Synthesis of hyperbranched dendrimers with hydrophobic surface group

Article abstract of DOI:10.1081/SCC-120037913

Hyperbranched dendrimers 1a, 1b, 2a, 2b, 3a, and 3b were synthesized with m-terphenyl end group, which can function as hydrophobic surface.

Full text of DOI:10.1081/SCC-120037913

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Synthesis of Hyperbranched Dendrimers with
Hydrophobic Surface Group
Perumal Rajakumar a & Kilivelu Ganesan a
a Department of Organic Chemistry , University of Madras , Guindy Campus, Chennai,
600 025, India
Published online: 20 Aug 2006.
To cite this article: Perumal Rajakumar & Kilivelu Ganesan (2004) Synthesis of Hyperbranched Dendrimers with
Hydrophobic Surface Group, Synthetic Communications: An International Journal for Rapid Communication of Synthetic
Organic Chemistry, 34:11, 2009-2017, DOI: 10.1081/SCC-120037913
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SYNTHETIC COMMUNICATIONSw
Vol. 34, No. 11, pp. 2009–2017, 2004
Synthesis of Hyperbranched Dendrimers
with Hydrophobic Surface Group
*
Perumal Rajakumar and Kilivelu Ganesan
Department of Organic Chemistry, University of Madras,
Guindy Campus, Chennai, India
ABSTRACT
Hyperbranched dendrimers 1a, 1b, 2a, 2b, 3a, and 3b were synthesized
with m-terphenyl end group, which can function as hydrophobic surface.
Key Words: Hyperbranched dendrimers; m-Terphenyl end group;
1
Hydrophobic surface group; H and 13C NMR spectra.
INTRODUCTION
Synthetic approaches and application of hyperbranched macromolecules
called dendrimer have grown into an interesting area of research during the
last decade.[1] Dendrimers are synthesized using either a divergent[2–4] or a
*
Correspondence: Perumal Rajakumar, Department of Organic Chemistry, University
of Madras, Guindy Campus, Chennai 600 025, India; E-mail: perumalrajakumar@
hotmail.com.
2009
DOI: 10.1081/SCC-120037913
0039-7911 (Print); 1532-2432 (Online)
www.dekker.com
Copyright # 2004 by Marcel Dekker, Inc.

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Rajakumar and Ganesan
convergent[5] approach. Novel approaches for the synthesis of various types of
dendrimers were reported in literature.[6–10] Synthesis of photodynamic,[11]
fluorescence sensing,[12] photoluminescent,[13] photoresponsive and water
soluble[14] dendrimers has been reported recently. Synthetic strategy[5,15] and
modification ofthesurface groups[9,16] haveplayedanimportantroleindendrimer
chemistry.
Dendrimer possess high surface area-to-volume ratio, and this ratio can
be further improved by hyperbranching with suitable surface end groups.[6]
By employing m-terphenyl as surface end group, hyperbranched dendrimers
could be easily achieved. Due to the presence of arene rings as terminal
groups, hydrophobic character can be enhanced in the super-branched dendri-
mer. We report, herein, the synthesis of novel hyperbranched dendrimers
1a, 1b, 2a, 2b, 3a, and 3b with m-terphenyl unit as hydrophobic surface
group.
In dendrimer 1a and 1b, the methylene unit could overlap into the aro-
matic p-electron cloud, and hence their 1H NMR could be interesting.
Further, due to the close packing in dendrimers 2a, 2b, 3a, and 3b benzene
rings could be as close as possible and hence can exhibit intramolecular
p–p interaction. Yet, another novelty in the present investigation is the

