A novel approach to stilbenoid dendrimer core synthesis

Article abstract of DOI:10.1055/s-0028-1083630

A new synthetic protocol for the one-pot, stereoselective synthesis of 1,3,5-tris[(E)-4-halostyryl]benzene and 1,2,4,5-tetrakis[(E)-4-halostyryl] benzene derivatives as stilbenoid dendrimer cores via palladium-catalyzed Hiyama cross-coupling of aryl trior tetrahalides with 1,3-bis[(E)-4-halostyryl] disiloxanes is described. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-0028-1083630

3026
LETTER
A Novel Approach to Stilbenoid Dendrimer Core Synthesis
Novel Approach to
S
tilb
i
enoid D
e
e
ndrimer
C
ore Synt
łhesis aw Prukała*
Department of Organometallic Chemistry, Faculty of Chemistry, Adam Mickiewicz University, Grunwaldzka 6, 60-780 Poznan, Poland
Fax +48(61)8291508; E-mail: wprukala@amu.edu.pl
Received 24 July 2008
Despite rapid advances in the potential application of den-
Abstract: A new synthetic protocol for the one-pot, stereoselective
drimers, their syntheses are still confined to the
synthesis of 1,3,5-tris[(E)-4-halostyryl]benzene and 1,2,4,5-tet-
rakis[(E)-4-halostyryl]benzene derivatives as stilbenoid dendrimer
divergent12a or convergent12b synthetic strategies. There
are two main convergent strategies reported for obtaining
(E)-stilbenyl-based dendrimers. The first strategy is based
on the Wittig10 or Horner–Wadsworth–Emmons-type
chemistry,13a–13c whereas the second is based on the
Heck9a,b,13d,e or Suzuki13f,g palladium-catalyzed-type chem-
istry. Recently, an alternative approach to stilbenyl den-
drons has been reported, which is based on the Ramberg–
Backlund reaction.14 Depending on the substituents, the
Wittig–Horner reaction in stilbenoid series sometimes ex-
hibits a low percent of Z isomer (2–5%).10
cores via palladium-catalyzed Hiyama cross-coupling of aryl tri- or
tetrahalides with 1,3-bis[(E)-4-halostyryl]disiloxanes is described.
Key words: cross-coupling, palladium catalyst, stilbenoid den-
drimer cores, silicon derivatives
Dendrimers are repeatedly branched, monodisperse mac-
romolecules built of a central core and repeated sequences
(dendrons).1 The number describing the dendrimer gener-
ation is defined by the number of branching points be-
tween the periphery and the core. The degree of branching
in a dendrimer increases with successive generations so
that high generation dendrimers are globular in shape and
possess high functional group densities at their periphery.
The palladium-catalyzed and fluoride-promoted cross-
coupling of unsaturated organosilicon compounds with
aryl halides (Hiyama coupling) has been recently em-
ployed as a mild and efficient alternative to the well-es-
tablished Heck, Stille and Suzuki reactions, taking into
account the commercial availability, high stability and
low toxicity of the silicon derivatives.15
Various dendritic systems have been studied in view of
their prospective applications as low-dielectric materials,2
catalysts,3 unimolecular micelles,4 sensors,5 efficient
light-harvesting antennae6 and potential drug-delivery
systems.7
Therefore, we wish to report herein the first, unprecedent-
ed, one-pot palladium-catalyzed Hiyama-type strategy for
the stereoselective synthesis of 1,3,5-tris[(E)-4-halosty-
ryl]benzene and 1,2,4,5-tetrakis[(E)-4-halostyryl]benzene
derivatives as stilbenoid dendrimer cores using aryl tri(or
tetra)halides and appropriate 1,3-bis[(E)-4-halostyryl]di-
siloxanes. These compounds can be easily transformed
into the first or higher generation dendrimers via the well-
known reactions like those of Heck, Hiyama, Stille or
Suzuki.
Dendrimers with polyconjugated branches represent an
important group within this class of materials and hence
they are interesting because of their electrical, optical,
nonlinear optical, electroluminescent and photophysical
properties. For example, such compounds have been suc-
cessfully used as charge-transporting8 and light-emitting
materials.9 Arylalkene-based dendrimers have been one
of the most widely studied families of dendrimers with
trans-stilbenyl moieties within the branching framework
playing a major role in the structures.10 Stilbene units can
make the core or can be elements of the dendrons or
both.1d,10 Stilbenoid dendrimers offer a number of advan-
tages over the linear poly(p-phenylenevinylene) (PPV).
