Snowflake-Like Dendrimers via Site-Selective Synthesis of Dendrons

Article abstract of DOI:10.1021/ol036011p

(Equation presented) Snowflake-shaped dendrimers were prepared via site-selective synthesis of dendrons, where an attachment of encapsulating dendritic branches and an extension of phenylacetylenic units were alternatively manipulated on the structure of

Full text of DOI:10.1021/ol036011p

ORGANIC
LETTERS
2004
Vol. 6, No. 4
485-488
Snowflake-Like Dendrimers via
Site-Selective Synthesis of Dendrons
Masatoshi Kozaki* and Keiji Okada*
Graduate School of Science, Osaka City UniVersity, 3-3-138 Sugimoto,
Sumiyoshi-ku, Osaka 558-8585, Japan
kozaki@sci.osaka-cu.ac.jp
Received October 15, 2003
ABSTRACT
Snowflake-shaped dendrimers were prepared via site-selective synthesis of dendrons, where an attachment of encapsulating dendritic branches
and an extension of phenylacetylenic units were alternatively manipulated on the structure of AB2 (diethyltriazeno for A and bromo for B)
substituted diphenylacetylene using a combination of Suzuki and Sonogashira cross-coupling reactions.
Highly branched and regularly repeating building blocks of
dendrimers effectively isolate a core and an interior space
in creating a specific microenvironment.1 An electron transfer
from/to an encapsulated redox-active core through the
building blocks has attracted much attention for a model for
both some biological systems and a charge injection into
isolated nanoscale devices.2 We have imagined a snowflake-
shaped dendrimer (Figure 1) that has an electron-carrying
path as an encapsulated π-conjugated system. The snowflake-
like dendrimer is different from any dendrimer reported so
far. It is difficult to synthesize this dendrimer by means of
the simple application of divergent and/or convergent
procedures. A site-selective synthesis of the branch (dendron)
structure is essential; the size of the encapsulating part in
the dendron is altered depending on the π-conjugation sites,
i.e., the larger ones in the inner and the smaller in the outer
region. Here, we report on such a procedure using a
dialkyltriazeno group as a key protecting group3 and
demonstrate the synthesis of snowflake-like dendrimers 1
and 2 possessing a linear oligo(phenylene ethynylene) as a
molecular wire4 inside branched poly(benzyl ether)s (Figure
1).5
(3) (a) Moore, J. S.; Weinstein, E. J.; Wu, Z. Tetrahedron Lett. 1991,
32, 2465-2466. (b) Patrick, T. B.; Juehne, T.; Reeb, E.; Hennessy, D.
Tetrahedron Lett. 2001, 42, 3553-3554 and references therein.
(4) For some selected papers for olgo(phenylene ethynylene)s as a
molecular wire, see: (a) Bumm, L. A.; Arnold, J. J.; Cygan, M. T.; Dunber,
T. D.; Burgin, T. P.; Jones, L., II; Allara, D. L.; Tour, J. M.; Weiss, P. S.
Science 1996, 271, 1705-1708. (b) Tour, J. M.; Kozaki, M.; Seminario, J.
M. J. Am. Chem. Soc. 1998, 120, 8486-8493. (c) Tour, J. M.; Rawlett, A.
M.; Kozaki, M.; Yao, Y. X.; Jagessar, R. C.; Dirk, S. M.; Price, D. W.;
Reed, M. A.; Zhou, C. W.; Chen, J.; Wang, W. Y.; Campbell, I. Chem.
Eur. J. 2001, 7, 5118-5134.
(1) Recent reviews for dendrimers, see: (a) Chow, H.-F.; Mong, T. K.-
K.; Nongrum, M. F.; Wan, C.-W. Tetrahedron 1998, 54, 8543-8660. (b)
Bosman, A. W.; Janssen, H. M.; Meijer, E. W. Chem. ReV. 1999, 99, 1665-
1688. (c) Hecht, S.; Fre´chet, J. M. J. Angew. Chem., Int. Ed. 2001, 40,
74-91. (d) Newcome, G. R.; Moorefield, C. N.; Vo¨gtle, F. Dendrimers
and Dendrons: Concepts, Syntheses, Applications; VCH: Weinheim,
Germany, 2001.
(2) For some recent papers concerning electron transfer in dendrimers,
see: (a) Gorman, C. B.; Smith, J. C.; Hager, M. W.; Parkhurst, B. L.; S.-
Gracz, H.; Haney, C. A. J. Am. Chem. Soc. 1999, 121, 9958-9966. (b)
Toba, R.; Quintela, J. M.; Peinador, C.; Roma´n, E.; Kaifer, A. E. Chem.
Commun. 2001, 857-858. (c) Stone, D. L.; Smith, D. K.; McGrail, P. T.
J. Am. Chem. Soc. 2002, 124, 856-864 and references therein.
(5) For dendrimer with both phenylene ethynylene and ester linkage,
see: (a) Zeng, F.; Zimmerman, S. C. J. Am. Chem. Soc. 1996, 118, 5326-
5327. (b) Kiang, Y.-H.; Gardner, G. B.; Lee, S.; Xu, Z. J. Am. Chem. Soc.
2000, 122, 6871-6883. For linear π-conjugated polymers and oligomers
with poly(benzyl ether) dendritic wedges, see: (c) Karakaya, B.; Claussen,
W.; Gessler, K.; Saenger, W.; Schlu¨ter, A. D. J. Am. Chem. Soc. 1997,
119, 3296-3301. (d) Stocker, W.; Karakaya, B.; Schu¨rmann, B. L.; Rabe,
J. P.; Schlu¨ter, A. D. J. Am. Chem. Soc. 1998, 120, 7691-7695. (e) Sato,
10.1021/ol036011p CCC: $27.50 © 2004 American Chemical Society
Published on Web 01/16/2004

