Synthesis of dendrimers containing 1,3,4-oxadiazoles

Full text of DOI:10.1021/jo005772s

4062
J . Org. Chem. 2001, 66, 4062-4064
Sch em e 1. Gen er a l Syn th esis of 1,3,4-Oxa d ia zoles
Syn th esis of Den d r im er s Con ta in in g
1,3,4-Oxa d ia zoles
Bert Verheyde and Wim Dehaen*
Department of Chemistry, Katholieke Universiteit Leuven,
Celestijnenlaan 200F, B-3001 Leuven, Belgium
wim.dehaen@chem.kuleuven.ac.be
Received December 15, 2000
Ta ble 1. Yield s of th e Differ en t Gen er a tion Den d r im er s
core
G0
G1
G2
17a
18a
19a
20a
60
64
85
70
74
48
71
75
In tr od u ction
47
75
56
Polymers1,2 containing 1,3,4-oxadiazoles are accessible
by several preparative methods and are known3-6 to be
useful as electron transporting layers in organic light
emitting diodes (LEDs). Charge transport through thin
layers of these materials occurs via a so-called hopping
mechanism. Because of their highly branched structures,
dendritic molecules can be even more advantageous as
the electrons have an enhanced probability to find an
energetically favored pathway to hop from one to another.
The usual way to prepare 2,5-diaryl-1,3,4-oxadiazoles
starts from an aroyl hydrazide 5. After the introduction
of a second aroyl group, the resulting 1,2-bis(aroyl)-
hydrazide 6 can be cyclized, for instance with POCl3 or
SOCl2, to form a 1,3,4-oxadiazole 4. This reaction se-
quence is rather difficult to apply to dendrimer synthesis
since the polar intermediates are not easy to purify. A
better way is the Huisgen route,7 which involves the
aroylation of 5-aryltetrazoles 2. These tetrazoles them-
selves are easily accessible starting from benzonitriles 1
(Scheme 1). However, even this reaction was not effective
enough to go beyond the first-generation stage.8-11 There-
fore, we decided to prepare the 1,3,4-oxadiazole ring prior
to the dendrimer propagation step and to use the nu-
cleophilic aromatic substitution reaction (NAS) as the key
step in the propagation of our dendrons. The well-known1
electron-withdrawing capacity of 1,3,4-oxadiazoles makes
this reaction possible. We used a convergent strategy to
build up higher generation dendrimers.
Ta ble 2. Ma ss Sp ectr om etr y Da ta 2
compd generation calcd mass
exptl mass (ESI-MS)
14
16
G1
G2
G0
G1
G0
G1
G2
G0
G1
G2
G0
G1
G2
606.36
1590.79
630.38
1430.76
1122.60
2323.17
5276.46
693.49
1894.06
4847.35
732.41
1532.79
3501.66
606.6a
1592.0b
630.6a
17b
17c
18b
18c
18d
19b
19c
19d
20b
20c
20d
1432.2b
1123.6b
2324.1b
694.5a
1896.1 (m/e); 948.8 (m/2e)b
2426.4 (m/2e); 1617.7 (m/3e)b
732.8a
1535.2b
3505.5 (m/e); 1752.9 (m/2e)b
a
b
Taken with EI-MS. Taken with ESI-MS.
oxyisophthalic acid chloride 7. The tetrazole 10 was
prepared from the known aldehyde12 via the nitrile
intermediate 8. Next, 4 equiv of BBr3 was needed for the
deprotection of the methoxy group of 12 to obtain
compound 14 in a quantitative yield. This product 14 will
be called the first-generation dendron because a first
branching point is introduced. A comparable synthetic
route starting from 5-(4-fluorophenyl)tetrazole 11 gave
rise to the monomer 13, which contains two fluorine
functions where NAS can take place. To prepare the
second-generation dendron 16, 2 equiv of the peripheral
product 14 were attached to the monomer 13 via a NAS
reaction. For the deprotection of the methoxy group of
compound 15, 10 equiv of BBr3 were necessary (Scheme
2).
Syn th esis
In a first step, the peripheral group 14 was prepared
from 5-(3,5-bis-tert-butylphenyl)tetrazole 10 and 5-meth-
With the G1- and G2-dendrons 14 and 16 it is possible
to carry out a final coupling reaction to a suitable core
molecule, obtaining G1- and G2-dendrimers. Model com-
pounds (G0) were prepared by coupling the commercially
available 3,5-bis(tert-butyl)phenol as a branch to different
core reagents and gave us an idea about the reactivity
of the core molecules. Bi- and tridirectional oxadiazole
cores, 2,5-bis-(4-fluorophenyl)-1,3,4-oxadiazole 17a and
1,3,5-tris-(2-(4-fluorophenyl)-1,3,4-oxadiazol-5-yl)ben-
zene 18a , were prepared from 5-(4-fluorophenyl)-tetrazole
(1) Hedrick, J . L.; Twieg, R. Macromolecules 1992, 25, 2021-2025.
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Met. 1995, 69, 603-604.
(5) Pei, Q.; Yang, Y. Chem. Mater. 1995, 7, 1568-1575.
(6) Chen, Z. K.; Meng, H.; Lai, Y. H.; Huang, W. Macromolecules
1999, 32, 4351-4358.
(7) Sauer, J .; Huisgen, R.; Sturm, H. J . Tetrahedron 1960, 11, 241-
251.
(8) Kraft, A. Chem. Commun. 1996, 77.
(9) Kraft, A.; Osterod, F. J . Chem. Soc., Perkin Trans. 1 1998, 1019-
1026.
(12) Newman, M. S.; Lee, L. F. J . Org. Chem. 1972, 37, 4468
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10.1021/jo005772s CCC: $20.00 © 2001 American Chemical Society
Published on Web 05/02/2001

