Ruthenium-catalyzed asymmetric n-acyl nitrene transfer reaction: Imidation of sulfide

Article abstract of DOI:10.1021/acs.orglett.0c01373

The asymmetric nitrene transfer reaction is a useful and strong tool for the construction of nitrogen functional groups such as N-sulfonyl amide and carbamic ester in a highly enantioselective manner. On the other hand, there is a substantial limitation in this filed: the transfer of N-acyl amide via the corresponding nitrene intermediates is still difficult because N-acyl nitrenes undergo undesired nitrene dimerization or Curtius rearrangement. Herein, we achieved highly enantioselective imidation of sulfides via catalytic N-acyl nitrene transfer with (OC)ruthenium-salen complex 2b as the catalyst and 3-substituted 1,4,2-dioxazol-5-ones 1 as the nitrene source. Complex 2b can decompose dioxazolones 1 to the desired N-acyl nitrene intermediates without any activation via heating or UV irradiation, or transfer generating nitrene intermediates to the sulfur atom of sulfides with good to excellent enantioselectivities (≤98% ee) without diazene and isocyanate contamination.

Full text of DOI:10.1021/acs.orglett.0c01373

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Letter
Ruthenium-Catalyzed Asymmetric N‑Acyl Nitrene Transfer Reaction:
Imidation of Sulfide
Masaki Yoshitake, Hiroki Hayashi, and Tatsuya Uchida*
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ABSTRACT: The asymmetric nitrene transfer reaction is a useful and strong
tool for the construction of nitrogen functional groups such as N-sulfonyl
amide and carbamic ester in a highly enantioselective manner. On the other
hand, there is a substantial limitation in this filed: the transfer of N-acyl amide
via the corresponding nitrene intermediates is still difficult because N-acyl
nitrenes undergo undesired nitrene dimerization or Curtius rearrangement.
Herein, we achieved highly enantioselective imidation of sulfides via catalytic
N-acyl nitrene transfer with (OC)ruthenium−salen complex 2b as the
catalyst and 3-substituted 1,4,2-dioxazol-5-ones 1 as the nitrene source.
Complex 2b can decompose dioxazolones 1 to the desired N-acyl nitrene
intermediates without any activation via heating or UV irradiation, or transfer
generating nitrene intermediates to the sulfur atom of sulfides with good to excellent enantioselectivities (≤98% ee) without diazene
and isocyanate contamination.
itrene transfer reactions such as imidation of heter-
H amination using 1 as the nitrogen source.11 In this reaction,
a cationic (pentamethylcyclopentadienyl)rhodium complex
induces the decomposition of 1 into the desired nitrene
intermediate without any activation method such as heating or
photoirradiation, and N-acyl amide groups are transferred to
C−H bonds in a chemoselective and site-selective manner.
After these findings had been published, the C−H amidation
reaction based on the transition metal/dioxazolone 1 system
has developed considerably, including asymmetric ver-
sions.12−14
However, the enantioselective N-acyl nitrene transfer
reaction is still limited to the C−H amination (Scheme
1a).15 This limitation could be overcome if the N-acyl nitrene
group could be transferred to heteroatoms and/or olefins with
chemo- and stereoselectivity, which would boost the
applicability of nitrene transfer reactions in organic synthesis.
Motivated by this, we were interested in the development of
the catalytic asymmetric N-acyl nitrene transfer reactions.
Herein, we describe highly chemo- and enantioselective N-acyl
imidation of sulfides catalyzed by (OC)ruthenium−salen
complexes 2 using dioxazolone 1 as the nitrene source
(Scheme 1b).
N
oatoms,1,2 aziridination of olefins,3 and amination of C−
H bonds4 are useful and powerful tools for the synthesis of
nitrogen-containing organic compounds, which are ubiquitous
in biologically active compounds such as alkaloids and amides.
Thus, these transformations have attracted continuing and
growing interest, leading to the development of various
powerful and useful methodologies in this field.5 Today, the
introduction of a nitrogen functional group can be readily
performed in a highly site-selective, chemoselective, and
enantioselective manner by using the appropriate catalyst
and nitrene source. However, there is a substantial limitation in
this field: the transfer of N-acyl amides via the corresponding
nitrene intermediates is still difficult. Unfortunately, N-acyl
nitrene species often undergo Curtius rearrangement and
produce the corresponding isocyanates or related products
under UV irradiation or thermal conditions.6 Although
catalytic N-acyl nitrene transfer reactions using azides have
been reported, the product is contaminated with diazene
derivatives as coproducts.7 On the other hand, the reaction
using metal(nitride) complexes as stoichiometric nitrene
source affords the desired product in a chemo- and
stereoselective manner.8 These results indicate that N-acyl
metal(nitrene), which can be obtained catalytically, would give
the desired N-acyl-amidated products without the formation of
isocyanates and diazenes as byproducts. Recently, Bolm and
co-workers demonstrated the high utility of 3-substituted 1,2,4-
dioxazol-5-ones 1 in the imidation of sulfides.9,10 Thus, the
imidation of sulfides and sulfoxides using dioxazolones 1 as the
nitrene source proceeds successfully with complete chemo-
selectivity. Subsequently, Chang et al. reported a catalytic C−
Received: April 21, 2020
https://dx.doi.org/10.1021/acs.orglett.0c01373
© XXXX American Chemical Society
Org. Lett. XXXX, XXX, XXX−XXX
A

