Silver-Catalysed Hydroarylation of Highly Substituted Styrenes

Article abstract of DOI:10.1002/anie.202016268

Hydroarylation is an effective strategy to rapidly increase the complexity of organic structures by transforming flat alkene moieties into three-dimensional frameworks. Many strategies have already been developed to achieve the hydroarylation of styrenes,

Full text of DOI:10.1002/anie.202016268

Angewandte
Communications
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How to cite: Angew. Chem. Int. Ed. 2021, 60, 8537–8541
International Edition: doi.org/10.1002/anie.202016268
German Edition: doi.org/10.1002/ange.202016268
Hydroarylation
Hot Paper
Silver-Catalysed Hydroarylation of Highly Substituted Styrenes
Toryn Dalton+, Steffen Greßies+, Mowpriya Das, Maximilian Niehues++, Malte L. Schrader++,
Christian Gutheil++, Bart Jan Ravoo, and Frank Glorius*
In memory of Dr. Steven Cooper
Abstract: Hydroarylation is an effective strategy to rapidly
increase the complexity of organic structures by transforming
flat alkene moieties into three-dimensional frameworks. Many
strategies have already been developed to achieve the hydro-
arylation of styrenes, however most of these reports examine
the hydroarylation of unpolar, b-mono- or b-unsubstituted
styrenes, while exploring mainly electron-rich benzene nucle-
ophiles. Herein, we report a mild and general catalytic system
for the selective hydroheteroarylation of multiply substituted
styrenes and heteroaromatic styrenes. Mechanistic analysis of
the reaction led to the discovery of commercially available
2,2’:5’,2’’-terthiophene as a key reagent.
S
tyrenes are important feedstock chemicals for a variety of
transformations. Many derivatives are cheap and commer-
cially available compounds, while more complex substitution
patterns are synthetically available by established chemistry.
Scheme 1. Hydroarylation of styrenes.
The styrene C C double bond is a versatile functional handle
and has been used extensively as an electrophile in directed,
transition-metal catalysed C H activation chemistry to give
as I2,[9] Brønsted acids,[10] or heterogeneous systems[11] have
been shown to facilitate the C C bond formation in the
À
À
À
presence of the appropriate nucleophile and electrophile
(Scheme 1B). While electron-rich benzene derivatives have
been well-explored as suitable nucleophiles, there are only
isolated scope entries involving heteroaromatic nucleophiles
in these reports, and in most such cases mixtures of C2 and C3
isomers are reported. To the best of our knowledge, there
have been no reports on the hydroheteroarylation of hetero-
aromatic styrenes as electrophiles. Furthermore, a general
method for the hydroheteroarylation of densely substituted
and sterically hindered styrenes, which tolerates a variety of
functional groups, has yet to be developed. Since aromatic
compounds play a central role across diverse chemical
disciplines and fields, new methods to introduce such moieties
under mild conditions are of high general interest.
Herein, we report our efforts to address these challenges.
We began our investigation using 1-(cyclohexylidenemethyl)-
4-methoxybenzene (2a) as a model substrate for styrenes
bearing a b,b-disubstituted moiety. 2-Chlorothiophene (3a)
was selected as the nucleophile, in order to investigate the
robustness of the halogen handle as well as the selectivity
between the three nucleophilic positions C3, C4, and C5. The
C3 position was judged to be less reactive owing to the steric
hindrance of the C2 substituent. We discovered that the
desired hydroheteroarylation product 4a was formed in 75%
yield of isolated product when 0.3 mmol 2a was mixed with
1.5 equivalents of 3a with 5 mol% AgClO4 in dichloro-
methane (0.2 m) at 608C. No regioisomers of the product were
detected by GC–MS or NMR analysis. We also observed that
the reaction mixture became dark and opaque. When we
both branched and linear products.[1,2] Other transformations
using prefunctionalised aryl halides or aryl nucleophiles, such
as aryl boronic acids, have been developed in parallel
(Scheme 1A).[3] Alternatively, functionalisation via Friedel-
Crafts-type hydroarylation or hydroheteroarylation of styr-
À
enes form new C C bonds with complete atom economy.
