Sulfoximidations of Benzylic C?H bonds by Photocatalysis

Article abstract of DOI:10.1002/anie.201801660

An efficient photocatalytic functionalization of compounds with benzylic C?H bonds by sulfoximidation in visible light is described. The mild reaction conditions allow the use of a broad array of substrates, including diarylmethane, alkyl arenes, arylacetonitrile, 2-arylacetate, and alkynyl aryl methanes. The sulfoximidation process is highly chemoselective and leads to the corresponding sulfoximines in generally good yields. Mechanistic investigations suggested the intermediacy of sulfoximidoyl radicals.

Full text of DOI:10.1002/anie.201801660

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Accepted Article
Title: Sulfoximidations of Benzylic C-H bonds by Photocatalysis
Authors: Han Wang, Duo Zhang, and Carsten Bolm
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10.1002/anie.201801660
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COMMUNICATION
Sulfoximidations of Benzylic C–H bonds by Photocatalysis
Han Wang, Duo Zhang, and Carsten Bolm*
Abstract: An efficient photocatalytic functionalization of compounds
with benzylic C–H bonds by sulfoximidation in visible light is
described. The mild reaction conditions allow the use of a broad
array of substrates, including diarylmethane, alkyl arenes,
arylacetonitrile, 2-arylacetate, and alkynyl aryl methanes. The
sulfoximidation process is highly chemoselective and leads to the
corresponding sulfoximines in generally good yields. Mechanistic
investigations suggested the intermediacy of sulfoximidoyl radicals.
proved inspiring.[5b] They had observed that the photochemical
degradation of an N-benzoxysulfoximine predominately led to
the corresponding NH-sulfoximine, which suggested that the
proposed sulfoximidoyl radical intermediate had
a good
reactivity in an intermolecular hydrogen atom transfer (HAT)
reaction and that, second, substrates with benzylic C–H bonds
were suitable H donors in such processes. Consequently, we
envisaged the following scenario (Scheme 2): Irradiation with
blue (LED) light would promote a photocatalyst PC [here, Ru(II)]
to its excited state (PC*), and its oxidative quench by
hypervalent iodine(III) reagent 1 leads to PC+ [here, Ru(III)] and
sulfoximidoyl radical B as key intermediate. Hydrogen atom
transfer (HAT) reaction of B with benzylic substrates C then
provides NH sulfoximine D and benzyl radical E. Oxidation of E
by PC+ transfers the photocatalyst to its original state (PC) and
leads to benzylic cation F, which reacts with NH sulfoximine D
giving product G upon deprotonation.[10]
Owing to their unique bioactivities, sulfoximines[1] have found
broad applications in medicinal and agrochemistry.[2] Our group
has continuous interest in such compounds with a focus on the
development of new approaches towards sulfoximidoyl moieties
and their incorporation into 3-dimensional heterocycles.[3] N-
Functionalizations of existing sulfoximines mostly involve
formally nucleophilic nitrogens. Recently, we reported on novel
sulfoximidoyl-containing hypervalent iodine(III) reagents such as
1 and their use in electrophilic sulfoximidoyl transfer reactions
(Scheme 1, top).[4] We now wondered about the behavior of 1 in
photoredox catalysis hypothesizing that 1 could eventually serve
as convenient source for nitrogen-centered sulfoximidoyl radical
B generated by electron-transfer (ET) processes via radical A
(Scheme 1, bottom). Species of type B are unusual and in terms
of synthetic applicability essentially unexplored.[5]
Me
N
Ph
O
H
C
O
N
OMs
S
S
I
SET
Ph
Me
Ar
R
Ph
B
1
PC+
PC*
HAT
Photoredox
Cycle
Ar
R
hν
SET
Me
E
O
S
PC
PC =
PF6
NH
D
Ph
Me
N
N
Ru
N
O
N
N
N
N
S
Ph
Previous work
Ph
O
N
Ar
R
S
Ar
R
N-alkinylation
– PhI,
– MsO
PF6
Me
F
G
Ph
N
OMs
S
I
O
Me
Ph
+ e
Scheme 2. Proposed mechanism for the sulfoximdation of benzylic C–H
bonds.
