Silicon in organic synthesis
Silicio: Presenza e distribuzione
Il suo nome è stato proposto da Thomson nel 1831 con la desinenza (inglese silicon) –on per
sottolineare l’analogia con il carbonio (carbon) e il boro (boron).
Silicio (27.2%): elemento più abbondante nella crosta terrestre dopo l’ossigeno (45.5%)
questi due elementi costituiscono i 4/5 di tutti gli atomi presenti sulla superficie del globo.
Nell’universo il silicio è “solo” al 7° posto come abbondanza.
Ulteriore differenziazione in concentrazione sulla terra.
core (31% circa della massa terrestre) sostanzialmente non contiene silicio
mantello (68% circa della massa terrestre) contiene probabilmente silicati densi (es. olivina)
crosta (0.4%) contiene minerali silicei leggeri.
Silicon: Physical and Atomical Information
Silicon is constituted aminly by 28Si (92.23%) together with 4. 67% of29Si and 3.10% of 30Si.
No other stable isotopes.
In ground state its electronic configuration is [Ne]3s23p2
Property
Electronic configuration
C
[He]2s22p2
Si
[Ne]3s23p2
Physical state (25°C, 1 atm)
Appearence/Properties
Melting point (°C)
solid
Diamond, graphite,
fullerens/Non-metal
non conductor
3550 (d)
solid
Solido lucente blugrigio/Metalloid
semiconductor
1420
Boiling point (°C)
4827 (d)
3280
Density (g/cm3)
3.51 (diamond), 2.22 2.336
(graphite)
Proprietà fisiche e atomiche
Proprietà
Atomic radius, pm
C
77
Si
117
Electronegativity
2.5
1.8
E°(volt)
Common oxidation states
pH=0
CO2 -0.1 CO -0.52
IV
II
pH = 14
CO32-1.15
IV
+4
Ionization energies (kJ/mol)
M(g) → M+(g) + eM+(g) → M2+(g) + eM2+(g) → M3+(g) + eM3+(g) → M4+(g) + e-
1086
2354
4622
6512
pH=0
C SiO2
-091
0 IV
pH = 14
C SiO32-1.7
0 IV
(+2), +4
786
1577
3231
4355
Si
0
Si
0
Silicon: Preparation and Industrial uses
Silicon (96-99% purity) is prepared by reduction of quartz or sand with high purity coke in
electric oven (1500 °C);
SiO2 + C  Si + 2 CO
Silicon at very high purity is prepared by conversion of Si in SiCl4 and reducing the cloride
with Zn, Mg or H2 higly pure then melted and crystallized in a single crystal
e successivamente fuso e cristallizzato in un cristallo singolo.
Si + 2Cl2 → SiCl4 (p.e. = 57.6 °C)
SiCl4 + 2Mg → Si + MgCl2 o SiCl4 + 2H2 → Si + 4HCl
A recent process: reduction of Na2SiF6 with Na metal.
So obtained silicon can be doped with B, Ga, P or As to produce electronic devices and for
space industry.
Silicon is an importand component of kinds of steel: Fe3Si alloys are used in metallurgy for
production of stainless steel.
Silicon Boule Top
Full boules
Chunk of 99.9999% crystal.
wafers
Anthophyllite asbestos
Mg7Si8O22(OH)2
Tourmaline
NaMg3Al6(BO3)3[Si6O18](OH)3(OH)
Quartz
SiO2
Aquamarine Beryl
Be3Al2Si6O18
Silicon structure
Silicon has a structure similar to that of diamond.
Fullerene
Grafite
Chemical properties
Silicon: Same group of carbon  4 valence electrons
However they differ for many aspects.
Silicon has also 3d orbitals in valence shell.
carbon: maximum coordination number 4
silicon can give higher coordination (e.g. SiF62- ).
 Less tendency to form multiple bonds:
Carbon has the peculiar property to form multiple bonds π-π as in C=C, C≡C, C=O,
C=S and C≡N
Silicon does not give multiple bonds π-π stable because the size of orbitals is too high
and they are much too diffused. Because of its size ,Si shows more tendency to form
chains and clusters of tetrahedra.
 As a consequence: carbon dioxide, CO2, is a gas containing discrete molecules
silicon dioxide (quartz or o silica), SiO2, is a solid containing thetrahedral repeating
units SiO4.
Chemical properties
Different bond energies (driving force = termodinamic) kJ/mole
C-C 368
Si-Si 340
forms many and very long chains
forms chains (up to Si6H14) but in a less extended fashion
C-O 360
Si-O 452
a strong bond involving silicon
C-F 453
Si-F 565
the strongest silicon bond
C-H 435
Si-H 395
weaker than C-H
Silicon compounds: silanes or silyl hydrides
Silyl hydrides are analogues to hydrocarbons but, for many reasons are less stable.
The only member stable undefinitely is silane (SiH4). The increase of complexity results in
instability.
