Cobalt-Catalyzed Oxidant-Free Spirocycle Synthesis by Liberation of Hydrogen

Article abstract of DOI:10.1021/acs.orglett.7b02266

The first example of oxidant-free cobalt-catalyzed synthesis of five-membered spirocycles is reported from benzimidates and maleimides utilizing nitrobenzene as promoter. In contrast to previously known cobalt-catalyzed oxidative C-H functionalization rea

Full text of DOI:10.1021/acs.orglett.7b02266

Letter
pubs.acs.org/OrgLett
Cobalt-Catalyzed Oxidant-Free Spirocycle Synthesis by Liberation of
Hydrogen
Ningning Lv,† Yue Liu,† Chunhua Xiong,‡ Zhanxiang Liu,† and Yuhong Zhang*,†,§
†Department of Chemistry, Zhejiang University, Hangzhou 310027, China
‡Department of Applied Chemistry, Zhejiang Gongshang University, Hangzhou 310012, China
§State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, China
S
* Supporting Information
ABSTRACT: The first example of oxidant-free cobalt-catalyzed synthesis of
five-membered spirocycles is reported from benzimidates and maleimides
utilizing nitrobenzene as promoter. In contrast to previously known cobalt-
catalyzed oxidative C−H functionalization reactions, this transformation occurs
efficiently in the absence of oxidant and is accompanied by liberation of
hydrogen. The spiro-lactams were readily achieved by the hydrolysis of as-
prepared spirocyclic compounds. The Cp*Rh(III) catalyst shows poor reactivity.
ransition-metal-catalyzed direct C−H functionalization
thesis of lactam from amines and water with concomitant H2
Thas experienced remarkable progress in the past decades, liberation.11a,b The Ru- and Rh-catalyzed oxidant-free alkeny-
1
and a wide variety of noble metal catalysts, including Pd,2 Rh,3
lation of aromatics with the liberation of hydrogen were
reported by the groups of Jeganmohan11c and Dong.11d Very
recently, Mn- and Ru-catalyzed oxidant-free dehydrogenative [4
+ 2] annulation of N−H imines and alkynes through a
hydrogen releasing process were developed by Wang11e and
Li.11f Our group has recently observed the liberation of
hydrogen in copper-catalyzed C−H olefination and decarbox-
ylative amination reaction promoted by the nickel powder.11g
These studies open up new routes for novel atom-economical
approaches to the transition-metal-catalyzed oxidative C−H
functionalization. Herein, we disclose the first protocol of
oxidant-free cobalt-catalyzed synthesis of five-membered spiro-
cycles from benzimidates and maleimides with the concomitant
liberation of H2 gas. Spirocyclic frameworks are fundamental
structural motifs that occur in a broad array of both natural
products and synthetic bioactive molecules.12
Ru,4 and Ir,5 have played the leading role in this field. From the
viewpoint of sustainable chemistry, the development of
improved procedures that involve cheap and environmentally
friendly catalysts can be of great significance in both laboratory
synthesis and industrial applications. Recently, the unique
reactivity and selectivity of cobalt catalysis in C−H activation
has attracted much attention, and led to exciting and fruitful
achievements.6 For instance, Kanai, Matsunaga and co-workers
discovered the more polarized nature of the Cp*Co(III)-C
bond than that of the Cp*Rh(III)-C bond, which results in not
only alkenylation products but also alkenylation/annulation
products in the reaction of N-carbamoylindole with the internal
alkynes.7a They also established an exceptional reactivity of the
Cp*Co(III) catalyst in addition to alkyne, in which the product
selectivity could be controlled by using different directing
groups and reaction conditions.7b A study by Hummel and
Ellman demonstrates that the reactivity of Cp*Co(III) in the
condensation of aldehydes with azobenzene derivatives is able
to be dramatically increased by the modification of the
counteranion species of Cp*Co(III).7c Afterward, the groups
of Glorius,7d Ackermann,7e Chang,7f Cheng,7g and others7h−l
have made progress in Cp*Co(III)-catalyzed C−H activation.
Despite these important progresses, stoichiometric external
oxidants (Ag2CO3, Ag2O, AgOAc, AgBF4, Cu(OAc)2, CuO, O2
etc.)8 or internal oxidizing group9 are typically required when a
transition-metal-catalyzed oxidative C−H activation process is
involved. The initial identifying oxidants and the disposal of
waste after the reaction are still the challenges lying ahead in
these reactions. In view of global concerns regarding the
environment and sustainable energy resources, the develop-
ment of greener and more economic alternatives without the
use of oxidant are highly desired.10 For this purpose, Milstein
and co-workers explored the oxidant-free Ru-catalyzed syn-
We commenced our investigations by using benzimidate 1a
and maleimide 2a as model substrates in the presence of 20 mol
% AgOTf, we initially examined the reactivity of different metal
catalysts, including [Cp*RhCl2]2, [Ru(p-cymene)Cl2]2, Pd-
