Synthesis and electrochemical properties of dendrimers containing meta-terphenyl peripheral groups and a 4,4′-bipyridinium core

Article abstract of DOI:10.1016/j.tet.2004.08.093

Fréchet-type poly(arylether) dendrons carrying m-terphenyl peripheral groups were synthesized up to second generation by convergent methodology. Simple quarternisation of 4,4′-bipyridine with the dendritic bromides afforded the corresponding dendrimers co

Full text of DOI:10.1016/j.tet.2004.08.093

Tetrahedron 60 (2004) 10285–10291
Synthesis and electrochemical properties of dendrimers
containing meta-terphenyl peripheral groups and a
4,40-bipyridinium core
*
Perumal Rajakumar and Kannupal Srinivasan
Department of Organic Chemistry, University of Madras, Guindy Campus, Chennai 600025, Tamil Nadu, India
Received 2 April 2004; revised 6 August 2004; accepted 26 August 2004
´
Abstract—Frechet-type poly(arylether) dendrons carryin0 g m-terphenyl peripheral groups were synthesized up to second generation by
convergent methodology. Simple quarternisation of 4,4 -bipyridine with the dendritic bromides afforded the corresponding dendrimers
containing a 4,40-bipyridine core. The electrochemical parameters were obtained for all the dendrimers and the half-wave potentials of both
the first and second redox processes shift to less-negative values as the dendrimer generation increases.
q 2004 Elsevier Ltd. All rights reserved.
1. Introduction
molecule if the core is electron-deficient. Dendrimers with
various types of encapsulated redox active cores have been
reported.7 Among them, dendrimers with a 4,40-bipyridi-
nium core have attracted significant interest as promising
materials for nanoelectronics8 and light harvesting.9 Hence
the incorporation of this group as the core of dendrimer
structure is of interest and importance. Herein we report
synthesis and electrochemical properties of dendrimers
containing a 4,40-bipyridinium core and m-terphenyl
peripheral groups.
The peripheral groups of dendrimers act as the primary
interface between the dendrimer and the external environ-
ment.1 The properties of dendrimers such as solubility,
chemical reactivity and glass transition temperature are
strongly influenced by the nature of the peripheral groups.
Hence there is a continuing interest in modifying the
peripheral groups of dendrimers. The systems end-capped
with organoruthenium moieties,2 naphthalene and pyrene
groups,3 ferrocene units4 and coumarin 2 (laser dye) units5
´
are some examples of peripherally modified Frechet-type
2. Results and discussion
2.1. Synthesis
dendrons and dendrimers. We have proven that meta-
terphenyls are ideal building blocks in cyclophane chem-
istry.6 In order to extend their utility to other supramolecular
systems, we planned to introduce m-terphenyls as peripheral
groups in dendrimers. A m-terphenyl carrying methoxy
groups at 4,400-positions was chosen to serve this purpose for
three reasons: (i) the methoxy groups will provide
appropriate solubility to permit dendritic growth through
the repetition of coupling and activation steps, (ii) since
the outer rings are orthogonal to the central ring in the
m-terphenyl, there will be electron rich pockets for
molecular recognition at the dendrimer surface and
(iii) the electron-rich m-terphenyl will allow intramolecular
charge transfer (CT) to take place from the periphery
through the dendrimer backbone to the center of the
The synthetic strategy for the preparation of the m-terphenyl-
capped dendrons utilized a convergent method.10 Hart’s
tandem aryne sequence reaction11 between 2,6-dibromo-4-
methyliodobenzene (1) and 4-anisylmagnesium bromide (2)
afforded the m-terphenyl 3 in 62% yield. Radical bromina-
tion of 3 with NBS in CCl4 gave the zeroth generation
m-terphenyl bromide 4, [G-0]-CH2Br, in 44% yield. The
presence of methoxy groups in the m-terphenyl significantly
lowers the yield of this step. Coupling of 2 equiv of 4 with
1 equiv of the monomer, methyl-3,5-dihydroxy benzoate, in
the presence of K2CO3 and 18-crown-6 in DMF at 80 8C
afforded the first generation ester 5, [G-1]-CO2Me, in 72%
yield. This ester was converted into the corresponding
alcohol 6, [G-1]-CH2OH, in 90% yield by reduction with
LAH in refluxing THF. Conversion of the alcohol 6 to the
corresponding bromide 7 using CBr4 and Ph3P in THF was
Keywords: Dendrimers; m-Terphenyl; 4,40 -Bipyridine; Redox potentials.
