Synthesis of shape-persistent polyal dendrimers - Facile entry to polyene and polyyne dendrimers

Article abstract of DOI:10.1002/ejoc.200500055

Polyaldehyde (polyal) dendrimers were synthesized by a divergent iterative method using 1,3,5-triethynylbenzene (1) as the core unit and 2-bromo-5-tert-butyl-1,3-benzenedicarbaldehyde (2) as the building block. The polyal dendrimers were further transform

Full text of DOI:10.1002/ejoc.200500055

FULL PAPER
Synthesis of Shape-Persistent Polyal Dendrimers – Facile Entry to Polyene and
Polyyne Dendrimers
Vijay Narayanan,[a] Sethuraman Sankararaman,*[b] and Henning Hopf[a]
Keywords: Dendrimers / Polyenes / Polyynes / Sonogashira coupling / Corey–Fuchs reaction
Polyaldehyde (polyal) dendrimers were synthesized by a di-
vergent iterative method using 1,3,5-triethynylbenzene (1) as
were further transformed into a new class of polyene and
polyyne dendrimers.
the core unit and 2-bromo-5-tert-butyl-1,3-benzenedicarbal- (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
dehyde (2) as the building block. The polyal dendrimers
Germany, 2005)
easily synthesized in multi-gram quantities from a readily
available and cheap starting material, namely 1-tert-butyl-
3,5-dimethylbenzene in three steps, it is very reactive in
Sonogashira coupling with several terminal acetylenes,
which offers a convenient synthetic methodology for the
synthesis of acetylenic dendrimers, the aldehyde functional
groups in the resulting dendrimers are amenable to further
functional-group transformations to a variety of derivatives,
and finally the innocuous tert-butyl group should impart
high solubility to the dendrimers.
Introduction
The past decade has witnessed an explosive growth in
the areas of cyclophynes,[1] cyclo(n)carbons,[2] graphyne and
graphdiyne segments,[3] carbon-rich organometallics,[4]
polyphenylenes,[5] and superacenes[6] with the revival of hy-
drocarbon chemistry[7] and, in particular, the chemistry of
acetylenes.[8] During the same period, the chemistry of den-
drimers has also evolved as a major area of research in view
of the synthetic challenges and wide range of applications
of these molecules in several areas.[9] Acetylenic dendrimers
are a special class of compounds with promising utility in
the areas of electronics and photonics.[10] The stiff acetyle-
nic units provide a persistent shape and dimension, and
acetylenic dendrimers of various shapes and sizes have been
reported.[11] Moore and co-workers have made seminal con-
tributions in the area of acetylenic dendrimers, in particular
phenylacetylene dendrimers.[12] Their synthetic approaches
based on convergent and divergent strategies along with the
SYNDROME concept have led to the synthesis of large
dendrimers of nanodimensions. Phenylacetylene dendri-
mers find applications in the areas of light-harvesting an-
tenna molecules,[13] nonlinear optical materials,[14] and or-
ganic light-emitting diodes.[15]
Results and Discussion
The Sonogashira coupling of 1,3,5-triethynylbenzene (1)
and 2-bromo-5-tert-butyl-1,3-benzenedicarbaldehyde (2)
proceeded smoothly to yield the first-generation hexaalde-
hyde 3 in 72% yield (Scheme 1). A small amount of poly-
In this paper we report the synthesis of first- and second-
generation polyaldehyde (polyal) dendrimers starting from
1,3,5-triethynylbenzene (1) as the core unit and 5-tert-butyl-
1,3-benzenedicarbaldehyde as the propagating unit. The
choice of 2-bromo-5-tert-butyl-1,3-benzenedicarbaldehyde
(2) as the building block is based on the following. It is
[a] Institute of Organic Chemistry, University of Braunschweig,
Hagenring 30, 38106, Braunschweig, Germany
Fax: +49-531-391-5388
E-mail: h.hopf@tu-bs.de
[b] Department of Chemistry, Indian Institute of
Technology Madras,
Chennai 600036, India
Scheme 1. Synthesis of first.generation polyal. Reagents and condi-
tions: (a) [PdCl2(PPh3)2], PPh3, CuI, NEt3, 70 °C, 8 h, 72%.
Fax: +91-44-2257-0545
E-mail: sanka@iitm.ac.in
2740
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DOI: 10.1002/ejoc.200500055
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Shape-Persistent Polyal Dendrimers
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mer formed from 1 was easily removed during the chroma- styryl derivative 6 as a colorless solid in 98% yield after
tographic purification of 3.
column purification (Scheme 3).
