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1 Journal of the Japan Petroleum Institute, 63, (1), 1-9 (2020) J. Jpn. Petrol. Inst., Vol. 63, No. 1, 2020 1. Introduction Hydrosilylation of olefins is an important process in the silicone industry. The reaction product, organo- silane is an amphiphilic compound having both organic functional groups derived from olefins and the silyl functional group (Eq. (1)). Owing to this, it is possible to tune the functionality of the organosilicon products by selecting appropriate organic groups in the olefins and an appropriate structure of the hydrosilane sub- strate. The unique property and highly functionality of the organosilicon compounds increase their applica- tion towards organic-inorganic oils, silicone rubbers, surface coating regents, sealants, cosmetics, and also pharmaceuticals 1) . 1At present, Pt complexes are widely used as catalysts for hydrosilylation 2) . However, the world Pt usage for this application amounts to 5.6 tons per year, with almost no recovery 3) . When using a noble metal cata- lyst, it is urgent to reduce the amount of catalyst used. Therefore, the development of highly active and reus- able industrial catalysts are desired. In this review, the basic chemistry of hydrosilylation of olefins using metal complex catalysts is introduced first. Next, based on the reactivity and mechanism of such metal complex catalysts, homogeneous catalysts used in the industry as well as the recent advances in the hydrosilylation of olefins using heterogeneous cata- lysts are presented. 2. Chemistry of Hydrosilylation Several mechanisms of hydrosilylation have been proposed 4)6) . Scheme 1 represents the Chalk-Harrod mechanism involving (1) the coordination of olefin to metal complex and oxidative addition of hydrosilane, (2) the insertion of olefin to M _ H bond, and (3) the reductive elimination of the hydrosilylation product, generating the organosilicon compound. This mecha- nism follows the anti Markovnikov rule. In the modified Chalk-Harrod mechanism shown in Scheme 2, the reaction proceeds by the insertion of an olefin into an M _ Si bond instead of a M _ H bond, and the final product obtained is a Markovnikov product. [Review Paper] Recent Advances on Heterogeneous Metal Catalysts for Hydrosilylation of Olefins Kyogo MAEDA and Ken MOTOKURA Dept. of Chemical Science and Technology, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8502, JAPAN (Received July 9, 2019) Since the discovery of hydrosilylation in 1947, the reaction has been continuously studied and is found to play an important role in the silicone industry, whose products have become an integral part of our lives today. Karstedt’s catalyst is a homogeneous Pt complex reported in the 70’s and has been used for hydrosilylation since then. It was long desirable to develop heterogeneous catalysts that are reusable, easy to separate, and highly active. Since 2000’s, heterogeneous catalysts with high activity and having excellent recyclability have been reported. Their activities are comparable with those of the currently used homogeneous catalyst. In particular, innovative catalysts such as the MOF immobilized complex and the single atom catalyst have been reported. Active species in these catalysts are precisely designed at the molecular or atomic level on surface, and all exhibit extremely high activity for hydrosilylation. In addition to these reports, rhodium complexes grafting on silica have been reported. The grafted Rh complex shows cooperative catalysis with an organic functionality. Keywords Hydrosilylation, Heterogeneous catalyst, Olefin, Silane, Organosilicon DOI: doi.org/10.1627/jpi.63.1 To whom correspondence should be addressed. E-mail: [email protected]
Transcript

1Journal of the Japan Petroleum Institute, 63, (1), 1-9 (2020)

J. Jpn. Petrol. Inst., Vol. 63, No. 1, 2020

1. Introduction

Hydrosilylation of olefins is an important process in the silicone industry. The reaction product, organo-silane is an amphiphilic compound having both organic functional groups derived from olefins and the silyl functional group (Eq. (1)). Owing to this, it is possible to tune the functionality of the organosilicon products by selecting appropriate organic groups in the olefins and an appropriate structure of the hydrosilane sub-strate. The unique property and highly functionality of the organosilicon compounds increase their applica-tion towards organic-inorganic oils, silicone rubbers, surface coating regents, sealants, cosmetics, and also pharmaceuticals1).

(1)

At present, Pt complexes are widely used as catalysts for hydrosilylation2). However, the world Pt usage for this application amounts to 5.6 tons per year, with almost no recovery3). When using a noble metal cata-lyst, it is urgent to reduce the amount of catalyst used. Therefore, the development of highly active and reus-able industrial catalysts are desired.

