Organometallics 2009, 28, 321–325
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Iridium and Ruthenium Complexes with Chelating N-Heterocyclic Carbenes: Efficient Catalysts for Transfer Hydrogenation, β-Alkylation of Alcohols, and N-Alkylation of Amines Dinakar Gnanamgari, Effiette L. O. Sauer, Nathan D. Schley, Chase Butler, Christopher D. Incarvito, and Robert H. Crabtree* Department of Chemistry, Yale UniVersity, 225 Prospect Street, P.O. Box 208107, New HaVen, Connecticut 06520-8107 ReceiVed August 22, 2008
Air-stable Ir and Ru complexes of a chelating pyrimidine-functionalized N-heterocyclic carbene were synthesized. The complexes were characterized by NMR spectroscopy and single-crystal X-ray diffraction and were found to be catalytically active for transfer hydrogenation, β-alkylation of secondary alcohols with primary alcohols, and N-alkylation of amines with primary alcohols. Notably, the Ir complexes were found to catalyze the N-alkylation of amines using the mild base NaHCO3. Introduction N-Heterocyclic carbenes (NHCs) have attracted much attention as stable ligands for homogeneous catalysis.1 They constitute a sterically and electronically tunable ligand set that supports catalysis. However, the principles regarding their structure-property relationships are underdeveloped. NHCs are more electron donating and tend to be sterically more demanding than phosphine ligands with the same substituents.2 NHCs have been used as ligands for a multitude of reactions including transfer hydrogenation,3 C-C coupling,4 olefin metathesis,1f and hydrosilylation.5 Functionalized NHCs6 having an additional donor group are an important class of ligands in organometallic catalysis. Many reports have appeared on NHCs functionalized with phosphine,7 * Corresponding author. E-mail:
[email protected]. (1) (a) Arduengo, A. J.; Dias, H. V. R.; Harlow, R. L.; Kline, M. J. Am. Chem. Soc. 1992, 114, 5530–5534. (b) Herrmann, W. A.; Kocher, C. Angew. Chem., Int. Ed. Engl. 1997, 36, 2163–2187. (c) Herrmann, W. A. Angew. Chem., Int. Ed. 2002, 41, 1290–1309. (d) Lappert, M. F. J. Organomet. Chem. 1988, 358, 185–214. (e) Bourissou, D.; Guerret, O.; Gabbai, F. P.; Bertrand, G. Chem. ReV. 2000, 100, 39–91. (f) Sanford, M. S.; Love, J. A.; Grubbs, R. H. Organometallics 2001, 20, 5314–5318. (g) Scott, N. M.; Nolan, S. P. Eur. J. Org. Chem. 2005, 1815–1828. (2) (a) Crabtree, R. H. J. Organomet. Chem. 2005, 690, 5451–5457. (b) Diez-Gonzalez, S.; Nolan, S. P. Coord. Chem. ReV. 2007, 251, 874– 883. (3) (a) Corberan, R.; Peris, E. Organometallics 2008, 27, 1954–1958. (b) Voutchkova, A. M.; Gnanamgari, D.; Jakobsche, C. E.; Butler, C.; Miller, S. J.; Parr, J.; Crabtree, R. H. J. Organomet. Chem. 2008, 693, 1815–1821. (c) Poyatos, M.; McNamara, W.; Incarvito, C.; Peris, E.; Crabtree, R. H. Chem. Commun. 2007, 2267–2269. (d) Gnanamgari, D.; Moores, A.; Rajaseelan, E.; Crabtree, R. H. Organometallics 2007, 26, 1226–1230. (4) (a) Dragutan, V.; Dragutan, I.; Delaude, L.; Demonceau, A. Coord. Chem. ReV. 2007, 251, 765–794. (b) Liu, Z.; Zhang, T.; Shi, M. Organometallics 2008, 27, 2668–2671. (c) Xi, Z.; Liu, B.; Chen, W. J. Org. Chem. 2008, 73, 3954–3957. (5) (a) Jimenez, M. V.; Perez-Torrente, J. J.; Bartolome, M. I.; Gierz, V.; Lahoz, F. J.; Oro, L. A. Organometallics 2008, 27, 224–234. (b) Poyatos, M.; Maisse-Francois, A.; Bellemin-Laponnaz, S.; Gade, L. H. Organometallics 2006, 25, 2634–2641. (6) (a) Kuhl, O. Chem. Soc. ReV. 2007, 36, 592–607. (b) Pugh, D.; Danopoulos, A. A. Coord. Chem. ReV. 2007, 251, 610–641. (c) Ray, L.; Shaikh, M. M.; Ghosh, P. Organometallics 2007, 26, 958–964. (7) (a) Lee, C. C.; Ke, W. C.; Chan, K. T.; Lai, C. L.; Hu, C. H.; Lee, H. M. Chem.-Eur. J. 2007, 13, 582–591. (b) Hahn, F. E.; Jahnke, M. C.; Pape, T. Organometallics 2006, 25, 5927–5936. (c) Yang, C. L.; Lee, H. M.; Nolan, S. P. Org. Lett. 2001, 3, 1511–1514.
