Gold-Catalyzed Annulation of 1,8-Dialkynylnaphthalenes: Synthesis and Photoelectric Properties of Indenophenalene-Based Derivatives

Article abstract of DOI:10.1002/chem.202004846

A simple gold-catalyzed annulation of 1,8-dialkynylnaphthalenes utilizing a cationic gold catalyst was developed. Such a peri-position of two alkynyl substituents has not been studied in gold catalysis before. Dependent on the substrate, the reactions either follow a mechanism involving vinyl cation intermediates or involve a dual gold catalysis mechanism which in an initial 6-endo-dig-cyclization generates gold(I) vinylidene intermediates that are able to insert into C?H bonds. Indenophenalene derivatives were obtained in moderate to high yields. In addition, the bidirectional gold-catalyzed annulation of tetraynes provided even larger conjugated π-systems. The optoelectronic properties of the products were also investigated.

Full text of DOI:10.1002/chem.202004846

Accepted Article
Accepted Article
Title: Gold-Catalysed Annulation of 1,8-Dialkynylnaphthalenes:
Synthesis and Photoelectric Properties of Indenophenalene-
Based Derivatives
Authors: Sara Tavakkoli Fard, Kohei Sekine, Kaveh Farshadfar, Frank
Rominger, Matthias Rudolph, Alireza Ariafard, and A. Stephen
K. Hashmi
This manuscript has been accepted after peer review and appears as an
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of the final Version of Record (VoR). This work is currently citable by
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to this Accepted Article as a result of editing. Readers should obtain
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content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.202004846
Link to VoR: https://doi.org/10.1002/chem.202004846
01/2020

10.1002/chem.202004846
Chemistry - A European Journal
Gold-Catalysed Annulation of 1,8-Dialkynylnaphthalenes: Synthesis and
Photoelectric Properties of Indenophenalene-Based Derivatives
Sara Tavakkoli Fard,[a] Kohei Sekine,[a,b] Kaveh Farshadfar,[c] Frank Rominger,[a] Matthias
Rudolph,[a] Alireza Ariafard*,[d] and A. Stephen K. Hashmi*,[a,e]
[a] M.Sc. S. Tavakkoli Fard, Dr. K. Sekine, Dr. F. Rominger, Dr. M. Rudolph, Prof. Dr. A. S.
K. Hashmi
Organisch-Chemisches Institut, Heidelberg University, Im Neuenheimer Feld 270, 69120
Heidelberg, Germany
E-mail: hashmi@hashmi.de
[b]
Dr. K. Sekine Department of Molecular and Material Sciences Interdisciplinary
Graduate School of Engineering Sciences Kyushu University, and Institute for Materials
Chemistry and Engineering Kyushu University, 6-1 Kasugakoen, Kasuga-shi, Fukuoka
816-8580, Japan
[c] Dr. K. Farshadfar, Department of Chemistry, Azad University Central Tehran Branch,
Poonak, Tehran, Iran
[d] Dr. A. Ariafard, School of Physical Sciences, University of Tasmania
Private Bag 75, Hobart TAS 7001, Australia
E-mail: alirezaa@utas.edu.au
[d]
Prof. Dr. A. S. K. Hashmi Chemistry Department, Faculty of Science King Abdulaziz
University (KAU), Jeddah 21589, Saudi Arabia
Keywords: annulation, alkynes, bidirectional synthesis, dual gold catalysis, extended -
systems
Abstract
A simple gold-catalyzed annulation of 1,8-dialkynylnaphthalenes utilizing a cationic gold
catalyst was developed. Such a peri-position of two alkynyl substituents has not been
studied in gold catalysis before. Dependent on the substrate, the reactions either follow
a mechanism involving vinyl cation intermediates or involve a dual gold catalysis
mechanism which in an initial 6-endo-dig-cyclization generates gold(I) vinylidene
1
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10.1002/chem.202004846
Chemistry - A European Journal
intermediates that are able to insert into C–H bonds. Indenophenalene derivatives were
obtained in moderate to high yields. In addition, the bi-directional gold-catalyzed
annulation of tetraynes provided even larger conjugated -systems. The optoelectronic
properties of the products were also investigated.
