DOI: 10.1002/ajoc.201600116
Full Paper
Allylic Alcohol Amination
Palladium-Catalyzed Direct Amination of Allylic Alcohols in Aqueous Media Zhengyin Du,* Yufei Yan, Ying Fu, and Kehu Wang[a] Abstract: Palladium chloride-catalyzed amination of allylic alcohols with aromatic amines in the presence of triphenylphosphine and trifluoroacetic acid in the aqueous phase gave a wide range of allylamines in good to excellent yields under mild conditions. This protocol has a wide substrate
scope and broad functional group tolerance. It is an environmentally friendly and highly atom-economical method for Nallylamine synthesis. A plausible mechanism involving a (pallyl)palladium complex intermediate is proposed.
Introduction
disclosed an N-alkylation reaction, including N-allylation reaction, involving palladium halides as catalysts. As we know, palladium chloride is a convenient and inexpensive commercial source of palladium. Using palladium chloride as a catalyst for carbon–carbon bond formation has been proved to be a practical method.[12] Since the palladium-catalyzed Tsuji–Trost-type N-allylation reaction was disclosed, it has been considered as one of the most important methods for the synthesis of N-allylamines.[13] In 2012, Han and co-workers[14] established a PdCl2-catalyzed C-allylation of heteroarenes with allyl acetate. In 2014, Varma et al. reported a chitosan-immobilized Pd0 catalyst, prepared by the reduction of palladium chloride with NaBH4 in an aqueous suspension of chitosan, which catalyzed the N-allylation of amines with allyl acetates.[15] However, these are not true PdCl2-catalyzed N-allylation reactions. Therefore, we hope to develop a green, simple, cheap, and highly efficient method for the amination of allylic alcohols by directly using PdCl2 as a catalyst. After several efforts, a facile method for the amination of allylic alcohols catalyzed by PdCl2 when using trifluoroacetic acid (TFA) as an additive and PPh3 as a ligand in aqueous phase was found. This is, to our knowledge, the first example of palladium chloride-catalyzed N-allylation.
Transition-metal-catalyzed N-allylic alkylation reactions to form allylamines play a significant role in allowing synthetic access to biologically important molecules and advanced intermediates in the total synthesis of natural products.[1] The process involves a coupling reaction between an amine and an allylic alcohol, which has emerged as the most attractive method because using allylic alcohols as alkylating agents is a practical, economical, and environmentally friendly process. The metal catalysts used in this N-allylation reaction include Ag/Au,[2] Ir,[3] Ru,[3a, 4] Rh,[3a] and Pt[5] simple compounds and complexes. FeBr3 was also used as a catalyst for the amination of allylic alcohol.[6] Compared with other transition metals, palladium is the most commonly used metal in this reaction. Pd-(p-allyl) complexes have good catalytic activities but they are not easy to prepare and some of them need water- and oxygen-free operation.[7] Pd(PPh3)4, a zero-valent palladium complex, can successfully catalyze the amination reaction of allylic alcohols with the addition of Et3B.[8] Some simple palladium compounds, such as Pd(OAc)2[9] and Pd(acac)2[10] combined with phosphorus ligands were also used as catalytic systems to achieve good results. Recently, Seayad and co-workers reported that the palladium chloride-catalyzed N-alkylation of amines by using three equivalents of saturated primary and secondary alcohols to obtain N-monoalkylation products.[11] However, allylic alcohols were not included in this method. To date, no other literature has