In this review, the basic chemistry of hydrosilylation of olefins using metal complex catalysts is introduced first. Next, based on the reactivity and mechanism of such metal complex catalysts, homogeneous catalysts used in the industry as well as the recent advances in the hydrosilylation of olefins using heterogeneous cata-lysts are presented.

2. Chemistry of Hydrosilylation

Several mechanisms of hydrosilylation have been proposed4)~6). Scheme 1 represents the Chalk-Harrod mechanism involving (1) the coordination of olefin to metal complex and oxidative addition of hydrosilane, (2) the insertion of olefin to M_H bond, and (3) the reductive elimination of the hydrosilylation product, generating the organosilicon compound. This mecha-nism follows the anti Markovnikov rule.

In the modified Chalk-Harrod mechanism shown in Scheme 2, the reaction proceeds by the insertion of an olefin into an M_Si bond instead of a M_H bond, and the final product obtained is a Markovnikov product.

[Review Paper]

Recent Advances on Heterogeneous Metal Catalysts for Hydrosilylation of Olefins

Kyogo MAEDA and Ken MOTOKURA*

Dept. of Chemical Science and Technology, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8502, JAPAN

(Received July 9, 2019)

Since the discovery of hydrosilylation in 1947, the reaction has been continuously studied and is found to play an important role in the silicone industry, whose products have become an integral part of our lives today. Karstedt’s catalyst is a homogeneous Pt complex reported in the 70’s and has been used for hydrosilylation since then. It was long desirable to develop heterogeneous catalysts that are reusable, easy to separate, and highly active. Since 2000’s, heterogeneous catalysts with high activity and having excellent recyclability have been reported. Their activities are comparable with those of the currently used homogeneous catalyst. In particular, innovative catalysts such as the MOF immobilized complex and the single atom catalyst have been reported. Active species in these catalysts are precisely designed at the molecular or atomic level on surface, and all exhibit extremely high activity for hydrosilylation. In addition to these reports, rhodium complexes grafting on silica have been reported. The grafted Rh complex shows cooperative catalysis with an organic functionality.

KeywordsHydrosilylation, Heterogeneous catalyst, Olefin, Silane, Organosilicon

DOI: doi.org/10.1627/jpi.63.1 * To whom correspondence should be addressed. * E-mail: [email protected]

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J. Jpn. Petrol. Inst., Vol. 63, No. 1, 2020

Seitz and Wrighton have reported that when Co is used as the catalyst, the reaction follows a different mechanism (Scheme 3)7). In the Seitz-Wrighton

mechanism, a Si_C bond is formed by the insertion of an olefin into the M_SiR3 bond originally contained in the metal complex. This is followed by the oxidative

Scheme 1●Chalk-Harrod Mechanism for Olefin Hydrosilylation

Scheme 2●Modified Chalk-Harrod Mechanism for Olefin Hydro silylation

Scheme 3●Seitz-Wrighton Mechanism for Olefin Hydrosilylation

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addition of hydrosilanes, reductive elimination of alkyl-silanes, and regeneration of the active species with silyl groups.

In general, when a Rh(I) complex is used in a reac-t ion fol lowing the Chalk-Harrod or modified Chalk-Harrod mechanism, the reaction proceeds via a Rh(I)/Rh(III) catalytic cycle. However, in 1985, Perutz et al. proposed the existence of a Rh(V) species in the hydrosilylation reaction using a Rh(I) complex7). As shown in Scheme 4, irradiating Rh(I) complex (η5-C5H5)Rh(C2H4)2 (1) with light at low temperature led to the formation of (η5-C5H5)Rh(C2H4)(SiR3)H (2). This compound was readily available for hydrosilylation and contained all the key ligands for a reaction obeying the Chalk-Harrod mechanism.

It was revealed by nuclear magnetic resonance (NMR) spectroscopy that 1 also reacted at room tem-perature to produce 2 and 3 (Scheme 5). 3 is a Rh(V) complex with hydride and silyl groups. In addition, similar Rh(V) complex prepared using triisopropyl-

silane (HSiiPr3) was identified by infrared spectroscopy, ultraviolet-visible spectroscopy, NMR spectroscopy, and mass spectrometry after its isolation and purifica-tion.

Perutz et al. speculated that the Rh(V) complex is an intermediate in the hydrosilylation reaction using com-plex 2, and proposed the reaction mechanism shown in Scheme 6. This is also a new class of reaction mech-anism for the metal complex-catalyzed hydrosilylation of olefins.

