Copper-Catalyzed Synthesis of β-Azido Sulfonates or Fluorinated Alkanes: Divergent Reactivity of Sodium Sulfinates

Article abstract of DOI:10.1021/acs.orglett.8b02735

A new and general method for the synthesis of β-azidosulfonates has been achieved through Cu(I)-mediated radical oxidative sulfonylation-azidation of alkenes with sodium sulfinates. Under identical conditions, azido fluoroalkyated products could be readily obtained instead using CF₃SO₂Na or CHF₂SO₂Na as reagents. The starting materials of sulfinate compounds, alkenes, and azidotrimethylsilane are stable and cheap. This method can be easily adapted for large-scale preparation.

Full text of DOI:10.1021/acs.orglett.8b02735

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Copper-Catalyzed Synthesis of β‑Azido Sulfonates or Fluorinated
Alkanes: Divergent Reactivity of Sodium Sulfinates
Yang Xiong, Youwen Sun, and Guozhu Zhang*
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Center for Excellence in Molecular
Synthesis, University of Chinese Academy of Sciences, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, P. R.
China
S
* Supporting Information
ABSTRACT: A new and general method for the synthesis of β-azidosulfonates has been achieved through Cu(I)-mediated
radical oxidative sulfonylation−azidation of alkenes with sodium sulfinates. Under identical conditions, azido fluoroalkyated
products could be readily obtained instead using CF3SO2Na or CHF2SO2Na as reagents. The starting materials of sulfinate
compounds, alkenes, and azidotrimethylsilane are stable and cheap. This method can be easily adapted for large-scale
preparation.
ulfones are key synthons in many synthetic trans-
1
Sformations, such as Junia−Lythgoe olefination, electro-
philic substitution,2 reductive cleavage of the sulfone group,3
and more recent fruitful exercises in photoredox and
fluorination chemistry.4 More importantly, sulfones display
diverse biological activities, making them one of the major
components in the pharmaceutical and agro-chemical agents.5
Despite many well established sulfone preparation methods,6
new strategies for expedient sulfonyl introduction are still
highly desired.
Figure 1. Representative active pharmaceutical compounds.
Difunctionalization of alkenes represents one of the most
powerful and straightforward methods for rapidly increasing
the molecular complexity as two chemical bonds are formed in
one step.7 One important class of such transformations is
amino difunctionalization, including diamination,8 amino-
oxygenation,9 amino-halogenation,10 and carbo-amination,11
with generation of valuable N-containing molecules. However,
examples of alkene sulfonyl amination for producing linear N-
containing sulfones are scarce and remain a challenge, despite
the fact that molecules bearing both amino and sulfone groups
have extensively been used in pharmaceuticals (Figure 1).12
Sulfonyl radicals are transient intermediates that undergo a
range of reactions, including those with alkenes and alkynes.13
Radical nature Cu-catalyzed azido difunctionalization of
alkenes has been well established using an azide reagent,14
leading to valuable organo-azides.15,16 We propose that a Cu-
catalyzed radical process, utilizing readily available sulfonyl and
azido radical precursors in the presence of a suitable oxidant,
would enable us to achieve the azido sulfonylation of alkenes.
We initiated our studies with the azido sulfonylation reaction
of styrene with readily available TMSN3 and NaSO2Me in the
presence of oxidants and a Cu catalyst. After extensive
experiments, the 1,2-azido sulfonate 1a was obtained in 20%
yield in 1 mL of CH3CN in the presence of CuCl using
NaSO2Me and TPBP (tert-butyl benzoperoxoate) (Table 1,
entry 1). In the hope of increasing the solubility of Na salt, an
equal amount of H2O was added and the product yield was
significantly improved (Table 1, entry 2). Fine-tuning of the
CH3CN/H2O ratio resulted in 3.3/1 being optimal. Examina-
tion of different oxidants revealed that TPBP was a unique
oxidant and radical initiator for this reaction;17a a combination
of Cu(I) and TBHP (tert-butyl hydroperoxide), often used in
organo-azido radical reactions,4a−d did not work well under
these conditions (Table 1, entry 5). Other oxidants, such as
BPO (benzoyl peroxide), DTBP (di-tert-butyl peroxide), and
NFSI (N-fluorobenzene-sulfonimide), were not effective
(Table 1, entries 6−8). No detectable amount of product
was found from reactions running in different solvents other
than CH3CN (Table 1, entries 9−11). CuCl exceeded other
Received: August 26, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b02735
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
aromatic ring, either electron-donating or electron-withdraw-
ing groups, were compatible with the current transformation.
