Chemistry Reference
In-Depth Information
R
2
CHO
OCHR
2
R
2
CHO
OCHR
2
Al
Al
O
O
O
O
•
C
C
R
′
C
C
R
′
R
R
H
H
R
R
R
′′
R
′′
As a consequence of the coordination of the carbonyl oxygen with aluminium
isopropoxide
R D Me
, a partial positive charge will be developed on the carbon
atom; this could be the rate-determining step in this reaction mechanism. This view
was supported experimentally by McGowan
998
who varied the electron density on the
carbonyl carbon atom by substituting an electronegative group X in the
para
-substituted
acetophenones XC
6
H
4
COCH
3
. However, this did not constitute a conclusive proof of
the mechanism. Furthermore, the postulated mechanism has the expected stereochem-
ical consequences, and this aspect has been demonstrated by Jackman
et al
.
995 - 997
and
Doering
et al
.
1001 - 1002
However, kinetic measurements
996
,
1005
,
1005a
are complicated by
the oligomeric nature of the aluminium alkoxides.
It is noteworthy that in some cases reduction with metal alkoxides, including alu-
minium isopropoxide, involves free-radical intermediates (single electron transfer (SET)
mechanism).
1006
The potentialities of the Meerwein - Ponndorf - Verley reaction as an alternative
method
for
preparation
of
metal
alkoxides
have
also
been
studied
(Eqs 2.341 -
2.345):
146
,
1007
,
1008
Ti
OPr
i
4
C 4Cl
3
CCHO ! Ti
OCH
2
CCl
3
4
C 4Me
2
C
D
O
146
2
.
341
Zr
OPr
i
4
C 4Cl
3
CCHO ! Zr
OCH
2
CCl
3
4
.
2Me
2
C
D
O C 2Me
2
C
D
O
146
2
.
342
2-Hydroxybenzaldehyde (salicylaldehyde) reacts with isopropoxides of tita-
nium
1007
,
1008
and zirconium
1008
with displacement of isopropoxo group(s) as isopropyl
alcohol to give mixed derivatives (Eqs 2.343 - 2.345), which on heating undergo reduc-
tion of the aldehyde function with elimination of acetone.
benzene
!
room temp
Ti
OR
4
C
x
HOC
6
H
4
CHO
RO
4
x
Ti
OC
6
H
4
CHO
x
C
x
ROH
2
.
343
where R D Et, Pr
i
,Bu
t
;
1007
x
D 1
,
2.
benzene
!
reflux
RO
42
x
Ti(OC
6
H
4
CH
2
O
x
C
x
R
0
CHO
RO
4
x
Ti
OC
6
H
4
CHO
x
2
.
344
where R D Et, Pr
n
;
1008
R
0
D Me, Et;
x
D 1
,
2.
The reaction of Ti
OPr
i
4
with salicylaldehyde in 1:2 molar ratio initially liberated
only one mole of acetone and an insoluble product (Eq. 2.345):
Ti
OPr
i
4
C 2HOC
6
H
4
CHO
toluene
!
reflux
Ti
OC
6
H
4
CH
2
O
OC
6
H
4
CHO
OPr
i
C
CH
3
2
C
D
O
2
.
345
