Reciprocal-Activation Strategy for Allylic Sulfination with Unactivated Allylic Alcohols

Article abstract of DOI:10.1021/acs.orglett.0c01747

A reciprocal-activation strategy for allylic sulfination with unactivated allylic alcohols was developed. In this reaction, the hydrogen bond interaction between allylic alcohols and sulfinic acids allowed for reciprocal activation, which enabled a dehydrative cross-coupling process to occur under mild reaction conditions. This reaction worked in an environmentally friendly manner, yielding water as the only byproduct. A variety of allylic sulfones could be obtained in good to excellent yields with wide functional group tolerance. In gram scale reactions, allylic sulfones could be conveniently isolated in high yield by filtration.

Full text of DOI:10.1021/acs.orglett.0c01747

pubs.acs.org/OrgLett
Letter
Reciprocal-Activation Strategy for Allylic Sulfination with
Unactivated Allylic Alcohols
Peizhong Xie,* Zuolian Sun, Shuangshuang Li, Xinying Cai, Ju Qiu, Weishan Fu, Cuiqing Gao,
Shisheng Wu, Xiaobo Yang, and Teck-Peng Loh*
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ABSTRACT: A reciprocal-activation strategy for allylic sulfination
with unactivated allylic alcohols was developed. In this reaction, the
hydrogen bond interaction between allylic alcohols and sulfinic
acids allowed for reciprocal activation, which enabled a dehydrative
cross-coupling process to occur under mild reaction conditions.
This reaction worked in an environmentally friendly manner,
yielding water as the only byproduct. A variety of allylic sulfones
could be obtained in good to excellent yields with wide functional
group tolerance. In gram scale reactions, allylic sulfones could be
conveniently isolated in high yield by filtration.
llylic sulfones are important moieties in both organic
conditions. For instance, the groups of Chandrasekhar, Tian,
and Sreedhar have pioneered investigations on the choice of
reaction additives, such as Et3B (2.0 equiv),9 B(OH)3 (4.0
equiv),10 and TMSCl (1.2 equiv),11 showing the reaction was
highly dependent on additive selection (Figure 1a). For
activated allylic alcohols [equipped with electron-withdrawing
groups or (multi)-aryl substituents and aryl-sulfinates], the
Reddy group reported that arenesulfonyl cyanides can realize
sulfonation in the presence of quantitative amines under metal-
A
synthesis1 and pharmaceutical development because they
are featured widely in many biologically and pharmaceutically
active molecules2 (such as anticancer agents,2a cysteine
protease inhibitors,2b antibacterial agents,2c etc.2d−j). There-
fore, great attention has been devoted to accessing this
framework. Common methods reported in the literature
usually employed the classical transition metal-catalyzed allylic
substitution reactions.3 Unfortunately, only functionalized
allylic compounds (allylic halides or their equivalents) and
active sulfonyl precursors could be incorporated into the
desired transformations.3 Moreover, the hazardous stoichio-
metric byproducts of these reactions are incongruous with the
principles of atom economy and are of environmental
concerns. Despite some impressive advancements in the
preparation of allylic sulfones, such as transition metal-
catalyzed hydrosulfination,4 SO2 insertion protocols,5 etc.,6,7
tedious preparation of the starting materials and the low atom
efficiency for the overall transformation limit the wide
application of these methods. To date, allylic alkylation of
sulfonyl precursors was the state of art to access allylic sulfones.
Therefore, an alternative catalytic reaction that utilizes
inexpensive, innocuous, and readily available starting materials
to yield allylic sulfones under mild reaction conditions is
extremely attractive and essential.
Figure 1. Strategies for direct allylic sulfination of unactivated allylic
alcohols.
Alcohols are abundant and easily available, and they are thus
a synthetically reliable feedstock.8 Incorporating allylic alcohols
into cross-coupling reactions with sulfonyl precursors should
be an ideal and practical strategy for the preparation of allylic
sulfones. A high temperature and/or a stoichiometric amount
of an acidic additive was required to overcome the high
activation barrier of C−OH scission. Unfortunately, only
aromatic sulfonyl sources were compatible with such
Received: May 23, 2020
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© XXXX American Chemical Society
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free conditions.12 In our continuing effort to develop catalysts
consistent with green and sustainable chemistry,13 our group
has developed a water-promoted dehydrative cross-coupling
strategy for preparing allylic sulfones from activated allylic
alcohols.14 Although remarkable progress has been made
toward this objective, a cross-coupling reaction with
unactivated allylic alcohols remains extremely challenging.
