Synthesis of Chiral Sulfonyl Lactones via Copper-Catalyzed Asymmetric Radical Reaction of DABCO?(SO2)

Article abstract of DOI:10.1002/adsc.201701532

In the present work, an asymmetric copper-catalyzed radical multi-component cascade reaction of an unsaturated carboxylic acid, aryldiazonium tetrafluoroborate, and DABCO?(SO₂)₂ (DABSO) has been developed for the enantioselective synthesis of sulfonyl lactones. In this reaction, this SO₂ surrogate, DABSO was applied for the first time in the construction of chiral compounds. This multiple-step asymmetric radical reaction was carried out under mild conditions and tolerated a wide range of substrates, resulting in the corresponding sulfonyl lactones with up to 95% chemical yields and 88% ee. The current reaction enriches the research contents of DABSO, and provides a new and efficient strategy to chiral functionalized lactones bearing quarternary stereogenic center. (Figure presented.).

Full text of DOI:10.1002/adsc.201701532

Title: Synthesis of Chiral Sulfonyl Lactones via Copper-Catalyzed
Asymmetric Radical Reaction of DABCO·(SO2)
Authors: Yang Wang, Lingling Deng, Jie Zhou, Xiaochen Wang, Haibo
Mei, Jianlin Han, and Yi Pan
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201701532
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10.1002/adsc.201701532
Advanced Synthesis & Catalysis
COMMUNICATION
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Synthesis of Chiral Sulfonyl Lactones via Copper-Catalyzed
Asymmetric Radical Reaction of DABCO·(SO2)
Yang Wang,[a] Lingling Deng,[a] Jie Zhou,[b] Xiaochen Wang,[a] Haibo Mei,[a] Jianlin
Han,*[a] and Yi Pan[a]
a
School of Chemistry and Chemical Engineering, State Key Laboratory of Coordination Chemistry, Jiangsu Key
Laboratory of Advanced Organic Materials, Nanjing University, 210093, China
phone number: 86-25-83686133; e-mail: hanjl@nju.edu.cn
Shenzhen Research Institute of Nanjing University, Shenzhen, 518057, China
b
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
reported, mainly because radicals are considered to be
disadvantaged in the formation of chiral centers because they
are highly reactive and short-lived.[11] In recent years,
enantioselective radical reactions attracted the attention of
many research groups.[12] For example, the Liu group has
developed a series of catalytic asymmetric difunctionalization
of alkenes via a radical process.[13] On the other hand, the Liu
group has also developed several novel reactions on direct
asymmetric radical difunctionalization of unactivated alkenes
with copper salts and chiral phosphoric acid as combined
catalyst.[14] Recently, Buchwald[15] and co-workers developed
Abstract. In the present work, an asymmetric copper-
catalyzed radical multi-component cascade reaction of an
unsaturated
carboxylic
acid,
aryldiazonium
tetrafluoroborate, and DABCO·(SO2)2 (DABSO) has been
developed for the enantioselective synthesis of sulfonyl
lactones. In this reaction, this SO2 surrogate, DABSO was
applied for the first time in the construction of chiral
compounds. This multiple-step asymmetric radical reaction
was carried out under mild conditions and tolerated a wide
range of substrates, resulting in the corresponding sulfonyl
lactones with up to 95% chemical yields and 88% ee. The
current reaction enriches the research contents of DABSO,
and provides a new and efficient strategy to chiral
functionalized lactones bearing quarternary stereogenic
center.
two examples
of asymmetric Cu-catalyzed radical
oxytrifluoromethylation of alkenes with Togni’s reagent or other
coupling partners. Considering that DABSO has never been
applied in asymmetric reactions and inspired by the above
asymmetric radical works,[13-16] we herein report the first
example of an asymmetric copper-catalyzed radical multi-
component reaction of unsaturated carboxylic acids,
aryldiazonium tetrafluoroborates,[17] and DABSO (Scheme 1b).
