Cobalt-Catalyzed Allylation of Amides with Styrenes Using DMSO as Both the Solvent and the α-Methylene Source

Article abstract of DOI:10.1021/acs.orglett.9b02462

An efficient synthesis of privileged allylic amines has been developed via cobalt-catalyzed allylation of amides with styrenes, in which DMSO was used as both the solvent and the α-methylene source. This transformation features high yields, and selectivity for the (E)-isomer of the linear product. Through the experimental and computational investigations, a sequential K₂S₂O₈-mediated oxidative coupling/cobalt-assisted regioselective alkene insertion/β-H elimination/alkene dissociation/hydride transfer process has also been deduced.

Full text of DOI:10.1021/acs.orglett.9b02462

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Cobalt-Catalyzed Allylation of Amides with Styrenes Using DMSO as
Both the Solvent and the α‑Methylene Source
Xu Zhang,†,§ Zhi Zhou,‡,§ Huiying Xu,‡,§ Xuefeng Xu,† Xiyong Yu,*,‡ and Wei Yi*,†,‡
†Engineering Technology Research Center of Henan Province for Solar Catalysis, College of Chemistry and Pharmaceutical
Engineering, Nanyang Normal University, Henan 473061, China
‡Key Laboratory of Molecular Target and Clinical Pharmacology & State Key Laboratory of Respiratory Disease, School of
Pharmaceutical Sciences & the Fifth Affiliated Hospital, Guangzhou Medical University, Guangzhou, Guangdong 511436, China
S
* Supporting Information
ABSTRACT: An efficient synthesis of privileged allylic amines has
been developed via cobalt-catalyzed allylation of amides with styrenes,
in which DMSO was used as both the solvent and the α-methylene
source. This transformation features high yields, and selectivity for the
(E)-isomer of the linear product. Through the experimental and
computational investigations, a sequential K2S2O8-mediated oxidative
coupling/cobalt-assisted regioselective alkene insertion/β−H elimi-
nation/alkene dissociation/hydride transfer process has also been deduced.
Scheme 1. One-Pot Synthesis of Allylic Amines by C−C
Couplings with π-Compounds
ecognizing the great importance of allylic amines in
Rorganic synthesis, medicinal chemistry, and materials
science,1 chemists have spent tremendous effort to develop
general and efficient methods toward their synthesis. As a
consequence, many versatile coupling reactions2−8 of relatively
active imines with π-compounds to construct the allylic
amines, including the classical Morita−Baylis−Hillman reac-
tion, Lewis-acid-catalyzed Friedel−Crafts alkylation reaction,
as well as recently documented metal-catalyzed C−H
activation/addition reaction, have been independently devel-
oped by the groups of Krische,3 Zhou,4 Jamison,5 Maruoka,6
Shi,7 and others.8 However, by reviewing these notable
achievements, one common feature was found that almost all
reactions are involved in employing an aryl part to stabilize the
imine species, thus giving rise exclusively to α-aryl-substituted
allylic amines (Scheme 1a). Undoubtedly, it limited the next-
stage synthetic application to some extent. To address this
disadvantage, more recently Rovis and Glorius have
independently developed an Ir-catalyzed intermolecular and
branch-selective amidation of allylic C−H bonds.9 However, a
highly active and synthetic dioxazolone species10 was still
involved as the amidating reagent. In addition, to the best of
our knowledge, there have been few reports that have shown
direct synthesis of more common allylic amines, especially for
those for which no substituent was installed at the α-position.11
In this context, exploring a new strategy for the efficient
construction of common allylic amines from the readily
accessible starting materials is still not only highly desirable but
also of great significance.
this topic, and a large number of examples have used DMSO as
the precursor for the introduction of a wide range of functional
groups, such as −O,13 −CN,14 −SMe,15 −CH2SMe,16
−SO2Me,17 −Me,18 and −CHO19 substituents.
