Organometallics 2009, 28, 2625–2628
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Notes A Water-Stable Organometallic Rhodium Quinone Catalyst and Its Recyclability Sang Bok Kim,† Chen Cai,† Marcus D. Faust,† William C. Trenkle,‡ and Dwight A. Sweigart*,† Department of Chemistry, Brown UniVersity, ProVidence, Rhode Island 02912, and National Institute of Diabetes and DigestiVe and Kidney Diseases, NIH, DHHS Bethesda, Maryland 20814 ReceiVed February 4, 2009 Summary: When the insoluble dimers [Rh(COD)X]2 (X ) OH-, Cl-) are in contact with a basic aqueous solution of hydroquinone dianion, the precipitate dissolVes to afford (benzoquinone)Rh(COD)- (2). Complex 2 is Very stable and can be recycled many times as a homogeneous catalyst for the conjugate addition of boronic acids to olefins. NMR spectra suggest that the catalytic species is indeed the quinone complex 2. We have reported previously1 that the rhodium complex [(η6-hydroquinone)Rh(η4-COD)][BF4] (1) is rapidly deprotonated with concomitant electron transfer upon addition of base such as LiOH or KOH (in water) or t-BuOK (in THF) to give the anionic η4-quinone complexes 2 · Li and 2 · K respectively, as shown in Scheme 1. The interest in 2 · M is due to its excellent ability to homogeneously catalyze Miyaura-Hayashi type2 1,4-conjugated addition of arylboronic acid to activated olefinic acceptors1band also to catalyze the 1,2-addition of arylboronic acids to benzaldehydes,1a as shown in Scheme 1. While we previously reported a plausible mechanism, it was not shown which Rh species exists under catalysis conditions. Using 1H NMR, we have been able to ascertain the likely species present during catalysis. Herein, we also report that the complex 2 · Li is formed spontaneously when the insoluble [Rh(η4-COD)OH]2 is in contact with an aqueous solution of doubly deprotonated hydroquinone at either room temperature or 50 °C and that 2 · Li is stable under catalytic conditions. We also show that the stability of 2 · Li allows it to be recycled many times as a homogeneous catalyst for conjugate addition reactions.
* To whom correspondence should be addressed. E-mail:
[email protected]. † Brown University. ‡ National Institute of Diabetes and Digestive and Kidney Diseases. (1) (a) Son, S. U.; Kim, S. B.; Reingold, J. A.; Carpenter, G. B.; Sweigart, D. A. J. Am. Chem. Soc. 2005, 127, 12238. (b) Trenkle, W. C.; Barkin, J. L.; Son, S. U.; Sweigart, D. A. Organometallics 2006, 25, 3548. (c) Son, S. U.; Reingold, J. A.; Kim, S. B.; Carpenter, G. B.; Sweigart, D. A. Angew. Chem., Int. Ed. 2005, 44, 7710. (2) (a) Sakai, M.; Hayashi, H.; Miyaura, N. Organometallics 1997, 16, 4229. (b) Sakuma, S.; Sakai, M.; Itooka, R.; Miyaura, N. J. Org. Chem. 2000, 65, 5951. (c) Itooka, R.; Iguchi, Y.; Miyaura, N. Chem. Lett. 2001, 722. (d) Itooka, R.; Iguchi, Y.; Miyaura, N. J. Org. Chem. 2003, 68, 6000. (e) Nakao, Y.; Chen, J.; Imanaka, H.; Hiyama, T.; Ichikawa, Y.; Duan, W.-L.; Shintani, R.; Hayashi, T. J. Am. Chem. Soc. 2007, 129, 9137.
