Rhodium-Catalyzed Enantioselective [2 + 2 + 1] Cycloaddition of 1,6

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Rhodium-Catalyzed Enantioselective [2 + 2 + 1] Cycloaddition of 1,6-Enynes with Cyclopropylideneacetamides Shunsuke Suzuki, Shuhei Nishigaki, Yu Shibata, and Ken Tanaka* Department of Chemical Science and Engineering, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-8550, Japan

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S Supporting Information *

ABSTRACT: It has been established that a cationic rhodium(I)/(R)-tolBINAP or (R)-BINAP complex catalyzes the enantioselective [2 + 2 + 1] cycloaddition of 1,6-enynes, possessing monosubstituted alkene units, with cyclopropylideneacetamides at room temperature through the elimination of ethylene to give bicyclic (cyclopent-2-en-1-ylidene)acetamides with high enantioselectivity.

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Scheme 1. Rhodium-Catalyzed Cycloadditions Involving Cyclopropylideneacetamides

yclopropylidene compounds have been used as three or two carbon units in transition-metal-catalyzed [3 + 2 + 2] and [2 + 2 + 2] cycloadditions, respectively.1 For example, Saito and co-workers reported the nickel(0)/phosphine complex-catalyzed [3 + 2 + 2] cycloaddition of two monoynes2a,b or a 1,6-diyne2c with cyclopropylideneacetates leading to 2-(cyclohepta-2,4-dien-1-ylidene)acetates through cleavage of the cyclopropane rings.3,4 They also reported the nickel(0)-catalyzed [2 + 2 + 2] cycloaddition of 1,6-diynes with cyclopropylideneacetates to produce spirocyclohexadienes retaining the cyclopropane rings when using sterically demanding phosphine ligands.4,5 On the other hand, our research group reported that cyclopropylideneacetamides are more suitable cycloaddition partners than the cyclopropylideneacetates in the cationic rhodium(I)/diphosphine complex-catalyzed cycloadditions due to their high coordination ability.6,7 For example, we reported that a cationic rhodium(I)/H8-BINAP complex catalyzes the [3 + 2 + 2] cycloaddition of internal 1,6-diynes with the cyclopropylideneacetamides to give the corresponding bicyclic 2-(cyclohepta2,4-dien-1-ylidene)acetamides through cleavage of the cyclopropane rings (Scheme 1a).6a On the contrary, the same rhodium(I) complex catalyzed the [2 + 2 + 2] cycloaddition of 1,6-enynes 1, possessing the 1,1-disubstituted alkene moieties, with the cyclopropylideneacetamides 2 to give the corresponding bicyclic spirocyclohexadienes A retaining the cyclopropane rings (Scheme 1c).6b,8 Surprisingly, the reactions of sterically demanding diaryl-substituted internal 1,6-diynes with cyclopropylideneacetamides did not afford the [3 + 2 + 2] cycloaddition products, but the [2 + 2 + 1] cycloaddition products (bicyclic fulvenes) in good yields through the elimination of ethylene (Scheme 1b).6c In this paper, we found that the reactions of 1,6-enynes 1, possessing the monosubstituted alkene moieties, with the cyclopropylideneacetamides 2 did not afford the [2 + 2 + 2] cycloaddition products A, but the [2 + 2 + 1] cycloaddition products 3 [bicyclic (cyclopent-2-en-1-ylidene)acetamides]9 in good yields through the elimination of ethylene10 (Scheme 1d). © XXXX American Chemical Society

We first examined the reaction of 1,6-enyne 1a (1 equiv), possessing the monosubstituted alkene moiety, and 2-cyclopropylidene-1-(pyrrolidin-1-yl)ethanone (2a) at room temperature in the presence of 20 mol % of the cationic rhodium(I)/ (R)-H8-BINAP complex (Table 1, entry 1). Unexpectedly, [2 + 2 + 1] cycloaddition product 3aa was obtained with high ee value, although the yield was low (Table 1, entry 1). Screening of axially chiral biaryl bisphosphine ligands (Figure 1, entries 1−3) revealed that the use of (R)-BINAP afforded 3aa with the highest yield and ee value (entry 2). The use of (R)-tolReceived: October 11, 2018

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DOI: 10.1021/acs.orglett.8b03235 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters

