Chemistry Reference
In-Depth Information
4.3.2
Phenol-Alcohol Exchange Reactions
The characteristic ability of metal alkoxides to undergo facile exchange reactions with
hydroxylic reagents has been extensively utilized for preparing homo- and heteroleptic
phenoxides of a large number of elements
652
by the general reaction (Eq. 2.198):
M
OR
x
C
y
PhOH ! M
OR
x
y
OPh
y
C
y
ROH "
2
.
198
M D Y
,
155
Pr
,
Nd
,
153
Sm
,
155
Gd
,
and Er;
157
y
D 1
,
2
,
3; R D Pr
i
;
x
D 3
.
M D U;
653
y
D 1
,
2
,
3
,
4; R D Et;
x
D 5
.
M D Ti
,
654
Zr;
655
y
D 4; R D Bu
t
,
x
D 4
.
M D Ti;
276
y
D 1
,
2
,
3
,
4; R D Et
,
Pr
i
;
x
D 4
.
M D VO;
656
y
D 3; R D Et;
x
D 3
.
M D Al;
657
y
D 1
,
2
,
3; R D Bu
t
;
x
D 3
.
M D Si
.
658
,
659
Ge;
660
y
D 1
,
2
,
3
,
4; R D Et;
x
D 4
This method tends to work well with simple phenol or phenols substituted with
electron-withdrawing groups (F, Cl, NO
2
), owing to the increased acidity of the phenols
and their lower volatility relative to alcohols.
In some cases, a control of stoichiometry allows the isolation of mixed alkoxide-
phenoxide derivatives.
153
,
155
,
157
,
276
,
652
In the case of uranium(
V
) ethoxide, only partial
substitution can be achieved even by reacting with an excess of phenol.
653
Reaction of Mo
2
OPr
i
6
with excess 2,6-dimethylphenol yields only partially substi-
tuted product
661
Mo
2
(OPr
i
)
2
(OC
6
H
3
Me
2
-2,6)
4
for steric reasons.
The important role of steric congestion around the central atom in determining the
composition of the final product has also been demonstrated
662 - 664
during the reactions
of other metal alkoxides [Ti
OPr
i
4
,Zr
OPr
i
4
.
Pr
i
OH, M
OPr
i
5
with M D Nb or Ta,
Al
OPr
i
3
,Sb(OPr
i
)
3
) with sterically hindered phenols (HOC
6
H
3
Me
2
-2,6; HOC
6
H
3
Pr
i
2
-
2,6; HOC
6
H
3
Bu
t
2
-2,6].
It is noteworthy that calix[
n
]arenes, which are a family of macrocyclic reagents
with
n
D 4
,
5
,
6
,
7, or 8 phenolic groups in the ring (2-XII) can also find the metal
ions
via
the deprotonated phenolate ligand.
665
,
666
For example, treating two equiva-
lents of Ti(OR)
4
(R D Pr
i
,Bu
t
) or Zr(OPr
i
)
4
with
p-tert
-butylcalix[8]arene (LH
8
when
n
D 8) in the presence of a base such as alkali metal (M
0
D Li, Na, K) alkoxides or
amines
R
0
NH
2
has led to the synthesis
665
of derivatives of the type [M
0
or R
0
NH
3
]
C
[(MOR)
2
(
p-tert
-butylcalix[8]arene)]
(M D Ti, Zr). Chisholm
et al
.
667
have shown
that the reaction of Mo
2
OBu
t
6
, with monomethylated calix[4]arene ligand
H
3
L
0
in benzene at room temperature affords an interesting derivative Mo
2
[
4
-L
0
2
.
Bu
t
(
n
=
4, 5, 6, 7, 8)
CH
2
OH
n
(2-XII)
4.3.3 Silanol-Alcohol Exchange Reactions
Like alcohols, silanols have been found to react with metal alkoxides to form the
corresponding homo- and heteroleptic metal siloxides (Eq. 2.199).
M
OR
x
C
y
R
0
3
SiOH
M
OR
x
y
OSiR
0
3
y
C
y
ROH "
2
.
199
