Synthesis of sulfimides and N-Allyl-N-(thio)amides by Ru(II)catalyzed nitrene transfer reactions of N-acyloxyamides

Article abstract of DOI:10.1021/acs.orglett.0c04043

The N-acyloxyamides were employed as effective N-acyl nitrene precursors in reactions with thioethers under the catalysis of a commercially available Ru(II) complex, from which a variety of sulfimides were synthesized efficiently and mildly. If an allyl group is contained in the thioether precursor, the [2,3]-sigmatropic rearrangement of the sulfimide occurs simultaneously and the N-allyl-N-(thio)amides were obtained as the final products. Preliminary mechanistic studies indicated that the Ru-nitrenoid species should be a key intermediate in the transformation.

Full text of DOI:10.1021/acs.orglett.0c04043

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Letter
Synthesis of Sulfimides and N-Allyl-N-(thio)amides by Ru(II)-
Catalyzed Nitrene Transfer Reactions of N-Acyloxyamides
Xinyu Zhang,† Bo Lin,† Jianhui Chen, Jiajia Chen, Yanshu Luo, and Yuanzhi Xia*
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ABSTRACT: The N-acyloxyamides were employed as effective N-acyl
nitrene precursors in reactions with thioethers under the catalysis of a
commercially available Ru(II) complex, from which a variety of
sulfimides were synthesized efficiently and mildly. If an allyl group is
contained in the thioether precursor, the [2,3]-sigmatropic rearrange-
ment of the sulfimide occurs simultaneously and the N-allyl-N-
(thio)amides were obtained as the final products. Preliminary
mechanistic studies indicated that the Ru-nitrenoid species should be a
key intermediate in the transformation.
any of the N−S bond containing compounds could be
substituted-1,4,2-dioxazol-5-ones under catalytic or photo-
chemical conditions (Scheme 1a).6 In these reactions the
nitrenoid intermediate could be used for C−H amidation,7
rearrange to the isocyanates,3e,8 or be trapped by sulfides to
form sulfoximines,9 as have been disclosed by different groups.
In mechanistic studies of Rh(III)-catalyzed C−H activations,
our group10 proposed that the metallo N-acyl nitrenes could be
key intermediates in reactions of N-acyloxybenzamides.11
However, the use of such precursors for nitrene transfer
reactions has not been disclosed thus far. Following our
interests in the field,10 in the current report we described a
mild trapping of the nitrenoid intermediates in Ru(II)-
catalyzed reactions of N-acyloxyamides with sulfides (Scheme
1b). This strategy provides a facile and efficient access to a
variety of sulfimides,9 from which sequential 2,3-sigmatropic
rearrangement occurs easily in cases of allylic sulfides to afford
N-allyl-N-(thio)amides as the final products.12,13
M
used as building blocks in organic synthesis and have
interesting biological properties in crop protection and
medicinal chemistry.1 Thus, methods for efficient construction
of nitrogen−sulfur bonds have been widely pursued.2 Among
the strategies, the nitrene transfer reactions have received
considerable attention. To this end, the azide derivatives have
been used as nitrene precursors via nitrogen elimination for a
long time (Scheme 1a),3 which were found to have important
Scheme 1. Generations and Applications of Metallo N-Acyl
Nitrenes
The study was initiated with the condition optimization of
the reaction between N-pivaloyloxybenzamide (1a) and
benzylphenylsulfane (2a). After screening different conditions,
we found the sulfimide 3aa could be obtained in 91% isolated
yield by reaction at 30 °C for 10 min with 5 mol %
RuCl2(PPh3)3 as the catalyst in the presence of 20 mol %
AgOAc and 10 mol % PhCONHOH in methanol solution
under an air atmosphere (Table 1, entry 1). No reaction was
observed if the RuCl2(PPh3)3 was not used or replaced by
Rh(III) or Pd(II) (entries 2−4). Other forms of Ru(II)
Received: December 6, 2020
Published: January 11, 2021
applications for the synthesis of various nitrogen-containing
compounds4 including compounds with a N−S linkage.3d,5
However, the methods with azides suffer from a potential
safety problem during preparation, storage, and use. More
recently, new methodologies based on N-acyl nitrene
intermediate were developed from a decarboxylate of 3-
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a
catalysts were tested. The use of [RuCl2(p-cymeme)]2 afforded
3aa in 84% yield (entry 5) while the Grubbs second generation
catalyst was much less effective (entry 6). Reducing the loading
of benzohydroxamic acid to 5 mol % gave a comparable yield
(entry 7); however, only 71% of 3aa was obtained in the
absence of it (entry 8). The replacement of AgOAc with AgF
resulted in a decreased yield (entry 9), but no 3aa was
observed when AgCN was used as the silver salt (entry 10).
