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2.3.3.5 Heterogeneous Catalysts

DOI: 10.1055/sos-SD-222-00068

Domínguez, G.; Pérez-Castells, J.Science of Synthesis: Metal-Catalyzed Cyclization Reactions, (20162135.

Efficient methodologies result from using homogeneous catalysts anchored to solids, as the resulting catalyst is reusable and easily removed from the reaction medium.[‌6‌] Different procedures are used to immobilize metals on the surface or within the interior structure of solid supports, such as polymers, silica gel, and zeolites. Table 11 summarizes the most efficient heterogeneous catalytic systems, which are tested with typical Pauson–Khand substrates; the results are generally excellent in terms of yield. However, harsh conditions are still required with some systems. Mesoporous silicas are applied as supports for cobalt metal with high pressure of carbon monoxide and the results are satisfactory with intramolecular examples (Table 11, entry 1).[‌113‌] The use of cobalt supported on charcoal gives excellent results, but a high carbon monoxide pressure is still required (Table 11, entry 2).[‌114‌] A different immobilization method, an entrapment of catalysts by the sol-gel process, is used with rhodium in order to obtain a catalyst that can be used under milder conditions. Silica sol-gel entrapped chloro(cycloocta-1,5-diene)rhodium(I) dimer [{RhCl(cod)}2] gives the Pauson–Khand product in 90% yield at 100 °C and at a carbon monoxide pressure of 5 atmospheres (Table 11, entry 3).[‌115‌] Cobalt nanoparticles, heterogeneous catalytic systems with high surface-to-volume ratio, also give good results under 5 atmosphere of carbon monoxide (Table 11, entry 4).[‌114‌] Combining the merits of conventional heterogeneous catalysts with the high catalytic activity of cobalt nanoparticles, cobalt nanoparticles on charcoal (CNC) give similar results to colloidal cobalt (Table 11, entry 5).[‌116‌] Polyethylene glycol stabilized cobalt nanoparticles are efficient in the Pauson–Khand reaction and they can be used in tetrahydrofuran or water (Table 11, entries 6 and 7).[‌117‌] Dodecacarbonyltriruthenium(0) is combined with cobalt nanoparticles on charcoal (CNC) to catalyze a Pauson–Khand-type reaction that uses 2-pyridylmethyl formate as the carbon monoxide source (Table 11, entry 8).[‌118‌] The use of immobilized heterobimetallic nanoparticle catalysts also catalyzes the Pauson–Khand reaction using either carbon monoxide (Table 11, entry 9)[‌119‌] or but-2-enal as a carbon monoxide source (Table 11, entry 10).[‌120‌] Raney cobalt also shows activity as a Pauson–Khand catalyst, albeit under high carbon monoxide pressure (Table 11, entry 11).[‌121‌]

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