Bifunctional dendrimers: From robust synthesis and accelerated one-pot postfunctionalization strategy to potential applications

Article abstract of DOI:10.1002/anie.200804987

A fourth wheel: Two sets of bifunctional AB₂C dendrimers having internal acetylene/azides and external hydroxy groups were constructed utilizing benign synthetic protocols. An in situ postfunctionalization strategy was successfully carried out to illustrate the chemoselective nature of these dendrimers. The dendrimers were also transformed into dendritic nanoparticles or utilized as dendritic crosslinkers for the fabrication hydrogels. (Figure Presented).

Full text of DOI:10.1002/anie.200804987

Communications
DOI: 10.1002/anie.200804987
Supramolecular Chemistry
Bifunctional Dendrimers: From Robust Synthesis and Accelerated
One-Pot Postfunctionalization Strategy to Potential Applications**
Per Antoni, Yvonne Hed, Axel Nordberg, Daniel Nystrꢀm, Hans von Holst, Anders Hult, and
Michael Malkoch*
Dendritic polymers including hyperbranched materials,
dendronized polymers, dendrigrafts, and dendrimers have
emerged as a promising family of macromolecules that
complement linear polymers. These structures are expected
to play a crucial role in future cutting-edge applications as
they display extraordinarily high functional group density per
macromolecule. Dendrimers are the flagship for dendritic
polymers and typically range from 1 to 10 nm in size. They are
monodispersed and constructed by using repetitive steps of
efficient synthetic protocols, either by the divergent[1,2] or
convergent approach.[3] Traditional dendrimers are synthe-
sized from ABx monomers, and the resulting structures can be
thought of as reactive scaffolds which comprise inactive
interiors and active exteriors having multiple functional
groups (Figure 1A). However, as dendritic materials migrate
into new research fields the demand on their structural
complexity is increasing. For example, recently proposed
delivery systems in the field of biotechnology include the
construction of sophisticated dendritic delivery vehicles
possessing two different functional groups (Figure 1B).[4–6]
The delivery systems are composed of a dendron wedge
having A-type functionality, which allows the target to reach
its destination, whereas the second dendron wedge having B-
type functionality expresses multiple active drug com-
pounds[4] or fluorescent dyes for quantitative measurements.[5]
The potential for bifunctional dendrimers is great; however,
the synthetic protocols utilized to obtain such structures are
tedious and typically require a minimum of 16 sequential
reaction steps to a obtain fourth generation dendrimer having
a total of 32 (16+16) reactive groups. For these sophisticated
dendrimers to become commercially viable, the number of
reaction steps needs to be reduced while maintaining a high
Figure 1. Dendrimer evolution. A comparison of different dendritic
architectures and functionalities.
[*] Dr. P. Antoni, Y. Hed, Dr. D. Nystrꢀm, Prof. A. Hult, Prof. M. Malkoch
Royal Institute of Technology
School of Chemistry and Chemical Science
number of functional groups. Unfortunately, only a limited
number of publications are available which report on
accelerated synthetic methodologies.[7] An alternative
approach was recently proposed for the construction of
peripheral bifunctional dendrimers.[8] The strategy included
the endcapping of 2,2-bis(methylol)propionic acid (bisMPA)
dendrimers with a cyclic carbonate monomer which was then
transformed to produce bifunctionality on the periphery. A
fourth generation dendrimer having 48 functional groups
(24+24) was obtained in eight steps (Figure 1C).
Division of Coating Technology
Teknikringen 56-58, 10044 Stockholm (Sweden)
Fax: (+46)790-8283
E-mail: malkoch@kth.se
A. Nordberg, Prof. H. von Holst
Royal Institute of Technology, School of Technology and Health
Neuronic Engineering, Huddinge (Sweden)
and Karolinska Institute
Department of Clinical Neuroscience, Stockholm (Sweden)
[**] We thank the Swedish Research Council (VR) Grant 2006-3617.
Supporting information for this article (including full experimental
data, 1H and 13C NMR spectra, GPC traces, and details on materials
and analytical techniques) is available on the WWW under http://
dx.doi.org/10.1002/anie.200804987.