* Corresponding author. Tel.: C91-44-22351269-213; fax: C91-44-
22352494; e-mail: perumalrajkumar2003@yahoo.co.in
0040–4020/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2004.08.093

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P. Rajakumar, K. Srinivasan / Tetrahedron 60 (2004) 10285–10291
1
unsuccessful as the removal of triphenylphosphine oxide
from the bromide was difficult. Hence the alcohol 6 was
treated with PBr3 in CH2Cl2 to afford the bromide 7, [G-1]-
CH2Br, in 68% yield. The second generation dendritic
bromide 10, [G-2]-CH2Br, was built in a similar way by the
repetition of the above three reactions namely O-alkylation,
reduction and bromination (Scheme 1).
In the H NMR spectra of the dendrons, the intra-annular
m-terphenyl protons (proton at 20-position of the m-terphenyl
unit) resonate in the region 7.58–7.69 ppm as singlets, the
m-terphenyl protons ortho to methoxy groups resonate at
6.88–6.99 ppm as doublets and the aromatic and methylene
protons of each layer of monomer units resonate in separate
regions. The resonances of methylene protons of CH2OH
Scheme 1. Reagents and conditions: (i) THF, reflux, 12 h, 62%; (ii) NBS, CCl4, Bz2O2, reflux, 48 h, 44%; (iii) methyl-3,5-dihydroxy benzoate, K2CO3, 18-
crown-6, DMF, 80 8C, 48 h; afforded 5 (72%) and 8 (70%); (iv) LAH, THF, reflux, 6 h; afforded 6 (90%) and 9 (86%); (v) PBr3, CH2Cl2, 0 8C to rt, 2 h;
afforded 7 (68%) and 10 (54%).

P. Rajakumar, K. Srinivasan / Tetrahedron 60 (2004) 10285–10291
10287
Scheme 2. Reagents and conditions: (i) CH3CN, reflux, 24 h; then NH4 PF6, H2O; afforded 11 (78%), 12 (63%) and 13 (50%).
group in 6 and 9 occur at 4.61 and 4.57 ppm, respectively,
whereas those of the CH2Br group in 7 and 10 occur at 4.44
and 4.27 ppm, respectively. In the 13C NMR spectra of the
dendritic alcohols and their bromides, however, a vast
difference in the chemical shifts (about 31.5 ppm) is found
between CH2OH and CH2Br carbons. All the dendrons
show absorption maxima around 268 nm and emission
maxima around 348 nm in CHCl3.
corresponding to the formation of a radical cation
(BIPY2C/BIPYC) and a neutral species (BIPYC/
BIPY).12 Fig. 1 shows the cyclic voltammogram of
dendrimer 12 in CH3CN at 50 mV/s.
The half-wave potentials (E1/2) and diffusion co-efficients
(D) obtained for the dendrimers are shown in Table 1.
The microenvironment around the 4,40-bipyridinium core
Heating 2 equiv of each of the dendritic bromides 4, 7 and 10
under reflux with 1 equiv of 4,40-bipyridine in CH3CN, followed
by counterion exchange and purification, afforded the dendri-
mers 11–13 in 78, 63 and 50% yields, respectively (Scheme 2).
Unlike the lower generation dendrimers 11 and 12,
dendrimer 13 is soluble in CH2Cl2 and CHCl3 despite its
dicationic nature, which indicates the influence of hydro-
1
phobic dendritic framework on solubility. In the H NMR
spectra, the bipyridinium protons of the dendrimers resonate
in the region 8.12–9.68 ppm as two doublets and the CH2N
protons resonate at 5.78–6.32 ppm as singlets.
2.2. Electrochemical properties
Figure 1. Cyclic voltammogram of dendrimer 12 in CH3CN at 50 mV/s.
