Thiyl radical promoted chemo- and regioselective oxidation of CC bonds using molecular oxygen: Via iron catalysis

Article abstract of DOI:10.1039/c8gc02369g

The first example of the thiyl radical promoted ligand-free iron-catalyzed oxidative cleavage of alkenes using molecular oxygen (1 atm) has been developed. The reaction proceeds under mild reaction conditions with high efficiency and high chemo- and regioselectivity. It features a broad substrate scope and excellent functional group compatibility, enabling facile access to valuable molecules for application in medicinal chemistry. Preliminary mechanistic studies reveal that a vital intermediate dioxetane might be involved in the reaction and a thiyl radical plays a synergistic role in facilitating the selective oxidation of the CC bond.

Full text of DOI:10.1039/c8gc02369g

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Thiyl Radical Promoted Chemo- and Regioselective Oxidation of
C=C Bonds by Molecular Oxygen via Iron Catalysis
Received 00th January 20xx,
Accepted 00th January 20xx
Baojian Xiong, Xiaoqin Zeng, Shasha Geng, Shuo Chen, Yun He*, and Zhang Feng*
DOI: 10.1039/x0xx00000x
The first example of thiyl radical promoted ligand-free iron-catalyzed oxidative cleavage of alkenes by molecular oxygen (1
atm) has been developed. The reaction proceeds under mild reaction conditions with high efficiency and high chemo- and
regioselectivity. It features a broad substrate scope and excellent functional group compatibility, enabling a facile access to
valuable molecules for application in medicinal chemistry. Preliminary mechanistic studies reveal that a vital intermediate
dioxetane might be involved in the reaction and thiyl radical plays a synergistic role in facilitating the selective oxidation of
www.rsc.org/
C=C
bond.
cost, and safety.7-8
Introduction
Very recently, a significant photocatalytic method for
oxidative cleavage of C=C bonds has been reported by Wang
and co-workers, in which terminal and internal alkenes could
convert into the corresponding aldehydes and ketones in
moderate to excellent yields via an olefin-disulfide charge-
transfer complex.9 Due to their abundance, low toxicity and
cost, the use of iron-based catalysts has become more
attractive, leading to a renewed interest in this area.10-11 To
date, however, iron-catalyzed methods for the oxidative
cleavage of C=C bonds using molecular oxygen have been
rare.12 A protocol involving carbonyl derivatives and other
oxidation products was reported by the Demessie group,
where the control of chemoselectivity was regulated by the
pressure of O2 in the presence of N2.12a Iron-based
heterogeneous systems could also cleave C=C bonds to access
the corresponding ketones, albeit in poor to moderate yields
under high pressure of O2.12b-d Thus far, these iron-catalyzed
oxidative cleavage reactions have generally suffered from low
chemo- and regioselectivity, low efficiency, and poor
functional-group tolerance. In addition, the competing
formation of other oxidation products, including diols,
epoxides, allylic alcohols have generally been observed
(Scheme 1).12a,12e,13 Recently, Xiao and coworkers described a
method to achieve the oxidation of styrene derivatives to the
corresponding carbonyls using an unusual bisimidazoline
ligand which is not commercially available nor readily
accessible. This catalytic system suffered from a propensity for
alkene migration, leading to poor regioselectivity in some
cases.12f Till now, the issue of highly regioselective cleavage of
C=C bonds has not been fully exploited. Thus, we hope to
develop a more general catalytic system for the highly
selective cleavage of C=C bonds, making a complementarity
with previous methods. Herein, we describe a homogenous
catalytic system for ligand-free iron-catalyzed oxidation of
alkenes to carbonyl compounds using cheap, commercially
The development of highly selective, environmentally friendly,
and sustainable systems for selective chemical transformations
are among the most important goals in fundamental
researches as well as potential-industrial applications.1 The
oxidative cleavage of alkenes to carbonyl derivatives plays a
role of paramount importance in synthetic organic chemistry,
owing to the versatility of the carbonyl group,2 which can
allow late-stage diversification of complex molecules via
derivatization of the C=C bonds.3 Moreover, alkenes are ideal
sources for the preparation of carbonyl derivatives due to their
diversity and both natural and industrial abundance.4 Despite
simplicity of the transformation, mild and highly selective
protocols for oxidative cleavage of alkenes with excellent
functional group compatibility are still in great demand. The
classical approaches for this strategy typically suffer from (i)
generation of stoichiometric waste from the oxidants, such as
KMnO4, OsO4, oxone, TBHP, NaIO4, and PhIO/HBF4;5 and (ii)
low chemo- and regioselectivity, along with the concomitant
low efficiency and poor functional group tolerance owing to
the strongly oxidative properties of the typical oxidants. A
more attractive strategy utilizes ozone as the oxidant, which
generates only molecular oxygen as a byproduct. However,
this strategy cannot be widely applied due to the toxicity and
associated safety issues associated with ozone.6 Therefore, a
protocol for the oxidative cleavage of alkenes using safe and
environmentally friendly oxidants is highly appealing.