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use of 3,5-dihydroxybenzoic acid rather than the conventional and expens-
ive 3,5-dihydroxybenzylalcohol, which is the usual precursor for the syn-
thesis of dendrimers.[5]
50-Methyl-1,10 : 30,100-terphenyl 4 was obtained by the application of Hart
reaction.[17] Reaction of 3.3 eq. of phenyl magnesium bromide with 3,5-
dibromo-4-iodotoluene gave 50-methyl-1,10 : 30,100-terphenyl 4 in 62% yield,
which on further reaction with NBS[18] gave 50-bromomethyl-1,10 : 30,100-
terphenyl 5 in 84% yield. Hydrolysis of 5 with CaCO3 gave 50-hydroxy-
methyl-1,10 : 30,100-terphenyl 6 in 80% yield. Reaction of 2.1 eq. of 6 with
1 eq. of o- and m-xylenyl dibromide gave 1a and 1b in 56% and 39% yield,
respectively (Sch. 1).
In 1H NMR, the methylene protons of 1a appeared at d 4.61 and 4.71 as
two singlets integrating for four proton each in addition to the aromatic
protons at d 7.31–7.70, and in 13C NMR the methylene carbons appeared
at d 70.1 and 72.5 in addition to aromatic carbons. Dendrimer 1a showed
the molecular ion at m/z 622 in mass spectrum and further characterized by
analytical data. Structure of the dendrimer 1b was also characterized by
spectral and analytical data.
In order to synthesis 2a and 2b, 2.1 eq. of 50-bromomethyl-1,10 : 30,100-
terphenyl 5 was reacted with 1 eq. of methyl 3,5-dihydroxybenzoate in DMF
in the presence of K2CO3 to give the carbomethoxy ester [G1]-CO2Me 7 in
58% yield. Reduction of 7 with LAH in THF[8] and after usual work-up
afforded the first generation dendritic alcohol [G1]-OH 8 in 98% yield. In
1H NMR, [G1]-OH 8 showed two singlets at d 4.42 and 4.92 for the methylene
protons in addition to the aromatic protons. Reaction of 2.1 eq. of [G1]-OH 8
with 1 eq. of o- and p-xylenyl dibromide gave the dendrimer 2a and 2b in 65%
and 35% yield, respectively (Sch. 2).
1
In H NMR spectrum, dendrimer 2a displayed three singlets at d 4.53,
4.67, and 5.13 each integrating for four, four and eight methylene protons.
However, in 13C NMR spectrum dendrimer 2a showed only two methylene
carbons at d 70.2 and 72.1 probably due to the overlap of one of the methylene
carbon with the other. Structure of the dendrimer 2a was further confirmed by
Scheme 1. Reagent and conditions: a) 3.1 eq. PhMgBr, THF, 12 hr, reflux; b) H3Oþ;
c) NBS, Bz2O2, CCl4, 12 hr, reflux, d) CaCO3, THF/H2O (3 : 2), 12 hr, reflux; e) o- and
m-xylenyl dibromide, NaH, THF, 12 hr, reflux.

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Scheme 2. Reagent and conditions: a) K2CO3, DMF, 608C, 48 hr; b) LAH, THF,
12 hr, reflux; c) o- and m-xylenyl dibromide, NaH, THF, 12 hr, reflux.
mass spectrum and analytical data. Similarly, structure of the dendrimer 2b
was confirmed based on spectral and analytical data.
We focused our attention on the synthesis of next generation dendrimer.
Dendritic alcohol [G1]-OH 8 was reacted with CBr4/PPh3 in THF,[8,9] and
2.1 eq. of the resulting dendritic bromide [G1]-Br 9 was then reacted with
1 eq. of methyl 3,5-dihydroxybenzoate in DMF in the presence of K2CO3 to
give the second generation of carbomethoxy dendritic ester [G2]-CO2Me 10
in 62% yield. Reduction of 10 with LAH in THF and after work-up as
usual afforded the dendritic alcohol [G2]-OH 11 in 94% yield. The resulting
second-generation dendritic alcohol [G2]-OH 11 was reacted with o- and m-
xylenyl dibromide in THF in the presence of NaH to give 3a and 3b in
27% and 29% yield, respectively (Sch. 3).
Dendrimers 3a and 3b were characterized based on spectral and analytical
data. In conclusion, we have synthesized hydrophobic hyperbranched dendri-
mers 1a, 1b, 2a, 2b, 3a, and 3b with m-terphenyl as surface group.
Scheme 3. Reagent and conditions: a) CBr4, PPh3, THF, r.t.; b) K2CO3, DMF, 608C,
48 hr; c) LAH, THF, 12 hr, reflux; d) o- and m-xylenyl dibromide, NaH, THF, 12 hr,
reflux.