To take advantage of the dendritic framework it is evident
that the typically dormant dendritic interior needs to be
activated by incorporating anchored functional groups which
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can undergo benign and efficient postfunctionalization reac-
tions. These structures require more sophisticated synthetic
protocols; the reports currently available include dendrimers
containing phosphorus groups[9] or predetermined interior
functionality,[10] or ones that utilize elaborate postfunctional-
ization strategies that require Ru catalysts, super bases, or
strong acids.[11]
We present herein a benign synthetic methodology for the
construction of a novel family of bifunctional dendrimers
which comprise active internal and external functional groups
(Figure 1D). Our strategy is based on the recent accomplish-
ments in chemoselective orthogonal reactions wherein tradi-
tional chemical reactions, such as esterification, amidation
etc., are compatible with the copper(I)-catalyzed cycloaddi-
tion reaction between primary alkynes and azides (CuAAC;
click reaction).[12] Initially, an ABxCy-type monomer was
designed, where x ꢀ 2 and y ꢀ 1, and the A functional group
can only react with the B functional group during dendritic
growth. The C group will decorate the dendritic interior for
postfunctionalization purposes. Trizma hydrochloride was
identified as a building block for the preparation of the
ABxCy monomer (Scheme 1). The resulting AB2C monomer
5, bearing one carboxylic group, one acetylene unit, and an
acetonide-protected diol (A = COOH, B = OH, C = Acet;
x = 2 and y = 1), was successfully obtained on a 30 gram scale.
A divergent growth approach from a trimethylol propane
(TMP) core was chosen for the construction of the multi-
functional dendrimers (Scheme 1). The synthetic method-
ology employed included the well-known esterification cou-
pling reagent dicyclohexylcarbodiimide (DCC) for dendritic
growth and the acidic Dowex resin for the activation/
deprotection of the diols. In the first step, a 1.2 excess of the
ABxCy monomer and DCC per active OH group was
sufficient to obtain the first generation dendrimer 6. The
activation/deprotection by using acidic conditions was accom-
plished in greater than 90% yield, generating TMP-G1-
(Acet)3-OH6 7 with six activated hydroxy groups and three
inert acetylene groups. Repetitive growth/activation reactions
led to the bifunctional dendrimer TMP-G3-(Acet)21-OH24 11
with a total yield of 57% and an approximate molecular
weight of 7300 gmolÀ1. Moreover, the fully activated dendri-
mer was efficiently synthesized in six steps and decorated with
21 acetylene and 24 hydroxy groups, which can undergo
robust postfunctionalizations. This synthesis is in contrast to
the bifunctional dendrimers (Figure 1B) which require a
minimum of 16 steps to obtain 16+16 active groups.[4,5] To
additionally illustrate the significance of our method, the
newly developed dendrimers are compared to different
dendritic scaffolds in a plot shown in Figure 2. The total
number of functionalities (ftot) is calculated by using the
equation in Figure 2, where z is the number of functional
groups in the core, x and y are the number of ABxCy
monomers, and n represents the number of generations.
Typically, a third generation dendrimer emanating from a
trifunctional core has 24 functional groups, or 12+12 groups
in the case of a peripheral bifunctional dendrimer. Our AB2C-
type dendrimer, for example, TMP-G3-(Acet)21-OH24 11, has
a total number of 45 functionlities (21 internal, 24 periph-
eral). For higher generations, the ftot for AB2C dendrimers
Scheme 1. Synthesis of bifunctional dendrimers comprising of acety-
lene groups on the interior and hydroxy groups on the periphery.
a) Succinic anhydride, DMAP, CH2Cl2; b) DCC, CH2Cl2, 08C;
c) 1. dimethoxypropane, DMF, p-TSA; 2. TEA; d) DMAP, CH2Cl2;
e) succinic anhydride, DMAP, CH2Cl2; f) 5, DCC, DMAP, DPTS,
pyridine, CH2Cl2; f) acidic Dowex resin, MeOH. DMAP=4-dimethyl-
aminopyridine, DPTS=4-(dimethylamino)pyridinium p-toluene-
sulfonate, TEA=triethylamine, p-TSA=toluene-p-sulfonic acid.
increases rapidly compared to peripheral bifunctional den-
drimers.