The generality of the reaction with regard to both cycloaddition partners was examined by using 5−20 mol % of the cationic rhodium(I)/(R)-tol-BINAP catalyst as shown in Scheme 2. With respect to the 1,6-enynes, not only tosylamide-

Table 1. Screening of Ligands for Rh-Catalyzed Enantioselective Cycloaddition of 1a with 2aa

Scheme 2. Substrate Scopea entry

ligand

yield (%)b

ee (%)

1 2 3 4 5

(R)-H8-BINAP (R)-BINAP (R)-Segphos (R)-tol-BINAP (R)-xyl-BINAP

32 55 52 48 11

93 95 89 97 97

a

[Rh(cod)2]BF4 (0.020 mmol), ligand (0.020 mmol), 1a (0.10 mmol), 2a (0.10 mmol), and CH2Cl2 (2.0 mL) were used. bIsolated yield.

Figure 1. Structures of axially chiral biaryl bisphosphine ligands.

BINAP improved the product ee value, although the product yield slightly decreased (entry 4). The use of (R)-xyl-BINAP significantly decreased the product yield (entry 5). Thus, (R)tol-BINAP and (R)-BINAP were selected as the best ligands. In these reactions, a complex mixture of products derived from 2a was formed instead of the corresponding [2 + 2 + 2] cycloaddition products A (90% ee’s), although the ee values were lower than 90% in two cases (3fa and 3ga). The reactions using the cationic rhodium(I)/(R)-BINAP catalyst were also examined, which revealed that similar or higher product yields were observed, while similar or lower ee values were observed except for 3ga. Finally, a preparative scale reaction (1.5 mmol of 1h and 1.0 mmol of 2a) was conducted to give the desired product 3ha with almost the same yield and ee value as those in a small scale. The absolute configuration of [2 + 2 + 1] cycloaddition product (+)-3ac was unambiguously determined as R by an Xray crystallographic analysis (CCDC 1871587; Figure 2).

Scheme 3. Possible Mechanism for Formation of 3

competitive process, the use of highly coordinative N,Ndialkylcyclopropylideneacetamides 2a−c is preferable for the selective synthesis of 3 through B. The [2 + 2 + 1] cycloaddition products 3 possess only one stereogenic center, but multiple stereogenic centers could be diastereoselectively constructed by hydrogenation as shown in Scheme 4. Treatment of (+)-3ac with Pd/C under the Scheme 4. Diastereoselective Hydrogenation of (+)-3ac

atmospheric pressure of hydrogen at room temperature afforded octahydrocyclopenta[c]pyrrole derivative (+)-4, possessing four contiguous stereogenic centers, as a single diastereomer in 33% yield without racemization.13 In summary, we have established that a cationic rhodium(I)/(R)-tol-BINAP or (R)-BINAP complex catalyzes the enantioselective [2 + 2 + 1] cycloaddition of 1,6-enynes, possessing monosubstituted alkene units, with cyclopropylideneacetamides at room temperature through elimination of ethylene to give bicyclic (cyclopent-2-en-1-ylidene)acetamides with high enantioselectivity. The [2 + 2 + 1] cycloaddition product, possessing one stereogenic center, could be converted to an octahydrocyclopenta[c]pyrrole derivative, possessing four contiguous stereogenic centers, as a single diastereomer without racemization.

Figure 2. X-ray crystallographic analysis of (R)-(+)-3ac with ellipsoids at 50% probability.

A possible mechanism for the formation of 3 is shown in Scheme 3. We believe that our previously reported mechanism of the rhodium(I)-catalyzed [2 + 2 + 1] cycloaddition of 1,6diynes with cyclopropylideneacetamides would be applied. Oxidative addition of cyclopropylideneacetamide 2 to rhodium generates rhodacyclobutane B.11 β-Carbon elimination generates ethylene-coordinated rhodium vinylidene C.12 The [2 + 2] cycloaddition of the alkyne moiety of 1,6-enyne 1 affords alkene-coordinated rhodacyclobutene D through releasing ethylene. Insertion of the pendant alkene generates rhodacyclohexene E. Reductive elimination affords 3 and regenerates the Rh(I)+ catalyst. However, when 1,6-enynes 1, possessing 1,1-disubstituted alkene units, are used, insertion of the pendant alkyne in rhodacyclobutene D to generate rhodacyclohexene E does not proceed due to steric hindrance. On the other hand, 1,6-enyne 1 reacts with rhodium to generate rhodacyclopentene F. Regioselective insertion of 2 into F generates rhodacycle G. Reductive elimination affords spirocyclohexene A. As the generation of rhodacyclobutane B from 2 and rhodacyclopentene F from 1 might be the

