Remarkable Positive Effect of Silver Salts on Asymmetric

802

Organometallics 2009, 28, 802–809

Remarkable Positive Effect of Silver Salts on Asymmetric Hydrogenation of Acyclic Imines with Cp*Ir Complexes Bearing Chiral N-Sulfonylated Diamine Ligands Shin-yo Shirai,† Hideki Nara,‡ Yoshihito Kayaki,† and Takao Ikariya*,† Department of Applied Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552, Japan, and Corporate Research & DeVelopment DiVision, Takasago International Corporation, 1-4-11 Nishiyawata, Hiratsuka, Kanagawa 254-0073, Japan ReceiVed September 24, 2008

A family of bifunctional Ir, Rh, and Ru complexes bearing chiral monosulfonylated diamine ligands has been evaluated for asymmetric hydrogenation of acyclic imines (1) to the corresponding amines (2). A chiral Ir complex, [Cp*IrCl{(S,S)-Tscydn}] (3a), combined with silver salts caused a marked improvement in the catalyst performance in terms of the activity and selectivity. The use of an excess amount of the silver salt, AgSbF6, increased enantioselectivity up to 72% ee in the asymmetric hydrogenation of N-(1-phenylethylidene)benzylamine (1a). A stoichiometric reaction of 3a with AgSbF6 in acetonitrile afforded an isolable cationic complex, [Cp*Ir(Tscydn)(CH3CN)]+SbF6- (4) which was fully characterized by NMR spectroscopy and X-ray crystallography. The resulting cationic complex 4 readily reacted with H2 under ambient conditions in the presence of triethylamine to give a hydridoiridium complex, [Cp*IrH{(S,S)-Tscydn}] (5) and showed comparable catalytic behavior to that for the catalyst system generated in situ from 3a and AgSbF6. On the basis of the additive effect on the outcome of the hydrogenation as well as the 13C{1H} NMR spectrum of a reaction mixture of imine 1a and AgSbF6, the mechanism of the imine hydrogenation including heterolytic bond cleavage of H2 on a cationic complex to generate a hydrido intermediate and the following H- transfer to the imine substrates activated by the silver cation was proposed. Introduction The catalytic enantioselective reduction of imines provides straightforward access to chiral amines as useful building blocks for biologically active compounds.1 Much efforts have been given to develop efficient molecular catalysts including chiral Ti,2 Zr,3 Ru,4,5 Rh,5d,6 Ir,7 and Pd8 systems for asymmetric hydrogenation of imines during the past decades. However, some drawbacks such as low productivity and/or catalyst * To whom correspondence should be addressed. E-mail: tikariya@ apc.titech.ac.jp. † Tokyo Institute of Technology. ‡ Takasago International Corporation. (1) (a) Gladiali, S.; Alberico, E. Chem. Soc. ReV. 2006, 35, 226–236. (b) Blaser, H.-U.; Malan, C.; Pugin, B.; Spindler, F.; Steiner, H.; Studer, M. AdV. Synth. Catal. 2003, 345, 103–151. (c) Kobayashi, S.; Ishitani, H. Chem. ReV. 1999, 99, 1069–1094. (2) (a) Willoughby, A. C.; Buchwald, S. L. J. Am. Chem. Soc. 1994, 116, 8952–8965. (b) Willoughby, A. C.; Buchwald, S. L. J. Am. Chem. Soc. 1994, 116, 11703–11704. (3) Ringwald, M.; Stru¨rmer, R.; Brintzinger, H. H. J. Am. Chem. Soc. 1999, 121, 1524–1527. (4) (a) Oppolzer, W.; Wills, M.; Starkemann, C.; Bernardinelli, G. Tetrahedron Lett. 1990, 31, 4117–4120. (b) Charette, A. B.; Giroux, A. Tetrahedron Lett. 1996, 37, 6669–6672. (c) Magee, M. P.; Norton, J. R. J. Am. Chem. Soc. 2001, 123, 1778–1779. (d) Cobley, J. C.; Henschke, J. P. AdV. Synth. Catal. 2003, 345, 195–201. (5) (a) Abdur-Rashid, K.; Lough, A. J.; Morris, R. H. Organometallics 2001, 20, 1047–1049. (b) Abdur-Rashid, K.; Guo, R.; Lough, A. J.; Morris, R. H. AdV. Synth. Catal. 2005, 347, 571–579. (c) Clarke, M. L.; DıázValenzuela, M. B.; Slawin, A. M. Z. Organometallics 2007, 26, 16–19. (d) Jackson, M.; Lennon, I. C. Tetrahedron Lett. 2007, 48, 1831–1834. (e) Li, T.; Bergner, I.; Haque, F. N.; Iuliis, M. Z.-D.; Song, D.; Morris, R. H. Organometallics 2007, 26, 5940–5949. (f) Cheruku, P.; Church, T. L.; Andersson, P. G. Chem. Asian J. 2008, 3, 1390–1394.

poisoning arising from the imine substrates and amine products have been realized in the hydrogenation of CdN bonds compared with that of CdC and CdO bonds.9 In addition, interconversion between E and Z isomers has been proposed to hamper control of enantioselectivity in the reduction of acyclic imine substrates. Pfaltz and co-workers developed a chiral analogue of Crabtree’s catalyst effective for asymmetric imine hydrogenation,7e and since then, a number of Ir(I) catalysts with P and N ligands have been applied to the reaction of acyclic N-aryl imines.7h-z Noyori and Ikariya found a prototype of conceptually new chiral Ru catalysts bearing N-sulfonylated 1,2-diamines for highly efficient asymmetric transfer hydrogenation of cyclic imines as well as ketones.10 Since this finding, the metal-NH bifunctional catalysis based on interconversion between amido and hydrido(amine) complexes has received much attention as a promising protocol for asymmetric redox transformations (Scheme 1).11 The proposed catalytic mechanism involves a cyclic transition state where H- and H+ equivalents are concertedly transferred from the hydrido(amine) complex to the CdO linkage without direct coordination to the metal center.12 Although such outer-sphere mechanism might be possible for (6) (a) Bakos, J.; Orosz, A.; Heil, B.; Laghmari, M.; Lhoste, P.; Sinou, D. J. Chem. Soc., Chem. Commun. 1991, 1684–1685. (b) Becalski, A. G.; Cullen, W. R.; Fryzuk, M. D.; James, B. R.; Kang, G.-J.; Rettig, S. J. Inorg. Chem. 1991, 30, 5002–5008. (c) Burk, M. J.; Feaster, J. E. J. Am. Chem. Soc. 1992, 114, 6266–6267. (d) Buriak, J. M.; Osborn, J. A. Organometallics 1996, 15, 3161–3169. (f) Lensink, C.; Rijnberg, E.; de Vries, J. G. J. Mol. Catal. 1997, 116, 199–207. (g) Spinder, F.; Blaser, H.-U. AdV. Synth. Catal. 2001, 343, 68–70. (h) Tararov, V. I.; Kadyrov, R.; Riermeier, T. H.; Fischer, C.; Bo¨rner, A. AdV. Synth. Catal. 2004, 346, 561–565.

