Letter Cite This: Org. Lett. XXXX, XXX, XXX-XXX
pubs.acs.org/OrgLett
Solvent-Controlled Switchable Domino Reactions of MBH Carbonate: Synthesis of Benzothiophene Fused α‑Pyran, 2,3-Dihydrooxepine, and Oxatricyclodecene Derivatives Jiru Jia, Aimin Yu,* Shanshan Ma, Youquan Zhang, Ke Li, and Xiangtai Meng* Tianjin Key Laboratory of Organic Solar Cells and Photochemical Conversion, School of Chemistry & Chemical Engineering, Tianjin University of Technology, Tianjin 300384, P. R. China S Supporting Information *
ABSTRACT: Solvent-controlled switchable domino reactions between 2alkylidenebenzothiophene-3(2H)-ones and Morita−Baylis−Hillamn (MBH) carbonate were developed. All domino reactions exhibited excellent regioselectivity, producing a broad spectrum of benzothiophene-fused α-pyran, 2,3-dihydrooxepine, and oxatricyclodecene derivatives. Furthermore, [4 + 2], [4 + 3], and related domino reactions from identical substrates can be controlled.
T
The reaction between methyl (Z)-2-(5-methyl-3-oxobenzo[b]thiophen-2(3H)-ylidene)acetate (1a)12 and MBH carbonate 2a in CHCl3 at 62 °C catalyzed by PPh3 (20 mol %) produced two new compounds, 3a and 4a, in 34% and 27% yields, respectively (Table 1, entry 1). After purification by column chromatography, 3a and 4a were characterized by NMR and HRMS. Conclusive evidence for the structure was obtained by single-crystal X-ray diffraction of 3a and 4j.13 To our surprise, [4 + 2] and [4 + 3] annulation occurred in this reaction. In view of the above interesting result, this domino reaction was optimized to improve the yield and selectivity. The results are summarized in Table 1. The effect of the solvent on the reaction was investigated first. When the reaction was carried out in CH2Cl2 and DCE (1,2dichloroethane), the selectivity of 3a and 4a was not improved (entries 2 and 3). To our satisfaction, good selectivity and yield for 3a were obtained when THF was used as the solvent (entry 4). In order to improve the yield and selectivity of 4a, we continued to examine other solvents. CH3CN, CH3OH, toluene, and 1,4dioxane were all found to be inferior to EtOH (entries 5−9). Therefore, EtOH was the best solvent for 4a. Surprisingly, when the reaction was carried out in DMSO, 4a and another new product, 5a, were isolated (entry 10). The structure of 5a was confirmed by NMR, HRMS, and X-ray analysis.13 A benzothiophene-fused oxatricyclodecene derivative was formed. This fantastic result promoted us to further optimization the reaction conditions. When the solvent was switched from DMSO to DMF, only 5a was formed, albeit with 25% yield (entry 11). The optimal results of the solvent effect encouraged us to further optimize the reaction conditions. First, for the 3a and 4a, we changed the ration of 1a:2a and the catalyst loading in their corresponding solvents. The best ratio of 3a and 4a was 1:2 and 1:4, respectively. The
he development of switchable intermolecular domino reactions from identical substrates in different regioselective methods is very challenging yet highly desirable in modern organic chemistry, especially organocatalytic switchable intermolecular domino reactions. Such reactions have many advantages: they are atom-economical, have fewer synthetic steps, and minimize the amount of purification required.1 Thus, considerable effort has gone into developing efficient domino reactions. Among the reported contributions, MBH carbonates possess multiple reaction modes and can occur in