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Organometallics 2010, 29, 928–933 DOI: 10.1021/om900975a
Bis(NHC)-Palladium(II) Complex-Catalyzed Dioxygenation of Alkenes Wenfeng Wang,† Feijun Wang,† and Min Shi*,†,‡ †
Laboratory for Advanced Materials and Institute of Fine Chemicals, School of Chemistry & Molecular Engineering, East China University of Science and Technology, 130 Mei-Long Road, Shanghai 200237, People’s Republic of China, and ‡State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 354 Fenglin Road, Shanghai 200032, People’s Republic of China. Received November 8, 2009
Bis(NHC)-Pd(II) complexes derived from 1,10 -binaphthyl-2,20 -diamine (BINAM) were successfully first used to catalyze the dioxygenation of alkenes under mild conditions tolerant of air and moisture. Cationic NHC-Pd2þ diaquo complex 1e showed the highest catalytic activity to give 1,2dioxygenation products with good syn-diastereoselectivity for 1,2-disubstituted alkenes. Introduction The osmium-catalyzed cis-dihydroxylation of alkenes and its asymmetric version developed by Sharpless and co-workers (known as Sharpless AD reaction) are the quintessential vicinal difunctionalization methods of alkenes.1 Recently, palladium-catalyzed alkene difunctionalization based on the Pd(II)/Pd(IV) catalytic cycle2 has also attracted considerable attention.3 This new approach represented a promising complement to the Sharpless method without use of toxic and expensive osmium complexes. For example, Song et al. reported an olefin hydroxyacetoxylation catalyzed by cationic palladium diphosphine complexes in the presence of PhI(OAc)2 as an oxidant to oxidize PdII to PdIV.3g Later on, Jiang’s group developed a process for the Pd-catalyzed diacetoxylation of alkenes directly using Pd(OAc)2 as palladium source and oxygen as the sole oxidant, wherein KI as *Corresponding author. E-mail:
[email protected]. Fax: 86-2164166128. (1) (a) Jacobsen, E. N.; Marko, I.; Mungall, W. S.; Schroder, G.; Sharpless, K. B. J. Am. Chem. Soc. 1988, 110, 1968–1970. (b) Kolb, H. C.; VanNieuwenhze, M. S.; Sharpless, K. B. Chem. Rev. 1994, 94, 2483–2547. (c) Johnson, R. A.; Sharpless, K. B. In Catalytic Asymmetric Synthesis, 2nd ed.; Ojima, I., Ed.; VCH: New York, 2000. (d) Li, G.; Wei, H.-X.; Kim, S. H. Org. Lett. 2000, 2, 2249–2252. (e) Li, G.; Wei, H.-X.; Kim, S.-H.; Carducci, M. D. Angew. Chem., Int. Ed. 2001, 40, 4277–4280. (f) Chen, D.; Timmons, C.; Wei, H.-X.; Li, G. J. Org. Chem. 2003, 68, 5742–5745. (2) For more recent selected reports about the mechanism of the PdII/ PdIV catalytic cycle, see: (a) Dick, A. R.; Hull, K. L.; Sanford, M. S. J. Am. Chem. Soc. 2004, 126, 2300–2301. (b) Desai, L. V.; Hull, K. L.; Sanford, M. S. J. Am. Chem. Soc. 2004, 126, 9542–9543. (c) Dick, A. R.; Kampf, J. W.; Sanford, M. S. J. Am. Chem. Soc. 2005, 127, 12790–12791. (d) Deprez, N. R.; Sanford, M. S. Inorg. Chem. 2007, 46, 1924–1935. (e) Fu, Y.; Li, Z.; Liang, S.; Guo, Q. X.; Liu, L. Organometallics 2008, 27, 3736– 3742. (f) Racowski, J. M.; Dick, A. R.; Sanford, M. S. J. Am. Chem. Soc. 2009, 131, 10974–10983. (g) Arnold, P. L.; Sanford, M. S.; Pearson, S. M. J. Am. Chem. Soc. 2009, 131, 13912–13913. (3) (a) Alexanian, E. J.; Lee, C.; Sorensen, E. J. J. Am. Chem. Soc. 2005, 127, 7690–7691. (b) Streuff, J.; Hovelmann, C. H.; Nieger, M.; Muniz, K. J. Am. Chem. Soc. 2005, 127, 