Hydrosilylation

Hydrosilylation, also called catalytic hydrosilation, describes the addition of Si-H bonds across unsaturated bonds. Ordinarily the reaction is conducted catalytically and usually the substrates are unsaturated organic compounds. Alkenes and alkynes give alkyl and vinyl silanes; aldehydes and ketones give silyl ethers.

The catalytic transformation represents an important method for preparing organosilicon compounds. An idealized transformation is illustrated by the addition of triethylsilane to diphenylacetylene: [James L. Fry, Ronald J. Rahaim Jr., Robert E. Maleczka Jr. "Triethylsilane", "Encyclopedia of Reagents for Organic Synthesis", John Wiley & Sons, 2007. DOI: 10.1002/ ISBN 047084289X .rt226.pub2.]

:Et₃SiH + PhC≡CPh → Et₃Si(Ph)C=CH(Ph)

The reaction is similar to hydrogenation, and similar catalysts are sometimes employed for the two catalytic processes. In industry "Speier's catalyst," H₂PtCl₆ is important. [C. Elschenbroich, "Organometallics" (2006) Wiley and Sons-VCH: Weinheim. ISBN 978-3-29390-6 ] The mechanism is usually assumes the intermediacy of a metal complex that contains a hydride, a silyl ligand (R₃Si), and the alkene or alkyne substrate.

Asymmetric hydrosilylation

Using chiral phosphines as spectator ligands, catalysts have been developed for catalytic asymmetric hydrosilation. A well studied reaction is the addition of trichlorosilane to styrene to give 1-phenyl-1-(trichlorosilyl)ethane::Cl₃SiH + PhCHCH₂ → PhCH(SiCl₃)CH₃Nearly perfect enantioselectivities (ee's) can be achieved using palladium catalysts supported by binaphthyl-subsituted monophosphine ligands. [T. Hayashi and K. Yamasaki, "C–E Bond Formation through Asymmetric Hydrosilylation of Alkenes", "Comprehensive Organometallic Chemistry III", Robert H. Crabtree and D. Michael P. Mingos, Editors. Amsterdam: Elsevier, 2007]

urface hydrosilylation

Silicon wafer can be etched in hydrofluoric acid (HF) to remove the native oxide, and form a hydrogen-terminated silicon surface. Then the hydrogen-terminated surfaces can react with unsaturated compounds (such as terminal alkenes and alkynes), to form a stable monolayer on the surface. For example:

H:Si(100) + CH=CH(CH₂)₇CH₃ → Si(100)-(CH₂)₉CH₃

The hydrosilylation reaction can be initiated with UV light at room temperature, or with applied heat (typical reaction temperature 120-200 degrees C), under moisture and oxygen free conditions. ["Photoreactivity of Unsaturated Compounds with Hydrogen-Terminated Silicon(111),” R. L. Cicero, M. R. Linford, C. E. D. Chidsey, Langmuir 16, 5688-5695(2000)] ["Alkyl Monolayers on Silicon Prepared from 1-Alkenes and Hydrogen-Terminated Silicon," M. R. Linford, P. Fenter, P. M. Eisenberger and C. E. D.Chidsey, J. Am. Chem. Soc. 117, 3145-3155 (1995).]

The resulting monolayer is stable and inert, and prevent oxidation of the base silicon layer. Surfaces of this kind could find applications in areas such as molecular electronics and biochemistry.

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