Notes
J . Org. Chem., Vol. 66, No. 11, 2001 4063
Sch em e 2. P r op a ga tion of G1- a n d G2-Den d r on s
Sch em e 3. Cor e Molecu les a n d G1-a n d
G2-Den d r im er sa
tively, the commercial cyanuric chloride 19a or the 1,10-
phenanthroline 20a can be used as reactive core molecules.
Yields of all dendrimers prepared are summarized in
Table 1.
Ch a r a cter iza tion
Because of the high symmetry of the dendritic mol-
1
ecules all H and 13C NMR spectra could be quite easily
interpreted. All prepared dendrimers were completely
characterized in this way. Electrospray mass spectrom-
etry (ESI-MS) gave in most cases good results (except
for 18d ), and data are summarized in Table 2.
Con clu sion s
The preparation of dendrimers containing the 1,3,4-
oxadiazole group, using the nucleophilic aromatic sub-
stitution reaction as the propagation step, was studied.
Dendrimers up to the second generation containing three
different oxadiazole layers were synthesized. All den-
drimers were characterized using NMR spectroscopy as
well as mass spectroscopy (EI-MS or ESI-MS).
a
OAr ) 3,5-bis(tert-butyl)phenoxy.
Ack n ow led gm en t. We thank the Ministerie voor
Wetenschapsbeleid, the FWO, the KU Leuven, and the
IWT for financial support, Ir. R. De Boer for taking the
11 and 4-fluorobenzoyl chloride or 1,3,5-benzenetricar-
boxylic acid chloride, respectively (Scheme 3). Alterna-

4064 J . Org. Chem., Vol. 66, No. 11, 2001
Notes
1
spectra of compounds 8, 10, 12, 14-17, 20a , 17b, 18b, 19b,
and 20b-d . This material is available free of charge via the
Internet at http://pubs.acs.org.
mass spectra, and Dr. S. Toppet for taking the H and
13C NMR spectra.
1
Su p p or tin g In for m a tion Ava ila ble: Printouts of the H
NMR spectra for compounds 8 and 10-20 and 13C NMR
J O005772S