Frequently encountered challenges are the regioselectivity of
the arene nucleophile, the poor reactivity of sterically
hindered styrenes, and the stability of heteroaromatic com-
pounds in the presence of transition metals, acidic conditions,
or elevated temperatures. Furthermore, substrates bearing
coordinating functional groups are usually incompatible with
this chemistry. Many researchers have developed useful
catalytic systems to achieve the Friedel-Crafts-type hydro-
arylation of styrenes. Metal catalysts based on Au,[4] Ca,[5]
Fe,[6] Bi,[7] and others,[8] as well as non-metallic catalysts such
[*] T. Dalton,[+] Dr. S. Greßies,[+] M. Das, M. Niehues,[++] M. L. Schrader,[++]
C. Gutheil,[++] Prof. Dr. B. J. Ravoo, Prof. Dr. F. Glorius
Organisch-Chemisches Institut
Westfꢀlische Wilhelms-Universitꢀt Mꢁnster
Corrensstrasse 40, 48149 Mꢁnster (Germany)
E-mail: glorius@uni-muenster.de
[+] These authors contributed equally to this work.
++] These authors contributed equally to this work.
[
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.202016268.
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attempted to record the kinetic profile, it was observed that
the reaction proceeded rapidly to completion at the time of
colour change, which occurred stochastically following an
induction period of between 2–4 h in repeat experiments.
During a preliminary scope investigation, we encountered
several limitations intrinsic to Friedel-Crafts-type hydroary-
lation. A majority of substrates explored, in particular those
bearing polar functionality, required a high reaction temper-
ature of 908C and up to 10 mol% loading of the more loosely
coordinated AgSbF6 salt to obtain sufficient conversion, or
exhibited no reactivity at all. Poor chemoselectivities were
frequently observed in the presence of labile functional
groups: for example, debromination was observed when 2-
bromo thiophene (3b) was used. When pivalate ester-bearing
substrate 2g was subjected to the reaction conditions, no
product formation was observed even under the most forcing
conditions and intriguingly, the aforementioned colour
change was absent (Scheme 2A).
in 74% yield. In addition, the same mild conditions allowed
the hydroarylation of 2b by 2-bromo thiophene (3b) to
furnish 5b in 72% yield of isolated product without dehalo-
genation (Scheme 2B). Further experiments showed
decreased yields when other chlorinated solvents were used,
and no colour change or reactivity was seen in the presence of
other solvents (Table 1 and for more details, see the Support-
ing Information).
Table 1: Selected deviations from the optimised conditions.[a]
Entry
AgX
[Ag]
[mol%]
Solvent
Yield
[%]
Note
We speculated that coordination of polar groups competes
with substrate activation and therefore deactivates the
catalyst. Control experiments showed that formation of the
coloured solution did not require the presence of the styrene.
When 2a was subsequently added to a solution of 3a and
AgClO4 (5 mol%) in CH2Cl2 that was preheated for several
hours until the colour change was observed, the reaction
proceeded to completion in 30 min at room temperature. The
coloured solution could be formed by heating 3a with AgSbF6
in < 15 min. A preliminary TEM image revealed the presence
of silver particles and higher-order organic material (for
details, see Supporting Information). In keeping with these
observations, at this point we speculated that thiophene
dimers or oligomers might be formed in situ under the
oxidative conditions and that they might be responsible for
the stabilization of a coloured, catalytically active species.
Commercially available 2,2’:5’,2’’-terthiophene (1) is an inex-
pensive yellow solid prepared on large scale as a common
building block for the synthesis of organic semiconductors.[12]
When 1 and AgSbF6 were simply combined in a 1:1 ratio in
dichloromethane at room temperature, a purple solution was
formed instantly. Addition of 3a and 1-methoxy-4-(2-meth-
ylprop-1-en-1-yl)benzene (2b) resulted in almost complete
conversion to 4b in 10 min (82% GC–FID calibrated yield).
Remarkably, when formerly inactive 2g was added to
a preactivated solution at room temperature, the reaction
showed complete conversion and 4g was successfully isolated
1
2
3
4
6
7
8
9
AgSbF6
AgSbF6
none
2
1
–
–
2
2
2
2
2
2
2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CHCl3
DCE
82
76
0
0
1
RT 10 min
RT 2 h
none
+5% NaSbF6
without 1
RT 16 h
RT 16 h
RT 16 h
AgSbF6
AgPF6
AgClO4
AgBF4
AgBF4
AgSbF6
AgSbF6
0
28
17
76
76
33
10
11
12
+5% NaSbF6
[a] GC–FID yields. DCE=1,2-Dichloroethane.