Consequently, few procedures have been reported because of
the much more robust C−OH bond. Herein, we report a
reciprocal-activation strategy for allylic sulfination synthesis
utilizing unactivated allylic alcohols and alkyl/aryl sulfonic
acids (Figure 1b). In this strategy, the hydrogen bond
interaction activates the alcohol toward oxidative addition,
and the same interaction activates the sulfinic acid by
preventing it from acting as an oxygen nucleophile and
pushing it toward being a stronger sulfur nucleophile. The
reciprocal activation enables the Pd/Ca-co-catalyzed dehy-
drative cross-coupling to proceed smoothly at room temper-
ature with water as the only byproduct. Using our method, a
wide variety of allylic sulfones can be obtained in good to
excellent yields with wide functional group tolerance (Figure
2).
other Lewis acids (ZnCl2, AlCl3, and FeCl3) result in
diminished yields (Table 1 of the Supporting Information,
entries 30−32, respectively).
Subsequently, the generalizability of this reaction was
evaluated using a wide variety of allylic alcohols. As shown
in Scheme 1, allylic alcohols bearing a wide variety of
Scheme 1. Dehydrative Cross-Coupling Concerning Allylic
a
Alcohols
Figure 2. Reaction between unactivated allylic alcohols and sulfinic
acids.
Initially, cinnamyl alcohol (1a) and benzenesulfinic acid
(2a) were selected as the model substrates to attempt to access
the desired (cinnamylsulfonyl)benzene (3a).15 Because of the
difficulty in cleaving the robust C−OH bond of 1a, 3a could
not be detected even though a wide variety of solvents, such as
EtOH/H2O, DCM (dichloromethane), THF, etc., were
screened (Table 1 of the Supporting Information, entries 1−
8). Subsequently, Ca(NTf2)2 was then introduced to activate
the C−OH bond16 (Table 1 of the Supporting Information,
entries 9−15). In the solvent DMA (N,N-dimethylacetamide),
a trace amount of 3a could be detected after 48 h at room
temperature (Table 1 of the Supporting Information, entry
15). In all test cases, transition metals, such as Cu and Ni, were
found to be ineffective at promoting this transformation (Table
1 of the Supporting Information, entries 16−19 and 21−24).
To our delight, 3a was obtained in 73% yield in the presence of
Pd(PPh3)4 (10 mol %) (Table 1 of the Supporting
Information, entry 20). Subsequent efforts demonstrated that
the yield of 3a (CCDC: 1949076) could be dramatically
improved to 90% by a combination of palladium and calcium
catalysts (Table 1 of the Supporting Information, entry 21). 3a
also can be isolated in 93% yield when the loads (Table 1 of
the Supporting Information, entries 25−29) of Pd(PPh3)4 and
Ca(NTf2)2 are decreased to 1 and 5 mol %, respectively (Table
1 of the Supporting Information, entry 27). 3a can also be
obtained in moderate yield with 0.5 mol % palladium (Table 1
of the Supporting Information, entries 28 and 29). However,
a
Experimental conditions: 1 (0.3 mmol), 2a (0.45 mmol), Ca(NTf2)2
(5.0 mol %), and Pd(PPh3)4 (1.0 mol %) in DMA (2.0 mL) under a
N2 atmosphere at room temperature (30 °C, oil bath). Isolated yield.
b
Yields in parentheses were generated from 1′. Pd(PPh3)4 (3.0 mol
c
%) and Ca(NTf2)2 (10.0 mol %) were used. Pd(PPh3)4 (5.0 mol %)
and Ca(NTf2)2 (10.0 mol %) were used
functional groups were found to be suitable substrates for
this transformation, delivering the corresponding allylic
sulfones in good to excellent yields. Both electron-withdrawing
(3b−3e) and electron-donating (3f−3h) aryl-substituted
allylic alcohols were amenable to this protocol. The positions
of the substituents (at the para or meta positions) on the
phenyl ring have limited effects on the overall transformation
(3f−3h). Even o-hydroxyl groups on the phenyl were well
B
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a
tolerated, albeit delivering the desired 3i in an only moderate
yield. In addition, allylic alcohols bearing heteroaryl sub-
stituents, such as pyri-dien-2-yl (3j) and furan-2-yl (3k), were
also well tolerated. Fused aromatic allylic alcohols can be
utilized in this reaction, generating 3l in a high yield. In
addition to monosubstituted allylic alcohols, multisubstituted
versions with substituents at position 1, 2, or 3 of allylic
alcohols were compatible with the reaction (3m−3r). In such a
case, the configuration of the double (3n, 3o, 3q, and 3r) was
determined by NOE spectrum (3m) or X-ray analysis (3n,
CCDC: 1949077). (R,E)-4-Phenylbut-3-en-2-ol was selected
to investigate the enantioselectivity of this process. Unfortu-
nately, only racemic 3p was obtained, which may be ascribed
to the epimerization of enantioenriched allylic alcohols in the
presence of Lewis acids.9 Remarkably, (Z)-2-nitro-3-phenyl-
prop-2-en-1-ol and (Z)-2-bromo-3-phenylprop-2-en-1-ol both
showed effective reactivity and yielded the functionalized allylic
sulfones highly efficiently (3q and 3r, respectively). These
results have the potential to greatly enhance the utility of the
product sulfones in further reactions.