Molecular oxygen is an ideal oxidant in terms of availability,
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available compounds, bismuththiol (0.3$/g) and 1,1'- To test this hypothesis, we initiated our study by subjecting 2-
bis(diphenylphosphino)ferrocene (dppf) as catalysts under one phenyl-1-propene 1a to 1 atm of O2 in the presence of various
atmosphere of O2. This protocol features high chemo- and thiols and iron catalysts. Cysteine was firstly tested since it
regioselectivity, high efficiency and excellent functional group appears in many iron-sulfur proteins and is known to facilitate
compatibility.
the SET process.17 Gratifyingly, we found that acetophenone
was produced in 32% yield when Fe(ClO4)2 was used (Table 1,
entry 1). Other iron sources, such as FeCl2, FeBr2, and Fe(acac)3
Scheme 1 Strategies for the oxidative cleavage of alkenes
In previous works,12f,12h,13b a dioxetane was considered as a
plausible intermediate in the oxidative cleavage pathway,
which would decompose to give the desired carbonyl product.
Therefore, lowering the kinetic barrier to dioxetane formation
is critical to improve the chemo- and regioselectivity of this
oxidative cleavage reaction. It is known that thiyl radicals,
which can be easily generated via a single-electron-transfer
(SET) process,14 will undergo reversible addition to double
bonds, generating a reactive carbon-centered radical, which
could be intercepted towards the desired cycloaddition
products.14 It is also widely known that in biological systems,
iron-containing enzymes can initiate oxidation reactions by SET,
such as rubredoxins (iron-sulfur proteins), heme and nonheme
oxygenases which can selectively oxidize alkenes to carbonyl
compounds.16
Scheme 2 Proposed mechanism for oxidative cleavage of alkenes via the synergy
of thiyl radicals and iron catalysis
furnished
1 in less than 10% yields. Switching the thiol from
cysteine to ethyl mercaptoacetate S2 gave the corresponding
ketone in a promising 44% yield (Table 1, entry 2). Given
ferrocene’s propensity to undergo SET,18 we employed
ferrocene Fe1 in the reaction, which demonstrated promising
catalytic efficiency (Table 1, entry 3).
Design plan
Wang reported that a thiyl radical would generate through the
olefin-disulfide charge-transfer complex under visible-light
(vide supra).9 Hence, we envisioned a thiyl radical could be
initiated by oxygen8i or an iron catalyst (for details, see
Supporting Information), providing both the selectivity and
reactivity in the transformation of the key dioxetane
intermediate. The detailed mechanistic description of this
transformation is shown in Scheme 2. We proposed that a thiyl
radical could be produced, and then the addition to a C=C
bond by the thiyl radical produces the intermediate II, which
would be subsequently trapped by oxygen and iron species
Owing to the high volatility of thiol catalysts used, an
unpleasant smell was produced during the operation of this
reaction; to further improve this procedure, various solid thiols
were investigated (Table 1, entries 4-5). We were pleased to
find that heteroaromatic thiol, bismuththiol S4 could promote
this reaction, leading to moderate yield (Table 1, entry 5).
