Palladium-Catalyzed Sulfinylation of Aryl- And Alkenylborons with Sulfinate Esters

Article abstract of DOI:10.1021/acs.orglett.1c01292

An efficient, direct sulfinylation of organoborons catalyzed by palladium is disclosed. Treatment of organoborons and sulfinate esters in the presence of a palladium precatalyst provided a broad range of sulfoxides. Various organosulfur compounds having oxidizable functional groups were successfully prepared through the sulfoxide synthesis.

Full text of DOI:10.1021/acs.orglett.1c01292

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Letter
Palladium-Catalyzed Sulfinylation of Aryl- and Alkenylborons with
Sulfinate Esters
Minori Suzuki, Kazuya Kanemoto, Yu Nakamura, Takamitsu Hosoya, and Suguru Yoshida*
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ABSTRACT: An efficient, direct sulfinylation of organoborons
catalyzed by palladium is disclosed. Treatment of organoborons
and sulfinate esters in the presence of a palladium precatalyst
provided a broad range of sulfoxides. Various organosulfur
compounds having oxidizable functional groups were successfully
prepared through the sulfoxide synthesis.
catalyzed by palladium, enabling the preparation of a wide
ulfoxides are a fundamental class of compounds in a broad
Srange of research fields such as synthetic organic variety of sulfoxides having easily oxidizable functional groups.
Conventional direct sulfoxide synthesis has been achieved
from nucleophilic carbanions with sulfinate esters as a sulfur
surrogate.4−6 A pioneering study on the sulfinylation of
organomagnesiums using sulfinate esters was reported by
Andersen and co-workers in 1962 (Figure 1B).4 Considering
that recent significant successes of modern organometallic
chemistry have greatly improved the availability of diverse
molecules including biaryls and amines, an efficient cross-
coupling reaction using sulfinate esters is highly demanded for
synthesizing diverse sulfoxides. With our recent achievements
in organosulfur chemistry using thiosulfonates catalyzed by
transition-metals in mind (Figure 1A, bottom),7 we envisioned
that a wide range of sulfoxides can be prepared by direct
sulfinylation of organoborons catalyzed by a transition-metal
complex using sulfinate esters as electrophilic sulfur surrogates
under mild conditions (Figure 1C).
chemistry, pharmaceutical sciences, agrochemistry, and materi-
als chemistry.1,2 Particularly, recent remarkable progress on
versatile transformations of sulfoxides have allowed us to
synthesize a wide variety of molecules.2 These recent advances
clearly enhanced the synthetic utility of not only chiral but also
achiral sulfoxides.2 Despite their great significance, accessible
sulfoxides by conventional methods through sulfanylation of
Grignard reagents, organic bromides, or organoborons and
following S-oxidation are limited since various functional
groups can be damaged in the oxidation step (Figure 1A).2,3
Thus, an efficient method for direct sulfinylation is highly
sought after. We herein describe a direct method for
sulfinylation of aryl- and alkenylborons with sulfinate esters
A sulfoxide synthesis from 4-tolylboronic acid (1a) and
methyl 4-methoxybenzenesulfinate (2a) was chosen as a model
reaction (Table 1). After a number of examinations, we found
that a catalytic amount of Pd(dba)2 with XPhos as a ligand
promoted the sulfinylation in the presence of potassium
carbonate in 1,4-dioxane and water (v/v = 10/1) at 80 °C
(entry 1). In contrast, the yields of sulfoxide 3a were
significantly decreased when the reaction was conducted
using triphenylphosphine or an N-hetero cyclic carbene ligand
(entries 2 and 3). Palladium precatalysts XPhos Pd G3 and
XPhos Pd G4 also catalyzed the synthesis of sulfoxide 3a
(entries 4 and 5).8 While the reaction performed at 100 °C
Received: April 15, 2021
Published: April 28, 2021
Figure 1. (A) Conventional methods for sulfoxide synthesis. (B)
Pioneering study. (C) This work.
