Electroactive, internal anthraquinonoid dendritic cores

Article abstract of DOI:10.1021/jo991278g

The modification of the extended 1 → 3 anthraquinonoid monomer 1 by substitution of the central aliphatic moiety with a more rigid, aromatic moiety has allowed the syntheses of the new branched building block 6 possessing multiple, internal redox sites. This monomer was used to generate the first tier, dendritic macromolecules 8 and 9, which incorporate different internal redox centers within the infrastructure as well as the traditional peripheral multifunctionality. Using cyclic voltammetry experiments, the electrochemical response and electronic interactions between the two electroactive sites (the anthraquinonoid and nitroaromatic moieties) of these dendritic constructs were explored.

Full text of DOI:10.1021/jo991278g

J . Org. Chem. 2000, 65, 1643-1649
1643
Electr oa ctive, In ter n a l An th r a qu in on oid Den d r itic Cor es1
George R. Newkome,*,† Venkatraj V. Narayanan,†,‡ and Luis A. God´ınez†,§
Center for Molecular Design and Recognition, Department of Chemistry, University of South Florida,
Tampa, Florida 33620
Received August 11, 1999
The modification of the extended 1 f 3 anthraquinonoid monomer 1 by substitution of the central
aliphatic moiety with a more rigid, aromatic moiety has allowed the syntheses of the new branched
building block 6 possessing multiple, internal redox sites. This monomer was used to generate the
first tier, dendritic macromolecules 8 and 9, which incorporate different internal redox centers
within the infrastructure as well as the traditional peripheral multifunctionality. Using cyclic
voltammetry experiments, the electrochemical response and electronic interactions between the
two electroactive sites (the anthraquinonoid and nitroaromatic moieties) of these dendritic constructs
were explored.
In tr od u ction
and fluorescence, has made them important tools in the
study and development of utilitarian dendrimers.14,15
Toward this goal Meijer and co-workers have demon-
strated16-18 the encapsulation and selective release of dye
molecules (e.g., Bengal Rose) from a “dendritic box”.
Fre´chet’s group has reported19-21 the synthesis of poly-
aryl-ether dendrimers wherein the terminally attached
chromophores (coumarin-based dyes) acted as antennas
absorbing incident light and subsequently transferring
energy to the central dye moiety, which in turn acted as
the fluorescent probe. Diederich’s research group22-27 and
others28-40 have used porphyrin-based dendrimers in
Since the experimental work of Friedrich Wo¨hler in
1828 in which ammonium cyanate was transformed to
an obvious organic material, urea, one of the most
significant aspirations of chemists has been to invent
novel organic materials with unusual, yet predetermined,
properties. Dendrimers, a relatively precise series of
oligomeric macromolecules, constitute an important ex-
ample of materials that can be synthetically tailored to
address specific chemicophysical needs.2-13 Thus, the
molecular incorporation, either by entrapment or inclu-
sion, of chromophoric substrates into their internal
regime offers an avenue to novel polymeric materials.
The high sensitivity even in dilute concentrations of
dye molecules to optical parameters, such as absorption
(14) Newkome, G. R.; Narayanan, V. V.; God´ınez, L. A.; Pe´rez-
Cordero, E.; Echegoyen, L. Macromolecules 1999, 32, 6782-6791.
(15) Newkome, G. R. Pure Appl. Chem. 1999, 70, 2337-2343.
(16) J ansen, J . F. G. A.; de Brabander-van den Berg, E. M. M.;
Meijer, E. W. Science 1994, 266, 1226-1229.
* E-mail: http://www.dendrimers.com.
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E. M. M. J . Am. Chem. Soc. 1995, 117, 4417-4418.
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E. M. M. Macromol. Symp. 1996, 102, 27-33.
† University of South Florida.
‡ Current address: Biomedical Magnetic Resonance Laboratory,
University of Illinois at Urbana-Champaign, 2100 South Goodwin
Avenue, Urbana, Illinois 61801.
(19) Gilat, S. L.; Adronov, A.; Fre´chet, J . M. J . Angew. Chem., Int.
Ed. 1999, 38, 1422-1426.
§ Current address: Departamento de Electroqu´ım´ıa, Centro de
Investigacio´n y Desarrollo Tecnolo´gico en Electroqu´ım´ıa S.C., Parque
Tecnolo´gico Quere´taro Sanfandila, Apdo Postal 064, C.P. 76700, Pedro
Escobedo, Quere´taro, Me´xico.
