * Corresponding author. address: akiasat@scu.ac.ir (A.R. Kiasat) /$ see front matter # 2009 Ali Reza Kiasat. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. doi: /j.cclet

2 1026 A.R. Kiasat et al. / Chinese Chemical Letters 20 (2009) Scheme 1. the b-hydroxythiocyanate [8 11], unfortunately, some of these methods are not always fully satisfactory and suffer from disadvantages such as relative low yields, long reaction time, difficulty in workup, high cost of reagent, lack of regioselectivity, lack of reusability of a catalyst, or involve high temperature reaction conditions to obtain ring-opened products. Therefore, it seems that there is still a need for development of novel methods that proceed under mild and economically appropriate conditions. Very recently we reported some acidic catalyst to obtain ring-opened products [12 14]. By considering all the above-mentioned points and as a part of our research to develop green chemistry by using water as reaction medium [12,15,16], we have decided to apply a multi-site phase-transfer catalyst containing the maximum possible number of active sites for the preparation of b-hydroxythiocyanates by a regioselective ring opening of epoxides with ammonium thiocyanatein in water (Scheme 1). 1. Experimental Epoxides and other chemical materials were purchased from Fluka and Merck in high purity. a,a 0,a 00 -Nhexakis(triethylammoniummethylene chloride)-melamine was prepared according to the previously reported procedure [2]. All of the b-hydroxythiocyanate compounds were prepared by our procedure, and their spectroscopic, and physical data were compared with those of authentic samples [12]. The purity determination of the products and reaction monitoring were accomplished by TLC on silica gel polygram SILG/UV 254 plates General procedure To a magnetically stirred mixture of epoxide (1 mmol) and NH 4 SCN (2 mmol) in water (5 ml), a,a 0,a 00 -Nhexakis(triethylammoniummethylene chloride)-melamine (0.2 mmol) was added. The resulting reaction mixture was stirred under reflux conditions for the appropriate time. After completion of the reaction as indicated by TLC, the product was extracted with ether (3 10 ml). The solution was dried with Na 2 SO 4 and evaporated under reduced pressure. The desired b-hydroxythiocyanate was obtained in good to excellent isolated yields (70 95%). For styrene oxide, further purification was achieved by preparative TLC or by silica gel column chromatography. 2. Results and discussion In 2004, a report by Balakrishnan [2] outlined the preparation of an efficient multi-site phase-transfer catalyst, a,a 0,a 00 -N-hexakis(triethyl-ammoniummethylene chloride)-melamine. They found that this MPTC catalyst have higher activity than the commercially available single-site phase-transfer catalyst in the alkylation of phenylacetonitrile with 1-bromobutane. Here we report that this MPTC catalyst can be efficiently used for regioselective ring opening of epoxides with ammonium thiocyanate in water. Scheme 2 described the three-step process for facile preparation of a,a 0,a 00 -N-hexakis(triethylammoniummethylene chloride)-melamine using inexpensive starting materials. Initially, ring opening of phenyl glycidyl ether was investigated with NH 4 SCN and in the presence of a,a 0,a 00 -Nhexakis(triethylammonium-methylene chloride)-melamine in various solvents. TLC analysis of the reaction mixture was clearly shown that in organic solvents and under reflux conditions the reaction did not complete after 5 h and contaminated by thiirane. Interestingly, the best result was obtained in water as reaction media. This is not only due to short time and high yield of the obtained b-hydroxythiocyanate, but also due to its inhabitation effect of water in the conversion of b-hydroxythiocyanate into the corresponding thiiranes. The obtained results are summarized in Table 1. To determine the optimum conditions, the conversion of phenyl glycidyl ether to the corresponding b-hydroxythiocyanate in the presence of multi-site phase-transfer catalyst in water was investigated. The optimum molar ratio of the catalyst to epoxide was found to be 0.2 mmol/1 mmol of epoxide.

3 A.R. Kiasat et al. / Chinese Chemical Letters 20 (2009) Scheme 2. The efficiency of a,a 0,a 00 -N-hexakis(triethylammoniummethylene chloride)-melamine as a multi-site phasetransfer catalyst in the conversion of epoxides to b-hydroxythiocyanates in water was definitely confirmed by reaction of styrene oxide with NH 4 SCN under similar reaction conditions and in the presence of tetrabutylammonium bromide (TBAB) as a single site phase-transfer catalyst, B-podants and polyethylene glycol as alternative to cyclic crown ethers and cryptands and Dowex resin SO 3 H as solid acid catalyst. As shown in Table 2, in these cases, the reaction did not complete after 5 h, and contaminated by diol and thiirane formation. It should be pointed out that in the absence of catalyst, the reaction was sluggish and a considerable amount of starting material was recovered unchanged and the reaction media was contaminated by diol and thiirane. Different types of oxiranes carrying activated and deactivated groups were cleanly, easily, and efficiently converted to the corresponding thiocyanohydrins in high isolated yields. The scope and generality of this process is illustrated with several examples and the results are summarized in Table 3. The structure of all the products were settled from their analytical and spectral (IR, 1 H NMR and 13 C NMR) data and by direct comparison with authentic samples. Under the optimized reaction condition, styrene oxide as an 2-arylepoxide underwent reaction with thiocyanate ion, in the presence of a,a 0,a 00 -N-hexakis(triethylammonium-methylene chloride)-melamine to afford the corresponding b-hydroxythiocyanate in high yield and regioselectivity. As shown in Table 3, the ring opening takes place in a regioselective manner with preferential nucleophilic attack at the benzylic position. 2-Alkylepoxides reacted under Table 1 Efficiency of solvents on thiocyanation of epoxides. Entry Solvent (5 ml) Time (min) Result 1 Dicholoromethane 300 Not completed 2 n-hexane 300 Not completed 3 Acetonitrile 300 Not completed 4 PEG Completed (only thiirane) 5 Water 15 Completed (only thiocyanihydrin) Table 2 Reaction of styrene oxide with NH 4 SCN in water under reflux condition. Entry Catalyst Time (h) Result 1 Without catalyst 5 Not completed (diol + thiirane) 2 TBAB (0.5 g) 5 Not completed (diol + thiocyanohydrin) 3 SSA (0.5 g) 5 Not completed (diol + thiocyanohydrin) 4 Dowex resin SO 3 H (0.5 g) 5 Not completed (diol + thiocyanohydrin) 5 B[PEG-OH] 3 (0.5 g) 2 Completed (diol + tiirane) 6 MPTC (0.2 mmol) 20 min Completed (only thiocyanohydrin) SSA: Silica sulfuric acid, TBAB: tetrabutyl ammonium bromide, MPTC: a,a 0,a 00 -N-hexakis(triethylammoniummethylene cholorid)-melamine.

4 1028 A.R. Kiasat et al. / Chinese Chemical Letters 20 (2009) Table 3 Reaction of various epoxides with ammonium thiocyanate in the presence of MPTC in water. Entry Substrate Products a Time (min) Yields (%) (95:5) b a Products were identified by compared of their physical and spectral data with those of authentic samples. b According to GC analysis. similar reaction conditions and their corresponding b-hydroxythiocyanates were produced in excellent yields and regioselectivity. In these cases, with the attack of the thiocyanate ion on the less substituted oxirane carbon, the regioselectivity is reversed. These observations demonstrated that in the former case, the product is generated through the formation of a stabilized benzylic cation during the reaction and in the second case; they are formed by predominant attack of the thiocyanate ion on the less hindered carbon of the epoxide. The structures of the products were established from their spectral ( 1 H and 13 C NMR and MS) data. In conclusion, we have developed a very easy way to operate, safe and cost-effective method for the regioselective preparation of b-hydroxythiocyanates using a,a 0,a 00 -N-hexakis(triethyl-ammoniummethylene chloride)-melamine as a multi-site phase-transfer catalyst in water. The present method is associated with several advantages including mild reaction conditions, high conversions, greater regioselectivity, short reaction time, clean reaction profiles, and high isolated yields which make it a useful and attractive process for the synthesis of b-hydroxy thiocyanates. Acknowledgment Authors acknowledge the partial support of this work by Shahid Chamran Research Council. References [1] C.M. Starks, J. Am. Chem. Soc. 93 (1971) 195. [2] T. Balakrishnan, E. Murugan, A. Siva, Appl. Catal. A: Gen. 273 (2004) 89. [3] E. Murugan, A. Siva, J. Mol. Catal. 235 (2005) 220. [4] A. Siva, E. Murugan, J. Mol. Catal. 241 (2005) 101. [5] (a) H. Eshghia, M. Rahimizadeh, A. Shoryabi, J. Iran. Chem. Soc. 2 (2005) 155; (b) B. Tamami, A.R. Kiasat, Synth. Commun. 26 (1996) 3953.

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