Chemistry Reference
In-Depth Information
5 mol% CuBr
2
, 5 mol% bpy
R
OH
R
O
(a)
5 mol% TEMPO, 5 mol%
t
-BuOK
CH
3
CN/H
2
O (2:1)
Air, 25
o
C
d
n
4
r
4
n
g
|
0
N
N
Cu
II
N
(b)
O
O
H
H
R
N
N
N
N
N
N
Cu
II
Cu
I
N
Cu
II
N
N
O
O
O
O
O
HO
R
R
H
(II)
(III)
H
H
H
R
R'
RCH
2
OH
N
O
2
N
N
N
Cu
I
Cu
II
N
N
O
O
(I)
Figure 2.5
(a) Copper-TEMPO catalyst system for the aerobic oxidation of primary
alcohols in the presence of bpy reported by Sheldon and co-workers;
(b) proposed mechanism for the (bpy)Cu-TEMPO catalyzed chemoselec-
tive oxidation of primary alcohols. Inset: possible intermediate respon-
sible for the lack of reactivity of secondary alcohols.
.
In the proposed mechanism, a ternary TEMPO-copper(
II
)-alcoholate inter-
mediate (II) is formed upon binding of alcoholate to the metal center of a
copper(
II
)-Z
2
-TEMPO species (I) (Figure 2.5b). X-ray structures of the com-
plexes of copper(
II
) halides with Z
2
-coordinated TEMPO were reported by Rey
and co-workers.
35,36
The rate of the reaction was reported to accelerate in the
presence of a base, indicating coordination of the alcoholate to the copper
center. Coordinated TEMPO in intermediate II abstracts the hydrogen atom to
form a copper(
I
)complex(III), aldehyde and TEMPO-H. For secondary alco-
hols, the coordinated TEMPO is unable to abstract the hydrogen atoms from
the alcoholate owing to steric hindrance associated with the methyl groups of
TEMPO and the secondary alcohol (Figure 2.5b, inset). TEMPO-H is oxidized to
TEMPO by O
2
and the copper(
I
)isoxidizedtoregenerateI in the catalytic cycle.
Further investigations revealed that a higher catalytic turnover number
(TON) could be achieved in the presence of electron-donating substituents at
para positions of bpy.
37
It was subsequently observed that the catalytic be-
havior of the Sheldon and co-workers' catalyst is dependent on the pH of the
reaction solution. Maximum catalytic activity was found with a smaller
amount of elemental copper (0.5 mol%) and 5 mol% TEMPO in the presence
of bpy (2.5 mo%) at pH 13-13.5, whereas the activity was lost at pH 8.
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