Visible-light induced oxidant-free oxidative cross-coupling for constructing allylic sulfones from olefins and sulfinic acids

Article abstract of DOI:10.1039/c6cc04109d

An oxidant-free dehydrogenative sulfonylation of α-methyl-styrene derivatives was developed for the construction of allylic sulfones by using eosin Y as a photosensitizer in conjunction with a cobaloxime catalyst. The process features a low-cost metal catalyst and atom economy, which provides an appealing strategy for future synthetic chemistry.

Full text of DOI:10.1039/c6cc04109d

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Visible-light Induced Oxidant-free Oxidative Cross-Coupling for
Constructing Allylic Sulfones from Olefins and Sulfinic Acids
Guoting Zhang,a Lingling Zhang,a Hong Yi,a Yi Luo,a Xiaotian Qi,d Chen-Ho Tung,b Li-Zhu Wub* and
Aiwen Lei*a,c
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
An oxidant-free dehydrogenative sulfonylation of α-methyl- However, the use of oxidants would bring the stoichiometric
styrene derivatives was developed for the construction of allylic amounts of chemical waste. The direct dehydrogenative
sulfones by using eosin Y as a photosensitizer in conjunction with coupling with liberation of H2 for the construction of new
a
cobaloxime catalyst. The process features low-cost metal chemical bonds is challenging but highly desirable.4 In this
catalyst and atomic economy, which provides an appealing context, several examples on the cross-coupling H2 evolution
strategy for future synthetic chemistry.
(CCHE) reaction have been developed via photocatalysis in
both heterogeneous5 and homogeneous6 systems. Through
the photochemical process, visible light energy is used to drive
the thermodynamically unfavorable reaction, giving rise to the
desired cross-coupling products. Nonetheless, this field
remains significantly limited in substrate scopes and reaction
patterns, especially for the formation of C‒heteroatom bonds.
Our lab recently demonstrated the external-oxidant-free
aromatic C‒H thiolation for intramolecular C‒S bond
construction by using Ru(bpy)3PF6 photosensitizer in
conjunction with a cobaloxime catalyst.7 We hope to expand
the general utility of this unique CCHE reaction to
intermolecular examples with the goal of achieving the
bioactive organosulfur compounds (Scheme 1C). Herein, we
report that the use of an organic photoredox catalyst and a
cobaloxime enables the direct cross-coupling of sulfinic acid
and α-methylstyrene derivatives for the construction of
sulfones, which exhibited interesting biological activities.8
Seeking efficient methods for the construction of sulfur-
containing compounds through simple C‒S bond-formation is
of great demand in general organic synthesis and
pharmaceutical industry.1 Recently, oxidative cross-coupling
reactions have been developed as one of the most powerful
strategies for
C−
S
bonds formation.2 Although the
dehydrogenative cross-coupling between C‒H/heteroatom‒H
bonds is considered as an ideal approach for C‒heteroatom
bond formation, releasing H2 from coupling process remained
elusive in most cases. Because the oxidation/dehydrogenation
of organic molecules is usually thermodynamically
unfavorable. For instance, the ΔG of directly dehydrogenative
coupling of the benzenesulfinic acid and α-methylstyrene is
positive (Scheme 1A, about 22.3 kcal/mol). Only are the
sacrificial oxidants present to turn the thermodynamically
uphill transformation to be feasible (Scheme 1B, ΔG < 0).3
oupling strategy for intermolecular C–S bond formation
O
∆ G = – 67.9 kcal/mol
or –19.6 kcal/mol
S
+
[Oxidant]
H2[O]
+
O
[oxidant] = PhI(OAc)2 or I2
dantꢀfree crossꢀcoupling, a lightꢀdriven thermodynamically uphill process
O
hv
Co
S
+
H2
O
visible light
oxidant free
Avoiding External Oxidants
Intermolecular CꢀS Bond Formation
Scheme 1. Dehydrogenative cross-coupling of sulfinic acid and α-methyltyrene.
