Rhodium-Catalyzed C-S and C-N Functionalization of Arenes: Combination of C-H Activation and Hypervalent Iodine Chemistry

Article abstract of DOI:10.1002/chem.201504179

Rhodium-catalyzed sulfonylation, thioetherification, thiocyanation, and other heterofunctionalizations of arenes bearing a heterocyclic directing group have been realized. The reaction proceeds by initial Rh^III-catalyzed C-H hyperiodination of

Full text of DOI:10.1002/chem.201504179

DOI: 10.1002/chem.201504179
Communication
&
Homogeneous Catalysis |Hot Paper|
Rhodium-Catalyzed CÀS and CÀN Functionalization of Arenes:
Combination of CÀH Activation and Hypervalent Iodine Chemistry
Fen Wang, Xinzhang Yu, Zisong Qi, and Xingwei Li*[a]
Abstract: Rhodium-catalyzed sulfonylation, thioetherifica-
tion, thiocyanation, and other heterofunctionalizations of
arenes bearing a heterocyclic directing group have been
realized. The reaction proceeds by initial RhIII-catalyzed
CÀH hyperiodination of arene at room temperature fol-
lowed by uncatalyzed nucleophilic functionalization. A
diaryliodonium salt is isolated as an intermediate, which
represents umpolung of the arene substrate, in contrast
to previous studies that suggested umpolung of the cou-
pling partner.
CÀH activation has been extensively explored and numerous
efficient catalytic systems have been developed.[1] The CÀH ac-
Scheme 1. Coupling of arenes with nucleophiles.
tivation of arenes has also allowed the development of valu-
able synthetic methods in the synthesis of natural products
and materials.[2] In the case of CÀH activation of arenes that
are not electronically activated, installation of a directing
group on the arene is typically required to offer chelation assis-
tance.[3] Thus, generation of the reactive MÀC bond offers
ample opportunities for further manipulation and eventual
functionalization of the arene substrate.
effecting the CÀH activation of arenes and subsequent func-
tionalization with electrophiles.[7] As a continuation of our in-
terest in RhIII-catalyzed CÀH activation of arenes,[1l] we pon-
dered the possibility of umpolung of arenes. This can be exe-
cuted by rhodium-catalyzed activation of an arene to afford an
ArÀRh species that further interacts with an oxidant, generat-
ing an oxidized form of the arene. This process can be regard-
ed as in situ umpolung of the arene, provided that the oxi-
dized arene is highly electrophilic and can react in situ with
a nucleophile. Recently, we reported the formation, isolation,
Despite the significant and diverse reactivity of MÀC(aryl)
species as a key intermediate in CÀH activation, the aryl group
is nucleophilic and is only compatible with an electrophile.
However, although incompatible nucleophiles are arguably
more abundant in nature, they are not directly applicable. To
address this limitation, two approaches have been adopted
(Scheme 1). In an oxidative coupling strategy,[4] reductive elimi-
nation of an aryl and an anionic ligand (nucleophile) consti-
tutes a product-forming step that proceeds via a low-valent
metal intermediate (Scheme 1a). Catalytic turnover can be ful-
filled when the low-valent metal is re-oxidized. In this strategy
the oxidant interacts directly with the metal. Alternatively, our
group[5] and others[6] have proposed an umpolung strategy
whereby a nucleophilic coupling partner is converted into an
electrophilic one (Scheme 1b). This strategy was rendered pos-
sible by the versatility of RhIII catalysts; RhIII complexes were
employed to effect the successful combination of CÀH activa-
tion and umpolung, given that RhIII catalysis is well known in
Table 1. Optimization studies[a]
Entry
Changes to the optimized conditions
Yield [%][b]
1
2
3
4
5
none
77
10
65
70
29
single-step procedure
MesCOOH was omitted.
PhI(OH)OTs was used.
acetonitrile was used as the solvent
[a] F. Wang, X. Yu, Z. Qi, Prof. Dr. X. Li
Dalian Institute of Chemical Physics, Chinese Academy of Sciences
Dalian 116023 (China)
[a] Optimized conditions: A mixture of the arene (0.3 mol), [{RhCp*Cl2}2]
(5 mol%), PhI(OH)OMs (1.6 equiv), and MesCOOH (20 mol%) were stirred
in acetone (3 mL) at room temperature for 20 min. Sodium sulfinate
(0.9 mmol) was then added, and the mixture was stirred at 808C for 16 h.
[b] Yield of product isolated by chromatography.
E-mail: xwli@dicp.ac.cn
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201504179.
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Scheme 2. Substrate scope.
and subsequent catalytic transformation of diaryliodonium
salts through rhodium-catalyzed CÀH activation of arenes.[5d]
We report herein that diaryliodonium salts can act as an acti-
vated form of arene as a result of CÀH umpolung (Scheme 1c),
thus enabling sulfonylation,[8] thiolation, and other heterofunc-
tionalization.
iodine reagent, and the RhIII catalyst in acetone at RT followed
by introduction of 2a afforded the desired sulfonylated prod-
uct 3aa in 70% yield (isolated product; 808C, see ref. [5d]).
