Chemistry Reference
In-Depth Information
all these problems has not yet been reported. The use of metallic cobalt(0) as a catalyst
had been anticipated from previous uses of
in situ
generated Co(0) species to generate
cobalt carbonyls.
9
For example, Co(acac)
2
with NaBH
4
acts as a catalytic system under
mild conditions.
10
This study suggested that such a system could be catalytic providing it
leads to
in situ
generation of low valency cobalt, which could then react with available CO
to afford cobalt carbonyl species. Based on this fact, the first practical heterogeneous PKR
catalyst of metallic cobalt was developed in 2000.
11
Since then, cobalt nanoparticles,
12
heterobimetallic cobalt and rhodium nanoparticles,
13
and useful variants of them
14
have
been reported.
9.2.1
Polymer-Supported Catalytic Systems
The immobilization of homogeneous catalysts on solid supports and polymer-supported
complexes has been actively studied in many catalytic systems.
15
However, only two
cases have been reported for the PKR.
7, 8
The first was reported by Gibson et al.
7
They prepared resin-bound phosphine-substitued cobalt carbonyl complexes via a re-
action of polymer-bound triphenylphosphine with Co
2
(CO)
8
in THF. The obtained
resin was much more stable than bare Co
2
(CO)
8
. Using 5 mol% of catalyst precursor,
they obtained products of an intramolecular PKR of terminal enynes in 50-60% yield
(Scheme 9.1).
Cat
Z
Z
O
70 °C
Ph
2
P
Yield(%)
Z
Co(CO)
3
Co(CO)
3
NTs
C(CO
2
Et)
2
61
49
P
Ph
2
Scheme 9.1
They presented only two examples. When the catalytic system was recycled, the yield of
allyl propargyl tosylamide dropped to 28%.
7b
According to their microanalysis of the resin
after catalysis, up to 13% of the cobalt was lost during the first cycle. Nevertheless, the
mild reaction conditions (70
◦
C and 1.05 bar CO) compared to those (130
◦
C and 20 atm
CO) of metallic cobalt system
11
is noteworthy.
Portnoy et al. reported
8
the building of a catalyst on polyether dendron-functionalized
polystyrene. They used Wang resin and three dendronized supports functionalized with
2- and 4-(diphenylphosphino)benzoic acid. Using cobalt complexes immobilized on the
second- and third-generation dendron-functionalized polystyrene as catalysts, they studied
an intramolecular PKR (Scheme 9.2).
