Organometallics 2009, 28, 4841–4844 DOI: 10.1021/om9004884
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Pt(II)/SnX2 (X=Cl, Br)-Catalyzed Cyclization: Completely Different Reactivity of the Platinum Complex toward 1,6-Haloenynes and 1,6-Enynes Min-Soo Jang, Xi Wang, Woo-Young Jang, and Hye-Young Jang* Division of Energy Systems Research, Ajou University, Suwon, 443-749, Korea Received June 10, 2009
Using a Pt catalyst generated from PtX2, SnX2, phosphines, and H2, the cyclizations of haloenynes and enynes without a halide substituent were investigated. Under the Pt catalyst conditions, the reaction routes were partitioned into the cycloisomerization of the haloenynes and the cycloreduction of the enynes without a halogen substituent. Through the optimization of the reaction conditions and control experiments, we proposed plausible catalytic cycles to account for the bifurcation of the reaction mechanisms in addition to the demonstration of the substrate scope of the cycloisomerization of the haloenynes.
Introduction Transition metal complex-catalyzed enyne cyclizations have received considerable attention due to their application to the synthesis of synthetically and biologically important carbo- and heterocyclic building blocks.1 Among the transition metal complexes used for such transformations, Pt complexes have been reported to promote the cycloisomerization (metathesis-type reaction) and cyclopropanation of enynes.2 In addition, enynes possessing allylsilanes or allylstannanes were also exposed to Pt-catalyzed reaction conditions, affording selective cyclization products without forming metathesis-type compounds or cyclopropanation compounds.3 As a working mechanism for these reactions, the coordination of the Pt complex to the enyne through the alkyne and the subsequent addition of a pendant alkene to the electron-deficient Pt-alkyne complex has been proposed. While Pt complexes have shown excellent catalytic activties in cycloisomerization, few investigations have been conducted of Pt-catalyzed reductive cyclization, including our methodology.4 In the hydrogen-mediated reductive Michael cyclization and aldol reaction, assuming that LnPtH (Ln=phosphine ligands) complexes are formed in situ from
PtCl2, phosphine, SnCl2, and H2, we proposed a mechanism involving the hydrometalation of LnPtH to the electrondeficient alkene and subsequent carbon-carbon formations.5 In 2006, Chung reported the (allyl)Pt-NHC (N-heterocyclic carbene)-catalyzed reductive cyclizations of 1,6-enynes under hydrogenation conditions, where the hydrometalation of LnPtH (Ln=NHC and allyl) to the alkyne was proposed.6 We are interested in developing various Pt-catalyzed reactions for diversity-oriented synthesis. Presuming that the different reaction modes can be controlled by modifying the catalyst or the structure of the reactant, in the present study, we employed commercially available Pt complexes, various phosphine ligands, and H2 (1 atm) for the cyclization of 1,6enynes and 1,6-haloenynes. Accordingly, it was found that the different cyclization routes (cycloisomerization and cycloreduction) were directed by the substituent of the enyne. Herein, we wish to report the first Pt-catalyzed haloenyne cycloisomerization and related mechanistic rationale accounting for the bifurcation of the enyne cyclizations to cycloisomerization and cycloreduction under Pt/SnX2 (X=Cl, Br)-catayzed cyclization conditions.