diverse transformations catalyzed by Lewis bases.2 For example, after the pioneering work of Lu’s [3 + 2] cycloaddition,3 significant advances were made in [4 + 1],4 [3 + 3],5 and [3 + 6]6 annulation reactions. Despite these contributions, MBH carbonates participating in [4 + 2],7 [4 + 3],8 and related domino reactions have been rarely reported in the literature. To the best of our knowledge, there are only two examples for each [4 + 2] or [4 + 3] annulation. Furthermore, the accomplishment of switchable [4 + 2], [4 + 3], and related domino reactions are also highly desirable because building small-molecule libraries with structural diversity from identical substrates plays an important role in organic chemistry. Benzothiophene and fused benzothiophene rings are privileged structural subunits present in many biologically active compounds and in materials science.9 As such, an efficient and expedient method to synthesize a series of these molecules would be highly desirable. Although numerous methods have been developed to form these molecules,10 there is still a demand for highly efficient and diversity-oriented synthesis of these molecules. As a continuation of our research on the organocatalyzed domino reactions,11 we report solvent-controlled switchable domino reactions between 2-alkylidenebenzothiophene-3(2H)-ones and MBH carbonate, which produce highly efficient synthesis of benzothiophene fused α-pyran, 2,3dihydrooxepine, and oxatricyclodecene derivatives. © XXXX American Chemical Society
Received: September 18, 2017
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DOI: 10.1021/acs.orglett.7b02916 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters Table 1. Optimization of the Domino Reactionsa
Table 2. Scope of the [4 + 2] Annulation Reactionsa
yieldb (%) entry
solvent
temp (°C)
time (min)
3a
4a
5a
1 2 3 4 5 6 7 8 9 10 11 12c 13d 14e 15f 16g 17e 18h 19i 20j 21k
CHCl3 CH2Cl2 DCE THF CH3CN CH3OH EtOH toluene 1,4-dioxane DMSO DMF THF THF THF EtOH EtOH EtOH DMF DMF DMF DEF
62 40 84 67 82 65 78 110 102 110 153 67 67 67 78 78 78 153 153 153 178
20 4.5 h 20 35 20 30 20 10 40 1h 10 1h 30 30 30 1h 4h 10 20 10 10
34 19 38 50 20 19 8 41 49 0 0 54 43 0 0 19 0 0 0 0 0
27 27 36 11 32 45 45 19 30 9 0 5 0 0 64 8 0 0 0 0 0
0 0 0 0 0 0 0 0 0 10 25 0 0 0 0 0 0 30 0 41 52
entry
R1
R2
time (min)
yieldb (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
5-CH3 (1a) 5-CH3 (1b) 5-CH3 (1c) H (1d) H (1e) H (1f) 5-Cl (1g) 5-Br (1h) 5-F (1i) 6-CH3O (1j) 7-Cl (1k) 5-CH3 (1l) 5-CH3 (1m) 5-CH3 (1n) 5-Cl (1o) 5-Br (1p) 5-Cl (1q) 5-Br (1r) 5-Cl (1s)
CO2Me CO2Et CO2Bn CO2Me CO2Et CO2Bn CO2Et CO2Et CO2Et CO2Me CO2Et COPh COPh-p-Me COPh-p-F Ph Ph p-CH3C6H5 p-CH3C6H5 p-ClC6H5
60 15 30 20 70 66 120 60 70 10 45 70 15 15 95 70 90 90 70
54 (3a) 88 (3b) 85 (3c) 48 (3d) 74 (3e) 75 (3f) 76 (3g) 75 (3h) 55 (3i) 43 (3j) 54 (3k) 70 (3l) 70 (3m) 86 (3n) 88 (3o) 80 (3p) 94 (3q) 74 (3r) 67 (3s)
a Reaction conducted on a 0.3 mmol scale of 1 in 4 mL of THF (molar ratio of 1/2a is 1:2). bIsolated yields.
a
Unless otherwise indicated, the reaction was performed on a 0.3 mmol scale in solvent (4 mL) in the presence of PPh3 (20 mol %), 1a/ 2a = 1:2.5. bIsolated yields. cThe ratio of 1a/2a was 1:2. dThe ratio of 1a/2a was 1:1.5. eThe catalyst loading was 10 mol %. fThe ratio of 1a/ 2a was 1:4. gThe ratio of 1a/2a was 1:5. hThe catalyst loading was 50 mol %. iDry DMF was used. jThe ratio of DMF/H2O was 7:1. kThe catalyst was tris(4-methoxyphenyl)phosphane.