Due to their highly branched structures, such dendrimers
are expected to be less aggregated and hence, should not
suffer from fluorescence quenching due to intermolecular
interactions.11 Dendrimers with photoreversible stilbene
core undergo mutual cis–trans isomerization in organic
solvents to give photostationary state mixture of cis and
trans isomers. The photocyclization reaction does not take
place in the second or higher generation dendrimers prob-
ably due to the steric effect of the bulky dendron groups.
Since 1,3-bis[(E)-4-halostyryl]disiloxanes 1–3 (Scheme 1)
as starting compounds can be easily prepared with high
yield from inexpensive 1,3-divinyltetramethyldisiloxane
and halostyrenes using the ruthenium-catalyzed silylative
cross-coupling reaction, this sequence of reactions, i.e. si-
lylative coupling (also called trans-silylation or silyl
group transfer)16–desilylative coupling (Hiyama) is ex-
pected to be an attractive alternative for the synthesis of
stilbenoid dendrimer cores.
The silylative coupling of 1,3-divinyltetramethyldisilox-
ane with para-halosubstituted styrenes was examined in
an open system under a gentle stream of argon. The reac-
tion was effectively catalyzed by the ruthenium catalyst
[RuHCl(CO)(PPh3)3] (1 mol% per silyl group) and CuCl
as a co-catalyst (2 mol%) in dioxane at 100 °C for 16
hours. The exclusive formation of the E-isomer of 1,3-
bis[(E)-4-halostyryl]disiloxanes 1–3 (Table 1) was con-
firmed by 1H NMR. The use of the catalyst or co-catalyst
SYNLETT 2008, No. 19, pp 3026–3030
x
x
.x
x
.2
0
0
8
Advanced online publication: 12.11.2008
DOI: 10.1055/s-0028-1083630; Art ID: G24008ST
© Georg Thieme Verlag Stuttgart · New York

LETTER
Novel Approach to Stilbenoid Dendrimer Core Synthesis
3027
Me Si Me
Me
Si
Me
Si
X
[Ru]/CuCl
O
+
2
O
dioxane
24 h, 100 °C
X
Me Si Me
Me
Me
X
1 X = H
2 X = Br 3 X = Cl
[Ru] = [RuHCl(CO)(PPh3)3]
Scheme 1
55 °C a low conversion of 1,3,5-tribromobenzene was de-
tected (ca. 20%). By increasing the temperature to 80 °C
the reaction was complete in 16 hours. Due to the pres-
ence of bromine substituents in both substrates, a mixture
of linear and branched polymers was isolated (Scheme 2,
Table 2).
Table 1 Synthesis of 1,3-Bis[(E)-4-halostyryl]tetramethyldisilox-
anes via Silylative Cross-Coupling of 1,3-Divinyltetramethyldisilox-
ane with 4-Halostyrenesa,17
Entry
Time (h) Product
Conversion (ArX) (%) Yield (%)b
1
2
3
4
5
24
8
1c
1
84
54
–
Under standard desilylative coupling conditions in the ab-
sence of 1,3,5-tribromobenzene, compound 2 underwent
at 80 °C desilylative homocoupling to poly(phenylene-
vinylene) derivatives with high yield (>99%) and stereo-
selectivity (>99% E).18 This unexpected result can be of
interest in the synthesis of (E)-poly(phenylenevinylene)
derivatives and needs further investigations.
–
16
16
16
1
>99
>99
>99
87
91
93
2
3
a Reaction conditions: [Si]/[ArX]/[TBAF]/[RuHCl(CO)(PPh3)3]/
CuCl = 1:4:2.4:0.01:0.02, dioxane, 100 °C.
b Isolated yields of products.