Concerning the development of inexpensive and earth-
abundant cobalt-catalyzed C−H activation transformations, we
have reported the Co(III)-catalyzed and DMSO-involved C−
H activation/cyclization of anilines with alkynes for the
synthesis of quinolines,20 where DMSO was employed as the
C1 source of the quinoline core. Continuing with our interest
in cobalt-catalyzed C−H functionalization and on the basis of
the elegant studies performed by the Maruoka group,6 we
envisaged the cobalt-catalyzed reaction of ubiquitous amides
On the other hand, recently, considerable attention has been
paid to the development of transition metal (TM) catalyzed
C−H functionalization by using DMSO as the versatile
reactant in organic synthesis, due to its low toxicity and easy
availability.12 Indeed, significant progress has been made on
Received: July 15, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b02462
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Letter
a
with alkenes in DMSO as an appealing synthetic route to the
synthesis of α-unsubstituted allylic amines (Scheme 1b). This
transformation was likely to involve a methylenebisamide
intermediate,21 which would evolve into the final products
through a cobalt-catalyzed regioselective alkene insertion
without the assistance of the directing groupa certainly
underexplored process in cobalt-catalyzed synthetic chemis-
try.22,23
Scheme 2. Scope of Amides
The feasibility of the proposed reaction was tested with
simple benzamide (1a) and styrene (2a) as model substrates,
AgSbF6 and K2S2O8 as benchmark additives, and Co(acac)2 as
the catalyst (Table 1). Delightfully, treatment of 1a with two
a
Table 1. Screening of Reaction Conditions
b
entry
catalyst
additive A
additive B
yield (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
Co(acac)2
Co(acac)2
Co(acac)2
Co(acac)2
Co(acac)2
Co(acac)2
Co(acac)2
Co(acac)2
Co(acac)2
Co(acac)2
Co(acac)2
Co(acac)2
Co(acac)2
Co(acac)2
---------
AgSbF6
AgNTf2
AgOTf
Ag2CO3
Ag2SO4
AgNO3
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
---------
AgOAc
---------
AgOAc
AgOAc
AgOAc
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
(NH4)2S2O8
Na2S2O8
H2O2
TBHP
DTBP
---------
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
36
41
34
0
0
0
94
0
40
0
0
0
0
34
0
0
38
0
a
---------
Reaction conditions: amide 1 (1.0 mmol), styrene 2a (0.5 mmol),
Co(acac)2 (5 mol %), AgOAc (10 mol %), and K2S2O8 (1.5 equiv) in
DMSO (2 mL) at 130 °C for 12 h under air; isolated yields were
Co(OAc)2
Cp*Co(CO)I2
CoI2
b
c
given. Performed on a 5.0 mmol scale. N-Hydroxybenzamide was
0
d
used instead of benzamide 1a. N-Acetoxybenzamide was used
instead of benzamide 1a. N-Methylbenzamide was used instead of
a
Reaction conditions: benzamide 1a (1.0 mmol), styrene 2a (0.5
e
mmol), Co(acac)2 (5 mol %), additive A (10 mol %), and additive B
(1.5 equiv) in DMSO (2 mL) at 130 °C for 12 h under air. Isolated
yields.
benzamide 1a.
b
delivering the corresponding allylic amines in good to excellent
yields regardless of the nature of the substituents on the amide
moiety. A series of electron-donating (Me, OCH3, Et, n-Bu, t-
Bu) or electron-withdrawing (F, Cl, Br, CF3, CO2Me) groups
at the para-position of benzamide were all well tolerated with
the optimized reaction conditions, giving the desired products
3b−3k in 81−98% yields. Similarly, ortho- and meta-
substituted benzamides also participated in this reaction,
affording the corresponding products 3l−3o with reasonable
yields (up to 92%). It was worth noting that the sole E-isomer
was detected in all cases. The reaction was also tolerant of
moderate steric bulk, as disubstituted benzamides could be
converted into the corresponding allylic amines 3p−3s in
decent yields (up to 90%). Interestingly, amides bearing either
heteroaromatic, polyaromatic, cycloalkyl, aliphatic, or alkyloxy
parts could be accommodated with such transformations,
generating the desired products 3t−3ac in 70−95% yields.
Finally, we were pleased to find that the reaction also worked
well with large-sized amides, N-substituted amides, and cyclic
sulfonamides as exemplified by the synthesis of 3a and 3ad−
equivalent amounts of 2a and an excess of K2S2O8 (1.5 equiv),
AgSbF6 (10 mol %, 0.1 equiv), and Co(acac)2 (5 mol %, 0.05
equiv) under air led to the anticipated 3a in 36% isolated yield,
thus supporting our initial hypothesis. Based on this
encouraging result, we next tested different additives (entries
2−12). The brief screening indicated that AgOAc and K2S2O8
were optimal additives, increasing the yield of 3a up to 94%
(entry 7). Control experiments revealed that the absence of
either the catalyst or additives led to a significant decrease in
the yield of the product, suggesting that both the cobalt(II)
species and AgOAc/K2S2O8 coadditives were essential for this
transformation (entries 13−16). Finally, switching Co(acac)2
to other cobalt species also obviously reduced the yield or
completely inhibited the reaction process (entries 17−19).
