Thiocarbamate-directed Cp*Co(III)-Catalyzed Olefinic C?H Amidation: Facile Access to Enamines with High (Z)-Selectivity

Article abstract of DOI:10.1002/ejoc.202001505

A thiocarbamate-directed Cp*Co(III)-catalyzed C?H oxidative amidation of olefins is achieved to synthesize a series of enamines. The key feature of this protocol is the use of earth-abundant cobalt as catalyst and thiocarbamate as directing group, which provides an efficient and simple manner to synthesize enamines in good yields with high (Z)-selectivity. This reaction proceeds smoothly under very mild conditions (rt and air), and a wide range of functionalized alkenes, as well as dioxazolones, were compatible with the standard reaction conditions.

Full text of DOI:10.1002/ejoc.202001505

European Journal of Organic Chemistry
Accepted Article
Accepted Article
Title: Thiocarbamate-directed Cp*Co(III)-Catalyzed Olefinic C-H
Amidation: Facile Access to Enamines with High (Z)-Selectivity
Authors: Ya-Ru Liang, Xiao-Ju Si, He Zhang, Dandan Yang, Jun-Long
Niu, and Mao-Ping Song
This manuscript has been accepted after peer review and appears as an
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.202001505
Link to VoR: https://doi.org/10.1002/ejoc.202001505
0
1/2020

European Journal of Organic Chemistry
10.1002/ejoc.202001505
COMMUNICATION
Thiocarbamate-directed
Cp*Co(III)-Catalyzed
Olefinic
C-H
Amidation: Facile Access to Enamines with High (Z)-Selectivity
Ya-Ru Liang, Xiao-Ju Si, He Zhang, Dandan Yang,* Jun-Long Niu* and Mao-Ping Song
Y.-R. Liang, X.-J. Si, H. Zhang, Dr. D. Yang, Prof. J.-L. Niu, Prof. M.-P. Song
Green Catalysis Center, and College of Chemistry
Zhengzhou University
Zhengzhou, Henan 450001, China
E-mail: yangdandan@zzu.edu.cn
niujunlong@zzu.edu.cn
Supporting information for this article is given via a link at the end of the document.((Please delete this text if not appropriate))
Abstract:
A
thiocarbamate-directed Cp*Co(III)-catalyzed C-H
oxidative amidation of olefins is achieved to synthesize a series of
enamines. The key feature of this protocol is the use of earth-
abundant cobalt as catalyst and thiocarbamate as directing group,
which provides an efficient and simple manner to synthesize
enamines in good yields with high (Z)-selectivity. This reaction
proceeds smoothly under very mild conditions (rt and air), and a
wide range of functionalized alkenes as well as dioxazolones were
compatible with the standard reaction conditions.
Enamines were usually served as useful precursors to
synthesize nitrogen heterocycles, which extensively exist in
biologicals, pharmaceuticals, and natural products.[1] Traditional
methods for synthesizing enamines were largely depended on
the cross-coupling of alkenyl halides with amines and the
condensation of aldehydes or ketones with amines (Scheme
1
a).[2] However, these methods generally occurred under harsh
conditions with a narrow substrate scope. Furthermore, another
obstacle is how to effectively control the selectivity of enamines，
[
3]
and this is the main problem and challenge we are facing now.
Different from the traditional methods for synthesizing
enamines, transition metal-catalyzed C-H functionalization[4] is
highly explored due to the tremendous potential in atom-
economy and high efficiency. Remarkable progress has been
Scheme 1. Background and project synopsis.
achieved by employing precious metals such as Pd, Ru, Rh
(
Scheme 1b).[ But one significant limitation existing in these
5]
resembled metal complex species to achieve olefinic amidation
will be challenging. In 2013, Colobert,[13] Shi,[14] You[15] and other
groups had achieved great progress on the sulfur-atom auxiliary
systems is that precious metals are few reserve and high cost.
