Palladium-Catalyzed Synthesis of Dibenzosilepin Derivatives via 1, n-Palladium Migration Coupled with anti-Carbopalladation of Alkyne

Article abstract of DOI:10.1021/jacs.0c12453

Catalytic reactions involving 1,n-metal migration represent a powerful method for the synthesis of complex molecules from simple precursors through the activation of C-H bonds. By utilizing this attractive feature, herein we devised a palladium-catalyzed synthesis of new members of 5H-dibenzo[b,f]silepins, a class of underexplored but potentially useful silicon-bridged π-conjugated compounds. The reaction sequence is composed of 1,n-palladium migrations and unusual anti-carbopalladation of alkynes, which was realized by the proper choice of ligand for palladium. A series of deuterium-labeling experiments were conducted toward an understanding of the reaction mechanism to propose plausible catalytic cycles. The newly obtained 5H-dibenzo[b,f]silepins exhibited tunable optical and electronic properties, demonstrating the power and importance of developing a new synthetic method based on 1,n-metal migration processes.

Full text of DOI:10.1021/jacs.0c12453

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Palladium-Catalyzed Synthesis of Dibenzosilepin Derivatives via
1,n‑Palladium Migration Coupled with anti-Carbopalladation of
Alkyne
Tomohiro Tsuda, Seung-Min Choi, and Ryo Shintani*
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ABSTRACT: Catalytic reactions involving 1,n-metal migration
represent a powerful method for the synthesis of complex
molecules from simple precursors through the activation of C−H
bonds. By utilizing this attractive feature, herein we devised a
palladium-catalyzed synthesis of new members of 5H-dibenzo-
[b,f ]silepins, a class of underexplored but potentially useful silicon-
bridged π-conjugated compounds. The reaction sequence is
composed of 1,n-palladium migrations and unusual anti-carbopal-
ladation of alkynes, which was realized by the proper choice of
ligand for palladium. A series of deuterium-labeling experiments
were conducted toward an understanding of the reaction
mechanism to propose plausible catalytic cycles. The newly
obtained 5H-dibenzo[b,f]silepins exhibited tunable optical and electronic properties, demonstrating the power and importance
of developing a new synthetic method based on 1,n-metal migration processes.
little progress has been made on the detailed study of this class of
compounds, and some scattered reports on their promising
features started to appear only recently.11 This slowness of the
research progress on the chemistry of 5H-dibenzo[b,f ]silepins is
most likely due to the lack of efficient synthetic methods, which
results in an insufficient diversity of accessible structures. In fact,
other than the pioneering synthesis of 5,5-dimethyl-5H-
dibenzo[b,f ]silepin,10 synthetic methods of 5H-dibenzo[b,f ]-
silepins are essentially limited to either a reaction of (Z)-1,2-
bis(2-lithioaryl)ethene with dichlorosilanes11a−c or a ring-
closing metathesis reaction of bis(2-vinylaryl)silanes,11e,12
both of which can only be used for the synthesis of 10,11-
unsubstituted compounds (Scheme 1a).13 In this context, herein
we describe a palladium-catalyzed synthesis of novel 5H-
dibenzo[b,f ]silepin derivatives, 13H-benzo[f ]fluoreno[1,9-bc]-
silepins and 1-alkylidene-2,7-dihydro-1H-dibenzo[b,f ]-
cyclobuta[d]silepins, through a reaction sequence triggered by
1,5-palladium migration followed by an unusual anti-carbopal-
ladation of alkyne (Scheme 1b).14,15 We also provide
mechanistic insights of the present catalysis as well as properties
of the obtained products.
