Palladium-Catalyzed Double Carbonylative Cyclization of Benzoins: Synthesis and Photoluminescence of Bis-Ester-Bridged Stilbenes

Article abstract of DOI:10.1021/acs.orglett.8b03169

A palladium-catalyzed double carbonylative cyclization of benzoins has been developed, which realizes the synthesis of bis-ester-bridged stilbenes just in two steps from aldehydes. Thus, the obtained fully fused tetracyclic π-systems have a pyrano[3,2-b]pyran-2,6-dione (PPD) core on their center, showing two reversible reductions at low potentials. In addition, their photoluminescence properties are strikingly affected by the aromatic rings fused to the PPD core; bis-thieno-fused PPDs are found to be excellent fluorophores with quantum yields up to 0.98.

Full text of DOI:10.1021/acs.orglett.8b03169

Letter
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Palladium-Catalyzed Double Carbonylative Cyclization of Benzoins:
Synthesis and Photoluminescence of Bis-Ester-Bridged Stilbenes
Yosuke Tani* and Takuji Ogawa*
Department of Chemistry, Graduate School of Science, Osaka University, Machikaneyama 1-1, Toyonaka, Osaka 560-0043, Japan
*
S Supporting Information
ABSTRACT: A palladium-catalyzed double carbonylative
cyclization of benzoins has been developed, which realizes
the synthesis of bis-ester-bridged stilbenes just in two steps
from aldehydes. Thus, the obtained fully fused tetracyclic π-
systems have a pyrano[3,2-b]pyran-2,6-dione (PPD) core on
their center, showing two reversible reductions at low
potentials. In addition, their photoluminescence properties
are strikingly affected by the aromatic rings fused to the PPD
core; bis-thieno-fused PPDs are found to be excellent
fluorophores with quantum yields up to 0.98.
4
olycyclic fused π-conjugated systems such as bridged
functionalized diaryl acetylenes into bridged stilbenes. In this
P
stilbenes, triphenylenes, and acenes are a privileged motif
in optical and electronic materials, including organic light-
context, benzoins can be a promising precursor for fused π-
systems, since they are available directly from aldehydes by
benzoin condensation and have a stilbene skeleton in the enol
form (Figure 1b). The development of a suitable transformation
of benzoins will expand the chemistry of bridged stilbenes that
are otherwise difficult to access. In particular, bis-ester-bridged
stilbenes (Y is CO in Figure 1b) are attractive because the
ester group is electron-withdrawing, chemically convertible, and
redox active, and it composes a weakly aromatic 2-pyrone
1
emitting diodes and organic photovoltaics. Therefore, efficient
2
synthetic strategies for such fused skeletons are of great interest.
Many of the established routes construct them from
preorganized, nonfused π-systems (Figure 1a), e.g. from
oligoarenes by Scholl reaction or direct arylation, or from
3
5,6
substructure. Interestingly, they are also found in some natural
7
products. Nonetheless, the synthesis and properties of bis-
8
ester-bridged stilbenes are virtually unexplored.
9
We envisioned that Pd-catalyzed carbonylation chemistry
would offer a powerful solution for the conversion of benzoins
into bis-ester-bridged stilbenes. In particular, carbonylative
1
0
cyclization has been utilized in the synthesis of heterocycles,1
1
although the construction of multiple rings is rarely achieved.
Herein, we have developed a palladium-catalyzed double
carbonylative cyclization of benzoins, which affords bis-ester-
bridged stilbenes having a pyrano[3,2-b]pyran-2,6-dione (PPD)
core on its center. The reaction reveals a stilbene skeleton, fixes
the planar π-system, and installs electron-withdrawing function-
ality at one time, thus providing them with unique electronic and
photophysical properties.
