Katsuaki
Kobayashi
a,
Norihisa
Fukaya
b and
Hiroshi
Nakazawa
*a
aDepartment of Chemistry, Graduate School of Science, Osaka Metropolitan University, Sumiyoshi-ku, Osaka 558-8585, Japan. E-mail: nakazawa@omu.ac.jp
bNational Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba 305-8565, Ibaraki, Japan
First published on 10th May 2023
The activation of a dibromo Co-terpyridine complex immobilized on a stationary phase (Co(tpy)Br2@SiO2) as a hydrosilylation catalyst was investigated. Inorganic salts that are sparingly soluble in an organic solvent were examined as activators, and K2CO3 was found to show advantages in activator ability, stability, cost, and ease of handling. Catalytic hydrosilylation using Co(tpy)Br2@SiO2 activated by K2CO3 afforded the products in good yield, in both the reactions of styrene with triethoxysilane (92%) and 1-octene with diphenylsilane (88%). Both Co(tpy)Br2@SiO2 and K2CO3 were easily separable from the hydrosilylated product, which contributed to achieving a reusable hydrosilylation system in both the catalyst and activator. Moreover, the Co(tpy)Br2@SiO2/K2CO3 system was found to be applicable in a continuous flow reactor as a catalyst in the stationary phase.
Catalyst reuse is one of the ways to make more effective use of specially synthesized catalysts. The immobilization of a catalyst on a stationary phase is a good method for reusing the catalyst because it can be separated readily from the liquid product or product solution. Pt catalysts immobilized on solid supports have been synthesized and their reuse has been reported.19 By contrast, base metal complexes for hydrosilylation that are immobilized on a stationary phase are limited.20,21 Recently, our group developed a Co complex supported on silica gel (Co(tpy)(X)2@SiO2, where X = OH, Br) in which a Co complex with terpyridine (tpy), being one of the simplest pincer ligands, was immobilized on the surface of silica gel or glassware.22Co(tpy)Br2@SiO2 (a rough image of the silica gel surface supporting Co(tpy)Br2 is shown in Fig. 1) exhibited good catalytic activity using NaBHEt3 as the activator for the hydrosilylation of 1-octene with diphenylsilane (Ph2SiH2). This Co-supported silica gel was reusable as a catalyst, but the activity gradually decreased with each reuse. The cause of this decrease in catalytic activity was considered to be the cleavage of the Si–O bond in Co(tpy)Br2@SiO2 by the highly active NaBHEt3. Next, we searched for a system that activates the Co complex without using NaBHEt3.
Recently, we found that [Co(tpy)Br2] was activated by inorganic salts which are stable under air.23 In particular, K2CO3 is a good activator for the hydrosilylation reaction in terms of its ability, cost and stability. It was interesting that K2CO3 activated [Co(tpy)Br2] even though the salt is sparingly soluble in the reaction system (a mixture of an olefin and hydrosilane).
This paper describes whether or not unprecedented combinations of a Co-complex immobilized on silica gel and activators, which include extremely sparingly soluble inorganic salts, result in catalytic activity for hydrosilylation; in addition, a flow reactor system is reported in which the hydrosilylation product is eluted simply by passing the reactants (the olefin and hydrosilane) through a column packed with this silica gel and K2CO3 powder.
KOAc, KOPv, KOtBu, K2CO3, KF, and K3PO4 were selected as the activators, considering our previous report on [Co(tpy)Br2].23 The hydrosilylation of styrene with (EtO)3SiH (eqn (1), Table 1) and that of 1-octene with diphenylsilane (Ph2SiH2) (eqn (2)) were used as model reactions. In the absence of an activator, Co(tpy)Br2@SiO2 did not show any catalytic activity for the hydrosilylation of styrene with (EtO)3SiH (Table 1, entry 1). The hydrosilylation of 1-octene with Ph2SiH2 using Co(tpy)Br2@SiO2 without an activator afforded moderate yields of the products, with the linear one (2l) as the major product and the branched one (2b) as the minor product (Table 1, entry 2).
