Metal-Free Visible-Light-Mediated Oxidative Cross-Coupling of Thiols with P(O)H Compounds Using Air as the Oxidant

Article abstract of DOI:10.1021/acs.orglett.6b02563

Visible light along with 5 mol % of rose bengal catalyzes the direct S-P(O) coupling between thiols and P(O)H compounds in the presence of air as the green oxidant. The protocol is operationally simple and amenable to gram-scale synthesis. A variety of S-

Full text of DOI:10.1021/acs.orglett.6b02563

Letter
pubs.acs.org/OrgLett
Metal-Free Visible-Light-Mediated Oxidative Cross-Coupling of
Thiols with P(O)H Compounds Using Air as the Oxidant
Jian-Guo Sun, Hua Yang, Ping Li,* and Bo Zhang*
State Key Laboratory of Natural Medicines, China Pharmaceutical University, 24 Tongjia Xiang, Nanjing 210009, China
S
* Supporting Information
ABSTRACT: Visible light along with 5 mol % of rose bengal
catalyzes the direct S−P(O) coupling between thiols and P(O)
H compounds in the presence of air as the green oxidant. The
protocol is operationally simple and amenable to gram-scale
synthesis. A variety of S−P(O) coupling products can be readily
prepared in moderate to excellent yields. The reaction features
good functional-group tolerance, operational simplicity, and excellent practicality.
rganophosphorus compounds bearing an S−P(O) bond
the presence of cheap organic dye, rose bengal, as the
photocatalyst and air as the green oxidant, providing various
S−P(O) coupling products in moderate to excellent yields
under very mild reaction conditions.
O
display important chemical and biological properties that
afford utility in various fields, including organic synthesis,
medicinal chemistry, and agrochemistry.1 Studies have
disclosed that many sulfur-containing organophosphorus
compounds exhibit excellent biological activities.2 As a
consequence, the construction of S−P(O) bonds is an essential
issue in organic chemistry and has gained significant attention.
Traditional routes for S−P(O) bond formation rely on the
nucleophilic substitution reaction of RSX or R2P(O)X directly
or indirectly.3,4 Nevertheless, the use of toxic and moisture-
sensitive P- or S-reagent, multistep reaction processes, and low
functional-group tolerance remain impediments of these
approaches. In light of the importance of the S−P(O)-
substituted moiety in various fields as mentioned above, great
efforts have recently been devoted to the development of
general and efficient approaches to construct S−P(O)
bonds.5−8 Among these methods, oxidative cross-coupling
reactions between an S−H bond and an P(O)−H bond are
particularly attractive, since they provide a straightforward and
atom-economic protocol for S−P(O) bond formation.8 Despite
significant advances in synthetic methods, the current
procedures are associated with one or more limitations such
as superfluous usage of strong oxidants, the need for transition-
metal catalysts, elevated temperatures, and so on. In this regard,
the development of convenient and “greener” protocol is still
highly desirable and remains a great challenge.
In recent years, photoredox catalysis enabled by visible light
has emerged as a powerful synthetic tool for mild and
environmentally benign organic transformations.9 A variety of
elegant photocatalyzed reactions for the construction of C−C
and C−heteroatom bonds have been well established.9−12
However, to the best of our knowledge, S−P bond formation
by photocatalysis has not been investigated to date. In this
context, we wondered whether an S−H bond could couple with
a P(O)−H bond to form an S−P(O) bond via synergistic
interactions of visible light stimulation and photosensitizer
sensitization. Herein, we report the first visible-light-mediated
oxidative cross-coupling of thiols with P(O)H compounds in
Our initial study was conducted by investigating the reaction
of a slight excess (1.2 equiv) of 4-methylbenzenethiol 1a with
diphenylphosphine oxide 2a in the presence of 5 mol % of
Ru(bpy)3Cl2 in CH3CN at room temperature under air
atmosphere. After 12 h of irradiation with 10 W blue LED
lights, the desired coupling product 3aa was obtained in 57%
yield with complete conversion of 2a (Table 1, entry 1).
