Chemistry Reference
In-Depth Information
2.10
Miscellaneous Methods of Alkoxide Synthesis (Method J)
2.10.1
By Oxidation Reactions (J-1)
There are other methods of synthesis of metal alkoxides. During the synthesis of
uranium tetraethoxide, Gilman and co-workers
379
,
380
observed that it could be oxidized
by an oxidizing agent such as dry air to uranium pentaethoxide. The exact mechanism
is not known but it is assumed that uranium(
IV
) is oxidized to uranium (
V
); the yield
of product was only 65%:
5U(OEt)
4
C O
2
! 4U(OEt)
5
C UO
2
2
.
117
Alternatively when uranium tetraethoxide was treated with bromine in 2:1 molar
ratio followed by the addition of 2 moles of sodium ethoxide, uranium pentaethoxide
was obtained in 90 - 95% yield:
378
,
379
2U(OEt)
4
C Br
2
! 2U(OEt)
4
Br
2
.
118
2U(OEt)
4
Br C 2NaOEt ! 2U(OEt)
5
C 2NaBr
2
.
119
It was also observed that uranium pentaethoxide was suceptible to oxidation by air or
molecular oxygen to give uranium hexaethoxide, but the yield of the product was very
poor. Gilman
et al
.
380
found that better yields were obtained by oxidizing the double
ethoxide of sodium and uranium, NaU(OEt)
6
, in place of uranium pentaethoxide,
U(OEt)
5
, with bromine, lead tetraacetate or benzoyl peroxide; the yields in these reac-
tions were 5 - 10, 20, or 40 - 60%, respectively:
U(OEt)
5
C NaOEt ! NaU(OEt)
6
2
.
120
2NaU(OEt)
6
C Br
2
! 2U(OEt)
6
C 2NaBr
2
.
121
2NaU(OEt)
6
C Pb(OAc)
4
! 2U(OEt)
6
C 2NaOAc C Pb(OAc)
2
2
.
122
2NaU(OEt)
6
C
C
6
H
5
CO
2
O
2
! 2U(OEt)
6
C 2NaO
2
CC
6
H
5
2
.
123
Chromium trimethoxide was obtained by the photooxidation of aryl tricarbonyl
chromium in methanol; the valency of chromium in the product was established by
absorption spectra and magnetic studies.
381
Hagihara and Yamazaki
382
synthesized
chromium tetra-
tert
-butoxide by heating a mixture of
bis
(benzene)chromium and di-
tert
-butylperoxide in the presence of benzene or petroleum ether in a sealed tube at
90
Ž
for about 20 h. The excess of solvent was removed and the green residue was
sublimed
in vacuo
. It was found that this residue was contaminated with a small
amount of biphenyl as an impurity which was removed as a complex by the addition
of 2,4,7-trinitrofluorenone. The blue product thus obtained showed no characteristic
absorption bands due to biphenyl and was characterized as chromium tetra-
tert
-butoxide
Cr
OBu
t
4
.
A much more convenient method for synthesis of Cr
OBu
t
4
could be the dispro-
portionation of [Cr
OBu
t
3
]
n
(prepared by the lithium alkoxide method with strict
exclusion of free Bu
t
OH in the reaction mixture) under reduced pressure.
7
A heteroleptic
tert
-alkoxide of cerium(
IV
),
Bu
t
3
CO
2
Ce
OBu
t
2
, has been reported
383
to be formed according to Eq. (2.124):
Ce(OCBu
t
3
3
C 2Bu
t
OOBu
t
!
Bu
t
3
CO
2
Ce(OBu
t
2
C
2
[Bu
t
3
COOCBu
t
3
]
2
.
124
