ortho-Oxygenative 1,2-Difunctionalization of Diarylalkynes under Merged Gold/Organophotoredox Relay Catalysis

Article abstract of DOI:10.1002/asia.201901275

Reported herein is an ortho-oxygenative 1,2-difunctionalization of diarylalkynes under merged gold/organophotoredox catalysis to access highly functionalized 2-(2-hydroxyaryl)-2-alkoxy-1-arylethan-1-ones. Detailed mechanistic studies suggested a relay process, initiating with gold-catalyzed hydroalkoxylation of alkynes, to generate enol-ether followed by a key formal [4+2]-cycloaddition reaction. The successful application of the present methodology was also shown for the synthesis of benzofurans.

Full text of DOI:10.1002/asia.201901275

CHEMISTRY
AN ASIAN JOURNAL
www.chemasianj.org
Accepted Article
Title: ortho -Oxygenative 1,2-Difunctionalization of Diarylalkynes under
Merged Gold/Organophotoredox Relay Catalysis
Authors: Nitin T. Patil, Shashank P. Sancheti, Manjur O. Akram,
Rupam Roy, Vaibhav Bedi, and Shubhankar Kundu
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To be cited as: Chem. Asian J. 10.1002/asia.201901275
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ortho-Oxygenative 1,2-Difunctionalization of Diarylalkynes under
Merged Gold/Organophotoredox Relay Catalysis
Shashank P. Sancheti,[a] Manjur O. Akram,[b],[c] Rupam Roy,[a] Vaibhav Bedi,[a] Shubhankar Kundu,[a]
and Nitin T. Patil*[a]
Abstract: Reported herein, is an ortho-oxygenative 1,2-
difunctionalization of diarylalkynes under merged
Au/Fe catalyst system for the synthesis of heterocyclic
aldehydes using molecular oxygen as an oxygenating agent
(Scheme 1a).[12] Mechanistically, an intriguing pathway involving
a Fe-assisted oxygen radical addition to in situ generated vinyl-
gold intermediates was found to be operating. In addition to
gold/organophotoredox catalysis to access highly functionalized 2-
(2-hydroxyaryl)-2-alkoxy-1-arylethan-1-ones. Detailed mechanistic
studies suggested a relay process, initiating with gold-catalyzed
hydroalkoxylation of alkynes to generate enol-ether followed by a
key formal [4+2]-cycloaddition reaction. The successful application
of the present methodology was also shown for the synthesis of
benzofurans.
molecular
oxygen,
oxygenation
reactions
utilizing
photochemically generated singlet oxygen (1O2) has also
evolved as an alternative method.[13] Banking on these reports
along with our ongoing interest in the field of merged
gold/photoredox catalysis;[14] we questioned the possibility of a
[4+2] cycloaddition reaction of photocatalytically generated 1O2
with the vinyl-gold species or enol-ethers generated in situ via
hydroalkoxylation of diarylalkynes with alcohols (Scheme 1b).[15]
A detailed investigation of the above-mentioned hypothesis
resulted into novel reaction involving ortho-oxygenative 1,2-
Owing to the inherent carbophilic nature, gold complexes
have emerged significantly as the best activators of C–C
multiple bonds over the past few decades.[1] In the majority of
cases, gold catalyst selectively activates alkynes, allenes and
alkenes towards an intra- or inter-molecular nucleophilic addition
forming an organo-gold species, which typically undergoes fast
difunctionalization
gold/organophotoredox catalysis to access functionalized 2-(2-
hydroxyaryl)-2-alkoxy-1-arylethan-1-ones. The detailed
of
diarylalkynes
under
merged
2
]
protodeauration to afford hydrofunctionalized products.[
Expanding the horizons of gold catalysis beyond the traditional
carbophilic activation chemistry, it has been coupled with various
other modes of activation. In this regard, binary catalysis[3] has
emerged as a pivotal tool allowing the merger of gold with other
development and implementation of this strategy is disclosed
herein.
catalytic systems including secondary amines,[ 4 ] Brönsted
[6]
acids,[5] and other metals.
In particular, reports on merging
gold with photoredox catalysis, have increased drastically in the
past decade.[ 7 ] However, merged gold/photoredox catalysis
remains confined to the use of Au/Ru binary catalytic system.[7]
Additionally, as far as the utility of an organophotoredox catalyst
with gold complexes is concerned, there exists only report by
Glorius and co-workers wherein a three component oxyarylation
reaction of olefin was reported.[ 8 ] To further expand the
repertoire of gold/organophotoredox catalysis, research in this
direction is highly necessary.