Entry | Olefin | Hydrosilane | Additive | Yielda (%) |
---|---|---|---|---|
a Determined via GC. b Not detected. | ||||
1 | Styrene | (EtO)3SiH | None | N.D.b |
2 | 1-Octene | Ph2SiH2 | None | 2l: 52 (2b: 0.3) |
3 | Styrene | (EtO)3SiH | KOAc | 1: 82 |
4 | Styrene | (EtO)3SiH | KOPv | 1: 90 |
5 | Styrene | (EtO)3SiH | KOtBu | 1: 80 |
6 | Styrene | (EtO)3SiH | K2CO3 | 1: 92 |
7 | Styrene | (EtO)3SiH | KF | 1: 98 |
8 | Styrene | (EtO)3SiH | K3PO4 | 1: 89 |
9 | 1-Octene | Ph2SiH2 | KOAc | 2l: 68 (2b: 3.0) |
10 | 1-Octene | Ph2SiH2 | KOPv | 2l: 86 (2b: 6.2) |
11 | 1-Octene | Ph2SiH2 | KOtBu | 2l: 83 (2b: 2.1) |
12 | 1-Octene | Ph2SiH2 | K2CO3 | 2l: 88 (2b: 2.1) |
13 | 1-Octene | Ph2SiH2 | KF | 2l: 84 (2b: 3.8) |
14 | 1-Octene | Ph2SiH2 | K3PO4 | 2l: 84 (2b: 2.4) |
The addition of KOAc to the reaction mixture of styrene and (EtO)3SiH containing Co(tpy)Br2@SiO2 resulted in a high catalytic activity and the formation of 1 (82%; Table 1, entry 3). Similarly, in the presence of other salts, 1 was produced in high yield. In particular, the addition of KOPv, K2CO3, and KF afforded the corresponding product in more than 90% yield (entries 4, 6, and 7, respectively). The hydrosilylation reaction between 1-octene with Ph2SiH2 was also improved in the presence of inorganic salts: the product yields were more than 80% with the addition of activators such as KOPv, KOtBu, K2CO3, KF, and K3PO4 (entries 10–14, respectively). GC-MS and 1H NMR measurements carried out on the supernatant after the reactions in Table 1 revealed that the hydrosilylation products (i.e., 1 for eqn (1), 2l and 2b for eqn (2)) and a small amount of the starting materials (olefin and hydrosilane) were detected, although no by-products were detected. These results are quite similar to the effect of the inorganic salts on the hydrosilylation of the catalytic system based on [Co(tpy)Br2].23
Among the inorganic additives, K2CO3 afforded good results for both the hydrosilylation of styrene with (EtO)3SiH and that of 1-octene with Ph2SiH2. Both Co(tpy)Br2@SiO2 and K2CO3 were barely soluble in organic solvents, showing the high reuse potential of the system. Thus, optimization of the reaction conditions was carried out for the Co(tpy)Br2@SiO2/K2CO3 system.
Entry | Amount of K2CO3 | Olefin | Hydrosilane | Yielda (%) |
---|---|---|---|---|
a Determined via GC. | ||||
1 | 0.5 | Styrene | (EtO)3SiH | 1: 87 |
2 | 1.0 | Styrene | (EtO)3SiH | 1: 88 |
3 | 1.5 | Styrene | (EtO)3SiH | 1: 88 |
4 | 2.0 | Styrene | (EtO)3SiH | 1: 91 |
5 | 2.5 | Styrene | (EtO)3SiH | 1: 92 |
6 | 0.5 | 1-Octene | Ph2SiH2 | 2l: 74 (2b: 1.8) |
7 | 1.0 | 1-Octene | Ph2SiH2 | 2l: 80 (2b: 2.3) |
8 | 1.5 | 1-Octene | Ph2SiH2 | 2l: 80 (2b: 2.2) |
9 | 2.0 | 1-Octene | Ph2SiH2 | 2l: 82 (2b: 2.5) |
10 | 2.5 | 1-Octene | Ph2SiH2 | 2l: 88 (2b: 2.1) |
Finally, the temperature dependence in the range of 25–100 °C for the hydrosilylation reaction catalyzed by Co(tpy)Br2@SiO2 was investigated in the presence of 2.5 mol% K2CO3 (eqn (5) and (6), Table 3). The hydrosilylation of styrene with (EtO)3SiH afforded a greater than 86% yield of 1 in the temperature range studied (Table 3, entries 1–5). By contrast, the hydrosilylation of 1-octene with Ph2SiH2 at 25 °C produced a low yield of 2l (14%, entry 6). The yield gradually increased as the reaction temperature was increased (entries 6–10), and reached 88% at 100 °C (2l: 88% and 2b: 2.1%; Table 3, entry 10). As reported before, the [Co(tpy)Br2]/K2CO3 system showed catalytic activity at 25 °C for the hydrosilylation of 1-octene with Ph2SiH2 (99% conversion).23 The low yield of the product for the hydrosilylation of 1-octene with Ph2SiH2 at 25 °C (entry 6) probably comes from the lack of activation energy for the reaction to proceed, since the unreacted substrates were observed via GC analysis without any by-product. Therefore, it seems that a high temperature is required to activate the Co complex supported on silica gel with K2CO3.
