Dendron rodcoils: Synthesis of novel organic hybrid structures

Article abstract of DOI:10.1021/ja020071t

Convergent and divergent syntheses of novel organic hybrid structures termed dendron rodcoils (DRC) containing dendritic, rodlike, and coillike segments are described. The aryl ester dendron masked with 32 trifluoromethyl groups is prepared via a converge

Full text of DOI:10.1021/ja020071t

Published on Web 04/24/2002
Dendron Rodcoils: Synthesis of Novel Organic Hybrid
Structures
Eugene R. Zubarev and Samuel I. Stupp*
Contribution from the Department of Chemistry, Department of Materials Science and
Engineering, Northwestern UniVersity, EVanston, Illinois 60208-3108
Received January 15, 2002
Abstract: Convergent and divergent syntheses of novel organic hybrid structures termed dendron rodcoils
(DRC) containing dendritic, rodlike, and coillike segments are described. The aryl ester dendron masked
with 32 trifluoromethyl groups is prepared via a convergent approach using 5-(tert-butyldimethylsiloxy)-
isophthalic acid as the monomer unit. The activation of the focal point of the dendron allows for successful
coupling between the dendron and the diblock rodcoil molecules synthesized separately. In another example,
the dendritic block is grown via divergent strategy from the terminus of rodcoil using 3,5-bis(tert-
butyldimethylsiloxy)benzoic acid as an AB2 monomer. A combination of catalyzed esterification reactions
and silyl deprotection chemistry proved to be a very efficient method for construction of these nanosized
structures with unusual molecular architecture. Both synthetic strategies allowed for the preparation of
DRCs with nearly monodisperse dendritic blocks as demonstrated by NMR, MALDI-TOF, and GPC
measurements.
Introduction
the literature contain flexible linear chain segments such as poly-
(ethylene oxide) or polystyrene and, therefore, cannot easily
Well-defined molecular architecture, specific conformation,
and shape persistence are known to be key structural elements
which define properties and biological function of natural
macromolecules. Their synthetic counterparts, on the other hand,
generally lack these particular features, and most conventional
polymers exist in random coil conformations. Therefore,
synthesis of functional well-defined macromolecular structures
represents a challenge for organic chemists. Some dendrimers1a
are believed to be one of the few examples of shape-persistent
nanostructures which differ drastically from linear analogues.1
Because of their compact spherical shape and the presence of
multiple functional groups on the periphery, dendrimers exhibit
unusual rheological behavior,2 enhanced catalytic activity,3 and
can be used for encapsulation of small molecules4 and delivery
of drugs.5
Recent advances in the area of dendrimer chemistry have
demonstrated that properties can be significantly improved if
dendritic segments are covalently linked to a structurally
different block to create a hybrid organic structure. Examples
include self-assembling dendrimers,6 dendronized polymers,7
dendritic-linear diblock structures,8 and triblock hybrid den-
drimers.9 However, all diblock hybrid structures described in
retain shape in solution or in the solid state. From a molecular
design standpoint, the incorporation of rigid aromatic spacers
between a dendritic block and a flexible coil can be interesting
for several reasons. First of all, these molecules would generally
have lower symmetry than would ordinary dendrimers and
could, therefore, be polar nanostructures. In molecules having
only dendritic and flexible coil segments, this low symmetry is
easily lost. Second, rigid aromatic segments as the middle block
could, depending on sterics, be useful in promoting aggregation
into larger nanostructures through π-π stacking.10 Dendron
rodcoil molecules could also improve the ability of dendrimers
to modify the surface properties of conventional polymers.
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Macromolecules 1998, 31, 4498-4510. (b) Sendijarevic, I.; McHugh, A.
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* To whom correspondence should be addressed. E-mail: s-stupp@
northwestern.edu.
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Dendron Rodcoils: Synthesis of Hybrid Structures
A R T I C L E S
Scheme 1
According to theoretical and experimental studies carried out
by A. M. Mayes and co-workers,11 highly branched structures
should undergo spontaneous migration to surfaces when mixed
with linear polymers. The presence of rod segments would
spatially separate the dendron and the coil, thus preventing the
latter from being buried inside the high generation dendron as
may be the case in diblock dendritic-linear hybrid structures
described in the literature. This way, flexible coil segments could
promote grafting and/or mixing of DRC molecules with linear
polymer matrixes (polybutadiene or polyisoprene in the case
of molecules described in this paper). Surface modification of
materials would occur by covalent capture of DRC molecules
through the coil segment’s reaction with the matrix following
their spontaneous migration to surface regions.
Results and Discussion
Convergent Synthesis. Over the past several years, we
synthesized a broad variety of novel hybrid structures called
dendron rodcoils (DRCs) which are composed of three major
blocks, a dendritic aryl ester block, a rigid biphenyl ester
segment, and a flexible oligoisoprene coil. We explored two
major strategies which can be used for design and preparation
of triblock DRC molecules. The first one involves convergent
growth of functionalized dendrons followed by a coupling
reaction with rodcoil molecules. This approach requires prepara-
tion of the dendron with a functional group at the focal point
that can be coupled to the terminus of a rodcoil synthesized
separately. Scheme 1 illustrates the synthesis of trifluoromethyl-
terminated aryl ester dendron (G4) via the convergent approach.
Unlike other groups, which previously reported the synthesis
of aryl ester demdrimers,12 we used the TBDMS-protected
derivative of 5-hydroxy-isophthalic acid (compound 2) as an
AB2 monomer for the preparation of dendron. The reaction
between 5-hydroxy-isophthalic acid (HIA) and 3 equiv of
TBDMSCl afforded triprotected compound 1 and was followed
by selective cleavage of silyl groups from two carboxyls under
mild acidic conditions. The esterification coupling reactions
involved in this synthesis were carried out under conditions
developed in our laboratory for the preparation of aryl ester
linear polymers.13 Diisopropyl carbodiimide (DIPC) and 4-(N,N-
(7) (a) Schlu¨ter, A. D.; Rabe, J. P. Angew. Chem., Int. Ed. 2000, 39, 864-
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Theriot, K. J.; Moorefield, C. N.; Rogers, L. E.; Miller, J. E.; Lieux, T. R.;
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8465. (b) Gitsov, I.; Wooley, K. L.; Fre´chet, J. M. J. Angew. Chem., Int.
Ed. Engl. 1992, 31, 1200-1202. (c) Chapman, T. M.; Hillyer, G. L.; Mahan,
E. J.; Shaffer, K. A. J. Am. Chem. Soc. 1994, 116, 11195-11196. (d)
Vanhest, J. C. M.; Delnoye, D. A. P.; Baars, M. W. P. L.; Vangenderen,
M. H. P.; Meijer, E. W. Science 1995, 268, 1592-1595. (e) Desal, A.;
Atkin, N.; Rivera, F.; Devonport, W.; Rees, I.; Branz, S.; Hawker, C. J. J.
Polym. Sci., Part A: Polym. Chem. 2000, 38, 1033-1044.
(9) Malenfant, P. R. L.; Groenendaal, L.; Fre´chet, J. M. J. J. Am. Chem. Soc.
1998, 120, 10990-10991.
(12) (a) Kwock, E. W.; Neenan, T. X.; Miller, T. M. Chem. Mater. 1991, 3,
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1 1992, 2459-2469. (c) Hawker, C. J.; Frechet, J. M. J. J. Am. Chem.
Soc. 1992, 114, 8405-8413. (d) Freeman, A. W.; Fre´chet, J. M. J. Org.
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Soc. 2001, 123, 4105-4106. (b) Zubarev, E. R.; Pralle, M. U.; Sone, E.
D.; Stupp, S. I. AdV. Mater. 2002, 14, 198-203.
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A R T I C L E S
Zubarev and Stupp
Figure 1. MALDI-TOF trace of the fourth generation dendron 10.
dimethylamino)pyridinium-4-toluenesulfonic acid (DPTS) lead
to very high yield for the esterification reactions between
aromatic acids and phenols at room temperature as described
previously.10 We decided to use the advantages of this catalytic
system for the synthesis of high generation dendritic structures,
which requires high yield and purity at intermediate stages.
These coupling conditions turned out to be very fast and versatile
for the preparation of aryl ester dendrons. The successful
esterification always proceeds with yields of greater than 80%
which suggests that these coupling conditions are activated
enough to overcome any steric hindrance through the fourth
generation. The convergent approach allows for incorporation
of the peripheral functional groups at the very beginning of the
synthesis.14 However, these groups have to be unreactive under
synthetic conditions. We have chosen trifluoromethyl groups
which are inert to DPTS and DIPC and can impart highly
hydrophobic properties to the final product. Coupling between
R,R,R-trifluoro-p-cresol and compound 2 afforded the first
generation of a fluorinated dendron 3. For activation of the focal
point, we used hydrofluoric acid which can cleave TBDMS
groups at room temperature. Surprisingly, we found that
tetrabutylammonium fluoride (TBAF) can only be used for
TBDMS deprotection when the generation of dendron does not
exceed 2. Reaction between TBAF and G3 dendron 7 even at
-78 °C results in nearly complete decomposition due to
destruction of ester bonds via nucleophilic attack by fluoride
anions. This instability of aromatic ester bonds toward even mild
base like TBAF is most likely due to a high local concentration
of esters in the interior of dendron. Hydrofluoric acid, on the
other hand, appears to be the best deprotecting agent, because
unlike TBAF or HCl it does not destroy ester linkages regardless
of size and generation of aryl ester dendritic molecules.
