Cp?Co(III)-Catalyzed C-H Acylmethylation of Arenes by Employing Sulfoxonium Ylides as Carbene Precursors

Article abstract of DOI:10.1021/acs.orglett.8b02796

A Cp?Co(III)-catalyzed C-H bond functionalization of a range of arenes by employing sulfoxonium ylides as carbene precursors instead of diazo compounds and other carbene precursors has been established. This reaction is highly efficient without any additi

Full text of DOI:10.1021/acs.orglett.8b02796

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Cp*Co(III)-Catalyzed C−H Acylmethylation of Arenes by Employing
Sulfoxonium Ylides as Carbene Precursors
Shuying Ji,† Kelu Yan,† Bin Li,† and Baiquan Wang*,†,‡,§
‡
†State Key Laboratory of Elemento-Organic Chemistry, College of Chemistry and Collaborative Innovation Center of Chemical
Science and Engineering (Tianjin), Nankai University, Tianjin 300071, People’s Republic of China
§State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences,
Shanghai 200032, People’s Republic of China
S
* Supporting Information
ABSTRACT: A Cp*Co(III)-catalyzed C−H bond functionalization of a
range of arenes by employing sulfoxonium ylides as carbene precursors
instead of diazo compounds and other carbene precursors has been
established. This reaction is highly efficient without any additive, possesses
high step and atom economies, and tolerates a range of functional groups.
Scheme 1. Transition-Metal-Catalyzed C−H
Functionalization Based on Metal Carbene Migratory
Insertion
n the past decades, the directed transition-metal-catalyzed
IC−H bond functionalization has flourished as a powerful
and atom- and step-economical method to construct a range of
complex molecules without preactivated substrates. Specifi-
cally, a nitrogen-containing directing-group-assisted C−H
functionalization catalyzed by numerous noble metals (such
as Ru, Rh, Pd, Ir) has been significantly developed.1 However,
these noble metals are low abundance and high cost, pressuring
scientists to seek more abundant, more affordable, and less
toxic first-row transition metals as the catalysts to take the
place of noble metals. In recent years, Cp*Co(III) (Cp* =
pentamethylcyclopentadienyl) catalyst systems have received
extensive attention as catalysts for C−H functionalization,
because of their higher Lewis acidity, smaller ionic radius, and
low cost, compared to precious metals.2 Since 2013, when
Kanai and Matsunaga3 first reported Cp*Co(III)-catalyzed
directed addition reactions of aryl C−H bonds to polar
electrophiles, many research groups such as those led by
Glorius,4 Ackermann,5 Ellman,6 Chang,7 Li,8 Cheng,9 Jiao,10
and others11 subsequently expanded the application of
Cp*Co(III) catalysts in C−H bond functionalization.
Recently, transition-metal-catalyzed C−H functionalization
based on metal carbene migratory insertion has been
significantly developed. α-Diazo carbonyls as carbene
precursors and coupling partners were extensively employed
in transition-metal-catalyzed C−H functionalization (Scheme
1a).12,13 Despite this significant progress, there are still many
limitations in the use of diazo compounds, such as the
potential danger of exothermic linked to the liberate of
nitrogen and inconvenience for reserve. In addition to diazo
compounds, other carbene precursors, such as hydrazones,14
triazoles,15 cyclopropenes,16 and ketone-functionalized
enynes,17 can also be used as coupling partners in C−H
functionalization reactions, but it is still necessary to develop
new reactants as surrogates of these carbene precursors.
