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Organometallics 2009, 28, 152–160
γ-Selective Cross-Coupling Reactions of Potassium Allyltrifluoroborates with Haloarenes Catalyzed by a Pd(0)/D-t-BPF or Pd(0)/Josiphos ((R,S)-CyPF-t-Bu) Complex: Mechanistic Studies on Transmetalation and Enantioselection Yasunori Yamamoto,* Shingo Takada, and Norio Miyaura* DiVision of Chemical Process Engineering, Graduate School of Engineering, Hokkaido UniVersity, Sapporo 060-8628, Japan
Tetsuji Iyama and Hiroto Tachikawa DiVision of Materials Chemistry, Graduate School of Engineering, Hokkaido UniVersity, Sapporo 060-8628, Japan ReceiVed August 27, 2008
Cross-coupling between bromoarenes and [(E)-CH3CH)CHCH2BF3]K (2a) by a catalyst prepared from Pd(OAc)2 and D-t-BPF selectively provided γ-coupling products via SE2′ substitution. Mechanistic study of transmetalation revealed a heretofore unknown process, namely, formation of a highly electrophilic [Pd(Ar)(D-t-BPF)]+ before transmetalation with 2a. Thus, kinetic data in coupling of 4-substituted bromoarenes with 2a showed a linear positive correlation (F ) -1.1) accelerated by donating substituents. This rate-determining role of transmetalation was further confirmed by kinetic data between oxidative adducts [Pd(Ar)(Br)(D-t-BPF)] and 2a that exhibited an analogous correlation with a negative F-value (-0.50). Theoretical study by density functional theory (DFT) calculation showed that transmetalation between [Ar-Pd]+ and 2a via an SE2′ (open) transition state is a slightly lower energy process than an SE2′ (closed) process. Allylic substitutions with chiral catalysts are the current topics for enantioselective C-C bond formation, but the catalysts that are effective for allylic nucleophiles have remained unexplored. Among the ligands screened, (R,S)-CyPF-t-Bu was found to achieve 77-90% ee for representative paraand meta-substituted bromoarenes and 2-bromo-1-alkenes in refluxing aqueous tetrahydrofuran (THF) or MeOH. To obtain mechanistic information on enantioselection, the mode of substrate coordination to a cationic phenylpalladium intermediate was calculated, that is, the reaction stage directly preceding the stereodetermining insertion step by DFT calculation. A stable adduct between [Pd(CyPF-t-Bu)(Ph)]+ and 2a located at the C-C double bond from its re-face yielding the experimentally observed R-product is preferred thermodynamically rather than the corresponding si-coordination. Introduction Transition-metal-catalyzed cross-coupling reactions have proved to be one of the most powerful methods for selective C-C bond formation.1 Among the possible combinations of electrophiles and nucleophiles, reactions of allylic metals with aryl, alkenyl, and allyl electrophiles or with their reversed combination provide an important class of compounds because of the frequent occurrence of allylic fragments in natural products.2 The coupling reactions of allylmagnesium,3 -zinc,4 -boron,5 -silicon,6 -tin,7 and -zirconium8 compounds with Ni and Pd catalysts have been extensively studied. Perfect control of the coupling position by phosphine ligands was first achieved by Hatanaka and Hiyama et al. using a combination of * To whom correspondence should be addressed. Tel/fax: +81-11-7066561; e-mail:
[email protected] (Y.Y.) and miyaura@ eng.hokudai.ac.jp. (1) For reviews, see: (a) Boronic Acids; Hall, D. G., Ed.; Wiley-VCH: Weinheim, Germany, 2005. (b) Metal-Catalyzed Cross-Coupling Reactions, 2nd ed.; de Meijere, A., Diederich, F., Eds.; Wiley-VCH: Weinheim, Germany, 2004. (c) Cross-Coupling Reactions. A Practical Guide; Miyaura, N., Ed.; Springer: Berlin, 2002. (d) Handbook of Organopalladium Chemistry for Organic Synthesis; Negishi, E.-I., Ed.; John Wiley & Sons: New York, 2002. (e) Metal-Catalyzed Cross-Coupling Reactions; Diederich, F., Stang, P. J., Eds.; Wiley-VHC: Weinheim, Germany, 1998.
