Synthesis and properties of carborane-functionalized aliphatic polyester dendrimers

Article abstract of DOI:10.1021/ja053730l

The incorporation of multiple p-carborane cages within an aliphatic polyester dendrimer was accomplished through the preparation of a bifunctional carborane synthon. A p-carborane derivative having an acid and a protected alcohol functionality was found t

Full text of DOI:10.1021/ja053730l

Published on Web 08/06/2005
Synthesis and Properties of Carborane-Functionalized
Aliphatic Polyester Dendrimers
Matthew C. Parrott,† Erin B. Marchington,† John F. Valliant,†,‡ and Alex Adronov*,§,†
Contribution from the Department of Chemistry, Medical Physics and Applied Radiation
Sciences, and the Brockhouse Institute for Materials Research (BIMR), McMaster UniVersity,
Hamilton, Ontario, Canada
Received June 7, 2005; E-mail: adronov@mcmaster.ca
Abstract: The incorporation of multiple p-carborane cages within an aliphatic polyester dendrimer was
accomplished through the preparation of a bifunctional carborane synthon. A p-carborane derivative having
an acid and a protected alcohol functionality was found to efficiently couple to peripheral hydroxyl groups
of low-generation dendrimers under standard esterification conditions. Deprotection of carborane hydroxyl
groups allowed for further dendronization through a divergent approach using the highly reactive anhydride
of benzylidene-protected 2,2-bis(hydroxymethyl)propanoic acid. This approach was used to prepare fourth-
and fifth-generation dendrimers that contain 4, 8, and 16 carborane cages within their interior. Upon
peripheral deprotection to liberate a polyhydroxylated dendrimer exterior, these structures exhibited aqueous
solubility as long as a minimum of eight hydroxyl groups per carborane were present. Several of the water-
soluble structures were found to exhibit a lower critical solution temperature. Additionally, irradiation of
these materials with thermal neutrons resulted in emission of gamma radiation that is indicative of boron
neutron capture events occurring within the carborane-containing dendrimers.
Introduction
Boron-10 delivery to biological tissues has been the subject
of longstanding research because of the potential of boron
neutron capture therapy (BNCT) in the treatment of diseases
such as cancer.1,2 BNCT is a binary method for radiation therapy
that involves the irradiation of 10B nuclei with thermal neutrons.
Upon neutron capture, the 10B nucleus undergoes fission
resulting in the localized emission of high linear energy transfer
Figure 1. Structures of some common polyhedral boranes.
C2B10H12, have attracted significant attention because of the high
boron content within each molecular cage (Figure 1).1 Carbo-
ranes are particularly attractive because of their high stability,
charge neutrality, and the relative ease with which they can be
chemically modified.6 For these reasons, the conjugation of
carboranes to small biologically relevant molecules, such as
nucleic acids, amino acids, sugars, and lipids, has been
extensively investigated.2 However, many of these studies have
been plagued by the modification of biomolecule structure
resulting from introduction of the carborane, which causes a
loss of function or receptor recognition of the hybrid com-
pounds.5 More recently, the need for high cellular 10B concen-
trations has prompted the conjugation of multiple carboranes
with biological and synthetic macromolecules capable of
specifically targeting cancer cells. For example, direct conjuga-
tion of monoclonal antibodies with carboranes and carborane-
functionalized polylysine resulted in heterostructures bearing
greater than 1300 boron atoms.7 Again, the success of this
approach was limited because of decreased in-vivo antigen
7
(LET) particles (4He and Li)3 having a penetration range of
less than 10 µm in biological tissues, which amounts to
approximately one cell diameter.4 Therefore, if high concentra-
tions of 10B are achieved in tumor cells relative to surrounding
healthy tissues, neutron irradiation should result in selective
tumor annihilation. Considering the high neutron capture cross
section of 10B relative to other light elements,2 BNCT is
theoretically an ideal method for targeted delivery of radiation
doses capable of selective tissue destruction. However, the
principal obstacle to mainstream application of BNCT for cancer
treatment has been the selective delivery of adequate boron
concentrations, requiring a minimum of 109 10B atoms per cell
within the target tissues.5 To address this issue, polyhedral
borane clusters, such as closo-[B10H10]2-, closo-[B12H12]2-, and
the isoelectronic icosahedral family of carboranes, closo-
† Department of Chemistry.
