Phenol dendron

CpFe+ -induced Activation of Ethoxytoiuene in the One-Pot Synthesis of a Phenol Dendron... 

Functionalization of the three allyl chains of the phenol dendron could be achieved by hydrosilylation, as catalyzed by the Karsted catalyst [70]. Indeed, it is very interesting that there is no need to protect the phenol group before performing these reactions. For instance, catalyzed hydrosilylation using ferrocenyldimethylsilane gives a high yield of the triferro-... 

Reasoning in this way turned out to be correct the cleavage of the arene intervenes rapidly at the 19-electron stage, because 19-electron complexes of this kind are not stable with a heteroatom located in the exocyclic position (most probably because the heteroatom coordinates to the metal from the labile 19-electron structure). After optimizing the reaction conditions, a 60%-yield of free phenol dendron from the ethoxytoluene complex could be obtained, and this reaction is now currently used in our laboratory to synthesize this very useful dendron as a starting material (Scheme 11.8). 

The supramolecular assembly of dendrimers via H-bonding has been accomplished by Astruc, Boisselier, and Ornelas in which commercial dendritic poly(propylenimine) cores are rapidly and reversibly H-bonded to triallyl-or tris-amidoferrocenyl phenol dendrons these novel supramolecular assemblies were used for the electrochemical recognition of H2P04 and adenosine-triphosphate anions. ... 

It was rationalized that one could gain control of the disassembly rate by changing the substituent R on the aromatic ring. Electron-withdrawing substituents should accelerate the cyclization step since phenol 7 will become a better leaving group in the acyl substitution of 6 to 7. Four dendrons were selected for this study (Fig. 5.5) two (9 and 11) with a methyl substituent and two others... 

The disassembly mechanism of dendron 27 is illustrated in Fig. 5.22. Cleavage of the trigger initiates the cyclization of a dimethylurea derivative to release phenol 28. The latter can undergo 1,8-elimination followed by decarboxylation to release one reporter unit and to generate quinone methide 29. In the next step, a nucleophile (most likely a solvent molecule) presumably reacts with the highly electrophilic quinone methide to generate the phenol 30. Similarly, we hypothesize that one more 1,8-elimination and four 1,6-eliminations take place to lead to the release of all six reporters. 

We also wanted to evaluate the disassembly of our dendritic system under physiological conditions. Thus, we synthesized a self-immolative AB6 dendron 32 with water-soluble tryptophan tail units and a phenylacetamide head as a trigger (Fig. 5.26) to evaluate disassembly in aqueous conditions. The phenylacetamide is selectively cleaved by the bacterial enzyme penicillin G amidase (PGA). The trigger was designed to disassemble through azaquinone methide rearrangement and cyclic dimethylurea elimination to release a phenol intermediate that will undergo six quinone methide elimination reactions to release the tryptophan tail units. 

As part of a program directed toward the synthesis of dendrimeric structures containing the 1,2,4-triazole moiety, 3,5-dichloro-4(4-methoxyphenyl)-4/7-l,2,4-triazole 69 was reacted with phenol 77 under basic conditions to give the dendron 78 in a yield of 80% (Equation 28) <2006T2677>. 

For the synthesis of perfectly dendronized sohd-phase polymers (Fig. 7.4) various dendritic structures were prepared based on amide connections [6]. For example, the naturally occurring amino acid lysine was used as a building block in creating a dendritic scaffold [33]. The synthesis of symmetrical tri-branching den-drimers on aminomethyl polystyrene macrobeads was also described in literature [34]. Recently, aryl ether dendrimers were prepared on hydroxymethyl polystyrene using a Mitsunobu reaction with 3,5-bis(acetoxymethyl)phenol [35]. 

The construction of the oiganometallic dendron required two synthetic transformations. Selective alkylations of the phenolic hydroxyl groups in the presence of potassium carbonate and I8-crown-6 afforded the ether dimetallic derivative 35. This first generation benzyl alcohol 35 was converted to the benzyl bromide 36 by... 

Between a Redox-active Dendronic Phenol and Dendritic Primary Amines also Recognize Oxo-Anions with Dendritic Effects. 128... 

Path B in Fig. 2 is the convergent method. It is the outside-inward method, proposed independently by Miller and Neenan [9] and by Hawker and Frechet [10]. This method is well suited when the branch point is an aromatic ring. As an example of the convergent process we show in Scheme 3 the preparation of poly(benzyl ether) dendrimers. The phenol functionality of 2,5-dihydroxyben-zyl alcohol is first protected by Williamson reaction with benzyl bromide to provide the first generation dendron [G-l]-OH. The benzyl alcohol in [G-l]-OH is then converted to the benzyl bromide form [G-1]-Br. This in turn reacts with... 

After debenzylation (Pd-C, cyclohexene) of the hypercores (e.g., 25) by transfer hydrogenation, they were treated with aryl branched, benzyl ether dendrons (Scheme 5.6) that were prepared by similar iterative transformations,1271 i.e., benzylic bromination and phenolic O-alkylation (See Section 5.4.2). Thus, hexaphenol core 27 was reacted with six equivalents of the benzylic bromide building block 28 to give the benzyloxy terminated dendrimer 29. Key features of these dendrimers include cores with flexible alkyl spacers and a three-directional, quaternary carbon branching center. 

Wooley et al.1511 have described the creation of fullerene-bound dendrimers (Scheme 5.14). Reaction of C60 with bis(p-methoxyphenyl)diazomethane 54 and subsequent cleavage of the methyl ethers afforded a 6-6 methano-bridged fullerene (55) possessing two phenolic moieties, as the major product.1521 Treatment of the bisphenolic fullerene 55 with 2.7 equivalents of the activated dendron (52) afforded the desired substituted fullerene (56) possessing two dendritic arms. 

Ester-based cascades (e.g., 107) have been prepared[77 80i by using 5-(tert-butyldime-thylsiloxy)isophthaloyl dichloride (108), which was synthesized in high yield from 5-hydroxy-isophthalic acid (Scheme 5.26). The dendron wedges were prepared by treatment of siloxane 108 with phenol to give bis(aryl ester) 109, which was hydrolyzed, or desilylated (HC1, acetone), to generate a new phenolic terminus. Treatment of this free phenolic moiety with monomer 108, followed by hydrolysis, afforded the next tier (110). Repetition of the sequence followed by reaction of the free focal phenols with a triacyl chloride core, (e.g., 86), afforded the fourth tier dendrimer 107 of the polyester aryl series. It was noted that the choice of base (N, A-dimethylaniline) used in the final esterification was critical, since with pyridine bases (pyridine or 4-(dimethylamino)pyridine) facile transesterification resulting in branch fragmentation occurred. 

Convergent construction (Scheme 7.14) began with the treatment of (25, 35)-(-)-l,4-di-O-tosyl-2,3-0-isopropylidene-L-threitol with one half equivalent of 4-fert-butylphenol under basic conditions to yield the mono-O-arylation product (62). Subsequent reaction of the chiral aryl ether 62 with one-half equivalent of dihydroxybenzene 42 63, followed by hydrogenolysis with palladium on charcoal gave the dendron 64. Attempted preparation of the disubstituted phenol 64 via reaction of the tosylate 62 with one third equivalent core 61 resulted in low yields (ca. 26 %). Poor solubility of the tris-phenolic trianion was suggested as a rationale for the low conversion. 

Scheme 23. One-pot syntheses of the pheno l-tria I lyl iron complex and metal-free dendron by variation of the experimental conditions. The iron complex can be demetalated by visible-light photolysis the metal-free phenol-triallyl dendron can be obtained more rapidly direct from the p-ethoxytoluene-iron complex.

---