Visible-Light-Promoted Photoredox Dehydrogenative Coupling of Phosphines and Thiophenols

Article abstract of DOI:10.1021/acs.orglett.0c02746

Herein, by applying visible-light photoredox catalysis, we have now achieved the first example of catalytic dehydrogenative coupling of phosphines and thiophenols that proceeds at room temperature. Key to our success is the use of benzaldehyde as a soft oxidant, which avoids the issue of phosphine oxidation. Furthermore, we observed the unexpected dealkylative coupling of secondary and tertiary alkylphosphine with thiophenols.

Full text of DOI:10.1021/acs.orglett.0c02746

pubs.acs.org/OrgLett
Letter
Visible-Light-Promoted Photoredox Dehydrogenative Coupling of
Phosphines and Thiophenols
Xianya Wang, Chungu Xia,* and Lipeng Wu*
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ABSTRACT: Herein, by applying visible-light photoredox catalysis,
we have now achieved the first example of catalytic dehydrogenative
coupling of phosphines and thiophenols that proceeds at room
temperature. Key to our success is the use of benzaldehyde as a soft
oxidant, which avoids the issue of phosphine oxidation. Furthermore,
we observed the unexpected dealkylative coupling of secondary and
tertiary alkylphosphine with thiophenols.
atalytic bond formation and cleavage at the carbon
centers play a pivotal role in the synthesis of bulk and
and agrochemistry.16 However, their direct synthesis from
phosphines and thiophenols is very rare. To the best of our
knowledge, there are only two examples reported in the
literature, using rhodium catalyst at 110 °C5b (Scheme 1A) or
C
fine chemicals.1 In contrast, catalytic chemistry that involves
the activation and transformation of main group substrates for
E−E′ (E = p-block elements other than carbon) bond
formation is considerably less developed.2 Traditionally, the
formation of main group element−element bonds has been
achieved using methods such as salt metathesis3 and Wurtz-
type reductive coupling.4 However, these approaches produce
large amounts of salts as side products, and the harsh reaction
conditions limit functional group tolerance. Consequently, the
development of catalytic methods for E−E′ bond formation
under mild reaction conditions with broad substrate tolerance
is required.5
Scheme 1. (A) Rhodium Phosphido Complex-Catalyzed
Dehydrocoupling of Phosphine with Thiophenols at 110
°C; (B) tBuOK-Catalyzed Dehydrocoupling of Phosphine
with Thiophenols in the Presence of H2 Acceptors at 130
°C; (C) Our Current Work Using Photoredox Catalyst
Catalyzed Dehydrocoupling of Phosphines with
a
Thiophenols under Visible-Light Condition
Over the past decade, visible-light photoredox catalysis has
emerged as an enabling platform for the development of new
organic reactions with high synthetic efficiency and broad
functional group tolerance.6 These reactions mainly focused on
the bonds formation of C−C and C−E (E = N, S, Cl, Br, Si, B,
Se).7 In contrast, the application of photoredox catalysis for
E−E′ bond formation is rare. In 2015, visible-light-catalyzed
Si−O bond formation between hydrosilane and water, using
rhodium porphyrin complexes, was reported.8 In 2016, a
visible-light-mediated oxidative cross-coupling of thiols with
(PVO)−H compounds using air as the oxidant was
developed.9 Furthermore, the use of photoredox catalysis for
S−S and NN bond formation has also been achieved.10
Notwithstanding these achievements, there is no application of
photoredox catalysis on bond formation involving primary and
secondary phosphines (PIII−H species: R1R2PH, R2 = H or R1)
to date.11 This is mainly because the PIII−H species is easily
oxidized by common oxidants such as air,9 dilauroyl
peroxide,12 or K2S2O8,13 which are otherwise required for
the (PVO)−H compounds. Molecules containing PIII have
numerous applications in coordination5d,14 and synthetic
chemistry.15 In addition, compounds containing PIII−S bonds
(thiophosphanes) are of great interest in medicinal chemistry
a
HA = hydrogen acceptor.