pyridine,8 oxazoline,9 amido,10 and ether11 donor functions. These ligands allow for potential hemilability, and several such cases have been reported.12 Transfer hydrogenation of CdO and CdN groups is a reaction for which NHC metal complexes3 have demonstrated particularly good activity. The CdO case has been most extensively studied, leading to important applications such as racemization13 of chiral alcohols and asymmetric reductions.14 In recent years, an alcohol activation strategy for C-C and C-N coupling reactions has received increased attention, in part because it replaces toxic halides as alkylating agents with relatively benign alcohols.15 In addition, reactions such as β-alkylation of secondary alcohols with primary alcohols16 and N-alkylation of amines with alcohols16b,17 produce water as the sole byproduct and are thus atom economical. We now report novel chelating NHC pyrimidine Ir and Ru complexes that can be synthesized in good yields even in the presence of air. They are catalytically active for three useful (8) (a) Danopoulos, A. A.; Tsoureas, N.; Macgregor, S. A.; Smith, C. Organometallics 2007, 26, 253–263. (b) Mas-Marza, E.; Sanau, M.; Peris, E. J. Organomet. Chem. 2005, 690, 5576–5580. (c) Mas-Marza, E.; Sanau, M.; Peris, E. Inorg. Chem. 2005, 44, 9961–9967. (9) Gade, L. H.; Bellemin-Laponnaz, S. Coord. Chem. ReV. 2007, 251, 718–725. (10) (a) Liao, C. Y.; Chan, K. T.; Zeng, J. Y.; Hu, C. H.; Tu, C. Y.; Lee, H. M. Organometallics 2007, 26, 1692–1702. (b) Spencer, L. P.; Winston, S.; Fryzuk, M. D. Organometallics 2004, 23, 3372–3374. (c) Arnold, P. L.; Liddle, S. T. Chem. Commun. 2006, 3959–3971. (11) Herrmann, W. A.; Goossen, L. J.; Spiegler, M. J. Organomet. Chem. 1997, 547, 357–366. (12) (a) Corberan, R.; Sanau, M.; Peris, E. Organometallics 2007, 26, 3492–3498. (b) Wang, R. H.; Zeng, Z.; Twamley, B.; Piekarski, M. M.; Shreeve, J. M. Eur. J. Org. Chem. 2007, 655–661. (c) Huynh, H. V.; Yeo, C. H.; Tan, G. K. Chem. Commun. 2006, 3833–3835. (d) Arnold, P. L.; Blake, A. L.; Wilson, C. Chem.-Eur. J. 2005, 11, 6095–6099. (e) Hahn, F. E.; Holtgrewe, C.; Pape, T.; Martin, M.; Sola, E.; Oro, L. A. Organometallics 2005, 24, 2203–2209. (f) Chen, J. C. C.; Lin, I. J. B. Organometallics 2000, 19, 5113–5121. (13) Yamaguchi, K.; Koike, T.; Kotani, M.; Matsushita, M.; Shinachi, S.; Mizuno, N. Chem.-Eur. J. 2005, 11, 6574–6582. (14) Gladiali, S.; Alberico, E. Chem. Soc. ReV. 2006, 35, 226–236. (15) (a) Skucas, E.; Ngai, M. Y.; Komanduri, V.; Krische, M. J. Acc. Chem. Res. 2007, 40, 1394–1401. (b) Hamid, M.; Slatford, P. A.; Williams, J. M. J. AdV. Synth. Catal. 2007, 349, 1555–1575. (c) Guillena, G.; Ramon, D. J.; Yus, M. Angew. Chem., Int. Ed. 2007, 46, 2358–2364. (d) Fujita, K.; Yamaguchi, R. Synlett 2005, 560–571.
