Photoinduced rearrangement of vinyl tosylates to β-ketosulfones

Article abstract of DOI:10.1039/c7gc01467h

We developed a photoinduced radical fragmentation and rearrangement of vinyl tosylates that enables efficient formation of β-ketosulfones. This process is based on the photoinitiated homolysis of vinyl tosylate to release a sulfinyl radical from the tosyl group and the subsequent addition of a sulfinyl radical to another vinyl tosylate to form the desired β-ketosulfones. This simple protocol features a broad scope with both aromatic and aliphatic substrates, convenient reagents and operating systems.

Full text of DOI:10.1039/c7gc01467h

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Photoinduced Rearrangement of Vinyl Tosylates to β-Ketosulfones
Lili Xie, Xiaomeng Zhen, Shuping Huang*, Xiaolong Su, Mai Lin and Yi Li*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
We developed
a
photoinduced radical fragmentation and
O
rearrangement of vinyl tosylates that enables efficient formation
of β-ketosulfones. The process is based on photoinitiated
homolysis of vinyl tosylate to release sulfinyl radical from the tosyl
group of and subsequent addition of sulfinyl radical to another
vinyl tosylate form the desired β-ketosulfones. This simple
protocol features board scope with both aromatic and aliphatic
substrates, convenient reagents and operating system.
+
X
R2
a)
RO2S
R1
(X = Cl, OR, benzotriazole)
Base
R2
d)
O
R1
RSO2X
+
b)
X
O
R1
(X = Cl, Na, OH, NHNH2)
SO2R
R1
(X = Cl, Br, I, OTs)
+
SO2R
O
R2
RSO2Na
hv
R1
e)
Ox
R2
OH
O
Organo sulfones are of considerable importance in synthetic
and medicinal chemistry.1 Particularly,
c)
R1
SO2R
or
SR
This work
R1
β-ketosulfones have
R2
R2
strongly attracted synthetic pursuit of chemists, not only due
to their versatile synthetic applications in organic synthesis,
but also because of their particular pharmaceutical relevance
exhibiting an extensive and broad range of biological
activities.2 Given their value in chemistry and biological
systems, intensive efforts have been devoted to the synthesis
Scheme 1 Pathways of sulfonylation.
ketosulfones.7a In 2016, Lei and co-workers also developed a
direct approach to -ketosulfones via aerobic oxysulfonylation
of vinyl phosphates with aromatic thiols.8b Recently, Wang et
al. disclosed sustainable synthetic approach to β-
β
a
of
-ketosulfones are acylation of alkyl sulfones with acid
chlorides, esters or N-acyl benzotriazoles (Scheme 1, a);3
β-ketosulfones. The representative protocols to synthesize
ketosulfones with alkenes and sulfonic acids.6d While
impressive progresses have been made in this area, many of
the reported examples generally required the addition of a
large excess amount of sulfonylation reagents and external
oxidants. Therefore, the development of new and efficient
β
alkylation of metallic arene sulfinates with
α
-halo- or
α
β
-
-
tosyloxy ketones (Scheme 1, b);4 and oxidation of
ketosulfides or
(Scheme 1, c).5
β-hydroxysulfones with stoichiometric oxidants
access to synthesize
β-ketosulfones still remains highly
desirable, especially with less or no use of oxidants, boarder
substrate scopes, and milder reaction conditions.