Furthermore, bismuththiol S4 could improve the reaction
efficiency and 67% yield was obtained using acetylferrocene
Fe2 (Table 1, entry 6). Encouraged by these results, several
ferrocene-based catalysts with different electronic properties
were evaluated (Table 1, entries 6-8). The best result, 81%
isolated yield was obtained (Table 1, entry 8) when 1,1'-
and generate an active intermediate III. The critical
intermediate dioxetane IV would be delivered when the thiyl
radical and iron species departs from III. Finally, the desired
ketone would be produced via the collapse of dioxetane.
Bis(diphenylphosphino)ferrocene Fe4
,
which has been
extensively used as a ligand in cross-coupling reactions, was
employed. Control experiments revealed the necessity of both
thiol and iron species. Only trace amount of desired product
was observed in the absence of thiol (Table 1, entry 9). Some
Results and discussion
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of the iron catalysts, such as ferrocene Fe1, Fe(ClO4)2 could substituents were also well-tolerated, producing the ketones
afford acetophenone in the absence of thiols, but in low yield in 62-72% yields. In addition, products 17 20 were formed in
,
with the concomitant formation of unidentifiable compounds good yields with no observed products deriving from olefin
(Table 1, entries 10-12). These results suggest that this isomerization, in contrast to previously-described methods.11f
reaction is indeed catalyzed by iron catalyst and the thiol Asymmetric diaryl ketones are multifaceted compounds in
might play a synergistic role in facilitating the selective organic chemistry, which are usually prepared from C−H
oxidation of C=C bonds.
arylation of aldehydes.19 Further, good yields were obtained,
when 1,1-diaryl alkenes were examined (18a 19a 23a 24a),
,
,
,
Table 1 Representative results for the optimization of the iron-catalyzed selective
oxidation of 2-Phenyl-1-Propene 1aa
showing that our protocol is an excellent alternative strategy
to access asymmetric diaryl ketones. Most remarkably,
naphthalene derivatives (15a
, 16a) and heterocyclic alkenes
(
22a-25a) were also suitable substrates, affording the
corresponding products in moderate to excellent yields (75-
76%, 53-82%, respectively). This is in a sharp contrast to
previous results,12f in which the substrates bearing a bulky
naphthyl group or a heterocyclic ring proceeded only in low
efficiency due to the steric and coordination effect between
the substrates and the catalyst.
Table 2 Scope of the iron-catalyzed oxidative cleavage of alkenesa
Fe SH
O
+
O2
Ar
R
MeCN, 80 o
C
Ar
R
2a-29a
2-29
Alkenes Scope
H2O
H2
O
O
O
O
O
MeO
Ph
MeO
5, 75%
3, 79%
4, 80%
8, 70%
2, 76%
O
O
O
O
OMe
6, 50%
MeO2C
F3C
NC
7, 81%
9, 85%
O
O
O
O
O2N
F
Br
Cl
10, 71%
11, 82%
12, 82%
13, 91%
aReaction conditions (unless otherwise specified): 1a (0.3 mmol, 1.0 equiv), O2 (1
atm), [Fe] (0.03 mmol, 0.1 equiv), thiol (0.03 mmol, 0.1 equiv), MeCN (0.5 mL), 80
oC, 15 h. bDetermined by 1H NMR using mesitylene as an internal standard. The
isolated yield is shown in parentheses.