1. Introduction
Over the last two decades, homogeneous gold catalysis[1] has been utilized for the
electrophilic activation of alkynes. Gold most commonly acts as a redox-neutral and
carbophilic π-acid that activates carbon-carbon multiple bonds towards nucleophilic
attack.[2] Alkynes are still the dominating class of substrates, among them diyne systems[3]
are very important as starting materials for gold-catalyzed annulation to extended -
systems. Different from the majority of gold-catalyzed reactions, diyne substrates lead to
high-energy intermediates, in the case of two internal alkynes to vinyl cations as
intermediates,[4] when at least one terminal alkyne is present, gold vinylidene
intermediates[5] are generated. Both types of intermediates are able to insert into non-
activated aliphatic C(sp3)–H or aromatic C(sp2)–H bonds. This provides easy access to a
diverse set of interesting organic scaffolds,[3,4] including π-extended structures. Polycyclic
aromatic hydrocarbons (PAHs), which might be the largest class of organic molecules,
have been intensively investigated, due to their small HOMO-LUMO gaps these highly
conjugated systems are promising for applications in the field of organic electronics.[6]
Therefore, the development of modular synthetic methods to access polycyclic π-
extended conjugated systems is a promising research topic.
1,8-Dialkynyl naphthalene structures are flexible and have been extensively been used
as substrates in intramolecular[7] and intermolecular[8] annulations. Recently Wang’s
group reported iodine-mediated electrophilic poly-cyclization in one step, which provides
two different products, probably generated from vinyl cation intermediates or radical
intermediates.[7i] Additionally, the synthesis of indeno[2,1-a]phenalene derivatives by an
iodine-mediated electrophilic cyclization of 1,8-dialkynylnaphthalenes was published
(Figure 1).[7j,7k] Tobe’s group elegantly designed a synthesis of a variety of rigid
carbocyclic platforms, which are stabilized by flanking aromatic groups.[7o] Such fused
five- or six-membered carbocyclic rings are key substructures of π-extended systems or
2
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10.1002/chem.202004846
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organic functional materials[7j,7k,9]. Gold catalysts represent mild carbophilic π-Lewis
acids,[10] we here for the first time use 1,8-dialkynyl naphthalenes with the parallel peri-
arrangement of the alknyes.
Previous work:
I
Ph
Radical
cyclization
Ar Ar
< 56%
Mes
I
Mes
Radical and cationic
cyclization
I
+
Ar = Mes
66%
45%
8%
This work:
cat.[Au]
R
R = Ph
Au
DAC PF6
H
Dual Activation
R = H
Figure 1. Known iodocyclization of 1,8-(bisalkynyl)naphthalene by a tandem
radical/cationic pathway and the new gold-catalyzed annulation reaction.
3
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Results and Discussion
The 1,8-diyne 1a was chosen as a model substrate (Table 1). 5 mol% of various gold
catalysts with sterically demanding ligands were examined in CDCl3. Even at room
temperature the conversions were fast (1 h, entries 1-5). With the IPr ligand the product
2a was obtained in 94% yield (entry 1).[11] With the triphenylphosphine ligand no
improvement in the yield was achieved, but the yield was slightly higher than in the case
of the electron-poor phosphine ligand (CH2CF3)3P (94% versus 90%) (entries 2 and 3).
Despite a complete conversion of all starting material, the SPhos-ligated cationic gold
catalyst gave a lower yield (88%, entry 4) In C6D6, the reaction was completed within 15
min to afford 2a in 96% yield (entry 5). (IPr)AuCl gave poor results even after prolonged
reaction times (4 h), (entry 6), and (IPr*)AuCl/AgSbF6 delivered 2a in 90% yield (entry 7).
With a silver(I) catalyst only low yields of 2a were obtained (entry 8).