SiH3
H3Si
H3Si
Si
H2
H2Si
Si
H2
SiH3
SiH3
SiH4
H3Si
SiH
Si
H2
SiH3
Silanes can be obtained in different ways, the most common being the reduction of
corresponding halides with LiAlH4.
SinCl2n+2 + LiAlH4 → SinH2n+2
Silicon compounds: Silicones
Compounds containing
Si-O and Si-C bonds.
Silicon oils: used as lubricants, additives, diathermic oils, in cosmetics etc.
O
Si
O
Si
O
Si
O
Si
O
Si
O
Si
O
Si
Si
Elastomers: like the previous ones but with longer chains
And loaded with ultrafine silica
Silicon resins:
Si
Si
O
Si
O
Si
O
Si
O
Si
O
O
O
Si
O
Si
Si
O
O
O
Si
Si
O
O
Si
O
Si
O
Si
O
Si
Si
Organosilicon compounds in synthesis
Silicon is electropositive with respect to carbon, making possible nucleophilic attack tat
silicon
For this reason silicon forms strong bonds with electronegative elements such as oxygen
or fluorine
Moreover, silicon can stabilize a negative charge on the adjacent carbon atom.
This and susceptibility to nucleophilic attack are ascribed to the presence of empty 3d
orbitals.
Finally, it is able to stabilize a positive charge on
the b carbon atom, with respect to silicon
Organosilicon compounds in synthesis: silyl enol ethers
Silyl enol ethers can be generated in a number of ways either under kinetic or
thermodinamic control, wich give rise to different silyl enol ethers in unsimmetrical
ketones.
Brownbridge, P. Synthesis, 1983, 1.
Organosilicon compounds in synthesis: silyl enol ethers
Silyl enol ethers can be used for alkylation or acylation of ketones, for directed aldol
condensation and for Michael addition:
Mukaiyama, T, Angew. Chem. Int. Ed., 1977, 16, 817.
Organosilicon compounds in synthesis: vinyl silanes
Vinyl silanes are useful in synthesis because they react with electrophiles regio- and
stereospecifically, and their epoxides serve as masked carbonyl compounds:
tandem
Chan, TH, Fleming, I. Synthesis, 1979, 761.
Organosilicon compounds in synthesis: vinyl silanes
Vinyl silanes can be used to in a tandem or in silicon directed Nazarov cyclization:
tandem
silicon directed
Chan, TH, Fleming, I. Synthesis, 1979, 761.
Organosilicon compounds in synthesis: epoxysilanes
Epoxysilanes can be easily converted to a variety of compounds; ketones, vinyl
halides, dicarbonyl compounds
Chan, TH, Fleming, I. Synthesis, 1979, 761.
Organosilicon compounds in synthesis: allylsilanes
Allylsilanes also react with electrophiles. Here reaction takes place at the g-carbon
with migration of the double bond and displacement of silicon from a-carbon.
Again, reaction is facilitated by silicon stabilization of a transient carbonium ion in
b position.
Chan, TH, Fleming, I. Synthesis, 1979, 761.
Organosilicon compounds in synthesis: a-silyl carbanions
Because of (p-d) p back-bonding between silicon and carbon, silicon can stabilize
an adjacent carbanion. Usually, the carbanion is flanked by another electron
withdrawing group. An example is the Peterson olefination:
Another interesting reaction is the additon of chlorotrimethylsilylmethyl lithium to
carbonyl compound resulting in epoxysilanes
Burford, C, Cooke, F, Roy, G, Magnus, P. Tetrahedron 1983, 39,867.
Organosilicon reagents: trimethylsilyl cyanide
Trimethylsilyl cyanide is a useful reagent which reacts readily with aldehydes and
ketones to give silylated cyanidrins., which can be further elaborated.
Evans, DA, Carroll, GL, Truesdale, LK. J. Org. Chem. 1974, 39, 914.
Hunig, S, Wehner, G. Chem. Ber. 1979, 112, 2062.
Organosilicon reagents : trimethylsilyl iodide and triflate
Other two useful silicon reagents are trimethylsilyl iodide and trimethylsilyl
trifluoromethanesulfonate (triflate). The iodide is commercially available but it is
expensive and very sensitive to light and moisture and is probably best prepared in situ
from chlorotrimethylsilane and sodium iodode in acetonitrile. It is an excellent reagent for
the cleavage of ethers, (methyl) esters and acetals.
Olah, GA, Narang, SC, Gupta, BG., Malhrotra , R. Synthesis. 1979, 61.
Olah, GA, Husain, A, Singh, BP., Mehrotra , AK. J. Org: Chem. 1983, 48, 3667.
Trimethylsilyl triflate is a powerful silylating agent and for organic compound and acts
either as Brönsted or Lewis acid in many reactions.
Schmidt, RR, Michel, J., Roos, M. Liebigs Ann. Chem. 1984, 1343 – 1357.

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