Hydrosilylation, also called catalytic hydrosilation, describes the addition of Si-H bonds across unsaturated bonds."Hydrosilylation A Comprehensive Review on Recent Advances" B. Marciniec (ed.), Advances in Silicon Science, Springer Science, 2009. Ordinarily the reaction is conducted catalytically and usually the substrates are unsaturated

organic compound In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millions of organic compounds are known. The ...

Alkene In organic chemistry, an alkene is a hydrocarbon containing a carbon–carbon double bond. Alkene is often used as synonym of olefin, that is, any hydrocarbon containing one or more double bonds.H. Stephen Stoker (2015): General, Organic, an ...

s and

alkyne \ce \ce Acetylene \ce \ce \ce Propyne \ce \ce \ce \ce 1-Butyne In organic chemistry, an alkyne is an unsaturated hydrocarbon containing at least one carbon—carbon triple bond. The simplest acyclic alkynes with only one triple bond and n ...

s give alkyl and

vinyl silane Vinylsilane refers to an organosilicon compound with chemical formula CH2=CHSiH3. It is a derivative of silane (SiH4). The compound, which is a colorless gas, is mainly of theoretical interest. Substituted vinylsilanes More commonly used than th ...

aldehyde In organic chemistry, an aldehyde () is an organic compound containing a functional group with the structure . The functional group itself (without the "R" side chain) can be referred to as an aldehyde but can also be classified as a formyl grou ...

s and

ketone In organic chemistry, a ketone is a functional group with the structure R–C(=O)–R', where R and R' can be a variety of carbon-containing substituents. Ketones contain a carbonyl group –C(=O)– (which contains a carbon-oxygen double bo ...

s give silyl ethers. Hydrosilylation has been called the "most important application of platinum in homogeneous catalysis."

Scope and mechanism

Hydrosilylation of alkenes represents a commercially important method for preparing

organosilicon compounds Organosilicon compounds are organometallic compounds containing carbon–silicon bonds. Organosilicon chemistry is the corresponding science of their preparation and properties. Most organosilicon compounds are similar to the ordinary organic co ...

. The process is mechanistically similar to the

hydrogenation Hydrogenation is a chemical reaction between molecular hydrogen (H2) and another compound or element, usually in the presence of a Catalysis, catalyst such as nickel, palladium or platinum. The process is commonly employed to redox, reduce or S ...

of alkenes. In fact, similar catalysts are sometimes employed for the two catalytic processes. The prevalent mechanism, called the Chalk-Harrod mechanism, assumes an intermediate metal complex that contains a

hydride In chemistry, a hydride is formally the anion of hydrogen( H−). The term is applied loosely. At one extreme, all compounds containing covalently bound H atoms are called hydrides: water (H2O) is a hydride of oxygen, ammonia is a hydride of ...

, a silyl ligand (R₃Si), and the alkene substrate. Oxidative addition proceeds by the intermediacy of a sigma-complex, wherein the Si-H bond is not fully broken. Hydrosilylation of alkenes usually proceeds via

anti-Markovnikov addition In organic chemistry, Markovnikov's rule or Markownikoff's rule describes the outcome of some addition reactions. The rule was formulated by Russian chemist Vladimir Markovnikov in 1870. Explanation The rule states that with the addition of a ...

, i.e., silicon is placed at the terminal carbon when hydrosilylating a terminal alkene Variations of the Chalk-Harrod mechanism exist. Some cases involve insertion of alkene into M-Si bond followed by reductive elimination, the opposite of the sequence in the Chalk-Harrod mechanism. In certain cases, hydrosilylation results in vinyl or allylic silanes resulting from

beta-hydride elimination β-Hydride elimination is a reaction in which an alkyl group bonded to a metal centre is converted into the corresponding metal-bonded hydride and an alkene. The alkyl must have hydrogens on the β-carbon. For instance butyl groups can undergo th ...

