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Recyclable hydroboration of alkynes using RuH@IL and RuH@IL/scCO2 catalytic systems Jakub Szyling, Adrian Franczyk, Kinga Stefanowska, Hieronim Maciejewski, and Jedrzej Walkowiak ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.8b02388 • Publication Date (Web): 11 Jul 2018 Downloaded from http://pubs.acs.org on July 14, 2018

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ACS Sustainable Chemistry & Engineering

Recyclable hydroboration of alkynes using RuH@IL and RuH@IL/scCO2 catalytic systems Jakub Szyling†,‡, Adrian Franczyk†, Kinga Stefanowska†,‡, Hieronim Maciejewski‡,§ , Jędrzej Walkowiak†,* †Centre for Advanced Technologies, Adam Mickiewicz University in Poznan, Umultowska 89c, 61-614 Poznan, Poland. ‡Faculty of Chemistry, Adam Mickiewicz University in Poznan, Umultowska 89b, 61-614 Poznan, Poland. § Poznan Science and Technology Park of Adam Mickiewicz University Foundation, Rubiez 46, 61-612 Poznan, Poland *[email protected]

ABSTRACT This paper reports on the first detailed studies on green and sustainable methods for the repetitive batch hydroboration of terminal and internal alkynes by the effective immobilization of Ru(CO)Cl(H)(PPh3)3 in various ionic liquids (RuH@IL) or in biphasic ionic liquid/supercritical

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CO2 (RuH@IL/scCO2) systems. The systems RuH(1mol%)@[EMPyrr][OTf](IL8) and RuH(1mol%)@[EMPyrr] [NTf2](IL9) were found to be the most effective immobilization approaches, with both allowing the completion of over 10 complete catalytic cycles in the hydroboration of a series of different alkynes. An increase in the catalyst content to 2 mol% (RuH(2mol%)@[EMPyrr][NTf2](IL9)) allowed for the completion of 25 repetitive batches, which proved the high utility of the developed system. In each case, high yields of products were obtained, and their purity was confirmed through NMR, MS, and ICP techniques. On the other hand, the use of RuH(1mol%)@/[EMPyrr][OTf](IL8)/scCO2 in the hydroboration of phenylacetylene with pinacolborane, permitted effective completion of 8 cycles, at a much lower temperature than that at which RuH@IL was used (40 vs.100 °C). Moreover, the application of such a system has a positive impact on the environment and process sustainability via the exchanging of organic solvent for scCO2 used for product extraction and separation when working with RuH@IL. The strategies presented in this work are, so far, the most effective and recyclable systems based on ILs for the hydroboration of alkynes with high TONs. In the future they will be applied in continuous flow regime.

KEYWORDS Hydroboration, alkynes, ionic liquids, supercritical CO2, catalyst immobilization, ruthenium, alkenyl boronates.

INTRODUCTION Due to negligible vapor pressure, incombustibility, enormous structural variability, and good solvent properties ionic liquids (ILs) are considered to be one of the most important media among


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the environmentally benign solvents in designing green and sustainable processes.1 This class of compounds, which are composed of bulky organic cation and organic or inorganic anion, have been widely used in academic laboratories and industry as a solvent in chemical syntheses,2 solution for metal extraction,3-6 battery electrolytes,7-9 solar cells,10-12 biological applications,13-14 and biomass processing.15-17 The application of ILs as green reaction solvents has been widely studied for a number of years in various chemical transformations and has led to the development of sustainable, environmentally friendly processes. Transition metal (TM) catalysis in ILs occupies a special place in organic synthesis due to the possibility of replacement of traditional organic solvents with nonvolatile media, the chance for the immobilization of molecular catalyst (often very expensive), and its multiple reuse in batch or continuous flow regimes.18-19 Furthermore, the TM-catalysts, especially ionic but also those with moderate polarity, are easily soluble in ILs. The polarity of IL, as well as its coordinative properties, can be controlled by the selection of cation and anion, and in many cases by the modification of one ligand in the catalyst structure. However, this last manipulation is often not required in order to obtain a homogeneous and catalytically active system. In contrast to ILs, the application of other green solvents as perfluorinated compounds or supercritical CO2 (scCO2) usually requires modification of the ligands (by incorporation of e.g. perfluorinated ponytail or silyl groups) to obtain homogeneous phase with these non-polar reaction media. Moreover, efficient immobilization of the catalyst in the ionic phase significantly reduces the content of the transition metal in the final product, which is of utmost importance in many branches of industry which require high purity e.g. in the pharmaceutical or food industries. These features are often crucial criteria for a large-scale use of homogeneous catalysis.20


