Rhodium-catalyzed directed sulfenylation of arene C-H bonds

Article abstract of DOI:10.1002/chem.201303730

The rhodium-catalyzed intermolecular direct C-H thiolation of arenes with aryl and alkyl disulfides was developed for the first time to provide a convenient route to aryl thioethers. This strategy is compatible with many different directing groups and exhibits excellent functional group tolerance. More significantly, mono- or dithiolation can be selectively achieved, thus providing a straightforward way for selective preparation of aryl thioethers and dithioethers.

Full text of DOI:10.1002/chem.201303730

DOI: 10.1002/chem.201303730
Communication
&
CÀH Activation
Rhodium-Catalyzed Directed Sulfenylation of Arene CÀH Bonds
Yaxi Yang, Wei Hou, Lihuai Qin, Juanjuan Du, Huijin Feng, Bing Zhou,* and Yuanchao Li*[a]
Abstract: The rhodium-catalyzed intermolecular direct CÀ
H thiolation of arenes with aryl and alkyl disulfides was
developed for the first time to provide a convenient route
to aryl thioethers. This strategy is compatible with many
different directing groups and exhibits excellent functional
group tolerance. More significantly, mono- or dithiolation
can be selectively achieved, thus providing a straightfor-
ward way for selective preparation of aryl thioethers and
dithioethers.
Aryl sulfides are ubiquitous structural motifs in numerous bio-
logically active natural products, pharmaceuticals, and materi-
als.[1] Metal-mediated cross-coupling reactions of aryl (pseudo)-
halides with sulfur nucleophiles, such as aryl thiols and aliphat-
ic thiols, has been developed as a powerful tool to give aryl
thioethers under a variety of conditions.[2] However, these
methods require the prefunctionalization of substrates. With
the increasing interest in transition-metal catalyzed CÀH bond
functionalization,[3] a direct thiolation of arene CÀH bonds
would provide an alternative method for the rapid preparation
Scheme 1. Different approaches toward CÀS bond formation through non-
acidic arene CÀH activation.
of aryl thioethers.
In the area of CÀH bond functionalization, much attention
has been paid to CÀC, CÀO, and CÀN bond-forming reac-
tions.[3] In sharp contrast, the formation of a CÀS bond through
transition-metal catalyzed CÀH activation is rare and most ex-
amples have been limited to the formation of aryl sulfones[4]
and benzothiazoles,[5] or the deprotonative sulfenylation of
acidic heterocycles.[6] The more challenging preparation of aryl
thioethers through non-acidic arene CÀH activation are signifi-
cantly rare,[7] presumably because the sulfur atom of thioethers
can tightly bind to metals, leading to the deactivation of the
metal catalyst.[8] Pioneering Cu-mediated direct thioetherifica-
tion of the arene CÀH bond was disclosed by Yu and co-work-
ers (Scheme 1a).[7a] Qing and co-workers developed a CuII-
mediated methylthiolation of arene CÀH bonds utilizing DMSO
as the methylthiolation reagent (Scheme 1b).[7b] Recently
Cheng et al. reported a CuI-catalyzed thiolation of the di- or tri-
methoxybenzene CÀH bond (Scheme 1c).[7c] However, these
methods suffer from low efficiency and a narrow scope of pos-
sible thiolation reagents or substrates. Very recently, Daugulis
and co-workers reported a beautiful CuII-catalyzed thiolation of
arene CÀH bonds using bidentate directing groups (Sche-
me 1d).[7d] However, in most cases, only dithioether products
were obtained. Therefore, the development of a general non-
acidic arene CÀH thioetherification reaction (especially one
using transition-metal catalysts other than Cu) that enables the
selective preparation of valuable aryl thioethers and dithioeth-
ers, and is compatible with a broad scope of substrates and
tolerant to a wide range of functional groups, would be of
prime synthetic value.
Recently, {Cp*RhIII} complexes have emerged as very useful
and efficient catalysts for CÀH bond activation[3a–c,i,m] and sub-
sequent CÀhalogen,[9] CÀN,[10] and especially CÀC bond-form-
ing reactions,[11–14] including alkynylation, olefination,[11] nucleo-
philic additions,[12] and coupling with carbene precursors[13]
and aziridines.[14] Indeed, it was also found that RhIII catalysts
could complement other transition metals in the functionaliza-
tion of CÀH bonds in terms of activity, substrate scope, selec-
tivity, and functional-group tolerance.[9–14] However, to the best
of our knowledge, there is no report on the more challenging
[a] Dr. Y. Yang,+ Dr. W. Hou,+ L. Qin, Dr. J. Du, H. Feng, Dr. B. Zhou, Dr. Y. Li
Shanghai Institute of Materia Medica
Chinese Academy of Sciences
555 Zu Chong Zhi Road, Shanghai 201203 (PR China)
E-mail: zhoubing2012@hotmail.com
ycli@mail.shcnc.ac.cn
[+] These authors contributed equally to this work.
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201303730.
