Efficient synthesis of porphyrin-containing, benzoquinone-terminated, rigid polyphenylene dendrimers

Article abstract of DOI:10.1021/jo026520p

A series of rigid polyphenylene, free-base porphyrin-containing dendrimers terminated with either dimethoxybenzene or benzoquinone end-groups were prepared by a combined divergent and convergent synthesis. Unlike previous routes for preparing polyphenylene dendrimers that are incompatible with end-groups bearing certain functional moieties, the synthetic methodology chosen for this work enables incorporation of functional groups on the dendrimer end-groups during preparation of the dendrimer wedges and during synthesis of the final dendrimer. The basic strategy utilized a convergent preparation of dendrimer wedges using Suzuki coupling conditions, which were then either attached to a porphyrin core in a divergent coupling step or cyclized to form the porphyrin dendrimer in a convergent step. The latter approach was found to be more general and resulted in higher yields and more readily separated products. Steady-state absorption measurements for these dendrimers showed Soret and Q-band absorptions typical of free-base porphyrins. Preliminary steady-state fluorescence measurements of these dendrimers indicate quenching of the S₁ state of the free-base porphyrin in all benzoquinone-containing dendrimers that is attributed to efficient electron-transfer from the excited porphyrin to the benzoquinone end-groups. The amount of fluorescence quenching was in good agreement with the number of benzoquinone groups at the dendrimer periphery and the distance between the porphyrin and benzoquinone groups as calculated by semiempirical (AM1) molecular orbital calculations.

Full text of DOI:10.1021/jo026520p

Efficien t Syn th esis of P or p h yr in -Con ta in in g,
Ben zoqu in on e-Ter m in a ted , Rigid P olyp h en ylen e Den d r im er s
Gregory J . Capitosti, Carol D. Guerrero, David E. Binkley, J r.,† Cheruvallil S. Rajesh, and
David A. Modarelli*
Department of Chemistry and The Center for Laser and Optical Spectroscopy,
Knight Chemical Laboratory, The University of Akron, Akron, Ohio 44325-3601
dam@chemistry.uakron.edu
Received October 3, 2002
A series of rigid polyphenylene, free-base porphyrin-containing dendrimers terminated with either
dimethoxybenzene or benzoquinone end-groups were prepared by a combined divergent and
convergent synthesis. Unlike previous routes for preparing polyphenylene dendrimers that are
incompatible with end-groups bearing certain functional moieties, the synthetic methodology chosen
for this work enables incorporation of functional groups on the dendrimer end-groups during
preparation of the dendrimer wedges and during synthesis of the final dendrimer. The basic strategy
utilized a convergent preparation of dendrimer wedges using Suzuki coupling conditions, which
were then either attached to a porphyrin core in a divergent coupling step or cyclized to form the
porphyrin dendrimer in a convergent step. The latter approach was found to be more general and
resulted in higher yields and more readily separated products. Steady-state absorption measure-
ments for these dendrimers showed Soret and Q-band absorptions typical of free-base porphyrins.
Preliminary steady-state fluorescence measurements of these dendrimers indicate quenching of
the S1 state of the free-base porphyrin in all benzoquinone-containing dendrimers that is attributed
to efficient electron-transfer from the excited porphyrin to the benzoquinone end-groups. The amount
of fluorescence quenching was in good agreement with the number of benzoquinone groups at the
dendrimer periphery and the distance between the porphyrin and benzoquinone groups as calculated
by semiempirical (AM1) molecular orbital calculations.
In tr od u ction
research is separation of the electronic charges for
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potential can be realized. Research in this area has
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assemblies,11 polymers,12 and, most recently, dendrim-
ers.13-16 These systems share a common theme in that
they generally contain two or more electron donor and
acceptor groups covalently attached to one another by
either flexible10 or rigid6,7 bridging groups (i.e., donor-
The photosynthetic reaction center has evolved over
time to facilitate photoinduced electron-transfer from an
electronically excited “special pair” of chlorophylls to a
quinone acceptor.1 Artificial mimics that duplicate the
electron-transfer relay in photosynthesis are interesting
for a variety of reasons, including understanding the
factors that influence photosynthesis1-4 and for use as
potential alternative fuel sources.5 A primary goal of such
† Project SEED (I and II) student.
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10.1021/jo026520p CCC: $25.00 © 2003 American Chemical Society
Published on Web 12/10/2002
J . Org. Chem. 2003, 68, 247-261
247

Capitosti et al.
bridge-acceptor). Photoinduced electron-transfer reac-
tions in these molecules occur through either a super-
exchange (through-bond) or through-space mechanism.
The chemistry of dendrimers has expanded dramati-
cally in recent years.17 This research is driven in part by
the desire to utilize the inherent structural properties of
these unique macromolecules for use in energy transfer,18
catalysis,19 sensor applications,20 and as drug delivery
agents.21 Not surprisingly, the composition of end-groups
in the dendrimer can influence solubility,22 conforma-
tion,23 and further synthetic transformations. It is clear
that the ability to perform transformations on terminal
functional groups is quite important. Our group13 and
others14-16 have used the dendritic architecture as a
bridging group in intramolecular photoinduced electron-
transfer. We have chosen to use a core electron-donor
group and electron-acceptor end-groups for two reasons.
First, this arrangement of donor and acceptor groups
should result in an increased yield of charge-separation
relative to monomeric analogues.7d Second, the reduction
of one or more of the electron-acceptor groups on the
dendrimer surface makes an intermolecular electron-
transfer to a secondary acceptor group, also present in
solution, competitive with charge recombination. In this
way, the dendrimer might act similar to the photosyn-
thetic reaction center, where the reduced ubiquinone is
removed through a transmembrane process, by removing
the electron from the porphyrin for potentially lengthy
periods of time.
We recently described photoinduced electron-transfer
reactions in flexible Newkome-type dendrimers that
contained tetraphenylporphyrin cores and anthraquinone
peripheral groups.13 Photoexcitation of the porphyrin core
resulted in efficient electron-transfer to the anthra-
quinone groups. The electron-transfer rate constants in
these dendrimers were found to be largely independent
of the generation number, indicating a through-space
electron-transfer mechanism. These results were ex-
plained in terms of the substitution pattern and flexible
nature of these dendrimers that resulted in extensive
backfolding of the dendrimer branches. Such backfolding
allowed both π-stacking of the terminal anthraquinone
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248 J . Org. Chem., Vol. 68, No. 2, 2003

Efficient Synthesis of Polyphenylene Dendrimers
SCHEME 1
groups with the porphyrin cores, and in the case of the
zinc-metalated analogues, ligation of the end-group with
the zinc porphyrin.13 Preliminary Monte Carlo stochastic
dynamics calculations on the G1 dendrimer indicate that
backfolding of the dendrimer branches place the an-
thraquinone groups very close to the porphyrin core.
Because of the close proximity of the oxidized porphyrin
and reduced anthraquinone units, however, charge re-
combination is suspected to be quite rapid in these
dendrimers. Experiments to verify this prediction are
ongoing. In an effort to better control the charge-
separation and charge-recombination steps occurring
upon irradiation of porphyrin-containing dendrimers, we
have begun to examine the photoinduced electron-
transfer chemistry in rigid dendrimers and starburst
macromolecules. The branches in these dendrimers are
composed of polyphenylene linkages and cannot backfold.
The electron-transfer rate constants in these dendrimers
are therefore expected to be highly dependent upon the
generation number, and electron transfer will likely
proceed by a superexchange mechanism.
The synthesis of polyphenylene dendrimers was first
reported by Miller24 and Kimura.25 These syntheses were
modeled after the work of Kim et al.,26 who reacted an
aryl boronic acid under Suzuki coupling conditions
(Scheme 1) to yield hyperbranched polymers. Miller et
al. modified this procedure to yield dendrimers by se-
quentially reacting 3,5-dibromo-1-(trimethylsilyl)benzene
with aryl boronic acids, also under Suzuki coupling
conditions. Conversion of the trimethylsilyl group to the
boronic acid was accomplished in one step with boron
tribromide. Repetition of this sequence resulted in the
controlled synthesis of dendrimers with well-defined
structures. Kimura and co-workers25 modified this pro-
cedure to make porphyrin-containing dendrimers FbTBP -
Gn (n ) 0-2) as shown in Scheme 2. While this synthesis
is efficient for the synthesis of unsubstituted polyphen-
ylene dendrimers, the necessity of the boron tribromide
addition to form boronic acids is incompatible with many
functional groups. The synthesis of porphyrin-containing
polyphenylene dendrimers requiring dimethoxybenzene
groups at the peripheral positions on the dendrimer by
a convergent route is, unfortunately, unsuitable with the
previously published approaches to polyphenylene den-
drimers, as phenyl methyl ethers are readily cleaved with
boron tribromide, the step necessary to convert the
(24) Miller, T. M.; Neeman, T. X.; Zayas, R.; Bair, H. E. J . Am. Chem.
Soc. 1992, 114, 1018.
(25) Kimura, M.; Shiba, T.; Yamazaki, M.; Haqnabusa, K.; Shirai,
H.; Kobayashi, N. J . Am. Chem. Soc. 2001, 123, 5636-5642.
(26) Kim, Y. H.; Webster, O. W. J . Am. Chem. Soc. 1990, 112, 4592.
trimethylsilyl groups to the boronic acid groups in the
syntheses used by Miller and Kimura. Thus, we sought
an alternate procedure for preparing these dendrimers.
J . Org. Chem, Vol. 68, No. 2, 2003 249

Capitosti et al.
phenylene boronic acid dendrons to meso-tetrakis(4-
tetrabromophenyl)porphyrin or the cyclization of large
polyphenylene aldehydes with pyrrole. Both of these
syntheses proceed in relatively high yields to generate
porphyrin-containing dendrimers of high purity. These
synthetic methods should be generally applicable to a
wide variety of dendrimer end-groups. In this work, we
also describe preliminary fluorescence quenching data for
both dimethoxybenzene and benzoquinone containing
dendrimers, as well as semiempirical calculations of the
dendrimer structures.
Resu lts a n d Discu ssion
Syn th esis. Although the syntheses of both F bP -G0-
p BQ4 and F bP -G0-m BQ4 have been previously re-
ported,27 we employed a different synthetic approach
because of problems with purification using the literature
methods. Thus, meso-tetrakis(4-bromophenyl)porphyrin
(1) was first synthesized under Lindsey28 conditions from
p-bromobenzaldehyde29 and pyrrole. Coupling 4 equiv of
2,5-dimethoxyphenylboronic acid (2) to tetrabromo-
phenylporphyrin 1 under Suzuki conditions30 (Pd(PPh3)4,
K2CO3, 3:1 toluene/ethanol, reflux) yielded meso-tetrakis-
((2′,5′-dimethoxyphenyl)-4-phenyl)porphyrin F b P -G0-
p DMB4 (Scheme 3) as a purple solid. Chan and co-
workers27 reported the purification of F bP -G0-p DMB4
using column chromatography on silica gel (3:1 CH2Cl2/
hexanes). Under these conditions, F bP -G0-p DMB4 could
not be removed from the silica column; successive changes
of the eluant to increasingly polar solvents were unsuc-
cessful in moving the product off the column as well. The
chromatographic conditions were therefore modified, and
successful purification was carried out on alumina (95:5
CH2Cl2/ethyl acetate). The dimethoxybenzene-terminated
porphyrin (F bP -G0-p DMB4) was subsequently converted
to the hydroquinone with boron tribromide and oxidized
to the benzoquinone-containing porphyrin (F bP -G0-
p BQ4) with 2,3-dichloro-4,5-dicyanoquinone (DDQ) in
43% yield. Porphyrin F bP -G0-p BQ4 was readily purified
by washing with methanol, which was found to remove
the excess DDQ, the reduced DDQ byproduct, and any
porphyrin still containing unoxidized hydroquinone moi-
eties.
Dendrimer F bP -G0-m BQ4 is also a G0 dendrimer but
has the benzoquinone groups positioned meta to the
porphyrin ring on the meso phenyl groups. This den-
drimer was prepared to examine the effects of regiochem-
istry on electron-transfer rate constants through a phenyl
bridging group (i.e., para vs meta). Two routes were
considered for the synthesis of F bP -G0-m BQ4: (a) a
divergent approach analogous to that used to prepare
FbP -G0-pBQ4, wherein 2,5-dimethoxyphenylboronic acid
would be added using Suzuki coupling conditions to meso-
tetrakis(3-bromophenyl)porphyrin, and (b) a convergent
approach where the dendrimer is assembled in one step
(27) Chan, C.-S.; Tse, A. K.-S.; Chan, K. S. J . Org. Chem. 1994, 59,
6084-6089.
Herein, we report the facile synthesis of polyphenylene,
porphyrin-containing dendrimers, and starburst poly-
mers having dimethoxybenzene end-groups that are then
readily converted into the corresponding benzoquinone
end-groups. The preparation of these dendrimers ulti-
mately involved either Suzuki coupling of large poly-
(28) Wagner, R. W.; Lawrence, D. S.; Lindsey, J . S. Tetrahedron Lett.
1987, 28, 3069-3070.
(29) Gradyushko, A. T.; Kozhich, D. T.; Solov’ev, K. N.; Tsvirko, M.
P. Dokl. Akad. Nauk Beloruss. SSR 1972, 16, 534-7. Kim, J . B.;
Leonard, J . J .; Longo, F. R. J . Am. Chem. Soc. 1972, 94, 3986-92.
(30) Bobe, F.; Tunoori, A. R.; Niestroj, A. J .; Gronwald, O.; Maier,
M. E. Tetrahedron 1996, 52, 9485.
250 J . Org. Chem., Vol. 68, No. 2, 2003

