J. Org. Chem. 2001, 66, 6021-6028
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Novel Synthesis of Alkenes via Triethylborane-Induced Free-Radical Reactions of Alkyl Iodides and β-Nitrostyrenes Ju-Tsung Liu, Yeong-Jiunn Jang, Yuh-Kuo Shih, Shin-Ru Hu, Cheng-Ming Chu, and Ching-Fa Yao* Department of Chemistry, National Taiwan Normal University, 88, Sec. 4, Tingchow Road, Taipei, Taiwan 116, Republic of China
[email protected] Received February 26, 2001
Reactions of (E)-β-nitrostyrenes 1 and triethylborane 2 or tricyclohexylborane 4 in THF solution at room temperature in the presence of oxygen in the air as radical initiator generate high yields of trans-alkenes (E)-3 or (E)-5. Medium to high yields of different (E)-alkenes (E)-5, 7, 10, 12, and 14 also can be prepared when 1 reacts with different radicals, prepared from secondary alkyl iodides 6 and 8 or tertiary alkyl iodides 9, 11, and 13, in the presence of 2 and air as radical initiator. The generation of the only product (E)-alkenes can be explained by the generation of the benzylic radical A and/or B as the intermediate only and the mechanism is similar to Scheme 1. Both (E)- and (Z)-16a-c are generated when (E)- and (Z)-15a-c are used to react with adamantyl radical under similar conditions. Only (Z)-16d was observed when either (E)- or (Z)-15d was used to react with adamantyl radical. The generation of the (E)- and/or (Z)-alkenes can be explained by the free rotation of the A and/or B to generate A′ and/or B′ and vice versa, and the mechanism is proposed to be a free-radical reaction via NO2/alkyl substitution and is shown as Scheme 2. Introduction Nitro olefins are useful intermediates in organic synthesis and are important structural units that can be used as starting materials for many classes of compounds.1 Reactions of β-nitrostyrenes with organometallic reagents such as dialkylzinc2a or organozinc halides,2b,c t-BuHgX/KI,3 organomanganese,4 trialkylgallium,5 trialkylaluminum or dialkylaluminum chloride,6 and Grignard reagents,7 respectively, generate alkenes and/or nitroalkanes or halooximes have been reported. Our previous study found that medium to high yields of alkenes can be generated when (E)-β-nitrostyrenes 1 react with triethylborane in THF solution under refluxing condition in the presence of a trace of oxygen in the (1) (a) Corey, E. J.; Estreicher, H. J. Am. Chem. Soc. 1978, 100, 6294. (b) Seebach, D.; Colvin, E. W.; Weller, T. Chimia 1979, 33, 1. (c) Barrett, A. G. M.; Graboski, G. G. Chem. Rev. 1986, 86, 751. (d) Rosini, G.; Ballini, R. Synthesis 1988, 833. (e) Nitroalkanes and Nitroalkenes in Synthesis. Tetrahedron 1990, 46, 7313. (f) Barrett, A. G. M. Chem. Soc. Rev. 1991, 20, 95. (2) (a) Seebach, D.; Schafer, H.; Schmidt, B.; Schreiber, M. Angew. Chem., Int. Ed. Engl. 1992, 31, 1587. (b) Hu, Y.; Yu, J.; Yang, S.; Wang, J.-X.; Yin, Y. Synlett. 1998, 1213. (c) Hu, Y.; Yu, J.; Yang, S.; Wang, J.-X.; Yin, Y. Synth. Commun. 1999, 29, 1157. (3) Russell, G. A.; Yao, C.-F. Heteroatom Chem. 1992, 3, 209. (4) Namboothiri, I. N. N.; Hassner, A. J. Organomet. Chem. 1996, 518, 69. (5) Han, Y.; Huang, Y.-Z.; Zhou, C.-M. Tetrahedron Lett. 1996, 37, 3347. (6) Chu, C.-M.; Liu, J.-T.; Lin, W.-W.; Yao, C.