The reaction performed with 1,3-bis[(E)-4-halostyryl]di-
siloxanes 1 and 3 and under the optimum conditions pro-
vided almost quantitative yields (90–96%) of the desired
1,3,5-tris[(E)-4-styryl]benzene (4) and 1,3,5-tris[(E)-4-
chlorostyryl]benzene (6; Table 2). Similarly to the reac-
tions with 1,3,5-tribromobenzene, the palladium-cata-
lyzed coupling of the 1,3-bis[(E)-4-halostyryl]disiloxanes
1–3 with 1,2,4,5-tetraiodobenzene in the presence of
TBAF and Pd2(dba)3 (1 mol% per silyl group) catalyst
proceeded stereoselectively to give satisfactory yields of
the desired 1,2,4,5-tetrakis[(E)-4-halostyryl]benzenes 7–
9 (Table 2).
c [Ru]:/[Si] = 1:200.
in lower amounts (0.5 mol% per silyl group) required
longer reaction time (24–48 h) and gave the desired prod-
uct in low yield (Table 1, entry 1). On the other hand, a
lower amount of catalyst led to significant amounts of
monosubstituted 1,3-divinyltetramethyldisiloxane (1-
styryl-3-vinyltetramethyldisiloxane). In the second step
of the one-pot silylative coupling–desilylative coupling
sequence, this monosubstituted compound yielded some
by-products.
The compounds 1–3 underwent the desilylative coupling
reaction (Hiyama coupling) with 1,2,4,5-tetraiodoben-
zene at 30 °C because of the higher reactivity of iodo-sub-
stituted substrates. This reaction required a small excess
of silyl compounds (at least 10% per iodo substituent) and
TBAF (20% per silyl group). The reaction needed at least
1 mol% catalyst loading ([Pd2(dba)3]) per silyl group for
exclusive formation of the expected products. The use of
a higher amount of palladium catalyst, or a higher temper-
ature of the reaction significantly reduced the reaction
time. Under the above specific reaction conditions, the de-
sired 1,2,4,5-tetrakis[(E)-4-halostyryl]benzenes 7–9 were
obtained with high yield and stereoselectivity (Table 2).
It is worth noting that the best results were observed when
the co-catalyst was added to the reaction mixture a few
minutes after beginning of the reaction.
The synthesized 1,3-bis[(E)-4-bromostyryl]disiloxane (2)
thus obtained was next used for the palladium-catalyzed
desilylative coupling reaction with 1,3,5-tribromoben-
zene in the synthesis of 1,3,5-tris[(E)-4-bromostyryl]ben-
zene (5) (Scheme 2). The reaction was performed in an
open system under a gentle flow of argon in the presence
of [Pd2(dba)3] (1 mol% per silyl group) and TBAF as an
activator at 30 °C for 24 hours.
Unfortunately, under these reaction conditions (30–40
°C), the expected product was not observed, whereas at
Me
Si
Me
Si
Br
Pd2(dba)3
Br
O
dioxane
n
Br
Me
Me
TBAF
80 °C
PPV >99%
n = 6–20
2
5
mixture of linear and
branched polymers
1,3,5-Br3C6H3
Scheme 2
Synlett 2008, No. 19, 3026–3030 © Thieme Stuttgart · New York

3028
W. Prukała
LETTER
The results of the catalytic study prompted us to perform uously confirmed by the appearance of FT-IR bands at-
experiments aiming at a design of the one-pot reaction tributable to the CH bending of trans-vinylene at 956–986
1
system without isolation of 1,3-bis[(E)-4-halostyryl]di- cm–1. Unfortunately, the corresponding H NMR signals
siloxanes 1–3 (Scheme 3). In the first step, the unsaturated (J = 15.9–19.2 Hz) fell in the region of aromatic protons
organosilicon precursors were synthesized using rutheni- and could not be assigned for some products (4, 6 and 7).
um hydride catalyzed silylative coupling of 1,3-di- The MALDI-MS spectra of poly(phenylenevinylene)
vinyltetramethyldisiloxane with styryl halides. Then, the (Scheme 2) in dithranol as a matrix revealed only short-
post-reaction mixture was treated with an appropriate chain polymers (6–20-mer).
