Gold-Catalyzed Regiospecific Annulation of Unsymmetrically Substituted 1,5-Diynes for the Precise Synthesis of Bispentalenes

Article abstract of DOI:10.1002/chem.201902381

Precise control of the selectivity in organic synthesis is important to access the desired molecules. We demonstrate a regiospecific annulation of unsymmetrically substituted 1,2-di(arylethynyl)benzene derivatives for a geometry-controlled synthesis of linear bispentalenes, which is one of the promising structures for material science. A gold-catalyzed annulation of unsymmetrically substituted 1,2-di(arylethynyl)benzene could produce two isomeric pentalenes, but both electronic and steric effects on the aromatics at the terminal position of the alkyne prove to be crucial for the selectivity; especially a regiospecific annulation was achieved with sterically blocked substituents; namely, 2,4,6-trimetyl benzene or 2,4-dimethyl benzene. This approach enables the geometrically controlled synthesis of linear bispentalenes from 1,2,4,5-tetraethynylbenzene or 2,3,6,7-tetraethynylnaphthalene. Moreover, the annulation of a series of tetraynes with a different substitution pattern regioselectively provided the bispentalene scaffolds. A computational study revealed that this is the result of a kinetic control induced by the bulky NHC ligands.

Full text of DOI:10.1002/chem.201902381

A Journal of
Accepted Article
Title: Gold-catalyzed Regiospecific Annulation of Asymmetric 1,5-Diyne
for Precise Synthesis of Bispentalenes
Authors: Sara Tavakkolifard, Kohei Sekine, Lisa Reichert, Mina
Ebrahimi, Ketevan Museridz, Elena Michel, Frank Rominger,
Rasool Babaahmadi, Alireza Ariafard, Brian Yates, Matthias
Rudolph, and A. Stephen K. Hashmi
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.201902381
Link to VoR: http://dx.doi.org/10.1002/chem.201902381
Supported by

10.1002/chem.201902381
Chemistry - A European Journal
Gold-catalyzed Regiospecific Annulation of Unsymmetrically Substituted
1,5-Diyne for Precise Synthesis of Bispentalenes
Sara Tavakkolifard,[a],+ Kohei Sekine,[a],+ Lisa Reichert,[a] Mina Ebrahimi,[a]
Ketevan Museridz,[a] Elena Michel,[a] Frank Rominger,[a] Rasool Babaahmadi,[b]
Alireza Ariafard,*,[b] Brian Yates,*,[b] Matthias Rudolph,[a] and A. Stephen K.
Hashmi*,[a,c]
[a] M.Sc. S. Tavakkolifard, Dr. K. Sekine, B.Sc. L. Reichert, B.Sc. M. Ebrahimi,
B.Sc. K. Museridz, B.Sc. E. Michel, Dr. F. Rominger, Dr. M. Rudolph, and Prof.
Dr. A. S. K. Hashmi
Organisch-Chemisches Institut, Heidelberg University, Im Neuenheimer Feld 270,
69120 Heidelberg, Germany
[b] R. Babaahmadi, Prof. A. Ariafard, Prof. B. Yates
School of Physical Sciences (Chemistry)
University of Tasmania
Private Bag 75, Hobart TAS 7001 (Australia)
[c] Prof. Dr. A. S. K. Hashmi
Chemistry Department, Faculty of Science
King Abdulaziz University, Jeddah 21589, Saudi Arabia
+ Equal contribution
Keywords: gold catalysis, pentalene, polycyclic aromatic hydrocarbons,
regiospecific
Abstract
Precise control of the selectivity in organic synthesis is important to access the
desired molecules. We demonstrate
a
regiospecific annulation of
unsymmetrically substituted 1,2-di(arylethynyl)benzene derivatives for a
geometry controlled synthesis of a linear bispentalene which is one of the
promising structures for material science. A gold-catalyzed annulation of
unsymmetrically substituted 1,2-di(arylethynyl)benzene possibly produces two
isomers of pentalenes, but both electronic and steric effects on the aromatics at
a terminal position of alkyne prove to be crucial for the selectivity; especially a
regiospecific annulation was achieved with sterically blocked substituents, 2,4,6-
trimetyl benzene or 2,4-dimethyl benzene. This approach enables the
1 / 17
This article is protected by copyright. All rights reserved.

