Gold-catalyzed synthesis of substituted 2-aminofurans

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Gold-catalyzed synthesis of substituted 2-aminofurans via formal [4+1]-cycloadditions on 3-en-1-ynamidesw Ramesh B. Dateer, Kamalkishore Pati and Rai-Shung Liu* Received 27th April 2012, Accepted 18th May 2012 DOI: 10.1039/c2cc33030j Gold-catalyzed formal [4+1]-cycloadditions between 3-en-1ynamides 1 and 8-methylquinoline oxide are reported. The success of this reaction relies on a hypothetic oxa-Nazarov cyclization on gold-stabilized allylic cations. Preliminary results on a new 1,2-difunctionalization of 3-en-1-ynamide are also reported. Transition-metal catalyzed [4+1]-cycloaddition reactions of 1,3-conjugated compounds with small molecules have found widespread applications in the synthesis of five-membered carbo- or heterocycles.1 Eqn (1) shows the general patterns of reported reactions, including enones,2 unsaturated imines,3 vinylallenes,4 diallenes,5 allenylketones6 and allenylimines6 serving as four-atom building units, and carbon monoxide, isocyanides and diazocarbonyl species acting as one-atom units (eqn (1)).2–6 3-En-1-ynes are readily available as 1,3-dienes, but their metal-catalyzed cycloadditions are much less explored. Only few [4+2]-reactions of 3-en-1-ynes with alkynes have been documented.7,8 We are aware of no reports on the [4+1]-cycloadditons of 3-en-1-ynes with a suitable one-atom donor. In this work, we report a new formal [4+1]-cycloaddition on activated 3-en-1-ynes using organic oxides as an oxygen donor,9 as depicted in eqn (2). Zhang reported10 that gold complexes catalyzed the intermolecular oxidations of alkynes with pyridine-based oxides, generating a-oxo gold carbenoids (eqn (3)) ð1Þ

ð2Þ

Scheme 1 shows a working mechanism to realize a [4+1]-cycloaddition of 3-en-1-ynamide 1a with an oxide. We selected compounds 1 as a target because its p-alkyne structure A is visualized as a vinyl ketenimine A0 ,11 which poses a framework similar to those in the literature (eqn (1)). We envisage that an intermolecular addition of organic oxide to species A0 is a viable route for the synthesis of highly substituted 2-aminofurans 2,12 providing that an oxa-Nazarov cyclization13 is operable on gold-stabilized allylic cation C 0 that is an important contribution of alkenyl gold carbenoids C.14 Herein, we report the success of this cycloaddition using a cationic gold catalyst. Table 1 presents the realization of an intermolecular [4+1]cycloaddition of 3-en-1-ynamide 1a using various oxides (1.2 equiv.) and gold catalysts (5 mol%). We isolated three products 2a–4a in various proportions with a complete consumption of starting 1a. Dicarbonyl compound 3a was generated from a second oxidation of gold carbenoid C whereas 3-alkenylamide 4a was derived from an alkyne hydration.15,16 We examined the reaction using PPh3AuCl/AgSbF6 and 8-methylquinoline oxide I in DCE (80 1C, 15 h), from which we obtained three compounds, i.e. 2a, 3a and 4a in 11, 27 and 55% yields, respectively (entry 1). To our satisfaction, the use of P(t-Bu)2(o-biphenyl)AuCl/AgSbF6 greatly improved the chemoselectivity to give 2-aminofuran species 2a (88%) exclusively (entry 2). We varied the gold catalysts with P(t-Bu)2(o-biphenyl)AuCl/AgNTf2 and IPrAuCl/AgSbF6 (IPr = 1,3-bis(diisopropylphenyl)imidazol-2-ylidene), which showed inferior chemoselectivity, giving additional dicarbonyl compound 3a in 10 and 40% yields, respectively (entries 3, 4). IPrAuSbF6 seems to show a strong alkenylgold carbenoid character C (Scheme 1) to give preferably dicarbonyl species 3a, whereas P(t-Bu)2(o-biphenyl)AuSbF6 tends to exhibit a Au(I)-stabilized allyl cation C 0 to achieve an oxaNazarov cyclization. We also tested other commonly used

ð3Þ

Department of Chemistry, National Tsing-Hua University, Hsinchu, Taiwan, ROC. E-mail: [email protected] w Electronic supplementary information (ESI) available: Experimental procedures and characterization data for new compounds. CCDC 876013. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c2cc33030j

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Scheme 1 A proposed mechanism for [4+1]-cycloaddition.

