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:^[1]

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.^[2] 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.

Idealized mechanism for metal-catalysed hydrosilylation of an alkene.

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 → (Ph)(CH₃)CHSiCl₃

Nearly perfect enantioselectivities (ee's) can be achieved using palladium catalysts supported by binaphthyl-substituted monophosphine ligands.^[3]

Surface 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.^{[4] [5]}

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.

References

1. ^ 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.
2. ^ C. Elschenbroich, Organometallics (2006) Wiley and Sons-VCH: Weinheim. ISBN 978-3-29390-6
3. ^ 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
4. ^ "Photoreactivity of Unsaturated Compounds with Hydrogen-Terminated Silicon (111),” R. L. Cicero, M. R. Linford, C. E. D. Chidsey, Langmuir 16, 5688-5695 (2000)
5. ^ "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).