Grignard-Reagent-Promoted Desulfonylation/Intramolecular Coupling for the Synthesis of 2-(1-Fluorovinyl)pyridines

Article abstract of DOI:10.1021/acs.orglett.0c03521

A novel process involving Grignard-reagent-promoted desulfonylation/intramolecular coupling of readily available α-fluoro-α,β-unsaturated-(2-pyridyl)sulfones was realized that provided a series of polysubstituted 2-(1-fluorovinyl)pyridines in good yields. The intrinsic coordination between pyridine and Mg(II) along with the "negative fluorine effect"of the substrates should play the key role for the smooth transformation in the absence of transition-metal catalysts.

Full text of DOI:10.1021/acs.orglett.0c03521

pubs.acs.org/OrgLett
Letter
Grignard-Reagent-Promoted Desulfonylation/Intramolecular
Coupling for the Synthesis of 2‑(1-Fluorovinyl)pyridines
Lei Kang, Jinlong Zhang, Huameng Yang, Jinlong Qian,* and Gaoxi Jiang*
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ABSTRACT: A novel process involving Grignard-reagent-promoted desulfonylation/intramolecular coupling of readily available α-
fluoro-α,β-unsaturated-(2-pyridyl)sulfones was realized that provided a series of polysubstituted 2-(1-fluorovinyl)pyridines in good
yields. The intrinsic coordination between pyridine and Mg(II) along with the “negative fluorine effect” of the substrates should play
the key role for the smooth transformation in the absence of transition-metal catalysts.
oupling chemistry is widely applied to carbon−carbon
polysubstituted 2-(1-fluorovinyl)pyridines still remains chal-
lenging and in great demand. Traditionally, 2-(1-fluorovinyl)-
pyridines could be obtained by the “HF” cross-addition of the
internal electron-deficient alkynes by using 2 equivalent of
silver fluoride reagent12a or through the fluoride Pd(0)/Cu(I)
system12b (Scheme 1C). However, the inherent instability and
difficulties encountered during the synthesis of 2-metalated
pyridines and fluorovinyl organometallic species severely limit
their application in cross-coupling reactions.13 Herein, we
reported the first example to synthesize 2-(1-fluorovinyl)-
C
bond formation in the synthesis of medicines, agro-
chemicals, and other functional materials.1 In recent years,
transition-metal-catalyzed cross-coupling of alkenylsulfones2,3
has proven effective for forging bonds to sp2 carbon centers as
the alkenylsulfones present three specific advantages. First, the
synthetic precursors (α-alkyl sulfones) can tolerate harsh basic
reaction conditions for their diversity-oriented synthesis via the
α-alkylation and fluorination;4,5 second, the corresponding
cross-coupling products can enable facile derivatization such as
cycloadditions and Michael/Giese-type additions;6 and last,
their stability and crystallinity should allow for the convenient
manipulation.1 Therefore, the literature is replete with
examples of alkenylsulfones serving a variety of different
roles in synthesis, from facilitating Michael addition7 and
variety of Co- and Ni-catalyzed desulfonylative cross-
coupling2,3,8 methodologies (see Scheme 1A).
The implantation of F atoms into organic molecules can
alter the biological properties of lipophilicity, bioavailability,
and metabolic stability. Consequently, development of new
access to fluorine-containing compounds has great potential in
the areas of pharmaceuticals, agrochemicals, and life sciences.9
Of the reported fluorinated structures, monofluoroalkene is a
prominent moiety.10 Significant progress has been made
toward the synthesis of such useful structures in the past few
decades.11 Despite the significant advances, limitations (such
as lower atom economy, using transition-metal catalysts and
lower Z/E-stereoselectivity) are still present. Indisputable,
among these, 2-vinylpyridines with the potential of fluorinated
modification and its fluorinated analogue 2-(1-fluorovinyl)-
pyridines are very important moieties, which are extensively
applied to luminescent materials and bioactive molecules
(Scheme 1B). Although considerable efforts have been
executed for this synthesis,12 an efficient method to prepare
2
pyridines through the desulfonylation/intramolecular Csp −
2
Csp coupling of α-fluoro-α,β-unsaturated-(2-pyridyl)sulfones
activated by Grignard reagents. Inspired by Hu’s “negative
fluorine effect”14 and our previous work,15 we recognized the
key role of the fluorine that is required at the α-position of α-
fluoro-α,β-unsaturated-(2-pyridyl)sulfones is to reduce β-
carbon electrophilicity for Michael addition, and the
coordination of magnesium and nitrogen could activate the
2
2-position of pyridine ring for productive intramolecular Csp −
Csp coupling.13 Intermediate A would undergo C−S bond
2
cleavage when exposed to the Grignard reagents, producing
intermediate B, thus leading to decomposition to form
methylpyridine (3), monofluoroethylene (4), and alkyne (5).