10.1021/om800926p CCC: $40.75  2009 American Chemical Society Publication on Web 01/06/2009

Effect of SilVer Salts on Asymmetric Hydrogenation

enantioselective transfer hydrogenation of imines with the metal-NH bifunctional (η6-arene)Ru10c,13 and Cp*Rh14 (Cp/ ) 1,2,3,4,5-pentamethylcyclopentadienyl) complexes bearing chiral N-sulfonylated diamine ligands, the precise mechanism of the hydrogen transfer from the metal hydride complex to imines is still controversial. Ba¨ckvall and co-workers have recently disclosed that preactivation of imine substrates by addition of acids to form iminium species is crucial for the smooth hydrogen transfer from the Ru hydride species in the (η6-arene)Ru catalyst system bearing the Tsdpen ligand (Tsdpen ) N-(p-toluenesulfonyl)-1,2-diphenylethylenediamine).15 We have found that a Cp*Ru system having the metal/NH moiety could facilitate heterolytic cleavage of H2 bound on a 16-electron [Cp*Ru{Me2N(CH2)2NH2}]+ fragment with the help of acidic compounds, leading to the hydrido(amine) species.11a,16 The possible mechanism for the H-H bond cleavage was further confirmed by Brandt and Andersson using theoretical calculations.17 Recently, we and other groups independently reported that the acidic compound-assisted H2 activation on the bifunctional catalyst systems can be applied to highly efficient hydrogenation of ketones, epoxides, and imides.11a,18,19 These results prompted us to study asymmetric hydrogenation of acyclic imines with our bifunctional complexes bearing N(7) (a) Spindler, F.; Pugin, B.; Blaser, H.-U. Angew. Chem., Int. Ed. 1990, 29, 558–559. (b) Ng Cheong Chan, Y.; Osborn, J. A. J. Am. Chem. Soc. 1990, 112, 9400–9401. (c) Morimoto, T.; Achiwa, K. Tetrahedron: Asymmetry 1995, 6, 2661–2664. (d) Sablong, R.; Osborn, J. A. Tetrahedron: Asymmetry 1996, 7, 3059–3062. (e) Schnider, P.; Koch, G.; Pre´toˆt, R.; Wang, G.; Bohnen, F. M.; Kru¨ger, C.; Pfaltz, A. Chem.;Eur. J. 1997, 3, 887–892. (f) Morimoto, T.; Suzuki, N.; Achiwa, K. Tetrahedron: Asymmetry 1998, 9, 183–187. (g) Bianchini, C.; Barbaro, P.; Scapacci, G.; Farnetti, E.; Graziani, M. Organometallics 1998, 17, 3308–3310. (h) Kainz, S.; Brinkmann, A.; Leitner, W.; Pfaltz, A. J. Am. Chem. Soc. 1999, 121, 6421– 6429. (i) Xiao, D.; Zhang, X. Angew. Chem., Int. Ed. 2001, 40, 3425– 3428. (j) Blaser, H.-U. AdV. Synth. Catal. 2002, 344, 17–31. (k) Wang, W.-B.; Lu, S.-M.; Yang, P.-Y.; Han, X.-W.; Zhou, Y.-G. J. Am. Chem. Soc. 2003, 125, 10536–10537. (l) Jiang, X.-B.; Minnaard, A. J.; Hessen, B.; Feringa, B. L.; Duchateau, A. L. L.; Andrien, J. G. O.; Boogers, J. A. F.; de Vries, J. G. Org. Lett. 2003, 5, 1503–1506. (m) Dorta, R.; Broggini, D.; Stoop, R.; Ruëgger, H.; Spindler, F.; Togni, A. Chem.-Eur. J. 2004, 10, 267–278. (n) Lu, S.-M.; Han, X.-W.; Zhou, Y.-G. AdV. Synth. Catal. 2004, 346, 909–912. (o) Blanc, C.; Agbossou-Niedercorn, F.; Nowogrocki, G. Tetrahedron: Asymmetry 2004, 15, 2159–2163. (p) Solinas, M.; Pfaltz, A.; Cozzi, P. G.; Leitner, W. J. Am. Chem. Soc. 2004, 126, 16142–16147. (q) Marie, P.; Deblon, S.; Breher, F.; Geier, J.; Bo¨hler, C.; Ruëgger, H.; Scho¨nberg, H.; Gru¨tzmacher, H. Chem.;Eur. J. 2004, 10, 4198–4205. (r) Trifonova, A.; Diesen, J. S.; Chapmen, C. J.; Andersson, P. G. Org. Lett. 2004, 6, 3825–3827. (s) Moessner, C.; Bolm, C. Angew. Chem., Int. Ed. 2005, 44, 7564–7567. (t) Dervisi, A.; Carcedo, C.; Ooi, L. AdV. Synth. Catal. 2006, 348, 175–183. (u) Vargas, S.; Rubio, M.; Sua´rez, A.; del Rıó, D.; ´ lvarez, E.; Pizzano, A. Organometallics 2006, 25, 961–973. (v) Trifonova, A A.; Diesen, J. S.; Andersson, P. G. Chem.;Eur. J. 2006, 12, 2318–2328. (w) Imamoto, T.; Iwadate, N.; Yoshida, K. Org. Lett. 2006, 8, 2289–2292. (x) Zhu, S.-F.; Xie, J.-B.; Zhang, Y.-Z.; Li, S.; Zhou, Q.-L. J. Am. Chem. Soc. 2006, 128, 12886–12891. (y) Reetz, M. T.; Bondarev, O. Angew. Chem., Int. Ed. 2007, 46, 4523–4526. (z) Cheemala, M. N.; Knichel, P. Org. Lett. 2007, 9, 3089–3092. (8) (a) Abe, H.; Amii, H.; Uneyama, K. Org. Lett. 2001, 3, 313–315. (b) Suzuki, A.; Mae, M.; Amii, H.; Uneyama, K. J. Org. Chem. 2004, 69, 5132–5134. (c) Yang, Q.; Deng, J. G.; Zhang, X. Angew. Chem., Int. Ed. 2006, 45, 3832–3835. (d) Wang, Y.-Q.; Lu, S.-M.; Zhou, Y.-G. J. Org. Chem. 2007, 72, 3729–3734. (9) Reviews: (a) James, B. R. Catal. Today 1997, 37, 209–221. (b) Clapham, S. E.; Hadzovic, A.; Morris, R. H. Coord. Chem. ReV. 2004, 248, 2201–2237. (c) Tang, W.; Zhang, X. Chem. ReV. 2003, 103, 3029– 3069. (d) Samec, J. S.; Ba¨ckvall, J.-E.; Andersson, P. G.; Brandt, P. Chem. Soc. ReV. 2006, 35, 237–248. (10) (a) Hashiguchi, S.; Fujii, A.; Takehara, J.; Ikariya, T.; Noyori, R. J. Am. Chem. Soc. 1995, 117, 7562–7563. (b) Fujii, A.; Hashiguchi, S.; Uematsu, N.; Ikariya, T; Noyori, R. J. Am. Chem. Soc. 1996, 118, 2521– 2522. (c) Uematsu, N.; Fujii, A.; Hashiguchi, S.; Ikariya, T.; Noyori, R. J. Am. Chem. Soc. 1996, 118, 4916–4917. (d) Haack, K.-J.; Hashiguchi, S.; Fujii, A.; Ikariya, T.; Noyori, R. Angew. Chem., Int. Ed. Engl. 1997, 36, 285–288. (e) Hashiguchi, S.; Fujii, A.; Haack, K.-J.; Matsumura, K.; Ikariya, T.; Noyori, R. Angew. Chem., Int. Ed. Engl. 1997, 36, 288–290.