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within Dendritic Structures. In Topics in Current Chemistry; Vo¨gtle,
F., Ed.; Springer-Verlag: Berlin/Heidelberg, 1998; Chapter 2, pp 19-
77.
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F. Helv. Chim. Acta 1997, 80, 2391-2417.
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sive Supramolecular Chemistry; Reinhoudt, D. N., Ed.; Pergamon:
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ecules: Concepts, Syntheses, Perspective; VCH: Weinheim, Germany,
1996.
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10.1021/jo991278g CCC: $19.00 © 2000 American Chemical Society
Published on Web 02/25/2000

1644 J . Org. Chem., Vol. 65, No. 6, 2000
Newkome et al.
F igu r e 1. Structures of the building block 1 and the first tier dendrimer 2.
designing synthetic models for globular proteins. Toward
their efforts in studying dendritic host-guest systems,
Watkins et al.41 have used the fluorescence emission
properties of the dye nile-red.
molecules covalently incorporated within the dendritic
infrastructure. The synthetic strategy to obtain these
macromolecules consisted of preparing the extended
building block 1, treating it with a four-directional core
to obtain the electroactive dodecaester 2, and then
divergently building subsequent tiers (Figure 1).
In this context, we reported6,14,42-46 the construction of
electroactive dendrimers based on diaminoanthraquinone
We herein report our efforts toward replacing the five-
carbon alkyl spacer unit in monomer 1 with a more
versatile aryl moiety for future internal attachments. The
spacer selected, apart from affecting the properties of the
resultant building block and the dendrimers subse-
quently constructed from it, must also be able to intro-
duce a useful synthon for future infrastructure modifi-
cations. 5-Nitroisophthaloyl chloride (3) was chosen,
because the nitro moiety could be subsequently reduced
to an amino functionality when desired, thus allowing
the incorporation of a new chemically active functional
group within the electroactive dendritic superstructure.
Initially, monomer 6 was synthesized, using a high-
dilution technique47 similar to that for the preparation
(37) Takasu, D.; Tomioka, N.; J iang, D.-L.; Aida, T.; Kamachi, T.;
Okura, I. J . Inorg. Biochem. 1997, 67, 242.
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Int. Ed. Engl. 1998, 37, 1531-1534.
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Horie, K.; Yashima, E.; Okamoto, Y. Macromolecules 1996, 29, 5236-
5238.
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1998, 8, 519-527.
(41) Watkins, D. M.; Sayed-Sweet, Y.; Klimash, J . W.; Turro, N. J .;
Tomalia, D. A. Langmuir 1997, 13, 3136-3141.
(42) Newkome, G. R.; Narayanan, V. V.; Patri, A.; Gross, J .;
Moorefield, C. N.; Baker, G. R. Polym. Mater. Sci. Eng. 1995, 73, 222-
223.
(43) Newkome, G. R.; Baker, G. R.; Moorefield, C. N.; Woosley, B.
D.; Shade, J . M. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.)
1996, 37, 415-416.
(44) Newkome, G. R.; Narayanan, V. V.; Echegoyen, L.; Pe`rez-
Cordero, E.; Luftmann, H. Macromolecules 1997, 30, 5187-5191.
(45) Newkome, G. R.; Baker, G. R.; Moorefield, C. N.; He, E.;
Epperson, J .; Weis, C. D. Polym. Mater. Sci. Eng. 1997, 77, 65-66.
(46) Newkome, G. R.; Moorefield, C. N.; Baker, G. R.. U.S. Patent
5,650,101, 1997.

Anthraquinonoid-Based Dendritic Cores
J . Org. Chem., Vol. 65, No. 6, 2000 1645
Sch em e 1. Con str u ction of a 1 f 3 Exten d ed Bu ild in g Block (6) w ith a n Ar om a tic Sp a cer a
a
(a) Et(i-Pr)2N, benzene, 0-25 °C, 8 h; (b) 1,4-DAAQ, Et(i-Pr)2N, THF, 40 °C, 12 h
of 1, from 1 equiv each of 3, 4 (Behera’s amine),48,49 and
It is interesting to note that monoamine 6 has a deep-
purple color in the solid state, similar to the color of 1,4-
DAAQ, but in a solution of CH2Cl2 it exhibits a deep red
coloration. In contrast, the building block 1 exhibits
similar but lighter shades of red.14 This could be at-
tributed to the extended conjugation (λmax at 530 nm for
6 vs λmax at 524 nm for 1) in 6 as compared to that of 1.
