Chemistry:Silylation

Short description: Process of addition of silyl group(s) to compounds

Not to be confused with silanization or sialylation.

Silylation is the introduction of one or more (usually) substituted silyl groups (R₃Si) to a molecule. Silylations are core methods for production of organosilicon chemistry. Silanization involves similar methods but usually refers to attachment of silyl groups to solids.

Of organic compounds

Bis(trimethylsilyl)acetamide, a popular reagent for silylation

Alcohols, carboxylic acids, amines, thiols, and phosphates can be silylated. The process involves the replacement of a proton or an anion with a trialkylsilyl group, typically trimethylsilyl (-SiMe₃), as illustrated by the synthesis of a trimethylsilyl ethers from alcohols and trimethylsilyl chloride (Me = CH₃):

ROH + Me
3SiCl → ROSiMe
3 + HCl

Generally a base is employed to absorb the HCl coproduct.

Bis(trimethylsilyl)acetamide ("BSA", Me₃SiNC(OSiMe₃)Me is an efficient silylation agent. The reaction of BSA with alcohols gives the corresponding trimethylsilyl ether, together with acetamide as a byproduct (Me = CH₃):

2 ROH + MeC(OSiMe
3)NSiMe
3 → MeC(O)NH
2 + 2 ROSiMe
3

Use of silylation

Silylation has two main uses: manipulation of functional groups and preparation of samples for analysis.

Manipulation of functional groups

Often silylation is employed to protect OH and NH groups. The derivatives, silyl ethers and silyl amides, are resilient toward many reagents that would attack their precursors. The other main role of silylation is to trap silyl enol ethers, which represent a reactive tautomer of many carbonyl compounds.

Silylation for analysis

The introduction of a silyl group(s) gives derivatives of enhanced volatility, making the derivatives suitable for analysis by gas chromatography and electron-impact mass spectrometry (EI-MS). For EI-MS, the silyl derivatives give more favorable diagnostic fragmentation patterns of use in structure investigations, or characteristic ions of use in trace analyses employing selected ion monitoring and related techniques.

Desilylation

Desilylation is the reverse of silylation: the silyl group is exchanged for a proton. Various fluoride salts (e.g. sodium, potassium, tetra-n-butylammonium fluorides) are popular for this purpose.

ROSiMe₃ + F⁻ + H₂O → ROH + FSiMe₃ + OH⁻

Of metals

CpFe(CO)₂Si(CH₃)₃, a trimethylsilyl complex.

Coordination complexes with silyl ligands are well known. An early example is CpFe(CO)₂Si(CH₃)₃, prepared by silylation of CpFe(CO)₂Na with trimethylsilyl chloride. Typical routes include oxidative addition of Si-H bonds to low-valent metals. Metal silyl complexes are intermediates in hydrosilation, a process used to make organosilicon compounds on both laboratory and commercial scales.

See also

• Silyl ether
• Hydrosilylation

External links

• Identification of Silylation Artifacts in Derivatization Reactions for Gas Chromatography
• Desilylation methods