Rhodium siloxide complex

Well-Defined Surface Rhodium Siloxide Complexes and Their Application to Catalysis... 

Our study on the synthesis, structure and catalytic properties of rhodium and iridium dimeric and monomeric siloxide complexes has indicated that these complexes can be very useful as catalysts and precursors of catalysts of various reactions involving olefins, in particular hydrosilylation [9], silylative couphng [10], silyl carbonylation [11] and hydroformylation [12]. Especially, rhodium siloxide complexes appeared to be much more effective than the respective chloro complexes in the hydrosilylation of various olefins such as 1-hexene [9a], (poly)vinylsiloxanes [9b] and allyl alkyl ethers [9c]. 

Figure 7.3 Proposed structures of 7C-allyl rhodium siloxide complexes immobilized on silica.

Scheme 7.2 Reactions of the surface 7t-allyl rhodium siloxide complex with trimethylphosphine.

Scheme 7.3 Reaction of surface Jt-allyl rhodium siloxide complex with carbon monoxide.

Scheme 7.4 Formation of surface rhodium siloxide complexes containing phosphine and hydride ligands.

Synthesis, Characterization and Catalytic Activity of Well-Defined Suface Rhodium Siloxide Complexes... 

I 7 Well-Defined Surface Rhodium Siloxide Complexes and Their Application to Catalysis Table 7.5 Hydrosilylation of 1-hexadecene and allyl ethers by polyhydrosiloxane (7.1) . 

Mechanism of HYdrosilylation Catalyzed by Surface versus Soluble Rhodium Siloxide Complexes j 309... 

Characterization of 6 by solid-state NMR spectroscopy as well as elemental analysis (only 28% decrease of rhodium content in samples after 20 cycles in the hydrosilylation) are convincing evidence of the high efficiency of surface rhodium siloxide complexes in the hydrosilylation of carbon-carbon multiple bonds as well as, presumably, in other reactions catalyzed by late transition metal siloxides syn-... 

The aim of this work was to examine the catalytic activity of rhodium siloxide complexes in the hydrosilylation of allyl glycidyl ether by triethoxysilane and hydro(poly)siloxanes, leading to optimization of procedures for the synthesis of epoxy-functional silanes and siloxanes. 

Our contribution to this field was the synthesis, isolation, and full characterization a few new rhodium siloxide complexes, both dimeric [13] and monomeric (see Fig. 1) [14]. 

High catalytic activity of dimeric complex 1 in the hydrosilylation of allyl esters and allyl ethers (as well allyl glycidyl ether) has been shown in Ref. [18]. The catalytic activity of rhodium-siloxide-phosphine complexes depends on the steric effects of the siloxy group and stereoelectronic effects of the trisubstituted phosphine [14]. Comparison of catalytic activity of dimeric (1) and monomeric, phosphine (2, 3) and non-phosphine (4) rhodium siloxide complexes in the examined reaction is presented in Table 1. 

The activity of rhodium siloxide complexes is not suppressed in the hydiosilylation process, which means that the same catalyst can be used a few times. Table 2 shows typical activities of [Rh(cod)(PCy3)(OSiMe3)] (2) three times reused in the reaction examined. 

Hydrosilylation is also one of the basic methods for synthesis of modifying silicones. The rhodium siloxide complexes have been used in the reaction between heptamethyltrisiloxane and allyl glycidyl ether, which is a modeling reaction of the polymeric system (Eq. 2). 

The use of rhodium siloxide complexes in a polymeric system (Eq. 3) has not affected the activity of the catalysts, as checked in the reaction with trisiloxane. The conversion of poly(hydro)siloxane was almost quantitative under very mild reaction conditions (25 - 40 °C, 2 h), and a valuable epoxy-ftmctional polysiloxane was formed selectively. 

Table 1. The hydrosilylation of allyl glycidyl ether by triethoxysilane catalyzed by rhodium siloxide complexes.

Catalytic Activity of Rhodium-Siloxide Complexes in Hydrosilylation of Allyl Ethers and... 

Rhodium-siloxide complex [ (diene)Rh(p-OSiMe3) 2] (I) appeared to be a very effective catalyst of the hydrosilylation of various allyl ethers for example, hydrosilylation of allyl glycidyl ether, allyl butyl ether, allyl phenyl ether and allyl benzyl ether [9] proceeds almost quantitatively even at room temperature (Table 1). The hydrosilylation of allyl glycidyl ether by triethoxysilane leads to glycidoxypropyltriethoxysilane, which is a commercially important silane coupling agent. The reaction of allyl ethers with hydrosiloxanes catalyzed by I also occurs with very high yield (Table 2) [10]. The hydrosilylation products have applications in the cosmetic industry [11]. 

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