Linker units

Star shaped macromolecules are polymers, where the one end of f > 2 (f functionality of the star) linear chains is chemically attached by covalent bonds to a small central linker unit, are the simplest form of branched polymers. Modern anionic polymerization techniques allow us to synthesize star systems with a large number of nearly monodisperse arms [133, 134],... 

Lactams were obtained in 80-95% conversion with high trans-selectivity (de >95%) but only moderate enantioselectivity ee ca. 30%). The stereoinduction results from the relatively remote stereocentres located at the linker units. For support recovery the authors performed nanofiltration experiments and it was found that the recovered dendrimers could be easily reused as substrate carriers. 

Given these requirements, it is obvious that the P-tetralin based N- and C-caps could not be used as position-independent templates. While the P-tetralin amino acid and the dipeptide units are compatible with a position-independent template design, the biphenyl ether linker unit clearly was not. In N- and C-caps, the linker unit forms a side chain (P-tetralin) to backbone constraint (either to the C-terminal carboxylate or the N-terminal amine), and only one attachment site for a peptide... 

The three different linker units used in N-caps, C-caps, and position-independent templates are shown in Figure 35. Figure 36 shows the structure of our first position-independent a-helix-inducing template 105 (HIT-1). In order to induce the a-helical conformation in peptides in a position-independent way, the template has to be able to adopt an a-helical conformation. This does not necessarily mean that the template is a-helical on its own. NMR measurements have revealed that our N-caps and the proline-based templates proposed by Kemp do not preferentially adopt the nucleating a-helical conformation unless they are linked to peptides.206-210... 

Figure 35. Linker units in N-caps, C-caps, position-independent templates.

By combining two bis-amido-3,4-dichloropyrroles into one molecule, bridged by an inflexible linker unit a new class of interlocking anionic polymer has been produced Figure ll).n Compounds 12 and 13 were synthesized and exhibited limited solubility in a variety of solvents although addition of tetrabutylammonium fluoride resulted in an increase solubility to a point where full characterization could be determined. 

These double rosettes can be easily extended to tetrarosettes by connecting two calix[4]arene dimelamines by a flexible linker unit X (Figure 4.3).11,12 A tetrarosette assembly consists of fifteen components, three tetramelamines 2 and twelve barbiturate/cyanurate derivatives held together by 72 hydrogen bonds and it has a size of 3.0 x 3.1 x 2.7 nm. The thermodynamic equilibrium for the assembly 23 (DEB)i2 is reached within seconds after mixing the two different components. Similarly, hexarosettes 33 (DEB)ig are prepared... 

In principle there are two positions in the ligand structure where appropriate linker units can be attached, either on one of the arene units or... 

High-throughput screening of the company sample collection in an AChE inhibition assay resulted in identification of a novel structural class exemplified by lead compound 5. Subsequent SAR studies were performed by a series of modification to the (a) phenyl group, (b) amide moiety, (c) linker unit, (d) piperidine ring, and (e) benzyl group. Para-... 

Advantage Transition metal complexes with chelating bis-carbenes featuring acidic methylene protons in the linker unit can be readily prepared, when the isolation of the free carbene using strong bases would be far more difficult. 

Catalytical experiments have not been reported with this ligand so far and we are left with experimental evidence for ligands that are unmodified in the linker unit. Powell et al. 

Another interesting class of bis-carbenes is derived from Trofimenko s bis-pyrazolato-borate ligands [338-340] and contains a BH -linker unit instead of methylene between the... 

The behaviour of this second ligand is absolutely analogous to the earlier one (equipped with a methylene linker unit). Deprotonation of the imidazolium salt is followed by... 

A third structural motif for silver(l) pincer carbene complexes is the dimer observed by Nielsen et al. [490] (see Figure 3.169). This dimeric structure becomes possible because of the flexibility introduced into the pincer structure by the linker unit, which acts as a hinge. It makes it possible for the NHC donor ligands to align themselves parallel to each other and perpendicular to the C-Ag-C vector of the silver coordination sphere. The dimer is considered to possess no argentophilic interactions as the Ag-Ag distance is 31 pm longer than the sum of the van der Waal s radii (340 pm) [20]. 

Figure 3.167 Transition metal pincer carbene complexes of Ag(l), Pd(ll) and Rh(l) featuring a pincer carbene ligand with a methylene linker unit.

The flexibility of the pincer carbene ligand system can be seen from the crystal structures of their palladium complexes. The ligand without linker units features the central pyridine ring and the flanking NHC units strictly coplanar [431] whereas the linker unit introduces twisting of these three ring systems [490]. The flanking NHC units are now... 

The method of preparation is seemingly irrelevant as demonstrated by Danopoulos et al. [504]. They used three different routes to synthesise the same C,C,C palladium(II) pincer carbene complexes and showed in the process that all three methods can be applied to the linker unit free ligand as well (see Figure 3.174). 

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