134
Organometallics 2010, 29, 134–141 DOI: 10.1021/om900894k
Synthesis of Electron-Rich CNN-Pincer Complexes, with N-Heterocyclic Carbene and (S)-Proline Moieties and Application to Asymmetric Hydrogenation Merce Boronat,† Avelino Corma,† Camino Gonzalez-Arellano,‡,§ Marta Iglesias,*,‡ and Felix Sanchez*,§ †
Instituto de Tecnologı´a Quı´mica, CSIC-UPV, Avenida de los Naranjos s/n, 46022 Valencia, Spain, ‡Instituto de Ciencia de Materiales de Madrid, CSIC, C/ Sor Juana In es de la Cruz 3, Cantoblanco 28049 Madrid, Spain, and §Instituto de Quı´mica Org anica, CSIC, C/ Juan de la Cierva 3, 28006 Madrid, Spain Received October 13, 2009
New chiral CNN-pincer-type gold, palladium, and rhodium complexes containing N-heterocyclic carbene substituent and (S)-N-tert-butyl-methylpyrrolidine-2-carboxamide as chiral auxiliary have been synthesized and studied for asymmetric hydrogenation. The complexes were prepared by the silver carbene transfer route from the respective silver complex. The reaction with [RhCl(cod)]2 (cod = cycloocta-1,5-diene), PdCl2(CH3CN)2, or K[AuCl4] affords the corresponding cationic [Rh(cod)(ligand)]Cl, [PdCl(ligand)]Cl, and [AuCl(ligand)]Cl2 complexes in which the ligand functions effectively in a CNN coordination mode. The complexes catalyze the enantioselective hydrogenation of prochiral alkenes. Enantioselectivity is very sensitive to the NHC N-substituent, resulting in a useful switch in the predominant enantiomer.
Introduction The chemistry of N-heterocyclic carbenes (NHCs) has developed significantly over recent years, encompassing their synthesis, reactivity, coordination chemistry, and application.1 Furthermore, the ease of functionalization of the imidazolium salt proligands led to incorporation of N-heterocyclic carbene donors in polydentate ligand structures, usually in combination with other classical donors. *Corresponding authors. (M.I.) E-mail:
[email protected]. es. Tel: þ34 913349000. Fax: þ34 913720623. (F.S.) E-mail: felix-iqo@ iqog.csic.es. Tel: þ34 915622900. Fax: þ34 915644853. (1) (a) Glorius, F. N-heterocyclic Carbenes (NHC) in Transition Metal Catalysis (Topics in Organometallic Chemistry); Springer-Verlag: Berlin, 2006. (b) Nolan, S. P. N-Heterocyclic Carbenes in Synthesis; Wiley-VCH, 2006. (c) Bourissou, D.; Guerret, O.; Gabbai, F. P.; Bertrand, G. Chem. Rev. 2000, 100, 39. (d) Hahn, F. E.; Jahnke, M. C. Angew. Chem., Int. Ed. 2008, 47, 3122. (e) Nair, V.; Bindu, S.; Sreekumar, V. Angew. Chem., Int. Ed. 2004, 43, 5130. (f) Marion, N.; Diez-Gonzalez, S.; Nolan, I. P. Angew. Chem., Int. Ed. 2007, 46, 2988. (g) Fremont, P. de; Marion, N.; Nolan, S. P. Coord. Chem. Rev. 2009, 253, 862. (2) (a) Hu, X. L.; Castro-Rodrı´ guez, I.; Meyer, K. Organometallics 2003, 22, 3016. (b) Mas-Marza, E.; Poyatos, M.; Sanau, M.; Peris, E. Organometallics 2004, 23, 323. (c) Hahn, F. E.; Langenhahn, V.; L€ugger, T.; Pape, T.; LeVan, D. Angew. Chem., Int. Ed. 2005, 44, 3759–3763. (d) McKie, R.; Murphy, J. A.; Park, S. R.; Spicer, M. D.; Zhou, Sh.-Z. Angew. Chem., Int. Ed. 2007, 46, 6525–6528. (e) Hahn, F. E.; Radloff, Ch. Chem.Eur. J. 2008, 14, 10900–10904. (f) Kaufhold, O.; Stasch, A.; Pape, T.; Hepp, A.; Edwards, P. G.; Newman, P. D.; Hahn, F. E. J. Am. Chem. Soc. 2009, 131, 306–317. (3) (a) Peris, E.; Loch, J. A.; Mata, J.; Crabtree, R. H. Chem. Commun. 