More coordinating silver salts, such as AgBF4 or AgClO4,
enabled only low conversions to 4b after extended reaction
times, although some reactivity could be restored upon the
addition of NaSbF6 as a loosely coordinating counterion. A
comparable yield was observed when organic-soluble reac-
tion components were added as a stock solution to the silver
salt, although in the latter case there was a delay of several
minutes before the reaction became coloured. This modifica-
tion was used to carry out the condition sensitivity screen
published recently by our laboratory[13] (for details see the
Supporting Information), which revealed that the transfor-
mation is insensitive to the factors of concentration, stir rate
and the presence or absence of O2, but the addition of 1%
H2O by volume inhibited the formation of the coloured
species and progress of the reaction. When the reaction was
initiated at 08C neither colour nor conversion were observed
until the reaction mixture was subsequently allowed to warm
to room temperature.
Satisfied with the optimality of our conditions and the
knowledge that we could conveniently run the reaction under
air, we began to explore the scope of the hydroarylation.
Although the system is also active for the hydroarylation of
terminal and disubstituted styrenes, which have been inves-
tigated extensively elsewhere, we decided to focus on more
challenging multisubstituted styrenes. The chemistry is selec-
tive for styrene double bonds, whereas aliphatic alkenes and
stilbenes remain untouched. First, we investigated the gen-
Scheme 2. Development of the preformed catalyst strategy.
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erality of the styrene coupling partner (Scheme 3). Different
increasing the loading of the thiophene coupling partner, we
cyclic alkyl chains as well as acyclic methyl substituents were
well-tolerated at the b-position (4a to 4e). Use of commer-
cially available 1-phenyl cyclohexene (2d) delivered the
congested quaternary carbon centre at 4d and the good
yield obtained was replicated on 20 mmol scale (4.60 g
isolated). Notably, a pyran ring was also tolerated, which
under the first set of conditions had inhibited the reaction,
delivering the desired product 4 f in good yield. Several useful
functional handles on the aromatic ring were likewise well-
preserved; products bearing -BPin, halogens, a free -OH
group, -OPh, the aforementioned ester, the potentially
reactive -O-allyl group as well as aryl and alkyl derivatives
were isolated in mostly excellent yields (4g to 4r). A sterically
hindered styrene 4s bearing a mesitylene moiety also reacted
smoothly. Tetrasubstituted styrenes are especially challenging
substrates owing to the steric repulsion of the a-substituent.
We were therefore especially pleased when tetrasubstituted
styrenes were found to deliver the products 4u to 4w in good
yields. Heteroaromatic styrenes have not yet been employed
in the Friedel-Crafts-like hydroheteroarylation chemistry.
These compounds are sensitive and unstable owing to their
propensity towards dimerization and oxidative decomposi-
tion, especially under harsh reaction conditions. By simply
were able to isolate two examples of these multi-heteroaryl-
substituted compounds (4x and 4y).
Next, we investigated the nucleophile scope of the
transformation (Scheme 4). Thiophenes bearing alkyl or
aryl substituents at the C2 or C3 position were suitable for
this reaction (5a to 5k). Notably, bromine and boron
substituents, which permit later derivatisation, were tolerated
(5b, 5 f, 5l). A benzothiophene, benzofurans, and especially
sensitive 2-Me furan were also convenient nucleophiles,
delivering the expected products in good yields (5p to 5s).
An electron-rich benzene, represented by 1,3,5-trimethoxy-
benzene, was a competent nucleophile, although the yield was
significantly lower (5o). We noted that the presence of basic
nitrogen in any of our other substrates resulted in no
conversion.
We were intrigued to understand the mode of action of
our system. While there are numerous reports of AgI ions
being exploited for their p-acidity towards triple bonds, they
are generally considered to have a much weaker affinity for
[14]
À
carbon carbon double bonds.
We therefore set out to
rationalise the unusually high activity of this system.
Terthiophenes are widely exploited for their fluorescent
properties. The UV/Vis and fluorescence spectra of 1 and the
catalytically active purple solution were measured and
Scheme 3. Styrene scope of the hydroarylation. Yields of isolated
product. Reaction conditions: 2 (0.3 mmol), 3a (1.5 equiv);
[a] 20 mmol scale; [b] 6:1 styrene a/b addition products; [c] 6 equiv of
3a.
Scheme 4. Heteroaryl scope of the hydroarylation. Yields of isolated
product. Reaction conditions: 2 (0.3 mmol), 3 (1.5 equiv); [a] Without
1, AgClO4 (5 mol%), 908C; [b] Without 1, AgClO4 (5 mol%), PhMe,
608C; [c] at 908C; [d] Without 1, AgClO4 (2.5 mol%), 608C.