Efficient Synthesis of Polyphenylene Dendrimers
SCHEME 2 a
a
Reaction conditions: (i) Pd(PPh3)4, Na2CO3, ∆; (ii) BBr3; (ii) KOH; (iv) 1, Pd(PPh3)4, Na2CO3, ∆.
SCHEME 3 a
a
Reaction conditions: (i) Pd(PPh3)4, K2CO3, 3:1 PhCH3/EtOH, ∆.
SCHEME 4 a
from a prefabricated wedge, having a benzaldehyde group
as the foci. Fre´chet and co-workers31 have demonstrated
the feasibility of both approaches in the synthesis of
porphyrin-containing Freche´t dendrimers. For reasons
related to ease of purification, the convergent approach
is generally considered a more convenient route for the
synthesis of pure dendrimers and was therefore chosen
for the preparation of F bP -G0-m BQ4. In addition, a
a
convergent synthetic approach appeared to be more
general for the preparation of the larger dendrimers in
this series (i.e., F b P -G1-p P h -m BQ4, F b P -G1-p P h -
m P h BQ8, and F bP -G2-p P h -m BQ16). The synthesis of
the dimethoxybenzene-containing precursor (F bP -G0-
m DMB4) of F bP -G0-m BQ4 required the preparation of
2′,5′-dimethoxyphenyl-3-benzaldehyde (3), which was
synthesized from 2,5-dimethoxyphenylboronic acid and
3-bromobenzaldehyde using Suzuki coupling conditions
Reaction conditions: (i) Pd(PPh3)4, K2CO3, 3:1 PhCH3/EtOH,
∆; (ii) BF3‚OEt2, pyrrole, CH2Cl2; (iii) DDQ.
(Scheme 4) as a pale yellow oil in 93% after column
chromatography (silica, 9:1 CH2Cl2/hexanes). Cyclization
of 3 with pyrrole under Lindsey conditions28 gave por-
phyrin F bP -G0-m DMB4 as a purple solid in 36% yield
after chromatography (alumina, 95:5 CH2Cl2/hexanes).
The dimethoxybenzene-terminated porphyrin (F bP -G0-
m DMB4) was subsequently converted to the hydro-
quinone with boron tribromide and oxidized to the
benzoquinone-containing porphyrin (FbP -G0-m BQ4) with
(31) Pollak, K. W.; Sanford, E. M.; Fre´chet, J . M. J . J . Mater. Chem.
1998, 8, 519-527.
J . Org. Chem, Vol. 68, No. 2, 2003 251

Capitosti et al.
SCHEME 5 a
also employed to synthesize 5 (Scheme 6b). This sequence
utilized the Suzuki coupling reaction of bromide 7 to
4-formylphenylboronic acid to give 5 after chroma-
tography (25% ethyl acetate/hexanes on silica) in 94%
yield. Condensation of aldehyde 5 under Lindsey condi-
tions resulted in formation of dendrimer F bP -G0-p P h -
m DMB4 as a purple solid in 46% yield (95:5 CH2Cl2/
hexanes on alumina). Porphyrin dendrimer F bP -G0-
p P h -m DMB4 was subsequently converted to the hydro-
quinone with boron tribromide, oxidized to the benzo-
quinone-containing porphyrin (FbP -G0-pP h -m BQ4) with
DDQ in 78% yield, and purified in the normal manner.
The next higher homologue in this rigid polyphenylene
series of dendrimers, F bP -G1-m BQ8, is a G1 dendrimer
similar to F bP -G0-p P h -m BQ4 but has bis(3′,5′-benzo-
quinone)-4-phenyl substitution at the meso phenyl (para)
positions on the porphyrin. Preparation of this dendrimer
(Scheme 7) used a (divergent) synthetic approach similar
to that used to synthesize dendrimer F bP -G0-p BQ4. For
the synthesis of dimethoxybenzene-containing precursor
F bP -G1-p P h -m DMB8, 3,5-bis(2′,5′-dimethoxyphenyl)-
phenyl boronic acid 8 functioned as the reactive dendritic
wedge and was to be coupled to tetrabromophenylpor-
phyrin 1 using Suzuki coupling conditions. The prepara-
tion of boronic acid wedge 8 involved the reaction of 1,3,5-
tribromobenzene with 2 equiv of 2,5-dimethoxyphenyl
boronic acid under Suzuki coupling conditions (Scheme
7). Purification by column chromatography on silica with
a
Reaction conditions: (i) Pd(PPh3)4, K2CO3, 3:1 PhCH3/EtOH,
∆; (ii) t-BuLi, -78 °C; (iii) B(OiPr)3; (iv) 1, Pd(PPh3)4, K2CO3, 3:1
PhCH3/EtOH, ∆.
DDQ. Porphyrin F bP -G0-m BQ4 was readily purified in
the usual fashion by washing with methanol and isolated
in 75% yield as a purple solid.
Dendrimer F bP -G0-p P h -m BQ4 is a G0 dendrimer
analogous to F bP -G0-m BQ4 but has a biphenyl linkage
between the porphyrin and benzoquinone groups. Ini-
tially a divergent approach (Scheme 5) was used to
synthesize the dimethoxybenzene-containing precursor,
F bP -G0-p P h -m DMB4. This reaction sequence required
the coupling of (2′,5′-dimethoxyphenyl)-3-phenylboronic
acid (4a ) with tetrabromophenylporphyrin 1 using Suzuki
coupling conditions (as per the successful synthesis of
F bP -G0-p DMB4). The synthesis of boronic acid 4a was
accomplished by first coupling 2,5-dimethoxyphenyl-
boronic acid (2) to 1-bromo-3-iodobenzene to give 3-(2′,5′-
dimethoxyphenyl) bromobenzene 4 in 85% yield as a
white solid (column chromatography on silica, 10% ethyl
acetate/hexanes). Subsequent treatment of this bromide
with tert-butyllithium followed by triisopropylborate
((iPrO)3B) yielded the corresponding boronic acid. While
preparation of this boronic acid was successful, all
attempts (i.e., crystallization, column chromatography,
and extraction) to isolate it in high purity were unsuc-
cessful. TLC data for all purification attempts showed
the presence of two different products. Impurities in
Suzuki couplings can lead to problems with preparation
and isolation of the final pure product. Because of the
problems isolating the pure boronic acid, this route was
abandoned.
The successful synthesis of porphyrin F bP -G0-m BQ4
by a convergent approach, wherein aldehyde 3 was
cyclized under Lindsey conditions, suggested that F bP -
G0-p P h -m DMB4 might also be successfully synthesized
in a similar fashion (Scheme 6). Thus, [1,1′:3′,1′′-(2′′,5′′-
dimethoxy)-terphenyl]-4-carboxyaldehyde (5) was pre-
pared by two separate routes (Scheme 6). The first route
(Scheme 6a) involved the Suzuki coupling reaction of
4-formylphenylboronic acid to 1-bromo-3-iodobenzene to
give aldehyde 6 in 61% yield as a pale yellow solid after
column chromatography (60:40 CH2Cl2/hexanes on silica).
The subsequent reaction of aldehyde 6 with boronic acid
2 under Suzuki reaction conditions gave aldehyde 5 as a
pale yellow oil in 70% yield (1:3 ethyl acetate/hexanes
on silica). A second, higher-yield reaction sequence was
a gradient of CH2
Cl2/hexanes (from 1:1 to 3:1) resulted
in the isolation of 1-bromo-3,5-(2′,5′-dimethoxyphenyl)-
benzene 9 in 68% yield as a white solid. The synthesis of
9 proved to be problematic because of formation of a low-
melting impurity suspected to be 1,3-(2′,5′-dimethoxy-
phenyl)benzene, the product obtained from replacement
of bromine with hydrogen during the coupling step. The
synthesis was initially carried out using 0.03 equiv of
Pd(PPh3
)4 per coupling site and a 24 h reaction time.
Upon reducing the time of the reaction to 3 h and the
amount of catalyst to a total amount of 0.01 equiv, 9 was
isolated without the low-melting impurity.
Reaction of bromide 9 with tert-butyllithium at -78
°C, followed by addition of triisopropyl borate, resulted
in the formation of 3,5-(2′,5′-dimethoxyphenyl)phenyl-
boronic acid (8) as a colorless oil (Scheme 7). Extraction
into aqueous base, followed by re-acidification and re-
extraction into ether gave the boronic acid as a white
solid (84% yield). Porphyrin-dendrimer F bP -G1-p P h -
m DMB8 was synthesized by reaction of 9 with tetra-
bromophenylporphyrin 1 using Suzuki coupling condi-
tions. Purification of the crude product by column chro-
matography on alumina (95:5 CH2Cl2/ethyl acetate)
resulted in the isolation of porphyrin F bP -G1-p P h -
m DMB8 as a purple solid in 75% yield. Porphyrin F bP -
G1-p P h -m DMB8 was subsequently converted to the
hydroquinone with boron tribromide and oxidized to the
benzoquinone-containing porphyrin (FbP -G1-pP h -m BQ8)
with DDQ in 79% yield. Porphyrin F bP -G1-p P h -m BQ8
was readily purified by washing with methanol in the
usual fashion.
The final two dendrimers in this rigid polyphenylene
porphyrin-containing dendrimer series, F bP -G1-p P h -
m P h BQ8 and F bP -G2-p P h -m BQ16, were assembled by
a convergent synthetic approach. The dendritic wedges
were prepared as aldehydes and then reacted under
252 J . Org. Chem., Vol. 68, No. 2, 2003

