Dendrimer Building Blocks

Dendrimers possess a central core with branches that more or less radiates out to a peripheral surface as a tree. The polymers have structures and properties different from linear polymers and have very high and narrow molecular weights. Relatively easy introduction of functional groups into them makes it possible to engineer a variety of functionalized polymers. For this reason many new molecular constructions are being carried out.

In the convergent strategy¹⁾, which is one of the main synthetic approaches, dendrimers are built from the periphery toward the central core. In here, we introduce building blocks for polyaryl ether and ester dendrimers, which are synthesized using the convergent strategy. Using these aryl ethers with a photoisomerizable azobenzene core, a highly branched dendrimer, which functions as an efficient photo-harvesting antenna, has been prepared by D.-L. Jiang and T. Aida (Scheme 1)²⁾.

Scheme 1.
Reagents: i) 3,5-Dihydroxybenzyl alcohol, 18-Crown-6, K₂CO₃, Acetone, ii) CBr₄, PPh₃, THF

A double-stage convergent synthesis has recently attracted much attention as a simplified and a fast method, as compared with the classical approach. The generation number is increased by two or three units in a one-pot reaction by simultaneous deprotection and alkylation (Scheme 2, 3)³⁾. Moreover, the periphery with tert-butyldiphenylsilyl as a protecting group can be functionalized at a later stage of the synthesis.

Scheme 3.
Reagents: i) KF, 18-Crown-6, Acetone

Dendrimers, which are nanometre-size macromolecules and have three-dimensional structure, are expected to have applications in wide areas.