Another unique feature to be noted for monoamine 6, as
compared to 1, is in its solubility characteristics. While
monoamine 1 is highly soluble in benzene or CH2Cl2 and
all organic solvents of higher polarity, monomer 6 is not
very soluble in either MeOH or MeCN; however, it is very
soluble in CH2Cl2 and CHCl3. This behavior may be
attributed, in part, to the rigidity introduced by the
aromatic spacer.The purified building block 6 was then
treated with the four-directional core (tetraacid chloride
7),50 prepared from the corresponding acid,51 in dry THF
to afford (60%) the desired dodecaester 8 (Scheme 2). The
13C NMR spectrum of 8 exhibited several novel features
in contrast to that of 2. As can be seen (Figure 1), the
anthraquinone moieties in dendrimer 2 are flanked by
1,4-diaminoanthraquinone (1,4-DAAQ). Unfortunately,
under these conditions, 5 (40%) was isolated as the major
product and the desired 6 was formed in lower (ca. 18-
20%) yields. The 13C NMR spectrum of purified 5 exhib-
ited a simplified set of eleven peaks, especially the
symmetrical four-peak pattern [124.1, 133.2, 136.5, 148.5
ppm] shown for the aryl carbons. It was then hypoth-
esized that Behera’s amine, although a hindered primary
amine, was more nucleophilic than 1,4-DAAQ when
reacted with bis-acid chloride 3, thus giving rise to 5 as
the major component. To address this problem, the
reaction was conducted in a (1:1) benzene/THF mixture
with the initial slow addition of 1 equiv of 4 in benzene
to the bis-acid chloride 3 in benzene with subsequent
addition of 1,4-DAAQ, which increased yields of 6 at the
expense of 5. Key signals (13C NMR) for 6 that appeared
at 57.4 (4° CNH), 171.8, 172.2, 172.8 (CONH) ppm, as
well as a spike at 829 amu (calcd 830 amu) in the
MALDI-TOF MS, provided evidence for its formation.
(47) Knops, P.; Sendhoff, N.; Mekelburger, H.-B.; Vo¨gtle, F. Top.
Curr. Chem. 1991, 161, 1-36.
(50) Newkome, G. R.; Young, J . K.; Baker, G. R.; Potter, R. L.;
Audoly, L.; Cooper, D.; Weis, C. D.; Morris, K. F.; J ohnson, C. S., J r.
Macromolecules 1993, 26, 2394-2396.
(51) Newkome, G. R.; Weis, C. D. Org. Prep. Proced. Int. 1996, 28,
242-246.
(48) Newkome, G. R.; Behera, R. K.; Moorefield, C. N.; Baker, G. R.
J . Org. Chem. 1991, 56, 7162-7167.
(49) Newkome, G. R.; Weis, C. D. Org. Prep. Proced. Int. 1996, 28,
485-488.

1646 J . Org. Chem., Vol. 65, No. 6, 2000
Sch em e 2. Syn th esis of Electr oa ctive Dod eca ester 8 a n d 9a
Newkome et al.
a
(a) monomer 6, Et(i-Pr)2N, THF, 60 °C, 30 min; (b) anhydrous HCO2NH4, 10% Pd-C, EtOH, reflux, 2 h
different but similar alkyl chains that give rise to two
very similar sets of seven peaks (including two signals
for the quinone carbonyl carbon atoms 186.2/3 ppm) in
the 13C NMR spectrum. In contrast, the aromatic region
of the 13C NMR spectrum of 8 exhibited 20 different
signals, 18 due to each of the electronically different
aromatic carbons (including those from the spacer nitro-
aromatic unit) and two well-separated signals (186.3 and
187.2 ppm) for the quinone carbonyl carbon atoms
(Figure 2). A peak at 3687 amu (M+ + Na peak, calcd
3663.9 amu) in the MALDI-TOF MS provided further
proof-of-structure for its formation.
