Synthesis of Isothiazoles and Isoselenazoles through Rhodium-Catalyzed Oxidative Annulation with Elemental Sulfur and Selenium

Article abstract of DOI:10.1021/acs.orglett.0c03674

A rhodium-catalyzed oxidative annulation of benzimidates with elemental sulfur for the direct construction of isothiazole rings is reported. The proposed reaction mechanism involving Rh(I)/Rh(III) redox is supported by a stoichiometric reaction of metallacycle species as well as DFT calculations. This method is also applicable to selenium cyclization to produce isoselenazole derivatives. The alkoxy substituent at C3 can be used for further functionalization of the azole core.

Full text of DOI:10.1021/acs.orglett.0c03674

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Letter
Synthesis of Isothiazoles and Isoselenazoles through Rhodium-
Catalyzed Oxidative Annulation with Elemental Sulfur and Selenium
Sanghun Moon, Yuji Nishii,* and Masahiro Miura*
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ABSTRACT: A rhodium-catalyzed oxidative annulation of benzi-
midates with elemental sulfur for the direct construction of
isothiazole rings is reported. The proposed reaction mechanism
involving Rh(I)/Rh(III) redox is supported by a stoichiometric
reaction of metallacycle species as well as DFT calculations. This
method is also applicable to selenium cyclization to produce isoselenazole derivatives. The alkoxy substituent at C3 can be used for
further functionalization of the azole core.
eteroaromatic compounds containing sulfur or selenium
atoms constitute the core structures of many functional
Scheme 1. Representative Synthetic Methods for
Isothiazoles
H
organic materials and biologically active compounds.1 In
particular, benzisothiazole derivatives have been of vital use
in a series of antipsychotic medicines such as tiospirone,
ziprasidone, perospirone, and lurasidone.2 Additionally, oxi-
dized and alkylated analogues of benzisothiazoles occupy an
important position for pharmaceutical and agrochemical
applications (Figure 1). Ebselen (2-phenyl-1,2-benzoselena-
Figure 1. Representative examples of benzisothiazole (benzisosele-
nazole) derivatives in bioactive compounds.
In contrast, the utility of S8 in direct C−H functionalization8
has rarely been explored despite the fact that atom transfer of
S, Se, and Te from elemental chalcogenides into the metal−
carbon bond of Ni(II) metallacycle species has been known for
more than 20 years.9 In 2014, Shi reported a Cu-mediated
benzoisothiazolone synthesis using the (pyridin-2-yl)-
isopropylamine (PIP-amine) bidentate directing group.10
Recently, Gong and Song developed a catalytic variant of
this using Ni with the aid of the 2-amino alkylbenzimidazole
zol-3-one) is a well-known oxidized selenium variant with
multiple medical uses.3 It is notable that this molecule exhibits
promising inhibitory activity against COVID-19 protease.4
Conventional synthetic methods for benzisothiazoles involve
the cyclization of ortho-disubstituted benzene derivatives,1,5
which would be prepared via cumbersome multistep processes
(Scheme 1a). The structural diversity in the cyclized products
is thus considerably limited by the availability of these
precursors, and a similar issue should be addressed for the
synthesis of selenium analogues. Moreover, the handling
difficulty of organosulfur and organoselenium compounds is
usually problematic in practical use. Elemental sulfur (also
known as S8) is one of the most important raw materials for
modern chemical production because of its low cost and user-
friendly nature (stable, nonvolatile, nonhygroscopic, nontoxic,
and nonodorous). A number of C−S bond-forming reactions
have been established exploiting this as the sulfur source.6,7
Received: November 4, 2020
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© XXXX American Chemical Society
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directing group.11 As a related example of the selenium
annulation, Nishihara achieved a Ni-catalyzed synthesis of
benzoisoselenazolones adopting an 8-quinolyl auxiliary.12 We
previously achieved the synthesis of benzo[b]thiophenes
through a Rh-catalyzed three-component coupling reaction
of arylboronic acids, alkynes, and S8 (Scheme 1b).13,14 To the
best of our knowledge, only these four reports successfully
utilized elemental chalcogenides with the C−H activation
strategy. As part of our continuous research interest in this
field, we envisioned utilizing this sulfur annulation protocol for
the concise assembly of isothiazole derivatives, and herein we
report a direct oxidative annulation of imidates with elemental
sulfur and selenium (Scheme 1c).
Scheme 2. Scaled-Up Reaction
a
Scheme 3. Substrate Scope
Our initial study was conducted to optimize the reaction
conditions using ethyl benzimidate (1a) as a representative
substrate (Table 1). To our delight, the desired product 2a was
a
Table 1. Optimization Study
b
entry
deviation from the standard conditions
yield
81%
n.d.
51%
n.d.
