Bifunctionalisation

Fig. 3.8 Convergent synthesis of a bifunctionalised dendrimer with two different peripheral groups in separate molecular segments (surface-block dendrimers)...

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). 

The preparation of peripherally bifunctionalised dendrimers by the method of non-covalenl local bifunctionalisation has hitherto remained restricted to individual cases. This method of bifunctionalisation can be applied to peripherally... 

Fig. 3.12 Synthesis of a locally bifunctionalised dendrimer (according to Shinkai et al.)...

Fig. 3.13 Method of local bifunctionalisation of oligo-/polyamine dendrimers...

Whereas the bifunctionalisation strategies presented so far are relatively straightforward, the synthesis of multifunctional dendrimers with more than two kinds of functional units requires considerable synthetic effort. Preparation of dendrimers with a functional core and additional functional units in the dendrimer scaffold and in the periphery requires de novo synthesis of the entire dendrimer scaffold, with the synthesis conditions having to be tolerable for all groups (Fig. 3.15). 

The above sections demonstrate that the dendrimer chemist meanwhile has recourse to an extensive range of possibilities for introducing specific functional units in one or more selected regions of the molecule (centre, periphery, interior branching scaffold) in order to customise dendrimers or dendrons for projected applications. It should, however, be borne in mind that the synthetic strategies for introduction of two or more different functional units (bifunctionalisation, multifunctionalisation) demand considerable synthetic effort as well as time and expense and their use is limited to certain dendrimers and dendrons, because special conditions have to be fulfilled (e.g. special end groups such as primary amino functions). 

Bifunctionalised dendrimer with perfectly alternating molecular periphery. 

Catalytic crossed olefin metathesis can also be used to stereoselectively mono-or bifunctionalise polyolefin dendrimers [6]. Grubbs et al. were already able to... 

An exciting recent development has been the synthesis and preliminary investigation of the first members of a new generation of poly(pyrazolyl)borate ligand that bear a hydrocarbyl chain in place of the hydride, and are also bifunctionalised with a phosphine donor. The first such ligand, [H2C = CHCH2B (CH2PPh2)(pz)2] (herein abbreviated (u-allyl)(P)Bp) was prepared (Scheme 77) as... 

The contribution from Iwao Tabushi (Kyoto) was based on his work with chemically-modified cyclodextrins as enzyme models. The asymmetrically bifunctionalised cyclodextrins (3a,3b) were synthesised in order to mimic the aminotransferase activity of Vitamin B5. The A-B regio-isomer (3a) was used to effect the transformation of keto-acids into L-amino acids with 96% ee. The elegance of the system was demonstrated by the fact that the B-A regioisomer (3b) performs the same reaction on keto-acids to give the corresponding D-amino acids acids with identical enantiomeric excesses. 

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