Organometallics, Vol. 28, No. 3, 2009 803 Scheme 1.

Hydrogen Transfer Catalysts with Metal/NH Bifunctionality

sulfonyldiamine ligands. Herein, we present the imine hydrogenation with the chiral bifunctional Ir catalyst systems in which the addition of silver salts was quite effective to enhance the catalytic activity and enantioselectivity.20 On the basis of the results of catalyst screening, isolation of catalyst intermediates including a cationic Cp*Ir-Tscydn complex (Tscydn ) N-(ptoluenesulfonyl)-1,2-cyclohexanediamime), and the NMR spectroscopy of the reaction mixture of N-(1-phenylethylidene)benzylamine (1a) and AgSbF6, the mechanism of the imine hydrogenation including heterolytic bond cleavage of H2 on a cationic complex to generate a hydrido intermediate and the (11) (a) Ito, M.; Ikariya, T. Chem. Commun. 2007, 5134–5142. (b) Ikariya, T; Blacker, A. J. Acc. Chem. Res. 2007, 40, 1300–1308. (c) Ikariya, T.; Murata, K.; Noyori, R. Org. Biomol. Chem. 2006, 4, 393–406. (d) Noyori, R.; Yamakawa, M.; Hashiguchi, S. J. Org. Chem. 2001, 66, 7931– 7944. (12) (a) Yamakawa, M.; Ito, H.; Noyori, R. J. Am. Chem. Soc. 2000, 122, 1466–1478. (b) Alonso, D. A.; Brandt, P.; Nordin, S. J. M.; Andersson, P. G. J. Am. Chem. Soc. 1999, 121, 9580–9588. (c) Petra, D. G. I.; Reek, J. N. H.; Handgraaf, J.-W.; Meijer, E. J.; Dierkes, P.; Kamer, P. C. J.; Brussee, J.; Schoemaker, H. E.; van Leeuwen, P. W. N. M. Chem.;Eur. J. 2000, 6, 2818–2829. (d) Casey, C. P.; Johnson, J. B. J. Org. Chem. 2003, 68, 1998–2001. (13) (a) Ahn, K. H.; Ham, C.; Kim, S.-K.; Cho, C.-W. J. Org. Chem. 1997, 62, 7047–7048. (b) Samano, V.; Ray, J. A.; Thompson, J. B.; Mook, R. A., Jr.; Jung, D. K.; Koble, C. S.; Martin, M. T.; Bigham, E. C.; Regitz, C. S.; Feldman, P. L.; Boros, E. E. Org. Lett. 1999, 1, 1993–1996. (c) Vedejs, E.; Trapencieris, P.; Suna, E. J. Org. Chem. 1999, 64, 6724–6729. (d) Meuzelaar, G. J.; van Vliet, M. C. A.; Maat, L.; Sheldon, R. A. Eur. J. Org. Chem. 1999, 2315–2321. (e) Roth, P.; Andersson, P. G.; Somfai, P. Chem. Commun. 2002, 1752–1753. (f) Williams, G. D.; Pike, R. A.; Wade, C. E.; Wills, M. Org. Lett. 2003, 5, 4227–4230. (g) Santos, L. S.; Pilli, R. A.; Rawal, V. H. J. Org. Chem. 2004, 69, 1283–1289. (h) Williams, G. D.; Wade, C. E.; Wills, M. Chem. Commun. 2005, 4735–4737. (i) Szawkalo, J.; Zawadzka, A.; Wojtasiewicz, K.; Leniewski, A.; Drabowicz, J.; Czarnocki, Z. Tetrahedron: Asymmetry 2005, 16, 3619–3621. (j) Wu, J.; Wang, F.; Ma, Y.; Cui, X.; Cun, L.; Zhu, J.; Deng, J.; Yu, B. Chem. Commun. 2006, 1766–1768. (k) Cavinet, J.; Suss-Fink, G. Green Chem. 2007, 9, 391–397. (14) (a) Mao, J.; Baker, D. C. Org. Lett. 1999, 1, 841–843. (b) Blackmond, D.; Ropic, M.; Stefinovic, M. Org. Process Res. DeV. 2006, 10, 457–463. (c) Matharu, D. S.; Martins, J. E. D.; Wills, M. Chem. Asian J. 2008, 3, 1374–1383. (15) Åberg, J. B.; Samec, J. S. M.; Ba¨ckvall, J.-E. Chem. Commun. 2006, 2771–2773. (16) Ito, M.; Hirakawa, M.; Murata, K.; Ikariya, T. Organometallics 2001, 20, 379–381. (17) Hedberg, C.; Ka¨llsto¨m, K.; Arvidsson, P. I.; Brandt, P.; Andersson, P. G. J. Am. Chem. Soc. 2005, 127, 15083–15090. (18) (a) Ito, M.; Hirakawa, M.; Osaku, A.; Ikariya, T. Organometallics 2003, 22, 4190–4192. (b) Ito, M.; Sakaguchi, A.; Kobayashi, C.; Ikariya, T. J. Am. Chem. Soc. 2007, 129, 290–291. (19) (a) Ohkuma, T.; Utsumi, N.; Tsutsumi, K.; Murata, K.; Sandoval, C.; Noyori, R. J. Am. Chem. Soc. 2006, 128, 8724–8725. (b) Sandoval, C.; Ohkuma, T.; Utsumi, N.; Tsutsumi, K.; Murata, K.; Noyori, R. Chem. Asian J. 2006, 1-2, 102–110. (c) Ohkuma, T.; Tsutsumi, K.; Utsumi, N.; Arai, N.; Noyori, R.; Murata, K. Org. Lett. 2007, 9, 255–257. (d) Ohkuma, T.; Utsumi, N.; Watanabe, M.; Tsutsumi, K.; Arai, N; Murata, K. Org. Lett. 2007, 9, 2565–2567. (20) Shirai, S.; Nara, H.; Ikariya, T. Abstr. Symp. Organomet. Chem. Jpn. 2005, 52, PB148.

804 Organometallics, Vol. 28, No. 3, 2009

Shirai et al.