Dendrimers 11–13 exhibited two sets of redox waves

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P. Rajakumar, K. Srinivasan / Tetrahedron 60 (2004) 10285–10291
Table 1. The half-wave potentials (E1/2) and diffusion co-efficients (D)
obtained for the dendrimers in CH3CN at 25 8C (the reported half-wave
potentials were measured at 50 mV/s)
3. Conclusions
´
We have synthesized Frechet-type poly(arylether) dendri-
Dendrimera
E11/2 (mV)
E21/2 (mV)
D (cm2/s)
mers carrying m-terphenyl peripheral groups and a redox
active 4,40-bipyridinium core up to second generation by
convergent methodology. The dendrimer 13 is soluble in
CH2Cl2 and CHCl3 despite its dicationic nature, which
indicates the influence of the hydrophobic dendritic frame-
work on solubility. They exhibit two sets of reversible redox
waves corresponding to the formation of a radical cation
(BIPY2C/BIPYC) and a neutral species (BIPYC/
BIPY). The half-wave potentials (E1/2) of both the first
and second redox processes shift to less-negative values as
the dendrimer generation increases and this indicates that
the hydrophobic character of the dendrimer backbone
dominates over its electron-rich character in determining
redox potentials.
11
12
13
K320
K291
K204
K755
K751
K688
2.5619!10K7
5.4984!10K8
—
a
E11/2 and E21/2 are the averages of the cathodic and anodic peak potentials
of the first and second redox processes respectively.
should become more electron-rich with increasing dendri-
mer generation due to the increasing number of electron-
rich m-terphenyl units. Hence the reduction should become
more difficult and the half-wave potentials should shift
to more-negative values as the dendrimer generation
increases.7a However, the half-wave potentials of both
first and second redox processes shift to less-negative values
with increasing dendrimer generation. As compared to the
half-wave potentials of dibenzyl viologen (E11/2ZK320 mV
and E21/2ZK758 mV vs. Ag/AgCl in CH3CN), the half-
wave potentials of the dendrimers (particularly for 12 and
13) shift to less-negative values. This trend observed in E1/2
values is attributable to the increasing hydrophobic
character of the dendrimer periphery, which makes it
progressively harder to solvate effectively the two positive
charges of the bipyridinium core and, therefore, reduction
becomes more favourable with increasing dendrimer
generation.8 Thus the hydrophobic character of the
dendrimer backbone dominates over its electron-rich
character in determining redox potentials.
4. Experimental
4.1. General
Melting points were determined by using a Toshniwal
melting point apparatus by open capillary tube method and
were uncorrected. Ultraviolet-Visible spectra (UV) were
recorded on Shimadzu 260 spectrophotometer and emission
spectra on Perkin–Elmer LS-5B spectrophotometer. IR
spectra were recorded on Shimadzu FTIR-8300 spectro-
photometer. 1H and 13C NMR spectra were recorded on Jeol
500 MHz, Jeol 400 MHz, Bruker 400 MHz, Bruker
300 MHz and Bruker ARX 200 MHz spectrometers. The
FAB-MS spectra were recorded on a Jeol SX 102/DA-6000
mass spectrometer using a m-nitro benzyl alcohol (NBA)
matrix, MALDI-TOF MS spectrum on Voyager-DE PRO
mass spectrometer using a a-cyano-4-hydroxy cinnamic
acid (CHCA) matrix and EI-MS spectra on Jeol DX-303
mass spectrometer. Elemental analyses were performed on a
Perkin–Elmer 240B elemental analyzer. Electrochemical
studies were carried out on a CH Instruments electro-
chemical analyzer.
Diffusion co-efficients (D) obtained for the dendrimers 11
and 12 using Randles–Sevcik equation13 indicate that the
diffusion of 12 is around five times slower than that of 11.
However, we could not able to obtain reliable D value for
the second generation dendrimer 13 due to the precipitation
of the one-electron reduced species.