Received: August 17, 2020
https://dx.doi.org/10.1021/acs.orglett.0c02746
© XXXX American Chemical Society
Org. Lett. XXXX, XXX, XXX−XXX
A

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tBuOK at 130 °C (Scheme 1B),17 both of which require high
temperatures and fall short in terms of substrate generality
studies. Consequently, the development of a milder and more
convenient approach for the direct synthesis of thiophosphanes
remains underdeveloped. Herein, we report the first example of
a general dehydrocoupling of phosphines with thiophenols
enabled by photoredox catalysis under visible-light irradiation
to produce a series of thiophosphanes (Scheme 1C). More
interestingly, we also observed an unexpected dealkylative
coupling process in the reaction of secondary and tertiary
alkylphosphine with thiophenols.
12), while it is worth mentioning that THF, dioxane, and
DMSO gave similar results (Table 1, entries 9−11).
With the optimized reaction conditions in hand (Table 1,
entry 6), we evaluated the generality of our methodology for
the synthesis of various thiophosphanes. First, thiophenols
with different substituents on the phenyl ring were applied in
the reaction with diphenylphosphine (1a) (Scheme 2). In
Scheme 2. Ir(ppy)3-Catalyzed Dehydrogenative Coupling of
Diphenylphosphine with Various Thiophenols under
Visible-Light Irradiation*
We chose diphenylphosphine 1a (0.2 mmol) coupled with
4-methoxythiophenol 2a (0.24 mmol) as the model reaction
and Ir(ppy)3 (1 mol %) as the photoredox catalyst (see Table
S1 for photoredox catalyst screening), under ambient
conditions, with a 22 W white LED plate as the light source.
The results of our initial investigations revealed that, without
any H2-acceptor (HA), only a trace amount of 3a was
produced (Table 1, entry 1). Then, in an effort to promote the
Table 1. Ir(ppy)3-Catalyzed Dehydrogenative Coupling of
Diphenylphosphine (1a) with 4-Methoxythiophenol (2a)
a
under Visible-Light: Condition Optimization
b
entry
HAs
solvent
yield (%)
1
2
3
4
5
6
7
8
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
EtOH
DCM
THF
DMSO
dioxane
toluene
2
29
32
trace
65
96
20
78
88
92
HA-1
HA-2
HA-3
HA-4
HA-5
HA-5
HA-5
HA-5
HA-5
HA-5
HA-5
*
9
Reactions were performed in a 20 mL vial with 1a (0.2 mmol),
thiophenols (0.24 mmol), benzaldehyde (0.2 mmol), Ir(ppy)3 (1 mol
10
11
12
%), and MeCN 1.0 mL under N2 and light irradiation for 12−24 h;
92
76
a
isolated yields are given. Isolated yields after complexation with BH3.
a
Reactions were performed in a 20 mL vial with 1a (0.2 mmol), 2a
general, good to excellent yields were obtained. For example,
thiophenols with −OMe, −Me, −Et, and −iPr groups on the
phenyl ring reacted smoothly with 1a and afforded the
corresponding thiophosphanes 3a−3h in yields ranging from
88% to 94%. We also found that the position of the
substituents on the phenyl ring had no obvious effect on the
results. Furthermore, fluoride- and chloride-substituted thio-
phenols also participated in the dehydrogenative coupling
reaction without side reactions arising from the C−X bonds (X
= F, Cl) (3i−3k). Apart from the monosubstituted
thiophenols, the multisubstituted thiophenols also worked
well in our system; 3l with 2,6-dimethyl groups and 3m with
3,4-dichloro groups were obtained in yields of 82% and 92%,
respectively. Finally, by changing the phosphine to 4-Me-
substituted diphenylphosphines we could obtain the corre-
sponding products in 78% and 75% yields when coupling with
different thiophenols (3n and 3o).