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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.8b03235. Experimental procedures and compound characterization data (PDF) Accession Codes

CCDC 1871587 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing C

DOI: 10.1021/acs.orglett.8b03235 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters

Sotome, I.; Komagawa, S.; Saito, S. Nickel-Catalyzed [3 + 2 + 2] Cycloaddition of Ethyl Cyclopropylideneacetate and HeteroatomSubstituted Alkynes: Application to Selective Three-Component Reaction with 1,3-Diynes. J. Org. Chem. 2010, 75, 480−483. (d) Bauer, R. A.; DiBlasi, C. M.; Tan, D. S. The tert-Butylsulfinamide Lynchpin in Transition-Metal-Mediated Multiscaffold Library Synthesis. Org. Lett. 2010, 12, 2084−2087. (e) Komagawa, S.; Takeuchi, K.; Sotome, I.; Azumaya, I.; Masu, H.; Yamasaki, R.; Saito, S. Synthesis of Vinylcycloheptadienes by the Nickel-Catalyzed ThreeComponent [3 + 2 + 2] Cocyclization. Application to the Synthesis of Polycyclic Compounds. J. Org. Chem. 2009, 74, 3323−3329. (f) Yamasaki, R.; Sotome, I.; Komagawa, S.; Azumaya, I.; Masu, H.; Saito, S. Ni-catalyzed [3 + 2 + 2] cycloaddition of ethyl cyclopropylideneacetate and 1,3-diynes. Application to the threecomponent cycloaddition. Tetrahedron Lett. 2009, 50, 1143−1145. (4) Saito, S.; Komagawa, S.; Azumaya, I.; Masuda, M. NickelCatalyzed Intermolecular [3 + 2 + 2] Cocyclization of Ethyl Cyclopropylideneacetate and Alkynes. Synthesis of Seven-Membered Carbocycles. J. Org. Chem. 2007, 72, 9114−9120. (5) For the transition-metal-catalyzed cycloaddition involving cyclopropylideneacetates, see: (a) Yamasaki, R.; Ohashi, M.; Maeda, K.; Kitamura, T.; Nakagawa, M.; Kato, K.; Fujita, T.; Kamura, R.; Kinoshita, K.; Masu, H.; Azumaya, I.; Ogoshi, S.; Saito, S. NiCatalyzed [4 + 3 + 2] Cycloaddition of Ethyl Cyclopropylideneacetate and Dienynes: Scope and Mechanistic Insights. Chem. - Eur. J. 2013, 19, 3415−3425. (b) Yamasaki, R.; Kato, K.; Hanitani, D.; Mutoh, Y.; Saito, S. Synthesis of monocyclic nine-membered compounds by the [4 + 3 + 2] cycloaddition-bond cleavage strategy. Tetrahedron Lett. 2013, 54, 3507−3509. (c) Ohashi, M.; Taniguchi, T.; Ogoshi, S. Nickel-Catalyzed Formation of Cyclopentenone Derivatives via the Unique Cycloaddition of α,β-Unsaturated Phenyl Esters with Alkynes. J. Am. Chem. Soc. 2011, 133, 14900−14903. (d) Saito, S.; Maeda, K.; Yamasaki, R.; Kitamura, T.; Nakagawa, M.; Kato, K.; Azumaya, I.; Masu, H. Synthesis of Nine-Membered Carbocycles by the [4 + 3 + 2] Cycloaddition Reaction of Ethyl Cyclopropylideneacetate and Dienynes. Angew. Chem., Int. Ed. 2010, 49, 1830−1833. (e) Saito, S.; Yoshizawa, T.; Ishigami, S.; Yamasaki, R. Ring expansion reactions of ethyl cyclopropylideneacetate and benzosilacyclobutenes: formal σ bond cross metathesis. Tetrahedron Lett. 2010, 51, 6028−6030. (f) Ohashi, M.; Taniguchi, T.; Ogoshi, S. [3 + 3] Cyclodimerization of Methylenecyclopropanes: Stoichiometric and Catalytic Reactions of Nickel(0) with Electron-Deficient Alkylidenecyclopropanes. Organometallics 2010, 29, 2386−2389. (g) Saito, S.; Takeuchi, K. Nickel-catalyzed [4 + 3] cycloaddition of ethyl cyclopropylideneacetate and 1,3-dienes. Tetrahedron Lett. 2007, 48, 595−598. (h) Kawasaki, T.; Saito, S.; Yamamoto, Y. Nickel(0)Catalyzed Dimerization of Ethyl Cyclopropylideneacetates. J. Org. Chem. 2002, 67, 4911−4915. (6) (a) Yoshida, T.; Tajima, Y.; Kobayashi, M.; Masutomi, K.; Noguchi, K.; Tanaka, K. Rhodium-Catalyzed [3 + 2 + 2] and [2 + 2 + 2] Cycloadditions of Two Alkynes with Cyclopropylideneacetamides. Angew. Chem., Int. Ed. 2015, 54, 8241−8244. (b) Yoshizaki, S.; Nakamura, Y.; Masutomi, K.; Yoshida, T.; Noguchi, K.; Shibata, Y.; Tanaka, K. Rhodium-Catalyzed Asymmetric [2 + 2 + 2] Cycloaddition of 1,6-Enynes with Cyclopropylideneacetamides. Org. Lett. 2016, 18, 388−391. (c) Yoshizaki, S.; Shibata, Y.; Tanaka, K. Fulvene Synthesis by Rhodium(I)-Catalyzed [2 + 2 + 1] Cycloaddition: Synthesis and Catalytic Activity of Tunable Cyclopentadienyl Rhodium(III) Complexes with Pendant Amides. Angew. Chem., Int. Ed. 2017, 56, 3590−3593. (7) Acrylamides show high reactivity in the cationic rhodium(I)/ bisphosphine complex-catalyzed C−C bond forming addition reactions. See: (a) Tanaka, K.; Hagiwara, Y.; Noguchi, K. RhodiumCatalyzed Regio- and Enantioselective Intermolecular [4 + 2] Carbocyclization of 4-Alkynals with N,N-Dialkyl Acrylamides. Angew. Chem., Int. Ed. 2005, 44, 7260−7263. (b) Tanaka, K.; Hagiwara, Y.; Hirano, M. Rhodium-Catalyzed Regio-, Diastereo-, and Enantioselective Intermolecular [4 + 2] Carbocyclization of 4Alkynals with Electron-Deficient Alkenes. Eur. J. Org. Chem. 2006,