Table 1. Optimization of the reaction conditions[a]
4
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MeO
OMe
MeO
[Au+] cat. (5 mol%)
CDCl3, rt
MeO
1a
Entrya
Catalysts
Time
Yield [%]d
1
(IPr)Au(NCMe)SbF6
Ph3PAuNTf2
94
1 h
1 h
2
94
3b
90e
(CH2CF3)3PAuCl/NTf2
(SPhos)AuCl/AgNTf2
(IPr)Au(NCMe)SbF6
(IPr)AuCl
2.5 h
1 h
4
88
5c
6
96
15 min
4 h
10 (89)f
90e
(IPr*)AuCl/AgSbF6
7
1 h
48 (36)f
8
AgNTf2
5 h
[a] Reactions were run on 0.01 mmol scale in CDCl3, and were afterwards directly analyzed by 1H NMR spectroscopy.
[b] 60 °C. [c] In d6-benzene. [d] Yield was determined by 1H NMR using hexamethylbenzene as the internal standard.
[e] Isolated yield. [f] Recovery of 1a.
SbF6
Ph
Ph
CF3
CF3
Ph
Ph
N
N
N
N
CF3
P
Me
Me
Au
N
Ph
Ph
Au
Cl
[(IPr*)AuCl]
Au
Cl
Ph
Ph
[(IPr)Au(NCMe)]SbF6
(CH2CF3)3PAuCl
Then we explored the reaction scope under the optimized conditions from Table 1, entry
1. A series of diynes with phenyl groups with various substituents at the aromatic system
were synthesized, all diynes provided the corresponding products (2a–2k, Table 2).
Electron-donating substituents (Me and OMe) at para- and ortho-positions of phenyl ring
gave yields between 89-95% (2a-2d), while for arene 2e the yield dropped significantly
(73%). The structure of 2e at the solid state was confirmed by the single-crystal X-ray
structural analysis.[12] Also substrates with fluorine substituents were smoothly converted
5
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to the corresponding phenalenes 2i and 2j in 89% and 84% yield, but for 2j heating to 50
°C for 48 h was necessary. The mesitylene-substituted substrate 1g was also converted
to the corresponding phenalene 2g in 94% yield via the carbocation rearrangement.
Amazingly, product 2h was directly obtained from the general procedure for the
Sonogashira-coupling to form 1h, no gold catalyst was needed, the palladium did the
conversion; for none of the other substrates this was observed. This probably is induced
by the significantly more electron-rich thiophene ring, which allows the palladium species
present during the cross coupling to interact with the alkyne units more strongly -- unlike
in the other substrates, which are dependent on the gold catalyst. Acenaphthene
substrates with different substituents (Me and F) at para-position on the phenyl groups
were successfully converted to the corresponding phenalene 2i and 2j in 89% and 84%
yield. The reaction of 5,6-diethynylacenaphthelene 1k didn’t give the corresponding
phenalene at all.
Table 2: Scope of the reaction.
6
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R
R
R
R
(IPr)Au(NCMe)SbF6 (5 mol%)
CHCl3, r.t., 1 h
1a-k
2a-k
tBu
Me
MeO
OMe
OMe
tBu
Me
MeO
2a: 89%
2b: 94%
2c: 95%
2d: 92%
F
F
molecular structure
2e: 73%
of 2e in the solid state
2f: 89%
2g: 94%
Me
F
Me
S
Me
F
Me
S
2h:[a] 95%
2i: 89%, 96 h[b]
2j: 84%, 48 h[b]
2k:
product could not
be isolated
Reaction conditions: 1a (0.05 mmol), catalyst (5 mol%), solvent (CHCl3, 1 mL), r.t.; [a]
product 2h was obtained directly from the palladium-catalyzed coupling which should
have provided 1h; [b] 50 °C).
7
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Then we tested the reactivity of the unsymmetrical diyne 1l to investigate the effect of the
mesitylethynyl and 2-benzo[b]thiopheneethynyl substituents (Table 3). First different
steric hindered ligands were examined. The reaction of 1l under the standard conditions
(entry 1) through 5 mol% of (IPr)Au(NCMe)SbF6 in 1,2-dichlororethane (DCE), resulting
in azuleno[1,2-a]acenaphthylene 3I (36%), phenalene 4I (18%) as a major products due
to a 5-exo pathway and a small amount of [b]fluorantheno[8,9-d]thiophene 2I (36%) due
to a 6-endo pathway gold catalyst annulation (Scheme 2). The structures of 2l and 4l at
the solid state were unambiguously confirmed by X-ray structural analysis.[12] The more
sterically hindered gold catalyst (IPr*)AuCl/AgNTf2 dramatically diminished the formation
of the phenalene 2I and improved regioselectivity of products 3I and 4I (entry 2). Switching
-
-
the counter ion from NTf2 to SbF6 were able to efficiently produce the compound 4I with
less of 3I, but the product 2l was not observed at all (entry 3). Changing the solvent to
benzene and toluene improved regioselectivity towards 3l (entries 4 and 5). Interestingly,
when the reaction was conducted with electron poor ligands, P(CH2CF3)3 and P(C6F5)3,
azuleno[1,2-a]acenaphthylene 3I was obtained as a major product (entries 6 and 9).
