Silver-Catalyzed Stereoselective Aminosulfonylation of Alkynes

Article abstract of DOI:10.1002/anie.201705122

A silver-catalyzed intermolecular aminosulfonylation of terminal alkynes with sodium sulfinates and TMSN₃ is reported. This three-component reaction proceeds through sequential hydroazidation of the terminal alkyne and addition of a sulfonyl radical to the resultant vinyl azide. The method enables the stereoselective synthesis of a wide range of β-sulfonyl enamines without electron-withdrawing groups on the nitrogen atom. These enamines are found to be suitable for a variety of further transformations.

Full text of DOI:10.1002/anie.201705122

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Accepted Article
Title: Ag-Catalyzed Stereoselective Aminosulfonylation of Alkynes
Authors: Xihe Bi, Qinghe Ji, Peiqiu Liao, Edward Anderson, and
Yongquan Ning
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201705122
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10.1002/anie.201705122
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Ag-Catalyzed Stereoselective Aminosulfonylation of Alkynes
Yongquan Ning, Qinghe Ji, Peiqiu Liao, Edward A. Anderson and Xihe Bi*
Abstract: A silver-catalyzed intermolecular aminosulfonylation of
terminal alkynes with sodium sulfinates and TMSN3 is reported.
This three component reaction proceeds through sequential
hydroazidation of the terminal alkyne, and addition of a sulfonyl
radical to the resultant vinyl azide. The method enables the
stereoselective synthesis of a wide range of β-sulfonyl enamines
without electron-withdrawing groups on the nitrogen atom, which
are found to be suitable for a variety of further transformations.
knowledge, this is the first example of intermolecular alkyne
aminosulfonylation,[10] resulting in stereoselective synthesis of
β-sulfonyl N-unprotected enamines
–
useful synthetic
intermediates whose applications are currently limited by a lack
of practical synthetic methods for their preparation.[11] Moreover,
only one report regarding the synthesis of N-unprotected
enamines by alkyne difunctionalization has been reported.[12]
Alkynes are one of the most common and versatile functional
groups in organic synthesis, and catalytic methods which
enable their efficient transformation into other useful
functionalities are therefore highly appealing in both academic
research and industrial applications.[1] Direct difunctionalization
reactions of alkynes, capable of affording tri- and
tetrasubstituted alkenes, have attracted much attention in
recent
years.[2]
Among
these,
radical-based
1,2-
Figure 1. Strategies for Radical Difunctionalization of Alkynes
difunctionalizations of alkynes offer a straightforward means to
construct functionalized alkenes by reaction with both carbon-[3]
and heteroatom-centered radicals[4] with excellent step- and
atom-economy.[5] Mechanistically, a common reaction pathway
is observed involving initiation of the reaction by radical addition
to the alkyne to generate a vinyl radical intermediate, which is
then coupled with another component to form the alkene
product (Figure 1a). However, such vinyl radical species are
highly reactive and readily undergo hydrofunctionalization by H-
Initial optimization of the reaction was performed using
alkyne 1a, TMSN3, and sodium p-toluenesulfinate 2a, with
variation of reaction parameters including metal catalyst,
solvent, and temperature (Table 1). Silver salts proved highly
effective in promoting the hydroazidation / sulfonation cascade;
Ag2CO3, Ag3PO4, and AgF all gave the aminosulfonylated
product 3a in high yields, with Ag3PO4 delivering an optimum
yield of 85% (entries 1-3). In contrast, other metal catalysts
such as Pd(OAc)2, CuI, and Mn(OAc)3 were ineffective,
presumably due to their inability to catalyze the hydroazidation
reaction (entries 4-6). Water was found to be an essential
additive, as a poor yield (30%) was obtained in its absence
(entry 7); the use of less than two equivalents of TMSN3 also
resulted in a decrease in product yield. The reaction solvent
also proved important, with DCE and 1,4-dioxane giving only
trace amounts of the desired product using Ag3PO4 as catalyst
(entries 8 and 9), compared to polar aprotic solvents such as
NMP (65%, entry 10) and DMSO. Finally, increasing or
reducing the reaction temperature led to a decrease in product
yield (entries 11 and 12).
atom abstraction,[6] which is
a significant challenge in
developing radical-based alkyne difunctionalization reactions.