The difference between anodic and cathodic potentials
(DEp) indicates that all these redox processes are reversible
at low scan rates even for the second generation dendrimer
13 (Table 2). This may be attributable to the angular nature
of the m-terphenyl units, which does not allow complete
steric burial of the core within the dendrimer structure.14 At
higher scan rates, however, all the redox processes become
quasireversible, particularly in case of 12 and 13. Some
distortion associated with precipitation of the one-electron
reduced species on the electrode surface is found in the
cyclic voltammogram of 13.
4.1.1. Synthesis of m-terphenyl 3. To a solution of 4-
anisylmagnesium bromide (2) [prepared from 4-bromo-
anisole (10.7 mL, 85.46 mmol) and magnesium (2.08 g,
85.46 mmol) in dry THF (150 mL)] heated at reflux under
nitrogen atmosphere was added dropwise a solution of 3,5-
dibromo-4-iodo toluene (1) (10.0 g, 26.61 mmol) in dry
THF (100 mL). After 12 h additional heating at reflux, the
reaction mixture was cooled to 20 8C and quenched by the
Table 2. The difference between the cathodic and anodic peak potentials (DEp) of the first and second redox processes of the dendrimers at different scan rates
in CH3CN at 25 8C
Scan ratea (mV/s)
11
12
13
DE1p (mV)
DE2p (mV)
DE1p (mV)
DE2p (mV)
DE1p (mV)
DE2p (mV)
25
50
100
200
500
750
66
68
69
68
74
79
70
74
68
75
82
87
60
60
66
73
92
102
70
69
72
82
112
121
66
75
88
102
—
—
59
65
97
127
—
—
a
DE1p and DE2p are the difference between the cathodic and anodic peak potentials of the first and second redox processes respectively.

P. Rajakumar, K. Srinivasan / Tetrahedron 60 (2004) 10285–10291
10289
addition of dil. HCl (6 M, 50 mL). Most of the solvent was
evaporated on a water bath; the reaction mixture was then
diluted with H2O and extracted with CHCl3 (2!200 mL).
The combined extracts were washed with water (2!
200 mL), satd. NaHCO3 solution (1!200 mL) and dried
(anhydrous Na2SO4). The crude product obtained after
solvent removal was chromatographed (SiO2; hexane/
CHCl3, 7:3, v/v) to give the m-terphenyl 3 as a colorless
solid. Yield 62%; mp 136 8C; 1H NMR (200 MHz, CDCl3) d
2.32 (s, 3H), 3.69 (s, 6H), 6.85 (d, JZ8.5 Hz, 4H), 7.19 (s,
2H), 7.42–7.46 (m, 5H); 13C NMR (50 MHz, CDCl3) d 22.2,
55.9, 114.8, 123.2, 126.6, 128.9, 134.5, 139.3, 142.0, 159.8;
EI-MS: m/z (%) 304 (MC, 100), 289 (47), 273 (11), 261
(34), 215 (30), 152 (31). Anal. Calcd for C21H20O2: C,
82.86, H, 6.62. Found: C, 82.78, H, 6.57.
4.2.2. Dendritic ester 8 {[G-2]-CO2Me}. The dendritic
ester 8 was obtained as a colourless solid from methyl-3,5-
dihydroxy benzoate (200 mg, 1.19 mmol) and 7 (2.02 g,
2.50 mmol). Yield 70%; mp 105 8C; IR (cmK1) 1720
(C]O); 1H NMR (500 MHz, CDCl3) d 3.83 (s, 24H), 3.88
(s, 3H), 5.00 (s, 4H), 5.11 (s, 8H), 6.67 (distorted t, 2H), 6.74
(distorted d, 4H), 6.80 (distorted t, 1H), 6.97 (d, JZ8.6 Hz,
16H), 7.29 (d, JZ2.3 Hz, 2H), 7.54–7.57 (m, 24H), 7.66 (s,
4H); 13C NMR (125 MHz, CDCl3) d 52.4, 55.4, 70.2, 70.4,
101.9, 106.6, 107.2, 108.5, 114.3, 124.5, 125.1, 128.4,
132.1, 133.5, 137.7, 139.0, 141.8, 159.4, 159.8, 160.3,
166.8; FAB-MS: m/z (%) 1620 (MC, 28). Anal. Calcd for
C106H92O16: C, 78.50, H, 5.72. Found: C, 78.68, H, 5.54.
