On the Gold-Catalyzed Generation of Vinyl Cations from 1,5-Diynes

Article abstract of DOI:10.1002/anie.201700057

Conjugated 1,5-diynes bearing two aromatic units at the alkyne termini were converted in the presence of a gold catalyst. Under mild conditions, aryl-substituted dibenzopentalenes were generated. Calculations predict that aurated vinyl cations are key intermediates of the reaction. A bidirectional approach provided selective access to the angular annulated product in high yield, which was explained by calculations.

Full text of DOI:10.1002/anie.201700057

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201700057
German Edition: DOI: 10.1002/ange.201700057
Gold Catalysis
On the Gold-Catalyzed Generation of Vinyl Cations from 1,5-Diynes
Thomas Wurm+, Janina Bucher+, Sarah B. Duckworth, Matthias Rudolph, Frank Rominger, and
A. Stephen K. Hashmi*
Abstract: Conjugated 1,5-diynes bearing two aromatic units at
the alkyne termini were converted in the presence of a gold
catalyst. Under mild conditions, aryl-substituted dibenzopen-
talenes were generated. Calculations predict that aurated vinyl
cations are key intermediates of the reaction. A bidirectional
approach provided selective access to the angular annulated
product in high yield, which was explained by calculations.
D
iyne systems are gaining importance as substrates for
gold-catalyzed[1] transformations. In most of these reactions,
one of the alkynes in these systems is converted into
a nucleophilic alkene/allene by initial intra- or intermolecular
attack onto the p-activated triple bond. Subsequently, the
reaction cascades are terminated by intramolecular reactions
with the second alkyne, providing access to a diverse set of
interesting organic scaffolds, often with complete atom
economy.[2]
Scheme 1. Diynes as precursors for high-energy intermediates. Left
and middle: known vinylidene precursors; right: potential vinyl cation
precursor, unknown.
(even worse, for gold “catalysts”, a yield of only 1% was
achieved, which corresponds to 0.02 turnovers, that is, a 50-
fold excess of gold with regard to the product formed); the
authors proposed a radical mechanism and thus subsequently
also investigated photochemical conversions without metal
catalyst, which delivered a different product in 3% yield.[6c] In
1991, Blum and Vollhardt succeeded in improving Mꢀllerꢁs
conditions, achieving a yield of 80% under quite forcing
conditions (1208C) in a biphasic water/dichloromethane
Another type of reactivity is induced by dual gold
catalysis. Instead of an external nucleophile, a gold acetylide
directly attacks a p-activated alkyne to give rise to highly
electrophilic intermediates, which can be used for fruitful
follow-up chemistry (Scheme 1, left).[3]
We demonstrated only recently that electrophilic species
can also be generated when gold acetylides are reacted in the
presence of appropriate leaving groups (Scheme 1, middle).[4]
In continuation of our studies on the gold-mediated gener-
ation of high-energy intermediates, we envisioned that
instead of gold acetylides, non-terminal alkynes could also
be applied as nucleophiles in combination with a second
p-activated alkyne (Scheme 1, right). The resulting vinyl
cation intermediates could then open up a new field of
chemistry as their generation does not require the presence of
at least one terminal alkyne in the diyne system as in dual gold
catalysis.[5] Earlier efforts by Mꢀller and co-workers[6a,b] with
these substrates using stoichiometric amounts of PdCl2, PtCl2,
PtCl4, AuCl3, AuCl, or Na[AuCl4] all showed low selectivity
mixture with
a phase-transfer catalyst and H2[PdCl4]
(5 mol%) after 16 h; again, no mechanistic insight was
obtained.[6d] With respect to the substrate, a similar publica-
tion by Liu and co-workers[6e] should be mentioned; while the
authors assumed that a different mechanism is operating, they
speculated about a possible mechanism involving vinyl
cations only in a footnote. In the light of the results reported
here, this mechanism should be re-investigated.