Moreover, aliphatic alkynes are generally unreactive in these
processes, likely due to the lack of a π-conjugation stabilizing
effects of intermediate alkyl-substituted vinyl radicals compared
to aryl-substituted analogues.[7] Consequently, conceptually
distinct approaches are in high demand. In the last years, the
nitrogenation of alkynes with trimethylsilylazide (TMSN3) has
attracted much attention, where the carbon-carbon triple bond
cleavage leads to a variety of nitrogen-containing molecules.[8]
Building from our recent efforts on the activation of alkynes by
silver catalysis,[9] we herein report a new strategy to effect
radical-based difunctionalization of terminal alkynes through an
Table 1. Optimization of the reaction conditions.[a]
unprecedented hydroazidation
/ radical addition cascade
(Figure 1b). The key point for this successful transformation is
that we discovered a mild and efficient approach to generate
sulfonyl radical from sodium sulfinate, thus avoiding the initial
competitive radical addition to alkynes.[6] To the best of our
Entry
1
2
3
4
5
6
7
8
[M] cat.
Ag2CO3
Ag3PO4
AgF
Pd(OAc)2
CuI
Mn(OAc)3
Ag3PO4
Ag3PO4
Ag3PO4
Ag3PO4
Ag3PO4
Ag3PO4
Amount
20 mol%
20 mol%
20 mol%
5 mol%
20 mol%
20 mol%
20 mol%
20 mol%
20 mol%
20 mol%
20 mol%
20 mol%
Solvent
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DCE
1,4-Dioxane
NMP
DMSO
DMSO
T (°C)
70
70
70
70
70
70
70
70
70
70
100
50
Yield (%)[b]
75
85
63
[a]
Y. Ning, Q. Ji, Dr. P. Liao, Prof. X. Bi
0[c]
Jilin Province Key Laboratory of Organic Functional Molecular
Design & Synthesis, Department of Chemistry
Northeast Normal University, Changchun 130024, China
E-mail: bixh507@nenu.edu.cn
0[c]
0[c]
30[d]
trace
trace
65
Homepage: http://www.bigroup.top/
9
10
11
12
[b]
[c]
Prof. X. Bi
62
55
State Key Laboratory of Elemento-Organic Chemistry
Nankai University, Tianjin 300071, China
Prof. E. A. Anderson
[a] Standard reaction conditions: 1a (0.5 mmol), TMSN3 (1.0 mmol), 2a (1.0
mmol), H2O (1.0 mmol), in DMSO (2 mL) at 70 ºC under air-open condition
for 4 h. [b] Yield of isolated product. [c] No 3a was detected by 1H NMR
Chemistry Research Laboratory
University of Oxford, 12 Mansfield Road, Oxford, OX1 3TA, U.K.
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10.1002/anie.201705122
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spectroscopic analysis of crude reaction mixture. [d] Without H2O. DCE = 1,2-
dichloroethane, DMSO = dimethylsulfoxide, NMP = N-methyl pyrrolidone.
Following this success with aryl-substituted alkynes, we
turned our attention to the reactivity of aliphatic alkynes, where
it is notable that alkyl-substituted β-sulfonyl enamines have not
been prepared by other methods. In the event, we were
delighted to find that reactions of alkyl-substituted terminal
alkynes generally proceeded with equal efficiency to aryl
The scope of the reaction with respect to the alkyne proved
broad, with a wide range of aryl- and heteroaryl-functionalized
terminal alkynes being suitable for this silver-catalyzed cascade
reaction, affording the corresponding β-sulfonyl enamines in
good to excellent yields (Scheme 1). For instance, a variety of
para-substituted phenylacetylenes 1a-1j underwent smooth
reaction with TMSN3 and 2a to give the products 3a-3j in 69-
87% yields. Pleasingly, common functional groups such as
alkoxy, alkyl, aryl, halogen, cyano, trifluoromethyl, aldehyde,
and ester were all well tolerated, with X-ray diffraction analysis
of 3c confirming the (Z)-configuration of the alkene. Similarly,
ortho-, meta-, and 3,4-disubstituted phenylacetylenes gave the
desired enamine products 3k-3p in high yields (75-83%).
