Dioxygen-triggered oxidative radical reaction: Direct aerobic difunctionalization of terminal alkynes toward β-keto sulfones

Article abstract of DOI:10.1021/ja4052685

An unprecedented dioxygen-triggered oxidative radical process was explored using dioxygen as the solely terminal oxidant, realizing aerobic oxidaitve difunctionalization of terminal alkynes toward β-keto sulfones with high selectivity. Operando IR experiments revealed that pyridine not only acts as a base to successfully surpress ATRA (atom transfer radical addition) process, but also plays a vital role in reducing the activity of sulfinic acids.

Full text of DOI:10.1021/ja4052685

Communication
pubs.acs.org/JACS
Dioxygen-Triggered Oxidative Radical Reaction: Direct Aerobic
Difunctionalization of Terminal Alkynes toward β‑Keto Sulfones
Qingquan Lu,† Jian Zhang,† Ganglu Zhao,† Yue Qi,† Huamin Wang,† and Aiwen Lei*,†,‡
†College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, P. R. China
‡College of Pharmacy, Wuhan University, Wuhan 430072, P. R. China
S
* Supporting Information
byproducts are usually generated concurrently. To the best of
ABSTRACT: An unprecedented dioxygen-triggered oxi-
dative radical process was explored using dioxygen as the
solely terminal oxidant, realizing aerobic oxidaitve
difunctionalization of terminal alkynes toward β-keto
sulfones with high selectivity. Operando IR experiments
revealed that pyridine not only acts as a base to
successfully surpress ATRA (atom transfer radical
addition) process, but also plays a vital role in reducing
the activity of sulfinic acids.
our knowledge, there have been no reported examples
concerning aerobic oxidation of terminal alkynes without
assistance of additional initiators or transition-metal catalysts.11
In this regard, it is desirable to seek exotic radical reactions in
this field. Owing to our continuous interest in radical chemistry
and dioxygen activation,12 we report our recent progress in this
field.
In principle, H-atom transfer process from the substrate
(RYH) to reactive vinyl radical intermediate is the key step to
generate stable unsaturated alkenes in Path I (Scheme 1).1 We
adical chemistry has played great roles in the development
Scheme 1. Dioxygen-Triggered Aerobic Difunctionalization
of Terminal Alkynes
R
of modern organic synthesis. To date, most of the radical
reactions are carried out by utilizing precious and/or toxic
metal complex such as tin hydride reagents and unfriendly
radical initiators including peroxides, diazines, etc.,1 leading to
the difficult purification of the desired products from a large
amount of unexpected byproducts and residual catalysts. These
disadvantages greatly restrict the application of radical
chemistry in the areas of pharmaceutical industry, etc.1g Thus,
it is challenging to update these traditional methods for green
and sustainable organic synthesis.2
envisaged that, if we can establish suitable strategies to suppress
the process of H-atom abstraction, the key intermediate, vinyl
radical, might be trapped by dioxygen due to its diradical
structure. On the basis of this hypothesis, we reasoned that
sulfinic acid, which can be easily activated by dioxygen via single
electron transfer (SET) and concomitant proton transfer (PT)
to produce reactive sulfonyl radical,7c,13 could be an ideal
substrate to achieve this goal. That is because H-atom transfer
process can be easily suppressed with the assistance of base to
trap the H-proton, and in contrast, the SET and PT process will
not be influenced. Furthermore, radical addition to carbon−
carbon triple bonds is believed to be the most convenient
method to generate the vinyl radical, which could be rapidly
and irreversibly trapped via subsequent transformation and thus
appears various synthetic possibilities.1e Herein, we communi-
cate a novel and convenient protocol of achieving valuable β-
keto sulfones using unfunctionalized materials via a dioxygen-
triggered radical process.14
As a result of its environmental friendliness, cleanliness, and
sustainability, radical process using dioxygen as an initiator
under metal-free conditions is undoubtedly the ideal and
promising route to addressing the aforementioned challenges.