Successive coupling and deprotection reactions shown in
Scheme 1 afford the fourth generation dendron 10 with the
hydroxyl group at the focal point. Solubility was found to
decrease rapidly and to pass through a minimum at generation
3. Compound 8 is only soluble in hot DMF, but the coupling
to monomer 2 can be carried out successfully even in dichlo-
romethane because the solubility of G4 dendron 9 is much
higher. Even the deprotected compound 10 is soluble in nonpolar
solvents such as dichloromethane or chloroform. Figure 1 shows
a MALDI-TOF spectrum of this compound which demonstrates
that G4 dendron is a defect-free molecule with a molecular
weight of 4782 daltons.
Although the solubility problem can be easily overcome in
this multistep synthesis once the generation of the dendron is
higher than 3, the rate of the deprotection reactions decreases
dramatically. As shown in Scheme 1, the cleavage of the
TBDMS group takes only 5 min for a G1 dendron and 24 h for
G4 molecules. Also, the yield becomes significantly lower for
G4 versus G2 dendrons (82 versus 99%). This is possibly due
to a combination of steric hindrance as well as the decreasing
relative concentration of TBDMS groups. As the size of the
dendron increases, accessibility of the TBDMS group to
hydrophilic HF becomes quite limited. To minimize this
problem, we prepared another derivative of HIA bearing one
biphenyl unit which would bring the focal point closer to the
periphery of the resultant G5 dendron, thus reducing its
hindrance. Scheme 2 illustrates the synthesis of this AB2
monomer (compound 15) and its successive coupling to afford
a G5 dendron. 4-Hydroxy-biphenylcarboxylic acid was first
protected on both sides with TBDMS group to give compound
11 which was then reacted with HF for 5 min to cleave the
silyl group only from the carboxyl group. On the other hand,
reaction of HIA with 2 equiv of TBDMSCl using morpholine
as a base allows for selective protection of carboxylic groups
leaving the hydroxyl terminus open (compound 13). Esterifi-
cation of 13 with 12 under standard conditions (DPTS/DIPS)
gives molecule 14 which is then reacted with acetic acid to
cleave the silyl groups from two carboxyls (compound 15). G4
dendron 10 was successfully coupled with 15, and the depro-
tection of the focal point with HF was found to proceed rapidly
(14) Hawker, C. J.; Frechet, J. M. J. J. Am. Chem. Soc. 1990, 112, 7638-7647.
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Dendron Rodcoils: Synthesis of Hybrid Structures
A R T I C L E S
Figure 2. 1H NMR spectrum of compound 17. Arrows indicate resonances of protons located in different generational layers of the molecule.
Scheme 2
to give 17 in very high yield (94% within 8 h). These findings
clearly demonstrate that incorporation of one biphenyl ester
fragment between the center of the dendron and the focal point
can significantly reduce steric barriers in the deprotection
reaction.
Interestingly, 1H NMR spectroscopy turned out to be a very
informative method for identification of the outer and inner
shells (generational layers) in these dendritic molecules. Figure
2 shows the 1H NMR spectrum of compound 17 (G5-biphenyl-
OH). Two doublets at 7.48 and 7.75 ppm represent two pairs
of protons in the ortho- and meta-position with respect to CF3
group in the peripheral G5 layer15 of the molecule (64H in each
doublet). As expected, the signals of protons from substituted
HIA units are shifted downfield. Protons at position 4 and 6 in
a given HIA monomeric unit generate a doublet, whereas the
proton at position 2 gives a triplet with a very small coupling
constant J (1.4-1.7 Hz). Most importantly, all the doublets and
triplets can be resolved which allows one to identify five
generational layers and even the core. For example, the doublet
(15) The outer layer (G5) is composed of trifluorocresol substituents, which
are slightly different from HIA fragments in the four inner layers of the
molecule.
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A R T I C L E S
Zubarev and Stupp
Scheme 3
at 8.44 ppm integrates as 32 protons, and the triplet at 8.87
shows 16 protons. These are two fully isolated signals coming
from the G4 layer. The next doublet integrates as 18 protons
(16H from G3 layer and 2H from the core), and the other two
doublets shifted downfield correspond to eight and four protons
from G2 and G1 layers, respectively. On the other hand, the
triplet from the core and that from the G3 layer are also isolated
and located at 8.95 ppm (1H) and 8.99 ppm (8H). The signal at
9.02 ppm integrates as six protons and represents the overlapped
resonances of protons from the G2 and G1 layers. Finally, three
doublets with J ) 8.4 Hz at 6.85, 7.57, and 8.21 ppm are also
visible in the spectrum which correspond to biphenyl protons
(the signal from two additional biphenyl protons is buried under
the doublet at 7.75 ppm). This compound and all the intermedi-
ate precursors were also characterized by 13C NMR, mass
spectrometry, elemental analysis, and gel permeation chroma-
tography (GPC).
The next part of the overall synthesis of DRC involves the
separate preparation of functionalized rodcoil molecules which
can be coupled to compound 17. We previously synthesized
and studied molecules that contain flexible oligoisoprene or
oligobutadiene blocks and three biphenyl ester units.16 Because
compound 17 already has one biphenyl with a hydroxyl group,
we prepared a carboxyl-terminated rodcoil with two biphenyl
esters. Scheme 3 shows the synthesis of the rodcoil and its
coupling to the G5 dendron. Compound 18, which is a building
block for the construction of the rod segment, was synthesized
in one step via the reaction of 4-hydroxy-biphenylcarboxylic
acid and TBDMSCl using morpholine as a base. Living anionic
polymerization of isoprene was used to grow oligoisoprene coil
with a low polydispersity index of 1.07, which contains on
average nine monomeric units (mostly 1,4- and 3,4-addition
according to NMR). Functionalization of the coil with a
hydroxyl group was achieved by reaction between the living
chains and ethylene oxide at room temperature. The Mitsunobu
reaction between 18 and the coil afforded intermediate precursor
20 which was deprotected with TBAF to give carboxyl-
terminated molecule 21. The second biphenyl was then coupled
under standard DPTS/DIPC conditions, and the following
deprotection of the silyl group yielded rodcoil 23 which was
isolated in a pure state simply by pouring the reaction mixture
into excess methanol. This is a particularly convenient way to
purify the product because the resultant rodcoil is not soluble
in methanol, unlike all the starting materials and byproducts of
the reaction. Finally, the esterification reaction between rodcoil
23 and compound 17 gave the first example of high generation
DRC 24 containing 32 CF3 groups on its periphery. The number
average molecular weight (Mn) of this structure is about 11 000
measured by MALDI-TOF. The polydispersity index determined
by GPC is similar to that of the starting rodcoil (1.08) prepared
via living anionic polymerization which inevitably generates a
small degree of structural diversity in the system. Figure 3 shows
the chemical structure and the corresponding molecular graphics
representation of DRC molecule 24. This nanosized molecule
measures 9 nm in length and 3.8 nm in diameter (dendritic
block). This material was fully characterized by various
techniques, and its properties will be reported elsewhere.
Divergent Synthesis. Analogous aryl ester dendron rodcoils
were also synthesized via a divergent approach outlined in
Scheme 4. We previously reported the synthesis of hydroxyl-
(16) (a) Zubarev, E. R.; Pralle, M. U.; Li, L. M.; Stupp, S. I. Science 1999,
283, 523-526. (b) Stupp, S. I.; Pralle, M. U.; Tew, G. N.; Li, L.; Sayer,
M.; Zubarev, E. R. MRS Bull. 2000, 25, 42-48.
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Dendron Rodcoils: Synthesis of Hybrid Structures
A R T I C L E S
Figure 3. Chemical structure (right) and the corresponding molecular graphics representation of dendron rodcoil molecule 24.
terminated G1 DRC 27 which has four OH groups on its
periphery.10 To convert 27 to high generation analogues, we
prepared a slightly different AB2 monomer (compound 26) based
on 3,5-dihydroxybenzoic acid (DHA) which can be used for
divergent growth of the dendritic block. Reaction between DHA
and 3 equiv of TBDMSCl gave the fully protected compound
25 which was converted to 26 via selective deprotection of the
carboxylic group under acidic conditions. Esterification reactions
and successive deprotection of TBDMS groups were carried
out under conditions similar to those used for convergent
synthesis. Repetition of these two types of reactions resulted in
formation of a hydrophobic silyl-terminated DRC series (from
G1 to G5) and a hydrophilic series of OH-terminated molecules.
For example, molecule 34 contains 64 TBDMS groups and has
a number average molecular weight (Mn) of 17 200 measured
by MALDI, whereas molecule 33 can be considered as a
dendritic amphiphile because it has 32 hydroxyl groups located
in the outer layer. Completion of the deprotection reactions was
monitored by NMR, GPC, and MALDI measurements for each
intermediate generation. The advantage of this approach lies in
the fact that TBDMS groups as well as hydroxyl groups are
located on the periphery of the dendritic shell, which makes
them easily accessible for deprotection reagents. As a result,
all the reactions in Scheme 4 proceeded fairly rapidly and with
nearly quantitative yields. Also, there was no solubility problem,
and molecules 28-34 were found to be highly soluble in organic
solvents.
out if this depends on the generation of the dendron. NMR
spectroscopy was very useful to monitor the completion of both
deprotection and esterification reactions (disappearance of
TBDMS and OH peaks, respectively). However, identification
of generational layers as described previously for the CF3-
terminated molecules was only possible for G2 and G3 products,
whereas high generation DRCs generate in this case rather broad
signals which partially overlap with those of protons from the
rod block. Therefore, measurements of absolute molecular
weight by mass spectrometry were necessary to confirm the
structure of high generation DRCs. Figure 4 shows MALDI-
TOF spectra of all five TBDMS-terminated molecules (G1
through G5). The average molecular weights of these structures
were found to be very close to theoretical values, and the
polydispersity indices calculated from MALDI spectra were
within the range from 1.004 to 1.01. As we mentioned
previously, DRC molecules are structurally diverse due to the
presence of the oligoisoprene coil segment synthesized by living
anionic polymerization. However, the polydispersity remains
nearly constant and does not depend on generation number. This
suggests that divergent growth of the dendritic block does not
result in the formation of a considerable amount of structural
defects. Similar MALDI spectra were obtained for a series of
hydrophilic DRC molecules terminated with hydroxyl groups.
