New, Highly Efficient, Simple, Safe, and Scalable Synthesis of [(Ph3P

Jul 30, 2013 - (17) It was clear, however, that 3 equiv of KOH per Ru would be needed to convert all of the chlorine in the system to KCl. We found th...
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New, Highly Efficient, Simple, Safe, and Scalable Synthesis of [(Ph3P)3Ru(CO)(H)2] Hamidreza Samouei and Vladimir V. Grushin* Institute of Chemical Research of Catalonia (ICIQ), Tarragona 43007, Spain S Supporting Information *

ABSTRACT: A new method has been developed to prepare [(Ph3P)3Ru(CO)(H)2] (1), an important homogeneous catalyst, directly from RuCl3·xH2O. Unlike previously reported procedures to make 1, the new method does not utilize toxic and hazardous materials such as formaldehyde and benzene and requires only a small excess of PPh3, while furnishing analytically and spectroscopically pure 1 in unprecedented >95% yield. The solvent (EtOH) is used in small quantities, thereby enabling scalability, as has been demonstrated by preparing >17 g of pure 1 in one batch.

T

Table 1. Selected Reported Methods for the Synthesis of 1

he title compound, [(Ph3P)3Ru(CO)(H)2] (1), was originally reported by Hallman, McGarvey, and Wilkinson 45 years ago.1 Since then, 1 has become one of the most important and widely used homogeneous catalysts for a broad variety of transformations, ranging from efficient C−H activation and functionalization to the borrowing hydrogen methodology and polymerization.2−6

Ru source

reaction conditions

[(Ph3P)3RuCl2]

benzene−EtOH or MeOH, NaBH4, H2, reflux PPh3, EtOH, KOH, reflux PPh3, EtOH, KOH, reflux PPh3, MeOH, KOH, reflux PPh3, EtOH, KOH, reflux

RuCl3·xH2O RuCl3·xH2O RuCl3·xH2O RuCl3·xH2O

CO source

yield of 1, %

ref

EtOH or MeOH

60a

1

CH2O

70b

8

CH2O

70b

9

CH2O

68c

10

CH2O

d

11

66

a

Analytically pure 1, as precipitated out of the reaction solution. Crude product prior to purfication; yield of purified 1 not specified. c Purified product. dBefore or after purification (not specified). b

The reported methods to synthesize 1 are, however, far from being on par with its excellence as a catalyst. Originally, 1 was prepared in 60% yield by the reaction of [(Ph3P)3RuCl2] with NaBH4 in benzene−ethanol or benzene−methanol under reflux in a hydrogen atmosphere.1 A few years later, Robinson’s group7 reported the synthesis of 1 from RuCl3·xH2O, PPh3, formaldehyde, and KOH in EtOH. After additional optimization, this procedure affording 1 in 70% yield was published in Inorganic Syntheses in 19748 and since then has remained the only widely used preparative method for this complex. The synthesis employs 6 equiv of PPh3, a ca. 130-fold excess of highly toxic formaldehyde, and large volumes of solvent. To make 20 g of crude 1 by this method, one would have to use over 300 mL of 40% aqueous formaldehyde and nearly 3 L of ethanol. In our hands, Robinson’s method7,8 consistently furnished crude 1 in ca. 65% yield. Numerous attempts have been made to improve the preparation of 1.9−12 Although a number of modifications of Robinson’s method7,8 have been made, the yield of 1 has remained in the range of 62−68%.9−11 A summary of the reported methods to synthesize 1 is presented in Table 1. Regardless of whether the original or a modified procedure is used, the precipitated crude product needs purification. The © XXXX American Chemical Society

latter incurs losses while requiring large quantities of solvents (often toxic benzene), filtration through alumina or silica, and/ or recrystallization. Wang 12 has recently reported an optimization study of the standard procedure,8 claiming higher yields for 1 (up to 84%) in the presence of even a larger excess of formaldehyde (160-fold). We have failed to reproduce these high yields of 1, apparently because of lack of sufficient experimental details in the report.12 Considering all of the above, there is a clear need for a higher-yielding, more convenient, and safer procedure to synthesize 1. Herein we report a new, highly efficient, safe, simple, and easily scalable method to prepare 1 in nearly quantitative yield directly from RuCl3·xH2O. Our method utilizes PPh3 in a minimal excess and, unlike the previously developed procedures, does not employ highly toxic formaldehyde and benzene, any complex hydrides such as NaBH4, Received: May 24, 2013