The use of other acetates instead of AgOAc, including NaOAc
and Cu(OAc)2, totally inhibited the reaction (entries 11 and
12). Reactions in other solvents, including ethanol, THF, and
acetonitrile, delivered the product only in moderate yields
(entries 13−15).
With the optimized conditions in hand, we explored the
generality of the reaction for synthesis of various sulfimides
(Scheme 2). First, the reactivity of different amides was
investigated by reactions with benzyl(phenyl)sulfane (2a).
Compared with the reaction of unsubstituted N-pivaloyloxy-
benzamide 1a for generation of 3aa, attaching an electron-
donating methyl group at the para- and meta-positions of the
phenyl ring led to a slightly higher yield of 3ba and 3ca,
respectively. However, a methyl group at the ortho-position
would result in an obviously decreased yield of 3da, which may
be attributed to steric effects. Interestingly, no such effect was
observed if the ortho-substitutent is a chloro, and good to
excellent yields of the sulfimides 3ea−3ja were obtained when
Table 1. Optimization of Reaction Conditions
a
b
entry
deviation of standard conditions
yield
1
none
91%
n.r.
2
without RuCl2(PPh3)3
3
4
[Cp*RhCl2]2 instead of RuCl2(PPh3)3
PdCl2(PPh3)3 instead of RuCl2(PPh3)3
n.r.
n.r.
5
6
7
8
[Ru(p-cymene)Cl2]2 instead of RuCl2(PPh3)3
Grubb-II instead of RuCl2(PPh3)3
with 0.05 equiv of PhCONHOH
without PhCONHOH
84%
20%
90%
71%
n.r.
9
AgCN instead of AgOAc
10
11
12
13
14
15
AgF instead of AgOAc
82%
n.r.
NaOAc instead of AgOAc
Cu(OAc)2 instead of AgOAc
C2H5OH instead of MeOH
THF instead of MeOH
n.r.
67%
50%
49%
CH3CN instead of MeOH
a
Reaction conditions: 1a (0.3 mmol), 2a (0.2 mmol), RuCl2(PPh3)3
(5 mol %), AgOAc (0.2 equiv), PhCONHOH (0.1 equiv), MeOH (1
b
mL) at 30 °C for 10 min in air. Isolated yield.
a
Scheme 2. Substrate Scope for the Synthesis of Sulfimides
a
Conditions: 1 (0.3 mmol), 2 (0.2 mmol), RuCl2(PPh3)3 (5 mol %), AgOAc (20 mol %), PhCONHOH (10 mol %), MeOH (1.0 mL), in air.
b
c
The yields for 3aa were 94% and 96% when N-acetoxy and N-benzoyloxybenzamides were used as precursors, respectively. The starting material
(2-chloroethyl)(phenyl)sulfane should be used with caution. This is an analogue of mustard gas.
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a
Scheme 3. Substrate Scope for the Synthesis of N-Allyl-N-(thio)amides
a
Conditions: 1 (0.2 mmol), 2 (0.26 mmol), RuCl2(PPh3)3 (5 mol %), AgOAc (20 mol %), PhCONHOH (10 mol %), MeOH (1.0 mL), in air.
fluoro, chloro, and bromo groups are attached at different
positions. The phenyl group in benzamide could be replaced
by 2-furanyl, 2-thienyl, and naphthyl, affording products 3ka−
3ma in moderate to good yields. The sulfimides 3na and 3oa
from reactions of 2a with 3-phenylpropanamide and
cyclohexanecarboxamide derivatives were obtained in 83%
and 54% yields, respectively, indicating aliphatic amides are
also suitable precursors.