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Furthermore, the catalysts immobilized in ILs can be combined with scCO2 to build a powerful method for repetitive batch production, which employs the special properties of both green solvents to carry out reactions, build an effective separation strategy, and allow catalyst recycling. Such a biphasic system permits development of an attractive approach benefiting from the advantages of both homogeneous and heterogeneous catalysis.21-22 The application of liquefied or supercritical CO2 reduces the viscosity of ILs and their melting points, thereby facilitating the mass transfer and efficient extraction of the products by compressed CO2 from ILs. The process occurs more often in milder conditions, without any organic solvent, and at higher reaction rates. The negligible solubility of ILs in scCO2 results in the use of these solvents in repetitive batch and continuous flow processes. In such systems the catalyst is immobilized in IL, maintain its homogeneous character, while reagents and products are transported by CO2.23-26 The limitation of the application of such catalyst system results from the volatility of reagents and their polarity. They need to be highly volatile to be easily extracted from the reactor. Our research interest perfectly matches the above-described area since we are focused on the synthesis and catalytic transformation of organoboron and organosilicon compounds performed in the presence of molecular catalysts and in one-pot procedures, immobilization of molecular catalysts, the use of non-conventional media, and separation techniques – e.g. IL, scCO2.27-37 Due to their high reactivity, low toxicity, ease of storage, the resulting B- or Si-containing products are considered as very valuable building blocks in organic synthesis and can be applied in many transformations e.g. Suzuki reaction,38-39 halodemetallation31, 40-41 and Hiyama coupling4243

etc. As such, the development of their synthetic and functionalization methods is highly

desirable.


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Investigation into hydroboration in ILs has been almost entirely neglected. Our literature survey indicates that reports on the synthesis of unsaturated organoboron compounds in ILs are limited to only the single study described by Vaultier et al., who reacted hex-1-yne with pinacolborane, in the presence of rhodium or zirconium catalysts immobilized mostly in imidazolium ILs – with pseudo binary, toxic and hygroscopic anions such as: chloroaluminate ([Cl] -/AlCl3) or chlorozincate ([Cl]-/ZnCl2]). In the reported protocol, the most effective composition of RhCl(PPh3)3 (0.8 mol%) with [BMIM][Cl][ZnCl2] was recycled six times. The authors only provided information about the yields and selectivity of isolated products from each batch, without a description of the post-reaction mixture composition. The obtained products were isolated via time-consuming and complicated bulb to bulb distillation, which can potentially generate problems with accumulation of high-boiling point by-products in the non-volatile ionic liquid phase. Moreover, a decrease in the catalyst activity was already observed in the second cycle, indicating low efficiency of the tested system.44 Likewise, the hydroboration of alkynes in IL/scCO2 thus far remains unexplored. There are only two examples of the application of scCO2 as a solvent for hydroboration. One study investigated alkene hydroboration under homogeneous conditions by employing Rh catalyst with modified perfluorinated phosphines as ligands45 and the second is our study on hydroboration of a series of terminal and internal alkynes catalyzed by self-dosing Ru(CO)Cl(H)(PPh3)3, which is gradually released to the reactor under high pressure in supercritical conditions (170-180 bar, 100 °C, d=0.40-0.43 g/cm3).32 The higher pressure the more soluble catalyst was in CO2, due to its stronger solvation power. During the extraction process of the products, low volatile catalyst remained in the reactor, giving the possibility for its reuse in the following repetitive batches with high efficiency. The method developed in that study proved to be highly efficient under repetitive batch


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mode. Up to the 8 catalytic cycles with high yield and selectivity in the presence of 1 mol% of Ru(CO)Cl(H)(PPh3)3) were achieved. An increase in the catalyst content to 2 mol% resulted in 16 cycles, with complete conversion of phenylacetylene. The above-described results have prompted us to develop new methods for the synthesis of alkenyl boronates and we therefore decided to investigate the activity of Ru(CO)Cl(H)(PPh3)3 immobilized in ILs (RuH@IL) as well as in a biphasic ionic liquid/supercritical CO2 system (RuH@IL/scCO2). Our main task was to develop highly active, efficient and long-lasting catalytic systems. These systems were based on the elimination or reduction of volatile organic solvents, effective catalyst immobilization, and the simplicity of the product separation from post-reaction mixtures for the hydroboration of alkyne, which in the future could be applied in continuous flow regime (Scheme 1).