Encouraged by this promising result, we next surveyed a range
of photocatalysts under the same conditions (Table 1, entries
2−7). When fac-Ir(ppy)3 was employed as the photocatalyst,
3aa was formed in 33% yield (Table 1, entry 2). Reactions with
various organic dyes including Mes-Acr+, eosin Y, eosin B,
rhodamine B, and rose bengal were also examined (Table 1,
entries 3−7). These results showed that rose bengal is an
effective photocatalyst for this transformation, which afforded
3aa in 56% yield. To develop a metal-free protocol, further
optimization of reaction conditions was conducted by using
rose bengal as the photocatalyst. A screening of solvents
showed that the yield of 3aa was significantly improved to 71%
when the reaction was performed in DMF (Table 1, entry 8).
Other solvents, such as THF, CH2Cl2, H2O, and CH3CH2OH,
resulted in lower yields (Table 1, entries 9−12). Poorer
conversion could be observed when the reaction was carried
out in the absence of either photocatalyst or light (Table 1,
entries 13 and 14). Notably, the yield was further increased to
74% or 79% by using 1.5 or 2.0 equiv of 1a (Table 1, entries 15
and 16). Lowering rose bengal loading to 3 or 1 mol %
decreased the yield to 69% or 36%, respectively (Table 1,
entries 17 and 18). When the reaction was run under 1 atm of
O2, the yield of 3aa was decreased to 60% due to the formation
of byproducts (Table 1, entry 19). The reaction did not work
Received: August 26, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b02563
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
a,b
Table 1. Optimization of Reaction Conditions
Scheme 1. Scope of Thiols
b
entry
photocatalyst (mol %)
solvent
CH3CN
yield (%)
1
Ru(bpy)3Cl2 (5)
fac-Ir(ppy)3 (5)
Mes-Acr+ (5)
eosin Y (5)
57
33
2
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
DMF
3
14
4
42
5
eosin B (5)
30
6
rhodamine B (5)
rose bengal (5)
rose bengal (5)
rose bengal (5)
rose bengal (5)
rose bengal (5)
rose bengal (5)
none
27
7
56
8
71
9
THF
50
10
11
12
13
CH2Cl2
H2O
35
22
CH3CH2OH
DMF
20
trace
trace
74
c
14
rose bengal (5)
rose bengal (5)
rose bengal (5)
rose bengal (3)
rose bengal (1)
rose bengal (5)
rose bengal (5)
DMF
a
b
c
d
Reaction conditions: see entry 16, Table 1. Isolated yields. 1.11 g of
15
DMF
d
e
3aa prepared. The reaction was conducted for 6 h.
16
DMF
79
69
e
17
DMF
e
phosphorylation of aromatic thiols. Aliphatic thiols could also
be phosphorylated under the optimized reaction conditions.
Thus, propane-1-thiol, phenylmethanethiol, and cyclohexane-
thiol were successfully converted to the targeted products 3ra−
ta in moderate yields. To show the practical usefulness of this
method, we carried out the reaction of 1a with 2a on a gram-
scale under the standard conditions and isolated 3aa in 68%
yield (1.11 g).
18
DMF
36
e
,
,
f
19
20
DMF
60
e
g
DMF
10
a
Reaction conditions: 1a (0.36 mmol), 2a (0.30 mmol), and
photocatalyst (5 mol %) in solvent (1.5 mL) were irradiated with
10 W blue LEDs at room temperature under air for 12 h. Isolated
yields. The reaction was performed in the dark. 1a (0.45 mmol) was
b
c
d
e
f
used. 1a (0.60 mmol) was used. The reaction was conducted under
g
O2 (1 atm). The reaction was conducted under N2. Mes-Acr+ = 9-
We next examined the scope of P(O)H compounds (Scheme
2). Diarylphosphine oxides bearing a methyl group at the para-
mesityl-10-methylacridinium perchlorate.
a
,b
Scheme 2. Scope of P(O)H Compounds
well under a nitrogen atmosphere, giving the product 3aa in
10% yield (Table 1, entry 20). These experimental results imply
that rose bengal, visible light, and O2 are all necessary for the
success of this transformation (Table 1, entries 13, 14, and 20).
