Polymer supports modification

There also have been other reports of polymer-supported catalysts with incorporated boron moieties resulting from multistep polymer modification reactions to incorporate the boron moiety.76... 

Highly efficient modifications of Mukaiyama s procedure, convenient for combinatorial syntheses, were reported recently, namely the polymer-supported synthesis of isoxazolines via nitrile oxides, starting from primary nitroalkanes, in a one-pot process (75) and by microwave activation of the process (73). 

This loading approach is successful for the relatively unhindered 6-hydroxyl, but proved to be inefficient for more sterically encumbered alcohols. A modification of the abovementioned method has been developed to enable attachments through these more hindered sites.9 This process is accomplished with inexpensive materials, and the linker is compatible with a variety of reaction conditions. Moreover, it allows for facile recycling of the polymer support for further use once the final carbohydrate assemblage is cleaved from the resin. 

Many other linkers besides those listed above have been developed for two-phase synthesis of oligosaccharides on insoluble supports, and it can be expected that at least some of them will be tested on soluble supports. It should be kept in mind that MPEG-supported syntheses can be easily scaled up therefore, any relationship between both types of polymer supports will be cooperative rather than mutually exclusive. Such linkers will most probably include dialkyl- or diaryl-silyl linkers,10,41 3 and linkers cleavable by photolysis such as the o-nitrobenzyl group and its modifications.44 16... 

A useful modification of the bis(p-methoxyphenyl) telluroxide reagent is its immobilization on a polymeric resin. This polymer-supported reagent, prepared from p-methoxyphenyl tellurocyanate and polylp-Uthiostyrene), exhibits several advantages over the monomeric reagent, such as easier product work-up and multiple recycle from the spent reagent. 

NAD glycohydrolases from rat liver nuclei, 66, 151 poly(ADP-ribose) synthetase from rat liver nuclei, 66, 154 poly(ADP-ribose) synthetase from calf thymus, 66, 159 extraction and quantitative determination of larger than tetrameric endogenous polyadenosine diphosphoribose from animal tissues, 66, 165 covalent modification of proteins by metabolites of NAD, 66, 168 coenzyme activity of NAD bound to polymer supports through the adenine moiety, 66, 176 use of differently immobilized nucleotides for binding NAD -dependent dehydrogenases, 66, 192. 

Techniques to discover the identity of such protein protein interactions are evolving. One approach involves protein affinity chromatography. Here, the purified protein of interest is immobilized on a solid polymer support and proteins that associate with it are identified by electrophoresis and MALDI. There exist a wide number of modifications to the affinity... 

The palladium-catalyzed, microwave-assisted conversion of 3-bromopyridine to 3-cyanopyridine using zinc cyanide in dimethylformamide (DMF) has been reported <2000JOC7984>. Substoichiometric quantities of copper or zinc species improve both conversion rate and efficiency of Pd-catalyzed cyanation reactions <1998JOC8224>. A modification of this procedure uses a heterogeneous catalyst prepared from a polymer-supported triphenylphosphine resin and Pd(OAc)2 the nitriles were obtained from halopyridines in high yields <2004TL8895>. The successful cyanation of 3-chloropyridine is observed with potassium cyanide in the presence of palladium catalysts and tetramethylethylenediamine (TMEDA) as a co-catalyst <2001TL6707>. 

Another approach to the preparation of polymer-supported metal Lewis acids is based on polymerization of functional monomers. If synthesis of the functional monomer is not difficult, polymerization should afford structurally pure functional polymers, because the polymer formed requires no further complicated chemical modification. A variety of substituted styrene monomers are now commercially available styrene monomers with an appropriate ligand structure can be prepared from these. Several other interesting functional monomers such as glycidyl methacrylate, 2-hydr-oxyethyl methacrylate, and other acrylics have also been used extensively to prepare functional polymers. 

The above mentioned polymer-supported oxazaborolidines are prepared from polymeric amino alcohols and borane. Another preparation of polymer-supported oxazaborolidines is based on the reaction of polymeric boronic acid with chiral amino alcohol. This type of polymer can be prepared only by chemical modification. Lithiation of the polymeric bromide then successive treatment with trimethyl borate and hydrochloric acid furnished polymer beads containing arylboronic acid residues 31. Treatment of this polymer with (li ,2S)-(-)-norephedrine and removal of the water produced gave the polymer-supported oxazaborolidine 32 (Eq. 14) [41 3]. If a,a-diphenyl-2-pyrrolidinemetha-nol was used instead of norephedrine the oxazaborolidine polymer 33 was obtained. The 2-vinylthiophene-styrene-divinylbenzene copolymer, 34, has been used as an alternative to the polystyrene support, because the thiophene moiety is easily lithiated with n-butyl-lithium and can be further functionalized. The oxazaborolidinone polymer 37 was then obtained as shown in Sch. 2. Enantioselectivities obtained by use of these polymeric oxazaborolidines were similar to those obtained by use of the low-molecular-weight counterpart in solution. For instance, acetophenone was reduced enantioselectively to 1-phe-nylethanol with 98 % ee in the presence of 0.6 equiv. polymer 33. Partial elimination of... 