(0.24 mmol), Ir(ppy)3 (1 mol %), HA (0.2 mmol), and solvents (1.0
b
mL) under N2 atmosphere with light irradiation. Yields were
determined by 31P NMR spectroscopy.
dehydrocoupling process, we added 1 equiv of various HAs to
the MeCN solution. To our delight, we obtained improved
results. With benzophenone (HA-1), we obtained 29% of the
dehydrocoupling product 3a (Table 1, entry 2). With an imine
(HA-2), we obtained 32% of 3a (Table 1, entry 3). With
styrene (HA-3), only a trace amount of 3a was observed; this
was due to the competing hydrophosphination of styrene
(Table 1, entry 4). With azobenzene (HA-4), we obtained 65%
of 3a (Table 1, entry 5). Finally, benzaldehyde (HA-5) was
found to be the best HA, which afforded 3a in 96% yield
(Table 1, entry 6, and Figure S1). Then, changing the reaction
solvent from MeCN to other solvents was conducted, and all
the tested solvents gave inferior results (Table 1, entries 7−
B
https://dx.doi.org/10.1021/acs.orglett.0c02746
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We then turned our attention to the use of a primary
phenylphosphine in the dehydrogenative coupling reactions.
Phenylphosphine (PhPH2) has two P−H bonds; therefore,
upon reaction with thiophenols, it is possible that a mixture of
mono- and bis-dehydrocoupling products will be formed. To
our delight, we selectively obtained the bis-dehydrocoupling
product by using 3 equiv of thiophenols in the reaction with
phenylphosphine. Scheme 3 shows that a series of thiophenols
Scheme 4. Mechanism Study Reactions
Scheme 3. Ir(ppy)3-Catalyzed Dehydrogenative Coupling of
Phenylphosphine with Various Thiophenols under Visible-
a
Light Irradiation
Scheme 5. Plausible Reaction Mechanism for Photoredox
Dehydrocoupling of Phosphines with Thiophenols
a
Reactions were performed in a 20 mL vial with 1b (0.2 mmol),
thiophenols (0.6 mmol), benzaldehyde (0.4 mmol), Ir(ppy)3 (1 mol
%), and MeCN 1.0 mL under N2 and light irradiation for 12−24 h;
isolated yields are given.
could readily react with phenylphosphine and afford the
corresponding bis-thiophosphanes in excellent yields (85−
93%). As in the case of diphenylphosphine, the position of the
substituents on the phenyl ring of thiophenol had hardly any
effect on the reaction outcome. Apart from the various alkyl
substituents, the halogen groups (such as fluoride and
chloride) remained intact.
To understand the reaction mechanism, we carried out
several control experiments. First, we observed that the
omission of benzaldehyde or light yielded only a trace amount
of 3a, hence indicating the crucial role of both the H2-acceptor
and light in the reaction (Scheme 4A, Figures S2 and S3).
Second, we observed that the reaction without Ir(ppy)3 gave
only 3% of the product (Scheme 4A, Figure S4), hence ruling
out the possibility of benzaldehyde itself acting as the
photoredox catalyst in the reaction.18 Furthermore, the
addition of 5 equiv of TEMPO to the reaction largely
suppressed the reaction with only 32% of 3a obtained (Scheme
4B). Further control experiments with only thiophenol in the
catalytic system showed that disulfide was formed in 79% yield
(Scheme 4C) but required the presence of benzaldehyde as
only 5% of disulfide was produced without benzaldehyde
(Scheme 4D). In addition, disulfide reacted with diphenyl-
phosphine to produce 3a in 43% yield (Scheme 4E).
From the control experiments and the fact that we could
detect disulfide and benzyl alcohol in the reaction (Figure S5),
we proposed the reaction mechanism as depicted in Scheme 5.