-F. J. Chem. Soc., Perkin Trans. 1 1999, 47. (7) (a) Kohler, E. P.; Stone, J. R. J. Am. Chem. Soc. 1930, 52, 761. (b) Buckley, G. D. J. Chem. Soc. 1947, 1494. (c) Buckley, G. D.; Ellery, E. J. J. Chem. Soc. 1947, 1497. (c) Ashwood, M. S.; Bell, L. A.; Houghton, P. G.; Wright, S. H. B. Synthesis 1988, 379. (d) Yao, C.-F.; Chen, W.-W.; Lin, Y.-M. Tetrahedron Lett. 1996, 37, 6399. (e) Yao, C.F.; Kao, K.-H.; Liu, J.-T.; Chu, C.-M.; Wang, Y.; Chen, W.-C.; Lin, Y.M.; Lin, W.-W.; Yan, M.-C.; Liu, J.-Y.; Chuang, M.-C.; Shiue, J.-L. Tetrahedron 1998, 54, 791. (f) Liu, J.-T.; Lin, W.-W.; Jang, J.-J.; Liu, J.-Y.; Yan, M.-C.; Hung, C.; Kao, K.-H.; Wang, Y.; Yao, C.-F. Tetrahedron 1999, 55, 7115.
nitrogen or by photolysis in the presence of tert-butyl peroxide as radical initiator.8 All these results indicate that β-nitrostyrenes can react with different organometallic reagents to generate nitroalkanes or alkenes under different conditions and workup procedures and the reaction mechanism is proposed to be a free-radical and/ or an ionic reaction.1-8 In this paper, we wish to report an improved and effective method, based on our previous study,8 to synthesize different alkenes by reaction of β-nitrostyrenes with different radicals that were prepared from alkyl iodides and triethylborane in the presence of oxygen in the air as radical initiator. Results and Discussion At room temperature, (E)-β-nitrostyrenes 1 reacted with triethylborane 2 or tricyclohexylborane 4 in THF solution in the presence of oxygen in the air to generate 87-100% of 1-aryl-1-butene 3 or 79-98% of 1-aryl-2cyclohexylethene 5 within 5-10 min (eq 1 and Table 1). All spectral data indicate that the alkenes have (E) configuration (Jtrans ) 16.0 Hz for vinyl hydrogen) after the mixture were purified by flash column chromatography and the spectral data of the known products are also consistent with literature report.2,8 Compared to our previous study to stimulate the reaction by refluxing or by photolysis the solution,8 the reaction condition of this method is mild and easy to follow and the reaction rate is also accelerated dramatically by the presence of the oxygen in the air so that the reaction time can be reduced from few hours to only 5-10 min. The mechanism is also proposed to be a free-radical reaction proceeding through an addition and elimination reaction to generate (E)alkenes that are similar to our previous report.8 (8) Yao, C.-F.; Chu, C.-M.; Liu, J.-T. J. Org. Chem. 1998, 63, 719.
10.1021/jo010213m CCC: $20.00 © 2001 American Chemical Society Published on Web 08/15/2001
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Table 2. Reaction of trans-β-Nitrostyrenes 1 (1 Equiv) with RI (Excess) and Et3B (3 Equiv) in THF in the Presence of Oxygen under an Air Atmosphere
Table 1. Reaction of trans-β-Nitrostyrenes 1 (1 Equiv) with R3B 2 or 4 (3 Equiv) in THF under an Air Atmosphere at Room Temperature entry
1
R3B
product
yield (%)a
1 2 3 4 5 6 7 8 9 10 11 12 13 14
1a 1b 1b 1b 1c 1d 1e 1f 1a 1b 1c 1d 1e 1f
2 2 2 2 2 2 2 2 4 4 4 4 4 4
3a 3b 3b 3b 3c 3d 3e 3f 5a 5b 5c 5d 5e 5f
96 100 trb 96c 89 98 87 99 79 97 85 90 98 92
a Isolated yields. b 50 mol % galvinoxyl was added and 92% unreacted 1b was recovered. c Pure oxygen was used.