These data gave us a rare opportunity to make a comparison
between molecular weights measured by two independent
methods such as MALDI and GPC. Figure 5 shows molecular
weight of silyl-terminated DRCs as a function of generation
number. The difference between GPC and MALDI data is
positive at low generation but decreases rapidly as the generation
number increases. At G4 both numbers practically coincide, and
for G5 molecules GPC molecular weight becomes even lower
than the absolute molecular weight measured by MALDI. It is
It is well known that formation of structural defects during
divergent synthesis can become a serious problem especially
for high generation molecules.17 We used different characteriza-
tion techniques to estimate the number of defects and to find
(17) Bosman, A. W.; Jansen, H. M.; Meijer, E. W. Chem. ReV. 1999, 99, 1665-
1688.
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Scheme 4
well known that molecular weight estimated by GPC relative
to polystyrene standards strongly depends on the shape and
flexibility of molecules. For example, rigid rods have greater
radii of gyration relative to polystyrene chains of the same
molecular weight. As a result, they can only pass through much
bigger pores of styrogel used in GPC columns, and that is why
the measured molecular weight of rigid molecules is always
overestimated by GPC. Low generation DRCs have fairly small
dendritic blocks, and their overall rigidity and anisometric shape
are still very high due to the presence of a much larger rodcoil
segment. As the generation increases, the contribution of the
dendron grows, and the difference between the GPC and
MALDI data becomes less significant. On the other hand, all
high generation dendrimers exhibit a significantly more compact
globular shape than do their linear counterparts.18 Therefore,
the density of high generation dendrimers is much greater than
that of linear polystyrene chains of the same molecular weight.
As a result, GPC measures a considerably lower molecular
weight as was previously described for various dendrimers.18,19
An underestimated molecular weight by GPC was observed here
only for G5 DRC molecules (Figure 5), indicating that the
contribution of the compact dendritic block becomes greater in
these molecules relative to that of the rigid biphenyl ester rod
segments.
Conclusions
A series of novel block structures containing dendritic,
rodlike, and coillike molecular segments termed dendron
rodcoils were successfully synthesized using either a convergent
or a divergent strategy. A combination of catalyzed esterification
reactions and silyl protection/deprotection chemistry proved to
be a very efficient and versatile method for construction of these
nanosized molecules with unusual molecular architecture. Both
synthetic strategies allowed for the preparation of nearly
monodisperse dendritic molecules as demonstrated by NMR,
MALDI-TOF, and GPC measurements. Properties of the highly
hydrophobic and amphiphilic DRC structures will be described
in an upcoming paper.
Experimental Section
General Procedures. Unless otherwise stated, all starting materials
were obtained from commercial suppliers and used without further
purification. Tetrahydrofuran (THF) and benzene were redistilled from
Na+/benzophenone ketyl, and 4-(N,N-dimethylamino)pyridinium-4-
toluenesulfonate (DPTS) was prepared as described elsewhere.13
Isoprene was redistilled under nitrogen and dried over calcium hydride
before use. The 1H NMR and spectra were recorded in THF-d8 or
DMSO-d6 solutions using a Varian Unity 500 (500 MHz) or Varian
(18) Xu, Z. F.; Moore J. S. Angew. Chem., Int. Ed. Engl. 1993, 32, 1354-
1357.
(19) (a) Mourey, T. H.; Turner, S. R.; Rubinstein, M.; Fre´chet, J. M. J.; Hawker,
C. J.; Wooley, K. L. Macromolecules 1992, 25, 2401. (b) Xu, Z. F.; Kahr,
M.; Walker, K. L.; Wilkins, C. L.; Moore, J. S. J. Am. Chem. Soc. 1994,
116, 4537-4550. (c) Murer, P.; Seebach, D. HelV. Chim. Acta 1998, 81,
603.
9
5768 J. AM. CHEM. SOC. VOL. 124, NO. 20, 2002

Dendron Rodcoils: Synthesis of Hybrid Structures
A R T I C L E S
Figure 4. MALDI-TOF traces of the TBDMS-terminated DRC molecules (G1 through G5) synthesized via divergent approach. The generation number of
the dendritic block is indicated in the upper left corner of each spectrum.
were collected on a Micromass 70-VSE spectrometer operating at 8
KV acceleration voltage and 4 KV extraction plate voltage. Matrix-
assisted laser desorption ionization (MALDI) mass spectra were
obtained on a VG TofSpec spectrometer using dithranol and silver
trifluoroacetate as a matrix for DRC molecules 28-34. Molecular
weights of compounds 9 and 10 were determined by MALDI using
trans-3-indoleacrylic acid (IAA) as a matrix. Elemental analysis was
performed using a Perkin-Elmer Model P2000 analyzer. Gel permeation
chromatography (GPC) was carried out on a Waters 486 chromatograph
in THF using styragel HMW 2 and styragel HMW 6E columns, 510
pumps, 486 UV-detector, and 410 RI detector.
General Procedure for Esterification Reactions (Convergent
Synthesis of Dendrons). These preparations were carried out on a scale
of 0.5-10 g depending on generation number. 5-tert-Butyldimethyl-
silyloxyisophthalic acid 2 (1 equiv), phenol (2.1 equiv), DPTS (2.2
equiv), and CH2Cl2 were all combined in an oven-dried flask with
stirring bar under N2. DIPC (2.2 equiv) was added via syringe, and the
reaction was allowed to stir for 5 h. Urea impurities were then removed
by precipitating the concentrated reaction mixture into methanol (three
times). The precipitate was dissolved in CH2Cl2, and the crude product
was purified by flash chromatography (silica gel).
Figure 5. Molecular weight of the TBDMS-terminated DRC molecules
as a function of the generation number of the dendritic segment measured
by MALDI-TOF and GPC, respectively.
Unity 400 (400 MHz) spectrometer using the solvent proton signal as
standard. 13C NMR spectra were recorded at 125 MHz on a Varian
Unity 500 spectrometer using THF-d8 or DMSO-d6 as solvents. The
following abbreviations are used: Ar refers to aromatic rings derived
from 5-hydroxyisophthalic acid, Ph refers to aromatic rings derived
from R,R,R-trifluoro-p-cresol, Ar′ refers to aromatic rings derived from
biphenyl fragments, and Ar′′ refers to aromatic rings derived from 3,5-
dihydroxybenzoic acid. High-resolution field desorption mass spectra
General Procedure for Silyl Deprotection (Convergent Synthesis
of Dendrons). These preparations were carried out on a scale of 0.5-
10 g depending on generation number. Silyl protected dendron (1 equiv)
was dissolved in THF in a plastic vessel. Hydrofluoric acid (49%
9
J. AM. CHEM. SOC. VOL. 124, NO. 20, 2002 5769

A R T I C L E S
Zubarev and Stupp
1
aqueous solution) was added via syringe (20 equiv). Depending on
generation number, the reaction mixture was allowed to stir for several
hours (1, 3, 8, 24, and 8 h for G1, G2, G3, G4, and G5 dendrons,
respectively). The reaction was diluted by CH2Cl2 and quenched by a
saturated solution of sodium bicarbonate. The resultant mixture was
washed several times with water, and the crude product was purified
by flash chromatography as outlined in the following text.
(mp 263 °C). Yield: 99%. H NMR (500 MHz, THF-d8): δ 7.50 (A
of ABq, d, 4H, J ) 8.4 Hz, CF3PhH), 7.80 (B of ABq, d, 4H, J ) 8.4
Hz, CF3PhH), 7.86 (d, 2H, J ) 1.5 Hz, ArH), 8.45 (t, 1H, J ) 2.1 Hz,
ArH), 9.29 (s, 1H, ArOH). 13C NMR (125 MHz, THF-d8): δ 122.53,
122.98, 123.46, 127.68, 128.61, 128.84, 132.18, 154.87, 159.50, 164.07.
FD-MS m/z 470.3 [M+], calcd M ) 470.3. Anal. Calcd for
C22H12F6O5: C, 56.17; H, 2.55; F, 24.26. Found: C, 56.29; H, 2.49; F,
24.19.
General Procedure for Esterification Reactions of Biphenyl
Derivatives. The acid (1 equiv), phenol (1 equiv), DPTS (1.2 equiv),
and CH2Cl2 were combined in a flask with stirring bar under nitrogen
atmosphere. DIPC (1.5 equiv) was added via syringe, and the reaction
was allowed to stir for 2 h. Urea impurities were then removed by
precipitating the concentrated reaction mixture into methanol (three
times). The precipitate was dissolved in CH2Cl2, and the crude product
was purified by flash chromatography as outlined in the following text.
