Science of Synthesis

Domínguez, G.; Pérez-Castells, J., Science of Synthesis: Metal-Catalyzed Cyclization Reactions, (2016) 2, 135.

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]

‌Meeee 88‌ Meeeee–Meeee Meeeeeeee eeee Meeeeeeeeeeee Meeeeeeee[‌888‌–‌888‌]

Meeee	Meeeeeee Meeeeeee	Meeeeeeee Meeeee	MM Meeeee	Meeeeeeeee	Meeeeee	Meeee (%)	Mee
8		Me/eeeeee, (8–88 ee%)	MM (88 eee)	MMM, 888 °M, 8.8 e		88	[‌888‌]
8		Me/eeeeeeee (88 ee%)	MM (88 eee)	MMM, 888 °M, 8–88 e		88	[‌888‌]
8		eeeeeeeee Me (8.8 eeeee)	MM (8 eee)	MMM, 888 °M, 88 e		88	[‌888‌]
8		eeeeeeeee Me (88 ee%)	MM (8 eee)	MMM, 888 °M, 88 e		88	[‌888‌]
8		Me eeeeeeeeeeeee ee eeeeeeee (88 ee%)	MM (8 eee)	MMM, 888 °M, 88 e		88	[‌888‌]
8		MMM8888-eeeeeeeeee Me eeeeeeeeeeeee (8 eee%)	MM (88 eee)	MMM, 888 °M, 88 e		88	[‌888‌]
8		MMM8888-eeeeeeeeee Me eeeeeeeeeeeee (88 eee%)	MM (88–88 eee)	M8M, 888 °M, 88 e		88	[‌888‌]
8		Me8(MM)88, Me eeeeeeeeeeeee ee eeeeeeee	8-eeeeeeeeeeeee eeeeeee (8.8 eeeee)	MMM, 888 °M, 88 e		88	[‌888‌]
8		Me8Me8	MM (8 eee)	MMM, 888 °M, 88 e		88	[‌888‌]
88		Me8Me8	MeMM=MMMMM (8.8 eeeee)	MMM, 888 °M, 88 e		88	[‌888‌]
88		Meeee Me (8 eee%)	MM (88 eee)	MMM, 888 °M, 88 e		88	[‌888‌]

M eeeeeee eeeeeeee ee e eeeeeeeeeeeee eee eeeeee eeeeeeee eeee Meeeee–Meeee eeeeeeee eeee e eeeeeeeeee eeeeeeeee eeeeee eeeeeee (eee-M8M8) ee e eeeee-eeee eeeeeeee eeee eeeeeee eee eeeeee eeeeeeeeee ee eeeeee eeeeeee eee eee eeeeeeeee ee eeeeeee ee eeeeee. Mee eeeeee eeeeee eeeeeeee ee eeee ee eeee eee eee eeeeeeeeee Meeeee–Meeee eeeeeeee.[‌888‌]

Meeeeeeeeeee Meeeeeeee

Meeeeeee 8-Mee-8,8e,8,8-eeeeeeeeeeeeeeeeeee-8,8(8M)-eeeeeeeeeeeee (‌Meeee 88‌, Meeee 8); Meeeeee Meeeeeeee:[‌888‌]

MMMMMMM: Meeeee eeeeeeee ee eeeeeeeee eeeeeeeee eee eeeee, eee eeeeeeee ee eeeeee eeeeeeeeeeeeee eee eeeeeee eeee ee e eeee.

Meeeeeeeeeeeee ee eeeeee eeeeeeeeeeeee ee eeeeeeee (MMM): Me 8,8-eeeeeeeeeeeeeee (88 eM) eeee eeeee eeeee eeee (8.88 eM) eee eeeeeeee eeeeeeeee (8.88 e). Mee eeee eee eeeeee ee 888 °M. Me eee eee eeee eee eeeee e eeee ee Me8(MM)8 ee 8,8-eeeeeeeeeeeeeee (8 eM). Mee eeeeeeeee eeee eee eeeeee ee 888 °M eee 8 e eee eeee eeeeeeeeeeee ee e eeeeee ee 8 eM. Mee eeeeeeeeeeee eeee eee eeeeee ee ee. Me eee eeeeee eeee eee eeeee MeMM (88 eM). Mee eeee eee eeee eeeeeee eee 88 eee, eee eeee eeeeeeee (8.88 e) eee eeeee. Mee eeeeeeeee eeeeeee eee eeeeeeee eee 88 e, eee eee eeeeeeeeeee eee eeeeeeeee ee eeeeeeeeee, eeeeee eeee MeMM (88 eM), MM8Me8 (88 eM), eee Me8M (88 eM), eee eeeee eeeee eeeeee ee eeee MMM ee e eeeee eeeee; eeeee: 8.88 e.

Meeeee–Meeee eeeeeeee: Meeeeeee 8-eeeee-8-(eeee-8-eeee)eeeeeeee (8.88 e, 8.88 eeee), MMM (8.88 e), eee MMM (8 eM) eeee eee ee e eeeeeeeee-eeeee eeee. Mee eeee eee eeeeeeeeeee eeeee 8 eee ee MM eee eeeeee ee 888 °M eee 88 e. Meeee eeeeeee eee eeeeeeeee eee eeeeeeee, eee eeee eee eeeeeeee eee eee eeeeeeee eee eeeeeeeeeeee ee eeeeeee. Mee eeeeeee eee eeeeeeee ee eeeee eeeeeeeeeeeeee (eeeeee eee, eeeeee/Me8M 8:8); eeeee: 8.888 ee (88%).

References