2001, 201. (b) Tulloch, A. A. D.; Danopoulos, A. A.; Tizzard, G. J.; Coles, S. J.; Hursthouse, M. B.; Hay-Motherwell, R. S.; Motherwell, W. B. Chem. Commun. 2001, 1270. (c) Chen, J. C. C.; Lin, I. J. B. Dalton Trans. 2000, 839–840. (d) Corberan, R.; Mas-Marza, E.; Peris, E. Eur. J. Inorg. Chem. 2009, 1700–1716. (e) Hahn, F. E.; Jahnke, M. C.; Gomez-Benitez, V.; Morales-Morales, D.; Pape, T. Organometallics 2005, 24, 6458–6463. (f) Hahn, F. E.; Jahnke, M. C.; Pape, T. Organometallics 2006, 25, 5927–5936. pubs.acs.org/Organometallics
Published on Web 12/11/2009
Among the polydentate ligand structures with carbene donors, tripodal2 and pincer systems3 have attracted much attention. Seminal work, with pincer ligands bearing phosphine and amine donors was carried out by Shaw,4 van der Boom and Milstein,5 Albrecht and van Koten,6 and others. The incorporation of NHC donors led to the synthesis of several kinds of NHC-containing pincer-type ligands: neutral3a,7 and monoanionic,8 with the NHC moiety as lateral donor function or as the backbone.9 The synthesis of new rigid tridentate ligands with modified topologies, functional groups, chirality, and hemilability is a challenge for future developments in these areas, and there are relatively few chiral pincer ligands known.10 In most of the reported systems one or more C-centered stereogenic centers are incorporated into the pincer framework via a chiral auxiliary. Gold complexes can have high activity in either homogeneous or heterogeneous catalysis, and a better understanding (4) Moulton, C. J.; Shaw, B. L. Dalton Trans. 1976, 1020–1024. (5) van der Boom, M. E.; Milstein, D. Chem. Rev. 2003, 103, 1759– 1792. (6) Albrecht, M.; van Koten, G. Angew. Chem., Int. Ed. 2001, 40, 3750–3781. (7) Gr€ undemann, S.; Albrecht, M.; Loch, J. A.; Faller, J. W.; Crabtree, R. H. Organometallics 2001, 20, 5485–5488. (8) (a) Andavan, G. T. S.; Bauer, E. B.; Letko, C. S.; Hollis, T. K.; Tham, F. S. J. Organomet. Chem. 2005, 690, 5938–5947. (b) Rubio, R. J.; Andavan, G. T. S.; Bauer, E. B.; Hollis, T. K.; Cho, J.; Tham, F. S.; Donnadieu, B. J. Organomet. Chem. 2005, 690, 5353–5364. (c) Hahn, F. E.; Jahnke, M. C.; Pape, T. Organometallics 2007, 26, 150–154. (9) Pugh, D.; Danopoulos, A. A. Coord. Chem. Rev. 2007, 251, 610– 641. (10) (a) The Chemistry of Pincer Compounds; Morales-Morales, D., Jensen, C. M., Eds.; Elsevier: Amsterdam, 2007. (b) Gosiewska, S.; Herreras, S. M.; Lutz, M.; Spek, A. L.; Havenith, R. W. A.; van Klink, G. P. M.; van Koten, G.; Gebbink, R. J. M. K. Organometallics 2008, 27, 2549–2557. r 2009 American Chemical Society
Article
of the role of ligands for these metal ions is required.11 In particular, ligands are needed that can prevent easy decomposition to gold metal while maintaining high catalytic activity and that can facilitate mechanistic studies of the catalytic reactions. For gold(III) catalysts, these ligands should also stabilize the higher oxidation state, thus ruling out soft donor ligands such as tertiary phosphines that tend to favor gold(I).12 Many simple nitrogen-donor ligands are easily displaced from gold(III), and decomposition to metallic gold tends to occur.12 Gold chemistry has the potential for reactions such