INTRODUCTION
■
Transition-metal-catalyzed reactions involving metal migration
from carbon to carbon within organic substrates in a 1,n-fashion
represent a powerful way of synthesizing complex molecules
from relatively simple precursors through activation of C−H
bonds that are difficult to functionalize directly.1 Among the
known such processes, reactions involving 1,4-migration of
palladium2 or rhodium3 have been most widely explored and
several effective examples of the corresponding 1,5-migrations
have also been developed.4,5 Despite the synthetic utility of these
1,n-metal migration processes, the transformations reported to
date usually include only one such migration during a catalytic
cycle, although a reaction involving multiple 1,n-metal
migrations in a cascade manner would be highly attractive
toward rapid construction of a complex molecular skeleton
through remote functionalization in a single operation.6
Recently, our group has been focusing on the development of
new synthetic methods of underexplored silicon-bridged
functional organic compounds such as 5,10-dihydrophenazasi-
lines,7 8H-benzo[e]phenanthro[1,10-bc]silines,8 and others9
through the use of 1,n-palladium or rhodium migrations to
realize an easy access to these potentially useful molecules. In
this regard, 5H-dibenzo[b,f ]silepins belong to a class of silicon-
bridged π-conjugated compounds and half a century has passed
since the first synthesis of 5,5-dimethyl-5H-dibenzo[b,f ]silepin
by the reaction of 1,2-bis(2-lithiophenyl)ethane with dichlor-
odimethylsilane followed by conversion of the carbon−carbon
single bond to a double bond (Scheme 1a).10 However, very
Received: November 30, 2020
Published: January 11, 2021
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Scheme 1. (a) Conventional Synthesis of 5H-Dibenzo[b,f ]silepins (Literature Precedents) and (b) New Synthesis of More
Complex 5H-Dibenzo[b,f]silepin Derivatives (This Work)
Scheme 2. Possible Reaction Pathways for the Production of 2a and 3a
novel silicon-bridged π-conjugated compound possessing a 5,7-
fused 5-siladihydroazulene structure, with no formation of
compound 2a. Among the ligands, binap (61% yield; entry 11)
and H8-binap (63% yield; entry 12) were more effective, and
through a brief examination of the reaction solvent using binap
as the ligand (entries 11 and 13−15), cyclopentyl methyl ether
(cPentOMe) was found to be better suited for the production of
compound 3a (77% yield (67% isolated yield); entry 15). The
present reaction presumably takes place via 1,5-palladium
migration (I → II; Scheme 2), which gives either 2a through
III or 3a through IV, indicating that the use of the isomeric
substrate 1a′ should provide the same products by direct
formation of II without 1,5-palladium migration. Indeed, as we
previously reported,8 the reaction of 1a′ in the absence of
phosphine ligands led to compound 2a in a much higher yield of
94% (entry 16) in comparison to the reaction of 1a via 1,5-
palladium migration (entry 1). On the other hand, the reaction
of 1a′ in the presence of binap unexpectedly showed a much
lower reactivity and product selectivity (entry 17) in comparison
to the reaction of 1a (entry 15).
RESULTS AND DISCUSSION
■
Synthesis of 13H-Benzo[f ]fluoreno[1,9-bc]silepins.
Reaction Development. As exemplified by the relationship
between naphthalene and azulene, replacing a 6,6-fused bicyclic
π-conjugated system by its 5,7-fused isomer can change the
overall properties, and this phenomenon has also been
incorporated into the field of graphene nanoribbons in recent
years.16 From a synthetic point of view, it is highly attractive and
convenient if the same precursor can be used to selectively
provide either a 6,6-fused system or a 5,7-fused system
depending on the reaction conditions. In this regard, we
recently found that 8H-benzo[e]phenanthro[1,10-bc]silines 2,
rarely explored silicon-bridged π-conjugated compounds
possessing a 6,6-fused 2-siladihydronaphthalene structure,17
can be synthesized from 2-(arylsilyl)-3-(arylethynyl)aryl triflates
1 under simple palladium catalysis.8 For example, as shown in
Table 1, the reaction of 1a proceeded smoothly in the presence
of Pd(OAc)2 (5 mol %) and Et2NH (2.0 equiv) in DMF at 100
°C without using any additional ligands to give benzophenan-
throsiline 2a in 59% yield (entry 1). The use of monophosphine
ligands such as PPh3, PCy3, and P(tBu)3 led to the selective
formation of 2a in 39−58% yield as well (entries 2−4). We then
decided to examine several bisphosphine ligands such as dppe,
dppb, dppf, and xantphos and found that 2a was still the major
product with somewhat lower efficiency (14−32% yield; entries
5−8). In stark contrast, as shown in entries 9−12, reactions
using bisphosphine ligands possessing a biaryl backbone were
Under the conditions in Table 1, entry 15, the scope of the
reaction toward benzofluorenosilepins 3 was found to be
reasonably broad, as summarized in Scheme 3. For example, in
addition to compound 3a having a (tert-butyl)phenylsilylene
bridging unit, benzofluorenosilepins 3b−d with other silylene
units could also be synthesized in 50−63% yield. With regard to
the substituents on the aromatic rings, introduction of a meta
substituent to Ar1 led to 2-substituted benzofluorenosilepins
found to provide 13H-benzo[f]fluoreno[1,9-bc]silepin 3a,18
a
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Most of the substrates employed here are prochiral, and the
corresponding products possess a silicon stereocenter.19
Although we have not achieved satisfactory results yet,
somewhat promising enantioselectivity can be induced by
simply employing enantiopure (R)-binap as the ligand, as
exemplified in the reaction of 1f to give 3f (eq 1).