1
2
3
4
−
without catalyst
[Cp*RhCl2]2 (2.0 mol %) as the catalyst
Cp*Co(CO)I2 (4.0 mol %) as the catalyst with
AgSbF6 (10 mol %)
5
6
7
8
9
[Ru(p-cymene)Cl2]2 (8.0 mol %) as the catalyst
Cu(OAc)2 (1.0 equiv) instead of AgOAc
S (2.0 equiv), average of three runs
PhCF3 as the solvent, S (2.0 equiv)
DMF as the solvent, S (2.0 equiv)
trace
7%
81%
94% (94%)
trace
a
Standard conditions: 1a (0.2 mmol), S8 (0.8 mmol as S), AgOAc
(0.4 mmol), and [Cp*Rh(MeCN)3][SbF6]2 (4.0 mol %) in solvent
a
b
Reaction conditions: 1 (0.2 mmol), S8 (0.6 mmol as S),
[Cp*Rh(MeCN)3](SbF6)2 (4.0 mol %), and Ag2O (0.4 mmol) in
PhCF3 (0.3 mL).
(DMF 0.1 mL + PhCF3 0.2 mL). Determined by NMR analysis.
Isolated yield is in parentheses. n.d. = not detected.
obtained in 81% yield under the standard conditions using
sulfur powder (4.0 equiv as S), [Cp*Rh(MeCN)3](SbF6)2 as
the catalyst (Cp* = pentamethylcyclopentadienyl), and AgOAc
as the oxidant in DMF/PhCF3 as the solvent (entry 1). No
competing polysulfide formation was observed. The reaction
did not proceed without the catalyst (entry 2). A chloride
complex [Cp*RhCl2]2 was moderately productive (entry 3),
whereas an analogous cobalt complex was totally inactive even
in the presence of AgSbF6 as an anion source (entry 4). The
ruthenium complex [Ru(p-cymene)Cl2]2 was also not an active
catalyst (entry 5). The product yield significantly dropped
when Cu(OAc)2 was used as the oxidant instead of AgOAc
(entry 6). The amount of sulfur could be reduced to 2.0 equiv
(entry 7), but the yield varied somewhat (66−97%). Thus, 3.0
equiv of sulfur was preferred to be used for the sake of better
reproducibility (see Schemes 2 and 3). After further
optimization, the target product was obtained in 94% isolated
yield employing PhCF3 as the solvent (entry 8), whereas DMF
was not a suitable solvent (entry 9). This reaction could be
conducted on a 2.0 mmol scale to give 2a in 70% isolated yield
(Scheme 2).
protocol was applicable to a series of benzimidates bearing a
substituent at the para (1b−1g), meta (1h, 1i), or ortho (1j)
position, producing the corresponding benzisothiazoles in
moderate to high yields. For the reaction of meta-substituted
substrates, the C−H bond at a sterically more accessible site
was preferentially activated to give C5-functionalized products.
It is notable that the bromo group (1g, 1i, 1j), which is hardly
tolerated in conventional synthetic methods, remained intact
under the present conditions. This is highly beneficial for
postfunctionalization of the benzisothiazole scaffold. 3,4-
Dimethoxyimidate 1k afforded a mixture of two isomers (2k
+ 2k′), whereas naphthyl imidates 1l and 1m gave 2l and 2m,
respectively, as single isomers. The present method was
suitable for the reaction of thienyl imidates 1n and 1o to give
the corresponding products in moderate yields. Additionally,
we examined the effect of substituents at the imine carbon.
Benzimidates with methyl (1p, 1q), isopropyl (1r), and n-octyl
(1s) groups at the oxygen atom were also applicable to the
reaction, whereas benzophenone imine (1t) was less
productive because of the competing hydrolysis. Unfortu-
nately, benzamidine derivatives were not applicable to the
present reaction system (not shown).
Under the optimized reaction conditions, the scope of
imidate derivatives was then evaluated (Scheme 3). This
B
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A proposed mechanism of the oxidative annulation is shown
in Scheme 4. A catalytically active Rh(III) species, assumed to
Scheme 4. Proposed Reaction Mechanism
Figure 2. Partial Gibbs free energy profile of the reaction. Values in
parentheses are relative enthalpies calculated at the ωB97X-D/6-
311+G(d,p)&SDD/PCM(CH2Cl2)//ωB97X-D/6-31G+(d,p)
&LANL2DZ level.
ligand is then replaced by an S8 molecule to give intD.
Subsequently, a sulfur atom directly bound to the metal inserts
into the Rh−C linkage with a free energy (enthalpy) barrier of
26.4 (27.2) kcal/mol (intD → intE). It is noteworthy that the
coordinated sulfur atom within intD carries a positive APT
charge of +0.427, thereby facilitating the nucleophilic attack of
the arylrhodium species onto it.19 A negative charge of −1.096
at the “leaving” sulfur atom within tsDE also indicates that this
process can be seen as an SN2-type reaction (Figure 3).
be Cp*Rh(OAc)(SbF6), undergoes coordination-assisted C−
H activation to afford five-membered metallacycle A. This step
would not be rate-limiting because no obvious KIE was
detected (see the Supporting Information). Insertion of a
sulfur atom into the Rh−C bond of the intermediate is effected
to form cationic six-membered complex B. The coupling
product would be liberated through N−S reductive elimination
and deprotonation, giving a Cp*Rh(I) complex (path A) or a
Cp*Rh(III) sulfide species (path B). The active Rh(III)
species is then regenerated through oxidation (path A) or
anion exchange (path B) with AgOAc, closing the catalytic
cycle.
