Chemistry Reference
In-Depth Information
2.10.5
By Insertion of Oxygen into Metal-Carbon Bonds (J-5)
The reactivity of molecular oxygen towards metal alkyls is very diverse; for example,
some metal alkyls react violently with O
2
and others are inert. There are only a few
well-documented examples of reactions which lead to the formation of alkoxides.
Since only one oxygen atom is required per metal - alkyl bond, the reactions require a
multistep process.
The high reactivity of certain metal alkyls is well known. The autoxidation of
dialkylzinc was investigated as early as 1864 by several workers and they showed
that dialkylzinc is oxidized rapidly to zinc dialkoxides
415 - 416
R
2
Zn C O
2
! Zn
OR
2
2
.
153
where R D Et, Pr
i
,C
5
H
11
.
Abraham
417
studied the oxidation of dialkylzinc and observed that rapid oxida-
tion leads to the formation of bis(alkylperoxy)zinc, Zn
OOR
2
. However, when the
oxidation was carried out slowly for several days under controlled conditions, zinc
dialkoxide was the final product, which on hydrolysis gave alcohol. The reduction of
bis(alkylperoxy)zinc with excess of dialkylzinc also afforded zinc dialkoxides:
Zn
OOR
2
C R
2
Zn ! 2Zn
OR
2
2
.
154
Similarly the oxidation of trialkylaluminium and trialkylboron with molecular oxygen
under controlled conditions also afforded trialkoxides.
418 - 420
2MR
3
C 3O
2
! 2M
OR
3
2
.
155
where M D B, Al.
When solutions of (Bu
t
3
CO
2
MMe
2
(M D Ti, Zr, Hf) were exposed to either an
excess (1 atm) or one equivalent of dry oxygen, the mixed alkoxide derivatives
Bu
t
3
CO
2
M
OMe
2
yields.
421
were
isolated
in
>
87%
The
addition
of
O
2
to
Bu
t
3
CO
TiMe
3
afforded three different crystalline derivatives
421
as represented by
the formula TiMe
3
x
OMe
x
OCBu
t
3
where
x
D 1, 2, 3.
Bu
t
3
CO
2
MMe
2
C O
2
!
Bu
t
3
CO
2
M
OMe
2
2
.
156
where M D Ti, Zr, Hf.
2.10.6 By Insertion of Aldehydes or Ketones Across a Metal-Carbon or
Metal-Hydrogen Bond (J-6)
Coates and Fishwick
385
as early as 1968 observed that dimethylberyllium adds to
acetaldehyde or acetone to form MeBe(OCHMe
2
) or MeBe(OBu
t
), respectively.
In 1986, Lappert and co-workers
422
carried out the reactions of Li
CH
2
PR
2
(tmeda)
(R D Me or Ph; tmeda D
N
,
N
,
N
0
,
N
0
-tetramethylethylenediamine) with Bu
t
2
CO in 1:1
(Eq. 2.157) and 1:2 (Eq. 2.158) molar ratios to yield highly hindered phosphino-
alkoxides of lithium:
n
-hexane
1
2
t
t
Li(CH
2
PMe
2
)(tmeda) + Bu CO
[Li(OCBu CH
2
PMe
2
]
2
+
tmeda
2
.
157