tri(or tetra)halobenzene under standard Hiyama cross-
In conclusion, we have developed a new, efficient, and
coupling conditions (Scheme 3, Table 2). A twofold ex-
highly stereoselective, one-pot synthetic methodology for
cess of 4-halostyrene relative to divinyltetramethyldi-
the construction of 1,3,5-tris[(E)-4-halostyryl]benzene
and 1,2,4,5-tetrakis[(E)-4-halostyryl]benzene derivatives
siloxane (per silyl group) was found enough to complete
with the silylative coupling reaction of both vinyl groups
as stilbenoid dendrimer core based on the palladium-cata-
and in these conditions homopolymerization of halosty-
lyzed Hiyama cross-coupling of 1,3-bis[(E)-4-halosty-
rene was not observed. These optimal conditions applied
ryl]disiloxanes or the sequential silylative coupling,
to the one-pot synthesis of 4, and 6–9 provided very high
Hiyama cross-coupling of 1,3-divinyltetramethyldisilox-
yields (72–95%) and stereoselectivity (>99%). Notice-
ane with halosubstituted arenes. The availability of the
ably, the yields were only slightly lower than those ob-
starting materials, simplicity of the experimental tech-
tained in the two-step process with isolation of 1,3-
nique and the possibility of the use of aryl bromides are fa-
bis[(E)-4-halostyryl]disiloxanes 1–3 (Table 2). 1,3,5-
vorable features of this new catalytic approach to obtain
the stereodefined stilbenoid dendrimer core.
Tris[(E)-4-halostyryl]benzene 4 and 6 and 1,2,4,5-tet-
rakis[(E)-4-halostyryl]benzene 7–9 were isolated via col-
umn chromatography with silica gel (THF–EtOAc).
Acknowledgment
The structure of the products 1–4 and 6–9 was confirmed
13
by FT-IR, 1H NMR, C NMR and EI-MS spectroscopy.
The author wishes to express his appreciation for the help from
Prof. Bogdan Marciniec. This work was made possible by a grant
The presence of (E)-vinylene functionality was unambig-
Table 2 Synthesis of 1,3,5-Tris(or 1,2,4,5-Tetrakis)[(E)-4-halostyryl]benzene Derivativesa,19
R2
Me
R1 = H; R2 = R3 =
Pd2(dba)3
R1
R1
Si
O
X = H, Cl
or
R2 = H; R1 = R3 =
X = H, Br, Cl
dioxane
X
Me
1,3,5-Br3C6H3
or
R3
R3
2
X = H, Br, Cl
1,2,4,5-I4C6H2
X
Entry
Aryl halide
X
Time (h)
Product
Yield (%)b
95 (93)c
polymer
96 (90)
1
2
3
4
5
6
1,3,5-C6H3Br3
1,3,5-C6H3Br3
1,3,5-C6H3Br3
1,2,4,5-C6H2I4
1,2,4,5-C6H2I4
1,2,4,5-C6H2I4
H
16
16
48
24
24
24
4
5
6
7
8
9
Br
Cl
H
77 (72)
Br
Cl
93 (95)
81 (78)
a Silylative coupling conditions: dioxane (0.25 M), 100 °C, 16 h, [Si]/[ArX]/[RuHCl(CO)(PPh3)3]/[CuCl] = 1:2:0.01:0.02;
Hiyama coupling conditions: dioxane (0.25 M), 80 °C, [Si]/[ArBr3]/[TBAF]/[Pd2(dba)3] = 1:0.25:1.2:0.01,
[Si]/[ArI4]/[TBAF]/[Pd2(dba)3] = 1:0.2:1.2:0.01, 30 °C.
b Isolated yields of products.
c The yield of the ‘one-pot’ reaction is given in parenthesis.
R2
R1 = H; R2 = R3 =
X = H, Cl
Pd2(dba)3
dioxane
R1
R3
R1
R3
Me Si Me
O
[Ru]
+
2
X
or
dioxane 1,3,5-Br3C6H3
R2 = H; R1 = R3 =
X = H; Br, Cl
Me Si Me
or
1,2,4,5-I4C6H2
X = H, Br, Cl
X
Scheme 3
Synlett 2008, No. 19, 3026–3030 © Thieme Stuttgart · New York

LETTER
Novel Approach to Stilbenoid Dendrimer Core Synthesis
3029
(NN 204 238734 and 204 162 32/4248) from the Ministry of Sci-
ence and Higher Education (Poland).