as hydrogenation of unsaturated groups (ketones, imines, etc.) or C-H activation.11,13 However, to date no studies of gold complexes with pincer ligands bearing an N-heterocyclic carbene moiety have been conducted. Our research is focused on the synthesis of neutral unsymmetrical pyridine pincer-type ligands with a lateral (S)-N-tert-butyl-methylpyrrolidine-2-carboxamide donor function and an N-heterocyclic carbene moiety. Previously, symmetrical NCN-pincer complexes with two ester prolinates or prolinols as the nitrogen donor substituents have been described.14,10b We also have reported the synthesis of chiral tridentate (1a, 1b in Figure 1)15 and pincer-type ONN (2a, 2b) (Figure 1), which contain Ophenol, Npyridil, and chiral Ncarboxamide donor group derivatives of the natural amino acid (S)-proline as N-donor groups on the pincer backbone, resembling coordination environments present in previously described Schiff base ligands (1a, 1b) (Figure 1).16 These are commercially available and enantiomerically pure building blocks for the synthesis of chiral ligands, and their stereogenic information can be maintained by choosing the appropriate reaction conditions during the synthesis and purification of the complexes. The stereogenic center (SC) at the prolinate ring plays an important role, together with the bulkiness of the ester group, in the introduction of stereogenicity on the nitrogen atoms (SN or RN) upon coordination to the metal center. We have now incorporated an N-heterocyclic carbene moiety (Figure 1, ligands 3, 4) into the CNN-pincer backbone. The purpose of incorporating a NHC functionality is to obtain unsymmetrical CNN pincer-type ligands, 2-[(3-aryl-2,3-dihydro-1H-imidazol-1-yl)methyl]6-(pyrrolidin-1-ylmethyl)pyridine (Figure 1, 3a, 4a) and chiral (2S)-N-tert-butyl-1-((6-((3-aryl-2,3-dihydro-1H-imidazol-1-yl)methyl)pyridin-2-yl)methyl)pyrrolidine-2-carboxamide (11) (a) Hashmi, A. S. K.; Hutchings, G. J. Angew. Chem., Int. Ed. 2006, 45, 7896–7936. (b) Bond, G. C.; Louis, C.; Thompson, D. T. Catalysis by Gold; Imperial College Press: London, 2006. (c) Dyker, G. Angew. Chem., Int. Ed. 2000, 39, 4237-4239, and references therein. (d) Ivanova, S.; Petit, C.; Pitchon, V. Gold Bull. 2006, 39, 3–8. (e) Corma, A.; Serna, P. Science 2006, 313, 332. (f) Gonzalez-Arellano, C.; Corma, A.; Iglesias, M.; Sanchez, F. Chem. Commun. 2005, 3451–3453. (12) (a) Schmidbaur, H. Gold: Progress in Chemistry, Biochemistry and Technology; Wiley: Chichester, U.K., 1999. (b) Puddephatt, R. J. The Chemistry of Gold; Elsevier: Amsterdam, 1978. (13) Clapham, S. E.; Hadzovic, A.; Morris, R. H. Coord. Chem. Rev. 2004, 248, 2201–2237. (14) (a) Gosiewska, S.; Veld, M. H.; Pater, J. J. M. de; Bruijnincx, P. C. A.; Lutz, M.; Spek, A. L.; van Koten, G.; Gebbink, R. J. M. K. Tetrahedron: Asymmetry 2006, 17, 674–686. (b) Gosiewska, S.; Martinez, S. H.; Lutz, M.; Spek, A. L.; van Koten, G.; Gebbink, R. J. M. K. Eur. J. Inorg. Chem. 2006, 22, 4600–4607. (15) Gonz alez-Arellano, C.; Gutierrez-Puebla, E.; Iglesias, M.; S anchez, F. Eur. J. Inorg. Chem. 2004, 1955–1962. (16) (a) Debono, N.; Iglesias, M.; Sanchez, F. Adv. Synth. Catal. 2007, 349, 2470–2476. (b) del Pozo, C.; Debono, N.; Corma, A.; Iglesias, M.; Sanchez, F. ChemSusChem 2009, 2, 650–657.