Other evidence to prove the free-radical reaction is that the reaction is retarded or inhibited by the addition of galvinoxyl, an efficient scavenger for free radicals.9 After addition of 50 mol % galvinoxyl under similar conditions, only a trace of 3b was observed and 92% unreacted 1b was recovered (Table 1, entry 3). Oxygen is not only known to be a free-radical initiator but also is known to be a radical scavenger. When the reaction was conducted in pure oxygen, 96% 3b (Table 1, entry 4) was obtained, which is lower than with the use of air (100%) under similar procedures (Table 1, entry 2). These results indicate that the use of air is good enough for these reactions. It has been reported that both (E)- and (Z)-1a react with diethylzinc to give the (E)-3a only.2a In addition to (E)-1a, (Z)-1a was also used to react with 2 under similar conditions as described above (eq 2). As expected, 90% yield of the same product (E)-3a was also isolated, and this result is actually consistent with literature report.2a
entry
1
RI (equiv)
product (yield %)a
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
1a 1b 1b 1b 1b 1c 1d 1e 1f 1a 1b 1c 1d 1e 1f 1a 1b 1c 1d 1e 1f
6 (12) 6 (12) 6 (12) 6 (12) 6 (12) 6 (12) 6 (12) 6 (20) 6 (20) 8 (12) 8 (12) 8 (12) 8 (12) 8 (12) 8 (12) 9 (6) 9 (6) 9 (6) 9 (6) 9 (6) 9 (6)
7a (85) 7b (97) 7b (100)b 7b (38)c 7b (64)d 7c (98) 7d (100) 7e (100) 7f (92) 5a (79)e 5b (97)e 5c (97)e 5d (94)e 5e (77)e 5f (57)e 10a (72) 10b (67) 10c (75) 10d (77) 10e (75) 10f (57)
a Isolated yields. b Pure oxygen was used. c One equivalent of Et3B was used and 62% unreacted 1b was recovered. d Two equivalents of Et3B was used and 36% unreacted 1b was recovered. e NMR yields.
Triethylborane not only can be served as one of the methods for the generation of radical species but also can be used as an effective radical initiator to generate different radicals by reaction of triethylborane with alkyl iodides under mild conditions.10,11 On the basis of these reports, we apply this methodology to β-nitrostyrenes by using triethylborane and air as the free radical initiator to induce alkyl iodide to generate different radicals. As expected, good to excellent yields of alkenes (E)-7, 5, or 10 were generated within 5-10 min when 1 (1 equiv) and excess alkyl iodides such as isopropyl iodide 6, cyclohexyl iodide 8, or tert-butyl iodide 9 (6-20 equiv) were put in the THF solution and then 2 (3 equiv) was added into the THF solution slowly by syringe at room temperature under similar conditions (eq 3 and Table 2). It has been found that 3 equiv of triethylborane 2 is necessary to generate enough ethyl radical as radical (9) Barlett, P. D.; Funahashi, T. J. Am. Chem. Soc. 1962, 84, 2596. (10) Smadja, W. Synlett 1994, 1. (11) (a) Suzuki, A.; Nozawa, S.; Harada, M.; Itoh, M.; Brown, H. C.; Midland, M. M. J. Am. Chem. Soc. 1971, 93, 1508. (b) Nozaki, K.; Oshima, K.; Utimoto, K. J. Am. Chem. Soc. 1987, 109, 2547. (c) Nozaki, K.; Oshima, K.; Utimoto, K. Tetrahedron Lett. 1988, 29, 1041. (d) Bertrand, M. P.; Feray, L.; Nouguier, R.; Perfetti, P. J. Org. Chem. 1999, 64, 9189. (e) Miyabe, H.; Ushiro, C.; Ueda, M.; Yamakawa, K.; Naito, T. J. Org. Chem. 2000, 65, 176. (f) Wu, B.; Avery, B. A.; Avery, M. A. Tetrahedron Lett. 2000, 41, 3797. (g) Miyabe, H.; Ueda, M.; Naito, T. J. Org. Chem. 2000, 65, 5043. (h) Miyabe, H.; Fujii, K.; Goto, T.; Naito, T. Org. Lett. 2000, 2, 4071. (i) Yorimitsu, H.; Nakamura, T.; Shinokubo, H.; Oshima, K.; Omoto, K.; Fujimoto, H. J. Am. Chem. Soc. 2000, 122, 11041.