Ph
O
Ph
O
1
(by photocatalysis)
N
N
S
I
S
– OMs
– PhI
Me
Me
Ph
Current strategy
A
B
For the proof-of-concept study, we selected sulfoximidoyl-
containing hypervalent iodine(III) reagent 1 and 4-benzylanisole
(2a) as representative model substrates. To our delight, both
reacted as hypothesized. Under irradiation with a blue LED strip,
using 1 mol % of Ir(dtbpy)(ppy)2PF6 as the photocatalyst in
MeCN, the reaction between 1 and 2a gave the desired product
N-[(4-methoxyphenyl)(phenyl)methyl]-S-phenyl-S-methylsulfox-
imine (3a) in 47% yield (Table 1, entry 1). Also other common
photocatalysts provided 3a, albeit mostly in only moderate yield
(Table 1, entries 2-6). The screening revealed Ru(bpy)3(PF6)2 to
be superior over all other photocatalysts, leading to 3a in 81%
yield (Table 1, entry 6). Neither changing the solvent from MeCN
to THF, DCE or PhCF3 (Table 1, entries 7-9) nor the addition of
bases (K3PO4, K2CO3, and NaHCO3; Table 1, entries 10-12) had
a positive effect on the yield of 3a. Control experiments
confirmed that both photocatalyst and light were essential for
product formation (Table 1, entries 13 and 14).
Under the optimal reaction conditions, the scope of the sulfox-
imidation process was examined. First, diarylmethanes were
applied (Scheme 3). In general, all substituted products 3a-l
were obtained in good yields. Neither electronic nor steric effects
played a significant role. For example, the yield of 3f bearing an
electron-withdrawing p-CF3 group at one of the arenes (80%)
was essentially identical to the one of 3a having an electron-
Scheme 1. Previous and current strategy for the use of hypervalent iodine
reagent 1.
Nitrogen-centered
radicals
(NCRs)
are
important
intermediates in preparative organic chemistry.[6] Using NCRs in
complex total synthesis, however, has proven difficult due to
their limited accessibility and high reactivity.[7] In recent years,
photoredox catalysis has emerged as powerful tool for the
generation of radicals under mild conditions.[8] Along these lines,
single-electron-transfer (SET) processes have been developed,
which allow the conversion of NCR precursors into their
corresponding N-centered radicals.[9]
For our concept design, a report by Toscano and co-workers
[*]
H. Wang, D. Zhang, Prof. Dr. C. Bolm
Institute of Organic Chemistry
RWTH Aachen University
Landoltweg 1, 52074 Aachen (Germany)
E-mail:carsten.bolm@oc.rwth-aachen.de
Homepage: http://bolm.oc.rwth-aachen.de/
Supporting information for this article is given via a link at the end of
the document.
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COMMUNICATION
Table 1: Optimization of the sulfoximidation conditions.[a]
blue LED (5 W)
Ru(bpy)3(PF6)2 (1 mol %)
Ph
O
Ph
O
H
S
N
OMs
N
S
I
Me
+
Ph
N
Ar
R
MeCN (0.1 M), rt, 15 h
Me
Ph
1
O
H
Ar
R
blue LED
PC (1 mol %)
S
Ph
O
N
2a-x
3a-x
OMs
Me
a-l: R = Ar’, m-x: R = non-aromatic
S
I
+
Me
Ph
solvent, rt, 15 h
MeO
S(O)MePh
S(O)MePh
S(O)MePh
MeO
1
2a
3a
N
N
OMe N
Entry
1
Photocatalyst
Ir(dtbpy)(ppy)2PF6
Ir(ppy)3
Solvent
Base
none
none
none
none
none
none
none
none
none
K3PO4
K2CO3
NaHCO3
none
none
Yield [%]
47
OMe
Me
3c, 80%
S(O)MePh
3a, 81%
3b, 76%
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
THF
S(O)MePh
S(O)MePh
N
N
N
2
n.d.
41
3
Flurescein
Me
3d, 63%
3f, 81%
3e, 69%
4
Rhodamine B
Eosin Y
31
S(O)MePh
S(O)MePh
S(O)MePh
Cl
N
N
N
5
32
F
Cl
6
Ru(bpy)3(PF6)2
Ru(bpy)3(PF6)2
Ru(bpy)3(PF6)2
Ru(bpy)3(PF6)2
Ru(bpy)3(PF6)2
Ru(bpy)3(PF6)2
Ru(bpy)3(PF6)2
none
81
3, 77%
3i, 76%
3h, 74%
7
n.d.