General Procedure for Silyl Deprotection of Biphenyl Deriva-
tives. The protected phenol (or arylacid) (1 equiv) was dissolved in
THF and cooled to -78 °C. TBAF (1.0 M solution in THF, 1.2 equiv)
was added via syringe, and the reaction was stirred for 15 min at -78
°C. The reaction was quenched with 10% AcOH/THF. The reaction
was diluted with CH2Cl2 and washed with H2O. The organic layer was
collected, dried, filtered, and concentrated in vacuo. The crude product
was purified by flash chromatography as outlined in the following text.
Compound 1. 5-Hydroxyisophthalic acid (1 equiv) and imidazole
(3.8 equiv) were dissolved in N,N-dimethylformamide under N2. tert-
Butyldimethylsilyl chloride (3.5 equiv) was added, and the solution
was stirred overnight. The resulting mixture was diluted with CH2Cl2,
washed with saturated aqueous sodium bicarbonate and water, and then
dried over MgSO4. The solvent was removed by rotary evaporation,
and the product was purified by column chromatography (CH2Cl2) to
give 1 as a colorless liquid. Yield: 98%. 1H NMR (500 MHz, DMSO-
d6): δ 0.25 (s, 6H, OSi(CH3)2C(CH3)3), 0.48 (s, 12H, COOSi(CH3)2C-
(CH3)3), 0.97 (br s, 27H, Si(CH3)2C(CH3)3), 7.63 (d, 2H, J ) 1.2 Hz,
ArH), 8.21 (t, 1H, J ) 1.3 Hz, ArH). 13C NMR (125 MHz, THF-d8):
δ -4.33, 0.24, 18.26, 19.13, 26.18, 121.15, 124.91, 133.17, 156.90,
167.41. FD-MS m/z 524.5 (M+), calcd M ) 524.9. Anal. Calcd for
C26H48O5Si3: C, 59.54; H, 9.16; Si, 16.03. Found: C, 59.83; H, 9.29;
Si, 16.36.
(CF3)4-[G-2]-OTBDMS (5). This compound was prepared from 4
and 2 and was purified by flash chromatography eluting with CH2Cl2
to give 5 as a colorless glass. Yield: 88%. 1H NMR (500 MHz, THF-
d8): δ 0.30 (s, 6H, OSi(CH3)2C(CH3)3), 1.03 (s, 9H, OSi(CH3)2C-
(CH3)3), 7.53 (A of ABq, d, 8H, J ) 8.6 Hz, CF3PhH), 7.81 (B of
ABq, d, 8H, J ) 8.6 Hz, CF3PhH), 8.03 (d, 2H, J ) 1.7 Hz, ArH),
8.46 (d, 4H, J ) 1.7 Hz, ArH), 8.67 (t, 1H, J ) 1.7 Hz, ArH), 8.89 (t,
2H, J ) 1.6 Hz, ArH). 13C NMR (125 MHz, THF-d8): δ -4.4, 18.93,
25.68, 122.77, 123.41, 124.12, 126.28, 127.75, 128.82, 129.06, 129.52,
132.01, 132.60, 152.58, 154.73, 157.57, 163.41, 164.43. FD-MS m/z
1200.4 [M+], calcd M ) 1201.0. Anal. Calcd for C58H40F12O13Si: C,
58.00; H, 3.33; F, 19.00; Si, 2.33. Found: C, 58.21; H, 2.41; F, 19.09;
Si, 2.31.
(CF3)4-[G-2]-OH (6). This compound was prepared from 5 and was
purified by flash chromatography eluting with CH2Cl2 gradually
increasing to 10% THF/CH2Cl2 to give 6 as a colorless glass. Yield:
98%. 1H NMR (500 MHz, THF-d8): δ 7.54 (A of ABq, d, 8H, J ) 8.4
Hz, CF3PhH), 7.81 (B of ABq, d, 8H, J ) 8.4 Hz, CF3PhH), 7.93 (d,
2H, J ) 1.5 Hz, ArH), 8.46 (d, 4H, J ) 1.7 Hz, ArH), 8.56 (t, 1H, J
) 1.6 Hz, ArH), 8.89 (t, 2H, J ) 1.6 Hz, ArH), 9.39 (s, 1H, ArOH).
13C NMR (125 MHz, THF-d8): δ 122.79, 123.44, 124.10, 126.26,
127.74, 128.81, 129.06, 129.51, 131.99, 132.60, 152.60, 154.72, 159.58,
163.40, 164.40. FD-MS m/z 1086.45 [M+], calcd M ) 1086.74. Anal.
Calcd for C52H26F12O13: C, 57.46; H, 2.39; F, 20.99. Found: C, 57.71;
H, 2.46; F, 20.79.
(CF3)8-[G-3]-OTBDMS (7). This compound was prepared from 6
and 2 and was purified by flash chromatography eluting with 5% THF/
CH2Cl2 to give 7 as a colorless glass. Yield: 93%. 1H NMR (500 MHz,
THF-d8): δ 0.32 (s, 6H, OSi(CH3)2C(CH3)3), 1.03 (s, 9H, OSi(CH3)2C-
(CH3)3), 7.53 (A of ABq, d, 16H, J ) 8.5 Hz, CF3PhH), 7.81 (B of
ABq, d, 16H, J ) 8.5 Hz, CF3PhH), 8.04 (d, 2H, J ) 1.8 Hz, ArH),
8.50 (d, 8H, J ) 1.6 Hz, ArH), 8.53 (d, 4H, J ) 1.7 Hz, ArH), 8.68 (t,
1H, J ) 1.7 Hz, ArH), 8.91 (t, 4H, J ) 2.1 Hz, ArH), 9.01 (t, 2H, J
) 2.1 Hz, ArH). 13C NMR (125 MHz, THF-d8): δ -4.1, 19.01, 25.61,
121.87, 123.39, 124.06, 126.23, 127.71, 128.37, 128.58, 128.84, 129.11,
129.40, 129.55, 132.29, 132.56, 152.30, 152.52, 154.63, 157.73, 163.33,
163.64, 164.34. FD-MS m/z 2432.4 [M+], calcd M ) 2433.83. Anal.
Calcd for C118H68F24O29Si: C, 58.22; H, 2.80; F, 18.75; Si, 1.15.
Found: C, 58.23; H, 2.96; F, 18.86; Si, 1.23.
5-tert-Butyldimethylsilyloxyisophthalic Acid (2). Compound 1 (20
g) was dissolved in 300 mL of THF/H2O/AcOH mixture (50:10:40)
and stirred for 24 h. The reaction mixture was concentrated under
reduced pressure and precipitated into 500 mL of ethanol. The
precipitate was washed several times with saturated aqueous solution
of sodium bicarbonate and water and was then recrystallized from 30%
hexanes/CH2Cl2 to give 2 as white needlelike crystals (mp 176 °C).
1
Yield: 78%. H NMR (500 MHz, DMSO-d6): δ 0.15 (s, 6H, OSi-
(CH3)2C(CH3)3), 0.92 (s, 9H, OSi(CH3)2C(CH3)3), 7.50 (d, 2H, J )
1.1 Hz, ArH), 8.05 (t, 1H, J ) 1.3 Hz, ArH). 13C NMR (125 MHz,
THF-d8): δ -4.36, 18.24, 26.09, 120.49, 124.91, 133.12, 155.82,
166.83. FD-MS m/z 296.4 [M+], calcd M ) 296.4. Anal. Calcd for
C14H20O5Si: C, 56.76; H, 6.76; Si, 9.46. Found: C, 56.81; H, 6.79; Si,
9.38.
(CF3)2-[G-1]-OTBDMS (3). This compound was prepared from
R,R,R-trifluoro-p-cresol and 2 and was purified by flash chromatography
eluting with CH2Cl2 to give 3 as a white solid (mp 158 °C). Yield:
95%. 1H NMR (500 MHz, THF-d8): δ 0.28 (s, 6H, OSi(CH3)2C(CH3)3),
1.03 (s, 9H, OSi(CH3)2C(CH3)3), 7.50 (A of ABq, d, 4H, J ) 8.4 Hz,
CF3PhH), 7.80 (B of ABq, d, 4H, J ) 8.4 Hz, CF3PhH), 7.97 (d, 2H,
J ) 1.6 Hz, ArH), 8.55 (t, 1H, J ) 2.0 Hz, ArH). 13C NMR (125
MHz, THF-d8): δ -4.36, 18.92, 26.07, 122.53, 123.03, 123.47, 127.68,
128.62, 128.81, 132.24, 154.87, 158.60, 164.10. FD-MS m/z 584.4
[M+], calcd M ) 584.8. Anal. Calcd for C28H26F6O5Si: C, 57.53; H,
4.45; F, 19.52; Si, 4.79. Found: C, 57.68; H, 4.53; F, 19.87; Si, 4.77.
(CF3)2-[G-1]-OH (4). This compound was prepared from 3 and was
purified by flash chromatography eluting with CH2Cl2 gradually
increasing to 10% THF/CH2Cl2 to give 4 as white needlelike crystals
(CF3)8-[G-3]-OH (8). This compound was prepared from 7 and was
purified by flash chromatography eluting with CH2Cl2 gradually
increasing to 30% THF/CH2Cl2 to give 8 as a colorless glass. Yield:
1
97%. H NMR (500 MHz, THF-d8): δ 7.53 (A of ABq, d, 16H, J )
8.4 Hz, CF3PhH), 7.80 (B of ABq, d, 16H, J ) 8.4 Hz, CF3PhH), 7.95
(d, 2H, J ) 1.5 Hz, ArH), 8.50 (d, 8H, J ) 1.5 Hz, ArH), 8.53 (d, 4H,
J ) 1.7 Hz, ArH), 8.57 (t, 1H, J ) 1.6 Hz, ArH), 8.91 (t, 4H, J ) 2.1
Hz, ArH), 9.01 (t, 2H, J ) 2.1 Hz, ArH), 9.40 (s, 1H, ArOH). 13C
NMR (125 MHz, THF-d8): δ 121.88, 123.40, 124.04, 126.20, 127.73,
128.37, 128.57, 128.84, 129.09, 129.39, 129.56, 129.71, 132.28, 132.54,
152.30, 154.60, 159.71, 163.35, 163.64, 164.39. FD-MS m/z 2318.9
[M+], calcd M ) 2319.3. Anal. Calcd for C112H54F24O29: C, 57.98; H,
2.33; F, 19.67. Found: C, 58.11; H, 2.46; F, 19.79.