Synthesis of Alkenes via Triethylborane-Induced Free-Radical Reactions
initiator to undergo further reactions. When 1 equiv of 2 was used, only 38% 7b and 62% unreacted 1b were found in the crude NMR analysis (Table 2, entry 4). Similarly, only 64% 7b and 36% unreacted 1b were found when 2 equiv of 2 was used (Table 2, entry 5). Similar to eq 2, 76% of 10a was also isolated when (Z)-1a reacted with 9 and 2 under similar conditions, and this result is also good enough to prove that both (E)-1a and (Z)-1a can react with the same radical to generate the same product trans-alkene. Compared to Table 1 and our previous study,8 we found that the same or similar alkenes also could be prepared easily just by adding alkyl iodides to the reaction mixture under similar conditions. The condition of this improved method is not only mild and convenient but this method also can be used to synthesize high yield of alkenes with three components in a one-pot reaction at room temperature. For example, 57-77% (E)-10a-f (Table 2, entries 16-21) were generated when 1 reacted with 9 and 2 as described above. In addition to (E)-10a-f, 78% of the same product (E)-3,3-dimethyl-1-(4-methylphenyl)-1butene that has been reported in our previous report8 also was isolated when (E)-2-(4-methylphenyl)-1-nitroethene reacted with 9 and 2 in THF solution in the presence of the oxygen in the air but only 50% yield was observed when the same starting material reacted with B-tertbutyl-9-BBN in THF solution under refluxing condition in the presence of a trace of oxygen in the nitrogen.8 It has been reported that different alkyl radicals including 1-adamantyl radical and aryl radical can be generated by the reduction of alkyl or aryl halides with (ethylenediamine)chromium(II) complexes in dry dimethylformamide.12 The palladium-catalyzed reaction of 1-bromoadamantane with styrene and donor-substituted styrenes give the corresponding Heck-type coupling product (E)-1-adamantyl-2-arylethene in 15-41% also has been reported by Bra¨se et al.13 Similarly, Yamataka and co-workers also have reported that both (Z)- and (E)1-adamantyl-2-arylethene mixture had been prepared from the Wittig reaction of substituted benzaldehydes with (1-adamantylmethylidene)triphenylphosphorane.14 According to these reports and eq 3, it might be possible to prepare similar products by reaction of 1 with 1-iodoadamantane 11 under similar conditions. As expected, when 1b (1 equiv) reacted with 11 (6 equiv) and 2 (3 equiv) in the presence of the oxygen within 5-10 min, the only product is (E)-1-adamantyl-2-(4-methoxyphenyl)ethene 12 and the yield is 100% when the crude mixture was analyzed by the proton NMR (eq 4). This result indicates that the same adamantyl radical actually could
J. Org. Chem., Vol. 66, No. 18, 2001 6023
Table 3. Preparation of 14 by Reaction of 1 (1 Equiv), 13 (6 Equiv), and 2 (3 Equiv) in the Presence of Oxygen
a
entry
1
14
yielda (%)
1 2 3 4 5 6
1a 1b 1c 1d 1e 1f
14a 14b 14c 14d 14e 14f
71 60 99 86 82 100
Isolated yields.