Efficient Synthesis of Polyphenylene Dendrimers
SCHEME 6 a
a
Reaction conditions: (i) Pd(PPh3)4, K2CO3, 3:1 PhCH3/EtOH, ∆; (ii) 2, Pd(PPh3)4, K2CO3, 3:1 PhCH3/EtOH, ∆; (iii) BF3‚OEt2, pyrrole,
CH2Cl2; (iv) DDQ.
SCHEME 7 a
SCHEME 8 a
a
Reaction conditions: (i) Pd(PPh3)4, K2CO3, 3:1 PhCH3/EtOH,
∆; (ii) t-BuLi, -78 °C; (iii) (i-PrO)3B; (iv) 1, Pd(PPh3)4, K2CO3, 3:1
PhCH3/EtOH, ∆.
a
Reaction conditions: (i) Pd(PPh3)4, K2CO3, 3:1 PhCH3/EtOH,
Lindsey conditions to directly provide the porphyrin
dendrimer. Dendrimer F bP -G1-p P h -m P h BQ8 is a first
generation dendrimer with eight 1,1′:3′,1′′-bis(4,4′′-benzo-
quinone)-terphenyl peripheral groups substituted at the
para carbons on the meso phenyl rings of tetraphenyl-
porphyrin. Dendrimer F bP -G2-p P h -BQ16 is the second
generation analogue of F bP -G1-p P h -m BQ8 and contains
sixteen peripheral benzoquinone groups.
Initially, a divergent approach was used to synthesize
F bP -G2-p P h -DMB16. In this synthesis (Scheme 8), bo-
ronic acid wedge 10 was targeted to be coupled in a final
step with tetrabromophenylporphyrin 1. The Suzuki
coupling reaction of boronic acid 8 with 1,3,5-tribromo-
benzene, the critical step necessary to prepare the
bromide precursor to 10, was unsuccessful, and only the
monosubstituted product was isolated, indicating un-
favorable steric interactions in the Suzuki coupling step
with 1,3,5-tribromobenzene.
∆; (ii) t-BuLi, -78 °C; (iii) (i-PrO)3B; (iv) 1, Pd(PPh3)4, K2CO3, 3:1
PhCH3/EtOH, ∆.
9). The best results were achieved when 1,3,5-tribromo-
benzene was in a 2.5 molar excess and the reaction
concentration was 0.033 M in the aldehyde. Under these
conditions, 12 was obtained as a white solid in 63% yield
after column chromatography employing 2:1 CH2Cl2/
hexanes followed by 95:5 CH2Cl2/ethyl acetate on silica.
The subsequent reaction of 12 with 2.2 equiv of 8 followed
by column chromatography resulted in formation of
aldehyde 11 in 65% yield (Scheme 9). Condensation of
11 with pyrrole under Lindsey conditions28 followed by
oxidation with DDQ resulted in the formation of porphy-
rin dendrimer F bP -G2-p P h -m DMB16. Isolation of por-
phyrin F bP -G2-p P h -m DMB16 as a purple solid in 54%
yield was achieved by column chromatography on alu-
mina (95:5 CH2Cl2/ethyl acetate). Porphyrin dendrimer
F bP -G2-p P h -m DMB16 was subsequently converted to
the benzoquinone-containing dendrimer F bP -G2-p P h -
m BQ16 after treatment with boron tribromide and oxida-
tion with DDQ. Porphyrin F bP -G2-p P h -m BQ16 was
readily purified in the usual fashion and isolated in 79%
yield as a purple solid. This dendrimer has only limited
As a result of this problem, a convergent route was
designed to synthesize the dimethoxybenzene-containing
dendrimer, F bP -G2-p P h -m DMB16, by the cyclization
reaction of aldehyde 11 with pyrrole (Scheme 9). Alde-
hyde 12 was first synthesized as a precursor to 11
through the carefully controlled coupling reaction of 1,3,5-
tribromobenzene with 4-formylphenylboronic acid (Scheme
J . Org. Chem, Vol. 68, No. 2, 2003 253

Capitosti et al.
SCHEME 9 a
F IGURE 1. Absorption spectra in CH2Cl2 measured for
dendrimers F bP -G0-p BQ4, F bP -G0-m BQ4, F bP -G0-p P h -
m BQ4, F bP -G1-p P h -m BQ8, F bP -G1-p P h -m P h BQ8, and
a
F bP -G2-p P h -m BQ16
.
Reaction conditions: (i) Pd(PPh3)4, K2CO3, 3:1 PhCH3/EtOH,
∆; (ii) 8, Pd(PPh3)4, K2CO3, 3:1 PhCH3/EtOH, ∆; (iii) BF3‚OEt2,
TABLE 1. Absor p tion Ma xim a of F r ee Ba se P or p h yr in
Den d r im er s F bP -G0-p BQ4, F bP -G0-m BQ4,
F bP -G0-p P h -m BQ4, F bP -G1-p P h -m BQ8,
F bP -G1-p P h -m P h BQ8, a n d F bP -G2-p P h -m BQ16 a n d th e
Resp ective Dim eth oxyben zen e (DMB)-Con ta in in g
Refer en ce Com p ou n d s
pyrrole, CH2Cl2; (iv) DDQ.
SCHEME 10 a
dendrimer
solvent
absorption maxima (nm)
F bP -G0-p DMB4
F bP -G0-p BQ4
CH2Cl2 422 518 554 591 648
CH2Cl2 419 517 556 591 648
CH2Cl2 421 518 554 592 647
CH2Cl2 420 517 553 590 623, 646
CH2Cl2 423 519 555 593 648
CH2Cl2 423 518 554 593 648
CH2Cl2 423 519 555 593 648
CH2Cl2 423 518 555 590 628, 644
F bP -G0-m DMB4
F bP -G0-m BQ4
F bP -G0-p P h -m DMB4
F bP -G0-p P h -m BQ4
F bP -G1-p P h -m DMB8
F bP -G1-p P h -m BQ8
F bP -G1-p P h -m P h DMB8 CH2Cl2 424 519 555 593 649
F bP -G1-p P h -m P h BQ8
F bP -G2-p P h -m DMB16
F bP -G2-p P h -m BQ16
CH2Cl2 424 518 555 594 648
CH2Cl2 423 518 555 594 649
CH2Cl2 424 518 556 593 650
a
a
This dendrimer was only moderately soluble in CH2Cl2.
sequently converted to the benzoquinone-containing den-
drimer F bP -G1-p P h -m P h BQ8 after treatment with
boron tribromide and oxidation with DDQ. Porphyrin
F bP -G1-p P h -m P h BQ8 was readily purified in the usual
fashion and isolated in 80% yield as a purple solid.
Stea d y-Sta te Absor p tion Sp ectr oscop y. The ab-
sorption spectra (Figure 1, Table 1) of the rigid dendrim-
ers synthesized in this work are largely typical of free
base porphyrins. In CH2Cl2, the Soret bands for these
dendrimers were observed at ∼420 nm, similar to the
Soret band observed for H2TPP (λmax ∼419 nm in CH2Cl2).
Neither the position nor the shape of the Soret transition
is substantially affected by increasing dendrimer genera-
tion or substitution pattern.
a
Reaction conditions: (i) Pd(PPh3)4, K2CO3, 3:1 PhCH3/EtOH,
∆; (ii) BF3‚OEt2, pyrrole, CH2Cl2; (iii) DDQ.
solubility and was difficult to characterize except through
absorption spectroscopy.
The final dendrimer (F bP -G1-p P h -m P h BQ8) in this
series was synthesized by the same convergent strategy
successfully used in the synthesis of F bP -G2-p P h -
m BQ16. The initial boronic acid (13) used in the prepara-
tion of F bP -G1-p P h -m P h DMB8 was synthesized by the
Suzuki coupling reaction of 2 with 1-bromo-4-iodobenzene
followed by conversion of the resulting bromide (13) to
4-(2′,5′-dimethoxyphenyl)-phenylboronic acid (14) using
tert-butyllithium followed by triisopropyl borate. Boronic
acid 14 was then reacted with aldehyde 12 (Scheme 10)
in a Suzuki coupling reaction to give the doubly branched
aldehyde 15 as a white solid in 64% yield (8:2 CH2Cl2/
hexanes on silica). Condensation of 15 with pyrrole under
Lindsey conditions yielded porphyrin F b P -G1-p P h -
m P h DMB8 as a purple solid in 64% yield after column
chromatography on alumina (95:5 CH2Cl2/ethyl acetate).
Porphyrin dendrimer F bP -G1-p P h -m P h DMB8 was sub-
Com p u ta tion a l Resu lts
Flexible dendrimers have been shown computation-
ally32 and experimentally13,33 to undergo extensive back-
folding with the end-groups positioned throughout the
structure of the dendrimer and not at the dendrimer
surface. The primary purpose of synthesizing the rigid
dendrimers presented in this work was to prevent this
254 J . Org. Chem., Vol. 68, No. 2, 2003

Efficient Synthesis of Polyphenylene Dendrimers
backfolding from occurring. Photoinduced electron-
transfer would then occur from a relatively fixed geom-
etry in these dendrimers, rather than one of a variety of
conformations.13 In this manner we had hoped to better
control electron-transfer rate constants in porphyrin- and
quinone-containing dendrimers. A second aspect of den-
drimer chemistry we seek to exploit in future work
relates to the accessibility of substrate molecules to the
core porphyrin. Large-generation flexible dendrimers
have been reported13,19 to isolate the core group from the
surrounding environment. We anticipated that the larger
rigid dendrimer(s) reported here might behave similarly.
To probe these structural issues we performed semi-
empirical calculations on the porphyrin dendrimers
reported. The following structural details were par-
ticularly interesting for these dendrimers: (a) the por-
phyrin-benzoquinone center-to-center distance, (b) the
total size of the dendrimer (outer diameter), and (c) the
size of the opening in the dendrimer above and below the
porphyrin core (inner diameter). Calculations were per-
formed at the semiempirical (AM1) level as implemented
by PCSpartan Plus.34 The results of these calculations
are shown in Figure 2 and summarized in Table 2.
Monosubstituted dendrimers, F bP -G0-p BQ4 and F bP -
G0-m BQ4, are found to be the smallest dendrimers
synthesized in this work, with F bP -G0-m BQ4 (the
smallest dendrimer) having a porphyrin-benzoquinone
center-to-center distance of 9.7 Å and outer diameter
distances of ∼2.2 nm (cis conformation) and ∼2.3 nm
(trans conformation). The para-substituted dendrimer
F bP -G0-p BQ4 is slightly larger in terms of both the
porphyrin-benzoquinone distance (11.4 Å) and the outer
diameter (∼2.6 nm). The additional phenyl spacer be-
tween the porphyrin and benzoquinone groups in den-
drimer F bP -G0-p P h -m BQ4 increases the porphyrin-
benzoquinone distance to 13.6 Å and increases the outer
diameter of the dendrimer to 3.0 nm. The doubly sub-
stituted first-generation analogue of this dendrimer,
F bP -G1-m BQ8, has a porphyrin-benzoquinone distance
of 13.8 Å that is approximately the same size as that in
F bP -G0-p P h -m BQ4. The outer diameter of this den-
drimer is ∼3.0 nm.
F IGURE 2. Space-filling models of the rigid polyphenylene
porphyrin-containing dendrimers synthesized in this work.
Optimized geometries are shown for all dendrimers: (a) F bP -
G0-p BQ4, (b) F bP -G0-m BQ4, (c) F bP -G0-p P h -m BQ4, (d)
F bP -G1-m BQ8, (e) F bP -G1-p P h -m P h BQ8, and (f) F bP -G2-
p P h -m BQ16
.
TABLE 2. Su m m a r y of Com p u ta tion a l Da ta for
Den d r im er s F bP -G0-p BQ4, F bP -G0-m BQ4,
F bP -G0-p P h -m BQ4, F bP -G1-p P h -m BQ8,
F bP -G1-p P h -m P h BQ8, a n d F bP -G2-p P h -m BQ16
porphyrin-
benzoquinone
distance (Å)
outer
diameter
(nm)
inner
diameter
(nm)
dendrimer
F bP -G0-p BQ4
F bP -G0-m BQ4
11.4
9.7
2.6
2.2 (cis),
2.3 (trans)
F bP -G0-p P h -m BQ4
F bP -G1-p P h -m BQ8
F bP -G1-p P h -m P h BQ8
F bP -G2-p P h -m BQ16
13.6
13.8
17.5
13.8, 17.9
3.0
3.0
3.4
3.7
More interesting in terms of their structural charac-
teristics are the larger dendrimers F bP -G1-p P h -m P h -
BQ8 and F bP -G2-p P h -m BQ16. The porphyrin-benzo-
quinone distance in G1 dendrimer FbP -G1-pP h -m P h BQ8
is 17.5 Å. Two orientations of the benzoquinone groups
exist in the G2 dendrimer F bP -G2-p P h -m BQ16, with the
shorter porphyrin-benzoquinone distance (13.8 Å) re-
sulting from the benzoquinone groups positioned above
or below the plane of the porphyrin core and the longer
distance (17.9 Å) resulting from the benzoquinone groups
2.4
positioned in more equatorial orientations (i.e., in-plane)
with the porphyrin ring. Both dendrimers are substan-
tially larger than any of the other dendrimers and have
outer diameters of ∼3.4 nm (F bP -G1-p P h -m P h BQ8) and
∼3.7 nm (F bP -G2-p P h -m BQ16). Only the largest G2
dendrimer, F bP -G2-p P h -m BQ16, has structural char-
acteristics that suggest potential sequestration of the
porphyrin core. The inner diameter in this dendrimer is
∼2.4 nm. It would therefore appear from the computa-
tional data that, while these dendrimers represent a
potential method of providing protection to the porphyrin
core, the critical size necessary for more complete isola-
tion has not been synthesized. An AM1 calculation on
F bP -G2-m BQ16 (not synthesized), an analogue of F bP -
G2-p P h -m BQ16 where the phenyl group between the
initial branching point of the dendrimer and the porphy-
rin core has been removed, indicates an inner diameter
of ∼1.5 nm and may be better suited for this purpose.
The computational results on the dendrimers prepared
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Bhalgat, M. K.; Roberts, J . C. Eur. Polym. J . 2000, 36, 647-651. (c)
Cavallo, L.; Fraternali, F. Chem. Eur. J . 1998, 4, 927-934. (d) Boris,
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Macromolecules 2000, 33, 6214-6216. (b) Baugher, A. H.; Goetz, J .
M.; McDowell, L. M.; Huang, H.; Wooley, K. L.; Schaefer, J . Biophys.
J . 1998, 75, 2574-2576. (c) Tasaki, K.; Klug, C. A.; Wooley, K. L.;
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(34) MacSpartan 4.0; Wavefunction, Inc.: Irvine, CA.
J . Org. Chem, Vol. 68, No. 2, 2003 255