‡ Medical Physics and Applied Radiation Sciences.
§ The Brockhouse Institute for Materials Research (BIMR).
(1) Hawthorne, M. F. Angew. Chem., Int. Ed. Engl. 1993, 32, 950-984.
(2) Soloway, A. H.; Tjarks, W.; Barnum, B. A.; Rong, F. G.; Barth, R. F.;
Codogni, I. M.; Wilson, J. G. Chem. ReV. 1998, 98, 1515-1562.
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Parrott et al.
specificity and decreased water solubility, resulting in greatly
diminished tumor-localizing capability.7 Additional promising
methods have also been reported, including boron-rich oligo-
meric phosphate diesters8 and carborane-loaded unilamellar
liposomes.9
poly(propylene imine),30 carbosilane,31 polylysine,32 metallo-
dendrimers,33 and the dendrimer-like closomers.34 Of these, only
the PAMAM and polylysine structures exhibited some degree
of aqueous solubility, though neither one proved to be an ideal
boron delivery agent. PAMAM dendrimers are cytotoxic
because of their polycationic nature,35 and the polylysine
scaffold, while clearly biocompatible, exhibited diminished
aqueous solubility upon carborane introduction, requiring aque-
ous-organic solvent mixtures for bioconjugation reactions.32
A more successful approach to producing water-soluble carbo-
rane-functionalized dendrimers, reported by Newkome and co-
workers, involved the reaction of alkyne moieties with decab-
orane to form ortho-carborane cages within the interior of
cascade macromolecules.36 Aqueous solubility over a wide pH
range was provided by peripheral sulfate groups, resulting in a
unimolecular micelle-type structure. However, the biocompat-
ibility and biodegradability of this hydrocarbon-based dendrimer
has not been reported.
Over the past two decades, the use of synthetic macromol-
ecules as drug-delivery agents has gained increasing momentum.
The idea of using water-soluble polymers to mimic transport
proteins was first introduced by Ringsdorf10,11 and Kopecek12,13
and has led to clinical trials of several polymer therapeutic agents
for cancer chemotherapy.14 Polymer-based drug delivery agents
exhibit improved solubility and increased vascular circulation
time because of a decreased rate of renal filtration, a process
that is abated by increasing the molecular size of the delivery
system.15,16 This prolonged circulation time enables macromo-
lecular drug delivery systems to passively target tumor tissues
as a result of increased permeability of tumor vasculature to
macromolecules and the limited lymphatic drainage away from
a tumor.17 Combined, these two factors allow the selective
accumulation of macromolecules in tumor tissue, a phenomenon
known as the enhanced permeation and retention (EPR)
effect.18-20
On the basis of these studies, it is clear that internal dendrimer
functionalization is advantageous, allowing peripheral hydro-
philic groups to impart aqueous solubility and to effectively
mask the presence of hydrophobic carborane cages within the
macromolecule. Additionally, the dendrimer scaffold must be
chosen such that it imparts the required solubility features while
also maintaining biocompatibility. Recently, Fre´chet and co-
workers developed an efficient divergent synthesis of aliphatic
polyester dendrimers based on 2,2-bis(hydroxymethyl)pro-
panoic acid (bis-MPA),37 originally prepared by Ihre et al. in a
convergent manner.38,39 These structures were found to be
promising as drug delivery agents, as they are biocompatible,
nonimmunogenic, nontoxic, water-soluble, and well-tolerated
in vivo.26,40 We have therefore undertaken the development of
similar aliphatic polyester dendrimers that incorporate an easily
controllable number of carboranes within the interior of the
dendrimer structure. Critical to this approach was the develop-
ment of a bifunctional carborane synthon that matches the dual
functionality of the bis-MPA monomer, allowing it to be inserted
within the dendrimer synthesis at any generation using traditional
carbodiimide esterification reactions. This flexibility in the
position of carborane insertion provides control over the boron
concentration within a specific dendrimer target compound.