. Alkynes also undergo hydrosilylation, e.g., the addition of

triethylsilane Triethylsilane is the organosilicon compound with the formula (C2H5)3SiH. It is a trialkylsilane. The Si-H bond is reactive. This colorless liquid is used in organic synthesis as a reducing agent and as a precursor to silyl ethers. As one of th ...

diphenylacetylene Diphenylacetylene is the chemical compound C6H5C≡CC6H5. The molecule consists of two phenyl groups attached to a C2 unit. A colorless solid, it is used as a building block in organic synthesis and as a ligand in organometallic chemistry. Prepar ...

: :Et₃SiH + PhC≡CPh → Et₃Si(Ph)C=CH(Ph)

Asymmetric hydrosilylation

Using

chiral Chirality is a property of asymmetry important in several branches of science. The word ''chirality'' is derived from the Greek (''kheir''), "hand", a familiar chiral object. An object or a system is ''chiral'' if it is distinguishable from ...

phosphines as spectator

ligand In coordination chemistry, a ligand is an ion or molecule (functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's electr ...

s, catalysts have been developed for catalytic asymmetric hydrosilation. A well studied reaction is the addition of

trichlorosilane Trichlorosilane is an inorganic compound with the formula HCl3Si. It is a colourless, volatile liquid. Purified trichlorosilane is the principal precursor to ultrapure silicon in the semiconductor industry. In water, it rapidly decomposes to pr ...

styrene Styrene () is an organic compound with the chemical formula C6H5CH=CH2. This derivative of benzene is a colorless oily liquid, although aged samples can appear yellowish. The compound evaporates easily and has a sweet smell, although high concen ...

to give 1-phenyl-1-(trichlorosilyl)ethane: :Cl₃SiH + PhCH=CH₂ → (Ph)(CH₃)CHSiCl₃ Nearly perfect enantioselectivities (ee's) can be achieved using palladium catalysts supported by binaphthyl-substituted monophosphine ligands.

Surface hydrosilylation

Silicon wafer In electronics, a wafer (also called a slice or substrate) is a thin slice of semiconductor, such as a crystalline silicon (c-Si), used for the fabrication of integrated circuits and, in photovoltaics, to manufacture solar cells. The wafer serv ...

s can be etched in

hydrofluoric acid Hydrofluoric acid is a Solution (chemistry), solution of hydrogen fluoride (HF) in water. Solutions of HF are colourless, acidic and highly Corrosive substance, corrosive. It is used to make most fluorine-containing compounds; examples include th ...

(HF) to remove the native oxide and form a hydrogen-terminated silicon surface. The hydrogen-terminated surfaces undergo hydrosilation with unsaturated compounds (such as terminal alkenes and alkynes), to form a stable monolayer on the surface. For example: :Si-''H'' + H₂C=CH(CH₂)₇CH₃ → Si-CH₂CH''H''-(CH₂)₇CH₃ The hydrosilylation reaction can be initiated with UV light at room temperature or with heat (typical reaction temperature 120-200 °C), under moisture- and oxygen-free conditions. The resulting monolayer, which is stable and inert, inhibits oxidation of the base silicon layer, relevant to various device applications.

Catalysts

Before introduction of platinum catalysts by Speier, hydrosilylation was not practiced widely. A peroxide-catalyzed process was reported in academic literature in 1947, but the introduction of Speier's catalyst ( H₂PtCl₆) was a big breakthrough.

Karstedt's catalyst Karstedt's catalyst is an organoplatinum compound derived from divinyl-containing disiloxane. This coordination complex is widely used in hydrosilylation catalysis. It is a colorless solid that is generally assumed to be a mixture of related Pt( ...

was later introduced. It is a lipophilic complex that is soluble in the organic substrates of industrial interest.C. Elschenbroich, ''Organometallics'' (2006) Wiley and Sons-VCH: Weinheim. Complexes and compounds that catalyze hydrogenation are often effective catalysts for hydrosilylation, e.g.

Wilkinson's catalyst Wilkinson's catalyst is the common name for chloridotris(triphenylphosphine)rhodium(I), a coordination complex of rhodium with the formula hCl(PPh3)3(Ph = phenyl). It is a red-brown colored solid that is soluble in hydrocarbon solvents such as ...

Scope and mechanism

Asymmetric hydrosilylation

Surface hydrosilylation

Catalysts

References

Further reading