Further screening showed that in the present of the bulky phosphine ligand tBuXPhos the
reaction proceeded faster and three isomers were obtained in 89% yield in a ratioof 2:2:1
(entries 7). In contrast, neither (IPr)AuCl, nor AgSbF6 alone promoted the desired
reaction, showing that a cationic gold species is necessary for the annulation (entries 9
and 10).
Table 3. Optimization of the reaction conditions for diyne 1k[a]
8
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S
S
S
Cat. (5 mol%)
Solvent, r.t., 50 °C
S
1l
2l
3l
Entrya
Cat.
Time [h]
Solvent
Ratiob 2I:3I:4I
Yield [%]c
1
(IPr)Au(NCMe)SbF6
(IPr*)AuCl/AgNTf2
(IPr*)AuCl/AgSbF6
(IPr*)AuCl/AgSbF6
(IPr*)AuCl/AgSbF6
(CH2CF3)3PAuCl/AgSbF6
(tBuXPhos)AuCl/AgSbF6
(IPr)AuCl/AgSbF6
[(C6F5)3PAuCl]SbF6
AgSbF6
DCE
DCE
DCE
PhH
2/2/1
90%
92%
94%
70%
70%
95%
89%
88%
73%
ND
2.5
5
2
3.4/1/1
3.5/1/0
3/1/0
3
5
4
3
5
Toluene
DCE
3/1/1
4
6
1/3/0.5
1/2/2
3
7
DCE
1.5
3
8
DCE
1/2/2
9d
10
11
DCE
1/2.5/0.5
ND
4
DCE
5
(IPr)AuCl
DCE
ND
ND
5
[a] General reaction conditions: 0.05 mmol 1l, 5 mol% Au catalyst, 5 mol% Ag catalyst, solvent (1 mL). [b] Ratio of 2I:3I:4I were
determined by 1H NMR analysis of the crude product. [c] Yield of the mixture. [d] 10 mol% [(C6F5)3PAuCl]SbF6 was used at 50 °C.
2l
4l
Figure 2. Solid state molecular structures of compounds 2l and 4l.
Recently, Zhang’s group[3a] and our group[3b-f,4a] showed that gold vinylidenes, accessible
from an internal and a terminal alkyne, are highly reactive intermediates and can
efficiently insert into C(sp2)-H bonds or C(sp3)-H bonds. Thus, we next tested our
naphtalene-linked diyne systems (1m, 1n) with a dual activation gold catalysts
(TDAC[PF6]) in DCE at 90 °C. These were efficiently converted into the targets 5m and
5n in 89 and 97% yields, respectively (Scheme 2). On the other hand, for yet unknown
9
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reasons no conversions were observed for the electron-rich substituents (R2 = OMe, tBu)
1o and 1p (Scheme 2).
R2
R2
-
+
R1
PF6
IPrAu
R1 = H, R2 = H, 5m, 89%
R1 = H, R2 = F, 5n, 97%
IPrAu
(5 mol%)
R1 = H, R2 = OCH3, 5o, NR
R1 = H, R2 = C(CH3)3, 5p, NR
R1 = CH2Ph, R2 = H, NR
DCE, 90 °C, 3 h
1m-p
dual
activation
+
-[Au]
R2
R2
R2
+
+
H
[Au]
vinylidine
pathway
C-H
insertion
[Au]
[Au]
[Au]
[Au]
[Au]
I
III
II
Scheme 2. Synthesis of 5m and 5n by dual gold-catalyzed cascade transformations.
