Photocatalytic Fluoro Sulfoximidations of Styrenes

Article abstract of DOI:10.1002/anie.202005844

Reactions of difluoroiodotoluene with NH-sulfoximines provide new hypervalent iodine(III) reagents, which photocatalytically transfer a fluoro and a sulfoximidoyl group onto styrenes with high regioselectivity. The substrate scope is broad with respect to

Full text of DOI:10.1002/anie.202005844

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Accepted Article
Title: Photocatalytic Fluoro Sulfoximidations of Styrenes
Authors: Chenyang Wang, Yongliang Tu, Ding Ma, and Carsten Bolm
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10.1002/anie.202005844
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COMMUNICATION
Photocatalytic Fluoro Sulfoximidations of Styrenes
Chenyang Wang, Yongliang Tu, Ding Ma, and Carsten Bolm*
Abstract: Reactions of difluoro iodotoluene with NH-sulfoximines
provide new hypervalent iodine(III) reagents, which photo-
catalytically transfer a fluoro and a sulfoximidoyl group onto
styrenes with high regioselectivity. The substrate scope is broad
with respect to both sulfoximines and olefins. Following an
catalytic amounts of such these interesting reagents.[12]
In the initial studies, difluoro iodotoluene (4a) and sulfoximine
5a were applied as substrates (Scheme 1, top). Combining
these two compounds in dichloromethane for 20 min led to a
new fluorine-containing species as revealed by 1H- and 19F-NMR
spectroscopy (see Supporting Information). Evaporation of the
solvent afforded an oil, which proved sensitive to standard
conditions of chromatography. ESI-MS analysis showed a signal
at m/z 357.97513 corresponding to a composition of [3a–F–].
Attempts to isolate analytically pure 3a failed.
operationally simple protocol,
a large library of fluorine-
containing N-functionalized sulfoximines can be accessed.
Results from mechanistic investigations revealed the importance
of radical intermediates.
Hypervalent iodine compounds are tremendously important in
modern organic chemistry, and recent developments have
elevated them from lab curiosities to indispensable synthetic
tools.[1] With the intention to advance preparative high-valent
sulfur chemistry, we introduced sulfoximidoyl-containing
iodine(III) reagents 1[2] and 2[3] and demonstrated their potential
in sulfoximidations of alkynes,[2] thiols,[4] olefins,[5] and benzylic
substrates.[6] Whereas in the first two transformations 1 and 2
reacted like electrophilic reagents (with a formal positive charge
at nitrogen), the latter two reactions proceeded via nitrogen-
centered radicals formed by photocatalysis. Intending to
progress this science we wondered about the accessibility and
reactivity of analogous fluoro-containing iodine(III) reagents 3.
The introduction of such molecule appeared attractive because
its activation could eventually generate intermediates prone to
lead to products with both moieties the sulfoximidoyl and the
fluoro group being incorporated.[7] Considering that either of
them is highly important in medicinal and crop protection
chemistry,[8,9] the realization of this idea appeared attractive. The
advance in this project is presented here.
Focusing on potential applications of 3a, an in-situ approach
was considered, thereby avoiding the need to isolate the
apparently sensitive reagent. Functionalizing alkenes[13] was set
into the focus with the goal of adding both the sulfoximidoyl and
the fluoro group to the double bond to give products such as 7
(Scheme 1, bottom). Such transformations would then
analogous to aminofluorinations developed by Liu,[14a] Zhang,[14b]
Pérez,[14c] Xu,[14d] and Xu,[14e] but contrasting them by neither
requiring a metal catalyst nor relying on an external fluoride
source such as Selectfluor as demonstrated by Studer[15a] and
Lectka.[15b]
F
NH
S
F
F
I
Me
I
+
Ph
Me
N
S
DCM, RT
30 min
R
Ph
Me
O
O
4 (a: R = pMe)
5a
3a
detected by
ESI-MS, 1H-, and 19F-NMR
F
?
