Cobalt(III)-Catalyzed Intermolecular Carboamination of Propiolates and Bicyclic Alkenes via Non-Annulative Redox-Neutral Coupling

Article abstract of DOI:10.1021/acs.orglett.9b02016

A cobalt(III)-catalyzed, redox-neutral, intermolecular carboamination of propiolates and bicyclic alkenes was developed. This non-annulative coupling strategy features atom economy, high regioselectivity, good yields, and functional groups tolerance. Such a carboamination reaction was applied to modified phenols from the corresponding phenols under mild conditions.

Full text of DOI:10.1021/acs.orglett.9b02016

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Cobalt(III)-Catalyzed Intermolecular Carboamination of Propiolates
and Bicyclic Alkenes via Non-Annulative Redox-Neutral Coupling
Yuelu Zhu,†,§ Feng Chen,† Xinyang Zhao,† Dingyuan Yan,† Wanxiong Yong,‡ and Jing Zhao*,†,§
†State Key Laboratory of Coordination Chemistry, Institute of Chemistry and BioMedical Sciences, School of Chemistry and
Chemical Engineering, Nanjing University, Nanjing 210093, China
‡College of Chemical Engineering, Nanjing Forestry University, Nanjing 210037, China
§Shenzhen Research Institute, Nanjing University, Shenzhen 518000, China
S
* Supporting Information
ABSTRACT: A cobalt(III)-catalyzed, redox-neutral, intermo-
lecular carboamination of propiolates and bicyclic alkenes was
developed. This non-annulative coupling strategy features
atom economy, high regioselectivity, good yields, and
functional groups tolerance. Such a carboamination reaction
was applied to modified phenols from the corresponding
phenols under mild conditions.
Despite significant progress, the metal catalysts, however, are
still primarily based on noble metals, especially the 4d and 5d
metals. Owing to the rising cost of noble metals and the
benefits of green chemistry, directed C−H bond activation via
the more available and more redox flexible 3d transition metal
catalysts has been receiving an increasing amount of attention
in recent years.7 Cobalt has been shown to exhibit good
activity and great functional group tolerance, rendering cobalt-
based catalytic systems competitive alternatives in exploring
new transformations.8 In 2013, the first direct C−H bond
functionalization activated by Cp*Co(III) was developed by
Kanai et al.9 The potent catalytic efficiency of Co(III) was
particularly impressive for C−H bond activation that was
comparable to that of the Cp*Rh(III) catalyst. Thereafter,
more cases of cobalt-catalyzed C−H bond functionalization
were reported. These cobalt catalysts displayed reactivity
analogous to that of their rhodium counterparts.10 However,
there were few reports of the complementary catalytic activity
of Co(III) to their Rh(III) congeners.11 In 2016, a significant
breakthrough by Glorius and co-workers showed that the
direct difunctionalization of alkenes could be constructed by
Cp*Co(III)-catalyzed carboamination (Scheme 1b). This
report demonstrated a complementary property of Cp*Co(III)
to its Rh(III) counterpart in O-NHAc-directed C−H bond
activation.12 Inspired by these reports, our group has been
working on sustainable C−H bond activation methods to
generate a variety of functional phenols.13 Herein, we report an
intermolecular carboamination of propiolates and bicyclic
alkenes catalyzed by Cp*Co(III) via non-annulative redox-
neutral couplings (Scheme 1c).
n internal oxidant-directed, transition metal-catalyzed C−
AH bond activation was discovered as an atom-economic
strategy for constructing the C−C bonds in the past decade.1
Notably, the N-phenoxyacetamide (PhO-NHAc) group could
utilize its unique redox-neutral reactivity for C−H bond
functionalization cascade reactions, yielding phenols after the
N−O bond cleavage.2 In 2013, Lu et al. pioneered the redox-
neutral olefination reactions in the synthesis of phenols by
Cp*Rh(III) catalysis using O-NHAc as a powerful directing
group.3 Subsequently, C−H bond activation/cyclization was
studied intensively.4 In 2018, Zhang et al. reported the first
Cp*Rh(III)-catalyzed isomerization and lactonization via C−
H bond activation for the synthesis of benzofuran-2(3H)-ones,
achieving great regio- and stereoselectivity (Scheme 1a).5
Recently, Yi et al. discovered the selective β-F elimination
directed by O-NHAc to generate monofluoroalkenyl
dihydrobenzo[d]isoxazoles via a Rh(III)-catalyzed [4+1]
annulation.6
Scheme 1. Carboamination of Alkynes and Alkenes
N-Phenoxyacetamide 1a was first reacted with 3-phenyl-
propiolate 2a in 2,2,2-trifluoroethanol (TFE) to obtain an
Received: June 12, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b02016
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Letter
intermolecular carboamination product 3aa with a yield of 19%
when [Cp*Co(CO)I2], AgSbF6, and NaOAc were used (see
Table S1). After an extensive screening of various acids and
bases, 3aa was obtained in 86% yield when [Cp*Co(CO)I2],
AgSbF6, K3PO4, and CsOAc were used. However, no reaction
occurred when CoBr2, CoCl2, and Co(acac)2 were used,
indicating an essential role of Cp*Co(III) in obtaining the
desired product.