Scheme 1. Recyclability of the catalyst in hydroboration reaction achieved by the application of RuH@IL or RuH@IL/scCO2 systems.


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EXPERIMENTAL SECTION General Information: Materials: Ionic liquids were purchased from Sigma-Aldrich or IoLiTech with purity at least 97% and were dried under vacuum at 40 °C over 16 hours before use. 4,4,5,5-Tetramethyl-1,3,2-dioxaborolane, alkynes and deuterium solvents were commercially available and were used as supplied. Liquid. CO2 4.5 tank (99.995%) was purchased from Messer. The Ru(CO)Cl(H)(PPh3)3 was synthesized according to the literature procedure.51 Methods: 1

H NMR spectra were recorded at 25 °C on Bruker UltraShield 300 MHz. Chemical shifts were

reported in ppm with reference to the residue portion solvent peak. The multiplicities are reported as follows: singlet (s), doublet (d), doublet of triplets (dt), multiplet (m), triplet (t) and broad resonances (br). 31P NMR spectra were recorded on Bruker Ascend 400 MHz NANOBAY with 85% H3PO4 as a reference. The mass spectra of the products were obtained by GC-MS analysis on a Bruker Scion 436-GC with a 30 m Varian DB-5 0.25 mm capillary column and a Scion SQ-MS mass spectrometry detector (60 °C (3 min), 10°C/min, 250 °C (30 min)). Metal content in the products were determined by inductively coupled plasma-mass spectroscopy (ICP-MS) with a NexION 350D (PerkinElmer). The products were isolated using column chromatography with the columns filled with silica 60. The separation was carried out with Flash Chromatograph Biotage IsoleraOne using cartridge 10 g, flow rate: 12 mL/min, time: 40 min, phase: hexane/ethyl acetate, gradient elution 1%/0.5 min (initial: hexane 100 %), UV detector (wavelength: 254 nm). General procedures: All manipulations were performed using standard Schlenk techniques.


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Hydroboration in RuH@IL system under repetitive batch mode The catalyst (0.01 mmol) and ionic liquid (4 mmol) were placed into a Schlenk vessel equipped with a stirring bar and dried under vacuum at 40 °C, for over 16 hours. Alkyne (1 mmol) and 4,4,5,5-tetramethyl-1,3,2-dioxaborolane (1.2 mmol) were added and the reactions were performed at 100 °C. After IL melted, the molar concentration of reagents was approximately 0.8M. After 15 minutes (terminal alkynes) or 3 hours (internal alkynes), the products were extracted with hexane (3x5 mL), purified using column chromatography with the columns filled with silica 60 and analyzed by GC-MS and 1H analyzes. Subsequently, the traces of n-hexane were removed under vacuum, a new loading of the substrates was added, and the process was repeated in the abovedescribed conditions. Extracted products from the next batches were analyzed by GC-MS without any purification steps. The leaching of the catalyst was determined by ICP-MS analysis, on the basis of measurements of the Ru content in the extract. Hydroboration in biphasic RuH@IL/scCO2 system The catalyst (0.01) and ionic liquid (4 mmol) were introduced into a stainless steel reactor (10 mL) equipped with a stirring bar and sapphire windows for visual inspection of phase behavior. They were then dried under vacuum at 40 °C, for over 16 hours. Then, the alkyne (1 mmol) and 4,4,5,5-tetramethyl-1,3,2-dioxaborolane

(1.2

mmol)

were

added.

Subsequently,

CO2

(approximately: 50-55 bar) was added and the setup was heated up to 100 °C (or 40 °C). When the temperature was stabilized, the system was pressurized to 180 – 190 bar. After 15 minutes (or 3 hours) products were extracted in a CO2 stream (pressure: 130 – 160 bar, CO2 flow: 9 mL/min) at 40 °C in 30 min into a bottle trap with a small amount of hexane cooled down to -78 °C. After evaporation of hexane, the products were analyzed by GC-MS analysis. Afterward, a new loading of substrates was added under an argon atmosphere and the process was repeated according to


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above described procedure. The leaching of the catalyst was determined by ICP-MS analysis, on the basis of measurements of the Ru content in the extract.

RESULTS AND DISCUSSION Reaction conditions optimization At the first stage of our investigations several ILs based on quaternary ammonium cations (imidazolium or pyrrolidinium) and inorganic or organic anions were tested in model hydroboration reaction between phenylacetylene (2a) and pinacolborane (1) at 100 °C, in the presence of ruthenium hydride catalyst – Ru(CO)Cl(H)(PPh3)3 (Scheme 2). In all cases, yellow or orange homogeneous solutions of the catalyst were obtained when the Ru-complex was dissolved in ILs. The reagents were easily soluble in the catalytic system and the reaction occurred in the monophasic system, without any diffusion limitations (usually caused by the process at the interface of two immiscible liquids).

