Diverse C-P Cross-Couplings of Arylsulfonium Salts with Diarylphosphines via Selective C-S Bond Cleavage

Article abstract of DOI:10.1021/acs.orglett.1c00748

Diverse C-P cross-couplings of arylthianthrenium salts with diarylphosphines producing various triarylphosphines via highly selective C-S bond cleavage are reported. In the absence of catalyst, the reaction of arylthianthrenium salts with diarylphosphines undergoes phosphinative ring opening exclusively via the cleavage of an endocyclic C-S bond of a thianthrene skeleton. The use of a palladacycle catalyst under otherwise the same conditions enables the phosphination via the cleavage of an exocyclic C-S bond with significantly higher speed.

Full text of DOI:10.1021/acs.orglett.1c00748

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Letter
Diverse C−P Cross-Couplings of Arylsulfonium Salts with
Diarylphosphines via Selective C−S Bond Cleavage
Yun Ye, Jie Zhu, and Yinhua Huang*
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ABSTRACT: Diverse C−P cross-couplings of arylthianthrenium
salts with diarylphosphines producing various triarylphosphines via
highly selective C−S bond cleavage are reported. In the absence of
catalyst, the reaction of arylthianthrenium salts with diary-
lphosphines undergoes phosphinative ring opening exclusively via
the cleavage of an endocyclic C−S bond of a thianthrene skeleton.
The use of a palladacycle catalyst under otherwise the same
conditions enables the phosphination via the cleavage of an
exocyclic C−S bond with significantly higher speed.
riarylphosphines (Ar1Ar2Ar3P) and their derivatives are
well-known for their high utility in synthetic chemistry
Scheme 1. Strategies for Direct Synthesis of
Triarylphosphines
T
and catalysis,1 biomedicines,2 and materials science.3 They are
conventionally synthesized by either addition of a Grignard/
organolithium reagent to a phosphine halide or reaction of a
phosphide anion with an electrophile (Scheme 1a).4 This
method is intolerant to a wide variety of functional groups due
to the harsh conditions. Alternatively, they can be prepared by
transition-metal-catalyzed cross-coupling reaction of aryl-
(pseudo)halides (Scheme 1b).5 Although the compatibility is
improved greatly by this method, special functional groups
such as (pseudo)halides must be preinstalled on the starting
materials. Indirect synthesis of triarylphosphines using
phosphine oxides, phosphine boranes, or phosphonates, etc.,
have also been well documented;6 however, the tedious
protection/deprotection, or reduction, involving multiple steps
is unavoidable. It is strongly desirable to develop direct and
greener approaches to introduce phosphino groups onto the
desired skeleton in a controlled manner. Herein, we report a
new approach for highly selective installation of diary-
lphosphino groups onto (hetero)arenes yielding various
triarylphosphines via diverse C−P cross-couplings of diary-
lphosphines with arylthianthrenium salts which can be readily
prepared by site-selective aromatic C−H thianthrenation
(Scheme 1c).
Sulfonium salts have been attracting increasing attention
owing to their broad applications in synthetic chemistry.7
Recently, arylsulfonium salts such as aryl-tetrafluoro-
thianthrenium (Ar-TFT) salts, arylthianthrenium (Ar-TT)
salts, and aryldibenzothiophenium (Ar-DBT) salts have been
developed as a class of versatile electrophile reagents for
various transformations by Ritter8 and others.9 An example of
Michaelis−Arbuzov phosphonate synthesis using an Ar-TFT
Received: March 4, 2021
Published: March 10, 2021
https://dx.doi.org/10.1021/acs.orglett.1c00748
© 2021 American Chemical Society
Org. Lett. 2021, 23, 2386−2391
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salt with triphenylphosphite (P(OPh)3) under photoredox
catalysis has been reported;8a however, the reaction of any
arylsulfonium salts with diarylphosphines (Ar1Ar2PH) is
unprecedented to the best of our knowledge. We have made
continuous efforts10 to develop new methods for construction
of chiral phosphines by hydrophosphination of electron-
deficient olefins.11 We envisaged that triarylphosphines can
be accessed by the reaction of arylsulfonium salts with
diarylphosphines.