Meanwhile, substrate 1 could eliminate hydrogen fluoride via
treatment with a Grignard reagent or strong base to generate
intermediate C, which also could go through C−S bond
Received: October 22, 2020
https://dx.doi.org/10.1021/acs.orglett.0c03521
© XXXX American Chemical Society
Org. Lett. XXXX, XXX, XXX−XXX
A

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pubs.acs.org/OrgLett
Letter
Scheme 1. MeMgBr-Activated Desulfonylation/
Intramolecular Csp −Csp Coupling Reaction
Table 1. Screening of the Conditions for Coupling of α-
Fluoro-α,β-unsaturated-(2-pyridyl)sulfone
a
2
2
temp
c
entry
RM (x equiv)
(°C)
solvent
THF
THF
THF
THF
THF
THF
THF
yield (%)
1
2
3
4
5
MeMgBr (1 M)/1.0
MeMgBr (1 M)/2.0
MeMgBr (1 M)/4.0
MeMgBr (1 M)/4.0
MeMgBr (1 M)/4.0
MeMgBr (1 M)/4.0
nBuMgCl (1 M)/4.0
EtMgBr (1 M)/4.0
C3H5MgBr (1 M)/4.0
Et2Zn (1 M)/4.0
nBuLi (2.5 M)/4.0
MeMgBr (1 M)/4.0
MeMgBr (1 M)/4.0
MeMgBr (1 M)/4.0
MeMgBr (1 M)/4.0
MeMgBr (1 M)/4.0
MeMgBr (1 M)/4.0
MeMgBr (1 M)/4.0
MeMgBr (1 M)/4.0
MeMgBr (1 M)/4.0
MeMgBr (3 M)/4.0
MeMgBr (3 M)/4.0
−20
−20
−20
0
57
61
65
53
50
20
49
51
trace
0
b
RT
6
7
−40
−20
−20
−20
−20
−20
−20
−20
−20
−20
−20
−20
−20
−20
−20
−20
−20
8
9
THF
THF
THF
THF
10
11
12
13
14
15
16
17
18
19
20
21
22
0
Et2O
43
74
38
31
70
64
73
77
81
toluene
n-hexane
p-xylene
xylene
benzene
toluene
toluene (1 mL)
toluene (2 mL)
d
e
toluene (2 mL) 85 (78)
toluene (3 mL) 78
a
Reaction conditions: 1a (0.05 mmol) reacted in different solvent
b
c
(0.5 mL) for 3 h. Room temperature (RT). Yields given by 19F
d
NMR spectroscopy using PhCF3 as an internal standard. The solvent
cleavage to produce 3 and 5. Both processes will consume 1
and RMgBr to generate several side products that erode the
efficiency of the desired coupling (Scheme 1D).
e
was degassed in liquid nitrogen. Isolated yields are given in
parentheses.
Our investigation began with the reaction of (E)-2-((1-
fluoro-2-(2-fluorophenyl)vinyl)sulfonyl)pyridine (1a) and
MeMgBr (1 M in tetrahydrofuran (THF)) in THF at −20
°C under a nitrogen atmosphere (Table 1, entry 1).
Delightedly, the desired product (Z)-2-(1-fluoro-2-(2-
fluorophenyl)vinyl)-pyridine (2a) was given in 57% yield.
The yields improved slightly with the increased dosage of
MeMgBr (Table 1, entries 2 and 3). Examination of the
reaction temperature revealed that −20 °C was ideal (Table 1,
entries 4−6). A high concentration of 1a remained if the
temperature decreased to −40 °C. Subsequently, several
Grignard reagents, including nBuMgCl, EtMgBr, and
C3H5MgBr (allylmagnesium bromide), were tested (Table 1,
entries 7−9). Unfortunately, Et2Zn (1 M in hexanes) and
nBuLi (2.5 M in hexanes), which is typically employed in the
Michael/Giese-type additions were completely inactive (Table
1, entries 10 and 11) for this reaction, indicating that the Mg
cation should be important for this transformation. The
solvents have obviously affected the reaction (Table 1, entries
12−17). However, the yield was lowered to 31% when using p-
xylene as a solvent (Table 1, entry 15). The reaction with
toluene as the solvent gave better results (see Table 1, entry
13). Degassed toluene provided results similar to those
observed for common anhydrous toluene (Table 1, entry
18). The yield was increased to 81% when the reaction
concentration was reduced from 0.1 M to 0.025 M.