Organic Letters
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Letter
Scheme 1. Asymmetric N-Acyl Nitrene Transfer Reactions
Table 1. Optimization of Imidation Using 3-Phenyl-1,4,2-
dioxazolone as a Nitrene Source
a
b
c
entry
catalyst
solvent
THF
THF
THF
1,4-dioxane
Et2O
AcOEt
toluene
CH2Cl2
MeOH
CH3CN
THF
yield (%)
ee (%)
d
e
1
2a
2a
2b
2b
2b
2b
2b
2b
2b
2b
2b
2b
NR
−
84
95
90
81
93
90
94
−
2
3
4
5
6
7
8
9
>99
>99
>99
73
>99
>99
>99
trace
46
10
76
95
97
f
11
g
>99
88
Recently, we reported that (OC)ruthenium−salen com-
plexes 2 are efficient catalysts in asymmetric nitrene transfer
reactions such as imidation of sulfides, aziridination of olefins,
and C−H aminations, using N-sulfonyl and N-alkoxycarbonyl
azides as nitrene sources (Figure 1).5c,16−18 These ruthenium-
catalyzed nitrene transfer reactions using azides could be
expected to prompt further development of N-acyl nitrene
transfer reactions.
12
THF
a
Reactions were carried out using 1a (0.13 mmol) and 3a (0.1 mmol)
with ruthenium complex 2 (2 mol %) at 25 °C for 24 h, unless
otherwise specified. Isolated yield, based on sulfide 3. Determined
by HPLC with a chiral stationary phase. N-Benzoyl azide (0.13
mmol) was used as the nitrene source instead of 1a. No reaction was
observed. Carried out with 2b (1 mol %) and 4 Å molecular sieves
(20 mg) for 18 h. Carried out on a 1.0 mmol scale with 2b (0.2 mol
b
c
d
e
f
g
%) and 4 Å molecular sieves (20 mg) for 24 h.
With the optimized conditions in hand, we conducted the
asymmetric imidation using 3-substituted 1,2,4-dioxazolones 1
as nitrene precursors with methyl phenyl sulfide in the
presence of complex 2b as the catalyst (Table 2). The reaction
of para- and meta-substituted 3-phenyl-1,2,4-dioxazolones 1b−
k gave better yields than the ortho-substituted derivatives 1l−o,
and high enantioselectivities were obtained in all cases (90−
98% ee) (entries 1−14). In addition, 3-alkyl- and 3-alkenyl-
1,2,4-dioxazolones 1p−u could be also successfully applied as
the nitrene source. 3-Methyl-, 3-pentyl-, and 3-(2-propyl)-
dioxazolones 1p−r gave desired products 4 with good
enantioselectivity and moderate to high yields (entries 15−
17, respectively). However, no reaction occurred when using
dioxazolone 1s as the nitrene source (entry 18). Furthermore,
3-(E)-propenyldioxazolones 1t also produced the correspond-
ing N-acyl sulfimides 4ta with good enantioselectivities (entry
19).
We further examined the asymmetric sulfimidation of
various sulfides 3 using the complex 2b/3-phenyldioxazolone
1a system (Scheme 2). In general, aryl methyl sulfides 3b−g
gave the desired products 4 with high enantioselectivities and
high yields, irrespective of the electronic nature and position of
the substituents. Ethyl phenyl sulfide 3j and benzyl phenyl
sulfide 3k also underwent the N-acyl imidation with moderate
stereoselectivities. Meanwhile, benzyl methyl sulfide 3l gave an
89% ee, albeit sluggish. Fortunately, the chemical yield of 4al
was improved to 99% by using a 4 mol % catalyst loading.
Dialkyl sulfides 3m−p also gave the products with good to
high enantioselectivities, and sterically hindered dialkyl sulfides
such as methyl 2-propyl sulfide 3n and methyl tert-butyl sulfide
3o afforded yields better than those of the less hindered
sulfides 3m and 3p. Ethyl propyl sulfide 3p also gave the
desired sulfimide 4ap with acceptable enantioselectivity (75%
ee).
Figure 1. Cross-selective oxidative coupling between two different
arenols.
We conducted the N-acyl imidation of methyl phenyl sulfide
3a using ruthenium−salen complex 2a as the catalyst (Table
1). No reaction was observed in the presence of N-benzoyl
azide (entry 1). Fortunately, 3-phenyl-1,4,2-dioxazol-5-one 1a
gave the desired sulfide 4aa with 84% ee in quantitative yield
without any activation such as heating or photoirradiation
(entry 2). Diazene and isocyanate derivatives were not
observed under those conditions. Encouraged by these results,
we conducted the N-acyl imidation of methyl phenyl sulfide 3a
using 3-phenyl-1,2,4-dioxazol-5-one 1a as the nitrene precursor
with a series of ruthenium complexes 2 as catalysts (Table 1,
entries 2 and 3, and Table S119). Complex 2b showed the best
enantioselectivity (95% ee) in tetrahydrofuran (THF) as the
solvent. The imidation with te complex 2b/dioxazolone 1a
system in nonpolar and less polar solvents proceeded with
good to high enantioselectivities (entries 3−8). On the other
hand, the reaction in methanol was sluggish (entry 9).
Acetonitrile could be used as the solvent, albeit with reduced
yield and enantioselectivity (entry 10). It was also possible to
reduce the catalyst loading to 0.2 mol % in THF without
affecting the chemical yield or the enantioselectivity (entries 11
and 12).
B
https://dx.doi.org/10.1021/acs.orglett.0c01373
Org. Lett. XXXX, XXX, XXX−XXX

