A copper-catalyzed insertion of sulfur dioxide: Via radical coupling

Article abstract of DOI:10.1039/d0cc00375a

A copper-catalyzed three-component reaction of O-acyl oximes, DABCO·(SO₂)₂, and 2H-azirines under mild conditions has been achieved. This protocol provides an efficient route for the construction of various tetrasubstituted β-sulfonyl N-unprotected enamines in moderate to good yields with excellent stereoselectivity and regioselectivity. Notably, this method represents a rare example of 2H-azirines as useful synthons for β-functionalized N-unprotected enamines. Preliminary mechanistic studies indicate that the reaction proceeds through coupling of a sulfonyl radical and α-carbon radical via copper-catalyzed ring-opening C-C bond cleavage of O-acyl oxime and C-N bond cleavage of 2H-azirine with the insertion of sulfur dioxide.

Full text of DOI:10.1039/d0cc00375a

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A copper-catalyzed insertion of sulfur dioxide via radical coupling
Jun Zhang,a Min Yang,b Jin-Biao Liu,c Fu-Sheng He,*a and Jie Wu*ad
Received 00th January 20xx,
Accepted 00th January 20xx
A copper-catalyzed three-component reaction of O-acyl oximes, DABCO·(SO2)2, and 2H-azirines under mild conditions has
been achieved. This protocol provides an efficient route for the construction of various tetrasubstituted β-sulfonyl N-
unprotected enamines in moderate to good yields with excellent stereoselectivity and regioselectivity. Notably, this
method represents a rare example of 2H-azirines as useful synthons for β-functionalized N-unprotected enamines.
Preliminary mechanistic studies indicate that the reaction proceeds through coupling of sulfonyl radical and α-carbon
radical via copper-catalyzed ring-opening C-C bond cleavage of O-acyl oxime and C-N bond cleavage of 2H-azirine with the
insertion of sulfur dioxide.
DOI: 10.1039/x0xx00000x
www.rsc.org/
Tetrasubstituted alkenes are found in many pharmaceuticals,
2H-Azirines are highly valuable synthetic intermediates for
natural products and material science, and can also be served the formation of heterocycles due to their high ring strain and
as versatile building blocks in organic synthesis.1 Owing to their easy ring opening.5 The high chemical reactivity of 2H-azirines
inherent steric-hindrance and difficulties in controlling leads to various transformations, such as nucleophilic addition
stereoselectivity, the preparation of tetrasubstituted alkenes to the C=N double bond,6 cycloaddition7 and ring-opening
remains a great challenge. On the other hand, N-unprotected reaction.8 Recently, the radical ring-expansion of 2H-azirines
enamines as ubiquitous structural elements are important in via a single electron transfer (SET) process has attracted
organic synthetic community.2 However, only few examples extensive attention.9 For instance, Guan and co-workers
toward the efficient synthesis of tetrasubstituted N- reported the synthesis of substituted pyrroles through an iron-
unprotected enamines have been reported so far, presumably catalyzed ring-opening radical cycloaddition of 2H-azirines
due to the sensitivity for hydrolysis. General strategies for their with enamides via homolytic C-N bond cleavage.9c However, to
construction involve the introduction of electron-withdrawing the best of our knowledge, the use of 2H-azirines as the N-
groups including sulfonyl and carbonyl at the β-position to unprotected enamines synthon to furnish the corresponding
confer stability.3,4 For example, Lautens and co-workers tetrasubstituted alkenes has not been achieved yet.
reported the rhodium(I)-catalyzed addition of arylboronic acids
In recent years, driven by the importance of sulfonyl-
to α-phenyl-substituted (arylsulfonyl)acetonitrile for the containing compounds in pharmaceuticals and agrochemicals,
synthesis of tetrasubstituted β-sulfonyl N-unprotected we have focused on the methods development with the
enamines.4e Jiang and co-workers described a route to sulfone insertion of sulfur dioxide, using DABCO·(SO2)2 and inorganic
derivatives through a copper-catalyzed oxidative coupling of sulfites as the sulfur dioxide surrogates.10 For instance, we
oxime acetates with sodium sulfinates.4b The metal-free demonstrated that 4-substituted Hantzsch esters could be
intermolecular aminoarylation of internal alkynes to access used as radical reservoirs in the reaction between sulfur
tetrasubstituted enamine derivatives was developed by the dioxide and vinyl azides under photoredox catalysis.4d As a
group of Greaney.3b Despite the progress in this area, practical result, only trisubstituted β-sulfonyl N-unprotected enamines
and efficient approaches for the preparation of were obtained through the addition of alkylsulfonyl radicals to
tetrasubstituted β-functionalized N-unprotected enamines are structurally limited vinyl azides. We also reported that the alkyl
still in high demand.