1
1
The H NMR spectrum of 3 is very simple and shows
The H NMR spectrum of 6 is complex and reveals that
four singlet resonances in the ratio of 2:2:1:9, corresponding 6 is formed as a mixture of (Z) and (E) isomers. From the
to the aldehyde, peripheral ring, core ring, and tert-butyl tert-butyl signals it is evident that 6 is a mixture of three
protons, respectively. The 13C NMR spectrum of 3 displays major isomers of almost equal proportions. Attempts to
six signals in the aromatic region, thereby confirming the separate these isomers by column chromatography on silica
overall threefold axis of symmetry of the molecule. The en- gel were unsuccessful as the retention times of the isomers
ergy-minimized structure of 3 based on semi-empirical are very similar. The hexastyryl derivative 6 was therefore
AM1 calculations[16] shows the molecule to be planar. The fully characterized as a mixture of geometrical isomers.
hexaaldehyde 3 reacts cleanly with methylenetriphenylphos-
The hexaaldehyde 3 was converted into the correspond-
phorane to yield the corresponding hexaene derivative 4 in ing hexaethynyl derivative 7 by two different methods. In
near quantitative yield (Scheme 2).
the first method ethynylation was carried out in a single
step using dimethyl (1-diazo-2-oxopropyl)phosphonate[17]
(Scheme 4). This procedure yielded 7, albeit in a poor yield
of only 29%. An alternative two-step methodology involv-
ing Corey–Fuchs dibromoolefination[18] followed by the
conversion of the dibromoolefin into acetylene by treatment
with lithium diisopropylamide was adopted (Schemes 2 and
4). When 3 was subjected to the modified Corey–Fuchs re-
action (using CBr4, PPh3, and Zn) it yielded the expected
hexakis(dibromoethenyl) derivative 5 in quantitative yield
(Scheme 2). In the 1H NMR spectrum of 5 the olefinic pro-
tons appear as a singlet at δ = 7.84 ppm along with two
other singlets at δ = 7.77 and 7.70 ppm for the peripheral
and core aromatic ring protons, respectively, in the ratio
2:2:1. In the 13C NMR spectrum the characteristic reso-
nance due to the vinylic CBr2 carbon appears at δ =
Scheme 2. Synthesis of first-generation polyenes. Reagents and con-
ditions: (a) Ph3PCH3Br, nBuLi, THF, –78 °C to 0 °C, 2 h, 79%; (b) 92.3 ppm. The MALDI-TOF mass spectrum of 5 shows the
CBr4, PPh3, Zn, CH2Cl2, 25 °C, 100%.
molecular ion peak at m/z = 1650 and peaks due to the
sequential loss of bromine atoms; each of these peaks ap-
pears as a cluster of isotope peaks. Peaks due to the loss of
tert-butyl groups are not observed in the spectrum. The
A pure sample of 4 was obtained as a colorless solid in
79% yield after column chromatographic purification. The
1H NMR spectrum of 4 confirmed the absence of aldehyde
bromo derivative 5 was converted into the hexaethynyl de-
protons. All vinyl protons appear as an AMX pattern, thus
rivative 7 by treatment with an excess of LDA (Scheme 4).
confirming the formation of the hexaene derivative 4. In
addition, the 13C NMR spectrum shows six signals due to
the aromatic carbons and two signals due to the vinylic car-
bons, indicating again the overall threefold axis of sym-
metry of 4. Treatment of the hexaaldehyde 3 with benzyli-
denetriphenylphosphorane yielded the corresponding hexa-
Scheme 4. Synthesis of first generation polyyne. Reagents and con-
ditions: (a) MeCOC(N2)P(O)(OMe)2, Cs2CO3, MeOH, 29%; (b)
LDA, THF, –78 °C, 10 min, 84%.
Compound 7 was isolated as a colorless crystalline solid
in 84% yield after chromatographic purification. The acety-
lenic protons appear as a sharp singlet in the 1H NMR
spectrum of 7 at δ = 3.37 ppm. The energy-minimized struc-
ture of 7 based on semi-empirical calculations (AM1) shows
the entire π system to be planar. In order to prepare the
hexaphenylethynyl derivative 8, the hexaethynyl derivative 7
Scheme 3. Synthesis of hexastyryl derivative 6. Reagents and condi-
tions: (a) Ph3PCH2PhCl, nBuLi, THF, 25 °C, 3 h, 98%.
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V. Narayanan, S. Sankararaman, H. Hopf
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was subjected to a sixfold Sonogashira coupling with iodo-
benzene (Scheme 5).
