Keith J. Weller

Iron catalysts for selective anti-Markovnikov alkene hydrosilylation using tertiary silanes.

An Iron Hand for Silicon Carbon-silicon bonds are integral to the structure of the silicone materials widely used in adhesives, cosmetics, and numerous other industrial and consumer products. Generally, platinum-based catalysts have performed best in the hydrosilylation reactions that form these bonds, but the expense of the precious metal and, in some cases, by-product formation have motivated a search for alternatives. Tondreau et al. (p. 567) now show that a class of iron compounds readily catalyzes hydrosilylation of certain commercially important substrates with rates and selectivities comparable to, or even exceeding, those associated with platinum. Iron catalysts offer a potentially cheaper route than platinum for certain commercially useful carbon-silicon compounds. Alkene hydrosilylation, the addition of a silicon hydride (Si-H) across a carbon-carbon double bond, is one of the largest-scale industrial applications of homogeneous catalysis and is used in the commercial production of numerous consumer goods. For decades, precious metals, principally compounds of platinum and rhodium, have been used as catalysts for this reaction class. Despite their widespread application, limitations such as high and volatile catalyst costs and competing side reactions have persisted. Here, we report that well-characterized molecular iron coordination compounds promote the selective anti-Markovnikov addition of sterically hindered, tertiary silanes to alkenes under mild conditions. These Earth-abundant base-metal catalysts, coordinated by optimized bis(imino)pyridine ligands, show promise for industrial application.

Bis(imino)pyridine Cobalt-Catalyzed Dehydrogenative Silylation of Alkenes: Scope, Mechanism, and Origins of Selective Allylsilane Formation

The aryl-substituted bis(imino)pyridine cobalt methyl complex, ((Mes)PDI)CoCH3 ((Mes)PDI = 2,6-(2,4,6-Me3C6H2-N═CMe)2C5H3N), promotes the catalytic dehydrogenative silylation of linear α-olefins to selectively form the corresponding allylsilanes with commercially relevant tertiary silanes such as (Me3SiO)2MeSiH and (EtO)3SiH. Dehydrogenative silylation of internal olefins such as cis- and trans-4-octene also exclusively produces the allylsilane with the silicon located at the terminus of the hydrocarbon chain, resulting in a highly selective base-metal-catalyzed method for the remote functionalization of C-H bonds with retention of unsaturation. The cobalt-catalyzed reactions also enable inexpensive α-olefins to serve as functional equivalents of the more valuable α, ω-dienes and offer a unique method for the cross-linking of silicone fluids with well-defined carbon spacers. Stoichiometric experiments and deuterium labeling studies support activation of the cobalt alkyl precursor to form a putative cobalt silyl, which undergoes 2,1-insertion of the alkene followed by selective β-hydrogen elimination from the carbon distal from the large tertiary silyl group and accounts for the observed selectivity for allylsilane formation.

The chemistry of methyl and ethyl radicals on Pt(111) from the decomposition of tri-alkyl bismuth compounds

Abstract The chemistry of trimethyl and triethyl bismuth compounds adsorbed on Pt(111) have been studied using thermal desorption spectroscopy, Auger electron spectroscopy, and high resolution electron energy loss spectroscopy. The tri-alkyl bismuth compounds adsorb molecularly on Pt(111) at 110 K. Upon heating, the R3Bi compound decomposes and alkyl radicals are introduced to the surface. Alkyl radicals and bismuth atoms are delivered to the Pt(111) surface in a 3 : 1 (alkyl : bismuth) ratio. In the presence of Bi, it is expected that the alkyl radical chemistry is not perturbed substantially. Below multilayer coverage, the methyl and ethyl radicals, delivered to the surface by thermal decomposition of the parent, show similar trends in their chemistry. There is some fraction of parent (BiR3) desorption at ~ 190 K The remaining methyl or ethyl radicals follow two reaction pathways. The first reaction channel is dehydrogenation of the methyl or ethyl radical leading to CH or ethylidyne species, respectively, remaining on the surface. The second channel is hydrogenation involving surface hydrogen produced by the dehydrogenation channel which leads to CH4 or CH3CH3 formation, respectively. This pathway leads to reaction rate limited desorption of CH4 with a peak temperature at 280 K and CH3CH3 with a peak temperature at 295 K.

Oxide-base adducts of aluminum: X-ray crystal structures of Me3Al(OPPh3), Me3Al(ONMe3) and [(CH3)3SiO]3Al(OPPh3)

Abstract The crystalline molecular structures of three Lewis adducts of aluminum are reported. The structures of the 1 : 1 adducts between trimethylaluminum and triphenylphosphine oxide, 1 : Me 3 Al(OPPh 3 ), trimethylaluminum and trimethylamine-N-oxide; 2 : Me 3 Al(ONMe 3 ); and 3 : [(CH 3 ) 3 SiO] 3 Al(OPPh 3 ) are discussed, and a comparison is made to the previously characterized aluminosilsesquinoxane complex ( c -C 6 H 11 ) 7 Si 7 O 12 Al(OPPh 3 ) ( 5 ).