Silicon
Silicon is located in the periodic table immediately below carbon. It is tetravalent and forms
tetrahedral compounds. Unlike carbon silicon does not form double bonds. There are also some
important differences in bond strength: the silicon-silicon bond (230 kJ mol-1) is weak in
comparison to the carbon-carbon bond (356 kJ mol-1) whereas the silicon-oxygen bond (368 kJ
mol-1) is stronger than the corresponding carbon-oxygen s-bond (336 kJ mol-1). In general, bonds
with electronegative elements are stronger with silicon than with carbon. In particular, the siliconfluorine bond is extremely strong (582 kJ mol-1). In comparison, bonds to electropositive elements
are weaker (i.e. triethylsilane (Et3SiH) is a reducing agent since the Si–H bond is relatively weak
(~323 kJ mol-1)). The carbon-silicon bond is strong enough for trialkyl silyl groups to survive a
wide variety of synthetic transformations, but it is weak enough to be selectively cleaved when
required using mild conditions. In particular, the hard nucleophilic fluoride group is used to
remove silicon groups with the driving force for the transformation being the formation of strong
Si–F bonds. The bonds between silicon and other atoms are in general longer than the equivalent
bonds between carbon and the corresponding atoms. For example, Si–C bonds are 1.89Å
whereas a typical C–C bond is 1.54Å. The increased bond lengths between silicon and other
atoms in comparison to the corresponding systems involving carbon enables hard nucleophiles
(in particular F-) to react at sterically hindered silicon centres. Silicon has a lower electronegativity
value (1.8), c.f. carbon (2.5) and consequently, carbon-silicon bonds are polarised, rendering the
silicon open to attack by nucleophiles.
The most effective nucleophiles for silicon are those which are strongly electronegative and that
upon reaction lead to the formation of strong bonds to silicon. A common source of fluoride ions is
tetrabutylammonium fluoride (TBAF). This reagent is soluble in a wide range of commonly
employed organic solvents. For example, silyl ethers can be cleaved by treatment with
tetrabutylammonium fluoride. The mechanism involved appears to involve a simple SN2 process,
however, this is not the case. The reaction proceeds via a pentacovalent silicon centre. Two
factors unique to silicon (in comparison to carbon) facilitate this process: i) the long silicon–
carbon bonds permit nucleophilic attack at what would appear to be a sterically congested silicon
centre and ii) the vacant d-orbitals of silicon permit nucleophilic attack via geometric approaches
not permitted by the bonding and anti-bonding orbitals of carbon.
This nucleophilic substitution process is referred to as the SN2-Si pathway and is an extremely
rapid process. Almost all acyclic halosilanes (apart from fluorosilanes) react with nucleophiles by
the SN2-Si pathway and this leads to the inversion of configuration at silicon. In theory, the above
reaction could proceed via an SN1 pathway, (trialkylsilane cations are frequently observed in
mass spectra), but this pathway is kinetically slower and leads to a 50:50 distribution of isomers.
ß-Effect
The stabilisation of cations at the carbon atom in β-position to silicon is referred to as the β-effect
of silicon. It is a result of the stabilisation of the positive charge by donation of electron density
from the filled s-orbitals of the adjacent C–H and C–C bonds to the vacant p-orbital of silicon.
Silicon is more electropositive in comparison to carbon and therefore the C–Si bonds is an even
more effective donor. In molecular orbital terms this can be described as the overlap between the
vacant p-orbital on the carbon ß to the silicon atom and the filled s-orbital between the silicon
atom and the a-carbon. Maximum stabilization only occurs if the vacant p-orbital and the s-orbital
of the carbon-silicon bond are in the same plane. This can readily occur in acyclic systems but
can be more difficult in cyclic systems.
Another consequence of the β-effect is the preference for Ipso-Substitution displayed by aryl
silanes in electrophilic aromatic substitution reactions. The only product formed in these reactions
is the one resulting from direct substitution of the silyl group on the aromatic ring. Due to the βeffect the most stable Wheland intermediate in this reaction is ß to the silicon atom. Subsequent
cleavage of the weakened C–Si bond by a nucleophile therefore affords the ipso product.