Table 1. Palladium-Catalyzed Reaction of 1a
a
yield (%)
conversn
ab
,
entry
ligand (x)
none
solvent
DMF
(%)
2a
3a
1
2
3
4
5
6
7
8
100
59
39
58
55
14
32
17
17
0
0
0
0
0
0
0
0
0
0
0
0
0
52
55
61
63
73
PPh3 (10)
DMF
DMF
DMF
DMF
DMF
DMF
100
100
100
33
68
100
46
100
100
100
100
100
100
100
100
38
c
PCy3 (10)
c
P(tBu)3 (10)
Mechanistic Studies. To understand how the reaction
proceeds for the synthesis of 13H-benzo[f ]fluoreno[1,9-bc]-
silepins 3, we decided to carry out some control experiments. In
this transformation, two C−H bonds of Ar1 and one C−H bond
of Ar2 in substrate 1 (see the equation in Scheme 3) are cleaved
to form two new carbon−carbon bonds, and two C−H bonds
are newly generated at the 8- and 12-positions of the resulting
benzofluorenosilepin 3. At first, we prepared substrate 1a-d10
having a (tert-butyl)bis(pentadeuteriophenyl)silyl group and
conducted the present reaction (eq 2). Compound 3a thus
dppe (5.5)
dppb (5.5)
dppf (5.5)
xantphos (5.5) DMF
9
biphep (5.5)
segphos (5.5)
binap (5.5)
DMF
DMF
DMF
10
11
12
13
14
15
H8-binap (5.5) DMF
binap (5.5)
binap (5.5)
binap (5.5)
none
toluene
dioxane
cPentOMe
DMF
cPentOMe
<1
74
d
0
77 (67 )
e
d
16
e
94
0
15
17
binap (5.5)
15
a
1
Determined by H NMR against an internal standard (MeNO2).
b
Multiple unidentified side products were observed when the yield of
c
2a/3a is low at a high conversion. PR3·HBF4/Et2NH was used.
d
e
Isolated yield. Compound 1a’ was used as the substrate.
obtained showed quantitative deuterium incorporation at the
12-position along with significant deuterium incorporation
(70% D) at the 7-position, but no deuterium incorporation was
observed at the 8-position. On the other hand, the reaction of
substrate 1a-d5 having a pentadeuteriophenylethynyl group gave
product 3a with a significant decrease in deuterium content at
the 7-position (55% D) and only a negligible amount of
deuterium incorporation at the 8-position (eq 3). These results
3e−g, and a substrate having a 2-naphthyl group as Ar1 gave the
π-extended 10H-benzo[f ]benzo[3,4]fluoreno[1,9-bc]silepin
3h. For substituents on Ar2, various functional groups such as
methoxy (3i), methoxycarbonyl (3l), acetyl (3m), and
trifluoromethyl (3n,p) groups were tolerated to give products
3 in up to 77% yield, and naphthyl and pyridyl groups could also
be used as Ar2 to give the corresponding products 3r,s, although
a thienyl group was not applicable. The carbon−carbon bond
formation between Ar1 and Ar2 took place selectively at the less
hindered position for the synthesis of 3o,p,r,s.
