Efficient synthesis of poly(phenylazomethine) dendrons allowing access to higher generation dendrimers

Article abstract of DOI:10.1021/ol049656d

Matrix presented. We have developed a novel synthetic method of phenylazomethine dendrons that uses 4,4′-methylenedianiline instead of 4,4′-diaminobenzophenone to synthesize the precursor of the phenylazomethine dendron and then oxidized the precursor to

Full text of DOI:10.1021/ol049656d

ORGANIC
LETTERS
2004
Vol. 6, No. 11
1709-1712
Efficient Synthesis of
Poly(phenylazomethine) Dendrons
Allowing Access to Higher Generation
Dendrimers
Kensaku Takanashi, Hiroshi Chiba, Masayoshi Higuchi, and Kimihisa Yamamoto*
Department of Chemistry, Faculty of Science & Technology, Keio UniVersity,
Yokohama 223-8522, Japan
yamamoto@chem.keio.ac.jp
Received February 26, 2004
ABSTRACT
We have developed a novel synthetic method of phenylazomethine dendrons that uses 4,4′-methylenedianiline instead of 4,4′-
diaminobenzophenone to synthesize the precursor of the phenylazomethine dendron and then oxidized the precursor to the next-generation
dendron. For this method, the productivity of the dendrons has been significantly increased. Furthermore, as the synthesis of high-generation
dendrons becomes easier, synthesis of DPA G5 was achieved.
Dendritic polyphenylazomethines (DPAs)1,2 containing a Cd
N-conjugated backbone have a conformational rigidity, and
their imine sites can trap metal ions. Particularly, the imines
on DPA show a stepwise radial complexation with SnCl2
from the core imines to the terminal imines based on the
gradients in the basicity of the imine groups, which can be
controlled by substituents on the core phenyl.3 Due to these
unique properties, DPAs are expected to be used as novel
nanomaterials such as charge accumulation devices, photo-
voltaic devices, and environmental catalysts.4,5 We revealed
that DPA with a cobalt-porphyrin core acts an efficient
catalyst for reduction of CO2,6 and organic light-emitting
(3) (a) Yamamoto, K.; Higuchi, M.; Shiki, S.; Tsuruta, M.; Chiba, H.
Nature 2002, 415, 509. (b) Enoki, O.; Imaoka, T.; Yamamoto, K. Org.
Lett. 2003, 5, 2547.
(4) For devices using dendrimers, see: (a) Freeman, A. W.; Koene, S.
C.; Malenfant, P. R. L.; Thompson, M. E.; Fre´chet, J. M. J. J. Am. Chem.
Soc. 2000, 122, 12385. (b) Hecht S.; Fre´chet J. M. J. Angew. Chem., Int.
Ed. 2001, 40, 74. (c) Setayesh, S.; Grimsdale, A. C.; Weil, T.; Enkelmann,
V.; Mu¨llen, K.; Meghdadi, F.; List, E. J. W.; Leising, G. J. Am. Chem.
Soc. 2001, 123, 946.
(5) For catalysts using dendrimers, see: (a) Astruc, D.; Chardac, F. Chem.
ReV. 2001, 101, 2991. (b) Crooks, R. M.; Zhao, M.; Sun, L.; Chechik, V.;
Yeung, L. K. Acc. Chem. Res. 2001, 34, 181. (c) van Heerbeek, R.; Kamer,
P. C. J.; van Leeuwen, P. W. N. M.; Reek, J. N. H. Chem. ReV. 2002, 102,
3717.
(6) Imaoka, T.; Horiguchi, H.; Yamamoto, K. J. Am. Chem. Soc. 2003,
125, 340.
(1) For reviews of dendrimers, see: (a) Zeng, F.; Zimmerman, S. C.
Chem. ReV. 1997, 97, 1681. (b) Bosman, A. W.; Janssen, H. M.; Meijer, E.
W. Chem. ReV. 1999, 99, 1665. (c) Fischer, M.; Vo¨gtle, F. Angew. Chem.,
Int. Ed. 1999, 38, 884. (d) Grayson, S. M.; Fre´chet, J. M. J. Chem. ReV.
2001, 101, 3819. (e) Newkome, G. R.; Moorefield, C. N.; Vo¨gtle, F.
Dendrimers and Dendrons: Concepts, Syntheses, Applications; VCH:
Weinheim, Germany, 2001. (f) Fre´chet, J. M. J.; Tomalia, D. A. Dendrimers
and Other Dendritic Polymers; Wiley: Chichester, UK, 2002.
(2) Higuchi, M.; Shiki, S.; Ariga, K.; Yamamoto, K. J. Am. Chem. Soc.
2001, 123, 4414.
10.1021/ol049656d CCC: $27.50 © 2004 American Chemical Society
Published on Web 04/27/2004

