Remarkable rate acceleration of SmI₃-mediated iodination of acetates of Baylis-Hillman adducts in ionic liquid: facile synthesis of (Z)-allyl iodides

Remarkable rate acceleration of SmI 3 -mediated iodination of acetates of Baylis-Hillman adducts in ionic liquid: facile synthesis of ( Z )-allyl iodides 1 first-author 1 1 1 †E-mail:CHRC@mail.hz.zj.cn; yminzhang@mail.hz.zj.cn †E-mail:CHRC@mail.hz.zj.cn; yminzhang@mail.hz.zj.cn CHRC@mail.hz.zj.cn 2 corresp ‡Corresponding Author ‡Corresponding Author †E-mail:CHRC@mail.hz.zj.cn; yminzhang@mail.hz.zj.cn †E-mail:CHRC@mail.hz.zj.cn; yminzhang@mail.hz.zj.cn CHRC@mail.hz.zj.cn State Key Laboratory Breeding Base of Green Chemistry-Synthesis Technology, Zhejiang University of Technology, Hangzhou 310014, China State Key Laboratory Breeding Base of Green Chemistry-Synthesis Technology, Zhejiang University of Technology, Hangzhou 310014, China Department of Chemistry, Zhejiang University, Hangzhou 310028, China Department of Chemistry, Zhejiang University, Hangzhou 310028, China Stereoselective transformation of Baylis-Hillman acetates 1 into corresponding ( Z )-allyl iodides 2 has been achieved by treatment of 1 with samarium triiodide in THF. Remarkable rate acceleration of samarium triiodide-mediated iodination of 1 was found when ionic liquid 1- n -butyl-3-methyl-imidazolium tetrafluroborate ([bmim]BF 4 ) was used as reaction media in stead of THF. This novel approach proceeds readily at 50 °C within a few minutes to afford ( Z )-allyl iodides 2 in excellent yields. A mechanism involving stereoselective iodination of the acetates of Baylis-Hillman adducts by samarium triiodide is described, in which a six-membered ring transition state played a key role in the stereoselective formation of 2 . Baylis-Hillman adduct ( Z )-allyl iodide Samarium triiodide Ionic liquid INTRODUCTION 1 The Baylis-Hillman reaction has drawn much attention as a useful carbon-carbon bond-forming reaction in the last decades (Basavaiah et al., 1996a 1996a ; 2003 2003 ). The adducts of the reaction, 3-hydroxy-2-methylene-alkanoates (derived from acrylate esters), have been widely utilized as important precursors for stereoselective synthesis of different multifunctional molecules (Cho and Krische, 2004 2004 ; Kabalka et al., 2003 2003 ; 2004 2004 ; Chung et al., 2001 2001 ). Among such transformations, the preparation of 2-(halomethyl)alk-2-enoates from Baylis-Hillman adducts has received much attention as these compounds are especially useful for the synthesis of various naturally occurring bioactive compounds and their analogs such as α -methylene- γ -butyrolactone, α -alkylidene- β -lactam and flavanoid (Hoffmann and Rabe, 1985 1985 ; Buchholz and Hoffmann, 1991 1991 ; Basavaiah et al., 1998 1998 ). The preparation of corresponding allyl chlorides and allyl bromides from Baylis-Hillman adducts is extensively studied up to now (Chavan et al., 1997 1997 ; Basavaiah et al., 1995 1995 ; 1996b 1996b ). In contrast, the synthesis of corresponding allylic iodide analogs were rarely investigated, which limited to much extent their application in organic synthesis (Yadav et al., 2001 2001 ). Moreover, the traditional synthetic procedures for preparation of allylic iodide obviously suffer from the use of strong acid (HI-H 3 PO 4 ) and long reaction time (4 d) (Ameer et al., 1985 1985 ). So development of an alternative synthetic method with high efficiency and operational simplicity is needed. On the other hand, the so-called room temperature ionic liquids (RTILs), especially those derived from 1- n -alkyl-3-methylimidazolium cations, have been shown to have versatile applications in organic synthesis as reaction media (Wasserscheid and Keim, 2000 2000 ; Welton, 1999 1999 ; Sheldon, 2001 2001 ). The attractiveness of RTILs as green solvents lies in their wide liquid range, good solvating ability, tunable polarity, highly thermal stability, negligible vapor pressure, and easy recycleability. As a continuation of our interest in Baylis-Hillman reaction (Li et al., 2004 2004 ; 2005a 2005a ; 2005b 2005b ; 2005c 2005c ) and application of