Practical and convenient synthesis of 1,6-Di- or 1,2,5,6-tetra-arylhexa-1, 3,5-trienes by the dimerization of Pd(0)-complexed alkenylcarbenes generated from π-allylpalladium intermediates

Article abstract of DOI:10.1021/ol501643x

Pd(0)-complexed 3-aryl or 2,3-diaryl propenylcarbenes generated from α-silyl-, α-germyl-, or α-boryl-σ-allylpalladium intermediates undergo self-dimerization to provide 1,6-di- or 1,2,5,6-tetraarylhexa-1,3,5-trienes in good to high yields. This method allows the use of a π-allylpalladium intermediate for a carbenoid precursor. Furthermore, the obtained 1,2,5,6-tetraarylhexa-1,3,5-trienes exhibit aggregation-induced emission enhancement property.

Full text of DOI:10.1021/ol501643x

Letter
pubs.acs.org/OrgLett
Practical and Convenient Synthesis of 1,6-Di- or 1,2,5,6-Tetra-
arylhexa-1,3,5-trienes by the Dimerization of Pd(0)-Complexed
Alkenylcarbenes Generated from π‑Allylpalladium Intermediates
Yoshikazu Horino,*,† Yu Takahashi,† Kaori Koketsu,† Hitoshi Abe,† and Kiyoshi Tsuge‡
‡
†Department of Applied Chemistry and Department of Chemistry, Graduate School of Science and Engineering, University of
Toyama, Gofuku, Toyama 930-855, Japan
S
* Supporting Information
ABSTRACT: Pd(0)-complexed 3-aryl or 2,3-diaryl propenylcar-
benes generated from α-silyl-, α-germyl-, or α-boryl-σ-allylpalla-
dium intermediates undergo self-dimerization to provide 1,6-di-
or 1,2,5,6-tetraarylhexa-1,3,5-trienes in good to high yields. This
method allows the use of a π-allylpalladium intermediate for a
carbenoid precursor. Furthermore, the obtained 1,2,5,6-tetraarylhexa-1,3,5-trienes exhibit aggregation-induced emission
enhancement property.
Scheme 1. Strategy for the Generation of Pd(0)-Carbene
ecently, significant research interest has been focused on
Rπ-conjugated compounds since they exhibit interesting
fluorescence properties. A number of π-conjugated hydro-
carbons are known to display intriguing fluorescence proper-
ties.1 1,6-Diarylhexa-1,3,5-trienes serve as fluorescent probes in
biological studies,2 and 1,3,4,6-tetraarylhexa-1,3,5-trienes have a
structure consisting of organic fluorophores, which exhibit
aggregation-induced emission enhancement.3 Although several
synthetic approaches to 1,6-diarylhexa-1,3,5-trienes have been
developed, substrate scope is narrow, and some methods
require elaborate substrates.4−9 Thus, more general protocols
for the synthesis of 1,6-diarylhexa-1,3,5-trienes are still of high
interest. Among their reported methods, the dimerization of the
Pd(0)-complexed 3-aryl propenylcarbene appears to be an ideal
approach due to its high functional group tolerance. Moreover,
in the past decade, Pd(II)-carbene complexes have emerged as
a promising intermediate for cross-coupling reactions;10,11
however, few studies have focused on the generation of
Pd(0)-carbene intermediates.12 To date, developments in the
generation of Pd(0)-carbene intermediates have been restricted
to diazo esters. Representative examples are transmetalation
from group 6 Fisher metal carbene complex to a Pd(0) complex
and decomposition of in situ generated diazo compounds by
the Pd(0) complex.12e−i Recently, a unique strategy involving a
Pd(0)-carbene intermediate to access to 1,6-diarylhexa-1,3,5-
trienes was developed by Fillion. Specifically, the strategy
involved the generation of Pd(0)-complexed 3-aryl propenyl-
carbenes generated from an γ-aryl-α-stannyl-σ-allylpalladium
intermediate.12a This method allows the utilization of readily
available substrates rather than diazo compounds to provide
1,6-diarylhexa-1,3,5-trienes and showed reasonable generality in
the substrate scope. On the other hand, our group reported the
Pd-catalyzed stereoselective cyclopropanation of strained
alkenes with 3-trimethylsilyl-1-arylallyl acetate 1 (Scheme
1A).13 The key step of this reaction appears to involve the
formation of a putative palladacyclobutene intermediate A from
the α-trimethylsilyl-σ-allylpalladium intermediate. Given our
result and Fillion’s finding, we hypothesized that reducing the
electrophilicity of the Pd atom in σ-allylpalladium intermediate
would preferentially induce the formation of Pd(0)-complexed
3-aryl propenylcarbene B, which subsequently leads to carbene
self-dimerization via disproportionation of B to provide the
desired product 2 (Scheme 1B). Herein, we report the
palladium-catalyzed practical and convenient synthesis of a
wide range of 1,6-diarylhexa-1,3,5-trienes 2 from readily
available starting materials 1. The application to the synthesis
of 1,2,5,6-tetraarylhexa-1,3,5-trienes to display the aggregation-
induced emission enhancement is considered, and the physical
properties of 1,2,5,6-tetraarylhexa-1,3,5-trienes are also dis-
cussed.