Recently, radical sulfonylation has drawn intense attention as
a useful approach to -ketosulfones. General strategies to
synthesize
-ketosulfones were applying alkenes,6 alkynes,7 or
β
Recently, visible-light promoted photoredox has leveraged
remarkable achievements.9 Among which, organic dyes as
photosensitizers showed their superiority to transition metal
catalysts, such as ruthenium and iridium complexes, with
regard to their lower cost and less toxicity.10 Considering these
advantages of reaction diversities and sustainable conditions,
possible application of photoredox in designing and identifying
β
activated alkenes8 to reactions with various radical
sulfonylation reagents (Scheme 1, d). For examples, in 2013,
Wang group reported a copper-catalyzed oxysulfonylation
reaction of alkenes with sulfonylhydrazides for the synthesis of
β
-ketosulfones.6a Lei et al. demonstrated an aerobic oxidative
difunctionalization of alkynes with sulfonic acids to generate β-
useful paradigms for alternative
β-ketosulfone synthesis is
particularly appealing. As shown in Scheme 1 e, we anticipated
that an oxidative fragmentation might be triggered on vinyl
tosylate under visible-light promoted photoredox, thereby
converting it into sulfinyl radical specie, which would
subsequently undergo a sequence of radical addition and
Department of Chemistry, Fuzhou University, Fuzhou 350116, China.
Corresponding author Email: liyi-chem@fzu.edu.cn
.
† Footnotes relaꢀng to the ꢀtle and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
rearrangement to afford β-ketosulfone.
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Table 1 Reaction optimizationa
Table 2 Reaction optimization for aliphatic vinyl tosylatea
O
OTs
photoinitiator (1 mol%)
5 W white LED
Ts
solvent, 12 h, rt.
F3
C
F3C
2a
1a
Cl
Cl
Cl
Cl
entry
initiator (X mol%)
eosin B (1 mol%)
conversionb
CO2Na
N
S
CO2Na
I
R
R
I
Me
Me
1
2
3
4
5
6
7
8
17%
NR
N
N
NaO
O
O
Cl
Me
Me
NaO
O
O
Br
Br
eosin Y (1 mol%)
I
I
R=Br, Eosin Y
R=NO2, Eosin B
Methylene Blue (MB)
Rose Bengal (RB)
methylene blue (1 mol%)
rose bengal (1 mol%)
fac-Ir(ppy)3 (1 mol%)
benzophenone (5 mol%)
acetophenone (5 mol%)
9-fluorenone (5 mol%)
NR
22%
NR
entry
1
initiator
eosin Y
solvent
conversionb
6%
tBuOH:H2O 1:1
tBuOH:H2O 1:1
tBuOH:H2O 1:1
tBuOH:H2O 1:1
tBuOH:H2O 1:1
tBuOH:H2O 1:1
CH3CN
NR
2
rose bengal
9-fluorenone
methylene blue
fac-Ir(ppy)3
eosin B
3%
NR
3
33%
99% (95%)
4
43%
aVinyl tosylate 1ad (0.2 mmol), 5 W white LED, solvent (1 mL), and initiator (X
mol%), under Ar at room temperature for 12 h. bDetermined by crude 1H NMR.
cIsolated yield was shown in parentheses.
5
41%
6
46%
7
eosin B
7%
While
aromatic
vinyl tosylates underwent these
8
eosin B
DMF
88%
transformations in excellent yields under optimized conditions
in Table 1, aliphatic derivatives did not, as tert-butyl vinyl
tosylate 1ad reacted to afford 2ad in a disappointing yield of
17% (Table 2, entry 1). Therefore, we set out to further
evaluate different photoinitiators for aliphatic substrates. To
our delight, reaction with the use of 9-fluorenone11 as
photoinitiator resulted 2ad in excellent yield (Table 2, entry 8).
With the optimized reaction conditions in hand, we then
evaluated the synthetic potential of this transformation.
9
eosin B
DMSO
96% (94%)c
50%
10
11
12d
13
14e
eosin B
H2O
eosin B
DMSO:H2O 1:1
DMSO
78%
eosin B
95%
-
DMSO
N.R.
eosin B
DMSO
N.R.
aVinyl tosylate 1a (0.2 mmol), 5 W white LED, solvent (1 mL), and initiator (1
mol%) at room temperature for 12 h. bDetermined by crude 19F NMR. cIsolated
yield was shown in parentheses. dReaction in 1 h. eReaction under daylight
lamp.