Within the area of polymer therapeutics, dendritic macro-
molecules exhibit several distinct advantages over their linear
counterparts. These include their precisely controlled architec-
ture, monodispersity, and the ability to incorporate specific
functional groups at the periphery or the interior of the
molecule.21-25 Dendrimers can therefore serve as highly versatile
drug delivery vehicles, allowing for control over solubility,
molecular weight, multiplicity of therapeutic agents, and
potentially the incorporation of active targeting moieties.17,26
In light of these advantages, several research groups have
already investigated the incorporation of carboranes within a
dendritic polymer architecture. Perhaps the first example was
Yamamoto’s ortho-carborane coupled to a cascade-type tetraol
that exhibited enhanced water solubility over the non-dendron-
functionalized starting material.27,28 Following this, other groups
investigated the coupling of multiple carborane cages to the
peripheral groups of various dendrimers, including PAMAM,29
(8) Nakanishi, A.; Guan, L. F.; Kane, R. R.; Kasamatsu, H.; Hawthorne, M.
F. Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 238-241.
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1995, 92, 1367-1370.
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123, 457-485.
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1995, 60, 1478-1478.
(12) Kopecek, J. Polym. Med. 1977, 7, 191-221.
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Bioconjugate Chem. 1994, 5, 58-66.
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2000, 50, 61-81.
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22, 4381-4383.
(14) Duncan, R. Nat. ReV. Drug DiscoVery 2003, 2, 347-360.
(15) Duncan, R. Anti-Cancer Drugs 1992, 3, 175-210.
(16) Maeda, H.; Seymour, L. W.; Miyamoto, Y. Bioconjugate Chem. 1992, 3,
351-362.
(31) Nunez, R.; Gonzalez, A.; Vinas, C.; Teixidor, F.; Sillanpaa, R.; Kivekas,
R. Org. Lett. 2005, 7, 231-233.
(32) Qualmann, B.; Kessels, M. M.; Musiol, H. J.; Sierralta, W. D.; Jungblut,
P. W.; Moroder, L. Angew. Chem., Int. Ed. Engl. 1996, 35, 909-911.
(33) Armspach, D.; Cattalini, M.; Constable, E. C.; Housecroft, C. E.; Phillips,
D. Chem. Commun. 1996, 1823-1824.
(17) Gillies, E. R.; Frechet, J. M. J. J. Am. Chem. Soc. 2002, 124, 14137-
14146.
(18) Matsumura, Y.; Maeda, H. Cancer Res. 1986, 46, 6387-6392.
(19) Maeda, H.; Wu, J.; Sawa, T.; Matsumura, Y.; Hori, K. J. Controlled Release
2000, 65, 271-284.
(34) Thomas, J.; Hawthorne, M. F. Chem. Commun. 2001, 1884-1885.
(35) Malik, N.; Wiwattanapatapee, R.; Klopsch, R.; Lorenz, K.; Frey, H.;
Weener, J. W.; Meijer, E. W.; Paulus, W.; Duncan, R. J. Controlled Release
2000, 68, 299-302.
(20) Seymour, L. W. Crit. ReV. Ther. Drug Carrier Syst. 1992, 9, 135-187.
(21) Frechet, J. M. J. Science 1994, 263, 1710-1715.
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(23) Tomalia, D. A.; Naylor, A. M.; Goddard, W. A. Angew. Chem., Int. Ed.
1990, 29, 138-175.
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G. H. Angew. Chem., Int. Ed. Engl. 1994, 33, 666-668.
(37) Ihre, H.; Padilla De Jesus, O. L.; Fre´chet, J. M. J. J. Am. Chem. Soc. 2001,
123, 5908-5917.
(24) Frechet, J. M. J.; Tomalia, D. A. Dendrimers and Other Dendritic Polymers;
John Wiley & Sons, Ltd.: West Sussex, UK, 2001.