radicals generated from O-acyl oximes11,12 through a SET
reduction could react with sulfur dioxide leading to sulfonyl
radicals for multicomponent sulfonylation process.10h
Encouraged by these results, we envisioned that 2H-azirines
might undergo C-N bond cleavage to generate α-carbon radical
in the presence of a copper catalyst, which would go through
selective coupling with sulfonyl radical, thus providing various
tetrasubstituted β-sulfonyl enamines (Scheme 1). Herein, we
describe the copper-catalyzed radical coupling of O-acyl
oximes and 2H-azirines with the insertion of sulfur dioxide,
which represents the first example for the conversion of 2H-
azirines into N-unprotected enamines.
a.School of Pharmaceutical and Materials Engineering, Taizhou University, 1139
Shifu Avenue, Taizhou 318000, China. Email: jie_wu@fudan.edu.cn;
0618006@zju.edu.cn
b.School of Pharmacy, Gannan Medical University, 1 Yixueyuan Road, Ganzhou
341000, China.
c.School of Metallurgical and Chemical Engineering, Jiangxi University of Science
and Technology, Ganzhou 341000, China.
d.State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic
Chemistry, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032,
China.
† Electronic Supplementary Information (ESI) available: [Experimental details and
spectral data, copies of 1H and 13C NMR spectra.]. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
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O
Ar
products 3 in moderate to good yields. Both electVrioewn-AdrtoiclneaOtnilninge
and electron-withdrawing substitutedDO2IH: 1-0a.z1i0r3in9e/Ds0C2C00w3e75rAe
suitable in this transformation. Notably, 2H-azirines bearing
thiophenyl and naphthyl substituents proceeded smoothly as
well to produce the corresponding products 3af and 3aj in 54%
and 32% yields, respectively. The structure of compound 3aj
was confirmed by the X-ray diffraction analysis,13 which
showed excellent stereoselectivity in the outcome.
R2
N
X
DABCO (SO2)2
N
Cu
R
R
R1
NH2
O
+
NC
X
S
copper catalysis
O
O
R1
R2
Ar = p-CF3C6H4
via radical coupling
X = C, O
NH
R1
"SO2"
NC
X
R2
Subsequently, various O-acyl oximes 1 were evaluated in the
reaction of DABCO·(SO2)2 and 2,3-diphenyl-2H-azirine 2a. The
results are summarized in Table 2. As expected, a range of
substituted O-acyl oximes 1 proceeded smoothly to provide
the corresponding products 3 in morderate to good yields. For
instance, functional groups including bromo, ester, and
nitrogen-containing heterocycle were well compatible giving
rise to the products 3da, 3fa and 3ka in 59-72% yields.
Additionally, O-acyl oximes with allyl or benzyl functional
groups at the 2-position could undergo selective C-C bond
cleavage to give more stable secondary alkyl radical, thus
leading to the products 3ia and 3ja in 55% and 70% yields,
respectively. Moreover, the desired product 3ha was obtained
in 29% yield when the oxygen-containing substrate 1h was
employed in the reaction.
Scheme 1. Strategy for the synthesis of N-unprotected β-sulfonyl
enamines with the insertion of sulfur dioxide.
Table 1. Scope exploration for the reaction of O-acyl oxime 1a,
DABCO·(SO2)2, and 2H-azirines 2
R2
O
Ar
CuOAc (20 mol%)
1,10-phen (20 mol%)
DABCO (SO2)2
N
N
R1
NH2
O
+
NC
S
CH3CN, r.t.
O
O
R1
R2
3
1a
: Ar = p-CF3C6H4
2
OMe
Ph
Ph
Ph
Ph
NC
S
NC
S
NC
S
O
O
NH2
O
O
O
O
NH2
NH2
3aa
3ab
3ac
, 71%
Ph
, 52%
, 63%
Ph
Br
NC
CF3
NC
Ph
S
NC
S
S
S
Table 2. Reaction of O-acyl oximes 1, DABCO·(SO2)2 and 2H-azirine 2a
O
O
O
O
O
O
NH2
NH2
NH2
O
Ar
Ph
N
CuOAc (20 mol%)
1,10-phen (20 mol%)
DABCO (SO2)2
N
R
3ad
3ae
3af
, 71%
Cl
, 68%
Cl
, 54%
F
R
Ph
O
+
NC
X
S
CH3CN, r.t.
