Chemistry Reference
In-Depth Information
R
R
•
[M]
R
H
H
H
8.140
8.141
8.142
Scheme 8.39
D
D
Ph
3
P(
p
-cymene)RuCl
2
NH
4
PF
6
,
Δ
O
O
8.143
8.144
D
D
•
[Ru]
[Ru]
O
O
8
1
8
Scheme 8.40
(OC)
5
W.THF
(W(CO)
5
•
I
I
I
8.149
8.147
8.148
Scheme 8.41
8.2 Vinylidene Complexes
Formally, vinylidene complexes
8.142
can be considered as complexes of vinylidene carbenes with transition-
metal fragments. Free, uncomplexed vinylidene carbenes
8.140
are postulated in organic chemistry as inter-
mediates on the way to terminal and some internal alkynes (the Fritsch-Buttenberg-Wiechell rearrangement)
(Scheme 8.39). They can, in some cases, be trapped by nucleophiles. It can be postulated that the equilibrium
between the alkyne and the vinylidene carbene, usually entirely in favour of the alkyne, would be less one
sided if complexation to a transition metal were involved.
Once generated, one way to trap a vinylidene complex could be in an electrocyclic reaction. Indeed,
treatment of an alkyne
8.143
, having a dienyl unit attached, with a ruthenium catalyst yields a tricyclic
product
8.144
(Scheme 8.40).
42
Deuterium-labeling experiments are consistent with the alkyne-vinylidene
isomerization. A tungsten catalyst, W(CO)
5
.THF, may also be used.
43
Hydrogen is not the only atom that can participate in this migration. Iodide can also migrate during the
generation of an intermediate vinylidene complex
8.148
(Scheme 8.41).
44
Vinylidene complexes
8.150
, generated
in situ
by the reverse Fritsch-Buttenberg-Wiechell rearrangement,
may also be trapped with heteroatom nucleophiles. Alkynols may be converted to oxygen heterocycles
8.153
,
or carbene complexes
8.152
, according to the conditions used (Scheme 8.42).
45
The organometallic reagent