The reactions of different sulfides with 1a were next
investigated for the synthesis of sulfimides containing different
groups on the sulfur moiety (Scheme 2). When the benzyl
group in 2a was replaced by other alkyl groups, including
methyl, ethyl, 2-chloroethyl, and cyclopropyl, the correspond-
ing products 3ab−3af were formed efficiently. The structure of
3ae was confirmed unambiguously by X-ray single crystal
diffraction (CCDC: 1911530, given in the Supporting
Information (SI)). The reactivity of sulfides containing
diversely substituted phenyl groups was explored, and
satisfactory yields of 3af−3al were obtained with both
electron-withdrawing (nitro, fluoro, and chloro) and elec-
tron-donating (methoxyl) substituents at different positions of
the phenyl ring. The diphenylsulfide was found to be much less
reactive, as the 3am was only isolated in 39% yield. However,
the yield of 3an could be increased dramatically to 82% when
the di(4-methoxylphenyl)sulfide was used, indicating the
electron-rich sulfides should be more reactive. Reactions of
dibenzylsulfide and dihexylsulfide with 1a respectively
generated 3ao and 3ap in 88% yield, suggesting the
dialkylsulfides are compatible substrates for the transformation.
When using N-acetoxy- and N-benzoyloxybenzamides as
precursors, the yields for 3aa were 94% and 96%, respectively.
The sulfimides 3 could be classified as an analog of the ylide
family. Inspired by the notable rearrangement reactions of
sulfur ylides in organic chemistry,12 we wondered if similar
migration of the alkyl or aryl group from sulfur to nitrogen in 3
could occur. We found that 3aa and 3ab are relatively stable
under thermal conditions, and no reaction was observed when
heating to 80 °C in different solvents. However, if an allyl
group is included in the sulfide precursor, smooth formation of
the nitrogen allylated and sulfidated product 4 was achieved
(Scheme 3).13 Reaction of 1a with allylphenylsulfide 2q
formed 4aq in 85% yield. No obvious substituent effect was
observed if methoxyl, methyl, bromo, or chloro was attached at
different positions of the phenyl moiety in 2, affording 4ar−
4ax in good to excellent yields. The crystal structure for 4ar is
shown in the SI (CCDC: 1892371). Replacing the phenyl with
2-naphthyl resulted in 4ay in 77% yield. Substitution of bromo
and methyl at the C2 position of the allyl moiety was studied,
which led to product 4az and 4aa′ in 62% and 80% yields,
respectively.
The scope of N-pivaloyloxy amides was also expanded for
the synthesis of 4 (Scheme 3). The R1 in 1 could be a
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monosubstituted phenyl group with methyl, methoxyl, cyano,
nitro, and various halogens at different positions, delivering
4bq−4sq in good yields. Generally, the examples with
electron-deficient amides are slightly more efficient. The
compatibility of halides, especially bromide and iodide, will
facilitate further modification of the product. Amides with aryl
groups other than phenyl, including 2-thienyl and 2-naphthyl,
deliver 4lq and 4mq in 78% and 50% yields, respectively.
Aliphatic amides are also reactive precursors, as exemplified by
the smooth formations of 4oq and 4tq.
Scheme 5. Control Experiments
The potential synthetic applications of the above methods
were evaluated. The synthesis of 3aa and 4aq could be carried
out at the gram scale with high efficiency (eqs 1 and 2, Scheme
4). Further transformations based on products 3 and 4 were
Scheme 4. Synthetic Applications
that the hydroximic acid is important for efficient trans-
formation. If this is replaced by benzamide or benzoic acid (eq
8), no promoting effect could be observed, showing the unique
function of hydroximic acid in the current catalytic system.
Both the N−H and N-pivaloyloxy moieties in amide 1 were
found to be essential for the transformation, as no product was
observed when 2a was mixed up with benzamide (1u),
hydroximic acid (1v), N-methoxyl benzamide (1w), or N-
methyl-N-(pivaloxy)benzamide (1x) (eq 9). The reaction of 1a
with 2f and 2i in a 1:1:1 molar ratio formed 3af and 3ai in 14%
and 73% yields (eq 10), respectively, in good agreement with
the higher reactivity of electron-rich sulfides.