Table 3 Scope of aromatic vinyl tosylatea, b
The feasibility of our exception was tested by exposing vinyl
tosylate (1a) in a mixed solvent (tBuOH:H2O 1:1) under visible
light irradiation from 5 W white light-emitting diodes (LEDs)
(Table 1). Performing the transformation in the presence of
the commercially available photosensitizer eosin Y (1 mol%)
for 12 h at room temperature, we were delight to observe that
2a R1 = CF3: 94%;
2b R1 = H: 99%;
2c R1 = F: 98%;
2d R1 = Cl: 86%;
2e R1 = Br: 82%;
2f R1 = I: 92%
2g R1 = CN: 95%
2h R1 = CO2Me:87%
2i R1 = Me: 99%
the corresponding β-ketosulfone 2a was formed, albeit in 6%
2j R1 = Cl: 93%
2m R1 = Cl: 83%
conversion (entry 1). Further screening of photoinitiators
revealed that eosin B was the best for this transformation with
moderate conversion of 46% (entry 6). A number of other
photoinitiators, including rose bengal (RB), methylene blue
(MB) and fac-Ir(ppy)3 were subsequently investigated (entries
2-5), and they were found to be less effective than eosin B.
Gratefully, of solvents tested, isolated yield of the desired
product 2a dramatically enhanced to 94% when the reaction
was performed in DMSO (entries 7-11). Furthermore, we were
delighted to find that one hour turns out to be a sufficient
reaction time with an equally matched conversion of 95%
(entry 12). In the control experiments, this transformation was
found to be completely suppressed in the absence of the
photoinitiator or irradiated by a daylight lamp (entries 13, 14).
2k R1 = Br: 94%
2n R1 = F: 90%
2l R1 = OMe: 99%
2o R1 = Me: 87%
2p R1n = 3-Br-4-F: 95%
2q R1n = 3,4-Me2: 94%
2r R1n = 2,4-Cl2: 99%
2s R = 2-naphthyl: 94%
2t R = 4-pyridyl: 92%
2u R2 = C6H5: 84%
2v R2 = 4-ClC6H4: 88%
2wR2 = 2,4-(Me)2C6H3: 99%
2x R2 = Me: 93%
2aa 92%
2ab 95%
2y R2 = Et: 85%
2z R2 = 10-camphoryl: 87%
aVinyl tosylate 1 (0.2 mmol), 5 W white LED, DMSO (1 mL), and eosin B (1 mol%),
under Ar at room temperature for 12 h. bIsolated yield.
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Table 4 Scope of aliphatic vinyl tosylatea, b
Scheme 2 Mechanistic studies.
O
Ts
2ac 97%
2ad 94%
2ae 93%
2af 89%
2ah n = 3: 77%
2ai n = 9: 35%
2ak 67%
single isomer
2ag 80%
2aj 66%
Figure 1 Proposed mechanism.
aVinyl tosylate 1 (0.2 mmol), 5 W white LED, DMSO (1 mL), and 9-fluoroenone (5
mol%), under Ar at room temperature for 12 h. bIsolated yield.