Having successfully demonstrated the synthesis of the
first-generation polyal dendrimer, namely the hexaaldehyde
3, and its conversion into several derivatives (4–8) in excel-
lent yields, synthesis of the second-generation dendrimer
was undertaken. The hexaethynyl derivative 7 was subjected
to Sonogashira coupling with the bromodialdehyde 2 in the
presence of [Pd(PPh3)4] as the catalyst (Scheme 5). The ex-
pected sixfold coupling occurred smoothly and the dodeca-
aldehyde 9 was isolated in 34% yield (84% for each coup-
ling step), after column chromatographic purification, as a
1
colorless amorphous solid. The H NMR spectrum of 9 is
simple, as expected, with six singlets in the ratio of
4:4:1:2:9:18 corresponding to the aldehyde protons (δ =
10.75 ppm), the second generation aryl protons (δ =
8.05 ppm), the core aromatic protons (δ = 7.73 ppm), the
first generation aryl protons (δ = 7.67 ppm), the first gener-
ation tert-butyl protons (δ = 1.42 ppm), and the second gen-
eration tert-butyl protons (δ = 1.20 ppm), respectively. The
MALDI-TOF mass spectrum of 9 shows the molecular ion
peak at m/z = 1818. The energy-minimized structure of 9
obtained by semi-empirical AM1-type calculations is shown
in Figure 1. The molecule is propeller shaped as in the case
of 8. More interestingly, however, six of the twelve aldehyde
groups are oriented towards the core of the dendrimer, as
dictated by the substitution pattern of the building block
used, namely 2. The dodecaaldehyde 9 underwent a 12-fold
Witting reaction with methylenetriphenylphosphorane to
yield the corresponding dodecaene derivative 10
(Scheme 6).
Scheme 5. Synthesis of second-generation polyal. Reagents and
conditions: (a) PhI (6.25 equiv.), [Pd(PPh3)4], PPh3, CuI, Et3N,
60 °C, 3 d, 36% (8); (b) 2 (7.0 equiv.), [Pd(PPh3)4], PPh3, CuI, Et3N,
60 °C, 3 d, 34% (9).
Compound 8 was obtained as a pale-yellow substance
in 36% yield (85% for each coupling step) after column
chromatographic purification of the crude product on silica
gel. The energy-optimized structure of 8 based on semi-em-
pirical AM1 calculations is propeller shaped. The dihedral
angle between the blades of the propeller and the shaft is
29° (Figure 1).
Scheme 6. Synthesis of second-generation polyenes. Reagents and
conditions: (a) Ph3PCH3Br (14.6 equiv.), nBuLi, THF, –78 °C to
25 °C, 24 h, 20%; (b) CBr4, PPh3, Zn, CH2Cl2, 25 °C, 38%.
Compound 10 was isolated as a pale-orange solid in 20%
yield (corresponding to Ͼ87% yield in each Wittig step).
The absence of signals due to the aldehyde protons and the
appearance of vinylic protons as an AMX pattern in the 1H
NMR spectrum confirmed the formation of 10. The
MALDI-TOF mass spectrum of 10 shows the molecular
Figure 1. Calculated energy-minimized structures of 8 (A) and 9
(B) shown without the tert-butyl groups.
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Shape-Persistent Polyal Dendrimers
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ion peak at m/z = 1794. When aldehyde 9 was subjected
to a Corey–Fuchs reaction the corresponding dibromovinyl
derivative 11 was obtained in 38% yield (Ͼ92% yield in
each step) as a red solid after column chromatographic pu-
rification (Scheme 6). The presence of four singlets in the
ratio 4:4:1:2 in the aromatic region and two singlets for the
tert-butyl protons in the ratio 2:1 in the 1H NMR spectrum
of 11 confirmed the formation of the second-generation
hexakis(dibromovinyl) derivative. The MALDI-TOF mass
spectrum of 11 shows peaks corresponding to the loss of
tert-butyl groups as well as bromine atoms from the mol-
ecular ion, in sharp contrast to the behavior of 5, where
loss of tert-butyl groups from the molecular ion was not
observed. Attempted synthesis of the corresponding dode-
caethynyl derivative by treatment of 11 with an excess of
LDA resulted in the formation of intractable material. It is
presumed that the dodecaethynyl derivative is unstable and
undergoes oxidative decomposition during the workup and
isolation process.
Figure 2. Fluorescence emission spectra of 4 (A), 6 (B), and 8 (C)
in cyclohexane (1.0×10–5 m).
