FURAN O The least aromatic 5-membered ring S O H H H H 7.1 ppm 127 ppm 7.2 ppm 126 ppm 6.3 ppm 110 ppm 7.4 ppm 144 ppm S O 1.71Å 1.37Å 1.42Å 1.37Å 1.35Å 1.44Å Reaction with electrophiles - Protonation O O H H Ring opening O H Much less basic than ordinary ethers O H H Major protonated form Conc. H 2 SO 4 Lewis acids (i.e. AlCl 3 ) Decomp. O H H H 2 O O OH Hemiacetal O O
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FURAN
O The least aromatic 5-membered ring
S O
H
H
H
H
7.1 ppm127 ppm
7.2 ppm126 ppm
6.3 ppm110 ppm
7.4 ppm144 ppmS O
1.71Å
1.37Å
1.42Å
1.37Å
1.35Å
1.44Å
Reaction with electrophiles - Protonation
O O
HH
Ring opening
OH
Much less basic than ordinary ethers
OHH
Major protonated form
Conc. H2SO4
Lewis acids (i.e. AlCl3)Decomp.
O
HH H2O
O OHHemiacetal
OO
Reaction with electrophiles - Nitration Cannot use conc. HNO3 / H2SO4
S
AcO NO2
conc. HNO3, Ac2O
X
X=S - ThiopheneX=O - Furan
XNO2H
AcO NO2X=S
X=O
ONO2H
AcOH
PyridineO NO2
Halogenation
OBr2
Dioxane, oC XBrH
Br O
BrH
BrH
Seen on low temp NMR
- HBr O BrBr2
O BrBrcan be isolated
OMeO2CBr2
OMeO2C
Br
Br O
Br
Br
HO OH Br2
HBr
Br
Br OH
HO
Br
H OH
HO
see 15.14.1.2O
Br
O
BrBr1. t-Buli2. H2O
1. n-BuLi2. DMF
O
Br
CHO
Reaction with electrophiles - Acylation
ODMF, POCl3
OO"Vilsmeyer"
FurfuralAlso very readily available by other routesO
R'COCl or(R'CO)2OBF3 . Et2O
R
R: H, MeO
R
OR'
Alkylation Generally not practical (polyalkylation, polymerisation)
Condensation with Aldehydes and Ketones
ORCHO, H+
O OHR
Further react.
O OHCCl3
stableFrom chloral
C.f.
NH
H+, RCHONHR=H, alkyl H
OHR
H±H+
NH R H
OH2- H2O N
H RH Polymer
Reaction with electrophiles - Condensation with imines / iminium ionsCH2O, R2NH, H+
NR
RO
Unsubst furan: iminium ion must be preformed
OH
NR
RH-
ON RR
X
B(OH)2
X=OX=S
Ph2CHNH2HO2CCHO
NHR
O2CX
X=OX=S
CO2HPh2HCHN
ipso subst.Reaction with oxidating agents
O R'R
(Ox)R'R O O
Div. ox. agents p 300E-isomer in most cases
O
Br2, MeOHOH H
OMeMeO
Pb(OAc)4
O OAcAcO
Reaction with nucleophilesSome ex. on furans activated with -NO2 group
Metallation and further react.
On-BuLi
Et2O, Δ O Li
On-BuLi
THF, -78oC
Br
O
Li
> -40 oC
n-BuLi TMEDAhexane, Δ O LiLi
O
Li
OLi
O
Br LDA
O
Br
Li
(ODG)n-BuLi
O CO2HLDA
O CO2LiLiO CO2Li
Li
O Li
B(OR)3 HydrolysisO B(OH)3
BR3
O BR
RR
Li
I-I ORBR2
HI
IO R
Pd-cat couplings
O
Li
O
Met Ar-X (or related)cat Pd
O
Ar
O
Br
O
Br
Br
Ar-Metcat Pd
O
Br
Ar
NR
Pd(OAc)2N
H PdOAc
R
OAc
NR
PdOAcEWG
NR
EWG
Heck
+ Pd(0)
Cu(II)
c.f.
O
EWGcat. Pd
O EWG
Ocat. Pd
Ar-X
O Ar
like an alkene in Heck
Heteroaryl-Heck
O ArAr-Pd-X
O O
H
ArHPdX
H-Pd-X
Cycloadditions
X+
O OO
XOO
OHH
X=O: r.t.; 80%X=S: 100 oC, 15kbar; 42%
Furanes as diene - one of the first DA examplesFuran reacts with many dienophiles (alkenes, alkynes, allenes)