To study this hypothesis, we considered 1,2-di(arylalky-
nyl)arenes as ideal test systems. Gold vinylidenes by a dual-
catalyzed pathway can easily be ruled out as no gold acetylide
can be formed. Symmetric substrates circumvent the forma-
tion of regioisomeric products. The diphenyl-substituted
system 1a was used for our initial experiments (Table 1).
Among the other ligands tested (for the full Table, see the
Supporting Information, Table S1), phosphorus-based ligands
only gave poor yields (entry 1). No reaction took place with
a nitrogen acyclic carbene (NAC) complex (entry 2), and
changing to a more flexible Tedii (1-cyclopentadecyl-3-(2,6-
diisopropylphenyl)-2,3-dihydro-1H-imidazol-2-ylidene) N-
heterocyclic carbene (NHC) ligand only delivered a moderate
yield of 63% (entry 3). As expected, no reaction occurred in
the complete absence of a catalyst (entry 4) or with a silver(I)
catalyst (entry 5). For reasons of practicality, we applied the
air-stable and storable nitrile-stabilized [IPrAu(NCMe)]SbF6
catalyst,[7] which does not require in situ activation with
[*] M. Sc. T. Wurm,[+] M. Sc. J. Bucher,[+] S. B. Duckworth,
Dr. M. Rudolph, Dr. F. Rominger,[++] Prof. Dr. A. S. K. Hashmi
Organisch-Chemisches Institut, Universitꢀt Heidelberg
Im Neuenheimer Feld 270, 69120 Heidelberg (Germany)
E-mail: hashmi@hashmi.de
Homepage: http://www.hashmi.de
Prof. Dr. A. S. K. Hashmi
Chemistry Department, Faculty of Science
King Abdulaziz University
Jeddah 21589 (Saudi Arabia)
[+] These authors contributed equally.
++] Crystallographic investigation.
[
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
http://dx.doi.org/10.1002/anie.201700057.
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Table 1: Screening with substrate 1a.[a]
advanced strategies towards these synthetically challenging
targets are based on transition-metal-catalyzed processes.
Mostly, the target compounds are obtained by homocoupling
reactions of ortho-alkynylaryl bromides or aryl acetylenes
under Pd or Ni catalysis.[11] Unsymmetrically substituted
systems can be obtained either through the use of tin-
[12b]
substituted precursors[12a] or by C H activation strategies,
À
but all of these reactions require temperatures of at least
1008C, and often other, non-desirable additives have to be
used. Di(arylalkynyl)arenes have also been used as precursors
for dibenzopentalene systems. One strategy uses substoichio-
metric amounts of platinum catalysts at high temperatures
(our results suggest that the dibenzopentalenes in fact slowly
decompose at such elevated temperatures; for examples, see
the Supporting Information, Table S1, entries 15–18),[6] and
only recently, Erker and co-workers showed that stoichio-
metric amounts of strongly electrophilic B(C6F5)3 are also
capable of initiating a related transformation that is intrinsi-
cally accompanied by a shift of one of the aromatic moieties at
the boron center.[13] Nevertheless, owing to the increasing
demand for pentalene derivatives as new potent compounds
for organic electronics, a simple method for the preparation of
these compounds at room temperature and under open-flask
conditions is highly desirable. To evaluate the applicability of
the gold-catalyzed approach, several diyne systems were
synthesized and subjected to the optimized reaction condi-
tions (Table 2).
Entry Gold complex
Activator
T
Additive[b] Yield [%][c]
[phosphiteAu]+ NTf2
NACAuCl
TediiAuCl
–
–
808C SiO2
808C SiO2
808C SiO2
808C SiO2
RT
RT
9 (–)
– (–)
63 (–)
– (–)
À
1
2
3
4
5
6
AgNTf2
AgNTf2
–
AgSbF6
–
–
–
– (–)
[IPrAuNCMe]+ SbF6
–
75 (74)
À
[a] Reactions were run on 3.6 mmol scale in 0.7 mL [D6]benzene for 24 h
and were afterwards directly analyzed by H NMR spectroscopy.
Hexamethylbenzene was used as an internal standard. [b] If silica gel
(50 mg) was added, the reactions were filtered through a short pad of
glass wool prior to analysis. [c] Determined by NMR spectroscopy. Yields
of isolated products (36 mmol scale) given in parentheses.