4.3. General procedure for the reduction of the dendritic
ester into the corresponding alcohol
4.1.2. Synthesis of m-terphenyl bromide 4 {[G-0]-
CH2Br}. NBS (3.22 g, 18.09 mmol) was added in four
equal portions at 4 h intervals to a solution of 3 (5.00 g,
16.43 mmol) in CCl4 heated at reflux. Each portion was
immediately followed by the addition of Bz2O2 (a few mg).
After heating under reflux for a total of 48 h, the mixture
was cooled and the precipitated succinimide was removed
by filtration. The residue obtained after evaporation of the
solvent was chromotographed (SiO2; hexane/CHCl3, 7:3,
v/v) to give 4 as a colorless solid. Yield 44%; mp 116 8C; 1H
NMR (200 MHz, CDCl3) d 3.76 (s, 6H), 4.46 (s, 2H), 6.88
(d, JZ8.7 Hz, 4H), 7.38–7.53 (m, 7H); 13C NMR (50 MHz,
CDCl3) d 34.3, 55.9, 114.9, 125.9, 126.3, 128.8, 133.7,
139.2, 142.5, 160.0; EI-MS: m/z (%) 384 (MC, 28), 382
(MC, 28), 304 (43), 303 (100), 258 (14), 217 (17), 192 (13),
152 (45). Anal. Calcd for C21H19BrO2: C, 65.81, H, 5.00.
Found: C, 65.52, H, 5.10.
To a stirred suspension of LAH (2.5 equiv) in dry THF
(20 mL), a solution of the corresponding dendritic ester
(1.0 equiv) in dry THF (30 mL) was added dropwise at
room temperature under nitrogen atmosphere. The reaction
mixture was heated under reflux for 6 h, after which it was
cooled to 0–10 8C and the excess LAH was quenched by
cautiously adding 10% NaOH solution dropwise. Anhy-
drous Na2SO4 was added to the reaction mixture, stirred and
filtered. THF (20 mL) was added to the residue, digested on
a steam bath and filtered. The process was repeated for four
or five times. The combined THF fractions were evaporated
to give the crude alcohol, which was purified by column
chromatography (SiO2).
4.3.1. Dendritic alcohol 6 {[G-1]-CH2OH}. The dendritic
alcohol 6 was obtained as a colourless solid by reducing 5
(2.5 g, 3.23 mmol) with LAH (306 mg, 8.08 mmol) and
then eluting the column with CHCl3. Yield 90%; mp 112 8C;
IR (cmK1) 3421 (O–H); 1H NMR (400 MHz, CDCl3) d 3.81
(s, 12H), 4.61 (s, 2H), 5.08 (s, 4H), 6.62 (distorted t, 1H),
6.66 (distorted d, 2H), 6.96 (d, JZ8.8 Hz, 8H), 7.52–7.56
(m, 12H), 7.65 (s, 2H); 13C NMR (100 MHz, CDCl3) d 55.3,
65.2, 70.2, 101.3, 105.7, 114.2, 124.3, 124.9, 128.2, 133.4,
137.7, 141.6, 143.5, 159.3, 160.2; FAB-MS: m/z (%) 744
(MC, 32). Anal. Calcd for C49H44O7: C, 79.01, H, 5.95.
Found: C, 79.33, H, 5.79.
4.2. General procedure for the synthesis of dendritic
esters
A mixture of the corresponding dendritic bromide
(2.1 equiv), methyl-3,5-dihydroxy benzoate (1.0 equiv),
dried potassium carbonate (3.0 equiv) and 18-crown-6
(0.1 equiv) in dry DMF (30 mL) was vigorously stirred at
80 8C for 48 h under nitrogen atmosphere. The reaction
mixture was then allowed to cool to room temperature and
poured into ice water. The resulting precipitate was filtered,
washed thoroughly with water and dissolved in CHCl3
(150 mL). The organic layer was separated, washed with
brine (1!150 mL), dried (anhydrous Na2SO4) and evapor-
ated to give the crude ester, which was purified by column
chromatography (SiO2) using hexane/CHCl3 (1:1–1:9, v/v)
as the eluent.