Entry | Temperature (°C) | Olefin | Hydrosilane | Yielda (%) |
---|---|---|---|---|
a Determined via GC. | ||||
1 | 25 | Styrene | (EtO)3SiH | 1: 86 |
2 | 40 | Styrene | (EtO)3SiH | 1: 88 |
3 | 60 | Styrene | (EtO)3SiH | 1: 91 |
4 | 80 | Styrene | (EtO)3SiH | 1: 95 |
5 | 100 | Styrene | (EtO)3SiH | 1: 92 |
6 | 25 | 1-Octene | Ph2SiH2 | 2l: 14 (2b: 0.2) |
7 | 40 | 1-Octene | Ph2SiH2 | 2l: 31 (2b: 0.9) |
8 | 60 | 1-Octene | Ph2SiH2 | 2l: 76 (2b: 3.5) |
9 | 80 | 1-Octene | Ph2SiH2 | 2l: 81 (2b: 2.9) |
10 | 100 | 1-Octene | Ph2SiH2 | 2l: 88 (2b: 2.1) |
The hydrosilylation of styrene with (EtO)3SiH catalysed by Co(tpy)Br2@SiO2/K2CO3 (eqn (5), Table 3) was also conducted in air. As a result, the yield of the hydrosilylated product dropped to 45%, which was almost half of that under N2. This result indicates that the catalytically active Co species produced from Co(tpy)Br2@SiO2 and K2CO3 is air-sensitive. The active species has not been identified since it is quite difficult to observe the Co complex on silica gel during the catalytic reaction. By contrast, in our previous report,23 the activation mechanism in the homogeneous system using [Co(tpy)Br2] was proposed in which the carbonate anion undergoes nucleophilic attack at the Si atom of hydrosilane causing the release of a hydride ion as a reductant of the Co complex. Considering the previous report, it can be thought, even in an immobilized catalytic system, that Co(tpy)Br2@SiO2 is activated via hydrosilane activation by K2CO3 to generate the catalytically active Co(I) or Co(0) species.
A comparison of the catalytic activity for the hydrosilylation reaction derived from discrete [Co(tpy)Br2] with that from immobilized [Co(tpy)Br2] is interesting. In the presence of 0.01 mol% [Co(tpy)Br2] relative to 5.4 mmol of each substrate, and in the presence of 30 mg Co(tpy)Br2@SiO2, which corresponds to 0.01 mol% immobilized-[Co(tpy)Br2], a mixture of the olefin and hydrosilane (both 5.4 mmol) to which 2.5 mol% K2CO3 had been added was heated at 100 °C for 1 h, and the products were examined. The results are summarized in Table 4. In the case of the hydrosilylation of styrene with (EtO)3SiH, the TOF of [Co(tpy)Br2] was 6910 h−1 (6282 h−1 per 0.01 mol% catalyst) and that of Co(tpy)Br2@SiO2 was 9120 h−1 (9702 h−1 per 0.01 mol% catalyst) (Table 4, entries 1 and 2). It is worth noting that our immobilized catalyst showed a 1.5-fold higher activity than the [Co(tpy)Br2] catalyst, even though the immobilization of a catalyst often lowers the catalytic activity. There are two possible reasons for this: (i) the tpy ligand was electronically affected due to the connection of an anchor (a triazole substituent) for immobilization, which enhanced the activity of the Co catalyst; and (ii) [Co(tpy)Br2] is reported to be in equilibrium in solution as shown in eqn (7).24 [Co(tpy)2]Br2 and CoBr2 show no catalytic activity. The immobilization of [Co(tpy)Br2] inhibits the bimolecular association and inhibits the shifting of the equilibrium to the right, resulting in the high catalytic performance. Although there is no experimental evidence, we tentatively think that (ii) is the main reason for the enhanced catalytic activity.
