Generation zero dendrimers

In order to functionalize the periphery of dendrimers of the type 69, van Koten and coworkers developed a strategy to cleanly obtain 4-lithioaryl substituted carbosi-lane dendrimers of the dendrimer generations zero (n = 4, 70a) and one (n = 12, 70b) (Scheme 24). The bromide-lithium exchange using n-butyllithium was successfully applied and the quantitative lithiation confirmed by hydrolysis or by conversion into functionalized dendrimers by reaction with various electrophiles. 

By the same method, the authors were able to synthesize dendrimers of the generations zero and one, bearing the para lithiated pincer ligands in their periphery. Compounds containing four and twelve lithium centres could be obtained cleanly and in almost quantitative yield (as an example, dodecalithiated carbosilane dendrimer 36 is shown in Figure 12). 

An early example of a dendritie eatalyst was reported by Knapen et al. 24), who functionalized GO (generation zero) and G1 earbosilane dendrimers with up to 12 NCN pincer-nickel(II) groups (7a). These dendrimers were applied as eatalysts in the Kharasch addition of organic halides to alkenes (Seheme 3). 

To a stirred 0.3 mM dispersion of PS-fi e rfr-(NH2) in a 0.01 M KCl solution, a 0.3 mM amphiphile solution in toluene was added dropwise. By measuring the conductivity of the system as a function of the ratio toluene-water, it could be estimated whether toluene or water was the continuous phase. At the point where the conductivity dropped to zero, the phase inversion point was reached and toluene became dispersing phase. The effect of dendrimer generation on the position of this inversion point was investigated with PS-c enrfr-(NH2) with n — 2-16. VS-dendr-(NH2)32 could not be measured in the same manner, because this product proved to be insoluble in toluene. The conductivity measurements show a distinct difference between PS-denc r-(NH2)i6 and the lower generations. For PS-cfendr-(NH2) with n — 2-8 there is a strong tendency to stabilize toluene as a continuous phase. PS-dendr-(NE.2)2 even showed a remarkable phase inversion at 2 vol% of toluene. This can be explained by the fact that polystyrene is the dominant part in the amphiphilic... 

New apphcations can be expected in the area of dendrimer-immobilized liquid niembranes. By using polyvmyhdene fluoride membrane films containing a generation zero PAMAM dendrimer, highly selective CO2 separation from gas mixtures is possible [26]. Dendrimers consisting of a cationic core shell and an apolar... 

In a subsequent paper (87), the same authors investigated the palladium-catalyzed allylic aminations with pyrphos-functionlized PPI and PAMAM dendrimers as multidentate ligands. Zero to four generation PPI-(pyrphosPdCl2)x and PAMAM-(pyrphosPdCl2)x neutral dendrimers (97) showed a strong positive dendritic effect on the selectivity of the allylic amination of 1,3-diphenyl-1-acetoxypropene with morpholine (at 45°C in DMSO). 

In contrast, the rotation per chiral end group of first- to fifth-generation POPAM dendrimers with sterically demanding Boc-protected amino acids as end groups (Fig. 4.75) decreased significantly with increasing generation number and approached the value zero [30]. 

Fig. 5. Kratky plot for SANS data taken from solutions of starburst polyamidoamine dendrimers with seven generations in deuterated water (extrapolated to zero concentration) and comparison with the gaussian blocks dendrimer model with b = 0.48 nm, n = 10, N = 7, Nb = 3 and f = 2. Salt was added to the solution in order to screen out the coulomb interactions...

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