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Indeed, the final product 1, like 3, bears three PPh3 ligands on the metal atom. We also reasoned that although intermediate 2 is a tetraphosphine species, the small amounts of extra PPh3 might be sufficient for its gradual formation and disappearance throughout the process. That was confirmed in a series of runs, thus allowing an approximately 1:4 RuCl3·xH2O to PPh3 molar ratio to be maintained in subsequent studies. Obviously, the precise stoichiometry of the formation of 1 from RuCl3·xH2O cannot be determined (see above).17 It was clear, however, that 3 equiv of KOH per Ru would be needed to convert all of the chlorine in the system to KCl. We found that excellent results in terms of both yield (>95%) and purity (>98%) of 1 could be achieved with just over 3 equiv of KOH, so long as the amount of EtOH used for the synthesis was approximately 85 mL per 1 g of 1 to be made. Lowering the volume of EtOH resulted in the formation of [(Ph3P)3Ru(H)(OAc)]18 (4), with ethyl acetate being detected in the liquid phase by 1H NMR and GC-MS. With 3 times less EtOH (25− 30 mL) under identical conditions, the product was heavily contaminated with 4 (up to 30%), as was revealed by 31P NMR analysis. Complex 4 was prepared independently and structurally characterized (Figure 1).19,20 Evidently, the

or a hydrogen atmosphere. No purification step is required, because at the end of the reaction 1 precipitates out analytically and spectroscopically pure. Two key findings provided the foundation for the development of the new synthetic method for 1: (1) It was found that 1 was formed quantitatively after a suspension of [(Ph3P)4Ru(H)2] (2)13 was stirred in EtOH under reflux for several hours. This transformation was apparently mediated by the formation of a Ru−OEt species that underwent β-H-elimination to produce acetaldehyde that served as the CO source. (2) We also found that [(Ph3P)3RuCl2] (3)14 generated in situ from RuCl3·xH2O and PPh3 in EtOH remained unchanged after agitation under reflux was continued for 4 days. Upon addition of KOH to this reaction mixture, however, the dark brown color from 3 gradually changed to yellow. 31P NMR analysis pointed to full conversion of 3 to a mixture of 2 and 1. Evidently, the addition of alkali prompted the formation of ethoxide, followed by its coordination to the metal center and β-H-elimination leading to the conversion of the Ru−Cl bonds to the Ru−H bonds. The findings described above suggested that formaldehydeand NaBH4-free synthesis of 1 directly from RuCl3·xH2O, PPh3, and KOH in EtOH (Scheme 1) should be possible. This Scheme 1. CH2O-Free, One-Pot Synthesis of 1 from RuCl3·xH2O

Figure 1. ORTEP drawing of 4 with thermal ellipsoids drawn at the 50% probability level and all H atoms of the AcO and PPh3 ligands omitted for clarity.

assumption was experimentally confirmed. In order to develop a truly practicable procedure, we then performed a series of experiments to study the reaction sequence, minimize the amounts of PPh3 and EtOH used, and establish an optimal temperature regime to achieve the highest possible yield and purity of 1. In the first step of the reaction, Ru(III) (RuCl3·xH2O) is reduced to Ru(II) (3)14 on heating with PPh3 in EtOH. In principle, both ethanol and phosphine could act as the reducing agent. A definitive answer to the question “which of the two (or both) actually reduces the Ru(III)?” is still not without controversy,15 despite the fact that the reaction has been widely known and used for nearly 50 years.16 We therefore analyzed by 31P NMR spectroscopy the mother liquor after the synthesis of 3 from RuCl3·xH2O and PPh3 (4 equiv) in EtOH under argon to find that both PPh3 and Ph3PO were present. Clearly, some of the phosphine must have been oxidized during the formation of 3. The PPh3 to Ph3PO ratio in the mother liquor was found to vary in a broad range, from ca. 1:1 to 1:4, depending on the source of RuCl3·xH2O used for the reaction. This is hardly surprising, since RuCl3·xH2O is a nonstoichiometric mixture of ruthenium species that is “called the “trichloride” because the Cl:Ru ratio is about 3:1” and that in fact can contain various quantities of Ru(IV).17 Importantly, however, the presence of unreacted PPh3 after the formation of 3 in the reaction of RuCl3·xH2O with 4 equiv of PPh3 pointed to the possibility of using RuCl3·xH2O and PPh3 in a 1:4 molar ratio for the entire reaction sequence leading to 1 (Scheme 1).