O
O
O
O
Et
Ph
Me
MeO
O
14, 78%
15, 76%
16, 75%
17, 62%
With the optimized reaction conditions in hand, we
examined the scope of this iron-catalyzed oxidative cleavage
reaction. As shown in Table 2, when α-alkyl aryl ethylenes
were used as substrates, the reaction proceeded smoothly,
delivering the corresponding products in good to excellent
yields. Functional groups, such as methoxy, halides, nitro,
cyano, alkoxycarbonyl, carbonyl, and trifluoromethyl were
well-tolerated. Alkenes bearing alkyl group, led to the desired
O
O
O
O
Ph
Ph
O
Br
18, 72%
19, 72%
20, 62%
21, 66%
O
O
O
O
S
S
S
S
F
22, 66%
23, 59%
24, 53%
25, 82%
products in 76-79% yields
(2-3). Both electron-rich and
MeO
MeO
Me
O
electron-poor alkenes proceeded smoothly without loss of
efficiency, furnishing the corresponding ketones in good to
MeO
O
27a, 60%[b]
27a, 60%
26a, 60%
28a, 50%
29a, trace%
excellent yields (
strong electron-donating group methoxy were excellent
substrates, and the transformations with 4a 5a proceeded in
4-10, 50-85%). The alkenes containing a
aReaction conditions: alkenes (0.3 mmol, 1.0 equiv), O2 (1 atm), 1,1'-
bis(diphenylphosphino)ferrocene (0.03 mmol, 0.1 equiv), bismuththiol (0.03
mmol, 0.1 equiv), MeCN (0.5 mL), 80 oC, 15 h.
-
75-80% yields. The efficiency of this reaction was somewhat
reduced in the case of 6a bearing an ortho-substituent on the
aromatic ring. Notably, this reaction was not only limited to α-
Substrate 26a possessing two styrenyl unsaturated bonds
methyl aryl ethylenes. Substrates 17a
-
21a with bulky exhibited good reactivity, delivering the desired compound (26
)
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aReaction conditions: alkenes (0.3 mmol, 1.0 equiv), O2 (1 atm), 1,1'-
bis(diphenylphosphino)ferrocene (0.03 mmol, 0.1 equiv), bismuththiol (0.03
mmol, 0.1 equiv), MeCN (0.5 mL), 80 oC, 15 h.
in 60% yield. It should be addressed that electron-rich styrene
derivative 4-methoxystyrene
(27a) produced the desired
product in a moderate yield. Interestingly, substrate 28a
containing a less accessible, internal C=C bond also proceeded
well. Moreover, we observed that aliphatic alkenes (29a) were
not reactive, leading only to trace amount of the cleavage
product, likely owing to the equilibrium of the thiyl/olefin
addition reaction favoring the S-centered radical. This result
encouraged us to further investigate the regioselectivity of this
transformation (vide infra).
Substrate possessing an allyloxy group was also tolerated,
furnishing 31 in good yield (78%). Furthermore, substrates
with prolonged carbon chains could proceed well, yielding the
ketones (32
olefins could also be tolerated, as the substrates were
converted to the desired compounds (33 34) in moderate
, 35, 36, 39) in 50-71% yields. Multi-substituted
,
yields (60-62%) without loss of selectivity. Interestingly,
alkenes bearing alkynyl group were excellent substrates,
To evaluate the regioselectivity of this transformation, we
subjected substrates containing multiple-unsaturated bonds to
the optimized reaction conditions (Table 3). Moderate to
excellent yields of the desired compounds were obtained
without over-oxidation. Notably, we found that in these
substrates bearing multiple unsaturated bonds as substrates,
the oxidative cleavage preferred the conjugated π-extended
backbones, yielding the mono-oxidized products with high
furnishing the corresponding products (40
-
43) in 57-75%
conjugated
yields. It is noteworthy that 43a bearing
a
unsaturated bond system also proceeded well. These results
demonstrate excellent regioselectivity inherent to this thiyl
radical-mediated approach, which is complementary to the
traditional approaches.5-6
The inherent value of this selective alkene cleavage protocol
was further demonstrated by its applicability to bio-relevant
compounds. As such, flavanone- and estrone-derived alkenes
underwent this C=C bond cleavage smoothly, delivering the
efficiency (30-43, 50%-78% yields). The high selectivity of this
transformation could be accredited to the new generated π-
extended carbon-centered radical is more stable than the
unconjugated carbon-centered radical. This transformation
could also tolerate a wide range of functional groups on the
side chains. Base-sensitive groups, such alkoxycarbonyl,
trimethylsilyl, allylsilyl, and carbonate were amenable to this
protocol. Substrate (30a) containing an allyl group on the
aromatic ring provided the corresponding product 30 in 63%
yield. In contrast, with a double bond tethered to the α-alkyl
substituent, the substrate reacted with low efficiency, yielding
30% yield of 44 under the standard conditions. The low
efficiency was likely due to intramolecular radical cyclization.