Results and Discussion The N-allylation reaction of aniline (1 a) with allyl alcohol (2 a) as a model reaction was first examined in the presence of PdCl2 and a phosphorus ligand in a mixture of 1,4-dioxane and water (2:1, v/v) at reflux temperature under air. The reaction proceeded for about 3 h until aniline was converted completely. The results are listed in Table 1. Considering that carboxylic acid in the catalytic system or attached to the substrate can promote the N-alkylation reaction,[9b, 10] we herein used cheap and commercially available haloacetic acids as promoters in this reaction. When PPh3 (5 mol %) and trifluoroacetic acid (TFA, 10 mol %) were used under these conditions; the N-allyla-
[a] Prof. Dr. Z. Du, Y. Yan, Dr. Y. Fu, Dr. K. Wang Key Laboratory of Eco-Environment Related Polymer Materials of Ministry of Education College of Chemistry and Chemical Engineering Northwest Normal University Lanzhou 730070 (P. R. China) E-mail:
[email protected] Supporting information for this article can be found under http:// dx.doi.org/10.1002/ajoc.201600116. Asian J. Org. Chem. 2016, 5, 812 – 818
812
Ó 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Full Paper yield of 3 a and 17 % yield of 4 a, which is no manifest change in the total yield and in molar ratio of 3 a/4 a (Table 1, entry 10). The molar ratio of 1 a and 2 a was also optimized. When five equivalents of 2 a were used, the yield of 4 a increased slightly and the total yield had almost no change (Table 1, entry 11). However, when decreasing the amount of 2 a to two equivalents, only 56 % of 3 a and 8 % of 4 a were obtained and simultaneously 24 % of aniline was recovered (Table 1, entry 12). To further examine the catalytic effect of palladium, palladium(II) chloride without phosphorus ligands and a palladium(0) complex were also applied in this reaction under similar reaction conditions. The results show that almost no product was obtained by using PdCl2 as the catalyst without any phosphorus ligands (Table 1, entry 13), whereas 47 % of the mono-N-allylation and 26 % of the di-N-allylation products were formed when using Pd(PPh3)4 as the catalyst and PPh3 as the ligand (Table 1, entry 14). Considering the conversion, the yield, and selectivity of the reaction, we finally selected the reaction conditions of entry 1 in Table 1 as the optimum conditions. With the optimized conditions in hand, we next explored the scope of aromatic amines and allylic alcohols. Electronic effects of the substituents attached on the aromatic amines were first examined by the reaction of various para- and metasubstituted aromatic primary amines with simple allyl alcohol 2 a. The results are described in Table 2. It can be seen that all the used electron-withdrawing and electron-donating groups can tolerate the reaction and 53 % to 88 % overall yield of the mono- and di-substituted allylation products were obtained. The difference is that the strong electron-withdrawing nitro group leads to a low overall yield (53 % for 4-NO2, 70 % for 3NO2) and some aniline recovery (36 % for 4-NO2, 15 % for 3NO2) but with high ratio of monoallylation to diallylation prod-
Table 1. Optimization of reaction of allyl alcohol with aniline.[a]
Entry Catalyst 1 2[b] 3[c] 4 5 6 7 8 9 10[e] 11[f] 12[g] 13 14
PdCl2/PPh3 PdCl2/PPh3 PdCl2/PPh3 PdCl2/dppp PdCl2/dppe PdCl2/PPh3/nanoFe3O4[d] PdCl2/PPh3/Cu(OAc)2[d] PdCl2/PPh3 PdCl2/PPh3 PdCl2/PPh3 PdCl2/PPh3 PdCl2/PPh3 PdCl2 Pd(PPh3)4/PPh3
Additive
Yield 3 aa [%] Yield 4 aa [%]
TFA TFA TFA TFA TFA TFA TFA ClCH2COOH Cl3CCOOH TFA TFA TFA TFA TFA
68 trace 25 45 48 65 67 39 53 69 64 56 trace 47
18 trace 9 7 9 17 22 17 18 17 24 8 trace 26
[a] Reaction conditions: 1 a (1 mmol), 2 a (3 mmol), PdCl2 (0.02 mmol), ligand (0.05 mmol), additive (0.1 mmol), 1,4-dioxane (2 mL) and water (1 mL) as mixed solvent, at reflux temperature for 3 h. Yield is the isolated yield. [b] 1,4-Dioxane (3 mL) was used instead of mixed solvent. [c] Water (3 mL) was used instead of mixed solvent. [d] The used amount is 5 mol %. [e] PdCl2 (5 mol %) was used. [f] Five equivalents of 2 a were used. [g] Two equivalents of 2 a were added and 24 % of aniline was recovered.
