Oligomeric Schiff bases

Ni(II) salts give rise to trimeric 31 and tetrameric 32 macrocycles (Fig. 16B).45 111 116 There was some speculation about the mechanism of the condensation, although when the reaction was run with cyclic intermediate 33114 there was a predominant formation of trimeric 31. Apparently, a rapid interconversion of oligomeric Schiff bases would in the presence of Ni(II) cations become directed to form thermodynamically stable and metallated 31 and 32. Since these pioneering discoveries, there have been many reports about using templating molecules for directing the outcome of reversible covalent processes.42,44,66... 

Figure 3 in Scheme 2.3-2 illustrates that Ni- or Pd-complexes prefer a different combination of elementary steps. Here, it is evident that Ni favors 2 1 co- oligomerization of butadiene with tddehyde or of a Schiff base with butadiene involving C -bond formation coupled with metalalogous 1,5-hydrogen transfer. On the other hand, Pd favors 0—C- or N—C- andC—C-bond formation. These processes seem to occur more frequently, as demonstrated by other catalytic processes and model reactions . ... 

Complexes of the acyclic Schiff base ligand are assumed to oligomerize due to coordinative unsaturation [189]. A dinuclear composition was proven for Ln(III) complexes of trivalent oligosilsesquisiloxane ligands of type T7(OH)3 (Fig. 28). Surprisingly, Si-O-Si frame sites can support additional coordination sites [190],... 

Material science can also benefit from oligomeric state of proteins. New retinal nano-ceramic materials with pillared hybrid micro-structures were fabricated for potential applications in optical holographic data storage. It was observed that the Schiff bases in retinal have substantial effect on optical properties of nano-ceramic films as well as diffraction efficiency for holographic storage. This study indicates feasibility of optimizing optical properties of nano-ceramic clay systems using Schiff bases for a variety of photonic applications.40... 

Various modern products may serve as sources for trace amounts of PAA. The origins, known hazards, release restrictions and environmental fate of PAA derived from azo dyes have been reviewed2. The metabolism of nitroarenes is closely linked to that of aromatic amines, as shown in a simplified way in equation 369. Gloeophyllum trabeum cultures spiked with TNT show formation of nitroaromatic amines such as 2-ami no-4,6-dinitrotoluene, 4-amino-2,6-dinitrotoluene and 2,4-diamino-6-nitrotoluene. Also, autoxi-dation of the methyl group of TNT or its metabolites may take part in the degradation process, as shown by the presence of Schiff bases and oligomeric and polymeric species detection and determination of the analytes after LLE and concentration were by HPLC-UVD or GC-MS70. 

A wide range of Schiff base macrocycles has now evolved from these early studies. Many involve the use of 2,6-diacetylpyridine (PDA) or 2,6-diformylpyridine (PDF) as building blocks and it is possible to find an oligomeric series of macrocycles based on the... 

Vincens, V. Le Borgne, A. Spassky, N. Aluminum complex of a Schiff base as new initiator for oligomerization of heterocycles. Makmmol. Chem., Macmmol. Symp. 1991,47, 285-291. 

The Schiff bases obtained by reaction of 4-methyl-2,6-diformylphenol with l- or D-phenylglycinol 52 (Fig. 16) were foimd to interact enantioselectively with the two enantiomers of mandelic acid on account of the appearance of a fluorescent component with a longer lifetime. A job plot of the complex formation, however, revealed a 2 3 guest host stoichiometry of the complex formed, thus suggesting an oligomeric form of the type reported in Fig. 16b. The good enantioselectivity observed (Ks/K = 4 for the (R,R)-sensor) also allowed these authors to calibrate the fluorescence response vs the enantiomeric compositions of mandelic acid in benzene solutions containing 10 " M of the sensor [81]. 

Fig. 16 Sensors used in enantioselective aggregation induced enhancement (a) Phenylglycinol-derived Schiff base 52 (b) proposed mechanism of oligomeric interaction of 52 with mandelic acid enantiomers (c) chiral derivatives of 2-phenyl-3-(p-ammophenyl)acrylonitrile 53,54 showing an AIEE effect in the presence of bases and acids respectively (d) fluorescence response as a function of enantiomeric excess of dibenzoyl tartrate observed using the amine 54 (adapted from [113]. Copyright (2010) with permission from Elsevier) (e) AIEE sensor 55, based on BINOL...

Le Borgne, A. Vincens, V Jouglard, M. Spassky, N. Ring-opening oligomerization reactions using aluminum complexes of Schiff s bases as initiators. Makromol. Chem., Macromol. Symp. 1993, 73, 37 6. 

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