(25) Newkome, G. R.; Moorefield, C. N.; Vogtle, F. Dendritic Macromol-
ecules: Concepts, Synthesis, PerspectiVes; VCH: Weinheim, Germany,
1996.
(26) Ihre, H. R.; Padilla De Jesus, O. L.; Szoka, F. C. J.; Frechet, J. M. J.
Bioconjugate Chem. 2002, 13, 443-452.
(38) Ihre, H.; Hult, A.; Soderlind, E. J. Am. Chem. Soc. 1996, 118, 6388-
6395.
(39) Ihre, H.; Hult, A.; Frechet, J. M. J.; Gitsov, I. Macromolecules 1998, 31,
4061-4068.
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Bioconjugate Chem. 2002, 13, 453-461.
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Carborane-Functionalized Aliphatic Polyester Dendrimers
A R T I C L E S
Figure 2. Schematic describing the strategy for incorporation of a carborane synthon into the polyester dendrimer.
Scheme 1
Here, we report the synthesis and properties of this novel class
of carborane-functionalized dendrimers.
Scheme 2
Results and Discussion
Dendrimer Synthesis. The general procedure for the diver-
gent synthesis of aliphatic poly(ester) dendrimers, utilizing
readily available and inexpensive 2,2-bis(hydroxymethyl) pro-
pionic acid (bisMPA) as the monomer, involves coupling of a
highly reactive bisMPA anhydride with nucleophiles such as
alcohols or amines.37 The bisMPA anhydride (1) was prepared
in two steps, according to literature procedures.37 This anhydride
was reacted with the 1,1,1-tris(hydroxyphenyl)ethane core using
a catalytic amount 4-(dimethylamino)pyridine (DMAP) in 96%
yield (Scheme 1). We have found that using a 3:2 mixture of
CH2Cl2 and pyridine and a 2-fold excess of 1 relative to each
alcohol functionality are optimal conditions for all generations.
The excess anhydride 1 was quenched with water, and the pure
first generation dendrimer was obtained after extraction and
washing with NaHSO4 (1 M), Na2CO3 (10% w/v), and brine.
The benzylidene-protecting groups of 3 were quantitatively
removed by hydrogenolysis using a catalytic amount of 10%
(w/w) Pd/C and H2 to produce 4. Iteration of these steps allowed
the production of a series of hydroxy-terminated dendrimer
generations, from G-1 to G-4.
The general strategy for the incorporation of carborane cages
into the polyester dendrimer synthesis involved the preparation
of a bifunctional carborane bearing a carboxylic acid and a
protected alcohol. The carboxylic acid of such a bifunctional
structure could be coupled to the peripheral alcohols of the
deprotected polyester dendrimer at any generation, and the
protected alcohols of the resulting product would subsequently
be deprotected to regenerate peripheral alcohol functionalities
(Figure 2). The new array of peripheral alcohols would then be
reacted with the bisMPA anhydride 1 again to produce higher
generation dendrimers. By doing this, the modified carborane
cage acts as a spacer between generations and can be inserted
at any stage of the dendrimer synthesis.
Preparation of the bifunctional carborane cage was ac-
complished by utilizing the relative acidity of the proton on
the carbon vertexes (pKa ) 26.8).41 A simple deprotonation of
para-carborane with one equivalent of n-butyllithium (n-BuLi)
was performed in dry THF (Scheme 2), leading to the formation
of a statistical mixture of three species, including a monoanion,
a dianion, and the starting material. The anions generated from
this reaction were treated with 1 equiv of trimethylene oxide
resulting in a hydroxypropyl group coupled directly to the
cage.42 The reaction was quenched with HCl (1 M), and the
product was purified by column chromatography in dichlo-
romethane giving 5 in 50% yield. The unreacted starting material
(25%) could be recovered and reused, while the diol byproduct
(25%) was a useful synthon in other reactions (vide infra). The
resulting alcohol (5) was subsequently protected using tert-
butyldiphenylsilyl chloride (TBDPS-Cl) to form 6 in 98% yield.