tion reaction proceeded smoothly and the monoallylation product (3 a) and diallylation product (4 a) were obtained with yields of 68 % and 18 %, respectively (Table 1, entry 1). It is surprising that the N-allylation reaction practically did not occur in pure 1,4-dioxane and only minor quantities of the products were obtained in water (Table 1, entries 2 and 3). So the mixed solvent manifests a very good synergy effect, which may be due to the good dissolution of 1,4-dioxane and activation of water for allyl alcohol through the hydration of the hydroxyl group to smoothly generate the p-allylpalladium intermediate.[16] Bidentate phosphorus ligands, such as 1,3-bis(diphenylphosphino)propane (dppp) and 1,2-bis(diphenylphosphino)ethane (dppe), were also compared with triphenylphosphine under the same conditions. However, no more than 50 % of the monoallylation product as well as 7–9 % of the diallylation products were obtained (Table 1, entries 4 and 5). The cause may be that strong the p-accepting capacity of PPh3 leads to a higher turnover number.[7c] To improve the catalytic efficiency, 5 mol % of a second metal compound, nano Fe3O4 or Cu(OAc)2 was added, but the product yields were not increased (Table 1, entries 6 and 7). In comparison with TFA, chloroacetic acid and trichloroacetic acid were also examined as acidic additives; however, inferior total yields (56 % for chloroacetic acid and 71 % for trichloroacetic acid versus 86 % for TFA) were obtained (Table 1, entries 8 and 9). It is clear that TFA is the superior acidic additive for this reaction. Increasing the amount of PdCl2 from 2 mol % to 5 mol %, gave a 69 % Asian J. Org. Chem. 2016, 5, 812 – 818
www.AsianJOC.org
Table 2. Amination of allyl alcohol with arylamines.[a]
Entry
R1, amine
t [h]
1[b] 2[c] 3 4 5 6 7 8 9[d]
4-NO2, 1 b 3-NO2, 1 c 4-Br, 1 d 4-CH3O, 1 e 4-CH3, 1 f 3-Cl, 1 g 4-Cl, 1 h 3-CH3, 1 i 1j
5 3 2 1.5 2 3 3 2 5
3 3 ba, 48 3 ca, 56 3 da, 67 3 ea, 64 3 fa, 58 3 ga, 54 3 ha, 45 3 ia, 59 trace
Yield of product [%] 4 3+4 4 ba, trace 4 ca, 14 4 da, 18 4 ea, 28 4 fa, 30 4 ga, 26 4 ha, 31 4 ia, 26
53 70 85 82 88 80 76 85
[a] Reaction conditions: amine 1 (1 mmol), PdCl2 (2 mol %), PPh3 (0.05 mmol), TFA (0.1 mmol), 2 a (3 mmol) in 1,4-dioxane (2 mL) and H2O (1 mL), reflux. Isolated yield. [b] 36 % of aniline was recovered. [c] 15 % of aniline was recovered. [d] 2-Aminopyridine was used instead of a substituted aniline.
813
Ó 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Full Paper uct (up to 9.6:1, Table 2, entries 1 and 2); whereas electron-donating groups, such as methyl and methoxyl, result in high overall yield (up to 88 % for 4-CH3) but with low ratio of 3/4 (Table 2, entries 4, 5, and 8). The cause may be that the electron-withdrawing groups on the benzene ring decrease the electron cloud density of the nitrogen atom of the amino group and make the nucleophilicity decrease. When 2-aminopyridine was used instead of anilines, only trace amounts of product were obtained (Table 2, entry 9). The reason is not yet clear. Steric hindrance effects of the aromatic amines were also studied in this reaction by using 2 a as the allylating reagent. Various ortho-substituted anilines 1 k–1 u were used in this allylation reaction and the results are summarized in Table 3. It can be seen that most of the substituents, such as methyl, ethyl, methoxyl, chloro, hydroxyl, and carboxyl, attached at the 2-position of aniline, can tolerate the N-allylation reaction to give the corresponding N-monoallylation products 3 ka–3 ua exclusively with yields of 67–85 % and no N,N-diallylation products were obtained (Table 3, entries 1–11, 13). When 1-naphthylamine was utilized as a substrate (1 v), the yield of N-allyl1-naphthylamine 3 va was 88 % (Table 3, entry 12). For 4chloro-2-nitroaniline and 6-ethyl-2-methylaniline, traces of the corresponding products were obtained (Table 3, entries 14 and 15). The former is due to the strong electron-withdrawing nature of the nitro and chloro groups, which renders the aniline less nucleophilic. The latter is due to the significant steric hindrance of the two ortho positions of the amino group, which prevents the combination of the amine and allyl alcohol. To further prove this point, 