4.3.2. Dendritic alcohol 9 {[G-2]-CH2OH}. The dendritic
alcohol 9 was obtained as a colourless solid by reducing 8
(1.0 g, 0.627 mmol) with LAH (60 mg, 1.57 mmol) and
then eluting the column with CHCl3/EtOAc (9.5:0.5, v/v).
1
Yield 86%; mp 110 8C; IR (cmK1) 3420 (O–H); H NMR
(400 MHz, CDCl3) d 3.82 (s, 24H), 4.57 (s, 2H), 4.96 (s,
4H), 5.10 (s, 8H), 6.54 (distorted t, 1H), 6.59 (distorted d,
2H), 6.65 (distorted t, 2H), 6.73 (distorted d, 4H), 6.96 (d,
JZ8.8 Hz, 16H), 7.53–7.57 (m, 24H), 7.66 (s, 4H); 13C
NMR (100 MHz, CDCl3) d 55.3, 65.2, 69.1, 70.3, 101.3,
101.8, 105.9, 106.5, 114.2, 124.3, 125.0, 128.3, 133.5,
137.7, 139.4, 141.7, 159.3, 160.1, 160.2; FAB-MS: m/z (%)
1592 (MC, 14). Anal. Calcd for C105H92O15: C, 79.12, H,
5.82. Found: C, 78.98, H, 5.87.
4.2.1. Dendritic ester 5 {[G-1]-CO2Me}. The dendritic
ester 5 was obtained as a colourless solid from methyl-3,5-
dihydroxy benzoate (1.0 g, 5.95 mmol) and 4 (4.79 g,
12.50 mmol). Yield 72%; mp 183 8C; IR (cmK1) 1720
(C]O); 1H NMR (300 MHz, CDCl3) d 3.85 (s, 12H), 3.92
(s, 3H), 5.17 (s, 4H), 6.89 (distorted t, 1H), 6.99 (d, JZ
8.7 Hz, 8H), 7.37 (d, JZ2.2 Hz, 2H), 7.25–7.59 (m, 12H),
7.68 (s, 2H); 13C NMR (75 MHz, CDCl3) d 55.4, 70.4,
107.3, 108.4, 114.2, 124.4, 125.1, 128.3, 132.1, 133.4,
137.3, 141.7, 159.3, 159.8, 166.8; FAB-MS: m/z (%) 772
(MC, 40). Anal. Calcd for C50H44O8: C, 77.70, H, 5.74.
Found: C, 77.58, H, 5.68.
4.4. General procedure for the conversion of the
dendritic alcohol into the corresponding bromide
To a stirred suspension of the corresponding dendritic

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P. Rajakumar, K. Srinivasan / Tetrahedron 60 (2004) 10285–10291
alcohol (1.0 equiv) in CH2Cl2 (10 mL) was added dropwise
a solution of PBr3 (3.0 equiv) in CH2Cl2 (20 mL) at 0 8C.
The reaction mixture was stirred at room temperature for 2 h
and then quenched by the addition of ice water. The organic
layer was separated, washed with water (2!25 mL), satd.
NaHCO3 solution (1!25 mL), dried (anhydrous Na2SO4)
and evaporated to give the crude dibromide, which was
purified by column chromatography (SiO2) using
hexane/CH2Cl2 (1:1–1:8 v/v) as the eluent.
159.5; FAB-MS: m/z (%) 907 (MCKPF6, 34), 762 (MCK
2PF6, 63). Anal. Calcd for C52H46F12N2O4P2: C, 59.32, H,
4.40, N, 2.66. Found: C, 59.49, H, 4.34, N, 2.78.
