No abstract

The mechanism of hydrosilylation using the highly active precatalyst Karstedt's precatalyst (Pt x -(M vinyl M vinyl ) y , M vinyl M vinyl ) divinyltetramethyldisiloxane) was investigated using extended X-ray absorption fine structure (EXAFS), small-angle X-ray scattering (SAXS), and ultraviolet-visible (UV-vis) spectroscopy. Contrary to previous reports suggesting colloidal catalysts, we find that regardless of the stoichiometric ratio of hydrosilane to olefin, the catalyst is a monomeric platinum compound with silicon and carbon in the first coordination sphere. The platinum end product, however, is a function of the stoichiometry of the reactants. At excess olefin concentration, the platinum end product contains only platinum-carbon bonds, whereas at high hydrosilane concentration, the platinum end product is multinuclear and also contains platinum-silicon bonds. The two end products can interconvert by adding additional aliquots of the stoichiometrically deficient reagent. An explanation of the "oxygen" effect is also given. In the absence of oxygen, hydrosilylation of certain olefins does not occur. Oxygen serves to disrupt multinuclear platinum species that are formed when poorly stabilizing olefins are employed. Finally, we discuss the olefin isomerization reaction that may accompany hydrosilylation of terminal olefins. When the hydrosilylation reaction proceeds slowly due to poorly reactive olefins, the olefin isomerization products become significant. EXAFS analysis of solutions after olefin isomerization has occurred shows the presence of platinum-platinum bonded compounds.

No abstract

No abstract

No 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.

Current fossil, genetic, and archeological data indicate that Homo sapiens originated in Africa in the late Middle Pleistocene. By the end of the Late Pleistocene, our species was distributed across every continent except Antarctica, setting the foundations for the subsequent demographic and cultural changes of the Holocene. The intervening processes remain intensely debated and a key theme in hominin evolutionary studies. We review archeological, fossil, environmental, and genetic data to evaluate the current state of knowledge on the dispersal of Homo sapiens out of Africa. The emerging picture of the dispersal process suggests dynamic behavioral variability, complex interactions between populations, and an intricate genetic and cultural legacy. This evolutionary and historical complexity challenges simple narratives and suggests that hybrid models and the testing of explicit hypotheses are required to understand the expansion of Homo sapiens into Eurasia.

No abstract