Scheme 1. Conversion of [(η6-hydroquinone)Rh(η4-COD)][BF4] (1) to [(η4-quinone)Rh(η4-COD)][Li] (2 · Li) by Deprotonation. 1,2-Addition of p-Tolylboronic acid to Benzaldehyde (3), and 1,4-Conjugate Addition of p-Tolylboronic acid to Cyclohexenone (5)
Experimental Section Materials. The complex [(η6-hydroquinone)Rh(η4-COD)][BF4] was synthesized according to literature procedures.1 All other commercial reagents were purchased from Aldrich, Strem Chemicals, and Fisher Scientific. All the reagents were used as received without further purification. Distilled water was used for catalysis. Catalysis. All catalysis reactions were performed under nitrogen. The procedures followed for reactions in Scheme 1 were virtually identical. The procedure for conjugate addition to cyclohexenone is as follows: [(η6-hydroquinone)Rh(η4-COD)][BF4] (20 mg, 0.05 mmol, 5 mol %)1,5 and LiOH · H2O (20 mg, 0.5 mmol) were dissolved in 7 mL of distilled water in a 20 mL vial. To this solution were added cyclohexenone (96 mg, 1.0 mmol) and p-tolylboronic acid (170 mg, 1.25 mmol). The reaction mixture was heated to 80 °C for 3 h and then cooled to room temperature. The reaction mixture was extracted with diethyl ether (3 × 7 mL). The combined organic layers were dried (MgSO4) and filtered through a silica gel plug. The organic solvent was removed with a nitrogen stream. The isolated product was confirmed by 1H NMR (CDCl3). The aqueous layer was filtered through a Celite plug to remove insoluble borate salts and used directly in the next cycle. Cyclohexenone (1.0 mmol), p-tolueneboronic acid (1.25 mmol), COD (50 µL), and LiOH · H2O (0.5 mmol) were added to the filtered aqueous layer, and the reaction was repeated. Characterization. 1H NMR spectra were recorded with a Bruker (300 MHz) spectrometer.
10.1021/om900084y CCC: $40.75 2009 American Chemical Society Publication on Web 03/27/2009
2626 Organometallics, Vol. 28, No. 8, 2009
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Figure 1. 1H NMR spectra: (a) hydroquinone in DME (100 µL) and D2O (1 mL); (b) hydroquinone dianion generated by LiOH in DME and D2O; (c) mixture of hydroquinone, [Rh(η4-COD)Cl]2 (0.0049 mmol) in THF (100 µL), 20 µL of LiOH in D2O (1 N, 0.02 mmole of LiOH), and D2O (1 mL) at 50 °C after 1 h; (d) mixture in (c) after 20 h; (e) mixture of [(η6-hydroquinone)Rh(η4-COD)][BF4] (0.0049 mmol) in DME (100 µL), 20 µL of LiOH in D2O (1 N, 0.02 mmol of LiOH), and D2O (1 mL) at 50 °C after 20 min; (f) mixture in (e) after 20 h; (g) mixture of [(η6-hydroquinone)Rh(η4-COD)][BF4] (0.0049 mmol) in DME (100 µL) and D2O (1 mL) at room temperature; (h) addition of 20 µL of LiOH in D2O (1 N, 0.02 mmol of LiOH) and D2O (1 mL) to mixture in (g) at room temperature after 1 h.
Results and Discussion We have previously shown that anhydrous solutions of 1 can be cleanly deprotonated by treatment with aqueous base to provide solutions of 2 · Li (Scheme 1, Figure 1e).1b When a neutral protic solvent such as D2O is added to a solution of 1 in dimethoxyethane (DME), the complex is rapidly and completely decomposed, producing free hydroquinone and a soluble cationic rhodium complex that retains COD. These two species were observed by 1H NMR. It is believed that the rhodium species formed is the simple hydrate Rh(η4-COD)(D2O)n+ (7), shown in Scheme 2 and Figure 1g. The addition of excess aqueous LiOH (4 equiv relative to the amount of Rh) to the solution causes an immediate precipitate with loss of the 1H NMR signals due to COD, while the free hydroquinone is converted to the dianionic form (Figure 1h). We believe the precipitate to be the well-known insoluble dimer [Rh(η4-COD)-
Scheme 2. Mechanism of the Conversion of [(η6-hydroquinone)Rh(η4-COD)][BF4] (1) into [(η4-quinone)Rh(η4-COD)][Li] (2 · Li)