indicate that C−H/C−D exchange seems to occur at the 7-
position of compound 3 and that H at the 8-position is not
directly derived from C−H bonds that engage in the carbon−
carbon bond formation, whereas H at the 12-position
quantitatively comes from the H at the ortho position of the
phenyl group on silicon of 1. In contrast, when the reaction of 1a
is conducted in the presence of nBu2ND (10 equiv) in place of
Et2NH, significant deuterium incorporation was observed at the
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Scheme 3. Scope of Palladium-Catalyzed Synthesis of 13H-Benzo[f]fluoreno[1,9-bc]silepins 3
7-, 8-, and 9-positions (eq 4), indicating that the hydrogens at
these positions are scrambled with an external hydrogen donor.
In addition, the same reaction was carried out in the presence of
nondeuterated benzofluorenosilepin 3l, and it was found that no
deuterium incorporation to 3l was observed under these
conditions (eq 5). This result establishes that the deuterium is
incorporated only during the formation of compound 3 from
substrate 1 and no C−H/C−D exchange occurs once
compound 3 is produced.
On the basis of these experiments, a proposed catalytic cycle
for the present reaction is illustrated in Scheme 4. Thus,
oxidative addition of aryl triflate 1 to palladium(0) gives
arylpalladium species A, which undergoes 1,5-palladium
migration4 with retentive migration of H to give another
arylpalladium species, B. Intramolecular anti-arylpalladation of
the alkyne14,15 then takes place to give alkenylpalladium C.
Successive 1,4-palladium migration2 with scrambling of H leads
to the arylpalladium species D, which further undergoes 1,5-
palladium migration with retentive migration of H to give
another arylpalladium species, E. C−H bond activation20 then
takes place, and reductive elimination of product 3 from the
resulting diarylpalladium species F closes the catalytic cycle to
regenerate palladium(0) species. Considering the deuterium
incorporation at the 7-position of 3a in eq 2, it is plausible to
propose the occurrence of 1,5-palladium migration from D to E
prior to the formation of F. In addition, on the basis of the
deuterium incorporation at the 9-position of 3a in eqs 4 and 5, it
is possible that alkenylpalladium C reversibly (and non-
productively) undergoes 1,3-palladium migration to and from
arylpalladium G with scrambling of H, although no example of
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a
Scheme 4. Proposed Catalytic Cycle for the Reaction of 1 to 3
a
Pd = Pd(binap), X = OTf. The amine base is omitted for clarity.
Scheme 5. Schematic Comparison of Initial Steps between
(a) 1a and (b) 1a′
Scheme 6. Palladium-Catalyzed Synthesis of 1-Alkylidene-
2,7-dihydro-1H-dibenzo[b,f]cyclobuta[d]silepins 6
1,3-palladium migration between two sp2-hybridized carbons
has been reported to date in the literature.21
In Table 1, we showed that, while the reaction of 1a
proceeded smoothly to give benzofluorenosilepin 3a selectively
(entry 15), isomeric compound 1a′ behaved quite differently
(entry 17). Oxidative addition of aryl triflate 1a′ should directly
form intermediate B, and one might think that it should lead to
the same outcome. A possible explanation for these seemingly
inconsistent results would be the difference in alkyne
orientations toward successive insertion. Thus, in the case of
1a, the alkyne moiety needs to flip away when 1,5-palladium
migration takes place (A1 → B1; Scheme 5a),22 but the alkyne
moiety of 1a′ can face toward palladium during the formation of
the oxidative adduct (B2; Scheme 5b), which could change the
reactivity and selectivity of the subsequent steps. With regard to
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Scheme 7. Proposed Reaction Pathway toward 6
Table 2. Optical Properties of Compounds 2a, 3a,o, 6a,c, 8,
and 9 in CH2Cl2 at 25 °C
Figure 1. UV−vis absorption spectra of compounds 2a (black solid
line), 3a (blue solid line), and 3o (red solid line) and fluorescence
spectra of compounds 2a (black broken line), 3a (blue broken line),
and 3o (red broken line) in CH2Cl2 ((3.3−4.8) × 10−5 M) at 25 °C.