14586–14587. (c) Liu, G. S.; Stahl, S. S. J. Am. Chem. Soc. 2006, 128, 7179–7181. (d) Desai, L. V.; Sanford, M. S. Angew. Chem., Int. Ed. 2007, 46, 5737–5740. (e) Muniz, K. J. Am. Chem. Soc. 2007, 129, 14542–14543. (f) Muniz, K.; Hovelmann, C. H.; Streuff, J. J. Am. Chem. Soc. 2008, 130, 763–773. (g) Li, Y.; Song, D. T.; Dong, V. M. J. Am. Chem. Soc. 2008, 130, 2962–2964. (h) Jensen, K. H.; Sigman, M. S. Org. Biomol. Chem. 2008, 6, 4083–4088. (i) Wang, A.; Jiang, H. F.; Chen, H. J. J. Am. Chem. Soc. 2009, 131, 3846–3847. pubs.acs.org/Organometallics
Published on Web 01/25/2010
the additive and 100 °C harsh conditions were indispensable.3i Further studies are obviously required to help understand details of the PdII/PdIV catalytic cycle and design proper achiral and chiral ligands to improve the diastereoselectivity and achieve enantiocontrol in the above dioxygenation of alkenes. Because of several typical features such as stability to air/moisture, being less toxic, and strong σelectron-donating properties,4 N-heterocyclic carbenes (NHCs) have been prevalent ligands in transition-metal catalysis.5 Previously, we have synthesized a series of axially chiral bis(NHC)-Pd(II) complexes from 1,10 -binaphthyl2,20 -diamine (BINAM) and successfully applied them in several asymmetric catalytic reactions.6 Herein we report the first example of bis(NHC)-Pd(II) complex-catalyzed dioxygenation of alkenes under mild conditions.
Results and Discussion Our initial experiments were carried out to investigate the catalytic abilities of bis(NHC)-Pd(II) complexes in the dioxygenation of trans-stilbene in acetic acid, which is commercially (4) (a) Jafarpour, L.; Nolan, S. P. Adv. Organomet. Chem. 2000, 46, 181–222. (b) Weskamp, T.; Bohm, V. P. W.; Herrmann, W. A. J. Organomet. Chem. 2000, 600, 12–22. (c) Hillier, A. C.; Nolan, S. P. Platinum Met. Rev. 2002, 46, 50–64. (d) Hillier, A. C.; Grasa, G. A.; Viciu, M. S.; Lee, H. M.; Yang, C.; Nolan, S. P. J. Organomet. Chem. 2002, 653, 69–82. (e) DiezGonzalez, S.; Nolan, S. P. Coord. Chem. Rev. 2007, 251, 874–883. (5) For selected reviews on NHC ligands, see: (a) Nolan, S. P., Ed. N-Heterocyclic Carbenes in Synthesis; Wiley-VCH: Weinheim, Germany, 2006. (b) Glorius, F. N-Heterocyclic Carbenes in Transition Metal Catalysis; Springer: Berlin, 2007. (c) Herrmann, W. A. Angew. Chem., Int. Ed. 2002, 41, 1290–1309. (d) Perry, M. C.; Burgess, K. Tetrahedron: Asymmetry 2003, 14, 951–961. (e) Kirmse, W. Angew. Chem., Int. Ed. 2004, 43, 1767–1769. (f) Cesar, V.; Bellemin-Laponnaz, S.; Gade, L. H. Chem. Soc. Rev. 2004, 33, 619–636. (g) K€uhl, O. Chem. Soc. Rev. 2007, 36, 592–607. (h) Kantchev, E. A. B.; O'Brien, C. J.; Organ, M. G. Angew. Chem., Int. Ed. 2007, 46, 2768–2813. (i) Diez-Gonzalez, S.; Marion, N.; Nolan, S. P. Chem. Rev. 2009, 109, 3612–3676. (6) (a) Xu, Q.; Duan, W. L.; Lei, Z. Y.; Zhu, Z. B.; Shi, M. Tetrahedron 2005, 61, 11225–11229. (b) Chen, T.; Jiang, J. J.; Xu, Q.; Shi, M. Org. Lett. 2007, 9, 865–868. (c) Zhang, T.; Shi, M.; Zhao, M. X. Tetrahedron 2008, 64, 2412–2418. (d) Zhang, T.; Shi, M. Chem.;Eur. J. 2008, 14, 3759–3764. (e) Ma, G. N.; Zhang, T.; Shi, M. Org. Lett. 2009, 11, 875–878. (f) Wang, W. F.; Zhang, T.; Shi, M. Organometallics 2009, 28, 2640–2642. r 2010 American Chemical Society
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Figure 1. Bis(NHC)-Pd(II) complexes 1a-e.