One interesting feature of 8 is that chemical as well
as electrochemical reactions can be conducted within the
dendritic superstructure to generate specific and chemi-
cal characteristics. For example, the reduction of the
internal nitro groups of dendrimer 8 to the corresponding
amines, which can be used for subsequent internal
attachments. Toward this end, we decided to prepare the
dendritic amino-derivative 9 by treating 8 with am-
monium formate52 and 10% Pd-C in ethanol as solvent
under reflux conditions. The reaction afforded a good
(72%) yield of 9 as a dark-green lustrous powder after
purification (Scheme 2). Evidence for the formation of 9
was obtained, in part, from a broad new peak (1H NMR)
at 4.2 (br, NH2, 8H) ppm. Key signals at 149.38 (NH2CAr),
162.55, 163.07 (COC6H5NO2CO), 171.17 (CONH), 173.45
(CO2), and 186.24, 187.28 (COquin) ppm also provided
evidence supporting its proposed structure. Additionally,
the UV spectrum showed a new absorption maximum at
436 nm that was absent in the nitro-precursor (Figure
3).
The electrochemical properties of these compounds
were also explored using cyclic voltammetry (CV) experi-
ments in N,N-dimethylformamide at 298 K. It is well
know that anthraquinones exhibit two reversible, one-
electron reduction processes during the negative scan and
the two associated reversible oxidations during the
positive potential scan.53-55 As can be seen in Figure 4a,
the monosubstituted 1,4-DAAQ (1) showed the expected
(53) Chambers, J . Q. Electrochemistry of Quinones. In The Chem-
istry of Quinonoid Compounds; Patai, S., Rappoport, Z., Eds.; J ohn
Wiley & Sons: London, 1988; Chapter 12, pp 719-757.
(54) Echegoyen, L.; Gustowski, D. A.; Gatto, V. J .; Gokel, G. W. J .
Chem. Soc., Chem. Commun. 1986, 220-223.
(52) Ram, S.; Ehrenkaufer, R. E. Tetrahedron Lett. 1984, 25, 3415-
3418.

Anthraquinonoid-Based Dendritic Cores
J . Org. Chem., Vol. 65, No. 6, 2000 1647
F igu r e 2. Comparison of the aromatic region of the 13C NMR
spectra of (a) dendrimer 2 and (b) dendrimer 8.
F igu r e 4. CV response of 1.0 mM solutions of compounds (a)
1, (b) 5, (c) 6, (d) 6 (see Table 1), (e) 8, and (f) 9 in 0.1 M of
Et4NTFB in DMF at 298 K. Scan rate 0.2 V s-1
.
derivative that in turn is easier to reduce than the singly
charged anion radical. As a consequence, a two-electron
reduction of the nitroso derivative to yield the hydroxyl-
amine takes place at the same potential of that corre-
sponding to the second wave. As a result of these coupled
chemical and electrochemical reactions, the second re-
duction wave is larger than the first one and the anodic
part of the voltammogram does not reveal the oxidation
of the anion radical formed during the negative scan.58
In Figure 4b, the CV of compound 5 in DMF showed
two cathodic waves of different sizes and two unsym-
metrical anodic peaks. Since this compound only bears
an electroactive aromatic nitro groupa, it is possible to
suggest that the electrochemical-chemical reduction
mechanism, noted above, may indeed be operable. (Con-
trolled experiments with a compound similar to 5, in
which the nitro group has been chemically reduced to the
amine, further showed that its hyperbranched structure
is electrochemically inert in the potential range under
study.) This being the case, the cathodic peak at -1455
mV would correspond to the formation of a radical anion,
and the large peak at -2000 mV would represent the
further electrochemical reduction that triggers the chemi-
F igu r e 3. UV-vis spectra of dendrimers 8 and 9.
two-wave electrochemical behavior in DMF; this will be
the reference for comparisons.
The other moiety that needs to be considered is that
of the widely studied aromatic nitro compounds.56 De-
pending on the solvent and other factors, the electro-
chemical reduction of the nitro functional group can be
coupled to specific chemical reactions to give anion
radicals, hydroxylamines, and even fully reduced amines.
According to Fry and co-workers,57 the reduction of
aromatic nitro-species in organic solvents can in fact be
represented by a two-wave voltammogram. The first
wave corresponds to a one-electron-transfer process, in
which an anion radical is formed in solution. At more
negative potentials, a second electron is transferred to
form a 2-anion radical that is a strong nucleophile.
Abstraction of a proton from the medium by this species
and further release of a hydroxyl group yields a nitroso
(55) Gustowski, D. A.; Delgado, M.; Gatto, V. J .; Echegoyen, L.;
Gokel, G. W. J . Am. Chem. Soc. 1986, 108, 7553-7360.