10.1002/chem.201902381
Chemistry - A European Journal
geometrically controlled synthesis of linear bispentalenes from 1,2,4,5-
tetraethynylbenzene or 2,3,6,7-tetraethynylnaphthalene. Moreover, the
annulation of a series of tetraynes with a different substitution pattern
regioselectively provided bispentalene scaffolds. A computational study revealed
that this is the result of a kinetic control induced by bulky NHC ligands.
Introduction
Antiaromatic molecules have received much attention due to unique
optoelectronic properties. Pentalene as well as the structurally similar
indenofluorene[1] is one of the important core structures for material science. Due
to the low stability of the pure pentalene core,[2] various syntheses of
dibenzo[a,e]pentalenes, which are stabilized by the fused aromatic moieties,
have been developed.[3] The properties of pentalene are of high interest for
organic semiconductors. Especially, -extended pentalenes prove to act as - or
n-type organic transistors, though tuning optoelectronic properties by the
introduction of functional groups are still desirable for higher performance.[4]
Therefore, the development of modular synthetic methods to access polycyclic -
extended pentalenes is a promising research topic.
Homogeneous gold catalysis has received much attention, and due to the mild
carbophilic  Lewis acidity of the gold catalyst, the field majorly contributed to the
intensive development of nucleophilic addition reactions to unsaturated carbon-
carbon multiple bonds.[5] For instance, the gold-catalyzed annulation of diyne
compounds enabled the synthesis of extended -conjugated compounds, such
as azahelicene, polycyclic indole and benzothiophen derivatives.[6] Our recent
work also contributed to the gold-catalyzed synthesis of dibenzo[a,e]pentalene
and quinoxaline/phenazine-fuzed pentalene derivatives from 1,5-diynes.[3n,t]
Moreover, the gold catalyst leads to both U-shaped and S-shaped bispentalenes
from the readily available tetra(arylethynyl)benzenes and -naphthalenes, but
lucky enough could be separated (Scheme 1, top).[4e] During that study, the
naphthalene-based linear bispentalene (S-shaped) turned out to be a highly
suitable scaffold for transistor applications. In these symmetric tetraynes,
however, the chemoselectivity towards U-shaped and S-shaped bispentalenes is
not controlled. Unsymmetrically substituted diynes or tetraynes as substrates
could be a method to selectively synthesize specific isomers, still it is unknown
whether the selectivity will be pronounced enough to deliver only one isomer
(Scheme 1, bottom). From the synthetic point of view, it would be interesting to
see if the reaction of unsymmetrically substituted 1,5-diynes can be controlled by
2 / 17
This article is protected by copyright. All rights reserved.

10.1002/chem.201902381
Chemistry - A European Journal
electronic or steric effects, which would allow to control the precise
regiochemistry and thus geometry in the synthesis of pentalene derivatives.
Herein, we report a study of selectivity control in the gold-catalyzed annulation of
unsymmetrically substituted 1,5-diynes, and the use of these principles in the
regiospecific synthesis of S-shaped benzene- and naphthalene-based
bispentalenes by an inside-out bidirectional approach. In addition, we
investigated an outside-in mode of cyclization, tetraynes with different
substitution patterns successfully provided bispentalenes, which with respect to
the total synthetic rout represents a much more convergent and thus flexible
approach. The opto-electronic properties of the obtained bispentalene derivatives
are also reported.
Scheme 1. Possible selectivity control by different aryl groups in the gold-
catalyzed annulation of unsymmetrically substituted 1,5-diynes.
Results and Discussion
During the annulation of a symmetric 1,5-diyne, one alkyne acts as a nucleophile
and the other alkyne as the electrophile. If electron-rich and electron-deficient
alkynes are arranged in an unsymmetrical substrate, the annulation could
proceed regiospecifically. We first prepared the unsymmetrically substituted
diynes 1 with methoxy- (1a), fluoro- (1b), and trifluoromethyl-substituents (1c),
and attempted the gold-catalyzed reactions with them (Table 1). The annulation
of methoxy-substituted diyne gave a mixture of isomers 2a and 3a in 40% yield
with a ratio of 4:96 (Entry 1). The fluoro-substituted diyne 1b afforded the isomers
2b and 3b in 62% yield in a ratio of 58:42 (Entry 2). When a trifluoromethyl group
3 / 17
This article is protected by copyright. All rights reserved.