Next, the bi-directional gold-catalyzed conversion of the 1,4,5,8-tetra-
(phenylethynyl)naphthalenes 6a and 6b was explored (Scheme 3). First we tested the
standard reaction conditions, when the solution of the tetrayne 6a in DCE was treated
with (IPr)Au(NCME)SbF6 (10 mol%) at 50 °C, the two dark purple isomers 7a and 8a were
isolated in 32% and 17% yield (Scheme 5). The lack of symmetry of the compound visible
from the 1H NMR spectra of these products indicated that neither of them are the expected
symmetrical products. Interestingly, an unexpected mode of intramolecular cyclization
was observed. The expected 6-endo-dig-cyclization proceeded in one side to construct a
phenalene moiety, but the unexpected 5-exo-dig-cyclization in the other side occurred to
10
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form a fluoranthane moiety. Regioisomers 7 and 8 were potentially produced via vinyl
cation intermediates Va and Vb (Scheme 3). The connectivity of 8a was unambiguously
confirmed by single-crystal X-ray diffraction analysis.[12]
Scheme 3. Gold-catalyzed conversions of the substrates 6a and 6b.
In the case of 1,4,5,8-tetrakis-(phenylethynyl)naphthalene 6a and 6b, additional mode of
distortion to reduce steric repulsion is effect to obtain the unsymmetrical products.
According the X-ray structure of phenylethynylnaphthalene derivatives,[8a] while the
structure of diene 1 is involved the expanding mode of distortion in the naphthalene core,
in the case of tetraiyne 6, both the twisting and expanding modes are observed together
with bending of phenylethynyl units, in particular the carbons attached directly to the
naphthalene core move away from each other compare to 1.[13]
11
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We used density functional theory (DFT) calculations at the SMD/M06/def2-
TZVP//SMD/B3LYP-D3/LANL2DZ,6-31G(d) level of theory in DCE to understand the
mechanistic details of the bi-directional gold-catalyzed reaction by considering 6c as a
model for 6b (Figure 3). This combination of methods and basis sets has been recently
confirmed by us to provide results having a better consistency with the experimental
data.[14] Accordingly, the reaction commences with coordination of the gold complex to
one of the alkyne moieties[15] of 6c in an endergonic fashion with ∆G = 3.5 kcal/mol. The
resultant intermediate VI is a branching point for two routes, A 6-endo-dig-cyclization via
TSVI-a and a 5-exo-dig-cyclization via TSVI-b. The calculations indicate that TSVI-a is
lower in energy than TSVI-b, implying that in agreement with the experimental finding, the
cyclization prefers the 6-endo-cycliztion mode. Once the first cyclization has occurred,
intermediate VII is formed by another cyclization via transition structure TSVII, furnishing
intermediate VIII. Since TSVII lies much lower in energy than TSVI-a and intermediate VIII
is highly stable with a relative free energy of -17.8, the transformation VI VII  VIII is
not reversible and thus the regioselectivity of the cyclization is determined by the energy
difference between TSVI-a and TSVI-b. Subsequently, the organic molecule IX is produced
with ∆G = -66.1 kcal/mol followed by a series of chemical steps from intermediate VII, as
shown in Figure 3.
The organic molecule IX has two different sites for coordination of the gold complex. If
the gold complex coordinates to IX to give X, the ensuing intermediate then is a bifurcation
point for two processes, 6-endo-dig-cyclization via TSX-a and 5-exo-dig-cyclization via
TSX-b. In line with the experimental observations, the calculations support favourability of
the 5-exo-dig-cyclization, as evidenced by the finding that TSX-b is 3.9 kcal/mol lower in
energy than TSX-a. The same is true if we assume that the cyclization occurs from the
other π-complex i.e. intermediate XI; similarly, for this case, transition structure of the 5-
exo-dig-cyclization (TSXI-b) lies 2.1 kcal/mol below that of the 6-endo-dig-cyclization
(TSXI-a), a result which agrees with the regioselectivity observed experimentally. Finally,
we want to note that due to the closeness of the relative free energies of TSX-b and TSXI-
b, both the syn- and anti-products are predicted computationally to be produced with a
nearly equal amount. This prediction is fully consistent with our experimental
observations, the results of which are given in Scheme 3.