desired products (45, 46) in reasonable to good yields (66%
and 40%, respectively). We could easily prepare fenofibrate 47
a pharmaceutical that is mainly used to reduce cholesterol
levels in people at risk of cardiovascular disease, from the
analogous olefin precursor in 67% yield. This late-stage
cleavage strategy could provide a versatile method to access
valuable molecules in medicinal chemistry. To demonstrate
,
the practicality of this transformation, the acetophenone
1
could be synthesized on a 25g scale, and was obtained in 62%
yield.
Table 3 Scope of the Iron-Catalyzed Regioselective Oxidative Cleavage of Alkenesa
Scheme 3 Synthesis of Biologically Active Compounds via Oxidative Cleavage of
Alkenes
Mechanism studies
To gain further insight into the mechanism of this oxidative
alkene cleavage reaction, a radical clock experiment was
carried out. Instead of ring opening product, compound 48
was obtained in 65% yield when (1-cyclopropylvinyl)benzene
was subjected to the standard reaction conditions.
Furthermore, to probe whether a free radical really exists in
this reaction, EPR studies of this reaction were conducted
using phenyl tert-butyl nitrone (PBN) as a spin-trapping agent,
and demonstrated that a free radical was involved in this
reaction (for details, see the Supporting Information).12f
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Additionally, we observed trace amount of vinyl thioether 49
on LC-MS; we speculate that compound 49 was produced from
intermediate II’ via a hydrogen abstraction process. So far, we
have not been able to directly observe the key intermediate
dioxetane owing to its instability. Inspired by Wang’s work, we
were pleased to observe that diol 50 was successfully isolated
when the reaction was performed in the presence of
stoichiometric amount of methionine and water.9, 20These
results suggest that the proposed mechanism illustrated in
Scheme 2 is reasonable. Continued studies are ongoing in our
laboratory to further elucidate the mechanism of this reaction,
including direct observation of key intermediates.
Acknowledgements
We are grateful for the financial support from National Natural
Science Foundation of China (Nos. 21572027, Nos. 21801029)
100 Talent Plan from Chongqing University (0247001104405).
We thank Prof. Xingang Zhang (Shanghai Institute of Organic
Chemistry), Dr. Gang Li (Princeton University) and Dr. Jeffrey
M. Lipshultz (Princeton University) for helpful discussions.
Notes and references
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Scheme 4 Mechanistic Studies
Conclusions
In conclusion, we have developed the first example of
oxidative cleavage of alkenes to carbonyl compounds via an
iron/thiol dual catalytic system. The reaction proceeds under
mild conditions at one atm of O2 with high chemo- and
regioselectivity and broad substrate scope, as well as excellent
functional group compatibility. A notable feature of this
method is its compatibility for late-stage oxidation of bioactive
compounds in good yields, providing good opportunities for
applications in drug discovery and development. Preliminary
mechanistic studies suggest that a dioxetane intermediate
might be involved in this reaction, and both iron and thiyl
radical play important roles in the catalytic cycle. Further
studies to expand this novel transformation are undergoing in
our lab and the results will be reported in due course.
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Conflicts of interest
There are no conflicts to declare.
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