We began by benzoin condensation of 2-bromoarylaldehydes
1
(
a−1e, which are readily available by several approaches
12
Scheme S1 in the Supporting Information (SI)). After the
corresponding benzoins 2a−2e were successfully synthesized,
double carbonylative cyclization of 2e was examined under
various conditions (Table 1). The reaction mainly produced
Figure 1. Synthetic strategy toward polycyclic fused π-systems utilizing
(
a) preorganized π-systems and (b) benzoins as an easily available,
Received: October 4, 2018
latent π-system (This work). FG = functional group.
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b03169
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Table 1. Pd-Catalyzed Double Carbonylative Cyclization of
Table 2. Synthesis of Fused PPDs 3a−3e from Benzoins 2a−
a
2
e
2e
a
b
entry
ligand
base
yield (%)
3e/4e
1
2
3
4
5
6
7
8
9
Xantphos
Xantphos
Xantphos
Xantphos
DPEphos
dppf
rac-BINAP
Xantphos
Xantphos
Cs CO
42
88
95
49
65
52
20
76
>95/5
85/15
87/13
>95/5
60/40
42/58
25/75
88/12
85/15
2
3
CsOAc
KOAc
NaOAc
KOAc
KOAc
KOAc
KOAc
c
d
d
e
KOAc
92
a
1
Combined yields of 3e and 4e determined by H NMR using 1,3,5-
b
1
trimethoxybenzene as an internal standard. Determined by H NMR.
c
d
PdCl (dppf) (6 mol %) was used as the catalyst. PdCl (PhCN) (6
2
2
2
e
mol %) was used instead of Pd (dba) ·CHCl . KOAc (3.0 equiv) was
used.
2
3
3
fully fused target 3e and its structural isomer 4e, and was
1
quantitatively evaluated by H NMR using an internal standard.
First, we tested the effect of a base by using Pd (dba) ·CHCl as
2
3
3
9
b
a catalyst precursor and Xantphos as a ligand under 1 atm of
carbon monoxide (CO) in toluene at 110 °C (entries 1−4).
When we used Cs CO , 3e was obtained in 42% yield (entry 1),
2
3
which was the highest among other tested carbonate bases and
KOtBu. These strong bases were expected to facilitate enolate
a
1
0b
Conditions: 2 (0.20 or 0.10 mmol), Pd (dba) ·CHCl (3 mol %),
formation. However, weakly basic acetates were found to be
far more effective for this transformation, possibly due to the
easily enolizable nature of benzoins. Using CsOAc, 3e was
obtained in 75% yield along with a non-PPD structural isomer
2
3
3
Xantphos (6 mol %), KOAc (2.0 equiv) in toluene (0.2 M) under CO
b
c
(
1 atm, closed) at 110 °C. On 1.0 mmol scale. At 90 °C. Si =
triisopropylsilyl.
13
4
e (13%, entry 2). The best yield for 3e was achieved by
employing KOAc; the combined yield of 3e and 4e reached 95%,
and the ratio was 87/13 (entry 3). Although the selectivity was
improved with NaOAc, the total yield dropped to 49% (entry 4).
The choice of ligand was also crucial. Other bidentate
phosphines DPEphos, dppf, and rac-BINAP were not as effective
as Xantphos was; the total yields decreased to 65%, 52%, and
efficiently converted to 3a and 4a in a ratio of 90/10 as judged
from H NMR of the crude mixture. Product 3a was isolated in
1
81% yield after silica-gel column chromatography (entry 1). The
same reaction was also performed in 1.0 mmol scale, affording
170 mg of 3a (entry 2). The syntheses of fused PPDs 3b, 3c, and
3e having alkyl chain, fluorine, and silyl substituents,
respectively, were also examined because these substituents
2
0%, respectively (entries 5−7). The selectivity (isomer ratio)
was much affected, even reversed in the case of dppf and rac-
BINAP, implying that the catalyst is involved in the selectivity-
determining step (see SI for further discussion). As for a catalyst
precursor, a divalent palladium source, PdCl (PhCN) complex,
1a,b,f,14
are often utilized to control crystal packing.