(56) Ketti, K. V.; Gali, H.; Smith, C. J .; Berning, D. E. Acc. Chem.
Res. 1999, 32, 9-17.
(57) Fry, A. J . Synthetic Organic Electrochemistry; J ohn Wiley &
Sons: New York, 1989.
(58) Schubert, U. S.; Newkome, G. R.; Go¨ldel, A.; Pemp, A.; Kersten,
J . L.; Eisenbach, C. D. Heterocycles 1998, 48, 2141-2148.

1648 J . Org. Chem., Vol. 65, No. 6, 2000
Newkome et al.
Ta ble 1. Red u ction P oten tia ls, E Va lu es (m V) for
Com p ou n d s 1, 5, 6, 8, a n d 9, Stu d ied in 0.1 M
Et4NBF 4/DMF a
6 may indeed correspond to the simultaneous incorpora-
tion of a total of four electrons in a process that involves
intramolecular electron transfer and associated chemical
reactions.
Figure 4 compd
Epa1
Epc1
Epa2
Epc2
Epa3
Epc3
a
b
c
d
e
f
1
5
6
6
8
9
-1240 -1307 -1748 -1808
-1095 -1455 -1350 -2000
-1230 -1200 -1298 -1450 -1602 -1715
-1230 -1200 -1298 -1450
This explanation is further supported by CV experi-
ments of 6 in which the potential was switched before
the reduction process defined by the third wave (Figure
4c) could take place. As can be seen in Figure 4d, the
two electrochemical waves in this potential window are
fully reversible. Since the two anodic peaks in Figure 4d
appear at potentials different from those presented in the
voltammogram of Figure 4c, it is possible to suggest that
the first two reduction waves correspond to stable anion
species and that the third reduction during the negative
scan is the event that triggers a series of associated
chemical and electrochemical coupled reactions that
ultimately result in the electrochemical irreversibility
observed in Figure 4c.
On the basis of the results discussed so far, the CV
response of the first tier dendrimer 8 was expected to be
similar to that of the extended monomer 6. As can be
observed by comparing Figures 4c and 4e, the shapes of
the voltammograms for 6 and 8 are in general terms
similar, but there are also important differences. The
reduction peaks of the anthraquinone moiety in 8 are
shifted to more positive potentials. This is expected
because of the electron-withdrawing effect that the
amidation of 6 to obtain 8 exerts on the electroactive
anthraquinone group and is also consistent with previous
electrochemical studies of similar compounds.
There is also a substantial broadening of the peaks
when the voltammograms of 6 and the corresponding first
tier dendrimer 8 are compared. This effect has also been
previously observed by us and by others and has been
attributed to a slow electron-transfer process due to the
confinement of the electroactive groups within the den-
dritic structure.25,26,59-61 Moreover, the peak broadening
effect that characterizes the electrochemistry of dendrim-
ers can also be observed (Figure 4f) where the CV
response of the first tier dendrimer 9 is presented. As
can be observed by inspecting its chemical structure, this
compound is the reduced form of dendrimer 8, and
therefore its electrochemical response should correspond
to that of the anthraquinone groups only. Comparison
of Figures 4a, 4c, and 4f for instance show that this is
the case, since the electrochemical response of 9 is much
closer to that of 1 than to that of the extended monomer
6. Once again, the main differences between the volta-
mmograms of 1 and 9 are a positive shift on the redox
potentials and a peak broadening due to confinement
effects. In any case, comparison of the CV responses of
the first tier dendrimers 8 and 9 and their corresponding
building blocks clearly show the influence that the nitro
group exerts on their electrochemical behavior.
-940
-998 -1517 -1425
-1173 -1279 -1760 -1869
Fc/Fc+ as internal standard. Compd ) a-f refer to Figure 4.
Epan ) Potential at anodic peak number n, and Epcn ) potential
at cathodic peak number n (n ) 1, 2, or 3).
a
cally coupled transformations previously discussed. In
this way, the anodic peak at -1350 mV would represent
the oxidation of the chemically unreacted anion, and the
peak at -1095 mV would correspond to the irreversible
signal due to the oxidation of the electrochemical-
chemically formed species at the large cathodic peak
positioned at -2000 mM (see Table 1).