Recently, sulfonyl radical-mediated processes for the rapid
synthesis of sulfones has made impressive achievements.9 The
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sulfonyl radical can be formed through the dioxygen-triggered Entries 7 and 8). A significant decrease in efficiency was
oxidation of sulfinic acid under transition-metal catalysis or observed with only low levels of conversion when the base
metal-free conditions, while oxysulfonylation of alkenes or was omitted (Table 1, Entry 9). There were small amounts of
alkynes are usually observed due to the presence of O2.10 The desired product forming in the absence of photosensitizer
generation of sulfonyl radical can also be achieved from the (Table 1, Entry 6).
treatment of sulfonylhydrazide with stoichiometric amounts of
Scheme 2. The generality of the oxidant-free Dehydrogenative Allylic Sulfonation
of Alkenes.a
peroxides, which need to be handled carefully on large
scales.11 Here, we demonstrated a green and atom economical
oxidant-free dehydrogenative allylic sulfonylation of α-
methylstyrene derivatives through the in-situ generated
sulfonic radical.
Table 1. Optimization of the reaction conditions.a
Entry
Photosensitizer (mol%)
3a(%)b
4a(%)b
H2(%)c
1d
2d
3
Ru(bpy)3(PF6)2 (3)
Ir(ppy)3 (3)
45
62
6
10
8
20
32
Na2-eosin Y (3)
TBA2-eosin Y (3)
TBA2-eosin Y (5)
none
60
45
4
61
10
10
0
46
5
76(71)
15
66
6
trace
0
7e
8f
9g
TBA2-eosin Y (5)
TBA2-eosin Y (5)
TBA2-eosin Y (5)
trace
0
0
0
0
29
0
trace
a Conditions: 1a (0.20 mmol), 2a (0.60 mmol), photosensitizer, Co(dmgH)2pyCl (10
mol %) and pyridine (2.0 equiv) in CHCl3 under argon atmosphere and irradiation
with green LEDs for 5 h. b Yields were determined by NMR; isolated yield was
shown in parentheses. c H2 was detected by GC-TCD using pure methane as an
internal standard. d Under the irradiation of blue LEDs. e Without Co(dmgH)2pyCl. f
In the dark. g Without pyridine.
a
Typical reaction conditions: 1 (0.20 mmol), 2 (0.60 mmol), TBA2-eosin Y (5 mol %),
Co(dmgH)2pyCl (10 mol %) and pyridine (2.0 equiv) in CHCl3 under argon atmosphere
and irradiation with green LEDs for 5 h. b a mixture of isomers (3c : 4c = 4 : 1) c sodium
sulfinate and TFA (0.2 mmol) were used to generate sulfinic acid in-situ.
We initially explored the transformation under the action of a
variety of photosensitizers in the presence of Co(dmgH)2pyCl
(dmgH = dimethylglyoximate, py = pyridine) as a proton-
reduction catalyst (Table 1). Interestingly, the allylic sulfone
Subsequently, the generality of this visible-light mediated
dehydrogenative allylic sulfonylation was investigated. Several
(
3a) was obtained 3a as the major product in 45% yield by
electronically diverse sulfinic acids
1 underwent reaction
using Ru(bpy)3(PF6)2 as the photosensitizer under the visible
light irradiation (Table 1, Entry 1). The reactions showed high
regioselectivity. The sulfonylations took place at the terminal
carbon of double bonds with the migration of the double
bonds as the major products.11a Meanwhile, H2 can be
smoothly with α-methyl styrene (2a) to give the corresponding
allylic sulfones Scheme 2, 3a‒3i). Among them, the sulfinic
acids with either electron-donating groups (R1 = Me, OMe) or
electron-withdrawing groups (R1 = Cl, Br, CF3) on the aryl ring
produced the corresponding products 3b‒3e in good yields.