Having the optimized conditions in hand, we sought to
probe the scope of the substrate and generality of this
transformation (Scheme 2). With regard to the amide scope,
we found that the reaction of styrene 2a with a wide range of
amides bearing different substituents proceeded smoothly,
B
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Letter
3ah. Taken together, these results further displayed the
versatility of the developed cobalt(II) catalysis.
Scheme 4. Mechanistic Studies
We subsequently turned our attention to the scope of
alkenes. As shown in Scheme 3, we found that this reaction was
a
Scheme 3. Scope of Styrenes
yield, respectively (Scheme 4b). Prolonging the reaction time
to 12 h resulted in an enhancement of the yield of 3a from 17%
to 92%, along with the complete disappearance of 5.
Subsequently, treatment of 5 with 2a under the standard
conditions gave the product 3a in 95% yield (Scheme 4c). The
results clearly indicated that 5 might be employed as one of the
active intermediates involved in the catalytic cycle. Moreover,
treatment of 1a with 2a in both DMSO and DMSO-d6 under
standard conditions gave a small KIE value (kH/kD = 1.2,
Scheme 4d). The result suggested that the K2S2O8-mediated
oxidation process in DMSO might not be involved in the rate-
limiting step. Further KIE was evaluated to shed more light on
the alkene-involved process. As demonstrated in Scheme 4e,
we performed a deuterium competition experiment between
substrate 2a and [d8]-2a and observed a kinetic isotope effect
(KIE) of kH/kD = 2.6.
Based on the above experimental results and to gain a better
mechanistic understanding, density functional theory (DFT)
calculations were further implemented with Gaussian 09 by
selecting the complex IM0 as the starting point (Figure 1).
The geometries of all reported species were optimized using
the B3LYP functional, and single-point energies were
calculated at the M06 level using the SMD solvation model
(see SI for details). First, the regioselective [2,1]-insertion of
the alkene via TS1 requires a barrier of 29.6 kcal/mol, leading
to the intermediate IM2 with a free energy of −3.0 kcal/mol.
Obviously, to proceed in a contrary alkene [1,2]-insertion
pathway, a higher energy profile via TS1′ with a barrier of 34.7
kcal/mol is detected, revealing that the [1,2]-insertion is
clearly disfavored. The results are also in line with our
experimental observation that the chain allylic amine backbone
presents as the preferential product. Thereafter, the species
IM2 undergoes a β-H elimination and subsequent hydride
transfer via TS2 with a barrier of 20.4 kcal/mol, providing the
E-type intermediate IM4. On the other hand, the Z-type
intermediate IM4′ can be formed via TS2′ with an energy
barrier of 23.4 kcal/mol, which is not favored compared to that
a
Reaction conditions: amide 1a (1.0 mmol), styrene 2 (0.5 mmol),
Co(acac)2 (5 mol %), AgOAc (10 mol %), and K2S2O8 (1.5 equiv) in
DMSO (2 mL) at 130 °C for 12 h under air; isolated yields were
given.
compatible with a broad scope of styrenes. Various
substituents with diverse electronic properties were tolerated
(F, Cl, Br, CH3, OCH3, i-Pr, t-Bu, and Ph), and the
corresponding allylic amines (4a−4l) were delivered smoothly
in good yields. Furthermore, di- and trisubstituted styrenes
were also competent in the reaction as products 4m−4p were
obtained in good to high yields (84−95%). Moreover, vinyl
naphthalene (1q) was found to couple with 1a productively,
providing the desired 4q in 95% isolated yield. Impressively, 2-
substituted styrenes with various groups, such as the methyl,
cyclohexyl, and phenyl substituents, which often display
relatively low reactivity in coupling reaction, can also react
with 1a, delivering the allylic amines 4r−4w in decent yields
(78−91%). To better probe the scope of this reaction, we
further explored other alkenes such as hept-1-ene and 1-
allylnaphthalene. However, no expected coupling products
were detected, revealing that the presence of a conjugated aryl
part was crucial for this reaction.