N
O
+
3a
R
R
S
Ph Me
7 (aa: R = Ph)
6 (a: R = Ph)
R
S
R
S
R
S
O
O
N
OMs(OTs)
R’’
N
O
Scheme 1. Preparation of monofluoro sulfoximidoyl iodoarene 3a and its
hypothesized addition to an olefin 6 to give difunctionalized product 7.
N
F
O
I
I
I
R’
R’
R’
O
R’’
R’’
3
1
2
reactions with
• alkynes
• thiols
reactions with
• olefins*
synthesis and reactivity?
The first test reactions along these lines were performed with
styrene (6a, R = Ph) as olefinic component. Reagent 3a was
prepared in-situ by mixing difluoro iodotoluene (4a, 1.5 equiv.)
and sulfoximine 5a (1.8 equiv.). An extensive screening of the
reaction conditions (for details see Table S2 in the Supporting
Information) revealed that the highest yield of 7aa was achieved
by following a stepwise process. First, 3a was prepared from 4a
and 5a in a sealed tube with DCM as solvent under argon at
ambient temperature, and then, after 20 min of stirring, 6a and
Ru(bpy)3(PF6)2 (1 mol %) were sequentially added. Irradiating
with a blue LED (24 W) for 12 h followed by standard aqueous
work-up and chromatography afforded 7aa in 83% yield
(Scheme 2).[16] Applying Ir(dtbpy)ppy)PF6, Rose Bengal, Eosin
yellowish, Rhodamine B, or Ru(bpy)3) instead of Ru(bpy)3(PF6)2
as photocatalyst proved less efficient.[17] Upon addition of CsF,
AgF, or CuF2 (2.0 equiv.) the amount of formed 7aa was lowered.
Without LED irradiation, the yield of 7aa was significantly
reduced (up to 25%) in a range of solvents (DCM, DCE, MeCN,
THF, and toluene).[17] Neither changing the addition mode nor
altering the substrate ratio was beneficial.
• benzylic substrates*
* = by photocatalysis
Figure 1. Reported sulfoximidoyl-containing iodine(III) reagents 1 and 2 and
targeted analogous compounds 3.
As starting materials for the preparation of fluoro-containing
iodine(III) reagents 3, difluoro iodoarenes (4) appeared most
promising.[10] The fluorination chemistry of 4 is rich, and various
ligand exchange reactions at the iodo core are known.[11]
Furthermore, the accessibility of 4 has recently been significantly
improved leading to efficient fluorination reactions with even
[a] C. Wang, Y. Tu, D. Ma, Prof. Dr. C. Bolm
Institute of Organic Chemistry, RWTH Aachen University
Landoltweg 1, 52074 Aachen (Germany)
Fax: (+49) 241-8092-391
E-mail: Carsten.Bolm@oc.rwth-aachen.de
Supporting information for this article is given via a link at the end of
the document.
This article is protected by copyright. All rights reserved.

10.1002/anie.202005844
Angewandte Chemie International Edition
COMMUNICATION
R
IF2
R
formation of 7sa (85%) and 7ta (81%) were noteworthy because
the former involved the use of dibenzothiophene sulfoximine 5s,
which in former studies had revealed a rather unusual nitrogen-
transfer behavior,[18] and the latter stemmed from sulfoximine 5t
representing the only substrate with an S-heteroaryl moiety. All
additions occurred with high regioselectivity leading to products
with the fluoro substituent in the benzylic position. In each case,
the diastereomeric ratio was about 1:1.
F
Ru(bpy)3(PF6)2 (1 mol %)
+
+
N
NH
blue LED
DCM, RT, Ar
S
S
O
Me
O
Me
Me
4a
5
6a
7
F
Cl
Br
F
F
F
F
N
N
N
N
S
Ph
S
Ph
S
Ph
S
Me
Ph
Ph
O
O
O
O
Me
Me
Me
7aa, 83%
7ba, 73%
7ca, 81%
7da, 81%
Me
NC
MeO
O2N
Ac
Next, the olefinic component was varied. As precursors for the
in-situ formed iodine(III) reagent, a combination of difluoro
iodotoluene (4a) and sulfoximine 5f was used (Scheme 3). The
applicability of 1-styrenes with mono-substituted aryl groups (6b-
r) was studied first. Also in this series the substrate scope was
broad. The yields varied, but generally, they were high. The best
results were observed with para-substituted styrenes as
exemplified by products 7ff (91%) and 7fh (93%) bearing a
para-tert-butyl and a para-phenyl group, respectively. Substrates
with meta- and ortho-substituents gave products (7fk-ft) in
somewhat diminished yields with 56% being the lowest as
observed in the formation of 7fk having a meta fluoro substituent.