As a result,
those fused PPDs were successfully synthesized and isolated in
good yields (65−72%). Thieno-fused PPD without any
substituents (3d) was obtained, albeit in low yield, partly
owing to its limited solubility.
We further extended the strategy to the synthesis of
unsymmetrically fused PPDs. Benzoin 2f was prepared by
cyanosilylation of 1a followed by deprotonation with lithium
diisopropylamide and trapping with 1e (see Supporting
Information (SI) for detail). Double carbonylative cyclization
of 2f proceeded efficiently under the standard conditions,
affording benzo-thieno-fused PPD 3f in 72% isolated yield (eq 1).
2
2
led to a slight decrease in the yield while maintaining the
selectivity, suggesting that the palladium complexes enter the
catalytic cycle as Pd(0) species (entry 7). The yield was restored
by increasing the amount of KOAc to 3 equiv (entry 8).
With the optimized conditions in hand, we completed the
synthesis of fused PPDs by the double carbonylative cyclization
of benzoins 2a−2e (Table 2). The conditions were demon-
strated to be effective not only for thiophene-derived benzoins
but also for benzene-derived benzoins. For example, 2a was
B
DOI: 10.1021/acs.orglett.8b03169
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
All the structures of fused PPDs 3a−3f were unambiguously
determined by single-crystal X-ray structure analysis (Figure 2
Figure 2. ORTEP drawing of the crystal structures for 3a (a) and 3d
(
b). Thermal ellipsoids are set at the 50% probability level. Selected
bond lengths for 3a: C1−C7, 1.437(2); C7−C7a, 1.341(2) Å. For 3d:
C4−C5, 1.435(7); C5−C5a, 1.343(6) Å.
Figure 3. UV−vis absorption (blue) and normalized PL (red) spectra
for 3e (top), 3f (middle), and 3a (bottom) in CH Cl . Oscillator
2
2
strengths of absorption (TD-DFT//B3LYP/6-31+G**) are depicted
15
as gray bars. Φ are relative to quinine sulfate. Left axis for UV−vis,
F
for 3a and 3d; see SI for others). Except 3f whose skeleton was
slightly bent in the crystal, all the other fused PPDs had planar
skeletons. All, except 3e, were packed very tightly (interplanar
distances of the parallel stacked pairs: 3.17−3.39 Å) with various
short contacts including hydrogen bonding on the carbonyl
oxygen. Moreover, their packing modes can be tuned by
substituents and the skeleton: herringbone stacking for 3a and
right axis for others.
Table 3. Photophysical Properties of 3a−3f and 4e
λabs
λem
(nm)
k
r
k
nr
a
ε (M−1 cm−1
b
c
d
−1
e
−1
e
compd (nm)
)
ΦF
(ns
)
f
(ns
)
f
3
3
3
3
3
3
4
a
b
c
f
d
e
e
373
382
364
394
408
420
419
16 000
19 600
18 100
26 900
33 700
40 800
45 900
384
395
370
403
413
424
0.009
0.049
0.004
0.32
0.90
0.98
0.29
n.d.
n.d.
0.37
0.38
0.38
0.41
32
n.d.
n.d.
0.79
0.042
0.0078
3
c, brick wall 2D stacking for 3b, 1D columnar stacking for 3d,
and antiparallel pairwise 1D stacking for 3f. We also performed
thermogravimetric analysis of 3a and 3d, which did not show
any indication of decomposition (Figure S1). Temperatures for
5
% weight loss were at 275 and 317 °C, respectively, both of
427
0.31
0.92
which corresponding to sublimation of the compounds. As for
a
b
c
The longest absorption maxima. At the global maxima. The
the electrochemical property, cyclic voltammograms of 3b and
d
15
shortest emission maxima. Determined relative to quinine sulfate.