acetaldehyde produced in the β-H-elimination step (see above) underwent the Tishchenko reaction that is known19 to be efficiently catalyzed by 2, a key intermediate involved in the reaction sequence (Scheme 1). Alkaline hydrolysis of the AcOEt thus produced gave rise to the acetate anion for the formation of the side product 4. We reasoned that the undesired Tishchenko reaction of the acetaldehyde could be suppressed by dilution and/or higher concentrations of KOH, prompting alkali-catalyzed aldol condensation. Both methods proved to be efficient. For the smaller scale preparation, one may use a larger quantity of EtOH with less KOH. For the synthesis on a larger scale, it is more convenient to keep a lower volume of EtOH at the expense of using a 2−4-fold excess of alkali. An increased concentration of ethoxide at a higher pH also facilitates the formation of Ru−OEt species involved in the β-elimination, including those originating from the side product 4. Adding KOH to a suspension of 3 in boiling EtOH was found to trigger side reactions leading to [(Ph3P)3Ru(CO)2]21 as a side product, which was identified by NMR and IR spectroscopy. We were delighted to find, however, that these side processes could be avoided altogether by adding KOH at room temperature. Therefore, after the formation of 3 in EtOH under reflux, the resultant mixture was first cooled to 25−30 °C. Solid KOH was then added and the reaction mixture was stirred for 1 h at room temperature and 1−1.5 h at 50−70 °C and finally brought back to reflux that was continued until full B

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conversion of 2 to 1 was reached after several hours (31P NMR). Running the reaction for a longer time only may have a beneficial effect on purity, while not affecting the yield of 1 that is stable under the reaction conditions. Two factors are important for shortening the time of the reaction that is mainly controlled by the transformation of 2 to 1 (Scheme 1). First, the amount of water in the system should be minimized by using absolute ethanol and fresh KOH. Larger quantities of water in the system have been found to not only slow down the formation of 1 but also favor the side formation of 4 (see above). Second, efficient agitation shortens the reaction time because the intermediates and the final product are poorly soluble in ethanol. At the end of the reaction, the precipitated 1 was isolated by filtration as a microcrystalline solid in over 95% yield. Although 1 is intrinsically white, the isolated off-white product was spectroscopically (>98%) and analytically pure (Figures 2 and

reaction times required in the one-pot synthesis of 1 described above (Scheme 1). The excess of PPh3 that is needed for the formation of 3 from RuCl3·xH2O in the first step (see above) remains in the reaction solution, slowing down the sequential transformations leading to 1.24 In conclusion, we have developed a new method to prepare [(Ph3P)3Ru(CO)(H)2] (1), an important catalyst for a broad variety of organic transformations and a useful starting material in inorganic and organometallic synthesis. Unlike the currently used procedure to make 1,8 our method does not utilize toxic and hazardous materials such as formaldehyde and benzene, uses PPh3 in the minimum possible excess, and produces 1 in unprecedented, nearly quantitative yield.25 For most purposes, no purification is needed for the product that precipitates out of the reaction mixture spectroscopically (>98%) and analytically pure. The reaction is conducted in ethanol, the “greenest” organic solvent. Importantly, the latter can be used in small quantities, thereby enabling scalability, as has been demonstrated by preparing up to over 17 g of pure 1 in one batch. The new method is likely to find use in both academic and industrial research.