With optimized reaction conditions in hand, the scope and
limitations of the title procedure were explored. We first
investigated the scope of thiols (Scheme 1). Thiophenol
derivatives bearing ethyl, tert-butyl, methoxyl, and amide groups
at the para-position of the aromatic ring underwent this
transformation smoothly to produce the desired coupling
products in good yields (3ba−ea). Meta-substituted congeners
of 1a and 1d provided similar results (3fa and 3ga). Slightly
lower but still good yields were obtained for the ortho-
substituted systems, probably for steric reasons (3ha and 3ia).
To probe the chemoselectivity of this reaction, aromatic thiols
with sensitive functional groups such as the free hydroxyl and
amine groups were tested. We found that the transformation
occurs efficiently with complete chemoselectivity, generating
the S−P(O) coupling products in good yields (3ja and 3ka).
Thiophenol derivatives carrying halogen substituents such as F,
Cl, and Br were shown to be compatible under the current
reaction conditions, and the corresponding products 3la−na
were isolated in moderate yields. Notably, the halogen
substituents such as Cl and Br are very useful for further
synthetic elaborations. The naphthyl derivatives coupled readily
with diphenylphosphine oxide 2a to give the corresponding
products in good yields (3oa and 3pa). Thiophenol also proved
to be a viable substrate and afforded 56% of 3qa. It is important
to note that the current approach is not restricted to the
a
b
c
Reaction conditions: see entry 16, Table 1. Isolated yields. The
reaction was conducted for 36 h.
or meta-position of the arene ring performed well with good
yields (3ab and 3ac). The ortho-substituted congener of 2a
exhibited less reactivity likely due to the steric hindrance effect
(3ad). Substrates with a tert-butyl group resulted in a low yield
due to some side reactions (3ae). Halogens like F, Cl, and Br
groups were well tolerated in the reaction, leading to the
corresponding products 3af−ah in moderate to good yields.
The phenyl and naphthyl derivatives were also suitable
B
DOI: 10.1021/acs.orglett.6b02563
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
substrates for this transformation, and the products 3ai and 3aj
were obtained in 71% and 55% yields, respectively. Besides
symmetric phosphine oxides, unsymmetric phosphine oxides
containing an alkyl group coupled readily with 4-methylbenze-
nethiol 1a to give the desired coupling products in good yields
(e.g., 3ak). Unfortunately, dialkylphosphine oxides failed to
afford the desired products under the applied reaction
conditions. Finally, we showed that ethyl phenylphosphinate
2l is also transformed into the corresponding product 3al in
30% yield.
(1O2),15 which would be the species responsible for this
oxidative coupling. In order to verify the presence of 1O2 in the
current reaction, we carried out trapping and quenching
experiments (eqs 5 and 6). The trapping experiment with
9,10-dimethylanthracene 816 under the standard conditions
provided the endoperoxide product 9 that was formed through
[4 + 2] cycloaddition involving 1O2, while the adduct product 9
was not observed in the dark. Moreover, the reaction was
completely quenched in the presence of 1,4-diazabicyclo[2.2.2]-
octane (DABCO), which is known as a strong physical
quencher of 1O2.17 These results indicated that 1O2 is produced
from O2 in our reaction system and is involved in the reaction
mechanism.
To shed light on the mechanism of this new visible-light-
mediated oxidative coupling, some preliminary mechanistic
studies were conducted (Scheme 3). Initially, the event for the
On the basis of the above experimental observations, we
proposed the following mechanism (Scheme 4). First, rose
Scheme 3. Mechanistic Experiments
Scheme 4. Proposed Reaction Mechanism
bengal (RB) is excited under visible light irradiation to produce
its excited state species RB*, which interacts with O2 to
15
1
generate O2 via the energy transfer. In this process, the
excited state RB* returns to its ground state (RB).
