Perfect Dendrimers as Supports in Organic Synthesis

In 1996, Kim and coworkers reported for the first time on the use of a polyami-doamine (PAMAM) dendrimer [Gl] as a soluble support for organic synthesis (Fig. 7.5) [37]. Advantages of PAMAM are its commercial availability and its high symmetry, which provides uniform site accessibility (in lower generations) and facilitates NMR interpretation. By attaching 4-hydroxymethylbenzoic acid (HMB) to 

Carbosilane dendrimer [G1] with 4-(hydroxymethyl)phenyl linker 

To demonstrate the applicability of this support in organic synthesis, the authors immobilized a (p-bromophenyl) acetic acid which was then transformed into the corresponding biaryl by means of the Suzuki cross-coupling reaction with p-methylphenylboronic acid. Quantitative NaOH cleavage of the ester linkage yielded the released biaryl, of which no purity was mentioned. 

Lactams were obtained in 80-95% conversion with high trans-selectivity (de 95%) but only moderate enantioselectivity ee ca. 30%). The stereoinduction results from the relatively remote stereocentres located at the linker units. For support recovery the authors performed nanofiltration experiments and it was found that the recovered dendrimers could be easily reused as substrate carriers. 

To synthesize diverse analog products in a combinatorial way, the authors provided the support with two different acid moieties which then were transformed with a 1 1 mixture of two different imine derivatives to receive four different / -lactams in equal amounts (overall yields 85%). 

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