Generally, reactions of aromatic vinyl tosylates containing
either electron-withdrawing or electron-donating groups
proceeded efficiently to provide the corresponding products in
good to excellent yields (Table 3, 2a-2o). The nature of the
substituents has almost no effect on the overall course of the
reaction. A wide range of functionalities such as halogen
(especially I (2f)), CN (2g), and ester (2h) groups were
compatible with this reaction, which were suitable for further
potential functionalization. Notably, bisubstitutional aromatic
vinyl tosylates are tolerated as well as substrates with meta-
four possible crossover products (2n, 2al, 2v and 2b) were
detected in a ratio of 1.4:1:1.4:1 (Scheme 2, b). Furthermore,
the apparent quantum efficiency of the model transformation
with 1a was calculated to be 9.4, which indicates that this
protocol probably processed via photo-induced chain
mechanism. In order to clarify the initiation mechanism, we
applied DFT calculations on the triplet energy of
photosensitizer and vinyl tosylate. 9-Fluorenone and (z)-but-2-
en-2-yl methanesulfonate were chosen as the simplified
models. The calculated triplet energy of 9-fluorenone is 219
kJ/mol (211 kJ/mol by literature)12 and the value of substrate is
121 kJ/mol. These results supported the possibility of energy
transfer between the initiator and the substrate. (more details
found in the SI)
para- or ortho-para-substitutents on the aromatic rings (2p
-
2r). Heterocyclic vinyl tosylates could also serve as suitable
substrates in this procedure and gave the desired products (2s
and 2t) in 94% and 92% yields. Furthermore, aromatic (2u
-2w)
or aliphatic substituents (2x-2z) on the sulfone-motifs were
also amendable to this protocol, exhibiting equally reactivity
and affording the expected products in mostly good to
excellent yields (84%-99%).
This protocol was next extended to the synthesis of aliphatic β-
ketosulfones (Table 4). Remarkably, the introduction of
sterically hindered substituents, such as iso-propyl (2ac), tert-
butyl (2ad) or adamantly (2ae), had almost no impact on the
On the basis of the above results and literature surveys, we
proposed a putative reaction cycles as shown in Figure 1.
Through energy transfer from the excited photosensitizer,
vinyl tosylate underwent homolytic decomposition to generate
enol radical and sulfinyl radical (initiation cycle). The addition
of sulfinyl radical to another vinyl tosylate affords the
reactivity; and the desired β-ketosulfones were well generated
in good to excellent yields (93%-97%). In addition, substrates
1af and 1ag, derived from cyclic ketones, also gave the
expected products in 89% and 80% yields. Linear aliphatic vinyl
tosylates with long chains reacted smoothly to give the
intermediate I. Radical reconstruction of I would eventually
corresponding
(Table 4, 2ah
β-ketosulfones in moderate to good yields
2ai and 2aj). We also examined this protocol to
form the desired product and another sulfinyl radical for next
reaction cycle (chain propagation).
,
the late-stage modification of natural product (2ak, 67%).
To gain preliminary inspect into this transformation, we
carried out radical-trapping experiments with 1b under the
standard conditions. The addition of the radical scavenger
TEMPO completely inhibited the reaction and 1b was
recovered in 99% yield, suggesting the involvement of a radical
mechanism (Scheme 2, a). The crossover experiment was
performed by applying an equimolar amount of 1n and 1v to
the standard conditions. As determined by the crude NMR,
Conclusions
In conclusion, we have described a new strategy to form β-
ketosulfones through visible-light initiated radical
rearrangement of vinyl tosylates. This protocol features mild
sustainable reaction conditions, broad substrate scope for
both aromatic and aliphatic compounds, and is compatible
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(e) F. Chen, N.-N. Zhou, J.-L. Zhan, B. Han and W. Yu, Org.
Chem. Front., 2017, 4, 135; (f) A. Shao, M. Gao, S. Chen, T.
Wang and A. Lei, Chem. Sci., 2017, 8, 2175.
(a) Q.-Q. Lu, J. Zhang, G.-L. Zhao, Y. Qi, H.-M. Wang and A.-
W. Lei, J. Am. Chem. Soc., 2013, 135, 11481; (b) S. Handa, J. C.
with various cyclic or sterically bulky skeletons. Further studies
to expand the applicability of the present reaction system are
currently underway in our laboratory.
7
Fennewald and B. H. Lipshutz, Angew. Chem., Int. Ed., 2014
,
Acknowledgments
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The authors gratefully acknowledge the financial support from
the National Natural Science Foundation of China (No.
21402028), Fuzhou University (510269) and Research Institute
of Photocatalysis, Fujian Provincial Key Laboratory of
Photocatalysis, Fuzhou University (52003426). We thank Prof.
Liang-Qiu Lu from Central China Normal University for
constructive discussions.
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