10.1021/om800821q CCC: $40.75 2009 American Chemical Society Publication on Web 11/26/2008
322 Organometallics, Vol. 28, No. 1, 2009 Scheme 1
Gnanamgari et al. Scheme 2
reactions dependent on transfer hydrogenation: the reduction of ketones and imines, β-alkylation of secondary alcohols with primary alcohols, and N-alkylation of amines with alcohols using the mild base NaHCO3.
Results and Discussion The present ligand design was adopted in the hope that the potentially labile pyrimidine would provide hemilability so that the free pyrimidyl group might then act as an internal base, perhaps obviating the need for added base in catalysis. As discussed below, we found no evidence of hemilability, but we were able to substitute the much weaker NaHCO3 for the usual strong base, KOH, in the catalytic N-alkylation of amines. Rather than this being a result of the anticipated internal base effect, however, this may instead be the result of the monocationic character of the catalyst induced by the bis-neutral donor structure of the present chelating NHC ligand compared with the monodentate NHCs more commonly encountered.16b The resulting monopositive ionic charge might be expected to facilitate deprotonation of the coordinated alcohol. The present catalysts have at least as good or better activity than previously reported systems.16b Synthesis of Precursors for Chelating NHC Pyrimidine Ligands. The imidazolium salts used as precursors for the chelating NHC pyrimidine ligands were synthesized by direct alkylation of 1-substituted imidazoles. The reaction of 2-chloropyrimidine with stoichiometric amounts of 1-n-butylimidazole or 1-mesitylimidazole in refluxing p-xylene gave the corresponding pyrimidine imidazolium salts (Scheme 1). The salts were isolated in moderate yields and were characterized by elemental analysis and 1H and 13C{1H} NMR spectroscopy. The 1 H NMR spectra of 1a and 1b in CDCl3 showed a very low field resonance in the range δ 10.3-11.3 ppm characteristic of the NCHN imidazolium proton. Synthesis of NHC Pyrimidine Ir(III) Salts. The Ir(III) salts 2a and 2b were prepared by in situ transmetalation from the silver carbene18 complexes of compounds 1a and 1b (Scheme 2). Treatment with Ag2O under light-free conditions in CH2Cl2 (16) (a) Cho, C. S.; Kim, B. T.; Kim, H. S.; Kim, T. J.; Shim, S. C. Organometallics 2003, 22, 3608–3610. (b) da Costa, A. P.; Viciano, M.; Sanau, M.; Merino, S.; Tejeda, J.; Peris, E.; Royo, B. Organometallics 2008, 27, 1305–1309. (c) Fujita, K.; Asai, C.; Yamaguchi, T.; Hanasaka, F.; Yamaguchi, R. Org. Lett. 2005, 7, 4017–4019. (d) Martinez, R.; Ramon, D. J.; Yus, M. Tetrahedron 2006, 62, 8982–8987. (e) Viciano, M.; Sanau, M.; Peris, E. Organometallics 2007, 26, 6050–6054. (f) Gnanamgari, D.; Leung, C. H.; Schley, N. D.; Hilton, S. T.; Crabtree, R. H. Org. Biomol. Chem. 2008, 6, 4442-4445. (17) (a) Fujita, K.; Li, Z. Z.; Ozeki, N.; Yamaguchi, R. Tetrahedron Lett. 2003, 44, 2687–2690. (b) Fujita, K. I.; Fujii, T.; Yamaguchi, R. Org. Lett. 2004, 6, 3525–3528. (c) Tillack, A.; Hollmann, D.; Michalik, D.; Beller, M. Tetrahedron Lett. 2006, 47, 8881–8885. (d) Hamid, M.; Williams, J. M. J. Chem. Commun. 2007, 725, 727. (18) (a) Chianese, A. R.; Li, X. W.; Janzen, M. C.; Faller, J. W.; Crabtree, R. H. Organometallics 2003, 22, 1663–1667. (b) Lin, I. J. B.; Vasam, C. S. Comments Inorg. Chem. 2004, 25, 75–129. (19) (a) Tanabe, Y.; Hanasaka, F.; Fujita, K.; Yamaguchi, R. Organometallics 2007, 26, 4618–4626. (b) Corberan, R.; Sanau, M.; Peris, E. J. Am. Chem. Soc. 2006, 128, 3974–3979. (c) Hanasaka, F.; Fujita, K.; Yamaguchi, R. Organometallics 2005, 24, 3422–3433. (d) Hanasaka, F.; Fujita, K. I.; Yamaguchi, R. Organometallics 2004, 23, 1490–1492.