As shown in Schemes 1, 2, and 6, the metal-catalyzed hydrosilylation proceeds through the Rh(I)/Rh(III) or Rh(III)/Rh(V) catalytic cycle without the formation of any Rh(0) monomeric species. Additionally, during the catalytic cycle, a ligand was always found to coor-dinate to the Rh center. These facts suggested that the immobilization of an active metal such as Rh onto a support surface was possible by covalent or coordina-tion bonding.

3. Industrial Process of Hydrosilylation

The first hydrosilylation reaction was reported in 1947 by Sommer et al.8). The reaction was based on a radical chain mechanism, in which the activated Si_H bond by methyl radical from acetyl peroxide reacted with olefin. However, the yield and selectivity of the reaction was poor (the hydrosilylation product was ob-tained in 24 % yield).

Subsequently, a heterogeneous Pt catalyst was reported by G. H. Wagner (1953)9). Pt particles supported on powdered carbon behaved as the active species, and good performances were obtained (Scheme 7). However, the research and development on Pt catalysts was again diverted to homogeneous catalysts such as Speier’s catalyst (H2PtCl6) (1957)10) and Karstedt’s cat-alyst (1973)11). Today, Karstedt’s catalyst is used for the industrial hydrosilylation. The catalyst exhibits very high activity and broad substrate scope and gener-ates no by-products (Scheme 8).

With the development of Pt catalysts, there are con-cerns about the reaction selectivity, catalyst cost, future catalyst supply, and reduction in the amount of usage of the catalysts. Therefore, reaction systems using other metal (Rh, Ir, Fe, Ru, Ni, Ti, Re) catalysts have been developed2),12). Recently, hydrosilylation reactions using base metal elements, such as Fe, has been actively studied, and one of the homogenous Fe catalysts resulted in a turnover number (TON) of over 100013).

Scheme 4●Generation of Rh(III) Intermediate from Rh(I) ComplexScheme 7●Hydrosilylation with Pt/Carbon Catalyst

Scheme 5●Generation of Rh(V) Intermediate from Rh(I) Complex

Scheme 6●Perutz Mechanism for Olefin Hydrosilylation

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4. Heterogeneous Catalysts for Hydrosilylation of Olefins

At present, the homogeneous Karstedt’s catalyst is primarily used for hydrosilylation. Although it has a high catalytic activity, the catalyst is not recoverable, and Pt is merely consumed in the reaction3). Consid-ering the future industrial usage, there is a huge demand for the development of solid catalysts that can be easily recovered. The recent examples of solid (hetero-geneous) catalysts used in the hydrosilylation of olefins are summarized in this section.

For hydrosilylation of alkynes and other unsaturated carbon_carbon bonds using heterogeneous metal cata-lysts, including supported metal complex and mono and bimetallic catalysts, have been also developed14). Recent examples are shown in reference 14) and cited therein.

In 2016, Sawano et al. reported a metal organic framework (MOF)-supported Rh catalyst (P1-MOF・Rh). The unit structure is shown in Fig. 115). In this catalyst, the Rh complex is immobilized onto the MOF, which includes a monophosphine ligand as a unit. When the Rh complex is immobilized onto the MOF with a repeating structure, each Rh complex is located at an appropriate distance interval, resulting in no direct interaction between the Rh complexes. The complex is stable throughout the reaction and does not undergo disproportionation. However, when the mono-phosphine Rh complex is used in solution, the complex disproportionates into inactive metal Rh and diphos-phine Rh complex. In the hydrosilylation reaction

shown in Scheme 9, P1-MOF・Rh catalyst showed high activity, and the TON reached 820,000 after 72 h.

In addition, it was confirmed that the reported cata-lyst exhibited high activity not only for the hydrosilyla-tion of olefins, but also for various other catalytic reac-tions such as the hydrogenation of ketones and olefins with H2 and C_H borylation of arenes. Most impor-tantly, the catalyst was stable, and the catalytic activity was maintained even after 10 cycles (Scheme 10). Corma and coworkers also reported hydrosilylation of styrene using recyclable supported Au complex and nanoparticles catalysts16).