2,4-dichloro-6-nitroaniline was used instead, however, no reaction occurred (Table 3, entry 16). The cause may be the co-effect of the large steric hindrance and strong electron-withdrawing nature of this substrate. It is clear that the presence of one ortho-substituent on the aromatic amines has a very important effect on the formation of N-monoallylation products with complete selectivity. To further investigate steric effects on this N-allylation reaction, various allylic alcohols 2 b–2 e were explored in combination with different anilines. The results are listed in Table 4. When 4-methylpent-3-en-2-ol (2 b), a secondary allylic alcohol, was subjected to this reaction with various anilines; moderate to excellent yields of up to 90 % of the N-monoallylation products were obtained exclusively (Table 4, entries 1–7). However, the conversions of nitro-containing anilines were low in the reactions with 2 b and the recoveries of 1 u and 1 b were 9 % and 20 %, respectively (Table 4, entries 1 and 6). The reason may be the decrease of nucleophilicity of the nitro-containing anilines and the large steric hindrance of 2 b. When cinnamyl alcohol (2 c), a primary alcohol, was used as the alkylating reagent, the mono- and dialkyl products of 4-chloroaniline were simultaneously obtained in a total yield of 83 % after 12 h (Table 4, entry 8). However, introduction of a nitro group at the 2-position of cinnamyl resulted in a significant decrease in the catalytic activity and, after 13 h, only the monoalkylated product was obtained in 70 % yield (Table 4, entry 9). 2-Methylbut3-en-2-ol (2 e), an allylic tertiary alcohol, was also attempted in the amination reaction. The results show that it can be easily Asian J. Org. Chem. 2016, 5, 812 – 818
www.AsianJOC.org
rearranged into a primary allylic group during the reaction. Mono- and diallylamines were found for less steric anilines (Table 4, entries 11 and 12) and only monoallylamines were achieved for sterically hindered anilines (Table 4, entries 10 and 13). To investigate the reaction mechanism, we used two isomers, 3-buten-2-ol (2 f) and 2-buten-1-ol (2 g, crotyl alcohol), as allylation reagents to react with 2,5-dichloroaniline (1 p) under the optimized conditions (Scheme 1). We found that the same
Scheme 1. Regio- and stereoselectivity in N-allylation reactions of 2,5-dichloroaniline. The regio- and stereoisomeric ratios were determined by 1 H NMR spectroscopy.
mono-N-allylation product of N-(2-butenyl)-2,5-dichloroaniline (3 pf) as the main product and N-(1-methyl-2-propenyl)aniline (3 pf’) as the minor product were obtained in total yields of 63 % and 69 %, respectively, with a ratio of about 9:1. In addition, the E/Z ratio of 3 pf in the two reactions was similar at 85:15. This means that the amination of allylic alcohols proceeds via the same p-allyl palladium(II) species. Based on our observations and previous literature, we propose a possible mechanism (Scheme 2). The amination of allylic alcohols may go via a p-allyl palladium complex intermediate.[9b] First, the p-allyl palladium(II) complex is formed by oxidative addition of the allylic alcohol with Pd0, which is generated
Scheme 2. Proposed mechanism.
in situ from PdCl2 and Ph3P. Subsequently, ligand exchange and reductive elimination occur and the Pd0 catalyst is released to obtain the monoallylation products. The monoallyl amines can be further transformed into diallylation products with the p-allyl palladium complex. In the reaction, the presence of water and TFA can promote allyl–OH bond cleavage remarkably to enhance the reaction rate and yield. Furthermore, it can 814
Ó 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Full Paper Table 3. Amination of allyl alcohol with ortho-substituted anilines.[a]
Entry
Amine
t [h]
1
Product
4 1k
2 1l
4 1o
1p 7 1q
8
1r
6
14
83 3 wa
6
trace
6
trace
6
NR
1x 70
15
3 qa
4
88 3 va
1w
3 pa 4
4
13
84
85
3 ua
1v
3 oa
5
6
12
78
82 3 ta
1u
3 na
67
5
11
Yield [%]
3 sa
1t
74
Product
5
10
3 ma
1n
t [h]
1s
67
4
6
9
Amine
3 la
1m
5
80
83
4
4
Entry
3 ka 6
3
Yield [%]
1y
74
16
3 ra
1z
[a] Reaction conditions: amine 1 (1 mmol), PdCl2 (2 mol %), PPh3 (0.05 mmol), TFA (0.1 mmol), 2 a (3 mmol) in 1,4-dioxane (2 mL) and H2O (1 mL), reflux. Isolated yield. NR = no reaction.