Representing substrates with more than one substituent on the
arene, 1-naphthyl- and mesityl styrene (6s and 6t) gave the
corresponding products (7fs and 7ft) in 86% and 50% yield. The
F
F
F
F
F
N
S
Me
N
S
N
S
Me
N
S
Me
Ph
Ph
Ph
O
O
O
O
Me
7ea, 85%
7fa, 90%
7ga, 64%
7ha, 55%
Cl
Br
Me
F
F
F
N
S
Me
N
S
Me
N
S
Me
N
S
Ph
Ph
Ph
Ph
O
O
O
O
Me
7ia, 68%
7ja, 69%
7ka, 66%
7la, 83%
MeO
F
F
F
F
N
S
Me
N
S
Me
N
S
Me
N
Ph Cl
Ph
MeO
Ph
S
Ph
O
O
O
O
Me
7ma, 85%
7na, 76%
7oa, 70%
MeO
7pa, 87%
F
O
F
F
N
S
N
F
N
Ph
N
S
S
Ph
Ph
N
O
O
S
Ph
O
Me
MeO
MeO
R2
IF2
OMe
R2
F
Ru(bpy)3(PF6)2 (1 mol %)
7qa, 84%
7ra, 85%
7sa, 85%
7ta, 81%
+
+
R1
N
NH
blue LED
DCM, RT, Ar
S
S
Me
O
R1
Me
O
Scheme 2. Substrate scope with respect to sulfoximines 5 in additions of in-
situ formed 3a to styrene (6a).
Me
R = 4-MeO-Ph
5f
4a
6
7
F
F
F
F
Me
O
Me
O
Me
N
Me
N
N
N
S
S
S
S
O
R
O
Consequently, the substrate scope was evaluated under the
aforementioned optimal conditions. First, various sulfoximines 5
were applied with styrene 6a as olefinic acceptor [and difluoro
iodotoluene (4a) as iodine(III) source]. Scheme 2 shows the
results.
R
R
R
F
Cl
Br
Me
7fb, 78%
7fc, 80%
7fd, 81%
7fe, 89%
F
F
F
F
Me
Me
Me
N
Me
O
N
N
N
S
S
S
S
S
O
O
O
R
R
R
R
Cl
tBu
OMe
Ph
Cl
7ff, 91%
7fg, 89%
7fh, 93%
7fi, 90%
The substrate scope with respect to the sulfoximine
component proved quite general, and a wide range of functional
groups including halo, alkyl, alkoxy, acetoxy and nitro
substituents were tolerated. In the series of S-aryl S-methyl
sulfoximines (5a-p), substrates with both electron-donating and -
withdrawing substituents on the arene reacted well affording the
corresponding products in yields ranging from 55% to 90%. A
more detailed analysis of the results in the series of para-
substituted products 7ea to 7ia suggested that sulfoximines with
electron-rich arenes (7ea and 7fa) gave slightly better results
than those with electron-poor aryl groups (7ga-ia). Comparing
the yields for products stemming from chloro-containing
sulfoximines 5c, 5j, and 5n and methoxy-substituted 5f, 5m, and
5o showed that in both cases the para-substituted derivatives
gave the highest yields of the corresponding products.
Nevertheless, although not on the same level, the results for
meta- and ortho-substituted products were good here too with
yields ranging from 69% (for 7ja) to 85% (for 7ma). S-2-
Naphthyl-containing product 7pa was obtained in 87% yield.