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Letter
Table 2. Scope and Limitation of 3-Substituted 1,4,2-
Dioxazolone 1
Scheme 3. Oxidation of Sulfimides 4 to Sulfoximines 5
a
b
c
entry
R
product
yield (%)
ee (%)
In conclusion, highly enantioselective N-acyl imidation of
sulfide 3 was achieved by using dioxazolones 1 as the nitrene
source in the presence of (OC)ruthenium−salen complex 2b
as the catalyst. It was thought that ruthenium complex 2b
decomposes various dioxazolones 1 to the corresponding N-
acyl ruthenium(nitrene) intermediates without any activation
such as heating and photoirradiation and produces the desired
sulfimides 4 in an excellent enantio- and chemoselective
manner. Fortunately, during these studies, undesired nitrene
dimerization and Curtius rearrangement were not observed.
Moreover, the reaction proceeded with a high yield even at a
0.2 mol % catalyst loading to give the corresponding products.
Obtained sulfimides 4 could be transformed into the
corresponding sulfoximines 5 with no erosion of the
enantiomeric excess.
1
2
3
4
5
6
7
8
4-MeC6H4 (1b)
4-MeOC6H4 (1c)
4-BrC6H4 (1d)
4-ClC6H4 (1e)
4-FC6H4 (1f)
4-CF3C6H4 (1g)
3-MeC6H4 (1h)
3-BrC6H4 (1i)
3-ClC6H4 (1j)
3-FC6H4 (1k)
2-MeC6H4 (1l)
2-BrC6H4 (1m)
2-ClC6H4 (1n)
2-FC6H4 (1o)
Me (1p)
4ba
4ca
4da
4ea
4fa
4ga
4ha
4ia
>99
99
99
93
>99
51
>99
>99
>99
98
46
75
86
>99
91
>99
52
NR
94
96
98
95
94
93
91
97
92
92
93
90
94
94
9
4ja
4ka
4la
10
11
12
13
14
15
16
17
18
19
4ma
4na
4oa
4pa
4qa
4ra
4sa
4ta
95
d
94 (R)
n-pentyl (1q)
iPr (1r)
tBu (1s)
(E)-PhCC- (1t)
93
96
−
ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c01373.
94
a
Unless otherwise specified, all reactions were carried out using 1
(0.13 mmol) and 3a (0.1 mmol) with complex 2b (1 mol %) and 4 Å
molecular sieves (20 mg) in THF at 25 °C. Isolated yield, based on
methyl phenyl sulfide 3a. Determined by HPLC with a chiral
Optimization, experimental procedure, characterization
data, HPLC conditions, and NMR spectra (PDF)
b
c
d
stationary phase. Determined by chiroptical comparison after 4pa
AUTHOR INFORMATION
Corresponding Author
■
was converted into sulfoximine 6pa.20
a
Tatsuya Uchida − Faculty of Arts and Science and International
Institute of Carbon-Neutral Energy Research (WPI-I2CNER),
Kyushu University, Fukuoka 819-0395, Japan; orcid.org/
0000-0002-6511-5752; Email: uchida.tatsuya.045@
m.kyushu-u.ac.jp
Scheme 2. Scope and Limitation of Sulfide 3
Authors
Masaki Yoshitake − Department of Chemistry, Graduate School
of Science, Kyushu University, Fukuoka 819-0395, Japan
Hiroki Hayashi − Faculty of Arts and Science, Kyushu
University, Fukuoka 819-0395, Japan
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c01373
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support from JSPS KAKENHI Grant 18H04264 in
Precisely Designed Catalysis with Customized Scaffolding and
The International Institute for Carbon-Neutral Energy
Research (WPI-I2CNER) from MEXT, Japan, is gratefully
acknowledged.
a
Unless otherwise specified, all reactions were carried out using 1a
(0.11 mmol) and 3 (0.1 mmol) with complex 2b (1 mol %) at 25 °C
b
in the presence of 4 Å molecular sieves. Carried out with 2b (4 mol
%).
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