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CÀS bond formation by Rh-catalyzed transformations. Given
the prevalence of aryl thioethers in medicinal chemistry and
pharmaceutical industries, here we describe the first example
of Rh-catalyzed arene CÀH thioetherification (Scheme 1e). Sig-
nificantly, this new CÀH thioetherification reaction is compati-
ble with many different directing groups (e.g., pyridine, pyrimi-
dine, pyrazole, and oxime) and tolerates various synthetically
useful functional groups, selectively providing mono- or di-
thioethers by simply changing the reaction conditions.
Table 1. Rh-catalyzed ortho mono-thiolation of arene CÀH bonds.[a]
We chose the reaction of 2-phenylpyridine (1a) with diphen-
yl disulfide (2a) as a model reaction (see Table S1 in the Sup-
porting Information). Although no coupling occurred when
using [RhCp*Cl2]2 (5 mol%) as a catalyst, addition of AgSbF6
(20 mol%) catalyzed the reaction to give the desired product
3a (for structure, see Table 1), albeit with low conversion. Addi-
tion of an oxidant (150 mol%) significantly affected this CÀS
coupling reaction and the use of Ag2CO3 as an oxidant gave
improved conversion and yield, providing dithioether 4a (for
structure, see Table 4) as a main product. Other halide-abstract-
ing reagents were also tested and AgOTf proved to be particu-
larly effective, giving a sharp increase in the conversion and
yield. Dithioether 4a was obtained in the greatest yield by
using Rh/AgOTf as the catalyst, Ag2CO3 as the oxidant, and tol-
uene as the solvent, heated to 1508C for 36 h. Selective mono-
thioetherification was achieved by using Rh/AgOTf as the cata-
lyst, Cu(OAc)2 as the oxidant, and t-AmOH (t-AmOH=2-
methyl-2-butanol) as the solvent, heated to 608C for 36 h. Con-
trol experiments revealed that the rhodium(III) complex is
essential.
With the optimal reaction conditions in hand, substrate
scope with respect to mono-thiolation was then surveyed
(Table 1). The reaction did not show pronounced electronic
preference, and introduction of various electron-rich (3b, 3d,
3e, 3g, 3h), electron-poor (3i–k), and halogen (3c, 3 f, 3l,
3m) groups at the ortho (3b, 3c), meta (3d–f), and para (3g–
m) positions of the phenyl ring were all well tolerated. The
meta-substituted derivatives showed excellent regioselectivity
in CÀH activation to give the less sterically crowded isomers
(3d–f). In particular, the 2-methyl derivative (3b) exhibited
good reactivity, thus showing high tolerance for steric hin-
drance. Furthermore, ester (3i), nitrile (3k), halogen (3 f, 3l and
3m), and even electrophilic carboxaldehyde (3j) groups were
compatible with this CÀS bond-forming reaction, making fur-
ther functionalization possible. Notably, the reaction was able
to thiolate the CÀH bond of heterocyclic substrates (3n). The
substrate scope was further extended to disulfides. To our de-
light, various diaryl disulfides showed good reactivities, irre-
spective of the electronic nature of the substituents on the
phenyl ring (3o–q). Fortunately, the alkyl (3r and 3s) and
benzyl (3t) disulfides were also compatible with the conditions
and afforded the corresponding thiolation products in good
yields, thus allowing for high diversity in the synthesis of aryl
thioethers.
[a] 0.2 mmol scale; yield of isolated products shown.
and the tolerance of halide groups (3ae) offers the opportuni-
ty for further functionalization. The steric bulk on the pyridine
ring had a limited effect on the reaction. Good reactivities
were seen when a methyl group was present on the pyridine
ring (3aa–ad), regardless of whether it was at the 3-, 4-, 5-, or
6-position. Multicyclic pyridine derivatives were also found to
be useful in this thiolation reaction, with tricyclic benzo[h]qui-
noline and bicyclic quinolone both giving the corresponding
products (3af and 3ag, respectively) in good yields. Notably,
other N-based groups could also serve as effective directing
groups. For example, by using pyrimidine or pyrazole as the di-
recting group, the reaction provided the corresponding thiola-
tion products (3ah–aj) and also showed high tolerance for
steric hindrance (3ai), thus expanding the scope of the present
thiolation method.
To demonstrate the scope of other possible directing
groups, various pyridine directing groups were investigated
(Table 2). Pyridine rings functionalized with electron-donating
(3aa–ad) or -withdrawing (3ae) groups were tolerated well
In addition to heterocycles, ketoximes also worked well as
a directing group to facilitate the thiolation reaction (Table 3).
Notably, no product was observed in the absence of Rh cata-
lyst, indicating that the rhodium(III) complex is essential.[15]
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Table 2. Scope of other directing groups.[a]
Table 4. Rh-catalyzed ortho dithiolation of arene CÀH bonds.[a]
[a] 0.2 mmol scale; yield of isolated products shown.
Table 3. Scope of aryl ketoximes.[a]
[a] Reaction conditions: 0.2 mmol of 1, 0.5 mmol of 2, 0.01 mmol
[RhCp*Cl2]2, 0.04 mmol of AgOTf, 0.4 mmol of Ag2CO3 and 0.3 mL of
PhMe at 1508C for 36 h. Yield of isolated products shown.