© 2021 The Authors. Published by
American Chemical Society
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Table 1. Optimization of the Reaction Conditions
a
b
b
entry
cat. Pd, (+ cat. ligand)
temp
3a/%
2a/%
1
2
3
4
5
6
7
8
9
10
11
Pd(dba)2 (10), XPhos (10)
Pd(dba)2 (10), PPh3 (20)
PEPPSI-IPr (10)
80
80
80
80
80
100
40
40
40
40
40
40
59
n.d.
7
22
<99
78
7
trace
14
28
40
4
c
XPhos Pd G3 (10)
XPhos Pd G4 (10)
XPhos Pd G4 (10)
XPhos Pd G4 (10)
XPhos Pd G4 (10)
XPhos Pd G4 (10)
XPhos Pd G4 (10)
XPhos Pd G4 (5)
55
63
48
61
48
d
e
f
78
g
18
58
78
0
39
n.d.
eh
,
f
12
XPhos Pd G4 (5)
a
b
1
Catalyst amount is shown in the parentheses. Yields based on H
c
d
NMR analysis. Not detected. The reaction was performed without
e
water. The reaction was performed in dioxane and water (v/v = 5/1).
f
g
Isolated yields. The reaction was performed in 1,4-dioxane and
h
water (v/v = 1/1). The reaction was performed using 1a (2.0 equiv)
at 0.05 M.
lowered the efficiency (entry 6), the yield of 3a based on
recovered starting material was improved by decreasing the
reaction temperature to 40 °C (entry 7). Although the reaction
without water decreased the yield of 3a (entry 8), we
accomplished the synthesis of sulfoxide 3a in high yield by
changing the ratio of solvents from 10/1 to 5/1 (entry 9).
Sulfoxide 3a was obtained in low yield when further increasing
the ratio of water (entry 10). We succeeded in decreasing the
catalyst loading from 10 to 5 mol % (entry 11). Further
improvement of the efficiency was achieved by increasing the
amount of 1a and decreasing the concentration of substrates
from 0.1 to 0.05 M, enabling us to prepare sulfoxide 3a in high
yield (entry 12). The reaction with a catalytic amount of
XPhos in the absence of palladium precatalysts did not afford
sulfoxide 3a, in which 94% of sulfinate ester 2a was recovered.
This result clearly showed that palladium catalyzed the
sulfinylation of 4-tolylboronic acid (1a) with sulfinate ester 2a.
A wide range of aryl- and alkenylborons were successfully
sulfinylated catalyzed by palladium under the optimized
conditions (Figure 2A,B). The reaction using phenylboronic
acid pinacol ester took place smoothly to afford sulfoxide 3b in
high yield. Sulfoxides 3c and 3d were prepared in moderate
yields by 4-methoxyphenylsulfinylation of 2-tolyl- and 2-
naphthylboronic acids, respectively. Of note, the sulfinylation
of electron-rich 4-methoxy-, 4-hydroxy-, 4-(dimethylamino)-,
and 4-(acetylamino)phenylboronic acids proceeded efficiently
to provide sulfoxides 3e−3h in good yields, leaving a broad
range of electron-donating groups untouched.9 Sulfoxides 3i
Figure 2. (A) General scheme. (B) Results using various organo-
borons 1. (C) Results using various sulfinate esters 2. See the
Supporting Information for the structures of 1 and 2. aPhenylboronic
b
acid pinacol ester was used. XPhos Pd G4 (25 mol %) was used.
d
cXPhos Pd G4 (10 mol %) was used. TMEDA (20 mol %) was
e
added. The reaction was performed using 1a (3.0 equiv) at 60 °C.
and 3j were also synthesized by the sulfinylation of electron-
deficient 4-chloro- and 4-acetylphenylboronic acid in moderate
to high yields. It is worth noting that we achieved the facile
preparation of sulfoxides 3k and 3l having a vinyl and a
methylthio group, which can be damaged by oxidation in the
conventional synthesis. Furthermore, efficient sulfinylation
took place to furnish alkenyl sulfoxide 3m or 3n when using
phenyl- or cyclohexyl-substituted alkenylboronic acid, respec-
tively. This broad substrate scope obviously demonstrated a
benefit of the palladium-catalyzed direct sulfinylation of
organoborons.