Interestingly, the yield of 2a was enhanced up to 85% by
using MeMgBr (3 M in 2-methyl THF), instead of MeMgBr (1
M in THF) (Table 1, entry 21).
With the optimized reaction conditions in hand, the above
method was applied to the synthesis of various 2-(1-
fluorovinyl)pyridines 2 from α-fluoro-α,β-unsaturated-(2-pyr-
idyl)-sulfones 1 (Scheme 2). Initially, the phenyl-substituted
substrates (1a−1k) were installed with moderate to good
yields (47%−78%). The benzene ring with ortho-substituted
(1a and 1b), meta- (1c, 1d, and 1e) and para- (1g−1j) gave
yields of 60%−78%, except for 1h, which had a yield that
obviously decreased to 47%. We also found that electron-
neutral (1f and 1l), electron-rich (1b, 1d, 1g, and 1h), and
electron-poor (1a, 1c, 1e, 1i, and 1j) α-fluoro-α,β-unsaturated-
(2-pyridyl)sulfones reagents were all viable under the current
desulfonylation/intramolecular coupling reaction conditions.
Heterocyclic, such as thiophene (1m) and furan (1n)
substrates, were also successfully transformed into products
2m and 2n in yields of 61% and 58%, respectively.
Furthermore, α-fluoro-α,β-unsaturated-(2-pyridyl)sulfone re-
agents bearing alkyl (1o and 1p) are also amenable to this
reaction with uneven yields of 67% and 47%. Interestingly, the
furanose derivative (1q) with a carbonyl group bears these
reaction conditions and furnished 2q in 46% yield. The
structure of (Z)-2-(1-fluoro-2-(naphthalen-2-yl)vinyl)pyridine
B
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slightly varied from 50% to 67%. 3-Methyl-substituted sulfone
(1w) gave an excellent yield of 80%. Indeed, the 2-((1-
fluorovinyl)sulfonyl)-3-methylpyridine 1zc was used as a
substrate, generating 2-(1-fluorovinyl)-3-methylpyridine 6zc
in 45% yield, which has the potential to be used in polymer
chemistry. Challenges emerged in moving to the scope of
different heteroaromatic ring systems (1zd); although also
tolerable under the optimal reaction conditions, the yield was
heavily compromised.
Scheme 2. Scope of β-Substituted α-Fluoro-α,β-unsaturated-
(2-pyridyl)sulfone
a
To gain more mechanistic insights into the current
2
2
MeMgBr-activated desulfonylation/intramolecular Csp −Csp
coupling reaction, control experiments were performed (see
Scheme 4A). When using 1r and 1zb as the substrates for one-
Scheme 4. Mechanistic Investigations
a
Reaction conditions: 1 (0.1 mmol), MeMgBr (0.4 mmol),
anhydrous toluene (4.0 mL), −20 °C, 3 h, isolated yields.
2l was confirmed via X-ray diffraction (XRD) analysis (CCDC
No. 2024903). It is notable that this reaction performs
efficiently in gram-scale experiments, making the present
method more attractive for large-scale preparation of 2-(1-
fluorovinyl)pyridines. We conducted the reaction on a 5.0
mmol scale without an appreciable decrease in product yield,
affording (Z)-2-(1-fluoro-2-phenylvinyl)pyridine 2f with 0.72 g
in 72% yield.
We then extended this MeMgBr-activated desulfonylation/
2
2
intramolecular Csp −Csp coupling reaction to other N-
heteroarenes-substituted β-phenyl-α-fluoro-α,β-unsaturated-
sulfones (1r−1zd; see Scheme 3). Different groups in the
pyridine ring barely influence yields; the yields of 6-substituted
(6z), 5-substituted (6r, 6s, and 6zb), 4-substituted (6t−6v), 3-
substituted (6x and 6y) and 4,6-dimethyl-substituted (6za) are
pot reaction, no crossover products were detected by both gas
chromatography-mass spectroscopy (GC-MS) and 19F nuclear
magnetic resonance (NMR) and the reaction was completely
transformed to the corresponding intramolecular coupling
products 6r and 6zb. A large amount of competing products
were detected by GC-MS during the reaction of 1zb with
PhMgBr (2.8 M in 2-methyl THF) under standard reaction
conditions. Among the products, bromobenzene, phenol, and
1,1′-biphenyl were produced from the decomposition of
PhMgBr. 2-(2-Fluorovinyl)naphthalene, 2-ethynylnaphthalene,
and phenyl-substituted 4-chloropyridine were formed from the
C−S bond cleavage process. Diphenylsulfane and sulfinyldi-
benzene were obtained from the desulfonylation reaction.