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EXPERIMENTAL SECTION
General
2013
All the melting points are uncorrected. The 1H and 13C NMR spectra of all
compounds were recorded in CDCl3 using Joel 90 (90 MHz), Jeol GSX 400
(400 MHz), and Jeol ECA 500 (500 MHz) NMR spectrometer. The mass
spectrum were recorded using Jeol DX-303, Jeol SX 102/DA-600 (6 kV,
10 mA), and MALDI-TOF LD. The column chromatography was performed
using silica gel (100–200 mesh).
50-Methyl-1,10: 30,100-terphenyl (4). To a solution of Grignard reagent
prepared from magnesium (2.10 g, 87.7 mmol) and bromobenzene (9.2 mL,
87 mmol) in dry THF (100 mL) at reflux under nitrogen was added dropwise
the solution of 3,5-dibromo-4-iodotoluene (10 g, 26.5 mmol) in dry THF
(50 mL). The reaction mixture was refluxed for 12 hr, cooled, followed by
quenching with 10% HCl (50 mL). The THF was removed on water bath,
and the residue was dissolved in CH2Cl2 (150 mL), washed with water
(100 mL). Evaporation of the organic layer after drying (MgSO4) gave 4 as
a crude material, which was then purified by column chromatography on silica
1
gel using hexane as eluent. Yield: 4.3 g, 62%; m.p.: 135–1388C; H NMR
(CDCl3): d 2.45 (s, 3H); 7.45–7.58 (m, 13H); MS: m/z 244 (Mþ); Anal.
calcd for C19H16: C, 93.44; H, 6.55; found: C, 92.98; H, 6.42.
50-Bromomethyl-1,10: 30,100-terphenyl (5). Freshly recrystallized N-bromo
succinimide (3.45 g, 19 mmol) was added in six portions 6 hr apart to a
solution of 4 (4.34 g, 17.6 mmol) in CCl4 (100 mL) at reflux, each addition
immediately followed by addition of a few milligrams of benzoyl peroxide.
After 18 hr of total reaction time at reflux, the mixture was cooled and the pre-
cipitated succinimide was removed by filtration. The solvent was removed and
residue was chromatographed on silica gel using Hexane/CHCl3 (9 : 1) as
eluent to give 5 as a colorless solid. Yield: 4.8 g, 84%; m.p.: 79–838C;
1H NMR (CDCl3): d 4.67 (s, 2H); 7.48–7.83 (m, 13H); MS: m/z 323 (Mþ);
Anal. calcd for C19H15 Br: C, 70.58; H, 4.64; found: C, 70.52; H, 4.58.
50-Hydroxymethyl-1,10: 30,100-terphenyl (6). A mixture of the bromide 5
(1.0 g, 3.0 mmol), calcium carbonate (3.0 g, 30 mmol) in 1,4-dioxane (60 mL),
and water (40 mL) was gently refluxed for 12 hr. The reaction mixture was
cooled, CaCO3 was filtered, and the solvent was evaporated in vacuum. The
crude reaction mixture was extracted using CHCl3 (3 ꢀ 50 mL). The organic
layer was washed with water, dried (MgSO4), and evaporated to give the alco-
hol 6, which was purified by column chromatography using CHCl3 as eluent.
1
Yield: 0.650 g, 80%; m.p.: 138–1408C; H NMR (CDCl3): d 4.73 (s, 2H);
7.31–7.78 (m, 13H); MS: m/z 260 (Mþ); Anal. calcd for C19H16O: C,
87.69; H, 6.15; found: C, 87.58; H, 6.12.

