Re duc tion of Ethyl t-cinnamates by Sa mar ium Diiodide

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1 Jour nal of the Chi nese Chem i cal So ci ety, 2001, 48, Re duc tion of Ethyl t-cinnamates by Sa mar ium Diiodide Tzuen-Yeuan Lin* ( ), Ming-Ren Fuh ( ) and I-San Chan ( ) De part ment of Chem is try, Soochow Uni ver sity, Tai pei, Tai wan, R..C. Rate con stants di rectly mea sured from the GC-analyzed method for SmI 2 re duc tion of ethyl t-cinnamate and its sub sti tuted de riv a tives in the pres ence of HMPA and t-butanol were ob tained. HMPA ex hib its a stron - ger cat a lytic ef fect than t-butanol. De pend ence of re ac tion rates on con cen tra tion of SmI 2 and tem per a ture were stud ied. Elec tron-donating groups re tard this re duc tion; the elec tron-withdrawing groups, on the other hand, en hance the re ac tion rates dra mat i cally. INTRDUCTIN In the se lec tive re duc tion of the car bon-carbon dou ble bond of, -un sat u rated ke tones, cat a lytic hy dro ge na tion and a se ries of re agents pre pared from hy dride re duc ing agents and cop per salts give pri mar ily the sat u rated ke tones. 1 Sim i lar re agents have been shown to re duce, -un sat u rated es ters 2 and nitriles 3 to the cor re spond ing sat u rated com - pounds. Sa mar ium diiodide (SmI2) has been used as a pow er - ful re duc ing agent in the past de cade. 4,5 The re duc tion of, -un sat u rated car bonyl sys tems was also achieved well by this mild but ef fi cient reductant. 4b In ad di tion, SmI2-me di ated intramolecular cyclization of alkyl halides with, -un sat u - rated car bonyl sys tems have been re ported. 6 In this study, we for the first time re port the ab so lute rate con stants of SmI 2 re duc tion of ethyl t-cinnamate in the pres ence of hexamethylphosphoramide (HMPA) or t-butanol which is con sid ered as a suit able pro ton source in this re ac - tion. sam ple by gas chro ma tog ra phy. The re sults in Ta ble 2 show that di lu tion of SmI2 with THF de creased the re duc tion rate. As we ex pected, it does not ren der a lin ear re la tion ship be tween the re ac tion rates and con cen tra tion of SmI2 even if the Sm metal pow der is re - moved by cen tri fuge prior to re ac tion to min i mize the ef fect of Sm metal dur ing di lu tion. These re sults are con sis tent with what we ob tained in our pre vi ous work. 7,8 Hexamethylphosphoramide (HMPA) cat a lyzes this re - duc tion ef fi ciently as ex pected. How ever, in the pres ence of HMPA, the pseudo-first-order lin ear plot can be ob tained only for fast re ac tions (within 10 min utes) while the cur va - ture ap peared for slow re ac tions. This can be ac counted for RE SULTS AND DIS CUS SIN For the re ac tion of CH CHCC 2H 5 + SmI 2 catalyst CH 2 CH 2 CC 2 H 5 pseudo-first-order rate con stants were ob tained by plot ting log(ct-c4) ver sus time for more than two half-lives. Here Ct and C rep re sent the con cen tra tion of ethyl t-cinnamate at time t and in fin ity, re spec tively (Fig. 1). The first or der rate of de cay of this sub strate was also tested by us ing dif fer ent ini - tial con cen tra tions of the re ac tant (Ta ble 1). The prod uct was char ac ter ized as ethyl hydrocinnamate with an au then tic Fig. 1. Pseudo first-order rate plot of SmI 2 re duc tion of ethyl t-cinnamate, [SmI 2] = M, [t-buh] = M, [ethyl t-cinnamate]o = M, T = 30 C.

