Chemistry Reference
In-Depth Information
O
R
L
R
S
MeO
M = W
OMe
8.123
(OC)
5
M
+
R
L
R
S
O
8.122
M = Cr
R
L
R
S
OMe
8.124
Scheme 8.34
8.1.4 Fischer Carbene Photochemistry
An alternative mode of reactivity for these carbenes is photochemical (Scheme 8.35).
31
This chemistry has
opened up an enormous number of possibilities. Chromium carbenes possess a metal to ligand charge-transfer
(MLCT) band in the near-ultraviolet. This excitation results in the promotion of an electron from a metal-
centred orbital to an orbital centred on the carbene ligand. This results in loss of electron density by the
chromium atom in the excited state
8.125
, already electron poor because of the five carbon monoxide ligands.
While one route to relieve this situation is relaxation to the ground state, another is by insertion of carbon
monoxide into the chromium-carbon double bond. The result is (formally) a metallacyclopropanone
8.127
,
which is a resonance structure of a coordinated ketene
8.126
. In fact, the photolysis solution behaves in many
ways like a ketene solution. If photolysis is carried out in the presence of a reagent that can trap a ketene,
then the expected ketene derived product is obtained.
32
As this method of ketene formation is reversible, by
relaxation back to the carbene, typical by-products that arise during classical ketene forming reactions are
not observed.
*
ν
r
elaxation
h
X
X
+
-
δ
δ
(OC)
5
Cr
(OC)
5
Cr
R
R
8.125
CO insertion
R
X
R
R'
X
R
R"
X
R'
N
(OC)
4
Cr
(OC)
4
Cr
•
O
N
O
R"
O
8.127
8.126
8.131
R'OH
R'
R'NH
2
R
X
R'
X
X
OR'
R
NHR'
R
O
O
8.128
8.130
O
8.129
Scheme 8.35
