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ORGANIC
LETTERS
2000
Vol. 2, No. 20
3079-3082
Novel Phenylazomethine Dendrimers:
Synthesis and Structural Properties
Masayoshi Higuchi, Satoshi Shiki, and Kimihisa Yamamoto*
Department of Chemistry, Faculty of Science & Technology, Keio UniVersity,
Yokohama 223-8522, Japan
Received June 22, 2000
ABSTRACT
The novel dendrimers consisting of a π-conjugated backbone, the dendritic polyphenylazomethines (DPAs), were synthesized by the convergent
method via the dehydration of aromatic ketones with aromatic amines in the presence of titanium(IV) tetrachloride. The obtained DPAs have
a high solubility unlike the conventional linear polyphenylazomethines. NMR studies revealed the conformational rigidity of DPA G4. DPAs
having many azomethine groups as the coordination site for metal ions are expected to be novel ligands.
Metal-ion collecting and/or endohedral metallodendrimers
are expected as novel nanomaterials for electronic devices
or catalysts.1 We synthesized these novel dendrimers with a
π-conjugated backbone, i.e., the dendritic polyphenylazo-
methines (DPAs).2 DPAs have a high solubility unlike the
conventional linear polyphenylazomethines, and the confor-
mational rigidity was confirmed by NMR. DPA provides the
potential for metal-ion collecting and/or endohedral metallo-
dendrimers based on the strong coordinating ability of the
imine sites, in which metal ions are incorporated into the
inner space of the DPA. These properties will be used to
develop nano-ordered hybrid organometallic materials for
electronic devices.
DPAs (DPA G1, G2, G3, and G4, designated as GX where
X is the generation number) were synthesized by the
convergent method as shown in Schemes 1 and 2. Benzo-
phenone was allowed to react with 4,4′-diaminobenzo-
phenone (1) in the presence of titanium(IV) tetrachloride and
1,4-diazabicyclo[2.2.2]octane (DABCO). Titanium(IV) tetra-
chloride is a more effective agent for the dehydration of
aromatic ketones with aromatic amines than p-toluenesulfonic
acid, which is a common dehydration agent.3 The DPA
dendron G2 was synthesized via dehydration and isolated
by silica gel column chromatography with a 48% yield. An
undesirable dehydration between two molecules of 1 lowered
the yield of the dendron.4 DPA dendrons G3 and G4 were
obtained in 64 and 20% yields by dehydration of the
dendrons G2 and G3 with 1, respectively. DPAs G1, G2,
G3, and G4 were obtained by the dehydration of benzo-
phenone, dendrons G2, G3, and G4 with p-phenylenedi-
amine, and isolated in 91, 62, 45, and 31% yields, respec-
tively. The isolated dendrons and dendrimers were identified
(1) (a) Petrucci-Samija, M.; Guillemette, V.; Dasgupta, M.; Kakkar, A.
K. J. Am. Chem. Soc. 1999, 121, 1968. (b) Balogh, L.; Tomalia, D. A. J.
Am. Chem. Soc. 1998, 120, 7355. (c) Zhao, M.; Sun, L.; Crooks, R. M. J.
Am. Chem. Soc. 1998, 120, 4877.
(2) For reported dendrimers with a π-conjugated backbone, see the
following. Polyphenylene dendrimers: (a) Berresheim, A. J.; Mu¨ller, M.;
Mu¨llen, K. Chem. ReV. 1999, 99, 1747. (b) Morgenroth, F.; Reuther, E.;
Mu¨llen, K. Angew. Chem., Int. Ed. Engl. 1997, 36, 631. Phenylacetylene
dendrimers: (c) Moore, J. S. Acc. Chem. Res. 1997, 30, 402. (d) Devadoss,
C.; Bharathi, P.; Moore, J. S. J. Am. Chem. Soc. 1996, 118, 9635. Stilbenoid
dendrimers: (e) Meier, H.; Lehmann, M. Angew. Chem., Int. Ed. 1998, 37,
643. (f) Deb, S. K.; Maddux, T. M.; Yu, L. J. Am. Chem. Soc. 1997, 119,
9079.
(3) Higuchi, M.; Yamamoto, K. Org. Lett. 1999, 1, 1881.
(4) The dendron G2 is easily removed from the side product arising by
the reaction of two molecules of 1 by means of silica gel column
chromatography, because the polarity of the side product, which has amino
groups, is higher than that of the dendron, which has no amino groups.
10.1021/ol006241t CCC: $19.00 © 2000 American Chemical Society
Published on Web 09/07/2000