(CF3)16-[G-4]-OTBDMS (9). This compound was prepared from 8
and 2 and was purified by flash chromatography eluting with CH2Cl2
to give 9 as a colorless glass. Yield: 88%. 1H NMR (500 MHz, THF-
d8): δ 0.32 (s, 6H, OSi(CH3)2C(CH3)3), 1.04 (s, 9H, OSi(CH3)2C-
(CH3)3), 7.52 (A of ABq, d, 32H, J ) 8.6 Hz, CF3PhH), 7.79 (B of
ABq, d, 32H, J ) 8.6 Hz, CF3PhH), 8.03 (d, 2H, J ) 1.8 Hz, ArH),
9
5770 J. AM. CHEM. SOC. VOL. 124, NO. 20, 2002

Dendron Rodcoils: Synthesis of Hybrid Structures
A R T I C L E S
8.48 (d, 16H, J ) 1.5 Hz, ArH), 8.54 (d, 4H, J ) 1.7 Hz, ArH), 8.56
(d, 8H, J ) 1.6 Hz, ArH), 8.69 (t, 1H, J ) 1.7 Hz, ArH), 8.90 (t, 8H,
J ) 2.1 Hz, ArH), 9.02 (t, 6H, J ) 2.1 Hz, ArH). 13C NMR (125
MHz, THF-d8): δ -4.4, 18.92, 25.72, 121.89, 123.40, 124.05, 125.30,
126.21, 127.75, 128.38, 128.60, 128.86, 129.12, 129.44, 129.61, 129.71,
132.24, 132.52, 132.65, 152.40, 152.47, 152.59, 154.65, 157.55, 163.36,
163.66, 163.75, 164.17. MALDI-MS m/z 4896.7 [M+H]+, calcd M )
4896.1. Anal. Calcd for C238H124F48O61Si: C, 58.33; H, 2.53; F, 18.63;
Si, 0.57. Found: C, 58.42; H, 2.56; F, 18.79; Si, 0.63.
(s, 6H, OSi(CH3)2C(CH3)3), 0.46 (s, 12H, COOSi(CH3)2C(CH3)3), 1.01
(br s, 27H, Si(CH3)2C(CH3)3), 7.04 (d, 2H, J ) 8.6 Hz, Ar′H), 7.67 (d,
2H, J ) 8.6 Hz, Ar′H), 7.81 (d, 2H, J ) 8.5 Hz, Ar′H), 8.15 (d, 2H,
J ) 1.5 Hz, ArH), 8.26 (d, 2H, J ) 8.6 Hz, Ar′H), 8.65 (t, 1H, J ) 1.5
Hz, ArH). 13C NMR (125 MHz, THF-d8): δ -4.36, 0.24, 18.91, 19.03,
26.09, 121.43, 127.39, 127.92, 128.31, 128.80, 129.25, 131.49, 133.73,
146.91, 152.29, 157.16, 157.47, 164.98, 166.33. FD-MS m/z 720.4
[M+], calcd M ) 721.1. Anal. Calcd for C39H56O7Si3: C, 65.00; H,
7.78; Si, 11.67. Found: C, 65.17; H, 7.89; Si, 11.71.
(CF3)16-[G-4]-OH (10). This compound was prepared from 9 and
was purified by flash chromatography eluting with CH2Cl2 gradually
increasing to 10% THF/CH2Cl2 to give 10 as a colorless glass. Yield:
Compound 15. Compound 14 (5 g) was dissolved in 100 mL of
THF/H2O/AcOH mixture (50:10:40) and stirred for 36 h. The reaction
mixture was concentrated under reduced pressure and precipitated into
500 mL of ethanol. The precipitate was washed several times with
saturated aqueous solution of sodium bicarbonate and water and was
then recrystallized from 30% hexanes/CH2Cl2 to give 15 as a white
solid. Yield: 92%. 1H NMR (500 MHz, THF-d8): δ 0.26 (s, 6H, OSi-
(CH3)2C(CH3)3), 1.03 (s, 9H, OSi(CH3)2C(CH3)3), 6.98 (d, 2H, J )
8.6 Hz, Ar′H), 7.66 (d, 2H, J ) 8.6 Hz, Ar′H), 7.81 (d, 2H, J ) 8.4
Hz, Ar′H), 8.11 (d, 2H, J ) 1.5 Hz, ArH), 8.24 (d, 2H, J ) 8.6 Hz,
Ar′H), 8.58 (t, 1H, J ) 1.4 Hz, ArH). 13C NMR (125 MHz, THF-d8):
δ -4.36, 18.91, 26.06, 121.41, 127.36, 127.92, 128.29, 128.80, 129.22,
131.48, 133.70, 146.92, 152.27, 157.16, 157.31, 164.94, 166.27. FD-
MS m/z 492.5 [M+], calcd M ) 492.6. Anal. Calcd for C27H28O7Si:
C, 65.85; H, 5.69; Si, 5.69. Found: C, 65.43; H, 5.87; Si, 5.73.
(CF3)32-[G-5]-1bp-OTBDMS (16). This compound was prepared
from 15 and 10 and was purified by flash chromatography eluting with
1
82%. H NMR (500 MHz, THF-d8): δ 7.51 (A of ABq, d, 32H, J )
8.4 Hz, CF3PhH), 7.78 (B of ABq, d, 32H, J ) 8.4 Hz, CF3PhH), 7.94
(d, 2H, J ) 1.5 Hz, ArH), 8.48 (d, 16H, J ) 1.3 Hz, ArH), 8.54 (d,
4H, J ) 1.7 Hz, ArH), 8.56 (d, 8H, J ) 1.6 Hz, ArH), 8.58 (t, 1H, J
) 1.6 Hz, ArH), 8.90 (t, 8H, J ) 2.1 Hz, ArH), 9.02 (t, 6H, J ) 2.1
Hz, ArH), 9.40 (s, 1H, ArOH). 13C NMR (125 MHz, THF-d8): δ
121.91, 123.38, 124.08, 126.23, 127.75, 128.37, 128.61, 128.86, 129.10,
129.44, 129.61, 129.74, 132.02, 132.20, 132.51, 132.65, 152.40, 152.47,
152.61, 154.67, 159.52, 163.37, 163.64, 163.75, 164.23. MALDI-MS
m/z 4782.7 [M + H]+, calcd M ) 4782.2. Anal. Calcd for
C
232H110F48O61: C, 58.22; H, 2.30; F, 19.07. Found: C, 58.29; H, 2.45;
F, 19.03.
Compound 11. A dry flask was charged with 4′-hydroxy-4-
biphenylcarboxylic acid (1 equiv), imidazole (2.2 equiv), and dichlo-
romethane. After stirring for 10 min, tert-butyldimethylsilyl chloride
(2.6 equiv) was added. The solution was stirred overnight at room
temperature, then diluted with dichloromethane and washed several
times with saturated aqueous sodium bicarbonate and water. The organic
layers were then dried over MgSO4 and filtered, and the solvent was
removed by rotary evaporation. The product was purified by flash
chromatography eluting with CH2Cl2 to give 11 as a colorless liquid.
1
CH2Cl2 to give 16 as a colorless glass. Yield: 89%. H NMR (500
MHz, THF-d8): δ 0.28 (s, 6H, OSi(CH3)2C(CH3)3), 1.04 (s, 9H, OSi-
(CH3)2C(CH3)3), 6.99 (d, 2H, J ) 8.6 Hz, Ar′H), 7.50 (A of ABq, d,
64H, J ) 10.2 Hz, CF3PhH), 7.68 (d, 2H, J ) 8.6 Hz, Ar′H), 7.77 (B
of ABq, d, 64H, J ) 10.2 Hz, CF3PhH), 7.83 (d, 2H, J ) 8.6 Hz,
Ar′H), 8.27 (d, 2H, J ) 8.6 Hz, Ar′H), 8.44 (d, 32H, J ) 1.5 Hz,
ArH), 8.52 (d, 18H, J ) 1.7 Hz, ArH), 8.55 (d, 8H, J ) 1.7 Hz, ArH),
8.59 (d, 4H, J ) 1.8 Hz, ArH), 8.88 (m, 16H, ArH), 8.96 (t, 1H, J )
1.6 Hz, ArH), 8.99 (m, 8H, ArH), 9.03 (m, 6H, ArH). 13C NMR (125
MHz, THF-d8): δ -4.46, 19.03, 26.09, 121.72, 123.27, 123.43, 123.68,
123.87, 124.10, 126.02, 126.26, 127.62, 128.20, 129.09, 129.49, 129.61,
131.52, 132.03, 132.21, 132.59, 132.66, 147.21, 151.26, 151.41, 151.77,
152.42, 152.58, 152.83, 153.82, 154.26, 154.68, 157.58, 160.71, 162.24,
162.39, 162.72, 163.40, 164.17. Anal. Calcd for C491H244F96O127Si: C,
58.80; H, 2.44; F, 18.20; Si, 0.28. Found: C, 58.69; H, 2.49; F, 18.14;
Si, 0.33.