Scheme 1. New Synthetic Method of DPA Dendrons
diodes (OLEDs) using DPA with a triphenylamine core
only a theoretical amount of the DPA dendron, because
reaction occurs with no undesired dehydration. Therefore,
all the dehydrations on each of the DPA dendrons had greater
than 80% yields. The pre-DPA dendrons are efficiently
oxidized by KMnO4 with greater than 80% yield on each of
the pre-DPA dendrons, because KMnO4 is a base; therefore,
the hydrolysis of the pre-DPA dendron hardly occurred and
the phenylazomethine is very strong for oxidation. Conse-
quently, this novel method allows remarkable progress in
the synthesis of the DPA dendron.
The precursor of the DPA dendron G2 (pre-DPA dendron
G2) was synthesized via dehydration of 4,4′-methylenedi-
aniline and benzophenone with an 85% yield and then
oxidized to the DPA dendron G2 by KMnO4 with an 85%
yield. Similarly, the pre-DPA dendron G3 and DPA dendron
G3 were obtained with 90 and 86% yields, and the pre-DPA
dendron G4 and DPA dendron G4 were obtained with 80
and 78% yields, respectively. The pre-DPA dendrons and
DPA dendrons were isolated by silica gel chromatography,
reprecipitation, or preparative-scale gel permeation chroma-
tography (GPC). The pre-dendrons and dendrons were
identified by MS, NMR, and TLC.
complexed with SnCl2 as a hole-transport material show a
significant EL performance improvement compared to non-
metal-complexed ones.7
DPA has very interesting properties, but its synthesis is
very difficult. Formerly, we synthesized DPA dendrons by
the convergent method, using the dehydration of 4,4′-
diaminobenzophenone (1) and the lower generation of the
dendron (or benzophenone) in the presence of titanium(IV)
tetrachloride.4 But this reaction has the following two
problems: first, undesired dehydrations arise because the
4,4′-diaminobenzophenone contains amines and a ketone.
Second, this reaction requires an excess of dendron to prevent
any undesired dehydrations. For such reasons, the yields are
not good, and the synthesis of the higher generation dendron
resulted in a lower yield, which makes it difficult to
synthesize the fifth-generation dendrimer. Therefore, we
developed a novel synthetic method to reduce such problems,
and the yields underwent a significant improvement. More-
over, we achieved the synthesis of DPA G5. For the
improved productivity of DPA dendrons by this novel
method, the use of DPAs has increased.
The novel method is described as follows. Precursors of
DPA dendrons (pre-DPA dendrons) were synthesized by
dehydration of 4,4′-methylenedianiline (2) instead of 4,4′-
diaminobenzophenone and DPA dendron (or benzophenone)
in the presence of titanium(IV) tetrachloride. The oxidation
of the benzyl methylene on the precursor to ketone by
KMnO4 then gave the DPA dendrons.8 This method requires
Table 1. Comparison between New and Conventional Method
new method
conventionala
benzophenone
G2 dendron
G3 dendron
G4 dendron
G4 dendrimer
4.80 g
5.15 g
4.64 g
3.10 g
1.00 g
847 g
121 g
36.0 g
3.10 g
1.00 g
(7) Satoh, N.; Cho, J.-S.; Higuchi, M.; Yamamoto, K. J. Am. Chem. Soc.
2003, 125, 8104.
(8) (a) Huntress, E. H.; Walter, H. C. J. Am. Chem. Soc. 1948, 70, 3702.
(b) Carroll, R. D.; Hamanaka, E. S.; Perie, D. K.; Welch, W. M. Tetrahedron
Lett. 1974, 16, 1515.
a For detailed conditions and yield of the conventional method, see ref
2.
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Org. Lett., Vol. 6, No. 11, 2004

Scheme 2. Synthesis of DPA G5 via New Method
To compare with the conventional synthesis, the weight
of the benzophenone and dendrons required to obtain 1 g of
DPA G4 are shown in Table 1. As can be seen in Table 1,
the novel method achieved a great reduction in the raw
material; for example, the amount of the benzophenone
required to obtain 1 g of DPA G4 becomes 1/176. Further-
more, a higher generation of the dendron occurs, thus
producing a higher synthesis efficiency; therefore, this novel
synthetic method is more available than the conventional
method, particularly for high-generation dendrons.
then oxidized to the DPA dendron G5 by KMnO4 with a
53% yield. DPA G5 was synthesized via dehydration of
p-phenylenediamine and the DPA dendron G5 with a 32%
yield. The products were isolated by silica gel chromatog-
raphy and preparative-scale GPC. The isolated pre-DPA
dendron G5 and DPA dendron G5 were identified by MS,
NMR, and TLC, and DPA G5 was identified by MS, NMR,
and triple-detector9 SEC (size exclusion chromatography).
The triple-detector SEC data (Mw/Mn ) 1.01) and MALDI-
TOF-MS spectrum of DPA G5 are shown in Figure 1.
In conclusion, we have developed a novel synthetic method
of phenylazomethine dendrons that uses 4,4′-methylenedi-
aniline instead of 4,4′-diaminobenzophenone to synthesize
the precursor of the phenylazomethine dendron and then
oxidized the precursor to the next-generation dendron. This
method significantly increased the productivity of the den-
Formerly, because the yield of the high-generation dendron
was very low, the generation limit was four, and a higher
generation did not occur. With this newly developed method,
the syntheses of DPA dendron G5 and DPA G5 were
achieved. In the same way as the lower generation dendrons,
the pre-DPA dendron G5 was obtained by the dehydration
of 4,4′-methylenedianiline and the DPA dendron G4 in the
presence of titanium(IV) tetrachloride with an 88% yield and
(9) System is equipped with refractive index, viscosity, and light
scattering detectors.
Org. Lett., Vol. 6, No. 11, 2004
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Figure 1. (a) MALDI-TOF MS spectrum of DPA G5 (calcd 11190.58 [M + H]+, found 11190.73). (b) Triple-detector SEC data of DPA
G5 monitored by RI (Mw/Mn ) 1.01, Mw ) 11 200).
drons. Furthermore, as the synthesis of high-generation
dendrons becomes easier, the synthesis of DPA G5 was
achieved. We intend to study the properties of DPA G5,
especially its metal-assembling property.
15036262) from MEXT, Japan, and KAST Research Grant
(Project 23).
Supporting Information Available: Detailed experi-
mental procedures and characterization data of pre-DPA
dendrons and DPA G5. This material is available free of
charge via the Internet at http://pubs.acs.org.
Acknowledgment. This work was partially supported by
the CREST project from JST and the 21st COE program
(Keio-LCC) and for Scientific Research (15655019, 15350073,
OL049656D
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Org. Lett., Vol. 6, No. 11, 2004