Polymer-bound Macrocycles

A good deal of work has been done on polymeric crown ethers during the last decade. Hogen Esch and Smid have been major contributors from the point of view of cation binding properties, and Blasius and coworkers have been especially interested in the cation selectivity of such species. Montanari and coworkers have developed a number of polymer-anchored crowns for use as phase transfer catalysts. Manecke and Storck have recently published a review titled Polymeric Catalysts , which may be useful to the reader in gaining additional perspective. 

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As soon as the novel ion-binding character of crown ethers was discovered, researchers sought ways to incorporate these characteristics into polymeric structures.An ion-binding site could be incorporated into the polymer backbone, or the site could be pendent from the polymer backbone. The ion-binding site could be either a macro-cyclic or acyclic multidentate ligand. A review of crown ethers, including polymer-bound macrocyclic polyethers, has recently appeared. 

First the protected oligopeptide is coupled with polymer-bound nitrophenol by DCC. N"-Deblocking leads then to simultaneous cycliiation and detachment of the product from the polymer (M. Fridkin, 1965). Recent work indicates that high dilution in liquid-phase cycli-zation is only necessary, if the cyclization reaction is sterically hindered. Working at low temperatures and moderate dilution with moderately activated acid derivatives is the method of choice for the formation of macrocyclic lactams (R.F. Nutt, 1980). 

Finally, the 1,3-dione systems prepared by Cram and Alberts deserve special note . These compounds, referred to as hexahosts are similar to the polymer-bound material illustrated as Compound 29 in Chap. 6. The synthesis is based on a methylene-bridged bis-dithiane unit. One of these may be cyclized with a polyethylene glycol, or more than one unit may be incorporated to give multiple 1,3-dione binding sites in the macrocycle. The former case is illustrated in Eq. (3.46). 

Functionalized macrocyclic all using polymer-bound stannanes. ... 

Functionalized macrocyclic alkenes can be synthesized by intramolecular Stille coupling using polymer-bound stannanes. The products are released during the coupling process. ... 

Carbonylative intramolecular Stille coupling to form macrocyclic molecules was investigated with a palladium complex of the polymer-bound ferrocenyl phosphine 21. One of the major problems encountered in the intramolecular... 

The similarity of the log K values for the silica gel-bound and unbound crown-cation interactions suggests that both crown entities are effectively solvated by the aqueous solution. Thus, the silica gel-bound macrocycles form complexes in the same manner as do the free crowns in water. On the other hand, the bonding of macrocycles to hydrocarbon polymers, such as polystyrene, causes a considerable modification in metal ion binding properties in both organic solvents and aqueous solutions. In particular, aqueous solutions cannot be treated effectively because the hydrocarbon polymers are not wetted by water [24, 25]. 

A boronic acid ester, which contains an aryl iodide moiety attached by an appropriate tether, can act as an inttamolecular arylation agent. Thns, Li and Burgess developed a polymer-bound precursor, which by a biaryl coupling ensning cleavage fnmished a macrocyclic constrained /3-tum peptide mimic (Scheme... 

Synthesis.—A review has recently appeared on the preparation of derivatives and analogues of macrocyclic oligomers of ethylene oxide " the article refers only briefly to basic crown ethers but gives an extensive survey of modified systems (fusion to aromatic rings, change of heteroatoms, chiral crowns, polymer-bound crowns, etc.) that will help prevent duplication in designed crown research. 

This chapter is divided into six subsections which cover the most prominent applications of the metathesis reaction in solid-phase organic chemistry RCM for cleavages from the resin synthesis of small rings to macrocycles on the resin via RCM the use of the metathesis reaction for dimerization of polymer-bound small molecules RCM to constrict peptide conformations (while this is formally an RCM, it is treated separately because of the unique considerations of peptide chemistry) CM between a polymer-bound alkene and an alkene in solution and ene-yne metathesis. As the field evolved relatively rapidly with major advances in the catalysts, each subsection is organized in chronological order for the sake of clarity. 

A poly(phenylquinoxaline) was prepared for electroluminescence applications <1996SM(76)105>. Crystallization of solution donor-acceptor complexes of 2,3-dimethylquinoxaline 1,4-dioxide or phenazine 5,10-dioxide with TCNE afforded two-component solids containing weakly bound 1-D donor-acceptor arrays <1997TL7665>. A pyrazine ladder polymer was constructed from two different pyrazine units, as an optical device <1999JA8783>. The new electron-deficient macrocycle tetrakis-2,3-[5,6-di(2-pyridyl)pyrazino]porphyrazine was prepared from l,2-di(2-pyridyl)ethanedione and 2,3-diaminomaleonitrile for a study of its electrochemical properties <2004IC8626>. 

Polyrotaxanes have been prepared by threading multiple a-cyclodextrin units onto a bulky benzimidazole-based linear chain polymer with an aliphatic spacer where the cyclodextrin resides. The rotaxane forms as the cyclodextrin-bound precursor amine is polymerised. Compared to the polymer in the absence of the cyclodextrins, the glass transition temperature is raised by some 20 °C even though only ca. 16 % of the aliphatic spacers in the polymer are rotaxanated.28 A novel approach to polyrotaxanes involved the use of a metal ion such as Zn2+ to thread a phenanthroline-based macrocycle onto a thiophene-based complementary monomer. The resulting pseudorotaxane can then be electropolymerised and the Zn2+ ions removed to give a polyrotaxane, Scheme 14.5. The redox and conductivity properties of the polymer are very much dependent on whether a metal ion is bound or not.29... 

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