Synthesis of Cored Dendrimers

Abstract

The unique structure of dendrimers gives rise to properties that have, in turn, led to applications ranging from bulk polymer additives to biomedical agents.1 Three structural components are common to all dendrimers: a core unit, peripheral groups, and the multiple branching units that span the two. The core unit is indispensable because it covalently links the dendritic “wedges” (dendrons). At the same time, the ability to “core” a dendrimer could greatly facilitate the development of new dendritic catalysts, delivery vehicles, and recognition systems. Maintaining the structural integrity of a cored dendrimer would, however, require an alternative connection between the dendritic wedges. Herein we report the first example of a dendrimer whose peripheral groups are extensively cross-linked and whose core is subsequently removed.2 Dendrimers 1a and 1b were designed with homoallyl ether groups on their periphery, which could be intramolecularly linked through a ring-closing metathesis (RCM) reaction.3 Model studies showed that homoallyl groups attached to the same benzene ring cannot undergo metathesis, increasing the probability of interwedge reactions. The required dendrons were synthesized from homoallyl alcohol and methyl 3,5-dihydroxybenzoate with, iteratively, the Mitsunobu etherification followed by lithium aluminum hydride reduction.4 Dendrimers 1a and 1b were produced, respectively, by a Mitsunobu esterification of trimesic acid with the appropriate third-generation alcohol, and by Mitsunobu esterification of a third-generation dendritic carboxylic acid and 1,3,5-tris(hydroxymethyl)benzene.5 The dendrimers, which are similar to those reported by Hawker and Frechet,6 contain three cleavable ester bonds at their core, but robust ether linkages throughout the remaining structure. The RCM reaction of dendrimer 1a with 4 mol % of Grubbs’ ruthenium alkylidene catalyst 23c per dendrimer was found to give exclusively the intramolecular product 3a7 when carried out in benzene under a nitrogen atmosphere at high dilution (10-5 M). The intramolecularity was established by size-exclusion chromatography (SEC) and matrix-assisted laser desorption ionization time-of-flight mass spectroscopy (MALDI-TOF-MS). At concentrations higher than 10-5 M, dimer and trimer were observed by both SEC and MALDI-TOF-MS. The polystyrene equivalent molecular weight of 3a determined by SEC is lower than the