1
Subsequently, the generated O2 abstracts a hydrogen atom
from thiol 1 to give the thiyl radical A,18 which undergoes
homocoupling to form the disulfide B. On the other hand, the
P-centered radical C is generated from the P(O)H compound 2
by a similar oxidation process. Finally, the formed P-centered
radical C couples with the thiyl radical A (path a) or reacts with
the disulfide B (path b) to afford the product 3. Alternatively,
the product 3 could be produced by nucleophilic attack of the
P(O)H compound 2 with the disulfide B (path c).
In summary, we have presented the first example of direct
oxidative cross-coupling of thiols with P(O)H compounds for
the construction of S−P(O) bonds via organic dye-sensitized
photocatalysis. In the presence of commercially available and
inexpensive organic dye, rose bengal, as the photocatalyst and
air as the green oxidant, a range of S−P(O) coupling products
can be readily obtained in moderate to excellent yields.
Reactions that are easy to conduct exhibit excellent chemo-
selectivity and good functional-group tolerance. All of these
features as well its mild and transition-metal-free conditions
make this protocol highly practical. Further studies on synthetic
application are ongoing in our laboratory.
generation of thiyl radicals from 4-methylbenzenethiol 1a in the
presence of the photocatalyst rose bengal was experimentally
supported by trapping the thiyl radicals with styrene 4 to afford
the thioether 5 in 40% yield.13 The background thiol−ene
reaction could be excluded because the adduct product 5 could
not be obtained in the absence of rose bengal (eq 1). The result
from a radical capture experiment with TEMPO on
diphenylphosphine oxide 2a suggested that P-centered radicals
are likely involved in the current reaction (eq 2). Moreover, we
found that disulfides are formed in all cases, which
demonstrated a radical reaction pathway as well.14 Further
investigation showed that the disulfide 7 could be generated
from 4-methylbenzenethiol 1a in 82% yield under the standard
conditions (eq 3). To demonstrate the role of disulfide 7, we
ran a reaction using 7 and 2a as substrates under the standard
conditions, leading to 3aa in 84% yield. This result indicated
that the disulfide 7 may be the key intermediate of this
transformation. A series of control experiments showed that the
formation of 3aa starting with 7 and 2a did not require any
photocatalyst, visible light, or O2 (eq 4). Additionally, it is well
recognized that rose bengal could react with molecular oxygen
under visible light irradiation to generate singlet oxygen
C
DOI: 10.1021/acs.orglett.6b02563
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.6b02563.
■
S
Experimental details and characterization data for the
products (PDF)
W. C. Chem. Rev. 2013, 113, 5322. (e) Hari, D. P.; Konig, B. Angew.
̈
AUTHOR INFORMATION
Corresponding Authors
Chem., Int. Ed. 2013, 52, 4734. (f) Beatty, J. W.; Stephenson, C. R. J.
Acc. Chem. Res. 2015, 48, 1474. (g) Romero, N. A.; Nicewicz, D. A.
Chem. Rev. 2016, DOI: 10.1021/acs.chemrev.6b00057. (h) Chen, J.-
R.; Hu, X.-Q.; Lu, L.-Q.; Xiao, W.-J. Chem. Soc. Rev. 2016, 45, 2044.
(10) For selected recent examples on transition-metal-mediated
photoredox catalysis, see: (a) Allen, L. J.; Cabrera, P. J.; Lee, M.;
Sanford, M. S. J. Am. Chem. Soc. 2014, 136, 5607. (b) Huang, H.; Jia,
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Cuthbertson, J. D.; Shurtleff, V. W.; MacMillan, D. W. C. Nature 2015,
■
*E-mail: liping2004@126.com.
*E-mail: zb3981444@cpu.edu.cn.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
524, 330. (d) Brachet, E.; Ghosh, T.; Ghosh, I.; Konig, B. Chem. Sci.
̈
This work is financially supported by the 111 Project (No.
B16046) and the Project Program of State Key Laboratory of
Natural Medicines, China Pharmaceutical University (No.
SKLNMZZYQ201602).
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DOI: 10.1021/acs.orglett.6b02563
Org. Lett. XXXX, XXX, XXX−XXX