4.5.2. Dendrimer 12. The dendrimer 12 was obtained from
the dendritic bromide 7 (259 mg, 0.320 mmol) and 4,40-
bipyridine (25 mg, 0.160 mmol). The precipitated solid was
filtered and washed thoroughly with diethyl ether (10 mL)
and CHCl3 (10 mL). The residue was suspended in CH3CN
(10 mL), solid NH4PF6 (0.1 g) was added, and the mixture
was stirred for 30 min and then filtered. The solvent was
removed under vacuum; the residue was washed thoroughly
with H2O and then dried. The crude product was
recrystallised from CH3CN to give the pure dendrimer 12
4.4.1. Dendritic bromide 7 {[G-1]-CH2Br}. The dendritic
bromide 7 was obtained as a white foam-like solid by the
bromination of 6 (2.0 g, 2.68 mmol) with PBr3 (0.77 mL,
1
8.10 mmol). Yield 68%; mp 202 8C; H NMR (500 MHz,
1
CDCl3) d 3.86 (s, 12H), 4.44 (s, 2H), 5.12 (s, 4H), 6.65
(distorted t, 1H), 6.71 (distorted d, 2H), 6.99 (d, JZ8.6 Hz,
8H), 7.54–7.59 (m, 12H), 7.69 (s, 2H); 13C NMR (125 MHz,
CDCl3) d 33.8, 55.5, 70.4, 102.4, 108.3, 114.4, 124.5, 125.2,
128.4, 133.5, 137.6, 140.0, 141.8, 159.4, 160.2; FAB-MS:
m/z (%) 808 (MC, 27), 806 (MC, 27). Anal. Calcd for
C49H43BrO6: C, 72.86, H, 5.37. Found: C, 72.66, H, 5.42.
as a pale yellow solid. Yield 63%; mp 134 8C; H NMR
(500 MHz, DMSO-d6) d 3.74 (s, 24H), 5.20 (s, 8H), 5.78 (s,
4H), 6.90 (distorted t, 2H), 6.93 (distorted d, 4H), 6.97 (d,
JZ8.6 Hz, 16H), 7.55 (distorted d, 8H), 7.62 (d, JZ8.6 Hz,
16H), 7.71 (s, 4H), 8.50 (d, JZ6.9 Hz, 4H), 9.68 (d, JZ
6.9 Hz, 4H); 13C NMR (125 MHz, DMSO-d6) d 55.7, 70.1,
108.9, 114.9, 124.3, 124.5, 127.5, 128.5, 132.7, 136.6,
138.4, 141.2, 146.1, 149.6, 149.8, 159.6, 160.6; FAB-MS:
m/z (%) 1755 (MCKPF6, 28), 1610 (MCK2PF6, 30). Anal.
Calcd for C108H94F12N2O12P2: C, 68.21, H, 4.98, N, 1.47.
Found: C, 68.39, H, 4.76, N, 1.52.
4.4.2. Dendritic bromide 10 {[G-2]-CH2Br}. The dendritic
bromide 10 was obtained as a white foam-like solid by the
bromination of 9 (500 mg, 0.314 mmol) with PBr3
1
(0.09 mL, 0.948 mmol). Yield 54%; mp 125 8C; H NMR
(400 MHz, CDCl3) d 3.75 (s, 24H), 4.27 (s, 2H), 4.88 (s,
4H), 5.03 (s, 8H), 6.46 (distorted t, 1H), 6.53 (distorted d,
2H), 6.59 (distorted t, 2H), 6.64 (distorted d, 4H), 6.88 (d,
JZ8.8 Hz, 16H), 7.45–7.49 (m, 24H), 7.58 (s, 4H); 13C
NMR (100 MHz, CDCl3) d 33.6, 55.3, 70.1, 70.3, 101.8,
102.2, 106.5, 108.3, 113.4, 114.2, 124.3, 125.0, 128.3,
133.5, 137.7, 139.2, 141.7, 159.3, 160.0, 160.2; FAB-MS:
m/z (%) 1656 (MC, 15), 1654 (MC, 15). Anal. Calcd for
C105H91BrO14: C, 76.12, H, 5.54. Found: C, 76.38, H, 5.56.