10.1002/chem.201902381
Chemistry - A European Journal
(1c), which is a strong electron-withdrawing group, was attached to aromatic ring,
the ratio of 2c and 3c was 6:94 (Entry 3). Overall, the results demonstrated the
selectivity can be controlled by the electronic properties and that an electron-rich
alkyne as nucleophile and electron deficient alkyne as electrophile, even though
the yield lower and the isomers are inseparable.
Table 1. Electronic effect on the selectivity.
To explore another mode of substituent control of the selectivity, two o-positions
of the aryl group on the alkyne were blocked by methyl groups, which probably
inhibits the approach of gold catalysts to one of the two alkynes, even though the
cyclization of vinyl cation and mesitylene, and 1,2-methyl transfer possibly
proceed to give a pentalene derivative. The gold catalysts promoted the reaction
of the diyne 1d and afforded the desired pentalene 2d in excellent yield, 94%
were obtained (Entry 1, Table 2). Methoxy- or trifluoromethyl-substituted diynes
1e and 1f were also converted to the pentalenes 2e and 2f in 63% and 41% yields,
respectively (Entries 2 and 3). The annulation of the bromo-substituted diyne 1g
gave the 60% yield of the pentalene 2g, which is useful for further transformations
by common coupling reactions (Entry 4). The mesityl group indeed enables a
control of the pentalene synthesis. It is interesting to note that the reaction of the
2,4-dimethylbenzene-substituted diyne 1h resulted in the clean formation of the
corresponding pentalene 2h in 91% yield and no generation of the pentalene 3h
(Entry 5). As shown in Scheme 3, the gold-catalysed reaction of diyne 1h possibly
4 / 17
This article is protected by copyright. All rights reserved.

10.1002/chem.201902381
Chemistry - A European Journal
leads to two intermediates Ia and Ib. Vinylcation intermediate Ib might be
unfavorable because the cyclization of Ib, which should proceed via planar
configuration is probably prohibited by the steric hindrance between the gold
catalyst and o-methyl substituent of aromatic moiety. This results in the selective
formation of 2h. This step could be crucial for controlling the reaction of the
mesitylene-substituted diyne 1d. Overall, those results indicated that
regiospecific annulation was achieved by the introduction of a mesityl group or
2,4-dimethylbenzene.
Table 2. Reaction of mesitylene-substituted diynes.
5 / 17
This article is protected by copyright. All rights reserved.

10.1002/chem.201902381
Chemistry - A European Journal
Scheme 2. Gold-catalyzed reaction to form pentalene 2h
Based on the results with the diynes being blocked at o-positions, the selective
synthesis of S-shape bispentalenes should be feasible. We then designed and
synthesized the benzene- and naphthalene-based tetraynes 4a and 4b with
mesitylene (Scheme 2). The tetraynes 4a and 4b were conveniently prepared by
sequential Sonogashira-coupling reactions of 1,4-dibromo-2,5-diiodobenzene or
3,7-dibromonaphthalene-2,6-diyl-bis(trifluoromethanesulfonate).
Using
(IPr)AuCl/AgNTf2, the benzene-based tetrayne 4a was completely consumed
within 2 h, the corresponding linear bispentalene 5a was obtained in 86% yield
as a reddish brown solid. In addition, the annulation of naphthalene-based
tetrayne 4b with (IPr)AuCl/AgNTf2 proceeded at room temperature and gave the
86% yield of the linear bispentalene 5b as a red solid.
6 / 17
This article is protected by copyright. All rights reserved.