Scheme 2. Hydrogenation of N-(1-Phenylethylidene)benzylamine

Scheme 3. Deuterium Labeling Experiments Using 2-Propanol-d8

Table 1. Optimization of Reaction Conditions for Asymmetric Hydrogenation of 1a Catalyzed by 3aa

Chart 1

entry

solvent

1 2 3 4 5 6d 7 8 9 10 11

2-propanol 2-propanol 2-propanol 2-propanol 2-propanol 2-propanol THF methanol ethanol t BuOH toluene

b

c

additive (equiv/Ir)

yield (%)

ee (%)

KOH (1) AgSbF6 (1) AgSbF6 (4) AgSbF6 (10) AgSbF6 (4) AgSbF6 (4) AgSbF6 (4) AgSbF6 (4) AgSbF6 (4) AgSbF6 (4)

89 78 85 92 94 90 97 86 89 35 3

41 45 59 70 72 70 71 41 48 7(R) -

a Conditions: 30 °C, 17 h, S/C ) 100, H2 3.0 MPa, 2-propanol 2 mL, MS4A 400 mg. b Determined by gas chromatography (GC). c Determined by high-performance liquid chromatography (HPLC). d Hydrogenation for 3 h.

following H- transfer to the imine substrates activated by the silver cation was proposed.

Table 2. Asymmetric Hydrogenation of 1a with Catalysts Bearing Chiral Diamine Ligandsa

Results and Discussion Asymmetric Hydrogenation of N-(1-Phenylethylidene)benzylamine Derivatives with Neutral Group 8 and 9 Metal Complexes Bearing Monosulfonylated Diamines. Our initial efforts concentrated on asymmetric hydrogenation of 1a (E:Z ) 92:8) to N-benzyl-1-phenethylamine (2a) with a chloroiridium complex, [Cp*IrCl{(S,S)-Tscydn}]21 (3a), with a substrate/catalyst molar ratio (S/C) of 100 in 2-propanol containing molecular sieves 4A22 under 3.0 MPa of H2 at 30 °C for 17 h (Scheme 2). In the absence of any additives, the reaction gave the corresponding (S)-2a with 41% ee and in 89% yield (Table 1, run 1). The possible catalyst, amido complex, generated in situ from the chloride complex 3a and KOH, exhibited a similar reactivity and selectivity to that of 3a (run 2). The addition of an equimolar amount of AgSbF6 was found to increase the enantioselectivity of the reaction, and further enhancement in the ee value of the product (up to 72% ee) was attainable by the treatment with an excess of AgSbF6 (runs 3-5). The reaction was also accelerated by the addition of AgSbF6, being almost complete after 3 h (run 6). Although the hydrogenation in tetrahydrofuran (THF) proceeded efficiently as in 2-propanol (run 7), other alcohols gave the product with lower ees (runs 8-10). The product yield dropped significantly in toluene, possibly due to its poor solubility for AgSbF6 (run 11). Valuable information on the reaction pathway was provided by isotope labeling experiments. Replacement of the solvent by 2-propanol-d8 in the hydrogenation gave the amine product 2a with no deuterium incorporation at the R-positions (HA and HB), however, H-D exchange at the methyl group (82%D for HC) was observed by 1H and 2H NMR spectroscopies (Scheme (21) Murata, K.; Ikariya, T.; Noyori, R. J. Org. Chem. 1999, 64, 2186– 2187. (22) The reaction conditions require rigorous removal of water to obtain reproducible results in terms of catalyst activity and enantioselectivity.

entry

catalyst

yield (%)b

ee (%)c

1 2 3 4 5 6

3a 3b 3c 3d 3e 3f

99 99 99 99 68 99

70 71 35 46 74 21

Conditions: 30 °C, 17 h, S/C ) 100, AgSbF6/cat ) 4, H2 3.0 MPa, 2-propanol 2 mL, MS4A 400 mg. b Determined by 1H NMR. c Determined by HPLC. a

3). This accounts for the fact that transfer hydrogenation of 1a with 2-propanol should not be involved in this system. The concurrent deuteration of HC was caused by imine/enamine isomerization of the imine substrate prior to the hydrogenation. Actually, separate experiments without the Ir catalyst in a 2-propanol-d8 solution containing 0.1 M of the unlabeled 1a or 2a at 30 °C showed that no deuteration of 2a at the HA-HC positions took place after the 2 h-reaction, whereas a deuterium incorporation into the methyl group on the imine 1a with an H/D ratio of 29/71 was observed. The structure of the diamine ligand and the choice of the central metal (Chart 1) affected enantioselectivity as shown in Table 2. The hydrogenation with the Ir-Tsdpen complex, [Cp*IrCl{(S,S)-Tsdpen}]21,23 (3b) led to a stereochemical outcome comparable to that obtained with the Ir-Tscydn system (entries 1 and 2). Changing the sulfonyl substituent in the diamine ligands to pentamethylbenzenesulfonyl (PMs) (3c) and methanesulfonyl (Ms) groups (3d) caused a serious decrease in the ee values to 35% and 46%, respectively (entries 3 and 4). A similar level of enantioselectivity (74% ee) was attained with the related Ru-Tscydn complex, [RuCl{(S,S)-Tscydn}(η6p-cymene)] (3e), although the unreacted substrate remained after the reaction under the standard conditions. It should be noted that the Rh version, [Cp*RhCl{(S,S)-Tscydn}], (3f) was found to have poor reactivity and selectivity as shown in the hydrogenation profiles using the Ir and Rh catalysts (Figures 1 (23) Mashima, K.; Abe, T.; Tani, K. Chem. Lett. 1998, 27, 1199–1200.


Organometallics, Vol. 28, No. 3, 2009 805

Figure 1. Plot of yield versus reaction time for the Ir (3a) and Rh (3f) catalyst systems.

Figure 2. Time dependence of enantioselectivity for the Ir (3a) and Rh (3f) catalyst systems. Table 3. Asymmetric Hydrogenation of Imines Catalyzed by 3a with AgSbF6a

Figure 3. ORTEP view of [Cp*Ir(CH3CN){(S,S)-Tscydn}]+ SbF6- · CH2Cl2 (4 · CH2Cl2). The counteranion, solvent molecule, and the hydrogen atoms other than the amine protons are omitted for clarity, and the ellipsoids represent 50% probability. Selected bond distances (Å): Ir(1)-N(1), 2.124(2); Ir(1)-N(2), 2.154(2); Ir(1)-N(3), 2.079(2); N(3)-C(15), 1.125(4); C(15)-C(14), 1.464(5). Selected bond angles (deg): N(1)-Ir(1)-N(2), 78.54(10); N(2)Ir(1)-N(3),83.80(11);N(3)-Ir(1)-N(1),83.70(11);N(3)-C(15)-C(14), 177.2 (4). Scheme 4. Synthesis of Cationic Ir-Tscydn Complex 4

entry imine 1 2 3 4 5 6 7 8 9 10

1a 1b 1c 1d 1e 1f 1g 1h 1i 1j

R1

R2

C6H5 C6H5 C6H5 C6H5 C6H5 C6H5 4-CF3C6H4 2-naphthyl C6H5 C6H5

C6H5CH2 4-CF3C6H4CH2 4-ClC6H4CH2 4-CH3C6H4CH2 2-CF3C6H4CH2 4-CH3OC6H4CH2 C6H5CH2 C6H5CH2 CH2CH)CH2 C6H5

yield (%)b % ee (config)c 99 99 99 99 99 0 98 99 66 99

70(S) 78(S) 72(S) 61(S) 39(S) 40d 69(S) 65(S) 0

a Conditions: 30 °C, 17 h, S/C ) 100, AgSbF6/3a ) 4, H2 3.0 MPa, 2-propanol 2 mL, MS4A 400 mg. b Determined by 1H NMR. c Determined by HPLC. d The absolute configuration was not determined.