Capitosti et al.
TABLE 3. F lu or escen ce Qu a n tu m Yield s (O) a n d
Rela tive F lu or escen ce Qu a n tu m Yield s (OBQ/ODMB) of F r ee
Ba se P or p h yr in Den d r im er s F bP -G0-p BQ4,
F bP -G0-m BQ4, F bP -G0-p P h -m BQ4, F bP -G1-p P h -m BQ8,
F bP -G1-p P h -m P h BQ8, a n d F bP -G2-p P h -m BQ16 a n d th e
Resp ective Dim eth oxyben zen e (DMB)-Con ta in in g
Refer en ce Com p ou n d s
(superexchange) electron-transfer mechanism having
strong electronic coupling between the donor and acceptor
groups. The center-to-center distance (see Computational
Results) between the porphyrin and benzoquinone groups
for F bP -G0-m BQ4 (9.7 Å) is shorter by ∼1.7 Å than that
calculated for F bP -G0-p BQ4 (11.4 Å), consistent with the
more pronounced amount of fluorescence quenching in
this dendrimer. An increase in the distance between the
donor and acceptor groups to 13.6 Å in F bP -G0-p P h -
m BQ4 leads to a large increase in the relative fluores-
cence quantum yield (φBQ/φDMB ∼0.0433). However, in-
creasing the number of electron-acceptor end-groups by
a factor of 2 leads to a 5-fold decrease in the relative
quantum yield of the analogous G1 dendrimer F bP -G1-
BQ8 (0.0094). These results are consistent with those of
Gust, Moore, and Moore,7d who demonstrated that an
increase in the number of acceptor groups attached to a
given donor has a scaling effect on electron transfer.
The porphyrin-benzoquinone distance in F bP -G1-p P h -
p P h m BQ8 is found to be ∼17.5 Å, while the shorter
distance in F bP -G2-p P h -m BQ16 is ∼13.8 Å. The shorter
distance and greater number of acceptor groups suggest
that F bP -G2-p P h -m BQ16 should have a greater amount
of fluorescence quenching resulting from electron transfer
than F bP -G1-p P h -p P h m BQ8. The relative quantum
yields obtained from the fluorescence spectra of F bP -G1-
p P h -p P h m BQ8 (φBQ/φDMB ≈ 0.1891) and F bP -G2-p P h -
m BQ16 (φBQ/φDMB ≈ 0.04913) confirm this prediction and
indicate more substantial quenching of the singlet excited
state in the latter dendrimer. Electron transfer in the
larger dendrimers likely occurs through a combination
of through-bond and through-space mechanisms, the
exact mechanism of which is probably highly dependent
upon the specific dendrimer. These results, together with
picosecond time-resolved fluorescence and femtosecond
time-resolved absorption measurements, will be reported
in greater detail in due course.
c
dendrimer
solvent
Φa,b
φBQ/φDMB
F bP -G0-p DMB4
F bP -G0-p BQ4
F bP -G0-m DMB4
F bP -G0-m BQ4
F bP -G0-p P h -m DMB4
F bP -G0-p P h -m BQ4
F bP -G1-p P h -DMB8
F bP -G1-p P h -BQ8
F bP -G1-p P h -p P h m DMB8
F bP -G1-p P h -p P h m BQ8
F bP -G2-p P h -m DMB16
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
0.149
0.00032
0.153
0.00009
0.168
0.0073
0.187
0.00176
0.187
0.0354
0.180
0.00883
0.11
0.0073
0.00057
0.0433
0.0094
0.1891
0.04913
d
F bP -G2-p P h -m BQ16
H2TP P
a
Excited at the Soret bands to avoid aggregation due to
b
concentration effects. Absolute fluorescence quantum yields
determined by ratio comparisons to the known yield of H2TPP.35
Quantum yields were calculated using standard methods. c Rela-
tive to the fluorescence of the corresponding dimethoxybenzene
d
(DMB)-containing dendrimer. Relatively higher error expected
due to poor solubility.
here indicate sequestration will occur only at larger
generations, although we expect that the synthesis of
these dendrimers will be difficult. Experimental work to
determine whether F bP -G2-p P h -m BQ16 can isolate the
porphyrin core from external substrates is underway.
Stea d y-Sta te F lu or escen ce Sp ectr oscop y. Prelimi-
nary steady-state fluorescence spectra for the dimethoxy-
benzene-containing reference dendrimers show fluores-
cence quantum yields (φFL, Table 3) that are slightly
higher than that observed for H2TPP in CH2Cl2. These
data are consistent with the data reported by Kimura et
al.25 for porphyrin-containing polyphenylene dendrimers
F bTBP -Gn (Scheme 2). Examination of the fluorescence
spectra of the benzoquinone-terminated dendrimers re-
veals a substantial decrease in the intensity of fluores-
cence relative to that of the dimethoxybenzene-termi-
nated dendrimers that is dependent upon both dendrimer
generation and substitution pattern. Because the sole
difference between these dendrimers is the replacement
of the dimethoxybenzene group with the benzoquinone
group, the decrease in the fluorescence of the benzo-
quinone-containing dendrimers relative to their dimeth-
oxybenzene-terminated analogues can be attributed to
electron-transfer from the porphyrin core to the periph-
eral benzoquinone groups. Steady-state fluorescence
measurements at a variety of dendrimer concentrations
indicate that interdendrimer electron-transfer is not
responsible for the fluorescence quenching, and fluores-
cence quenching thus occurs via intramolecular ET. To
compare the fluorescence quantum yield data from the
various dendrimers without biasing in generation-
Con clu sion s
In conclusion, a rigid series of free-base porphyrin- and
benzoquinone-containing polyphenylene dendrimers have
been prepared by a combination of divergent and con-
vergent synthetic approaches. Unlike previous routes for
preparing polyphenylene dendrimers that are incompat-
ible with end-groups bearing certain functional moieties,
the synthetic methodology presented here enables incor-
poration of functional groups on the dendrimer end-
groups during preparation of the dendrimer wedges.
These syntheses have been performed in a manner that
enables further manipulation of the functionalized end-
groups. Yields in all steps of the syntheses were relatively
high. Steady-state absorption measurements for all den-
drimers are typical for free-base porphyrins. Steady-state
fluorescence measurements indicate efficient quenching
of the S1 state of the free-base porphyrin in all dendrim-
ers. This quenching is attributed to efficient electron-
transfer from the singlet excited state of the porphyrin
core to the benzoquinone end-groups. The electron trans-
fer likely occurs through a through-bond (superexchange),
a through-space mechanism, or a combination of both
mechanisms, the exact mechanism of which is probably
highly dependent upon the specific dendrimer. Future
work on this project includes time-resolved fluorescence
dependent effects, the relative quantum yields (φBQ/φDMB
)
were calculated as described in the Experimental Section.
The relative quantum yields of G0 dendrimers F bP -G0-
p BQ4 and F bP -G0-m BQ4 are the smallest of the den-
drimers reported here and have φBQ/φDMB values of 0.0073
and 0.00057, respectively. Excited state quenching in
these molecules presumably arises from a through-bond
256 J . Org. Chem., Vol. 68, No. 2, 2003