The unsymmetrical extended monomer 6 is an inter-
esting electroactive species since it incorporates both
anthraquinone and nitro functionalities in close juxta-
position. The CV response of 6, however, does not show
the overlapped signals of the two analogues 1 and 5
(Figure 4c). This clearly suggests that both electroactive
groups in 6 influence the electrochemical behavior of each
other. Comparison of Figures 4a-c shows for instance
that during the cathodic scan the first reduction wave
for 6 appears at more positive potentials than that
corresponding to monomer 1. The roughly 100 mV peak
potential shift is in fact consistent with the electron-
withdrawing effect that the presence of the aromatic nitro
group should impose on the anthraquinone part of the
molecule.44 The second reduction peak, however, appears
at the same potential as that of the first reduction of the
nitro model 5. This suggests that the second reduction
peak corresponds to the one-electron reduction of the
nitro group in 6 and that the delocalized electron in the
anthraquinone moiety does not influence this process. In
this way, the first two one-electron reduction processes
in Figure 4c correspond to the formation of a stable 2-
anion, in which one electron is localized into each of the
electroactive moieties of 6.
Incorporation of an additional electron to 6 at more
negative potential results in the third cathodic wave
observed in Figure 4c. According to the reduction poten-
tials of the models 1 and 5, this electron-transfer process
should correspond to the formation of a 2- anion in the
anthraquinone moiety of 6. This is further supported by
the cathodic peak potential that is again about 100 mV
more positive than that of the second reduction peak of
1. The size of the peak corresponding to the third
reduction process and the virtual disappearance of the
second reduction wave for the nitro functional group of
6 suggests that the one-electron reduction of the an-
thraquinone radical anion is not the only process taking
place at this potential. On the basis of the mechanism
described for the reduction of model 5, it is possible to
speculate that once the 2- anthraquinone anion is formed,
an intramolecular electron transfer from the anthraquino-
ne to the nitro group occurs to generate the 2- nitro
anion. As was previously discussed for 5, this doubly
charged nitro anion is a very strong nucleophile that
readily undergoes chemical and electrochemical coupled
reactions. In this way, the third large reduction peak of
The preparation and study of dendrimers bearing
internal moieties that can undergo intramolecular or
intermolecular chemical reactions as a consequence of an
induced excitation signal are relevant for the design and
synthesis of molecular devices and catalysts. This par-
(59) Gorman, C. B.; Parkhurst, B. L.; Su, W. Y.; Chen, K.-Y. J . Am.
Chem. Soc. 1997, 119, 1141-1142.
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Macromolecules 1998, 31, 815-822.
(61) Newkome, G. R.; Gu¨ther, R.; Moorefield, C. N.; Cardullo, F.;
Echegoyen, L.; Pe´rez-Cordero, E.; Luftmann, H. Angew. Chem., Int.
Ed. Engl. 1995, 34, 2023-2026.

Anthraquinonoid-Based Dendritic Cores
J . Org. Chem., Vol. 65, No. 6, 2000 1649
133.9, 134.6, 148.3, 171.8, 172.2, 172.8, 183.6, 187.1; IR 3348,
3295 (NH2), 1719 (ester CdO), 1640 (amide CdO), 1310 (CN),
1290 (ester C-O) cm-1; UV λmax 530 (ꢀ ) 8292), 255 (ꢀ ) 23217)
nm; MALDI-TOF MS m/z 829, calcd mass 828.9 amu. Anal.
Calcd for C44H52N4O12: C, 63.75; H, 6.32; N, 6.75. Found: C,
63.50; H, 6.48; N, 6.41.
ticular line of research is under intense investigation in
our laboratory.
Exp er im en ta l Section
All starting reagents were obtained from Aldrich Chemical
Co. THF was dried from LiAH4 before use. All melting points
were taken in capillary tubes and are uncorrected. 1H and 13C
NMR spectra were determined on a Bruker 250 MHz spec-
trometer, using CDCl3 as solvent except when noted, with Me4-
Si, as the internal standard. Cyclic voltammograms at 20 °C
were obtained using a PAR (Princeton Applied Research)
model 175 universal programmer, a model 173 potentiostat,
and a Houston Instruments X-Y recorder. IR compensation
was performed in all experiments by means of a PAR model
179 module. A conventional three-electrode configuration 2 cm3
cell (Cypress Systems, Lawrence, Kansas) was used with a 1
mm diameter glassy-carbon working electrode, a platinum wire
as the counter electrode, and a silver wire as a pseudo-
reference electrode. The working electrode was polished before
the experiments in sequential stages using diamond polishing
compound of 0.25 µm particle size and a 0.05 µm alumina-
water mixture on a felt surface. All of the solutions were
carefully deoxygenated by bubbling dry N2 for at least 10 min
before obtaining the CV data. The solvent used in all CV
experiments was a 0.1 M electrochemical grade Et4NBF4
solution in HPLC grade DMF. The electrochemical response
of the internal ferrocene/ferrocenium couple was used as a
reference against which the potentials reported in this work
were measured.