Scheme 2. Substrate Scope of Sulfinic Acids
Notably, this method also allows access to diverse 2,4-dien-
1-yl sulfone skeletons (3s−3u) from vinyl-substituted allylic
alcohols. This access will increase the opportunities for further
structural modification. For allylic alcohols with adjacent C−H
bonds, preparing alkyl-substituted allylic sulfone skeletons can
be a challenge due to the competitive self-dehydration process
that often yields the undesired 1,3-diene byproduct. In this
investigation, both cyclic and straight chain alkyl-substituted
allylic alcohols can be successfully incorporated into this
transformation (3u−3y). A citronellal derivative also served as
a productive reaction partner to afford 3z in a 95% yield.
Moreover, alcohols containing alkynyl groups were feasible
under the reaction conditions, giving 3aa in a moderate yield.
Remarkably, a steroid moiety with another active hydroxyl
group was also well tolerated (3ab) (Scheme 2). These results
further highlight the utility of this method in pharmaceutical
studies.
a
Experimental conditions: 1 (0.3 mmol), 2a (0.45 mmol), Ca(NTf2)2
(5.0 mol %), and Pd(PPh3)4 (1.0 mol %) in DMA (2.0 mL) under a
N2 atmosphere at room temperature (30 °C, oil bath). Isolated yield.
b
c
Pd(PPh3)4 (3.0 mol %) was used. Pd(PPh3)4 (5.0 mol %) was used.
Then, we focused our attention on exploring functionaliza-
tion of the sulfinic acids. In addition to benzenesulfinic acid 1a,
many different kinds of aromatic sulfinic acids possessing
different electronic effects were suitable substrates for this
transformation (3ac−3af). Both electron-withdrawing (3ac)
and electron-donating groups (3ad−3af) on the phenyl group
were tolerated. Moreover, thiophene-2-sulfinic acid and
naphthalene-2-sulfinic acid can also react well and deliver the
corresponding allylic sulfones in high yields (3ag and 3ah). To
date, it has been a challenge to incorporate aliphatic sulfinic
acids into allylic sulfones due to their instability and lower
reactivities. Remarkably, either cyclic (3ai and 3aj) or acyclic
(3ah−3an) alkyl sulfinic acids were compatible with our
developed transformation. An even bulkier acid, camphorsul-
finic acid, can be incorporated into the corresponding allylic
sulfone (3ao) with an excellent yield.
The synthetic utility of this transformation was then
preliminarily studied. It has been noted that 1-{4-
[(trimethylsilyl)ethynyl]phenyl}prop-2-en-1-ol reacted well
with benzenesulfinic acid, and the alkynyl group was well
tolerated. This observation allowed for further functionaliza-
tion by a Cu-catalyzed azide−alkyne cycloaddition. After the
TMS moiety had been removed from 3ap, the allylic sulfones
(4) can be conveniently linked with an antivirus drug
Zidovudine17 via click reaction, delivering new compound 5
in 85% yield (Figure 3). Moreover, the reaction can be scaled
Figure 3. Utility of this protocol and synthetic transformation of
allylic sulfones.
up to 6.0 mmol without significant erosion of the outcome, and
product 3a can be isolated without chromatography (in 70%
yield, by filtration). Further functionalization of 3a via
nucleophilic allylic substitution18 with PhMgBr was realized
by using an FeII catalyst, affording 6 in good yield.
