Chemistry Reference
In-Depth Information
O
CH
3
O
Fe(CO)
3
CH
3
COCl
AlCl
3
(OC)
3
Fe
(OC)
3
Fe
CH
3
migratio
n
acylation
on iron
H
10.35
10.34
10.36
Fe(CO)
3
Fe(CO)
3
O
- H
+
O
CH
3
CH
3
10.38
exo
10.37
endo
isomerisation
Scheme 10.11
(OC)
2
LF
e
F
e
(
CO)
2
L
CH
3
COCl
AlCl
3
(OC)
2
LF
e
COCH
3
COCH
3
- H
a
H
a
H
b
1
0
.
3
1
.
4
0
10.1
L = CO
10.41
L = PPh
3
Scheme 10.12
group
endo
or
exo
. Under the right work-up conditions, and with enough time, the more-stable
exo
-product
10.38
becomes the exclusive product. Stereochemical and other studies have shown that acylation occurs
on iron, as this is where the HOMO is located. The acetyl group subsequently migrates to carbon and the
products are obtained via loss of a proton from an unstable
3
-intermediate
10.36
.
4
-cyclohexadienyl complex
10.1
(Scheme 10.12). As acyla-
tion is initially on iron, the iron and the acetyl group are
cis
in the
This has an interesting consequence with the
3
-intermediate
10.39
. The proton that is
lost must be
cis
to iron (perhaps via transfer to iron in a reverse of the acylation sequence). The more acidic
proton, H
b
,
- to the newly installed acetyl group is
trans
to iron and, therefore, not available. The proton
lost, therefore, is H
a
on the other side resulting in net movement of the diene system to give diene com-
plex
10.40
. Better yields are obtained with the more electron rich monotriphenylphosphine complex
10.41
(L
CO).
15
In the cycloheptatriene complex
10.43
, one of the three double bonds is uncomplexed. This double
bond is more reactive towards electrophiles (Scheme 10.13). Thus, acylation of the complex gives the
Friedel-Crafts product as an
=
PPh
3
), than the tricarbonyl complex (L
=
5
-dienyl complex
10.44
that can be converted to the
4
-diene complex
10.45
by
addition-elimination of methoxide.
16
Intramolecular molecular acylation of iron complexes is also possible
(Scheme 10.14).
17
Another example of electrophilic chemistry is provided by the myrcene complex
10.48
(Scheme 10.15).
The iron tricarbonyl complex
10.48
undergoes cyclization via a carbocation
10.49
on acid treatment.
18
The
16e
-allyl complex
10.50
produced may then lose a proton (in a way similar to benzene in electrophilic
cyclization) to give a
4
-diene complex
10.51
or gain an additional ligand, CO if supplied, to give a stable
18e
-allyl complex
10.52
.