In this context, several examples of its use have recently been
developed, including hydroacylation of α,β-unsaturated esters,3
radical addition of Grignard reagents to olefins,4 autoxidative
coupling reactions,5 oxidative difunctionalization of alkenes,6
oxidative cleavage of C−C double bonds.7,8 Despite these
advances, radical reactions initiated solely by dioxygen still
remain an outstanding challenge. A typical example is the
aerobic C−C triple bond functionalization. Up to now, only
rare examples have been reported to realize aerobic oxidation of
terminal alkynes using transition-metals as catalysts, and the
homocoupling of terminal alkynes from the Glaser-Hay
coupling can hardly be dodged, which greatly hinders its
application in synthetic chemistry.9,10 Additionally, the major
reason for the difficulty of this aerobic transformation is
attributed to the fact that the vinyl radical intermediate
generated in situ, which belongs to σ radicals, is highly reactive
and unstable; thus, it is easier to afford the stable unsaturated
alkenes through H-atom abstraction and difficult to be captured
by dioxygen.1e Because of their high reactivity, these reactions
are not always as selective as initially desired and unexpected
To probe the feasibility of our proposed assumption, we
started our investigation by reacting phenylacetylene (1a) with
benzenesulfinic acid (2a) as the model reaction (Scheme 2).
Received: June 2, 2013
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ja4052685 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
Scheme 2. O18 Isotope Labeling Experiment
Table 1. Substrate Scope of Aerobic Difunctionalization of
Terminal Alkynes
a
On the basis of our hypothesis, the feasibility of this strategy
relies on the selectivity after the initial radical addition step, and
the oxidation process under metal-free conditions has very few
reports, whereas ATRA (atom transfer radical addition) process
has been well studied.1 Not surprisingly, the reaction gave a
complex result when we directly treated 1a with 2a in
dichloroethane (DCE) at 45 °C under air conditions. Large
amounts of unexpected byproducts were formed, presumably
due to the ATRA process that was favored under present
conditions, which led to the very poor selectivity. Subsequently,
we chose pyridine as a base to inhibit the possible ATRA
process based on our initial assumption, and we found that the
expected 1-phenyl-2-(phenylsulfonyl)ethanone (3aa) could be
isolated in 28% yields with high selectivity and no other
byproducts including homocoupling of 1a and vinyl sulfone
generated through H-atom abstraction process were observed.
After a series of further screening conditions, we found that
bases and the concentration of dioxygen have significant impact
on the efficiency of this transformation; pyridine and air were
proved as the best choice (for details, see Supporting
Information (SI), Table S1). Finally, 1a with 5-fold 2a in the
presence of pyridine and air at 45 °C using DCE as the solvent
was screened as the optimized aerobic reaction conditions.
Furthermore, the reaction between 1a and 2a in the presence
of 18O2/N2 afforded the 18O-labeled product 3aa′ in 72% yield
(Scheme 2), demonstrating the carbonyl oxygen atom of the β-
keto sulfone originated from dioxygen as our initial assumption.
Subsequently, having optimized the reaction conditions, we
then investigated the scope of the reaction with respect to 1a
and different sulfinic acids. As can be seen in Table 1, a variety
of aryl sulfinic acids, bearing either electron-donating groups (R
= OMe, Me) or electron-withdrawing groups (R = F, Cl, Br) on
the aryl ring, reacted smoothly with phenylacetylene, affording
the corresponding β-keto sulfones 3aa−3af in good to excellent
yield. The sterically hindered ortho-methyl substituted
benzenesulfinic acid gave the corresponding 3ag in 66% yield.
Moreover, 2-naphthylsulfinic acid was also effective to give the
corresponding desired product 3ah in good yield (80%).