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(17) Typical Procedure for the Synthesis of 1,3-Bis[(E)-4-
halostyryl]disiloxanes: The glass reactor (10-mL, two-
necked, round-bottomed flask equipped with a magnetic
stirring bar, reflux condenser, argon bubbling tube and
thermostated oil bath) was evacuated and flushed with
argon. [RuH(Cl)(CO)(PPh3)3] (47.6 mg, 0.05 mmol), 1,3-
divinyltetramethyldisiloxane (0.5 g, 2.5 mmol), styrene or
4-bromo(or chloro)styrene (10 mmol) and anhyd dioxane
(5 mL) were added to the reactor. Then the reaction mixture
was stirred and heated at 100 °C under argon flow. After
5 min, copper(I) chloride (CuCl; 9.9 mg, 0.1 mmol) was
added as a co-catalyst. The synthesis process was carried out
for the next 16 h. After the reaction was completed (GC–MS
analysis) the volatiles were evaporated under vacuum and
the crude product was chromatographed on silica gel (eluent:
hexane–EtOAc, 10:1) to afford the analytically pure
products.
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1,3-Bis[(E)-4-bromostyryl]tetramethyldisiloxane (2): mp
56–60 °C. IR (KBr): 799.5, 844.8, 985.3, 1055.5, 1253.5,
1485.7, 1604.6, 2956.9, 3020.3 cm–1. 1H NMR (CDCl3): d =
0.24 (s, 12 H, SiMe), 6.41 (d, J = 19.1 Hz, 2 H, SiCH=), 6.86
(d, J = 19.2 Hz, 2 H, PhCH=), 7.27 (d, J = 7.6 Hz, 4 H,
BrC6H4), 7.43 (d, J = 8.3 Hz, 4 H, BrC6H4). 13C NMR
(CDCl3): d = 0.9, 121.9, 127.9, 129.4, 131.5, 136.9, 142.9.
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133 (100), 117 (37), 73 (50). HRMS: m/z [M+] calcd for
C20H2479Br81BrOSi2: 495.9712; found: 495.9685.
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1,3-Bis[(E)-4-chlorostyryl)tetramethyldisiloxane (3): mp
51–54 °C. IR (KBr): 800.6, 845.1, 985.6, 1056.4, 1254.2,
1488.9, 1564.5, 1606.2, 2957.6, 3024.3 cm–1. 1H NMR
(CDCl3): d = 0.25 (s, 12 H, SiMe), 6.40 (d, J = 19.2 Hz, 2 H,
SiCH=), 6.88 (d, J = 19.2 Hz, 2 H, PhCH=), 7.28 (d, J =
8.8 Hz, 4 H, ClC6H4), 7.34 (d, J = 8.8 Hz, 4 H, ClC6H4). 13
C
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406.0746.
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Tonogaki, K.; Nokami, T.; Ohashi, Y.; Yoshida, J. Angew.
Chem. Int. Ed. 2006, 45, 2404.
(18) Synthesis of PPV from 1,3-Bis[(E)-4-bromostyryl]tetra-
methyldisiloxane: [Pd2 (dba)3] (9.16 mg, 0.01 mmol),
dioxane (4 mL), 1,3-bis[(E)-4-bromostyryl]tetramethyl-
disiloxane (2; 248 mg, 0.5 mmol), and tetrabutylammonium
fluoride (320 mg, 1.2 mmol) were placed in an evacuated
and flushed with argon, 10-mL flask. The mixture was
heated at 80 °C for 12 h under an argon atmosphere. The
degree of conversion of the substrates was estimated by GC
and TLC analyses. Then the reaction mixture was cooled and
the precipitated solid was filtered and washed extensively
with acetone.
Synlett 2008, No. 19, 3026–3030 © Thieme Stuttgart · New York

3030
W. Prukała
LETTER
(19) Typical Procedure for the One-Pot Synthesis of 1,3,5-
Tris- or 1,2,4,5-Tetrakis[(E)-4-halostyryl)benzenes and
Spectroscopic Data of Selected Products: The glass
reactor (10-mL, two-necked, round-bottomed flask
(CDCl3): d = 7.09 (d, J = 15.9 Hz, 3 H, C6H3CH=), 7.20–
7.45 (m, 15 H, ClC6H4CH=), 7.65 (s, 3 H, C6H3). 13C NMR
(CDCl3): d = 124.8, 125.4, 127.6, 128.1, 128.8, 133.3, 135.5,
137.7. MS (EI): m/z (%rel. int.) = 486 (8) [M+], 364 (56), 350
(47), 220 (49), 205 (100), 73 (57). HRMS: m/z [M+] calcd for
C30H2135Cl3: 486.0709; found: 486.0694.