Compound 6 was deprotonated with n-BuLi and the resulting
anion was quenched with CO2 to produce the desired carboxylate
functionality. Acidic workup, followed by column chromatog-
raphy in 9:1, DCM:methanol resulted in acid 7 (84% yield).
To investigate the esterification chemistry between acid 7 and
the eventual hydroxyl-functionalized dendrimer, a model study
(41) Leites, L. A. Chem. ReV. 1992, 92, 279-323.
(42) Herzog, A.; Knobler, C. B.; Hawthorne, M. F.; Maderna, A.; Siebert, W.
J. Org. Chem. 1999, 64, 1045-1048.
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A R T I C L E S
Scheme 3
Parrott et al.
Scheme 4
Scheme 5
was performed using benzylated bisMPA (8) as a dendrimer
mimic (Scheme 3). Compound 8 was prepared in a single
quantitative step by reacting bisMPA with benzyl bromide in
the presence of DMAP.39 Unfortunately, all attempts to couple
7 and 8 using carbodiimide chemistry were unsuccessful,
resulting only in the isolation of starting material. The apparent
lack of reactivity is likely due to steric hindrance of the proximal
acid group caused by the bulky carborane cage. To reduce this
deactivating effect, it was necessary to introduce a spacer
between the carborane cage and the acid functionality.
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Carborane-Functionalized Aliphatic Polyester Dendrimers
A R T I C L E S
Scheme 6
fication. The bifunctional carborane 10 was subsequently used
as a synthon in the preparation of carborane-functionalized
dendrimers.
The EDC/DPTS couplings between 10 and the 1,1,1-tris-
(hydroxyphenyl)ethane core 2, the first-generation bisMPA
dendrimer 4, and the second-generation bisMPA dendrimer 12
were carried out in 93%, 84%, and 79% yield, respectively
(Scheme 5). In each case, a small excess of the carborane acid
10 was used (1.25 equiv per alcohol) to ensure complete
functionalization. Compounds 13-15 were easily purified by
column chromatography using various mixtures of hexanes and
ethyl acetate as the eluent. These structures were fully character-
The original synthesis of the bifunctional carborane was easily
modified by deprotonating the protected alcohol 6 using
n-butylithium and reacting with a second equivalent of trim-
ethylene oxide (Scheme 4). After an acidic workup, compound
9 was purified by column chromatography using 100% DCM
as the eluent and was isolated in 87% yield. Alcohol 9 was
then oxidized using iodobenzene diacetate (IBDA) and TEMPO
in dichloromethane under ambient conditions.43 This nitroxyl
radical mediated oxidation was chosen for its mild and selective
oxidation of primary alcohols, allowing us to avoid the more
aggressive chromium(VI) oxides that deprotect the TBDPS
group.44 Compound 10 was precipitated from hexanes and was
isolated in 85% yield. The reactivity of acid 10 was tested by
coupling to 8 using EDC and DPTS.45 This reaction produced
11 in quantitative yield, indicating that the spacer between the
carborane and the acid group was indeed required to impart the
necessary acid reactivity in the carbodiimide mediated esteri-
1
ized by H NMR, 13C NMR, and MALDI-TOF MS to ensure
that complete functionalization of all peripheral alcohols on the
dendritic precursors was obtained. In each case, lower mass
structures corresponding to incompletely carborane-functional-
ized dendrimers were not observed, indicating that the coupling
of 10 to the dendrimer periphery is a highly efficient process.
To add dendrimer generations at the periphery of these
molecules, it was necessary to remove the TBDPS protecting
groups, followed by coupling with anhydride 2. Deprotection
of the TBDPS groups was attempted under standard conditions
using tetrabutylammonium fluoride (TBAF) in THF. However,
both thin-layer chromatography and 1H NMR indicated exten-
sive degradation of the dendrimers during the course of this
deprotection reaction. It was postulated that the alkoxides
generated from removal of the TBDPS groups attacked the
various esters in the dendrimer backbone, especially the
Scheme 7
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Parrott et al.