The requirement of a two-stage reaction is in contrast to our
previously reported nitration reaction of the same arene sub-
strate.[7a] Further improvement of the efficiency (to 77% yield)
was realized when a catalytic amount of 2,4,6-trimethylbenzoic
acid[8b,10] (0.2 equiv) was introduced. However, acetic acid or
PivOH (2 equiv) additives proved to be less efficient. The reac-
tion occurred in slightly lower yield when the hypervalent
iodine reagent was changed to its tosylate analogue
(PhI(OH)OTs). The reaction performed in other solvents [MeCN,
1,2-dichloroethane (DCE), and dichloromethane] afforded the
We initiated our studies with the coupling of 2-phenylpyri-
dine (1a) and sodium p-toluenesulfinate (2a) in the presence
of [{RhCp*Cl2}2] catalyst with PhI(OH)OMs as oxidant[9] at 808C
(Table 1). The reaction was found to proceed with poor effi-
ciency when all reagents and the catalyst were premixed prior
to the addition of acetone solvent. In contrast, a two-stage
process with pre-stirring of a mixture of 1a, the hypervalent
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product in poor yields. Control experiments also confirmed
that the rhodium catalyst is necessary.
tion was detected (by TLC and HPLC) when it was allowed to
react for 30 min, which is line with the failure of the catalytic
reaction under our standard conditions.[13]
The scope and limitations of this coupling system were next
explored by using the optimized conditions. Installation of vari-
ous electron-donating, -withdrawing, and halogen groups at
the para position of the phenyl ring had limited effects on the
outcome of the coupling system (3ba–ia; Scheme 2), and
there was no direct correlation between the electronic effect
of the substituent and the reaction yield. The compatibility of
halogen (3ia), ester (3ga), and aldehyde (3ha) groups should
allow further manipulation of the coupled product. An excep-
tion was found for the coupling of 2-(4-fluorophenyl)pyridine,
from which essentially no desired product was detected under
the standard conditions. However, prolonging the pre-stirring
in the first stage to 15 h increased the yield of product 3ja to
35%. Introduction of electron-withdrawing and -donating sub-
stituents into the pyridine ring of 2-phenylpyridines was well
tolerated and the sulfonylated product was isolated in moder-
ate yields (3qa–sa). Discrepancies in site selectivity of different
meta-substituted substrates were observed. The coupling oc-
curred at the less hindered site for m-methyl and -bromo sub-
stituted arenes (3ka–ma). Decreasing the substituent size to
a m-OMe group caused formation of a mixture of two re-
gioisomers (3na and 3na’). In contrast, only sulfonylation at
the more hindered ortho position was detected for m-fluoro-
substituted arene 3oa, a trend that was also observed in our
previous studies.[5a] In addition, an o-chloro-substituted sub-
strate exhibited comparable activity (3pa). The arene scope
was not limited to phenyl rings and the reaction of a thiophene
substrate afforded product 3ta in moderate yield.
Although most 2-phenylpyridines reacted efficiently under
the standard conditions, essentially no desired reaction oc-
curred for 1-phenylpyrazole. To address this limitation, 1-phe-
nylpyrazole was hyperiodinated to afford 5 by following our
previously reported protocol.[5d] Treatment of 5 with 2a afford-
ed the corresponding product 6 in 59% yield [Eq. (3)].
In addition to identification of the intermediate hypervalent
iodine species, the mechanism of the sulfonylation reaction
was explored. Cyclometalated RhIII sulfinate complex 7a was
prepared and tested as a catalyst [Eq. (4)]. However, no desired
catalytic reaction occurred. This suggests that it is not an
active catalyst, which stands in stark contrast to the efficacy of
the previously reported cyclometalated RhIII nitrate complex in
the nitration of 2-phenylpyridines with NaNO2.[5a] Thus, reduc-
tive CÀS bond formation from 7a is likely irrelevant and an oxi-
dative coupling process that involves the oxidation of a low-
valent RhI species to RhIII is not plausible (Scheme 1a). This
result also suggests that the sulfinate anion exhibits an inhibi-
tive effect at the hyperiodination stage.
Variation of the sulfinate coupling partner was also well tol-
erated. Introduction of (hetero)arenesulfinate bearing different
electron-donating and -withdrawing substituents at different
positions were viable, and the coupled products were isolated
in consistently moderate to high yields (3ac–ai; Scheme 2).
Furthermore, the sulfinate scope could be extended to (tri-
fluoro)methanesulfinate with no obvious deterioration of the
reaction efficiency (3aj, 3ak).
