backbone during the second reaction (grafting reaction), and
purification of the product from the reaction mixture. We
investigated the reaction conditions for the model com-
pounds. The macromonomers (DPA-Br2 Gn, n ) 1-3) were
obtained by dehydration between excess 2,5-dibromoaniline
and the corresponding phenylazomethine dendrons8 (Scheme
1). In our study, 2,5-substituted phenylazomethine dendrim-
Scheme 2. Synthesis of DPA-DSB Gna
Scheme 1. Synthesis of DPA-Br2 Gna
a Reagents and conditions: (i) styrene, Pd(OAc)2, P(o-tol)3, (i-
Pr)2NH, dioxane.
Br2 G3 does not proceed because of the incomplete capping
of the metal-collecting sites and the inhibition of the
catalyst.12 In the same way, we can apply this to other
palladium-catalyzed reactions such as the Suzuki coupling
and Sonogashira coupling reactions by initially complexing
with rare earth metal ions.13
The addition of SnCl2 to a chloroform/acetonitrile solution
of DPA-DSB Gn (n ) 1-3) resulted in complexation with
a stepwise spectral change, similar to that for the previously
reported phenylazomethine dendrimers.5a,9,14 During the
addition of SnCl2, the solution color of DPA-DSB Gn
changed from light to a deeper yellow. We observed that
the complexation was complete within 10 min by the spectral
change after the addition of SnCl2, that is, the complexation
equilibrium is reached within at least several minutes. Using
UV-vis spectroscopy to profile the complexation, changes
in the position of the isosbestic points were observed,
indicating that the complexation proceeds not randomly but
stepwise. This result suggests that three different complexes
are successively formed during the SnCl2 addition.
a Reagents and conditions: (i) TiCl4, DABCO, PhCl.
ers were obtained in relatively low yields (<50%) as a result
of steric hindrance between the bulky dendrons and substit-
uents at the core.9 We have refined the dehydration condi-
tions, utilizing a large excess of 2,5-dibromoaniline, and
succeeded in preparing the corresponding macromonomers
in very high yields (>90%).
We then examined the palladium-catalyzed Heck reactions
for the polymerizations.1d,7a,10 The macromonomers were
simply allowed to react with styrene, but the corresponding
compounds were not obtained under the normal synthetic
conditions. This result suggested that the phenylazomethine
dendron unit acted as a metal-collecting site and a catalytic
amount of palladium was trapped and prohibited the oxida-
tive insertion into the monomer. The complexed macro-
monomers with metal ions were then employed to avoid the
deactivation of the palladium catalyst by complexation and
reacted with styrene.11 The Heck reaction successfully
proceeded and produced the reaction compounds (DPA-DSB
Typically, the spectra of DPA-DSB G2 gradually changed,
with an isosbestic point at 337 nm up to the addition of 1
equiv of SnCl2 (Figure 1). The isosbestic point then shifted
upon the further addition of SnCl2 and appeared at 339 nm
1
Gn) as confirmed by H and 13C NMR analyses (Scheme
2). Eu(OTf)3 in the reaction mixture was removed by silica
gel column chromatography. However, the reaction of DPA-
(7) (a) Bao, Z.; Amundson, K. R.; Lovinger, A. J. Macromolecules 1998,
31, 8647. (b) Karakaya, B.; Claussen, W.; Gessler, K.; Saenger, W.; Schlu¨ter,
A.-D. J. Am. Chem. Soc. 1997, 119, 3296. (c) Sato, T.; Jiang, D.-L.; Aida,
T. J. Am. Chem. Soc. 1999, 121, 10658. (d) 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. (e) Marsitzky, D.; Vestberg,
R.; Blainey, P.; Tang, B. T.; Hawker, C. J.; Carter, K. R. J. Am. Chem.
Soc. 2001, 123, 6965.
(8) Higuchi, M.; Shiki, S.; Yamamoto, K. Org. Lett. 2000, 2, 3079.
(9) Higuchi, M.; Tsuruta, M.; Chiba, H.; Shiki, S.; Yamamoto, K. J.
Am. Chem. Soc. 2003, 125, 9988.
(10) (a) Kim, J. H.; Lee, H. Chem. Mater. 2002, 14, 2270. (b) Izumi,
A.; Nomura, R.; Masuda, T. Macromolecules 2000, 33, 8918.
(11) Eu3+ ion was selected for the complexation with the phenylazo-
methine unit in the reaction because of the electrochemical stability.
Figure 1. UV-vis spectra changes of DPA-DSB G2 upon stepwise
addition of SnCl2 in CH3CN/CHCl3 ) 1/2. (Insert) Enlargements
showing isosbestic points.
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Org. Lett., Vol. 6, No. 7, 2004