To gain more insight into the reaction mechanism, control
experiments were then conducted under the identified reaction
conditions. As is shown in Figure 4, no reaction occurred when
benzenesulfinic acid (Figure 4a) was replaced by sodium
benzenesulfinate (Figure 4b); even 2.0−4.0 equiv of HCl
(aqueous, 37%) was introduced (Figure 4c). This result
indicated that the benzenesulfinic acid was probably crucial for
C
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Experimental procedures, screening reaction conditions,
analytical data for all new compounds, and NMR spectra
of products (PDF)
Accession Codes
CCDC 1949076−1949077 contain the supplementary crys-
tallographic 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 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Authors
■
Peizhong Xie − School of Chemistry and Molecular Engineering,
Institute of Advanced Synthesis, Nanjing Tech University,
Nanjing 211816, P. R. China; orcid.org/0000-0002-6777-
0443; Email: peizhongxie@njtech.edu.cn
Teck-Peng Loh − School of Chemistry and Molecular
Engineering, Institute of Advanced Synthesis, Nanjing Tech
University, Nanjing 211816, P. R. China; Division of Chemistry
and Biological Chemistry, School of Physical and Mathematical
Sciences, Nanyang Technological University, Singapore
637371; orcid.org/0000-0002-2936-337X;
Figure 4. Control experiments and proposed mechanism.
Email: teckpeng@ntu.edu.sg
Authors
C−OH bond activation. In addition, no desired product was
obtained if the hydroxyl group of cinnamyl alcohol was
methylated (Figure 4d). Furthermore, an increase in the
acidity of the sulfinic acids (Figure 4e) resulted in diminished
yields, which was also consistent with the outcome concerning
3ac−3af. Thus, we came to the conclusion that C−OH bond
activation was insufficient for the overall transformation, but a
suitably nucleophilic sulfonyl anion was also required.
Hydrogen bonding interaction of the sulfinic acid activates
the alcohol toward oxidative addition, and the same interaction
activates the sulfinic acid by preventing it from acting as an
oxygen nucleophile and pushing it toward being a stronger
sulfur nucleophile. Thus, in this model, the reciprocal
activation was realized. The calcium salt was not indispensable
for this process but might facilitate the generation of
intermediate A, which was followed by the convenient
palladium insertion that delivered the key intermediate B.
Then the desired 3 was afforded with the regeneration of
palladium.
In summary, we developed a reciprocal-activation strategy
for allylic sulfination of unactivated allylic alcohols in an
environmentally friendly manner. In this protocol, the
hydrogen bond interactions between the sulfinic acids and
allylic alcohols allow for the reciprocal activation, which
enabled the dehydrative cross-coupling reaction to proceed
smoothly at room temperature with water as the only
byproduct. A variety of allylic sulfones can be isolated in
good to excellent yields with wide-spectrum functional groups
tolerated. Remarkably, the reaction can be performed on a
gram scale where allylic sulfones can be isolated without
chromatography.
Zuolian Sun − School of Chemistry and Molecular Engineering,
Institute of Advanced Synthesis, Nanjing Tech University,
Nanjing 211816, P. R. China
Shuangshuang Li − School of Chemistry and Molecular
Engineering, Institute of Advanced Synthesis, Nanjing Tech
University, Nanjing 211816, P. R. China
Xinying Cai − School of Chemistry and Molecular Engineering,
Institute of Advanced Synthesis, Nanjing Tech University,
Nanjing 211816, P. R. China
Ju Qiu − School of Chemistry and Molecular Engineering,
Institute of Advanced Synthesis, Nanjing Tech University,
Nanjing 211816, P. R. China
Weishan Fu − School of Chemistry and Molecular Engineering,
Institute of Advanced Synthesis, Nanjing Tech University,
Nanjing 211816, P. R. China
Cuiqing Gao − Co-Innovation Center for the Sustainable
Forestry in Southern China, College of Forestry, Nanjing
Forestry University, Nanjing 210037, China
Shisheng Wu − CNPC Northeast Refining & Chemical
Engineering Company, Ltd., Shenyang Company, Shengyang
110167, P. R. China
Xiaobo Yang − School of Chemistry and Molecular Engineering,
Institute of Advanced Synthesis, Nanjing Tech University,
Nanjing 211816, P. R. China
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c01747
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c01747.
■
The authors gratefully acknowledge the financial support from
the National Natural Science Foundation of China
(21702108), the Natural Science Foundation of Jiangsu
Province, China (BK20160977), the Six Talent Peaks Project
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