2[Co(tpy)Br2] ⇄ [Co(tpy)2]Br2 + CoBr2 | (7) |
Entry | Catalyst | Co amount (mol%) | Substrates | Yielda (%) | TOF (h−1) (TOF per 0.01 mol%) |
---|---|---|---|---|---|
a Determined via GC. b Total yield of 2l and 2b. | |||||
1 | [Co(tpy)Br2] | 0.011 | Styrene (EtO)3SiH | 76 | 6910 (6282) |
2 | Co(tpy)Br2@SiO2 (30 mg) | 0.0094 | Styrene (EtO)3SiH | 86 | 9120 (9702) |
3 | [Co(tpy)Br2] | 0.013 | 1-Octene Ph2SiH2 | 87b | 6690 (5146) |
4 | Co(tpy)Br2@SiO2 (30 mg) | 0.0094 | 1-Octene Ph2SiH2 | 34b | 3620 (3851) |
The TOF for the hydrosilylation of 1-octene with Ph2SiH2 using [Co(tpy)Br2] was 6690 h−1 (5146 h−1 per 0.01 mol% catalyst), whereas the TOF using Co(tpy)Br2@SiO2 was 3620 h−1 (3851 h−1 per 0.01 mol% catalyst), showing that immobilization of the Co complex caused a decrease in the catalytic activity, in contrast to the above case. The reason for this is not clear, but one possibility is shown below. First, a comparison of the steric accessibility of the olefin and hydrosilane to the catalytically active Co center was carried out. The olefin substituent is a planar phenyl group in styrene, whereas it is a hexyl group in 1-octene. Therefore, it is sterically more difficult for 1-octene to approach the Co center than it is for styrene. Concerning the hydrosilane substrate, Ph2SiH2 appears to be less able to approach the Co center than (EtO)3SiH, because the Ph group in the former hydrosilane is considered to make the Si environment more crowded than the OEt group in the latter hydrosilane. Therefore, when Co(tpy)Br2 is used as the catalyst, the hydrosilylation of 1-octene with Ph2SiH2 is considered to be sterically less likely than that for styrene with (EtO)3SiH, which is consistent with the results of entries 1 vs. 3 in Table 4. This tendency is more pronounced in the catalytic system with Co(tpy)Br2 immobilized on silica gel. This is because one side of the catalytically active Co species is the silica gel surface, which limits the space that is accessible to the substrates. Therefore, in the Co(tpy)Br2@SiO2 system, the reaction of styrene with (EtO)3SiH, which is less sterically constrained by the immobilization of the complex, can effectively receive the benefit of (ii) shown above (entries 1 vs. 2 in Table 4), whereas the reaction of 1-octene with Ph2SiH2 suffers from more steric constraints due to immobilization of the complex compared with the benefit of (ii) (entries 3 vs. 4 in Table 4).