An alternative process can occur that would also involve the generation of a cation ß to silicon.
However, this cation is not as stable as the vacant p-orbital is orthogonal to the C-Si bond and
there cannot interact with it.
Synthesis of Organosilanes
Alkylsilanes
Nucleophilic displacement of a Halogen from a Halosilane by an Organometallic Reagent
Grignard reagents and alkyl lithium systems can react with trimethylsilyl chloride to afford the
corresponding tetrasubstituted silanes. The carbon-silicon bonds are sufficiently robust to
withstand these reaction conditions and thus the trimethylsilyl group remains intact.
Vinyl- or Alkenylsilanes
Addition of Silanes to Multiple Bonds (Hydrosilation): Alkenes and alkynes can be converted into
organosilanes by hydrosilation, typically involving the use of a catalyst such as hexachloroplatinic
acid (H2PtCl6). Silanes that possess either one or more halogen substituent are more reactive
than trialkylsilanes.
Nucleophilic displacement of a Halogen from a Halosilane by an Organometallic Reagent
Alkynyl Silanes
Terminal alkynes possess acidic protons (pKa ~25) and are therefore removed by strong bases
(Grignards, alkyl lithiums). If there are two alkynyl protons present one of them is protected
("masked") by trimethylsilylation. After the reaction the TMS protecting group can be readily
cleaved by treatment with TBAF.
Alkynyl silanes can undergo electrophilic attack followed by elimination. The
determines the regiochemistry of the electrophile attack.
-effect of silicon
Silicon-based Protecting Groups
Silicon-based protecting groups are used extensively as a direct consequence of their versatility.
They are formed efficiently by treatment of an halosilane with an alkoxide and the silicon group is
easily removed by nucleophilic displacement with either fluoride or oxygen nucleophiles. The rate
of removal is related to the steric bulk of the silicon-protecting group. The simplest protecting
group is the trimethylsilyl (TMS) unit.Trimethylsilylation can be carried out using TMS-Cl and base
or other related reagents in which silicon is activated towards nucleophilic attack (e.g. NTrimethylsilylimidazole (TMSI)). As it is the least sterically hindered silyl ether it is cleaved using
water with either trace of acid or base.
In general, the bulkier the alkyl substituents on the silicon the harsher the conditions required for
removal of the protecting group. In use are the tertiary-butyldimethylsilyl (TBDMS) group, the
triisopropylsilyl (TIPS) group and the tertiary-butyldiphenylsilyl (TBDPS). The bulkier the group
the more difficult the formation of the silylether is. Imidazole is added to increase the reaction rate
(nucleophilic catalysis). The TBDMS group is easily introduced by treating the alcohol with
TBDMSCl in DMF in conjunction with imidazole. With bulkier groups it is possible to distinguish
between primary and secondary alcohols.
The most common deprotection procedure of sterically demanding Si-protecting groups involves
flouride cleavage with TBAF (tetrabutylammonium fluoride) as reagent.
Silyl Enol Ethers
Silyl enol ethers are of enormous importance in organic chemistry as stable enolate equivalents.
They are frequently employed in directed aldol reactions. Silyl enol ethers are produced
classically by quenching enolate anions with TMS-Cl as electrophile.
The regiochemistry of enolate formation can be controlled using kinetic conditions (low
temperature, strong, sterically demanding base) or thermodynamic conditions (weaker,
"equilibrating" base, room or higher temperatures). Silyl enol ethers can also be prepared by
either conjugate addition followed by trapping of the intermediate enolate ion with TMS-C.
silylation or by direct hydrosilylation of a,ß-unsaturated ketones.
Silyl enol ethers are relatively inert towards reaction with electrophiles. To achieve this reaction
the silyl enol ether is first "activated" by conversion into a more reactive enolate (lithium enolate
by reaction with methyl lithium or quaternary ammonium enolate with quaternary ammonium
fluoride.
Silyl enol ethers can also react with strong electrophiles if the electrophilicity of the alkylating
agent is enhanced by the presence of a Lewis acid. Under such Lewis acid catalysis,
electrophiles such as aldehydes and ketones may be added to trimethylsilyl enol ethers. This
reaction is sometimes referred to as the Mukayama Aldol reaction.