Results and Discussion
*Corresponding author. E-mail:
[email protected]. (1) Selected review articles for transition metal-catalyzed enyne cyclization: (a) Trost, B. M. Acc. Chem. Res. 1990, 23, 34. (b) Ojima, I.; Tzamarioudaki, M.; Li, Z.; Donovan, R. J. Chem. Rev. 1996, 96, 635. (c) Trost, B. M.; Kriche, M. J. Synlett 1998, 1. (d) Aubert, C.; Buisine, O.; Malacria, M. Chem. Rev. 2002, 102, 813. (e) Fairlamb, I. J. S. Angew. Chem., Int. Ed. 2004, 43, 1048. (f) Shen, H. C. Tetrahedron 2008, 64, 7847. (2) Selected review articles for Pt-catalyzed enyne cycloisomerization: (a) Michelet, V.; Toullec, P. Y.; Gen^et, J.-P. Angew. Chem., Int. Ed. 2008, 47, 4268. (b) Zhang, L.; Sun, J.; Kozmin, S. A. Adv. Synth. Catal. 2006, 348, 2271. (c) Lee, S. I.; Chatani, N. Chem. Commun. 2009, 371. (3) (a) Fern andez-Rivas, C.; Mendez, M.; Echavarren, A. M. J. Am. Chem. Soc. 2000, 122, 1221. (b) Mendez, M.; Muoz, M. P.; Nevado, C.; Cardenas, D. J.; Echavarren, A. M. J. Am. Chem. Soc. 2001, 123, 10511. (c) Fernandez-Rivas, C.; Mendez, M.; Nieto-Oberhuber, C.; Echavarren, A. M. J. Org. Chem. 2002, 67, 5197. (d) Mendez, M.; Echavarren, A. M. Eur. J. Org. Chem. 2002, 15. (4) (a) Lee, H.; Jang, M.-S.; Hong, J.-T.; Jang, H.-Y. Tetrahedron Lett. 2008, 49, 5785. (b) Lee, H.; Jang, M.-S.; Song, Y.-J.; Jang, H.-Y. Bull. Korean Chem. Soc. 2009, 30, 327.
Initially, N-(4-bromobut-2-enyl)-4-methyl-N-(3-phenylprop-2-ynyl)benzenesulfonamide (1a) was expected to form a cycloreduction product under hydrogenation conditions.6 Unexpectedly, the Pt catalyst formed from PtBr2 (5 mol %), P(Ph-pCF3)3 (5 mol %), and SnBr2 (25 mol %) under 1 atm of H2 provided the cycloisomerization product 1b in 74%
r 2009 American Chemical Society
Published on Web 07/30/2009
(5) Selected review articles for hydrogen-mediated coupling reactions: (a) Jang, H.-Y.; Krische, M. J. Acc. Chem. Res. 2004, 37, 653. (b) Ngai, M.-Y.; Krische, M. J. Chim. Oggi/Chem. Today 2006, 24, 12. (c) Skucas, E.; Ngai, M.-Y.; Komanduri, V.; Krische, M. J. Acc. Chem. Res. 2007, 40, 1394. (d) Ngai, M.-Y.; Kong, J.-R.; Krische, M. J. J. Org. Chem. 2007, 72, 1063. (e) Iida, H.; Krische, M. J. Top. Curr. Chem. 2007, 279, 77. (f) Krische, M. J.; Cho, C.-W. In Handbook of Homogeneous Hydrogenation; De Vries, H., Elsevier, K. Eds.; Wiley-VCH: Weinheim, 2007; Vol 2, p 713. (6) Jung, I. G.; Seo, J.; Lee, S. I.; Choi, S. Y.; Chung, Y. K. Organometallics 2006, 25, 4240. pubs.acs.org/Organometallics
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Table 1. Optimization of Cycloisomerizationa
entry 1 2 3 4 5 6 7 8 9 10 11 12 13
Pt complex PtBr2 (5 mol %) PtBr2 (5 mol %) PtBr2 (5 mol %) PtBr2 (5 mol %) PtBr2 (5 mol %) PtBr2 (5 mol %) PtBr2 (5 mol %) PtBr2 (5 mol %) Pt(acac)2 (5 mol %) Pt(acac)2 (5 mol %) PdBr2 (5 mol %) DMSAuCl (5 mol %)
ligand
additives
yield (Z/E)
P(Ph-pCF3)3 (5 mol %) P(Ph-pCF3)3 (5 mol %) P(Ph-pCF3)3 (5 mol %) P(Ph-pCF3)3 (5 mol %) P(Ph-F5)3 (5 mol %) P(Ph-2,4,6-OMe3)3 (5 mol %)
SnBr2 (25 mol %) SnBr2 (25 mol %) SnBr2 (25 mol %) SnCl2 (25 mol %) SnBr2 (25 mol %) SnBr2 (25 mol %) SnBr2 (25 mol %)
74% (1:0.5)b 41% (1:0.6)c 77% (1:0.6) 56% (1:0.8)d 80% (1:0.7) 85% (1:0.6) 75% (1:0.7) N.R. N.R. 76% (1:0.5) 80% (1:0.5) N.R. N.R.