Although the mechanistic details for the sulfur atom
migration into the metallacycle species has not been
established, this process would be experimentally supported
by a stoichiometric reaction (Scheme 5). We used a literature
Scheme 5. Preparation and Stoichiometric Reaction of
Rhodacycle Complex 4 with Elemental Sulfur
Figure 3. (a) Optimized molecular geometry of the transition state
tsDE with APT charges. Atom colors: carbon (gray), hydrogen
(white), oxygen (red), nitrogen (bulue), sulfur (yellow), and rhodium
(green). (b) Atom labeling with selected bond lengths. The Cp*
ligand has been omitted for the sake of clarity.
Unfortunately, all attempts to optimize the transition state
structure for the N−S reductive elimination were unsuccessful
because of the flexible conformational change of the sulfur
chain.
procedure15 to prepare Rh complex 4, which is structurally
relevant to the proposed reaction intermediate, and treated it
with elemental sulfur in PhCF3 as the solvent. As expected, 2p
was obtained in 97% yield. This result suggested that the silver
salt did not participate in the sulfur insertion and reductive
elimination steps, ruling out the occurrence of higher-valent
Rh(IV) or Rh(V) species within the reaction mechanism.15,16
To gain further insight into the mechanism, we conducted a
computational study adopting 1p as a model compound.17
Here the composition of elemental sulfur was assumed to be a
cyclic S8 molecule,18 and dichloromethane (DCM) was
employed as the solvent because its electric permittivity is
similar to that of PhCF3. Figure 2 displays an energy profile
relative to a cationic [Cp*Rh(OAc)(MeCN)]+ complex
(intA). After the coordination of 1p, the C−H activation
step was calculated as a concerted metalation−deprotonation
(CMD) process to output an activation free energy (enthalpy)
barrier of 19.6 (18.3) kcal/mol (intB → intC). The AcOH
To our delight, the present method was applicable to the
synthesis of isoselenazoles using elemental selenium (Scheme
6). Upon treatment with 3.0 equiv of selenium powder under
the standard conditions, imidate 1a was successfully converted
to the desired product 3a in 75% yield. Para-substituted
imidates (1c, 1g) also reacted smoothly. The selenium
cyclization exhibited lower regioselectivity, as the meta-Br
imidate 1i afforded a mixture of two isomers (3i + 3i′).
Imidates bearing an ortho substituent (1j), a naphthyl ring
(1m), or a thiophene ring (1n, 1o) were applicable as well,
producing the corresponding isoselenazoles in high yields.
Analogous methoxyimidates 1p and 1u were also productive,
and the reaction of 1u could be conducted on a 1.0 mmol
scale.
Finally, we examined derivatization of the coupling products
to highlight the synthetic utility of the developed method
C
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a
Scheme 6. Direct Annulation with Elemental Selenium
ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c03674.
• Experimental procedures, computational method, prod-
uct identification data, and copies of NMR spectra
(PDF)
• Atomic coordinates of all calculated molecules (XYZ)
AUTHOR INFORMATION
Corresponding Authors
■
Yuji Nishii − Department of Applied Chemistry, Graduate
School of Engineering, Osaka University, Suita, Osaka 565-
0871, Japan; orcid.org/0000-0002-6824-0639;
Email: y_nishii@chem.eng.osaka-u.ac.jp
Masahiro Miura − Department of Applied Chemistry,
Graduate School of Engineering, Osaka University, Suita,
Osaka 565-0871, Japan; orcid.org/0000-0001-8288-
6439; Email: miura@chem.eng.osaka-u.ac.jp
a
Reaction conditions: 1 (0.2 mmol), Se (0.6 mmol), [Cp*Rh-
(MeCN)3](SbF6)2 (4.0 mol %), and AgOAc (0.4 mmol) in PhCF3
(0.3 mL).
Author
Sanghun Moon − Department of Applied Chemistry,
Graduate School of Engineering, Osaka University, Suita,
Osaka 565-0871, Japan
(Scheme 7). The alkoxy group at C3 of 2a was a suitable
leaving group for the direct substitution with a lithium amide
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c03674
Scheme 7. Derivatization of the Coupling Products
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by JSPS KAKENHI Grants JP
19K15586 (Grant-in-Aid for Young Scientists) to Y.N. and JP
17H06092 (Grant-in-Aid for Specially Promoted Research) to
M.M. The authors thank Dr. Hiroaki Iwamoto (Osaka
University) for his assistance with DFT calculations.
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