Manganese-Catalyzed Aromatic C-H Allylation of Ketones

Article abstract of DOI:10.1021/acs.orglett.9b02554

Manganese-catalyzed aromatic C-H allylation of ketones is reported. The reaction proceeded in a monoselective allylation manner to provide various ortho C-H allylated ketones in high yields. With challenging allylic electrophiles bearing substituents at t

Full text of DOI:10.1021/acs.orglett.9b02554

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Manganese-Catalyzed Aromatic C−H Allylation of Ketones
Shaukat Ali,† Jiaqi Huo,†,‡ and Congyang Wang*,†,‡,§
†Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Molecular Recognition and Function, CAS
Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing,
100190, China
‡University of Chinese Academy of Sciences, Beijing 100049, China
§Physical Science Laboratory, Huairou National Comprehensive Science Center, Beijing 101400, China
S
* Supporting Information
ABSTRACT: Manganese-catalyzed aromatic C−H allylation of
ketones is reported. The reaction proceeded in a monoselective
allylation manner to provide various ortho C−H allylated
ketones in high yields. With challenging allylic electrophiles
bearing substituents at the α-, β-, or γ-position, excellent SN2′
regioselectivity was achieved under mild conditions (rt to 35
°C). Mechanistic studies revealed a possible turnover-limiting
C−H bond cleavage step affording a five-membered mangana-
cycle followed by reaction with allylic electrophiles to give the
C−H allylation product.
Scheme 1. Transition-Metal-Catalyzed Aliphatic and
Aromatic C−H Allylation of Ketones
irect functionalization of inert C−H bonds is a powerful
D
tool to construct new chemical bonds in organic
synthesis. Since the pioneering Ru-catalyzed C−H alkylation
of aryl ketones with olefins,1 ketones have been explored for a
range of synthetically useful C−H functionalization reactions.2
Nevertheless, the predominant use of precious transition
metals (Ru, Rh, Pd, and Ir) in these reactions raises the
question of sustainability. Development of earth-abundant
base-metal catalyzed C−H functionalization of ketones is
highly desirable. Manganese, owing to its abundance, low cost,
and low toxicity, has recently received considerable attention in
organometallic C−H activation reactions.3,4 Despite impres-
sive progress, the arene substrates for the most manganese-
catalyzed C−H functionalization contain strong-coordination
nitrogen-directing groups. Although stoichiometric C−H
activation reactions of aryl ketones giving manganacycles
have been long reported,5 the development of catalytic C−H
functionalization of readily available ketones has not been
reported until our recent contributions.4k,o The prominent
challenges of using the ketone moiety as an effective directing
group for C−H activation derive from the weak coordinating
ability of ketones with metals, thus decreasing the thermody-
namic stability of the resulting transition states and/or
metallacycle intermediates.6 Also, active C(sp3)−H bonds α
to the carbonyl of ketones could undergo facile Mannich, aldol,
and/or Michael-addition types of reactions thus competing
with the inert C−H activation pathway in catalysis.7
The only elegant example was reported recently by Maji
demonstrating a cobalt-catalyzed ortho-allylation of aryl
ketones.9 Nevertheless, it showed lack of reactivity and/or
poor regioselectivity for β- and γ-substituted allylic electro-
philes. As part of our ongoing interest in sustainable
manganese catalysis,3,4 herein we describe the first manga-
nese-catalyzed aromatic C−H allylation of ketones with broad
substrate scopes, sole SN2′ regioselectivity, and high E/Z
selectivity (Scheme 1b).10
We started with the reaction of p-methoxyacetophenone 1a
and allyl bromide in the presence of MnBr(CO)5 and Me2Zn/
ZnBr2.4k,o No desired C(sp2)−H monoallylated product 3a
was detected in diethyl ether; instead, aldol-type product 4a
and α-C(sp3)−H allylated product 5a were observed in 18%
and 15% NMR yields, respectively (Table 1, entry 1). To our
Specifically, Pd-, Ir-, and Rh-catalyzed α-C−H allylic
alkylation of ketones or ketone enolates has been studied
extensively, which provides an efficient way to construct
tertiary or quaternary α-carbon centers of ketones and even
enantioselectivity (Scheme 1a).8 However, direct allylation of
aromatic C−H bonds of ketones has rarely received attention.
