Pd/Norbornene Collaborative Catalysis on the Divergent Preparation of Heterocyclic Sulfoximine Frameworks

Article abstract of DOI:10.1021/acs.orglett.8b00776

Pd/Norbornene cocatalyzed tandem C-H activation/annulation reactions of free NH-sulfoximines with aryl iodides to produce diverse polyheterocyclic sulfoximines in highly chemoselective models are reported. The reaction tolerated a broad range of functiona

Full text of DOI:10.1021/acs.orglett.8b00776

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Pd/Norbornene Collaborative Catalysis on the Divergent Preparation
of Heterocyclic Sulfoximine Frameworks
Hao Zhou, Weihao Chen, and Zhiyuan Chen*
Key Laboratory of Functional Small Organic Molecules, Ministry of Education, and College of Chemistry & Chemical Engineering,
Jiangxi Normal University, 99 Ziyang Road, Nanchang, Jiangxi 330022, P. R. China
S
* Supporting Information
ABSTRACT: Pd/Norbornene cocatalyzed tandem C−H
activation/annulation reactions of free NH-sulfoximines with
aryl iodides to produce diverse polyheterocyclic sulfoximines
in highly chemoselective models are reported. The reaction
tolerated a broad range of functional groups under external
oxidant-free conditions. The preliminary mechanistic studies using density functional theory (DFT) calculations highlighted the
key role of a PdIV intermediate.
ulfoximidoyl-based molecules have important biological
properties; therefore, they are widely applied in practical
Scheme 1. Strategies To Prepare the Polyheterocyclic
Sulfoximines
S
pharmaceutical industries, including clinical drugs such as
Suloxifen from Godecke GmbH,1 Roniciclib (BAY 1000394)
̈
from Bayer,2 AZD-6738 from AstraZeneca,3 and the insecticide
Sulfoxaflor from Dow Chemical Corporation.4 In organic
synthesis, sulfoximine derivatives have been used as key
intermediates to prepare biologically active molecules,5 in
directed C−H activations,6 and asymmetric catalysis.7
As a result of intensive efforts in the past decade, novel
approaches that employ free NH-sulfoximines as readily
available starting materials have been developed for the
preparation of acyclic sulfoximine derivatives.8 However,
despite the achievements, procedures for the construction of
fused heterocyclic sulfoximine frameworks are limited. Several
studies have focused on the synthesis of bicyclic sulfoximine
derivatives, such as benzothiazines and benzoisothiazines.9 Our
group developed a microwave assisted Cp*CoIII-catalyzed C−
H activation/double C−N bond formation reaction of free NH-
sulfoximines with 1,4,2-dioxazol-5-ones to produce bicyclic
thiadiazine-1-oxides.10 Based on this result and in connection
with our interest in the construction of polycyclic aromatic
molecules,11 we conceived that the fused polycyclic sulfox-
imines could be prepared in similar C−H activation/C−N
bond formation strategies.
A limited number of methods are available for the synthesis
of polycyclic sulfoximine derivatives (Scheme 1). For instance,
in 2015, Jeganmohan et al. developed the generation of tricyclic
dibenzothiazines by Ru-catalyzed ortho arylation of NH-
sulfoximines with aryl boronic acids, followed by Pd-catalyzed
intramolecular cyclization sequences in two consecutive steps
(eq 1).12 Later, Bolm and co-workers disclosed the formation
of functionalized dibenzothiazines starting from N-acylsulfox-
imines through RhIII-catalyzed ortho halogenation, Pd-catalyzed
Suzuki coupling reaction, and a subsequent Pd(OAc)2-PhI-
(OAc)2-mediated oxidative annulation cascade (eq 2).13
Enlightened by the results, we herein present a facile
construction of polycyclic sulfoximine derivatives in a one-pot
and one-step annulation reaction, which employs the readily
available free NH-sulfoximines and aryl iodides as the starting
materials, and Pd(OAc)2/norbornene (NBE) as catalysts, to
rapidly deliver divergent dibenzothiazines or eight-membered
fused heterocyclic sulfoximines (eq 3).
We recently reported Pd/NBE-mediated Catellani-type C−
H amination reactions of aryl iodides with O-benzoyl
hydroxylamines to produce biaryl tertiary amines in a
chemoselective-controlled fashion.14 The Catellani reaction is
an effective sequential protocol to construct structurally
divergent polycyclic aromatic scaffolds, and the reaction has
been optimized through seminal contributions by the groups of
Catellani15 and Lautens16 among others.17 Conventionally, the
Catellani reaction employs two or more distinct reactive
molecules as substrates, such as aryl iodides with alkyl halides
or pseudohalides.18 Nevertheless, the combination of two aryl
halides as substrates has been studied,19 which indicate that the
Received: March 8, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b00776
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
inherent competitive chemical selectivity problem could be
suppressed when taking the reaction parameters such as
substrates, catalyst, ligand, etc. into consideration.