To test our hypothesis, we initially examined the effect of
ligands and additives, as shown in Table 1. The reaction of 1a
in the presence of 5 mol % of [(η3-C3H5)PdCl]2 did not
provide the desired product 1,6-diphenylhexa-1,3,5-triene 2a
(entry 1); however, to our delight, the treatment of 1a with 5
Received: March 26, 2014
Published: June 12, 2014
© 2014 American Chemical Society
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Organic Letters
Letter
a
Table 1. Optimization of Reaction Conditions
Table 2. Leaving Group and Silyl Group Effects on Pd-
a
Catalyzed Dimerization of 1
b
c
entry
ligand
additives
time (h)
2a (%)
3 (%)
b
entry
1
LG
[Si]
time (min)
2a (%)
1
2
3
4
5
6
7
8
9
−
DPPE
−
CsF
12
3.5
6
trace
36
0
1
2
3
4
5
6
7
8
1b
1c
1d
1e
1f
OCO2Et
OCONHPh
OCONHBz
OCSNHPh
Br
SiMe3
SiMe3
SiMe3
SiMe3
SiMe3
SiMe2Ph
30
84
65
57
0
d
41
0
50
e
TMEDA
CsF
trace
74
60
f
phen
CsF
1
10
22
0
50
g
2,2′-bpy
2,2′-bpy
2,2′-bpy
2,2′-bpy
2,2′-bpy
2,2′-bpy
2,2′-bpy
CsF
0.5
0.5
19
1
76
60
0
CsF, 4 Å MS
4 Å MS
TBAF
88
1g
1h
1i
OAc
45
88
81
45
48
52
0
t
OAc
SiMe2 Bu
SiiPr3
180
0
OAc
240
h
TBAT
1
34
10
0
a
Conditions: 1 (1 mmol), [(η3-C3H5)PdCl]2 (5 mol %), 2,2′-bpy (10
mol %), CsF (3 mmol), 4 Å MS (500 mg), DMF (5 mL), 60 °C. 2a
was isolated as a (E,E,E)-isomer.
i
10
KF, 4 Å MS
CsF, 4 Å MS
24
5.5
40
b
11
76
10
a
Conditions: 1a (1 mmol), [(η3-C3H5)PdCl]2 (5 mol %), ligand (10
mol %), MF (M = Cs or K, 3 mmol), DMF (5 mL), 60 °C. 4 Å MS
(500 mg) was used. 2a was isolated as a (E,E,E)-isomer. DPPE =
b
a
Table 3. Substrate Scope and Limitations
c
d
e
1,2-bis(diphenylphosphino)ethane. TMEDA = N,N,N′,N′-tetrame-
f
g
thylethylenediamine. phen = 1,10-phenanthroline. 2,2′-bpy = 2,2′-
h
bipyridine. TBAT = tetrabutylammonium difluorotriphenylsilicate.
i
[(η3-C3H5)PdCl]2 (2.5 mol %) and 2,2′-bpy (5 mol %) were used.
b
entry
1
R
time (h)
2
yield (%)
1
1j
2-Me-C6H4
2-allyl-C6H4
2-(i-propenyl)-C6H4
2-Ph-C6H4
0.5
0.5
0.5
0.5
3
2b
2c
2d
2e
2f
88
60
70
68
86
88
92
91
50
77
62
mol % of [(η3-C3H5)PdCl]2, 10 mol % of DPPF, and 3.0 equiv
of CsF in DMF at 60 °C for 3.5 h gave desired product 2a in
36% yield along with cinnamyl acetate 3 in 41% yield (entry 2).