The extended monomer 1 and the corresponding dendrimer
2 [13C NMR (see peaks in Figure 2a) δ 116.3-138.1 (CAr), 170.9,
171.6, 172.2 (CONH), 172.8 (CO2), 186.3, 186.4 (COQuin)] were
reported14 elsewhere and utilized herein for comparative
purposes.
Syn th esis of 3-Ca sca d e:1-a m in oa n th r a qu in on e[1,4]:(1-
oxo-2-a za et h ylid yn e):5-n it r op h en ylen e[2-1,3]:(3-oxo-2-
a za p r op ylid yn e):ter t-bu tyl P r op a n oa te (6). To a stirred,
ice-cold solution of benzene (300 mL) containing freshly
prepared bis-acid chloride62 3 (2.39 g, 9.63 mmol) were added
amine49 4 (2.0 g, 4.82 mmol) and di(isopropyl)ethylamine
(DIEA; 1.7 mL, 9.64 mmol) in dry benzene (50 mL) over a
period of 1 h. The mixture was warmed to 25 °C and then
stirred for an additional 7 h. A solution of excess 1,4-DAAQ
(1; 5.04 g, 21.2 mmol) and DIEA (1.7 mL, 9.64 mmol) in THF
(50 mL) was added at once to the mixture and stirred for 12 h
at 40 °C.
Syn t h esis of 12-Ca sca d e:m et h a n e[4]:(2-oxo-5-oxa -1-
a za h exylid yn e):a n th r a qu in on e[2-1,4]:(1-oxo-2-a za eth yli-
d yn e):5-n itr op h en ylen e[2-1,3]:(3-oxo-2-a za p r op ylid yn e):
ter t-bu tyl P r op a n oa te (8). To a stirred THF solution (20 mL)
of the extended amine 6 (5.97 g, 7.2 mmol) with DIEA (1.3
mL, 7.20 mmol) under nitrogen was added tetraacid chloride
7 (750 mg, 1.50 mmol), freshly prepared63,64 from the corre-
sponding tetraacid.51 After 30 min of stirring at 60 °C, the THF
was evaporated in vacuo, and the residue was dissolved in CH2-
Cl2. The organic layer was sequentially washed with aqueous
HCl (10%, 2×), aqueous NaHCO3 solution (10%, 2×), deionized
water (2×), and saturated brine. The organic layer was dried
(MgSO4), followed by concentration in vacuo to afford a dark
orange residue, which was chromatographed (SiO2) eluting
with an EtOAc/CH2Cl2 (1:1) mixture to afford (60%) dendrimer
8, as a deep-orange, microcrystalline powder: 3.29 g, 900 µmol;