10.1002/chem.201902381
Chemistry - A European Journal
Scheme 3. Gold-catalyzed annulation of 1,2,4,5-tetra(ethynyl)benzene 4a (top)
and 2,3,6,7-tetra(ethynyl)naphthalene 4b (bottom).
The connectivity of 5b at a solid state was confirmed by X-ray crystallography
(Figure 1). Due to the mesityl group, the previously reported n-pentyl substituted
S-shape bispentalene B (Figure 2)[4e] shows significantly smaller torsion angles,
(34.2°–37.9°) between the pentalene core and the peripheral aryl group than
pentalene 5b (63.5°–69.3°).
Figure 1. Solid state molecular structure of 5b.
Figure 2. Previously reported bispentalenes A and B.
We considered the teterayne 6, having a different substitution pattern, as another
approach for forming a bispentalene scaffold. It potentially affords two isomers, 7
and 8, as the second annulation can occur on both carbon atoms of the
7 / 17
This article is protected by copyright. All rights reserved.

10.1002/chem.201902381
Chemistry - A European Journal
intermediate II-a (Scheme 4). Indeed, the intramolecular annulation with 5 mol %
of the gold catalysts in dichloroethane afforded the pentalenes 7a and 8a in 20%
and 72% yields, respectively. A single crystal of 8a was obtained and its structure
was unambiguously confirmed by X-ray crystal structure analysis (Figure 3).
Interestingly, the pentalene core and the peripheral mesityl group are nearly
vertical and the two mesityl groups are parallel.
Scheme 4. Gold-catalyzed reaction to form bispentalenes 7a and 8a.
Figure 3. Solid-state molecular structure of 8a.
At the above experiment with (IPr)AuCl/AgNTf2, isomers 7a and 8a produced in
the ratio of 22:78. In the previous report,[4e] the bulkiness of the ligand had a
significant effect on the ratio of the bispentalene isomers. Therefore, we now
explored a set of differently sized ligands on the catalysts in order to prove this
effect on the selectivity of isomeric bispentalenes 7a and 8a. We applied pre-
activated [IPrAu(NCMe)]SbF6 and [IPrAu]NTf2 catalysts,[7] which gave the same
results as (IPr)AuCl/AgNTf2 (entries 2 and 6). On the other hand, the PPh3 ligand
only gave poor yields of the products 7a and 8a (entry 3). No reaction took place
with a nitrogen acyclic carbene (NAC) complex (entry 4). As expected, no
8 / 17
This article is protected by copyright. All rights reserved.

10.1002/chem.201902381
Chemistry - A European Journal
reaction occurred in the complete absence of a gold catalyst, using only AgNTf2
(entry 7). The short ligand screening finally revealed that the sterically bulky IPr*
ligand[8] increases the ratio of pentalen 8a (entry 5) (Table 3). This can be
rationalized by the steric hindrance between the mesitylen substituent and the
gold complex on the intermediate IIa, which ultimately leads to the formation of
the products 8a.
Table 3. Examination of different ligands on the gold catalysts with substrate 6a.
We investigated the selective formation of 8a by using M06-2X-
CPCM/BS2//B3LYP-CPCM/BS1 calculations and found that the selectivity is
mainly controlled by a steric repulsion between the mesityl substituent and the
IPr ligand in TS-1b (Figure 3). This destabilizing interaction is absent in TS-1a,
causing this transition structure to be 0.3 kcal/mol lower in energy than in TS-1b.
To support this assertion, we replaced the IPr carbene ligand by the IMe carbene
ligand, in which the bulky N-substituents of IPr are replaced by methyl groups.
9 / 17
This article is protected by copyright. All rights reserved.