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Table 2 [4+1]-cycloaddition reactions on various 3-en-1-ynamidesa

Table 1

Catalytic reactions on various gold catalysts

Entry

[Au]b

Oxide

t/h (T/1C)

2ac

3a

4a

1 2 3 4 5 6 7

PPh3AuSbF6 LAuSbF6 LAuNTf2 IPrAuSbF6 LAuSbF6 LAuSbF6 LAuSbF6

I I I I II III IV

15 10 12 10 20 15 8

11 88 78 30 75 58 45

27 — 10 40 — 5 10

55 — — — 20 — 20

(80) (80) (80) (80) (80) (28) (28)

a

Oxides: I = 8-methylquinoline oxide; II = 3,5-dichloropyridine oxide; III = N-benzylideneaniline oxide; IV = diphenyl sulfoxide. b [1a] = 0.01 M, L. = P(t-Bu)2(o-biphenyl), IPr = 1,3-bis(diisopropylphenyl)imidazol-2-ylidene. c Product yields are reported after separation from a silica column.

organic oxides (1.2 equiv. entries 5–7) at optimized conditions; only 3,5-dichloropyridine oxide (II) gave a satisfactory yield (75%) of 2-aminofuran 2a whereas N-benzylideneaniline oxide (III), and diphenyl sulfoxide (IV) gave 2-aminofuran 2a in decreased yields (58–45%), together with undesired products 3a and 4a in 5–20% yields. The frequent occurrence of dicarbonyl compound 3a among our tests reveals the intermediacy of gold carbenoids C. We prepared various 3-en-1-ynamides 1b–o to assess the scope of substrate and results are presented in Table 2. For substrates 1b–g bearing various sulfonamides (entries 1–6), we obtained their 2-aminofuran products 2b–g in satisfactory yields (68–91%), but their optimized conditions have various temperature ranges (28–80 1C). Room-temperature reactions were achieved for 2-aminofuran products 2d, 2e and 2g bearing N-phenylmethanesulfonamide, N-cyclopropylmethanesulfonamide and butan-4-sultam, whereas the other substrates were run with elevated temperatures (60–80 1C). The molecular structure of 2-aminofuran product 2d was determined by X-ray diffraction studyw. We speculate that sulfonamides bearing an electronwithdrawing group (R1 = Ph, cyclopropyl) enhance the electrophilicity of vinyl ketenimine A 0 to accelerate the cycloaddition. For butane-4-sultam species 1g, its cyclic sulfonamide geometry around the electrophilic ketenimine generates a less hindered environment to facilitate an oxide attack. We prepared 3-en-1-ynamide 1h bearing R2 = n-propyl, giving desired product 2h in 56% yield (entry 7). This reaction is compatible with substrates 1i–l bearing both electron-deficient and rich benzenes Ar = 4-XC6H4 (X = F, Cl, Me, MeO, entries 8–11), giving expected 2-aminofurans 2i–l in satisfactory yields (76–83%). This new cycloaddition is efficient for the room-temperature synthesis of heterobiaryl compounds including 2-(furanyl)thiophene (2m, 67%), 2-(furanyl)benzothiophene (2n, 74%) and 2-(furanyl)benzofuran 2o (71%, entries 12–14). The presence of an alkyl at the C(3)-carbon of 3-en-1ynamides increases the character of gold-stabilized allylic cation C 0 to facilitate an oxa-Nazarov cyclization. As shown in Scheme 2, we prepared 4-phenyl-3-en-1-ynamide 1p bearing a reactive NPhMs substituent, but its gold-catalyzed oxidative This journal is

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a L = P(t-Bu)2(o-biphenyl), [enyne] = 0.01 M; product yields are reported after separation from a silica column.

Scheme 2 Reactions on 3-en-1-ynamides 1p and 1q.

cyclization in DCE (28 1C, 12 h) gave dicarbonyl compound 3p (52% yield) and unreacted 1p (28% recovery), reflecting that the intermediate has a strong gold carbene character C. We also prepared 2-cyclohexenyl-1-ethynylamide 1q, which upon gold-catalyzed oxidative cyclization gave dienylamide 5q in 65% yield via a rapid loss of a proton from the goldstabilized allylic cation C 0 . Scheme 3 shows a new development of the difunctionalization of 3-en-1-ynamide 1a via an initial [4+1]-cycloaddition, followed by iodination with N-iodosuccinimide (NIS, 1.1 equiv.); the resulting 2-amino-3-iodofuran derivative 6 was obtained in 63% yield. This reaction proceeded rapidly in DCE at 28 1C (3 h) whereas the [4+1]-cycloaddition of starting 1a was only achieved at 80 1C (10 h); accordingly, protodeauration Chem. Commun., 2012, 48, 7200–7202

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Scheme 3

Gold-catalyzed difunctionalizations of 3-en-1-ynamides.

of species E appears to be very slow.17 In a separate experiment, the treatment of 2-aminofuran 2a with NIS (1.1 equiv.) in DCE 28 1C (6 h) gave 2-en-1,4-dione 7 in 81% yield via hydration of oxidized cationic species F. As depicted in Scheme 4, this new synthetic method is applicable also to an efficient synthesis (76%) of new oligomer 9 with 76% yield, via a two-fold [4+1]-cycloaddition of benzene compound 8 bearing two para-3-en-1-ynamide groups.