The proposed strategy for the preparation of fused tricyclic
dibenzothiazines is depicted in Figure 1. We expected that the
optimize the reaction. After an extensive study of halogenated
sulfoximines, along with metal catalyst, ligands, additives, and
solvents, we were pleased to find that 2-bromo-NH-sulfoximine
1a, which is easily available from 2-bromophenyl methyl
sulfide,23 was an optimal substrate for the desired trans-
formation (Table S1). Indeed, in a highly chemoselective
conversion in DMF at 105 °C, the expected tricyclic product
3aa was isolated and identified, albeit in relatively low yield
(entry 1). The other 2-bromo substituted substrates, such as 1-
bromo-2-(ethylsulfonimidoyl)benzene (1b) and 1-bromo-2-
(phenylsulfonimidoyl)benzene (1c), gave less than desirable
results under the reaction conditions, whereas a complex
mixture was observed when 1-(methylsulfonimidoyl)-2-iodo-
benzene was used as a reaction partner.24
Once the optimal reaction conditions were established, we
defined the scope and limitation of this Pd/norbornene
cocatalyzed tandem annulation reaction (Scheme 2). The
tandem cyclization reaction of 2-bromo-NH-sulfoximine 1a
with various aryl iodides 2a−2o proceeded smoothly to afford
products 3aa−3ao in yields ranging from 45% to 95%.
Serviceable yields of 3ab, 3ac, and 3ae were obtained when
substrate 2 was attached with electron-donating groups, such as
MeO− or alkyl groups. A low yield of 3ad was isolated for the
reaction of 1a with 1-iodo-2-isopropylbenzene 2d, which
should be attributed to the steric hindrance of the starting
material. The reaction of 2-bromo-NH-sulfoximine 1a with 1-
iodonaphthalene 2f was successful, affording the tetracyclic
framework 3af in 71% yield. Good to excellent yields were
obtained when aryl iodides 2 were attached with electron-
withdrawing substituents, as the Cl- and CF3-substituted
tricyclic dibenzothiazines 3ag and 3ah were obtained in 90%
and 78% yields, respectively. The production results of the
methodology was not compromised when reactive electron-
withdrawing substituents, such as COOMe or NO2 groups,
were attached, as the corresponding products 3am, 3an, and
3ao were all delivered in up to 95% yields. These results
highlighted the possibility that further modulation of these
tricyclic heterocycles could be achieved by utilizing the alkyl or
reactive substitutions as synthetic handles.
Figure 1. Proposed reaction design to prepare tricyclic dibenzothia-
For variation on the S-linkage of the NH-sulfoximines
(Scheme 2, below), the reaction of ethyl substituted 2-bromo-
NH-sulfoximine substrate 1b with 2a and 2f produced the
desired product 3ba and 3bf without difficulty in acceptable
yields. The tandem cyclization reactions of 1c with various
electron-rich (2a) or electron-poor (2g and 2m) aryl iodides all
reacted smoothly, resulting in the corresponding dibenzothia-
zines 3ca, 3cg, and 3cm in 67%−79% yields. A tetracyclic
heterocycle 3cf could be chemoselectively isolated from the
reaction of 1c with 1-iodonaphthalene.
Interestingly, for the aryl iodides 2 bearing an electron-
withdrawing substituent located at the meta-position of the
phenyl ring, eight-membered bridged heterocycles 4 can be
generated (Scheme 3). For instance, the reaction of NH-
sulfoximine 1a with methyl 3-iodobenzoate under the standard
conditions delivered product 4a in 86% yield. The structure of
compound 4a was verified unambiguously through X-ray
crystallography analysis (CCDC 1819922). Besides the ester
group, many synthetically useful substitutions, including nitro
(4b), cyano (4c), and trifluoromethyl (4d) groups, were all well
tolerated, resulting in corresponding polycyclic sulfoximine
derivatives 4 in up to 94% yield. Unfortunately, for the meta-
MeO substituted electron-rich aryl iodide, the reaction became
complex, and the desired product 4e was not detected, which
zines and possible byproducts.