54
S(O)MePh
S(O)MePh
S(O)MePh
N
N
N
DCE
8
CF3
F
F
MeO
OMe
PhCF3
MeCN
MeCN
MeCN
MeCN
MeCN
9
n.d.
81
3k, 85%
3j, 80%
3l, 60%
S(O)MePh
Me
S(O)MePh
Me
S(O)MePh
Me
10
11
12
13
14[b]
N
N
N
76
Me
MeO
MeO
MeO
3m, 83%
3n, 69%
3o, 58%
74
S(O)MePh
S(O)MePh
S(O)MePh
OAc
n.d.
n.d.
N
N
N
OBz
Me
Me
Ru(bpy)3(PF6)2
MeO
MeO
Ph
MeO
MeO
MeO
MeO
3q, 69%
3r, 60%
3p, 68%
[a] Reaction conditions: 1 (0.2 mmol), 2a (0.4 mmol), photocatalyst (1
mol %), base (0.2 mmol), solvent (2 mL), sealed tube. [b] No irradiation.
S(O)MePh
S(O)MePh
Cl
S(O)MePh
NPhth
N
N
N
donating p-MeO substituent (81%). If the MeO group was
located in ortho position as in 3c, the yield was 80%. Only for
tolyl-containing products 3d (63%) and 3f (69%) and difluoro-
substituted 3l (60%) the yields were somewhat lower. With the
exception of 3b all products were formed as ca. 1:1 mixtures of
diastereomers.
3s, 40%
3t, 70%
3u, 35%
S(O)MePh
S(O)MePh
Me
S(O)MePh
Me
N
N
N
MeO
MeO
Me
S
3x, 21%
3v, 72%
3w, 89%
Next, substituted alkyl arenes were used as substrates
(Scheme 3). In this series, the yields of the corresponding
products 3m-x varied significantly, ranging from 21% (for 3x) to
89% (for 3w). Anisols with linear or branched alkyl substituents
proved to be good substrates, leading to 3m-p in yields between
58% (for 3o) and 89% (for 3m). Similar results were obtained
with anisols having heteroatoms in the alkyl substituent leading
to products 3q-t. Of particular interest was the conversion of 3-
butenyl-containing anisol 2u. Albeit the yield of 3u was only 35%,
the outcome was significant because it demonstrated that the
HAT process was fast (compared to a competing olefin addition)
allowing unsaturated molecules to be functionalized as well.
Thus in that respect, the sulfoximidoyl radical proved superior
over other nitrogen-centered radicals, which often show low
addition/substitution selectivity in conversions of compounds
with C–C double bonds.[6,7,11] Also the applying ethyl arenes with
modified aryl groups was possible, as confirmed by the
formations of 3v and 3w, which were obtained in 72% and 89%
Scheme 3. Substrate scope: diarylmethanes (2a-l) and substituted alkyl
arenes (2m-x).
yield, respectively. 3-Ethylthiophene (2x) reacted sluggish
leading to 3x in only 21% yield. Also in this series (3m-x) the
products were mixtures of diastereomers in ratios of ca. 1:1.
Chart
reactions of
1
shows products 4a-h obtained in photocatalytic
with substrates having electron-withdrawing
1
groups (CN or CO2R) directly bound to the benzylic carbon
where the radical was generated. Although the yields were only
moderate to good (39-52%), the transformations are noteworthy
because they represent a rather unusual protocol for the
preparation of a-amino acid derivatives. The same is true for
propagylic amide-type compounds 5a-d, which were
photocatalytically prepared from
propagylic arenes.
1 and the corresponding
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S(O)MePh
CN
S(O)MePh
CN
S(O)MePh
a)
Ph
MeO
N
N
N
standard
conditions
O
O
6
7
n.r.
CN
Ph
EtO
MeO
MeO
PhO
8
4a, 50%
S(O)MePh
4c, 52%
S(O)MePh
4b, 44%
S(O)MePh
9
Ph
O
standard conditions
N
N
N
S
PMP
N
PMP
Me
11, 38%
OEt
OMe
O
10
O
O
O
MeO
MeO
PMP
PMP
4d, 39%
4e, 42%
4f, 45%
standard
conditions
S(O)MePh
S(O)MePh
O
b)
c)
N
N
PMP
Me
+
3a
OEt
O
OEt
in air
PMP
Me
n.d.
