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Synthesis

Ironing Out Alkene Hydrosilylation

Catalysis: Iron compound could supplant more expensive catalysts

by Bethany Halford
February 6, 2012 | APPEARED IN VOLUME 90, ISSUE 6

Chemists could soon have a cheaper way to make silicone-based surfactants, fluids, release coatings, and pressure-sensitive adhesives, thanks to an iron catalyst that spurs alkene hydrosilylation reactions (Science, DOI: 10.1126/science.1214451). Alkene hydrosilylation—in which a silicon hydride adds across a carbon-carbon double bond—is typically done with expensive metal catalysts based on platinum or rhodium, most of which is not recovered.

By tinkering with the substituents of a bis(imino)pyridine iron dinitrogen catalyst they’d previously developed for olefin hydrogenations (Inorg. Chem., DOI: 10.1021/ic902162z), researchers led by Princeton University’s Paul J. Chirik were able to get tertiary silanes to add across terminal olefins in exclusively anti-Markovnikov fashion. That is, the silicon group adds only to the less substituted carbon at the end of the chain, producing the terminal alkyl silanes used in various commercial applications.

The iron compounds can complete the reaction in as little as 15 minutes, and they don’t produce internal isomers that are unwanted by-products of the reaction with Pt or Rh catalysts. “We knew from our olefin hydrogenation work that these compounds were fast, but we were still amazed at their performance in hydrosilylation,” Chirik tells C&EN.

“The results of Chirik’s fine-tuning have created an excellent catalyst for specific formation of a silicon-terminated alkyl while utilizing quite low catalyst loading as well as tertiary silane,” comments Joyce Y. Corey, an expert on organosilicon chemistry at the University of Missouri, St. Louis. Corey also notes that the hydrosilylation reactions take place at room temperature and in the absence of solvent. “These features fit in well with the tenets of green chemistry,” she says.

Chirik has teamed up with specialty chemical company Momentive, with the hope of using the iron catalysts to make products on an industrial scale. “Ideally, the sensitivity to moisture and oxygen of these catalysts needs to be improved to make them more practical for large-scale chemical processes,” says Keith J. Weller, Momentive’s group manager for engineered materials. “We firmly believe that these catalysts have the potential for use in the production of a wide range of organofunctional silanes and silicones.”

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