Chemistry Reference
In-Depth Information
4.12
Reactions with Unsaturated Substrates
4.12.1
Insertion Reactions into M—OR
4.12.1.1 With OCO, R
0
NCO , or R
0
NCNR
0
An interesting feature of metal alkoxides is the tendency to react stoichiometrically with
some isoelectronic unsaturated molecules like carbon dioxide, alkyl or aryl isocyanates/
isothiocyanates, and carbodiimides (X
D
C
D
Y) to form metal derivatives as shown in
Eq. (2.283):
Y
M(OR)
x
−
y
(X
COR)
x
M(OR)
x
+
y
XCY
(2.283)
Y
COR
M(OR)
x
−
y
X
x
(X,Y
=
O or NR)
The bidentate ligating mode of the resulting ligand appears to provide significant
driving force for these types of reactions, some of which have been shown to be
reversible.
Alkoxides of many metals react readily with CO
2
to give carbamates and alkylcar-
bonates(bicarbonates). The reversible absorption of approximately one mole of carbon
dioxide by solutions (benzene) of some early transition metal alkoxides
931
has been
reported to proceed according to Eq. (2.284):
O
(2.284)
M(OR)
x
+
CO
2
ROCOM(OR)
x
−1
M D Ti, Zr; R D Et, Bu
n
;
x
D 4. M D Nb; R D Et;
x
D 5. M D Fe; R D Et;
x
D 3.
The final products could not be isolated in a pure state and their formulation was
inferred from infrared spectroscopic data, the observed stoichiometry of the reaction,
and their reaction with ethyl iodide which gave diethylcarbonate.
931
Molybdenum alkoxides, Mo
2
OR
6
, react in hydrocarbon solvents reversibly with
two moles of CO
2
to yield the corresponding bis (alkyl carbonates) (Eq. 2.285).
932
,
933
R
O
C
O
OR
O
O
C
Mo
2
(OR)
6
+
CO
2
(2.285)
∆
(90
°
C)
Mo
O
Mo
RO
O
RO
O
R
R
The mechanism proposed for this reaction in solution involves catalysis by adventitious
free alcohol and the appropriate monoalkyl carbonic ester as the effective reagent, as
