Fe-catalyzed dithiane radical induced C?S bond activation?addition to α, β-unsaturated ketones

Article abstract of DOI:10.1002/adsc.201800241

An efficient and clean strategy to construct organosulfur compounds has been developed via a Fe-catalyzed dithiane C?S bond activation/addition process with α, β-unsaturated ketones. This C?S activation protocol exhibits excellent reactivities, and up to 92% yield of the corresponding thioether-thioester derivatives could be obtained under the mild conditions, allowing the ready preparation of a number of synthetically valuable S-linked conjugates. (Figure presented.).

Full text of DOI:10.1002/adsc.201800241

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DOI: 10.1002/adsc.201800241
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Fe-catalyzed dithiane radical induced CÀS bond
activationÀaddition to a, b-unsaturated ketones
a
a
a
a,
a, b,
Deng Min, Xinyu Yuan, Teng Liu, Jian Liu, * and Shouchu Tang *
a
School of Pharmacy, Lanzhou University, Lanzhou 730000, People’s Republic of China
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E-mail: tangshch@lzu.edu.cn; jianliu@lzu.edu.cn
State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, Peopleꢀs Republic of China
b
Received: February 14, 2018; Revised: March 13, 2018; Published online: &&&, &&&&
Supporting information for this article is available on the WWW under https://doi.org/10.1002/adsc.201800241
version of suitable coupling partners to the thioethers
or/and thioesters would be worthwhile.
Abstract: An efficient and clean strategy to con-
[
5]
struct organosulfur compounds has been developed
via a Fe-catalyzed dithiane CÀS bond activation/
addition process with a, b-unsaturated ketones.
This CÀS activation protocol exhibits excellent
reactivities, and up to 92% yield of the correspond-
ing thioether-thioester derivatives could be ob-
tained under the mild conditions, allowing the
ready preparation of a number of synthetically
valuable S-linked conjugates.
Dithianes, introduced by Corey and Seebach are
attractive building blocks for construction of CÀC
[
6]
bonds in organic synthesis (Scheme 1). Extensive
research in the unique umpolung field have resulted in
the development of nucleophilic addition of a, b-
unsaturated alkenes through the anionation of di-
[
7]
thiane as an acyl anionic synthon. In 2014, Denmark
group developed a novel addition of dithiane deriva-
tives to unactivited a, b-unsaturated ketones and
esters using a fluorodesilylation strategy via a dithian-
Keywords: Dithiane; Organosulfur Compounds;
CÀS Addition; Conjugates; Iron
[
8]
yl anion process. Next year, Gaggero reported an
efficient organocatalytic conjugate addition of 2-
[
9]
carboxythioesters-1,3-dithiane to nitroalkenes. While
these transformations are important, radical processes
Organosulfur compounds such as thioethers, thioest- of 1,3-dithianes to alkenes can offer a feasible
ers, and their derivatives are widely present in a alternative to anionic reactions that can lead to similar
[
10]
variety of biologically active molecules and play products. Among these, there are several reports on
important roles in areas of bioconjugate chemistry, the radical atom transfer additions of alkenes with
[
1]
polymer science, and pharmaceutical chemistry.
dithianyl radicals through the use of radical initiator
[10b] [10c]
Organosulfur compounds also serve as useful synthetic groups such as phenylseleno,
intermediates with applications in organic and medic-
xanthates,
and
[2]
inal chemistry. Generally, reliable protocols to access
the organosulfur compounds include thiol-Michael
additions and thiol-ene reactions of diverse alkenes
[3,4]
with free thiols.
These are mechanistically distinct
transformations that have been widely recognized as
powerful methods for the generation of a wide variety
of organosulfur compounds. However, these available
processes use highly volatile and stinky free thiols,
leading to serious environmental, safety problems as
well as the issues of the side reactions owing to the
oxidation of free thiols. In addition, common thiol-
addition methodologies require harsh conditions and
the use of rare, expensive, and toxic metal-photo-
catalysis. In order to minimize the encountered
problems, the development of an environmental,
efficient and concise protocol that enables the con-
Scheme 1. Development of coupling reactions of dithianes.