and 2). The ee value of 2a obtained with 3f decreased with the reaction time, possibly due to racemization24 of the product amine, in contrast to that obtained with the Ir catalyst, 3a. Using optimal reaction conditions for the asymmetric imine hydrogenation with the combined catalyst system of 3a and AgSbF6 under the standard conditions of 3.0 MPa and 30 °C, the substrate scope was examined. The hydrogenation was almost completed for the N-(1-arylethylidene)benzylamine derivatives listed in Table 3 to give the corresponding products in a range of 39-78% ee (entries 1-8) except for the reaction of 1f, which underwent elimination of the p-methoxybenzyl group (entry 7). The reaction of N-allyl imine afforded the desired amine product with a moderate ee, and the CdC bond or allyl-N bond remained intact during the reaction (entry 9). The catalytic system also promoted the hydrogenation of N-(1phenylethylidene)aniline, but not in an enantioselective manner (entry 10). (24) Dorta, R.; Broggini, D.; Kissner, R.; Togni, A. Chem.;Eur. J. 2004, 10, 4546–4555.

Synthesis and Properties of Cationic Cp*Ir(Tscydn) Complex as a Model for the Catalytic Intermediate. In order to gain further insight into the roles of the silver salt on the hydrogenation, we tried to isolate the cationic Ir species from a stoichiometric reaction of 3a and AgSbF6. The treatment of an acetonitrile solution of 3a with an equimolar amount of AgSbF6 at room temperature gave a pale yellow cationic complex, [Cp*Ir(CH3CN){(S,S)-Tscydn}]+SbF6- (4), in 66% isolated yield (Scheme 4). The isolated product was fully characterized by NMR spectroscopy, elemental analysis, and X-ray crystallographic analysis. The 1H NMR spectrum of 4 in CD2Cl2 exhibits a singlet signal due to the coordinated CH3CN at 2.29 ppm, in addition to two broad signals due to amine protons at 3.64 and 4.23 ppm. Figure 3 shows the ORTEP diagram of 4, along with the selected bond distances and angles. The cationic complex 4 displays distorted octahedral geometry with coordination of three nitrogen atoms of the bidentate (S,S)Tscydn and CH3CN. Whereas the bond distances between the Ir center and Tscydn are similar to those of the related Cp*IrTscydn complexes,21,25 4 has a slightly longer Ir-NCCH3 distance of 2.079(2) Å than that of the related [Cp*Ir(CH3CN)(Tsdpen)]+ complex, which has been recently reported by Rauchfuss and co-workers.26 A similar cationic carbonyl complex, [Cp*Ir(Tscydn)(CO)]+, was found to be crystallized as a pair of diastereomers arising from the chirality of the metal center.26b The crystal structure and 1H NMR

(25) Murata, K.; Konishi, H.; Ito, M.; Ikariya, T. Organometallics 2002, 21, 253–255.


Shirai et al.

Table 4. Comparison of Neutral and Cationic Catalyst Precursors Bearing (S,S)-Tscydna entry

Ir cat

Ag/Ir

yield (%)b

ee (%)c

1 2 3 4 5 6

3a 3a 4 3a 4 5

0 1 0 4 3 3

99 99 99 99 99 99

41 59 57 70 73 73

Scheme 6. Stoichiometric Reaction of 6 and H2

Scheme 7. Reduction of Acetophenone with 3a

Conditions: 30 °C, 17 h, S/C ) 100, H2 3.0 MPa, 2-propanol 2 mL, MS4A 400 mg. b Determined by 1H NMR. c Determined by HPLC. a

Scheme 5. Formation of Hydridoiridium Complex 5

spectrum in a CD2Cl2 solution revealed that the cationic (S,S)Tscydn complex 4 has the (R)-Ir configuration and exists as a single diastereomer in solution. The catalyst performance of the preformed cationic complex 4 for the hydrogenation was comparable to that attained with in situ-generated catalytic species from 3a with an equimolar amount of AgSbF6 as listed in Table 4 (entries 2 and 3). Again, the positive effect of the silver salt on the enantioselectivity of the reaction with 4 was observed, the ee value of (S)-2a being increased up to 73% by extra addition of the silver salt to 4 (Ir:Ag ) 1:3), as attained with the combined system of 3a and AgSbF6 (entries 4 and 5). These results clearly indicate that the additive, AgSbF6, should have a role to enhance the enantioselectivity of the hydrogenation in addition to generate the cationic active intermediate from the neutral catalyst precursor 3a (Vide infra). Notably, the hydrogenation catalyzed by a well-defined complex, [Cp*Ir(H){(S,S)-Tscydn}] (5) also led to a similar result to that obtained with 4 (Table 4, entry 6). The addition of the silver salt is also crucial for activation of H2 on the present bifunctional catalyst. NMR monitoring of reaction of the cationic complex 4 with H2 (0.1 MPa) in the presence of an equimolar amount of N(C2H5)3 in THF-d8 at room temperature showed a rapid increase in the intensity of a singlet signal at -11.2 ppm ascribed to the hydrido-Ir complex 5 (Scheme 5). In contrast, the parent neutral complex 3a did not react with H2 to give the corresponding hydride complexes under these conditions. The difference in the reactivity of the cationic and neutral complexes toward H2 might be attributed to a coordination site available for the formation of an η2-hydrogen complex, which undergoes heterolytic cleavage of molecular hydrogen on the metal center with the aid of the base,27 leading to the hydrido intermediate. The cationic complexes with weakly (26) Analogous cationic Ir complexes bearing Tsdpen ligand exist as a coordinatively unsaturated species with a planar geometry and its Lewis base adducts with a three-legged piano-stool structure: (a) Heiden, Z. M.; Rauchfuss, T. B. J. Am. Chem. Soc. 2006, 128, 13048–13049. (b) Heiden, Z. M.; Gorecki, B. J.; Rauchfuss, T. B. Organometallics 2008, 27, 1542– 1549. (27) (a) Chu, H. S.; Lau, C. P.; Wong, K. Y.; Wong, W. T. Organometallics 1998, 17, 2768–2777. (b) Lee, D. H.; Patel, B. P.; Clot, E.; Eisenstein, O.; Crabtree, R. H. Chem. Commun. 1999, 297–298. (c) Custelcean, R.; Jackson, J. E. Chem. ReV. 2001, 101, 1963–1980. (d) Gruet, K.; Clot, E.; Eisenstein, O.; Lee, D. H.; Patel, B. P.; Macchioni, A.; Crabtree, R. H. New J. Chem. 2003, 27, 80–87. (e) Jaloń, F. A.; Manzano, B. R.; Caballero, A.; Carrioń, M. C.; Santos, L.; Espino, G.; Moreno, M. J. Am. Chem. Soc. 2005, 127, 15364–15365.