This Cu-catalyzed asymmetric cyclization reaction is conducted
at room temperature, and proceeds through a four-step radical
pathway, affording the chiral sulfonyl lactones with up to 95%
yields and 88% ee. Furthermore, this radical multi-component
reaction provides a new method for the synthesis of chiral
sulfonyl lactones. It should be mentioned that the Wu group
Keywords: asymmetric catalysis; radical reaction; Cu-
catalysis; DABSO; lactones
Sulfones represent an important type of organic compounds
and widely exist in many natural products and pharmaceutical
molecules.[1] Also, they are useful organic intermediates, and
such sulfonyl structural units have been widely applied in many
organic transformations.[2] Traditional methods for the
synthesis of sulfones usually focus on oxidation of sulfides[3] or
sulfonylation of C(sp2)–H and C(sp3)–H bonds.[4] Recently,
many elegant methods using of DABCO·(SO2)2 (DABSO) as an
SO2 surrogate for the preparation of sulfonyl compounds have
been developed independently by the Willis,[5] Wu[6] and other
groups,[7] which allows the direct introduction of a sulfonyl
group through SO2 insertion under simple conditions. Although
great achievements have been made on this SO2 surrogate
(Scheme 1a),[5-8] this useful reagent has never been explored
in enantioselective synthesis.
recently reported
a related visible light-promoted radical
cyclization reaction of 2-vinylbenzoic acids, aryldiazoniumtetra
fluoroborates and sulfur dioxide surrogate of DABCO•(SO2)2
for the preparation of sulfonated isobenzofuran-1(3H)-ones.[18]
Chiral structures play an important role in natural products,
pharmaceuticals, and other bioactive compounds.[9,10] In the
past decades enantioselective ionic reactions have been well
developed. However, asymmetric radical reactions remain less
Scheme 1. Radical coupling reaction of DABSO.
1
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10.1002/adsc.201701532
Advanced Synthesis & Catalysis
To test the possibility of this strategy, we chose 4-
7
NiI2
DCM
DCM
trace -
L1
L2
phenylpent-4-enoic acid 1a, 4-methylbenzenediazonium
tetrafluoroborate 2a and DABSO as model substrates for the
initial study. The reaction was carried out in the presence of
2,6-di-tert-butylpyridine with Cu(OAc) as catalyst, bis((S)-4-
(tert-butyl)-4,5-dihydrooxazol-2-yl)methane (L1) as chiral ligand
under argon atmosphere at room temperature. The reaction
did happen, and the corresponding sulfonyl lactone 3aa was
obtained in 46% yield and 36% ee after 12 h (entry 1, Table 1).
Other catalysts, such as CuBr and Cu(OTf)2, also catalyzed
this transformation resulting in similar yields and
enantioselectivities (entries 2 and 4). An obvious improvement
on yield was found when Cu(OAc)2 was used (60% yield, entry
3). [Cu(MeCN)4]PF6 was demonstrated to be the best one, and
furnished the product 3aa in 80% yield and 56% ee (entry 5).
The reaction did not take place with other metal catalysts, such
as NiCl2 and NiI2 (entries 6 and 7). Then, the chiral ligand was
scanned for this reaction (for details on the optimization of
chiral ligands, see Supporting Information). There was an
obviously reduction on ee value when the chiral ligand was
replaced by bis((S)-4-isopropyl-4,5-dihydrooxazol-2-yl)methane
(L2) (entry 8, 71% yield and 21% ee). The similar result was
observed with bis((S)-4-phenyl-4,5-dihydrooxazol-2-yl)methane
(L3) as ligand (60% yield and 26% ee, entry 9). Other
bisoxazoline ligands also have been tried, however, no
improvement on the enantioselectivity was found (entries 10
and 11). The solvent also showed obvious effect on the
reaction outcome. Reaction in CH2Br2 provided the same level
of yield and ee (entry 14, 75% yield, 52% ee). Other solvents,
such as DCE, CHCl2CH2Cl and 1,4-dioxane, gave obviously
lower yields and enantioselectivities (entries 12, 13 and 15).
Finally, different bases were tested as additives for this
reaction. When DABCO was added instead of 2,6-di-tert-
butylpyridinein (DTBP), similar ee was obtained, but with
dramatically decreased yield (entry 16, 56% ee and 34% yield).
Inorganic base, like Ag2CO3, could promote the reaction to give
the corresponding product, but still no improvement on yield
and enantioselectivity was obtained (entry 18, 65% yield and
54% ee).
8
Cu(MeCN)4PF6
71
21
9
Cu(MeCN)4PF6
Cu(MeCN)4PF6
Cu(MeCN)4PF6
Cu(MeCN)4PF6
Cu(MeCN)4PF6
DCM
DCM
DCM
DCE
60
76
61
58
50
26
30
26
53
47
L3
L4
L5
L1
L1
10
11
12
13
1,4-
dioxane
CH2Br2 75
14
15
Cu(MeCN)4PF6
Cu(MeCN)4PF6
52
54
L1
L1
CHCl2
CH2Cl
DCM
DCM
DCM
64
16[d] Cu(MeCN)4PF6
17[e] Cu(MeCN)4PF6
18[f] Cu(MeCN)4PF6
34
52
65
56
52
54
L1
L1
L1
[a] Reaction conditions: 1a (0.2 mmol), 2a (0.4 mmol, 2.0
equiv), DABSO (0.2 mmol, 1.0 equiv), additive (0.4 mmol,
2.0 equiv), catalyst (0.02 mmol, 10 mol %), ligand (0.024,
12 mol %), 2,6-di-tert-butylpyridine (0.4 mmol, 2.0 equiv),
under argon atmosphere for 12 h. [b] Isolated yields. [c]
Enantiomeric excess (ee) was determined by HPLC on a
chiral stationary phase. [d] DABCO as additive. [e] DBU
as additive. [f] Ag2CO3 as additive.