Adv. Synth. Catal. 2018, 360, 1–6
1
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ÞÞ
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TEMPO . More recently, Leow and co-workers been used provided unsatisfactory yields of the
[11b]
reported a visible-light photocatalytic addition of 1,3- desired product.
The use of a solvent such as
was found to be critical in obtaining high
tionally, Koike and Akita reported the reaction of yields. As compared with photocatalyzed dithianyl
[
10e]
[11a–c]
benzodithianes to several Michael acceptors.
Addi- DCE
[
10e–g]
potassium 1,3-dithian-2-yltrifuoroborate with terminal radical coupling chemistry
olefins bearing electron withdrawing groups.
2017, Cozzi and Sambri have developed an effective radical reaction is highly unusual.
photocatalytic addition of dithiane-2-carboxylates to
Michael acceptors by Iridium (III) photocatalyst.
Over the past years, our group has a long-standing
interest in the radical oxidative coupling reactions
from the use of commercially available dithiane. We
recently developed a radical oxidative coupling reac-
tion of aryl alkenes with 2-chloro-dithiane for the
construction of complex dithiane derivatives by in situ
generation of dithianyl radical. As part of an ongoing
project on the development of new transformation of
dithiane, we examined the reaction of a, b-unsaturated
ketones with 1,3-dithiane via the corresponding radi-
cal oxidative process. Interestingly, the iron catalyzed
dithiane radical route with a, b-unsaturated ketones
was unique in comparison with the results of tradi-
tional cross-coupling addition reactions of a, b-unsatu-
rated alkenes via a dithianyl radical process. Herein,
we report the first example of the direct synthesis of
thioether-thioformates via a CÀS bond activation of
, the formation of
[
10f]
In thioether-thioester molecule in this type of oxidative
[
10g]
Table 1. Optimization of addition of enone 1a with dithia-
1
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2
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[a]
ne.
[
11]
[b]
Entry
Variation
none
Yield (%)
1
2
3
4
5
6
7
8
9
89
trace
78
62
45
33
52
70
FeCl (10 mol%), under Ar, 12 h
3
FeCl (10 mol%), 24 h
3
Fe(acac) (10 mol%), 12 h
3
FeS (10 mol%), 24 h
FeSO (10 mol%), 24 h
4
FeSO ·7H O (10 mol%), 12 h
4
2
FeCl ·4H O (10 mol%)
2
2
BF ·Et O (10 mol%), 24 h
trace
30
3
2
[
c]
1,3-dithiane and an addition to a, b-unsaturated 10
ketones by an inexpensive iron catalyst.
Accordingly, we began our study by evaluating the 12
coupling of enone 1a with 1,3-dithiane 2 (Table 1).
Previously, FeCl has been successfully proved to be
optimal in these dithianyl radical oxidative coupling
reactions; thus, it was utilized in this system.
Surprisingly, there was no dithianyl cross-coupling
product observed under this typical conditions (Ta-
ble 1, entry 2). In particular, we found that by
increasing the reaction time and by exposing the
reaction mixture to air, a modest yield of thioether-
thioformate coupled product 3a could be isolated. In
contrast, the expected cross-coupling product 4a was explored the substrate scopes with respect to the aryl
not observed. To the best of our knowledge, the enone component with 1,3-dithiane 2 to provide a
process of the direct formation of thioether and range of thioether-thioester derivatives (Scheme 2).
thioester simultaneously has not yet been reported. Consequently, this transformation tolerates a variety
The significance of this result from the viewpoint of of substituents on the aryl ring of enones. Aryl enones
conjugate chemistry, as well as sulfur chemistry, containing both electron-withdrawing groups (F, Cl
prompted us to carry out further study. As expected, and Br) and electron-donating groups (Me and OMe)
the inclusion of water in the reaction mixture on the aromatic rings were all found suitable for this
DTBP (200 mol%), 24 h
NBS instead of NCS, 24 h
DCE instead of DCM
[d]
11
-
81
1
3
THF instead of DCM, 12 h
trace
[a]
3
Reaction condition: 2 (30 mg, 0.25 mmol), NCS (40 mg,
0
0
.30 mmol), 1a (30 mg, 0.21 mmol), FeCl ·6H O (7 mg,
.025 mmol), DCM (2 mL), room temperature.