Also S,S-diaryl sulfoximines reacted well as exemplified by the
results for products 7qa and 7ra, which were obtained in 84%
and 85% yield, respectively. Finally, the high yields in the
F
F
F
F
Me
O
Me
O
Me
O
Me
O
N
N
F
N
N
Br
S
S
S
R
R
R
R
OAc
Me
7fj, 79%
7fk, 56%
7fl, 73%
7fm, 78%
F
F
F
F
Me
O
Me
O
Me
O
Me
O
N
N
N
N
S
S
S
S
S
S
R
R
R
R
F
Cl
Br
7fn, 86%
7fo, 62%
7fp, 65%
7fq, 61%
F
F
Me
Me
O
Me
O
N
N
Me
Me
O
S
N
S
R
S
O
F
N
R
R
R
Me
Me
Me
Me F
7fr, 70%
7fs, 86%
7ft, 50% (dr = 4:1)
7fu, 83%
O
Me
O
Me
N
S
N
F
R
R
F
7fv, 85%
7fw, 73%
Scheme 3. Substrate scope with respect to olefinic component 6 in additions
of the in-situ formed iodine(III) reagent starting from difluoro iodotoluene (4a)
and sulfoximine 5f.
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10.1002/anie.202005844
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COMMUNICATION
latter result was particularly interesting because the dr was 4:1
thereby contrasting all other reactions with dr ratios of about 1:1.
Using 1,1-disubstituted styrenes 6u and 6v afforded 7fu and 7fv
in 83% and 85% yield, respectively. Finally, to our surprise and
delight, also dihydronaphthaline 6w reacted well leading to the
corresponding addition product 7fw in 73% yield. This result was
remarkable because it showed that also 1,2-disubstituted
styrene derivatives could react selectively.
ground state of the photocatalyst by visible light. As a
consequence, an N-centered sulfoximidoyl radical A, a fluoride
ion, p-tolyl iodide, and the oxidized photocatalyst (PC+) are
formed. Subsequently, A adds to the double bond of 6a leading
to radical B. The benzylic stabilization is critical for the entire
process and the basis for its high regioselectivity.[19] Oxidation of
B by SET from PC+ provides benzylic cation C and closes the
catalytic cyclic by regenerating the ground state photocatalyst
PC. Finally, cation C reacts with fluoride to give product 7aa.[20]
In order to elucidate the reaction pathway, various process
modifications were studied (Scheme 4, top). As noted before,
iodine reagent 3a could be identified by mass spectrometry and
NMR spectroscopy. The step-wise protocol involving an addition
of 6a to a solution of in-situ formed 3a followed by irradiation of
the resulting mixture with a blue LED in the presence of the
photocatalyst gave 7aa in 83% yield. If all reagents were mixed
without the separate preformation of 3a, the yield of 7aa was
reduced to 68% suggesting that under these conditions parts of
the starting materials reacted differently. (Specific by-products
formed in traces have not been identified.) This observation was
in line with the result from an experiment with altered mixing
order. Thus, an initial stirring of iodine reagent 4a in the
presence of olefin 6a for 20 min followed by the addition of
sulfoximine 5a and the photocatalyst with subsequent blue LED
irradiation did not lead to any detectable amounts of 7aa.
Apparently, other (unidentified) reaction pathways dominated.
Following the original protocol but generating 3a in the presence
of base (3 equiv. of K2CO3) reduced the yield of 7aa to 55%
indicating a decisive role of the intermediately formed HF. No
product was observed when the reaction (after the in-situ
formation of 3a) was performed in the presence of TEMPO (2
equiv.) suggesting an involvement of radicals as relevant
intermediates.
In summary, we developed a new in-situ formed hypervalent
iodine(III) reagent, which photocatalytically adds two of its
iodine-bound substituents to styrenes providing fluorine-
containing N-functionalized sulfoximines in a single operational
step. The fluoro sulfoximinations show a pronounced functional
group tolerance and occur with are highly regioselectivity.
Mechanistic studies suggest the intermediacy of radicals.
Acknowledgements
C.W., Y.T., and D.M. are grateful to the China Scholarship
Council for predoctoral stipends.
Keywords: Fluorination • hypervalent iodine • photoredox
cataysis • radical • sulfoximine.
