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1
Chapter 13 Silicon reagents
General features
Two other highly important properties of silicon
Reactions of organosilanes 1,2 rearrangements (Brook rearrangement)
Silicon as a protecting group for OH
Peterson olefination(Si-stabilised carbanions)
Silylenolethers
Alkylsilanes
Vinyl- or Alkenylsilanes
Allyl Silanes
Aryl Silanes
2
General Features
•Silicon is directly below carbon in the periodic table, and shows
some similarity in bonding. It forms 4 bonds in neutral molecules
and is tetrahedral.
• Silicon does not form very stable multiple bonds, as the large 3p
orbital on silicon does not overlap well with the 2p orbital on carbon,
oxygen or nitrogen.
• Carbon is more electronegative than silicon
•Silicon is a very versatile element, and you will find silicon reagents
in 2 major roles;
•As protecting groups for OH
•In reactions for C-C and C=C bond formation
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4
Two other highly important properties of silicon
Si-stabilised carbanions are important in the Peterson olefination reaction (see later)
Donation of the negative charge into a * antigonding orbityal of an adjacent Si-C bond stabilises the anion.
This donation is rendered more effective due to silicon’s lower electronegativity compared to carbon. Therefore the * antibonding orbital has a greater orbital coeffient on silicon. What this really means is that the antibonding orbital is a bit larger on the silicon side and leads to better overlap with the orbital containing the negative charge.
Stablisation of α-carbanions
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Two other highly important properties of silicon
Stablisation of β-cations (-effect of silicon )
C C
Si
+
E.g allyl and vinyl silanes react with electrophiles via the following mechanisms
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ipso-Substitution: a consequence of -effect of silicon
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Reaction of organosilanes (Brook rearrangemen
t)
When silylcarbinols are treated with base or active metals the silyl gro
up is known to migrate from the C to a neighbouring O atom e.g. Bro
ok rearrangement, generating silyl ethers
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• Various protecting groups are available and the resulting silyl ethers
can be cleaved under a variety of conditions.
• Silyl protecting groups are typically put on under basic conditions
Reaction of organosilanes (Silicon as a protecting grou
p for OH)
R OHTBSClBase R OTBS
Base = Et3N, imidazole
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The stability of the silyl ether towards cleavage depends on a numbe
r of factors:
Increasing steric bulk raises stability.
EWGs on silicon increase acid stability and decrease base stability and v
ice versa.
The ease of cleavage with F- parallels the ease of basic hydrolysis.
Reaction of organosilanes (Silicon as a protecting
group for OH)
With bulkier groups, such as TBDMS, it is possible to distinguish between primary
and secondary alcohols.
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Reaction of organosilanes (Silicon as a protecting group for O
H)Deprotection:
Selective deprotection
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Reaction of organosilanes (Peterson olefination)
Generation of Si-stabilised carbanions
Deprotonation with a strong base
Metal-halogen exchange
Addition of organometallics to vinyl silanes
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Reaction of organosilanes (Peterson olefination)
• Key compounds are β-hydroxysilanes generated from the reactio
n of Si-stabilised carbanions with carbonyl compounds
13
Reaction of organosilanes (Peterson olefination)
The next step is the elimination of OH and SiMe3, which can
generally be done in two ways. Each method is stereospecifi
c.
Acidic hydrolysis proceeds via
an anti-elimination
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Reaction of organosilanes (Peterson olefination)
Basic conditions - syn elimination
Therefore, in principle the Peterson olefination can be used to make single
geometric isomers of alkenes. However, this is complicated by the difficult
y in preparing pure diastereomers of the β-hydroxysilanes.
15
Example:
The Peterson olefination is often less sterially demanding than the Wittig reaction
W. Adam, C. M. Ortega-Schulte, Synlett, 2003, 414-416.
Recent Literature:
16
M. Iguchi, K. Tomioka, Org. Lett., 2002, 4, 4329-4331.
A. Barbero, Y. Blanco, C. Garcia, Synthesis, 2000, 1223-1228.
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Reaction of organosilanes (Silyl Enol Ethers )
• Some reactions of silyl enol ethers
• Preparation of silyl enol ethers
OSiMe3 OBr
OLi+
MeLi
Me4Si
OSiMe3 O O
I
Me3
N+CH2
Ph
PhCH2N+Me3F
-
FSiMe3
Activated enolate
O
OSiMe3
OSiMe3
1. Me2CuLi2. Me3SiCl
Et3SiHCat. (Ph3P)3RhCl
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In the presence of a lewis acid
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Genaration:
Silyl Enol Ethers: 2-Trimethylsilyloxybuta-1,3-dienes
O O
SiMe3
Me3SiCl
NEt3
O O
SiMe3
Me3SiCl
NEt3
OMe OMe
ZnCl2
Danishefsky's diene
O
SiMe3
OMe
+ O
O
O
O
O
OH
HMe3SiO
OMe
Via endo TS
H3O+
O
O
OH
HO
Reaction:
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Alkylsilanes
• Almost all acyclic Cl, Br, and I silanes react with all nucleophi
les by an SN2 type mechanism leading to an inversion of confi
guration at the Si atom.
• In many cases the pentacovalent Si is thought to be an interme
diate rather than a transition state.
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Vinyl silanes
• Preparation using a variety of methods
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Vinyl silanes
• The reactions of vinyl silanes: electrophilic substitution of the silyl gr
oup. This is highly regioselective and the substitution occurs with reten
tion of alkene geometry.
23
Vinyl silanes
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Allyl silanes
Preparation:
The reaction of allyl organometallic reagents with silylating agents.
The Wittig Reaction
25
The reactions of allyl silanes
Allyl silanes are more reactive than vinyl silanes and much more reactive th
an simple alkenes
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The reactions of allyl silanes
Allyl silanes react with electrophiles with high regioselectivity. The electrophile
attacks at the other end of the allylic system, and no bond rotation is required.
27
Allyl silanes react with a wide range of electrophiles in the presence of
Lewis acids such as TiCl4
28
29
Aryl silanes
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Exercise 4
• Drawing the structures of the intermediate and the final products.
• Outline at least two kinds of enzymes or whole-cell systems used in the redu
ction of ketones to secondary alcohols.
1.O
H
+ TMSClEt3N/C6H6 CO2Et
2.+
O
HPh
OTMS i) TBAFii) H2O