3 2
[
11a]
[
[
[
b]
c]
d]
Isolated yield.
DTBP=di-tert-butyl peroxide.
Not detected.
With the optimized protocol in hand, we then
[
3a,d,4g]
improved the yields.
An examination of different reaction. Substrates with ortho (1b–1d), meta (1e and
iron sources was carried out. Gratifyingly, the use of 1f), and para (1g and 1h) substituents were tolerated
easily available catalyst FeCl ·6H O gave the best with comparable efficiencies. Chalcone and the corre-
3
2
result. The reaction proceeded to completion in just sponding chalcone derivatives were also viable sub-
over 3 h at room temperature in the presence of strates for this reaction (1n–1q). Increasing the
10 mol % FeCl ·6H O, and the product 3a was bulkiness of R of aryl enones resulted in comparable
obtained in 89% yield. Interestingly, the addition of yields. Good yields (68–89%) and the corresponding
base or the use of NBS as a promoter did not coupled type products were observed within 3–5 hours
significantly improve the coupling process. Reactions in all cases. These thioester-thioether derivatives could
employing Lewis acid BF ·Et O that had previously
3
2
3
2
Adv. Synth. Catal. 2018, 360, 1–6
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be challenging to access via the existing CÀS bond Table 2. Substrate scope of simple alkyl enones.
methodologies.
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[a]
Scheme 2. Scope of aryl enones. Unless otherwise specified
(30 mg, 0.25 mmol), NCS (40 mg, 0.30 mmol),
(0.21 mmol), FeCl ·6H (7 mg, 0.025 mmol) in DCM
2
1
O
2
3
(
2 mL) was stirred for 3–5 h at room temperature under air.
Isolated yield. 0.3 mmol of alkene was used.
[
b]
[c]
[a]
Reaction conditions: 2 (30 mg, 0.25 mmol), NCS (40 mg,
.30 mmol), (0.21 mmol), FeCl ·6H O (7 mg,
0.025 mmol) in DCM (2 mL) at room temperature under
The substrate scope of the alkyl enones was briefly
0
1
3
2
examined under standard reaction conditions as
shown in Table 2. Not surprisingly, cyclic enones could
be effectively coupled with 1,3-dithiane under the
same conditions. Excellent yields (78–92%) were
obtained for additions to a variety of cyclic enones,
including five- and seven-membered rings (1aa and
1ad, respectively). In fact, these coupling reactions
air, 3 h.
Isolated yield.
[
[
b]
c]
5
mol % of catalyst was used, 1 h.
In the interest of expanding the reaction scope, we
proceeded with lower quantities of catalyst (5 mol%) focused on the other electron-deficient alkenes as
in shorter reaction times, were still providing the substrates. Unexpectedly, a variety of a, b-unsaturated
desired products in comparable yields. Sterically Michael acceptors did not participate successfully
encumbered enone (1ac) was also viable substrates. under the optimized reaction conditions (Scheme 3).
Particularly noteworthy is the reaction of the deriva- In these cases, a, b-unsaturated esters (5e–5g) remain
tive of cycloheptyl enone, which proceed to afford challenging substrates for this methodology, which is
product 3ae as single product in 83% yield. Interest- most likely due to differences in the relative coordina-
ingly, alkyl substituents of enones (1af and 1ag) had tion of a, b-unsaturated esters and enones with
no significant impact on its reactivity. The above dithianyl and iron species. It was noted that attempts
procedure proved to be general for the synthesis of to extend this method to 2-substituted dithiane
organosulfur compounds bearing a 1,3-bithiopropanyl substrates met with difficulty under the current
linkage that would provide a useful complement to reaction conditions.
the common S-linked conjugates.