P(Ph-2,4,6-OMe3)3 (5 mol %)
SnBr2 (25 mol %) SnBr2 (25 mol %) SnBr2 (25 mol %) SnBr2 (25 mol %) SnBr2 (25 mol %)
a Procedure: To a premixed solution of Pt(II) (5 mol %), phosphine (5 mol %), and SnX2 (25 mol %) under H2 (1 atm) in dichloroethane (0.1 M) was added substrate 1a under N2 (1 atm) at room temperature. The resulting mixture was allowed to run at 80 °C until the starting material was completely consumed. b Under H2 (1 atm). c Under N2 (1 atm). d 9% of the corresponding chloride was also obtained.
yield within an hour (entry 1, Table 1). The product 1b was obtained as a mixture of the Z- and E-isomers, and their ratio was determined by 1H NMR spectroscopy. The major isomer was found to be Z by comparing the 1H NMR spectra of the product with those of previously reported compounds.7 On the basis of the structure of 1b, it was unlikely that the reaction occurred through the reductive cyclization path. Thus, the reaction was carried out under a nitrogen atmosphere for a longer reaction time (4 h), providing 1b in diminished yield (41%) (entry 2, Table 1). Hydrogen appears to be essential to obtain a higher yield and shorter reaction time by forming highly active Pt catalysts. Accordingly, the solution containing PtBr2, phosphine, and SnBr2 was purged with hydrogen prior to the substrate addition, and the reaction mixture with the substrate was stirred for 1 h under nitrogen, providing the product 1b in 77% yield (entry 3, Table 1). This became the standard procedure for all of the cycloisomerizations of haloenynes in this study. Next, to gain information regarding its role, SnBr2 was replaced by SnCl2, leading to the formation of the product 1b in 56% yield with the concomitant formation of the chlorineadded product in 9% yield (entry 4, Table 1).7c In Barbiertype carbonyl allylations, the allyl anion equivalents (allyl trichlorotin or allyl tribromotin) were formed from the reaction of allyl bromides and SnX2 (X=Cl, Br).8 It is likely that the allyltribromotin or allylbromodichlorotin is generated from the reaction of 1a and SnX2 (X=Cl, Br) and behaves as a nucleophile added onto the Pt-alkyne complex. When 5 mol % of SnBr2 was used with 5 mol % of PtBr2 and 5 mol % of P(Ph-2,4,6-OMe3)3 in dichloroethane, (7) (a) Minami, T.; Niki, I.; Agawa, T. J. Org. Chem. 1974, 39, 3236. (b) Ma, S.; Lu, X. J. Org. Chem. 1991, 56, 5120. (c) Zhu, G.; Zhang, Z. J. Org. Chem. 2005, 70, 3339. (8) (a) Yamamoto, Y.; Asao, N. Chem. Rev. 1993, 93, 2207. (b) Kundu, A.; Prabhakar, S.; Vairamani, M.; Roy, S. Organometallics 1997, 16, 4796. (c) Ito, A.; Kishida, M.; Kurusu, Y.; Masuyama, Y. J. Org. Chem. 2000, 65, 494. (d) Tan, X.-H.; Shen, B.; Liu, L.; Guo, Q.-X. Tetrahedron Lett. 2002, 43, 9373. (e) Tan, X.-H.; Hou, Y.-Q.; Huang, C.; Liu, L.; Guo, Q.-X. Tetrahedron 2004, 60, 6129. (f) Chaudhuri, M. K.; Dehury, S. K.; Hussain, S. Tetrahedron Lett. 2005, 46, 6247. (g) Tang, L.; Ding, L.; Chang, W.-X.; Li, J. Tetrahedron Lett. 2006, 47, 303. (h) Zhao, X.-L.; Liu, L.; Chen, Y.-J.; Wang, D. Tetrahedron 2006, 62, 7113.