1,2,4,5-Tetrakis[(E)-4-bromostyryl]benzene (8): mp 265–
268 °C. IR (KBr): 798.9, 845.4, 956.6, 1008.5, 1072.1,
1258.5, 1487.9, 1587.4, 1682.4, 1725.1, 2924.1, 2957.9,
3049.3 cm–1. 1H NMR (THF-d8): d = 7.19 (d, J = 16.0 Hz,
4 H, C6H2CH=), 7.50–7.58 (m, 16 H, BrC6H4), 7.67 (d, J =
16.1 Hz, 4 H, BrC6H4CH=), 7.98 (s, 2 H, C6H2). 13C NMR
(THF-d8): d = 121.9, 125.0, 127.2, 129.1, 130.9, 132.4,
136.3, 137.6. MS (EI): m/z (%rel. int.) = 802 (5) [M+], 633
(14), 308 (20), 196 (32), 185 (64), 91 (95), 57 (100). Anal.
Calcd for C38H26Br4: C, 56.89; H, 3.27. Found: C, 56.58; H,
3.03.
1,2,4,5-Tetrakis[(E)-4-chlorostyryl]benzene (9): mp 242–
246 °C. IR (KBr): 808.2, 853.3, 960.3, 1012.2, 1091.7,
1492.4, 1592.4, 1667.7, 1686.8, 2924.6, 2955.8, 3027.2
cm–1. 1H NMR (C6D6): d = 6.96 (d, J = 16.2 Hz, 4 H,
C6H2CH=), 7.06–7.17 (m, 16 H, ClC6H4), 7.44 (d, J = 16.2
Hz, 4 H, ClC6H4CH=), 7.87 (s, 2 H, C6H2). 13C NMR (C6D6):
d = 126.4, 127.1, 129.3, 129.7, 130.2, 134.2, 136.1, 138.9.
MS (EI, %rel. int.): m/z = 622 (6) [M+], 248 (32), 178 (51),
139 (68), 125 (100). Anal. Calcd for C38H26Cl4: C, 73.09; H,
4.20. Found: C, 72.81; H, 4.01.
equipped with a magnetic stirring bar, reflux condenser,
argon bubbling tube and thermostated oil bath) was
evacuated and flushed with argon. [RuH(Cl)(CO)(PPh3)3]
(9.52 mg, 0.01 mmol), 1,3-divinyltetramethyldisiloxane (0.1
g, 0.5 mmol), styrene or 4-bromo(or chloro)styrene (2.0
mmol) and anhyd dioxane (2 mL) were added to the reactor.
Then the reaction mixture was stirred and heated at 100 °C
under argon flow. After 5 min, CuCl (1.98 mg, 0.02 mmol)
was added as a co-catalyst. The synthesis process was
carried out for the next 24 h. After the reaction was
completed (GC–MS or GC and TLC analyses), palladium
catalyst [Pd2 (dba)3] (9.16 mg, 0.01 mmol), TBAF (320 mg,
1.2 mmol), dioxane (3 mL) and respective haloarene [1,3,5-
tribromobenzene (78.7 mg, 0.25 mmol) or 1,2,4,5-tetra-
iodobenzene (116 mg, 0.2 mmol)] were added and the
mixture was heated at 80 °C (30 °C for 1,2,4,5-tetra-
iodobenzene) for 16–48 h under an argon atmosphere. The
degree of conversion of the substrates was estimated by GC
and TLC analyses. The final product was separated using
chromatography column with silica (THF–EtOAc).
1,3,5-Tris[(E)-4-chlorostyryl]benzene (6): mp 216–220
°C. IR (KBr): 806.3, 841.7, 960.2, 1090.4, 1490.7,
1585.1, 1668.2, 2924.2, 2957.7, 3024.9 cm–1. 1H NMR
Synlett 2008, No. 19, 3026–3030 © Thieme Stuttgart · New York