at room temperature formed the presumed silver carbene, which was filtered and directly added to [Cp*IrCl2]2 and KPF6. The mixture was stirred for 2 h, then filtered through Celite and concentrated to yield yellowish-orange solids 2a and 2b in good yields. After recrystallization from CH2Cl2/pentane, compounds 2a and 2b were characterized by 1H and 13C{1H} NMR, elemental analysis, and X-ray diffraction. Both 2a and 2b lack the NCHN proton resonance of the precursor imidazolium ion but show the resonances for the backbone protons of the imidazolium fragment in the range δ 8.8-8.9 ppm. The chelate formation was initially suggested by the appearance of three distinct peaks in the 1H NMR for the pyrimidine ring that are absent in the free imidazolium salt, showing that the symmetry of the pyrimidine is lost on binding. The molecular structures of 2a (Figure 1) and 2b (Figure 2), determined by single-crystal X-ray diffraction, confirm the formation of a chelate. The Ir-Ccarbene distances of 2a and 2b are 2.146 and 2.044 Å, respectively, lying in the expected range.19 To explore potential hemilability in 2a, we undertook a variable-temperature NMR experiment. No broadening of the three pyrimidine 1H resonances was observed, even at 110 °C. This result suggests that the chelate is stable and that no hemilability occurs under these conditions. Synthesis of NHC Pyrimidine Ru(II) Salt. The Ru(II) salt 3a was made in an analogous manner to the syntheses described above. Treatment of 1a with Ag2O and transmetalation of the resulting silver carbene to [Ru(p-cymene)2Cl2]2 in the presence of KPF6 gave a yellow solid 3a (Scheme 3) in good yield. As expected, the compound 3a lacked the NCHN proton and
Figure 1. Molecular structure of 2a. Selected bond lengths (Å) and angles (deg): Ir(1)-cent 1.821(6), Ir(1)-C(15) 2.146(5), Ir(1)-Cl(1) 2.4028(15), Ir(1)-N(1) 2.103(5), C(15)-Ir(1)-Cl(1) 86.78(16), C(15)-Ir(1)-N(1) 75.7(2).
Ir and Ru Complexes with Chelating NHCs
Organometallics, Vol. 28, No. 1, 2009 323 Table 2. β-Alkylation of Secondary Alcohols with Primary Alcoholsa
entry
catalyst
Ar
R
t (h)
% conv
% alc
% ket
1 2 3 4 5 6 7 8
2a 2b 3a 2ab 2a 2a 2a 2a
Ph Ph Ph Ph Ph Ph Ph Ph
Ph Ph Ph Ph 4-Cl(C6H4) 4-Cl(C6H4) 4-Me(C6H4) n-Pr
3 3 3 3 3 6 3 5
93 86 10 64 96 100 90 82
100 100 100 100 100 80 100 60
0 0 0 0 0 20 0 22
a 2.00 mmol of secondary alcohol and primary alcohol, 2.0 mmol of KOH (100 mol %), 1 mol % catalyst, 0.5 mL of toluene at 110 °C. Conversions determined by 1H NMR using 1,3,5-trimethoxybenzene as an internal standard. b 0.5 mol % of catalyst.
Table 3. N-Alkylation of Amines with Alcoholsa
Figure 2. Molecular structure of 2b. Selected bond lengths (Å) and angles (deg): Ir(1)-cent 1.830(6), Ir(1)-C(15) 2.044(7) Ir(1)-Cl(1) 2.3927(19), Ir(1)-N(1) 2.109(6), C(15)-Ir(1)-Cl(1) 86.98(19), C(15)-Ir(1)-N(1) 76.7(3). Scheme 3
Table 1. Catalytic Transfer Hydrogenationa
entry
catalyst
R
R′
% amine
% imine
1 2 3 4 5 6 7 8
2a 2b 3a 2a 2a 2a 2a 2a
Ph Ph Ph Ph Ph n-Pr n-Pr >n-Pr
PhCH2 PhCH2 PhCH2 Ph PhCH2CH2 Ph PhCH2 PhCH2CH2
86 62 53 98 55 43 56 25
2 23 2