1
Yield: 91%. H NMR (500 MHz, THF-d8): δ 0.26 (br s, 12H, OSi-
(CH3)2C(CH3)3), 1.04 (br s, 18H, OSi(CH3)2C(CH3)3), 6.96 (d, 2H, J
) 8.5 Hz, Ar′H), 7.60 (d, 2H, J ) 8.6 Hz, Ar′H), 7.69 (d, 2H, J ) 8.6
Hz, Ar′H), 8.16 (d, 2H, J ) 8.4 Hz, Ar′H). FD-MS m/z 442.4 [M+],
calcd M ) 442.7. Anal. Calcd for C25H38O3Si2: C, 67.87; H, 8.60; Si,
12.67. Found: C, 67.72; H, 8.71; Si, 12.35.
4′-tert-Butyldimethylsilyloxy-4-biphenylcarboxylic Acid (12). This
compound was prepared from 11 and was purified by flash chroma-
tography eluting with 30% THF/CH2Cl2 and recrystallization from 70%
hexane/CH2Cl2 to give 12 as white crystals (mp 245 °C). Yield: 87%.
1H NMR (500 MHz, THF-d8): δ 0.24 (s, 6H, OSi(CH3)2C(CH3)3), 1.02
(s, 9H, OSi(CH3)2C(CH3)3), 6.94 (d, 2H, J ) 8.6 Hz, Ar′H), 7.58 (d,
2H, J ) 8.6 Hz, Ar′H), 7.66 (d, 2H, J ) 8.4 Hz, Ar′H), 8.06 (d, 2H,
J ) 8.4 Hz, Ar′H). FD-MS m/z 328.4 [M+], calcd M ) 328.5. Anal.
Calcd for C19H24O3Si: C, 69.51; H, 7.32; Si, 8.54. Found: C, 70.12;
H, 7.55; Si, 8.39.
(CF3)32-[G-5]-1bp-OH (17). This compound was prepared from 16
and was purified by flash chromatography eluting with 10% THF/CH2-
1
Cl2 to give 17 as a colorless glass. Yield: 94%. H NMR (500 MHz,
THF-d8): δ 6.85 (d, 2H, J ) 8.6 Hz, Ar′H), 7.48 (A of ABq, d, 64H,
J ) 10.2 Hz, CF3PhH), 7.57 (d, 2H, J ) 8.4 Hz, Ar′H), 7.75 (d, 66H,
B of ABq, CF3PhH (64), J ) 10.2 Hz and Ar′H (2)), 8.21 (d, 2H, J )
8.6 Hz, Ar′H), 8.44 (d, 32H, J ) 1.5 Hz, ArH), 8.52 (d, 19H, J ) 1.7
Hz, ArH(18) and Ar′OH (1)), 8.55 (d, 8H, J ) 1.7 Hz, ArH), 8.59 (d,
4H, J ) 1.8 Hz, ArH), 8.87 (t, 16H, ArH), 8.95 (t, 1H, J ) 1.6 Hz,
ArH), 8.99 (t, 8H, ArH), 9.02 (t, 6H, ArH). 13C NMR (125 MHz, THF-
d8): δ 121.69, 123.28, 123.42, 123.70, 123.86, 124.08, 126.02, 126.24,
127.62, 128.21, 129.10, 129.47, 129.63, 131.52, 132.01, 132.19, 132.59,
132.66, 147.23, 151.25, 151.41, 151.75, 152.42, 152.56, 152.85, 153.82,
154.22, 154.65, 159.52, 160.71, 162.21, 162.39, 162.71, 163.40, 164.18.
GPC (254 nm, THF) PDI ) 1.03, Mn ) 12 121. Anal. Calcd for
Compound 13. Morpholine (2 equiv), 5-hydroxy-isophthalic acid
(1 equiv), and N,N-dimethylformamide were placed in a flask and
stirred. tert-Butyldimethylsilyl chloride (2 equiv) was added, and the
solution was stirred at room temperature for 3 min. The resulting
mixture was diluted with dichloromethane, washed three times with
water, and dried over MgSO4. The solvent was removed by rotary
evaporation, and the product was purified by flash chromatography
eluting with 5% THF/CH2Cl2 to give 13 as a colorless liquid. Yield:
75%. 1H NMR (500 MHz, THF-d8): δ 0.42 (s, 12H, Si(CH3)2C(CH3)3),
1.05 (s, 18H, Si(CH3)2C(CH3)3), 7.41 (d, 2H, J ) 1.2 Hz, ArH), 8.12
(t, 1H, J ) 1.3 Hz, ArH), 9.12 (s, 1H, ArOH). 13C NMR (125 MHz,
THF-d8): δ 0.21, 19.08, 26.08, 121.12, 124.87, 133.15, 156.85, 167.35.
FD-MS m/z 410.6 [M+], calcd M ) 410.6. Anal. Calcd for C20H34O5-
Si2: C, 58.54; H, 8.29; Si, 13.66. Found: C, 58.78; H, 8.41; Si, 13.38.
Compound 14. This compound was prepared from 13 and 12 and
was purified by flash chromatography eluting with CH2Cl2 to give 14
C
485H230F96O127: C, 58.75; H, 2.32; F, 18.41. Found: C, 58.59; H, 2.42;
F, 18.13.
Compound 18. Morpholine (1 equiv), 4′-hydro-4-biphenylcarboxylic
acid (1 equiv), and N,N-dimethylformamide were placed in a flask and
stirred. tert-Butyldimethylsilyl chloride (1 equiv) was added, and the
solution was stirred at room temperature for 3 min. The resulting
mixture was diluted with CH2Cl2 and washed three times with water.
The solvent was removed by rotary evaporation, and the crude product
was purified by flash chromatography eluting with 5% THF/CH2Cl2
1
as a white solid. Yield: 86%. H NMR (500 MHz, THF-d8): δ 0.26
9
J. AM. CHEM. SOC. VOL. 124, NO. 20, 2002 5771

A R T I C L E S
Zubarev and Stupp
1
to give 18 as a white solid. Yield: 75%. H NMR (500 MHz, THF-
d8): δ 0.44 (s, 6H, OSi(CH3)2C(CH3)3), 0.98 (s, 9H, OSi(CH3)2C-
(CH3)3), 6.98 (d, 2H, J ) 8.6 Hz, Ar′H), 7.52 (d, 2H, J ) 8.6 Hz,
Ar′H), 7.63 (d, 2H, J ) 8.6 Hz, Ar′H), 8.23 (d, 2H, J ) 8.5 Hz, Ar′H),
8.44 (s, 1H, Ar′OH). FD-MS m/z 328.3 [M+], calcd M ) 328.5. Anal.
Calcd for C19H24O3Si: C, 69.51; H, 7.32; Si, 8.54. Found: C, 69.27;
H, 7.61; Si, 8.08.
Hz, ArH), 8.87 (t, 16H, ArH), 8.97 (t, 1H, J ) 1.6 Hz, ArH), 9.00 (t,
8H, ArH), 9.03 (t, 6H, ArH). 13C NMR (125 MHz, THF-d8): δ 14.43,
19.23, 23.52, 24.98, 28.10, 30.01, 32.79, 40.21, 43.05, 45.82, 63.42,
65.23, 67.93, 108.91, 108.96, 109.03, 121.56, 121.72, 123.27, 123.40,
123.68, 123.76, 124.10, 126.02, 126.13, 127.67, 128.05, 128.24, 129.10,
129.43, 129.67, 130.24, 131.52, 131.98, 132.03, 132.19, 132.59, 132.63,
147.26, 150.79, 151.25, 151.41, 151.75, 152.42, 152.56, 152.85, 153.82,
154.22, 154.65, 157.58, 160.71, 162.34, 162.39, 162.77, 163.42, 164.18.
Calcd M ) 11 032. Found MALDI-TOF: Mn ) 11 000. GPC (254
nm, THF) PDI ) 1.08, Mn ) 13 450.
General Procedure for Esterification Reactions (Divergent
Synthesis of the Dendritic Part of Dendron Rodcoils). These
preparations were carried out on a scale of 4-12 g depending on
generation number. A mixture of the appropriate dendron rodcoil (HO)m-
[G-n]-3bp-PI9h (1 equiv), 3,5-bis(tert-butyldimethylsilyloxy)benzoic acid
26 ((m + 1) equiv), and DPTS ((m + 1) equiv) was dissolved in CH2-
Cl2 and stirred under nitrogen. DIPC ((m + 1) equiv) was added via
syringe, and the reaction was allowed to stir for 3 h. Urea impurities
were then removed by precipitating the concentrated reaction mixture
into methanol (three times). The precipitate was dissolved in CH2Cl2,
and the crude product was purified by flash chromatography as outlined
in the following text.
General Procedure for Silyl Deprotection (Divergent Synthesis
of the Dendritic Part of Dendron Rodcoils). These preparations were
carried out on a scale of 4-12 g depending on generation number.
Silyl protected dendron rodcoil (TBDMSO)m-[G-n]-3bp-PI9h (1 equiv)
was dissolved in THF in a plastic vessel. Hydrofluoric acid (49%
aqueous solution) was added via syringe ((m + 10) equiv). Depending
on generation number, the reaction mixture was allowed to stir for
several hours (from 24 to 36 h). The reaction was diluted by CH2Cl2
and quenched by a saturated solution of sodium bicarbonate. The
resultant mixture was washed several times with water, and the crude
product was purified by flash chromatography as outlined in the
following text.