10.1002/chem.201902381
Chemistry - A European Journal
The calculation results starting from this new model system is shown in Figure 4.
In contrast to the real system in which TS-1a is calculated to be slightly lower in
energy than TS-1b, for the less bulky model system, the energy order of the
transition structures becomes reversed. In this case, TS-1b-M is calculated to be
2.0 kcal/mol lower in energy than TS-1a-M (Figure 4). The energy order of
transition structures TS-1b-M and TS-1a-M is most likely set by the
thermodynamic aspects of the transformations. Indeed, 7a is about 6 kcal/mol
more stable than 8a, resulting in TS-1b-M lying lower in energy than TS-1a-M. It
follows that although formation of 7a is thermodynamically favoured over 8a, the
steric interaction between IPr ligand and the Mes substituent in TS1-b leads to
less 7a than 8a being formed.
Mes
Mes
first cyclization
(siminlar to the second one)
Mes
(IPr)Au(NCMe)SbF6
(5 mol %)
AuL
DCE, 50 °C
Int1
selectivity determining step
here the second cyclization starts
6a
-55.9 kcal/mol
-57.6 kcal/mol
0.0 kcal/mol
Mes
H
Mes
Mes
H
Mes
AuL
TS-1b
-35.5
AuL
TS-1a
-35.8
Mes
H
Mes
Mes
H
20.4
20.1
AuL
int-2a
Mes
L = IPr
AuL
-55.0
-55.9
int-2b
-61.2
AuL
Mes
Mes
AuL
-85.4
Mes
-91.8
Mes
Mes
AuL
int1
8a
71% yield
M062X
Mes
7a
21% yield
Figure 3. The selectivity-determining step with the IPr carbene ligand (energies
in kcal/mol).
Figure 3 shows quite similar energy values of TS-1a and TS-1b with former being
lower in energy by 0.3 kcal/mol. This is consistent with the experimental 22:78
10 / 17
This article is protected by copyright. All rights reserved.

10.1002/chem.201902381
Chemistry - A European Journal
ratio of the two products. Figure 4 shows a higher difference in energy for TS-1a-
M and TS-1b-M with a reverse oder, now with TS-1b-M being lower. Since the
repulsive interaction of the two aryl substituents in TS-1a and TS-1a-M is almost
identical, this clearly indicates that the ligand-aryl steric interaction in TS-1b is
stronger than that in TS-1b-M.
Figure 4. The selectivity-determining step with the smaller IMe carbene ligand
(energies in kcal/mol).
Next, we investigated the several electron-withdrawing and electron-donating
substituents on the aromatic moieties with (IPr)Au(NCMe)SbF6. In the case of
tetrayne 6a, the gold-catalyzed reaction afforded the bispentalenes 7a and 8a in
a ratio of 22:78 (Entry 1). The fluoro-substituted substrate 6b led to separable
bispentalene isomers 7b and 8b with a ratio of 35:65 in 25% and 55% yield,
respectively (entry2). Tetrayne 6c with dimethyl substituents on outer aromatic
moieties gave rise in the high selectivity toward 8c (91:9) in 36% yield and 7c in
3% (Entry 3), though the combined yield significantly dropped. These results
indicate that the substituents on outer aromatics have an effect on the selectivity
of two isomers.
Table 4. Bispentalene derivatives
11 / 17
This article is protected by copyright. All rights reserved.

10.1002/chem.201902381
Chemistry - A European Journal
The optical properties of bispentalenes 5a, 5b, 7a and 7b were examined by
UV/Vis absorption spectroscopy in dichlromethane (Figure 6). Based on the
previous work,[4e] the two characterisitic absorptions (λ = 450-550 nm) of 5a, 7a,
and 7b might be assigned to HOMO → LUMO+1 and HOMO-1 → LUMO
transitions. Introduction of mesityl groups has significant effect on the absorption;
the absorption peaks (λmax= 474 and 502 nm) of mesithylene-substituted 5a were
red-shifted (30 nm) compared to S-shaped benzene-based bispentalenes A
(λmax= 496 and 532 nm). Similar tendency was observed between naphthalene-
based bispentalenes 5b (λmax = 490 and 525 nm) and B (λmax= 510 and 550 nm).
In addition, from the comparison of 5a (λmax= 474 and 502 nm) and 7a (λmax = 460
and 490 nm), different substitution position of mesityl groups caused significant
blue-shift.
12 / 17
This article is protected by copyright. All rights reserved.