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General Procedure for the Synthesis of Dendritic
Esters 7 and 10
Mixture of methyl 3,5-dihydroxybenzoate (1 mmol) and m-terphenyl
bromide 5/dendritic bromide 9 (2.1 mmol) was stirred with K2CO3 (5 mmol)
in DMF (20 mL) at 608C for 48 hr. The reaction mixture was poured into water
and extracted with CH2Cl2 (3 ꢀ 150 mL). The combined organic extract was
washed with brine, dried over MgSO4. Evaporation of organic layer gave a
residue, which was chromatographed on silica gel using Hexane/CHCl3
(1 : 9) as eluent.
1
Dendritic ester [G1]-CO2Me 7. Yield: 5.40 g, 58%; m.p.: 1488C; H
NMR (CDCl3): d 3.83 (s, 3H), 5.11 (s, 4H), 7.15–7.68 (m, 29H); 13C
NMR: d 52.3, 70.3, 107.2, 108.4, 125.3, 125.9, 127.2, 127.5, 128.8, 132.1,
137.4, 140.7, 142.2, 159.7, 166.7; MS (FAB): m/z 652 (Mþ); Anal. calcd
for C46H36O4: C, 84.66; H, 5.52; found: C, 84.03; H, 5.32.
Dendritic ester [G2]-CO2Me 10. Yield: 1.30 g, 62%; m.p.: 95–988C;
1H NMR (CDCl3): d 3.85 (s, 3H), 4.99 (s, 4H), 5.12 (s, 8H), 7.21–7.73
(m, 61H); 13C NMR: d 52.2, 70.1, 101.1, 106.4, 107.1, 108.3, 125.3, 127.2,
127.5, 128.7, 137.7, 138.9, 140.7, 142.1, 159.6, 160.1, 166.6; MS (FAB):
m/z 1380 (Mþ); Anal. calcd for C98H76O8: C, 85.21; H, 5.51; found: C,
85.19; H, 5.48.
General Procedure for the Synthesis of Dendritic
Alcohols 8 and 11
To a solution of ester 7/10 (1 mmol) in dry THF (100 mL) was added
LAH (5 mmol) in portion at room temperature. The reaction mixture was stir-
.
red at reflux for 12 hr and then run into Na2SO4 10H2O and stirred. It was then
digested on steam bath 20 min and filtered. The inorganic residue was further
extracted (soxhlet) with THF (100 mL). The combined organic layer on
evaporation gave the corresponding alcohol, which was chromatographed
on silica gel using Hexane/CHCl3 (1 : 9) as eluent.
1
Dendritic alcohol [G1]-OH 8. Yield: 5.10 g, 98%; m.p.: 62–658C; H
NMR (CDCl3): d 4.42 (s, 2H), 4.92 (s, 4H), 7.18–7.58 (m, 29H): 13C
NMR: d 65.5, 70.1, 102.2, 108.2, 125.9, 127.2, 127.5, 128.7, 137.5, 139.8,
140.7, 142.1, 160.0; MS (FAB): m/z 624 (Mþ); Anal. calcd for C45H36O3:
C, 86.53; H, 5.76; found: C, 86.43; H, 5.72.
Dendritic alcohol [G2]-OH 11. Yield: 1.02 g, 94%; m.p.: 98–1008C; 1H
NMR (CDCl3): d 4.58 (s, 2H), 4.98 (s, 4H), 5.14 (s, 8H), 7.25–7.77 (m, 61H);
13C NMR: d 65.3, 70.0, 70.2, 101.7, 105.8, 106.5, 128.4, 126.0, 127.4, 127.6,

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128.9, 140.2, 160.1, 160.2; MS (FAB) m/z: 1352 (Mþ); Anal. calcd for
C97H76O7: C, 71.74; H, 5.62; found: C, 71.68; H, 5.58.
Synthesis of Dendritic bromide [G1]-Br 9. To a solution of the dendri-
tic alcohol [G1]-OH 8 (3.60 g, 5.7 mmol) in THF (30 mL), a solution of CBr4
(2.29 g, 6.9 mmol, 2 ꢀ 1.2 eq.) in PPh3 (1.8 g, 6.9 mmol, 2 ꢀ 1.2 eq.) in THF
(20 mL) was added during 10 min. Upon complete addition, the mixture turned
bright yellow and was subsequently quenched by the addition of H2O and
extracted with CH2Cl2 (3 ꢀ 150 mL). The combined organic layer was
dried (MgSO4) and evaporated to dryness, and the residue purified by
1
(SiO2) column chromatography. Yield: 2.2 g, 56%; m.p.: 142–1458C; H
NMR (CDCl3): d 4.41 (s, 2H), 5.12 (s, 4H), 7.20–7.75 (m, 29H); 13C
NMR: d 69.5, 70.1, 102.2, 108.2, 125.9, 127.2, 127.5, 128.7, 137.5, 139.8,
140.7, 142.1, 160.0; MS (FAB): m/z 687 (Mþ); Anal. calcd for C45H35
BrO2: C, 78.60; H, 5.09; found: C, 78.58; H, 5.02.
General Procedure for the Preparation of Dendrimer by
O-Alkylation: 1a, 1b, 2a, 2b 3a, and 3b
To a solution of the alcohol 6/8/11 (2.2 eq.) in dry THF (30 mL) NaH
(10 eq) was added at once and the reaction mixture was stirred for 30 minutes
under nitrogen. A solution of o/m-xylenyl dibromide (1 eq.) in THF (50 mL)
was added to the reaction mixture and refluxed for 12 hr. After completion of
the reaction, dil. HCl (2 N, 20 mL) was added to the reaction mixture to
quench excess NaH, and then the reaction mixture was evaporated to dryness.
The residue obtained was extracted with CH2Cl2 (3 ꢀ 100 mL); washed with
water (100 mL) followed by brine (100 mL), and dried (MgSO4). Evaporation
of organic layer gave a residue, which was chromatographed over SiO2 using
Hexane/CHCl3 (1 : 9).
Dendrimer 1a. Yield: 0.132 g, 56%; Colorless liquid. 1H NMR (CDCl3):
d 4.61 (s, 4H), 4.71(s, 4H), 7.31–7.70 (m, 30H); 13C NMR d 70.1, 72.5, 125.6,
125.7, 127.4, 127.6, 128.1, 128.9, 141,1, 142.0; MS: m/z 622 (Mþ); Anal.
calcd for C46H38O2: C, 88.74; H, 6.10; found: C, 88.72; H, 6.08.
Dendrimer 1b. Yield: 0.092 g, 39%; Colorless liquid; 1H NMR
(CDCl3): d 4.66 (s, 4H), 4.70 (s, 4H), 7.36–7.75 (m, 30H); 13C NMR: d
70.2, 72.4, 125.6, 125.7, 127.3, 127.4, 127.6, 128.9, 141.1, 142.1; MS: m/z
622 (Mþ); Anal. calcd for C46H38O2: C, 88.74; H, 6.10; found: C, 88.72;
H, 6.08.
Dendrimer 2a. Yield: 0.168 g, 65%; m.p.: 80–848C; 1H NMR (CDCl3):
d 4.53 (s, 4H), 4.67 (s, 4H), 5.13 (s, 8H), 7.32–7.78 (m, 62H); 13C NMR; d
70.2, 72.3, 101.6, 106.8, 125.4, 126.0, 127.4, 127.6, 128.9, 138.0, 140.9,