Received: July 22, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b02554
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Letter
a
a
Table 1. Screening of Reaction Parameters
Scheme 2. Manganese-Catalyzed Aromatic C−H Allylation
c
Yield
b
entry
solvent
LG
T (°C)
3a
4a
5a
d
1
Et2O
PhMe
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
Br
Br
Br
Br
120
120
120
120
120
80
80
80
80
80
−
18
23
25
−
−
−
−
−
−
−
15
−
−
−
−
−
−
−
−
−
−
−
−
−
−
d
2
11
17
34
10
52
62
67
−
58
53
61
91(89)
−
d
3
4
5
6
OPiv
OPiv
OPiv
OAc
OBoc
OCOCF3
Cl
OAc
Cl
Cl
e
7
e
8
e
9
e
10
11
12
e
80
80
80
80
−
−
−
30
f
f
g
13
fh
14 ,
fi
15 ,
Cl
80
32
28
a
Reaction conditions: 1a (0.2 mmol), 2 (0.4 mmol), MnBr(CO)5 (10
mol %), 1.5 equiv of Me2Zn (1.2 M in toluene), 1.0 equiv of
b
c
Cu(OTf)2, DCE (0.1 M), 80 °C, 17 h. LG = leaving group. NMR
yields using 1,3,5-trimethoxybenzene as standard. 1.0 equiv of ZnBr2
instead of Cu(OTf)2. 1a:2 = 2:1. 1a:2 = 3:1 and 0.6 equiv of
Cu(OTf)2. Isolated yields, almost quantitative recovery of excess 1a
d
e
f
g
h
i
after column chromatography. No MnBr(CO)5. No Cu(OTf)2.
PMP = p-methoxyphenyl.
delight, 3a was obtained by using toluene or 1,2-dichloro-
ethane (DCE) as solvents, albeit in low yields (entries 2, 3).
Screening of additives revealed that Cu(OTf)2 not only
increased the yield of 3a but also inhibited the formation of 4a
(entry 4).11 When O-pivalate was used as a leaving group, the
yield of 3a increased to 52% at 80 °C (entries 5, 6). Further
variations on the ratios of substrates and types of leaving
groups showed that 3a could be isolated in 89% yield by using
Cl as the optimal leaving group (entries 7−13). No reaction
occurred without the manganese catalyst (entry 14). The yield
of 3a decreased dramatically, and the aldol-type product 4a
emerged in the absence of Cu(OTf)2 (entry 15).
a
Reaction conditions: 1 (1.5 mmol), 2 (0.5 mmol), MnBr(CO)5
(0.05 mmol), Me2Zn (0.75 mmol, 1.2 M in toluene), Cu(OTf)2 (0.3
mmol), DCE (0.1 M), 80 °C, LG = Cl, 17 h. Major regioisomer was
shown; regioisomeric ratio was given in parentheses. 1 (2.0 mmol),
b
c
With the optimized conditions in hand, various ketones were
first tested with allyl chloride (2a) as a model substrate
(Scheme 2). Both electron-donating and -withdrawing
substituents were tolerated on the benzene ring giving the
expected products smoothly (3a−g). Despite increased steric
hindrance, ortho-methoxy acetophenone delivered the corre-
sponding product in moderate yield (3h). The secondary
directing effect, rather than the steric, governed the
regioselectivity when two C−H bonds were available for
allylation (3i). Linear and cyclic primary alkyl aryl ketones
were both suitable substrates for this reaction (3j, k).
Secondary alkyl aryl ketones worked equally well (3l−o).
tert-Butyl aryl ketones bearing electron-varied substituents
were also amenable to the reaction conditions in spite of the
enhanced steric hindrance (3p−z). A sterically less congested
C−H bond was preferentially allylated with high regioselec-
tivity for the meta-substituted substrate (3A). The naphthalene
moiety could also be allylated smoothly (3B). Benzophenone
derivatives delivered exclusively the mono-C−H allylation
no other regioisomers were detected in the crude reaction mixture by
d
e
NMR. No Cu(OTf)2. RT, Me2Zn (0.5 mmol, 1.2 M in toluene),
f
g
h
Cu(OTf)2 (0.1 mmol), LG = OCOCF3. LG = Br. 35 °C. E/Z ratio
in parentheses.
products out of four available C−H bonds (3C−E).
Remarkably, the electron-rich benzene ring was selectively
allylated in an unsymmetrical diaryl ketone (3F), and selective
C−H allylation on the unsubstituted benzene ring was found
in the diaryl ketone bearing an o-methyl group (3G). In
addition, heterocycles such as indole, thiophene, and furan
were also successfully allylated (3H−J). Of note, a variety of
functional groups such as ester, amine, ether, thioether,
sulfonyl, trifluoromethyl, and halides, which are valuable
synthetic handles for further elaborations, were well tolerated
in the current reaction.
