G4-dendron

Repetition of the sequential divergent-convergent protocol with TMS-protected nonithiophene 5.80 led to boronic ester 5.82 (82 %) on the one hand and the tetraiodinated derivative 5.83 (98 %) on the other. Pd-catalyzed cross-coupling of the latter molecule with 4 equivalents of boronic ester 5.82 directly resulted in G4-dendron 5.84 (45T-Si) in one step (80%, Scheme 1.74). 

Figure36 (a) Fourth generation [G4] dendron, based on 3,5-dihydroxy benzyl alcohol core, (b) Isotherms of [G = 4] and [G = 5] dendrons showing differences due to compression rate and pause time before compression. (Courtesy of J. Phys. Chem. 1993,97,294. Copyright 1993 American Chemical Society.)... 

Miller and co-workers have recently prepared nanometer-scaled molecular dumbbells based on poly(benzyl ether) dendrons (G1-G4) and oligoimide spacers [64], Their synthetic approach involved the coupling of amine-terminated oligoimides to dendrons with a carboxylic acid focal point. The resulting hybrid materials were found to be quite soluble thus allowing their analysis by cyclic voltammetry in DMF. Consistent with Roncali s observation, the kinetics of reduction of the oligoimide core was not found to be limited by the presence of the dendritic wedges. 

Fig. 1. Chemical structure of a dendrimer of fourth generation (G4). Starting with a focal group in the center of the molecule, a fractal-like structure is built up by branches which emanate from three-functional groups. The outer ends of the branches are terminated by end-groups [3]. The molecule may be divided into dendrons designating the substructures originating from a branch point. Hence, the dendrimer shown here may be viewed upon as composed of four dendrons emanating from a central ethylenediamine group...

On use as homogeneous catalysts in the asymmetric reductive alkylation of benzaldehyde with diethylzinc to form secondary alcohols, the corresponding dendritic titanium-TADDOL complexes having either chiral or achiral dendrons gave enantiomeric excesses (ee) of up to 98.5 1.5 at a conversion of 98.7% (for the catalyst with GO dendrons). With larger dendrons the reduction of the ee to 94.5 5.5 (G4) remained within reasonable limits, while the drop in conversion to 46.8% (G4) proved to be drastic. In comparison, the unsubstituted Ti-TAD-DOL complex gave an ee of 99 1 with complete conversion. This negative den-... 

Measurements of molecular density and intrinsic viscosity of PAMAM dendrimers indicate an unusual variation with dendrimer generation. Minimum density and maximum intrinsic viscosity were observed at around G4, which suggests that the fully developed dendrimers have a high accessible internal surface area in a solvent-filled intramolecular free volume that may consist of internal cavities and channels. Similar findings were reported by Mourey et al. for PBE monodendrons (based on dihydroxybenzyl alcohol) and tridendrons produced by coupling these dendrons to a trifunctional core, l,l,l-tris(4 -hydro-xyphenyl)ethane prepared by the convergent method. A maximum intrinsic viscosity occurred at G3 for tridendrons and G5 for monodendrons, consistent with the model developed by Lescanec and Muthukumar... 

Scheme 37.2 Step-wise (attach-to or divergent) synthesis of a polymer G4 starting from poly(t-lysine), and subjecting it to a series of dendronization and deprotection steps.

But, with thickness being so important in so many aspects of this research, has the true potential of dendronized polymers, at the G3 and G4 levels of thickening. 

G2, and G4) was observed [72]. From time-resolved studies on energy and molecular volume changes, it was found that the conformational change completes with the decay of the Ti state for G0-G2. The dynamics was slightly slower for G4, which is attributed to the conformational change of the dendron part. 

Advincula et al. decorated conventional dendrimers with conjugated dendrons at the periphery [499]. Formylated terthiophene dendron 5.54, which was prepared by Vilsmeyer-Haack formylation of dendron 5.73a (see below), was linked to the periphery of a 64-amine terminated G4-PAMAM dendrimer by reductive amination. The incorporation of Pd or Au nanoparticles in the dendrimer created new hybrid nanomaterials 5.55 (Scheme 1.67). As a consequence of the dendritic architecture, the emission properties... 

Some features of the dendronized polymers obtained from the macromonomers of Table 1 will become important later in this article and are therefore emphasized in the following (1) Most of the macromonomers carry G1 or G2 dendrons. Exceptions are only (12) (G3), (16) (G3), (18) (G3 and G4), (23) (G4), (25) (G3), and (27) (G3 and G4) (2) Many dendrons are of the benzyl ether type whose terminal phenyl rings are either unsubstituted or have one to three long alkoxy or fluoroalkoxy chains. The corresponding polymers are important for a variety of reasons but are unreasonable candidates, of course, whenever chemical modification becomes an issue (3) This is where dendrons (7), (8), (10), (11), and potentially also (13) come into play which carry protected... 

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