Iridium(III)-Catalyzed Selective and Mild C-H Amidation of Cyclic N-Sulfonyl Ketimines with Organic Azides

Article abstract of DOI:10.1002/adsc.201700785

A general protocol for iridium catalyzed direct C?H amidation of cyclic N-sulfonyl ketimines using sulfonyl, acyl and aryl azides as nitrogen source is reported herein. The reaction takes place at room temperature with acyl and aryl azides, while an elevated temperature needed with sulfonyl azides to furnish aminated sultams in excellent yields with complete chemo and regioselectivity, thus providing a robust and environmentally benign process to the synthesis of aminosultams. (Figure presented.).

Full text of DOI:10.1002/adsc.201700785

Title: Iridium(III)-Catalyzed Selective and Mild C-H Amidation of Cyclic
<i>N</i>-Sulfonyl Ketimines with Organic Azides
Authors: Manikantha Maraswami, Gang Chen, and Teck-Peng Loh
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201700785
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10.1002/adsc.201700785
Advanced Synthesis & Catalysis
COMMUNICATION
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Iridium(III)-Catalyzed Selective and Mild C-H Amidation of
Cyclic N-Sulfonyl Ketimines with Organic Azides
Manikantha Maraswami,b Gang Chen,b* and Teck-Peng Loha,b,c*
aInstitute of Advanced Synthesis, College of Chemistry and Molecular Engineering, Jiangsu National Synergetic
Innovation Center for Advanced Materials, Nanjing Tech University, Nanjing, Jiangsu, P. R. China, 210009.
b Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological
University, Singapore 637371.
c Hefei National Laboratory for Physical Sciences at the Microscale and Department of Chemistry, University of Science
and Technology of China, Hefei, China, 230026.
Fax: (65) 6515-8229; email: teckpeng@ntu.edu.sg
(65) 6791-1961; email: rnachen@ntu.edu.sg
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))
Abstract. A general protocol for iridium catalyzed direct
C-H amidation of cyclic N-sulfonyl ketimines using
sulfonyl, acyl and aryl azides as nitrogen source is reported
herein. The reaction takes place at room temperature with
acyl and aryl azides, while an elevated temperature needed
with sulfonyl azides to furnish aminated sultams in
excellent yields with complete chemo and regioselectivity,
thus providing a robust and environmentally benign process
to the synthesis of aminosultams.
amine source and widely applied them for an array of
organic compounds, as azides are environmentally
friendly reagents releasing non-toxic nitrogen gas as
the only byproduct.
Being a ubiquitous motif in many pharmaceuticals
and biologically active molecules, cyclic N-sulfonyl
ketimines, are considered as an important class of
sulfur and nitrogen containing heterocyclic
molecules.[7] N-sulfonyl ketimines are valuable
synthons to access a myriad of prevalent nitrogen
containing molecules.[8] Although cyclic N-sulfonyl
ketimines are commonly used building blocks[9] and
chiral auxillaries[10] in many organic transformations,
Keywords: amidation; sultams; azides; C-H activation;
Iridium
In modern synthetic chemistry transition metal
catalyzed amination represents one of the significant
and elemental C-N bond formation reactions.[1] By
facilitating introduction of versatile synthetic amine
fragments onto a great diversity of arenes and
their use as
a
directing group for C-H
functionalization is still in a primitive stage. In
contrast, acyclic N-sulfonyl ketimines are well
explored as directing groups.[11] The limited
exploration of these sultams as directing groups may
be attributed to the lower coordination ability of
nitrogen atom, due to the electron withdrawing effect
of conjugated sulfonyl moiety.
heteroarens,
these
reactions
find immense
applications in medicinal chemistry, agrochemistry,
natural product synthesis and material chemistry.[2]
Thus, it is not surprising that, in the past decades
tremendous efforts have been paid by synthetic
chemists to develop reliable and advanced synthetic
routes to construct C-N bond.[3] Among them, cross-
coupling reactions developed by Buchwald-Hartwig
and Ullmann-Goldberg using palladium and copper
respectively, have been studied extensively for
efficient construction of C-N bonds.[4] Despite the
efficiency and versatility, to react with amine source,
these coupling reactions require pre-functionalized
species such as aryl halides or pseudohalides. More
recently, Chang’s[5] and other groups[6] have
developed amination methods using organic azides as
Though Li et al.,[12] reported the synthesis of
indenamines from N-tosyl aldimines and internal
alkynes by C-H activation using Ru catalyst, the first
C-H functionalization of cyclic N-sulfonyl ketimines
to form spirocyclic sultams with 1,3 dienes and
alkynes was reported by Nishimura et al.,[13] and
Dong et al.,[14] using Ir and Rh catalyst respectively.