4.5.3. Dendrimer 13. The dendrimer 13 was obtained from
the dendritic bromide 10 (106 mg, 0.064 mmol) and 4,40-
bipyridine (5 mg, 0.032 mmol). The crude product was
dissolved in DMF (1 mL) and a satd. aq. solution of NH4PF6
(1 mL) was added dropwise and stirred for 30 min. The
resulting precipitate was filtered, washed thoroughly with
H2O and dried. The product was recrystallised from CH2Cl2
to give the dendrimer 13 as a yellow solid. Yield 50%; mp
1
122 8C; H NMR (400 MHz, CDCl3) d 3.74 (s, 48H), 5.12
(s, 16H), 5.22 (s, 8H), 6.32 (s, 4H), 6.40 (s, 4H), 6.51–6.56
(m, 14H), 6.83 (d, JZ8.3 Hz, 32H), 7.39–7.44 (m, 48H),
7.53 (s, 8H), 8.12 (d, JZ6.8 Hz, 4H), 8.47 (distorted d, 4H);
MALDI-TOF-MS: m/z (%) 3306 (MCK2PF6, 11).
4.5. General procedures for the synthesis of dendrimers
with a 4,40-bipyridine core
A mixture of the corresponding dendritic bromide (2.0 equiv)
and 4,40-bipyridine (1.0 equiv) in dry CH3CN (25 mL) was
heated at reflux for 24 h under nitrogen. After cooling the
reactionmixture toroomtemperature, the solvent was reduced
to one-fifth of its volume under vacuum. The precipitated
crude product was filtered and purified by recrystallisation or
column chromatography (SiO2).
4.6. Electrochemical properties
Electrochemical experiments were carried out in nitrogen-
purged CH3CN solutions at room temperature. The
solutions for electrochemistry were held at a concentration
of 0.25 mM of the electroactive species. TBAPF6 (0.1 M)
was included as a supporting electrolyte. A glassy carbon
electrode (0.052 cm2) was used as the working electrode; its
surface was routinely polished with a 0.05 mm alumina–
water slurry on a felt surface prior to use. All potentials were
recorded against a saturated Ag/AgCl electrode and a
platinum wire was used as a counter electrode. Cyclic
voltammograms were obtained at scan rates of 25, 50, 75,
100, 200, 300, 400, 500, 600 and 750 mV/s. A plot of
cathodic peak currents of first redox processes vs. (scan
rate)1/2 was linear. From the slope of the plot, the diffusion
co-efficients (D) were calculated using Randles–Sevcik
equation.13
4.5.1. Dendrimer 11. The dendrimer 11 was obtained from
the dendritic bromide 4 (491 mg, 1.28 mmol) and 4,40-
bipyridine (100 mg, 0.640 mmol). The crude product was
purified by column chromatography over SiO2 using
CH3OH/H2O/satd. aq. NH4Cl (6:3:1) as the eluent. The
dendrimer-containing fractions were combined and the
solvent was evaporated under vacuum. The residue was
dissolved in H2O (25 mL) and solid NH4PF6 (0.25 g) was
added, then the precipitate was filtered, washed thoroughly
with H2O and dried to give the pure dendrimer 11 as a pale
yellow solid. Yield 78%; mp 170 8C (dec.); 1H NMR
(400 MHz, DMSO-d6) d 3.78 (s, 12H), 5.99 (s, 4H), 7.03 (d,
JZ8.3 Hz, 8H), 7.71 (d, JZ8.8 Hz, 8H), 7.84 (s, 2H), 7.89
(s, 4H), 8.71 (d, JZ6.4 Hz, 4H), 9.68 (d, JZ6.4 Hz, 4H);
13C NMR (100 MHz, DMSO-d6) d 55.4, 64.0, 114.6, 125.2,
125.6, 127.5, 128.4, 131.9, 135.5, 141.7, 145.8, 149.5,
Acknowledgements
K. S. thanks CSIR, India for financial support. The authors

P. Rajakumar, K. Srinivasan / Tetrahedron 60 (2004) 10285–10291
10291
9749–9754. (e) Rajakumar, P.; Srinivasan, K. Eur. J. Org.
Chem. 2003, 1277–1284.
thank SAIF, Madras, for NMR spectra and MALDI-TOF
MS; and RSIC, Lucknow, for FAB-MS.
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