Scheme 2. The hydroboration of alkynes (2a-e) with pinacolborane (1) and the scope of ionic liquids (IL1-IL9) used in the studies for RuH@IL system.


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To determine the yield of the reaction, the products were extracted with non-polar hexane and subjected to GC-MS and NMR analyses. During the extraction process, the biphasic hexane-IL system was observed until most of the ILs had become solidified due to the heat exchange with the solvent used for the extraction. A biphasic solid-liquid system was obtained, in which the upper hexane-phase bearing products was easily decanted. The choice of organic solvent is crucial for the development of effective extraction process. It should have low polarity in order to be immiscible with ILs and to prevent catalyst leaching, but should also readily dissolve the obtained products. Moreover, in this method the products can be separated from the catalyst immobilized in ionic liquid without heating. Through this process thermal deactivation is avoided and all potential by-products are removed from the extraction mixture. Cross-contamination of ILs through extractant can be avoided by simple evaporation of the organic phase under vacuum. When more polar solvents as THF, CH2Cl2 or toluene were used for extraction, the yellow color of the extract revealed from the catalyst was visible, clearly indicating its leaching. Excellent, almost quantitative reaction yields were observed for all tested RuH@IL systems, apart from IL1 and IL3. The presence of chloride anions in their structures deteriorates the catalyst activity. Similar observations have been previously made for the other transformations.46 In light of above, the application of ionic liquids with weakly coordinating anions like [BF4]-, [PF6]-, [NTf2]-, [OTf]- or [TOS]- seems to be crucial for the effectiveness of the applied catalytic system, which retain the initial reactivity of the metal complex and are not affected by the interactions with ILs. To exclude non-catalytic hydroboration of 2a or the catalytic activity of IL, the reaction was performed without the catalyst in IL8 at 100 °C. Formation of the desired unsaturated organoboron compounds was not detected by GC-MS analysis after 15 minutes. Furthermore, an extension of


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the reaction time to 120 hours resulted in only 22 % reaction yield. When toluene was used as a solvent, the products were not observed in the reaction mixture, after 15 minutes in the same reaction conditions in the presence of the RuH catalyst. The total conversion of reagents was achieved in 3 hours. It should be underlined, that the application of ILs as solvents in this reaction allowed for complete conversion of reagents in 15 minutes, making this system much more efficient in comparison to the hydroboration carried out in toluene or scCO2. This phenomenon is ambiguous and requires detailed mechanistic studies, what will be the subject of our forthcoming reports. To measure metal content in the final product, 15-20 mg of the extract were analyzed by ICP-MS. In all samples, the amount of ruthenium was at a very low level – over 100 times lower than in the analogous reaction carried out in toluene, regardless of the ionic liquid used (Figure 1). Ru [ppm]

0.04

0.03

0.03

0.05

0.06

0.08

0.06

0.09

0.11

10

100

Yields of the reaction [%]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80

60

40

20

0 IL 1

IL 2

IL 3

IL 4

IL 5

IL 6

IL 7

IL 8

IL 9 Toluene

Solvent 3a

4a

5a

Figure 1. The yields of the hydroboration reaction of phenylacetylene (2a) with pinacolborane (1) carried out in RuH(1mol%)@IL1–IL9 systems at 100 °C in 15 min. Ru content (in ppm) in the final products is written above each bar. Hydroboration in toluene was performed at 100 °C in 3 hours.


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Negligible catalyst leaching from the reaction system can result from: i) residual solubility of the catalyst in the extractant or ii) residual solubility of the catalyst in final products. Although the risk of the first can be eliminated or reduced by application of another co-solvent (e.g. supercritical CO2), the second depends on the nature of the process. Nevertheless, a significant reduction of the metal content in the final products, with simple and fast extraction, is no doubt an advantage of this work and confirms effective immobilization of Ru-catalyst in ILs. There is no need for the time-consuming and costly modification of the catalyst coordination sphere of the complex, which should be considered as further advantage of the system presented here. Repetitive batch hydroboration of alkynes in RuH@ILs catalytic system High reaction and extraction yields and selectivities, as well as very low catalyst leaching in all the tested ILs, provided the basis for the development of a protocol for the synthesis of unsaturated organoboron compounds in repetitive batch experiments with multiple reuse of the catalyst (Figure 2). Hydroboration of 2a in IL1–IL3 under repetitive batch mode resulted in a significant decrease in the reaction yield in the 2nd and 3rd catalytic cycle, respectively. Although this phenomenon for IL1 and IL3 bearing [Cl]- anions can be explained through deactivation of ruthenium hydride catalyst from the nucleophilic chloride, it is quite surprising for IL2 bearing non-coordinating [PF6]- anion, which is stable under the applied anhydrous conditions. The application of IL4, containing tetrafluoroborate with similar properties to hexafluorophosphate, resulted in excellent reaction yield until the 4th catalytic cycle. Decidedly better results were obtained for IL6 and IL7, for which very good reaction efficiency, complete reagents conversion, and high products yield and selectivities, were observed until the 7th cycle.