The selective oxidation of organic molecules is a formidable
challenge in synthetic organic chemistry. Molecular oxygen, also
referred as triplet oxygen (3O2), is the most abundant and least
expensive oxidant available, and thus used extensively in
organic transformations.[9] The redox stability of gold-complexes
towards oxygen makes it unique and allows the development of
new modes of reactivities.[10] Several reports have emphasized
on the use of molecular oxygen in gold catalysis to effect
oxygenation of multiple bonds, including oxidative cleavage.[11]
As far as binary catalysis is concerned, Jiao and Shi employed a
Scheme 1: Merging gold with other catalysts for oxygenation reactions using
molecular oxygen: known and present work.
To test our hypothesis, diphenylacetylene (1a) was used as
a model substrate (Table 1). In light of the well-established
reactivity of Eosin Y as a organophotocatalyst,[16] especially in
combination with molecular oxygen; we decided to use Eosin Y
in combination with gold catalysts. To our delight, when 1a was
treated with
5 mol% Ph3PAuNTf2, 5 mol% Eosin Y as
photocatalyst in O2 purged DCM/MeOH (1:1) under the
irradiation of a 32 W CFL bulb for 24 h; product 3a was obtained
in 21% yields (entry 1). The structure of the compound 3a was
confirmed by X-ray crystallography. Encouraged by the results,
we then began the optimization process to achieve the most
productive outcome. Considering the initial gold-catalyzed
hydroalkoxylation as the limiting factor for the subsequent
cycloaddition, we began screening of various gold catalysts.
Accordingly, when gold catalysts such as Ph3PAuOTf,
Ph3PAuSbF6, and Ph3PAu(NCMe)SbF6 were examined (entries
2-4), the latter was found to be better giving 3a in 33% yield.
Excited by this lead, we turned our attention to investigate
[a]
Shashank P. Sancheti, Vaibhav Bedi, Shubhankar Kundu, Rupam
Roy, Dr. Nitin T. Patil
Department of Chemistry,
Indian Institute of Science Education and Research Bhopal, Bhauri,
Bhopal – 462 066 (India)
E-mail: npatil@iiserb.ac.in
Manjur O. Akram, Division of Organic Chemistry, CSIR-National
Chemical Laboratory, Dr. Homi Bhabha Road, Pune – 411 008
(India)
Academy of Scientific and Innovative Research (AcSIR), New Delhi
– 110 025 (India)
[b]
[c]
[d]
Supporting information for this article is given via a link at the end of
the document.
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various ligands associated with the gold(I) (entries 5-7). To our
delight an improved yield was observed when Au-1 was used as
catalyst; giving the product 3a in 62% yield. Further variation of
photocatalysts like rose bengal, methylene blue, etc. had
detrimental effect and resulted in either no reaction or afforded
desired product in traces (entries 8-12). Interestingly, when one
of our previously reported pyridinium-oxazole dyad[17] was used
as a photocatalyst, we obtained the desired product, albeit in
30% yield (entry 13). Unfortunately, any change from the
formerly employed solvent system exhibited detrimental
effect,[18] which led us to the final optimized conditions as: 1
equiv. of 1a to be stirred with 5 mol% of Au-1 and Eosin Y in O2
purged DCM/MeOH (1:1) solution under 32 W CFL bulb for 24 h.
Next the necessity of both Au-1 and Eosin Y was confirmed as
the reaction did not proceed in absence of either of the catalyst
(entries 14 and 15). Further, performing the reaction in a one-pot
sequential fashion did not improve the outcome.[18]
diarylalkynes having −F and −Cl at the para-position gave the
corresponding products 2b and 2c in 68 and 58% yields,
respectively. Further, diarylalkynes containing electron-donating
–Me, −tBu, –OBoc and –OTf also gave the corresponding
products 2d-g in 34-56% yields. Unfortunately, stronger
electron-donating diarylalkynes bearing –OMe gave a complex
mixture of products (cf. 2h). On the other hand, symmetrical
diarylalkynes bearing electron-withdrawing –CF3 and –CO2Et at
para-position gave the corresponing products 2i and 2j in 61 and
54% yields, respectively. Unfortunately, highly electron-
withdrawing –NO2 group was found to be unreactive (2k); as the
hydroalkoxylation step did not work under the standard reaction
conditions. Next, when we varied the substituents at the meta-
position of the symmetrical diarylalkynes, compound 2l, 2m and
2n were obtained in 31, 30 and 34% yields, respectively. The
drop in the yield was possibly due to the high dependancy of the
hydroalkoxylation reaction on steric encumbrance around the
ethylene-moiety. For the same reason, we did not observe any
product formation for the symmetrical diarylalkynes bearing the
substituents at the ortho-position (2o and 2p).