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EXPERIMENTAL SECTION

Ruthenium trichloride hydrate, ca. 40% Ru (Precious Metals Online, PMO Pty Ltd., Australia), PPh3 (Alfa Aesar), KOH (85% min; Carlo Erba), and absolute ethanol (Panreac) were used as received. Deuterated solvents were purchased from Aldrich. NMR spectra were recorded on Bruker Avance Ultrashield 400 and 500 MHz spectrometers. An Agilent Technologies 7890A chromatograph equipped with a 5975C MSD unit was used for GC-MS analysis. Single-crystal X-ray diffraction studies were performed using a BrukerNonius diffractometer equipped with an APEX II 4K CCD area detector. Elemental analyses were carried out by the Microanalysis Center at the Complutense University of Madrid. An inert atmosphere was maintained throughout the synthesis of 1. Isolation of 1 was performed in air. Preparation of [(Ph3P)3Ru(CO)(H)2] (1). Large Scale. A 1 L twoneck round-bottom flask equipped with a gas inlet, a reflux condenser, and a Teflon-coated magnetic stir bar was charged with absolute ethanol (600 mL) and RuCl3·xH2O (5.0 g). The mixture was stirred at reflux under an argon atmosphere for 10 min for deaeration, then PPh3 (20.0 g, 76.2 mmol) was added, and vigorous agitation at reflux under argon was continued for 1 h. The resultant dark brown suspension of [(Ph3P)3RuCl2] was cooled to room temperature and treated with KOH (pellets; 13.58 g; 205.7 mmol). The mixture was agitated first at room temperature for 1 h and then at 50−60 °C for 1 h. The resultant dark green suspension was then vigorously stirred at reflux for 24 h, during which time the reaction mixture turned light yellow. 31P NMR analysis of a representative sample diluted with benzene under argon indicated full conversion to the final product. After the mixture was cooled to room temperature, the precipitate was separated by filtration in air, washed with ethanol (2 × 50 mL), deionized water (3 × 50 mL), and ethanol (3 × 50 mL), and dried under vacuum. The yield of [(Ph3P)3Ru(CO)(H)2] (1) as a pale yellowish microcrystalline solid was 17.54 g (97%). No NMR-detectable impurities were present. NMR (benzene-d6, 25 °C): 1H, δ 7.6−6.8 (m, 45H, PPh3), −6.5 (tdd, 1H, J = 31, 15, and 6 Hz, Ru−H trans to C), −8.3 (dtd, 1H, J = 74, 29, and 6 Hz, Ru−H trans to P); 31P{1H}, δ 60.7 (d, 2P, J = 17 Hz), 48.6 (t, 1P, J = 17 Hz). Anal. Calcd for C55H47OP3Ru: C, 71.9; H, 5.2. Found: C, 71.7; H, 5.1. Medium Scale. A 100 mL two-neck round-bottom flask equipped with a gas inlet, reflux condenser, and a Teflon-coated magnetic stir bar was charged with absolute ethanol (50 mL) and RuCl3·xH2O (0.50 g). The mixture was stirred at reflux under an argon atmosphere for 10 min for deaeration, then PPh3 (2.00 g, 7.62 mmol) was added, and vigorous agitation at reflux under argon was continued for 1 h. The resultant dark suspension of [(Ph3P)3RuCl2] was cooled to room

Figure 2. 1H NMR spectrum of 1 in benzene-d6 (hydride region).

Figure 3. 31P{1H} NMR spectrum of 1 in benzene-d6.

3). We have also confirmed the solid-state structure of 1 by single-crystal X-ray diffraction (Figure 4)22 and demonstrated the facile scalability of the method by preparing over 17.5 g of 1 in one batch using only 400−600 mL of EtOH.

Figure 4. ORTEP drawing of 1 with thermal ellipsoids drawn at the 50% probability level and all H atoms of the PPh3 ligands omitted for clarity.

Alternatively, the synthesis of 1 can be carried out in two steps. First, RuCl3·xH2O was reacted with PPh3 in EtOH under reflux for 1 h to give 314 quantitatively. The dark brown product was isolated by filtration and treated with KOH (2 equiv) in ethanol under reflux to produce white 1 in 95% yield after only 2 h.23 We found that extra PPh3 slows down the formation of 1 from 3. This observation accounts for the longer C