Significantly, transition-metal-catalyzed X−H insertions of
sulfoxonium ylides were used industrially to form C−N and
C−O bonds as safe alternatives to diazo compounds.18
However, transition-metal-catalyzed C−H insertion reactions
of sulfoxonium ylides are rarely studied. Only very recently,
̈
Aıssa, Li, and others reported the Cp*Rh(III)-catalyzed C−H
functionalization of arenes with sulfoxonium ylides as carbene
precursors and coupling partners (Scheme 1b).19
Based on our previous study of transition-metal-catalyzed
C−H/diazo compounds coupling reactions,20 we commenced
our investigation by probing the reaction conditions for the
envisioned C−H bond acylmethylation of 2-phenylpyridine 1a
and α-benzoyl sulfur ylide 2a with Cp*Co(III) as catalyst (see
Table 1). First, reactions of 1a (0.2 mmol) with 2a (0.3 mmol)
in the presence of Cp*Co(CO)I2 (10 mol %) and AgSbF6 (20
mol %) in a variety of solvents at 100 °C under argon
atmosphere for 12 h were studied, and 1,2-dichloroethane
(DCE) (Table 1, entry 1) was demonstrated to be the most
Received: August 31, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b02796
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Letter
a
a
Table 1. Optimization of Reaction Conditions
Scheme 2. Substrate Scope of Sulfoxonium Ylides
b
entry
catalyst/Ag salt
solvent
yield
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Cp*Co(CO)I2/AgSbF6
Cp*Co(CO)I2/AgSbF6
Cp*Co(CO)I2/AgSbF6
Cp*Co(CO)I2/AgSbF6
Cp*Co(CO)I2/AgSbF6
Cp*Co(CO)I2/AgSbF6
Cp*Co(CO)I2/AgBF4
Cp*Co(CO)I2/AgOTf
Cp*Co(CO)I2/AgNTf2
Cp*Co(CO)I2/-
DCE
TFE
67
21
14
21
39
0
43
43
50
toluene
dioxane
DCM
CH3CN
DCE
DCE
DCE
DCE
DCE
0
89
[Cp*Co(CH3CN)3](SbF6)2
[Cp*Co(CH3CN)3](SbF6)2
[Cp*Co(CH3CN)3](SbF6)2
cd
,
DCE
DCE
DCE
96 (91)
e
54
0
a
Reaction conditions: 1a (0.2 mmol), 2a (0.3 mmol), and
Cp*Co(CO)I2 (0.02 mmol)/Ag salt (0.04 mmol) or [Cp*Co-
a
(MeCN)3](SbF6)2 (0.02 mmol) were stirred in solvent (1.5 mL) at
Reaction conditions: 1a (0.2 mmol), 2 (0.3 mmol) Cp*Co-
(CH3CN)3(SbF6)2 (10 mol %), DCE (1.5 mL), 120 °C, under Ar,
b
100 °C for 12 h under Ar. Yields were determined by 1H NMR using
c
b
1,1,2,2-tetrachloroethane as an internal standard. At 120 °C.
12 h, isolated yield. 1.0 mmol scale.
d
e
Isolated yield is shown in brackets. Under an air atmosphere.
a
Scheme 3. Substrate Scope of 2-Arylpyridines
effective, affording the desired product 3aa in 67% yield. When
other solvents such as 2,2,2-trifluoroethanol (TFE), toluene,
dioxane, CH3CN, dichloromethane (DCM) were used for this
reaction, the yields were only 0−39% (Table 1, entries 2−6).
Co-catalytic amounts of a silver(I) salt were momentous for
the C−H acylmethylation reaction, and AgSbF6 gained the
best performance (Table 1, entries 1 and 7−10). Delightfully,
the use of cationic [Cp*Co(MeCN)3](SbF6)2 dramatically
improved the yield of 3aa to 89% (Table 1, entry 11).
Moreover, 3aa was formed in 96% yield (91% isolated yield)
when the temperature was increased to 120 °C (Table 1, entry
12). When the reaction was proceeded under air, it still
afforded 3aa in 54% yield (Table 1, entry 13). No reaction was
observed in the absence of the cobalt catalyst (Table 1, entry
14).
With the establishment of the optimized conditions, the
substrate scope was further investigated. Initially, this
acylmethylation protocol was applied to a variety of α-benzoyl
sulfur ylides (see Scheme 2). Benzoyl-substituted sulfoxonium
ylides bearing a variety of important functional groups, such as
electron-donating groups (CH3, OMe), the halogens (F, Cl,
Br), and electron-withdrawing group (CF3) at the para, ortho,
and meta positions of the phenyl ring reacted smoothly with 1a
to afford the corresponding products (3ab−3ai) in moderate
to high yields (39%−89%). Delightfully, the reactants could
contain a thiophene/furan ring or naphthalene and the
corresponding products (3aj−3al) were obtained in 71%−
82% yields. The sulfoxonium ylides were not limited to
(hetero)aryl-substituted substrates, several alkyl substrates
were also suitable and the corresponding products were
isolated in moderate yields (3am−3ao, 40%−61%).
a
Reaction conditions: 1a (0.2 mmol), 2 (0.3 mmol), Cp*Co-
(CH3CN)3(SbF6)2 (10 mol %), DCE (1.5 mL), 120 °C, under Ar, 12
h, isolated yield. py = pyridyl, pym = pyrimidyl.