the substrate, presumably due to the tautomerization of the
enamine and the imine and the rotation of the C−C single
bond.5 Moreover, 3-alkyl propiolates gave 3an and 3ao with
40% and 67% yields, respectively.
We then examined the carboamination of alkenes and found
that the coupling reaction of 1a with 1,4-dihydro-1,4-
methanonaphthalene (4a) proceeded smoothly by fine-tuning
of the reaction conditions (for details, see Table S2). The
range of N-phenoxyacetamide was investigated under opti-
mized reaction conditions (Scheme 3). First, we focused on
Next, we explored the range of substrates under the
carboamination reaction (Scheme 2). N-Phenoxyamides with
Scheme 2. Variations of N-Phenoxyacetamides and
Propiolates
Scheme 3. Variations of N-Phenoxyacetamides and
Alkenes
a,
b
a,b
a
Reaction conditions: 1 (0.2 mmol), 4 (0.22 mmol), [Cp*Co(CO)-
I2] (10 mol %), AgSbF6 (20 mol %), K3PO4 (0.5 equiv), KOAc (0.5
equiv), TFE (1.0 mL), 40 °C, 24 h. Isolated yields.
b
a
Reaction conditions: 1 (0.2 mmol), 2 (0.22 mmol), [Cp*Co(CO)-
I2] (10 mol %), AgSbF6 (20 mol %), K3PO4 (0.5 equiv), CsOAc (0.5
equiv), TFE (1.0 mL), 40 °C, 24 h. Isolated yields.
b
the acyl-substituted group of N-phenoxyamides and obtained
the desired products (5aa−5ac) in 68−86% yields; N-
phenoxyacetamides with electron-rich and -poor substituents
gave 39−82% yields (5da−5qa). With disubstituted N-
phenoxyacetamides, the C−H bond that possessed less steric
hindrance exhibited excellent selectivity, and the reaction
delivered 5ra and 5sa with 74% and 32% yields, respectively. 2-
N-(Naphthalen-2-yloxy)acetamide delivered carboamination
product 5ta with a 56% yield. Various bicyclic olefins also
gave the corresponding carboamination products. 7-Azabenzo-
norbornadienes with various protecting groups, such as tosyl
(Ts), mesyl (Ms), and tert-butyloxycarbonyl (Boc), were also
performed well, giving the corresponding products in 46−71%
yields. Derivatives of 1,4-dihydro-1,4-epoxynaphthalene and 7-
oxabicyclo 2,5-diene delivered the desired products in 55−81%
yields (5ae, 5af, and 5ag). Moreover, using a norbornadiene
derivative as the substrate, the carboamination product (5ah)
was obtained in 77% yield.
different acyl substituents were found to provide the
carboamination products (3aa−3ca) in 51−86% yields. para-
Substituted N-phenoxyacetamides bearing electron-rich and
-poor moieties provided the corresponding products with 43−
83% yields (3da−3la). When the N-phenoxyacetamides were
meta-occupied, carboaminations occurred regioselectively at
the less steric position (3ma and 3na). Additionally, ortho
substituents were used to produce 3oa and 3pa as desired
products with 54% and 45% yields, respectively. For the
disubstituted N-phenoxyacetamides, the reaction provided
excellent selectivity for the C−H bond with less steric
hindrance, giving 3qa and 3ra with yields of 64% and 52%,
respectively. A range of propiolates were explored by tolerating
the electron-rich and -poor moieties on aromatic rings, giving
the carboamination products in 48−78% yields (3ab−3ak). In
addition, 3-phenylpropiolates with a carbethoxy group afforded
3al in a 69% yield. Interestingly, lactonated product 3am was
formed in 58% yield when using 3-thiophen-2-yl propiolate as
Phenols are the pivotal scaffolds in pharmaceuticals and
many natural products.14 Advantages of the mild reaction
conditions encouraged us to apply this strategy to the natural
B
DOI: 10.1021/acs.orglett.9b02016
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Letter
bioactive scaffolds (Scheme 4). Under the optimized
conditions, substrates derived from nature products, such as
faster than the deuteration of 1a. The parallel experiments of
1a and 1a-d5 with 2a were also carried out for the
measurement of the kinetic isotope effect (KIE), giving a
KIE value of 2.45, and a KH/KD ratio of 1.61, which indicated
that the rate-determining step could be influenced by the C−H
bond cleavage (Scheme 5b).15 Under the standard reaction
conditions, when compounds 1a and 1d were used as
reactants, high-resolution mass spectrometry (HRMS) de-
tected only the product of intramolecular amide migration, but
mixed amide migration products 3ca′ and 3da′ were not
detected. This result suggested that the formation of the C−N
bond might occur intramolecularly (Scheme 5c).