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Figure 2. Hydroboration of phenylacetylene (2a) with pinacolborane (1) under repetitive batch mode using RuH(1mol%)@IL1–IL9 at 100 °C in 15 min. The

best

results

were

obtained

with

RuH(1mol%)@[EMPyrr][OTf](IL8)

and

RuH(1mol%)@[EMPyrr] [NTf2](IL9) containing non-coordinating, very stable, and highly hydrophobic anions. These compositions produced the desired borylated styrene 3a with excellent yields up to the 10th and 13th cycles, respectively. It is worth reiterating that such good results for the multiple reuse of the cat@ILs system for hydroboration of alkynes are reported here for the first time and this system should be considered as the most effective one. To check whether the catalyst loading effect on the process efficiency is detectable for hydroboration of 2a, the reaction was performed with 2 mol% of Ru(CO)Cl(H)(PPh3)3 in IL9 (RuH(2mol%)@IL9). The 100% increase in the catalyst content allowed for carrying out 25 full catalytic runs with the 1345 accumulative TON values. The time of the run was doubled after the


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100 90 80 70

Yield of 3a [%]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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60 50 40 30 20 10 0 1

2

3

4

5

6

7

8

9

10 ... 25 26 27 28

Cycle

Figure 3. Yields of 3a obtained in the hydroboration of phenylacetylene (2a) with pinacolborane (1) in the presence of RuH(2mol%)@IL9 at 100 °C in 15 min or 30 min (from batch 16th). first 15 cycles, because of the slight decrease of the reaction rate, which was monitored by GC analysis. This procedure permitted to obtain full conversion of reagents up to the 25th batch (Figure 3). The Ru leaching in the same cycles was at a very similar level as that for the reaction with 1 mol% of the catalyst, suggesting that the increased amount of the catalyst did not influence the efficiency of the complex immobilization in IL9. Lower catalyst loading (0.5 mol%) permitted to complete only 4 runs, with accumulative TON 997, whereupon rapid decrease of the conversion of 2a was observed. This clearly shows that higher catalyst concentration in IL allows for carrying out more repetitive batches and positively influence the process productivity. Due to the fact that the catalyst leaching is not the issue of the loss of the catalytic system activity after several batches when IL8 and IL9 were used for its immobilization, we were tried to


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determine, whether an inactive catalytic species is formed during the following batches. The gradual decrease in the reagents conversion can be resulted also from the catalyst decomposition by impurities of the compounds and solvents used for extraction (e.g. moisture), which might accumulate in IL phase and deactivate or slow down the catalyst. To check the possibility for the formation of inactive species we have carried out the hydroboration of phenylacetylene with pinacolborane in the presence of RuH(5mol%)@[EMPyrr][NTf2](IL9) with a low coordinating anion, which was characterized by high stability and good activity in the repetitive batches. The process was monitored by GC-MS and 31P NMR. The catalyst content in the IL was increased to 5 mol % because the reaction with 1 or 2 mol% gave the low intensity of signals coming from the Ru-complex in

31

P NMR spectra or the signals were invisible. The spectrum of the initial

[Ru(CO)Cl(H)(PPh3)3] complex contains signals from coordinated phosphines at 39.02 and 12.96 ppm, as well as, the signal from triphenylphosphine oxide at 29.00 ppm, which occurred in the catalyst as its contamination. The initial