We began our investigation with the reaction of readily
accessible sulfonium salt 1a with Ph2PH (2a). In the presence
of a base, the reaction underwent phosphinative ring opening
exclusively (Table 1, entries 1−2). The use of K2CO3 leads to
achieved by employing the C,P-palladacycle L2-Pd as the
catalyst under the given conditions (14 h), with thianthrene
(TT) being recovered in 90% yield (entry 12). When the
reaction time was shortened to 1 h, 4aa was formed in 89%
yield accompanied by 2% of 3aa (entry 13). The C,N-
palladacycle L1-Pd did not give satisfactory selectivity (entry
11). The cationic analogue L3-Pd can also work producing 4aa
in a slightly lower yield. The use of the platinum analogue L1-
Pt did not give 4aa at all.13 Screening of solvents evidenced
that dioxane can give similar results as THF, while MTBE and
toluene are not the appropriate ones. Several other types of
arylsulfonium salts including 1a2−1a5 were also examined
indicating that only the Ar-TT salt can undergo the divers C−
P cross-couplings in a highly selective manner.
a
The high selectivity of 3aa/4aa (4/96) is attributed to the
high catalytic activity of L2-Pd which produces 4aa with
significantly higher speed than the formation of 3aa. Control
experiments for conversion versus time curves for the reaction
of 1a and 2a were performed (Figure 1). Under the catalytic
Table 1. Optimization of Conditions
c
yield (%)
b
entry
catalyst
base
solvent
3aa
86
96
4aa
1
2
3
4
5
6
7
8
none
none
none
CuCl
Cs2CO3
KOH
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
dioxane
MTBE
toluene
0
0
0
−
0
21
65
85
94
90
67
96(95)
89
85
0
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
99(99)
54
68
4
5
15
6
8
33
4
2
7
Figure 1. Profiles for the reaction of 1a and 2a.
Ni(cod)2
Pd(OAc)2
Pd2(dba)3
Pd(PPh3)4
Pd(OAc)2/binap
Pd(OAc)2/segphos
L1-Pd
conditions, the production of 4aa catalyzed by L2-Pd is about
24 times faster than that of 3aa. For 50% conversion of 1a, it
takes only about 7 min with L2-Pd compared to around 80
min without the catalyst.
9
10
11
12
Under the optimized conditions for the phosphinative ring
opening, we assessed the scope of Ar-TT salts with Ph2PH, and
the results were summarized in Scheme 2. A variety of para-,
meta-, ortho-substituted Ar-TT salts successfully underwent
phosphinative ring opening, tolerating a broad range of
functional groups giving 3aa−3pa in mild to excellent yields.
Notably, this transformation exclusively gave the ring-opening
product 3, the formation of 4 being not observed. Ring
openings of cyclic sulfonium salts by a nucleophile8j,14 (ArSNa,
ArONa, NaBAr4, NMe4F) have been reported; however, high
selectivity remains a challenge for the ring opening.
L2-Pd
L2-Pd
L3-Pd
L1-Pt
L2-Pd
L2-Pd
L2-Pd
d
13
14
15
16
17
18
97
6
4
94
12
4
3
a
Reaction conditions: 1a (0.12 mmol), 2a (0.14 mmol), base (3.0
b
equiv), solvent (2 mL), at 60 °C for 14 h. Catalyst (5 mol % of M)
c
was loaded if applicable. The yields were obtained by 31P NMR
Next, the scope of Ar-TT salts and diarylphosphines for the
L2-Pd-catalyzed phosphination under the optimized condi-
tions was investigated (Scheme 3). A variety of para-, meta-,
ortho-substituted Ar-TT salts were successfully coupled to give
products 4aa−4ya in generally good to excellent yields,
tolerating a wide range of functional groups such as ethers,
esters, amines, amides, halogens, olefins, alcohols, heterocycles,
etc. The presence of sensitive functionalities such as Br (1b),
OTf (1g), and OH (4ta) did not hamper the reaction;
however, I (1f) cannot be well tolerated and can give the
double phosphination product.15 Thianthrenation takes place
at the 2-position on the thiophene ring forming 2-thienyl-
thianthrenium salts (1u−1w) which can react smoothly with
Ph2PH to give the phosphination products (4ua−4wa), while
analysis of the crude reaction mixture after quenching with sulfur
using (PhO)3P(O) as an internal standard. Isolated yields in
parentheses. The reaction was quenched after 1 h. THF =
d
tetrahydrofuran; MTBE = methyl tert-butyl ether.
product 3aa12 in a quantitative yield (entry 3). The X-ray
analysis of 3aa (CCDC-2025189) unambiguously confirmed
its structure. Next, a few transition metal catalysts were
examined (entries 4−8), and it was found that Pd(OAc)2 can
inhibit the ring-opening product 3aa significantly and yield
another cross-coupling product 4aa in 21% yield. Further
screening revealed that the chelation ligands (binap, segphos)
can improve its yield greatly (entries 9−10). The best yield of
4aa (95%) with high selectivity (3aa/4aa = 4/96) was
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Scheme 2. Phosphinative Ring Opening of Ar-TT Salts with
Ph2PH
Scheme 3. Palladium-Catalyzed Phosphination of Ar-TT
Salts with Diarylphosphines
a
a
a
Reaction conditions: 1a (0.12 mmol), 2a (0.14 mmol), K2CO3 (3.0
equiv), THF (2 mL), at 60 °C for 14 h. Isolated yield of sulfide 3 after
quenching of the reaction with sulfur.