no product was formed. During the catalytic reaction, SnBr2 is thought to react with the Pt complex and hydrogen, forming an active Pt species, as well as reacting with the allyl bromide of 1a to afford the allyltribromotin intermediate. Therefore, more than 5 mol % but less than the stoichiometric amount of SnBr2 is required to perform this reaction in good yield. By employing a catalytic amount of SnBr2 for the cycloisomerization of haloenynes, we were able to avoid the preparation of stoichiometric amounts of allyltrihalotin compounds.3 To investigate their effect, we screened the phosphine ligands and found that the reaction yields were slightly increased in their presence, compared to the conditions without phosphines (entries 3, 5, 6, and 7, Table 1).9 In the absence of either SnBr2 or Pt complexes, no product was formed, indicating that the generation of the allyltribromotin from the allyl bromide and SnBr2 and coordination of the Pt complex on the alkyne are essential for the cycloisomerization (entries 8 and 9, Table 1). We next examined other Pt catalysts for this reaction. The reaction conditions using Pt(acac)2 with/without phosphine ligands provided 1b in good yields (entries 10 and 11, Table 1). In contrast to the Pt complexes, neither Pd nor Au complexes catalyzed the cycloisomerization of 1a (entries 12 and 13, Table 1). A diverse range of substrates participated effectively under the optimized conditions described in Table 2. The nitrogentethered substrate 2a, having the gem-dimethyl group at the propargylic position, was exposed to the optimized conditions, providing only the Z-isomer 2b in 49% yield. The thiophene-substituted bromo enyne 3a underwent cycloisomerization to produce the Z-isomer 3b in 75% yield. The naphthalene-substituted bromoenyne 4a participated in the cycloisomerization to form 4b in 62% yield as a mixture of 1:1 isomers. In accordance with the results regarding 2b, 3b, and 4b, the selectivity of the cycloisomerization of the haloenynes might be controlled by the steric and coordination effect of the substituent. The carbon-tethered haloenyne 5a participated in the reaction smoothly, affording product (9) 31P NMR spectra of phosphine and in situ mixture of phosphine and PtBr2 are included in the Supporting Information.
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Organometallics, Vol. 28, No. 16, 2009 Table 2. Cycloisomerization of Haloenynes
a
No phosphine was used.
5b in 72% yield. Due to the facile cleavage of the allylic carbon-oxygen bond by the Pt catalyst, the yields of 6b and 7b were slightly lower than those of the nitrogen- or carbontethered compounds.7b,10 Compound 6b was formed in 55% yield with a 1:0.7 mixture of the Z- and E-isomers being obtained. In the case of 7b, only the Z-isomer was observed in 37% yield, which is similar to that of 2b. In addition to the examples in Table 2, lactone-tethered haloenynes were also tested. Although the use of lactone-tethered haloenynes has often been reported in the Pd-catalyzed cyclization of haloenynes,7b,c,11 no product was formed under our Pt catalyst conditions. To compare the catalytic activities of Pt(II)/SnX2 (X=Cl, Br) in the presence of nonhalogenated 1,6-enynes, N-allyl-4methyl-N-(3-phenylprop-2-ynyl)benzenesulfonamide (8a) was subjected to a dichloroethane solution containing PtBr2 (5 mol %) and SnBr2 (25 mol %) after hydrogen purging, providing the cycloreduction product 8b in 13% yield (Scheme 1). Neither the halogen-incorporated product nor isomer 8b which was observed in the (allyl)Pt-NHC-catalyzed (10) The reaction of the platinum hydride complex with allylic ether was reported. Clark, H. C.; Kurosawa, H. Inorg. Chem. 1973, 12, 1566. (11) (a) Kaneda, K.; Uchiyama, T.; Fujiwara, Y.; Imanaka, T.; Teranishi, S. J. Org. Chem. 1979, 44, 55. (b) Ma, S.; Lu, X. J. Org. Chem. 1993, 58, 1245. (c) Zhang, Z.; Lu, X.; Xu, Z.; Zhang, Q.; Han, X. Organometallics 2001, 20, 3724. (d) Zhang, Q.; Xu, W.; Lu, X. J. Org. Chem. 2005, 70, 1505. (e) Tong, X.; Li, D.; Zhang, Z.; Zhang, X. J. Am. Chem. Soc. 2004, 126, 7601. (f) Goeta, A.; Salter, M. M.; Shah, H. Tetrahedron 2006, 62, 3582.