1
First we varied the substituents at the aromatic system
(Table 2). Electron-donating substituents in the para or meta
position were tolerated well, and the corresponding products
could be isolated in good to moderate yields (2b–2e). For the
ortho-substituted arene 2 f, the yield dropped significantly.
Electron-withdrawing substituents led to yields of around
20% (2g–2i); we assume that the decreased nucleophilicity of
the aromatic moieties in these substrates slows down the
formation of the desired product so that unwanted side
reactions of the vinyl cation dominate; in addition, the
stability of the vinyl cation is decreased by this substituent,
which again accelerates side reactions. Among the applied
heteroaromatic systems, a benzothiophene turned out to be
a suitable substrate (2j) whereas in the case of methylthio-
phene, the formed product (visible by TLC) turned out to be
unstable during isolation. Then we focused on variations of
the aromatic backbone of the diyne system. All of the tested
backbones could be converted with good efficiency regardless
of whether electron-donating (2m, 2n) or electron-withdraw-
ing substituents (2o, 2p; Figure 1) were present. A substrate
with a naphthyl backbone was also successfully converted,
giving rise to the p-extended pentalene 2q in acceptable yield.
A clean reaction was observed with tetrayne 3; out of the
two conceivable regioisomers, only the angular annulated 4a
was obtained in remarkable yield considering the complexity
of the obtained carbon scaffold (Scheme 2). The connectivity
of the obtained product was confirmed by X-ray single-crystal
structure analysis (Figure 2). The reason for the selectivity is
probably an unfavorable interaction in intermediate II;
Scheme 2 also shows the two different intermediates IIa
and IIb. As clearly visible from the optimized geometries, in
IIb (which would lead to the non-observed 4b), there is
a silver salt, and gave the best yield (entry 6). The structure of
the product was unambiguously assigned by X-ray single-
crystal structure analysis.[8] A preparative reaction delivered
the product in 74% yield. The product was the expected
dibenzopentalene 2a (Figure 1), and is thus structurally
strongly related to the dibenzopentalene derivatives obtained
from terminal alkynes by dual catalysis.[3f] However, the
Figure 1. Solid-state molecular structures of 2a and 2o. Thermal
ellipsoids set at 50% probability.
presence of the second aryl substituent in the vinyl cation
intermediate leads to an aryl-substituted product in this case.
This clearly underlines the fact that aside from gold acety-
lides, non-terminal alkynes can also serve as nucleophiles in
diyne cyclizations, giving rise to high-energy intermediates
with potential for fruitful follow-up chemistry. The obtained
dibenzopentalene scaffold has been known for over
100 years,[9] but p-extended pentalenes, in particular, came
into the focus of materials science only recently.[10] The most
2
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Table 2: Substrate scope.[a]
Scheme 2. A bidirectional approach for the facile and constitutional-
isomer-selective synthesis of p-extended pentalenes.
[a] On 20–25 mmol scale in technical-grade benzene (c=0.1m) using
5 mol% of [IPrAuNCMe]SbF6. The reactions were stopped after TLC
analysis showed complete conversion (typically 0.5 to 7 h). [b] Owing to
the rather poor solubility of the starting material in benzene, 1,2-
dichloroethane was used instead.
a repulsion between the IPr ligand on the vinylgold inter-
mediate and the phenyl substituent on the already formed
dibenzopentalene substructure. This unfavorable interaction
is probably also present in the corresponding alkyne
p-complexes. To explore all factors contributing to this very
useful selectivity, we are currently conducting a full-scope
computational study that considers all steric and electronic
parameters directing the selectivity.
Figure 2. Solid-state molecular structure of 4a. Thermal ellipsoids set
at 50% probability.