reaction of 5a having a 2-propenyl group instead of an aryl group
on the alkyne of 1 led to selective formation of a
methylenecyclobutene-fused dibenzosilepin, 1-methylene-2,7-
dihydro-1H-dibenzo[b,f ]cyclobuta[d]silepin 6a, in 70% yield
under the same reaction conditions (eq 7).18 Similarly, the
UV−vis absorption λmax (nm)
(ε (104 M−1 cm−1))
fluorescence
λmax (nm)
compd
ΦF
a
2a
229 (4.2), 254 (3.9), 268 (4.3), 325 (2.6), 371, 391, 411 0.19
336 (2.3)
228 (3.4), 250 (4.0), 278 (2.4), 298 (0.6), 523
309 (0.6), 359 (2.2), 376 (1.9)
228 (4.1), 273 (3.8), 303 (7.3), 339 (3.7), 529
353 (3.8), 379 (3.9), 398 (4.3)
252 (3.6), 342 (1.8)
262 (4.4), 355 (2.2)
b
3a
0.01
diisopropylsilylene-bridged product 6b as well as the 9-
phenylated compound 6c were also obtained in 59−63%
yield, and these results represent rare examples of methyl-
enecyclobutene formation under palladium catalysis.23 The
scope of this new transformation was briefly investigated, as
shown in Scheme 6. For example, substrate (E)-5d (E/Z = 97/
3) having a 2-butenyl group gave product 6d, where a methyl
group was installed exclusively on the cyclobutene ring rather
than on the exo methylene, indicating that C1′, not C3′, was
selectively engaged in the four-membered ring-formation. The
same product was obtained from (Z)-5d (E/Z = 20/80),
although that the reactivity was somewhat lower. This position
selectivity was also observed for the reaction of (E)-5e to give 6e,
having a phenyl group on the four-membered ring. C1′-selective
ring formation was further confirmed by the reaction of 5f
having a 1-buten-2-yl group and 5g having a 3-phenyl-1-propen-
2-yl group, giving the products 6f having a methyl group and 6g
having a phenyl group on the exo methylene without forming
isomeric 6d and 6e, respectively. It is worth noting that
substrates 5h−j having a cycloalkenyl group also followed the
same reaction pathway toward the formation of cyclobutene
rings to give 6h−j in up to 65% yield. Unlike 6i,j, 6h was
obtained as a mixture of isomers having a carbon−carbon
double bond at different positions within the six-membered ring,
presumably due to the ring strain of the initially formed 4,6-
fused product, and this was isolated as 6h′ in 48% overall yield
after hydrogenation of the double bond.
c
3o
0.05
d
6a
383, 403, 425 0.52
399, 421 0.67
e
6c
f
8
250 (2.9), 282 (2.5), 345 (5.9), 485 (0.6),
513 (0.7), 553 (0.6)
300 (1.4)
g
9
349, 366, 383 0.08
a
b
c
At 4.8 × 10−5 M, λex = 336 nm. At 3.4 × 10−5 M, λex = 376 nm. At
d
e
3.3 × 10−5 M, λex = 398 nm. At 4.0 × 10−5 M, λex = 342 nm. At 2.6
f
g
× 10−5 M, λex = 355 nm. At 2.6 × 10−5 M, not emissive. At 5.8 ×
10−5 M, λex = 300 nm.
the conversion from C to product 3 (Scheme 4), we prepared
the model compound 4, which could enter the catalytic cycle
from C via oxidative addition (eq 6). Indeed, the reaction of 4
proceeded smoothly to give the corresponding benzofluor-
enosilepin 3d in 78% yield, supporting the intermediacy of
alkenylpalladium C.