Figure 3. 1H NMR spectra of 21-23.
followed by ring opening of trimethylene oxide to produce diol
16 in 92% yield after acidic workup and crystallization from
CHCl3. This diol was monoprotected using benzyl bromide
under basic conditions to produce 17 in 48% yield. The
remaining free alcohol of 17 was subsequently oxidized using
TEMPO and IBDA to yield acid 18 in 91% yield.
Scheme 8
Figure 4. MALDI-TOF MS of 21-23 with calculated m/z values in
parentheses.
relatively labile phenolic ester linkages between the core and
the dendrons.
To avoid the TBAF deprotection and the ensuing degradation
of the dendrimer, we decided to utilize a benzyl ether group to
monoprotect the carborane diol 16 (Scheme 6). The benzyl ether
was chosen for its stability to slightly acidic and basic media
and for its mild and efficient deprotection conditions. Consider-
ing that benzyl ethers are quantitatively removed by hydro-
genolysis with a catalytic amount of 10% (w/w) Pd/C and H2,44
identical to the conditions used to remove the benzylidene-
protecting groups of the dendrimer, this deprotection is conve-
nient and highly compatible with the dendrimer synthesis.
Starting with para-carborane, deprotonation of both carbon
vertices using two equivalents of n-butyllithium (n-BuLi) was
As an additional precaution in the dendrimer synthesis, it was
decided to substitute the 1,1,1-tris(hydroxyphenyl) ethane core
with Pentaerythritol. This modification not only eliminates the
weak phenolic ester linkages between the core and the dendrons
but also increases the number of dendrons, and therefore the
number of carboranes, within each dendrimer. The G-1 and G-2
protected dendrimers (19 and 20, respectively) having Pen-
taerythritol cores were easily prepared in high yields using the
(43) deNooy, A. E. J.; Besemer, A. C.; vanBekkum, H. Synthesis 1996, 1153-
1174.
(44) Greene, T. W.; Wuts, P. G. W. ProtectiVe Groups in Organic Synthesis,
3rd ed.; Wiley-Interscience: New York, 1999.
(45) Moore, J. S.; Stupp, S. I. Macromolecules 1990, 23, 65-70.
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A R T I C L E S
Figure 5. 1H NMR before (A) and after (B) benzyl ether deprotection of 23. Solvents used for spectra A and B were CDCl3 and MeOD, respectively.
aforementioned procedures. As depicted in Scheme 7, carbo-
diimde coupling reactions were carried out on Pentaerythritol,
the G-1 bisMPA dendrimer (19), and the G-2 bisMPA dendrimer
(20) in 99%, 93%, and 93% yield, respectively. In each of these
reactions, a small excess of the carborane acid 18 (1.25 equiv
per alcohol) was required for complete functionalization.
Compounds 21-23 were easily purified by column chroma-
tography.
reaction occurred without any degradation of the dendritic
backbone, unlike the analogous reaction with TBDPS protecting
groups.
Successful deprotection of the benzyl ether groups resulted
in the regeneration of peripheral alcohols on the dendrimer,
allowing for further divergent growth. These peripheral func-
tionalities were reacted with the bisMPA anhydride 1 to further
dendronize each molecule, allowing the carborane cages to be
internalized within higher generation dendrimers. Scheme 9
depicts the deprotection and dendronization of 21 to the third-
generation deprotected structure. Similar reaction sequences
were used to produce hydroxyl-terminated dendrimers of varying
generation containing 8 and 16 carborane cages. The structures
of the largest members of these two dendrimer families are given
in Figure 6. To name each of these molecules, we refer to the
number of carborane cages, the overall dendrimer generation
number, and the total number of peripheral functional groups.
Thus, a structure containing eight carboranes attached to a first-
generation core and G-4 dendrons coupled to each of the carbor-
anes would be named 8-[G-5]-OH128, as depicted in Figure 6.