OD]2 (8) (Scheme 2). To our surprise, heating the reaction mixture at 50 °C for 20 min resulted in dissolution of the solid and the clean formation of the deprotonated quinone complex
Notes
Organometallics, Vol. 28, No. 8, 2009 2627
Table 1. Catalysis of 1,2-Addition of p-Tolylboronic Acid to Benzaldehyde by 2 · Li
a
Table 2. Catalysis of 1,4-Addition of p-Tolylboronic acid to Cyclohexenone by 2 · Li
no. of recycles
yield (%)b
no. of recycles
yield (%)b
0a 1 2 3 4 5 6 7
93 91 77 88 86 82 89 98
0a 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
94 96 95 98 94 91 96 88 91 94 89 83 85 86 86 83 88 80 80
The first catalytic reaction. b Yield determined by 1H NMR.1a,4
2 · Li (Scheme 2, Figure 1e). In stark contrast to the behavior of 1, the anionic complex 2 · Li is very stable in protic solvents under basic conditions and shows no observable decomposition after 20 h at 50 °C in D2O (Figure 1 and Figure S3 (Supporting Information)). It was found that the insoluble dimer [Rh(η4-COD)Cl]2 can also be converted to 2 · Li. Thus, treatment of [Rh(η4-COD)Cl]2 dimer (1 equiv) combined with hydroquinone (5 equiv) and LiOH (10 equiv) in 1:2 DME/D2O or THF/D2O cleanly forms 2 · Li after ca. 24 h at 50 °C (Figure 1c f Figure 1d). No reaction is observed when phenol is used in place of hydroquinone. These experiments demonstrate the great stability of the quinone complex 2 · Li under basic conditions in aqueous media. Several Rh dimers, including [Rh(η4-COD)OH]2 and [Rh(η4-COD)Cl]2, are commonly used for heterogeneous and homogeneous catalyzed C-C coupling reactions.2,3 Complex 2 · Li is freely soluble in aqueous solution and functions as a homogeneous catalyst (Scheme 1). Water is a convenient and environmentally benign solvent and allows product extraction with diethyl ether. Significantly, it is shown herein that the aqueous solution of 2 · Li can be recycled in catalyzing these reactions due to the stability of 2 · Li under catalytic conditions. To demonstrate this, we chose the 1,2-addition reaction (3 f 4) and the 1,4-conjugate addition reaction (5 f 6). The results for the 1,2-addition of p-tolylboronic acid to benzaldehyde are given in Table 1. The general procedure was to dissolve the catalyst precursor 1 (0.05 mmol, 5 mol %) and LiOH (0.5 mmol) in deaerated water, add benzaldehyde (1.00 mmol) and the p-tolylboronic acid (1.25 mmol), heat the reaction mixture to 80 °C for 3 h, cool to room temperature, and extract with diethyl ether. The conversion of benzaldehyde into the product, (4-methylphenyl)phenylmethanol, was determined by 1H NMR analysis of the organic extract (Figure S1 (Supporting Information)).1a,4 The aqueous layer was filtered to remove insoluble borate salts, then COD (50 µL), LiOH (0.5 mmol), benzaldehyde (1.0 mmol), and p-tolylboronic acid (1.25 mmol) were added to the filtrate, and the procedure was repeated. As seen in Table 1, even after seven recycles (eight reactions in total), there is good conversion to product. However, when the 1,2-addition reaction was (3) (a) Fagnou, K.; Lautens, M. Chem. ReV. 2003, 103, 169. (b) Furstner, A.; Krause, H. AdV. Synth. Catal. 2001, 343, 343. (c) Kakuuchi, A.; Taguchi, T.; Hanzawa, Y. Tetrahedron 2004, 60, 1293. (d) Oi, S.; Sato, T.; Inoue, Y. Tetrahedron Lett. 2004, 45, 5051. (e) Wadsworth, K. J.; Wood, F. K.; Chapman, C. J.; Frost, C. G. Synlett 2004, 2022. (f) Miura, T.; Shimada, M.; Murakami, M. Angew. Chem., Int. Ed. 2005, 44, 7598. (g) de la Herran, G.; Mba, M.; Murcia, M. C.; Plumet, J.; Csaky, A. G. Org. Lett. 2005, 7., 1669. (h) Ukai, K.; Aoki, M.; Takaya, J.; Iwasawa, N. J. Am. Chem. Soc. 2006, 128, 8706. (i) Siewert, J.; Sandman, R.; von Zezschwitz, P. Angew. Chem., Int. Ed. 2007, 46, 7122. (4) Chen, D.-W.; Ochiai, M. J. Org. Chem. 1999, 64, 6804. (5) [(η6-hydroquinone)Rh(η4-COD)][BF4] is sold by Strem Chemicals, Inc.
a
The first catalytic reaction. b Yield of isolated product.