allyltrifluorosilanes and haloarenes.6a,c The coupling selectively occurred at the γ-carbon when using PPh3 or bisphosphines possessing a large bite angle such as 1,4-bis(diphenylphosphino)butane (dppb), whereas bisphosphines possessing a relatively small angle such as 1,3-bis(diphenylphosphino)propane (dppp) underwent selective coupling at the R-carbon. Such an effect of phosphine ligands was also demonstrated in regioselective Stille coupling of aryl halides with Me3SiCH2CH)CHCH2SnBu3 at the R- or γ-carbon.7g There have been very few attempts to employ allylboron compounds, but boron reagents have various advantages, including availability, air and moisture stability, low toxicity, and easy removal of boron-derived byproducts. We have reported the efficiency of potassium allyltrifluoroborates for perfect γ-selective coupling with bro(2) (a) Heck, R. H. J. Am. Chem. Soc. 1968, 90, 5531. (b) Negishi, E.-i.; Chatterjee, S.; Matsushita, H. Tetrahedron Lett. 1981, 22, 3737. (c) Legros, J.-Y.; Flaud, J.-C. Tetrahedron Lett. 1990, 31, 7453. (d) MarenoMan˜as, M.; Pajuelo, F.; Pleixats, R. J. Org. Chem. 1995, 60, 2396. (e) Chung, K.-G.; Miyake, Y.; Uemura, S. J. Chem. Soc., Perkin Trans. 1 2000, 2725. (f) Bouyssi, D.; Gerusz, V.; Balme, G. Eur. J. Org. Chem. 2002, 2445. (g) Ikegami, R.; Koresawa, A.; Shibata, T.; Takagi, K. J. Org. Chem. 2003, 68, 2195. (h) Kayaki, Y.; Koda, T.; Ikariya, T. Eur. J. Org. Chem. 2004, 4989. (i) Tsukamoto, H.; Sato, M.; Kondo, Y. Chem. Commun. 2004, 1200.
10.1021/om800832r CCC: $40.75 2009 American Chemical Society Publication on Web 11/24/2008
Transmetalation and Enantioselection Studies
moarenes and bromoalkenes9 and their asymmetric coupling with bromoarenes using (R,S)-CyPF-t-Bu as the chiral auxiliary.10 Such a selective γ-coupling was also achieved by Szabo and co-workers in the synthesis of allylboronic acids via crosscoupling between diboronic acid and allylic alcohols or vinylcyclopropanes.11 The proposed mechanisms of those γ-selective coupling reactions of allylic nucleophiles were the SE2′ (open or closed) process reported for allylsilicon compounds6b and the addition-elimination process reported for allylboronic acids,5a,11c,12,13 whereas formation of (π-allyl)palladium(II) intermediate, which predominates R-coupling, was ruled out.5a In this paper, we report transmetalation and enantioselection mechanisms involved in cross-coupling between potassium (E)crotyltrifluoroborate and bromoarenes catalyzed by a Pd(0)/Dt-BPF or Pd(0)/Josiphos ((R,S)-CyPF-t-Bu) complex. Formation of a cationic palladium(II) intermediate such as [ArPd]+ before transmetalation was proposed on the basis of results of kinetic and theoretical studies.
Organometallics, Vol. 28, No. 1, 2009 153 Scheme 1. γ-Selective Coupling Giving 3 (Table 1)