available and inevitably contains minor water, using PhI(OAc)2 as the terminal oxidant. Bis(NHC)-Pd(II) complexes 1a-e,6f bearing different counterions on the palladium center, were examined under the above conditions to seek out the most effective catalyst (Figure 1). The results of these experiments are summarized in Table 1. In the absence of palladium(II) catalyst, no background reaction was observed even with prolonging the time to 84 h (Table 1, entry 1). Using 4 mol % NHCPd(II) 1a as catalyst after 2.5 days at 50 °C afforded a mixture of products 4 and 5a in 82% yield (syn:anti = 95:5) and 7% yield (syn:anti = 40:60) in wet acetic acid, respectively (Table 1, entry 2). Other NHC-Pd(II) complexes such as 1b, 1c, and 1d were not very effective in this reaction (Table 1, entries 3-5), although they were better catalysts than 1a in umpolung allylation of aldehydes with cyclohexenyl acetate6f or conjugate addition of arylboronic acids to cyclic enones.6d Cationic NHC-Pd2þ diaquo complex 1e6e was found to be more reactive than 1a to consume trans-stilbene completely within 12 h, giving 4 in 63% yield with good syn-diastereoselectivity (syn: anti = 90:10), along with 5a in 8% yield (Table 1, entry 6). Similar results were observed at room temperature, albeit after a longer time (Table 1, entry 7). High catalytic ability of NHC-Pd(II) catalyst 1e is possibly attributed to its cationic character, facilitating activation of the double bond of alkenes and enhancing the electrophilicity of the subsequent palladated alkyl group.3f,g Surprisingly, catalyst 1a, bearing an iodide counterion, was also effective despite a longer reaction time required to complete the transformation. Titration experiments suggested that 1a in CDCl3 solution was converted to a new complex by addition of stoichiometric PhI(OAc)2, with the 1H NMR spectroscopic signal at 3.82 ppm for the N-CH3 of NHCPd(II) complex 1a slightly downfield shifted to 3.88 ppm after 5 min (see the Supporting Information). Formation of reddish I2 was observed simultaneously, which possibly originated from oxidation of the coordinating iodide counterion on the palladium center by PhI(OAc)2. Prompted by these results, we assumed that the new complex generated from 1a with PhI(OAc)2 featured a cationic palladium center bearing noncoordinating counterions. Although details remain to be elucidated further, this assumption is consistent with our experimental observations.3i Without the addition of PhI(OAc)2 (Table 1, entry 8), no detectable product was formed, even with 20 mol % of NHCPd(II) catalyst 1e. Using benzoquinone as the oxidant was (7) Herein, H2O2 (30 wt%, aqueous) tested to be an effective oxidant. For example, by using H2O2 (6 equiv) instead of PhI(OAc)2, catalyst 1a exclusively afforded hydroxyacetate 4 in 70% yield (syn:anti = 90:10) at ambient temperature for 72 h. A PdII/IV-catalyzed oxidative cyclization of enyes with H2O2 as the oxidant: Yin, G. Y.; Liu, G. S. Angew. Chem., Int. Ed. 2008, 47, 5442–5445.