Presumably, the enhanced Brønsted base character of the
carbon atom between bidentate phosphine-ligated palladium
and a trimethylsilyl group led to protodesilylation to produce 3.
While the screening of phosphine ligands did not improve the
yield of 2a, we were pleased to find that use of 2,2′-bipyridine as
a ligand increased the yield of 2a (entries 3−5). Among the
additives tested, cesium fluoride furnished 2a in high yield
(entries 6−10). When the catalyst loading was reduced to 2.5
mol %, 3 was formed in considerable yield (entry 11).14
Although 2a was obtained as a mixture of (E,E,E)-2a and
(E,Z,E)-2a ((E,E,E)/(E,Z,E) = 4/1 in entry 6) in all cases, the
isomerization of 2a consisted of a mixture of stereoisomers was
occurred easily with (E,E,E)-2a during the acidic work up and
silica-gel column chromatography.15
2
1k
1l
3
4
1m
1n
1o
1p
1q
1r
5
2,6-Me2-C6H3
4-F-C6H4
6
0.5
0.5
0.5
1
2g
2h
2i
7
4-CF3-C6H4
2-MeO-C6H4
3-BnO-C6H4
3-MOMO-C6H4
4-MOMO-C6H4
4-MeO-C6H4
2-thienyl
8
9
2j
10
11
12
13
14
15
16
17
1s
0.5
0.5
0.5
0.5
12
0.5
2
2k
2l
1t
c
1u
1v
1w
1x
1y
1z
2m
2n
2o
2p
2q
36
73
d
3-thienyl
40
2-furyl
75
35
75
3-furyl
3-pyridyl
0.5
2r
b
a
The same reaction conditions with those in Table 2. 2 was isolated
as a (E,E,E)-isomer. Yield was determined after hydrogenation. 1w
c
d
We then explored the effect of leaving groups and sustituents
on the silyl group (Table 2). Importantly, the formation of gem-
dimetallic species in α-trimethylsilyl-σ-allylpalladium intermedi-
ate plays a crucial role in the formation of B because the
reaction of a cinnamyl acetate 3 under optimal reaction
conditions resulted in no reaction.5b Furthermore, the intra-
molecular coordination of a carbonyl oxygen atom to the
silicon atom was likely to be involved in the formation of B
(entries 1−5). As expected, the use of more robust silyl groups
such as DMPS, TBDMS and TIPS required a prolonged
reaction time and diminished the yield of 2a (entries 6−8).
Under the optimized reaction conditions, substitution effects
on the phenyl ring of 1 were probed (Table 3). Alkyl-, alkenyl-,
and phenyl-substituted substrates at the ortho-position provided
the corresponding products in good to high yields (entries 1−
5). In addition, the present reaction was found to proceed
independently of the electronic nature of the substituent on the
phenyl ring, and the corresponding products were obtained in
good to high yields (entries 6−11). On the other hand, the
was recovered in 60% yield.
introduction of methoxy group at the para-position resulted in
diminished yield (entry 12).
Furthermore, furyl-, thienyl-, and 3-pyridyl-substituted
substrates also participated in the present reaction (entries
13−17). However, reaction with 2-pyridyl-substituted substrate
was rather difficult, and (Z)-1-(2-pyridyl)-1-propenyl acetate
was obtained in 27% yield.
Next, insights into the nature of the reactive species were
gathered through the reaction of deuterium-labeled 1a-d1 and
1a-d3. The scrambling of the deuterium atom between C-1 and
C-3 in the respective products 2a-d1 and 2a-d3 was not
observed in either case (Scheme 2).16
Furthermore, a crossover experiment was conducted using a
1:1 mixture of 1a and 1r to clarify the dimerization mechanism
in the dimerization of present Pd(0)-complexed 3-aryl
propenylcarbene (Scheme 3). Under optimal reaction con-
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Organic Letters
Letter
Scheme 2. Deuterium Labeling Experiments
Scheme 5. Application to the Synthesis of 1,2,5,6-
Tetraarylhexa-1,3,5-trienes 9a−9c
moderate yield.20 The structure of 9a was unambiguously
confirmed by X-ray diffraction analysis (Figure 1).21 The triene
ditions, products 4a, 4b, and crossover product 4c were isolated
respectively after hydrogenation.