Compound 25. 3,5-Dihydroxybenzoic acid (1 equiv) and imidazole
(3.5 equiv) were dissolved in N,N-dimethylformamide under nitrogen
atmosphere. tert-Butyldimethylsilyl chloride (3.5 equiv) was added, and
the solution was stirred overnight. The resulting mixture was diluted
with CH2Cl2, washed with water, and dried over MgSO4. The solvent
was removed by rotary evaporation, and the product was purified by
flash chromatography (CH2Cl2) to give 25 as a colorless liquid. Yield:
96%. 1H NMR (500 MHz, THF-d8): δ 0.23 (s, 12H, OSi(CH3)2C-
(CH3)3), 0.47 (s, 6H, COOSi(CH3)2C(CH3)3), 1.01 (br s, 27H, Si-
(CH3)2C(CH3)3), 6.60 (t, 1H, J ) 2.2 Hz, Ar′′H), 7.22 (d, 2H, J ) 2.4
Hz, Ar′′H). 13C NMR (125 MHz, THF-d8): δ -4.31, 0.17, 18.98, 19.19,
26.17, 115.65, 117.31, 133.97, 157.96, 167.40. FD-MS m/z 496.5 [M+],
calcd M ) 496.9. Anal. Calcd for C25H48O4Si3: C, 60.48; H, 9.68; Si,
16.94. Found: C, 60.63; H, 9.74; Si, 16.69.
PI9h-OH (19). THF (100 mL) was placed in a flask under nitrogen
atmosphere, and n-BuLi (1 equiv) was added followed by the addition
of isoprene (9 equiv). The reaction was stirred for 30 min, then
quenched by bubbling ethylene oxide through the solution for 15 min
followed by addition of 10 mL of 3 N HCl/THF (1/2). The solvent was
removed by rotary evaporation. The crude material was purified by
flash chromatography eluting with 50% CH2Cl2/hexane gradually
increasing to CH2Cl2 to give 19 as a colorless liquid. Yield: 73%. 1H
NMR (500 MHz, THF-d8): δ 0.5-2.8 (br m, 70H aliphatic), 3.61 (br
m, 2H, CH2OH), 4.5-5.2 (br m, 14H vinyl). FD-MS m/z 730, 663,
596, 797, 864. Anal. Calcd for C51H86O2: C, 83.83; H, 11.78. Found:
C, 85.57; H, 12.42.
PI9h-1bp-COOTBDMS (20). Compound 18 (1.3 equiv), oligoiso-
prene coil 19 (1 equiv), and triphenyl phosphine (1.5 equiv) were
dissolved in CH2Cl2. After stirring for 5 min, diethyl azodicarboxylate
(1.5 equiv) was added dropwise, and the reaction mixture was allowed
to stir for 2 h. The resulting mixture was diluted with CH2Cl2 and
washed three times with water. The solvent was removed by rotary
evaporation, and the crude product was purified by flash chromatog-
raphy eluting with CH2Cl2 to give 20 as a colorless liquid. Yield: 60%.
1H NMR (500 MHz, THF-d8): δ 0.46 (s, 6H, OSi(CH3)2C(CH3)3), 0.5-
2.8 (br m, 80H aliphatic), 4.03 (br m, 2H, CH2OAr′), 4.5-5.2 (br m,
14H vinyl), 7.01 (d, 2H, J ) 8.7 Hz, Ar′H), 7.58 (d, 2H, J ) 8.6 Hz,
Ar′H), 7.67 (d, 2H, J ) 8.6 Hz, Ar′H), 8.21 (d, 2H, J ) 8.5 Hz, Ar′H).
GPC (254 nm, THF) PDI ) 1.09, Mn ) 1260.
PI9h-1bp-COOH (21). This was prepared from 20 and purified by
flash chromatography eluting with CH2Cl2 gradually increasing to 30%
THF/CH2Cl2 to give 21 as a colorless liquid. Yield: 87%. H NMR
(500 MHz, THF-d8): δ 0.5-2.8 (br m, 70H aliphatic), 4.01 (br m, 2H,
CH2OAr′), 4.5-5.2 (br m, 14H vinyl), 7.01 (d, 2H, J ) 8.6 Hz, Ar′H),
7.55 (d, 2H, J ) 8.6 Hz, Ar′H), 7.65 (d, 2H, J ) 8.6 Hz, Ar′H), 8.14
(d, 2H, J ) 8.5 Hz, Ar′H). GPC (254 nm, THF) PDI ) 1.08, Mn )
1109.
1
PI9h-2bp-COOTBDMS (22). This was prepared from compounds
21 and 18 and purified by flash chromatography eluting with CH2Cl2
1
to give 22 as a tacky solid. Yield: 86%. H NMR (500 MHz, THF-
d8): δ 0.46 (s, 6H, OSi(CH3)2C(CH3)3), 0.5-2.9 (br m, 80H aliphatic),
4.04 (br m, 2H, CH2OAr′), 4.5-5.2 (br m, 14H vinyl), 7.04 (d, 2H, J
) 8.6 Hz, Ar′H), 7.42 (d, 2H, J ) 8.4 Hz, Ar′H), 7.70 (d, 2H, J ) 8.5
Hz, Ar′H), 7.82 (m, 6H, Ar′H), 8.21 (d, 2H, J ) 8.6 Hz, Ar′H), 8.25
(d, 2H, J ) 8.7 Hz, Ar′H). GPC (254 nm, THF) PDI ) 1.07, Mn )
1670.
PI9h-2bp-COOH (23). This was prepared from 22 and purified by
flash chromatography eluting with CH2Cl2 gradually increasing to 30%
THF/CH2Cl2 to give 23 as a white solid. Yield: 96%. H NMR (500
MHz, THF-d8): δ 0.5-2.9 (br m, 70H aliphatic), 4.04 (br m, 2H, CH2-
OAr′), 4.5-5.2 (br m, 14H vinyl), 7.03 (d, 2H, J ) 8.6 Hz, Ar′H),
7.41 (d, 2H, J ) 8.4 Hz, Ar′H), 7.68 (d, 2H, J ) 8.6 Hz, Ar′H), 7.81
(m, 6H, Ar′H), 8.16 (d, 2H, J ) 8.6 Hz, Ar′H), 8.24 (d, 2H, J ) 8.5
Hz, Ar′H). GPC (254 nm, THF) PDI ) 1.07, Mn ) 1410.
Compound 26. Compound 25 (18 g) was dissolved in 300 mL of
THF/H2O/AcOH mixture (50:10:40) and stirred for 24 h. The reaction
mixture was concentrated under reduced pressure and precipitated into
500 mL of ethanol. The precipitate was washed several times with
saturated aqueous solution of sodium bicarbonate and water and then
was purified by flash chromatography eluting with CH2Cl2 gradually
increasing to 40% THF/CH2Cl2 and recrystallization from 40% hexanes/
CH2Cl2 to give 26 as a white solid. Yield: 98%. 1H NMR (500 MHz,
THF-d8): δ 0.22 (s, 12H, OSi(CH3)2C(CH3)3), 1.00 (s, 18H, Si(CH3)2C-
(CH3)3), 6.54 (t, 1H, J ) 2.2 Hz, Ar′′H), 7.14 (d, 2H, J ) 2.4 Hz,
Ar′′H). 13C NMR (125 MHz, THF-d8): δ -4.29, 18.88, 26.10, 115.47,
117.09, 133.80, 157.41, 167.21. FD-MS m/z 382.4 [M+], calcd M )
382.6. Anal. Calcd for C19H34O4Si2: C, 59.69; H, 8.90; Si, 14.65.
Found: C, 59.73; H, 8.99; Si, 14.61.
1
(CF3)32-[G-5]-3bp-PI9h (24). This was prepared from compound 17
and 23 and purified by flash chromatography eluting with CH2Cl2 to
1
give 24 as a colorless glass. Yield: 89%. H NMR (500 MHz, THF-
d8): δ 0.5-2.8 (br m, 70H aliphatic), 4.04 (br m, 2H, CH2OAr′), 4.5-
5.2 (br m, 14H vinyl), 7.42 (d, 6H, J ) 8.6 Hz, Ar′H), 7.48 (A of
ABq, d, 64H, J ) 9.1 Hz, CF3PhH), 7.75 (d, 66H, B of ABq, CF3PhH
(64) and Ar′H (2)), 7.83 (m, 6H, Ar′H), 7.90 (d, 4H, J ) 7.8 Hz, Ar′H),
8.27 (m, 6H, Ar′H), 8.44 (d, 32H, J ) 1.5 Hz, ArH), 8.52 (d, 18H, J
) 1.6 Hz, ArH), 8.55 (d, 8H, J ) 1.6 Hz, ArH), 8.60 (d, 4H, J ) 1.4
(TBDMSO)8-[G-2]-3bp-PI9h (28). This was prepared from com-
pounds 27 and 26 and was purified by flash chromatography eluting
1
with CH2Cl2 to give 28 as a white solid. Yield: 97%. H NMR (500
MHz, THF-d8): δ 0.26 (s, 48H, OSi(CH3)2C(CH3)3), 0.5-2.8 (br m,
9
5772 J. AM. CHEM. SOC. VOL. 124, NO. 20, 2002

Dendron Rodcoils: Synthesis of Hybrid Structures
A R T I C L E S
142H aliphatic), 4.30 (br m, 2H, CH2OCOAr′), 4.5-5.2 (br m, 14H
vinyl), 6.67 (t, 4H, J ) 2.3 Hz, Ar′′H), 7.32 (d, 8H, J ) 2.4 Hz, Ar′′H),
7.43 (m, 6H, Ar′H), 7.61 (t, 2H, J ) 2.1 Hz, Ar′′H), 7.78 (m, 5H,
Ar′H(4) and Ar′′H(1)), 7.85 (d, 4H, J ) 8.6 Hz, Ar′H), 7.89 (d, 4H, J
) 8.4 Hz, Ar′H), 8.07 (d, 4H, J ) 2.2 Hz, Ar′′H), 8.09 (m, 2H, Ar′H),
8.13 (d, 2H, J ) 2.2 Hz, Ar′′H), 8.29 (d, 4H, J ) 8.2 Hz, Ar′H). Calcd
M ) 3166. Found MALDI-TOF: Mn ) 3200. GPC (254 nm, THF)
PDI ) 1.06, Mn ) 3890.