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Scheme 1. Cycloreduction of 8a
cycloreduction of 8a was obtained under our reaction conditions.6 Changing the halogen anion from bromide to chloride in PtX2 and SnX2 and adding P(Ph-2,4,6-OMe3)3 increased the yields of 8b to 61% (Scheme 1). As proposed in the (allyl)Pt-NHC-catalyzed cycloreduction of enynes,6 PtCl2 did not promote the cycloreduction of 8a in the absence of SnCl2. The premixing of SnCl2 and PtCl2 seems to be crucial for the formation of catalytically competent species. To obtain confirmation of the mechanism involving the hydrometalation of LnPtH (Ln=Ligands) onto the alkyne followed by the addition to the alkene, deuterium labeling studies were performed. Under the deuteration conditions, 50% of deuterium was incorporated at the vinyl position of deuterio-9b, implying that the reaction might go through intermediate A (Scheme 2).3b Plausible catalytic cycles accounting for both the cycloisomerization of the haloenynes and cycloreduction of the enynes are shown in Scheme 3. Depending on the alkene substituents, the reaction pathways are partitioned. The presence of the allyl bromide in the enyne directs SnX2 to react with the allyl bromide, producing the organotin intermediate I. Subsequently, Pt-promoted intramolecular cyclization and halogen transfer provide the product 1b and regenerate the Pt complex. The platinacyclopropene II would open by preferred stereoelectronic pathways to form the observed E- and Z-products. Upon introducing the gem-dimethyl group at the propargylic position of the haloenynes (2a and 7a), only the Z-isomer was observed, due to the steric hindrance imposed by the phenyl and dimethyl groups. In addition to the steric effect, the electronic effect of the substituent plays an important role, as shown in 3a. In the absence of the allyl bromide in the enyne, SnCl2 reacts only with the Pt complexes, promoting the cycloreduction. The catalytic cycle is initiated by the formation of HPt(SnCl3) from PtCl2, H2, and SnCl2. SnCl3 is known as a key component in effective Pt-catalyzed hydrogenation and hydrogenative coupling reactions.4,6,12 Once (12) Selected articles for Pt-catalyzed hydroformylations and hydrogenations: (a) Clark, H. C.; Dixon, K. R. J. Am. Chem. Soc. 1969, 91, 596. (b) Hsu, C.-Y.; Orchin, M. J. Am. Chem. Soc. 1975, 97, 3553. (c) Pittman, C. U., Jr.; Kawabata, Y.; Flowers, L. I. J. Chem. Soc., Chem. Commun. 1983, 612. (d) Ruegg, H. J.; Pregosin, P. S.; Scrivanti, A.; Toniolo, L.; Botteghi, C. J. Organomet. Chem. 1986, 316, 233. (e) Parrinello, G.; Stille, J. K. J. Am. Chem. Soc. 1987, 109, 7122. (f) Kollar, L; Consiglio, G.; Pino, P. J. Organomet. Chem. 1987, 330, 305. (g) Kollar, L; Bakos, J.; Toth, I.; Heil, B. J. Organomet. Chem. 1989, 370, 257. (h) Holt, M. S.; Wilson, W. L.; Nelson, J. H. Chem. Rev. 1989, 89, 11. (i) Stille, J. K.; Su, H.; Brechot, P.; Parrinello, G.; Hegedus, L. S. Organometallics 1991, 10, 1183. (j) Enholm, E. J.; Kinter, K. S. J. Am. Chem. Soc. 1991, 113, 7784. (k) Toth, I.; Elsevier, C. J.; Kegl, T.; Kollar, L. Inorg. Chem. 1994, 33, 5708. (l) Farkas, E.; Kollar, L; Moret, M.; Sironi, A. Organometallics 1996, 15, 1345.