Considering the remarkably good substrate tolerance and
specific regioselectivity displayed by the cobalt(II) catalysis, we
were then intrigued to uncover the reaction mechanism. As
shown in Scheme 4a, we first changed DMSO to DMSO-d6 to
verify the role of DMSO. The result showed that the
deuterated product [d2]-3a was obtained in 89% yield with
almost 100% incorporation of deuterium, suggesting that
DMSO played crucial roles acting as both the solvent and the
α-methylene carbon source for this transformation. Further-
more, the reaction of 1a and 2a was carried out under standard
conditions for 4 h (Scheme 4b), and both allylic amine 3a and
methylenebisamide species 5 were detected with 17% and 81%
C
DOI: 10.1021/acs.orglett.9b02462
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Letter
Figure 1. Computed Gibbs free energy changes of the reaction pathways using the IM0 as the starting point.
of IM4. Finally, with the aid of the PhC(O)NH anion24 from
the reaction solution, the Co−alkene dissociation/hydride
transfer occurred via TS3 (ΔΔG⧧ = 20.0 kcal/mol, from IM5
to TS3) to deliver the desired product with the regeneration of
the complex IM0. It was worth noting that the present DFT
calculations preliminarily disclosed the cobalt-mediated
processes, while the detailed mechanism regarding the roles
of other metal cations (e.g., Ag+) and the rate-determining step
of this transformation still need to be addressed.
On the basis of these experimental and computational
observations, and inspired by the mechanistic studies in the
precedent literature, we proposed a plausible catalytic cycle for
this transformation (Scheme 5). Initially, DMSO was activated
In summary, by taking advantage of DMSO as both the
solvent and the α-methylene source, we have developed a new
and practical protocol for direct allylation of simple amides
with styrenes catalyzed by the earth-abundant and inexpensive
Co(acac)2. This protocol features broad substrate scope and
good to excellent yields, thus rendering it as a highly versatile,
straightforward, and atom-economical alternative to the
existing methods for the synthesis of privileged allylic amines,
in which no additional substituent was attached at the α-
position. Through the detailed experimental and computa-
tional investigations, a possible reaction pathway was rationally
deduced. Further studies on the mechanism and application of
this catalytic reaction are currently underway in our
laboratories.
Scheme 5. Proposed Catalytic Cycle
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.9b02462.
Experimental procedures, characterization of products,
1
and copies of H and 13C NMR spectra (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: yiwei@gzhmu.edu.cn.
*E-mail: yuxycn@aliyun.com.
ORCID
Xu Zhang: 0000-0002-3588-3240
Zhi Zhou: 0000-0002-6521-8946
Wei Yi: 0000-0001-7936-9326
Author Contributions
§X.Z., Z.Z. and H.X. contributed equally.
Notes
by K2S2O8 to generate the reactive electrophilic thionium ion
A, which is then attacked by 1a to yield the intermediate B. A
classical nucleophilic substitution of B with 1a occurred to
provide the methylenebisamides 5. Subsequently, intermediate
C is easily formed from B or 5. The following cobalt(II)-
mediated reaction of C delivers the five-membered cobalt(III)
species D. Regioselective migratory insertion of 2a to the Co−
C bond of D (IM0) via IM1 gives the intermediate E, which
undergoes the β-H elimination to produce the intermediate F
(IM4), followed by the alkene dissociation to yield the desired
product 3a along with the release of HCoX2 species, which
reacts with PhC(O)NH− or MeS− via hydride transfer to
regenerate the active Co(II) catalyst for the next catalytic cycle.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the NSFC (81502909, 21502100, and 21877020),
Guangdong Natural Science Funds for Distinguished Young
Scholar (2017A030306031), the Science and Technology
Programs of Guangdong Province (2015B020225006), Med-
D
DOI: 10.1021/acs.orglett.9b02462
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Organic Letters
Letter
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E
DOI: 10.1021/acs.orglett.9b02462
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
(24) The MeS anion was also proposed to abstract the proton from
HCoX2 for catalyst regeneration via TS3′ (ΔΔG⧧ = 19.9 kcal/mol,
from IM5′ to TS3′). Please see the Supporting Information for
details.
F
DOI: 10.1021/acs.orglett.9b02462
Org. Lett. XXXX, XXX, XXX−XXX