determined in 20% yield, relatively lower to the sulfone 3a
,
Notably, halogen substituents including Cl, Br were proved to
be well tolerated under the standard reaction conditions,
providing more chance for further functionalization or
modification of these molecules. In addition, naphthalene-2-
sulfinic acid 1g was viable for allylic sulfonylation in good yield
(67%). A heterocyclic sulfinic acid was also was also tested as a
substrate for this transformation, affording the desired
product 3h in 63% yield. Aliphatic sulfinic acid, such as
methanesulfinic acid, also can provide the corresponding vinyl
sulfone 3i in good yield. We further applied the optimized
reaction conditions to different α-methylstyrene derivatives to
prepare corresponding allylic sulfones using p-toluenesulfinic
which resulted from the in-situ reduction of 2a (small amounts
of cumene can be detected by GC-MS). Ir(ppy)3 would promote
the desired coupling (Entry 2). Moreover, a similar result can
be obtained by using eosin Y, an organic dye, which has been
used as an attractive alternative to the expensive transition
metal complexes in photoredox catalysis (Table 1, Entries 3‒4).
Further improvement in efficiency of coupling and H2
generation would be achieved by increase the loading of
organic photosensitizer (Entry 5), since eosin
Y might
decompose during the photoredox catalyzed H2 production
process.
The control reactions were performed in the absence of Co
catalyst or light, resulted in no product formation (Table 1,
acid (1b) as the reaction partner (Scheme 2, 3j−3t). Electron-
2 | J. Name., 2012, 00, 1-3
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donating substituents on the phenyl ring of α-methylstyrenes, When (1-cyclopropylvinyl)benzene 2u was used as the radical
such as p-methyl (3j), and p-methoxyl (3k) groups, provided acceptor, the rearrangement product 3u resulted from the
the corresponding allylic sulfones in good efficiency. Electron- rapid ring opening process of cyclopropylmethyl radical was
withdrawing group (CF3) was tolerated in this transformation observed, which revealed the involvement of benzyl radical in
in 81% yield (3n). 1-phenyl-4-(prop-1-en-2-yl)benzene reacted the transformation (Scheme 4B). Furthermore, the radical
smoothly with 1b to give corresponding sulfone (3o) in 50% mechanism of this transformation was also proven by electron
yield. In addition, the reaction can be applied to α-methyl paramagnetic resonance (EPR) study. Under the irradiation of
styrene derivatives bearing halogen atoms such as Cl (3l), Br green light, a radical intermediate was captured by using 5,5-
(
3m). The reaction of 2-(prop-1-en-2-yl)naphthalene also dimethyl-1-pyrroline N-oxide (DMPO) as the spin trapping
proceeded well, yielding the desired product in 84% yield (3p). reagent (Scheme 4C). The parameters observed here for the
Other α-methylstyrene, having steric hindrance at the ortho, spin adduct are g factor = 2.00749, a (N,NO) = 14.06 G, and a(H,-
meta substituent, were applicable to the reaction for forming (CR3)CH) = 19.06 G. The spin Hamiltonian parameters observed
the desirable products 3q and 3r in good yield. α-ethyl styrene for this spin adduct is in good agreement with the literature
resulted in the formation of two isomeric double bonds of values for a carbon-centered radical.12 By contrast, this EPR
sulfone 3s in 72% yield (Z : E = 2 : 1).
signal was hardly observed in the absence of alkene 2a or p-
toluenesulfinic acid 1b. We proposed this radical is belonging
to the carbon-centered radical, resulted from the addition of
sulfonyl radical to α-methylstyrene.
Scheme 3. Oxidant-free vinylic sulfonation of alkene.
O
3 mol% Ir(ppy)3
Ts
S
10 mol% Co(dmgH)2pyCl
OH
+
2 equiv py
CH2Cl2, blue LEDs
1b
4t, 95%a
2t
a1b (0.20 mmol), 2p (0.60 mmol), Ir(ppy)3 (3 mol %), Co(dmgH)2pyCl (10 mol%) and
pyridine (2.0 equiv) in CH2Cl2 under irradiation with blue LEDs for 12 h.