of sulfoxonium ylides. The substrate with an electron-donating
group (OMe) at the para position of the phenyl ring
performed better (3ca, 76%) than that with an electron-
withdrawing group (CF3) (3ga, 52%). The halogen substituted
Then, the substrate scope of 2-arylpyridines was extended
(Scheme 3). Interestingly, the electronic effect of various
functional groups at 2-phenylpyridine was different from that
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DOI: 10.1021/acs.orglett.8b02796
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2-phenylpyridines afforded the expected products (3da−3fa)
in good yields (74%−79%). It was noteworthy that 3ha was
only obtained in 9% yield. One possible reason is that the steric
hindrance of methyl group at the ortho position of phenyl ring
makes it difficult to form cyclo-cobalt intermediate. When 1i
was employed as coupling partner, selective C−H acylmethy-
lation occurred only at the C6 position to give the desired
product 3ia in 44% yield. Moreover, polycyclic and
heterocyclic arenes were also suitable for the acylmethylation
reaction, to give the corresponding products 3ja and 3ka in
moderate yields (71% and 49%). When N-phenylpyrazole was
used as a substrate, the acylmethylation product 3la was
obtained in 63% yield. Encouraged by these initial findings, the
scope of the substrate was further extended to N-pyridyl- or N-
pyrimidyl-indoles, in which the pyridyl or pyrimidyl group can
act as a removable directing group, the corresponding products
(3ma−3ua) were obtained in 58%−83% yields.
d5-1a illustrated a KIE of kH/kD = 1.3 (see Scheme 4b). The
result shows that the cleavage of the C−H bond may not be
included in the rate-determining step. A intermolecular
competition reaction between 2-(4-methoxyphenyl)pyridine
(1c) and 2-(4-(trifluoromethyl)phenyl)pyridine (1g) with 2a
was performed in a one-pot fashion. The NMR yield of 3ca
was higher than that of 3ga, indicating a higher reactivity for
the electron-rich substrate (see Scheme 4c).
Based on the above experimental results and the literature
reports,19,21 a rational mechanism was proposed (Scheme 5).
Scheme 5. Proposed Mechanistic Pathway
The substrates with other directing groups such as amide,
ketone, oxime, azobenzene, imine, and 2-phenyl imidazole
have been also tested under the standard conditions.
Unfortunately, they all failed to give the desired products. In
some cases, a byproduct 4 was isolated in 18%−38% yields
(see the Supporting Information). In contrast to the formation
of 4 by Cp*Rh(III)-catalyzed C−H/2a coupling and tandem
cyclization,19c for our Cp*Co(III) catalyst system, compound
4 should be formed via the self-coupling of 2a. These results
show the different reactivities of the cobalt catalyst, in
comparison with that of a rhodium catalyst.19
To gain more insight in the mechanism of this reaction,
some experiments have been conducted. First, an H/D
exchange experiment was performed, when 2-phenylpyridine
(1a) was subjected to the optimized reaction conditions using
[Cp*Co(MeCN)3](SbF6)2 in the presence of CD3OD. 2-
Phenylpyridine was recovered in 84% yield and the NMR
analysis disclosed H/D exchange occurred at the ortho position
(70% D) of the phenyl ring (see Scheme 4a), suggesting that
the Cp*Co(III)-catalyzed C−H activation was reversible. The
kinetic isotope effect (KIE) experiment was implemented, and
a deuterium competition experiment between substrate 1a and
The first step is likely the coordination of pyridine with cobalt
to replace a CH3CN of [Cp*Co(MeCN)3](SbF6)2 and
subsequent ortho C−H bond activation process to form the
cobaltacycle intermediate I. The ligand exchange between 2a
and I affords intermediate II, which undergoes elimination of
DMSO to acquire a cobalt carbene intermediate III. Migratory
insertion of the cobalt-carbene into the Co−C bond leads to
intermediate IV. Finally, protonation of IV delivers the final
product 3aa and releases the active catalyst.
In conclusion, we have achieved the Cp*Co(III)-catalyzed
C−H bond functionalization with sulfoxonium ylides as
carbene precursors and coupling partners. The reaction
conditions are simpler without additional additives, in
comparison with the rhodium catalyst systems. This catalyst
systems may be used industrially for the synthesis of related
complex compounds instead of diazo compounds in the future.
Scheme 4. Mechanism Study Experiments
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b02796.
1
Full experimental procedures, characterization and H,
13C, and 19F NMR spectra of products (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: bqwang@nankai.edu.cn.
ORCID
Bin Li: 0000-0003-3909-3796
C
DOI: 10.1021/acs.orglett.8b02796
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the National Natural Science Foundation of China
(No. 21672108) and the Natural Science Foundation of
Tianjin (No. 16JCZDJC31700) for financial support.
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DOI: 10.1021/acs.orglett.8b02796
Org. Lett. XXXX, XXX, XXX−XXX