10.1002/chem.201902381
Chemistry - A European Journal
Figure 6. UV absorption of 5a, 5b, 7a, and 7b.
The HOMO and LUMO levels of the series of bispentalenes 5a, 5b, 7a, and 7b
in CH2Cl2 solution were estimated by cyclic voltammetry (Table 5). Compared to
the previously synthesized benzen-based bispentalene A and naphthalene-
based bispentalene B (Figure 2), the HOMO-levels of 5a (-5.24 eV) and 5b (-5.50
eV) are lower than the HOMO-levels of the corresponding compounds A (-5.20
eV) and B (-5.38 eV). The LUMO-levels of 5a (-3.11 eV) and 5b (-2.93 eV) are
significantly higher than the LUMO-level of the corresponding compounds A (-
3.23 eV) and B (-3.09 eV), which resulted in the larger HOMO-LUMO energy gap
of 5a and 5b. Based on the solid state structure of 5b (Figure 1), the peripheral
mesitylene might less contribute to the core π-system, which could have an effect
on HOMO and LUMO energy levels. The HOMO and LUMO levels of the S-
shaped bispentalenes 7a (HOMO= -5.30 eV, LUMO = -3.18 eV) and 7b (HOMO
= -5.40 eV, LUMO = -3.25 eV) are lower compared to 5a. The HOMO-LUMO
gaps for 7a and 7b (Egap = 2.15 eV) are not significantly different from that of 5a
(Egap = 2.13 eV).
Table 5. Cyclic voltammetry data and estimated HOMO and LUMO energies
13 / 17
This article is protected by copyright. All rights reserved.

10.1002/chem.201902381
Chemistry - A European Journal
c
Eox1
[V]
Ered1
[V]
EHOMO ELUMO
Egap
[eV]
c
[eV]
[eV]
Aa
5aa
Ba
5ba
7ab
7bb
0.40
0.44
0.58
0.70
0.51
0.60
-1.57
-1.69
-1.71
-1.87
-1.65
-1.55
-5.20
-5.24
-5.38
-5.50
-5.30
-5.40
-3.23
-3.11
-3.09
-2.93
-3.15
-3.25
1.98
2.13
2.29
2.57
2.15
2.15
a Cyclic voltammetry in CH2Cl2 containing 0.1 M nBu4NPF6 with ferrocene on a Pt
working electrode, a Pt/Ti counter electrode, and a Ag reference electrode at a
scan rate of 0.2 Vs−1. All potentials are given versus the Fc+/Fc couple used as
an internal standard. b Electrochemical data obtained at a scan rate of 0.2 Vs−1
in CH2Cl2 containing 0.1 M nBu4NPF6 on a glassy carbon working electrode, a
Pt/Ti counter electrode, and Ag reference electrode. c HOMO and LUMO energy
levels in eV were approximated using the equation HOMO = -(4.80 + Eox), LUMO
= -(4.80 + Ered), Egap = LUMO-HOMO.
Conclusion
We demonstrated the regiospecific annulation of unsymmetrically substituted 1,2-
di(arylethynyl)benzene derivatives. Both electronic and steric effects on the
aromatics of substrates are crucial for the selectivity; especially introduction of
sterically blocked substituents, 2,4,6-trimetylbenzene or 2,4-dimethylbenzene,
enabled the regiospecific annulation. This method provided the geometrically
controlled synthesis of S-shaped bispentalenes from 1,2,4,5-tetraethynlbenzene
or 2,3,6,7-tetraethynlnaphthalene. Moreover, annulation of a series of tetraynes
with a different substitution pattern regioselectively provided bispentalenes. The
computational study showed that 7a is the thermodynamic product of the
reaction, 8a is the kinetic product formed with bulky NHC ligands (like IPr, but
better IPr*).
References
[1] C. K. Frederickson, B. D. Rose, M. M. Haley, Acc. Chem. Res. 2017, 50, 977.
[2] T. Bally, S. Chai, M. Neuenschwander, Z. Zhu, J. Am. Chem. Soc. 1997, 119,
1869.
[3] Review for the synthesis and optoelectronic properties of pentalenes, please
see a) M. Saito, Symmetry 2010, 2, 950. For recent publications about
14 / 17
This article is protected by copyright. All rights reserved.

