Chemistry Reference
In-Depth Information
X = CH
2
, SO
2
, NAc, NCO
2
Me
X
X
H
H
h
ν
(OC)
3
Cr
11.175
11.174
Scheme 11.58
O
O
SiMe
3
S
h
ν
(vycor filter
)
h
ν
(uranium glass filter
)
O
2
S
SiMe
3
-SO
2
SiMe
3
(OC)
3
Cr
11.174a
11.176
11.177
Scheme 11.59
ring system
11.175
with retention of the stereochemistry of the diene double bonds (Scheme 11.58).
80
The
reaction conditions, heat or photolysis, are required to open up a vacant site on the chromium for coordination
of the diene. An interesting question is whether this is by dissociation of CO, or by a change from
6
-to
4
-coordination. For synthetic purposes, the reaction is remarkably tolerant of diene substituents, which can
be varied from strongly electron-donating alkoxy and silyloxy groups to electron-withdrawing ester groups.
The reaction may also be intramolecular.
81
An alkyne may also be used in place of the diene component. With the cyclic sulfone complex
11.174a
,this
leads to an interesting synthesis of cyclooctatetraenes as the initial cycloadduct
111.76
is photochemically
unstable and readily loses SO
2
(Scheme 11.59).
82
The adduct may be isolated if uranium glass filters are
employed to exclude the high-energy wavelengths. Subsequent photolysis with the more transparent vycor
filters causes chelotropic elimination of sulfur dioxide to give the tetraene
11.177
.
The intramolecular cycloaddition is particularly convenient, as the starting material may be prepared by
the addition of an organozinc compound to the stable
7
-complex
11.178
(Scheme 11.60). The initial 7-alkyl
SiMe
3
M
SiMe
3
M = ZnI.CuCN.LiCl
(OC)
3
Cr
(OC)
3
Cr
11.178
11.179
SiMe
3
Δ
Me
3
Si
Cr(CO)
3
11.180
11.181
Scheme 11.60