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142.2, 160.2; MS (FAB): m/z 1350 (Mþ); Anal. calcd for C98H78O6: C, 87.11;
H, 5.77; found: C, 87.08; H, 5.72.
Dendrimer 2b. Yield: 0.080 g, 35%; m.p.: 78–828C; 1H NMR (CDCl3):
d 4.53 (s, 4H), 4.56 (s, 4H), 5.14 (s, 8H), 7.25–7.76 (m, 62H); 13C NMR: d
70.2, 72.1, 101.5, 106.8, 125.4, 125.9, 127.4, 127.6, 128.9, 138.0, 140.9,
142.2, 160.2; MS (FAB): m/z 1350 (Mþ); Anal. calcd for C98H78O6: C,
87.11; H, 5.77; found: C, 87.08; H, 5.72.
Dendrimer 3a. Yield: 0.060 g, 27%; m.p.: 116–1208C; 1H NMR
(CDCl3): d 4.45 (s, 4H), 4.62 (s, 4H), 4.92 (s, 8H), 5.09 (s, 16H), 7.25–7.75
(m, 126H); 13C NMR: d 70.0, 70.2, 72.5, 101.7, 106.5, 106.7, 125.4, 127.4,
127.6, 128.9, 137.9, 140.9, 142.2, 160.0, 160.2; MS (FAB): m/z 2806
(Mþ); Anal. calcd for C202H158O14: C, 86.38; H, 5.60; found: C, 85.98; H,
5.88.
Dendrimer 3b. Yield: 0.065 g, 29%; m.p.: 106–1108C; 1H NMR
(CDCl3): d 4.58 (s, 8H), 4.98 (s, 8H), 5.15 (s, 16H), 7.25–7.76 (m, 126H);
13C NMR: d 70.0, 70.2, 72.5, 105.8, 106.5, 125.4, 126.0, 127.4, 127.6,
128.9, 137.8, 140.9, 142.2, 160.2; MS (FAB): m/z 2806 (Mþ); Anal. calcd
for C202H158O14: C, 86.38; H, 5.60; found: C, 85.98; H, 5.88.
ACKNOWLEDGMENT
The authors thank CSIR, New Delhi for financial assistance and RSIC,
CDRI, Lucknow for spectral data. KG thanks CSIR, New Delhi for fellowship,
U. Senthilkumar and M. Karthikeyan, CLRI, Adyar for help.
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Received in India January 5, 2004

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