Scheme 4. Diversification of Natural Products
On the basis of the experiments described above and related
reports,5,11,16 a possible cycle of reaction mechanism was
proposed (Scheme 6). First, the active species (A) was formed
Scheme 6. Plausible Mechanism
a
Reaction conditions: 1 (0.2 mmol), 2a (0.22 mmol), [Cp*Co-
(CO)I2] (10 mol %), AgSbF6 (20 mol %), K3PO4 (0.5 equiv), CsOAc
(0.5 equiv), TFE (1.0 mL), 40 °C, 24 h. Reaction conditions: 1 (0.2
b
mmol), 4a (0.22 mmol), [Cp*Co(CO)I2] (10 mol %), AgSbF6 (20
mol %), K3PO4 (0.5 equiv), KOAc (0.5 equiv), TFE (1.0 mL), 40 °C,
24 h.
tyrosine, tyramine, and estrone, were also tested in this
reaction. The catalytic carboamination proceeded smoothly,
yielding the corresponding products with 64−72% yields.
Several control experiments were conducted to explore the
reaction mechanisms (Scheme 5). First, to investigate the
Scheme 5. Mechanistic Experiments
by reacting [Cp*Co(Co)I2] with AgSbF6 and a base. A
coordinated with 1a, activating the o-C−H bond by
elimination of HOAc to form a five-membered cobaltacyclic
species (B). The seven-membered intermediate (C) was
generated after the insertion of alkyne into intermediate B.
Next, the formation of the Co(I) intermediate (D) delivered
the desired C−N bond by reductive elimination of C(sp2)−
metal species (C). Subsequently, an oxidative addition by the
O−N bond to intermediate D led to the seven-membered
cobaltacyclic species (E). Lastly, E regenerates active catalytic
species A, which was available for the next catalytic cycle.
Thus, the carboamination of alkenes was achieved through C−
H bond metalation, olefin insertion, reductive elimination, and
oxidative addition processes.
To conclude, we described an efficient, highly regioselective,
and atom-economic process of o-C−H bond functionalization
of phenols by Cp*Co(III)-catalyzed intramolecular carboami-
nation of the propiolates and bicyclic alkenes via non-
annulative redox-neutral couplings. This strategy possesses
the advantages of the appropriate tolerance of the functional
groups and mild reaction conditions. This cobalt-catalyzed
reactivity is complementary to the rhodium(III)-catalyzed
route to intermolecular carboamination.
reversibility of C−H bond activation, the reaction in
deuterated TFE was performed without addition of the
1
propionate substrate (2a). The H NMR spectrum showed
ASSOCIATED CONTENT
* Supporting Information
■
that ∼76% protons on the ortho positions of 1a were
deuterated after 24 h, indicating a reversibility of the C−H
activation process (Scheme 5a). However, <5% deuteration of
3aa was achieved when 2a was added, and this might suggest
that the migratory insertion reaction rate of 2a with 1a was
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b02016.
C
DOI: 10.1021/acs.orglett.9b02016
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Experimental procedures, characterization, and spectral
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data (PDF)
Accession Codes
CCDC 1878905, 1900206, and 1921289 contain the
supplementary crystallographic data for this paper. These
data can be obtained free of charge via www.ccdc.cam.ac.uk/
data_request/cif, or by emailing data_request@ccdc.cam.ac.
uk, or by contacting The Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44
1223 336033.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: jingzhao@nju.edu.cn.
ORCID
Jing Zhao: 0000-0001-5177-5699
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank the National Science Foundation of China
(21571098, 21622103, and 91753121), the Shenzhen Basic
Research Program (JCYJ20170413150538897), the Natural
Science Foundation of Jiangsu Province (BK20160022), and
the Fundamental Research Funds for the Central Universities
(020514380139).
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