1
The H NMR spectra for dendrimers 21-23 exhibit all of
the expected resonances attributed to the branched polyester
core, the carborane linker, and the peripheral-protecting groups
(Figure 3). The broad carborane B-H resonances observable
in these spectra prevent accurate integration of signals in the
range of 0.8-3.8 ppm. However, the appearance of methyl and
methylene signals at 1.1-1.3 ppm and 4.0-4.3 ppm (J, L and
I, K, respectively) provides a clear indication of dendrimer
growth (Figure 3).
MALDI-TOF MS provided the critical evidence for successful
preparation of compounds 21-23 (Figure 4). The mass of the
observed molecular ion clearly corresponded to the Na+ adducts
of each respective dendrimer, with no observable lower mo-
lecular weight fragments or incompletely functionalized materi-
als. Again, this illustrates the efficiency of the esterification
between the carborane acid 18 and the multitude of hydroxyl
groups at the dendrimer periphery.
Aqueous Solubility. Considering the potential therapeutic
applications of carborane-containing compounds, it was impor-
tant to evaluate the aqueous solubility of each of these structures.
Specifically, we were interested in determining the minimum
number of peripheral alcohol groups required per carborane to
impart water solubility at a level of 1 mg/mL or higher. Table
1 summarizes these data and clearly indicates that an alcohol:
carborane ratio of 4:1 or lower is not sufficient for aqueous
solubility. A ratio of 8:1 was also not sufficient for complete
water solubility in the structures containing four and eight
carboranes but did impart solubility to the fifth-generation
dendrimer containing 16 carboranes. Finally, an alcohol:
carborane ratio of 16:1 allowed rapid dissolution of the structures
containing four and eight carboranes to concentrations in excess
of 5 mg/mL. It is expected that a cooperative effect between
the hyrophilicity of the multiple hydroxyl groups and the overall
globular shape of the dendrimers beyond generation four must
dictate overall solubility.
Deprotection of the peripheral benzyl ether groups was
accomplished quantitatively by hydrogenolysis using a catalytic
amount of 10% (w/w) Pd/C and H2 (Scheme 8). The reaction
conditions were essentially identical to those of the benzylidene
deprotections discussed above. The success of this reaction could
1
easily be confirmed by H NMR because of complete disap-
pearance of the aromatic signals at 7.2-7.4 ppm as well as by
the benzylic proton signals at 4.4 ppm. As an example, the 1H
NMR spectra of 23 and 24 are depicted in Figure 5. It can clearly
be seen that the strong aromatic signals at 7.3 ppm and the
methylene protons at 4.4 ppm completely disappear, while the
signals due to the dendrimer backbone remain unchanged. This
result was significant as it confirmed that the deprotection
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A R T I C L E S
Scheme 9
Parrott et al.
Figure 6. Structures of largest synthesized dendrimers containing 8 and 16 carborane cages.
Table 1. Aqueous Solubility of Carborane Functionalized
LCST up to the boiling point of water (Table 1). LCST behavior
within dendrimers has previously been reported by Kono and
co-workers, but their PAMAM dendrimers were peripherally
functionalized with isobutyramide groups that are known to
impart LCST behavior in poly(N-vinylisobutyramide).46 In our
Dendrimers
# of OHs per carborane
8
# of carboranes
2
4
16
4
8
16
no
no
no
no
no
no
<1 mg/mLa,b
<1 mg/mLa,b
1 mg/mLb
8 mg/mL
6 mg/mL
a Exhibits LCST. b Completely soluble in 50/50 (v/v) MeOH/H2O.
Interestingly, during the course of these measurements, we
found that heating the aqueous suspensions of 4-[G-3]-OH32
and 8-[G-4]-OH64 did not improve solubility. In fact, these
molecules precipitated from solution at elevated temperatures,
indicating that they exhibit a lower critical solution temperature
(LCST). Quantitative measurements of this behavior were made
by % transmittance measurements as a function of temperature
(Figure 7) and indicated an onset of precipitation at 52 °C and
83 °C for 4-[G-3]-OH32 and 8-[G-4]-OH64, respectively. Den-
drimer 16-[G-5]-OH128, which was soluble at room temperature,
was also found to precipitate at a temperature of 63 °C, although
its solubility transition is more gradual than for the other two
structures (Figure 7). However, the more water-soluble structures
containing a 16:1 ratio of alcohols to carborane exhibited no
Figure 7. Plots of % transmittance as a function of temperature, indicating
the LCST behavior of carborane-functionalized dendrimers 4-[G-3]-OH32
(2), 8-[G-4]-OH64 (b), and 16-[G-5]-OH128 (9).