Next, the scope of the hydrosilane and siloxane substrates was investigated. Styrene was employed as the olefin substrate (eqn (9), Table 6), since hydrosilylation using styrene solely produced the anti-Markovnikov type product (3) with Co(tpy)Br2@SiO2/K2CO3 in good yield (Table 5, entry 1). When PhSiH3 was used as the primary silane, the corresponding product (7) was obtained in 90% yield (Table 6, entry 1). In the case of the tertiary silane, hydrosilylation using (EtO)2MeSiH afforded the hydrosilylated product (8) in good yield (84%, entry 2). However, the similar tertiary silane Ph2MeSiH was not reactive at all (entry 3). These three hydrosilanes were quantitatively converted into the corresponding products by [Co(tpy)Br2]/K2CO3.23 This indicates that Co(tpy)Br2@SiO2/K2CO3 is affected by the steric hindrance around the Si atom of the hydrosilane. This tendency was also observed in the hydrosilylation reaction using siloxane substrates with Co(tpy)Br2@SiO2/K2CO3. 1,1,1,3,3-Pentamethyldisiloxane (PMDS) and 1,1,1,3,5,5,5-heptamethyltrisiloxane (MD′M) were employed as the siloxane substrates. When PMDS and MD′M were used as the substrates the products were obtained in 46% (9) and 36% (10) yield, respectively. Thus, the larger framework of MD′M afforded the hydrosilylation product in a lower yield.
A mixture of 5.4 mmol each of 1-octene and Ph2SiH2 containing 50 mg of Co(tpy)Br2@SiO2 and 2.5 mol% K2CO3 was stirred using a mechanical stirrer (not a magnetic stirrer) to avoid crushing the silica gel (Co(tpy)Br2@SiO2). After 2 h of reaction at 100 °C, Co(tpy)Br2@SiO2 and K2CO3 were separated via centrifugation, washed with hexane, dried, and used for the second reaction. These sequential operations were repeated five times, and the yield of 2l was measured for each cycle (the yield of 2b was lower than 2% in each cycle). From the results shown in Fig. 2, it was found that the Co(tpy)Br2@SiO2/K2CO3 system is reusable at least 5 times without any reduction in the catalytic activity.
![]() | ||
Fig. 2 Repeated hydrosilylation of 1-octene with Ph2SiH2 catalyzed using the Co(tpy)Br2@SiO2/K2CO3 system. |
When toluene was used as the solvent and the flow was carried out at 100 °C, the flow rate was maintained at 0.03 mL min−1 and the conversion rate was around 45% for 24 h (Fig. 3A). When THF was used and the flow was carried out at 60 °C, the conversion rate improved to more than 95% for the first 4 h, which then settled down to ca. 80% after 24 h (Fig. 3B). The real active species is not clear, but it is probably a coordinatively unsaturated Co complex, such as [Co(tpy)], which is air-sensitive. As THF is known to be a coordinative solvent, whereas toluene is not, it is likely that THF coordinates to the real Co active species and prevents decomposition, resulting in the high catalytic activity. Actually, the batch reaction of Co(tpy)Br2@SiO2/K2CO3 in THF was superior to that in toluene (Table S2, ESI†). Thus, the continuous flow reaction in the presence of THF in total afforded 21.6 g of the product (2l) (88% conversion) with 90% purity after removal of the solvent, in which only small amounts of the starting materials and 2b (<2%) were found.
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Fig. 3 Time course of the conversion rate of hydrosilylation of 1-octene with Ph2SiH2 by continuous flow reaction using Co(tpy)Br2@SiO2/K2CO3 in the presence of toluene (A) or THF (B) as a solvent. |
These results show that a flow reactor using Co(tpy)Br2@SiO2 and K2CO3 can be adapted for hydrosilylation reactions between olefins with hydrosilanes, and the reactor can operate for at least 24 h whilst maintaining a high catalytic activity. Therefore, our reaction system, which combines an air-stable immobilized catalyst and an inorganic salt, is a step towards the practical application of a flow reactor.
In many hydrosilylation catalyzed reactions, as the catalytically active species is air-sensitive, it is required to conduct the reaction under an inert atmosphere and to separate the product from the catalyst residues after the reaction has been completed. Since this paper reports that a solid catalyst and a solid activator are packed in a column under air and the deaerated reaction reagents (olefin and hydrosilane) are simply flowed through the column to continuously generate the hydrosilylation product that contains no catalyst residue, our results can be said to be a step towards the realization of an ideal catalytic system. The present research suggests an environmentally friendly catalytic system for hydrosilylation.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3nj01062g |
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2023 |