Figure 2. Isolated yield of 1,4-addition catalysis at each cycle: (black square) recycle with addition of COD (0.5 mL/reaction); (red circle) recycle with addition of COD (50 µL/reaction); (blue triangle) recycle without addition of COD.
performed without the addition of COD, the conversion for the first recycle was only 20%, representing a large 73% dropoff from the initial catalyzed reaction (Figure S2 (Supporting Information)). It was found that the catalytic activity of the system was maintained by the addition of COD to each recycle in the 1,2-addition reaction. The isolated yields for the 1,4-addition of p-tolylboronic acid to cyclohexenone catalyzed by 2 · Li (Scheme 1) are given in Table 2. As with the 1,2-addition reaction, it was necessary to add COD (50 µL) and LiOH (0.5 mmol) after each run to prevent a substantial dropoff in yield. Without additional COD, the yield of product after the first recycle was 68% and just 29% for the 10th recycle (Figure 2). The addition of COD allows us to use the same aqueous solution of organometallic catalyst for over 18 recycles, with the final reaction providing an 80% isolated yield of product. The slow dropoff in yield may be due to the necessary change in viscosity and ionic strength accompanying each recycle and/or trace oxygen, which is known to destroy the catalyst 2 · Li. In order to probe the effects of COD and the potential existence of bis-COD species, the catalytic activity and recy-
2628 Organometallics, Vol. 28, No. 8, 2009
Notes
Table 3. Catalysis of 1,4-Addition of p-Tolylboronic Acid to Cyclohexenone by [Rh(η4-COD)2][BF4] no. of recycles
yield (%)b
yield (%)c
0a 1 2 3
88 63 61 52
83 54 46 36
a The first catalysis. b Yield of isolated product; catalysis with COD between runs. c Yield of isolated product; catalysis without COD between runs.
clability of [Rh(η4-COD)2][BF4] was examined. The use of independently synthesized [Rh(η4-COD)2][BF4] in the 1,4addition of p-tolylboronic acid to cyclohexenone gave the results in Table 3. At the first recycle, both catalysis with COD and catalysis without COD showed a significant decrease in yield compared to the original (zeroth) reaction. These results do not support the proposal that [Rh(η4-COD)2]+ is involved in the recycle reactions that utilize the rhodium hydroquinone 1 as the catalyst precursor to 2 · Li. Additionally, a 1H NMR spectrum of the aqueous layer (D2O) after the first recycle of the reaction (5 f 6) with catalyst derived from complex 1 (Figure S4 (Supporting Information)) showed that complex 2 · Li is the principal species in solution. We conclude from these results that complex 2 · Li is indeed the catalyst for the reactions in Scheme 1.
Conclusion In summary, we have shown that [(η4-quinone)Rh(η4-COD)][Li] (2 · Li) can function as a recyclable, water-stable, and soluble homogeneous catalyst in C-C forming reactions. It was observed that the addition of COD in the recycling reactions with 2 · Li was necessary to maintain high levels of catalytic activity and yields. Furthermore, it is shown from 1H NMR studies that [(η4-quinone)Rh(η4-COD)][Li] can be formed spontaneously from [Rh(η4-COD)OH]2 and [Rh(η4-COD)Cl]2 when these insoluble complexes are heated with basic aqueous solutions of hydroquinone dianion. Present work in our group is focused on extending this system to recyclable, water-stable, chiral catalysts.
Acknowledgment. We are grateful to the donors of the Petroleum Research Fund, administered by the American Chemical Society, to the intramural research program at the NIH, National Institute of Diabetes and Digestive and Kidney Diseases (W.C.T.), and to the National Science Foundation (No. CHE-0308640) for support of this research. Supporting Information Available: Figures giving 1H NMR experimental data for the 1,2-addition reaction and hydrolytic stability of 2 · Li. This material is available free of charge via the Internet at http://pubs.acs.org. OM900084Y