31

P NMR spectrum of RuH(5mol%)@IL9, carried out

after its dissolution in IL in the process temperature (100oC) did not show any significant difference in the signals intensity and their chemical shifts. Any new signals were not observed after catalyst immobilization, suggesting that there are no chemical interactions between the Ru complex and IL9. Then, the reagents were added and each run was carried out for 15 minutes, under optimized conditions. The extract was injected to the GC-MS analysis to monitor the formation of the products and reagents conversion, while the RuH(5mol%)@IL9 phase was analyzed by 31P NMR. The spectra for the first 15 cycles, taken after the 1st, 2nd, 5th and 15th cycles looked similarly. The signals from the Ru(CO)Cl(H)(PPh3)3 catalyst were visible. Moreover, the total conversion of phenylacetylene and formation of borylated products was monitored by GCMS analyses. The decrease of the reaction yield was noticeable from the 19th cycle. The formation


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of the new catalytic species was observed in this spectrum at 38.0 ppm, while the intensity of the signal of the initial complex decreases. In the 32nd cycle, the signals from the Ru(CO)Cl(H)(PPh3)3 were not monitored in the 31P NMR analysis and the conversion of phenylacetylene was at really low level – 3%, which means that the catalyst was completely deactivated. Moreover, it needs to be underline that in the above-described experiment, even 2.5 or 5.0-times higher catalyst concentration was used than in the typical catalyst test, a similar number of catalytic cycles were obtained. The reason of that was the reduction of the catalyst content in the reactor by its subsequent sampling for the 31P NMR analysis. Further research focused on the reaction mechanism and kinetics based on the stoichiometric experiments, as well as DFT calculations are now under investigation to fully support the problems with the catalyst activity and stability and will be the subject of another more specific paper focused on hydroboration reaction mechanism in IL environment. Encouraged by the excellent results for hydroboration of phenylacetylene (2a) with pinacolborane in IL8 and IL9 at 100 °C for 15 minutes in the presence of 1 mol% of Ru(CO)Cl(H)(PPh3)3, we investigated two different terminal alkynes: triethylsilylacetylene (2b) and 1-heptyne (2c), as well as two internal alkynes: 1,2-diphenylacetylene (2d) and 4-octyne (2e) (Figure 4). The same alkynes have been previously tested for the reactions in monophasic scCO 2 system.32 For terminal alkynes, 10 catalytic runs were performed in IL8 and IL9 without a significant decrease in the reaction yield. In contrast to the model reaction, hydroboration of nonaromatic compounds results in deterioration of selectivity, which was also identified in our previous work.32 The major products were (E)-substituted alkenyl boronates in all batches, irrespective of the ionic liquid used (approximately: 85 % for silyl-substituted and 55 % for alkylsubstituted alkenyl boronate). On the other hand, for internal alkynes, the major products were (Z)-


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substituted alkenyl boronates. Hydroboration of 2d and 2e performed in the same time as for terminal alkynes resulted in low alkynes conversions (33 and 41% respectively). It is probably caused by the lower reactivity of internal alkynes as well as less access to the more shielded C≡C triple bond. Thus, extension of the reaction time from 15 minutes to 1.5 hour was necessary for complete conversion of 2e in the first cycle for reaction carried out in IL8 and IL9.

a)

b)

Figure 4. Yields in hydroboration of 2a-e with pinacolborane (1) at 100 °C in the a) RuH(1mol%)@[EMPyrr][OTf](IL8) and b) RuH(1mol%)@[EMPyrr] [NTf2](IL9) systems. Hydroboration of terminal alkynes (2a-c) was performed in 15 min while hydroboration of internal alkynes was performed in 1.5 h (2d-e)


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Deactivation of the catalytic system after the first cycle was only observed for 2e in IL9. Conversion of 2d after 1.5 h reached over 90 % for both solvents up to the 8th cycle and maintained unchanged even if the reaction time was extended to 3 hours. Application of IL8 and IL9 allows for conversion of much higher amounts of reagents using the single loading of the catalyst, than in typical homogeneous process utilizing volatile organic solvents. This was proved by the accumulative TON values (Figure 4). The application of IL permitted to reduce the reaction time of the single batch to 15 minutes in comparison to toluene and scCO2 (3 hours), which showed the high efficiency of RuH@IL system. Moreover, such an environmentally aware approach permitted multiple uses of the expensive transition metal molecular complexes, whilst also simplifying the separation procedures. Hydroboration of alkynes in RuH@ILs/scCO2 catalytic system Although the proposed protocol for hydroboration of alkynes in ILs with subsequent extraction of the products with hexane is efficient and meets green chemistry requirements, we tried to improve and make it even “greener” through the elimination of the hexane and application of biphasic solvent system cat@IL/scCO2. Our previous research has shown that the borylated alkenes obtained in the hydroboration of terminal and internal alkynes can be easily extracted in this green solvent because of the presence of boryl group, which has a positive impact on their solubility in scCO2.32 On the basis of the results obtained for hydroboration of 2a in ILs under repetitive batch mode, we thus selected IL8 and IL9 as the most promising ILs for further investigation. Additionally, the presence of the fluorine atoms in IL structures has a positive impact on the solubility of scCO2, facilitating the mass transfer in IL through a reduction in its viscosity.47 Hydroboration in RuH(1mol%)@IL8/scCO2 and RuH(1mol%)@IL9/scCO2 systems was performed in a high-pressure reactor equipped with a sapphire window allowing visual inspection


