Polymer-supported ligands, preparation

A polymer-supported version of our optimal ligand was also developed [52]. Its preparation involves attachment of aziridine carbinols to polymer-bound triphenylchloromethane (Scheme 40). This polymer-bound ligand 53 was almost equally effective in the enantioselective addition of diethylzinc to aromatic and aliphatic aldehydes with ee s ranging from 77-97% for the latter type of substrate [52]. It is of practical interest that this polymer-supported ligand could be reused without losing much of its efficiency. 

N. E. Leadbeater, M. Marco, Preparation of polymer-supported ligands and metal complexes for use in catalysis, Ghent. Rev. 102 (2002) 3217. 

Study of catalysts prepared from polymer-supported ligands containing 1,2- or 1,4-diol functionalities is interesting as this structural factor would favor the formation of very stable chelate rings and provide, if using chiral auxiliaries, a well defined steric environment. Additionally, it has been shown that catalytic activity in solution of some of those catalysts is higher than that reported for aminoalcohol derivatives. 

Polymerization of ligand monomers is a useful tool for preparing polymer-supported ligands. The cross-linked polystyrene bound ferrocenyl bisphos-phine ligand 21 was prepared by the copolymerization of styrene, divinylben-zene,and l,r-bis(diphenylphosphino)-2-vinylferrocene (20) (Scheme 8) [46]. The loading density of the catalyst on the support was readily controlled by the ratio of the monomers used. 

Itsuno has described the first enantioselective allylation of aldehydes using crosslinked polymer-supported A-sulfonylated aminoalcohols as chiral ligands [146], Polymer-supported ligands 220 and 221 were prepared by copolymerisation of the norephedrine-derived monomer 218 or D-camphor-derived 219 with styrene and divinylbenzene (DVB) in a [chiral monomer]/[styrene]/[DVB] molar ratio of 1/8/1 (Scheme 90). 

Moberg focused her attention on the immobilzation of oxazoline-derived ligands for the enantioselective allylic alkylation [159]. Pyridine-containing oxazolines 251 were first prepared and were supported on TentaGel resin to afford the eorresponding polymerie oxazolines 252 (Scheme 102). These polymer-supported ligands were then tested in the... 

Kamer and co-workers" reported an interesting extension of this chemistry to prepare polymer-supported ligands (Scheme 4.31). 

Scheme 7.27 Representative methods for the preparation of polymer-supported ligands and their use for enantioselective alkylation under flow conditions.

IV. 10. N.E. Leadbeater, M. Marco - Preparation of Polymer-Supported Ligands and Metal Complexes for Use in Catalysis, Chem. Rev. 102,3217,2002. 

Electropolymerization is also an attractive method for the preparation of modified electrodes. In this case it is necessary that the forming film is conductive or permeable for supporting electrolyte and substrates. Film formation of nonelectroactive polymers can proceed until diffusion of electroactive species to the electrode surface becomes negligible. Thus, a variety of nonconducting thin films have been obtained by electrochemical oxidation of aromatic phenols and amines Some of these polymers have ligand properties and can be made electroactive by subsequent inincorporation of transition metal ions... 

Moberg et al. [146] modified further the bis(pyridylamide) ligand described by Trost for the preparation of a polymer-supported pyridylamide (113 in Scheme 60) for the microwave-accelerated molybdenum-catalyzed al-lylic alkylation. TentaGel resin was tested in the presence of high concentrations of reactants and gave, after a 30 min reaction, total conversion in the... 

The Dupont method to prepare polymer-supported bidentate phosphorus-containing ligands makes use of Merrifield s resin (Scheme 10).69... 

Soluble polymer-supported (i )-BINAP ligands were employed for the preparation of the Ru -bearing catalysts (54) and (55) which are shown in Scheme 4.33 [126]. Both these catalysts exhibited high activity and enantioselectivity in the asymmetric hydrogenation of 2-(6 -methoxy-2 -naphthyl)propenoic add. 

Kragl 13) pioneered the use of membranes to recycle dendritic catalysts. Initially, he used soluble polymeric catalysts in a CFMR for the enantioselective addition of Et2Zn to benzaldehyde. The ligand a,a-diphenyl-(L)-prolinol was coupled to a copolymer prepared from 2-hydroxyethyl methyl acrylate and octadecyl methyl acrylate (molecular weight 96,000 Da). The polymer was retained with a retention factor > 0.998 when a polyaramide ultrafiltration membrane (Hoechst Nadir UF PA20) was used. The enantioselectivity obtained with the polymer-supported catalyst was lower than that obtained with the monomeric ligand (80% ee vs 97% ee), but the activity of the catalyst was similar to that of the monomeric catalyst. This result is in contrast to observations with catalysts in which the ligand was coupled to an insoluble support, which led to a 20% reduction of the catalytic activity. 

A polymer-supported Sharpless epoxidation catalyst was prepared using linear poly(tartrate ester) catalyst ligands 43.65 This catalyst system was used in the reaction of tranA-hex-2-en- l-ol with titanium tc/ra-isopropoxide and tert-butyl hydroperoxide to afford the desired epoxide in high chemical yield and moderate enantiomeric excess. 

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