With the optimal conditions in hand, we then investigated the
substrate scope of this Cu-catalyzed asymmetric reaction by
using various unsaturated carboxylic acids, and the results are
shown in Scheme 2.
Table 1. Optimization of the reaction conditions.[a]
Entry Catalyst
Solvent Yield Ee
(%)[b] (%)[c]
L
1
2
3
4
Cu(OAc)
CuBr
DCM
DCM
DCM
DCM
DCM
DCM
46
39
60
41
80
36
35
40
40
56
L1
L1
L1
L1
L1
L1
Scheme 2. Substrate scope of unsaturated carboxylic acids.
Cu(OAc)2
Cu(OTf)2
Cu(MeCN)4PF6
NiCl2
First, several 4-arylpent-4-enoic acid substrates were tested
for the construction of five-member ring lactones (3aa-3ha).
The p-tolyl substituted unsaturated carboxylic acids provided
the corresponding product 3ba in good yield (81%) and ee
(64%). The substrate with strong electron-donating group
5
6
trace -
2
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10.1002/adsc.201701532
Advanced Synthesis & Catalysis
substituted phenyl, 4-methoxyphenyl unsaturated carboxylic
acids 1c, afforded the product 3ca in excellent yield (95%), but
with low ee (22%). In contrast, when the substrate with para-
phenyl substituted phenyl, the reaction showed an opposite
outcome, and the desired product 3da was obtained in
excellent ee (88%) but poor yield (25%). The substrates with
electron-withdrawing groups also worked well in this reaction
(3ea-3ga). Even the 4-fluorophenyl substituted one 1g was
tolerated, affording 3ga in 60% yield and 44% ee. To
investigate the steric effect of the current system, one
substrate with two phenyl groups on the α-position, 4-methyl-
2,2-diphenylpent-4-enoic acid 1h, was tried. Unfortunately, the
corresponding product 3ha was obtained with only 6% ee and
56% chemical yield. Interestingly, 5-phenylhex-5-enoic acid
was also a suitable substrate, affording the six-membered
lactone derivative 3ia in 60% yield and 12% ee. Finally, several
ortho-vinyl benzoic acid derivatives were tried in this reaction
due to the importance of isobenzofuran-1-ones motif in
bioorganic chemistry. The substrates without a substituted
group or with an electron-donating group on the aromatic ring,
reacted well with DABSO and 4-methylbenzenediazonium
tetrafluoroborate to give the desired product with moderate
yields (3ja-3ka). However, in the case of the substrate bearing
electron-withdrawing group substituted phenyl, the reaction
gave very poor yield (10%, 3la).
reaction. However, both the yield and ee were poor because of
the steric hindrance (30% yield and 42% ee for 3ai, 55% yield
and 44% ee for 3aj). It should be mentioned that the reaction
of disubstituted benzenediazonium salt also proceeded
smoothly, affording the corresponding product in good yield
and enantioselectivity (3ak, 88% yield and 78% ee). Finally, a
benzyl diazonium salt was examined in this reaction, which did
not work at all with most of the starting materials remained.
The absolute configuration of these sulfonyl lactones has been
determined by the x-ray analysis of 3ak, which discloses its (S)
absolute configuration (see SI file).[19] Accordingly, all the other
products 3 were assigned (S) stereochemistry.
To get insight into the mechanism of this Cu-catalyzed
asymmetric reaction, two control experiments were carried out
(Scheme 4). First, a radical trapping experiment was carried
out under standard conditions with the addition of TEMPO
[(2,2,6,6-tetramethylpiperidin-1-yl)oxyl] (Scheme 4a). The
reaction was totally suppressed and only a trace amount of the
desired product 3aa was found. To verify radical process,
another radical scavenger 1,1-diphenylalkene was added to
the reaction under standard reaction conditions (Scheme 1b).
The similar result was found and almost no 3aa was obtained.
After careful isolation of the reaction mixture, 1,1-
diphenylalkene-toluene radical adduct 4 was obtained in 56%
isolated yield.