Synthesis of 1-Alkylidene-2,7-dihydro-1H-dibenzo-
[b,f]cyclobuta[d]silepins. In the aforementioned catalysis,
we further examined the effect of substituents and found that the
A similar four-membered-ring formation was observed for the
reaction of substrate 7 having an electron-deficient azulenyl
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Table 3. Electrochemical Data for Compounds 2a, 3a,i,l,o, and 8
ab
,
ac
,
de
,
e f
,
e
compd
Eoxonset (V)
Eredonset (V)
EHOMOonset (eV)
ELUMOonset (eV)
Egonset (eV)
2a
3a
3i
3l
3o
8
0.96
1.02
0.72
1.06
0.91
−2.25
−2.05
−2.04
−1.90
−1.91
−5.76 [−5.50]
−5.82 [−5.54]
−5.52 [−5.26]
−5.86 [−5.72]
−5.71 [−5.26]
−2.55 [−1.41]
−2.75 [−1.87]
−2.76 [−1.78]
−2.90 [−2.13]
−2.89 [−2.14]
−3.65 [−2.73]
3.21 [4.09]
3.07 [3.67]
2.76 [3.48]
2.96 [3.59]
2.82 [3.12]
1.76 [2.39]
0.61
b
−1.15
d
−5.41 [−5.12]
a
c
e
Values are against Fc/Fc+. In CH2Cl2. In THF. EHOMOonset = −(4.8 + Eoxonset). Values in brackets are calculated at the B3LYP/6-31G(d) level
of theory. ELUMO
f
= −(4.8 + Eredonset).
onset
sertion steps toward compounds 3 from compounds 1 (Scheme
4 in Synthesis of 13H-Benzo[f]fluoreno[1,9-bc]silepins),
alkenylpalladium species J would be generated (Scheme 7).
On consideration of the position-specific carbon−carbon bond
formation (at C1′ over C3′) as observed in Scheme 6, concerted
deprotonation−allylation may be operative to give the alkenyl-
(σ-allyl)palladium species K, and reductive elimination would
lead to compound 6.24
Physical Properties of Newly Synthesized Dibenzosi-
lepin Derivatives. With the new members of dibenzosilepins
in hand, we first compared the optical properties of 8H-
benzo[e]phenanthro[1,10-bc]siline 2a and isomeric 13H-
benzo[f]fluoreno[1,9-bc]silepin 3a (Table 2 and Figure 1).
While compound 2a showed an absorption maximum at 336 nm
in its UV−vis absorption spectrum in CH2Cl2, a significant red
shift was observed for compound 3a (λmax = 376 nm), and these
absorption peaks mainly consist of the HOMO−LUMO
transition on the basis of time-dependent density functional
theory (TD-DFT) calculations, indicating a narrower HOMO−
LUMO energy gap for compound 3a.25 In addition, both of
these compounds were found to be somewhat emissive in the
solution state, and while compound 2a showed an emission
maximum in the UV range, compound 3a emitted a green light
(λmax = 523 nm), albeit in a low quantum yield. The narrower
HOMO−LUMO energy gap of compound 3a was also
confirmed by an electrochemical analysis using cyclic
voltammetry, and it was found that the lower LUMO level is
mostly responsible for this outcome (Table 3).25 Thus,
compound 3a showed a reversible reduction potential at
−2.05 V, which is 0.20 V higher than the irreversible reduction
potential of compound 2a. It is also worth noting that, through
extension of the π-conjugation, a larger absorption coefficient
and a longer absorption wavelength (λmax = 398 nm) can be
observed, as shown for compound 3o possessing two phenyl-
ethynyl substituents (Table 2 and Figure 1). The nature of the
extended π-conjugation of compound 3o was confirmed by the
electrochemical analysis, which showed a higher HOMO and a
lower LUMO in comparison to the unsubstituted compound 3a
(Table 3). In addition, a selective increase in the HOMO energy
level can be achieved by introduction of an electron-donating
Figure 2. UV−vis absorption spectra of compounds 6a (blue solid
line), 6c (red solid line), and 9 (black solid line) and fluorescence
spectra of compounds 6a (blue broken line), 6c (red broken line), and 9
(black broken line) in CH2Cl2 ((2.6−5.8) × 10−5 M) at 25 °C.