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of the phase behavior at 100 °C for 15 minutes under 180-190 bar of CO2. The pressure and temperature conditions were previously determined for the reaction in pure scCO2.32 Similar to the reaction in RuH@ILs systems, the formation of orange, homogenous solution constituting lower phase bearing immobilized catalyst was observed. After 15 minutes, the products were extracted in CO2 stream (130 – 160 bar, 0.74 – 079 g/cm3) at 40 °C for 30 minutes with 90-95% efficiency and subjected to further ICP-MS analysis to determine catalyst leaching. The Ru contents found in the final products for both RuH(1mol%)@IL8/scCO2 and RuH(1mol%)@IL9/scCO2 systems were slightly lower than that obtained in RuH(1mol%)@IL8 and RuH(1mol%)@IL9 systems (0.06 vs 0.09 and 0.07 vs 0.11 ppm, respectively). Encouraged by the promising results for the hydroboration of 2a in IL8 and IL9, the activity of the system under repetitive batch mode was subsequently checked. After each complete run, another portion of the substrates was added and the reactor was heated and pressurized to the required reaction conditions. Conversions and selectivities for hydroboration of 2a in RuH(1mol%)@IL8 and RuH(1mol%)@IL8/scCO2 systems were comparable up to the 8th catalytic cycle. On the other hand, the use of the RuH(1mol%)@IL9/scCO2 system already resulted in low yield after just the second cycle. Due to this fact RuH(1mol%)@IL8/scCO2 system was chosen for further investigation. During the research, under 55 bar of CO2 we noticed the melting of IL8 already at room temperature (for detailed phase behavior description see supporting information).


19


For RuH(1mol%)@IL8 the IL melted at approx. 100 °C. Depression of the melting point for organic molecular solids via pressurization with CO2 is a well-known phenomenon.48-50 However, it has yet to be described for alkyne hydroboration. It was therefore possible to carry out the hydroboration of 2a in RuH(1mol%)@IL8/scCO2 at a much lower temperature (40 °C under 180 bar). Formation of the orange homogeneous solution of catalyst dissolved in IL8 was observed through the sapphire window during the reaction. However, lowering the reaction temperature resulted in low 2a conversion (18 %). Thus, extending reaction time from 15 min to 3 hours was essential for complete conversion. The process gave the desired products with an excellent yield up until the 6th cycle. Extraction yields in all batches were high, with values in the 90 – 95 % range. The reaction of 2a with 1 in RuH(1mol%)@IL8/scCO2 at 40 °C resulted in exclusive formation of 3a in the first three cycles. At 100 °C, for RuH(1mol%)@IL8 and RuH(1mol%)@IL8/scCO2 systems, lower selectivity was observed (Figure 5). a)

b)

100

100

80

80

60

60

Yield [%]

Yield [%]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0 1

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7

8

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Cycle

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Figure 5. Yields for hydroboration of phenylacetylene (2a) with pinacolborane (1) in IL8/scCO2 a) at 100 °C for 15 minutes and b) at 40 °C in 3 hours in the presence of 1mol% of Ru(CO)Cl(H)(PPh3)3.