However, the reaction is limited to the
derivatives under present conditions. Other alkenes, like
styrene and -methyl styrene, only can afford the
α-methyl-styrene
β
corresponding vinyl sulfone in relatively low yield. When 1,2-
dihydronaphthalene 2t was used as coupling partner, the
corresponding vinyl sulfones could be obtained by using
Ir(ppy)3 as the photosensitizer in CH2Cl2 (Scheme 3, 4t).
Scheme 5. Proposed Mechanism.
According to the mechanistic studies of our previous
reports,10a an acid–base reaction between sulfinic acid and
pyridine would occur rapidly. The consumption of 1b and
1
generation of pyridinium sulfinate were also monitored by H
NMR studies (see the ESI†, Fig. S2), which indicated that the
deprotonation of 1b might occur first before the electron
transfer processes. Considering that the oxidative potential of
sodium benzenesulfinate was –0.37 V vs SCE, the reductive
quenching of the excited eosin Y would be possible.9j
*
*
*
*
*
*
Based on these, a plausible mechanism was proposed (Scheme
5). Under visible light irradiation, eosin Y is excited to its
excited state [eosin Y]* (E1/2 red [eosin Y*/eosin Y•−] = +0.83 V
vs SCE),13 which could be reductively quenched by free
3320
3340
3360
3380
3400
Field (G)
Scheme 4. Radical Trapping Experiment. (A) Radical inhibition experiment using TEMPO
as a radical inhibitor. (B) A test on a "radical-clock" substrate. (C) EPR measurements of
a solution in CHCl3 of eosin Y, Co(dmgH)2pyCl, pyridine, 1b and 2a in the presence of
DMPO under the irradiation of green LEDs for 30 min.
sulfinate 9, released form the deprotonation of aryl sulfinic
acid, to generate eosin Y radical anion and sulfonyl radical. The
radical addition of sulfonyl radical to α-methylstyrene
Then the mechanism of the reaction was investigated. Radical
trapping experiment provided an evidence on the involvement
of radical species in the reaction. No desired product was
observed when the reaction was performed by addition of a
derivatives
2 produces carbon-centered radical 12, which is
further oxidized by Co(dmgH)2pyCl (E1/2 red [CoIII/CoII] = −0.61 V
vs SCE)14 to induce the formation of a Co(II) species 15 and a
cation intermediate 13. The elimination of the cation releases
radical
scavenger
2,2,6,6-tetramethyl-1-piperidinyloxy
a proton to produce allylic sulfones
3 or vinyl sulfones 4.
(TEMPO) under the standard reaction condition (Scheme 4A).
Furthermore, the single-electron reduction of Co(II) species 15
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by eosin Y radical anion affords a Co(I) species (E1/2 red [CoII/CoI] 4. K.ꢀH. He and Y. Li, ChemSusChem, 2014,
7
, 2788ꢀ2790.
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,
[eosin Y•‒/ eosin Y] = −1.06 V vs SCE).13 Protonation of the 1459ꢀ1465; (b) T. Mitkina, C. Stanglmair, W. Setzer, M. Gruber, H. Kisch
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formed Co(I) species, on the other hand, provides a CoIII−H and B. Konig, Orga. Bio. Chem., 2012, 10, 3556ꢀ3561.
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a
reaction J. Li, K. Feng, B. Chen, C.ꢀH. Tung and L.ꢀZ. Wu, J. Am. Chem. Soc., 2013,
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This work was supported by the 973 Program (2011CB808600,
2012CB725302, 2013CB834804), the National Natural Science
Foundation of China (21390400, 21272180, 21302148,
91427303 and 21402217), the Key Research Programme of the
Chinese Academy of Sciences (KGZD-EW-T05) and the
Research Fund for the Doctoral Program of Higher Education
of China (20120141130002) and the Ministry of Science and
Technology of China (2012YQ120060) and the Program for
Changjiang Scholars and Innovative Research Team in
University (IRT1030).The Program of Introducing Talents of
Discipline to Universities of China (111 Program) is also
appreciated.
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