In 2017, Beller et al. developed a heterogeneous sin-gle atom catalyst for the hydrosilylation reaction17). Recently, there has been an increasing number of reports on single atom catalysts, which include metals dispersed at the atomic level. Such catalysts are known to exhibit improved activity as compared with bulk and nanoparticle catalysts18). This catalyst was the first example of a single atom catalyst for the hydro-silylation reaction. For preparing the single atom plat-inum catalyst (Pt/NR_Al2O3

_IP), an aluminum nanorod (NR_Al2O3) with a high specific surface area (SBET=330 m2/g) was added to an aqueous solution of hexa-chloropratinic acid (H2PtCl6・6H2O aq.) and acetone, stirred at atmospheric pressure, and dried in vacuum. The single atom Pt catalyst prepared by impregnation precipitation (Pt/NR_Al2O3

_IP) exhibited excellent cata-lytic activity that was comparable with that of Karstedt’s catalyst (Table 1). On the other hand, the Pt nano-particle catalyst prepared by reductive precipitation (Pt/NR_Al2O3

_RP) showed low activity.The stability of the catalysts was confirmed from the

recycling experiments. The yield of the product was 92 % with Pt/NR_Al2O3

_IP even in the sixth cycle (total

Fig. 1●Structures of P1-MOF·Rh

Scheme 9●Hydrosilylation with P1-MOF·Rh

Scheme 10● Recyclability of P1-MOF·Rh in the Hydrosilylation of Octene and Triethylsilane

Scheme 8●Hydrosilylation Reaction with Karstedt’s Catalyst

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TON of 1.18×106). High-resolution scanning trans-mission electron microscopy and X-ray absorption fine structure (XAFS) measurements confirmed that the state of the catalyst did not change after the recycle reactions. Very recently, single atom Pt catalyst on N-doped graphene has also been reported as a hetero-geneous catalyst for hydrosilylation of terminal olefins with triethoxysilane19). Pt(0) nanoparticle stabilized by silica network was also reported by Ciriminna and coworkers20).

In 2008, Marciniec and coworkers reported that silica-supported Rh complex shows high activity for the hydrosilylation reaction (Scheme 11)21a). The catalyst was a grafted Rh mononuclear complex formed by reacting Rh binuclear complexes with silanol groups. In-situ NMR measurements confirmed that the reaction proceeded without leaching of the Rh complex.

Following this, they developed a catalyst in which an originally mononuclear Rh complex was immobi-lized21b). Rh phosphine siloxide complexes ([(SiR’3)Rh(cod)(PR3)], R=Cy, Ph, iPr) were used as precursors and were immobilized on the silica surface. The struc-tures of the synthesized catalysts were determined by solid state NMR (1H, 31P). The catalysts showed excellent activity and recyclability for hydrosilylation reaction (Scheme 12).

Examination of the NMR spectrum of the catalyst in the presence of a substrate suggested that the reaction followed the mechanism shown in Scheme 13, and no Rh was leached. The proposed cycle follows the Chalk-Harrod mechanism.

In subsequent studies, anticipating the industrial use, they tried to modify the reaction of olefin substrates using octakis(hydridodimethylsiloxy)octasilsesquioxane as a model compound instead of siloxide polymer (Scheme 14)21c). Rh catalyst 1 and Rh catalyst 2 showed excellent yields. Both the catalysts could be used repeatedly for 10 reactions when [silane] : [olefin] : [Rh]=1 : 1.2 : 5×10-5 (total TON of 19.8×104). Moreover, the total TON in these reactions reached 200,000.

Several research groups, including ours, have devel-oped catalysts with multiple active sites on a solid sur-face22). Organic moieties were immobilized close to the metal complex on the same support surface, and the catalytic activity was enhanced by the “concerted effect.”

In 2017, our group reported a catalyst in which the Rh complex and a tertiary amine were present on the same SiO2 surface (SiO2/Rh_NEt2). The activity of the catalyst for hydrosilylation was higher compared with only Rh complex on SiO2 (SiO2/Rh) (Scheme 15)23). Moreover, the TON of SiO2/Rh_NEt2 approached 1,900,000 (Scheme 16). The substrate scope of the reaction was also broad (Table 2); not only alkyl-silanes, alkoxysilanes, and phenylsilanes but also allyl-benzene derivatives, cyanoolefin, and epoxy olefin could be used as reactive substrates.

We investigated the effect of amines on the Rh com-plex by dynamic nuclear polarization-enhanced (DNP) solid-state 15N NMR spectroscopy and Rh K-edge

Table 1 Hydrosilylation with Pt/NR_Al2O3_IP

(CH2)4CH3 (CH2)4CH3+ HOTMS

OTMSSiMe

OTMS

OTMSSiMe

Pt catalyst(Pt: 1.4×10-5 mmol)

neat, 100 °C, 2 h

3.0 mmol (1.0 eq.)