10.1002/chem.201902381
Chemistry - A European Journal
monobenzopentalene, see b) P. Rivera-Fuentes, M. von Wantoch Rekowski, W.
B. Schweizer, J- P. Gisselbrecht, C. Boudon, F. Diederich, Org. Lett. 2012, 14,
4066; c) G. London, M. von Wantoch Rekowski, O. Dumele, W. B. Schweizer, J.-
P. Gisselbrecht, C. Boudon, F. Diederich, Chem. Sci. 2014, 5, 965; d) S. Kato, S.
Kuwako, N. Takahashi, T. Kijima, Y. Nakamura, J. Org. Chem. 2016, 81, 7700.
For dibenzopentalene, see e) K. Katsumoto, C. Kitamura, T. Kawase, Eur. J. Org.
Chem. 2011, 4885; f) M. Saito, Y. Hashimoto, T. Tajima, K. Ishimura, S. Nagase,
M. Minoura, Chem. Asian J. 2012, 7, 480; g) A. S. K. Hashmi, M. Wieteck, I.
Braun, P. Nosel, L. Jongbloed, M. Rudolph, F. Rominger, Adv. Synth. Catal.
2012, 354, 555; h) C. Chen, M. Harhausen, R. Liedtke, K. Bussmann, A.
Fukazawa, S. Yamaguchi, J. L. Petersen, C. G. Daniliuc, R. Frçhlich, G. Kehr, G.
Erker, Angew. Chem. Int. Ed. 2013, 52, 5992; Angew. Chem. 2013, 125, 6108; i)
T. Maekawa, Y. Segawa, K. Itami, Chem. Sci. 2013, 4, 2369; j) J. Zhao, K. Oniwa,
N. Asao, Y. Yamamoto, T. Jin, J. Am. Chem. Soc. 2013, 135, 10222; k) J. Shen,
D. Yuan, Y. Qiao, X. Shen, Z. Zhang, Y. Zhong, Y. Yi, X. Zhu, Org. Lett. 2014,
16, 4924; l) C. Chen, M. Harhausen, A. Fukazawa, S. Yamaguchi, R. Frçhlich, C.
G. Daniliuc, J. L. Petersen, G. Kehr, G. Erker, Chem. Asian J. 2014, 9, 1671; m)
B. Wei, H. Li, W.-X. Zhang, Z. Xi, Organometallics 2016, 35, 1458; n) T. Wurm,
J. Bucher, S. B. Duckworth, M. Rudolph, F. Rominger, A. S. K. Hashmi, Angew.
Chem. Int. Ed. 2017, 56, 3364; Angew. Chem. 2017, 129, 3413; o) K. Takahashi,
S. Ito, R. Shintani, K. Nozaki, Chem. Sci. 2017, 8, 101; p) H. Oshima, A.
Fukazawa, S. Yamaguchi, Angew. Chem. Int. Ed. 2017, 56, 3270; Angew. Chem.
2017, 129, 3318; q) A. Konishi, Y. Okada, M. Nakano, K. Sugisaki, K. Sato, T.
Takui, M. Yasuda, J. Am. Chem. Soc. 2017, 139, 15284; r) D. C. Grenz, M.
Schmidt, D. Kratzert, B. Esser, J. Org. Chem. 2018, 83, 656; s) T. Wurm, E. C.
Rediger, J. Schulmeister, S. Koser, M. Rudolph, F. Rominger, U. H. F. Bunz, A.
S. K. Hashmi, Chem. Eur. J. 2018, 24, 2735; t) K. Sekine, F. Stuck, J.
Schulmeister, T. Wurm, D. Zetschok, F. Rominger, M. Rudolph, A. S. K. Hashmi,
Chem. Eur. J. 2018, 24, 12515.
[4] 4p-type: (a) T. Kawase, T. Fujiwara, C. Kitamura, A. Konishi, Y. Hirano, K.
Matsumoto, H. Kurata, t. Kubo, S. Shinamura, H. Mori, E. Miyazaki K. Takimiya,
