Aerobic oxysulfonylation of alkenes leading to secondary and tertiary β-hydroxysulfones

Article abstract of DOI:10.1002/anie.201301634

New channel! A novel and attractive dioxygen activation by sulfinic acids was explored that is capable of performing efficiently without the assistance of transition metals or radical initiators. This reaction furnishes secondary and tertiary β-hydroxysulfones under mild conditions; β-hydroperoxysulfone was isolated as an important intermediate. Copyright

Full text of DOI:10.1002/anie.201301634

Angewandte
Chemie
Communications
Dioxygen Activation
Q. Lu, J. Zhang, F. Wei, Y. Qi, H. Wang,
Z. Liu, A. Lei*
&&&& — &&&&
Aerobic Oxysulfonylation of Alkenes
Leading to Secondary and Tertiary b-
Hydroxysulfones
New channel! A novel and attractive
dioxygen activation by sulfinic acids was
explored that is capable of performing
efficiently without the assistance of tran-
sition metals or radical initiators. This
reaction furnishes secondary and tertiary
b-hydroxysulfones under mild conditions;
b-hydroperoxysulfone was isolated as an
important intermediate.
Angew. Chem. Int. Ed. 2013, 52, 1 – 5
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DOI: 10.1002/anie.201301634
Dioxygen Activation
Aerobic Oxysulfonylation of Alkenes Leading to Secondary and
Tertiary b-Hydroxysulfones**
Qingquan Lu, Jian Zhang, Fuliang Wei, Yue Qi, Huamin Wang, Zhiliang Liu, and Aiwen Lei*
Dioxygen is not only a green oxidant but also an ideal oxygen
source for the functionalization of organic molecules.[1]
Dioxygen activation has been of long-standing interest and
has fascinated organic chemists owing to its tremendous
potential usage in synthetic chemistry, bioinorganic chemistry,
enzymology, and so on.[1,2] Over the past few decades,
significant progress has been achieved. Many transition
metals, such as Pd,[3a,b] Cu,[3c–i] Ag,[3j] Mn,[3k–m] Fe[3n–j] and
others,[1,4] have been successfully applied to activate dioxygen,
whereas organic molecules has been seldom studied in this
area (Scheme 1).[4]
promising field.[6] In this regard, it is extraordinarily impor-
tant to find the new reactions to broaden this area. Owning to
our continuous interests in dioxygen activation,[7] we present
herein our recent progress in dioxygen activation mediated by
sulfinic acids, which illustrates an convenient method towards
the synthesis of secondary and tertiary b-hydroxysulfones
using simple materials under transition-metal-free conditions.
The chemical structure of b-hydroxysulfone is not only
a basic scaffold of numerous pharmaceutically important
molecules and synthetically fine chemicals, such as the hugely
successful and valuable bicalutamide, Sch42427 and SSY726
etc,[8] but is also an important precursor in the synthesis of
a series of useful biologically active molecules.[9,10] Generally,
b-hydroxysulfones are prepared by the opening of epoxides
with sulfinate salts,[11] the reaction of phenylsulfonylmethyl-
metalltic reagents with carbonyl compounds,[12] and chemical
or bioreduction of b-ketosulfones.[13] All of these methods
require multistep processes to synthesize starting materials
that often produce large amounts of unwanted byproducts,
which make them environmentally unfavorable and result in
poor functional-group tolerance. As such, tremendous limi-
tations remain in the synthetic scope of b-hydroxysulfone, and
efficient approaches towards tertiary b-hydroxysulfone is very
rare because of the difficult preparation of starting materi-
als.[14] Therefore, the development of direct, mild, and
environmentally benign processes to access b-hydroxysul-
fones, and in particular for tertiary b-hydroxysulfones from
basic chemical materials, are always highly desirable. To the
best of our knowledge, no examples in which b-hydroxysul-
fones were prepared from direct oxidative difunctionalization
of simple alkenes through dioxygen activation by organic
molecules have been reported.[15]
Scheme 1. Dioxygen activation by sulfinic acids for the formation of b-
hydroxysulfones.
Seeking dioxygen activation by organic molecules is an
extremely attractive and sustainable approach for introducing
oxygen functional groups in synthetic chemistry. Moreover,
such procedures obviate the need for purification of the
products for the residual catalysts and vastly broaden the
potential practicality for green synthesis of fine chemicals,
especially in the pharmaceutical industry. However, up to
now, only a few examples of dioxygen activation by organic
molecules have been reported, which mainly focused on the
oxidation of simple substrates, such as phenols, organic
sulfides, and alkenes.[5] Great challenges still remain in this
We initially chose a-methylstyrene (1a) and benzenesul-
finic acid (2a) as the substrates to test the reaction (Support-
ing Information, Table S1). To our delight, 1a and 2a reacted
smoothly to afford the desirable 2-phenyl-1-(phenylsulfonyl)-
propan-2-ol (3aa) in 46% yield in chloroform under air
atmosphere, without any additives. Subsequently, various
parameters were screened to optimize the reaction condi-
tions. The experiments showed that bases as an additive
seemingly played a key role in promoting the efficiency and
pyridine dramatically enhanced the yield to 98%. When other
bases, such as DBU, Et3N, Et2NH, and LiOH, were applied
instead of pyridine, the yield of the desired product more or
less decreased. Further screening of the solvents revealed that
chloroform was the best solvent for this reaction. It is
noteworthy that this reaction could also work well at room
temperature, although a longer reaction time was required.