annulation reaction with the corresponding products having moderate to good yields (entries 7−9). The C6- and C7substituted substrates also reacted smoothly under the optimized conditions, delivering the corresponding products in 43% and 54% yields, respectively (entries 10 and 11). Furthermore, substrates 1 bearing benzoyl groups were also screened. We first examined the tolerance of various benzoyl groups under the optimized conditions. A series of 1 with electron-donating and -withdrawing benzoyl groups at the 4-position produced the desired products in good yields (entries 12−14). Moreover, thioaurones (1o−s) were examined in this [4 + 2] annulation reaction. Importantly, halogen-substituted thioaurones (R1) can achieve the desired products in good to high yields regardless of electron-donating and -withdrawing substitutions (entries 15− 19). We also observed that the amount of the 3 was gradually decreased, while the amount of 4 increased after lengthening of the reaction time. However, we cannot push it all the way to 4 by lengthening the reaction time. Furthermore, the substrate scope of the [4 + 3] annulation reaction was also examined. The results are summarized in Table 3. For 5-CH3-substituted substrates, the ester group on the double bond did not affect the yield dramatically (entries 1−4); in particular, bulky tert-butyl ester led to 4d in 71% yield. For the nonsubstituted substrates, similar product yields were achieved from the reaction of 1d, 1e, and 1f with 2a (entries 5−7). In addition, halogen substituents at the 5 position of the benzene ring of 1 were also tolerated and achieved the corresponding products in moderate to good yields (entries 8−10). Substrates 1, bearing a 6-MeO group, also reacted well regardless of the ester groups (entries 11−14). In addition, 7-Cl-substituted substrate 1k was also examined in this [4 + 3] annulation reaction, with a 60% yield of the desired product obtained (entry 15). Similar to [4 + 2] annulation reactions, benzoyl groups were also introduced to the substrates (1l−n,x). The corresponding products were
catalyst loadings for both were 20 mol % (entries 12−17). We then continued to optimize the reaction conditions for 5a. When the catalyst loading was further increased from 20 to 50 mol %, the yield of 5a can be slightly improved from 25% to 30% (entry 11 vs entry 18). It should be noted that desired product was not obtained using dehydrated DMF as solvent, indicating the trace amount of water in commercially available DMF might assist formation of 5a (entry 19).14 After carefully screening the different amount of water in the mixture solvents, we found the best ratio of DMF/water was 7/1 (see Table S1, entry 20). Furthermore, we also investigated the ratio of reactants 1a and 2a as well as other phosphine catalysts (see Table S1). Ultimately, the best reaction conditions used tris(4-methoxyphenyl)phosphane (MOTPP) as catalyst and a 1:2.5 ratio of reactants 1a:2a, and the yield of 5a was improved to 52% in DEF/H2O (DEF = N,N-diethylformamide) (7:1) at higher temperature (entry 21). With the best reaction conditions in hand, we next studied the scope of the domino reactions with regard to substitution at 1. For the [4 + 2] annulation, we first screened the effect of the ester group. Substrates containing methyl ester gave products in fairly good yields (Table 2, entries 1, 4, and 10). However, ethyl- and benzyl-substituted substrates led to the corresponding products, achieving good to excellent yields (entries 2, 3, 5, and 6). Moreover, for substrates 1 bearing an ethyl ester group, halogensubstituted derivatives 1g−i were also tolerated in the [4 + 2] B
