Compounds with extremely branched molecular structures (for example 1) that are formed in iterative syntheses are penetrating many disciplines of chemistry. Compound 1 has 36 carboxyl groups on its periphery and can be considered a unimolecular micelle. Recently dendrimers have found their way into coordination chemistry through a compound which contains up to 22 metal centers per molecule (see also Balzani et al. Angew. Chem. Int. Ed. Engl. 1992, 31, 1493).
Dendrimers with bulky repeating units containing up to 43 benzene rings (in 9) have been obtained by using a repetitive divergent synthetic strategy (three generations). The new functional dendrimer 13 containing six azobenzene units at the periphery was synthesized allowing a reversible switching of the shape and size of the molecule upon irradiation. An X-ray structure analysis of the dendritic molecule 3a shows the inclusion of acetonitrile.Since our first synthesis of multi-branched cascade molecules in 1978 dendrimers have gained widespread attention in organic, supramolecular, and polymer chemistryL2]. The research groups of Denkewalteri3], N e w k~m e [~-~] , Tomaliai8-1'1, Masamunei"I, and others used the divergent synthetic method, whereas Frk~het['~-'~], Neenan"'], Shinkai[lE1, and employed the convergent methodology. Recently we described a synthetic scheme of a new family of monodisperse dendritic molecules using large and bulky repeating units i201. Due to obvious disadvantages of the current nomenclature system with regard to length of names and their missing relation to the structure Newkome proposed a new systematic nomenclature for cascade molecules[211 which is used below.
Synthetic StrategyIn this paper we report on the extension of our repetitive synthetic concept using modified monomers to give a "functional dendrimer", a reversibly photoactive molecule functionalized with six azobenzene units at the periphery. After divergent synthesis the monomeric units are connected in a more or less rigid fashion, yet the dendrimers of all generations (with one exception) are still very soluble in common organic solvents. These dendrimers were designed to incorporate a balanced mixture of aromatic and aliphatic parts. This is not only advantageous for the characterization of these products by 'H-and 13C-NMR spectroscopy but also for the solubility in organic solvents.The building blocks can be prepared in a single step from commercially available starting materials. The core 1,3,5-tris(bromomethy1)benzene (l), was synthesized by N-bromosuccinimide (NBS) bromination[221 and the dimethyl 5-aminoisophthalate was tosylated to afford building block 2aiZ3]. The reaction of 1 with three equivalents of 2a in DMF using K2C03 as base gave the hexaester 3a in 81% yieldiz3]. After reduction of 3a with LiAlH4 in THF, alcohol 4a was obtained in a moderate yield of 49% after recrystallization. This is probably due to the low solubility of 4a in THF; however, attempts to carry out the reduction in ether failed. After transformation of hexaalcohol 4a with PBr3 to give hexabromide 5a in 55% yield, one cycle of the repetitive strategy was completed. Chem.
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