it takes place at the 3-position on the benzothiophene ring to
give 1x which can react with diarylphosphines to give products
4xa−4xc. The X-ray analysis of 1x (CCDC-2057925) and 4xa
(CCDC-2025190) unambiguously confirmed their struc-
tures.15 This protocol can be used to manipulate the complex,
biologically relevant scaffold derived from estrone (4oa, 95%)
and salicin pentaacetate (4ya, 90%) via selective incorporation
of a diphenylphosphino group. In addition, substituted
diarylphosphino groups can be efficiently installed (4ab, 4xb,
4ac). Unsymmetrical diarylphosphine Ph(Mes)PH (2d) can
be also coupled to give 4ad in 63% yield. Optically pure (S)-
L2-Pd as a chiral catalyst was investigated for this trans-
formation. Unfortunately, no ee (ee% = 0) was observed under
the conditions.
a
Reaction conditions: 1a (0.12 mmol), 2a (0.14 mmol), L2-Pd (5
To gain insights into the reaction mechanism, radical
trapping experiments using TEMPO or BHT as the scavenger
were performed, and the results indicated that an aryl radical
intermediate is not formed during the reaction (Scheme 4a).
The reaction of sterically hindered Mes-TT salt (1z) with
Ph2PH only gives the ring-opening product 3za (Scheme 4b).
The congestion from the mesityl group inhibits the formation
of 4za completely even with the L2-Pd. When sterically more
hindered (Mes)2PH was used instead of Ph2PH, no reaction
takes place at all.
mol % of Pd), base (3.0 equiv), THF (2 mL), at 60 °C for 14 h.
Isolated yield of sulfide 4 after quenching of the reaction with sulfur.
The ratio of the corresponding 3/4 determined by 31P NMR analysis
of the crude reaction mixture is indicated in parentheses.
Scheme 4. Control Experiments
Tertiary triarylphosphine products can be obtained without
sulfurization under the standard conditions. An example (eq 1)
was shown to prepare 5aa (90% isolated yield). Slight
oxidation was observed during the isolation of the product if
not protected. To demonstrate the scalability of this protocol,
1.20 g of salicin pentaacetate (pharmaceutical) was used for
the synthesis of 4ya with L2-Pd (2 mol % Pd). The target
product 4ya was isolated in 85% overall yield (two steps), and
thianthrene (TT) was recovered in 91% overall yield (eq 2).
In summary, we have disclosed a new approach for the
synthesis of various triarylphosphines via diverse C−P cross-
couplings of arylthianthrenium salts with diarylphosphines.
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Plan Foundation of Hangzhou Normal University for financial
support. We are also grateful to Prof. T. Hayashi (Department
of Chemistry, National Tsing Hua University, Taiwan 30013)
and P.-H. Leung (Division of Chemistry and Biological
Chemistry, School of Physical and Mathematical Sciences,
Nanyang Technological University, Singapore 637371) for
their insightful discussions which contributed to this work.
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ASSOCIATED CONTENT
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The Supporting Information is available free of charge at
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Experimental details, characterization data including H
NMR, 13C NMR, and 31P NMR spectra, X-ray data
(PDF)
Accession Codes
CCDC 2025189−2025190 and 2057925 contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge via www.ccdc.cam.ac.uk/data_request/
cif, or by emailing data_request@ccdc.cam.ac.uk, or by
contacting The Cambridge Crystallographic Data Centre, 12
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AUTHOR INFORMATION
Corresponding Author
■
Yinhua Huang − College of Materials, Chemistry and
Chemical Engineering, Hangzhou Normal University,
Hangzhou 311121, China; orcid.org/0000-0002-5523-
6286; Email: yhhuang@hznu.edu.cn
Authors
Yun Ye − College of Materials, Chemistry and Chemical
Engineering, Hangzhou Normal University, Hangzhou
311121, China
Jie Zhu − College of Materials, Chemistry and Chemical
Engineering, Hangzhou Normal University, Hangzhou
311121, China
Complete contact information is available at:
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We gratefully acknowledge Natural Science Foundation of
Zhejiang Province (ZJNSF) (LY19B020005) and the Pandeng
■
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Organic Letters
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(12) For the convenience of isolation and characterization, the
reaction is quenched with sulfur.
(13) The L1-Pt catalyst did not affect the ring-opening reaction
(97% yield of 3aa); however, some palladium catalysts such as
Pd(OAc)2 and Pd2(dba)3 resulted in consumption of Ph2PH leading
to lower total yields of 3aa and 4aa.
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(15) For more details, see the Suporting Information.
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