According to literature precedence and the above exper-
imental results, a plausible mechanism for the formation of 3
and 4 was proposed in Scheme 6. First of all, Ru(II) acetate A
Scheme 6. Plausible Reaction Mechanism
explored. Oxidation of 3ab with NaIO4 produced the
sulfoximine 5 in 86% yield (eq 3).14 Such compound has
been found wide applications as a directing group for
transition-metal-catalyzed C−H activations.15 4av could be
oxidized to N-allyl-N-(phenylsulfinyl)benzamide 6 in the
presence of DEA and oxone (eq 4).16 N−S bond cleavage of
4aa occurs easily under basic conditions to give rise to the
desulfurization product 7 in 87% yield (eq 5).17 When NIS was
used, the desulfurization of 4av leads to iodized 4,5-
dihydrooxazole 8 efficiently (eq 6).18
To probe the plausible mechanism of the transformations,
sulfide 2b′ containing the 3,3-dimethylallyl group was
subjected to the reaction with 1a under the conditions in
Scheme 5. After 10 min, the crude NMR indicated that 3ab′
and 4ab′ were formed in a 5:1 ratio (eq 7). However, isolation
of 3ab′ by column chromatography failed, as it is not stable.
Net conversion of 3ab′ to 4ab′ was observed after the mixture
was put in the NMR tube for 14 h. These results indicated that
the sulfimide 3 is an intermediate for the formation of 4, and
this process could be slowed by substituents at the allyl
terminal carbon. During condition optimization, it was found
could be formed as a catalytic active species through ligand
exchange. The hydroximic acid has been known as a prevalent
bidentate ligand in coordination chemistry and chemical
biology,19 due to its high binding affinity to a range of
transition metal ions. Possibly, the promoting effect of
hydroximic acid could be attributed to the formation of a
hydroxamato Ru(II) complex in the current reaction. The
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Author Contributions
anionic acetate ligand in A could be involved for N−H
deprotonation of 1 to form B. From B, the Ru-nitrenoid
intermediate C could be formed by pivaloyloxy elimination.
From C, the incorporation of thioether 2 by nucleophilic
addition gives rise to intermediate D. This is in agreement with
the observed outcomes in reactions of substrates of different
electronic properties. Following D, the generation of sulfimide
3 could be realized by Ru−N bond cleavage and concurrent
regeneration of A. When allyl is contained in 3, the [2,3]-
sigmatropic rearrangement occurs to give rise to product 4.
In conclusion, an efficient and mild synthesis of a variety of
sulfimides and N-allyl-N-(thio)amides was developed by
Ru(II)-catalyzed reactions of N-acyloxyamides with thio-
ethers.20 The reaction shows a tolerance of various functional
groups in both substrates, and the chemoselectivity is
determined by the structure of the sulfide precursor.
Preliminary mechanistic studies suggested that the Ru-
nitrenoid species could be formed as a key intermediate to
deliver the sulfimides, from which the N-allyl-N-(thio)amides
are generated by [2,3]-sigmatropic rearrangement in cases of
allylated sulfides. Further exploration of the novel reactivity of
N-acyloxybenzamides under transition metal catalysis is
underway in our laboratory.
†X.Z. and B.L. contributed equally to this work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support was provided by NSFC (Nos. 21572163,
21801191, and 21873074) and the Wenzhou Science &
Technology Bureau (G20180016). X.Z. is supported by the
Graduate Innovation Fundation of Wenzhou University
(3162019025).
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ASSOCIATED CONTENT
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* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c04043.
Experimental procedures, data, and NMR spectroscopy
(PDF)
Accession Codes
CCDC 1892371 and 1911530 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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Corresponding Author
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Yuanzhi Xia − College of Chemistry and Materials
Engineering, Wenzhou University, Wenzhou 325035, China;
orcid.org/0000-0003-2459-3296; Email: xyz@
wzu.edu.cn
Authors
Xinyu Zhang − College of Chemistry and Materials
Engineering, Wenzhou University, Wenzhou 325035, China
Bo Lin − College of Chemistry and Materials Engineering,
Wenzhou University, Wenzhou 325035, China
Jianhui Chen − College of Chemistry and Materials
Engineering, Wenzhou University, Wenzhou 325035,
China; orcid.org/0000-0002-5080-1880
Jiajia Chen − College of Chemistry and Materials Engineering,
Wenzhou University, Wenzhou 325035, China
Yanshu Luo − College of Chemistry and Materials
Engineering, Wenzhou University, Wenzhou 325035, China
̈
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