Polyethylene-bound catalysts

Polyethylene-supported catalysts that are initially insoluble but that become soluble on heating and are separated as insoluble materials on cooling are also used as catalysts in polymerization reactions. Infact, this was the first way a polyethylene-bound catalyst was used (Eq. 11) [34]. However, soluble polymers used in this manner appear to have several deficiencies. First, separation of the products from the catalyst may notalways be as simple as was the case with catalysts like 11 or 12 and low molecular weight products. For example, while a hot solution of a polyethylene-bound neodymium salt was successfully used in the stereoselective polymerization of butadiene to form high molecular weight (Z)-poly( 1,4-butadiene), the product mixture after cooling was a thick, viscous sus-... 

An unresolved problem with these polyethylene-bound ruthenium clusters is their thermal decomposition. This decomposition, which was noted by Drago, was confirmed by UV-visible studies of solutions of these polyethylene-bound catalysts and is the principle limit to catalyst recyclability in the case of this cluster catalyst. 

Janda, Bolm and Zhang generated soluble polymer-bound catalysts for the asymmetric dihydroxylation by attaching cinchona alkaloid derivatives to polyethylene glycol monomethyl ether (MeO-PEG) [84—87]. Since these polymeric catalysts like (24) are soluble in many common solvents they are often as effective as their small homogenous counterparts. Janda et al. prepared catalyst (24) in which two dihydroquinidine (DHQD) units were linked together by phthalazine and finally were attached to MeO-PEG via one of the bicyclic ring system moieties (Scheme... 

A polyethylene-bound soluble recoverable dirhodium(II) tetrakis(2-oxapyrrolidine-(55 )-carb-oxylate) was also highly efficient in enantioselective intramolecular cyclopropanation of allyl diazoacetates and could be used repeatedly without significant loss of enantiocontrol. Some enantiomerically pure, secondary allylic diazoacetates showed the expected substrate-induced diastereofacial selectivity in intramolecular cyclopropanation, when they were decomposed with bis(A-n-r/-butylsalicylamidinato)copper(II). ° This selectivity could be significantly enhanced or reversed with the chiral catalyst 30 or its antipode. Furthermore, catalysts 30 and 32 allowed a highly efficient kinetic resolution of racemic secondary allylic diazoacetates. 

The new amphiphilic polyethylene-bound BINAP ligand 35 was used to generate Ru catalysts for the asymmetric hydrogenation of 2-(6-methoxy-2-naphthyl)-acrylic acid to naproxene [61]. The enantioselectivity in the biphasic decreased from 77 to 88% compared to the methanolic system [61]. 

Non-cross-linked polymers can be used in this way just as cross-linked polymers can. For example, we have used polyethylene supports with surface grafts to support Pd(0) catalysts [133,134]. In these cases, the polymer-immobilized catalyst is used in exactly the same way as an insoluble polymer-bound catalyst. Such supported catalysts do require that the insoluble polymer be swollen or permeable to substrates or that the catalysts be within a solvent-permeable, thin immobilized graft. While this approach can be useful, it takes no advantage of the polymer s solubility. It is an approach that conceptually is no different than that used with insoluble inorganic supports or with polymers that are by design insoluble by virtue of cross-linking, and is an approach to catalyst immobilization that is not further discussed since this review is focused on polymer-immobilized catalysts that are used under solution-state conditions. 

However, although this polyethylene chemistry is general, it has disadvantages. Its principle disadvantage is that it requires elevated temperatures to dissolve the polymer. Below about 70 °C, these catalysts have zero or minimal reactivity. If the PEoiig-bound catalyst doesn t go into solution, the reaction does not work. 

Further efficient polymeric palladium complexes containing Pd(OAc)2, Pd(dba), Pd(MeCN)Cl2, Pd(PhCN)Cl2 and (7i-allyl)PdCl anchored onto the polyethylene-bounded triphenylphosphine are called FibreCat catalysts [141] ... 

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