2 844 J. Chin. Chem. Soc., Vol. 48, No. 5, 2001 Lin et al. Table 1 a. Dependence of Pseudo-first-order Rate Constants on Initial Concentration of Ethyl t-cinnamate [Ethyl t-cinnamate] 0, M k ps b, sec a [t-buh] = M; [SmI 2 ] = M; T = 30 C. b k ps represents pseudo-first-order rate constant by the slow re ac tion be tween SmI2 and HMPA. 5g Un der our re ac tion con di tion, we solved this prob lem sim ply by tak ing the ini tial rates for slow re ac tions since at the be gin ning of the re ac tion, the lin ear ity for the first-order re la tion ship still can be ob tained. Fig. 2 shows the cat a lytic ef fect of HMPA on SmI 2 re duc tion of ethyl t-cinnamate. The in duc tion pe riod of this fig ure can be ex plained by the slow re ac tion be tween SmI 2 and HMPA, i.e., a small amount of HMPA is con sumed by SmI2. The fol low ing re la tion ship can be de rived as k ps = k 1 [HMPA] for the lin ear part of this graph. Since Rate = kps[ethyl t-cinnamate] there fore, Table 2 a. Dependence of SmI 2 Concentration on Reduction Rates [SmI 2 ], M k ps b, sec a [ethyl t-cinnamate] 0 = M; [t-buh] = M; T = 30 C. Pro tons play an im por tant role on SmI2 re duc tion of, -un sat u rated car bonyl sys tem. 4b We there fore ex am ined the re la tion ship be tween pro ton source and re duc tion rates. t-buh was cho sen to sup ply pro tons be cause it is eas ier to be deoxygenated. Ta ble 3 ex hib its the mod er ate cat a lytic ef - fect by t-buh which is def i nitely less pow er ful than HMPA. Ac cord ing to data in Ta ble 3, k ps can be plot ted against con cen tra tion of t-buh as shown in Fig. 3. The re la tion ship of Rate = k2[t-buh][ethyl t-cinnamate] is valid and k2 can be ob tained as sec -1 M -1 where k2 is a Rate = k1[hmpa][ethyl t-cinnamate] for the lin ear part of this graph, and the slope ob tained can be used to cal cu late k 1 which is de pend ent on [SmI 2] and tem per - a ture. Un der our re ac tion con di tion, k 1 is ap prox i mated to be 0.56 sec -1 M -1. The mech a nism thus can be pro posed as fol - lows: SmI 2 + HMPA Complex Complex + CH CHCC 2 H 5 rds SmI 2 CH CH CC 2 H 5 CH CH CC 2H 5 SmI 2 SmI 2 + Complex CH CH CC2H5 SmI 2 SmI 2 H + H CH CH CC 2H 5 CH 2CH CC 2H 5 SmI 2 CH 2CH 2 CC 2H 5 Fig. 2. Cat a lytic ef fect of HMPA, [SmI2] = M, [ethyl t-cinnamate]o = M, T = 30 C.

3 Re duc tion of Ethyl t-cinnamates by Sa mar ium Diiodide J. Chin. Chem. Soc., Vol. 48, No. 5, Table 3 a. Dependence of t-buh Concentration on Reduction Rates [t-buh], M k ps b, sec a [ethyl t-cinnamate] = M; [SmI 2 ] = M; T = 30 C. func tion of [SmI 2 ] and tem per a ture. The cat a lytic ef fect of t-buh can be at trib uted by its protonation of the car bonyl group of ethyl t-cinnamate to make it eas ier to be re duced, CH CHCC 2 H 5 + t-buh CH CH C A se ries of dif fer ent sub sti tuted cinnamate es ters were ex am ined to study the elec tronic ef fect and steric ef fect on H C 2H 5 Table 4 a. Structural Effect on Reduction Rates of Cinnamates Substrate k ps b, sec -1 Ethyl t- -methyl-cinnamate Ethyl 4-hydroxy-3-methoxy-cinnamate Ethyl t-cinnamate Ethyl t-4-bromo-cinnamate Ethyl 4-nitro-cinnamate Too fast to measure a [cinnamate] 0 = M; [SmI 2 ] = M; T = 30 C; [t-buh] = M. this re duc tion (Ta ble 4). It is clear that elec tron with draw ing groups are ef fec - tively ac cel er at ing the re ac tion and steric hin drance is ob vi - ously un fa vor able to this re duc tion as seen in ethyl t- - methyl-cinnamate. This re sult fully sup ports the mech a nisms pro posed above in which the elec tron trans fer is the rate de - ter min ing step. Finally, tem per a ture ef fect was ex am ined (Ta ble 5) which shows a sim i lar re sult to that of our pre vi ous work. 7,8 CNCLUSIN In sum mary, we have ob tained the first-order rate con - stants of the SmI2 re duc tion on, -un sat u rated es ters which play an im por tant role on SmI 2 -me di ated intramolecular cyclization of alkyl halides. EXPERIMENTAL Chemicals Sa mar ium io dide, dec ane, ethyl t-cinnamate, ethyl hy - dro cinnamate, ethyl t- -methyl-cinnamate, ethyl t-4-bromocinnamate, ethyl 4-nitrocinnamate, ethyl 4-hydroxy-3- meth - Fig. 3. De pend ence of t-butanol con cen tra tion on pseu do first-order rate con stant. [SmI2] = M, [ethyl t-cinnamate]o = M, T = 30 C. Table 5 a. Temperature Effect on Reaction Rate T, C k ps b, sec a [cinnamate] 0 = M; [SmI 2 ] = M; T = 30 C; [t-buh] = M.