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temperature and treated with KOH (pellets; 1.07 g; 16.2 mmol). The mixture was agitated first at room temperature for 1 h and then at 50− 60 °C for 1 h, during which time the originally dark brown reaction mixture first turned dark green and then yellow. The mixture was then vigorously stirred at reflux for 10 h, at which point 31P NMR analysis of a representative sample diluted with benzene under argon indicated full conversion to the final product. After the mixture was cooled to room temperature, the precipitate was separated by filtration in air, washed with ethanol (2 × 20 mL), deionized water (3 × 20 mL), and ethanol (3 × 20 mL), and dried under vacuum. The yield of [(Ph3P)3Ru(CO)(H)2] (1) as a pale yellowish microcrystalline solid was 1.77 g (98%). NMR data are as described above. Anal. Calcd for C55H47OP3Ru: C, 71.9; H, 5.2. Found: C, 71.4; H, 5.1. Small Scale. A 100 mL two-neck round-bottom flask equipped with a gas inlet, a reflux condenser, and a Teflon-coated magnetic stir bar was charged with EtOH (75 mL) and RuCl3·xH2O (0.25 g). This solution was stirred at reflux under an argon atmosphere for 10 min for deaeration. Triphenylphosphine (1.00 g, 3.81 mmol) was added, and the mixture was stirred at reflux under an argon atmosphere for 1 h to produce [(Ph3P)3RuCl2] as a dark brown solid and cooled to room temperature. KOH (pellets; 0.21 g; 3.2 mmol) was added, and the mixture was agitated at room temperature for 1 h to give a greenish precipitate and then at 60 °C for 1 h to produce a yellow solid. The mixture was then stirred at reflux for 5 h to reach full conversion to [(Ph3P)3Ru(CO)(H)2] (31P NMR). After the mixture was cooled to room temperature, the slightly yellowish precipitate was separated by filtration, washed with ethanol (2 × 10 mL), deionized water (2 × 10 mL), and ethanol (2 × 10 mL), and then dried under vacuum. The yield was 0.89 g (98%). NMR data are as described above. Anal. Calcd for C55H47OP3Ru: C, 71.9; H, 5.2. Found: C, 71.5; H, 5.1.