Conclusions
In the present study it is clearly shown that dialdehyde 2
is a very useful building block for divergent iterative synthe-
sis of polyal dendrimers. These dendrimers, in turn, are use-
Thermal Stability Studies
During the melting-point determination of the hexaene ful for the synthesis of polyene and polyyne dendrimers
derivative 4 we observed that it did not melt on heating up and, presumably, other classes of dendrimers containing
to 200 °C but underwent a rapid thermal decomposition to functional groups such as imines, acetals etc. The majority
a black soot that is insoluble in all common organic sol- of the polyphenylacetylene-based dendrimers reported by
vents. Similarly, the hexaacetylene derivative 7 also rapidly Moore[12] and others[20] contain 1,4 and 1,3,5 connectivity
decomposed above 195 °C without any sign of melting. The of the aromatic units. As a result of this connectivity such
differential scanning calorimetric studies of both of these dendrimers are dense on the periphery and relatively less
compounds showed a single, strong, sharp exotherm at dense at the core (e.g. starburst[21] and snowflake[22] type
202 °C for 4 and at 158 °C for 7. In contrast, the hexastryryl structures). The noteworthy feature of the present contri-
derivative 6 is quite stable even in refluxing diphenyl ether bution is that the arene connectivity is 1,2,6 or 1,2,3 (com-
when heated up to 250 °C under nitrogen. It is well known pounds 4–11). This is due to the fact that the reactive 2-
that polyacetylenes undergo thermal decomposition to yield bromo-5-tert-butyl-1,3-benzenedicarbaldehyde (2) is used
fullerenes[19] and other interesting carbon materials. We are as the repeating building block. Therefore in this class of
currently investigating this aspect in detail. Although the dendrimers both the periphery and the core get progress-
hexakis(dibromovinyl) derivative 5 is quite stable, the do- ively crowded due to the encroachment of one arm of the
decakis derivative 11 decomposes at room temperature and building block into the core (Figure 1). Such steric crowd-
under exposure to light.
ing will surely strongly impose restrictions on the synthesis
of higher generations of this class of dendrimers and their
stability. We are currently exploring the synthesis of imine-
based dendrimers from these polyals and also the use of
longer spacers for the synthesis of higher generations of this
class of dendrimers to overcome the steric effects. The use
of other aromatic core units is also underway.
Fluorescence Emission Studies
The fluorescence emission spectra of the first generation
dendrimers 4, 6, and 8 were measured in cyclohexane. In
all cases blue fluorescence emission could be observed un-
der UV light. The emission spectra are shown in Figure 2.
Compounds 4, 6, and 8 were excited at 308, 261, and
292 nm, respectively. The hexastyryl derivative 6 has emis-
sion maxima at 408 and 426 nm. The emission maxima of
4 [369, 382, and 395 (sh) nm] and 8 [371, 388, 401 (sh), 421
(sh)] are nearly the same and are blue-shifted in comparison
to that of 6.
Experimental Section
General Remarks: 1H and 13C NMR spectroscopy: Bruker AM-400
at 400 MHz and 100 MHz or Bruker AM-200 at 200 MHz and
50 MHz, respectively, in CDCl3 solution, unless otherwise stated,
with TMS as internal standard. All reactions were carried out un-
der argon in flame-dried Schlenk flasks unless indicated otherwise.
Column chromatography was performed on silica gel (60–120 or
240–400 mesh). TLCs were run on Macherey–Nagel polygram sil
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V. Narayanan, S. Sankararaman, H. Hopf
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G/UV254 plates. Melting points are uncorrected. THF was distilled
from potassium. Fluorescence spectra were recorded on a Jobin–
Yvon Fluorolog spectrometer.
rated, dried over anhydrous MgSO4, and the solvent was removed.
The crude product was dissolved in CH2Cl2 (100 mL) and filtered
through a short column of silica gel to remove the Ph3PO formed
in the reaction. Compound 4 (0.70 g, 79%) was obtained as a color-
less solid that was recrystallized from a mixture of CH2Cl2/hexane.
2-Bromo-5-tert-butyl-1,3-dimethylbenzene was synthesized from 1-
tert-butyl-3,5-dimethylbenzene according to
a literature pro-
M.p. 214–215 °C (decomp). IR (neat): ν = 2204, 1576 cm–1 (C=C).
˜
cedure.[23] Oxidation of 2-bromo-5-tert-butyl-1,3-dimethylbenzene,
according to a literature procedure,[24] with CrO3 in acetic anhy-
dride, yielded the corresponding acetate of 2 in 65% yield. This
was hydrolyzed with 6 n HCl to yield 2 in quantitative yield. 1,3,5-
Triethynylbenzene was prepared from 1,3,5-tribromobenzene by a
Sonogashira coupling.[25]
UV/Vis (hexane): λmax (log ε) = 205 nm (4.883), 263 (5.117), 308
1
(4.993), 326 (4.933). H NMR (CDCl3): δ = 7.69 (s, 3 H), 7.58 (s,
6 H), 7.35 (dd, J = 17.6, 11.0 Hz, 6 H), 5.85 (dd, J = 17.6, 0.98 Hz,
6 H), 5.42 (dd, J = 11.0, 0.94 Hz, 6 H), 1.37 (s, 27 H) ppm. 13C
NMR (CDCl3): δ = 151.6, 139.5, 135.6, 133.6, 124.3, 121.2, 117.5,
115.7, 96.8, 87.3, 35.0, 31.1 ppm. MS (70 eV, EI): m/z (%) = 702
(10) [M+], 688 (24), 687 (46), 533 (24), 57 (100). HRMS calcd. for
C54H54: 702.4225; found 702.4225.