Efficient Synthesis of Polyphenylene Dendrimers
g, 0.05405 mmol) was added. The resulting solution was
brought to reflux, where it was allowed to stir under argon
for 3 h. The solution was cooled to room temperature and
diluted with chloroform and then washed with 10% NaOH (3
× 50 mL), water (50 mL), and brine (50 mL). The organic layer
was separated and dried over Na2SO4. Concentration of the
solvent resulted in the crude product. Purification by column
chromatography on silica using 9:1 CH2Cl2/hexanes yielded 3
as a pale yellow oil (1.2187 g, 93%): 1H NMR (CDCl3, δ) 3.78
(s, 3H), 3.83 (s, 3H), 6.93 (m, 3 H), 7.58 (t, 1H, J ) 7.6 Hz),
7.84 (m, 2H), 8.06 (t, 1H, J ) 1.5 Hz) 10.07 (s, 1H); 13C NMR
(CDCl3, δ) 56.02, 56.43, 112.82, 114.03, 116.77, 128.35, 128.90,
130.18, 131.22, 135.77, 136.53, 139.57, 150.85, 154.04, 192.64;
IR (neat) 1692 cm-1 (CdO).
and absorption measurements of electron-transfer on the
picosecond and femtosecond time scales, the results of
which will be reported in due course.
Exp er im en ta l Section
Gen er a l Meth od s. All reactions were run under a nitrogen
atmosphere unless indicated otherwise. THF was distilled from
the sodium benzophenone ketyl, while all other solvents were
used as received. All starting materials were used as received.
Flash chromatography was carried out on silica gel (230-400
mesh) or neutral alumina (60-325 mesh). All melting points
were taken using a capillary melting point apparatus and are
uncorrected. Proton NMR spectra were acquired at either 300
or 750 MHz. Proton-decoupled 13C NMR spectra were acquired
at 75 MHz and referenced to chloroform. MALDI-TOF-MS
were obtained using R-cyano-4-hydroxycinnamic acid (CCA)
as the matrix. Samples were prepared by mixing 1.0 mL of a
solution of the dendrimer in CH2Cl2 with 1 mL of a solution
of the matrix in 50:45:5 MeCN/EtOH/H2O. Steady-state fluo-
rescence measurements were run on an ISA J obin Yvon-SPEX
Fluorolog 3-22 fluorometer having dual input and output
monochromators. Fluorescence spectra were collected using
argon-saturated solutions by exciting at the Soret maxima of
optically matched samples (OD ) 0.20) in the S/R mode to
correct for changes in the lamp output intensity. Quantum
yield measurements were made relative to H2TPP. Relative
quantum yields (φBQ/φDMB) were calculated in order to compare
the fluorescence quantum yield data from the various den-
drimers without biasing in generation-dependent effects and
were calculated as defined in eq 1,13 where I(DMB) and I(BQ)
P r ep a r a t ion of 3-(2,5-Dim et h oxyp h en yl)b r om ob en -
zen e (4). 1-Bromo-3-iodobenzene (1.500 g, 5.304 mmol), 2,5-
dimethoxyphenylboronic acid (0.9653 g, 5.304 mmol), and
anhydrous potassium carbonate (0.9744 g, 7.054 mmol) were
placed in a 250 mL round-bottom flask. Toluene (75 mL) and
ethanol (25 mL) were then added, and the resulting solution
was purged with argon for 10 min. After 10 min, Pd(PPh3)4
(0.06144 g, 0.05304 mmol) was added, and the mixture was
brought to reflux where it was allowed to stir under argon for
3 h. The solution was then cooled to room temperature and
diluted with chloroform and then washed with 10% NaOH (3
× 50 mL), water (50 mL), and brine (50 mL). The organic layer
was dried over Na2SO4 and evaporated. The crude product was
purified by column chromatography on silica using 10% ethyl
acetate/hexanes, yielding 4 as a white solid (1.3276 g, 85%):
1
mp 72-74 °C; H NMR (CDCl3, δ) 3.78 (s, 3H), 3.82 (s, 3H),
6.91 (m, 3H), 7.29, (t, 1H, J ) 8 Hz), 7.47 (d, 2H, J ) 8 Hz),
7.69 (s, 1H); 13C NMR (CDCl3, δ) 56.04, 56.49, 112.85, 113.94,
116.74, 122.27, 128.33, 129.72, 130.22, 132.60, 140.67, 150.83,
153.96. Anal. Calcd for C14H13O2Br (293.17): C, 57.35; H, 4.48.
Found: C, 57.40; H, 4.65.
φBQ
I(BQ) A(DMB)
I(DMB) A(BQ)
)
(1)
φDMB
P r ep a r a tion of [1,1′:3′,1′′-(2′′,5′′-d im eth oxy)-ter p h en yl)-
4-ca r boxya ld eh yd e (5). Bromide 4 (1.500 g, 5.116 mmol),
4-formylphenylboronic acid (0.9207 g, 6.140 mmol), and an-
hydrous potassium carbonate (0.9399 g, 6.805 mmol) were
placed in a 250 mL round-bottom flask. Toluene (105 mL) and
ethanol (35 mL) were added, and the resulting solution was
purged with argon for 10 min. After 10 min, the Pd(PPh3)4
(0.0593 g, 0.05116 mmol) was added, and the resulting solution
was brought to reflux where it was allowed to stir under argon
for 3 h. The solution was cooled to room-temperature diluted
with CHCl3 and then washed with 10% NaOH (3 × 50 mL),
water (50 mL), and brine (50 mL). The organic layer was dried
over Na2SO4 and evaporated. Purification of the crude product
by column chromatography on silica using 25% ethyl acetate/
hexanes yielded 5 as a pale yellow oil (1.533 g, 94%): 1H NMR
(CDCl3, δ) 3.80 (s, 3H), 3.84 (s, 3H), 6.95 (m, 3H), 7.58 (m,
3H), 7.81 (d, 3H, J ) 8 Hz), 7.98 (d, 2H, J ) 8 Hz), 10.08 (s,
1H); 13C NMR (CDCl3, δ) 56.01, 56.51, 112.82, 113.53, 117.01,
126.27, 127.98, 128.75, 128.90, 129.82, 130.45, 131.28, 135.38,
139.39, 139.69, 147.47, 150.93, 154.00, 192.12; IR (neat) 1693
cm-1 (CdO). Anal. Calcd for C21H18O3 (318.39): C, 79.21; H,
5.71. Found: C, 78.86; H, 5.81.
P r epar ation of 3′-Br om o-4-for m ylbiph en yl (6). 1-Bromo-
3-Iodobenzene (2.166 g, 7.659 mmol), 4-formylphenylboronic
acid (1.148 g, 7.659 mmol), and anhydrous potassium carbon-
ate (1.407 g, 10.19 mmol) were placed in a 250 mL round-
bottom flask. Toluene (105 mL) and ethanol (35 mL) were
added, and the resulting solution was purged with argon for
10 min. After 10 min, Pd(PPh3)4 (0.0887 g, 0.07659 mmol) was
added. The resulting solution was brought to reflux, where it
was allowed to stir under argon for 3 h. The solution was cooled
to room temperature and diluted with CHCl3 and then washed
with 10% NaOH (3 × 50 mL), water (50 mL), and brine (50
mL). The organic layer was dried over Na2SO4 and evaporated.
Purification of the crude product by column chromatography
using 60:40 CH2Cl2/hexanes resulted in the isolation of 7 as a
pale yellow solid (1.213 g, 61%): 1H NMR (CDCl3, δ) 7.36 (t,
represent the fluorescence intensity of the appropriate den-
drimer, and A(DMB) and A(BQ) represent the optical density
of the sample used in the measurement. Slits were set to 0.75
mm each for the entrance, exit, and intermediate excitation
slits and 4.00 mm each for the emission slits. All dendrimers
synthesized in this work were fully characterized by NMR (1H
and 13C) and MALDI-TOF MS, except F bP -G2-p P h -m BQ16
,
which was not sufficiently soluble for any characterization
except UV-vis.
P r ep a r a t ion of m eso-Tet r a k is-(4-b r om op h en yl)p or -
p h yr in (1).29 Into a 2000 mL round-bottom flask containing
p-bromombenzaldehyde (3.000 g, 16.21 mmol) in 1621 mL of
CHCl3 was added pyrrole (1.088 g, 16.21 mmol). After the
solution was purged with argon for 5 min, BF3‚OEt2 (0.6721
mL, 5.351 mmol) was added, and the resulting solution was
stirred under argon for 1 h. After 1 h, DDQ (2.761 g, 12.16
mmol) was added rapidly as a solid. The solution was stirred
for an additional 1 h, at which time the solvent was evapo-
rated. The crude solid was placed on a filter and washed with
methanol until the filtrate became colorless, which resulted
in the isolation of 1 as a bright purple solid (2.033 g, 58%):
1H NMR (CDCl3, δ) -2.86 (s, 1H), 7.92 (d, 4H, J ) 8 Hz), 8.08
(d, 4H, J ) 8 Hz), 8.85 (s, 4H); 13C NMR (CDCl3, δ) 119.20,
122.86, 130.21, 136.04, 141.04; UV/vis (λmax, nm, CH2Cl2) 420.0,
515.5, 551.0, 589.5, 646.5.
P r ep a r a tion of 3-(2,5-Dim eth oxyp h en yl)ben za ld eh yd e
(3).36 3-Bromobenzaldehyde (1.000 g, 5.405 mmol), 2,5-
dimethoxyphenylboronic acid (1.180 g, 6.486 mmol), and
anhydrous potassium carbonate (0.9929 g, 7.188 mmol) were
added to a 100 mL round-bottom flask. Toluene (30 mL) and
ethanol (10 mL) were added, and the resulting solution was
purged with argon for 10 min. After 10 min, Pd(PPh3)4 (0.06260
(35) Strachan, J . P.; Gentemann, J . S.; Kalsbeck, W. A.; Lindsey, J .
S.; Holten, D.; Bocian, D. F. J . Am. Chem. Soc. 1997, 119, 11191.
(36) Malesani, G.; Galiano, F.; Pietrogrande, A.; Rodighiero, G.
Tetrahedron 1978, 34, 2355-2359.
J . Org. Chem, Vol. 68, No. 2, 2003 257

