Hexene

Abstract: Linear and cyclic alkanes can be selectively dehydrogenated to the corresponding alkenes both thermally and photochemically (254 nm) with iridium complexes as catalysts. In the photochemical case, a sacrificial hydrogen acceptor is not required and H/sub 2/ is evolved directly. Preferential initial formation of the least stable alkene (e.g., methylenecyclohexane from methylcyclohexane) is explained by attack at unhindered C-H bonds by an oxidative addition to the metal. A subsequent ..beta..-elimination gives the alkene. A key feature of the catalyst, (IrH/sub 2/(eta/sup 2/-O/sub 2/CCF/sub 3/)(PR/sub 3/)/sub 2/) (4), is that the chelating acetate group can open up to allow ..beta..-elimination to take place in the alkyl hydride intermediate (IrH(R)(eta/sup 2/-O/sub 2/CCF/sub 3/)(PR/sub 3/)/sub 2/). Prolonged reaction times lead to progressive isomerization of the alkene to give the thermodynamic product (e.g., 1-methylcyclohexene from methylcyclohexane). Up to 32 turnovers of dehydrogenation are seen. Deactivation of the catalyst takes place by P-C hydrogenolysis of the PAr/sub 2/ ligand in the thermal case; the rise in the amount of ArH formed parallels the fall-off in activity of the catalyst. P-C cleavage does not take place in the photochemical system (R = cyclohexyl).

Abstract: The anionic ruthenium cluster carbonylates [Ru 6 C(CO) 16 ] 2– or [H 2 Ru 10 (CO) 25 ] 2– interspersed with bis(triphenylphosphino)iminium counterions (PPN + ) are incorporated from solution into the pores of MCM-41 mesoporous silica (3 nanometers in diameter), where they form tightly packed arrays. These arrays were shown by high-resolution transmission electron microscopy, Fourier transform optical diffraction, and computer simulations to be well ordered both along and perpendicular to the axis of the cylindrical pores. In their denuded state produced by gentle thermolysis, the cluster carbonylates yield nanoparticles of ruthenium that are less well ordered than their assimilated precursors but show good activity as hydrogenation catalysts for hexene and cyclooctene. In both their as-prepared and denuded states, these encapsulated clusters are likely to exhibit interesting electronic and other properties.

Abstract: Development of shear-induced crystallization precursor structure was studied by in-situ rheo-SAXS (small-angle X-ray scattering) and rheo-WAXD (wide-angle X-ray diffraction) techniques using binary polymer blends of high and low molecular weight polyethylenes near their nominal melting temperatures (120 °C). Two low molecular weight polyethylene copolymers, containing 2 mol % hexene, with weight-average molecular weights (Mw) of 50 000 (MB-50K) and 100 000 (MB-100K), and polydispersity of about 2, were used as the noncrystallizing matrices. A high molecular weight polyethylene homopolymer with Mw of 250 000 (MB-250K) and polydispersity of about 2 was used as the crystallizing minor component. Two series of model blends, MB-50K/MB-250K and MB-100K/MB-250K, each containing weight ratios of 100/0, 97/3, 95/5, and 90/10, were prepared by solution blending to ensure thorough mixing at the molecular level. At the chosen shear conditions (rate = 60 s-1, duration = 5 s, T = 120 °C), while no flow-induced structur...

Abstract: Alkene hydrosilylation is typically performed with Pt catalysts, but inexpensive base-metal catalysts would be preferred. We report a Co catalyst for anti-Markovnikov alkene hydrosilylation that can be used without added solvent at low temperatures with low loadings, and can be generated in situ from an air-stable precursor that is simple to synthesize from low-cost, commercially available materials. In addition, a mixture of Co catalysts performs a tandem catalytic alkene isomerization/hydrosilylation reaction that converts multiple isomers of hexene to the same terminal product. This regioconvergent reaction uses isomerization as a benefit rather than a hindrance.