CAUTION: The polyenes and polyynes described herein are poten-
tially sensitive to heat and shock and can undergo violent decomposi-
tion; they should be handled with caution.
First-Generation Hexastyryl Derivative 6: The reaction was carried
out as described for the synthesis of 4. A mixture of 3 (0.5 g,
0.7 mmol), the ylide from benzyltriphenylphosphonium chloride
(1.8 g, 4.62 mmol), and nBuLi (2.88 mL of a 1.6 m hexane solution,
4.62 mmol) was allowed to react for 3 h at 20 °C. Column chroma-
tographic purification of the crude product on silica gel using
CH2Cl2/pentane (1:4) yielded 6 (0.809 g, 98%) as a colorless solid,
as a mixture of three major products that are geometric isomers.
Attempted separation of the isomers by column chromatography
with CH2Cl2/diethyl ether/pentane as eluent were unsuccessful. IR
4-tert-Butyl-2,6-bis(diacetoxymethyl)bromobenzene: M.p. 165 °C.
1
IR (neat): ν = 1751 cm–1 (C=O). H NMR (CDCl3, 400 MHz): δ
˜
= 7.97 (s, 2 H), 7.61 (s, 2 H), 2.15 (s, 12 H), 1.35 (s, 9 H) ppm.
13CNMR (CDCl3): δ = 168.2, 151.2, 135.3, 126.2, 119.6, 89.1, 34.8,
31.1, 20.6 ppm. MS (EI, 70 eV): m/z (%) = 474 (2) [M+], 472 (2)
[M+], 393 (5), 291 (44), 255 (98), 253 (100), 227 (32), 225 (30),
207 (66). HRMS calcd. for C20H25O8 [M – Br]: 393.1549; found
393.1550.
2-Bromo-5-tert-butyl-1,3-benzenedicarbaldehyde (2): M.p. 93–95 °C.
(neat): ν = 2198 cm–1, 1575. UV/Vis (hexane): λmax (log ε) = 192 nm
IR (neat): ν = 1685 cm–1 (C=O). UV/Vis (CH2Cl2): λmax (log ε) =
˜
˜
(5.148), 229 (4.945), 261 (sh, 4.886), 308 (5.234). 1H NMR
(CDCl3): δ = 7.70–7.85 (m, 6 H), 7.69 (s, 3 H), 7.45–7.65 (m, 9 H),
7.05–7.40 (m, 27 H), 6.90–7.00 (m, 3 H), 6.60–6.80 (m, 3 H), 1.45,
1.16, 0.88 (all singlet, total relative integration 27 H) ppm. 13C
NMR (CDCl3): δ = 118–151 (57 signals due to aromatic and ole-
finic carbons of the three isomers), 97.8, 97.7, 97.4, 97.3, 88.0, 87.9,
87.8, 35.1, 34.7, 34.3, 31.2, 30.8, 30.5 ppm. MS (70 eV, EI): m/z =
1160 (12) [M+ + 2], 1159 (15), 1158 (17) [M+], 1145 (8), 1144 (16),
1143 (17), 1103 (6), 1102 (10), 1101 (12), 438 (26), 362 (24), 305
(70), 180 (38), 91 (22), 78 (60), 57 (100). C90H78 (1158.6): calcd. C
93.26, H 6.73; found C 93.05, H 6.80.
240 nm (4.54), 320 (3.24). 1H NMR (CDCl3): δ = 10.52 (s, 2 H),
8.17 (s, 2 H), 1.37 (s, 9 H) ppm. 13C NMR (CDCl3): δ = 190.9,
151.9, 133.9, 132.5, 128.1, 34.9, 30.8 ppm. MS (EI, 70 eV): m/z (%)
= 270 (29) [M+], 268 (30) [M+], 255 (98), 253 (100), 227 (44), 225
(48), 118 (22), 117 (20), 115 (28). HRMS calcd. for C12H13O279Br:
268.00989; found 268.00950. C12H13BrO2 (268): calcd. C 53.73, H
4.89; found C 53.61, H 4.88.
First-Generation Polyal (Hexaaldehyde) 3: A Schlenk flask was
charged with
2 (4.03 g, 15.0 mmol), [PdCl2(PPh3)2] (0.53 g,
0.75 mmol), PPh3 (0.195 g, 0.75 mmol), CuI (0.14 g, 0.75 mmol),
and anhydrous Et3N (150 mL). The mixture was stirred and heated
to 70 °C. A solution of 1 (0.75 g, 15.0 mmol) in anhydrous THF
(15 mL) was then added dropwise. After 60 min a copious amount
of precipitate was observed. The reaction mixture was stirred for
8 h at 70 °C and allowed to stand at room temperature overnight.
The solvent was removed under reduced pressure in a rotary evapo-
rator. The dark-brown crude product (4.6 g) thus obtained was dis-
solved in CH2Cl2 and washed with water followed by saturated
brine solution. After removal of the solvent the crude product was
purified by flash chromatography on silica gel with CH2Cl2 to yield
3 (2.6 g, 3.6 mmol, 72%) as a colorless amorphous solid. M.p.