Conversely, the earth-abundant 3d transition metal cobalt
catalysts have attracted considerable attention due to the low
cost and unique activity compared to other counterparts.[6] But
there are few examples to realize the Co-catalyzed olefinic
selective C-H functionalization.[7] Therefore, it is highly desirable
to develop an efficient and convenient strategy to construct C-N
bond with high selectivity by using cobalt catalyst in current
methodological research.
On the other hand, the directing group strategy that takes
advantage of the coordination of chelating atom with transition
metal for forging new C–C or C–X bonds seems to be an
excellent choice.[8] In previous works, oxygen[9] and nitrogen[10]
atoms were commonly used as the electron donors in directing
groups, whereas utilizing sulfur atom as directing group in C-H
activation was relatively rare for a long time.[11] This may be
2
C(sp )-H functionalization catalyzed by transition metal (Scheme
16]
[17]
1
c). In addition, Yu[ and Gong
groups had published the
3
thioamide-directed regioselective C(sp )-H activation and
realized the enantioselectivity version. In 2015,
a cobalt-
catalyzed C-H amidation of arenes with doxazolones as new
amidating reagents was originally reported by the Chang group
(Scheme 1d). However, sulfur atom-directed olefinic C-H
oxidative amidation has been rarely exploited. Enlightened by
the previous work and our ongoing cobalt catalysis studies,[
herein we report a thiocarbamate-directed Cp*Co(III) catalyzed
olefinic selective C-H amidation. The sulfur atom of
thiocarbamate plays a key role in the coordination of the
transition metal. And this protocol possesses the advantages of
exclusive (Z)-selectivity, a wide substrate scope and high
isolated yields (Scheme 1e).
18]
attributed to its strong-coordinating and potential poison with
transition metals.[
12]
Consequently, employing the sulfur-
We commenced a preliminary investigation by using model
substrates thiocarbamate 1a and dioxazolone 2a in the
chelating groups that coordinate with the metal to generate
1
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European Journal of Organic Chemistry
10.1002/ejoc.202001505
COMMUNICATION
presence of [Cp*Co(MeCN)
Table 1). Encouragingly, the reaction proceeded smoothly to
afford the C-H amidated product 3aa in 46% yield (entry 1, Table
). It was noteworthy that the exclusive Z-isomers product 3aa
was observed. Next, subsequent investigation on solvents was
conducted (Table S1), and 1,4-dioxane was found to be most
effective as compare to other solvents (70% yield, entry 2). The
3
](SbF
6
)
2
in DCE under air for 12 h
were afforded in good yields (55%-70%). The aliphatic substrate
1q also reacted well with dioxazolone to afford the desired 3qa
successfully. It should be noted that all the products were
exclusive Z-isomers in this reaction, which indicated that this
protocol possessed a characteristic of high selectivity.
(
1
Scheme 2. Cp*Co (III) catalyzed olefinic C-H amidationa
yield
Cp*Co(MeCN)
Subsequently, various additives such as K
decreased
obviously
was replaced by Cp*Co(CO)I
CO , Na CO and
(entry
3)
when
the
2
.
[
3
](SbF
)
6 2
2
3
2
3
acids were evaluated (entries 4-9). It was found that the yield
was remarkably improved to 83% with 0.5 equiv of AcOH as the
additive (entry 9). Further investigation revealed that the yields
decreased while increasing the temperature (Table S5).