Organometallics, Vol. 29, No. 1, 2010
135
Figure 1
(Figure 1, 3b, 4b). These ligands react with gold, palladium, and rhodium precursors to afford square-planar gold(III), palladium(II), and pentacoordinate rhodium(I) complexes. We have also studied the catalytic activity of the resulting products for hydrogenation reactions.
Results and Discussion Preparation of Imidazolium Salts. The new neutral N-heterocyclic carbene pincer-type ligands were designed with the objective of stabilizing metal complexes with enhanced catalytic properties. In this regard two anionic ONN-tridentate unsymmetrical pincer-type ligands, 2-(6-(pyrrolidin-1ylmethyl)pyridin-2-yl)phenol (2a) and N-tert-butyl-1-((6-(2hydroxyphenyl)pyridin-2-yl)methyl)pyrrolidine-2-carboxamide (2b),16 have been recently reported, and their gold and Pd complexes showed activity toward several organic reactions. In this work we have obtained the unsymmetrical CNN pincer-type ligands 2-[(3-aryl-2,3-dihydro-1H-imidazol-1-yl)methyl]-6-(pyrrolidin-1-ylmethyl)pyridine (3a, 4a) and (2S)-N-tert-butyl-1-((6-((3-aryl-2,3-dihydro-1H-imidazol-1-yl)methyl)pyridin-2-yl)methyl)pyrrolidine-2-carboxamide (3b, 4b). Specifically, the new 1-aryl-3-{[6-(pyrrolidin1-ylmethyl)pyridin-2-yl]methyl}-2,3-dihydro-1H-imidazol-3-ium bromide ([3a]Br, [3b]Br) were prepared in two successive nucleophilic substitutions with 1-aryl-1H-imidazole and 2,6bis(bromomethyl)pyridine and the corresponding pyrrolidine in 70% yield with full retention of the SC configuration of the carbon stereogenic center (Scheme 1). The 1H NMR spectra (CDCl3) of compounds [3a]Br, [3b]Br, [4a]Br, and [4b]Br show that imidazolium C(2)-H characteristically resonates at 10.10 and 10.40 ppm. The bridging methylene (NimCH2C) moiety appeared as a singlet at 5.99 ([3a]Br) and 6.23 and 5.98 ([3b]Br) ppm, CCH2Npyrr ([3a]Br) at 3.73, and CCH2Npro ([3b]Br) at 3.77 and 3.67 ppm in the 1H NMR spectrum and at 59.77 (CCH2Npyrr); 51.44 (NimCH2C) ([3a]Br) or 61.44 (CCH2Npro); and 54.25 (NimCH2C) ([3b]Br) ppm in the 13C NMR spectrum. In the electrospray mass spectrum the imidazolium cation appeared as molecular peaks at m/z 361 ([3a]Br) or 460 ([3b]Br), respectively, in 100% abundance. Synthesis of Complexes. Salts [3]Br and [4]Br are precursors to NHCs, and their gold, rhodium, and palladium complexes were synthesized by Lin’s method of transmetalation from intermediate silver(I) complexes 3Ag and 4Ag
136
Organometallics, Vol. 29, No. 1, 2010
Boronat et al.
Scheme 1. Synthesis of Ligand Precursors
Figure 2. Optimized geometry of complexes 3bPd (left) and 4bPd (right).