(4e), and halogen (4 f and 4g) groups were tolerated, offering
the opportunity for further functionalization. Contrasting with
previous Rh-catalyzed CÀH functionalization reactions,[9–14]
meta-substituted substrates successfully provided difunctional-
ized products, enabling the preparation of 1,2,3,4-tetrasubsti-
tuted benzene derivatives (4h and 4i). Both electron-rich and
electron-poor diaryl disulfides were successfully coupled with
2-phenylpyridine to give products 4j–l in good yields. More
gratifyingly, the present dithiolation reaction tolerated other N-
based directing groups (4m–o) even when 6-methylpyridine
was used as the directing group (4m), indicating the high
level of steric tolerance of this system. Notably, double activa-
tion of arene CÀH bonds is rare with rhodium catalysis,[9,16] and
most reported examples involved CÀC bond formation as
well.[16]
[a] 0.2 mmol scale; yield of isolated products shown.
Electronically modified aryl ketoximes were viable for this thiol-
ation reaction (3ak–ap) and a bicyclic ketoxime also success-
fully underwent thiolation to give 3aq. More gratifyingly, aro-
matic heterocycles were also successfully reacted (3ar). The
above features indicate that the present method is a general
reaction that can be extended to more directing groups and it
is complementary to previous Cu-catalyzed CÀH thiolation
reactions.[7]
To probe the catalytic mechanism, we carried out several ex-
periments: First, cyclometalated RhIII complex 5 was found to
successfully catalyze the thiolation reaction of 2-phenylpyridine
to give dithiolation product 4a in 60% yield, indicating the
plausible intermediacy of a rhodacycle complex in the catalytic
cycle (Scheme 2). A significant primary kinetic isotope effect
(kH/kD =4.0) was observed for an intermolecular competitive
Next, the scope of the double CÀH activation/thioetherifica-
tion reaction was examined (Table 4). Various substrates con-
taining both electron-donating and electron-withdrawing
groups at the para-position of the phenyl ring were successful-
ly coupled with 2a, providing dithioether products in moder-
ate to good yields (4a–g). Notably, ester (4d), carboxaldehyde
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product 3a and RhIII B (path b). Intermediate B can continue to
react with substrate 1a through a CÀH activation step to form
a five-membered rhodacycle intermediate (D), which would
undergo reductive elimination to afford 3a together with a rho-
dium(I) species. Oxidation of RhI gives RhIII to complete the cat-
alytic cycle.
In summary, we have reported the first example of Rh-cata-
lyzed direct CÀH thiolation by using readily available disulfides
as thiolation reagents. This protocol shows a broad substrate
scope and many different directing groups can be used in the
CÀH thiolation reaction. More significantly, this method allows
Scheme 2. Preliminary mechanistic study.
coupling of 2a with a 1:1 mixture of 1a and 1a-D5 at a low
conversion (Scheme 3). This KIE value of 4 is typical for the CÀ for selective mono- or dithiolation and exhibits excellent toler-
H activation processes.[17]
ance of functional groups, enabling the straightforward and se-
lective preparation of valuable and versatile aryl thioethers and
dithioethers. Owing to its high selectivity and broad substrate
scope, this CÀH thiolation reaction should be of high synthetic
value. Detailed mechanistic studies and synthetic applications
of the Rh-catalyzed thiolation reaction are underway.
Experimental Section
Scheme 3. Intermolecular KIE experiment.
General Procedure
An oven-dried reaction vessel was charged with [Cp*RhCl2]2
(6.1 mg, 0.01 mmol), AgOTf (10.2 mg, 0.04 mmol), 2-phenylpyridine
(1a; 31 mg, 0.2 mmol), diphenyl disulfide (2a; 43 mg, 0.2 mmol),
Cu(OAc)2 (20 mg, 0.1 mmol), and 2 mL of t-AmOH (0.1m). The
vessel was sealed and heated at 608C (oil bath) for 36 h. The re-
sulting mixture was cooled to room temperature, filtered through
a Celite pad and then transferred directly to an alumina column
and eluted with petroleum ether and acetone (15:1) to give prod-
ucts 3a (40.5 mg, 77% yield).
Although the mechanism of the reaction is unclear at this
moment, we tentatively propose the following mechanistic
pathway on the basis of the above data (Scheme 4). First, the
[RhCp*Cl2]2 precursor reacts with the AgOTf additive to provide
a cationic RhIII species, which facilitates the CÀH bond activa-
tion of substrate 1a to form an Ar–RhIII rhodacycle (A). Inter-
mediate A can undergo a nucleophilic-addition-type reaction
with diphenyl disulfide to afford the desired product 3a and
PhSRhIII species B (path a). Alternatively, oxidative addition of
the disulfide bond to the RhIII center could occur to give a RhV
intermediate (C),[18] followed by reductive elimination to give
Acknowledgements
Support of this work by the National Natural Science Founda-
tion of China (No. 21302200) is gratefully acknowledged.
Keywords: arenes · CÀH activation · CÀS bond formation ·
rhodium · thiolation
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Received: September 24, 2013
Published online on November 28, 2013
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