Scheme 1
group of 4 was converted to a diethyltriazeno group by
treating it with tert-butyl nitrate, trifluoroborane etherate, and
then diethylamine to give key compound 5.7 The Sonogashira
coupling of 5 with phenylacetylene or trimethylsilylacetylene
using Pd(0)/Cu(I) gave the compounds 6-18 (I, A ) N3Et2,
B ) Br) or 7, respectively. A deprotection of 7 gave 8 (IV,
A ) N3Et2, B ) Br). In addition, an encapsulating 10-n
(PBEn-X (X ) B(OH)2) was synthesized from bromophenol
through condensation with benzyl bromides Pn-Br (n ) 1,
2)9 followed by treatment with n-butyllithium-(trimethyl
borate)-H+.10
With these key compounds in hand, snowflake-like den-
drimers 1 and 2 were synthesized in a convergent way
(Scheme 3). A Suzuki coupling of diphenylacetylene 6-1 with
10-1 (2 equiv) produced fir tree-like dendritic branch 11-1
(II in Scheme 1) in 88% yield. Dendritic branch 11-1 was
heated to 120 °C with iodomethane in a sealed tube to give
first generation dendritic branch 12-1 (III, A* ) I) in 87%
yield3 with no detectable cleavage of benzyl ethers. A second
cycle of a series of reactions gave second generation dendritic
branch 12-2 in 57% yield (3 steps). A Sonogashira coupling
Figure 1. Snowflake-shaped dendrimers.
A snowflake has six branch structures. Scheme 1 illustrates
the strategy of the site-selective synthesis of the branched
dendrons. The synthesis is accomplished by repeating three
steps: introducing branched poly(benzyl ether) branches
(PBEn, where n is the number of generation) in place of B
on the AB2-diphenylacetylene I (step 1), activating A to the
A* leading to III (step 2), and extending the phenylacetylenic
unit in place of A* giving V (step 3). The generation number
(n) of the incorporating PBEn increases as the repetition
cycles.
Key intermediates were prepared according to Scheme 2.
A reduction of the nitro group in 36 using iron powder
afforded 4 in a moderate yield. We used diethyltriazene as
a key in the protection-activation of group A. The amino
(7) Friedman, L.; Chlebowski, J. F. J. Org. Chem. 1968, 33, 1636-
1638.
T.; Jiang, D.-L.; Aida, T. J. Am. Chem. Soc. 1999, 121, 10658-10659. (f)
Apperloo, J. J.; Janssen, R. A. J.; Malenfant, P. R. L.; Groenendaal, L.;
Fre´chet, J. M. J. J. Am. Chem. Soc. 2000, 122, 7042-7051. (g) Schenning,
A. P. H. J.; Arndt, J.-D.; Ito, M.; Stoddart, A.; Schreiber, M.; Siemsen, P.;
Martin, R. E.; Boudon, C.; Gisselbrecht, J.-P.; Gross, M.; Gramlich, V.;
Diederich, F. HelV. Chim. Acta 2001, 84, 296-334. (h) Miller, L. L.;
Schlechte, J. S.; Zinger, B.; Burrell, C. J. Chem. Mater. 2002, 14, 5081-
5089. (i) Furuta, P.; Fre´chet, J. M. J. J. Am. Chem. Soc. 2003, 125, 13173-
13181.
(8) In the following discussion about newly developed iterative
convergent method, the notation X-n will be used for some compounds
where n refers to the generation number.
(9) Hawker, C. J.; Fre´chet, J. M. J. J. Am. Chem. Soc. 1990, 112, 7638-
7647.
(10) (a) Percec, V.; Johansson, G. J. Mater. Chem. 1993, 3, 83-96. (b)
Trollsås, M.; Hult, A.; Percec, V. Macromol. Chem. Phys. 1995, 196, 1821-
1837. (c) Imrie, C.; Loubser, C.; Engelbrecht, P.; McCleland, C. W. J. Chem.
Soc., Perkin Trans. 1 1999, 2513-2523. (d) Donnelly, D. M. X.; Finet,
J.-P.; Guiry, P. J.; Rea, M. D. Synth. Commun. 1999, 29, 2719-2730.
(6) Schoutissen, H. A. J. J. Am. Chem. Soc. 1933, 55, 4531-4534.
486
Org. Lett., Vol. 6, No. 4, 2004