In particular, product 7a, bearing two alkoxy groups, could
serve as the advanced intermediate for the synthesis of drug
Apremilast.12
Table 1. Optimization of the Reaction Conditions
yield
b
For the 1,1-disubstituted substrate, the reaction proceeded
well to give the β-Ms-substituted tertiary alkyl azide 9a in 68%
yield. In addition, (E)-prop-1-en-1-ylbenzene with an internal
alkene was also suitable for this transformation, thus giving the
product 10a in 67% yield with good dr. A valuable pyridine
moiety was successfully incorporated (11a). The 1,3-dienes are
capable substrates as well, and an arrangement of substituted
dienes are engaged in this azido sulfonylation reaction to
provide the desired products in moderate yields with good 1,2-
selectivity (12a−14a). Notably, better 1,2-regioselectivity was
observed for aryl substituted 1,3-dienes by lowering the
reaction temperature to 30 °C likely under kinetic control. No
detectable 1,4-isomer was found for 14a, possibly due to
increased steric hindrance.
Various aliphatic and aromatic sodium sulfinates were
proven to be suitable sulfonylation reagents as well, leading
to diversified precursors for drug analogues (15a−17a).
Success in using sodium fluoromethanesulfinate would offer
a potent entry to tuning the properties of related molecules for
medicinal purposes by introduction of a F atom. Interestingly,
for simple aliphatic 1,3-butadiene and cyclohexa-1,3-diene,
thermodynamically more stable 1,4-addition regioisomers were
observed (18a−20a). Notably, alkenes with aliphatic sub-
stitutions did not react well under the current reaction
conditions.
The use of fluorinated alkyl groups including CF3 and CHF2
has become increasingly important in a variety of research
fields18 due to the versatile beneficial effects rendered by the
presence of a F atom on the materials.19 Langlois reagent,
NaSO2CF3, has been found to undergo oxidative trifluor-
omethylation in many events through SO2 extrusion, but to be
limited in the difunctionalization of alkenes.20 We envisioned
that the successful extension of the above-established strategy
to induce 1,2-azido fluoroalkylation of alkenes with the
NaSO2CF3 would offer new opportunities for readily available
Langlois reagent, and also enrich the alkene difunctionalization
research field. The desired trifluoromethylated azide indeed
formed under the standard conditions using NaSO2CF3
instead of nonfluorinated sulfinate salt.
a
entry
catalyst
oxidant
solvent
CH3CN
CH3CN/H2O (1/1)
CH3CN/H2O (2/1)
CH3CN/H2O (3.3/1)
(%)
1
2
3
4
5
6
7
8
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuBr
TBPB
TBPB
TBPB
TBPB
20
40
72
78
trace
ND
ND
ND
trace
trace
trace
54
TBHP CH3CN/H2O (3.3/1)
BPO CH3CN/H2O (3.3/1)
DTBP CH3CN/H2O (3.3/1)
NSFI
CH3CN/H2O (3.3/1)
EtOAc
toluene
9
TBPB
TBPB
TBPB
TBPB
TBPB
TBPB
TBPB
TBPB
10
11
12
13
14
15
DMF
CH3CN/H2O (3.3/1)
CH3CN/H2O (3.3/1)
CH3CN/H2O (3.3/1)
CH3CN/H2O (3.3/1)
(CH3CN)4CuPF6
Cu(OTf)2
no
72
68
ND
c
d
16
CuCl
CH3CN/H2O (3.3/1)
76
a
b
1
Reactions were carried out in 0.2 mmol scale. Measured by H
c
NMR analysis using diethyl phthalate as internal standard. 0.4 mmol
scale. Isolated yield.
d
copper salts (Table 1, entries 12−14). The reaction did not
occur without a Cu catalyst (Table 1, entry 15). Therefore,
heating the reactants in CH3CN/H2O (3.3/1) at 60 °C for 16
h using 10 mol % CuCl as the catalyst and TPBP as the oxidant
was chosen as the optimized reaction conditions.
The substrate scope of azido sulfonylation was promptly
examined, and the results are summarized in Scheme 1.
Substrates 2a−8a, having various substituents (R) on the
a b
,
Scheme 1. Scope Studies
Under almost identical conditions, the generality of the
method with alkenes was explored and an even broader
substrate scope was revealed (Scheme 2). Activated alkenes
such as monoarylated alkenes, including naphthalene, sub-
stituted phenyl, and pyridine were suitable substrates for this
trifluoromethyl azidation reaction (1b−3b). 1-Aryl and 1,2-
dialkyl trisubstituted alkene also reacted well to provide the
desired product in good dr (4b). A range of alkylated alkenes
proved to be capable substrates (5b−8b). Notably, ester and
amide groups are tolerated under current reaction conditions.