Surprisingly, when the loading of 2a was decreased to 1.0
equivalent, an extremely high yield of 94% was observed at
room temperature (entry 11). A control experiment confirmed
that the reaction didn’t occur in the absence of the cobalt
catalysts (entry 12). Meanwhile, exchanging the S with O atom
in the directing group gave no desired product, demonstrating
the necessity of sulfur for this transformation.[19]
Table 1. Optimization of reaction conditionsa
o
yield (%)a
46
entry
1b
2
catalyst
T ( C)
60
60
60
60
60
60
60
60
60
rt
additive
[Co*Co(MeCN)
[Co*Co(MeCN)
3
](SbF
6
6
)
)
2
2
-
-
3
](SbF
70
3c
Cp*Co(CO)I
AgBF
26
2
4
4
[Co*Co(MeCN)
[Co*Co(MeCN)
[Co*Co(MeCN)
[Co*Co(MeCN)
[Co*Co(MeCN)
[Co*Co(MeCN)
[Co*Co(MeCN)
[Co*Co(MeCN)
-
3
3
3
3
3
3
3
3
](SbF
](SbF
](SbF
](SbF
](SbF
](SbF
](SbF
](SbF
6
6
6
6
6
6
6
6
)
)
)
)
)
)
)
)
2
2
2
2
2
2
2
2
K
2
CO
3
72
5
NaHCO
1-AdCOOH
PhCO
3
61
6
77
7
2
H
73
8
PivOH
AcOH
AcOH
AcOH
AcOH
AcOH
74
9
83
10
87
aReaction conditions: 1 (0.2 mmol), 2a (1.0 equiv), [Co*Co(MeCN)
mol%), 1,4-dioxane (1 mL), air atmosphere, rt, 12 h, isolated yield. 2a (1.5
3
6 2
](SbF ) (5
1
1d
rt
94
b
2 4
equiv), [Cp*Co(CO)I ] (5 mol%), AgBF (10 mol%), AcOH (1.0 eq), HFIP (1
12
rt
N.R.
82%
mL), air atmosphere, 12 h, rt. cThe Z/E ratio was determined by 1H NMR of
1
3e
[Co*Co(MeCN)
3
](SbF
6
)
2
rt
d
crude product mixture. The Z/E ratio was determined by isolated yield.
aReaction conditions: 1a (0.1 mmol), 2a (1.5 equiv), [Co*Co(MeCN)
5 mol%), 1,4-dioxane (0.5 mL), air atmosphere, rt, 12 h, isolated yield. DCE
3
6 2
](SbF )
b
(
Meanwhile, the substrate 1r as internal alkene was
performed to discuss the applicability of this method.
Surprisingly, the amidation product 3ra with oxygen and sulfur
exchanged was observed under standard conditions, [ albeit
with a low yield (Table S9). When 1,4-dioxane was replaced by
HFIP in the system (Table S9), there was an obvious yield
enhancement of 3ra (33%).[21] Further optimization on cobalt
catalysts and additives was conducted (Tables S9-S16), and the
yield of 3ra was improved to 60% in the presence of
c
d
e
as solvent. Cp*Co(CO)I
as solvent.
2 4
(5 mol%), AgBF (10 mol%). 2a (1.0 equiv). HFIP
With the optimal reaction conditions in hand, we firstly
examined the scope of 1 (Scheme 2). The substrates bearing
EDG or EWG groups (-Cl, -OMe) at the ortho-position of the
phenyl groups reacted smoothly, affording the corresponding
products in good yields (3ba, 3ca). Next, the thiocarbamate
substrates with para-substituted groups on the aromatic ring
were further investigated, both electron-donor (3da, 3ea, 3fa)
and electron-receptor (3ga, 3ha, 3ia, 3ja) ones were feasible
with good yields (up to 99%). The diverse meta-substituted
thiocarbamate substrates also participated in the reaction
favorably to deliver the desired amidation products in excellent
yields (3ka, 3la, 3ma, 3na). Besides, when the phenyl was
replaced by naphthyl groups, the desired products 3oa and 3pa
20]
2 4
Cp*Co(CO)I and AgBF in HFIP. Next, several internal olefinic
substrates were investigated. The para-methoxyl substituted
substrate was well-tolerated and gave the O/S exchanged
product 3sa in 49% yield with high (Z)-selectivity. The
naphthalene-substituted internal olefinic substrate 1t was
suitable for this transformation as well, giving the amidated
product 3ta in 68% yield with Z/E ratio of 5:1. And the Z/E-
2
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European Journal of Organic Chemistry
10.1002/ejoc.202001505
COMMUNICATION
isomers structures of 3ra (Z isomer: CCDC 2038667; E isomer:
CCDC 2038666) were further confirmed by X-ray
crystallography analysis (Tables S18-S19).
Then, competitive experimental results demonstrate that the
electron-rich substrates have better reactivity. (Scheme 4c).
