On the mechanism of metal colloid catalyzed hydrosilylation: proposed explanations for electronic effects and oxygen cocatalysis

Abstract

Several aspects of the platinum-catalyzed hydrosilylation reaction, R3SiH + R'CH=CH2, are described and a mechanism based on the intermediacy of colloids is proposed. New features of this mechanism include (1) formation of a Pt colloid/R3SiH intermediate 2 from the reaction of the Pt colloid 1 and R3SiH, (2) consideration of the olefin as a nucleophile and thus intermediate 2 being an electrophile in this reaction, (3) hydrosilylation dependence on cocatalysis by dioxygen where no 0-0 bond breakage occurs and dioxygen action to electronically modify 2 by making it more electrophilic, (4) hydrosilylation being but one case of the reactivity of 2 with nucleophiles; the reaction with R"OH where R" = H or alkyl is discussed. The effect of the electronic nature of the substituents on the rate of hydrosilylation was measured. Electron withdrawing substituents, R, on R3SiH accelerate the rate of addition to olefins, e.g. the rate of addition of (EtO)3SiH to olefins proceeds at a higher rate than the addition of Et3SiH to olefins. Electron donating groups, R', on R'CH=CH2 greatly accelerate the rate of R3SiH to olefins, e.g. the Et3SiH addition occurs at a faster rate to Me3SiCH=CH2 than to Cl3SiCH==CH2. The relative rate of addition of (EtO)3SiH to a series of para-substituted styrenes was studied which confirmed the trend that higher rates of addition of R3SiH occurs to olefins, R'CH=CH2 with more electron donating substituents, R'. The origin of the «¡catalytic effect of dioxygen in hydrosilylation was studied by generating Pt colloid under an atmosphere containing 1602 and l802 and noting that the 0-0 bond is not broken and reformed under these conditions. It was demonstrated that the proposed intermediate 2 behaves as an electrophile by showing that Me3SiCH2CH=CH2 exchanges with Et3SiH in the presence of Pt to give trapped products based on the rearranged products Me3SiH and Et3SiCH2CH=CH2 in the presence of an electrophile (in this case Pt/Et3SiH). The reaction of water with R3SiH in the presence of a Pt catalyst in commercial silicone foams produces H2, and this reaction is described in the context of hydrosilylation where the water nucleophile replaces the olefin. up the NMR. Dr. Elizabeth Williams and Ms. Joanne Smith ran the MSi NMRs, Ralph May, Steve Dorn, and Hans Grade carried out the mass spectrometry analyses, and Dr. Pete Codella carried out the Raman measurements. Professor Sam Danishefsky, Yale University, is thanked for suggesting the allyltrimethylsilane experiments. Cindy Herderich is thanked for help in preparing this manuscript. Supplementary Material Available: Figure showing relative rate of addition of (EtO)3SiH to Me3Si(vinyl) and styrene (1 page).Ordering information is given on any current masthead page.

Hydrosilation in Supercritical CO2: Synthesis of Fluorinated Polysiloxanes

“…The reactor was not purged prior to pressurization with CO 2 , since oxygen is necessary for hydrosilation to occur. [15] This was verified during an experiment where the reactor was purged with CO 2 prior to pressurization; this resulted in no reaction at all. Without purging, any moisture in the air remains in the reactor, and could conceivably participate in side reactions, espe- cially at such high pressures.…”

Design, characterization, and evaluation of peptide arrays allowing the direct monitoring of MMP activities

“…Hydrosilylation of a mixture (1:1) of compounds (4) and (6) (Electronic Supplementary Material Scheme S1) was then performed with trichlorosilane using Kärsted's catalyst to give compounds (5) and (7) [45,46]. Grafting of hydrosilylation products was then performed at low concentration (6 mM) (Scheme 1).…”

Hydrosilylation conditions applied on alkenyl benzylated xyloses: selective reduction and isomerization

“…Comments concerning the formation of the reduction and isomerization products are required. According to literature, [9,19,21,22] the reaction of Et 3 SiH with transition metals (M) can lead to the generation of Et 3 SiMH and hydrogen. Thus, some metal-catalyzed hydrogenation of the C C bond can occur.…”