group to give compound 8 as the major product in a moderate
yield (eq 8),18 although the reaction of a related azulenyl
substrate having no ester groups resulted in a complex mixture
(data not shown). This outcome is in stark contrast to the
reaction of substrates 1 having a benzenoid substituent on the
alkyne, which selectively gave benzofluorenosilepins 3 (Syn-
thesis of 13H-Benzo[f]fluoreno[1,9-bc]silepins).
With regard to the reaction pathway from 5 to 1-alkylidene-
2,7-dihydro-1H-dibenzo[b,f ]cyclobuta[d]silepins 6, after the
same oxidative addition−1,5-palladium migration−alkyne in-
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Authors
group at the 5-position of compound 3, as shown by an
electrochemical analysis of compound 3i, whereas a selective
decrease in the LUMO energy level can be realized by
introduction of an electron-withdrawing group, as shown in
compound 3l.
Tomohiro Tsuda − Division of Chemistry, Department of
Materials Engineering Science, Graduate School of
Engineering Science, Osaka University, Toyonaka, Osaka 560-
8531, Japan
With regard to the optical properties of 1-alkylidene-2,7-
dihydro-1H-dibenzo[b,f ]cyclobuta[d]silepin 6a, we compared
it with unsubstituted 5H-dibenzo[b,f ]silepin 9 (Table 2 and
Figure 2).25 Although their structural differences are relatively
subtle, a significant red shift was observed in the UV−vis
absorption spectrum of compound 6a, and it showed a much
higher quantum yield in the photoluminescence (ΦF = 0.52 vs
ΦF = 0.08 for 9). Introduction of a phenyl group at the 9-
position led to an even higher quantum yield, as observed in the
emission of compound 6c (ΦF = 0.67).11a
Seung-Min Choi − Division of Chemistry, Department of
Materials Engineering Science, Graduate School of
Engineering Science, Osaka University, Toyonaka, Osaka 560-
8531, Japan
Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.0c12453
Notes
The authors declare no competing financial interest.
It is also worth noting that the azulene-fused compound 8
showed absorption peaks at a much longer wavelength (Table 2)
with a broad shoulder at 600−820 nm, which mostly consists of
the HOMO−LUMO transition.25 The HOMO−LUMO energy
gap of this compound was estimated to be as narrow as 1.76 eV
by the electrochemical analysis (Table 3).
These results clearly demonstrate that the optical and
electronic properties of 5H-dibenzo[b,f ]silepins can be
effectively modified by devising a synthetic method of
structurally new members with various functional groups.
ACKNOWLEDGMENTS
■
Support has been provided in part by the JSPS KAKENHI Grant
Numbers JP20H02741 (Grant-in-Aid for Scientific Research
(B)) and JP20H04821 (Hybrid Catalysis). We thank Prof.
Takeshi Naota and Prof. Shuichi Suzuki at Osaka University for
the measurement of high-resolution mass spectra.
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CONCLUSIONS
■
We developed a palladium-catalyzed synthetic method of new
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/jacs.0c12453.
■
sı
Experimental procedures, characterization data, X-ray
crystallographic data, NMR spectra, and theoretical
calculations (PDF)
X-ray crystallographic data (CIF)
X-ray crystallographic data (CIF)
X-ray crystallographic data (CIF)
AUTHOR INFORMATION
Corresponding Author
■
Ryo Shintani − Division of Chemistry, Department of Materials
Engineering Science, Graduate School of Engineering Science,
Osaka University, Toyonaka, Osaka 560-8531, Japan;
orcid.org/0000-0002-3324-9393; Email: shintani@
chem.es.osaka-u.ac.jp
1648
https://dx.doi.org/10.1021/jacs.0c12453
J. Am. Chem. Soc. 2021, 143, 1641−1650

Journal of the American Chemical Society
pubs.acs.org/JACS
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