Chemistry Reference
In-Depth Information
SCHEME 2.4
Effective synthesis of triazole dendrimers using click chemistry.
of high purity and excellent yield (Scheme 2.4). In this approach, they utilized the
click-chemistry transformation using copper-catalyzed ligation of azides and al-
kynes. A variety of functional groups are compatible with this process, and the only
major by-products formed in the reaction are NaCl. Although this approach utilizes
the conventional stepwise convergent method, it is remarkable that, in some cases,
filtration or solvent extraction is the only method required for purification in this
highly efficient construction of the triazole units of the dendrimers.
More recently, they have developed a robust, efficient orthogonal synthesis of
poly(thio-ether) dendrimers using thiol-ene click chemistry via divergent ap-
proach [19]. The overall synthetic strategy is shown in Scheme 2.5, starting from
2,4,6-triallyloxy-1,3,5-triazine
5
as a core and 1-thioglycerol
6
as a AB
2
-building
5
6
block. The thiol-ene reaction between
yields the first-generation, hexahy-
droxy dendrimer [G1]-OH
6
in quantitative yield under photochemical initiation and
subsequent esterification of [G1]-OH
6
with 4-pentenoic anhydride
and
gives the ene-
functional dendrimer [G1]-ene
6
. The all of the thiol-ene generation growth steps is
compatiblewith water and oxygen and can be performed in the absence of solvent at
room temperature, which allows for an environmental-friendly process. Although
the purification of ene-functional dendrimers after esterification of hydroxy-func-
tional dendrimers with 4-pentenoic anhydride needs chromatographic separations,
the efficient nature of thiol-ene chemistry and the lack of by-products allow all
generation hydroxyl-terminated dendrimers to be purified by simple precipitation
into diethyl ether in 90
7
% yields.
One of the most challenging approaches in this region is the formation of
dendrimers by only mixing the building blocks in one pot all at once, without
multiple-addition reaction systems. Hirsch and coworkers [20] have demonstrated the
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