(TBDMSO)32-[G-4]-3bp-PI9h (32). This was prepared from 31 and
was purified by flash chromatography eluting with CH2Cl2 to give 32
as a white solid. Yield: 97%. H NMR (500 MHz, THF-d8): δ 0.26
1
(s, 192H, OSi(CH3)2C(CH3)3), 0.5-2.8 (br m, 358H aliphatic), 4.30
(br m, 2H, CH2OCOAr′), 4.5-5.2 (br m, 14H vinyl), 6.67 (t, 16H, J
) 2.3 Hz, Ar′′H), 7.32 (d, 32H, J ) 2.2 Hz, Ar′′H), 7.43 (m, 6H,
Ar′H), 7.60 (t, 8H, J ) 2.1 Hz, Ar′′H), 7.78 (t, 6H, J ) 2.1 Hz, Ar′′H),
7.80 (m, 5H, Ar′H(4) and Ar′′H(1)), 7.86 (m, 4H, Ar′H), 7.90 (m, 4H,
Ar′H), 8.07 (d, 16H, J ) 2.2 Hz, Ar′′H), 8.09 (d, 2H, J ) 2.2 Hz,
Ar′′H), 8.11 (m, 2H, Ar′H), 8.16 (m, 12H, Ar′′H), 8.30 (d, 4H, J )
8.0 Hz, Ar′H). Calcd M ) 9167. Found MALDI-TOF: Mn ) 9200.
GPC (254 nm, THF) PDI ) 1.06, Mn ) 9190.
(HO)8-[G-2]-3bp-PI9h (29). This was prepared from 28 and was
purified by flash chromatography eluting with 30% THF/CH2Cl2 to
1
give 29 as a white solid. Yield: 98%. H NMR (500 MHz, THF-d8):
δ 0.5-2.8 (br m, 70H aliphatic), 4.30 (br m, 2H, CH2OCOAr′), 4.5-
5.2 (br m, 14H vinyl), 6.50 (t, 4H, J ) 2.2 Hz, Ar′′H), 7.09 (d, 8H, J
) 2.3 Hz, Ar′′H), 7.42 (m, 6H, Ar′H), 7.59 (t, 2H, J ) 2.2 Hz, Ar′′H),
7.78 (m, 5H, Ar′H(4) and Ar′′H(1)), 7.84 (d, 4H, J ) 8.4 Hz, Ar′H),
7.88 (d, 4H, J ) 8.3 Hz, Ar′H), 8.02 (d, 4H, J ) 2.2 Hz, Ar′′H), 8.09
(m, 2H, Ar′H), 8.12 (d, 2H, J ) 2.2 Hz, Ar′′H), 8.29 (d, 4H, J ) 8.0
Hz, Ar′H), 8.56 (s, 8H, Ar′′OH). Calcd M ) 2254. Found MALDI-
TOF: Mn ) 2265. GPC (254 nm, THF) PDI ) 1.07, Mn ) 3040.
(TBDMSO)16-[G-3]-3bp-PI9h (30). This was prepared from 29 and
was purified by flash chromatography eluting with CH2Cl2 to give 30
(HO)32-[G-4]-3bp-PI9h (33). This was prepared from 32 and was
purified by flash chromatography eluting with 50% THF/CH2Cl2 to
1
give 33 as a white solid. Yield: 92%. H NMR (500 MHz, THF-d8):
δ 0.5-2.8 (br m, 70H aliphatic), 4.30 (br m, 2H, CH2OCOAr′), 4.5-
5.2 (br m, 14H vinyl), 6.48 (t, 16H, J ) 2.2 Hz, Ar′′H), 7.07 (d, 32H,
J ) 2.2 Hz, Ar′′H), 7.42 (m, 6H, Ar′H), 7.58 (t, 8H, J ) 2.2 Hz, Ar′′H),
7.77 (m, 7H, Ar′′H), 7.92-7.79 (m, 12H, Ar′H), 8.02 (d, 16H, J ) 2.2
Hz, Ar′′H), 8.08 (d, 2H, J ) 2.2 Hz, Ar′′H), 8.11 (m, 2H, Ar′H), 8.15
(d, 12H, J ) 2.2 Hz, Ar′′H), 8.29 (m, 4H, Ar′H), 8.56 (s, 32H, Ar′′OH).
Calcd M ) 5519. Found MALDI-TOF: Mn ) 5528. GPC (254 nm,
THF) PDI ) 1.08, Mn ) 6230.
1
as a white solid. Yield: 96%. H NMR (500 MHz, THF-d8): δ 0.26
(s, 96H, OSi(CH3)2C(CH3)3), 0.5-2.8 (br m, 214H aliphatic), 4.30 (br
m, 2H, CH2OCOAr′), 4.5-5.2 (br m, 14H vinyl), 6.68 (t, 8H, J ) 2.3
Hz, Ar′′H), 7.33 (d, 16H, J ) 2.2 Hz, Ar′′H), 7.44 (m, 6H, Ar′H),
7.61 (t, 4H, J ) 2.2 Hz, Ar′′H), 7.79 (m, 7H, Ar′H(4) and Ar′′H(3)),
7.86 (m, 4H, Ar′H), 7.90 (m, 4H, Ar′H), 8.08 (d, 8H, J ) 2.2 Hz,
Ar′′H), 8.12 (m, 4H, Ar′H(2) and Ar′′H(2)), 8.17 (d, 4H, J ) 2.2 Hz,
Ar′′H), 8.30 (d, 4H, J ) 8.0 Hz, Ar′H). Calcd M ) 5167. Found
MALDI-TOF: Mn ) 5181. GPC (254 nm, THF) PDI ) 1.06, Mn )
6280.
(TBDMSO)64-[G-5]-3bp-PI9h (34). This was prepared from com-
pounds 33 and 26 and was purified by flash chromatography eluting
1
with CH2Cl2 to give 34 as a white solid. Yield: 91%. H NMR (500
MHz, THF-d8): δ 0.26 (s, 384H, OSi(CH3)2C(CH3)3), 0.5-2.8 (br m,
646H aliphatic), 4.30 (br m, 2H, CH2OCOAr′), 4.5-5.2 (br m, 14H
vinyl), 6.64 (t, 32H, J ) 2.2 Hz, Ar′′H), 7.30 (d, 64H, J ) 2.4 Hz,
Ar′′H), 7.41 (m, 6H, Ar′H), 7.58 (t, 16H, J ) 2.1 Hz, Ar′′H), 7.76 (t,
15H, J ) 2.0 Hz, Ar′′H), 7.80 (m, 8H, Ar′H), 7.89 (m, 4H, Ar′H),
8.05 (d, 32H, J ) 2.1 Hz, Ar′′H), 8.10 (m, 2H, Ar′H), 8.14 (m, 30H,
Ar′′H), 8.29 (d, 4H, J ) 8.0 Hz, Ar′H). Calcd M ) 17 168. Found
MALDI-TOF: Mn ) 17 195. GPC (254 nm, THF) PDI ) 1.06, Mn )
13 570.
(HO)16-[G-3]-3bp-PI9h (31). This was prepared from 30 and was
purified by flash chromatography eluting with 50% THF/CH2Cl2 to
1
give 31 as a white solid. Yield: 98%. H NMR (400 MHz, THF-d8):
δ 0.5-2.8 (br m, 70H aliphatic), 4.30 (br m, 2H, CH2OCOAr′), 4.5-
5.2 (br m, 14H vinyl), 6.50 (t, 8H, J ) 2.2 Hz, Ar′′H), 7.09 (d, 16H,
J ) 2.2 Hz, Ar′′H), 7.43 (m, 6H, Ar′H), 7.59 (t, 4H, J ) 2.2 Hz, Ar′′H),
7.79 (m, 7H, Ar′H(4) and Ar′′H(3)), 7.85 (m, 4H, Ar′H), 7.90 (d, 4H,
J ) 8.0 Hz, Ar′H), 8.03 (d, 8H, J ) 2.2 Hz, Ar′′H), 8.10 (m, 2H,
Ar′H), 8.13 (d, 2H, J ) 2.2 Hz, Ar′′H), 8.17 (d, 4H, J ) 2.2 Hz, Ar′′H),
8.30 (d, 4H, J ) 8.1 Hz, Ar′H), 8.58 (s, 16H, Ar′′OH). Calcd M )
3343. Found MALDI-TOF: Mn ) 3362. GPC (254 nm, THF) PDI )
1.06, Mn ) 3950.
Acknowledgment. This work was supported by the Depart-
ment of Energy, the Army Research Office MURI, and the
National Science Foundation.
JA020071T
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J. AM. CHEM. SOC. VOL. 124, NO. 20, 2002 5773