12
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Ar
Ar
Ar
Ar
6c; Ar = 4-Me-C6H4
6c
0.0
(IPr)Au(NCMe)+
(IPr)Au+
MeCN
Au(IPr)
Au(IPr)
TSVIi-b
TSVI-a
VI
3.5
20.2
21.8
Au(IPr)
Au(IPr)
Au(IPr)
H
TSVII
11.0
VII
VIII
-17.8
3.1
(IPr)Au+
deprotonation
protodeauration
TSXa
(IPr)Au+
-41.5
(IPr)Au+
MeCN
MeCN
(IPr)Au(NCMe)+
(IPr)Au(NCMe)+
MeCN
X
-65.1
syn-product
TSX-b
-140.7
-45.4
IX
-66.1
(IPr)Au(NCMe)+
MeCN
(IPr)Au+
(IPr)Au(NCMe)+
anti-product
MeCN
TSXI-b
-140.4
-45.1
(IPr)Au+
XI
-66.5
(IPr)Au+
TSXI-a
-43.0
13
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Figure 3. Calculated mechanism for cyclization of 6c catalysed by [(IPr)AuNCMe]+. The
relative free energies (in red) are given in kcal/mol.
The optical properties of phenalenes 2a-2k were examined by UV–Vis absorption
spectroscopy in dichloromethane (Figure 3). In view of maximum absorption wavelengths
of 2a, 2f and 2l, they show strong absorption at long wavelength (about 460 nm).
Comparison with phenalenes structures, azulenoacenaphthylene 3l exhibits the distinct
broad peak between 500~750 nm. The
π-extention has a significant effect on the
absorption of 7a and 8a (λ= 510 and 550 nm). The HOMO and LUMO levels of 2a, 2f, 7a,
and 8a were estimated by cyclic voltammetry (Table 4). The HOMO energy level of 2a
containing electron donor group (-5.33 eV) was higher than 2f containing electron
acceptor (-5.47 eV). Based on the compounds 7a and 8a, π-extention with fluoranthane
provided lower LUMO levels and smaller HOMO-LUMO energy gaps.
Figure 2. UV absorption of 2a, 2f, 4l, 7a and 8a.
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Table 4. Cyclic voltammetry data and estimated HOMO and LUMO energies[a]
[b]
[b]
[c]
Eox
Ered
[V]
EHOMO
[eV]
ELUMO
[eV]
Egap
[V]
[eV]
2a 0.53
-1.93
-1.74
-1.20
-1.16
-5.33
-5.47
-5.21
-5.34
-2.87
-3.06
-3.60
-3.64
2.46
2f 0.67
7a 0.41
8a 0.54
2.41
1.61
1.70
n
[a] Cyclic voltammetry in CH2Cl2 containing 0.1 M Bu4NPF6. [b] HOMO and LUMO energy levels in eV
were approximated using the equation HOMO = -(4.80 + Eox), LUMO = -(4.80 + Ered). [c] Egap = LUMO-
HOMO.
In conclusion, an efficient gold-catalyzed annulation protocol through gold–vinyl or –
vinylidene intermediates via a selective shift towards a 6-endo-dig-cyclization mode for
the indeno[2,1-a]phenalenes synthesis was developed. In addition, the bi-directional
gold-catalyzed annulation of tetraynes provided even larger conjugated -systems, the
interesting different mode of the first and the second cyclization, leading to unsymmetrical
products, was explored by a computational study.
Acknowledgements
The author S. Tavakkoli Fard is grateful for Ph.D funding from the Hans-Böckler-Stiftung.
We gratefully acknowledge the generous allocation of computing time from the Australian
National Computational Infrastructure and University of Tasmania, and the Australian
Research Council (grant number DP180100904) for financial support.
References
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7936; Angew. Chem. 2006, 118, 8064–8105; c) A. Fürstner, P. W. Davies, Angew. Chem.
15
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Chemistry - A European Journal
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TOC graphic
The 6-endo-cyclization of the first diyne unit (blue) with two alkynes in peri-position
induces a switch to a 5-exo-cyclization for the second peri-diyne unit (red) in the fast
modular synthesis of new extended -systems.
20
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