Conclusions
be seen from this mechanism that a catalytic amount of TFA is enough for this reaction. Strong p-acceptor ligands, that is, PPh3,act as a better donor than alkylphosphorus ligands for the Pd-catalyzed allylation of anilines.[7a, c]
Asian J. Org. Chem. 2016, 5, 812 – 818
www.AsianJOC.org
We have developed an efficient, direct, catalytic amination of allylic alcohols with aromatic amines in the aqueous phase under mild conditions, which provides moderate to excellent 815
Ó 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Full Paper Table 4. N-Allylation of aromatic amines with various allylic alcohols.[a]
Entry
Allylic alcohol 2
1[b]
Amine 1
Product
1u
2b 2
2b
t [h]
Yield [%]
6
88
4
72
6
71
7
88
6
90
8
77
4
68
12
50
12
33
13
70
3 ub 1o 3 ob
3
2b
1v
3 vb 4
2b
1d 3 db
5
2b
1z 6[c]
2b
3 zb
1b 3 bb
7
2b
1f 3 fb
8
1h 2c
3 hc
2c
4 hc 9
1h 2d
Asian J. Org. Chem. 2016, 5, 812 – 818
3 hd
www.AsianJOC.org
816
Ó 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Full Paper Table 4. (Continued) Entry
Allylic alcohol 2
10
Amine 1
Product
1u 2e
11
2e
t [h]
Yield [%]
10
83
8
61
6
65
10
35
8
67
3 ue 1f 3 fe
12
2e
1i 3 ie
13
2e
1p 3 pe
14
2e
1h 3 he
[a] Reaction conditions: amine 1 (1 mmol), PdCl2 (2 mol %), PPh3 (0.05 mmol), TFA (0.1 mmol), 2 (3 mmol) in 1,4-dioxane (2 mL) and H2O (1 mL), reflux. Isolated yield. [b] 9 % of aniline was recovered. [c] 20 % of aniline was recovered.
yields of the desired N-allylated amines. The PdCl2/PPh3 catalytic system used is simple, inexpensive, and easily available. It is an environmentally friendly and economical protocol for N-allylamines synthesis. The method has a wide substrate scope and broad functional group tolerance. Steric hindrance at the ortho position of anilines and allylic secondary alcohols has an effect on the selective formation of N-monoallylation products. A plausible mechanism involving a (p-allyl)palladium complex intermediate is proposed.
by TLC. Then, the reaction media was removed by evaporation under reduced pressure and the residue was washed with saturated sodium carbonate solution. The resulting mixture was extracted with diethyl ether (10 mL Õ 3) and the combined organic layers were dried over anhydrous MgSO4. The solvent was removed by evaporation under reduced pressure to afford the crude products, which were further purified by column chromatography on silica gel (200–300 mesh) by using petroleum ether (60–90 8C) and ethyl acetate (1:3) as the eluent.
Acknowledgments
Experimental Section
We gratefully acknowledge the National Natural Science Foundation of China (20702042, 21262028) for financial support.
General remarks All reagents were used as obtained from commercial sources without further purification. The 1H and 13C NMR spectra were recorded with a Bruker MERCURY-PLUS 400 MHz NMR spectrometer. Chemical shifts are reported in parts per million (ppm, d). IR spectra were measured with an Alpha Centauri FTIR spectrometer and low resolution MS was analyzed by using a QP-1000 A GC-MS with EI sources. HRMS of some products were obtained by using ESI ionization. Melting points were determined with an XT-4 electrothermal micro-melting-point apparatus. Elemental analyses were performed with a Carlo–Erba 1106 Elemental Analysis instrument.
