Copper-Catalyzed Synthesis of N-Formyl/Acylsulfenamides and -thiosulfonamides

Article abstract of DOI:10.1002/ejoc.201500857

Recent remarkable success of copper-catalyzed oxidative bond formations led us to develop an environmentally benign copper-catalyzed sulfur-nitrogen bond-forming reaction to provide synthetically and industrially useful electron-deficient sulfenamides. The low nucleophilicity of N-formyl/acyl- and sulfonylamides was overcome by introducing a 1,5,7-triazabicyclo[4.4.0]dec-5-ene additive. A variety of N-formyl/acylsulfenamides and -thiosulfonamides were synthesized in good yields without over-oxidation of sulfur. Related mechanistic studies supported the proposed reaction mechanism.

Full text of DOI:10.1002/ejoc.201500857

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201500857
Copper-Catalyzed Synthesis of N-Formyl/Acylsulfenamides and
-thiosulfonamides
Chan Lee,[a] Yu Na Lim,[a] and Hye-Young Jang*[a,b]
Keywords: Homogeneous catalysis / Copper / S–N bond formation / Sulfur / Oxidation / Thiosulfonamides / Sulfenamides
Recent remarkable success of copper-catalyzed oxidative
1,5,7-triazabicyclo[4.4.0]dec-5-ene additive. A variety of N-
bond formations led us to develop an environmentally be- formyl/acylsulfenamides and -thiosulfonamides were synthe-
nign copper-catalyzed sulfur–nitrogen bond-forming reac-
tion to provide synthetically and industrially useful electron-
deficient sulfenamides. The low nucleophilicity of N-formyl/
acyl- and sulfonylamides was overcome by introducing a
sized in good yields without over-oxidation of sulfur. Related
mechanistic studies supported the proposed reaction mecha-
nism.
of electron-deficient amides with aromatic and aliphatic
thiols to resolve the limitations of previously reported
methods. Although electron-deficient sulfenamides show
biological activity, prodrug activity, synthetic utility, and
interesting properties during vulcanization processes (Fig-
ure 1),[3b,8,9] the low nucleophilicity of amines possessing an
electron-withdrawing group and the high oxidation suscep-
tibility of sulfur during the oxidative coupling reactions im-
pede the progress of various catalytic methods for electron-
deficient sulfenamides.[10,11] The reaction of amides with
sulfenyl chloride [Scheme 1, Equation (1)], the reaction of
amides with disulfide in the presence of a large excess
amount of NaH [Scheme 1, Equation (2)],[10n] and the reac-
tion of N-chlorosuccinimide with metal thiolates[10k] are
representative methods of forming N-acylsulfenamides
through sulfur–nitrogen bond formation (Scheme 1). In this
study, we present the first catalytic aerobic coupling of
amides (formyl/acyl- and sulfonylamides) with various
thiols to afford a range of sulfenamides [Scheme 1, Equa-
tion (4)]. We believe this protocol will be a general strategy
to synthesize biologically, synthetically, and industrially
useful electron-deficient sulfenamides.
Introduction
The development of efficient and environmentally benign
protocols to form sulfur–nitrogen bonds has been an im-
portant issue, considering the abundance of synthetic proto-
cols for N-sulfenyl-/sulfinyl-/sulfonylamine derivatives and
their pharmaceutical and industrial utilizations.[1–3] Com-
pared to sulfonyl- and sulfinylamine derivatives, catalytic
systems for the synthesis of sulfenylamine derivatives are
rare. Owing to the oxidation susceptibility of sulfur, a con-
trolled and selective catalytic oxidative coupling of amines
and thiols to form sulfenylamine derivatives is challenging.
Recently, we reported the direct oxidative coupling of
amines and thiols to form N-sulfenyl imines by using cop-
per catalysts and oxygen as the oxidant, for which over-
oxidation of the sulfur atom was not observed.[4] In
conjunction with our previous results, the selective and
catalytic synthesis of sulfenamides shifted our attention to
developing controllable copper-catalyzed oxidative sulfur–
nitrogen coupling protocols.
The classical method to synthesize sulfenamides involves
the the reaction of amines with sulfenyl chlorides.[5,6] Con-
sidering the toxicity of chlorinating agents used to form
sulfenyl chlorides, alternative catalytic methods have been
investigated for the synthesis of sulfenamides. Recently, the
copper-catalyzed aerobic coupling of amines and thiols to
afford sulfenamides was reported by Taniguchi.[7] Inspired
by Taniguchi’s work, we decided to investigate the reaction
[a] Division of Energy Systems Research, Ajou University,
Suwon, Republic of Korea
E-mail: hyjang2@ajou.ac.kr
http://ajou.ac.kr/~hyjang2
[b] Korea Carbon Capture & Sequestration R&D Center,
Deajeon 305-343, Korea
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201500857.
Figure 1. Interesting N-acylsulfenamides and -thiosulfonamides.
© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Eur. J. Org. Chem. 2015, 5934–5938
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Synthesis of N-Formyl/Acylsulfenamides and -thiosulfonamides
Table 1. Optimization of the synthesis of N-formylsulfenamide
(1c).[a]
Entry Catalyst
Base (mol-%)
Solvent
Yield [%]
1
2
3
4
5
6
7
8
CuI
CuI
CuI
CuI
CuI
CuI
CuOAc
CuBr
CuBr2
–
TBD (10)
TBD (5)
–
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
1,4-dioxane
89
89
DBU (10)
K2CO3 (10)
KOtBu (10)
TBD (10)
TBD (10)
TBD (10)
TBD (10)
TBD (10)
TBD (5)
53
14
35
37
63
53
3
9
10
11
12
CuI
CuI
–
[b], 65[c]
92
Scheme 1. Synthesis of electron-deficient sulfenamides.
[a] Conditions: Copper salt (5 mol-%) and TBD (5 mol-%) were
added to a solution of 1a (1.0 mmol) and 1b (0.5 mmol) in toluene
(0.5 m). The mixture was stirred at 100 °C for 12 h under an O2
atmosphere. [b] The reaction was run under an atmosphere of
nitrogen. [c] The reaction was run in air.
Results and Discussion
The optimization results are given in Table 1. N-Methyl-
formamide (1a, 2 equiv.) and thiophenol (1b, 1 equiv.) were
subjected to copper-catalyzed aerobic oxidation conditions. formylsulfenamides 2c–6c in good yields (81–89%). Ali-
In the presence of CuI (5 mol-%) and 1,5,7-triazabi- phatic thiols also participated in the reaction to afford
cyclo[4.4.0]dec-5-ene (TBD, 10 mol-%), a solution of 1a and products 7c–10c. In particular, 2-mercaptoethanol reacted
1b in toluene (0.5 m) was stirred at 100 °C under an atmo- with N-methylformamide to afford sulfur–nitrogen de-
sphere of oxygen (1 atm) to afford N-formylsulfenamide 1c hydrogenative coupling product 7c in 57% yield, even in the
in 89% yield (Table 1, entry 1). In the previously reported presence of a free hydroxy group. Primary aliphatic thiols
reaction using CuCl (1 equiv.) and Cu(OAc)2 (1 equiv.), smoothly afforded products 8c and 9c in yields of 90 and
thiophenol and formamides underwent deformylation fol- 75%, respectively. The reaction of cyclohexanethiol with N-
lowed by oxidative sulfur–nitrogen bond formation to af- methylformamide showed a lower yield, owing to the steric
ford sulfonamides.[12] Unlike the above reactions, our reac- effect of the cyclohexyl group. Next, N-formylamide deriva-
tion uses catalytic amounts of metal salts and base addi- tives were subjected to the reaction conditions. Ethyl-,
tives, which leads to the formation of different products. benzyl-, and cyclohexyl-substituted formylamides partici-
With reduced loadings of TBD (5 mol-%), the yield of 1c pated in the reaction to afford corresponding products 11c,
was retained (Table 1, entry 2). Without TBD, the reaction 12c, and 13c in yields of 78, 72, and 73%, respectively.
did not proceed (Table 1, entry 3). We screened several Formamide without a substituent was subjected to the reac-
bases; 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), K2CO3, tion conditions to afford 14c in low yield (12%). In addition
and KOtBu showed inferior activity (Table 1, entries 4–6). to formyl-substituted amides, N-acylamides and sulfon-
Presumably, a strong organic base enhances the reactivity amides were tested. Sulfenamide 15c possessing two carb-
of the amides. Next, we tested copper salts; CuOAc, CuBr, onyl groups proximal to the nitrogen atom was formed in
and CuBr2 generated lower yields of 1c than CuI (Table 1, 63% yield. Five-, six-, and seven-membered lactams 16c–
entries 7–9). Without copper salts, 1c was isolated in 3% 18c were formed in good yields (92, 97, and 94%, respec-
yield (Table 1, entry 10). To investigate the effect of oxygen, tively). p-Tolylsulfonylamines were subjected to the reaction
the reaction was performed under a nitrogen atmosphere, conditions to afford phenylthiosulfonates 19c and 20c in
which did not result in the formation of 1c (Table 1, en- yields of 75 and 66%, respectively. The reaction of octan-
try 11). The reaction run in air afforded 1c in 65% yield ethiol with a sulfonamides provided 21c in 30% yield.
(Table 1, entry 11). By changing the solvent to dioxane, the
As mentioned at the beginning of this report, sulfen-
yield of 1c increased slightly (Table 1, entry 12).
amides containing a weakly acidic amide group can be used
To investigate the substrate scope of the reaction, various as potential prodrugs.[8] Upon exposure of a sulfenamide to
thiols and amides were subjected to the aerobic oxidation glutathione (GSH), an abundant thiol in vivo, the sulfur–
conditions in the presence of CuI and TBD (Figure 2). nitrogen bond of the sulfenamide is cleaved by nucleophilic
First, N-methylformamide and various thiols were exposed attack of the thiol to release the amide drug into the blood.
to the optimized reaction conditions. Thiophenols with F, One of the synthesized sulfenamides was tested to examine
Cl, Br, CH3, and CH3O groups were converted into N- whether an amide could be released by reaction with GSH.