First-Generation Hexakis(dibromovinyl) Derivative 5: A Schlenk
flask was charged with CBr4 (2.79 g, 8.4 mmol), PPh3 (2.20 g,
8.4 mmol), Zn (0.55 g, 8.4 mmol), and dry CH2Cl2 (100 mL). The
slurry was stirred at room temperature overnight to yield a straw-
berry colored suspension. Hexaaldehyde 3 (0.25 g, 0.35 mmol) was
added in one portion and stirring was continued for 3 d at 20 °C.
The reaction mixture was filtered through a pad of silica gel and
the solvent was removed to obtain the crude product as an orange
solid. It was further purified by column chromatography on silica
gel eluting with a CH2Cl2/pentane mixture (1:3, v/v) to yield 5 as
a pale-orange solid (0.57 g, 100%). The substance obtained from
the column chromatographic purification was washed twice more
with pentane to obtain a nearly colorless solid that was analytically
245 °C (decomp). IR (neat): ν = 2207, 1688 cm–1 (C=O). 1H NMR
˜
(CDCl3): δ = 10.73 (s, 6 H), 8.26 (s, 6 H), 7.88 (s, 3 H), 1.41 (s, 27
H) ppm. 13C NMR (CDCl3): δ = 190.2, 153.5, 153.0, 136.6, 135.1,
130.1, 125.2, 123.4, 115.2, 99.4, 82.9, 35.4, 30.8 ppm. MS (EI,
70 eV): m/z (%) = 715 (54) [M+ + 1], 714 (100) [M+], 687 (30),
686 (50), 57 (16). HRMS calcd. for C48H42O6: 714.29814; found
714.29818.
pure. M.p. 219–220 °C. IR (neat): ν = 2207 cm–1, 2165, 1576. UV/
˜
Vis (CHCl3): λmax (log ε) = 279 nm (5.0168), 319 (4.874). 1H NMR
(CDCl3): δ = 7.84 (s, 6 H), 7.78 (s, 6 H), 7.70 (s, 3 H), 1.36 (s, 27
H) ppm. 13C NMR (CDCl3): δ = 151.5, 137.7, 135.8, 134.2, 125.7,
123.9, 118.4, 98.3, 92.3, 86.8, 35.2, 31.0 ppm. MALDI-TOF MS:
First-Generation Hexaene 4: nBuLi (5.2 mL of a 1.6 m hexane solu-
tion, 8.32 mmol) was added, at –78 °C, to a stirred suspension of
methyltriphenylphosphonium bromide (2.97 g, 8.32 mmol) in THF
(50 mL), After addition the solution was warmed to 0 °C and stir-
ring was continued for 60 min to obtain the ylide as a clear, bright-
yellow solution, which was added to a solution of 3 (0.9 g,
1.26 mmol) in THF (200 mL) at –78 °C. After addition the mixture
was warmed to 0 °C and stirred for 2 h. The reaction was quenched
by addition of saturated brine solution. The organic layer was sepa-
m/z = 1650 [M+] 1570 [M+ – Br], 1490 [M+ – 2Br], 1410 [M+
–
3Br], 1330 [M+ – 4Br], 1250 [M+ – 5Br], 1170 [M+ – 6Br]; each
peak appeared as a cluster of isotope peaks. C54H42Br12 (1650.3):
calcd. C 39.31, H 2.57; found C 39.34, H 2.63.
First-Generation Hexaethynyl Derivative 7. Method A: A Schlenk
flask was charged with MeOH (100 mL, dried over molecular
sieves) and was degassed by bubbling argon for 30 min. Hexaalde-
hyde 3 (2.0 g, 2.8 mmol) and Cs2CO3 (10.95 g, 33.6 mmol) were
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Shape-Persistent Polyal Dendrimers
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added and the mixture was stirred at room temperature for 5 min
and then cooled to –40 °C. A solution of dimethyl-1-diazo-2-oxo-
propyl phosphonate (3.55 g, 18.5 mmol) in MeOH (10 mL) was
added slowly to the reaction mixture with stirring over 5 min. The
reaction mixture was then allowed to attain room temperature over
a period of 4 h, after which time it was diluted with diethyl ether
(100 mL), washed with saturated NaHCO3, and dried over MgSO4.