Diverse sulfinate esters participated in the catalytic
sulfinylation of organoborons allowing us to synthesize a
wide variety of sulfoxides 3o−3w (Figures 2A,C).10 Phenyl-
ation of 4-tolylboronic acid (1a) with methyl benzenesulfinate
was facilitated by the palladium catalysis to furnish 3o in good
yield, in which the addition of N,N,N′,N′-tetramethylethyle-
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Figure 3. Control experiments and plausible reaction mechanisms.
(A) Reaction using a catalytic amount of base. (B) Control
experiments from 2a. (C) Plausible reaction mechanisms. (a)
Catalytic cycle via Pd(0) and Pd(II). (b) Catalytic cycle via Pd(II).
nediamine (TMEDA) slightly improved the efficiency.
Sulfoxides 3p and 3q were prepared in moderate yields by
the reaction using 4- and 3-toluenesulfinic acid methyl esters.
Additionally, S-tolylation of bulky methyl 2-toluenesulfinate
took place albeit in low yield. The palladium-catalyzed
sulfinylation with 2-naphthalene- and 4-chlorobenzenesulfinic
acid methyl esters was achieved to provide sulfoxides 3s and 3t
in moderate yields. It is worthy to note that a variety of alkyl
aryl sulfoxides 3u−3w were successfully synthesized using
primary and secondary sulfinate esters. In particular, we
accomplished the catalytic synthesis of sulfoxide 3v without
damaging a (tert-butoxycarbonyl)amino group.
To obtain insights into the reaction mechanism, we
conducted a number of control experiments (Figure 3). For
example, a mixture of 1a and 2a was treated with XPhos Pd G4
as a precatalyst in the presence of a catalytic amount of
potassium carbonate (Figure 3A). As a result, sulfoxide 3a was
obtained in moderate yield even when using only 5 mol % of
base. Treatment of sulfinate ester 2a with an equimolar
amount of XPhos Pd G4 and potassium carbonate followed by
the addition of 1a and potassium carbonate resulted in
affording a complex mixture of products, in which sulfoxide 3a
was not detected (Figure 3B, upper). In contrast, sulfoxide 3a
was synthesized in high yield when the palladium precatalyst
loading was reduced to 10 mol % (Figure 3B, lower). A
plausible reaction mechanism on the basis of these results is
Figure 4. Application of the palladium-catalyzed sulfoxide synthesis.
(A) Sequential cross-couplings. (B) One-pot synthesis of sulfoxide 6b.
(C) Aryne reaction of sulfoxide 3j or 3m. See the Supporting
Information for the details.
illustrated in Figure 3C-a. First, the oxidative addition of
sulfinate esters to XPhos-ligated Pd(0) I generated in situ
would proceed leading to Pd(II) complex II.11,12 Then,
transmetalation between II and borates III and subsequent
reductive elimination will provide sulfoxides, where liberating
methoxide from borate V can facilitate the reaction. Another
mechanism through transmetalation between Pd(II) complex
VI and borates III followed by σ-bond metathesis of Pd(II)
intermediate VII with sulfinate esters through transition state
VIII is also possible (Figure 3C-b).11 Although further
mechanistic studies should be performed to reveal the reaction
pathway, it is worth noting that sulfinate esters successfully
served as sulfur building blocks without C−S cleavage.12,13
An advantage of the palladium-catalyzed sulfoxide synthesis
was showcased by consecutive cross-coupling reactions using
bromo-substituted sulfinate ester 4 (Figure 4A,B). Bromide-
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selective Suzuki−Miyaura cross-coupling of 4 catalyzed by
palladium with a variety of arylboronic acids proceeded
efficiently keeping the sulfinate moiety unreacted (Figure
4A). Then, following S-arylation with arylboronic acids
realized the synthesis of diverse sulfoxides 6a−6d without
damaging hydroxy, formyl, dimethylamino, acetylamino,
methylthio, and vinyl groups. Furthermore, we succeeded in
the synthesis of sulfoxide 6b by the consecutive coupling of 4
with arylboronic acids in a one-pot manner (Figure 4B). Since
sequential coupling reactions were realized even in the
presence of reactive functional groups including formyl and
dimethylamino groups owing to the good functional group
tolerance, this one-pot procedure will contribute to the
modular synthesis of diverse sulfoxides from bromo-substituted
sulfinate esters and easily available organoboron derivatives.