The first report of a polymer-supported approach to this reaction appeared in 1987 [48]. Enantiopure amino alcohols such as ephedrine, prolinol, and 3-exo-amino-isoborneol were attached to Merrifield polymer. The use of polymer-supported 3-exo-aminoisoborneol 40 resulted in quite high enantioselectivity ( 95 % ee) in the ethylation of aldehydes with diethylzinc (Eq. 15), a result comparable with those obtained from the corresponding low-molecular-weight catalyst system (Eq. 16). A similar system was also reported in 1989, this time using ephedrine derivatives (41,42) and prolinol derivative (43) [49]. A methylene spacer was introduced between the polymer and the amino alcohol to improve activity [50]. Despite this the selectivity was always somewhat lower than that obtained from the low-molecular-weight catalyst (44). These chiral polymers were all prepared by the chemical modification method using Merrifield polymer. 

The same reaction has also been catalyzed by chiral oxazaborolidinones derived from amino acids and boranes. They proved to be efficient catalysts for enantioselec-tive Diels-Alder reaction [91,92]. The polymer-supported chiral oxazaborolidinones 75 were reported to be efficient catalysts [93]. These polymer-supported chiral oxaza-borolidinone ligands were prepared both by chemical modification and by the copolymerization shown in Sch. 5 [94]. The polymer-supported chiral ligands were then reacted with borane to give the oxazaborolidines which were used as catalysts in Diels-Alder reaction of cyclopentadiene with methacrolein. 

Polymer-supported TADDOL-Ti catalyst 79 prepared by chemical modification was poorly active in the Diels-Alder reaction of 3-crotonoyloxazolidinone with cyclo-pentadiene (Eq. 24) whereas polymeric TADDOL-Ti 81 prepared by copolymerization of TADDOL monomer 80 with styrene and divinylbenzene had high activity similar to that of the soluble catalyst. In the presence of 0.2 equiv. 81 (R = H, Aryl = 2-naphthyl) the Diels-Alder adduct was obtained in 92 % yield with an endolexo ratio of 87 13. The enantioseleetivity of the endo product was 56 % ee. The stability and recyclability of the catalyst were tested in a batch system. The degree of conversion, the endolexo selectivity, and the enantioseleetivity hardly changed even after nine runs. Similar polymer-supported Ti-TADDOLate 82 was prepared by the chemical modification method [99]. Although this polymer efficiently catalyzed the same reaction to give the (2R,2S) adduct as a main product, asymmetric induction was less than that obtained by use of a with similar homogeneous species. 

Rare earth metal triflates are recognized as a very efficient Lewis acid catalysts of several reactions including the aldol reaction, the Michael reaction, allylation, the Diels-Alder reaction, the Friedel-Crafts reaction, and glycosylation [110]. A polymer-sup-ported scandium catalyst has been developed and used for quinoline library synthesis (Sch. 8) [111], because lanthanide triflates were known to be effective in the synthesis of quinolines from A-arylimines [112,113]. This catalyst (103) was readily prepared from poly(acrylonitrile) 100 by chemical modification. A variety of combinations of aldehydes, amines, and olefins are possible in this reaction. Use of the polymer-supported catalyst has several advantages in quinoline library construction. 

The use of the Wittig reaction and its modifications for the synthesis of vinylchalcogenides, ketene chalcogenoacetals and related species has been reviewed. New developments on this topic include the synthesis of a series of polymer-supported alkyltriphenylphosphoranes (46) which have been utilised in the preparation of vinylic selenides. The synthesis of vinylic selenides, such as 1-chloro-l-phenylselenoalkenes, has also been accomplished in a one-pot reaction which proceeds through the formation of selenoylphosphorane (47) intermediates. ... 

Different conditions (including additives and solvent) for the reaction have been reported,often focusing on the palladium catalyst itself," or the ligand." Catalysts have been developed for deactivated aryl chlorides," and nickel catalysts have been used." Modifications to the basic procedure include tethering the aryl triflate or the boronic acid to a polymer, allowing a polymer-supported Suzuki reaction. Polymer-bound palladium complexes have also been used." " The reaction has been done neat on alumina," and on alumina with microwave irradiation." Suzuki coupling has also been done in ionic liquids," in supercritical... 

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