oxidative addition (O.A.) of PdII intermediate B with a suitable
halogenated free NH-sulfoximine 1 would selectively generate
the PdIV intermediate C,20 which undergoes reductive
elimination (R.E.)/expulsion of norbornene/Buchwald−Hart-
wig C−N bond formation cascades to give the desired tricyclic
product 3. Nonetheless, we encountered several challenges in
this catalytic cycle: (1) Pd-catalyzed direct cross-coupling of
NH-sulfoximine 1 with aryl iodide or bromide 2 to give N-
arylated product 3_I is already known.21 (2) The oxidative
addition of aryl iodide 2 with intermediate B to form
intermediate C′ should occur more readily than the formation
of aryl bromide analogue 1. (3) Direct reductive elimination of
B is possible to give undesired compound 3_II,22 and (4) the
intramolecular Buchwald−Hartwig cross-couplings may occur
to give biaryl sideproducts 3_IV, 3_V, and 3_VI, respectively
(Figure 1).
Motivated by the above-mentioned challenges, and encour-
aged by the potential usefulness of the cyclic sulfoximine
derivatives in biological evaluations, we aimed toward exploring
the possibility of this reaction. Initially, 2-iodotoluene (2a) was
employed as a substrate, and PPh3 was selected as a ligand to
B
DOI: 10.1021/acs.orglett.8b00776
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Organic Letters
Letter
a
Scheme 2. Substrate Scope Studies
Scheme 3. Synthesis of Medium-Sized Sulfoximine
Heterocycles
a
a
Reaction conditions: NH-sulfoximine 1a (0.30 mmol), aryl iodide 2
(0.36 mmol) in 3.0 mL of CH3CN at 105 °C for 24 h.
Scheme 4. Synthesis of Medium-Sized Heterocycles Bearing
a
Two Sulfoximine Moieties
a
Reaction conditions: Table S1, entry 17, NH-sulfoximine 1 (0.30
mmol), arene iodide 2 (0.36 mmol) in 3.0 mL of CH3CN at 85−105
a
Reaction conditions: NH-sulfoximine 1 (0.72 mmol, 2.4 equiv), aryl
iodide 2 (0.30 mmol, 1.0 equiv) in 3.0 mL of N,N-dimethylacetamide
b
c
°C for 12 h. 1.0 mmol of 1a scale reaction. Temp = 105 °C, time =
b
c
at 105 °C for 18 h. 1-Chloro-3-iodobenzene as the substrate. 1-Iodo-
24 h.
3-methylbenzene as the substrate.
indicated the electronic effect should play a crucial role in the
tandem C−H activation/cyclization reaction.
constitutional isomers 5c was formed in 54% yield when 1a
reacted with 1-iodo-3-methylbenzene 2s.
Recently, organic molecules bearing a 3D-type heterocyclic
framework have attracted notable attention. Heterocycles
containing sp3-carbon atoms possess explicit spatial config-
urations, which enable powerful creative applications in porous
materials, chemical probes, medicinal drugs, and chiral
monomers.25 When increasing the amount of 2-bromo-NH-
sulfoximine 1a to 2.4 equiv, a fused medium-sized polyhetero-
cycle 5, which bears two sulfoximine moieties, could be isolated
under slightly modified conditions (Scheme 4). The Pd/NBE
cocatalyzed direct cyclization of 1a with iodobenzene 2q
reacted smoothly to afford compound 5a in 64% yield. The
structure of compound 5a was established via 2D NMR.
Likewise, the reaction of 1a with 1-chloro-3-iodobenzene 2r
afforded product 5b in 42% yield. An inseparable mixture of
Based upon these studies and the preliminary DFT
calculations on the bond formation Gibbs free energy,23 we
proposed a possible mechanism for the formation of product
5a. As shown in Scheme 5, the reductive elimination of PdIV-
intermediate C3 (C refer to Figure 1) first occurred to give a
biaryl species H, which was followed by a C−H activation
reaction under the assistance of a base to give the pentapallada-
cycle I. Oxidative addition reaction of I with ortho-bromo-NH-
sulfoximine 1a to give another PdIV-intermediate J occurred,
followed by the intramolecular nucleophilic substitution to
release HBr and two reductive elimination reactions to deliver
the final product 5a, along with the regeneration of Pd0 to
complete the catalytic cycle.
C
DOI: 10.1021/acs.orglett.8b00776
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Organic Letters
Letter
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ASSOCIATED CONTENT
* Supporting Information
■
S
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The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b00776.
1
Experimental details, characterizations, and copies of H
and 13C NMR spectra for new compounds (PDF)
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1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
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Corresponding Author
*E-mail: zchen@jxnu.edu.cn.
ORCID
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Zhiyuan Chen: 0000-0002-5159-7287
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the National Natural Science Foundation of China
(21462022, 21672085) and the Talented Youth Project of
Jiangxi Province (20171BCB23038) for financial support.
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