4 846 J. Chin. Chem. Soc., Vol. 48, No. 5, 2001 Lin et al. yl cinnamate and hexamethyl phosphoramide (HMPA) were pur chased from Aldrich Chem i cal Com pany, Inc. (Mil wau - kee, WI, USA). Hy dro chlo ric acid and io dine were pur chased from Nacalai Tesque, Inc. (Kyoto, Ja pan) and Wako Pure Chem i cal In dus tries, Ltd. (saka, Ja pan). Hex ane was pur - chased from Tedia Com pany (Fairfield, H, USA). The chem i cals were used as re ceived with out fur ther pu ri fi ca tion. Tetrahydrofuran (THF) was ob tained from Labscan Ltd. (Dub lin,ire land). THF was freshly dis tilled and im me di ately trans ferred to a glove box be fore use. General Method Both ex ter nal and in ter nal stan dards were nec es sary for quan ti ta tive mea sure ments of cinnamates. A HP 6890 se ries GC sys tem with a flame ion iza tion de tec tor (FID), a HP auto - sampler (Hewlett Packard Com pany, CA, USA), HP-5890 seriesii GC sys tem (Hewlett Packard Com pany, CA, USA), HP 5971A MS (Hewlett Packard Com pany, CA, USA), and a split/splitless in jec tor were used for anal y sis. The SISC chro - ma tog ra phy data sys tem (Sci en tific In for ma tion Ser vice Corp., Tai pei, Tai wan) and a per sonal com puter were used for data ac qui si tion and pro cess ing. An HP-5 col umn (30 m., 0.53 mm i.d., 1.5 m film thick ness, Hewlett Packard Com - pany, CA, USA) was used for sep a ra tion. Most op er a tions were car ried out in the glove box. All the re ac tions pro ceeded in a tem per a ture con trolled com part ment in the glove box. In or der to min i mize the ef fect of free Sm metal in the SmI2 so - lu tion, each set of ex per i ments was con ducted by us ing the same batch of stock so lu tion. 1k The con cen tra tion of SmI2 was de ter mined by I2 ti tra tion prior to re ac tion. In a typ i cal ex per i ment, ml of the cinnamate in hex ane or THF was in jected into 5.0 ml of the SmI 2 so lu tion and mixed thor - oughly. Sub se quently, at each time in ter val, 0.50 ml of the re ac tion mix ture was trans ferred into 0.50 ml of hex ane that con tained dec ane and I2. This pro ce dure was to en sure that the unreacted Sm(II) was quenched by I 2. The I 2 quench ing so lu tion was freshly pre pared with an equal con cen tra tion of SmI 2. Af ter the mix ture was re moved from the glove box, 0.50 ml of a 0.12 M HCl so lu tion was added to each quenched so lu tion and was mixed for three min utes by a Virtex mixer. The so lu tion was set at room tem per a ture un til the or ganic and aque ous lay ers were well sep a rated. The clear or ganic layer was then in jected into GC for anal y sis. A cal i - bra tion curve was con structed by plot ting the ra tio of the peak area of the sub strate to the peak area of dec ane vs. the con cen - tra tion of sub strates. Ex trac tion