bonded ligands served as the efficient catalysts for hydrogenation as previously discussed by Ru and Ir complexes.19,28 It should be noted that an isolable amido-Ir complex (6), derived from (S,S)-Tsdpen, did not readily react with atmospheric pressure of hydrogen gas in THF-d8 at 30 °C within 15 min, in contrast to reactivity of some other amido metal complexes.29 However, 6 was transformed to the corresponding hydrido(amine) complex, [Cp*IrH{(S,S)-Tsdpen}] (7)30 with 25% conversion under the forced conditions using pressurized hydrogen (3.0 MPa) for 10 min. Elongation of the reaction time to 17 h was required for the complete conversion of 6 to 7 (Scheme 6). These results are consistent with the relatively poor catalytic activity of the amido complex generated in situ as discussed in Table 1 (run 2). Thus, the cationic species generated by the treatment of the neutral complexes with AgSbF6 can be easily converted to the metal hydrides possibly through the heterolytic cleavage of H2 on the metal center under mild conditions. The facile heterolytic splitting of coordinating hydrogen with an external base was also demonstrated by asymmetric hydrogenation of acetophenone with 3a and AgSbF6. Although the reaction with a substrate/AgSbF6/catalyst ratio of 100:4:1 in the presence of molecular sieves 4A under 3.0 MPa of H2 at 30 °C for 17 h gave no reduction product at all, the reaction in the presence of the external base, N(C2H5)3, (amine/catalyst ) 10) under otherwise identical conditions proceeded to give (S)-1phenylethanol in 47% yield and 96% ee (Scheme 7). In the imine hydrogenation, the base substrates and/or products would participate in activation of the coordinated H2 molecule as discussed previously, leading to catalytically active hydrido intermediates. Unprecedented Effect of Silver Salts. As discussed above, AgSbF6 has two major roles, including the facile activation of H2 gas with the cationic complexes and the marked improvement in the enantioselectivity in the imine hydrogenation. To evaluate the effect of the extra amount of AgSbF6 on the hydrogenation, the stereochemical outcome of the reaction in the presence of other silver salts was examined. As shown in Table 5, the (28) (a) Martıń, M.; Sola, E.; Tejero, S.; Andre´s, J. L.; Oro, L. A. Chem.-Eur. J. 2006, 12, 4043–4056. (b) Kayaki, Y.; Ikeda, H.; Tsurumaki, J.; Shimizu, I.; Yamamoto, A. Bull. Chem. Soc. Jpn. 2008, 81, 1053–1061. (29) (a) Fryzuk, M. D.; Montgomery, C. D.; Rettig, S. J. Organometallics 1991, 10, 467–473. (b) Abdur-Rashid, K.; Clapham, S. E.; Hadzovic, A.; Harvey, J. N.; Lough, A. J.; Morris, R. H. J. Am. Chem. Soc. 2002, 124, 15104–15118. (c) Sandoval, C. A.; Ohkuma, T.; Mun˜iz, K.; Noyori, R. J. Am. Chem. Soc. 2003, 125, 13490–13503. (d) Maire, P.; Bu¨ttner, T.; Breher, F.; Le Floch, P.; Gru¨tzmacher, H. Angew. Chem., Int. Ed. 2005, 44, 6318–6323. (e) Hedberg, C.; Ka¨llstro¨m, K.; Arvidsson, P. I.; Brandt, P.; Andersson, P. G. J. Am. Chem. Soc. 2005, 127, 15083–15090. (30) Mashima, K.; Abe, T.; Tani, K. Chem. Lett. 1998, 27, 1201–1202.


Organometallics, Vol. 28, No. 3, 2009 807

Table 5. Effect of Additives on the Catalyst Performancea entry

additive

yield (%)b

ee (%)c

1 2 3 4 5 6 7 8

AgSbF6 AgPF6 AgOTf Ag2CO3 Ag2O Ag3PO4 AgOCOCH3 HBF4

99 99 99 99 99 99 99 90

73 71 70 67 65 58 50 41

a Conditions: 30 °C, 17 h, S/C ) 100, additive/3a ) 3, H2 3.0 MPa, 2-propanol 2 mL, MS4A 400 mg. b Determined by 1H NMR or GC. c Determined by HPLC.

enantiomeric excesses of 2a obtained by the hydrogenation of 1a with the cationic complex 4 in the presence of silver salts (Ir:Ag ) 1:3) were ranged from 50% to 73%. Although the silver salts with PF6 and OTf anions provided comparable enantioselectivities, the counteranion with stronger coordinating ability resulted in deterioration of the ee value of 2a. The addition of HBF4 · O(CH3)2 as the acid activator leading to a reactive iminium ion caused no significant effect on the outcome the reaction. (Table 5 entry 8). These results might be explained by the activation of the imine substrate through its coordination to Ag+ bearing weakly coordinating anions. In fact, 13C{1H} NMR experiments using a THF-d8 solution of the substrate 1a revealed that a characteristic downfield shift of the signal ascribed to the imine carbon from 165.8 ppm to 179.1 ppm was observed by addition of an equimolar amount of AgSbF6, implying a strong interaction of the imine nitrogen with the Lewis acidic silver center (Figure 4). Mechanistic Consideration. On the basis of the additive effect on the catalyst performance and the reactivity of the isolable cationic complex 4, a possible mechanism for the imine hydrogenation with the present cationic bifunctional catalyst was envisaged as illustrated in Scheme 8. The first key step should be the activation of H2 with the cationic active catalyst 4: As mentioned with the role of additional triethylamine, which participates in the activation of H2, the coordination of H2 to the cationic species A affords the η2-H2 complex B. The resulting complex B undergoes the intermolecular heterolytic splitting by the basic compounds, including the imine substrate and/or amine product to generate the hydrido species C. The involvement of both the cationic and the hydrido intermediates in the catalytic cycle is compatible with the fact that a series of the binary system of 3a/Ag+, the cationic complex 4, and the hydrido complex 5 catalyze the imine hydrogenation with identical performance. The active species C should reduce the imine substrates via an iminium intermediate D activated by the Lewis acidic Ag center. The electrophilic nature of the highly polarized imine fragment in D is susceptible to the hydride attack from the intermediate C, which releases the amido-Ag species E to reproduce the cationic intermediate A.4c,31 The proton exchange of E with H+ · base affords the product amines with the concomitant formation of activated imine by the Ag+ center. Whereas the origin of the enantioface discrimination still remains unclarified, the formation of the N-Ag bond facilitates the hydride attack to the Re face of the imine to achieve higher enantioselection. The enhancement of enantioselectivity in the hydrogenation with excess AgSbF6 implies the presence of the competitive Ag-free pathway, exhibiting less selectivity. (31) Guan, H.; Iimura, M.; Magee, M. P.; Norton, J. R.; Zhu, G. J. Am. Chem. Soc. 2005, 127, 7805–7814.