Scheme 4

Two-fold [4+1]-cycloaddition reactions.

In summary, we report here a new gold-catalyzed [4+1]cycloaddition reaction between 3-en-1-ynamides 1 and 8-methylquinoline oxide. This reaction is efficient for the synthesis of highly substituted furans from readily available 3-en-1-ynamides. The success of this reaction relies on a hypothetic oxa-Nazarov cyclization on gold-stabilized allylic cation C 0 . The efficiency of the reaction is influenced by the types of sulfonamide and aryl substituents on 3-en-1-ynamides. This work will help to design new cycloaddition reactions on 3-en-1-ynes which has been largely ignored in organic synthesis. This work was supported by National Science Council, National Tsing-Hua University and the Ministry of Education in Taiwan.

Notes and references 1 For general reviews on metal-catalyzed cycloadditions, see: (a) M. Lautens, M. Klute and W. Tam, Chem. Rev., 1996, 96, 49; (b) I. Ojima, M. Tzamarioudaki and Z. Li, and R. J. Donovan, Chem. Rev., 1996, 96, 635; (c) H.-W. Fruhauf, Chem. Rev., 1997, 97, 523. 2 (a) M. Oshita, K. Yamashita, M. Tobisu and N. Chatani, J. Am. Chem. Soc., 2005, 127, 761; (b) N. Chatani, M. Oshita, M. Tobisu, Y. Ishii and S. Murai, J. Am. Chem. Soc., 2003, 125, 7812; (c) S. Son and G. C. Fu, J. Am. Chem. Soc., 2007, 129, 1046. 3 T. Morimoto, N. Chatani and S. Murai, J. Am. Chem. Soc., 1999, 121, 1758. 4 (a) T. Mandai, J. Tsuji, Y. Tsujiguchi and S. Saito, J. Am. Chem. Soc., 1993, 115, 5865; (b) M. Murakami, K. Itami and Y. Ito, J. Am. Chem. Soc., 1997, 119, 2950; (c) M. Murakami, K. Itami and Y. Ito, J. Am. Chem. Soc., 1999, 121, 4130; (d) J. L. Loebach, D. M. Bennett and R. L Danheiser, J. Am. Chem. Soc., 1998, 120, 9690.