Later Wei group[15] demonstrated olefination,
arylation, allylation and vinylation of sultam
derivatives utilizing Rh catalysts and cyclic N-
sulfonyl ketimines as effective directing groups. In
1
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10.1002/adsc.201700785
Advanced Synthesis & Catalysis
early 2016, Cramer[16] reported the synthesis of chiral
spirocyclic sultams using Rh complexes. These
approaches are generally limited to C-C bond
formation and only one example features C-N bond
construction.[17] Our continuous research interest in C-
N bond forming reactions,[18] prompted us to
investigate cyclic N-sulfonyl ketimine assisted
amidation of C-H bonds. Described herein is a
Cp*Ir(III)-catalyzed mild and effective diect C-H
amidation of sultam derivatives using organic azides,
which furnishes monoamidation products in a highly
chemo and regioselective manner(Scheme 1).
aReaction conditions: To a mixture of catalyst (4 mol %), additive (16
mol%) were added 1a (0.3 mmol), 2a (0.36 mmol), solvent (0.5mL) and
allowed to stir for particular time at indicated temperature under air
atmosphere. bisolated yield.
Scheme 1. Ir(III)-catalyzed directed C-H amidation
In preliminary experiments, readily accessible sultam
derivative 1a, was selected as the model substrate.
Initial reaction of 1a with p-toluenesulfonyl azide 2a,
in the presence of [Cp*IrCl2]2 and AgNTf2 in DCE at
room temperature resulted in no conversion. To our
With the established conditions in hand, we next
tested the scope of the reaction with respect to various
sulfonyl azides using 1a as the model substrate.
Sulfonyl azides bearing both electron donating and
electron withdrawing groups at para- position gave
mono sulfamidated products in excellent yields except
2f with p-cyano group.
o
surprise, when the reaction was heated at 60 C,
complete conversion was observed within 4 hours and
product 3aa was isolated in 80% yield, without the
aid of any external oxidants (Table 1, entry 2). Other
comparative catalysts RhIII, CoIII were ineffective for
this conversion, while RuII showed some activity, but
in inferior effectiveness. Test reactions justified vital
roles of the Iridium catalyst and the additive. The
effects of reaction temperature and additives were
also examined (see Supporting Information for
details). It was found that decreasing the temperature
resulted in lower conversion, while elevated
temperatures furnished monoamidated sultam 3aa as
sole product in quantitative yield in shorter time
(Table 1, entry 8). AgNTf2 provided optimal results
among a representative set of silver additives. With
this exciting results in hand, a systematic screening of
solvents was performed. The reaction displayed a
prominent solvent dependence characteristic. While
THF, 1,4-dioxane, DCM, t-AmOH, MeOH and
acetone afforded secondary results compared to DCE.
The solvents possessing coordinating ability, such as
DMF and MeCN, completely retarded the reaction.
To our delight, when reaction was performed in
CF3CH2OH, product 3aa was isolated in excellent
yield (Table 1, entry 15). Related imidation
reaction[19] was also tried in vain (See Supporting
Information).
Table 2. Substrate Scope for Sulfonyl Azides with 1a
under Optimized Conditiona
Under
the
optimized
condition,
3-
methylbenzenesulfonyl azide furnished product 3ah
in 97% yield. Arylsulfonyl azide having bromo group
at ortho position could readily participate in this
sulfamidation reaction, providing 3ai in moderate
yield. To our surprise, reaction with sulfonyl azide
bearing trifluoromethyl groups at ortho- and meta-
position, yielded 3aj in 94% yield, while the substrate
with the same substituent at 3- and 5- position
provided 3ak in only 77% yield. In addition,
amidation reaction of 1a with 2-naphthalenesulfonyl
azide occurred efficiently to furnish corresponding
Table 1. Optimization of Reaction Conditions for Iridium
Catalyzed Amidation of Cyclic N-sulfonyl Ketimines.a
2
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10.1002/adsc.201700785
Advanced Synthesis & Catalysis
aminosultam 3al in high yield. Moreover, azide derived from naphthalene occurred efficiently to
alkanesulfonyl azides were also compatible under the furnish 6al in high yield. Moreover, the acyl azides of
standard amidation condition.
cyclobutane and 3-thiazole displayed only moderate
We then set out to expand the scope of this efficiencies in present catalytic system. Even aryl
transformation to substituted cyclic N-sulfonyl azide, derived from 3,5-bis(trifluoromethyl)benzene
ketimines with respect to sulfonyl azide 2a. We were furnished the corresponding aminated product 6ao
pleased to observe that the amidation occurred albeit in moderate yield.
smoothly irrespective of the position and electronic
nature of substituents, thus giving rise to the Table 4. Scope of acyl azidesa
corresponding C-6 sulfamidated sultams in high
efficiency and regioselectivity. Indeed, substrates
bearing methyl (1b and 1j), alkoxy (1d, 1e, 1i and
1k), trifluoromethyl (1c), phenyl (1g), chloro (1f) and
fluoro (1h) groups at the C2-, 3-, 4-and 5- position of
cyclic N-sulfonyl ketimines were all well suited for
the present sulfamidation. Sultam bearing naphthyl
group 1l, also reacted effectively to produce 4al in
excellent yield. Furthermore ketimine derived from
substituted saccharin 1m was also suitable substrate
for C-H amidation reaction, furnishing 4am in 93%
yield.
To probe the generality of cyclic N-sulfonyl ketimines
towards amidation, a set of derivatized sultams 1 were
tested. The para- and ortho-substituted sultams were
Table 3. Substrate Scope of Ir-Catalyzed C-H Amidation
of Cyclic N-sulfonyl Ketiminesa
selectively transformed into corresponding amide
products 7ai-hi. The catalytic system exhibited
phenomenal chemo selectivity and hence tolerated a
plethora of valuable functional groups such as Cl, F,
OMe, OPh, Ph and CF3. Even under steric control,
meta- substituted sultams furnished 7ii-ki in moderate
yield with an excellent site selectivity. Under the
optimized conditions, sultam with naphthyl group,
scaccharin derived ketimine and cyclic ketimine
reacted exceptionally well to yield respective
aminosultams 7li, 7mi and 7ni.
Table 5. Scope of sultams with acyl azide 5ia
Next we turned our attention to exploring the
compatibility of acyl azides for our reaction. Under
the optimized amidation conditions for sulfonyl
azides, acyl azide 5a was unreactive. Pleasingly, the
reactivity of acyl azide was dramatically enhanced
when an oxidant AgOAc was added to the reaction
performed in DCE at room temperature to give
amidated compound 6aa in 89% yield after 24 h.
Only DCE and AgOAc furnished optimal results
among a representative set of solvents and oxidants
screened (see the Supporting Information). With this
optimized iridium catalytic system, we tested scope of
acyl azides with cyclic N-sulfonyl ketimines.
Finally, to acquire some insights into mechanistic
pathway of our current amidation reaction,
intermolecular competition and parallel experiments
between 1a and its penta-deuterated analog 1a-d5 with
tosyl azide 2a (eq. 1 and 2) as well as acyl azide 5i
An array of para-, meta- and ortho-substituted azides
5 bearing electron-withdrawing or electron-donating
groups were successfully converted into desired
amide products 6. In addition, reaction of 1a with acyl
3
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10.1002/adsc.201700785
Advanced Synthesis & Catalysis
(eq. 3 and 4) were carried out separately. The kinetic
isotope effect values (KIE) of 2.7 and 3.2 were
observed, after the reactions were stirred for 3 min
and 60 min, respectively. Notable KIE values for
C(sp2)-H amidation suggest that C-H bond cleavage is
plausibly involved in the rate determining step.[20]
Inter
Scheme 2. Proposed Plausible Mechanism for the Iridium-
Catalyzed Amidation of Sultams.
atom on sultams. Then the interaction of azide with
iridium metal centre of II generates III, which forms
IV upon migratory insertion of an imido group into
Iridium-Carbon bond with simultaneous expulsion of
N2. The complex IV, after protodemetallation
furnishes amidated product with the regeneration of
active catalyst.
In conclusion, we have described an iridium-
catalyzed mild and regioselective amidation protocol
for cyclic N-sulfonyl ketimines with a range of
organic azides to furnish the amidated products in
moderate to excellent yields, which offers an efficient
route to the synthesis of biologically relevant
aminosultam compounds.
Experimental Section
To a screw capped vial with stir bar were added cyclic N-
sulfonyl ketimine (1a, 0.12 mmol), sulfonyl azide (2, 0.14
mmol), [IrCp*Cl2]2 (3.9 mg, 0.004 mmol, 4 mol %),
AgNTf2 (7.6 mg, 0.019 mmol, 16 mol %) and 2, 2, 2-
trifluoroethanol (0.5 ml) under atmospheric conditions. The
molecular competition experiments with various
substituted sultams 1 (eq. 5) and acyl azides 5 (eq. 6),
unveiled electron rich sultams and acyl azides to be
intrinsically reactive substrates. The relatively faster
o
reaction mixture was stirred at 80 C for 2 h, filtered
through
a
pad of celite and then washed with
dichloromethane (10 mL x 3). Organic solvents were
rate of acyl azide 5e may be due to more electron removed under reduced pressure and the residue was
purified by chromatography on silica gel (n-hexane/EtOAc)
to give the desired product 3.
richness of 5e compared to 5f. Based on these studies
21]
and previous results,[5,
a plausible pathway for
amidation reaction is proposed as shown in scheme 2.
Initially, the dimeric iridium species forms a cationic
species I with AgNTf2, which in turn initiates the
directed C6-H bond activation of cyclic N-sulfonyl
ketimines via the formation of five membered
cyclometallated iridium complex by chelation
assistance of iminyl nitrogen
Acknowledgements
We are grateful to Nanjing Tech University, Nanyang
Technological University, Singapore Ministry of Education
Academic Research Fund [Tier 1: MOE2015-T1-001-070
(RG5/15), MOE2014-T1-001-102 (RG9/14) (to T.P.L.) and
MOE2015-T1-001-098 (RG42/15) (to G.C.)], and Singapore
National Research Foundation [(NRF2015NRF-POC001-024) (to
T.P.L.)] for generous financial support.
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10.1002/adsc.201700785
Advanced Synthesis & Catalysis
COMMUNICATION
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