Results and Discussion γ-Selective Coupling Catalyzed by Pd(0)/D-t-BPF. Reaction between potassium allyltrifluoroborates (2) and bromoarenes gave γ-coupling products in high yields with perfect regioselectivities for variously functionalized bromoarenes (Scheme 1).9 Air- and water-stable allyltrifluoroborates (2), synthesized by treatment of allylboronic acids or esters with KHF2, were advantageous over triallylboranes and allylboronic acids or esters in preparation and handling of pure and water-stable crystalline materials.14,15 These K+ salts are highly insoluble in common organic solvents, but the reaction smoothly proceeded in aqueous (3) (a) Tsuji, T.; Yorimitsu, H.; Oshima, K. Angew. Chem., Int. Ed. 2002, 41, 4137. (b) Ohmiya, H.; Tsuji, T.; Yorimitsu, H.; Oshima, K. Chem. Eur. J. 2004, 10, 5640. (4) (a) Peet, W. G.; Tam, W. J. Chem. Soc., Chem. Commun. 1983, 853. (b) Campbell, J. B.; Firor, J. W.; Davenport, T. W. Synth. Commun. 1989, 19, 2265. (5) (a) Nilsson, K.; Hallberg, A. Acta Chem. Scand. 1987, B41, 569. (b) Miyaura, N.; Suzuki, A. Chem. ReV. 1995, 95, 2457. (c) Kalinin, V. N.; Denisov, F. S.; Bubnov, Y. N. MendeleeV Commun. 1996, 202. (d) Fu¨rstner, A.; Seidel, G. Synlett 1998, 161. (e) Fu¨rstner, A.; Leitner, A. Synlett 2001, 290. (f) Occhiato, E. G.; Trabocchi, A.; Guarna, A. J. Org. Chem. 2001, 66, 2459. (g) Kotha, S.; Behera, M.; Shah, V. R. Synlett 2005, 1877. (6) (a) Hatanaka, Y.; Ebina, Y.; Hiyama, T. J. Am. Chem. Soc. 1991, 113, 7075. (b) Hatanaka, Y.; Goda, K.-i.; Hiyama, T. Tetrahedron Lett. 1994, 35, 1279. (c) Hatanaka, Y.; Goda, K.-i.; Hiyama, T. Tetrahedron Lett. 1994, 35, 6511. (d) Hiyama, T.; Matsuhashi, H.; Fujita, A.; Tanaka, M.; Hirabayashi, K.; Shimizu, M.; Mori, A. Organometallics 1996, 15, 5762. (7) (a) Kosugi, M.; Sasazawa, K.; Shimizu; Y.; Migita, T. Chem. Lett. 1977, 6, 301. (b) Trost, B. M.; Keinan, E. Tetrahedron Lett. 1980, 21, 2595. (c) Godschalx, J.; Stille, J. K. Tetrahedron Lett. 1980, 21, 2599. (d) Echavarren, A. M.; Stille, J. K. J. Am. Chem. Soc. 1987, 109, 5478. (e) Martorell, G.; Garcı´a-Raso, A.; Saa´, J. M. Tetrahedron Lett. 1990, 31, 2357. (f) Farina, V.; Krishnan, B. J. Am. Chem. Soc. 1991, 113, 9585. (g) Obora, Y.; Tsuji, Y.; Kobayashi, M. J. Org. Chem. 1995, 60, 4647. (h) Cotter, W. D.; Barbour, L.; McNamara, K. L.; Hechter, R.; Lachicotte, R. J. J. Am. Chem. Soc. 1998, 120, 11016. (8) Hirano, K.; Yorimitsu, H.; Oshima, K. Synlett 2005, 1787. (9) Yamamoto, Y.; Takada, S.; Miyaura, N. Chem. Lett. 2006, 35, 704. (10) Yamamoto, Y.; Takada, S.; Miyaura, N. Chem. Lett. 2006, 35, 1368. (11) (a) Sebelius, S.; Olsson, V. J.; Szabo´, K. J. J. Am. Chem. Soc. 2005, 127, 10478. (b) Olsson, V. J.; Sebelius, S.; Selander, N.; Szabo´, K. J. J. Am. Chem. Soc. 2006, 128, 4588. (c) Sebelius, S.; Olsson, V. J.; Wallner, O. A.; Szabo´, K. J. J. Am. Chem. Soc. 2006, 128, 8150. (d) Dutheuil, G.; Selander, N.; Szabo´, K. J. Synthesis 2008, 2293. (12) Miyaura, N.; Suzuki, A. J. Organomet. Chem. 1981, 213, C53. (13) (a) Karabelas, K.; Westerlund, C.; Hallberg, A. J. Org. Chem. 1985, 50, 3896. (b) Karabelas, K.; Hallberg, A. J. Org. Chem. 1986, 51, 5286. (c) Karabelas, K.; Hallberg, A. J. Org. Chem. 1989, 54, 1773. (d) Albe´niz, A. C.; Espinet, P.; Lo´pez-Ferna´ndez, R. Organometallics 2006, 25, 5449. (14) (a) Darses, S.; Geneˆt, J.-P. Eur. J. Org. Chem. 2003, 4313. (b) Molander, G. A.; Ellis, N. Acc. Chem. Res. 2007, 40, 275. (c) Darses, S.; Geneˆt, J.-P. Chem. ReV. 2008, 108, 288.