The solvent was removed on a rotary evaporator at 25 °C under
reduced pressure. The crude product thus obtained was purified by
column chromatography on silica gel and eluted with a 1:4 (v/v)
mixture of CH2Cl2 and pentane. The desired hexaacetylene 7 was
obtained (0.56 g, 29%) as a colorless solid.
rated brine solution. After removal of the solvent the crude product
was purified by flash chromatography on silica gel and eluted with
CH2Cl2 to yield 9 (0.26 g, 0.14 mmol, 34%) as a colorless amorph-
ous solid. M.p. Ͼ272 °C (decomp). IR (neat): ν = 2199 cm–1, 1687
˜
(C=O). UV/Vis (CH2Cl2): λmax (log ε) = 246 nm (5.33), 308 (5.09),
1
323 (5.08). H NMR (CDCl3): δ = 10.75 (s, 12 H), 8.05 (s, 12 H),
7.73 (s, 3 H), 7.67 (s, 6 H), 1.42 (s, 27 H), 1.20 (s, 54 H) ppm. 13C
NMR (CDCl3): δ = 190.6, 152.8, 152.1, 136.5, 130.5, 129.4, 126.3,
125.3, 123.1, 100.5, 96.5, 87.5, 84.7, 35.2, 34.9, 30.99, 30.74 ppm.
MALDI-TOF MS: m/z = 1818 [M+]. C126H114O12 (1818.9): calcd.
C 83.14, H 6.31; found C 82.59, H 6.21.
Second-Generation Dodecaene Derivative 10: The procedure de-
scribed for the synthesis of 4 was followed using 9 (0.1 g,
0.055 mmol), Ph3PCH3Br (0.26 g, 0.73 mmol), and nBuLi (1.6 m
solution in hexane, 0.73 mmol, 0.45 mL). The reaction was carried
out for 24 h at room temperature. Chromatographic purification of
the crude product on silica gel with a 1:4 (v/v) CH2Cl2/pentane
mixture as eluent yielded the desired product 10 (0.02 g, 20%).
Method B: A solution of LDA [freshly prepared from iPr2NH
(0.52 g, 5.1 mmol) and nBuLi (1.6 m solution in hexane, 3.18 mmol,
5.1 mL) in THF (10 mL) at –78 °C] was added to a solution of 5
(0.4 g, 0.24 mmol) in dry THF (20 mL) at –78 °C. After 10 min the
reaction was quenched with saturated NH4Cl solution (10 mL) and
the mixture was allowed to attain room temperature. The organic
layer was removed and the aqueous layer was extracted with
CH2Cl2. The combined organic phase was washed with saturated
brine solution and dried over MgSO4. Removal of the solvent un-
der reduced pressure on a rotary evaporator yielded a crude pro-
duct which, after chromatographic purification over silica gel with
a 1:4 (v/v) CH2Cl2/pentane mixture as eluent, yielded the desired
hexaacetylene 7 as a colorless solid (0.14 g, 84%). M.p. 195 °C (de-
M.p. Ͼ145 °C (decomp). IR (neat): ν = 2200 cm–1, 1579, 908, 879.
˜
UV/Vis (CH3CN): λmax (log ε) = 269 nm (5.17), 300 (5.03), 316
1
(4.98). H NMR (CDCl3): δ = 7.73 (s, 3 H), 7.56 (s, 6 H), 7.42 (s,
12 H), 7.35 (dd, J = 17.5, 11.0 Hz, 12 H), 5.67 (d, J = 17.5 Hz, 12
H), 5.12 (d, J = 11.0 Hz, 12 H), 1.39 (s, 27 H), 1.21 (s, 54 H) ppm.
13C NMR (CDCl3): δ = 151.2, 151.17, 139.4, 135.5, 135.3, 128.9,
126.7, 123.6, 123.4, 121.0, 117.7, 115.8, 97.1, 94.8, 90.2, 88.6, 34.8,
34.7, 31.1, 31.0 ppm. MALDI-TOF MS: m/z = 1794 [M+].
composed violently!). IR (neat): ν = 3267 cm–1, 2960, 2207, 2107,
˜
1574. UV/Vis (CH3CN): λmax (log ε) = 245 nm (5.0174), 250
(5.0176), 256 (5.001), 264 (4.9335), 309 (4.9051), 329 (4.90). 1H
NMR (CDCl3): δ = 7.76 (s, 3 H), 7.56 (s, 6 H), 3.38 (s, 6 H), 1.32
(s, 27 H) ppm. 13C NMR (CDCl3): δ = 151.5, 134.7, 130.2, 125.6,
124.8, 123.9, 95.2, 87.7, 82.1, 81.1, 34.7, 30.9 ppm. MS (70 eV, EI):
m/z (%) = 691 (6) [M+ + 1], 690 (12) [M+], 603 (1), 292 (9), 202
(4), 101 (28), 72 (52), 57 (88). C54H42 (690.32): calcd. C 93.87, H
6.13; found C 92.12, H 6.12.
