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.
The aryl-substituted bis(imino)pyridine cobalt methyl complex, (MesPDI)CoCH3 (MesPDI = 2,6-(2,4,6-Me3C6H2-NCMe)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.
Iron dialkyl complexes, N3Fe(CH2SiMe3)2, with three different classes of tridentate, nitrogen-based “N3” ligands, aryl-substituted bis(imino)pyridines, terpyridine, and pyridine bis(oxazoline), have ...been synthesized and evaluated in the catalytic hydrosilylation of olefins with tertiary silanes. The 2,2′:6′,2″-terpyridine (terpy) complex, (terpy)Fe(CH2SiMe3)2, was prepared either via alkylation of (terpy)FeCl2 with LiCH2SiMe3 or by pyridine displacement from (pyridine)2Fe(CH2SiMe3)2 by free terpyridine. The aryl-substituted bis(imino)pyridine compounds, (RPDI)Fe(CH2SiMe3)2 (RPDI = 2,6-(2,6-R2-C6H3NCMe)2C5H3N), with smaller 2,6-dialkyl substituents (R = Et, Me) or a 2- i Pr substituent (2‑iPrPDI)Fe(CH2SiMe3)2 (2‑iPrPDI = 2,6-(2- i Pr-C6H4NCMe)2C5H3N, are effective precursors (0.5 mol %) for the anti-Markovnikov hydrosilylation of 1-octene with (Me3SiO)2MeSiH and (EtO)3SiH over the course of 1 h at 60 °C. No hydrosilylation activity was observed with Et3SiH. The most hindered member of the series, ( iPrPDI)Fe(CH2SiMe3)2, and the pyridine bis(oxazoline) iron compound, (R,R)-( iPrPybox)Fe(CH2SiMe3)2 ( iPrPybox = 2,6-bisisopropyl-2-oxazolin-2-ylpyridine), were inactive for the hydrosilylation of 1-octene with all tertiary silanes studied. By contrast, the terpyridine precursor, (terpy)Fe(CH2SiMe3)2, reached >95% conversion at 60 °C with Et3SiH and (Me3SiO)2MeSiH. In addition, the hydrosilylation of vinylcyclohexene oxide was accomplished in the presence of 1.0 mol % (terpy)Fe(CH2SiMe3)2, demonstrating functional group compatibility unique to this compound that is absent from bis(imino)pyridine iron compounds. The electronic structures of all three classes of iron dialkyl compounds have been evaluated by a combination of X-ray diffraction, magnetochemistry, Mössbauer spectroscopy, and density functional theory calculations. All of the compounds are best described as high-spin iron(III) compounds with antiferromagnetic coupling to chelate radical anions.
Aryl-substituted bis(imino)pyridine iron dinitrogen complexes are active for the hydrosilylation of 1,2,4-trivinylcyclohexane with tertiary alkoxy silanes, a process used in the manufacture of low ...rolling resistance tires. The iron compounds exhibit unprecedented selectivity for the monohydrosilylation of the desired 4-alkene that far exceeds results obtained with commercially used platinum compounds.
We report the preparation, structure, and reactions of a stable metallapyridine complex of tantalum prepared in the course of model studies of hydrodenitrogenation (HDN) reactions. is isolated upon ...thermolyzing the η2(N,C)-pyridine complex η2(N,C)-2,4,6-NC5 tBu3H2Ta(OAr)2Me (2) in the presence of THF, while the metallapyridine is isolated when this reaction is carried out in benzene. Complete NMR characterization of 5·THF is described, along with its conversion into 6. The bis(pyridine) is also described. is shown to react with tBuNCO and iPrNCNiPr to afford the σ (η1) and π (η3) insertion products and TaNCtBuCHCtBuCH(η3-C(NiPr)2)(OAr)2 (8). The molecular structure of the was detemined by X-ray crystallography and shown to adopt a trigonal-bipyramidal configuration with aryl oxide oxygens and the metallacyclic carbon occupying equatorial positions. The TaNC4 metallacycle is very nearly planar, and discrete single and double bonds are evident around the ring. This π localization clearly favors the imido than a carbene structure. The relevance of these compounds to hydrodenitrogenation catalysis is described.