Heteroaryl acetylenes including 2- and 3-pyridyl, 2- and 3-
thienyl, as well as ferrocenyl acetylene were also evaluated,
and the corresponding products 3s-3v were obtained with high
efficiency. A more elaborate estrone-derived terminal alkyne
could also be successfully transformed into the corresponding
β-sulfonyl enamine 3x (79%), underlining the robust nature of
the methodology.
alkynes, affording
a number of functionalized β-sulfonyl
enamines 4 in high yields. In addition to simple alkyl acetylenes
(4a-d, 69-81%), cyclopropyl, hydroxyl, and phthalimide
substituents were all tolerated, giving the functionalized sulfonyl
enamines 4e-h (59-82%). Further, enyne systems such as 1-
cyclohexenyl and styryl acetylenes also participated efficiently
in this three-component reaction, giving the 1,3-butadienes 4i
and 4j in reasonable yields. The presence of an internal alkyne
did not affect the formation of β-sulfonyl enamine 4k (79%),
illustrating the exquisite chemoselectivity between internal and
terminal alkynes. All of these heteroaryl- and alkyl-substituted
β-sulfonyl enamines in Schemes 1 and 2 are novel compounds
which could prove useful as intermediates in organic and
medicinal chemistry research.
We next set out to evaluate the reaction scope with respect
to the sulfinate component in reactions with 4-phenyl
phenylacetylene 1y (Table 2). Whether electron-rich or
electron-deficient, aryl sulfinates afforded the corresponding β-
sulfonyl enamines in high yields (5a-5c, 75-82%). Alkyl sulfinate
salts also proved suitable reaction partners, giving products 5d-
5f with similar efficiency.
Table 2. Reaction scope: sodium sulfinates.
Entry
R'
4-MeC6H4
C6H5
4-ClC6H4
Me
Product 5
Isolated yield (%)
1
2
3
4
5
6
5a
5b
5c
5d
5e
5f
75
82
75
75
85
87
Et
Cyclopropyl
To gain insight into the reaction mechanism, the reaction of
4-ethynyltoluene 1b was monitored by 1H NMR spectroscopy
under standard reaction conditions in d6-DMSO (Figure 2a). By
comparison with authentic samples, signal A at 4.1 ppm was
assigned as the acetylenic proton of 1b. After 40 min, this
signal had almost completely disappeared, and new doublets at
4.97 and 5.61 ppm were assigned as the olefinic hydrogens of
vinyl azide B, which reached a maximum intensity after 20 min.
The singlet at 5.07 ppm was assigned as the olefinic hydrogen
of product C, which appeared at ~20 min, and was almost the
sole reaction component by 180 min. This result clearly
suggests initial rapid alkyne hydroazidation, followed by slower
addition of sulfonyl radical to the in situ generated vinyl azide,
and also illustrates the clean conversion of the terminal alkyne
to the β-sulfonyl enamine. Further information on the radical
addition step was obtained by submission of vinyl azide (VA) to
various reaction conditions (Figure 2b), which revealed a dual
role of TMSN3: in the absence of TMSN3, no reaction took place,
whereas in the presence of 1.5 equivalents, product 3b was
afforded in high yield (85%) after a short reaction time (40 min).
Scheme 1. Reaction scope: aromatic and aliphatic alkynes.
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This implies that TMSN3 plays a critical role in generation of the
sulfonyl radical. That this second step indeed involves a radical
intermediate was supported by the formation of only a trace
amount of 3b in the presence of TEMPO.