Entry Catalyst Yield [%] TON [104]

1 Pt/NR_Al2O3_IPa) 95 20.6

2 Pt/NR_Al2O3_IPb) 73 31.6

3 Pt/NR_Al2O3_RPc) 33 7.1

4 Karstedt’s catalyst 96 20.6

a) IP=impregnation precipitation. b) 30 mmol of olefin, 30 mmol of silane, 5.0 × 10–6 mmol of Pt. c) RP=reductive precipitation.

Scheme 11●SiO2 Supported Rh CatalystScheme 12●SiO2 Supported Rh Phosphine Catalyst

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XAFS measurement at 20 K. It was found that the amine sterically interacts with the COD ligand in the Rh complex immobilized on the silica surface. In-situ FT-IR revealed that the reactivity of the COD ligand with hydrosilane was enhanced. It was concluded that the presence of an amine facilitated the COD dissocia-tion from the Rh center24).

5. Conclusion

The basics of metal-catalyzed hydrosilylation of ole-fins, catalytic systems for industry, and recent advances of heterogeneous metal catalysts were reviewed. In the history of the hydrosilylation reaction, the catalysts used for the industrial purposes contain Pt and are often homogeneous catalysts. Recently, several types of heterogeneous metal catalysts have been developed, including supported metal complexes and single atom catalysts. Since heterogeneous catalysts have many merits, they should be seriously targeted for develop-ment. It is also suggested here that the performance of the heterogeneous catalysts are comparable with that of

the currently used industrial catalysts.Both homogenous and heterogeneous catalysts often

contain precious metals (Pt, Rh, Ir, Ru). Though in-creasing efforts are being taken to replace such precious metals by Fe, Ni, and Co in homogeneous catalysts, no remarkable development is seen for heterogeneous cat-alysts in this regard. In future, improvement in the activity of these catalysts and development of hetero-geneous catalysts are expected.

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Scheme 14●Model Reaction of Hydrosilylation of Olefins with Spherosilicates

Scheme 15●Effect of Amine on the Same Surface

Scheme 16● Maximum TON of SiO2/Rh_NEt2-catalyzed Hydro-silylation

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Table 2●Substrate Scope of SiO2/Rh_NEt2-catalyzed Hydrosilylation

R R+ H─Si SiSiO2/Rh-NE2

(Rh: 1.5 µmol)neat, 40 °C4.0 mmol 1.0 eq.

R= (CH2)13CH3

MeTMSO

SiR

OTMS

t=15 min99 %, [99 %]

EtEt

SiR

Et

t=15 min99 %, [99 %]

BuBu

SiR

Bu

t=15 min99 %, [99 %]

EtOEtO

SiR

OEt

t=30 min76 %

PhMe

SiR

Me

t=30 min82 %

PhMe

SiR

Ph

t=2 h71 %

PhPh

SiR

Ph

t=2 htrace

MeSi=TMSO

SiOTMS

t=1 h83 %, [82 %]

Si

t=2 h94 %, [91 %]

Si

Me

t=24 h98 %, [95 %]

Si

OMe

(CH2)5CH3Si

t=1 h99 %

100 °C, t=24 h99 %, [97 %]

Si CN

t=2 h99 %, [98 %]

SiO

[Isolated yield].

9

J. Jpn. Petrol. Inst., Vol. 63, No. 1, 2020

要   旨

不均一系金属触媒によるオレフィンのヒドロシリル化反応

前田 恭吾,本倉  健

東京工業大学物質理工学院応用化学系,226-8502 横浜市緑区長津田町4259 G1-19

オレフィンのヒドロシリル化反応は1947年に発見され,今日では私たちの生活を支える基幹技術になっている。ヒドロシリル化反応に現在使用されている触媒は均一系 Pt錯体触媒であり,再利用可能かつ高活性な不均一系金属触媒の開発が長く望まれてきた。本総説では,ヒドロシリル化反応の基礎的な反応機構と現行の工業利用について概要をまとめるとともに,不均一系金属触媒の最近の研究動向について解説する。2000年代以降,オレフィンのヒドロシリル化反応において優れたリサ

イクル性を有する不均一系金属触媒が報告されている。これらの不均一系触媒は,均一系触媒に匹敵する活性を示す。特に,MOF固定化錯体触媒や,金属単原子触媒といった革新的な触媒が次々に発表されており,これらの触媒では,表面に存在する触媒活性種が分子あるいは原子レベルで精密にデザインされている。これらの報告に加え,シリカ表面に Rh錯体をグラフトした触媒と,表面での協同触媒作用によるヒドロシリル化反応における高活性発現についても併せて述べる。


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