Conclusions In summary, we have shown appreciable positive effects of silver salts on the bifunctional catalyst-promoted asymmetric hydrogenation of acyclic imines. The use of AgSbF6 as the additive has proven to remarkably improve catalyst performance in terms of both the activity and enantioselectivity. The isolable novel cationic Cp*Ir-Tscydn complex 4, obtained from the reaction of [Cp*IrCl(Tscydn)] 3a and AgSbF6, readily reacted with H2 in the presence of an external base under mild conditions to give the hydrido(amine) complex, and exhibited comparable catalyst performance to that attained with in situgenerated catalytic species from 3a with an equimolar amount of AgSbF6. The NMR study of the solution of 1a and AgSbF6 indicated that the silver salts can activate the imines by the appreciable interaction between the imine nitrogen and the Lewis acidic silver center. Thus, the addition of the silver salts is crucial for the facile formation of the active catalyst from the neutral catalyst precursor and effective activation of H2 and the substrate imine. The characteristic dual roles of silver salts presented here may have an extensive potential to delicately tune the outcome of imine hydrogenation.

Experimental Section General. Reactions requiring air-sensitive manipulations were conducted under argon atmosphere following standard Schlenk techniques. Dichloromethane and acetonitrile were distilled under argon after drying over P2O5. Diethyl ether, THF, and toluene were dried and deoxygenated by refluxing and distilling from sodium benzophenone ketyl under argon. Alcoholic solvents including 2-propanol, methanol, ethanol, and tBuOH were distilled and dried over CaH2 under argon and stored in Schlenk tubes in the presence of molecular sieves. Other reagents were purchased and used as delivered unless otherwise noted. Cp*IrCl[(S,S)-Tscydn] (3a),21 Cp*IrCl[(S,S)-Tsdpen] (3b),21 [RuCl{(S,S)-Tscydn}(η6-p-cymene)] (3e),32 Cp*RhCl[(S,S)-Tscydn] (3f),21 Cp*IrH[(S,S)-Tscydn] (5),21 and Cp*Ir[(S,S)-TsNCH(C6H5)CH(C6H5)NH] (6)30 were prepared according to the literature procedures with modifications. 1H, 13C, and 2H NMR spectra were recorded on JEOL JNM-LA300 and JNM-ECX400 spectrometers. Chemical shifts in 1H and 13C spectra were referenced to SiMe4 by using the residual signals of solvents. 2 H NMR shifts were referenced to Si(CD3)4 via residual solvent deuterium. Analytical gas chromatography was performed with a GL Science GC-353 equipped with a G-100 column (1.2 mm i.d. × 40 m), using N2 as carrier gas. Enantiomeric excesses of chiral compounds were determined by HPLC analysis using a Daicel Chiralcel OD or AD-H column with hexane/2-propanol as the eluent where baseline separation was obtained. Preparation of Cp*IrCl[(S,S)-PMscydn] (3c). To a mixture of [Cp*IrCl2]2 (198.6 mg, 0.249 mmol) and (1S,2S)-N-pentamethylbenzenesulfonyl-1,2-cyclohexanenediamine (PMscydn; 163.6 mg, 0.504 mmol) in CH2Cl2 (20 mL), triethylamine (0.083 mL, 0.599 mmol) was added. After the reaction mixture was stirred for 16 h at room temperature, the solvent was removed under reduced pressure. The resulting solid was washed with water and then with ether, dried under vacuum, and isolated as a yellow powder (127.8 mg, 37% yield): mp 203-205 °C (dec). 1H NMR (300.4 MHz, CD2Cl2): δ 0.82-2.59 (br, 9H, cyclohexane) 1.68 (s, 15H, C5(CH3)5), 2.20 (s, 6H, C6(CH3)5), 2.25 (s, 3H, C6(CH3)5), 2.73 (s, 6H, C6(CH3)5), 2.83 (m, 1H, CHNSO2), 3.62 (br, 1H, NH), 3.88 (br, 1H, NH). Anal. Calcd for C27H42ClIrN2O2S: C, 47.25; H, 6.17; N, 4.08. Found: C, 47.19; H, 6.36; N, 3.84. Preparation of Cp*IrCl[(S,S)-Msdpen] (3d). To a mixture of [Cp*IrCl2]2 (113 mg, 0.142 mmol) and (1S,2S)-N-methanesulfonyl(32) Canivet, J.; Labat, G.; Stoeckli-Evans, H.; Su¨ss-Fink, G. Eur. J. Inorg. Chem. 2005, 4493–4500.


Shirai et al.

Figure 4. A section of 13C{1H} NMR spectra of 1a and a 1:1 mixture of 1a and AgSbF6 in THF-d8. Scheme 8. A Plausible Hydrogenation Mechanism for the Ir-Tscydn System

1,2-diphenylethylenediamine (Msdpen; 82.2 mg, 0.283 mmol) in CH2Cl2 (10 mL), triethylamine (79 µL, 0.57 mmol) was added. After the reaction mixture was stirred for 3 h at room temperature, the solvent was removed under reduced pressure. The resulting solid was extracted with a mixture of CH2Cl2 and ether. Removal of solvents gave the crude product. Recrystallization from THF and ether afforded a yellow powder (43.0 mg, 46% yield): mp 194-196 °C (dec). 1H NMR (399.8 MHz, CDCl3): δ 1.77 (s, 15H, C5(CH3)5), 2.42 (s, 3H, CH3SO2), 3.75 (m, 1H, CHNH2), 4.10 (br, 1H, NH), 4.52 (d, JHH ) 11.2 Hz, 1H, CHNSO2), 4.58 (br, 1H, NH), 6.95-7.24 (m, 10H, C6H5). 13C{1H} NMR (75.6 MHz, CDCl3): δ9.4 (C5(CH3)5), 43.7 (CH3SO2N), 69.2 (NCH2), 73.7 (NCH2), 85.5 (C5(CH3)5), 127.0 (C6H5), 127.4 (C6H5), 128.1 (C6H5), 128.4 (C6H5), 128.78 (C6H5), 128.84 (C6H5), 138.4 (C6H5), 140.5 (C6H5). Anal. Calcd for C25H32ClIrN2O2S: C, 46.03; H, 4.94; N, 4.29. Found: C, 46.24; H, 5.36; N, 3.89. Preparation of [Cp*Ir{(S,S)-Tscydn}(CH3CN)]+SbF6- (4). To an acetonitrile solution (5.0 mL) of 3a (500 mg, 0.793 mmol), AgSbF6 (272 mg, 0.794 mmol) was added at room temperature. After the reaction mixture was stirred for 12 h at room temperature, a yellow suspension was filtrated through a celite pad and dried under reduced pressure. The remaining solid was recrystallized from CH2Cl2 and CH3CN to give pale yellow crystals as CH2Cl2 solvate (460 mg, 73% yield): mp 116-118 °C (dec). 1H NMR (399.8 MHz, CD2Cl2): δ 0.66-0.79 (m, 1H, cydn), 0.92-1.16 (m, 2H, cydn), 1.30-1.44 (m, 2H, cydn), 1.51-1.60 (m, 1H, cydn), 1.73 (s, 15H, C5(CH3)5), 1.88-1.96 (m, 1H, cydn), 2.08-2.16 (m, 1H, cydn), 2.21-2.33 (m, 1H, cydn), 2.40 (s, 3H, CH3CN), 2.44 (s, 3H, CH3C6H4), 2.68 (ddd, JHH ) 3.5 10.7 Hz, and 10.7 Hz, 1H, CHNTs), 3.68 (dd, JHH ) 10.7 Hz, 1H, NH), 4.39 (d, JHH ) 10.7 Hz, 1H, NH), 7.26 (AA’BB’ pattern, JHH ) 8.0 Hz, 2H, C6H4), 7.68

