Schiff Base Polymers

Eujii Y, Matsutani K, Kikuchi K. Formation of a specific coordination cavity for a chiral amino-acid by template synthesis of a polymer Schiff-base cobalt(III) complex. Chem Commun 1985 415-417. 

The ability of monomeric Schiff base complexes or polymeric Schiff base ligands to absorb solvent molecules is well-known. Sawodny et al.27) found that the polymer Schiff base complexes exhibit this property much more markedly. The polymers were prepared by the polycondensation reaction of polymer ligands of the Schiff base (29) with metal salts (Eq. 8). The adsorption of solvent is reversible and the ability is dependent upon the nature of the central metal atom and of the ring size around the metal. This profile suggests that the above polymers can be used as a molecular sieve. 

Polymer Schiff Base ligands containing a nitrogen atom in the bridge are easily synthesized by N-alkylation from various types of linear or crosslined ddoromethylated polystyrenes and low molecular N302-ligands Afterwards Ck>(n) was introduced to obtain the chelates with 2-4% Co (Eq. 25) and 60) is isolated. 

The preparation of polymer bound Ni-chelates (54) of the acacen type started from chloromethylated polystyrene and pentane-2,4-dione leading with catalytic amounts of sodium ethoxide to the polymer diketone (53) (Eq. 21)m Seven % of pendent diketone groups of soluble linear (55) were converted with 1,2-diaminoethane and Ni(II) to the polymer Schiff base cbelate (54). In the reaction of (55) to (54) two pendent diketones must react. But surprisingly no network formation was reported. 

Fujii, Y. Kikuchi, K. Matsutani, K. Ota, K. Adachi, M. Syoji, M. Haneishi, I. Kuwana, Y. Template synthesis of polymer schiff base Cobalt (III) complex and formation of specific cavity for chiral amino acid. Chem. Lett. 1984,13, 1487-1490. 

Reactions with Ammonia and Amines. Acetaldehyde readily adds ammonia to form acetaldehyde—ammonia. Diethyl amine [109-87-7] is obtained when acetaldehyde is added to a saturated aqueous or alcohoHc solution of ammonia and the mixture is heated to 50—75°C in the presence of a nickel catalyst and hydrogen at 1.2 MPa (12 atm). Pyridine [110-86-1] and pyridine derivatives are made from paraldehyde and aqueous ammonia in the presence of a catalyst at elevated temperatures (62) acetaldehyde may also be used but the yields of pyridine are generally lower than when paraldehyde is the starting material. The vapor-phase reaction of formaldehyde, acetaldehyde, and ammonia at 360°C over oxide catalyst was studied a 49% yield of pyridine and picolines was obtained using an activated siHca—alumina catalyst (63). Brown polymers result when acetaldehyde reacts with ammonia or amines at a pH of 6—7 and temperature of 3—25°C (64). Primary amines and acetaldehyde condense to give Schiff bases CH2CH=NR. The Schiff base reverts to the starting materials in the presence of acids. 

Primary aromatic amines react with aldehydes to form Schiff bases. Schiff bases formed from the reaction of lower aUphatic aldehydes, such as formaldehyde and acetaldehyde, with primary aromatic amines are often unstable and polymerize readily. Aniline reacts with formaldehyde in aqueous acid solutions to yield mixtures of a crystalline trimer of the Schiff base, methylenedianilines, and polymers. Reaction of aniline hydrochloride and formaldehyde also yields polymeric products and under certain conditions, the predominant product is 4,4 -methylenedianiline [101 -77-9] (26), an important intermediate for 4,4 -methylenebis(phenyhsocyanate) [101-68-8], or MDI (see Amines, aromatic amines, l thylenedianiline). 

A mixture of 1.4 g (10 mmol) of 4-chlorobenzaldehyde and 0.71 g (5 mol %) of the chiral polymer E is stirred in 10 mL of dry toluene for 15 h, under a dry nitrogen atmosphere, to form the Schiff base. After cooling to 0lC, 15 mL (15 mmol) of 1 M diethyl/inc in hexane is added and the mixture is stirred for a further 24 h at O C. 1 N HC1 is then added dropwise at O C, and the chiral polymer is removed by filtration. The polymer is washed several times with 11,0 and Et,0. The aqueous layer is separated and extracted with Et20. The combined organic layer is dried over MgS04 and concentrated under reduced pressure. The crude product is purified by column chromatography (silica gel, CHC1,) yield 1.61 g (95 %) 99 % ee [a]2,0 —23.9 (r = 4.93, benzene). 

The interaction of PCSs with nucleophilic reagents was studied by us we took the reactions of hydrazine and phenylhydrazine with polyazines, poly(schiff base)s, and other polymers containing conjupted C=N bonds as an example40,41, U7,258. ... 