Furthermore, when the reaction was carried out under a N2
atmosphere, only trace amount of desired product was
[*] Q. Lu, J. Zhang, Y. Qi, H. Wang, Z. Liu, Prof. A. Lei
College of Chemistry and Molecular Sciences, Wuhan University
Wuhan 430072 (P. R. China)
E-mail: aiwenlei@whu.edu.cn
Homepage: http://www.chem.whu.edu.cn/GCI_WebSite/
Prof. A. Lei
State Key Laboratory for Oxo Synthesis and Selective Oxidation
Lanzhou Institute of Chemical Physics
Chinese Academy of Sciences
Lanzhou 730000 (P. R. China)
F. Wei
School of Materials Science & Engineering
Nanjing University of Posts and Telecommunications (P. R. China)
[**] This work was supported by the “973” Project from the MOST of
China (2012CB725302) and the National Natural Science Founda-
tion of China (21025206, 20832003, and 20972118).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201301634.
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formed, which demonstrated the importance of the presence
of O2.
bulky substrate, 2-naphthylsulfinic acid, also efficiently
reacted with 1a giving the product in 80% yield (3ag).
Subsequently, we applied our method to a range of
alkenes to prepare an array of tertiary b-hydroxysulfones
using p-toluenesulfinic acid (2b) as the reaction partner. As
shown in Table 1, a variety of a-methylstyrene derivatives,
including methyl, tert-butyl, bromo, and trifluoro groups on
the aromatic ring, readily worked well in this reaction (3bb–
3 fb). Notably, a nitro substituent, which is usually unfavor-
able in reactions with radical participation, was also well
tolerated,[16] affording the corresponding tertiary b-hydrox-
ysulfone 3eb in 89% yield. The reaction of 2-(prop-1-en-2-
yl)naphthalene also proceeded well, giving the desired
product in 80% yield (Table 1, 3gb). Furthermore, this
reaction was also applicable to other a-alkylstyrenes, which
have steric hindrance at the substituent, as demonstrated by
the formation of the desirable products 3hb and 3ib in good
yields. Encouraged by these promising results, we further
applied our developed procedure to synthesize secondary b-
hydroxysulfones using styrene and its derivatives as sub-
strates. Pleasingly, p-toluenesulfinic acid (2b) reacted
smoothly with both electron-rich (methyl) and electron-
deficient (bromo, chloro, fluoro) group substituted styrenes,
and afforded the expected secondary b-hydroxysulfones in
excellent yield under slightly modified conditions (3jb–3ob).
Non-terminal and cyclic styrene derivatives, such as b-methyl
styrene and 1.2-dihydronaphthalene, were also effective to
give the expected products 3pb and 3qb in excellent yield
respectively. Delightfully, activated aliphatic alkenes, as
a case study of ethyl methacrylate, was also tested, affording
the desired product 3rb in 82% yield. Nevertheless, non-
activated aliphatic alkenes such as 1-butene is not amendable
to this procedure, which is probably due to the unfavorable
factors for the generation of the unstable intermediate.[14]
Furthermore, the identity of product 3 fb was unequivocally
established by its X-ray single-crystal structure (see the
Supporting Information for details).
With the optimized conditions in hand, we next explored
the scope of the reaction between various sulfinic acids and a-
methylstyrene 1a, and the results are summarized in Table 1.
Gratifyingly, a series of aryl sulfinic acids bearing both
electron-donating groups (R = OMe, Me) and electron-with-
drawing groups (R = F, Cl, Br) furnished to the desired
tertiary b-hydroxysulfones in excellent yields (3ab–3af). It is
noteworthy that Cl and Br substituents on the phenyl ring
were well tolerated, which enable a potential application in
further functionalization (3ad–3ae). Additionally, a more
Table 1: Substrate scope of the aerobic oxysulfonylation of alkenes with
various sulfinic acids.[a]
3aa 98%
3ab 92%
3ac 83%[b]
3ad 98%[b]
3ag 80%[b]
3ae 90%[b,c]
3bb 91%
3af 98%[b]
3cb 83%
3 db 83%
3eb 89%
3 fb 60%
To gain insight into the reaction mechanism, we designed
the following experiments. An isotope labeling experiment
was first conducted to elucidate the origination of the hydroxy
oxygen atom of the b-hydroxysulfones. The reaction of 1a
with 2a under an 18O2/N2 (1:4) atmosphere under the standard
conditions generated the 18O-labeled product in 97% yield
(Scheme 2). This result demonstrated that O2 took part in this
reaction and the oxygen atom in the product came from O2.