Angew. Chem. Int. Ed. 2010, 49, 7728; (b) C. Liu, X. Xu, W. Zhu, X. Zhu, W. Hu,
Z. Li Z. Wang, Chem. Eur. J. 2015, 21, 17016; (c) G. Dai, J. Chang, W. Zhang,
S. Bai, K.-W. Huang, J. Xu, C. Chi, Chem. Commun. 2015, 51, 503; (d) C. Li, C.
Liu, Y. Li, X. Zhu, A. Wang, Chem. Commun. 2015, 51, 693. (e) K. Sekine, J.
Schulmeister, F. Paulus, K. P. Goetz, F. Rominger, M. Rudolph, J. Zaumseil, A.
S. K. Hashmi, Chem. Eur. J. 2019, 25, 216; n-type: f) M. Nakano, I. Osaka, K.
Takimiya, T. Koganezawa, J. Mater. Chem. C 2014, 2, 64; g) M. Nakano, I.
Osaka, K. Takimiya, J. Mater. Chem. C 2015, 3, 283; h) G. Dai, J. Chang, L. Jing,
C. Chi, J. Mater. Chem. C. 2016, 4, 8758; i) D. C. Grenz, M. Schmidt, D. Kratzert,
B. Esser, J. Org. Chem. 2018, 83, 656.
[5] a) D. Pflästerer, A. S. K. Hashmi, Chem. Soc. Rev. 2016, 45, 1331; b) R.
Dorel, A. M. Echavarren, Chem. Rev. 2015, 115, 9028; c) L. Zhang, Acc. Chem.
Res. 2014, 47, 877; d) L. P. Liu, G. B. Hammond, Chem. Soc. Rev. 2012, 41,
3129; e) H. Ohno, Isr. J. Chem. 2013, 53, 869; f) A. Ferstner, Chem. Soc. Rev.
2009, 38, 3208; g) N. Bongers, N. Krause, Angew. Chem. Int. Ed. 2008, 47, 2178;
Angew. Chem. 2008, 120, 2208; h) A. Ferstner, P. W. Davies, Angew. Chem. Int.
Ed. 2007, 46, 3410; Angew. Chem. 2007, 119, 3478; i) A. S. K. Hashmi, G. J.
Hutchings, Angew. Chem. Int. Ed. 2006, 45, 7896; Angew. Chem. 2006, 118,
8064; j) A. S. K. Hashmi, Chem. Rev. 2007, 107, 3180; k) D. J. Gorin, F. D. Toste,
Nature 2007, 446, 395; l) for early results, see: A. S. K. Hashmi, L. Schwarz, J.-
H. Choi, Ta. M. Frost, Angew. Chem. Int. Ed. 2000, 39, 2285; Angew. Chem.
2000, 112, 2382; m) A. S. K. Hashmi, T. M. Frost, J. W. Bats, J. Am. Chem. Soc.
2000, 122, 11553.
15 / 17
This article is protected by copyright. All rights reserved.

10.1002/chem.201902381
Chemistry - A European Journal
[6] a) A. M. Asiri, A. S. K. Hashmi, Chem. Soc. Rev. 2016, 45, 4471; b) A. S. K.
Hashmi, Acc. Chem. Res. 2014, 47, 864; c) I. Braun, A. M. Asiri, A. S. K. Hashmi,
ACS Catal. 2013, 3, 1902.
[7] P. de Frémont, N. Marion, S. P. Nolan, J. Organomet. Chem. 2009, 694, 551.
[8] A. Collado, A. Gómez-Suárez, A. R. Martin, A. M. Z. Slawin, S. P. Nolan,
Chem. Commun. 2013, 49, 5541.
16 / 17
This article is protected by copyright. All rights reserved.

10.1002/chem.201902381
Chemistry - A European Journal
TOC
17 / 17
This article is protected by copyright. All rights reserved.