Adv. Synth. Catal. 2018, 360, 1–6
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ÞÞ
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activation/addition process. At present, our results
suggest that a coordination transition state II formed
[13]
predominately under the reaction conditions. This
mechanism is not similar to that of the previous
photocatalyzed addition reaction processes.
In conclusion, we have developed a protocol for
the preparation of organosulfur compounds via a
tandem process involving an iron-catalyzed CÀS bond
activation of dithiane followed by an addition to a, b-
unsaturated ketones. This operationally simple meth-
od uses a readily available iron catalyst in an efficient
manner, particularly the absence of thiols, to access an
array of valuable thiolether-thioester compounds in
excellent yields and with short reaction times. Impor-
tantly, the CÀS bond activation method provides a
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Scheme 3. Unsuccessful a, b-unsaturated acceptors.
In order to aid interpretation of the mechanism general complementary for the efficient and safe
process, radical trapping experiments and oxygen- generation of a range of novel organosulfur conju-
labeling experiments were performed. The addition gates. Further investigations into the asymmetric
of TEMPO as a radical scavenger quenched the version of dithiane induced activation, as well as the
addition reaction, demonstrating that a dithiane radi- application in biological conjugates area are ongoing
cal process might take place under the iron catalyzed in our laboratory.
[
12]
1
8
reaction conditions. Meanwhile, the O-labeled prod-
1
2
8
uct was formed in the presence of H O, suggesting
Experimental Section
On the basis of previous studies in radical oxidative General procedure for compound 3: To a flame-dried 10 mL
that water was involved in the thioformates formation.
[11]
flask 1,3-dithiane 2 (30 mg, 0.25 mmol) and NCS (40 mg,
.30 mmol), DCM (2 mL) after dissolved the mixture was
stirred at 08C, then 1 (0.21 mmol) and FeCl ·6H O (7 mg)
couplings and above experimental results, a plau-
sible mechanism for this process is depicted in
Scheme 4. Firstly, the dithianyl radical is generated via
the activation of 2-H-dithiane with NCS under air
0
3
2
were added at room temperature. Reaction mixture was
stirred at room temperature for 3–5 h until the reaction was
completed (as determined by TLC analysis). Then reaction
[11b]
atmosphere.
The generated dithiane radical under-
goes a single-electron transfer (SET) with Fe (III)
catalyst to facilitate ionization for the formation of
ionic species I, which then reacts with water originated
mixture was diluted with ethyl acetate (10 mL) and H O
2
(5 mL). The organic layer was separated, and the aqueous
phase was re-extracted with ethyl acetate (33 3 mL). The
from FeCl ·6H O or air, affording the corresponding
3
2
combined organic extracts were washed with H O (10 mL),
2
2-OH-dithiane. Meanwhile, iron catalyst also acts as a
and dried over anhydrous MgSO . The solvent was removed
4
Lewis acid to facilitate the coordination process with under vacuum, and then the residue was purified by column
[
11c]
the carbonyl group of a, b-unsaturated ketone 1.
chromatography on silica gel with petroleum/ethyl acetate
Subsequently, the dithianyl ring opening and conju- (EA/PE=1: 50~1:5) to yield the product 3.
gate addition would take place to furnish the thio-
ether-thioformate products via a dithianyl CÀS bond
Acknowledgements
We thank the National Natural Science Foundation of China
21772074, 21602185), the Fundamental Research Funds for
(
the Central Universities (861098) and the Natural Science
Foundation of Fujian Province of China (2016J05047) for
financial support. We thank Prof. Xiaolei Wang of Lanzhou
University for helpful discussions.
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ꢁ 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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COMMUNICATIONS
Fe-catalyzed dithiane radical induced CÀS bond
activationÀaddition to a, b-unsaturated ketones
Adv. Synth. Catal. 2018, 360, 1–6
D. Min, X. Yuan, T. Liu, J. Liu*, S. Tang*
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