re cov er ies were also done. Pseudo-first-order rate con stants were ob tained by plot ting logc ver sus time for more than two half-lives. Here C rep re - sents the con cen tra tion of the mon i tored sub strate. ACKNWLEDGMENT This work was fi nan cially sup ported by the Na tional Sci ence Coun cil of the Re pub lic of China (NSC M ). Re ceived March 26, Key Words Sa mar ium diiodide; Ethyl cinnamates. REFERENCES 1. (a) Masamune, S.; Bates, G. S.; Georghiou, P. E. J. Am. Chem. Soc 1974, 96, (b) Ashby, E. C.; Lin, J.-J.; Kover, R. J. rg. Chem 1939, 41, (c) Ashby, E. C.; Lin, J.-J.; Goel, A. B. J. rg. Chem. 1978, 43, 183. (d) Mahoney, W. S.; Brestensky, D. M.; Stryker, J. M. J. Am. Chem. Soc. 1988, 110, 291. (e) Brestensky, D. S.; Huseland, D. E.; McGettigan, C.; Strylker, Y. M. Tet ra he - dron Lett. 1988, 29, Semmethack, M. F.; Stauffer, R. D.; Yamashita, A. J. rg. Chem. 1977, 42, sborn, M. E.; Pegues, J. F.; Paguette, L. A. J. rg. Chem. 1980, 45, (a) Namy, J. L.; Gi rard, P.; Kagan, H. B. New L. Chem. 1997, 1, 5. (b) Gi rard, P.; Namy, J. L.; Kagan, H. B. J. Am. Chem. Soc. 1980, 102, (c) Namy, J. L.; Gi rard, P.; Kagan, H. B.; Caro, P. E. New J. Chem. 1981, 5, 479. (d) Curran, D. P.; Totleben, M. J. J. Am. Chem. Soc. 1992, 114, (e) Molander, G. A.; Mckie, J. M. J. rg. Chem. 1992, 57, (e) Molander, G. A.; McKie, J. J. rg. Chem. 1992, 57, (f) Curran, D. P.; Jasperse, C. P.; Totleben, M. J. J. rg. Chem. 1991, 56, (g) Hasegawa, E.; Curran, D. P. J. rg. Chem. 1993, 58, (h) Inanaga, J.; Ishikawa, M. Chem. Lett. 1987, (i) Molander, G. A.; Har ris, C. R. J. Am. Chem. Soc. 1995, 117, (j) Lange, G. L.; Gottardo, C. Tet - ra he dron Lett. 1994, 35, (k) gawa, A.; Nanke, T.; Takami, N.; Sumino, Y.; Ryu, I.; Sonoda, N. Chem. Lett. 1994, For re views see: (a) Kagan, H. B.; Namy, J. L. Tet ra he - dron 1986, 42, (b) Curran, D. P. Syn the sis 1988, 417. (c) Curran, D. P. Synthesis 1988, 489. (d) Jasperse, C. P.; Curran, D. P.; Fevig, T. L. Chem. Rev. 1991, 91, (e) Molander, G. A. Chem. Rev. 1992, 92, 29. (f) Curran, D. P.; Fevig, T. L.; Jasperse, C. P.; Totleben, M. J.

5 Re duc tion of Ethyl t-cinnamates by Sa mar ium Diiodide J. Chin. Chem. Soc., Vol. 48, No. 5, Synlett. 1992, 943. (g) Brandukova, N. E.; Vygodskii, Y. S.; Vinogradova, S. V. Rus sian Chem. Re view 1994, 63, 345. (h) Molander, G. A.; Har ris, C. R. Chem. Rev. 1996, 96, (a) Currar, D. P.; Wolin, R. L. Synlett. 1991, 317. (b) Zhou, Z.; Larouche, D.; Bennett, S. M. Tet ra he dron. 1995, 51, (c) Zhou, Z.; Benntett, S. M. Tet ra he - dron Lett. 1997, 38, (d) Molander, G. A.; Har ris, C. R. J. rg. Chem. 1997, 62, Lin, T. Y.; Fuh, S. M.; Chang, S. C.; Lin, Y. S. J. Chin. Chem. Soc. 1998, 45, Lin, T. Y.; Fuh, S. M.; Lin, Y. S.; Wen, C. R.; Lin, J. Y.; Hu, C. L. J. Chin. Chem. Soc. 2000, 47, 1223.