Journal of the Chinese Chemical Society, 2008, 55,

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1 Journal of the Chinese Chemical Society, 2008, 55, Dowex as Reusable Acidic Polymeric Catalyst in the Efficient and Regioselective Conversion of Epoxides into -Hydroxy Thiocyanates under Solvent Free Conditions Ali Reza Kiasat* and Mehdi Fallah-Mehrjardi Department of Chemistry, Faculty of Sciences, Shahid Chamran University, Ahvaz , Iran A convenient and efficient procedure for the regioselective ring opening of epoxides in the presence of Dowex (strongly acidic cationic exchange resin), as reusable eco-friendly catalyst under solvent free conditions is described. Thus, several -hydroxy thiocyanates, useful intermediates toward pharmaceutical and biologically active molecules, are easily obtained in high isolated yields. Keywords: Dowex; Epoxide; -Hydroxy thiocyanate; Solvent free; Ring opening. INTRDUCTIN Thiocyanates are important intermediates in agricultural and pharmaceutical chemistry. 1 -Hydroxy thiocyanates represent an interesting subclass having multiple modes of reactivity. Synthetic access by epoxide ring opening with thiocyanate ion has been limited by a further reaction to give thiiranes. 2 The formation of thiiranes from epoxides and thiocyanate ion has been explained by the intermediacy of the corresponding -hydroxy thiocyanate. However, -hydroxy thiocyanates have not been isolated due to their rapid conversion into the corresponding thiiranes. 3 Thus, very few methods are reported in the literature for the preparation of -hydroxy thiocyanates. 4 For these syntheses it has been reported that the presence of some hydroquinone or DDQ is required to stabilize the produced -hydroxy thiocyanates and to inhibit their conversion to thiiranes. 5 Few reagents such as 3 P() 2, 6 [Pd(P 3 ) 4 /NH 4 ], 7 [Ti(- i Pr) 4 /NH 4 ], 8 [TiCl 3 (or ZnCl 2 )/K], 9 TMSNCS, 10 [cryptants/nh 4 ], 11 [PTC/ NH 4 ], 12 [TPP/NH 4 ], 13 [BABMB/NH 4 ], 14 [Co II TPP/NH 4 ], 15 [[P(TPP)Cl 2 ]Cl/NH 4 ] 16 and [Selectfluor/NH 4 ] 17 have been reported for the ring opening of epoxides to the -hydroxy thiocyanates. Each of the above methods has at least one of the following drawbacks: (1) restricted only to specific epoxides, (2) required to high temperature (3) long reaction time, (4) low regioselectivity, (5) using of organic solvent and (6) using of expensive catalysis. In organic syntheses and reactions, increasing attention is being focused on green chemistry, using environmentally benign reagents and conditions, particularly solvent free procedures, which often lead to clean, ecofriendly, and highly efficient procedures involving simplified workups. 18 Solvent free reactions under heterogeneous conditions that are facilitated by supported reagents on various solid inorganic surfaces have received attention in recent years. 19 The advantage of these methods over conventional homogeneous reactions is that they provide greater selectivity, enhanced reaction rates, cleaner products and manipulative simplicity. In view of the emerging importance of polymeric catalyst as efficient and recyclable catalyst, we wish to report a simple, efficient and practical approach for the synthesis of -hydroxy thiocyanates using Dowex as eco-friendly catalyst with high catalytic activity under solvent free conditions. RESULTS AND DISCUSSIN Dowex has been used as an efficient solid acid catalyst in organic synthesis. 