[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yu Shibata: 0000-0003-1017-0436 Ken Tanaka: 0000-0003-0534-7559 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported partly by ACT-C (No. JPMJCR1122YR) from the Japan Science and Technology Agency (JST), Japan. We thank Mr. Yu Nakamura (Tokyo University of Agriculture and Technology) and Dr. Koji Masutomi (Tokyo Institute of Technology) for their preliminary experiments. We are grateful to Takasago International Corporation for the gift of tol-BINAP, xyl-BINAP, H8BINAP, and Segphos, and Umicore for generous support in supplying the rhodium complex.

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REFERENCES

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(12) The stoichiometric formation of osmium vinylidenes from alkylidenecyclopropanes through releasing ethylene has been reported. See: Castro-Rodrigo, R.; Esteruelas, M. A.; Lopez, A. M.; Lopez, F.; Mascareñas, J. L.; Olivan, M.; Onate, E.; Saya, L.; Villarino, L. Cleavage of Both C(sp3)−C(sp2) Bonds of Alkylidenecyclopropanes: Formation of Ethylene-Osmium-Vinylidene Complexes. J. Am. Chem. Soc. 2010, 132, 454−455. (13) In this reaction, a dehydrogenated product (shown below) was generated (ca. 30% yield) in addition to the tetrahydrogenated product 4. Unfortunately, increasing the catalyst loading did not improve the yield of 4.

E

DOI: 10.1021/acs.orglett.8b03235 Org. Lett. XXXX, XXX, XXX−XXX