Chemistry Reference
In-Depth Information
The reductive PKR provides a straightforward approach to the bicyclic and tricyclic
ketone frames which appear as core skeletons in many natural syntheses, such as those of
the linear and angularly fused triquinane sesquiterpenes (Scheme 9.9).
32
CNC
TsN
Ts N
O
5 atm CO, 5 atm H
2
130 °C, 18 h, THF
H
H
H
n
n
n=1, 92%
n=2, 87%
Scheme 9.9
Poly(ethylene glycol)-Stabilized Cobalt Nanoparticles
14b
9.3.3
The use of insoluble polymeric materials as a support in heterogeneous catalysis often
involves problems that include lowered reactivities, extended reaction times, diffusion-
limited reactivity and reagent leaching. To alleviate these problems, Leitner et al. reported
14 b
the use of PEG-stabilized cobalt nanoparticles as readily available, highly active and partly
recyclable catalysts for the PKR (Scheme 9.10).
EtO
2
C
EtO
2
C
PEG-stab Co NP
O
CO, 130
°
C, 16 h, THF
EtO
2
C
EtO
2
C
Press. (bar)
Conv. (%)
Select. (%)
Co (mol%)
3
3
50
50
23
10
10
5
>99
35
97
85
98
90
98
82
Scheme 9.10
As expected, the catalytic system was quite active for intramolecular PKRs of carbon-
tethered enynes and for intermolecular PKRs of norbornene with phenylacetylene, 4-
pentyn-1-ol, 1-pentyne and 1-octyne (Table 9.5). At high cobalt loading (50 mol%), the
pressure of CO can be lowered 5-10 bar at 130
◦
C, but at low cobalt loading (3 mol%)
suitable for the practical application, a high CO pressure (23-35 bar) was inevitable. Most
reactions were thus carried out at 130
◦
C under high CO pressure and a reaction time of
16 h. Contrary to their expectations, PEG-stabilized cobalt nanoparticles were only partly
recyclable due to the deactivation of the catalytic system resulting from the noticeable
leaching (up to 0.74% per cycle based on the contamination in the product) and changes in
the particle morphology.
