Chemistry Reference
In-Depth Information
t
-Bu
t
-Bu
t
-Bu
t
-Bu
t
-Bu
t
-Bu
t
-Bu
t
-Bu
t
-Bu
N
t
-Bu
t
-Bu
N
t
-Bu
t
-Bu
N
t
-Bu
t
-Bu
N
t
-Bu
O
O
O
O
O
O
O
O
M
M
M
M
M(
ISQ
L
15
BQ
)
e
−
+e
−
e
−
+e
−
−
e
−
+e
−
−
−
M(
AP
L
15
BQ
)
M(
AP
L
15
AP
)
M(
AP
L
15
ISQ
)
Figure8.18
OxidativebehaviorofH
3
AP
L
15
AP
inthepresenceofmetal.
137 cm
−
1
. It could be reduced into [Cu
II
AP
L
15
AP
]
−
the radical spin has been estimated to be
−
(
)
(NEt
3
)
at
0.14 V into [Cu
II
AP
L
15
BQ
]
+
.
−
1.06 or oxidized at
−
(
)
(NEt
3
)
Both [Cu
II
2
(L
15
S
)
••
2
] and [Cu
II
AP
L
15
ISQ
)
•
(NEt
3
)
], and [Cu
II
(L
15
Se
)
•
(NEt
3
)
(
] (the selenium analogue
of
the
latter
complex),
are
efficient
catalysts
for
the
aerobic
oxidation
of
alcohols.
For
instance
[Cu
II
AP
L
15
ISQ
)
•
(NEt
3
)
(
] oxidizes aerobically a 0.125 M ethanol solution in an overall yield of 55%
10
−
5
M in catalyst) with release of H
2
O
2
. The mechanism is very similar to that performed by GO,
the reduced catalyst being a copper(I) - phenol species. Interestingly, this is not its sole catalytic activity
of the complex of H
2
Ph
L
15
Se
, as it also reacts with benzylamine to form benzylidinebenzylamine.
132
(2
.
65
×
The
reaction is believed to be initiated by aerobic oxidation of the dimer [Cu
II
2
(L
15
Se
)
2
(NH
2
CH
2
Ph)
2
] into
a radical complex. A simplified mechanism is depicted below (Equations 8.19 - 8.21), but note that for
clarity only one half of the dimeric complex is shown. The reaction product PhCHO reacts with PhCH
2
NH
2
to form benzylidinebenzylamine, whereas the reduced form of the catalyst (copper(I) complex) is
oxidized by air into the radical dimer.
[Cu
II
L
15
Se
[Cu
II
L
15
Se
)
•
(
(
)(
NH
2
CH
2
Ph
)
]
→
(
NH
2
CH
2
Ph
)
2
] in the presence of O
2
and NH
2
CH
2
Ph
(8.19)
[Cu
II
L
15
Se
)
•
(
[Cu
II
HL
15
Se
NH
2
CH
•
Ph
(
)
→
(
)(
)(
)
(
(8.20)
NH
2
CH
2
Ph
2
]
NH
2
CH
2
Ph
]
H abstraction as GO
[Cu
II
HL
15
Se
NH
2
CH
•
Ph
[Cu
I
HL
15
Se
(
)(
)(
NH
2
CH
2
Ph
)
]
→
(
)(
NH
=
CHPh
)(
NH
2
CH
2
Ph
)
]
[Cu
I
HL
15
Se
+
H
2
O
→
(
)(
NH
3
)(
NH
2
CH
2
Ph
)
]
+
PhCHO
(8.21)
The copper complex of the tetradentate ligand H
4
Ph
L
16
N
could be obtained in five oxidation states
)
•
]
+
are diamagnetic species due to antiferro-
magnetic coupling of the copper unpaired electron in a d
x2
−
y2
orbital and the radical, whereas the other
compounds are paramagnetic with (S
t
)
•
]
−
and [Cu
II
(Figure 8.19).
133
[Cu
II
ISQ
L
16
N
BQ
L
16
N,ISQ
(
(
.[Cu
II
ISQ
L
16
N
)
•
]
−
=
S
Cu
=
/
)
(
can be oxidized by oxygen to give
1
2
[Cu
II
BQ
L
16
N,ISQ
)
•
]
+
and one equivalent of H
2
O
2
, whereas [Cu
II
BQ
L
16
N,ISQ
)
•
]
+
(
(
reacts in the presence of
triethylamine with alcohols to give the corresponding aldehyde and [Cu
II
)
•
]
−
. Oxidation of alco-
hols by these complexes is thus catalytic in the presence of oxygen (and base), with 5000 turnovers achieved
in 50 hours for the oxidation of ethanol (turnover frequency of 0.03 s
−
1
ISQ
L
16
N
(
. Interestingly, the first half-
reaction involves substrate binding and further rate limiting hydrogen abstraction (Equations 8.22 - 8.23),
as observed in GO. Nevertheless, in contrast with GO the reduced form of the catalyst is a copper(II)
coordinated radical and not a copper(I) complex.
[Cu
II
)
BQ
L
15
N,ISQ
)
•
]
+
+
[Cu
II
BQ
L
15
N,ISQ
)
•
(
RCH
2
O
−
)
H
+
(
RCH
2
OH
(
]
+
ISQ
L
15
N
)
•
]
−
+
2H
+
[Cu
II
→
(
RCH
2
O
+
(8.22)
[Cu
II
ISQ
L
15
N
)
•
]
−
+
2H
+
+
[Cu
II
BQ
L
15
N,ISQ
)
•
]
+
+
(
O
2
→
(
H
2
O
2
(8.23)
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