S(O)MePh
12, 7%
2m
N
O
S
MeS
4h, 42%
4g, 37%
Me
N
O
S
R
standard
conditions
MeO
5a : R = Ph
S(O)MePh
Ph
N
OMs
+
PMP
PMP
71%
39%
38%
PMP
3y, 33%
5b: R = 4-CF -Ph
5c: R = COOMe
3
13, 23%
2y
CF3
5d, 39%
PMP
PMP
PMP
rearrangement
PMP
Chart 1. Products obtained from 1 by photocatalysis.
Me
O
d)
Me
Ph
Bz
standard
conditions
S
N
H
N
N
Ph
+
With the goal to validate the hypothesized reaction path
presented in Scheme 2, various control experiments were
performed. First, couplings of 1 with olefinic substrates were re-
assessed. The formation of 3u had already indicated that
compounds with C–C double bonds could be applied and that
the sulfoximidation occurred with high preference over radical
addition. To further explore the inherent reactivity and selectivity
of the proposed sulfoximidoyl radical, reactions with 1-hexene
(6), propyl vinyl ether (7), styrene (8), ethyl acrylate (9), and 1-
allyl-4-methoxybenzene (10) were attempted (Scheme 4, part a).
While the first four substrates remained untouched (allowing to
recover 1 almost quantitatively), doubly (allyl/benzyl) activated
10 was reactive enough to convert leading to substituted product
11 in 38% yield. Presumably, the latter transformation
proceeded via the highly stabilized allyl radical and an attack of
the sulfur component at its sterically less hindered terminal
carbon. When 4-ethylanisole (2m) was applied under standard
conditions in air (instead of argon), ketone 12 was isolated in 7%
yield, which was consistent with the intermediacy of a radical
species and its reaction with dioxygen. Sulfoximidation product
3m was not observed (Scheme 4, part b). A "radical clock
experiment" with cyclopropyl-containing 2y as substrate led to
3y and 13 in 33% and 23% yield, respectively (Scheme 4, part
c). Both products indicated the presence of an α-cyclopropyl
radical, which in part ring-opened and reacted further by
substitution reactions via the resulting cations. Finally, N-
methylbenzamide (14) was added to a standard photocatalysis
with diphenylmethane (2b) (Scheme 4, part d). As a result, a
mixture of substitution products 3b (19%) and 15 (18%) was
formed, which we interpreted as evidence for the existence of
benzylic cations as intermediate.[12]
Ph
Ph
+
Me
Bz
Ph
Ph
Ph
14
2b
3b, 19%
15, 18%
Scheme 4. Control experiments.
of mechanistic studies suggest a sulfoximidoyl radical and
benzylic radicals/cations as intermediates.
Acknowledgements
H.W. and Y.C. thank the China Scholarship Council for pre-
doctoral stipends.
Keywords: hypervalent iodine(III) reagent • photocatalysis •
radicals • sulfoximidation • sulfoximine
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In summary, we developed
a photoredox catalysis for
sulfoximidation reactions using hypervalent iodine(III) reagent 1.
Under mild reaction conditions, various substrates with benzylic
C–H bonds react chemoselectively at room temperature. Results
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[12] Treatment of N-methylbenzamide 14 with 1 in CD3CN for 24 h did not
lead to any detectable change of the 1H NMR spectrum, suggesting that
15 did not interfere in the radical pathways (for details, see Supporting
Information).
TOC
Me
O
Ph
Photoredox
Catalysis
H
S
[9]
N
N
I
Ph
Me
+
R
S
OMs
Ar
R
Ph
Ar
O
HAT
R = aryl, alkyl, CN, COOR, alkynyl
.
This article is protected by copyright. All rights reserved.

10.1002/anie.201801660
Angewandte Chemie International Edition
COMMUNICATION
COMMUNICATION
Han Wang, Duo Zhang, and Carsten
Bolm*
Page No. – Page No.
Sulfoximidations of Benzylic
C–H bonds by Photocatalysis
Benzylic C–H bonds have selectively been functionalized by photoredox catalysis
starting from a sulfoximidoyl-containing hypervalent iodine(III) reagent. The process
is highly chemoselective and allows converting a broad range of substrates.
Mechanistic studies suggest the intermediacy of a sulfoximidoyl radical.
Conceptionally, a new approach towards sulfoximines has been developed.
This article is protected by copyright. All rights reserved.