Three arylated 1,3-dienes were successfully transformed into
products in good 1,2-selectivity (9b−11b). Again, for an
aliphatic diene, a 1,4-disubstituted adduct was isolated (12b).
We then turned our attention to the unprecedented
difluoro-methyl azidation using NaSO2CHF2 under the
optimized conditions.21 Again, difluoromethyl azidation
proceeded smoothly to provide the desired products in good
yields (Scheme 3).
a
All reactions were performed in CH3CN/H2O (3.3/1, 0.02 M) at 60
b
c
°C. Isolated yield. The temperature was 30 °C, ratio of regioisomer
(rr) and major isomer are shown. Configuration was assigned by
NOE.
d
B
DOI: 10.1021/acs.orglett.8b02735
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a b
,
Scheme 2. Scope Studies
Scheme 4. Preliminary Mechanistic Study
was obtained in 3:1 dr. These results are consistent with a
stepwise radical process.16,17
A plausible reaction mechanism is illustrated in Figure 2. As
previously suggested, CuCl first reduces TBPB to provide a
a
All reactions were performed in CH3CN/H2O (3.3/1, 0.02 M),
except 6b performed in CH3CN/H2O (3.3/1, 0.03 M) at 60 °C;
b
Isolated yield. Configuration was assigned by comparison with
c
reported example. Isolated yield of triazole over two steps, for second
step [CuI (12.1 mg, 0.06 mmol), phenylacetylene (66 μL, 0.6 mmol)
d
in THF (2 mL)], stirred at 60 °C for 6 h]. The temperature was 30
°C, ratio of regioisomer.
a b
,
Scheme 3. Scope Studies
Figure 2. Proposed reaction mechanism.
tert-butoxyl radical and a Cu(II)OBz species.17a A radical relay
process then occurs between RSO2Na and the tert-butoxyl
radical to afford a sulfonyl radical that adds to the olefin,
generating an internal radical.22 The Cu(II)OBz species
undergoes ligand exchange with TMSN3 to give Cu(II)N3.17a
In path a, rejoining of Cu(II)N3 and an internal radical would
provide a Cu(III) complex,17b which undergoes reductive
elimination to eventually deliver product with regeneration of
the Cu(I) catalyst. In path b, a direct SET process possibly
leads to the same products.
Alternatively, the internal radical is oxidized to a cationic
intermediate that is trapped by TMSN3 to form the same
product,14f which is, however, less likely, considering the good
dr value for internal alkenes.23 In the case of CF3SO2Na and
CHF2SO2Na, fluorinated methyl radicals are formed through
SO2 expulsion and are added onto the alkene to continue the
catalytic cycle.4,20,21
To illustrate the synthetic utility of the azidosulfonylation
products, the synthesis of Apremilast was pursued. As shown in
Scheme 5, Apremilast could be obtained in a three-step
sequence in gram scale.12
a
All reactions were performed in CH3CN/H2O (3.3/1, 0.02 M),
except 3c and 5c performed in CH3CN/H2O (3.3/1, 0.03 M) at 60
b
c
°C. Isolated yield. Configuration was assigned by comparison with
reported example. Isolated yield of triazole over two steps, for second
step [CuI (12.1 mg, 0.06 mmol), phenylacetylene (66 μL, 0.6 mmol
in THF (2 mL), stirred at 60 °C for 6 h]. The temperature was 30
°C, ratio of regioisomer (>15:1).
d
Investigation of the alkene scope exhibits broad generality.
Both aryl and alkyl substituted alkenes reacted readily to afford
the desired products in moderate to good yields (1c−5c).
Good 1,2-regioselecvitities were observed for a range of 1,3-
dienes tested (6c−14c).
Scheme 5. Approach for the Synthesis of Apremilast
With addition of TEMPO, the reaction was inhibited and no
desired product was observed [Scheme 4, eq 1]. To further
understand this reaction, the reaction of 3-(allyloxy)prop-1-ene
was investigated [Scheme 4, eq 2], and cyclized product 1d
C
DOI: 10.1021/acs.orglett.8b02735
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b02735.
■
S
Research details, experimental procedures, full character-
ization of products, and NMR spectra (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: guozhuzhang@sioc.ac.cn.
ORCID
Guozhu Zhang: 0000-0002-2222-6305
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to NSFC-21772218, 21421091,
XDB20000000, the “Thousand Plan” Youth Program, State
Key Laboratory of Organometallic Chemistry, Shanghai
Institute of Organic Chemistry, and the Chinese Academy of
Sciences.
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(23) A low ee was observed in a preliminary asymmetric catalysis;
see the Supporting Information for detailed information.
E
DOI: 10.1021/acs.orglett.8b02735
Org. Lett. XXXX, XXX, XXX−XXX