Notably, Br, Cl and F substituents can be tolerated in this
reaction, thereby, facilitating further modifications at halo-
genated positions (3ad−3af). Most importantly, aliphatic
sulfinic acid, as a case study of n-octylsulfinic acid, was also a
suitable substrate for this reaction, giving the desired product
3ai in moderate yield.
a
Unless otherwise specified, all reactions were carried out using 1
(0.20 mmol), 2 (1.0 mmol), and pyridine (0.82 mmol) in DCE (2.0
mL) at 45 °C for 4 h under 1 atm of air (balloon). Isolated yield. 2
(1.2 mmol) and pyridine (0.98 mmol) were employed. CHCl3 (2.0
mL).
b
c
product 3fa was isolated in moderate yield. These results are
significant since aryl halides, in particular for Br and I, are
reactive and, thus, are difficult to be retained in many transition
metal participated or organocatalytic reactions,15 which offer
opportunities for further functionalization. Most importantly,
aryl alkynes with carbonyl groups such as ketone and ester
substituents on phenyl ring were also compatible reaction
substrates, affording the corresponding products (3ja, 3ka) in
good yields, which are difficult to be prepared by other
methods. The heterocyclic alkynes which usually are sensitive
to oxidative conditions, as exemplified by 2-ethynylthiophene,
could also serve as suitable reaction partner in this protocol and
gave the desired product 3la in 67% yield. Internal alkyne, as an
example of prop-1-ynylbenzene, was also mendable to this
protocol, although exhibiting relatively low reactivity and
affording the expected 3ma in moderate yield.16
We next explored the scope of the reaction between a set of
functionalized alkynes and benzenesulfinic acid 2a. As shown in
Table 1, a range of alkynes, with either electron-rich or
electron-poor substituents on aromatic ring, were viable in this
transformation. Electron-donating aryl alkynes (3ba, 3da)
displayed higher reactivity than those bearing electron-with-
drawing groups (3ja, 3ka). ortho-Methoxyl substituted phenyl-
acetylene was found to marginally affect the efficiency of this
reaction, providing the desired product 3ca in 70% yield.
Notably, halo substituents including F, Cl, Br were proved to be
well tolerated under the standard reaction conditions (3ga−
3ia). Even aryl iodides remained intact after reaction and
Additionally, radical trapping experiments were also con-
ducted by employing 2,2,6,6-tetramethyl-1-piperidinyloxy
(TEMPO) and 2,4-di-tert-butyl-4-methylphenol (BHT) to
elucidate whether the reaction involves radical species.3,5 The
reactions were found to be mostly inhibited by TEMPO and
BHT (see SI).
B
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Journal of the American Chemical Society
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To further investigate the mechanism in detail, we
investigated these reactions using operando IR to monitor
the reaction of 1a with 2b under the optimized conditions. As
can be seen from the kinetic profiles of relative absorbance
(ConcIRT vs time) for individual species in Figure 1A, the
Scheme 3. Proposed Mechanism
sulfinyl anion I in the presence of pyridine. Subsequently, the
pathway was triggered by the autoxidation of I with dioxygen
via single electron transfer (SET) process, affording an oxygen-
centered radical II which could resonate with sulfonyl radical
III. Thereafter, sulfonyl radical addition to alkynes produces the
reactive vinyl radical IV, which could be trapped by dioxygen
under present conditions and forms intermediate V. Afterward,
the intermediate V goes through SET and PT process
successively with I and pyridinium, generating II/III and
affording peroxide VII. Finally, VII undergoes subsequent
reduction by benzenesulfinic acid, produces VIII, which
isomerizes and furnishs 3aa.
On the basis of the speculation, besides the desired product
3ab from the reaction of 1a, 2b and O2, we posited that p-
toluenesulfonic acid should be the other reaction product. In
fact, from Figure 1, we did identify the formation of pyridinium
p-toluenesulfonate from the reaction mixture. However, from
Figure 2, we knew pyridinium p-toluenesulfonate could also be
generated from an autoxidation of 2b and O2. Actually, the
autoxidation of 2b had been also reported to form sulfonate,
although there is less detailed kinetic investigation.17 To clarify
whether the sulfonate generated from autoxidation or from the
oxidative radical reaction of 1a, 2b, and O2, the in situ IR
investigations were further conducted, and the results were
shown in the Figure 3. It was clear that the formation of
pyridinium p-toluenesulfonate from the oxidative difunctional-
ization of 1a, 2b, and O2 was faster than the autoxidation of 2b
with O2. This result indicated that the embodied oxygen atom
(at least parts of) of pyridinium p-toluenesulfonate from
pyridinium p-toluenesulfinate was not directly from O2, but
from one of the intermediates of the oxidative radical reaction.