samarium reagents in organic synthesis (Zhang and Liu, 2001 2001 ; Chen and Zhang, 2004 2004 ; Lu and Zhang; 1999 1999 ; Zhou and Zhang, 1997 1997 ), we wish to report here that SmI 3 (Fan and Zhang, 2002 2002 ; Ma and Zhang, 2003 2003 ) as a Lewis acid can efficiently promote the conversion of Baylis-Hillman acetates 1 into the corresponding ( Z )-allylic iodides 2 in ionic liquid. METHODS 1 When Baylis-Hillman acetates 1 were treated with SmI 3 generated in situ in the ionic liquid 1- n -butyl-3-methyl-imidazolium tetrafluoroborate ([bmim]BF 4 ) at 50 °C for 10 min, ( Z )-allyl iodides 2 were obtained in excellent yields (Fig. 1 1 ). The Z -stereoconfiguration of the product was assigned by comparing the chemical shifts in 1 H NMR with the reported ones (Yadav et al., 2001 2001 ) and no E -isomer was observed from the spectra. RESULTS AND DISCUSSION 1 The results are summarized in Table 1 1 . Our previous study found that samarium and iodine can efficiently promote the iodination of the Baylis-Hillman acetates (Li et al., 2005c 2005c ). We propose that the above iodination reaction may proceed via the Sm(III) species, which prompted us to directly use SmI 3 as reagent for iodination of the Baylis-Hillman acetates. At first, when the reaction was carried out in THF, it gave very low yield of product even for long time at 50 °C (Entry 1). However, when ionic liquid 1- n -butyl-3-methyl-imidazolium tetrafluoroborate ([bmim]BF 4 ) was used as reaction media, in which remarkable acceleration of iodination of the Baylis-Hillman adducts was found to complete the reaction within 10 min at 50 °C. http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=23536428&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=23536428&type=small http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=23536428&type=middle http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=23536428&type=small http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=23536432&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=23536436&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=23536432&type=middle A series of substrates were used in this reaction to establish the generality, and most reactions proceeded efficiently under similar conditions. The experimental results showed that the present method was effective for substrates bearing either electron-donating or electron-withdrawing groups. Besides good yields, this process also exhibited excellent stereoselectivity. Only Z -stereoconfiguration of the products was obtained while no E -isomers were observed from the spectra. In the following discussion, we will give some explanation for the excellent Z -stereoselectivity. The reaction mechanism could be explained by a six-membered ring transition state. According to Concellón et al.( 1999 1999 ), chelation of Sm(III) with the carbonyl oxygen and another oxygen atom of the OAc group led to the formation of a six-membered ring (Fig. 2 2 ). Subsequently, the attack of iodine anion on the olefin methylene carbon followed by the elimination of OAc group resulted in the formation of new carbon-carbon double bond. Thus, the products allyl iodides 2 provided good Z -stereoselectivity. CONCLUSION AND EXPERIMENTAL DETAILS 1 In conclusion, we have provided a new method to prepare ( Z )-allylic iodides 2 from the acetates of Baylis-Hillman adducts 1 mediated by SmI 3 in ionic liquid media. The remarkable advantages of this process are its short reaction time, experimental simplicity, environmentally benign process, excellent yields and good stereoselectivity. General procedure: Under a nitrogen atmosphere, Sm powder (0.15 g, 1.0 mmol) and I 2 (0.37 g, 1.5 mmol) were added to a 10-ml flask charged with 5 ml [bmim]BF 4 . The mixture was stirred at room temperature for 30 min. Then to the in situ generated SmI 3 was added Baylis-Hillman acetates 1 (1.0 mmol). The resulting mixture was stirred at 50 °C for 10 min. At the end of the reaction, the resulting mixture was extracted with ether (3×20 ml). The organic phase was successively washed with saturated Na 2 S 2 O 3 (15 ml), brine (15 ml) and dried over anhydrous Na 2 SO 4 . The solvent was removed under reduced pressure to give the crude products, which were purified by preparative TLC (thin layer chromatography) using ethyl acetate and cyclohexane (1:5) as eluent. 2a : solid, m.p. 49~51 °C; δ H (×10 −6 ) 3.88 (s, 3H), 4.34 (s, 2H), 7.40~7.57 (m, 5H), 7.73 (s, 1H); ν max (cm −1 ) 2843, 1716, 1620, 1269, 1069, 770; m / z (%) 303 (37.59, M + +1), 175 (100, M + −I), 143 (26.26); Anal. calcd. for C 11 H 11 IO 2 : C 43.73; H 3.67. Found: C 43.61; H 3.74. 2b : solid, m.p. 52~53 °C; δ H (×10 −6 ) 2.39 (s, 3H),3.88 (s, 3H), 4.37 (s, 2H), 7.28 (d, 2H, J =8.0 Hz), 7.47 (d, 2H, J =8.0 Hz), 7.71 (s, 1H); ν max (cm −1 ) 1704, 1605, 1508, 1066, 811, 761; m / z (%) 317 (0.17, M + +1), 189 (58.94, M + −;I), 129 (100), 115 (38.51); Anal. calcd. for C 12 H 13 IO 2 : C 45.59; H 4.14. Found: C 45.78; H 4.08. 2c : oil; δ H (×10 −6 ) 3.89 (s, 3H), 4.20 (s, 2H), 7.33~7.70 (m, 4H), 7.82 (s, 1H); ν max (cm −1 ) 2952, 1722, 1627, 1437, 1074, 735; m / z (%) 337 (14.53, M + +1), 209 (66.96, M + −I), 149 (55.74), 115 (100); Anal. calcd. for C 11 H 10 ClIO 2 : C 39.26; H 2.99. Found: C 39.40; H 2.95. 2d : solid, m.p. 37~39 °C; δ H (×10 −6 ) 3.78 (s, 3H), 4.17 (s, 2H), 7.31 (d, 2H, J =8.8 Hz), 7.37 (d, 2H, J =8.8 Hz), 7.54 (s, 1H); ν max (cm −1 ) 2950, 1716, 1620, 1592, 1067, 767; m / z (%) 337 (23.36, M + +1), 209 (100, M + −I), 149 (29.32), 115 (59.34); Anal. calcd. for C 11 H 10 ClIO 2 : C 39.26; H 2.99. Found: C 39.13; H 3.03. 2e : solid, m.p. 71~72 °C; δ H (×10 −6 ) 3.87 (s, 3H), 4.36 (s, 2H), 6.04 (s, 2H), 6.89~7.10 (m, 3H), 7.63 (s, 1H); ν max (cm −1 ) 2949, 1708, 1602, 1511, 828; m / z (%) 347 (4.15, M + +1), 219 (100, M + −I), 189 (95.57), 159 (85.47), 115 (48.69); Anal. calcd. for C 12 H 11 IO 4 : C 41.64; H 3.20. Found: C 41.80; H 3.25. 2f : oil; δ H (×10 −6 ) 3.92 (s, 3H), 4.28 (s, 2H), 7.68 (t, 1H, J =8.0 Hz), 7.74 (s, 1H), 7.89 (d, 1H, J =8.0 Hz), 8.26 (d, 1H, J =8.0 Hz), 8.41 (s, 1H); ν max (cm −1 ) 3063, 2919, 1662, 1605, 1573, 1447, 1410; m / z (%) 348 (1.09, M + +1), 220 (53.68, M + −I), 174 (60.60), 115 (100). 2g : solid, m.p. 112~114 °C; δ H (×10 −6 ) 3.92 (s, 3H), 4.26 (s, 2H), 7.71 (d, 2H, J =8.0 Hz), 7.74 (s, 1H), 8.33 (d, 2H, J =8.0 Hz); ν max (cm −1 ) 1722, 1593, 1514, 1273, 874; m / z (%) 348 (1.51, M + +1), 220 (15.39, M + −I), 174 (100), 115 (90.34); Anal. calcd. for C 11 H 10 INO 4 : C 38.06; H 2.90. Found: C 38.23; H 2.84. 2h : oil; δ H (×10 −6 ) 3.78 (s, 3H), 4.60 (s, 2H),6.48~6.74 (m, 2H), 7.35 (s, 1H), 7.61 (s, 1H); ν max (cm −1 ) 1714, 1614, 1529, 1437, 1351, 1209; m / z (%) 293 (0.07, M + +1), 254 (100), 165 (28.68, M + −I), 127 (59.51); Anal. calcd. for C 9 H 9 IO 3 : C 37.01; H 3.11. Found: C 36.91; H 3.16. 2i : solid, m.p. 93~95 °C; δ H (×10 −6 ) 3.90 (s, 3H), 3.91 (s, 3H), 4.35 (s, 2H), 6.96~7.73 (m, 4H), 7.97 (s, 1H); ν max (cm −1 ) 2943, 1707, 1597, 1025, 817; m / z (%) 333 (0.83, M + +1), 205 (75.69, M + −I), 145 (100), 115 (33.66); Anal. calcd. for C 12 H 13 IO 3 : C 43.39; H 3.95. Found: C 43.47; H 3.88. 2j : oil; δ H (×10 −6 ) 1.44 (t, 3H, J= 7.2 Hz), 4.37~4.42 (m, 4H), 7.45~7.62 (m, 5H), 7.78 (s, 1H); ν max (cm −1 ) 2978, 1706, 1609, 1510, 1022, 814; m / z (%) 317 (2.71, M + +1), 189 (100, M + −I), 145 (18.57), 117 (71.45); Anal. calcd. for C 12 H 13 IO 2 : C 45.59; H 4.14. Found: C 45.38; H 4.09. 2k : oil; δ H (×10 −6 ) 1.39 (t, 3H, J =7.0 Hz), 4.20 (s, 2H), 4.35 (q, 2H, J =7.0 Hz), 7.33~7.70 (m, 4H), 7.82 (s, 1H); ν max (cm −1 ) 2981, 1717, 1599, 1508, 1369, 1205, 1053; m / z (%) 351 (26.91, M + +1), 223 (67.66, M + −I), 151 (55.33), 115 (100); Anal. calcd. for C 12 H 12 ClIO 2 : C 41.11; H 3.45. Found: C 41.24; H 3.53. 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A novel cyclodimerization of arylidenecyanoacetate promoted by samarium diiodide 10.1631/jzus.2006.B0193 Project (No. 2004C21032) supported by the Key Technology R & D Program of Zhejiang Province, China 2005-11-21 2005-12-21 2006-02-27 2022-01-05 Copyright © 2006, Journal of Zhejiang University Science 2006 Journal of Zhejiang University-SCIENCE B (Biomedicine and Biotechnology) 03 07