be generated by using 11, 2, and air at room temperature.13,14 5-Substituted-2-adamantanones and their derivatives are useful and interesting compounds for the study of diastereoselectivities in the reactions of sterically unbiased ketones.15 Many 5-substituted-2-adamantanones with different withdrawing and donor groups such as CN, COOCH3, CF3, F, Cl, Br, I, OH, NMe2, OCH3, OCOCH3, CH3, t-Bu, SiMe3, and SnMe3 have been prepared.15 On the basis of eq 4, compounds 1 were also used to react with 5-iodoadamantan-2-one 13 and 2 to yield 60-100% of (E)-1-(4-oxoadamantyl)-2-arylethene (E)-14 under similar conditions (eq 5 and Table 3). The generation of 14 indicates that not only the alkyl radicals but the γ-keto radical also could be produced by this useful methodology. Steric hindrance is one of the effects to affect reactions and always plays an important role in determining of the intermediate and final product stability. After observation substrate 1, we then focus our study on the reactions of steric hindered R-alkyl-β-nitrostyrenes such as (E)-15 and (Z)-15, prepared from the substitution of the R-hydrogen of the β-nitrostyrenes 1 by different alkyl groups,16 with 11 and 2 as described above (eq 6 and Table 4). Surprisingly, not only (E)-16a but also (Z)-16a was generated when either (E)-15a or (Z)-15a reacted with 11 and 2 under similar conditions, and the total yields were 74% and 90%, respectively. The gas chromatography analysis of the crude mixture found that the ratio of (E)-16a to (Z)-16a approximately equal to 96:4 or 92:8 (12) (a) Tashtoush, H. I.; Sustmann, R. Chem. Ber. 1992, 125, 287. (b) Tashtoush, H. I.; Sustmann, R. Chem. Ber. 1993, 126, 1759. (13) Bra¨se S.; Waegell, B.; de Meijere, A. Synthesis 1998, 148. (14) Yamataka, H.; Takatsuka, T.; Hanafusa, T. J. Org. Chem. 1996, 61, 722. (15) (a) Gung, B. W.; le Noble, B. Chem. Rev. 1999, 99, 1067. (b) Tomoda, S. Chem. Rev. 1999, 99, 1243. (c) Cieplak, A. S. Chem. Rev. 1999, 99, 1265. (d) Ohwada, T. Chem. Rev. 1999, 99, 1337. (e) Gung, B. W. Chem. Rev. 1999, 99, 1377. (f) Kaselj, M.; Chung, W.-S.; le Noble, W. J. Chem. Rev. 1999, 99, 1387. (g) Adcock, W.; Trout, N. A. Chem. Rev. 1999, 99, 1415. (h) Mehta, G.; Chandrasekhar, J. Chem. Rev. 1999, 99, 1437. (i) Wipf, P.; Jung, J.-K. Chem. Rev. 1999, 99, 1469. (j) Kao, K.-H.; Sheu, R.-S.; Chen, Y.-S.; Lin, W.-W.; Liu, J.-T.; Yao, C.-F. J. Chem. Soc., Perkin Trans. 1 1999, 283. (k) Tsai, T.-L.; Chen, W.-C.; Yu, C.-H.; le Noble, W. J.; Chung, W.-S. J. Org. Chem. 1999, 64, 1099.
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Table 4. Preparation of 16 by Reaction of 15 (1 Equiv), 11 (6 Equiv), and 2 (3 Equiv) in the Presence of Oxygen entry
15
16
yielda (%)
E/Zb
1 2 3 4 5 6 7 8
(E)-15a (Z)-15a (E)-15b (Z)-15b (E)-15c (Z)-15c (E)-15d (Z)-15d
16a 16a 16b 16b 16c 16c 16d 16d
74 90 59 50 73 63 35 32
96:4 92:8 32:68 37:63 18:82 13:87