Scheme 2. Synthesis of Complexes
(Scheme 2).17 Metalation of the imidazolium salts [3]Br and [4]Br using Ag2O was monitored by 1H NMR spectroscopy, but the silver complexes were not isolated. Instead, PdCl2(CH3N)2 or [RhCl(cod)]2 (cod = cycloocta-1,5-diene) was added directly to the dichloromethane solution of 3Ag or 4Ag. Precipitation of silver chloride was observed immediately, but the mixtures were allowed to stir at room temperature for 2-24 h before workup. Consistent with the formation of the Ag complex, the 1H NMR spectra showed the absence of an imidazolium (NCHN) resonance at ∼10.10 ppm owing to the loss of the acidic imidazolium proton as a result of the reaction with Ag2O along with the appearance of a diagnostic silver-bound carbene (NCN-Ag) peak at ∼174 ppm in the 13C NMR spectrum of 3Ag and 4Ag. Palladium Complexes. Addition of an equal molar amount of PdCl2(CH3N)2 to a CH2Cl2 solution of 3Ag or 4Ag immediately afforded a precipitate (75%, 95% yield). The 1 H and 13C NMR spectra obtained for complexes 3bPd and (17) (a) Garrison, J. C.; Youngs, W. J. Chem. Rev. 2005, 105, 3978– 4008. (b) Chianese, A. R.; Li, X. W.; Janzen, M. C.; Faller, J. W.; Crabtree, R. H. Organometallics 2003, 22, 1663–1667. (c) Simons, R. S.; Custer, P.; Tessier, C. A.; Youngs, W. J. Organometallics 2003, 22, 1979–1982. (d) Wang, H. M. J.; Lin, I. J. B. Organometallics 1998, 17, 972–975.
4bPd displayed only one set of signals and indicated the presence of a single diastereoisomer in solution. In the 1 H NMR spectrum of the Pd complexes, the bridging methylene (CH2) moieties appeared at 6.19 ppm (CCH2Nim), 4.33 ppm (CCH2Npyrr) (3aPd), 5.67 ppm (CCH2Nim), 4.37 ppm (3bPd), 5.6 ppm (CCH2Nim), 4.05 ppm (CCH2Npyrr) (4aPd), and 5.83 ppm (CCH2Nim), 5.46 ppm (CCH2Npro) (4bPd). The characteristic palladiumbound carbene (NNC-Pd) peak appeared at 160.25 (3aPd), 169.23 ppm (3bPd), 160.85 (4aPd), and 175.63 ppm (4bPd) in the 13C NMR and falls well within the range, ca. 175-145 ppm, observed for other reported Pd-NHC complexes. Unfortunately we have not obtained crystals of adequate size to determine the structure by X-ray. In order to study the nature of the ligand-metal interactions, the molecular structure of complexes 3bPd and 4bPd was obtained from DFT calculations (see Figure 2). In both pincer-type complexes the Pd center is bonded to the imino and pyrrolidine N atoms, to the carbene ligand (with Pd-Ccarb distances of 2.060 and 2.070 A˚, respectively), and to a chloride atom. A hydrogen bond is formed between the Cl atom and the amino proton of the CONHtBu group, which preferentially stabilizes the exo isomer in the transmetalation step. Rhodium Complexes. Reaction of 3Ag and 4Ag with [RhCl(cod)]2 at 40 °C gave the rhodium complexes (3Rh, 4Rh) in high yields. The 1H and 13C NMR spectra are as expected for complexes of the general formula Rh(NHCligand)(cod)Cl. In the 1H NMR spectra the cyclooctadiene resonances are observed as four broad lines between 1.8 and 4.8 ppm due to fluxionality in the conformation of the cod chelate. Complexes 3Rh and 4Rh exhibit a doublet carbene signal in the 13C spectrum at 171-181 ppm in addition to four other signals attributable to the carbon atoms of the COD ligand. Elemental analysis and the ESI-MS spectrum are consistent with the proposed formulation shown in Scheme 2, but a single crystal structure has yet to be obtained. Gold Complexes. Chelated organogold(III) complexes can readily be prepared by transmetalation using organomercury(II) reagents.18 We have used a similar route from the silver compounds described before. Treatment of the silver complexes 3Ag and 4Ag with KAuCL4 in ethanol gives the chlorogold(III) complexes 3Au(III) and 4Au(III), respectively, in >90% yield, as yellow-orange solids. The 1H NMR spectra showed the absence of an imidazolium (NCHN) (18) (a) Li, C. K.-L.; Sun, R. W.-Y.; Kui, S. Ch.-F.; Zhu, N.; Che, Ch.-M. Chem.-Eur. J. 2006, 12, 5253–5266. (b) Parish, R. V.; Wright, J. P.; Pritchard, R. G. J. Organomet. Chem. 2000, 596, 165–176. (c) Bonnardel, P. A.; Parish, R. V.; Pritchard, R. G. Dalton Trans. 1996, 3185–3193. (d) Wong, K. H.; Cheung, K. K.; Chang, M. C. W.; Che, C. M. Organometallics 1998, 17, 3505–3511.