tetrahydrofuran (THF) or MeOH in the presence of a palladium catalyst prepared in situ from Pd(OAc)2 and phosphine ligands. Since the reaction was very slow in the absence of a base, it was carried out in a basic solution as was previously reported in cross-coupling reactions of potassium aryl- and 1-alkenyltrifluoroborates.15 The use of K2CO3 (3 equivalents) in anhydrous THF gave the best results (85-99% yields), whereas the use of stronger bases, such as K3PO4 and KOH, resulted in lower yields (9-30%). The yields and regioselectivities of the coupling position were highly sensitive to phosphine ligands employed for palladium acetate in the reaction of 4-bromoanisole with potassium (E)crotyltrifluoroborate (2a) (Table 1). Monodentate PPh3 and bidentate dppm and dppe possessing a relatively small bite angle resulted in a mixture of two isomers (entries 1-3), whereas bisphosphines that have a bite angle larger than dppp yielded a γ-coupling product (3) as the sole product (entries 4-8). This effect of bite angle was almost the same as that reported for allyltrifluorosilanes.6a,c Although these catalysts suffered from low yields mainly because of formation of anisole derived by hydrogenolysis of bromoanisole, D-t-BPF16 was finally found to be the best ligand to achieve high yields and high γ-selectivities for 4-bromoanisole and methyl 4-bromobenzoate (entries 9-11). This ligand also worked well for a series of allylboronates including (Z)-crotyl- (2b), (E)-cinnamyl-(2c), (E)-3methyl-2-pentenyl (2d), (E)-3-methyl-2-heptenyl (2e), allyl (2f), (15) (a) Darses, S.; Geneˆt, J.-P.; Brayer, J.-L.; Demoute, J.-P. Tetrahedron Lett. 1997, 38, 4393. (b) Darses, S.; Michaud, G.; Geneˆt, J.-P. Tetrahedron Lett. 1998, 39, 5045. (c) Darses, S.; Michaud, G.; Geneˆt, J.-P. Eur. J. Org. Chem. 1999, 1875. (d) Batey, R. A.; Thadani, A. N.; Smil, D. V. Tetrahedron Lett. 1999, 40, 4289. (e) Batey, R. A.; Thadani, A. N.; Smil, D. V. Synthesis 2000, 990. (f) Batey, R. A.; Quach, T. D. Tetrahedron Lett. 2001, 42, 9099. (g) Molander, G. A.; Rivero, M. R. Org. Lett. 2002, 4, 107. (h) Thadani, A. N.; Batey, R. A. Org. Lett. 2002, 4, 3827. (i) Molander, G. A.; Katona, B. W.; Machrouhi, F. J. Org. Chem. 2002, 67, 8416. (j) Molander, G. A.; Bernardi, C. R. J. Org. Chem. 2002, 67, 8424. (k) Molander, G. A.; Biolatto, B. J. Org. Chem. 2003, 68, 4302. (l) Molander, G. A.; Yun, C.-S.; Ribagorda, M.; Bilatto, B. J. Org. Chem. 2003, 68, 5534. (m) Barder, T. E.; Buchwald, S. L. Org. Lett. 2004, 6, 2649. (n) Kabalka, G. W.; Dong, G.; Venkataiah, B. Tetrahedron Lett. 2004, 45, 5139. (o) Kabalka, G. W.; Dong, G.; Venkataiah, B. Tetrahedron Lett. 2005, 46, 763. (p) Molander, G. A.; Felix, L. A. J. Org. Chem. 2005, 70, 3950. (q) Kabalka, G. W.; Al-Masum, M. Org. Lett. 2006, 8, 11. (r) Molander, G. A.; Sommers, E. M.; Baker, S. R. J. Org. Chem. 2006, 71, 1563. (s) Molander, G. A.; Yokoyama, Y. J. Org. Chem. 2006, 71, 2493. (t) Molander, G. A.; Ham, J.; Seapy, D. G. Tetrahedron 2007, 63, 768. (16) (a) Itoh, T.; Sato, K.; Mase, T. AdV. Synth. Catal. 2004, 346, 1859. (b) Itoh, T.; Mase, T. Tetrahedron Lett. 2005, 46, 3573.
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Table 1. γ-Selective Coupling of Potassium Allyltrifluoroborates (2) with Haloarenes Giving 3 (Scheme 1) entry
2
ArBr, X )
ligand
yield/%a (3/4)
1b 2b 3b 4b 5b 6b 7b 8b 9b 10c 11c 12c 13c 14c 15c 16c,d 17c
2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2b 2c 2d 2e 2f 2g
4-MeO 4-MeO 4-MeO 4-MeO 4-MeO 4-MeO 4-MeO 4-MeO 4-MeO 4-MeO 4-CO2Me 4-CO2Me 4-CO2Me 4-CO2Me 4-CO2Me 4-CO2Me 4-CO2Me
PPh3 dppm dppe dppp dppf DPEphos Xantphos DBFphos D-t-BPF D-t-BPF D-t-BPF D-t-BPF D-t-BPF D-t-BPF D-t-BPF D-t-BPF D-t-BPF
42 (78/22) 35 (78/22) 17 (37/63) 37 (>99/1) 68 (>99/1) 21 (>99/1) 10 (>99/1) 10 (>99/1) 87 (>99/1) 85 (>99/1) 99 (>99/1) 91 (>99/1) 99 (>99/1) 96 (>99/1) 99 (>99/1) 99 (-) 99 (