Next, we concentrated on the study of substrate generality of
aryl diazonium salts, and the results are shown in Scheme 3.
Scheme 4. Radical trapping experiment.
Based on the above results and previous reports,[5-7,15]
a
possible mechanism is illustrated in Scheme 5. Initially,
phenyldiazonium tetrafluoroborate 2b is reduced by Cu(I) via
single-electron-transfer to generate phenyl radical and Cu(II)
complex A via release of nitrogen.[17c] This phenyl radical is
trapped by DABCO·(SO2)2 and generates the benzene sulfonyl
radical. At the same time, unsaturated carboxylic acid 1a
reacts with A in the presence of 2,6-di-tert-butylpyridine in to
give the Cu(II)-unsaturated carboxylic acids complex B. Then,
intermediate B couples with benzene sulfonyl radical leading to
the radical intermediate C.[15b] Although the radical nature of
the unobservable transient intermediate is an disadvantage,
the possible interaction between Cu(II) and carbon radical may
be helpful for the enantioselective control of this reaction.
Scheme 3. Substrate scope of aryl diazonium salts.
Benzenediazonium salt 2b was suitable substrate for this
reaction, which gave the lactone 3ab in moderate yield (54%)
and ee (54%). On the other hand, varieties of functional groups
in para position of phenyl ring, such as t-butyl (3ac), methoxyl
(3ad), fluoro (3ae), nitro (3af), chloro (3ag) and trifluoromethyl
(3ah), could be well tolerated in the reaction, resulting in the
desired product with up to 71% yield and 74% ee. Notably,
even the substrates containing strong electron-donating or
electron-withdrawing group could work well in this system,
affording the desired product in the same level of yield and ee
(71% yield and 56% ee for 3ad, 64% yield and 54% ee for 3ae,
50% yield and 74% ee for 3ah respectively). The substrate
bearing ortho-substituted group has also been tested in the
Subsequently,
a
single electron oxidation process of
intermediate C happens, affording the Cu(III) complex as well
as the formation of chiral center. Finally, reductive elimination
of this Cu(III) intermediate D to give the final chiral product 3ab
and regenerate the Cu(I) for the next catalytic cycle. It should
be mentioned that aryl sulfonyl radical may also generate from
the reaction between aryldiazonium tetrafluoroborate and
DABCO•(SO2)2.[20]
3
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10.1002/adsc.201701532
Advanced Synthesis & Catalysis
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In conclusion, we have developed a novel method to prepare
sulfonyl lactones bearing quanternary stereogenic center via
copper-catalyzed asymmetric radical cascade multi-component
reaction of unsaturated carboxylic acids. In this reaction,
DABSO was used as sulfonyl precursor and aryldiazonium
tetrafluoroborate was used as aryl radical, to give the
corresponding product with good chemical yields and
enantioselectivities. In particular, this SO2 surrogate was used
for first time in synthesis of functionalized chiral compounds,
which enriches the research contents of reactions on DABSO
and asymmetric radical reactions.
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Experimental Section
General procedures for copper-catalyzed asymmetric
radical reaction of unsaturated carboxylic acids,
aryldiazonium tetrafluoroborates, and DABSO.
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The bisoxazoline ligand L1 (6.4 mg, 0.024 mmol) and
Cu(MeCN)4PF6 (7.5 mg, 0.020 mmol) were dissolved in 2
mL of anhydrous DCM under argon at room temperature
and stirred for 2 h. Then the solution was inject into a
bottle which was filled with argon, unsaturated carboxylic
acids 1 (0.2 mmol), aryldiazonium salts 2 (0.4 mmol),
DABSO (0.2 mmol) and 2,6-di-tert-butylpyridine (0.4
mmol). The mixture was stirred for another 12 h at room
temperature. The solvent was removed under reduced
pressure, and the residue was purified by silica gel column
chromatography (PE:EA = 10:1) to afford products 3.
Acknowledgements
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a) C. C. Chen, J. Waser, Org. Lett. 2015, 17, 736-
739; b) B. Du, Y. Wang, W. Sha, P. Qian, H. Mei, J.
Han, Y. Pan, Asian J. Org. Chem. 2017, 2, 153-156; c)
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We gratefully acknowledge the financial support from the
National Natural Science Foundation of China (No. 21772085).
The support from Collaborative Innovation Center of Solid-State
Lighting and Energy-Saving Electronics, Shenzhen Virtual
University Park and Changzhou Jin-Feng-Huang program (for
Han) are also acknowledged.
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10.1002/adsc.201701532
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