(AA’BB’ pattern, JHH ) 8.0 Hz, 2H, C6H4). 13C{1H} NMR (100.5 MHz, CD2Cl2): δ3.6 (CH3CN), 9.4 (C5(CH3)5), 21.4 (CH3C6H4), 24.8 (cydn), 25.0 (cydn), 33.4 (cydn), 34.9 (cydn), 66.6 (cydn), 66.7 (cydn), 89.6 (C5(CH3)5), 120.8 (CH3CN), 126.8 (C6H4), 129.5 (C6H4), 142.0 (C6H4), 143.7 (C6H4). Anal. Calcd for C26H39Cl2F6IrN3O2SSb: C, 32.65; H, 4.11; N, 4.39. Found: C, 32.49; H, 4.19; N, 4.40. Hydrogenation of Imines. In a typical experiment, a 50-mL stainless steel autoclave equipped with a pressure gauge and a magnetic stirrer was charged with the imine substrate (1.00 mmol) and molecular sieves 4A (0.4 g) under argon atmosphere. After loading of a mixture of the catalyst precursor (1.0 × 10-2 mmol) and additives (silver salts) in 2-propanol (2 mL), the autoclave was flushed with H2 and then pressurized to 3.0 MPa. The reaction mixture was stirred in an oil bath at 30 °C for 17 h. After carefully venting hydrogen, a sample of the resulting mixture was passed through a small amount of alumina and subjected to GC (using tridecane as internal standard), NMR, and HPLC analyses. In Situ NMR Experiments for Reaction of Cationic Complex 4 and H2. Complex 4 (10.9 mg, 1.3 × 10-2 mmol) and N(C2H5)3 (1.8 µL, 1.3 × 10-2 mmol) was dissolved in 0.50 mL of THF-d8 in a 5 mm NMR tube. H2 was gently bubbled into the solution with a long syringe needle for 5 min. The 1H NMR spectrum, immediately acquired at room temperature, showed the formation of 5. Reaction of Amidoiridium Complex 6 and H2. A solution of Cp*Ir[(S,S)-Tsdpen] 6 (15.6 mg, 2.3 × 10-2 mmol) in THF-d8 (0.5 mL) was placed into a 50-mL stainless steel autoclave equipped with a pressure gauge and a gas inert tube. H2 (3.0 MPa) was introduced into the autoclave. The reaction mixture was stirred at 30 °C for 17 h. After carefully venting hydrogen, the resulting

Effect of SilVer Salts on Asymmetric Hydrogenation solution was transferred to a 5 mm NMR tube under argon atmosphere. 1H NMR, immediately acquired at room temperature, showed the formation of 7. Asymmetric Hydrogenation of Acetophenone with 3a. A 50mL stainless steel autoclave equipped with a pressure gauge and a magnetic stirrer was charged with acetophenone (120.2 mg, 1.00 mmol) and molecular sieves 4A (0.4 g) under argon atmosphere. After loading of a mixture of 3a (6.3 mg, 1.0 × 10-2 mmol), AgSbF6 (13.7 mg, 4.0 × 10-2 mmol), and triethylamine (14 µL, 0.10 mmol) in 2-propanol (2 mL), the autoclave was flushed with H2 and then pressurized to 3.0 MPa. The reaction mixture was stirred in an oil bath at 30 °C for 17 h. After carefully venting hydrogen, the resulting mixture was passed through Florisil and subjected to NMR and HPLC analyses. In Situ NMR Experiments for Reaction of 1a and AgSbF6. AgSbF6 (16.6 mg, 4.8 × 10-2 mmol) was added under a stream of Ar to a 5 mm NMR tube containing a THF-d8 (0.45 mL) solution of 1a (9.9 mg, 4.7 × 10-2 mmol). Then NMR analysis was immediately carried out at room temperature. X-ray Structure Determination of 4. Data were corrected on a Rigaku Saturn CCD area detector equipped with graphitemonochromated Mo-KR radiation (λ ) 0.71070 Å) under nitrogen stream at 93 K. Indexing was performed from seven images. The crystal-to-detector distance was 45.04 mm. The data were collected to a maximum 2θ value of 55.0°. A total of 720 oscillation images were collected. A sweep of data was carried out using ω scans from -110.0 to 70.0° in 0.5° steps, at χ ) 45.0° and φ ) 0.0°. A second sweep was performed using ω scans from -110.0 to 70.0° in 0.5° steps, at χ ) 45.0° and φ ) 90.0°. Intensity data were collected for Lorentz-polarization effects as well as absorption. Structure solution and refinements were performed with the Crystal Structure program package. The heavy atom positions were determined by a direct program method (SIR2002), and the remaining non-hydrogen atoms were found by subsequent Fourier techniques (DIRDIF99). An empirical absorption correction based on equivalent reflections was applied to all data. All non-hydrogen atoms were refined anisotropically by full-matrix least-squares

Organometallics, Vol. 28, No. 3, 2009 809 Table 6. Crystallographic Data for 4 · CH2Cl2a empirical formula formula weight crystal color crystal system space group a, Å b, Å c, Å V, Å3 Z Dcalcd, g cm-3 F000 µ, cm-1 (Mo KR) exposure rate, sec/° no. of reflections measured no. of unique reflections no. variables R1(I > 2.00σ(I)) wR2 (All reflections) GOF on F2 Flack parameter a

C26H39Cl2F6IrN3O2SSb 956.54 pale yellow orthorhombic P212121 (#19) 10.2343(18) 13.632(3) 23.778(5) 3317.5(11) 4 1.915 1856.00 51.177 4.0 26001 7515 419 0.0351 0.1035 1.000 0.023(3)

R1 ) Σ |Fo| - |Fc|/Σ |Fo|; wR2 ) [Σ(w(Fo2 - Fc2)2)/ Σw(Fo2)2]1/2.

techniques based on F2. All hydrogen atoms were constrained to ride on their parent atom. Relevant crystallographic data are compiled in Table 6.

Acknowledgment. This work was financially supported by a grant-in-aid from the Ministry of Education, Science, Sports and Culture of Japan (No. 18065007) and partially supported by The 21st Century COE and G-COE Programs. Supporting Information Available: Experimental details for preparation and characterization of 1 and 2 and X-ray crystallographic data of 4. This material is available free of charge via the Internet at http://pubs.acs.org. OM800926P

Remarkable Positive Effect of Silver Salts on Asymmetric

Recommend Documents