Article
Organometallics, Vol. 29, No. 1, 2010
Scheme 3. Synthesis of Gold(III) Complexes, Route A
Table 1. Catalytic Hydrogenation of Diethyl Itaconate and Diethyl 2-Benzylidenesuccinate with Rh(I), Pd(II), and Au(III) Catalystsa diethyl itaconate
resonance at 10.11 ([3a]Br), 10.39 ([3b]Br), 10.20 ([4a]Br), or 10.42 ([4b]Br) ppm owing to the loss of the acidic imidazolium proton along with the appearance of a diagnostic goldbound carbene (NCN-Au) peak at 192.18 ppm (3aAu(III)), 181.29 ppm (3bAu(III)), or 197.62 (4bAu(III)) in the 13C NMR spectrum, although it was possible to obtain the desired gold(III) complex by direct reaction of the imidazolium salt [3a]Br or [3b]Br with the potassium gold salt (KAuCl4) (Scheme 3). The 1H NMR spectra showed the presence of an imidazolium (NCHN) resonance at ∼10.00 ppm owing to the acidic imidazolium proton of [3a]Br and [3b]Br. This intermediate, in the presence of a base such as [(CH3)3Si]2NK yields the formation of 3aAu(III) and 3bAu(III), respectively. Transmetalation from the silver salt was much more efficient, giving cleaner and quicker reactions. Subsequent abstraction of the chloride anion from all complexes by treatment with AgPF6 yielded the cationic complexes in good yields. One new stereogenic center was generated upon coordination of ligand 3b or 4b to metal (palladium, gold, rhodium). In all cases the cationic complexes were obtained as single diastereoisomers. Catalytic Activity: Hydrogenation of Olefins. Table 1 shows data for the hydrogenations of diethyl itaconate and diethyl 2-benzylidenesuccinate with Au, Rh, and Pd complexes carried out under standard conditions (EtOH as the solvent, 4 atm PH2, 40 °C). The nature of the metal center influences the catalytic activities. In general, rhodium(I) catalyst is more active than gold(III) or palladium(II) with the same ligand. In all cases no metal was detected during the hydrogenation reaction. Low enantiomeric excesses were achieved with the 3bPd compound (80 and 70%, respectively, for the hydrogenation of diethyl 2-benzylidenesuccinate, as we could detect from the HPLC analysis of hydrogenated products. Table 1 shows the effect that ligand substituents have on activity and enantioselectivity. Thus, when we have a fairly rigid skeleton (ligand 1b), resulting complexes are very active catalysts but lead to poor enantioselectivity. When we have a rigid skeleton, such as pyridine, selectivity in the case of palladium does not improve, but we have observed a significant increase in the gold derivative case. When we replace the phenolate group by a NHC carbene substituent, the catalysts show a significant improvement on all metals. Table 1 also shows the effect of modifying the NHC N-substituent, indicating that the rate and particularly the enantioselectivity are very sensitive to changes at this position. The catalytic activity diminishes when the bis-3,5diisopropyl phenyl derivative (4b) is used as catalyst, but this decrease is accompanied by a significant increase of the enantioselectivity (>90%) and inversion of stereochemistry. Hydrogenation of methylenesuccinates catalyzed by derivatives 3b (N-mesityl) leads to the S isomer, while the derivative
137
b
entry catalyst
TOF
1 2 3 4 5 6 7 8 9 10
4470 4962 3372 3500 4000 2000 3200 2800 4920 3368
3bAu 3bRh 3bPd 4bAu 4bRh 4bPd 2bAu 2bPd 1bAu 1bPd
c
diethyl 2-benzylidene succinate
ee %
TOFb
ee %d
10 (S)