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Not only the process conditions, especially temperature, may have the influence on the selectivity but also the catalyst itself. It is generally known that ruthenium complexes catalyze the isomerization reaction of olefins. Therefore to check, whether the decrease in the reaction selectivity occurred from the isomerization of borylated product (3a), which is formed in the majority during the hydroboration of phenylacetylene (2a) with pinacolborane, we have added the pure isolated product (3a) to the reactor with a new load of RuH(1mol%)@IL8 and carried out the reaction in scCO2 for 15 minutes at 100 °C. Then the reaction mixture was extracted in CO2 stream and analyzed by 1H NMR. The formation of the second isomer (4a) was observed in the ratio (3a):(4a)=20:1. It proves, that the lowering of the process selectivity resulted also in the side isomerization reaction, which occurred with the lower rate. The same observations were made in monophasic IL system carried out at 100 °C. Moreover, at 40 °C using the RuH(1mol%)@IL8/scCO2 system the formation of additional isomers in the first three cycles was not detectable, because the isomerization reaction in these conditions is sluggish. The formation of new catalytic species in the following cycles can be more active in the isomerization process and therefore the amount of isomers 4a and 5a increased in the next batches, when the reaction was carried out at lower temperature. It is worth mentioning that the application of compressed CO2 in the biphasic solvent system as an extractant, not only eliminates the use of organic solvent, but also allows a significant reduction of the process and extraction temperature, which is not possible in the classical path, due to the high melting points of ILs used. The yields of the products and conversion of reagents were high and similar to those of the analogous reactions carried out in scCO2 at 100 °C, using the self-dosing [RuH] catalyst previously reported in our own research.32 It should be pointed out that lowering the temperature to 40 °C for the reaction of 2a with 1 in the presence of Ru(CO)Cl(H)(PPh3)3 (1


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mol%) performed for 3 hours in monophasic – scCO2 system resulted only in 52 % reaction yield. Thus, application of IL8 in biphasic RuH(1mol%)@IL8/scCO2 system, beside lowering the melting point of IL8, has a positive impact on the reaction efficiency. The catalyst was very well soluble in IL and remained active in subsequent catalytic runs.

CONCLUSIONS In the above-described studies, a highly efficient synthetic protocol leading to alkenyl boronates via catalytic hydroboration of various alkynes by the immobilization of the commercially available catalyst Ru(CO)Cl(H)(PPh3)3 in ionic liquids (RuH@IL) or in the biphasic IL/supercritical CO2 system (RuH@IL/scCO2) was developed. The presented approach permitted the reuse of the molecular complex dissolved in IL, making these catalytic systems the most productive for the recyclable hydroboration of alkynes according to the methods previously described in the literature. Moreover, the time of the reaction was shortened just to 15 minutes in a single batch, comparing to the same reactions carried out in toluene or scCO2, where the full conversion of alkyne was observed after 3 h. The mechanistic aspects of these phenomenon are now under investigations

and

will

be

the

subject

of

a

separate

publication.

RuH(1mol%)@[EMPyrr][NTf2](IL9) was found to be the most effective system for repetitive batch hydroboration, which converts a diverse number of alkynes to (E)-alkenyl boronates with high selectivity and yield, without a significant loss of its activity for up to 10 catalytic cycles. The increase in the catalyst concentration to 2 mol%, allowed for the completion of 25 catalytic cycles which, according to our knowledge, is the best ever recyclable system in hydroboration of alkynes. The high selectivity and product yields were observed throughout the entire process, with the accumulative TON values up to 1345. This clearly proved the utility of the developed method. On


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the other hand, the application of the RuH(1mol%)@/[EMPyrr][OTf](IL8)/scCO2 system permitted the carrying out of up to 8 repetitive batches at even a much lower temperature than that used with RuH@IL (40 vs 100 °C). This is due to the decrease in the melting point of IL when the reactor is pressurized with CO2. The selectivities were even better and the (E)-isomer was exclusively formed in the first three runs. Moreover, the application of CO2 has a positive impact on process sustainability through the elimination of hexane used for extraction of the products when RuH@IL is applied, as well as a simplification and shortening of the separation method (release of CO2 gave the pure products). Both protocols, based on a single or double solvent system, are effective alternatives to those that use organic solvents. Furthermore, the elimination or reduction of metal content in the final products means that the above proposed systems easily meet green chemistry requirements. The presented approach, which is characterized by high stability and residual catalyst leaching, is a promising perspective for our further investigations on continuous flow hydroboration using scCO2 as a mobile phase.

ASSOCIATED CONTENT Supporting Information. General information and products characterization are available free of charge at http://pubs.acs.org AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]


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Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors acknowledge the financial support from the National Centre for Research and Development in Poland – Lider Programme No. LIDER/26/527/L-5/13/NCBR/2014 and National Science Centre (Poland) – Project Opus, No. UMO-2014/15/B/ST5/04257 REFERENCES (1) Wasserscheid, P.; Keim, W. Ionic liquids—new “solutions” for transition metal catalysis. Angew.

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ABSTRACT GRAPHIC

SYNOPSIS Efficient immobilization of [Ru(CO)Cl(H)(PPh3)3] in ILs provides the environmentally benign route to alkenyl boronates by multiple reuses of the solvent and catalyst as well as by omitting waste-generating purifications steps.


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Recyclable Hydroboration of Alkynes Using RuH ... - ACS Publications

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