DOI: 10.1021/acs.orglett.7b02916 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters Table 3. Scope of the [4 + 3] Annulation Reactionsa
Table 4. Scope of the Domino Reactions of Formation of 5a
entry
R1
R2
time (min)
yieldb (%)
entry
R1
R2
time (min)
yieldb (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
5-CH3 (1a) 5-CH3 (1b) 5-CH3 (1c) 5-CH3 (1t) H (1d) H (1e) H (1f) 5-Cl (1g) 5-Br (1h) 5-F (1i) 6-CH3O (1j) 6-CH3O (1u) 6-CH3O (1v) 6-CH3O (1w) 7-Cl (1k) 5-CH3 (1l) 5-CH3 (1m) 5-CH3 (1n) 6-CH3O (1x) 5-Br (1r)
CO2Me CO2Et CO2Bn CO2But CO2Me CO2Et CO2Bn CO2Et CO2Et CO2Et CO2Me CO2Et CO2Bn CO2But CO2Et COPh COPh-p-Me COPh-p-F COPh p-CH3C6H5
30 20 20 10 17 20 20 10 20 10 30 30 20 20 20 80 20 30 15 30
64 (4a) 77 (4b) 63 (4c) 71 (4d) 65 (4e) 71 (4f) 66 (4g) 51 (4h) 66 (4i) 74 (4j) 72 (4k) 75 (4l) 85 (4m) 79 (4n) 60 (4o) 81 (4p) 75 (4q) 72 (4r) 76 (4s) 31 (4t)
1 2 3 4 5 6 7c 8c 9c 10 11 12 12 13c
5-CH3 (1a) 5-CH3 (1b) 5-CH3 (1c) H (1d) H (1e) H (1f) 5-Cl (1g) 5-Br (1h) 5-F (1i) 6-CH3O (1j) 6-CH3O (1u) 6-CH3O (1v) 6-CH3O (1v) 7-Cl (1k)
CO2Me CO2Et CO2Bn CO2Me CO2Et CO2Bn CO2Et CO2Et CO2Et CO2Me CO2Et CO2Bn CO2Bn CO2Et
20 15 10 5 10 5 15 10 15 10 10 15 15 40
52 (5a) 44 (5b) 57 (5c) 28 (5d) 54 (5e) 41 (5f) 35 (5g) 33 (5h) 43 (5i) 58 (5j) 53 (5k) 56 (5l) 56 (5l) 28 (5m)
a
Reaction conducted on a 0.3 mmol scale of 1 in 4 mL of DEF/H2O (7:1) (molar ratio of 1:2a is 1:2.5). bIsolated yields. cReaction was carried out in DMF/H2O (7:1) at 153 °C.
transformations are outlined in Scheme 1 on the basis of our experimental results and some related literature.2,3,15 Initially, the
a
Reaction conducted on a 0.3 mmol scale of 1 in 4 mL of EtOH (molar ratio of 1/2a is 1:4). bIsolated yields.
Scheme 1. Proposed Mechanism
obtained in good yields under the optimized reaction conditions (entries 16−19). We also examined thioaurones in this [4 + 3] annulation reaction (1r); however, only 31% of yield 4t was isolated accompanied by 60% of 3q. It should be noted that the yield of 4t was not improved by prolonging the reaction time (entry 20). Finally, the scope of the domino reaction of formation of 5 was screened using MOTPP as catalyst. For the 5-CH3-substituted substrates, moderate yields were obtained no matter which ester group was used (Table 4, entries 1−3). Similar results were observed for the nonsubstituted substrates, except the methyl ester group, where only 28% of 5d was produced (entries 4−6). Halogen-substituted analogues 5g−i were obtained in 33−43% yields from the corresponding starting materials 1g−i (entries 7− 9). Substrates 1 bearing a 6-CH3O group were also examined, resulting in methyl, ethyl, and benzyl ester groups not affecting the reaction efficiency, and good yields were obtained (entries 10− 12). The substrate bearing 7-Cl group 1k underwent the reaction smoothly to give the desired product 5m in a 28% yield (entry 13). Controlled experiments showed that reaction of 3a proceeded with 2 equiv of 2a catalyzed by PPh3 (20 mol %) in EtOH at 78 °C. Compound 4a was obtained in 52% yield. Furthermore, reaction of 4a catalyzed by MeOTPP in DEF/H2O did not result in 5a. This result indicated that 4a was not the intermediate for formation of 5a. We also performed the asymmetric reaction catalyzed by chiral phosphines. Unfortunately, no ee values were observed for 3a, 4a, and 5a (see the SI). In addition, MBH carbonate bearing aromatic substitution (2b) was also examined. However, only [4 + 2] annulation occurred under various reaction conditions. The structure of 6 was confirmed by X-ray crystal structure analysis (see the SI).13 Although the full mechanistic details of these transformations remain to be further elucidated, plausible mechanisms for these