O
O
X
NH2
X = C, O
Ph
Ph
1
: Ar = p-CF3C6H4
2a
3
Cl
Ph
Ph
Ph
NH2
Ph
Ph
Ph
Ph
S
NC
S
NC
S
NC
S
NC
S
NC
O
O
O
O
O
O
O
O
Ph
NH2
NH2
O
O
NH2
NC
NC
S
NH2
O
O
3ag
3ah
3ai
, 64%
, 62%
Cl
, 65%
NH2
Br
3ba
3ca
3da
, 59%
, 66%
Ph
, 65%
Ph
Ph

CO2tBu
Ph
O
S
Ph
NC
S
O
O
Ph
Ph
NH2
NH2
O
O
NC
S
NC
NC
S
NH2
O
O
O
NH2
3aj
, 32%
Ph 3ea
3fa
3ga
, 58%
, 64%
Ph
, 70%
Ph
Bn Ph
S
Initially, cyclobutanone O-(4-(trifluoromethyl)benzoyl) oxime
1a, DABCO·(SO2)2, and 2,3-diphenyl-2H-azirine 2a were chosen
as the model substrates for condition optimization (Table S1,
in ESI). To our delight, the reaction occurred smoothly in
acetonitrile in the presence of Cu(OAc)2 and 1,10-
phenanthroline at room temperature, giving rise to the desired
product 3aa in 47% yield (Table S1). Screening of other
solvents including toluene, 1,4-dioxane and dichloromethane
showed that acetonitrile was the best choice. The use of
CuOAc in the presence of 2,3-diphenyl-2H-azirine 2a (1.5 equiv)
gave rise to a good result (71% yield). Control experiments
revealed that no reaction occurred in the absence of copper
catalyst, and the yield of 3aa was dramatically decreased to
26% without the use of 1,10-phen (see ESI). Having established
the optimized conditions, we started to explore the scope
generality for the reaction of O-(4-(trifluoromethyl)benzoyl)
oxime 1a, DABCO·(SO2)2, and 2H-azirines 2. As shown in Table
1, various substituents on the aromatic ring of 2H-azirines 2
did not affect the reaction efficiency, affording the desired
Ph
Ph
Ph
NC
NC
O
S
NC
S
O
O
O
O
O
O
NH2
NH2
, 55%
NH2
3ja
, 70%
3ha
3ia
, 29%
Ph
Ph
NH2
S
O
O
N
Boc
3ka
, 62%
To gain more insight of the reaction mechanism, several
control experiments were conducted (Scheme 2). Initially, 2.0
equiv of 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) was
added as a radical scavenge to the reaction of O-acyl oxime 1n,
DABCO·(SO2)2 and 2,3-diphenyl-2H-azirine 2a under the
standard conditions (Scheme 2, eqn a). The β-sulfonyl enamine
3ea was obtained in 28% yield and the radical trapping
products 4 and 5 were isolated in 18% and 27% yields,
respectively. Additionally, the formation of 3ea was
completely inhibited when the amount of TEMPO was
increased to 3.0 equiv, revealing that a radical coupling
2 | J. Name., 2012, 00, 1-3
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Journal Name
process might be involved in the process. Subsequently, a E would give rise to the intermediate F, which would further
View Article Online
reaction of O-acyl oxime 1b, DABCO·(SO2)2 and TEMPO was experience tautomerization to provide tDhOeI:fi1n0a.1l0p39r/oDd0uCcCt030.37O5nA
carried out in acetonitrile at room temperature, and the the other hand, Cu(II) generated in this system could be
TEMPO-trapped product 6 was obtained in 20% yield in the reduced to Cu(I) by DABCO·(SO2)2 at room temperature to
presence of CuOAc and ligand. However, only a trace amount finish the catalytic cycle. The (Z)-selectivity of tetrasubstituted
of product 6 was obtained without the addition of CuOAc β-sulfonyl enamines should be attributed to the intramolecular
(Scheme 2, eqn b). Similarly, the reaction of 2H-azirine 2a, hydrogen bonding effect. For the role of 1,10-phenanthroline,
DABCO·(SO2)2 and TEMPO under copper catalysis gave rise to we reasoned that it would stabilize the copper catalyst during
the TEMPO-trapped product 5 in 39% yield (Scheme 2, eqn c). the reaction process via coordination with the metal.