We have already pointed out that the reduction in conjugation efficiency in PCSs is followed by a short-wave shift of the CTC transfer band. This accounts for the fact that poly(schiff base)s and polyazines having conjugated sections separated by oxygen and sulfur atoms are characterized by a short-wave shift of the transfer band of CTC with all acceptors compared to the respective polymers having no interruption of the conjugated chain. This shift may reach 20-50 nm. 

One method of solving the kinetics dilemma is well known in coordination chemistry that is, start with a labile metal ion and render it inert during the course of the synthetic reaction. We have accomplished this in the case of zirconium(IV) by starting with tetrakis(salicylaldehydo)zirconium(IV), which is quite labile, and polymerization with 1,2,4,5-tetraaminobenzene in a Schiff-base condensation reaction in situ (6). The polymeric product contains a "double-headed" quadridentate ligand, which is much more inert to substitution. However, 1,2,4,5-tetraaminobenzene has become very expensive. Therefore, the synthesis of a zirconium polymer with 3,3, 4,4 -tetraaminobiphenyl (commercially 3,3 -diami nobenzidine) with zirconium salicylaldehyde, Zr(sal)4 (7) has been undertaken as shown below ... 

Other examples of this synthetic strategy are known for example, a recent zirconium polymer by Illingsworth and Burke (8), who joined amine side groups of a zirconium bis(quadridentate Schiff-base) with an acid dianhydride to give amide linkages. Once again, caution is necesary, as Jones and Power (2) learned when they attempted to link metal bisO-diketonates) with sulfur halides that is, they obtained insoluble metal sulfides because the p-diketone complexes which they used were fairly labile and the insolubility drove the reactions to completion in the wrong direction. 

Infrared and thermal studies also add to our knowledge of these polymers. For example, the aromatic C-0 stretch of the Schiff-base ligands is shifted above 1300 cm-1 when coordinated, and coordinated carboxylate shows no CO stretch above 1600 cm-1 when bidentate. The... 

A range of tetradentate Schiff-base ligands have also been employed to prepare discrete aluminum alkoxides. The most widely studied system is the unsubstituted parent system (256), which initiates the controlled ROP of rac-LA at 70 °C in toluene. The polymerization displays certain features characteristic of a living process (e.g., narrow Mw/M ), but is only well behaved to approximately 60-70% conversion thereafter transesterification causes the polydispersity to broaden.788 MALDI-TOF mass spectroscopy has been used to show that even at low conversions the polymer chains contain both even and odd numbers of lactic acid repeat units, implying that transesterification occurs in parallel with polymerization in this system.789... 

Proteins may be modified with oxidized dextran polymers under mild conditions using sodium cyanoborohydride as the reducing agent. The reaction proceeds primarily through e-amino groups of lysine located at the surface of the protein molecules. The optimal pH for the reductive amination reaction is an alkaline environment between pH 7 and 10. The rate of reaction is greatest at pH 8-9 (Kobayashi and Ichishima, 1991), reflecting the efficiency of Schiff base formation at this pH. 

Another example where PEG played the role of polymeric support, solvent, and PTC was presented by the group of Lamaty [72]. In this study, a Schiff base-proteded glycine was reacted with various electrophiles (RX) under microwave irradiation. No additional solvent was necessary to perform these reactions and the best results were obtained using cesium carbonate as an inorganic base (Scheme 7.64). After alkylation, the corresponding aminoesters were released from the polymer support by transesterification employing methanol in the presence of triethylamine. 

The Klotz group has also found rate enhancements of decarboxylation reactions with PEI derivatives. Catalysis of decarboxylation of j -keto acids by small amines goes via a Schiff base intermediate. Mine s group has shown that unmodified PEI catalyzes dedeuteration effectively and that the reactions involve Schiff base intermediates 34, and references therein). Dodecyl-PEI containing free amino groups and quaternized nitrogens, dodecyl-PEI-Q-NHj, was found to be an effective catalyst for the decomposition of oxaloacetate (reaction 12) (92). At pH 4.5 the polymer is 10 times as effective as ethylamine. was found to be 3.5 x 10 " M at pH 4.5. 

Other examples involved the use of chiral Schiff base-zinc complexes as catalysts [33 a] and polymer-supported chiral N-tritylaziridino alcohols as catalysts. The stereoselectivity was reported to be up to 97% ee for aUphatic and up to 96% ee for aromatic aldehydes [103]. 

The binding of pyridoxal 5 -phosphate (vitamin Be) to enzymes has been modelled using homo- and co-polypeptides containing L-lysine as a source of reactive amino groups. This has now been extended to reaction of pyridoxal with polyallylamine, with the polymer acting as a control that cannot provide amido -CO- or -NH- functions to stabilize the Schiff base products, as occurs in enzymes and polypeptides. Rate constants for the formation and hydrolysis of the imines have been measured and interpreted in terms of formation of the carbinolamine (in its neutral or zwitterionic form). 

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