Next, we turned our attention to varying structures of allylic
electrophiles. While 3-chloro-1-butene reacted with p-
methoxyacetophenone (1a) under previously optimized
B
DOI: 10.1021/acs.orglett.9b02554
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conditions giving 45% linear and 9% branched products, the
linear product (3K) was obtained from the reaction of 1a and
α-methylallyl trifluoroacetate (2b) at room temperature in
74% isolated yield with 100% SN2′ regioselectivity and high E/
Z selectivity. Similarly, the corresponding linear products could
be accessed by varying the allylation reagent (2c), directing
group, and arene moiety (3L−N). β-Substituted allyl bromides
(2d, 2e) reacted with ketone 1a at room temperature to afford
the expected products in good yields (3O, 3P). When γ-
methylallyl trifluoroacetate (2f) was used, the branched
product was obtained in 64% yield at 35 °C with excellent
regioselectivity (3Q). Of note, no double-bond migrated
styrene derivatives or bis-C−H allylation products were
observed in the reaction for all cases.
in good yields respectively (Scheme 3b). The addition of either
Me2Zn or Cu(OTf)2 gave inferior results. Meanwhile,
manganacycles could also catalyze the C−H allylation
reactions of ketones in comparably high yields of products
(Scheme 3c). A competition reaction between electron-rich
and -deficient aceto-phenones (1a and 1e) with 2a revealed
that 1a reacted more favorably than 1e with a ratio of 2.3:1
(Scheme 3d). A kinetic isotope effect value of 2.2 was obtained
from two parallel reactions of 1p and [D5]-1p with 2a
respectively, suggesting a possible turnover-limiting C−H
cyclomanganation step (Scheme 3e).
Based on these clues and previous reports,4e,k a plausible
reaction mechanism is proposed in Scheme 4. The initial
To study the possible reaction pathways, stoichiometric
reactions of ketones (1a and 1q) with MnBr(CO)5 were
conducted (Scheme 3a). While no reaction occurred without
Scheme 4. Proposed Reaction Mechanism
Scheme 3. Mechanistic Studies
reaction of ketone 1 and MnBr(CO)5 gave five-membered
manganacycle Mn-I with the aid of Me2Zn and Cu(OTf)2
(Scheme 3a). Coordination of allylic electrophile 2 with
mangangese through releasing a CO ligand led to intermediate
Mn-II, which was followed by insertion of the CC bond
affording seven-membered manganacycle Mn-III. Then β-
elimination of the leaving group in Mn-III gave product-
coordinated species Mn-IV. The ensuing ligand exchange
between product 3 and substrate 1 on the manganese center
and further transmetalation with Me2Zn produced 3 and Mn-
V. A step of C−H activation in Mn-V occurred to release
methane4e and regenerate Mn-II. The liberation of CO and
methane was confirmed by GC analysis.11 Of note, the
presence of Cu(OTf)2 not only promoted the C−H activation
step4o but also prohibited the aldol-type side reactions (Table
1). Interestingly, Fairlamb and Lynam uncovered the
formation of MnI carbonyl clusters in manganese catalysis,
which may provide hints regarding the possible catalyst
deactivation pathways in our reactions.12
In summary, we have developed the first manganese-
catalyzed aromatic C−H allylation of ketones, which features
mild reaction conditions, a wide scope of substrates, and high
regio-/stereoselectivity. Our approach allows for facile access
to diversely substituted ortho-C−H allylated ketone products
through use of challenging allylic electrophiles bearing
substituents at the α-, β-, or γ-position. Further investigations
on the detailed reaction mechanism and other Mn-catalyzed
C−H activation reactions of ketones are underway in our
laboratory.
an additive or with Cu(OTf)2, the expected manganacycles
Mn-Ia and Mn-Iq were obtained in 20% and 21% isolated
yields respectively by adding Me2Zn.4k,o Further addition of
Cu(OTf)2 increased the yields of the corresponding mangana-
cycles, which indicated the positive role of Cu(OTf)2 in the
C−H activation step.4o Next, stoichiometric reactions of
manganacycles Mn-Ia and Mn-Iq with allylic electrophiles
were tested and the allylation products 3a and 3q were isolated
C
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b02554.
■
S
Experimental details, characterization data, and NMR
spectra for all new compounds (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: wangcy@iccas.ac.cn.
ORCID
Congyang Wang: 0000-0002-8053-8251
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
Financial support from the National Natural Science
Foundation of China (21772202, 21831008), Beijing Munic-
■
ipal Science
& Technology Commission (No.
Z181100004218004), and Beijing National Laboratory for
Molecular Sciences (BNLMS-CXXM-201901) is gratefully
acknowledged. We also thank the Chinese Academy of
Sciences for the PIFI fellowship to S.A.
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