Capitosti et al.
1H, J ) 8 Hz), 7.55 (t, 1H, J ) 2 Hz), 7.58 (s,1H), 7.73 (d, 2H,
J ) 8 Hz), 7.79 (t, 1H, J ) 2 Hz), 7.98 (d, 2H, J ) 8 Hz), 10.08
(s, 1H); 13C NMR (CDCl3, δ) 123.35, 126.19, 127.94, 130.53,
130.63, 130.73, 131.59, 135.84, 142.03, 145.77, 191.97; IR
(Nujol) 1692 cm-1 (CdO). Anal. Calcd for C15H14O3 (242.28):
C, 74.36; H, 5.82. Found: C, 74.59; H, 6.11.
were added, and the resulting solution was purged with argon
for 10 min. After 10 min, Pd(PPh3)4 (0.6813 g, 0.05882 mmol)
was added. The resulting solution was brought to reflux, where
it was allowed to stir under argon for 24 h. The solution was
cooled to room temperature, diluted with CHCl3, and then
washed with 10% NaOH (3 × 50 mL), water (50 mL), and brine
(50 mL). The organic layer was dried over Na2SO4 and
evaporated. Purification of the crude product by column
chromatography on silica initially using 9:1 CH2Cl2/hexanes
and changing to 95:5 CH2Cl2/ethyl acetate resulted in 11 as a
white solid (3.3642 g, 65%): mp 114-117 °C; 1H NMR (CDCl3,
δ) 3.80 (s,12H), 3.82 (s, 12H), 6.96 (m, 12H), 7.76 (s, 2H), 7.89
(m, 8H), 8.01, (d, 3H, J ) 8 Hz), 10.10 (s, 1H); 13C NMR (CDCl3,
δ) 56.04, 56.54, 112.77, 113.62, 117.02, 125.67, 127.00, 127.58,
128.11, 130.33, 130.58, 131.56, 135.58, 138.99, 140.54, 140.92,
143.03, 147.40, 151.06, 153.98, 192.17; IR (Nujol) 1693 cm-1
(CdO); FAB-MS 881.00 (calculated for C57H50O9: 879.07).
Anal. Calcd for C57H50O9 (879.07): C, 77.87; H, 5.74. Found:
C, 76.37; H, 5.70.
P r ep a r a tion of 4-F or m yl-3′,5′-d ibr om obip h en yl (12).
1,3,5-Tribromobenzene (4.200 g, 13.34 mmol), 4-formylphenyl-
boronic acid (0.8000 g, 5.335 mmol), and anhydrous potassium
carbonate (0.9801 g, 7.096 mmol) were placed in a 250 mL
round-bottom flask. Toluene (120 mL) and ethanol (40 mL)
were added, and the resulting solution was purged with argon
for 10 min. After 10 min, Pd(PPh3)4 (0.0618 g, 0.05335 mmol)
was added. The resulting solution was brought to reflux, where
it was allowed to stir under argon for 3 h. The solution was
cooled to room temperature, diluted with CHCl3, and then
washed with 10% NaOH (3 × 50 mL), water (50 mL), and brine
(50 mL). The organic layer was dried over Na2SO4 and
evaporated. Purification of the crude product by column
chromatography on silica using a hexane/CH2Cl2 gradient from
2:1 to 1:1 resulted in 12 as a white solid (1.1369 g, 63%): mp
117-120 °C; 1H NMR (CDCl3, δ) 7.70 (m, 5H), 7.98, (d, 2H, J
) 8 Hz), 10.08 (s, 1H); 13C NMR (CDCl3, δ) 123.75, 127.97,
129.40, 130.58, 133.94, 136.24, 143.44, 144.25, 191.79; IR
(Nujol) 1697 cm-1 (CdO). Anal. Calcd for C13H8OBr2 (340.02):
C, 45.92; H, 2.38. Found: C, 43.92; H, 2.37.
P r ep a r a tion of 3-(2′,5′-Dim eth oxyp h en yl)br om oben -
zen e (7). 1-Bromo-3-iodobenzene (1.500 g, 5.304 mmol), 2,5-
dimethoxyphenylboronic acid (0.9653 g, 5.304 mmol), and
anhydrous potassium carbonate (0.9744 g, 7.054 mmol) were
placed in a 250 mL round-bottom flask. Toluene (75 mL) and
ethanol (25 mL) were then added, and the resulting solution
was purged with argon for 10 min. After 10 min, Pd(PPh3)4
(0.06144 g, 0.05304 mmol) was added, and the mixture was
brought to reflux where it was stirred under argon for 3 h.
The solution was then cooled to room temperature and diluted
with chloroform and then washed with 10% NaOH (3 × 50
mL), water (50 mL), and brine (50 mL). The organic layer was
dried over Na2SO4 and evaporated. The crude product was
purified by column chromatography on silica using 10% ethyl
acetate/hexanes, yielding 7 as a white solid (1.3276 g, 85%):
1
mp 72-74 °C; H NMR (CDCl3, δ) 3.78 (s, 3H), 3.82 (s, 3H),
6.91 (m, 3H), 7.29, (t, 1H, J ) 8 Hz), 7.47 (d, 2H, J ) 8 Hz),
7.69 (s, 1H); 13C NMR (CDCl3, δ) 56.04, 56.49, 112.85, 113.94,
116.74, 122.27, 128.33, 129.72, 130.22, 132.60, 140.67, 150.83,
153.96. Anal. Cald for C14H13O2Br (293.17): C, 57.35; H, 4.48.
Found: C, 57.40; H, 4.65.
P r ep a r a tion of 3,5-Bis(2′,5′-d im eth oxyp h en yl)p h en yl
Bor on ic Acid (8). A 250 mL round-bottom flask containing
9 (3.5000 g, 8.152 mmol) in 40 mL of THF was cooled to -78
°C. tert-Butyllithium (1.7 M in pentane, 9.60 mL, 16.30 mmol)
was added by cannula, turning the mixture a blue-green color.
This solution was allowed to stir at -78 °C for 30 min.
Triisopropyl borate (7.666 g, 40.76 mmol) was added, and the
mixture turned cloudy and pale yellow. The solution was
allowed to warm slowly to room temperature over the course
of 3 h. After 3 h, 3 M HCl was added until two clear layers
formed. The biphasic solution was washed with ether (3 × 50
mL). The combined ether layers were then extracted with 10%
NaOH. The base layer was re-acidified with 10% HCl to a pH
of ∼5 and extracted with ether. The ether layer was dried over
Na2SO4 and evaporated to yield 8 as a white solid (2.723 g,
P r ep a r a tion of (4′-Br om o)-2,5-d im eth oxybip h en yl (13).
1-Bromo-4-iodobenzene (1.682 g, 5.950 mmol), 2,5-dimethoxy-
phenylboronic acid (0.9836 g, 5.404 mmol), and potassium
carbonate (1.093 g, 7.906 mmol) were all placed in a 100 mL
round-bottom flask. Toluene (30 mL) and ethanol (10 mL) were
added, and the resulting solution was purged with argon for
10 min. After 10 min, Pd(PPh3)4 (0.2066 g, 0.1783 mmol) was
added, and the solution was brought to reflux where it was
stirred under argon for 3 h. After 3 h, the solution was diluted
with CHCl3, washed with 20% NaOH (4 × 50 mL), water (50
mL), and brine (50 mL). The organic layer was dried over
Na2SO4 and evaporated to dryness. The crude residue was
purified by column chromatography on silica using 10% ethyl
acetate/hexanes, which resulted in isolation of (4′-bromo)-2,5-
dimethoxybiphenyl as a white solid (1.318 g, 83%): mp 59-
1
85%). H NMR (acetone-d6, δ) 3.74 (s, 6H), 3.80 (s, 6H), 5.80
(s, 2H), 6.97 (m, 6H), 7.79 (t, 1H, J ) 1.8 Hz), 7.97 (d, 2H, J
) 1.7 Hz); 13C NMR (acetone-d6, δ) 56.00, 56.68, 113.89, 113.93,
117.52, 132.87, 133.61, 134.69, 135.66, 138.36, 151.89, 154.95.
P r ep a r a tion of 1-Br om o-3,5-(2′,5′-d im eth oxyp h en yl)-
ben zen e (9). 1,3,5-Tribromobenzene (4.000 g, 12.70 mmol),
2,5-dimethoxyphenylboronic acid (4.625 g, 25.40 mmol), and
anhydrous potassium carbonate (4.668 g, 33.80 mmol) were
placed in a 500 mL round-bottom flask. Toluene (216 mL) and
ethanol (72 mL) were added, and the mixture was purged with
argon for 10 min. After 10 min, Pd(PPh3)4 (0.05887 g, 0.05083
mmol) was added. The resulting solution was brought to reflux,
where it was allowed to stir under argon for 3 h. The solution
was cooled to room temperature and diluted with chloroform
and then washed with 10% NaOH (3 × 50 mL), water (50 mL),
and brine (50 mL). The organic layer was separated and dried
over Na2SO4. Evaporation of the solvent resulted in the crude
product, which was purified by column chromatography on
silica with a CH2Cl2/hexane gradient from 1:1 to 3:1. The
purification resulted in isolation of 9 as a white solid (3.693 g,
1
60 °C; H NMR (CDCl3, δ) 3.77 (s, 3H), 3.82 (s, 3H), 6.90 (m,
3H), 7.42 (d, 2H, J ) 7 Hz), 7.54 (d, 2H, J ) 7 Hz); 13C NMR
(CDCl3, δ) 55.3, 55.8, 112.2, 113.9, 117.0, 120.7, 130.0, 131.6,
131.2, 137.9, 150.1, 153.2. Anal. Calcd for
C14H13O2Br
(293.17): C, 57.35; H, 4.48. Found: C, 57.32; H, 4.63.
P r ep a r a t ion of 4-(2′,5′-Dim et h oxyp h en yl)-p h en yl-
bor on ic Acid (14). A 50 mL round-bottom flask containing
(4′-bromo)-2,5-dimethoxybiphenyl (1.3000 g, 4.4343 mmol) in
13.5 mL of THF was cooled to -78 °C. tert-Butyllithium (1.7
M in pentane, 5.22 mL, 8.869 mmol) was added by cannula,
turning the mixture a deep blue color. This solution was
allowed to stir at -78 °C for 30 min. Triisopropyl borate (2.500
g, 13.30 mmol) was added, and the mixture turned cloudy and
pale yellow. The solution was allowed to warm slowly to room
temperature over the course of 3 h. After 3 h, 3 M HCl was
added until two clear layers had formed. The biphasic solution
was washed with ether (3 × 50 mL). The combined ether layers
were then washed with 3 M HCl, dried over Na2SO4, filtered,
1
68%): mp 134-136 °C; H NMR (CDCl3, δ) 3.79 (s, 3H), 3.82
(s, 3H), 6.91 (m, 3H), 7.64 (d, 1H, J ) 12.6 Hz); 13C NMR
(CDCl3, δ) 55.80, 56.27, 112.57, 113.74, 116.55, 121.53, 129.35,
130.09, 130.96, 139.87, 150.67, 153.72. Anal. Calcd for C22H21O4-
Br (429.33): C, 61.54; H, 4.94. Found: C, 61.30; H, 5.13.
P r ep a r a t ion of 4-F or m yl-3′,5′-(3,5-(2,5-d im et h oxy-
p h en yl)p h en yl)bip h en yl (11). Aldehyde 12 (2.000 g, 5.882
mmol), 8 (5.102 g, 12.94 mmol), and anhydrous potassium
carbonate (2.161 g, 15.65 mmol) were placed in a 250 mL
round-bottom flask. Toluene (120 mL) and ethanol (40 mL)
258 J . Org. Chem., Vol. 68, No. 2, 2003

