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Leitner reported cobalt nanoparticles stabilized on poly(ethylene glycol) (PEG). 30 They
were prepared by thermal decomposition of Co 2 (CO) 8 in PEG. The heterogeneous catalyst
operated as an efficient catalyst for intra- and intramolecular reactions under the harsh
conditions (Scheme 3.29). Also in this case, a significant decrease of catalytic activity was
observed by twice recycling.
R
PEG 5000 stabilized Co-nanoparticles
(3 mol%)
R
Z
O
Z
THF, 130 °C, CO (23 atm)
Z = C(CO 2 Et) 2 , R = H: 89%
Z = C(CO 2 Et) 2 , R = Me: 90%
Z = C(CH 2 OH) 2 , R = H: 74%
R
O
PEG 5000 stabilized Co-nanoparticles
(3-5 mol%)
R
+
THF, 130 °C, CO (23 atm)
R = Ph: 88%
R = n-C 3 H 7 : 72%
R = (CH 2 ) 3 OH: 82%
Scheme 3.29
The polymer-bound triphenylphosphines could immobilize Co 2 (CO) 8 to give hetero-
geneous catalysts. For example, polystyrene cross-linked with 1% divinylbenzene 31 and
polystyrene functionalized by second or third generation dendron 32 were reported as carri-
ers, and the heterogeneous catalysts were used in the Pauson-Khand reaction. The reaction
surely proceeded, but was far from practically useful, because the yield was generally low
to moderate and the substrate scope was narrow.
3.8 Catalytic Reaction in Other Than Conventional Solvents
Hydrocarbons such as benzene and toluene, and etherate solvents such as THF and 1,2-
dimethoxyethane are the usual solvents for the catalytic Pauson-Khand reaction. Jeong used
a supercritical fluid for the catalytic Pauson-Khand reaction. 33 Co 2 (CO) 8 was submitted
under 30 atm pressure of CO (at 23 C) in supercritical carbon dioxide (final pressure was
110-120 atm at 36- 39 C), and the reaction mixture was heated at 90-95 C. Intra- and
intermolecular reactions proceeded to give cyclopentenones (Scheme 3.30). The reaction
of nitrogen-tethered enynes did not proceed, because they were not sufficiently soluble in
supercritical carbon dioxide
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