Keywords: allylic alcohols · amination · aqueous-phase reaction · aromatic amines · N-allylation · palladium chloride [1] a) B. M. Trost, M. L. Crawley, Chem. Rev. 2003, 103, 2921 – 2944; b) M. Johannsen, K. A. Jørgensen, Chem. Rev. 1998, 98, 1689 – 1708; c) S. Bhn, S. Imm, L. Neubert, M. Zhang, H. Neumann, M. Beller, ChemCatChem 2011, 3, 1853 – 1864. [2] X. Giner, P. Trillo, C. Njera, J. Organomet. Chem. 2011, 696, 357 – 361. [3] a) C. Liu, S. Liao, Q. Li, S. Feng, Q. Sun, X. Yu, Q. Xu, J. Org. Chem. 2011, 76, 5759 – 5773; b) C. Defieber, M. A. Ariger, P. Moriel, E. M. Carreira, Angew. Chem. Int. Ed. 2007, 46, 3139 – 3143; Angew. Chem. 2007, 119, 3200 – 3204. [4] Z. Sahli, B. Sundararaju, M. Achard, C. Bruneau, Org. Lett. 2011, 13, 3964 – 3967. [5] a) M. Utsunomiya, Y. Miyamoto, J. Ipposhi, T. Ohshima, K. Mashima, Org. Lett. 2007, 9, 3371 – 3374; b) G. Mora, O. Piechaczyk, R. Houdard, N. M¦zailles, X.-F. Le Goff, P. Le Floch, Chem. Eur. J. 2008, 14, 10047 – 10057;
General procedure for the amination of allylic alcohols Aniline (1 mmol), allylic alcohol (3 mmol), F3CCOOH (0.1 mmol), triphenylphosphine (0.05 mmol), PdCl2 (0.02 mmol), 1,4-dioxane (2 mL), and H2O (1 mL) were added to a 50 mL round-bottom flask. The mixture was stirred at reflux temperature for the desired time until complete consumption of the starting material was detected Asian J. Org. Chem. 2016, 5, 812 – 818
www.AsianJOC.org
817
Ó 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Full Paper [6] [7]
[8] [9]
c) T. Ohshima, Y. Miyamoto, J. Ipposhi, Y. Nakahara, M. Utsunomiya, K. Mashima, J. Am. Chem. Soc. 2009, 131, 14317 – 14328. Y. Zhao, S. W. Foo, S. Saito, Angew. Chem. Int. Ed. 2011, 50, 3006 – 3009; Angew. Chem. 2011, 123, 3062 – 3065. a) F. Ozawa, H. Okamoto, S. Kawagishi, S. Yamamoto, T. Minami, M. Yoshifuji, J. Am. Chem. Soc. 2002, 124, 10968 – 10969; b) F. Ozawa, T. Ishiyama, S. Yamamoto, S. Kawagishi, H. Murakami, M. Yoshifuji, Organometallics 2004, 23, 1698 – 1707; c) C. Thoumazet, H. Grìtzmacher, B. Deschamps, L. Ricard, P. Le Floch, Eur. J. Inorg. Chem. 2006, 3911 – 3922; d) R. Ghosh, A. Sarkar, J. Org. Chem. 2011, 76, 8508 – 8512; e) Y. Tamaru, J. Organomet. Chem. 1999, 576, 215 – 231. M. Kimura, M. Futamata, K. Shibata, Y. Tamaru, Chem. Commun. 2003, 234 – 235. a) S. C. Yang, C. W. Hung, Synthesis 1999, 1747 – 1752; b) H. Hikawa, Y. Yokoyama, J. Org. Chem. 2011, 76, 8433 – 8439; c) H. Bricout, J.-F. Carpentier, A. Mortreux, J. Mol. Catal. A 1998, 136, 243 – 251; d) S. C. Yang, Y. C. Tsai, Organometallics 2001, 20, 763 – 770; e) Y. S. Wagh, D. N. Sawant, K. P. Dhake, B. M. Bhanage, Catal. Sci. Technol. 2012, 2, 835 – 840; f) D. Banerjee, R. V. Jagadeesh, K. Junge, H. Junge, M. Beller, Angew. Chem. Int. Ed. 2012, 51, 11556 – 11560; Angew. Chem. 2012, 124, 11724 – 11728.
Asian J. Org. Chem. 2016, 5, 812 – 818
www.AsianJOC.org
[10] S. C. Yang, Y. C. Hsu, K. H. Gan, Tetrahedron 2006, 62, 3949 – 3958. [11] T. T. Dang, B. Ramalingam, S. P. Shan, A. M. Seayad, ACS Catal. 2013, 3, 2536 – 2540. [12] a) F. Gang, G. Xu, T. Dong, L. Yang, Z. Du, Chin. J. Org. Chem. 2015, 35, 1428 – 1440; b) G. Xu, Y. Zhang, K. Wang, Y. Fu, Z. Du, J. Chem. Res. 2015, 39, 399 – 402; c) Z. Du, W. Zhou, L. Bai, F. Wang, J. X. Wang, Synlett 2011, 369 – 372; d) Z. Du, W. Zhou, F. Wang, Tetrahedron 2011, 67, 4914 – 4918. [13] J. Tsuji, I. Shimizu, I. Minami, Y. Ohashi, T. Sugiura, K. Takahashi, J. Org. Chem. 1985, 50, 1523 – 1529. [14] F.-Q. Yuan, F.-Y. Sun, F.-S. Han, Tetrahedron 2012, 68, 6837 – 6842. [15] R. B. N. Baig, B. R. Vaddula, M. A. Gonzalez, R. S. Varma, RSC Adv. 2014, 4, 9103 – 9106. [16] H. Kinoshita, H. Shinokubo, K. Oshima, Org. Lett. 2004, 6, 4085 – 4088.
Manuscript received: March 10, 2016 Revised: April 6, 2016 Accepted Article published: April 8, 2016 Final Article published: May 24, 2016
818
Ó 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