Furthermore, it is well-known that activation of dioxygen
mostly proceeded by a radical process.[3,4] Therefore, when
investigating the mechanism of oxidative difunctionalization
of alkenes, a radical pathway was also supposed involving in
this transformation, and radical trapping experiments sup-
ported this assumption. This reaction was extremely inhibited
in the presence of 2,2,6,6-tetramethyl-1-piperidinyloxy
3gb 80%
3jb 97%[e]
3hb 70%[b]
3kb 94%[e]
3ib 66%[d]
3lb 97%[e]
3mb 84%[e]
3nb 93%[e]
3ob 95%[e]
3rb 82%[b]
3pb 97%[e]
erythro/threo 20:1
3qb 90%[e]
trans/cis 35:1
[a] Unless otherwise specified, all reactions were carried out using
1 (0.20 mmol), 2 (0.40 mmol), and pyridine (0.18 mmol) in CHCl3
(4.0 mL) at 458C for 80 min under 1 atm of air (balloon). Yield of isolated
product after PPh3 workup. [b] p-Toluenesulfinic acid (b) (0.80 mmol),
pyridine (0.37 mmol), and 4 h were employed. [c] 6 h. [d] 2b (1.0 mmol)
and pyridine (0.9 mmol) were used. [e] 2b (2.0 mmol) and pyridine
(0.92 mmol) were employed at 608C.
Scheme 2. 18O isotope labeling experiment.
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(TEMPO) or 2,4-di-tert-butyl-4-methylphenol (BHT; see
radical trapping experiments in the Supporting Information),
which were well-known radical scavengers. These results
suggested that the reaction presumably underwent a radical
pathway.
To acquire further understanding of the reaction mecha-
nism, we next attempted to use in situ IR to monitor the
reaction between 1a and 2b. The profiles of relative
absorbance (ConcIRT) versus time for individual species is
shown in Figure 1. Clearly, when pyridine was added, the
absorbance of 2b immediately decreased sharply, which
Figure 1. The 2D kinetic profiles of the reaction of 2b (0.8 mmol),
pyridine (0.36 mmol), and 1a (0.4 mmol) added to CHCl3 (4.0 mL) at
458C in succession; the reaction was monitored by in situ IR spectros-
copy.
Figure 2. A) The 3D-FTIR profile of the reaction of 2b (0.4 mmol) and
pyridine (0.4 mmol). B) ConcIRT spectra of the new component (black
curve) and authentic sample (gray curve).
meant that an acid–base neutralizing reaction happened
first. When 1a was added, the signal of 3ab and a new
component increased gradually in intensity with the con-
sumption of 1a. The IR band of the new component was
assigned to pyridium p-toluenesulfonate by comparison with
an authentic sample (see the Supporting Information), these
results revealed that p-toluenesulfinic acid may be also
serving as a reductant during the reaction.
Scheme 3. Proposed mechanism.
The reaction was also monitored by in situ IR spectros-
copy in the absence of 1a (Figure 2). The kinetic profile
clearly shows that the signal of 2b disappeared immediately
while pyridine was added. Thereafter, two new bands at
1009 cmÀ1 and 1032 cmÀ1, which were unambiguously
assigned to pyridinium p-toluenesulfonate by comparison
with an authentic sample, increased proportionally. These
results indicated that sulfinic acid was easily oxidized to the
corresponding sulfonic acid under the standard reaction
conditions and pyridine may also serve as a base to neutralize
the sulfonic acid generated in situ.
Based on the aforementioned results and previous
reports,[15,17] a possible reaction pathway is proposed for the
formation of b-hydroxysulfones (Scheme 3). First, benzene-
sulfinic acid reacts with pyridine to release free sulfinyl anion
I, which could be further oxidized by dioxygen by single
electron transfer (SET) to induce the formation of an oxygen-
centered radical II resonating with sulfonyl radical III.
Subsequently, the radical addition of III to a-methylstyrene
(1a) affords carbon-centered radical IV, which reacts with
dioxygen by a redox-transfer process to produce alkylhydro-
peroxy radical intermediate V. And then the generated
intermediate Vaffords b-peroxylsulfone VI by single-electron
transfer (SET) and concomitant proton transfer (PT) from I
and pyridium benzenesulfonate, furnishes II/III. Finally,
subsequent reduction by benzenesulfinic acid or workup by
PPh3, gives the oxysulfonylation product 3aa.
To probe the feasibility of the pathway and prove the key
precursor VI is involved in this transformation, we attempt to
verify the production of b-peroxylsulfone by isolation and
characterization. To our delight, b-peroxylsulfone 4ab could
be readily isolated in excellent yield by simply changing
conditions at room temperature (Scheme 4) and it could be
quantitatively transformed to 3ab after PPh3 work up.
In conclusion, we have developed a highly attractive and
operationally simple intermolecular oxysulfonylation method
to construct secondary and tertiary b-hydroxysulfones with
Scheme 4. Isolation of peroxide 4ab.
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À
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Received: February 25, 2013
Revised: April 3, 2013
Published online: && &&, &&&&
Keywords: alkenes · dioxygen activation ·
.
oxidative difunctionalization · sulfinic acids · b-hydroxysulfones
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