20 Recently, we have reported the use of Dowex polymer in the protection of carbonyl compounds. 21 In conjunction with the ongoing work on the new methods for the preparation of -hydroxy thiocyanates, 22 we found that Dowex polymer in the presence of silica gel efficiently catalyzed the addition of ammonium thiocyanate to epoxides to form -hydroxy thiocyanates with high regioselectivity under solvent free conditions (Scheme I). We started to study this ring opening reaction by examining the amount of polymeric catalyst and silica gel for the reaction involving 2,3-epoxypropyl phenyl ether (1

2 1120 J. Chin. Chem. Soc., Vol. 55, No. 5, 2008 Kiasat et al. Scheme I Regioselectivity in ring opening of epoxides by NH 4 R NH 4 Dowex H silica gel R R I H II mmol) and ammonium thiocyanate (2 mmol). The best result which we can get was obtained with 0.1 g polymeric catalystand1gofsilica gel under solvent free conditions gives 3-phenoxy-2-hydroxypropyl thiocyanate in high isolated yield. To ascertain the scope and limitation of the present reaction, different types of epoxides carrying electron donating and withdrawing groups were cleanly, easily and efficiently converted to the corresponding -hydroxy thiocyanates, in high isolated yields without the formation of any of the corresponding thiiranes (Table 1). The structure of each product was confirmed from its analytical and spectral (IR, 1 Hand 13 C-NMR) data and by direct comparison with an authentic sample. 5,9 As shown in Table 1, except for the reaction of styrene oxide (entry 2) which it produced -hydroxy thiocyanates I and II as a mixture with the ratio of 90:10, the reaction of other epoxides were found to be highly regioselective, and only one isomer was obtained. Also in case of cyclohexene oxide (entry 3) trans product was obtained (Scheme II). bviously, in these reactions, the nucleophilic attack by thiocyanate ion was proceed faster at the primary Table 1. Reaction of various epoxides with ammonium thiocyanate in the presence Dowex polymer/silica gel under solvent free conditions Entry Substrate Product (s) a Time (min) 1 H CH 2 CH2 Yields (%) H H H 6 89 (9:1) b H (CH 3 ) 2 CHCH 2 (CH3 ) 2 CHCH 2 H CH 2 =CHCH 2 CH 2 H2 C=CHCH 2 CH 2 H CH 3 (CH 2 ) 2 CH 2 CH 2 CH3 (CH 2 ) 2 CH 2 CH H 2 C CH 3 H 2 C CH 3 H 5 90 a Products were identified by compared of their physical and spectral data with those of authentic samples. b According to GC analysis.

3 Conversion of Epoxides into Thiocyanohydrins J. Chin. Chem. Soc., Vol. 55, No. 5, Scheme II Stereoselectivity in ring opening of cyclohexene oxide NH 4 Dowex silica gel H carbon atom of the epoxide ring. The direction of ring opening is that characteristically observed for reactions of monoalkyl-substituted epoxides under SN 2 conditions and is probably dictated by steric factor. It is noteworthy that no evidence for the formation of thiirane as by-product of the reaction was observed, and the products were obtained in pure form without further purification. The effect of acidic polymeric catalyst was definitely confirmed by the reaction of styrene oxide with ammonium thiocyanate. In the absence of polymeric catalyst, thiirane was produced as only product (Table 2, entry 2). The advantage of using Dowex as catalyst for the synthesis of thiocyanohydrins using thiocyanate ion is shown by comparing our result with those previously reported in the literature (Table 2). It is worthy to note that the polymeric catalyst can be reused several times without loss of activity. The catalyst (mixture of Dowex and silica gel) recovered by filtering and washing with water and methanol, and then it was reused four times for facile conversion of cyclohexene oxide to the corresponding thiocyanohydrin. The catalyst did not show any loss in its activity, and produced trans-2-hydroxycyclohexyl thiocyanate in 85%, 82%, 83% and 81% yield, respectively. In conclusion, Dowex polymer has proved to be a highly efficient solid acid catalyst for the regioselective ring opening of epoxides to -hydroxy thiocyanates by thiocyanate ion under solvent free conditions. This method offers several advantages including mild reaction conditions, greater regioselectivity, short reaction times, clean reaction profiles, ease of work-up, environmentally benign, and use of inexpensive reagent, which makes it a useful, easy, mild, efficient and general method for the synthesis of -hydroxy thiocyanates. EXPERIMENTAL General Dowex 50 X 8 (strongly acidic cationic exchange resin) was purchased from Fluka. Epoxides and other chemicals were purchased from Fluka and Merck in high purity. All of the thiocyanohydrin compounds were prepared by our procedure, and their spectroscopic and physical data were compared with those of authentic samples. 1 Hand 13 C NMR spectra were recorded in CDCl 3 on a Bruker Advanced DPX 400 MHz spectrophotometer using TMS as internal standard. IR spectra were recorded on a BMEM MB-Series 1998 FT-IR spectrometer. The purity determination of the products and reaction monitoring were accomplished by TLC on silica gel polygram SILG/UV 254 plates. General procedure for the preparation of -hydroxy thiocyanates Reaction was carried out by mixing the epoxide (1 mmol), NH 4 (2 mmol), Dowex polymer (0.1 g) and silica gel (1 g). The reaction mixture was ground for the time specified in Table 1. The progress of reaction was monitored by TLC using n-hexane-ether (4:1). n completion of reaction, the reaction mixture was poured into CH 2 Cl 2 (20 ml). The solid was filtered off and the filtrate washed with water (2 20 ml). The organic layer was separated and dried over anhydrous calcium chloride. Then solvent was evaporated under reduced pressure. The desired thiocyanohydrines were obtained in good to excellent isolated yields (78-90%). For styrene oxide, further purification was achieved by preparative TLC or by silica gel column chromatography. 3-enoxy-2-hydroxypropyl Thiocyanate (1) 13 IR: 2157 cm -1 (). 1 H-NMR (CDCl 3, 400 MHz): =3.30(2H,d,CH 2 ),3.78(1H,s,H), 4.04 (2H, d, CH 2 ), 4.29 (1H, m, CHH), 6.9 (2H, m, Ar-H), 7.01 (1H, m, Ar-H), 7.27 (2H, m, Ar-H). 13 C-NMR (CDCl 3, 100 MHz): = 37.4 (CH 2 ), 68.0 (CHH), 69.5 (CH 2 ), (), (o-ch), (p-ch), (m-ch), (C). 2-Hydroxy-2-phenylethyl Thiocyanate (2) 12,13,17 IR: 2155 cm -1 (). 1 H-NMR (CDCl 3, 400 MHz): =3.18(1H,s,H), (2H, m, CH 2 H), 4.79 (1H, m, CH),7.25(5H,m,Ar-H). 13 C-NMR (CDCl 3, 100 MHz): = 43.5 (CH 2 ), (CH 2 H), (), (p-ch), (o-ch), (m-ch), (C). 2-Hydroxycyclohexyl Thiocyanate (3) 12,13 IR: 2153 cm -1 (). 1 H-NMR (CDCl 3, 400 MHz): = (4H, m, CH 2 CH 2 CH), 1.69 (2H, m, CH 2 CH 2 CHH), 1.98 (2H, m, CH 2 CHH), 2.35 (1H, s,

4 1122 J. Chin. Chem. Soc., Vol. 55, No. 5, 2008 Kiasat et al. Table 2. Reaction of styrene oxide with NH 4 in the presence of the representative catalysts Entry Catalyst Reaction conditions Product (s) 1 Dowex a NH 4 /silica gel solvent free 2 NH 4 /silica gel solvent free H H Reaction time (min) Yields (%) 6 89 (9:1) S DDQ 5b NH 4 /CH 3 CN H (8:1) 4 Pd(P 3 ) 4 7 NH 4 /N 2 /THF H S Ti( i Pr) 4 8 NH 4 /THF H ZnCl 2 9 K/THF H S PTC 12 NH 4 /CH 3 CN r.t. H (9:1) 8 T(4-H P)P 13 NH 4 /CH 3 CN H H BABMB 14 NH 4 /CH 3 CN H H (1:4) 10 BABMB 14 K/CH 3 CN H S Co II T(p-HP)P 15 NH 4 /N 2 /CH 3 CN H (1:5) H

5 Conversion of Epoxides into Thiocyanohydrins J. Chin. Chem. Soc., Vol. 55, No. 5, [PTPPCl 2 ]Cl 16 NH 4 /N 2 /CH 3 CN H (1:4) 13 Selectfluor 17 NH 4 /CH 3 CN r.t. H H (1:5) H a Present method H), 3.14 (1H, m, CH),3.34(1H,m,CHH). 13 C- NMR (CDCl 3, 100 MHz): = 22.2 (CH 2 CH 2 ), 25.1 (CH 2 CH 2 ), 30.5 (CH 2 CH), 31.5 (CH 2 CHH), 52.6 (CH), 76.4 (CHH), (). 2-Hydroxy-3-isopropoxypropyl Thiocyanate (4) 13,17 IR: 2156 cm -1 (). 1 H-NMR (CDCl 3, 400 MHz): =1.15(6H,d,CH 3 ), 3.16 (1H, s, H), 3.34 (2H, d, CH 2 ),3.61(3H,m,CH 2 CHH),3.75(1H,m, CHCH 3 ). 13 C-NMR (CDCl 3, 100 MHz): = 22.0 (CH 3 ), 35.7 (CH 2 ), 68.9 (CH), 71.4 (CHH), 75.8 (CH 2 ), (). 3-Allyloxy-2-hydroxypropyl Thiocyanate (5) 13 IR: 2156 cm -1 (). 1 H-NMR (CDCl 3, 400 MHz): =3.04(1H,s,H), 3.24 (2H, d, CH 2 ),3.53(2H,d, CH 2 ), 4.05 (3H, m, CH 2 CHH), (2H, m, =CH 2 ), 5.86 (1H, m, =CH). 13 C-NMR (CDCl 3, 100 MHz): = 37.3 (CH 2 ), 69.1 (CH 2 ), 70.7 (CHH), 71.6 (CH 2 ), (), (CH 2 =), (=CH). 3-Butoxy-2-hydroxypropyl Thiocyanate (6) 22 IR: 2157 cm -1 (). 1 H-NMR (CDCl 3, 400 MHz): =0.91(3H,t,CH 3 ), 1.37 (2H, six, CH 3 CH 2 CH 2 ), 1.56 (2H, quin, CH 2 CH 2 CH 2 ), 3.01 (1H, s, H), 3.25 (2H, d, CH 2 ),3.47(2H,t,CH 2 CH 2 ),3.71(2H,m,CH 2 ), 3.83 (1H, m, CHH). 13 C-NMR (CDCl 3, 100 MHz): = 13.5 (CH 3 ), 18.7 (CH 3 CH 2 ), 31.5 (CH 3 CH 2 CH 2 ), 35.0 (CH 2 ), 69.9 (CH 2 ), 71.4 (CHH), 72.7 (CH 2 ), (). 2-Hydroxy-3-thiocyanatopropyl Methacrylate (7) 22 IR: 2156 cm -1 (). 1 H-NMR (CDCl 3, 400 MHz): =1.85(3H,m,CH 3 ), 2.89 (1H, s, H), 3.16 (2H, d, CH 2 ),3.40(1H,m,CHH),4.41(2H,d,CH 2 ), 5.55 (1H, m, =CH 2 ), 6.06 (1H, m, =CH 2 ). 13 C-NMR (CDCl 3, 100 MHz): = 18.4 (CH 3 ), 37.3 (CH 2 ), 66.1 (CH 2 ), 68.0 (CHH), (), (CH 2 =), (=CH), (C=). ACKNWLEDGMENT The authors are thankful to the Shahid Chamran University Research Council for the financial support of this work. Received March 28, REFERENCES 1. Newman, A. A. Chemistry and Biochemistry of Thiocyanic Acid and Its Derivatives,1 st ed.; Academic Press: New York, Tamami,B.;Kiasat,A.R. Synth. Commun. 1996, 26, (a) Vedejs, E.; Krafft, G. A. Tetrahedron 1982, 38, (b) Iranpoor, N.; Zeynizadeh, B. Synth. Commun. 1998, 28, (a) Van-Tamelen, E. E. J. Am. Chem. Soc. 1951, 73, (b) Kawashima, K.; Eshiguro, T. Chem. arm. Bull. 1978, 26, 951. (c) Gao, Y.; Sharpless, K. B. J. Am. Chem. Soc. 1988, 110, (a) Wagner-Jauregg, Th. Liebigs Ann. 1949, 561, 87. (b) Iranpoor, N.; Kohmareh, G. A. osphorus, Sulfur Silicon Relat. Elem. 1999, 152, Tamura, Y.; Kawasaki, T.; Kita, Y. J. Chem. Soc., Perkin Trans , Choudary, B. M.; Shobha, S.; Kantam, M. L. Synth. Commun. 1990, 20, Najera, C.; Sansano, J. M. Tetrahedron 1991, 47, lszewski, A.; Gros, P.; Fort, Y. Tetrahedron Lett. 1997, 38, Tanabe, Y.; Mori, K.; Yoshida, Y. J. Chem. Soc., Perkin Trans , Sharghi, H.; Nasseri, M. A.; Niknam, K. J. rg. Chem. 2001, 66, Tamami, B.; Mahdavi, H. Tetrahedron Lett. 2002, 43, Sharghi, H.; Nasseri, M. A.; Nejad, A. H. J. Mol. Catal. A:

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