Efficient Synthesis of Polyphenylene Dendrimers
and concentrated under vacuum. The crude product was
recrystallized from 50% CH2Cl2/hexane and then again from
hexane, which resulted in 14 as a white solid (0.9270 g, 81%):
mp 165-172 °C; 1H NMR (CDCl3, δ) 3.80 (s, 3H), 3.85 (s, 3H),
6.95 (m, 3H), 7.71 (d, 2H, J ) 8 Hz), 8.33 (d, 2H, J ) 8 Hz);
13C NMR (CDCl3, δ) 56.05, 56.60, 113.06, 113.76, 116.92,
129.31, 131.67, 135.62, 142.97, 151.09, 154.02.
120.40, 126.69, 128.73, 131.63, 133.93, 136.40, 136.94, 141.96,
151.36, 154.12; MALDI-TOF-MS 1158.21 (calcd for C76H62O8N4,
1159.42); UV/vis (λmax, nm, CH2Cl2) 306.5, 421.5, 517.0, 553.0,
590.5, 647.0.
P r ep a r a t ion of m eso-Tet r a k is-[4-(3-(2,5-d im et h oxy-
ph en yl)ph en yl)ph en yl]por ph yr in (FbP -G0-pP h -m DMB4).
To a 1000 mL round-bottom flask containing 5 (1.5000 g, 4.713
mmol) in 471 mL of CHCl3 was added pyrrole (0.3162 g, 4.713
mmol). After purging the solution with argon for 5 min, BF3‚
OEt2 (0.1954 mL, 1.555 mmol) was added, and the resulting
solution was stirred under argon for 1 h. After 1 h, DDQ
(0.8024 g, 3.535 mmol) was added rapidly as a solid. The
solution was stirred for an additional 1 h, at which time the
solvent was evaporated. The crude solid was placed on a filter
and washed with methanol until the filtrate became colorless.
The resulting purple product was purified by column chroma-
tography on alumina using 95:5 CH2Cl2/ethyl acetate. The first
purple band was collected and evaporated to dryness to yield
F bP -G0-p P h -m DMB4 as a purple solid (0.7997 g, 46%): 1H
NMR (CDCl3, δ) -2.63 (s, 1H), 3.88 (s, 6H), 3.89 (s, 6H), 7.02
(m, 6H), 7.68 (s, 4H), 7.92 (s, 2H), 8.07 (d, 4H, J ) 8 Hz), 8.13
(s, 2H), 8.35 (d, 4H, J ) 8 Hz), 9.01 (s, 4H); 13C NMR (CDCl3,
δ) 56.11, 56.71, 113.00, 113.56, 117.10, 120.18, 125.75, 126.35,
128.81, 128.93, 129.01, 131.86, 135.35, 139.38, 140.71, 140.84,
141.39, 151.14, 154.09; MALDI-TOF-MS 1463.38 (calcd for
P r epar ation of 4-For m yl-3′,5′-(4-(2,5-dim eth oxyph en yl)-
p h en yl)bip h en yl (15). Aldehyde 12 (0.4217 g, 1.240 mmol),
14 (0.7402 g, 2.728 mmol), and anhydrous potassium carbonate
(0.4557 g, 3.299 mmol) were placed in a 40 mL round-bottom
flask. Toluene (30 mL) and ethanol (10 mL) were added, and
the resulting solution was purged with argon for 10 min. After
10 min, Pd(PPh3)4 (0.1436 g, 0.1240 mmol) was added. The
resulting solution was brought to reflux, where it was allowed
to stir under argon for 24 h. The solution was cooled to room
temperature and diluted with CHCl3. The solution was then
washed with 10% NaOH (3 × 50 mL), water (50 mL), and brine
(50 mL). The organic layer was dried over Na2SO4 and
evaporated under vacuum. Purification of the crude product
by column chromatography on silica initially using 8:2 CH2Cl2/
hexanes resulted in 15 as a white solid (0.4858 g, 65%): mp
1
94-98 °C; H NMR (CDCl3, δ) 3.82 (s, 6H), 3.85 (s, 6H), 6.95
(m, 6H), 7.70 (d, 4H, J ) 8 Hz), 7.78 (d, 4H, J ) 8 Hz), 7.88 (s,
2H), 7.90 (d, 2H, J ) 8 Hz), 7.96 (t, 1H, J ) 2 Hz), 8.03 (d, 2H
J ) 8 Hz), 10.11 (s, 1H); 13C NMR (CDCl3, δ) 56.04, 56.54,
112.85, 113.47, 116.91, 125.41, 126.41, 127.22, 128.14, 130.24,
130.58, 131.22, 135.63, 138.15, 139.66, 141.06, 142.65, 147.40,
151.05, 154.03, 192.14; IR (Nujol) 1692 cm-1 (CdO); FAB-MS
606.00 (calcd for C41H34O5, 606.75). Anal. Calcd for C41H34O5
(606.75): C, 81.16; H, 5.66. Found: C, 79.64; H, 5.69.
C
100H78O8N4, 1463.82); UV/vis (λmax, nm, CH2Cl2) 305.0, 424.0,
518.5, 556.0, 592.0, 648.5.
P r ep a r a t ion of m eso-Tet r a k is[4-(3,5-(2,5d im et h oxy-
ph en yl)ph en yl)ph en yl]por ph yr in (FbP -G1-pP h -m DMB8).
Porphyrin 1 (0.1020 g, 0.1096 mmol), 8 (0.2593 g, 0.6578
mmol), and anhydrous potassium carbonate (0.1211 g, 0.8768
mmol) were added to a 25 mL round-bottom flask. Toluene (9
mL) and ethanol (3 mL) were added, and the resulting solution
was purged with argon for 10 min. After 10 min, Pd(PPh3)4
(0.0140 g, 0.01206 mmol) was added. The mixture was brought
to reflux where it was stirred for 48 h. The reaction mixture
was diluted with chloroform and washed with 20% NaOH (4
× 50 mL), water (50 mL), and brine (50 mL). The organic layer
was dried over Na2SO4 and concentrated. The crude product
was purified by column chromatography on alumina using 95:5
CH2Cl2/ethyl acetate. The first purple band was collected and
evaporated to dryness to yield F bP -G1-p P h -m DMB8 as a
purple solid (0.1720 g, 78%): mp 294-297 °C; 1H NMR (CDCl3,
δ) -2.63 (s, 1H), 3.89 (s, 12H), 3.90 (s, 12H), 7.05 (m, 12H),
7.85 (s, 2H), 8.10 (t, 8H, J ) 3 Hz), 8.36 (d, 4H, J ) 8 Hz),
9.02 (s, 4H); 13C NMR (CDCl3, δ) 56.11, 56.72, 112.96, 113.65,
117.12, 120.23, 125.88, 127.69, 130.18, 131.88, 135.38, 139.05,
140.57, 140.82, 141.36, 151.21, 154.08; MALDI-TOF-MS 2008.79
(calcd for C132H110O16N4: 2008.46); UV/vis (λmax, nm, CH2Cl2)
309.0, 424.5, 519.5, 556.5, 592.0, 649.0.
P r epar ation of meso-Tetr akis-[4-(3,5-(4-(2,5-dim eth oxy-
p h en yl)p h en yl)p h en yl)p h en yl]p or p h yr in (F bP -G1-p P h -
m P h DMB8). To a 250 mL round-bottom flask containing 15
(0.5000 g, 0.8241 mmol) in 82 mL of CHCl3 was added pyrrole
(0.05528 g, 0.8241 mmol). After the solution was purged with
argon for 5 min, BF3‚OEt2 (0.03416 mL, 0.2719 mmol) was
added, and the resulting solution was stirred under argon for
1 h. After 1 h, DDQ (0.1403 g, 0.6180 mmol) was added rapidly
as a solid. The solution was stirred for an additional 1 h, at
which time the solvent was evaporated. The crude solid was
placed on a filter and washed with methanol until the filtrate
became colorless. The resulting purple product was purified
by column chromatography on alumina using 95:5 CH2Cl2/
ethyl acetate. The first purple band was collected and evapo-
rated to dryness to yield F bP -G1-p P h -m P h DMB8 as a purple
solid (0.3470 g, 64%): 1H NMR (CDCl3, δ) -2.56 (s, 1H), 3.86
(s, 12H), 3.88 (s, 12H), 6.99 (m, 12H), 7.78 (d, 8H, J ) 8 Hz),
7.95 (d, 8H, J ) 8 Hz), 8.07 (s, 2H), 8.20 (d, 4H, J ) 8 Hz),
8.25 (s, 4H), 8.45 (d, 4H, J ) 8 Hz), 9.09 (s, 4H); 13C NMR
(CDCl3, δ) 56.05, 56.57, 112.94, 113.54, 116.96, 120.23, 125.64,
125.96, 127.42, 130.32, 131.43, 135.55, 138.07, 140.19, 140.74,
141.79, 142.24, 142.66, 151.16, 154.10; MALDI-TOF-MS 2614.96
P r epar ation of meso-Tetr akis-[4-(2,5-dim eth oxyph en yl)-
p h en yl]p or p h yr in (F bP -G0-p DMB4). Porphyrin 1 (0.5000
g, 0.5732 mmol), 2,5-dimethoxyphenyl boronic acid (0.6259 g,
3.439 mmol), and anhydrous K2CO3 (0.6338 g, 4.586 mmol)
were added to a 200 mL round-bottom flask. Toluene (48 mL)
and ethanol (16 mL) were added, and the resulting solution
was purged with argon for 10 min. After 10 min, Pd(PPh3)4
(0.0738 g, 0.06368 mmol) was added. The mixture was brought
to reflux where it was stirred for 2 days. The reaction mixture
was diluted with chloroform and washed with 20% NaOH (4
× 50 mL), water (50 mL), and brine (50 mL). The organic layer
was dried over Na2SO4 and concentrated. The crude product
was purified by column chromatography on alumina (95:5
CH2Cl2/ethyl acetate). The first purple band was collected and
evaporated to dryness to yield F bP -G0-p DMB4 as a purple
solid (0.4886 g, 74%): 1H NMR (CDCl3, δ) -2.65 (s, 1H), 3.96
(s, 6H), 3.98 (s, 6H), 7.00 (m, 2H), 7.09 (d, 2H, J ) 9 Hz), 7.30
(d, 2H, J ) 3 Hz), 8.00 (d, 4H, J ) 8 Hz), 8.32 (d, 4H, J ) 8
Hz), 9.05 (s, 4H); 13C NMR (CDCl3, δ) 55.93, 56.47, 112.92,
113.51, 116.96, 120.09, 127.76, 131.24, 134.53, 137.67, 140.89,
151.06, 153.99; UV/vis (λmax, nm, CH2Cl2) 306.5, 422.5, 518.0,
554.5, 591.5, 648.0.
P r epar ation of meso-Tetr akis[3-(2,5-dim eth oxyph en yl)-
p h en yl]p or p h yr in (F bP -G0-m DMB4). To a 500 mL round-
bottom flask containing 3 (0.5000 g, 2.064 mmol) in 206 mL
of CHCl3 was added pyrrole (0.1384 g, 2.064 mmol). After
purging the solution with argon for 5 min, BF3‚OEt2 (0.0860
mL, 0.0681 mmol) was added, and the resulting solution was
stirred under argon for 1 h. After 1 h, DDQ (0.3514 g, 1.548
mmol) was added rapidly as a solid. The solution was stirred
for an additional 1 h, at which time the solvent was evapo-
rated. The crude solid was placed on a filter and washed with
methanol until the filtrate became colorless. The resulting
purple product was purified by column chromatography on
alumina (95:5 CH2Cl2/ethyl acetate). The first purple band was
collected and evaporated to dryness to yield F bP -G0-m DMB4
as a purple solid (0.2183 g, 36%): 1H NMR (CDCl3, δ) -2.68
(s, 1H), 3.83 (s, 6H), 3.89 (s, 6H), 6.90 (d, 2H, J ) 8.8 Hz),
6.99 (d, 2H, J ) 8.8 Hz), 7.23 (s, 2H), 7.83 (t, 2H, J ) 7.6 Hz),
7.98 (d, 2H, J ) 7.7 Hz), 8.22 (s, 2H), 8.52 (s, 2H), 9.07 (s,
4H); 13C NMR (CDCl3, δ) 56.06, 56.72, 113.06, 113.58, 117.18,
J . Org. Chem, Vol. 68, No. 2, 2003 259