Scheme 2
Scheme 3
of dendritic branches 12-n (n ) 1 or 2) with 1,3,5-
triethynylbenzene (13) provided target dendrimers 1 (70%)
and 2 (48% yield) as white and yellow powders, respectively.
The dendrimers 1 and 2 were soluble in various organic
solvents such as chloroform, dichloromethane, THF, and
toluene and were characterized by standard spectroscopic
measurements. The 1H NMR spectrum (CDCl3) of 1 showed
sharp and well-resolved signals at room temperature. On the
other hand, the signals of 2 were broad at room temperature
because of the slow conformational change of the dendritic
chain and became sharp at 55 °C (Figure 2). A sharp singlet
signal due to three equivalent protons on the core benzene
ring appeared at 7.67 ppm for 1 and 7.65 ppm (*) for 2 at
55 °C. The benzylic protons appeared around 5.0 ppm as a
singlet for 1 and three overlapping singlets with 2:1:1 ratio
(**) for 2. These results indicate the expected symmetric
conformations for 1 and 2. The molecular weights were
determined by FAB-MS for 1 and MALDI-TOF-MS
for 2.
These dendrimers had absorptions at 292, 342, and 362
nm for 1 and at 287, 370, and 392 nm for 2 in THF (Figure
3). The shortest absorption is mainly ascribed to the dendritic
cone structure, and the longer two absorptions are due to
the conjugated main chain structure, oligo(phenylene ethy-
nylene)s.5,9 Direct excitation of the conjugated chain (342
nm for 1 and 370 nm for 2) gave strong fluorescence spectra
(Figure 3). Sensitized excitation of the dendritic cone
structure (292 nm for 1 and 287 nm for 2) resulted in the
same fluorescence pattern. No emission from the cone
(11) Hamai, S.; Hirayama, F. J. Phys. Chem. 1983, 87, 83-89.
Org. Lett., Vol. 6, No. 4, 2004
487

molecular energy transfer from the cone to the conjugated
chain structures in both dendrimers (Table 1).5e
Table 1. Fluorescence Quantum Yields of Dendrimers 1 and 2
a
compd
λ
exc (nm)
λmax (nm)
φf
em
1
292
342
287
370
411
408
409
409
0.50
0.53
0.67
0.76
2
a 9,10-Diphenylanthracene was used as actinometer (φf ) 0.90 in
cyclohexane).11
Figure 2. 1H NMR spectra of 2 in CDCl3 at room temperature
(bottom) and 55 °C (top).
In summary, we have established a synthetic route to the
snowflake-shaped dendrimers 1 and 2. A developed site-
selective synthesis of dendrons is quite general and could
be applied to higher analogues. In these dendrimers, oligo-
(phenylene ethynylene)s should provide unique electron
circuit paths covered by bulky dendritic cones. Research is
currently progressing toward (1) electron-transfer processes
of this type of dendrimers involving redox-active molecules
at the core and at terminals, and (2) introduction of metal-
molecule anchored interfaces at the ends of conjugated
chains.
Acknowledgment. This work was partially supported by
a Grant-in-Aid for Young Scientists (B) (no.14740354) from
the Ministry of Education, Culture, Science, Sports, and
Technology Japan. Financial support from Asahi glass
foundation is also gratefully acknowledged by M.K.
Figure 3. Absorption spectra (plain lines) [(a) for 1 and (b) for 2,
[1] ) [2] ) 4.0 × 10-6 M] and emission spectra (bold lines) [(c)
for 1 (342 nm excitation) and (d) for 2 (370 nm excitation), [1] ≈
[2] ) 8.0 × 10-8 M] in THF.
Supporting Information Available: Synthetic procedures
and characterization of compounds. This material is available
free of charge via the Internet at http://pubs.acs.org.
structure was observed. Fluorescence quantum yields of the
direct and sensitized excitations showed efficient intra-
OL036011P
488
Org. Lett., Vol. 6, No. 4, 2004