Figure 1. (A) The 2D-Kinetic profile of the reaction of p-
toluenesulfinic acid 2b (2.0 mmol), pyridine (1.64 mmol), and 1a
(0.4 mmol) added to CHCl3 (4.0 mL) at 45 °C in succession; the
reaction was monitored by operando IR. (B) ConcIRT spectra of the
new component 1 (black curve) and authentic sample (pyridinium p-
toluenesulfonate, red curve). (C) ConcIRT spectra of the new
component 2 (black curve) and authentic sample (pyridinium p-
toluenesulfinate, red curve).
absorbance of 2b dropped to baseline as soon as pyridine was
added. In the meantime, a new component 2 was formed
immediately. This component was assigned to be pyridinium p-
toluenesulfinate when compared with authentic sample (Figure
1C). When 1a was added, the pyridinium p-toluenesulfinate
was consumed as the product 3ab generated from the reaction.
Besides the formation of 3ab, another formation of new
component 1 was observed accordingly. From authentic sample
(Figure 1B), this component was assigned as pyridinium p-
toluenesulfonate.
Additionally, to acquire further understanding of the effect of
the pyridine in this reaction, autoxidation of p-toluenesulfinic
acid (2b) was also monitored by in situ IR in the presence or
absence of pyridine. The kinetic profiles in Figure 2 clearly
Figure 2. Kinetic profiles of the autoxidation of 2b monitored by in
situ IR under different conditions: (A) 2b (0.5 mmol) in CHCl3 (4.0
mL) at 45 °C, monitored by 2b; (B) 2b (0.5 mmol), pyridine (0.41
mmol) in CHCl3 (4.0 mL) at 45 °C, monitored by pyridinium p-
toluenesulfonate.
show that the existence of pyridine prolongs the reaction time
of autoxidation of 2b, which means that the role of the pyridine
as a base is to reduce the reactivity and obviate the accelerated
oxygenation of the sulfinic acids.
On the basis of the results described above and previous
studies,7c,13 a proposed reaction pathway is depicted in Scheme
3. Initially, benzenesulfinic acid is quickly transformed into
Figure 3. Kinetic profiles of the pyridinium p-toluenesulfonate
monitored by in situ IR under different conditions: (A) 2b (2.0
mmol), pyridine (1.64 mmol), and 1a (0.4 mmol) in CHCl3 (4.0 mL)
at 45 °C; (B) autoxidation of 2b under same conditions without 1a.
C
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Journal of the American Chemical Society
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In conclusion, we have developed an environmentally
challenging dioxygen-triggered oxidative radical process using
dioxygen as the only oxidant, and offered an unprecedented
sustainable radical method for highly selective synthesis of
valuable β-keto sulfones via aerobic difunctionalization of
terminal alkynes. Notably, this reaction exhibits a wide range of
functional-groups tolerance. Preliminary mechanism revealed
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organic reactions. Our further efforts in this area are currently
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ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental procedure, characterization data, and copies of 1H
and 13C NMR spectra. This material is available free of charge
via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
aiwenlei@whu.edu.cn
■
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
Dedicated to Professor Zhi-Tang Huang on the occasion of his
85th birthday. This work was supported by the 973 Program
(2012CB725302), the National Natural Science Foundation of
China (21025206 and 21272180), the Program for Changjiang
Scholars and Innovative Research Team in University
(IRT1030) and the Research Fund for the Doctoral Program
of Higher Education of China (20120141130002).
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W. Angew. Chem., Int. Ed. 2012, 51, 4666.
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dx.doi.org/10.1021/ja4052685 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX