NIS-assisted aza-Friedel-Crafts reaction with α-carbamoysulfides as precursors of N-carbamoylimines

Article abstract of DOI:10.1002/chem.201400117

A general and practical N-iodosuccinimide (NIS)-promoted aza-Friedel-Crafts reaction of various aromatic nucleophiles with N-acylimines generated in situ from α-amidosulfides to give a rapid access to highly functionalized amines is described. The newly developed methodology is very mild, fast, efficient, and complementary.

Full text of DOI:10.1002/chem.201400117

DOI: 10.1002/chem.201400117
Communication
&
Organic Synthesis
NIS-Assisted Aza-Friedel–Crafts Reaction with a-Carbamoysulfides
as Precursors of N-Carbamoylimines
Nicolas George,[a] Mathieu Bekkaye,[a] Aurꢀlien Alix,[a] Jieping Zhu,[b] and Gꢀraldine Masson*[a]
an oxocarbenium ion A that can react with various acceptors
Abstract: A general and practical N-iodosuccinimide (NIS)-
to form a glycosidic bond (Scheme 1a).[9] While early methods
promoted aza-Friedel–Crafts reaction of various aromatic
utilized metal salts as promoters,[10] recent work has allowed
nucleophiles with N-acylimines generated in situ from
the development of milder and efficient strategies, using, for
a-amidosulfides to give a rapid access to highly function-
instance, sulfonium (RSÀS+R2)[9,11] and halonium (X+)[9,12] ion
alized amines is described. The newly developed method-
activators. Inspired by these seminal contributions, we envi-
ology is very mild, fast, efficient, and complementary.
The N-acyl and N-carbamoylimines are one of the most impor-
tant reactants for the preparation of nitrogen-containing build-
ing blocks.[1] However, their instability renders extremely labori-
ous the handling of these activated imines. In this context,
a number of researchers have reported the use of stable N-acyl
and N-carbamoylimine precursors as efficient alternative strat-
egy. Among them, a-amidoethers,[1e,2] a-amidosulfones,[3]
a-amidoalkyl benzotriazoles,[4] bisamides,[5] and a-amidosu-
fides[6,7] have been widely used to generate imines (or iminium
Scheme 1. Proposed work hypothesis.
ions) under basic or acidic conditions. Although above-men-
tioned elegant methods are effective, application of these pro-
tocols to sensitive, acid/base-labile or polyfunctional imines is
limited. Therefore, the development of a mild method for the
generation of imines from stable precursors is still required.
Recently, we have developed a novel one-pot three-compo-
nent synthesis of a-aminosulfides (or N,S-acetals). We have
demonstrated that phosphoric acids in catalytic amounts were
able to promote the N-acyliminium formation as well as the
Friedel–Crafts alkylation of 3-susbtituted indoles.[8] However,
while the conditions used are quite mild, the presence of acid
catalyst restricts its application. We therefore decided to ex-
plore the feasibility of a one-pot imine formation/Friedel–
Crafts reaction under acid/base-free conditions.
sioned that a soft electrophilic reagent, such as halonium acti-
vators X+ could be efficient candidates to promote the forma-
tion of N-acyliminium 4 from a-aminosulfides 1 under neutral
conditions (Scheme 1b). Indeed, the electrophilic ion might
react with the sulfur atom of 1 forming an activated halosulfo-
nium ion 2. This reactive complex then could collapse into an
N-acyliminium ion 4, which can proceed with nucleophiles to
form the addition product 6 and a sulfenyl halide compound
3. For this study, arene nucleophile 5 was selected to allow
a novel efficient access to monoalkylated aza-Friedel–Crafts
product 6, which is found in numerous biologically active mol-
ecules. Herein, we report a novel aza-Friedel–Crafts reaction
protocol of a-aminosulfides under mild and neutral conditions,
using N-iodosuccinimide (NIS) in stoichiometric or catalytic
amounts.
Interestingly, thioglycosides (S,O-acetals) are currently one of
the most commonly used glycosyl donors in glycosidation
chemistry. Various promoter systems have been developed for
efficient chemoselective activation of S,O-acetals to generate
To validate our hypothesis, we initially examined the reac-
tion of tert-butyl N-[(ethylthio)phenylmethyl]carbamate (1a)
with 1,3,5-trimethoxybenzene (5a) in the presence of several
N-halosuccinimides as halonium sources in dichloromethane
(Table 1). Results (Table 1, entry 1) showed that the use of NIS
at 08C promoted only a degradation of 1a and partial iodina-
tion of 5a was observed.[12f–h,13] However, to our delight, the
reaction was complete in less than 5 min at À788C and fur-
nished the desired product 6a in 94% yield. Although CH2Cl2
is presently the preferred solvent, the reaction also gives good
results when other solvents are used, such as THF and CHCl3.
In addition, N-chlorosuccinimide (NCS, entry 3) and N-bromo-
[a] Dr. N. George, M. Bekkaye, Dr. A. Alix, Dr. G. Masson
Centre de Recherche de Gif, Institut de Chimie des Substances Naturelles
CNRS, 91198 Gif-sur-Yvette Cedex (France)
Fax: (+33)1-69077247
E-mail: geraldine.masson@cnrs.fr
[b] Prof. Dr. J. Zhu
Institute of Chemical Sciences and Engineering
Ecole Polytechnique Fꢀdꢀrale de Lausanne (EPFL)
EPFL-SB-ISIC-LSPN, 1015 Lausanne (Switzerland)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201400117.
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Table 1. Optimization of the activation/aminoalkylation reaction
sequence.[a]
Table 2. Substrate scope of the activation/aminoalkylation reaction of
a-amidosufides.[a]
Entry
Halonium source
T [8C]
Solvent
Yield [%][b]
[c]
1
2
3
4
5
6
7
NIS
NIS
NIS
NIS
NCS
NBS
–
0
À78
À50
À78
0
CH2Cl2
CH2Cl2
CHCl3
THF
CH2Cl2
CH2Cl2
CH2Cl2
–
94
92
94
94
À35
25
90
trace
[a] Reaction conditions: amidosulfide 1a (0.131 mmol), 1,3,5-trimethoxy-
benzene 5a (0.157 mmol), and the halonium source (0.144 mmol) in 1 mL
of solvent for 5 min. [b] Yields refer to chromatographically pure product.
[c] Iodinated products of 5a were formed. Boc=tert-butoxycarbonyl;
NIS=N-iodosuccinimide; NBS=N-bromosuccinimide; NCS=N-chlorosuc-
cinimide.
succinimide (NBS, entry 4) were also effective promoters at 0
and À358C, respectively, to conduct to 6a with excellent iso-
lated yields. In control reactions, in which N-halosuccinimide
was omitted, a trace amount of product 6a was observed
(<5%), and the starting material was mainly recovered. It is
noteworthy that the iodination of 1,3,5-trimethoxybenzene 5a
as well as the nucleophilic addition of succinimide were not
observed, which demonstrates the high chemoselectivity of
the reaction.
[a] Isolated yields. [b] From 1b (R3 =SPh). [c] Reaction conducted at
À358C with only 0.67 equivalents of indole derivative.
Encouraged by the excellent reactivity of a-amidosulfides as
imine surrogates, we evaluated the scope of the Friedel–Crafts
reaction to various nucleophilic aromatic partners. Under the
optimum reaction conditions, aryls bearing electron-donating
groups, such as 1,3-dimethoxybenzene (6l, 78% yield, Table 2)
and N,N-dimethylaniline (6m, 87% yield) led to the formation
of Fridel–Crafts products in excellent yields. Moreover, a range
of heterocycles could be employed in the reaction system. Var-
ious indoles and pyrazole were examined with 1a, and the re-
sults showed a good tolerance in this system (6n–q), although
C3-substituted indole and pyrazole led only to N-alkylated
products 6r and 6t. Fortunately, the N-protected C3-alkylated
indole afforded the N-alkylated product 6s in good yield. This
reaction was not limited to aromatic nucleophile, acetylace-
tone was also tested, and it turned out that they could react
with 1a to give Mannich product 6u, which demonstrated the
broad scope of this transformation.
Scheme 2. Putative mechanism.
On the basis of the result that only a trace of product was
obtained during the aza-Friedel–Crafts reaction of a-amidosul-
fides in the absence of NIS (Table 1, entry 7), we envisioned
two plausible pathways for the formation of 6 (Scheme 2).
Both are initiated by NIS activation of the a-amidosulfide to
afford cationic amido-iodosulfonium intermediate 2.[9,12] In one
case, aryl might react with alkyl halides via an SN2 mechanism.
Alternatively, irreversible elimination of iodosulfonium ion and
hydrogen abstraction by 7 might form the imine 4 which
could then be activated by iodine and/or ethyl-iodo(alkylthio)-
sulfonium iodide 8 generated in situ from the decomposition
of sulfenyl iodide compound 3. Then, these species could act
as Lewis acids to promote the Friedel–Craft reaction at this low
temperature. In a control experiment, optically active a-amido-
sulfide afforded the Friedel–Craft product in a racemic form
(see the Supporting Information). This experiment supports
the SN1 mechanism. A parallel experiment showed that no re-
action takes place between the preformed imines and 5a at
À788C in the absence of NIS. Finally, the isolation of disulfides
as well as purple coloration of the reaction medium provides
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sults seem to validate the catalytic cycle wherein the process is
initiated by NIS and propagated by ethyl-iodo(alkylthio)sulfoni-
um iodide 8 or I2 (Scheme 3).[9,12j,l]
After having identified the optimal catalytic conditions of
the process, the substrate scope was then investigated. As
shown in Table 4, the process can be applied to aromatic a-
amidosulfides in excellent yields (6a–c, 6e and 6s). We were
Scheme 3. Proposed catalytic mechanism.
Table 4. Substrate scope of the catalytic activation/aminoalkylation reac-
tion of a-amidosufides.[a]
support for the formation of iodine or sulfonium ion 8. As
these species are also highly electrophilic,[9,11] we hypothesized
that they should also be able to promote the imine formation
via reactive intermediates 2 and/or 9 (Scheme 3).[9,12j,l] In this
case, the proton of 2 would be abstracted by sulfenyl iodide
species 3 forming iodine and thiol as final products.
To verify the feasibility of these assumptions, an experiment
with 1a and 1,3,5-trimethoxybenzene (5a) in the presence of
10 mol% of NIS was performed (Table 3). While no reaction oc-
curred at À788C (Table 3, entry 1), the reaction at 08C provid-
ed the desired product 6a in 67% yield together with the sul-
Table 3. Optimization of the catalytic process.[a]
[a] Isolated yields. [b] From 1b (R3 =SPh). [c] 5 mol% of NIS. [d] Reaction
with 3 equivalents of indole derivatives.
Entry Halonium
source
Loading
[mol%]
T
Yield 6a
Yield 10a
[8C] [%][b]
[%][b]
pleased to find that various aliphatic a-amidosulfides were tol-
erated in the reaction. In this case, increases in catalyst loading
were required to obtain full conversion (6h and 6j). Next, the
reactivity of other aromatic nucleophiles 5 was evaluated. To
our satisfaction, indoles underwent nucleophilic addition to 1a
affording products 6n and 6p in good yield. With a less-elec-
tron-rich aryl, the corresponding Friedel–Craft product 6m was
isolated in a low yield even after prolonged reaction times.
This can possibly be attributed to the interruption of the cata-
lytic cycle because of the consumption of iodine, although at
this time the exact reason is unclear.
1
2
3
4
5
6
7
8
NIS
NIS
NIS
NIS
NIS
I2
I2
10
10
5
1
0.1
1
À78
0
–
–
29
7
67
80
90
94
82
84
89
0
0
0
0
0
0
4
trace
6
5
0.1
0.5
I2/PhSSPh[c]
trace
[a] Reaction conditions: a-amidosulfide 1a (0.131 mmol), 1,3,5-
trimethoxybenzene 5a (0.157 mmol), and the halonium source in 1 mL of
CH2Cl2 for 5 min. [b] Yields refer to chromatographically pure product.
[c] Iodine and diphenyl disulfide were premixed together at 08C for
10 min before addition of 1a and 5a.
In summary, we have developed a novel NIS-promoted aza-
Friedel–Crafts-type reaction, allowing a rapid access to highly
functionalized amines from readily available a-amidosulfides. It
is noteworthy that the present reaction proceeded in the ab-
sence of any strongly acidic or basic conditions. In this process,
NIS initiates both the formation of N-carbamoylimine as well as
the aza-Friedel–Crafts reaction. Furthermore, mechanism stud-
ies allow us to develop an efficient catalytic version, employing
only 0.1 mol% of NIS. This simple system offers a powerful
method for the utilization of various unstable N-carbamoyli-
mines. Further investigations into an enantioselective version
of the developed reaction, as well as application to the synthe-
sis of biologically active compounds, are in progress.
foalkylated product 10a (29%). Much to our delight, when
using only 0.1 mol% of NIS, the reaction was completed within
5 min to provide the desired product in 94% isolated yield
(entry 5). We found that molecular iodine was also able to cat-
alyze the reaction, although with slightly lower yields (en-
tries 6–7). In addition, a very similar result was obtained when
the reaction was performed in the presence of a catalytic
amount of iodine and disulfide (entry 8). Moreover, contrary to
our previous work (described above), unpleasant odor of thiol
was observed at the end of the catalyzed reaction. These re-
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Acknowledgements
We wish to thank the ICSN for financial support, ANR for a post-
doctoral fellowship (to A.A.), MESR for doctoral fellowships,
and University Paris-Sud (to M.B.) and ICSN for doctoral fellow-
ships (to N.G.).
Keywords: Friedel–Crafts
reaction
·
imines
·
N-iodosuccinimide
·
multicomponent reactions ·
tandem
reactions
ˇ
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