These results revealed that copper catalyst might play a vital Additionally, it could accelerate the reaction from the control
role in the radical ring opening of O-acyl oximes and 2H- experimental result (Table S1, entry 14).
O
Ar
azirines. Furthermore, β-keto sulfone 7 was isolated in 29%
yield when 10 equiv of trimethyl((1-phenylvinyl)oxy)silane was
subjected to the model reaction, whereas another possible
product 8 was not detected. This outcome showed that the
alkyl radical generated from O-acyl oxime 1a would prefer to
combine with sulfur dioxide to form sulfonyl radical (Scheme 2,
eqn d). These experiments confirmed that alkyl radical,
sulfonyl radical and α-carbon radical were involved in this
transformation.
N
N
O
O
R
R
O
[Cu]
SET
SO2
NC
X
NC
X
S
X
R
R
X = C, O
: Ar = p-CF3C6H4
A
B
C
1
Cu(II)
Ph
N
H+
Ph
N
D
NH
R1
R2
R1
cross-coupling
2
R2
Cu(II)
Cu(I)
E
C6H4p-Ph
a)
NC
O
DABCO (SO2)2
R2
N
O
Ar
TEMPO (2.0 equiv)
CuOAc (20 mol %)
1,10-phen (20 mol %)
N
R2
R
R
DABCO (SO2)2
N
4
R1
R1
NH
O
, 18%
NC
X
S
NC
X
S
+
3ea
+
O
O
O
O
NH
NH2
CH3CN, rt
28%
C6H4p-Ph
3
Ph
Ph
F
O
2a
Ph
N
Ph
, 27%
1e
: Ar = p-CF3C6H4
3ea
with TEMPO (3.0 equiv):
, 0%
NC
Scheme 3. Proposed mechanism
5
b)
O
Ar
N
Ph
In conclusion, we have developed a copper-catalyzed
three-component reaction of O-acyl oximes, DABCO·(SO2)2 and
TEMPO (1.2 equiv)
O
O
1,10-phen (20 mol %)
N
+
DABCO (SO2)2
CH3CN, rt
2H-azirines
for
the
stereoselective
synthesis
of
6
Ph
tetrasubstituted β-sulfonyl enamines. Notably, this is an
example of 2H-azirines as novel and useful synthons for β-
functionalized N-unprotected enamines. Radical trapping
experiments show that alkyl radical, sulfonyl radical and α-
carbon radical are involved in the transformation. Preliminary
mechanistic studies indicate that the reaction proceeds
through coupling of sulfonyl radical and α-carbon radical via
copper-catalyzed ring-opening C-C bond cleavage of O-acyl
oxime and C-N bond cleavage of 2H-azirine with the insertion
of sulfur dioxide.
with CuOAc (20 mol %)
without CuOAc
20%
trace
1b
: Ar = p-CF3C6H4
c)
NH
TEMPO (1.2 equiv)
N
O
1,10-phen (20 mol %)
+
Ph
N
DABCO (SO2)2
Ph
Ph
CH3CN, rt
Ph
2a
5
with CuOAc (20 mol %) 39%
without CuOAc
trace
O
O
O
d)
S
CN
Ph
+
O
Ar
CuOAc (20 mol %)
1,10-phen (20 mol %)
DABCO (SO2)2
N
N
7
, 29%
3aa
+
O
CH3CN, rt
OTMS
O
NH2
15%
O
O
Ph
Ph
S
2a
Ph
Ph
1a
: Ar = p-CF3C6H4
(10 equiv)
Ph
Conflicts of interest
Ph
8
, n.d.
There are no conflicts to declare.
Scheme 2. Mechanistic investigation
On the basis of above results and previous reports,9c,10,11
a
plausible radical coupling mechanism is depicted in Scheme 3.
Single-electron reduction of O-acyl oxime 1 by Cu(I) led to the
formation of iminyl radical intermediate A, which would
convert to the alkyl radical B via C-C bond cleavage. Then, the
resulting radical B would react with DABCO·(SO2)2 leading to
sulfonyl radical C. Meanwhile, the Cu(I)-promoted reductive
Acknowledgements
Financial support from National Natural Science Foundation of
China (Nos. 21871053 and 21532001) is gratefully
acknowledged. We thank Prof. Pornchai Rojsitthisak for English
review.
radical ring opening of 2H-azirine
2 with sequential
protonation would afford α-carbon radical E and Cu(II).
Subsequently, selective cross-coupling of radical species C and
Notes and references
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this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif
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