Chemistry Reference
In-Depth Information
9.4.2.1
Co
2
Rh
2
-catalyzed PKR under 1 atm of CO
The use of heterobimetallic Co/Rh nanoparticles as catalysts in the intramolecular PKR of
allyl propargyl ether was studied (Scheme 9.12).
Ph
Ph
Cat.
O
O
O
1 atm CO, 130
°
C
18 h, THF
Cat.
Yield (%)
Co
2
Rh
2
Co
3
Rh
Co NP
Rh NP
A mixture of Co and Rh NP
87
65
0
23
12
Scheme 9.12
Under 1 atm CO, no reaction was observed with
CNC
, and 23% of the reaction product
was obtained with rhodium nanoparticles on charcoal. When
Co
3
Rh
nanoparticles (derived
from Co
3
Rh(CO)
12
) were used, the expected product was obtained in 65% yield. The use of
Co
2
Rh
2
nanoparticles (derived from Co
2
Rh
2
(CO)
12
) as a catalyst gave 87% of the reaction
product. Interestingly, when a mixture of colloidal cobalt and rhodium nanoparticles was
used as a catalyst under the same reaction conditions, only 12% of the product was obtained.
This study showed that the catalytic activities towards specific products can often be tuned
by changing the composition of heterobimetallic nanoparticle catalysts. Using
Co
2
Rh
2
as
a catalyst, the optimized reaction conditions were established as 1 atm of CO, 130
◦
C, THF,
and 18 h. The catalytic system maintained its high level of activity even after being recycled
five times.
The catalytic system was effective in the inter- and intramolecular PKR (Table 9.7). All
the substrates including heteroatom-tethered enynes gave high yields (81-92%). Even in the
intermolecular PKRs, moderate yields (59-68%) were obtained. This result is remarkable
because many useful rhodium catalysts are generally less effective for the intermolecular
PKR
34
and cannot be reused. Thus, the
Co
2
Rh
2
catalytic system seems to overcome the
disadvantages of homogeneous rhodium catalysts.
9.4.2.2
Co
2
Rh
2
-catalyzed Pauson-Khand-type reaction in the presence of aldehydes
instead of carbon monoxide
13
Owing to the recent attention given to green chemistry, in many cases instead of car-
bon monoxide aldehyde,
37
normally prepared from the corresponding alcohol, was used
as the substitute for carbon monoxide. Very recently, Chung
38
reported the use of an al-
cohol as a source of carbon monoxide in the presence of [Rh(CO)Cl(dppp)]
2
(dppp
=
bis(diphenylphosphino)propane).