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5 (a) B. E. Eaton and B. Eollman, J. Am. Chem. Soc., 1992, 114, 6245; (b) M. S. Sigman and B. E. Eaton, J. Am. Chem. Soc., 1996, 118, 11783. 6 (a) M. S. Sigman, C. E. Kerr and B. E. Eaton, J. Am. Chem. Soc., 1993, 115, 7545; (b) M. S. Sigman, B. E. Eaton, J. D. Heise and C. P. Kubiak, Organometallics, 1996, 15, 2829; (c) M. S. Sigman and B. E. Eaton, J. Org. Chem., 1994, 59, 7488. 7 (a) V. Gevorgyan, A. Takeda and Y. Yamamoto, J. Am. Chem. Soc., 1997, 119, 11313; (b) R. L. Danheiser, A. E. Gould, R. Fernandez de la Predilla and A. L. Helgason, J. Org. Chem., 1994, 59, 5514; (c) R. C. Burwell, K. J. Daoust, A. Z. Bradley, K. J. DiRico and R. P Johnson, J. Am. Chem. Soc., 1996, 118, 4218. 8 For the [4+2]-cycloadditions of arenynes, see: (a) R.-S. Liu, R. B. Dateer and B. S. Shaibu, Angew. Chem., Int. Ed., 2012, 51, 113; (b) R.-S. Liu, J. J. Lian, P. C. Chen and H. C. Ting, J. Am. Chem. Soc., 2006, 128, 11372; (c) V. L. Carrillo and A. M Echavarren, J. Am. Chem. Soc., 2010, 132, 9292. 9 Zhang reported the use of pyridine-based oxide as a one-oxygen donor in a gold-catalyzed [2+2+1]-cycloaddition reaction, see: W. He, C. Li and L. Zhang, J. Am. Chem. Soc., 2011, 133, 8482. 10 (a) L. Ye, L. Cui, G. Zhang and L. Zhang, J. Am. Chem. Soc., 2010, 132, 3258; (b) L. Ye, W. He and L. Zhang, J. Am. Chem. Soc., 2010, 132, 8550; (c) L. Ye, W. He and L. Zhang, Angew. Chem., Int. Ed., 2011, 50, 3236; (d) W. He, C. Li and L. Zhang, J. Am. Chem. Soc., 2011, 133, 8482. 11 (a) P. W. Davies, A. Cremonesi and N. Martin, Chem. Commun., 2011, 47, 379; (b) S. Kramer, Y. Odabachian, J. Overgaard, M. Rottander, F. Gagosz and T. Skrydstrup, Angew. Chem., Int. Ed., 2011, 50, 5090; (c) A. S. K. Hashmi, M. Bu¨hrle, M. Wo¨lfle, M. Rudolph, M. Wieteck, F. Rominger and W. Frey, Chem.–Eur. J., 2010, 16, 9846; (d) A. Cremonesi, L. Dumitrescua and P. W. Davies, Angew. Chem., Int. Ed., 2011, 50, 8931. 12 For synthesis of 2-aminofuran derivatives, see selected examples: (a) J. Oppenheimer, W. L. Johnson, M. R. Tracey, R. P. Hsung, P.-Y. Yao, R. Liu and K. Zhao, Org. Lett., 2007, 9, 2361; (b) C. H. Oh, V. T. Reddy, A. Kim and C. Y. Rhim, Tetrahedron Lett., 2006, 47, 5307; (c) K. Steffen and G. Himbert, Chem. Ber., 1987, 120, 71–77; (d) A. Padwa, M. Dimitroff, A. G. Waterson and T. Wu, J. Org. Chem., 1997, 62, 4088; (e) R. H. Schlessinger and C. P. Bergstrom, Tetrahedron Lett., 1996, 37, 2133; (f) B. T. Freure and J. R. Johnson, J. Am. Chem. Soc., 1931, 53, 1142. 13 Unlike aza-Nazarov cyclizations,13b,c gold-catalyzed oxa-Nazarov reaction was just reported when we prepared this manuscript. See ref. 13a for a completely different reaction. (a) S. Kramer and T. Skrydstrup, Angew. Chem., Int. Ed., 2012, DOI: 10.1002/anie201200307; (b) D. A. Klumpp, Y. Zhang, M. J. O’Connor, P. M. Esteves and L. S. de Almeida, Org. Lett., 2007, 9, 3085; (c) J. Dieker, R. Frohlich and E. U. Wurthwien, Eur. J. Org. Chem., 2006, 5339. 14 (a) A. S. K. Hashmi, Angew. Chem., Int. Ed., 2008, 47, 6754; (b) S. Bhunia and R.-S. Liu, J. Am. Chem. Soc., 2008, 130, 16488; (c) D. Benitez, N. D. Shapiro, E. Tkatchouk, Y. Wang, W. A. Goddard III and F. D. Toste, Nat. Chem., 2009, 1, 482; (d) G. Seidel, R. Mynott and A. Fu¨rstner, Angew. Chem., Int. Ed., 2009, 48, 2510; (e) E. JimeńezNuń˜ez, C. K. Clavarie, C. Bour, D. J. Cardenas and A. M. Echavarren, Angew. Chem., Int. Ed., 2008, 47, 7892; (f) P. Mauleoń, R. M. Zeldin, A. Z. Gonza´lez and D. D. Toste, J. Am. Chem. Soc., 2009, 131, 6348. 15 In the presence of external water (1.0 equiv.), the use of PPh3AuSbF6 (Table 1, entry 1) gave species 4a with increased yield 4a (76%) whereas products 2a and 3a were obtained in 5 and 13% yields, respectively. 16 (a) N. Marion, R. S. Ramoń and S. P. Nolan, J. Am. Chem. Soc., 2009, 131, 448; (b) A. Leyva and A. Corma, J. Org. Chem., 2009, 74, 2067. 17 Selected examples; see (a) L. Ye and L. Zhang, Org. Lett., 2009, 11, 3646; (b) B. Crone and S. F. Kirsch, J. Org. Chem., 2007, 72, 5435; (c) A. Buzas and F. Gagosz, Org. Lett., 2006, 8, 515; (d) A. Buzas, F. Istrate and F. Gagosz, Org. Lett., 2006, 8, 1957; (e) H.-H. Liao and R.-S. Liu, Chem. Commun., 2011, 47, 1339.

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