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fluorescence bands of 620–640 nm and 670–690 nm were
observed, respectively. Since our class of styrene substrates
are known to absorb in the region around 450 nm, we
concluded that the fluorescence energy was too low to
facilitate a photochemical excitation.[15] This result was
confirmed by running the reaction with rigorous exclusion
of light. When butylated hydroxy toluene was added as
a radical trapping agent, full completion of the standard
reaction was seen with no adducts detected, suggesting no
radical species are present.
Next, we considered whether the catalyst might be
heterogeneous.[16] Upon standing, settling of black solids
from the active purple solution was visible. When AgSbF6, 1,
and 2d were combined in dichloromethane, TEM revealed
Ag particles in the range of 20–100 nm and oligomers of
1 were detectable by MALDI–MS. During the scope inves-
tigation, traces of alkyl arenes were observed by GC–MS
analysis. We initially hypothesised that silver nanoparticles,
supported by terthiophene derivatives, were the catalytically
active species. Literature-known heterogeneous AgI and Ag0
catalysts were tested, along with a commercial Ag0 nano-
powder, and none gave any conversion. Addition of a mercury
drop, a method used to agglomerate and deactivate hetero-
geneous species, did not prevent reaction completion. When
the purple solution was centrifuged at 6000 rpm for 5 min, the
supernatant was shown to retain its activity and no particles
were observable by TEM. In contrast, the black pellet was
inactive when resuspended in dichloromethane. Taken
together, these results suggest that the formation of particles
is a side reaction during oligomerisation of 1 and the active
species is most likely homogeneous. An unsymmetrical 2,2-
disubstituted styrene 2z (3:2 E/Z ratio) subjected to the
standard reaction conditions gave a dr of 4:1 (Scheme 5A, see
Supporting Information for further details). When the
blocked, C5,C5’’-dimethylated terthiophene 1* was used
instead of 1, the previously seen particles were not observed
by eye nor by TEM, and a GC–FID yield identical to the
standard reaction was achieved. We propose that terthio-
phene-type ligands offer a loosely bound environment for the
AgI ion in solution, with MLCT also responsible for the
intense purple colour, rendering it highly suitable for
catalysing the hydroarylation reaction.
Finally, we completed our present studies with mecha-
nistic investigations. In a competition experiment, substrate
2b was observed to react slightly faster than the analogous
para-fluorinated 2n’ (Scheme 5B-1), however a clear prefer-
ence for the more electron-rich thiophene was seen when 3e
and 3i were together reacted with 2b (Scheme 5B-2). The
standard reaction was estimated to bear a primary kinetic
isotope effect of 3.0 when C5-deuterated D-3d was used as
nucleophile in the standard reaction with 2b in parallel
experiments (Scheme 5C). Positional scrambling of the
deuterium label of D-3d was seen in the hydroarylation
product (Scheme 5D-1,2,3), in the recovered starting material
and also when no styrene (Scheme 5D-4) was added.
Together, these observations suggest that a reversible electro-
philic aromatic substitution (SEAr) may be the rate-limiting
step in a Friedel-Crafts-type catalytic cycle (Scheme 6).
Scheme 5. Mechanistic investigations.
Counter-intuitively, we observed the deuterium label in
both the a- and b-positions of the product, which could
possibly result from a reversible b-hydride elimination/
reinsertion process (for full details, see the Supporting
Information).
In conclusion, we have discovered and developed a new
system for the regioselective hydroheteroarylation of multi-
ply substituted styrenes, providing complex, three-dimen-
sional, multi-ring building blocks from flat precursors. High-
lighted applications include the hydroheteroarylation of both
quaternary and heterocyclic styrenes. The reaction is cata-
lysed by a simple silver salt in combination with commercial
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Acknowledgements
Generous financial support by the Deutsche Forschungsge-
meinschaft (CRC 1459, SFB 858, IRTG 2027, Leibniz Award)
is gratefully acknowledged. We thank Dr. Klaus Bergander,
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Conflict of interest
The authors declare no conflict of interest.
Keywords: catalysis · heteroarenes · hydroarylation · silver ·
styrenes
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Manuscript received: December 7, 2020
Revised manuscript received: January 21, 2021
Accepted manuscript online: January 25, 2021
Version of record online: March 5, 2021
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