Capitosti et al.
(calcd for C180H142O16N4, 2617.26); UV/vis (λmax, nm, CH2Cl2)
er F bP -G0-p P h -m DMB4 (0.1500 g, 0.1025 mmol) dissolved
in the minimum amount of CH2Cl2 was added via a dropping
funnel into BBr3 (1.25 mL, 1.25 mmol) in CH2Cl2 (10 mL) at
-78 °C under nitrogen. The solution was stirred at -78 °C
for 1 h. After 1 h, the solution was warmed to room temper-
ature where it was stirred overnight. The solution was cooled
to 0 °C, and water was added to hydrolyze the excess BBr3.
The mixture was then washed with triethylamine to neutralize
the porphyrin dication. The purple organic layer was sepa-
rated, dried over Na2SO4, and evaporated to dryness under
reduced pressure. The remaining residue was dissolved in
methanol, and DDQ (0.4652 g, 2.049 mmol) was added. The
mixture was then refluxed for 30 min. After 30 min, the
mixture was filtered and the solid was washed with methanol,
resulting in the isolation of the product as a purple solid
(0.1076 g, 78%): 1H NMR (CDCl3, δ) -2.79 (s, 1H), 6.89 (d,
2H, J ) 9.9 Hz), 6.94 (d, 2H, J ) 9.9 Hz), 7.06 (s, 2H), 7.59 (d,
2H, J ) 7.8 Hz), 7.70 (t, 2H, J ) 7.8 Hz), 8.03 (d, 8H, J ) 7.5
Hz), 8.35 (d, 4H, J ) 7.9 Hz), 8.98 (s, 4H); 13C NMR (CDCl3,
δ) 120.05, 124.88, 126.72, 127.67, 128.45, 129.60, 130.31,
132.59, 133.84, 134.63, 136.27, 137.88, 138.80, 139.93, 141.58,
141.88, 146.21, 186.88, 187.81; UV/vis (λmax, nm, CH2Cl2) 422.5,
517.5, 554.5, 588.0, 628.0.
311.0, 423.5, 517.5, 554.5, 592.0, 647.5.
P r ep a r a tion of m eso-Tetr a k is-[4-(3,5-(3,5-(2,5-d im eth -
oxyp h en yl)p h en yl)p h en yl)p h en yl]p or p h yr in (F bP -G2-
p P h -m DMB16). To a 250 mL round-bottom flask containing
11 (0.5000 g, 0.5688 mmol) in 57 mL of CHCl3 was added
pyrrole (0.03816 g, 0.5688 mmol). After the solution was
purged with argon for 5 min, BF3‚OEt2 (0.02358 mL, 0.1877
mmol) was added, and the resulting solution was stirred under
argon for 1 h. After 1 h, DDQ (0.09684 g, 0.4266 mmol) was
added rapidly as a solid. The solution was stirred for an
additional 1 h, at which time the solvent was evaporated. The
crude solid was placed on a filter and washed with methanol
until the filtrate became colorless. The resulting purple product
was purified by column chromatography on alumina using 95:5
CH2Cl2/ethyl acetate. The first purple band was collected and
evaporated to dryness to yield F bP -G2-p P h -m DMB16 as a
purple solid (0.2828 g, 54%): 1H NMR (CDCl3, δ) -2.63 (s,
1H), 3.85 (s, 48H), 7.00 (m, 25H), 7.81 (s, 4H), 8.00 (s, 8H),
8.10 (s, 2H), 8.15 (d, 4H, J ) 8 Hz), 8.24 (s, 4H), 8.38 (d, 4H,
J ) 8 Hz), 9.02 (s, 4H); 13C NMR (CDCl3, δ) 56.06, 56.10, 56.62,
56.67, 112.89, 113.66, 117.04, 117.10, 120.13, 125.82, 126.22,
127.72, 130.22, 131.78, 135.48, 138.99, 140.60, 141.00, 141.71,
142.06, 142.97, 151.17, 154.04; MALDI-TOF-MS 3705.15 (calcd
for C244H206O32N4, 3706.54); UV/vis (λmax, nm, CH2Cl2) 307.5,
423.5, 518.5, 554.5, 593.5, 647.5.
P r ep a r a tion of m eso-Tetr a k is-[4-(2,5-ben zoqu in on yl)-
p h en yl]p or p h yr in (F bP -G0-p BQ4). Dendrimer F bP -G0-
p DMB4 (0.0740 g, 0.06728 mmol) dissolved in the minimum
amount of CH2Cl2 was added via a dropping funnel into BBr3
(0.6728 mL, 0.6728 mmol) in CH2Cl2 (15 mL) at -78 °C under
nitrogen. The solution was stirred at -78 °C for 1 h. After 1
h, the solution was warmed to room temperature where it was
stirred overnight. The solution was cooled to 0 °C, and water
was added to hydrolyze the excess BBr3. The mixture was then
washed with triethylamine to neutralize the porphyrin di-
cation. The purple organic layer was separated, dried over
Na2SO4, and evaporated to dryness. The remaining residue
was dissolved in methanol, and DDQ (0.2795 g, 1.231 mmol)
was added. The mixture was then refluxed for 30 min. After
30 min, the mixture was filtered and the solid was washed
with methanol, resulting in the isolation of F bP -G0-p BQ4 as
a purple solid (0.0297 g, 43%): 1H NMR (CDCl3, δ) -2.80 (s,
1H), 6.94 (m, 2H), 7.00 (d, 2H, J ) 10 Hz), 7.21 (s, 2H), 7.90
(d, 4H, J ) 8 Hz), 8.31 (d, 4H, J ) 8 Hz), 8.89 (s, 4H); IR (Nujol)
1653, 1590 cm-1; UV/vis (λmax, nm, CH2Cl2) 419.0, 517.0, 556.5,
591.0, 648.5.
P r ep a r a tion of m eso-Tetr a k is[4-(3,5-(2,5-ben zoqu in o-
n yl)p h en yl)p h en yl]p or p h yr in (F bP -G1-p P h -m BQ8). Den-
drimer F bP -G1-p P h -m DMB8 (0.1500 g, 0.07468 mmol) dis-
solved in the minimum amount of CH2Cl2 was added via a
dropping funnel into BBr3 (1.80 mL, 1.80 mmol) in CH2Cl2 (10
mL) at -78 °C under nitrogen. The solution was stirred at
-78 °C for 1 h. After 1 h, the solution was warmed to room
temperature where it was stirred overnight. The solution was
cooled to 0 °C, and water was added to hydrolyze the excess
BBr3. The mixture was then washed with triethylamine to
neutralize the porphyrin dication. The purple organic layer
was separated, dried over Na2SO4, and evaporated to dryness
under reduced pressure. The remaining residue was dissolved
in methanol, and DDQ (0.6782 g, 2.987 mmol) was added. The
mixture was then refluxed for 30 min. After 30 min, the
mixture was filtered and the solid was washed with methanol,
resulting in the isolation of the product as a purple solid
(0.1046 g, 79%): 1H NMR (CDCl3, δ) -2.80 (s, 1H), 6.69 (d,
8H, J ) 9.2 Hz), 6.94 (s, 4H), 7.57 (s, 2H), 8.03 (s, 8H), 8.33
(d, 4H, J ) 5.9 Hz), 8.94 (s, 4H); 13C NMR (CDCl3, δ) 119.94,
125.96, 129.26, 130.12, 133.44, 133.92, 135.44, 136.51, 137.08,
139.28, 141.96, 141.19, 145.21, 186.48, 187.37; UV/vis (λmax
nm, CH2Cl2) 422.5, 517.5, 554.0, 590.5, 647.0.
,
P r ep a r a t ion of m eso-Tet r a k is-[4-(3,5-(4-(2,5-b en zo-
q u in on yl)p h en yl)p h en yl)p h en yl]p or p h yr in (F b P -G1-
p P h -m P h BQ8). Dendrimer F bP -G1-p P h -m P h DMB8 (0.1554
g, 0.05938 mmol) dissolved in the minimum amount of CH2Cl2
was added via a dropping funnel into BBr3 (1.40 mL, 1.40
mmol) in CH2Cl2 (10 mL) at -78 °C under nitrogen. The
solution was stirred at -78 °C for 1 h. After 1 h, the solution
was warmed to room temperature where it was stirred
overnight. The solution was cooled to 0 °C, and water was
added to hydrolyze the excess BBr3. The mixture was then
washed with triethylamine to neutralize the porphyrin di-
cation. The purple organic layer was separated, dried over
Na2SO4, and evaporated to dryness under reduced pressure.
The remaining residue was dissolved in methanol, and DDQ
(0.5392 g, 2.375 mmol) was added. The mixture was then
refluxed for 30 min. After 30 min, the mixture was filtered
and the solid was washed with methanol, resulting in the
isolation of the product as a purple solid (0.1123 g, 80%): 1H
NMR (CDCl3, δ) -2.68 (s, 1H), 6.90 (m, 12H), 7.66 (d, 8H, J )
7.1 Hz), 7.91 (d, 8H, J ) 7.5 Hz), 7.96 (s, 2H), 8.12 (d, 4H, J
) 7.1 Hz), 8.19 (s, 4H), 8.36 (d, 4H, J ) 6.6 Hz), 8.99 (s, 4H);
13C NMR (CDCl3, δ) 120.02, 125.66, 125.87, 126.16, 127.77,
130.17, 132.25, 132.69, 135.50, 136.56, 137.27, 140.22, 141.94,
142.56, 142.92, 145.57, 186.89, 187.72; UV/vis (λmax, nm,
CH2Cl2) 423.5, 517.5, 554.5, 592.5, 647.5.
P r ep a r a tion of m eso-Tetr a k is-[3-(2,5-ben zoqu in on yl)-
p h en yl]p or p h yr in (F bP -G0-m BQ4). Dendrimer F bP -G0-
m DMB4 (0.1400 g, 0.1208 mmol) dissolved in the minimum
amount of CH2Cl2 was added via a dropping funnel into BBr3
(1.45 mL, 1.45 mmol) in CH2Cl2 (10 mL) at -78 °C under
nitrogen. The solution was stirred at -78 °C for 1 h. After 1
h, the solution was warmed to room temperature where it was
stirred overnight. The solution was cooled to 0 °C, and water
was added to hydrolyze the excess BBr3. The mixture was then
washed with triethylamine to neutralize the porphyrin di-
cation. The purple organic layer was separated, dried over
Na2SO4, and evaporated to dryness under reduced pressure.
The remaining residue was dissolved in methanol, and DDQ
(0.5486 g, 2.416 mmol) was added. The mixture was then
refluxed for 30 min. After 30 min, the mixture was filtered
and the solid was washed with methanol, resulting in the
isolation of the product as a purple solid (0.0937 g, 75%): 1H
NMR (CDCl3, δ) -2.78 (s, 1H), 6.82 (d, 2H, J ) 9.7 Hz), 6.89
(d, 2H, J ) 9.7 Hz), 7.13 (s, 2H), 7.87 (d, 4H, J ) 6.8 Hz), 8.36
(d, 4H, J ) 7.3 Hz), 8.97 (s, 4H); 13C NMR (CDCl3, δ) 119.63,
127.31, 128.91, 131.54, 133.43, 135.43, 136.40, 136.56, 137.28,
142.48, 146.01, 186.86, 187.67; UV/vis (λmax, nm, CH2Cl2) 420.5,
516.5, 553.0, 588.0, 645.5.
P r epar ation of meso-Tetr akis-[4-(3-(2,5-ben zoqu in on yl)-
p h en yl)p h en yl]p or p h yr in (F bP -G0-p P h -m BQ4). Dendrim-
260 J . Org. Chem., Vol. 68, No. 2, 2003

Efficient Synthesis of Polyphenylene Dendrimers
P r ep a r a tion of m eso-Tetr a k is-[4-(3,5-(3,5-(2,5-ben zo-
q u in on yl)p h en yl)p h en yl)p h en yl]p or p h yr in (F b P -G2-
p P h -m BQ16). Dendrimer F bP -G2-p P h -m DMB16 (0.1500 g,
0.04876 mmol) dissolved in the minimum amount of CH2Cl2
was added via a dropping funnel into BBr3 (2.35 mL, 2.35
mmol) in CH2Cl2 (10 mL) at -78 °C under nitrogen. The
solution was stirred at -78 °C for 1 h. After 1 h, the solution
was warmed to room temperature where it was stirred
overnight. The solution was cooled to 0 °C, and water was
added to hydrolyze the excess BBr3. The mixture was then
washed with triethylamine to neutralize the porphyrin di-
cation. The purple organic layer was separated, dried over
Na2SO4, and evaporated to dryness under reduced pressure.
The remaining residue was dissolved in methanol, and DDQ
(0.8854 g, 3.900 mmol) was added. The mixture was then
refluxed for 30 min. After 30 min, the mixture was filtered
and the solid was washed with methanol, resulting in the
form of a CAREER Award (NSF-9875489) and a CRIF
grant (NSF-9816260), the donors of the Petroleum
Research Fund, administered by the American Chemi-
cal Society (ACS-PRF-34812-G6), the Ohio Board of
Regents, and The University of Akron for a Faculty
Research Grant (FRG-07480). D.E.B. acknowledges the
American Chemical Society and the Project SEED
endowment for summer support. Prof. Chrys Wes-
demiotis, Mr. Mark Arnould, and Ms. Kathleen
Wollyung of the Mass Spectroscopy Center at The
University of Akron are acknowledged for obtaining
MALDI spectra (C.W. thanks NSF-DMR-9703946 for
support).
1
Su p p or t in g In for m a t ion Ava ila b le: 1H NMR and 13C
NMR spectra of aldehydes 11 and 12 and all dendrimers
synthesized in this work. This material is available free of
charge via the Internet at http://pubs.acs.org.
isolation of the product as a purple solid (0.0995 g, 79%). H
NMR, 13C NMR: the dendrimer was not sufficiently soluble
to obtain NMR spectra; UV/vis (λmax, nm, CH2Cl2) 421.5, 517.0,
570.5, 659.5.
Ack n ow led gm en t. D.A.M. gratefully acknowledges
the support of the National Science Foundation in the
J O026520P
J . Org. Chem, Vol. 68, No. 2, 2003 261