1
mp 168.0-169.6 °C; H NMR δ 1.42 (br s, 108H), 2.21, 2.39
(48H), 2.64 (8H), 3.56, 3.74 (16H), 7.71 (4H), 8.05-9.02 (m,
24H), 12.85, 13.49 (8H); 13C NMR δ 28.23, 30.14, 30.38, 39.68,
45.83, 58.71, 67.33, 70.07, 81.07, 116.58, 117.09, 124.39,
125.30, 127.08, 127.68, 128.62, 129.15, 132.23, 132.80, 132.97,
134.77, 134.92, 136.26, 137.80, 137.97, 138.90, 148.68, 162.95,
163.47, 171.07, 173.33, 186.27, 187.18; IR 3295 (NH), 1712
(ester CdO), 1640 (amide CdO), 1500 (N-O), 1310 (CN), 1290
(ester C-O) cm-1; UV λmax 484 (ꢀ ) 25650), 265 (ꢀ )1.92 ×
105) nm; MALDI-TOF MS m/z 3687.15 (M+ + Na), calcd mass
3663.9. Anal. Calcd for C193H224N16O56: C, 63.27; H, 6.16; N,
6.12. Found: C, 63.10; H, 6.35; N, 5.99.
Syn t h esis of 12-Ca sca d e:m et h a n e[4]:(2-oxo-5-oxa -1-
a za h exylid yn e):a n th r a qu in on e[2-1,4]:(1-oxo-2-a za eth yli-
dyn e):5-am in oph en ylen e[2-1,3]:(3-oxo-2-azapr opylidyn e):
ter t-bu tyl P r op a n oa te (9). To a solution of the dodecaester
8 (1.00 g, 2.73 × 10-4 mol) in EtOH was added 10% Pd-C
(150 mg), and then the mixture was stirred for 5 min at 35
°C. Ammonium formate (500 mg, 7.94 × 10-3 mol) was added,
and then the mixture was refluxed for 2 h. After cooling to 25
°C, the solution was filtered through Celite. The filtrate was
concentrated in vacuo to afford a residue, which was dissolved
in CH2Cl2, washed with deionized water, and dried (MgSO4).
Concentration in vacuo, followed by flash chromatography on
a short SiO2 column eluting with an EtOAc/CH2Cl2 (1:1)
mixture afforded (72%) 9, as a dark-green lustrous powder:
690 mg, 193 µmol; mp 140-145 °C; 1H NMR δ 1.44 (br s,
108H), 2.21, 2.25 (48H), 2.74 (8H), 3.46, 3.54 (16H), 4.2 (br,
8H), 7.71 (4H), 8.05-9.02 (m, 24H), 12.80, 13.39 (8H); 13C NMR
δ 28.21, 30.0, 30.28, 39.48, 45.33, 57.71, 67.33, 69.57, 80.87,
116.58, 116.63, 124.39, 124.45, 127.08, 127.21, 128.52, 129.61,
132.13, 132.23, 132.97, 134.77, 134.92, 136.26, 137.80, 137.97,
138.90, 149.38, 162.55, 163.07, 171.17, 173.45, 186.24, 187.28;
IR 3310, 3295 (NH2), 1717 (ester CdO), 1645 (amide CdO),
1290, 1120 (ester C-O) cm-1; UV λmax 436 (ꢀ ) 35442), 265 (ꢀ
) 1.95 × 105) nm. Anal. Calcd for C193H232N16O48: C, 65.41;
H, 6.60; N, 6.32. Found: C, 65.26; H, 6.75; N, 6.15.
The solvent was evaporated in vacuo to give a residue, which
was dissolved in CH2Cl2. The organic layer was washed with
aqueous HCl (20%) and then filtered. The filtrate was sequen-
tially washed with aqueous NaHCO3 (10%, 2×), deionized
water (3×), and brine solution. The organic layer was dried
(MgSO4), and the solvent was removed in vacuo to afford a
residue, which was chromatographed (SiO2) eluting with an
EtOAc/CH2Cl2 (1:1) mixture. The crude product was rechro-
matographed on a short silica column eluting with an EtOAc/
cyclohexane (1:3) mixture to remove traces of hexaester 5 [13C
NMR δ 28.1 (CH3), 29.3, 29.4 (CH2CH2), 58.1 (4° CNH), 80.91
(CMe3), 124.1, 133.2, 136.5, 148.2 (CAr), 164.9 (CONH), 174.2
(CO2). Anal. Calcd for C52H83N3O16: C, 62.07; H, 8,32; N, 4.18.
Found: C, 62.11; H, 8.19; N, 4.19.] Concentration of the eluent
in vacuo afforded (30%) the pure 6, as a purple solid: 2.39 g,
Ack n ow led gm en t. Support for this work was pro-
vided in part from the National Science Foundation
(DMR-96-22609), the Army Research Office (DAAH04-
95-1-0373; DAAH04-96-1-0306), and the Office of Naval
Research (N00014-99-1-0082).
1
mp 232.0-232.6 °C; H NMR δ 1.45 (s, 27H), 2.00 (br d, 6H),
2.15 (m, 2H), 2.3 (br d, 6H), 2.5 (t, J ) 7.2 Hz, 4H), 6.05 (s,
1H), 7.05, 7.70, 8.25, 8.90 (ArH, 6H), 7.20 (br s, 2H), 12.35 (s,
1H); 13C NMR δ 21.6, 27.9, 29.9, 30.2, 36.3, 37.7, 57.4, 80.5,
110.4, 116.1, 126.3, 126.6, 126.7, 126.8, 132.8, 133.2, 133.8,
J O991278G
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