trimethylsilanol with release of a proton, which could affect
protodemetalation of intermediate A' to give the observed vinyl
azide (VA). This in turn is readily attacked by the sulfonyl free
radical C, leading to carbon-centered radical D,[16] which rapidly
converts to iminyl radical E with release of N2. Following
sequential reduction and protonation, an imine intermediate (G)
is formed. Product 3a is obtained by tautomerization of this
imine.[17] Stereochemistry of the product should be ascribed to
the intramolecular hydrogen-bonding effect. Note that the
screening of a variety of potential radical precursors, such as
Togni’s reagent, diphenylphosphine oxide, and potassium 2-
oxo-2-phenylacetate,[18] were not successful in the desired
aminofunctionalization of terminal alkynes, and therefore
demonstrated the generation of sulfonyl radical under above
mild conditions appears to play a crucial role in controlling the
reaction sequence.[9b][19]
(a) Reaction monitoring by 1H NMR spectroscopy[a]
Finally, a gram scale reaction of phenylacetylene 1c, TMSN3
and sodium p-toluenesulfinate 2a was tested; delightfully, this
reaction could be performed on 20 mmol scale and proceeded
smoothly to give product 3c with only a modest decrease in
yield (3.11 g, 57%) (Scheme 3). Next, further synthetic
manipulations were conducted to explore its reactivity. As
reported by Jiang,[11] β-ketosulfone 6 could be prepared by
treatment of 3c with silica gel (95%), while 2H-azirine 7 was
obtained in 65% yield through hypervalent iodine-induced
oxidative cyclization.[20] Unexpectedly, dihydropyrrole 8 was
obtained under Bao and Guan’s K2S2O8-mediated oxidative
cyclization conditions, instead of a pyrrole as observed with
analogous β-keto or β-ester enamines.[21] Similarly, a new
reaction pattern was discovered on treatment of 3c with NBS,
which afforded polybrominated imine 9 in 72% yield. N-
brominated imines have rarely been reported in the literature.[22]
(b) The crucial role of TMSN3 in the sulfonyl radical addition to vinyl
azides
Figure 2. Mechanistic investigations. [a] Reaction process of 1b was
monitored by 1H NMR spectroscopy (600 MHz, d6-DMSO). A: acetylenic
hydrogen of 1b; B: two olefinic hydrogens of vinyl azide (VA); C: olefinic
hydrogen of product 3b.
Scheme 3. Gram scale synthesis and further transformations. [a]
Reaction conditions: (a) Stir in CH2Cl2 with silica gel at room temperature,
overnight; (b) PhIO (2.0 mmol) in TFE (5 mL) stirred at rt for 15 min, then
substrate 3c (1 mmol in 5 mL of TFE) was added dropwise; (c) 3c (0.5 mmol),
K2S2O8 (0.6 mmol), DMSO (5 mL), 5 h; (d) 3c (0.5 mmol), NBS (1.65 mmol ),
DCE (3 mL), at rt for 12 h. [b] Isolated yields.
Scheme 2. Proposed reaction mechanism.
On the basis of above experimental results, a possible
mechanism is illustrated in Scheme 2. Initially, AgN3 is
generated by anion exchange of TMSN3 with Ag3PO4.[13] Its
subsequent addition to the terminal alkyne 1a produces
vinylsilver intermediate A'.[8a] Meanwhile, TolSO2TMS (A) is
generated from sulfinate salt 2a (possibly promoted by Ag(I)).
Such intermediates are known to be somewhat unstable,[14] and
could be oxidized by Ag(I) to give a radical cation B,[15] which
could then release sulfonyl radical C and a trimethylsilyl
cation.[9g] The latter is captured by water to produce
In summary, a convenient and functional-group tolerant Ag-
catalyzed three-component reaction of terminal alkynes,
TMSN3, and sodium sulfinates has been developed, which
shows broad substrate scope with respect to both the alkyne
and sulfinate. The reaction proceeds through an unprecedented
sequence of alkyne hydroazidation, and radical addition of a
sulfonyl radical to the in situ generated vinyl azide. This
strategy represents an appealing means to achieve alkyne
aminofunctionalization under mild reaction conditions;
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Acknowledgements
XB thanks NSFC (21522202, 21502017), and EAA thanks the
EPSRC for support (EP/M019195/1).
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Keywords: alkyne • aminosulfonylation • silver catalysis •
radical reaction • stereoselective
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COMMUNICATION
Yongquan Ning, Qinghe Ji, Peiqiu Liao,
Edward A. Anderson and Xihe Bi*
Page No. – Page No.
Ag-Catalyzed Stereoselective
Aminosulfonylation of Alkynes
Controllable Assemble: The first intermolecular aminosulfonylation of terminal
alkynes with sodium sulfinates and TMSN3 is reported. This reaction proceeds
through sequential hydroazidation of the terminal alkyne, and addition of a sulfonyl
radical to the resultant vinyl azide, which enables the stereoselective synthesis of a
wide range of β-sulfonyl N-unprotected enamines.
This article is protected by copyright. All rights reserved.