In a mechanistic experiment, deuterated substrate 1r was
prepared and subjected to the standard reaction conditions
(Scheme 3). After the reaction, approximately 45% of 2r
showed deuterium incorporation at the vinylic position while
no exchange was observed for all of the remaining aromatic
hydrogen atoms. The deuterium labeling of the vinylic
position strongly indicates that a gold atom was bound to
that position during the reaction. The fact that only less than
half of the vinylic positions were deuterated indicates an
electrophilic attack of the vinyl cation at the p-system; the
corresponding s-complex then eliminates a D+, which parti-
ally exchanges for H+ with residual H2O or the benzene in
Scheme 3. Conversion of [D10]-1r into dibenzopentalene 2r.
À
solution. Insertion into the C H s-bond and subsequent 1,2-
shifts would have led to complete intramolecular transfer of
the deuterium without D/H exchange.
A computational study was conducted for the trans-
formations of 1a, 1e, and 1i; these substrates were chosen to
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These can be exploited for the synthesis of unsymmetrically
substituted dibenzopentalenes, interesting targets for materi-
als science as demonstrated by a bidirectional approach to
a dibenzopentalene with extended conjugation in one step.
First quantum-chemical calculations indicate a rapidly inter-
converting valence tautomer equilibrium between the gold
alkyne complex and the vinyl cation. Further studies on the
exploitation of diyne-derived vinyl cations in organic syn-
thesis are ongoing in our laboratories.
Acknowledgements
J.B. is grateful for a Chemiefonds scholarship of the Fonds der
Chemischen Industrie. We thank Umicore AG & Co. KG for
the donation of gold salts.
Conflict of interest
Scheme 4. Mechanistic picture for the transformation of 1 into 2
obtained by calculations.
The authors declare no conflict of interest.
Keywords: extended p-systems · gold catalysis ·
include the electronic parameters of the arene moiety into our
considerations. We thus obtained a mechanistic picture
(Scheme 4) and thermodynamic data for the intermediates
(see the Supporting Information). Our mechanistic pro-
posal[14] starts with p-coordination of a cationic gold fragment
to one of the alkyne units of 1, leading to alkyne complex I.
Complex I is transformed into its valence tautomer II, the
postulated vinyl cation, in an endergonic process. Our
calculations predict an activation energy of 10–15 kcalmolÀ1
for this process depending on the substitution pattern of the
diyne. This rather low barrier is comparable with the barrier
found for the valence isomerization processes of molecules
such as bullvalenes that fluctuate at room temperature.[15]
Vinyl cation II is then trapped by an irreversible nucleophilic
attack of the proximate aryl substituent onto the electrophilic
cationic vinyl carbon atom, which generates a strongly
resonance-stabilized cationic species III. As the calculated
energies of both transition states are in the same range (TS I
vs. TS II), we cannot unequivocally determine with the
applied methods which of the two steps is turnover-limiting
for the formation of 2. The calculations show that the
formation, the trapping, and the stability of the vinyl cation
intermediate are enhanced by an electron-donating para-
methoxy substituent whereas an electron-withdrawing para-
trifluoromethyl substituent has the opposite effect. These
findings are qualitatively in line with our experimental finding
of a dramatically increased reaction time for the formation of
2i. After the formation of III, rearomatization to the arene
system and subsequent irreversible protodeauration (a pro-
cess that was not explicitly included into this computational
study owing to its obvious complexity) lead to IV; after ligand
exchange, product 2 is released with subsequent regeneration
of I.
high-energy intermediates · pentalenes · vinyl cations
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4
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Manuscript received: January 3, 2017
Final Article published: && &&, &&&&
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Communications
Gold Catalysis
T. Wurm, J. Bucher, S. B. Duckworth,
M. Rudolph, F. Rominger,
A. S. K. Hashmi*
&&&& — &&&&
On the Gold-Catalyzed Generation of
Vinyl Cations from 1,5-Diynes
Positive intermediates: The combination
of non-terminal 1,5-diynes with cationic
gold complexes enables the generation of
highly reactive vinyl cations that can be
used for the synthesis of unsymmetrically
substituted dibenzopentalenes. Quan-
tum-chemical calculations indicate a fast
valence tautomer equilibrium between
a gold alkyne complex and the vinyl
cation.
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Angew. Chem. Int. Ed. 2017, 56, 1 – 6
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