Table 1: Optimization of the reaction conditions.[a]
Table 2: Scope of symmetrical diarylalkynes.[a],[b]
Entry
1
[Au] cat
Photo cat
Yield %[b]
21
Ph3PAuNTf2
Ph3PAuOTf[c]
Eosin Y
Eosin Y
Eosin Y
2
NR[d]
[c]
3
Ph3PAuSbF6
29
4
Ph3PAu(NCMe)SbF6
33
Eosin Y
5
Au-1
Au-2
Au-3
Au-1
Au-1
Au-1
Au-1
Au-1
Au-1
Au-1
--
Eosin Y
62
6
Eosin Y
Eosin Y
50
7
41
8
Rose bengal
Methylene blue
[Mes-Acr][ClO4]
[Ru(bpy)3](PF6)3
[Ru(phen)3]Cl2
PC-1
traces[d]
traces[d]
NR[d]
traces[d]
traces[d]
30
9
[a] Reaction conditions: 0.20 mmol 1a, 5 mol% Au-1, 5 mol% Eosin Y, 0.05 M
DCM/MeOH (1:1), O2 balloon, 32 W CFL bulb, rt, 24 h. [b] Isolated yields. [c]
Complex reaction mixture. [d] Starting material was recovered in quantitative
yields.
10
11
12
13
14
15
Moving on to unsymmetrical diarylalkynes (Table 3), we
tested several substituents on one of the aryl ring keeping the
other aryl ring as phenyl. Diarylalkynes having –F and –Cl at
para-position afforded desired products 2q and 2r in 50 and
54% yields, respectively, as a mixture of two inseparable
isomers. Further, we tested the effect of various electron
donating groups at para-position. In this regard, diarylalkynes
bearing –Me group also worked to afford 2s in 41% yields.
Unfortunately, highly electron donating –OMe only provided a
complex mixture of products (cf. 2t) under the standard reaction
conditions. Moving on to electron withdrawing substituents, –
CF3, –CO2Et, –COMe, –CN and –NO2 were well-tolerated and
produced the desired products 2u-y in 48-59% yields. Further,
diarylalkynes bearing –CO2Me at meta-position afforded 2z in
48% yield. Next, the applicability of the methodology was
extended to heteroaryl consisting unsymmetrical diarylalkynes.
To our delight, compound 2aa was obtained in 43% yield. Be
noted that, alkyl or –CO2Me substituted arylacetylenes were not
tolerated under the present reaction conditions (cf. 2ab and
2ac). Next, when ethanol was used as the solvent, 2ad was
--
NR[e]
NR[d]
Eosin Y
[a] Reaction conditions: 0.10 mmol 1a, 5 mol% [Au] cat, 5 mol% photocat, 0.05
M DCM/MeOH (1:1), O2 balloon, 32 W CFL bulb, rt, 24 h. [b] Isolated yields. [c]
Generated in situ by mixing 5 mol% Ph3PAuCl and 5 mol% AgX (X = OTf,
SbF6). [d] Unreacted 1a was recovered in quantitative yields. [e] Compound 1a
was fully consumed and 1aʹ was isolated in 52% yield.
With the optimum reaction conditions (Table 1, entry 5),
several symmetrical diarylalkynes were evaluated to find the
scope of the reaction (Table 2). To our delight, the reaction was
found to work satisfactorily when we varied the substituents at
the para-position of the diarylalkynes. For instance, symmetrical
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obtained in 49% yield. Unfortunately, n-propanol did not afford
any product 2ae, supposedly due to the slow hydroalkoxylation.
and DPBF was observed when Eosin Y was added in presence
of dissolved molecular oxygen in 1:1 DCM/MeOH solution.[18]
These experiments suggest that 9,10-DPA and DPBF react with
in situ generated singlet oxygen (1O2), as a result of energy
transfer process between Eosin Y and 3O2.[18] To further
vindicate the generation of singlet oxygen by chemical methods;
1a was reacted with oxygen in the presence of Eosin Y and
triphenylphosphine (Scheme 3a).[ 19 ] As anticipated, a drastic
drop in the yield was observed with increase in equivalence of
Table 3: Scope with unsymmetrical diarylalkynes.[a],[b]
triphenylphosphine
along
with
the
formation
of
triphenylphosphine oxide as the major product. The results
clearly validate the generation of singlet oxygen and its crucial
role in driving the reaction forward. Next, when an 18O-labelling
experiment was performed under the standard conditions using
18O2, 2af was observed with >35% 18O-incorporation; validating
molecular oxygen to be the source of oxygen in the product
(Scheme 4b). To gain insights into the subsequent ring-opening
pathway being followed after a formal [4 + 2] cycloaddition,
deuterium labeling experiments were performed (Scheme 3c).
First, 1ag was subjected to standard reaction conditions, product
2ag was obtained in 36% yield without any deuterium migration
at -position of carbonyl. Next, when 1a was subjected to
standard reaction conditions in presence of deuterated
methanol, product 2ah was obtained in 41% yield. The presence
of –D (–H) and –OCD3 (–OCH3) on the same carbon suggested
a possible methanol elimination-addition pathway operating.