Second-Generation Dibromovinyl Derivative 11: The procedure de-
scribed for the synthesis of 5 was followed. Compound 9 (0.22 g,
0.12 mmol) was treated with CBr4 (1.93 g, 5.81 mmol), PPh3
(1.52 g, 5.81 mmol), and Zn powder (0.38 g, 5.81 mmol) in CH2Cl2
(100 mL). The mixture was stirred at 25 °C overnight. Chromato-
graphic purification of the crude product on silica gel with a 1:4
(v/v) CH2Cl2/pentane mixture as eluent yielded 11 (0.17 g, 38%) as
a red solid. M.p. 80 °C. IR(neat): ν = 2188 cm–1, 2166, 1580, 829,
˜
First-Generation Hexaphenylethynyl Derivative 8: A Schlenk flask
was charged with iodobenzene (0.46 g, 2.25 mmol), [Pd(PPh3)4]
(0.13 g, 0.11 mmol), PPh3 (0.03 g, 0.11 mmol), CuI (0.021 g,
0.11 mmol), and anhydrous Et3N (50 mL). The mixture was stirred
at 60 °C and a solution of hexaacetylene 7 (0.25 g, 0.36 mmol) in
THF (15 mL) was added. The reaction was protected from room
light. After 3 d the mixture was cooled to room temperature and
worked up as described in the synthesis of 3. Chromatographic pu-
rification of the crude product on silica gel with a 1:3 (v/v) CH2Cl2/
pentane mixture as eluent yielded the desired product 8 as a pale-
743. UV/Vis (CH2Cl2): λmax (log ε) = 285 nm (5.27), 320 (4.998).
1H NMR (CDCl3): δ = 7.80 (s, 12 H), 7.73 (s, 12 H), 7.72 (s, 3 H),
7.59 (s, 6 H), 1.39 (s, 27 H), 1.26 (s, 54 H) ppm. 13C NMR (CDCl3):
δ = 151.6, 151.1, 137.4, 135.8, 129.8, 125.8, 125.7, 123.6, 123.55,
118.7, 98.6, 95.4, 92.1, 89.1, 88.1, 35.1, 34.8, 31.01, 30.97 ppm.
MALDI-TOF MS: m/z = 3633 [M+ – 57 (tBu)], 3576 [M+ – 114
(2×tBu)], 3481, 3401, 3321, 3241, 3081 (due to sequential loss of
Br atoms, each peak appears as a cluster of isotope peaks).
yellow solid (0.15 g, 36%). M.p. 204–205 °C. IR (neat): ν =
˜
2209 cm–1, 1596, 750, 685. UV/Vis (CHCl3): λmax (log ε) = 292 nm
(5.23). 1H NMR (CDCl3): δ = 7.84 (s, 3 H), 7.57 (s, 6 H), 7.47–
7.45 (m, 12 H), 7.25–7.13 (m, 18 H), 1.39 (s, 27 H) ppm. 13C NMR
(CDCl3): δ = 151.5, 134.3, 131.6, 128.7, 128.5, 128.4, 126.2, 124.8,
124.5, 122.8, 95.2, 93.7, 88.8, 88.2, 34.9, 31.0 ppm. MS (70 eV, EI):
m/z (%) = 1148 (15) [M+ + 2], 1147 (18) [M+ + 1], 1146 (20) [M+],
1091 (5), 1090 (8), 1089 (10), 277 (16), 178 (16), 167 (36), 149 (72),
78 (100), 57 (60). MALDI-TOF MS: m/z = 1146 [M+], 2292 [2M+].
C90H66 (1146.5): calcd. C 94.20, H 5.80; found C 93.38, H 5.76.
Acknowledgments
We gratefully acknowledge financial support from the Volkswagen
Stiftung and the Fonds der Chemischen Industrie, Germany. We
thank Dr. T. Pradeep and Mr. D. M. David Jaba Singh (IIT Ma-
dras) for MALDI-TOF MS measurements, Ms. T. Shyamala (IIT
Madras) for fluorescence measurements, and Mr. Harald Berger
(TU-BS, Germany) for DSC measurements.
Second-Generation Polyal (Dodecaaldehyde) 9: A Schlenk flask was
charged with
2
(0.79 g, 2.94 mmol), [Pd(PPh3)4] (0.17 g,
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0.15 mmol), PPh3 (40 mg, 0.15 mmol), CuI (28 mg, 0.15 mmol),
and anhydrous Et3N (50 mL). The mixture was stirred and heated
to 60 °C. A solution of 7 (0.29 g, 0.42 mmol) in anhydrous THF
(15 mL) was added dropwise. The reaction mixture was stirred for
3 d at 60 °C then cooled and the solvent removed under reduced
pressure on a rotary evaporator. The crude product thus obtained
was dissolved in CH2Cl2 and washed with water followed by satu-
Eur. J. Org. Chem. 2005, 2740–2746
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2745

V. Narayanan, S. Sankararaman, H. Hopf
FULL PAPER
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Received: January 24, 2005
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Eur. J. Org. Chem. 2005, 2740–2746