Ir(ppy)3 is first excited under visible light to yield its excited-
state species, [Ir(ppy)3]*, which then undergoes a single-
electron transfer with benzaldehyde to yield [Ir(ppy)3]+ and
radical anion of benzaldehyde (A), which produced ketyl
radical (B) with H+, the ketyl radical then interacted with
thiophenols to yield the S-based radical (C) and benzyl
alcohol. On the other hand [Ir(ppy)3]+ was reduced by
another thiophenol to regenerate Ir(ppy)3 and produce radical
cation of thiophenol (D), which produced S-based radical after
deprotonation. Coupling of S-based radical generated ArS−
SAr, which reacted with Ph2PH to generate ArS−PPh2 and
thiophenol.
We then carried out experiments to determine whether an
alkylphosphine could undergo the catalytic photoredox
dehydrocoupling reaction with thiophenols. Di-tert-butylphos-
phine 1c was used in the above-described system. To our
surprise, we found that only ∼30% dehydrogenative coupling
product 5 was formed with its further dealkylcoupling product
6 as the major product (Scheme 6). This unexpected
transformation is applicable to various thiophenols with an
alkyl substituent on the phenyl ring to produce the dealkylative
coupling products 6a−6d in 50−68% yields. Halides such as
−Cl and −F were also tolerant as demonstrated by products
C
https://dx.doi.org/10.1021/acs.orglett.0c02746
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Letter
orcid.org/0000-0003-0583-4832; Email: lipengwu@
licp.cas.cn
Scheme 6. Photoredox Dealkylcoupling of Alkyl Phosphines
with Thiophenols
Author
Xianya Wang − State Key Laboratory for Oxo Synthesis and
Selective Oxidation, Suzhou Research Institute of LICP,
Lanzhou Institute of Chemical Physics (LICP), Chinese
Academy of Sciences, Lanzhou 730000, P.R. China; University
of Chinese Academy of Sciences, Beijing 100049, P.R. China
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c02746
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to the Natural Science Foundation of Jiangsu
Province (BK20180246) and the National Natural Science
Foundation of China (21901247, 91845108) for generous
financial support and the National Program for Young
Investigators of China for support to start the lab. We also
thank Dr. Liang-Qiu Lu and Dr. Quan-Quan Zhou for helpful
suggestions.
REFERENCES
■
6e−6g. More interestingly, this dealkylative coupling approach
could extend to tri-tert-butylphosphine 1d, in which case the
dealkylative coupling of one −tBu group was found to be the
major products, and substituents on the para-, ortho-, and
meta-position of the phenyl ring were all applicable with yields
ranging from 55 to 76% (7a−7e).
In conclusion, we have developed the first visible-light-
promoted photoredox dehydrogenative coupling of phosphines
with thiophenols. The application of visible light and the use of
benzaldehyde as H2-acceptor are key to our success. Our
system offers the mildest conditions (25 °C) for the
dehydrogenative coupling of phosphines with thiophenols.
Prior to this, the only two existing catalytic thermal reactions
required temperatures >110 °C. A series of thiolphosphanes
was successfully synthesized in excellent yields. Moreover, the
unexpected dealkylative coupling of alkylphosphine with
thiophenols was observed.
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ASSOCIATED CONTENT
* Supporting Information
■
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AUTHOR INFORMATION
Corresponding Authors
■
Chungu Xia − State Key Laboratory for Oxo Synthesis and
Selective Oxidation, Suzhou Research Institute of LICP,
Lanzhou Institute of Chemical Physics (LICP), Chinese
Academy of Sciences, Lanzhou 730000, P.R. China;
Email: cgx@licp.cas.cn
Lipeng Wu − State Key Laboratory for Oxo Synthesis and
Selective Oxidation, Suzhou Research Institute of LICP,
Lanzhou Institute of Chemical Physics (LICP), Chinese
Academy of Sciences, Lanzhou 730000, P.R. China;
D
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