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the Role of Silicon inSubstitution of Furan
By: Andy Arifin Tjeng
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Outline Introduction
Role of Silicon in Furan substitution
Conclusion
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Carbon-Silicon bond Bond Length: 1.89
Bond strength: ~ 300 kJ/mol
Partly polarized bond
Carbon-Silicon bonds does not occurs
naturally. No natural product containing silicon has
been found.
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-Effects Stabilization of carbocation at -position.
Possibility from donation of C-Si bond to empty p
orbital. Maximum stabilization at 90 angle from cations
Other possibiltiy: hyperconjugation
Si Si+
+
SiR3
R'
+
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-Effects Stabilization of carbanion at -position.
Possible reason: backbonding interaction,
similar to Si-O bond (full p orbitalinteraction with empty d
orbital from Si)
SiR3R'
..
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Arylsilanes Synthesis: Grignard reaction
Cycloaddition reaction
Removal: Fluoride ion
SiMe3
SiMe3
SiMe3
SiMe3
+
CpCo(Co)2
SIR3
MgX SiR3
+ SiR3Cl
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ipso-substitution Ipso-Substitution with Electrophiles
Reason: stabilization from -effects
SiMe3
E+
E SiMe3
E
+
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Substitution of Furan Attack of electrophiles are C-2 or C-5
site
Substitution at C-3 results in regioisomers of C-2
and C-5 position.
Silyl group in substitution of Pyrroles
O
N
Si(iPr)3
N
Si(iPr)3
Br
N
BrNBS Bu4N
+ F-
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ipso-substitution of Furan First reported in 1948 by Benkeser
and Currie
O O SiMe3 O SiMe3HOOC
O BrHOOC
1) BuLi2) SiMe3Cl
1) BuLi2) CO2
Br2
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O
CH2OSiR
3
O
CH2OH
SiR3
O
CH2OH
SiR3
R'
O
CH2OSiR
3R'
O
CH2OHR'
BuLi, HMPA
THF, rt
2.2 eq. BuLielectrophiles
NaH, DMF Bu4N+ F-
3,4-substituted Furan
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3,4-substituted Furan
O
O
OSiR3
O
O
OH
SiR3
O
O
OH
SiR3
R'
O
R'
O
OMe
1) BuLi, HMPA
THF, rt
2) 10% HCl
1) 2.2 eq. BuLi
2) e lectrophile
3) 10% HCl
1) n-Bu4NF, THF
2) CH2N2
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2,4-Substituted Furan
O
CH2OSiEt
3
Si(t-Bu)Me2
O Si(t-Bu)Me2
CH2OH
RO
CH2OSi(t-Bu)Me
2
R
O
CH2OSiEt
3
Si(t-Bu)Me2
R O
CH2OH
R
1) BuLi, HMPA
THF, -40o
2) electrophilesAc
2O-THF-H
2O
(8:8:1)
Bu4N+ F-
CH2N2
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Synthesis of 3,4-Furan
O
N
Ph SiMe3
SiMe3
O
SiMe3
SiMe3
COOMeMeOOC
O
COOMeMeOOC
+
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3,4-substituted Furan
O
SiMe3
SiMe3
O
SiMe3 I
O
SiMe3
R
BO
B
OB
O
O
O
O
R
R
R
O
RR'
O
RI
O
R
R'
I2, AgBF
4
1) BCl3, -78oC
2) 5% HCl
I2, CF
3COOAg
R'CCH, PdCl2,
CuI, Et2NH
Heck, Stille,
Suzuki coupling
R'X,
Pd(PPh3)4
120oC, 14-18 h
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2,3-substituted benzene
O
SiMe3
SiMe3
O
SiMe3
SiMe3
O
SiMe3
SiMe3R
OR
SiMe3
O
SiMe3
IR
OR
R'
BuLi, THF
RX
I2
CF3COOAg
LiAlH4
THF
1) BCl3
2) R'X, Pd(PPh3)4
toluene-MeOH
120oC, 14-18 h
sealed tube, 160o
(CF3CO)
2O
cat CF3COOH,
CCl4, 24 h
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2,3,4-substituted Furan
O
SiMe3
I
O
SiMe3
Bu
SiMe3 Bu
I Bu
Br
BuMgCl
Ni(PPH3)2Cl
2
THF, 80
o
C, 3 h
tBuLi, THF
I2, AgBF
4
THF, -78oC
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2,3,5-substituted furan
Me3Si
IPhCH2
Me3Si
PhCH2 C6H4Me-p C6H4Me-pPhCH2
Arp-C6H4MgBr
Ni(dppe)Cl2
Et2O, 24 h
1) BCl3
2) ArX, Pd(PPh3)4
toluene-MeOH
120o
C, 1-3 h
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Conclusion
The properties of Silyl group: bulkiness, ease ofattachment and
removal, migration tendency andipso-substitution has been used to
control
substitution in furan ring.
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References
Keay, B. A. Chem. Soc. Rev., 1999, 28, 209-215