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12088 J. AM. CHEM. SOC. VOL. 127, NO. 34, 2005

Carborane-Functionalized Aliphatic Polyester Dendrimers
A R T I C L E S
Figure 8. Gamma energy profile resulting from neutron capture events occurring within a 10 mg/mL solution of 16-[G-4]-OH64.
case, the hydrophobic carborane functionality is encapsulated
within the interior of the dendrimer, yet surprisingly imparts
LCST behavior to an otherwise fully water-soluble structure.
A detailed investigation into the LCST behavior of these
dendrimers will be reported elsewhere.
cages within an aliphatic polyester dendrimer. The insertion of
4, 8, or 16 carboranes was accomplished in high yield using a
previously reported divergent synthesis. It was subsequently
possible to further dendronize the macromolecular periphery
to install a controllable number of hydroxyl functionalities that
imparted aqueous solubility to the final structures. We found
that a minimum of eight alcohols was required to achieve water
solubility. Additionally, it was found that all structures having
an alcohol-to-carborane ratio of 8:1 exhibited a lower critical
solution temperature, which varied with the total number of
carboranes in the structure. Neutron activation experiments
indicated that neutron capture was occurring within the syn-
thesized dendrimers. This family of dendrimers has proven to
be extremely versatile, allowing complete control over location
and number of carborane moieties, as well as overall solubility.
These structures should therefore serve as potential BNCT
agents, and their investigation for this purpose will be investi-
gated soon.
Neutron Activation. To examine neutron activation of the
dendrimer-encapsulated carborane cages, 10 mg/mL solutions
of the hydroxy-terminated dendrimers 4-[G-3]-OH32, 8-[G-4]-
OH64, and 16-[G-4]-OH64 in THF were placed in a thermal
neutron beam at the Prompt Gamma Facility of the McMaster
Nuclear Reactor. The energy profile of the emitted gamma
radiation is given in Figure 8, clearly showing the characteristic
gamma radiation signature at an energy of 480 keV, corre-
sponding to boron neutron capture events. The small signal
observed at 511 keV is due to gamma emission caused by
annihilation of the positron byproduct from the boron neutron
capture event. Irradiation was conducted for a period of 6 h at
a flux of 107 neutrons/cm2‚s, during which no changes in signal
intensity or energy were observed from the dendrimer samples.
Additionally, NMR analysis of the dendrimer samples before
and after neutron activation indicated no degradation during the
course of these experiments. However, the detection of sample
degradation because of neutron capture events at this neutron
flux would not be expected, as the annihilation of only ∼0.01%
of the sample’s 10B atoms occurs during the 6-h experiment.
Acknowledgment. We would like to thank Dr. Dyanne
Brewer for help with MALDI MS as well as Jovica Atanackovic
and Jean Johnson for help with neutron activation experiments.
Financial support for this work was provided by the Natural
Science and Engineering Research Council of Canada (NSERC),
the Canada Foundation for Innovation (CFI), the Ontario
Innovation Trust (OIT), and the Premier’s Research Excellence
Award (PREA).
Conclusion
We have shown that a bifunctional carborane derivative
bearing an acid group and a benzyl ether protected alcohol serves
as a highly effective synthon for the incorporation of carborane
Supporting Information Available: Full experimental details
and characterization for all compounds. This material is available
free of charge via the Internet at http://pubs.acs.org.
(46) Haba, Y.; Harada, A.; Takagishi, T.; Kono, K. J. Am. Chem. Soc. 2004,
126, 12760-12761.
JA053730L
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