Furthermore, H/D exchange could be detected when 1a was
Subsequently, the scope of dioxazolones 2 was evaluated
and the results were listed in Scheme 3. Dioxazolones with
electron-donating and -withdrawing groups at the ortho, meta
positions of the phenyl groups were well tolerated. And the
desired products 3ab-3af were obtained in extremely high yields
with excellent (Z)-selectivity. Additionally, para-phenyl-
substituted dioxazolones also worked well, leading to 3ag-3al in
good yields (56%-94%). And the structure of 3ag (CCDC
stirred under standard conditions with 5 eq D
the C-H cleavage was reversible (Scheme 4d). However, when
the deuterated D -1a was added to the model condition with 5
2
O, indicating that
2
eq H
2
O, the D/H exchange product was not observed according
1
to the H NMR analysis. Then we put the deuterated substrate
2
D -1a and dioxazolone into the standard conditions, but it was
found that the reaction could not react at all (Scheme 4d). This
result indicates that the C−H bond cleavage may be involved in
the rate-limiting step.
2038668) was further confirmed by X-ray crystallography
analysis. Moreover, when the phenyl was replaced with a
naphthalene or thiophene moiety (3am, 3an), the reaction also
proceeds smoothly (82% and 98%). Notably, the olefinic and
aliphatic dioxazolones were compatible with good functional
tolerance, affording the corresponding enamine products 3ao-
Scheme 4. Large scale and mechanistic experiments
3
aq in moderate to high yields.
Scheme 3. Substrate scope of dioxazolonesa
Based on the above mechanistic experimental results, a
plausible mechanism is proposed. First, the Cp*Co(III)
undergoes ligand exchange with AcOH, followed by
thiocarbamate-assisted concerted-metalation-deprotonation
CMD)[22] to generate intermediate I. Then the nitrogen of
a
(
dioxazolone coordinates with intermediate I to afford the Co(III)
intermediate II. Subsequently, intermediate II undergoes
aReaction conditions: 1a (0.2 mmol), 2 (1.0 equiv), [Co*Co(MeCN)
3
6 2
](SbF ) (5
2
migratory insertion and releases CO to deliver intermediate III.
mol%), 1,4-dioxane (1 mL), air atmosphere, rt, 12 h, isolated yield.
Finally, demetalation of intermediate III affords the 3aa and
regenerates Co(III) catalyst for a new catalytic cycle.
To demonstrate the synthetic utility of the Co-catalyzed
amidation reaction, a large-scale reaction was performed with a
high yield (86%). Notably, further derivatization of enamines
products could be carried out smoothly. The biologically
important 2,5-disubstituted oxazole could be synthesized by
In conclusion, an efficient Cp*Co(III)-catalyzed olefinic C-H
oxidative amidation strategy using thiocarbamate as directing
group to synthesize (Z)-selective enamines has been developed.
The exclusive (Z)-selectivity, earth-abundant cobalt catalyst,
excellent functional group tolerance and mild conditions make
this reaction to be an attractive protocol for the construction of
using Pd(OAc)
thus promoted the potential application value in synthesis.
Scheme 4a). In order to gain further insight into the mechanism,
2
with BQ, TsOH, and AcOH in one-pot, which
(
2
C(sp )−N bonds of olefins. This strategy also provides a simple
following experiments were performed. Upon addition of
stoichiometric amounts of radical scavengers, the formation of
and practical method to access structurally diverse enamine
derivatives. Further applications in other related types of olefinic
C−H functionalization are actively ongoing in our laboratory.
3
aa was not suppressed except for the BHT, which suggested
that this process might not involve a free radical (Scheme 4b).
3
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European Journal of Organic Chemistry
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COMMUNICATION
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Keywords: C-H activation • cobalt catalysis • enamines •
thiocarbamate • (Z)-selectivity
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Entry for the Table of Contents
A Thiocarbamate-directed earth-abundant Cp*Co(III)-catalyzed C-H amidation of olefins to synthesize (Z)-selective enamines was
explored. And the mild reaction conditions and a yield of up to 99% make the strategy more practical and attractive for constructing a
new C-N bond.
6
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