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Huang, D.; Gupta, S.; Londergan, T. M.; Sargent, J. R.; Mabry, J. M. ACS Symp. Ser. 2000, 760, 24. (7) (a) Robinson, S. D.; Levison, J. J. J. Chem. Soc. A 1970, 2947. (b) Ahmad, N.; Robinson, S. D.; Uttley, M. F. J. Chem. Soc., Dalton Trans. 1972, 843. (8) Ahmad, N.; Levison, J. J.; Robinson, S. D.; Uttley, M. F. Inorg. Synth. 1974, 15, 45. (9) (a) Kakiuchi, F.; Sekine, S.; Tanaka, Y.; Kamatani, A.; Sonoda, M.; Chatani, N.; Murai, S. Bull. Chem. Soc. Jpn. 1995, 68, 62. (b) Kakiuchi, F.; Kochi, T.; Mizushima, E.; Murai, S. J. Am. Chem. Soc. 2010, 132, 17741. (10) (a) Owston, N. A.; Parker, A. J.; Williams, J. M. J. Org. Lett. 2007, 9, 3599. (b) Maytum, H. C.; Tavassoli, B.; Williams, J. M. J. Org. Let. 2007, 9, 4387. (c) Owston, N. A.; Parker, A. J.; Williams, J. M. J. Chem. Commun. 2008, 624. (d) Blacker, A. J.; Farah, M. M.; Hall, M. I.; Marsden, S. P.; Saidi, O.; Williams, J. M. J. Org. Lett. 2009, 11, 2039. (11) Grounds, H.; Anderson, J. C.; Hayter, B.; Blake, A. J. Organometallics 2009, 28, 5289. (12) Zhang, C.; Wang, B.; Wang, Y. Jingxi Shiyou Huagong 2007, 24, 1. (13) (a) Young, R.; Wilkinson, G. Inorg. Synth. 1977, 17, 75. (b) Young, R.; Wilkinson, G. Inorg. Synth. 1990, 28, 337. (14) Hallman, P. S.; Stephenson, T. A.; Wilkinson, G. Inorg. Synth. 1970, 12, 237. (15) (a) Linn, D. E., Jr. J. Chem. Educ. 1999, 76, 70. (b) Arnáiz, F. J. J. Chem. Educ. 1999, 76, 1484. (c) Linn, D. E., Jr. J. Chem. Educ. 1999, 76, 1485. (16) Stephenson, T. A.; Wilkinson, G. J. Inorg. Nucl. Chem. 1966, 28, 1945. (17) Rard, J. A. Chem. Rev. 1985, 85, 1. (18) Rose, D.; Gilbert, J. D.; Richardson, R. P.; Wilkinson, G. J. Chem. Soc. A 1969, 2610. (19) Ito, T.; Horino, H.; Koshiro, Y.; Yamamoto, A. Bull. Chem. Soc. Jpn. 1982, 55, 504. (20) A lower quality crystal structure of 4 has been reported: Skapski, A. C.; Stephens, F. A. Chem. Commun. 1969, 1008. (21) [(Ph3P)3Ru(CO)2] has been reported to form from 1 upon treatment with PhCOOMe in the presence of an olefin at elevated temperatures: Hiraki, K.; Kira, S.-i.; Kawano, H. Bull. Chem. Soc. Jpn. 1997, 70, 1583. (22) The structure is a solvate with 0.8 molecule of toluene and 0.2 molecule of hexane, modeled with disorder over three positions with a 40:40:20 occupancy ratio. An X-ray structure of 1·CH2Cl2 has been reported: Junk, P. C.; Steed, J. W. J. Organomet. Chem. 1999, 587, 191. (23) The formation of 1 from [(Ph3P)3Ru(CO)(H)(Cl)] and NaOH in boiling methyl cellosolve has been reported: Boniface, S. M.; Clark, G. R.; Collins, T. J.; Roper, W. R. J. Organomet. Chem. 1981, 206, 109. (24) Attempts were made to shorten the reaction time to full conversion to 1 by performing the synthesis at a higher temperature, in boiling n-BuOH (bp 118 °C) in place of EtOH. Under these conditions, however, the reaction was poorly selective, giving rise to a complex mixture of products. In MeOH, the reaction gave large quantities of [(Ph3P)3Ru(CO)2] as a side product.21 (25) The formation of 1 in the reaction of premade 3 with KOH in toluene−glycerol−water at 80 °C has recently been reported. The yield of 1, the conversion of 3, and reaction selectivity are not specified in this article. See: Dibenedetto, A.; Stufano, P.; Nocito, F.; Aresta, M. ChemSusChem 2011, 4, 1311.

ASSOCIATED CONTENT

S Supporting Information *

Text, figures, and CIF files giving full details of synthetic and crystallographic studies. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for V.V.G.: [email protected]. Notes

The authors declare no competing financial interests.

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ACKNOWLEDGMENTS We thank Drs. Marta Martı ́nez Belmonte, Eddy Martin, and Eduardo C. Escudero-Adán for crystallographic studies, Fedor M. Miloserdov for checking the procedure, and Prof. Michael K. Whittlesey for valuable comments. The ICIQ Foundation, Consolider Ingenio 2010 (Grant CSD2006-0003), and the Spanish Government (Grant CTQ2011-25418) are thankfully acknowledged for support of this research.

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REFERENCES

(1) Hallman, P. S.; McGarvey, B. R.; Wilkinson, G. J. Chem. Soc. A 1968, 3143. (2) (a) Kakiuchi, F.; Murai, S. Acc. Chem. Res. 2002, 35, 826. (b) Kakiuchi, F.; Murai, S. Top. Organomet. Chem. 1999, 3, 47. (3) Arockiam, P. B.; Bruneau, C.; Dixneuf, P. H. Chem. Rev. 2012, 112, 5879. (4) Naota, T.; Takaya, H.; Murahashi, S.-I. Chem. Rev. 1998, 98, 2599. (5) Nixon, T. D.; Whittlesey, M. K.; Williams, J. M. J. Dalton Trans. 2009, 753. (6) (a) Mabry, J. M.; Runyon, M. K.; Paulasaari, J. K.; Weber, W. P. ACS Symp. Ser. 2003, 838, 50. (b) Weber, W. P.; Paulasaari, J. K.; D

dx.doi.org/10.1021/om400461w | Organometallics XXXX, XXX, XXX−XXX