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F
F
I
pTol
Me
NH
S
F
F
6a, PC
N
S
N
O
Ph
Me
I
pTol
+
Ph
S
DCM, RT
20 min
Ph
blue LED
DCM, Ar, 12 h
O
Ph Me
O
3a
4a
5a
7aa
step 2
step 1
yield of 7aa
• standard protocol:
• initial mixing of 4a, 5a, and 6a:
83%
68%
• initial mixing of 4a and 6a, then 5a:
• addition of K2CO3 in step 1:
• addition of TEMPO in step 2:
—
55%
—
[2]
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F
Ph
O
Ph
N
H. Wang, D. Zhang, H. Sheng, C. Bolm, J. Org. Chem. 2017, 82,
11854-11858.
+
+
SET
O
F
S
I
S
N
pTol-I
•
Me
Me
6a
A
Me
[4]
[5]
[6]
H. Wang, D. Zhang, M. Cao, C. Bolm, Synthesis 2019, 51, 271-275.
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3a
*
PC
PC+
Me
O
N
S
Ph
SET
Ph
hν
B
PC
[7]
For reviews on fluorinated sulfoximines, see: a) V. Bizet, R. Kowalczyk,
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J. Org. Chem. 2014, 4437-4451; c) A.-L. Barthelemy, E. Magnier, C. R.
Chimie 2018, 21, 711-722.
F
Me
O
Me
N
N
F
S
S
Ph
O
Ph
Ph
C
7aa
[8|
For overviews on the use of sulfoximines in medicinal and crop
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Biochem. Phys. 2013, 107, 1-7.
Scheme 4. Standard protocol and variations thereof (top); suggested
mechanistic pathway (bottom).
Taking all observations into account, the following mechanistic
scenario is suggested: After the initial formation of iodine(III)
reagent 3a (Scheme 4, step 1), the N–I bond of 3a is cleaved by
single electron transfer (SET) from the excited state of
Ru(bpy)3(PF6)2 (PC*), which had been generated from the
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10.1002/anie.202005844
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COMMUNICATION
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[16] When the LED irradiation was stopped after 2 h, 7aa was obtained in
36% yield after 12 h.
[17] DCE = dichloroethane, dtbpy = 4,4’-di-tert-butyl-2,2’-dipyridyl, ppy =
phenylpyridine, bpy = 2,2’-bipyridine.
[18] Z. Li, H. Yu, C. Bolm, Angew. Chem. 2017, 129, 9660-9663; Angew.
Chem. Int. Ed. 2017, 56, 9532-9535.
[19] Attempts to use olefinic substrates other than styrenes (vinyl-
cyclohexan, 2-vinylisoindoline-1,3-dione, 4-phenyl-1-butene, and vinyl
hexanoate) under standard reaction conditions remained unsuccessful.
[20] In the presence of TMSCF3 or TMSCN as potential sources for other
nucleophiles to react with suggested cation C, no other products were
observed.
This article is protected by copyright. All rights reserved.

10.1002/anie.202005844
Angewandte Chemie International Edition
COMMUNICATION
COMMUNICATION
Chenyang Wang, Yongliang Tu, Ding
Ma, and Carsten Bolm.*
Page No. – Page No.
Photocatalytic Fluoro
Sulfoximidations of Styrenes
Combining a fluoro and a sulfoximidoyl group by iodine: New hypervalent
iodine(III) reagents are formed by reactions of difluoro iodotoluene with NH-
sulfoximines. When applied in-situ, they photocatalytically react with styrenes by
transfering a fluoro and a sulfoximidoyl group onto the olefin with high regio-
selectivity. The operationally simple process provides a variety of fluorine-
containing N-functionalized sulfoximines, which are difficult to prepare otherwise.
F
F
R''
F
Ar
I
Ar
F
NH
R S R'
O
I
Ar
photocatalyst
O
N
N
Ar
S
blue LED
DCM, Ar, 12 h
DCM, 20 min
R''
R S R'
O
R'
R
This article is protected by copyright. All rights reserved.