Chemistry Reference
In-Depth Information
Me
3
Si
CpCo(CO)
2
-2 CO
SiM
e
3
CoCp
CoCp
11.29
11.30
11.31
Me
3
Si
SiMe
3
SiMe
3
SiMe
3
SiMe
3
Co
Cp
Co
SiMe
3
Cp
11.33
11.32
11.34
Scheme 11.14
For synthesis, the problem has always been how to control the reaction so that three different alkynes can
be coupled together selectively. One answer is to arrange for two of the alkynes to be connected by a tether,
so that part of the reaction is intramolecular.
19,20
The third alkyne should then be one that it reluctant to self-
trimerize, such as bis(trimethylsilyl)acetylene, which is quite hindered, or other trimethylsilyl-substituted
alkynes.
21
It is likely that the cobalt catalyst coordinates the two alkynes of the diyne (Scheme 11.14).
Oxidative cyclization then gives a metallacyclopentadiene
11.31
. Coordination of bis(trimethylsilyl)acetylene
is followed by insertion and reductive elimination. The reaction conditions, involving heating, are required
for the first step, which is dissociation of CO from the cobalt.
The final product
11.34
also contains two silyl groups. These are also very useful as they undergo facile
electrophilic
ipso
-substitution. As two of the alkynes are part of a diyne, the reaction produces a bicyclic
compound. Remarkably, even benzocyclobutenes
11.35
can be formed in good yield (Scheme 11.15). These
are especially useful as thermolysis results in an electrocyclic ring opening to an
o
-xylylene, which can be
trapped
in situ
in a (classical) Diels-Alder reaction.
This strategy was used in an outstanding synthesis of estrone
11.42
(Scheme 11.16).
22
The diyne
11.38
,
with a pendant alkene ready for the Diels-Alder step, was constructed by specific enolate chemistry. Heating
diyne
11.38
with bis(trimethylsilyl)acetylene gave the benzocyclobutenes
11.39
as a mixture of diasteroiso-
mers, accompanied by some of the Diels-Alder product. Further heating of the benzocyclobutene mixture
resulted in complete conversion to the Diels-Alder product
11.41
. Both diastereoisomeric benzocyclobutenes
gave the same
o
-xylyene
11.40
, and, hence, the same Diels-Alder product. Conversion to estrone
11.42
was achieved by protio-desilylation at C2 with good, but not complete selectivity, followed by oxida-
tive cleavage of the C3 carbon-silicon bond. Another application of
o
-xylylene chemistry can be found
in Scheme 3.65.
CpCo(CO)
2
SiMe
3
Me
3
Si
SiMe
3
SiMe
3
11.35
Scheme 11.15