These control experiment results might suggest that this
Scheme 3. Scope of β-Phenyl-α-fluoro-α,β-unsaturated-
a
sulfones
2
2
reaction proceeds via an intramolecular Csp −Csp coupling
pathway and that C−S bond cleavage is disadvantageous. The
reaction of 1f-D1 with MeMgBr generated the deuterated (Z)-
2-(1-fluoro-2-phenylvinyl)pyridine 2f-D1 with an increased
yield to 91% (Scheme 4B). This result shows that the
mechanism might involve the elimination of hydrogen fluoride
as an adverse event. As shown in Scheme 1, organometallic
reagents such as RMgBr were effective Michael addition
reagents for nonfluorinated α,β-unsaturated-(2-pyridyl)-
sulfones,7g and, when testing (E)-(2-fluoro-2-(phenylsulfonyl)-
vinyl)benzene under standard reaction conditions, has failed to
produce the corresponding product (Scheme 4C); this
a
Reaction conditions: 1 (0.1 mmol), MeMgBr (0.4 mmol),
b
anhydrous toluene (4.0 mL), −20 °C, 3 h, isolated yields. Reaction
temperature up to room temperature (RT). Reaction temperature
decreased to −60 °C for 8 h.
c
C
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Letter
indicates that the fluorine and 2-pyridyl group are necessary to
Jinlong Zhang − State Key Laboratory for Oxo Synthesis and
Selective Oxidation, Center for Excellence in Molecular
Synthesis, Suzhou Research Institute of LICP, Lanzhou Institute
of Chemical Physics (LICP), Chinese Academy of Sciences,
Lanzhou 730000, People’s Republic of China
Huameng Yang − State Key Laboratory for Oxo Synthesis and
Selective Oxidation, Center for Excellence in Molecular
Synthesis, Suzhou Research Institute of LICP, Lanzhou Institute
of Chemical Physics (LICP), Chinese Academy of Sciences,
Lanzhou 730000, People’s Republic of China
this transformation. Taken together, these results suggest that
2
2
these intramolecular Csp −Csp coupling products are generated
2
2
by desulfonylation and intramolecular Csp −Csp coupling
concerted reaction pathways. Meanwhile, the elimination of
hydrogen fluoride and C−S bond cleavage paths are the major
side reaction processes.
In summary, we have developed the first Grignard-reagents-
2
2
activated desulfonylation/intramolecular Csp −Csp coupling of
α-fluoro-α,β-unsaturated-(2-pyridyl)sulfone to synthesize 2-(1-
fluorovinyl)pyridines. Preliminary mechanistic investigations
disclose that the introduction of fluorine should reduce the
electrophilicity of the β-carbon via the “negative fluorine
effect” and the coordination of magnesium to nitrogen could
activate the 2-position of the pyridine ring to make the reaction
occur. The current method is complementary to the
transformation of unsaturated sulfones and represents a
valuable addition to the synthetic toolbox for organofluorine
chemistry. Our study not only provides a new protocol to
obtain 2-(1-fluorovinyl)pyridines but also gives new insights
into the new reactivity of fluoride unsaturated-(2-pyridyl)-
sulfones without a transition-metal catalyst. Further study in
this direction is underway in our laboratory.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c03521
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support from the National Natural Science
Foundation of China (No. 21602231), Chinese Academy of
Sciences “Light of West China” Program, and the Natural
Science Foundation of Jiangsu Province (Nos. BK20160396
and BK20191197) is gratefully acknowledged.
ASSOCIATED CONTENT
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REFERENCES
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* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c03521.
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Experimental procedures and spectral data (PDF)
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CCDC 2024903 contains the supplementary crystallographic
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Corresponding Authors
Jinlong Qian − State Key Laboratory for Oxo Synthesis and
Selective Oxidation, Center for Excellence in Molecular
Synthesis, Suzhou Research Institute of LICP, Lanzhou Institute
of Chemical Physics (LICP), Chinese Academy of Sciences,
Lanzhou 730000, People’s Republic of China; orcid.org/
0000-0003-0390-848X; Email: q124802q@163.com
Gaoxi Jiang − State Key Laboratory for Oxo Synthesis and
Selective Oxidation, Center for Excellence in Molecular
Synthesis, Suzhou Research Institute of LICP, Lanzhou Institute
of Chemical Physics (LICP), Chinese Academy of Sciences,
Lanzhou 730000, People’s Republic of China; orcid.org/
0000-0003-1555-1107; Email: gxjiang@licp.cas.cn
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