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Hydrosilylation Reaction of Olefins RSC Advances REVIEW View Article Online View Journal | View Issue Hydrosilylation reaction of olefins: recent advances and perspectives Cite this: RSC Adv.,2015,5, 20603 Y. Nakajima and S. Shimada* This review covers the recent advances in hydrosilylation chemistry mainly published in the last decade. Hydrosilylation of olefins is an important reaction for the production of various organosilicon compounds such as industrially important silicone products. Although the utility of platinum catalysts, Speier's and Karstedt's catalysts, has been widely recognized in this field for decades, development of more efficient, selective, and cheaper catalyst systems are still desired for more economical production of organosilicon materials having superior properties. In these contexts, much progress has been made in recent years. In the platinum catalysis systems, continuous progress has been made to further improve selectivity and activity. Several non-precious metal catalysts, such as Fe and Ni catalysts, with good Received 30th December 2014 efficiency and selectivity have been developed. Furthermore, unique chemo- and regioselectivity have Accepted 12th February 2015 been achieved not only by precious metal catalysts but also by non-precious metal catalysts. The utility DOI: 10.1039/c4ra17281g of non-transition metal catalysts including early main group metals, Lewis acidic alane, borane and www.rsc.org/advances phosphonium salts as well as N-heterocyclic carbenes has also been disclosed. 1. Introduction diverse commodities including lubricating oils, pressure- sensitive adhesives, liquid injection moulding products and The hydrosilylation reaction, which enables the addition of paper release coatings, etc. silicon hydrides across C–C multiple bonds, is an efficient In spite of the high utility in industry, however, it is known method for the formation of organosilicon compounds and that the platinum catalysts still suffer from a number of draw- represents one of the most important reactions in silicon backs. For example, platinum-catalysed hydrosilylation of chemistry. The process is widely applied in industry to produce olens is oen accompanied by side reactions, such as dehy- Published on 12 February 2015. Downloaded 9/26/2021 6:54:18 AM. silane coupling agents and silicone polymers such as oils, drogenative silylation, hydrogenation of olens, isomerization rubbers and resins.1 The hydrosilylation reactions can also of olens, olen oligomerization and redistribution of hydro- produce various organosilicon reagents, which are used in ne silanes (Scheme 1). chemical synthesis for stereospecic oxidation, cross-coupling These side reactions lead to signicant yield losses of the reactions, etc.2–4 desired product. Furthermore, additional processes to remove The rst hydrosilylation reaction was reported in 1947 by the undesirable impurities are necessary, which would cause Sommer in which trichlorosilane and 1-octene reacted in the the deleterious effects on the quality or properties of the nal presence of peroxide in a free-radical mechanism although the silicone materials. Therefore, development of a well-designed reaction selectivity was quite low.5 In the late 1950s, hexa- catalytic system with higher selectivity and activity is still an chloroplatinic acid [H2PtCl6]$H2O was revealed to be a very intensive research topic to attain more economical production effective homogeneous transition metal catalyst (Speier's cata- of silicone materials with superior quality and/or properties. lyst), in which system the selectivity was improved to a large So far, numerous transition metal-based catalysts have been extent.6 An important breakthrough has been made in early developed mainly using groups 8, 9, and 10 metals, and many 70's. Thus, in 1973, Karstedt developed the platinum(0) reviews on these studies have been already reported especially 8,9 complex containing vinyl-siloxane ligands (Karstedt's catalyst, in the last 20 years. Most recently, Troegel and Stohrer Fig. 1),7 which exhibits extremely improved activity and selec- tivity as well as high solubility in polysiloxane compositions. As a result, the nding lead to the wider application of this process in the silicone chemistry, for example, the manufacture of National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan. E-mail: s-shimada@ Fig. 1 Karstedt's catalyst. aist.go.jp This journal is © The Royal Society of Chemistry 2015 RSC Adv.,2015,5, 20603–20616 | 20603 View Article Online RSC Advances Review Fig. 2 Structures of 1a–c. monomer silane, bis(trimethylsiloxy)methylsilane (MD0M) (eqn (1)). Although these catalysts are slightly less active than Kar- stedt's catalyst, good reaction rates were obtained even at a catalyst