During pre-stirring of a mixture of 2-phenylpyridine, the RhIII
catalyst, the 2-mesitylenecarboxylic acid (MesCOOH), and
PhI(OH)OTs in acetone,
a white precipitate was formed
[Eq. (1)], which may indicate formation of a reactive intermedi-
ate. Indeed, this intermediate (4) was isolated in 75% yield
after simple filtration and was identified as an unsymmetrical
diaryliodonium salt on the basis of NMR spectroscopy and
HRMS analyses. The NMR spectra of 4 are in close agreement
with those of its tosylate analogue.[5d] To probe the intermedia-
cy of this hypervalent iodine species, a mixture of 4 (recrystal-
lized) and 2a (recrystallized) was stirred in acetone. Interesting-
ly, the sulfonylated product 3aa was isolated in 91% yield
from a reaction at 608C, even in the absence of any catalyst
[Eq. (2)].[11,12] Thus, the overall catalytic reaction comprises
a RhIII-catalyzed hyperiodination of the arene followed by an
uncatalyzed nucleophilic sulfonylation reaction. Furthermore,
the intermediacy of a hypervalent iodine species seems in
agreement with the poor efficiency of the coupling of
2-(4-fluorophenyl)pyridine, because essentially no hyperiodina-
To probe the CÀH activation process, rhodacyclic complex
7b was also applied as a catalyst [Eq. (4)]. It follows that com-
plex 7b exhibited comparable activity to the real catalyst
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system. Thus, a CÀH activation mechanism is followed and the
chloride ligand seems necessary for an active catalyst system.
To further investigate the details of the CÀH activation process,
a kinetic isotope effect (KIE) experiment was performed on
equimolar amounts of 1a and [D5]1a in a competitive reaction
with sodium p-toulenesulfinate at low conversion [Eq. (5)].
1H NMR spectroscopy of the product mixture gave a KIE value
of 3.8, indicating that CÀH activation is likely turnover-limiting
in the catalytic cycle. This value should reflect the KIE of the
hyperiodination stage.
uct 9 in good yield (room temperature, Scheme 3).[15] Further
reaction of 9 with acetone at 808C generated the correspond-
ing thioether 8a in 68% yield. In addition, thiocyanate 9a
could be readily transformed into the corresponding trifluome-
thylthioether 10 in good yield by following a previously report-
ed protocol.[14a] Thus, the formation of 8i proceeded by initial
thiocyanation followed by nucleophilic functionalization with
acetone and the thioether could be prepared in one pot start-
ing from either the arene or the hypervalent iodine reagent. In
addition to this thioetherification reaction via a thiocyanate in-
termediate, uncatalyzed nucleophilic thioetherification of 4
with sodium thiophenolate readily occurred to afford diaryl-
thioether 11 in high yield.[16]
In addition to the CÀS bond formation reactions, the nucleo-
philic functionalization of a hypervalent iodine was extended
to amidation, azidation, nitration, and iodination reactions
(products 12–15; Scheme 4). In the previous studies of RhIII-cat-
alyzed one-pot nitration, azidation, and halogenation of 2-aryl-
pyridines with the corresponding sodium salts as the coupling
partner in combination with phenyliodine(III) diacetate (PIDA)/
TsOH as an oxidant, our group[5a] and others[6d] postulated that
the coupling occurred through umpolung of the coupling
partner so that the nucleophilic RhÀAr bond might interact
with an electrophilic nitro, azido, or halogen species. On the
basis of the results of our diversified CÀH functionalizations via
an established IIII intermediate, the previously proposed mech-
anism warrants correction. These reactions most likely proceed
by initial hyperiodination followed by nucleophilic functionali-
zation and, in most cases, this nucleophilic functionalization is
uncatalyzed.
Inspired by the sulfonylation system, CÀH thiocyanation[14]
was next explored by using sodium thiocyanate. Interestingly,
the two-stage coupling between 1i and NaSCN afforded thio-
ether 8i, in which an acetone molecule was incorporated, in
moderate yield [Eq. (6)].
In summary, we have developed an efficient heterofunction-
alization of arenes using sodium salts as the coupling partners.
The reactions proceeded by CÀH activation under relatively
mild conditions and tolerated a broad scope of substrates.
Mechanistic studies, particularly isolation of the hypervalent
Several experiments were performed to understand the
mechanism. The copper-catalyzed reaction of hypervalent
iodine reagent 4 and NaSCN afforded the thiocyanation prod-
Scheme 3. Thiocyanation and thioetherification of arenes.
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previous report on the umpolung of the coupling partner.
Functionalization of other arenes by using this strategy is cur-
rently underway in our laboratory and will be reported in due
course.
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Acknowledgements
Financial support from the NSFC (Grant Nos. 21272231,
21402190, 21472186, and 21525208) and the Dalian Institute
of Chemical Physics, Chinese Academy of Sciences, is gratefully
acknowledged.
Keywords: CÀH activation
·
homogeneous catalysis
·
hypervalent iodine · rhodium · umpolung
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[15] The uncatalyzed nucleophilic thiocyanation of 4 with NaSCN could
occur at a higher temperature (608C), but a mixture of 9 and 8a was
obtained.
Received: October 16, 2015
Published online on December 3, 2015
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