The palladium-catalyzed sulfoxide synthesis significantly
improved the accessibility of diaryl sulfides by oxythiolation
of aryne intermediates IX (Figure 4C).14 Treatment of o-
silylaryl triflates 7 and sulfoxide 3j or 3m with potassium
fluoride and 18-crown-6 in hot dioxane provided a range of
diaryl sulfides 8a−8c via selective oxythiolation of arynes IX
and subsequent O-arylation, where an electron-deficient aryl or
alkenyl group was selectively migrated. Of note, the synthesis
of highly functionalized diaryl sulfides was achieved by virtue
of the enhancing the accessibility of sulfoxides developed in
this study. Since various functional groups were tolerated in
the palladium-catalyzed sulfinylation and this aryne reaction, a
modular synthesis of a wide range of diaryl sulfides will be
realized from easily available sulfinate esters, organoborons,
and o-silylaryl triflates.
Kazuya Kanemoto − Laboratory of Chemical Bioscience,
Institute of Biomaterials and Bioengineering, Tokyo Medical
and Dental University (TMDU), Chiyoda-ku, Tokyo 101-
0062, Japan; orcid.org/0000-0002-3958-7369
Yu Nakamura − Laboratory of Chemical Bioscience, Institute
of Biomaterials and Bioengineering, Tokyo Medical and
Dental University (TMDU), Chiyoda-ku, Tokyo 101-0062,
Japan
Takamitsu Hosoya − Laboratory of Chemical Bioscience,
Institute of Biomaterials and Bioengineering, Tokyo Medical
and Dental University (TMDU), Chiyoda-ku, Tokyo 101-
0062, Japan; orcid.org/0000-0002-7270-351X
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.1c01292
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank Dr. Yuki Sakata at Tokyo Medical and
Dental University for HRMS analyses. This work was
supported by JSPS KAKENHI Grant Nos. JP19K05451 (C;
S.Y.); the Naito Foundation (S.Y.); the Japan Agency for
Medical Research and Development (AMED) under Grant
Nos. JP20am0101098 (Platform Project for Supporting Drug
Discovery and Life Science Research, BINDS); and the
Cooperative Research Project of Research Center for
Biomedical Engineering.
In summary, we have developed an efficient catalytic method
for sulfinylation of organoborons. A wide variety of sulfoxides
were synthesized from sulfinate esters and organoborons,
keeping easily oxidizable functional groups unreacted. Further
studies including detailed mechanistic studies and applications
to the synthesis of bioactive organosulfurs are ongoing.
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ASSOCIATED CONTENT
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The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.1c01292.
Experimental procedures and characterization of new
compounds including NMR spectra (PDF)
AUTHOR INFORMATION
Corresponding Author
■
Suguru Yoshida − Laboratory of Chemical Bioscience,
Institute of Biomaterials and Bioengineering, Tokyo Medical
and Dental University (TMDU), Chiyoda-ku, Tokyo 101-
0062, Japan; Department of Biological Science and
Technology, Faculty of Advanced Engineering, Tokyo
University of Science, Katsushika-ku, Tokyo 125-8585,
Japan; orcid.org/0000-0001-5888-9330; Email: s-
yoshida@rs.tus.ac.jp
Authors
Minori Suzuki − Laboratory of Chemical Bioscience, Institute
of Biomaterials and Bioengineering, Tokyo Medical and
Dental University (TMDU), Chiyoda-ku, Tokyo 101-0062,
Japan; Department of Biological Science and Technology,
Faculty of Advanced Engineering, Tokyo University of
Science, Katsushika-ku, Tokyo 125-8585, Japan
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