Table 1. Bis(NHC)-Pd(II) Complex-Catalyzed Dioxygenation of trans-Stibenea
4 b
entry catalyst time (h) yield (%) 1 2 3 4 5 6 7e 8f 9g 10g,h
none 1a 1b 1c 1d 1e 1e 1e 1e 1e
84 61 72 68 49 12 35 70 22 20
0 82 35 35 11 63 61 0c 59 0c
5a c
syn:anti 95:5 92:8 89:11 82:18 90:10 92:8 90:10
yield (%)d syn:antic 0 7 2 2 1 8 10 0 12 73b
40:60 53:47 62:38 47:53 69:31 73:27 69:31 85:15
a Reaction conditions: All reactions were performed with 0.3 mmol of 2, 4 mol % of NHC-Pd(II) catalyst, 1.1 equiv of PhI(OAc)2, and 3.0 equiv of H2O in 3 mL of acetic acid (available commercially) at 50 °C. b Isolated yield. c Determined by 1H NMR integration of the crude product. d Determined by 1H NMR integration of crude product 5a compared with 4. e At room temperature. f No oxidant PhI(OAc)2. g Without the addition of 3.0 equiv of H2O. h Anhydrous acetic acid was used.
also ineffective in this reaction.7 On the basis of these results, a process based on a Pd(II)/Pd(0) catalysis appeared improbable here. To evaluate the influence of water for the distribution of products 4 and 5a, the control experiments were carried out. It was found that the yield of hydroxyacetate 4 (59% yield) slightly decreased along with the increase of 5a (12% yield) without the additional 3 equiv of H2O (Table 1, entry 9), and moreover, using anhydrous AcOH as solvent (without H2O) the dioxygenation of trans-stilbene exclusively afforded diacetate 5a in 73% yield (syn:anti = 85:15) (Table 1, entry 10). These results suggested that water was required for the formation of hydroxyacetate 4. Possible Mechanism for the Dioxygenation of trans-Stilbene. By an isotopic labeling study using 97% 18O-enriched water, Song et al. have proposed a Pd(II)/Pd(IV) catalytic mechanism involving hydrolysis of an acetoxonium intermediate to lead to hydroxyacetate 4.3g To investigate the origin of hydroxyacetate 4 herein, the 18O-labeling experiment was repeated by treatment of trans-stilbene with 3.0 equiv of H218O (97% 18O-enriched) in anhydrous acetic acid (see the Supporting Information). Dioxygenation products 4 and 5a were isolated as isotopic mixtures in 67% and 10% yield, respectively. As shown in Table 2, the formation of
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Table 2. Results of Isotopic Labeling Experiment with 18 O-Enriched Water
Scheme 1. Hydrolysis of Isotopic Mix-4
a
isotopic mix-4
m/za
abundance (%)b
unlabeled 18 O-labeled doubly 18O-labeled
279.1 281.1 283.1
73.1 26.9 0
a
ESI/MS, [M þ Na]þ. b Integration of peak area.
doubly 18O-labeled hydroxyacetate 4 was not observed. The relative amounts of unlabeled 4 (73.1%) and 18O-labeled 4 (26.9%) were determined using mass spectrometry (ESI, [M þ Na]þ). As shown in Scheme 1, hydrolysis of the isotopic mixture of hydroxyacetate 4 resulted in significant removal of the 18O-labeling. This result demonstrated that the 18O-labeling was selectively incorporated into the carbonyl group of the acetate rather than the hydroxyl group of 4. On the basis of this 18O-labeling experiment and the syn-diastereoselectivity observed here, NHC-Pd(II) complex-catalyzed dioxygenation of alkenes to give hydroxyacetate 4 showed the same mechanism as that published by Song and co-workers (Figure 2, a).3g However, this mechanism (Figure 2, path a) cannot clearly illustrate the formation of diacetate 5a, which was always observed as a concomitant product in wet AcOH and, moreover, as the exclusive product in anhydrous AcOH. Several possible scenarios were discussed to explain the observed diacetoxylation product 5a, as below. First, the control experiment showed that hydroxyacetate 4 was converted into 5a in only very low yield (