All reactions were monitored by using MALDI-TOF
analysis to ensure complete substitution of the growth sites,
and the dendrimers were purified by using flash chromatog-
raphy. The purity was analyzed by using NMR, GPC, and
MALDI-TOF techniques. A typical NMR spectrum for the
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hydrophobic interior and a hydrophilic exterior was challeng-
ing for MALDI-TOF analysis and efforts are now underway
to identify suitable matrices to confirm the purity of higher
generation materials.
To provide insight into the chemoselective and orthogonal
nature of these activated dendritic scaffolds, two different
model postfunctionalization reactions were performed. A
robust postfunctionalization strategy is the hallmark of
synthetic efficiency and is essential for future scientific
exploitation. Recent reports have described elegant examples
of expedient methodologies in which the click reaction was
compatible with chemical reactions including controlled
radical polymerization, esterification, and amidation reac-
tions.[13] Consequently, an in situ strategy was employed in
both cases, targeting the efficient functionalization of the
anchor groups on the interior and the groups on the exterior
by using esterification and CuAAC reactions (Scheme 2). In
the first model system, the first generation dendrimer TMP-
G1-(Acet)1-(OH)6 7 and AB2C monomer 5 were dissolved in
DMF and the esterification reagents (DCC/DMAP) were
added (Scheme 2A). The reaction was monitored with
MALDI-TOF methods, and upon completion of the reaction
benzyl azide and the CuAAC reagents (CuBr/PMDETA)
were added. During the postfunctionalization reaction den-
drimer 7 underwent an in situ generation growth and the
internal functionalization with benzylic groups generated
TMP-G2-(Benzyl)9-(Ac)6 13, which was isolated after column
chromatography. The second step involved the in situ reaction
of the second generation dendrimer TMP-G2-(Acet)9-(OH)12
9 with AB2C monomer 5 and an azide functional initiator (3-
azidopropyl 2-bromo-2-methylpropanoate, 15), suitable for
atom transfer radical polymerization (ATRP) techniques. By
using a procedure similar to that used for the first model
system, TMP-G3-(Initiator-Br)21-(Ac)12 16 was effectively
obtained, having an approximate molecular weight of
13000 gmolÀ1, in 77% yield (Scheme 2B). This one-pot
accelerated and benign postfunctionalization strategy opens
the door for a highly tailored functionalization, which could
deliver dendritic materials with increased sophistication as
required for advanced applications, such as optical devices or
drug delivery systems.
Figure 2. Increase in total number of functional groups. Comparison
between functionality and the chemical composition of the dendritic
architecture (three-armed core for dendrimers and two-armed core for
*
the didendron). Bifunctional didendron ( ); traditional dendrimer (&);
bifunctional dendrimer having internal and peripheral functional
~
groups ( ).
third generation multifunctional dendrimer 10, TMP-G3-
Acet21-Ac12, can be seen in Figure 3. Additional evidence of
the monodispersity is shown in the MALDI-TOF spectra
(Figure 4). Interestingly, this technique was less suitable for
the examination of higher generation multifunctionality
dendrimers (beyond third generation). The increased com-
plexity of higher generation dendritic frameworks having a
Dendritic materials are also candidates for multifunc-
tional hydrogel crosslinkers. Hydrogels derived from AB2C
dendritic crosslinkers with anchored peptides can, for exam-
ple, act as artificial extracellular matrices to allow accelerated
interaction with body cells, promoting growth of new bone.[14]
We believed that the AB2C dendrimers, including biodegrad-
able ester and amide bridges, would exhibit promising
cytotoxic results for use in physiological environments.