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Jang et al. Scheme 2. Deuterium Labeling Study
Scheme 3. Plausible Catalytic Cycles
HPt(SnCl3) is exposed to 8a, the cycloreduction proceeds with the intermediacy of the hydrometalated intermediate V. Unlike the cycloisomerization of the haloenynes, only the Z-isomer was formed, indicating that the mechanism involving the transfer of the hydrogen atom at the vinyl position of 8b might not be the same as that involving the installation of the halide at the vinyl position of 1b.
Experimental Section All reactions were run under an atmosphere of argon, unless otherwise indicated. Anhydrous solvents were transferred by an oven-dried syringe. Flasks were flame-dried and cooled under a stream of nitrogen. Dichloroethane was distilled from calcium hydride. Products 8b and 9b exhibited spectral properties consistent with previous literature reports.13 The products were obtained as an inseparable mixture of Z/E-isomers. The relative stereochemical assignments were made in analogy with those previously reported.7c,14 Representative Experimental Procedure for Cycloisomerization of Haloenynes. To a premixed solution of Pt(II) (5 mol %), phosphine (5 mol %), and SnBr2 (25 mol %) under H2 (1 atm) in dichloroethane (0.1 M) was added haloenynes under N2 (1 atm) at room temperature. The resulting mixture was allowed to run at 80 °C until the starting material was completely consumed. Representative Experimental Procedure for Cycloreduction of Enynes. To a premixed solution of Pt(II) (5 mol %), phosphine (5 mol %), and SnCl2 (25 mol %) in dichloromethane (0.1 M) was added enynes under H2 (1 atm) at room temperature. The resulting mixture was allowed to run at 40 °C until the starting material was completely consumed. (13) Jang, H.-Y.; Krische, M. J. J. Am. Chem. Soc. 2004, 126, 7875. (14) Cook, G. R.; Hayashi, R. Org. Lett. 2006, 8, 1045.
Conclusions We reported new catalytic systems involving Pt(II) and SnX2 (X=Cl, Br) that promote enyne cyclizations. In the case of haloenynes, the first Pt-catalyzed cycloisomerization of enynes possessing allyl bromide substituents was discovered. This catalytic system provides bromide-incorporated products in modest to good yields using nitrogen-, carbon-, and oxygen-tethered haloenynes. Under similar reaction conditions, nonhalogenated enynes were transformed to the cycloreduction products, without forming cycloisomerization products or halogen-incorporated products. On the basis of the control experiments and deuterium labeling studies, it was found that under conditions involving the use of PtX2 and SnX2 the reaction routes appear to be directed by the substituent at the allylic position of the enynes. The bromide at the haloenyne forces the reaction route to proceed exclusively by cycloisomerization, while the enyne with no halogen undergoes cycloreduction. Currently, detailed mechanistic studies along with efforts to improve the selectivity are in progress.
Acknowledgment. We thank Young-Jin Song for the preliminary works and the Korean Basic Science Institute (Daegu) for the mass spectra. This work was supported by the Korea Science and Engineering Foundation (grant no. R01-2007-000-20223-0 and 2009-0072421) and the Korea Research Foundation (grant no. KRF-2007-412-J04003). Supporting Information Available: Representative experimental procedures and spectral characterization of compounds 1b-6b. This material is available free of charge via the Internet at http://pubs.acs.org.