nucleophilic attack of PR3 on the 2a and then elimination of CO2 and tert-butoxide anion generates an intermediate A. Deprotonation of the intermediate A gives the phosphorus ylide B. Subsequently, sterically favored γ-addition of the ylide B to 1 generates the intermediate C. Intermediate D, which is the resonance structure of C, is subsequently transformed into intermediate E via an intramolecular enolate- or tert-butanolassisted proton transfer (see the SI for full details),11h and then intermediate F is formed via another intramolecular tert-butanolassisted proton transfer (see the SI for full details). Next, releasing PR3 affords an intermediate G. Using THF as solvent, the C
DOI: 10.1021/acs.orglett.7b02916 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters
20120502). X.M. is grateful for the support from the 131 Talents Program of Tianjin. We thank Dr. Xuguang Liu, Yin Wei, and ZhiXiang Yu for the revision and discussion.
intermediate G undergoes keto−enol tautomerization to form H (see the SI for full details). 6π electrocyclization of H can produce α-pyran 3 (blue path). On the other hand, in the presence of EtOH at higher temperature, the intermediate G is deprotonated by tert-butoxide anion to form intermediate I (red path), which undergoes an intramolecular O-Michael addition to afford intermediat J. Then Michael addition of J to A produces intermediate K, followed by release of PPh3 to produce 2,3dihydrooxepine 4.16 Using DEF/H2O as solvent at high temperature, intermediate L, which is the resonance structure of I, undergoes Michael addition to another molecule of A, producing intermediate M (black path). Subsequently, intramolecular 1,2-addition of carbon anion to the carbonyl group in M furnishes intermediate N, which undergoes another intramolecular O-Michael addition and kicks off MOTPP to form 5 via intermediate O. On the basis of our experimental results, the solvent polarity might play an important role for the selective formation of different products 3, 4 and 5. In the slight less polarity solvent THF, the reaction favors formation of 3 via [4 + 2] annulations. On the other hand, intermediates J and K might be more stable in the more polar solvent EtOH, leading to formation of 4 via [4 + 3] annulation. In the case of formation of 5 (black path, Scheme 1), the strong polarity solvent DEF/H2O and high reaction temperature might be helpful for the formation of 5 via three zwitterionic intermediates M, N, and O. In conclusion, solvent-controlled switchable domino reactions between 2-alkylidenebenzothiophene-3(2H)-ones and MBH carbonate were developed. Three types of domino reaction including [4 + 2], [4 + 3], and the related domino reaction were demonstrated from identical substrates via controlling the solvents. Moreover, [4 + 2] and [4 + 3] annulations of MBH carbonate have rarely been reported before. The formation of benzothiophene fused oxatricyclodecene derivatives is the first reported in through one-step reaction.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.7b02916. Full experimental details, characterization, and NMR spectra of all new compounds (PDF) Crystallographic data for compound 3a (CIF) Crystallographic data for compound 4j (CIF) Crystallographic data for compound 5a (CIF) Crystallographic data for compound 6 (CIF)
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REFERENCES
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Xiangtai Meng: 0000-0003-2713-0078 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grant No. 21403154), the Natural Science Foundation of Tianjin (Grant No. 13JCYBJC38700), and the Tianjin Municipal Education Commission (Grant No. D
DOI: 10.1021/acs.orglett.7b02916 Org. Lett. XXXX, XXX, XXX−XXX