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C. Lee, Y. N. Lim, H.-Y. Jang
SHORT COMMUNICATION
Figure 2. Examples of sulfenamides.
In Scheme 2, the reaction of 1c with GSH in [D6]DMSO is
shown. Analysis by NMR spectroscopy showed that thio-
phenol was cleaved from 1c to generate N-methylform-
amide. A liberated thiophenol was delivered to GSH to
form glutathione–SPh, which was confirmed by mass spec-
trometry analysis (see the Supporting Information).
To probe a catalytic mechanism, CuSPh was used as both
the catalyst and the substrate [Scheme 3, Equations (1) and
(2)].[7] With CuSPh instead of CuI, the yield of 1c decreased
to 41%, which implies that CuI is more efficient than
CuSPh. Upon employing a stoichiometric amount of
CuSPh in the absence of CuI, 1c was obtained in 30% yield
in the presence of O2 and in 4% yield in the absence of
O2. These results indicate that CuSPh is not a catalytically
Scheme 2. Reactions of 1c with glutathione.
competent species for sulfur–nitrogen bond formation. The yield in the absence of O2 [Scheme 3, Equations (3) and (4)].
reaction of diphenyl disulfide with 1a afforded desired In the absence of copper salts, the desired sulfur–nitrogen
product 1c in 61% yield in the presence of O2 and in 27% bond formation did not occur, which implies that sulfur–
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Synthesis of N-Formyl/Acylsulfenamides and -thiosulfonamides
nitrogen bonds are mediated by copper species [Scheme 3, reaction of diphenyl disulfide affords a sulfenamide, albeit
Equation (5)]. Finally, 2,2,6,6-tetramethylpiperidin-1-yloxyl in low yield. To probe the active copper species, the electron
(TEMPO) was added to the reaction mixture to trap radical paramagnetic resonance (EPR) spectrum of a copper-cata-
intermediates; however, the reaction did not stop and no lyzed reaction mixture of 1a and 1b was taken by cooling
intermediate was trapped by TEMPO. Upon increasing the the mixture in liquid nitrogen; however, no significant sig-
loading of TEMPO, the yield of 1c decreased [Scheme 3, nal corresponding to a CuII species was observed (see the
Equation (6)].
Supporting Information).[14]
Scheme 4. Plausible reaction mechanism for the formation of N-
sulfenylamides.
Conclusions
We reported the efficient copper-catalyzed aerobic cou-
pling of electron-deficient amines (N-formyl/acyl- and sulf-
onylamides) and thiols to afford N-formyl/acylsulfenamides
and -thiosulfonamides in good yields without over-oxid-
ation of sulfur. The various synthesized sulfenamides and
thiosulfonamides can be used as synthetic, pharmaceutical,
and industrial building blocks. The facile delivery of thiol
units from synthesized sulfenamides to biologically impor-
tant glutathione shows that the synthesized compounds
have potential in prodrug applications. A reaction mecha-
nism was proposed on the basis of control experiments and
electron paramagnetic resonance spectroscopy studies.
Experimental Section
General Procedure for the Reaction: Copper(I) iodide (4.8 mg,
0.025 mmol) and TBD (3.5 mg, 0.025 mmol) were added to a solu-
tion of N-methylformamide (1a; 58 μL, 1.0 mmol) and thiophenol
(1b; 51 μL, 0.5 mmol) in toluene (0.5 m, 1 mL). A slow stream of
O2 was passed through this solution for 10 min. Then, the mixture
was stirred at 100 °C for 12 h under an O2 atmosphere. The solvent
was removed under reduced pressure, and the residue was purified
by flash column chromatography (silica gel, 3% ethyl acetate/hex-
Scheme 3. Control experiments.
A plausible catalytic cycle is described in Scheme 4. On
the basis of the results of the control experiments, 1a coor-
dinates to a copper ion to form amide–CuI complex I,
which reacts with the disulfide to afford intermediate II. At
this stage, copper complex II undergoes reductive elimi-
nation to provide 1c and generates PhS–CuI complex ane) to obtain N-formylsulfenamide 1c (74.1 mg, 89%).
III.[11f,13] Under aerobic oxidation conditions, complex III
is converted into a catalytically more reactive CuI complex,
Acknowledgments
whereas diphenyl disulfide is formed from the thiophenol-
ate.[7a] Oxygen plays an important role in the regeneration
of the catalytically active species as well as the oxidative
dimerization of the thiophenols. Without O2, the reaction
This study was supported by the Korea Research Foundation
(grant number 2013008819), the Human Resources Development
of the KETEP grant from the Korea Government (Ministry of
of thiophenol does not provide a sulfenamide; however, the Trade, Industry and Energy), and a Korea CCS R&D Center
Eur. J. Org. Chem. 2015, 5934–5938
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C. Lee, Y. N. Lim, H.-Y. Jang
SHORT COMMUNICATION
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Received: June 29, 2015
Published Online: August 14, 2015
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