Functionalisation terminal group

The convergent synthesis strategy proceeds in the opposite direction, from the periphery to the core, that is from the outside inwards. (Functionalised) dendri-mer components ( dendrons ) are bonded to the reactive terminal groups linked to a focal point of a multifunctional core unit. The CFP symbolism is again used to illustrate the principle of the synthesis (Fig. 2.2). 

Various strategies for the synthesis of peripherally functionalised dendrimers are presented and evaluated in the following sections. The expression peripheral group is used here as a neutral term. It can refer both to a peripheral function-bearing unit and to a peripheral functionality. Terminal groups linked covalently to the periphery can also be - partly - backfolded (see Sections 1.2 and 7.63). 

However, these bifunctionalisation methods are comparatively laborious and applicable only in special cases, since the monofunctionalisation step is limited to substrates possessing an additional coupling site in protected form for the second functional unit. A more versatile method of local bifunctionalisation, which has no need of a deprotection step and also utilises commercially available dendrimer scaffolds, consists in the functionalisation of POPAM dendrimers bearing amine terminal groups with sulphonyl chlorides and subsequent substitution of the sulphonamide proton with other sulphonyl chlorides [63] or with alkyl- or (dendritic) benzyl bromides [64] (see Fig. 3.13). 

They functionalised up to second-generation polyether amide dendrimers (ar-borols see Chapter 1) with enantiomerically pure tryptophan units (Fig. 4.74). The measured optical activities per chiral terminal group were roughly constant for all molecules of this dendrimer series. 

Fig. 6.10 Glycinylurea guest molecules (green) dock onto the urea-functionalised, niche-like periphery (red) of a POPAM dendrimer (shown schematically as grey sphere, with 32 terminal groups) owing to tailor-made supramolecular interactions (red bonds) (according to Meijer el al.)...

Through van der Waals and hydrophobic interactions, CNTs were functionalised and made water soluble by the strong adsorption of phospholipids (PLs) grafted onto amino-terminated polyethylene glycol (PEG). The group of Dai bound nucleic acids (DNA and RNA) and proteins to CNTs for specific detection of antibodies (Chen et al., 2003 Kam et al., 2005a, b Liu et al., 2007b). 

The retrosynthesis involves the following transformations i) isomerisation of the endocyclic doble bond to the exo position ii) substitution of the terminal methylene group by a more stable carbonyl group (retro-Wittig reaction) iii) nucleophilic retro-Michael addition iv) reductive allylic rearrangement v) dealkylation of tertiary alcohol vi) homolytic cleavage and functionalisation vii) dehydroiodination viii) conversion of ethynyl ketone to carboxylic acid derivative ix) homolytic cleavage and functionalisation x) 3-bromo-debutylation xi) conversion of vinyl trimethylstannane to methyl 2-oxocyclopentanecarboxylate (67). 

Modification and functionalisation of natural polyenes, the carotenoids, is an efficient way for the molecular engineering of polyenic chains. Terminal bis-pyridinium carotenoids, termed caroviologens, represent an approach to electron conducting molecular wires (5). Fitting polyconjugated chains with an electron donor group on one end and an electron acceptor on the other end yields push-pull systems of type 1 that may be considered as polarized, unidirectional (oriented) molecular wires and also possess marked NLO properties. 

In 1993 Meijer et al. and Miilhaupt et al. almost simultaneously accomplished the preparative synthesis of higher generation monodisperse POPAM dendrimers on the basis of this principle [6]. Thanks to their terminal amino groups, POPAM dendrimers can readily be per-functionalised. Thus Balzani, De Cola,... 

Diaminobutyl dendrimers (DAB-POPAM) were functionalised with terminal diphenylphosphanyl groups and employed as catalysts in the Heck coupling of bromobenzene and styrene to give stilbene. Owing to their greater thermal stability, these dendritic palladium catalysts afforded higher yields than conventional palladium catalysts. In addition, the dendritic catalyst could be completely recovered by precipitation after addition of diethyl ether [7]. 

Oligolysine dendrimers with terminal carbohydrate structures were functionalised by solid-phase synthesis for well-defined construction of glycoclusters . Synthesis proceeded via coupling of lysine molecules. Eight amino groups are attached by peptide bonding to the periphery via a spacer-functionalised sialic acid derivative (Fig. 8.8). 

Functionalisation of the polymer backbone through reaction of the hydroxyl groups (esterification and etherification), nucleophilic displacement reactions, oxidation and selective reactions at the terminal reducing ends... 

Amino-terminated telechelic polybutadiene was prepared by LiAlH4 reduction of amidino end-group in polybutadiene, which was polymerised by a water-soluble initiator, 2,2 -azobis(amidinopropane)dihydrochloride. The structure was analysed by 1H- and 13C-NMR, but functionality of 2.0 was obtained by a titration method [70]. Synthesis of co-epoxy-functionalised polyisoprene was carried out by the reaction of 2-bromoethyloxirane with living polymer that was initiated with sec-butyl lithium. The functionality of the resulting polyisoprene was 1.04 by 1H-NMR and 1.00 by thin layer chromatography detected with flame ionisation detection [71]. 

Functionalisation of the complexes with reactive groups a number of metal complexes have been modified with (1) an NHS ester, isothiocyanate, and aldehyde, which can react readily with amines of lysine and the A-terminal of proteins and (2) iodoacetamide and maleimide that can react with sulfhydryls of the cysteine residue [3-5, 10]. These facile reactions lead to covalent attachment of luminescent complexes to the target proteins and amine-/sulfhydryl-modified oligonucleotides. 

When a C2 spacing group is required to be C-functionalised, the most obvious precursor is an a-amino acid derivative. Such a strategy has been developed for the synthesis of the [9]-N3 and [12]-N4 derivatives 8,9 and 10. Both 8 and 9 were synthesised from (25)-lysine16 while 10 has been prepared from the enantiopure p-nitrophenylalanine.17 In the synthesis of the [9]-N3 derivative, a key step (Scheme 1.11) involved protection of the diethylenetriamine moiety by copper(II) prior to selective acylation of the terminal primary amine and subsequent Richman-Atkins8 cyclisation. Detosylation was effected with concentrated sulfuric acid, leaving the benzamide intact. 

The longer bonds occur at the end of the side chains (245 pm) and between the atoms of the central one dimensional backbone (249 pm). The "functionalisation" of the silicon atoms is not only reflected but seemingly initiated by the distribution of the Mg and Sr atoms. Clearly the latter coordinate exclusively the Si atoms of the central backbone of the polyanion. Those silicon atoms which are in the bridging side chain have a 4-1-3 coordination of Sr and Mg atoms while the terminal (lb)Si groups have an outer coordination of four Mg atoms. The whole one dimensional polyanion looks like a blossomed CaSi chain" with its syndiotactic side chains. 

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