loading of 30 ppm or less. Furthermore, no colloidal platinum species was formed and the amount of undesired by- products was greatly reduced with 1a–c. The reactivity and selectivity of these catalysts is dependent on the steric and Scheme 1 Reactions of olefin with hydrosilane in the presence of a electronic properties of the carbene substituents. An optimum catalyst. was reached with cyclohexyl substituted 1b, leading to the selective formation of primary alkylsilanes with less than 1% reviewed the advances of hydrosilylation processes covering the yield of side-products, while Karstedt's catalyst provides the period from 2000 to 2010, in particular from industrial point of alkylsilane in only 78% yield with isomerized or reduced view.10 Active studies have been continued until now in this eld products in ca. 20%. These Pt–NHC catalysts is tolerant toward using various transition metal catalysts including earth- various reactive functionalities, such as free alcohols, protected abundant and environmental benign metals, which recently alcohols, silyl ethers, ketones, and esters. It is to be noted that attract strong interest due to the high cost, environmental 1b catalysed hydrosilylation of vinylcyclohexeneoxide, which is concerns, and uncertainty of the long-term supply of platinum. sensitive to side reactions such as ring-opening and polymer- In addition, new types of catalysts are recently intensively ization reactions, gave the corresponding hydrosilylated investigated including not only transition metal catalysts but product in excellent yield (eqn (2)). The reaction is of signi- also other metals or non-metal catalyst systems. Thus, the cant importance relevant to the preparation of an polyepoxy present review mainly deals with important relevant contribu- silicone oil from vinylcyclohexeneoxide and Me3SiO(Me2- tions appearing in the last 10 years. SiO)80(MeHSiO)7SiMe3. Based on detailed kinetic studies, a plausible catalytic cycle, which is initiated by the dissociation 2. Precious metal catalysts of divinyltetramethyldisiloxane, was proposed (Scheme 2).13 2.1. Platinum catalysts Published on 12 February 2015. Downloaded 9/26/2021 6:54:18 AM. The platinum catalysts have been utilized in industry for more (1) than half a century due to the high catalytic activity and selec- tivity, and the high stability towards heat, oxygen and moisture. One of the most industrially important platinum catalysts, Kar- stedt's catalyst, has been established as the benchmark for other hydrosilylation catalysts in industrial hydrosilylation processes. Therefore, it seems that the chemistry of platinum catalysts has already reached a high level of sophistication. On the other hand, the platinum catalysts are known to possess some disad- vantages as mentioned in the introduction section. One of them (2) is the formation of platinum black during the hydrosilylation reaction, which is responsible for some of the side products and leads to a contamination of the hydrosilylated product. Also the platinum catalyst used for the curing of silicone products cannot be recovered, which leads to signicant consumption of plat- inum metal in silicone industry (5.6 t in 2007,11 3% of annual worldwide platinum production). Therefore, the development of a new Pt-catalyst system, which attains remarkable efficiency and selectivity, still draws lots of attentions. Recently, the efficiency of Pt(0) complexes supported by the N-heterocyclic carbene (NHC) as a robust s-donor ligand was reported (Fig. 2).12 The catalytic activities of 1a–c were examined by performing hydrosilylation of 1-octene with a commercial Scheme 2 Proposed hydrosilylation mechanism catalysed by 1. 20604 | RSC Adv.,2015,5,20603–20616 This journal is © The Royal Society of Chemistry 2015 View Article Online Review RSC Advances Related to the above examples, many platinum complexes comparative or even higher catalytic activity towards hydro- with a carbene ligand have been examined and revealed to be silylation of 1-hexene or vinylmethylbis(trimethylsiloxy)-silane active towards catalytic hydrosilylation of styrenes.14 Cavell and with MD0Mat50C compared to Karstedt's catalyst. co-workers reported Pt(0) complexes 2a–c coordinated with sterically demanding and stronger s-donating 6-membered NHC ligands (Fig. 3).15 Although these complexes catalysed olen isomerization in the presence of Et3SiH to give internal olens, 1-octene was (4) successfully hydrosilylated with MD0Mat72C in the presence of 0.005 mol% catalyst, resulting in the selective formation of the corresponding b-hydrosilylated product. The catalytic activity was revealed to be dependent on the steric hindrance of the complexes; i.e. the least sterically hindered 2b is the most As a part of a longstanding
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