Scheme 3. Crossover Experiment
Figure 1. X-ray structure of 9a (hydrogens are omitted for clarity):
side view of the molecule (left) and view from the long axis of the
molecule (right).
moiety of 9a was found to adapt a completely planar
conformation, whereas the terminal benzene rings were
distorted with dihedral angles of 27.4°. Another dihedral
angle of the remaining 2,5-diphenyl groups to the plane of the
triene moiety was 71.8°.
Next, we extend the present reaction to investigate carbon−
metalloid bond activation such as C−Ge and C−B bonds
(Scheme 4). Substrate 5 with a trimethylgermyl group instead
Interestingly, compounds 9a−9c in the solid state emitted
blue light, although the solution of 9a−9c in CHCl3 did not
exhibit fluorescence.20−22 Next, the fluorescence spectra of 9a
were measured in a mixed solvent of THF and water to confirm
aggregation-induced emission enhancement.20 As a result, a
THF solution of 9a did not exhibit luminescence, whereas the
fluorescence intensity increased when up to 70 vol % water was
added to the THF solution. Further addition of water resulted
in a decrease in fluorescence intensity. This result demonstrated
that 9a exhibits an aggregation-induced emission enhancement
property. The crystal packing of 9a also supported this result.20
In conclusion, we developed a convenient and practical
method for the synthesis of 1,6-diarylhexa-1,3,5-trienes by the
self-dimerization of Pd(0)-complexed 3-aryl propenylcarbene
generated from α-silyl-, α-germyl-, or α-boryl-σ-allylpalladium
intermediates. Furthermore, we demonstrated that a π-
allylpalladium intermediate could be used for cyclopropanation
of strained alkenes,13 as well as to serve as a carbenoid
precursor. Finally, the present reaction could be applied to the
synthesis of 1,2,5,6-tetraarylhexa-1,3,5-trienes, which exhibit
aggregation-induced emission enhancement. We are currently
synthesizing a series of 1,2,5,6-tetraarylhexa-1,3,5-trienes that
could be applied to display highly fluorescent organic solids.
Scheme 4. Pd-Catalyzed Dimerization of Substrates 5−7
Possessing a Germyl and Boryl Group at the Alkene
Terminus
of the trimethylsilyl group at the alkene terminus provided the
corresponding product 2a in excellent yield. On the other hand,
the use of substrate 6 with a pinacolatoboryl group at the
alkene terminus produced 2a in 35% isolated yield in the
presence of Cs2CO3 as a base. Interestingly, although the boryl
group masked by 1,8-diaminonaphthalene is known to serve as a
robust protecting group, particularly under basic conditions,17
the reaction of substrates 7a and 7b proceeded under milder
conditions than that of 6 and gave 2a and 2h in 70 and 67%
isolated yield, respectively.
Finally, application for the synthesis of 1,2,5,6-tetraarylhexa-
1,3,5-trienes 9a−c was examined (Scheme 5). Substrates 8a−c
were easily prepared by three steps involving silylstannylation18
and modified Stille coupling19 from readily available 1-aryl-2-
propyn-1-ol. The Pd-catalyzed reaction of 8a−c in the presence
of CsF and 18-crown-6 afforded the 9a−c, respectively, in
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental procedures and analytical data. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: horino@eng.u-toyama.ac.jp.
Notes
The authors declare no competing financial interest.
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Organic Letters
Letter
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ACKNOWLEDGMENTS
■
We thank Prof. Ryuta Miyatake (University of Toyama) for his
assistance in X-ray crystallographic analysis and Prof. Makoto
Genmei (University of Toyama) for his helpful discussion. This
work was financially supported by a Grant-in-Aid for Scientific
Research (C) (No. 24550115) from the Japan Society for the
promotion of Science and The Public Foundation of Chubu
Science and Technology Center.
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