Chemistry Reference
In-Depth Information
OMe
OMe
Ce(NH
4
)
2
(NO
3
)
6
(OC)
5
Cr
O
n
-Bu
n
-Bu
8.35
8.36
Scheme 8.13
OMe
OMe
(OC)
5
Cr
(OC)
5
Cr
H
N
N
H
8.35
8.37
reductive
elimination
H
OMe
OMe
(OC)
5
Cr
(OC)
5
CrPy
8.38
8.39
Scheme 8.14
8.1.1 Demetallation
The principle method for demetallation is by oxidation, for instance with cerium(IV). The product is the
corresponding carboxylic compound (Scheme 8.13).
Alternatively, demetallation to give enol ethers
8.39
can be achieved by treatment with pyridine
(Scheme 8.14).
12
Under these conditions, as pyridine is a weak base, an equilibrium is established be-
tween the carbene
8.35
and its anion
8.37
. The anion, however, can also reprotonate on chromium to give
a chromium hydride
8.38
. This is followed by reductive elimination. The enol ether
8.39
is obtained as its
Z
-isomer, a consequence of the carbene anion having
E
-geometry to keep the alkyl group away from the
bulky Cr(CO)
5
moiety; the chromium is converted into a pyridine complex.
8.1.2 The Dotz Reaction
In addition to the carboxylate-like reactivity of carbenes, these complexes also display a rich and quite
remarkable metal-based chemistry. This can be divided into two types: thermal and photochemical. The
principle thermal reaction is the Dotz reaction with alkynes (Scheme 8.15).
13
This involves heating an
-
unsaturated carbene
8.40
with an alkyne and results in the formation of a phenol
8.41
. The phenolic carbon
is derived from CO. The
,
-unsaturation may be a part of a benzene ring, in which case a naphthol will
be formed. If the reaction is run in the presence of acetic anhydride and a base, the corresponding acetate is
,
R
S
R
L
OMe
Δ
R
L
(OC)
5
Cr
R
S
MeO
OH
8.40
8.41
Scheme 8.15