3
e in tetrahydrofuran were measured. They showed two
e
Calculated with Φ and τ according to the formula k = Φ /τ and k
F
r
F
nr
reversible reduction peaks at −2.71 and −2.31 V for 3b and
f
+
= (1 − Φ
F
)/τ. Calculated with λem, oscillator strength, and Φ
. See
F
−
2.37 and −1.91 V for 3e (versus Fc/Fc ), which demonstrated
Table S2 in SI for detail. n.d. = not determined.
their electrochemical stability and low lying LUMO (−2.5 eV for
5
,6
3
b and −2.9 eV for 3e, Figures S2 and S3). These thermal and
electrical properties as well as tunable molecular arrangements
in crystal indicate that fused PPDs are promising materials for
the application in organic electronics.
total, suggesting that thieno-fused systems have a smaller
HOMO−LUMO gap and more delocalized π-system than
benzo-fused ones do. Indeed, according to time-dependent
density functional theory (TD-DFT) calculation at the B3LYP/
6-31+G(d,p) level of theory, the strongest transitions in 3a, 3f,
and 3e are of S0−S1 (HOMO−LUMO 94% or more) and are
essentially a π−π* transition in nature, for both absorption and
emission.
UV−vis absorption and photoluminescence (PL) spectra as
15
well as photoluminescence quantum yields (Φ ) of fused
F
PPDs were measured in degassed dichloromethane (Figure 3
and Table 3 for representative data; see SI for full details). For all
fused PPDs 3a−3f, UV−vis and PL spectra are mirror images of
each other with vibrational structure, and the Stokes shifts are
very small. These features imply their rigidness and structural
similarity in the ground state (S0) and the first singlet excited
state (S1). More specifically, along with replacing the peripheral
fused ring from benzene to thiophene (i.e., 3a → 3f → 3e, from
bottom to top in Figure 3), the vibrational structure became
sharper with progression of 0−0 band, and the Stokes shifts
became smaller (11 nm for 3a, 9 nm for 3f, and 4 nm for 3e).
These trends clearly indicate that the structural displacement in
S1 becomes much smaller in this order. In addition, both UV−
vis and PL spectra are bathochromically shifted about 40 nm in
The peripheral aromatic rings most substantially effected Φ .
F
Thieno-fused PPDs 3d and 3e exhibited much higher Φ (≥0.9)
F
than benzo-fused PPDs 3a−3c did (Φ < 0.1). The highest Φ
F
F
was observed for 3e (0.98) while 3d still showed a high Φ of
F
0.90, suggesting that the highly fluorescent nature is attributed
16
to the core skeleton itself, rather than to the silyl substituent.
This is also evident from a comparison of 3e and its structural
isomer 4e. Their UV/PL spectra were similar in peak positions
and molar extinction coefficient (ε); however, 4e showed
somewhat broadened spectra (Figure S4), as well as a much
diminished quantum yield of 0.31. This emphasizes the
C
DOI: 10.1021/acs.orglett.8b03169
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
superiority of the PPD core for the structural rigidity even in the
excited states.
We further evaluated the radiative and nonradiative decay rate
Naota (Osaka University) for their assistance with the lifetime
and low-temperature photoluminescence measurement. The
theoretical calculations were partly performed using the
Research Center for Computational Science, Okazaki, Japan.
constants (k and k ) of 3a, 3d−3f, and 4e (Table 3; see SI for
r
nr
details). According to the equation Φ = k /(k + k ), it is clearly
F
r
r
nr
revealed that the strong dependency of Φ on their fused
F
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E-mail: y-tani@chem.sci.osaka-u.ac.jp.
E-mail: ogawa@chem.sci.osaka-u.ac.jp.
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ORCID
(
d) Kawasumi, K.; Zhang, Q.; Segawa, Y.; Scott, L. T.; Itami, K. A
Yosuke Tani: 0000-0003-1326-5457
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■
This work was supported by the JSPS KAKENHI (Grant
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DOI: 10.1021/acs.orglett.8b03169
Org. Lett. XXXX, XXX, XXX−XXX