116
4.1.4 Coupling of C sp2 -H with C sp2 -H Bonds
Although the oxidative coupling of phenols to afford biphenols has been known for
several decades, the cross-coupling of two different phenols is not well studied and
very challenging due to the formation of many side products, such as dimerization
of the same phenols.
+
up to 88%
10 mol% FeCl 2
. 4H 2 O
3 equiv tBuOOtBu
100
o
C
R
3
R 1
H
O
O
R
4
Ar
H
O
O
R
4
Ar
R
3
R 1
O
O
EtO
Ph
88%
7 7%
82%
20 equiv.
R 2
R 2
O
O
EtO
Ph
O
O
EtO
Ph
Fe(II)Cl 2
t BuOOtBu
One-electron
oxidation
tBuO-Fe(III)Cl 2 +
+
R 3
R
1
H
O
O
R
4
Ar
H
R
2
tBuOH
Hydrogen
abstraction
+
R
3
R
1
O
O
R 4
Ar
R
2
Fe(III)
O
O
R
4
Ar
R 3
R 1
R
2
Addition/Single
electron
transfer
tBuO
Scheme 4.29 Fe-catalyzed alkylation of activated methylenes
10 mol% CuBr
1.2 equiv TBHP
60 °C, 12 h, air
O
X
H +
R
O
X
O
R
R = 4-Me, 4-OMe,
4-Br, etc.
X = CH 2
or O
Scheme 4.30 Oxidative coupling between vinylarenes and cyclic ethers
W. Liu