Consequently, TMP-G2-(Acet)9-(OH)12 9 was assessed for
potential cytotoxicity. An indirect contact test using MTT (3-
[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide)
staining on a MG-63 osteoblast cell line at both 4 mgmLÀ1
and 12 mgmLÀ1 was performed. Absorbance measurements
to determine cell viability were performed at 0, 24, 48, and
72 hour time points. The dendrimer showed either no or low
toxic effect at each of the two concentrations tested. The
nontoxic nature of the dendrimer encouraged the study of
their crosslinking ability in polyethylene glycol (PEG) based
Figure 3. Representative 1H NMR spectrum of an AB2C dendrimer with
internal acetylenes, TMP-G3-Acet21-Ac12 10. G=generation.
Figure 4. MALDI-TOF spectra of AB2C dendrimers with internal acety-
lenes.
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Scheme 2. One-pot in situ postfunctionalizations of AB2C dendrimer interior and periphery. A) 2ꢁ3ꢁ6 acetonide end groups and 3+6ꢁ9 benzylic
groups: a) DCC, DMAP, DMF, RT; b) PMDETA, CuBr. B) 2ꢁ2ꢁ3ꢁ12 acetonide end groups and 3+6+12ꢁ21 initiators for ARTP: c) DCC, DMAP,
THF, RT; d) CuSO4, NaAsc, H2O. C) PEG hydrogel 28 based on second generation dendritic crosslinker.
networks. By using a click-based hydrogel procedure,[15] an
equimolar amount of the second generation dendrimer 9 and
N3-PEG8000-N3 27 were dissolved in ethanol (50:50 mass%).
CuSO4/NaAsc (aq) was then added to the solution, and the
reaction was run in a Teflon mold (Scheme 2C). Crosslinking
was observed within 30 minutes, and after 24 hours the
yellowish hydrogel was acquired. After extraction of the
copper using aqueous EDTA (ethylenediaminetetracetic
acid), the transparent hydrogel 28 having dendritic cross-
linking junctions was obtained. The swollen hydrogel was
found to contain 96% water and could be degradaded within
1 hour at pH 11 or 4 days at pH 4.
In contrast to the acetylenes, the azide groups can be
exposed to UV light to release N2 to generate imine and
nitrene groups, which can additionally react within the
dendritic framework.[16] From this perspective, multifunc-
tional dendrimers equipped with internal azides can be
exposed to UV light and theoretically undergo intramolecular
crosslinking to generate collapsed nanoparticles or encapsu-
lators, making them candidates for the delivery of agents or
molecular sensors. An exciting report by Zimmermann and
co-workers[17] reveals the intramolecular crosslinking of allyl
decorated dendrimers using the Grubbs catalyst. Unfortu-
nately, the reported nanoparticles are based on the toxic
benzyl ether backbone. Therefore, a simple and alternative
approach was investigated using the second generation
dendrimer TMP-G2-(N3)9-(Ac)6 22, which has nine internal
azides. The dendrimer was diluted to 0.5 mgmLÀ1 in THF and
divided equally (1 mL) into four quartz cuvettes. The
solutions were exposed to UV irradiation with 1 to 4 pulses
(scans) for 2 seconds at an intensity level of 0.537 JcmÀ2. A
sample from each solution was analyzed by GPC analysis to
monitor the collapse efficiency (Figure 5). The decrease in the
hydrodynamic volume from 11.7 ꢀ to 9.6 ꢀ confirmed the
intramolecular collapse of the dendrimer into a more
Next we made a second set of multifunctional dendrimers
bearing internal azides instead of acetylene groups. 2-
(Bromomethyl)-2-(hydroxymethyl)propane-1,3-diol
was
effectively converted into the AB2C monomer 18 having
one carboxylic group, one azide unit, and a protected diol
(A = COOH, B = OH, C = N3; x = 2 and y = 1). By adapting
the same divergent growth/activation strategy, a third gen-
eration fully activated dendrimer, TMP-G3-(N3)21-OH24 24,
was obtained in 54% yield and had an approximate molecular
weight of 5400 gmolÀ1 (Scheme 3).
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Keywords: click chemistry · dendrimers ·
.
gels · nanoparticles · synthetic methods
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Published online: December 29, 2008
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