[a] Reaction conditions: 0.20 mmol 1a, 5 mol% Au-1, 5 mol% Eosin Y, 0.05 M
DCM/MeOH (1:1), O2 balloon, 32 W CFL bulb, rt, 24 h. [b] Isolated yields. [c]
Combined yield. [d] Complex reaction mixture. [e] Starting material was
recovered in quantitative yields.
Demonstrating the synthetic potential of the reaction, we
successfully transformed our product into benzofurans 3a and
3b in 54 and 45% yields, respectively (Scheme 2).
Scheme 2: Utility of the products.
The control experiments outlined in Scheme 3 were set up to
gain further insights into the mechanism. As control experiments
supported that the reaction follows a relay path via the
intermediacy of enol-ether 1aʹ; and gold(I)-catalyst has
essentially no role in the photocatalytic process,[18] we sought to
understand the energy transfer process between intermediate
1aʹ, Eosin Y and O2. The absorption maxima of 1aʹ and Eosin Y
are 294 and 521 nm, respectively, suggesting that the radiated
visible light was exclusively absorbed by the Eosin Y.[18] To
explore the elementary step following the excitation of the Eosin
Y, a photoluminescence quenching experiment was performed.
However, we did not observe any fluorescence quenching of
Eosin Y with 1aʹ, indicating a possible energy transfer from
photoexcited Eosin Y to oxygen.[18]] To confirm this, we
performed another fluorescence quenching experiment using
traditionally used singlet oxygen quenching probes 9,10-
diphenylanthracene (9,10-DPA) and 1,3-diphenylisobenzofuran
(DPBF). A drastic photochemical degradation of both 9,10-DPA
Scheme 3: Mechanistic studies.
Based on the fluorescence quenching studies and control
experiments, a plausible mechanism is outlined in Scheme 4. At
first, a gold-catalyzed hydroalkoxylation reaction[
of 1y
]
20
generates an enol-ether 1yʹ. In the meantime, photoexcited
Eosin Y undergoes an energy transfer event with triplet oxygen
to sponsor the necessary singlet oxygen. The enol-ether reacts
with singlet oxygen to furnish perepoxide intermediate I[21] which
eventually undergoes intramolecular rearrangement[16] followed
by rearomatization (cf. II) to form intermediate III. The whole
process from the conversion of 1y' to II could be considered as a
formal [4+2] cycloaddition reaction. Further, addition of methanol
to III would result in the formation of product after O-O bond
cleavage.
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Scheme 4: A plausible mechanism.
In conclusion, an ortho-oxygenative 1,2-difunctionalization of
diarylalkynes under merged gold/organophotoredox catalysis
has been reported. The method provided substituted 2-(2-
hydroxyaryl)-2-alkoxy-1-arylethan-1-ones in moderate yields.
Mechanistic investigations revealed a relay process initiating
with the in situ formation of enol-ether followed by formal [4+2]-
cycloaddition reaction with singlet oxygen. The mechanism of
the reaction was investigated through meticulously designed
experiments and fluorescence quenching studies. The
successful application of the present methodology was also
shown for the synthesis of benzofurans.
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Acknowledgements
Generous financial support by the Science and Engineering
Research Board (SERB), New Delhi (File No. EMR/2016/
007177 and DIA/2018/000016) is gratefully acknowledged.
M.O.A and S.P.S thank CSIR and IISER Bhopal respectively for
their fellowships. V.B. and S.K. thank IISER-Bhopal for providing
summer internships. Dr. Saibal Bera and Mr. Aditya Upadhyay
are gratefully acknowledged for the help in X-ray
Crystallography. We thank Dr. Apurba Lal Koner for providing an
access to the instruments which are required for fluorescence
quenching studies. Dr. T. K. Paine (IACS Kolkata) is
acknowledged for his help in 18O-labelling studies.
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Keywords: gold catalysis • organocatalyst • photoredox • binary
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Entry for the Table of Contents
Shashank Sancheti, Manjur O. Akram,
Rupam Roy, Vaibhav Bedi, Shubhankar
Kundu, and Nitin T. Patil*
Page No. – Page No.
ortho-Oxygenative 1,2-
Difunctionalization of Diarylalkynes
under Merged
Gold/Organophotoredox Relay
Catalysis
An ortho-oxygenative 1,2-difunctionalization of diarylalkynes under merged
gold/organophotoredox catalysis to access highly functionalized 2-(2-hydroxyaryl)-
2-alkoxy-1-arylethan-1-ones has been reported. The key features of the mechanism
includes the formal [4+2]-cycloaddition reaction between in situ generated enol-
ether with singlet oxygen. The mechanism of the reaction was investigated through
meticulously designed experiments and fluorescene quenching studies.
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