(OAc)2, and Cp*Co(CO)I2 in dichloroethane (DCE).
Preliminary inspection revealed that the Cp*Co(CO)I2 proved
uniquely effective for the desired selective reaction: spirocycle
3a was obtained in 55% yield as major product (Table 1, entries
1−4). A byproduct quinoline 3a′ was formed in 10% yield
(Table 1, entry 4, parentheses). No reaction was observed in
the absence of cobalt catalyst, and a very low yield was ob-
tained in the absence of additives (entries 5−6). Among the
various silver additives tested, AgOTf and AgBF4 showed high
efficiency to the reaction (entries 7−9). The reaction
atmosphere has much impact on the reactivity of this oxidative
Received: July 24, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b02266
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Organic Letters
Letter
ab
,
a b
,
Table 1. Optimization of Reaction Conditions
Scheme 1. Scope of Benzimidates
b
entry
catalyst
additive
solvent
yield (%)
f
1
[Cp*RhCl2]2
[Ru(p-cymene)Cl2]2
Pd(OAc)2
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
−
AgSbF6
AgBF4
AgOAc
AgOTf
AgOTf
AgOTf
AgOTf
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
trace
trace
nr
2
3
4
5
6
7
8
9
Cp*Co(CO)I2
−
55(10)
nr
Cp*Co(CO)I2
Cp*Co(CO)I2
Cp*Co(CO)I2
Cp*Co(CO)I2
Cp*Co(CO)I2
Cp*Co(CO)I2
Cp*Co(CO)I2
Cp*Co(CO)I2
20
40
54
nr
c
10
39
d
11
20
e
12
15
f
13
95(0)
a
Reaction conditions: 1a (0.3 mmol), 2a (0.2 mmol), catalyst (10 mol
%), additive (20 mol %) in solvent (1.0 mL), under vacuum at 100 °C
b
c
d
for 11 h. Isolated yield. Under N2 atmosphere. Under O2
atmosphere. Under air atmosphere. 1 equiv PhNO2 was used. nr =
no reaction.
e
f
reaction. Unexpectedly, the reaction was very sluggish under
oxygen atmosphere or air but worked well under nitrogen
atmosphere (entries 10−12). Further study showed that the
reaction gave the best yield in vacuum conditions (entry 4). On
the other hand, the use of stoichiometric oxidants such as
AgOAc or Cu(OAc)2 proved to be deleterious to the reaction
(see the Supporting Information, SI), which is unusual for a
process involving oxidative C−H activation. Encouragingly, the
subsequent GC analysis of the captured gas from the reaction
indeed confirmed the liberation of hydrogen gas (see the SI). It
was significant that 1 equiv PhNO2 promoted the reaction
remarkably to give the spirocycle 3a in 95% yield, and the
generation of byproduct quinoline 3a′ was suppressed
drastically (Table 1, entry 13).
With the optimal conditions in hand, we studied the scope of
benzimidate substrates as shown in Scheme 1. The
benzimidates with both electronic-donating and electronic-
withdrawing groups can carried out the reaction efficiently to
give the corresponding spirocyclic compounds in good to
excellent yields (3a−3m). Significantly, the transformation
could be carried out on 1 mmol scale in good yields (3e, 3m).
A wide variety of functional groups, including alkyl, methoxy,
fluoride, chloride, bromide, iodide, methyl ester, acetyl,
trifluoromethyl, and nitro groups, were well tolerated.
Importantly, aryl C−Br and C−I bonds are well compatible
with the reaction conditions, which may allow further
construction of complex molecules by coupling reactions. It is
important to note that the high yield was obtained in the
absence of nitrobenzene with the nitro substituted benzimidate
substrate (3m). The ortho-substituted benzimidate gave the
corresponding spirocycle in lower yield (3n), suggesting that
the steric hindrance has negative effects on the reaction. The
benzimidates with meta-substituents showed comparable
reactivity to regioselectively afford the only isomer of
spirocycles in good yields (3b, 3d, 3o−3q). We also tested
the variation of the alkoxyl groups in the benzimidates to the
reaction. It was found that the diverse alkoxyl groups could be
a
Reaction conditions was optimal conditions shown in Table 1.
Isolated yields. Without PhNO2. 1 mmol scale.
b
c
d
tolerated the reaction well to give the spiralcycles in good yields
(3r−3u). The structure of the product 3e was further
confirmed by single X-ray diffraction analysis (see the SI).13
Ethyl 2-naphthimidate participated in the reaction smoothly to
give the spiralcyclic compound in 65% yield (3v). Ethyl
thiophene-2-carbimidate and ethyl thiophene-3-carbimidate
carried out the reaction well to give the corresponding
spirocyclic compounds 3w and 3x as the major product.
We next investigated the substrates scope with a range of
maleimides bearing different substitution patterns (Scheme 2).
The reaction system displayed good tolerance toward a free
NH group of imide (4a). Diverse substituents in the N atom of
maleimide, such as ethyl, tert-butyl, cyclohexyl, phenyl, and
benzyl, were tolerated to provide the corresponding spirocycles
in good to excellent yields (4b−4f). Excellent yields could be
obtained in the absence of nitrobenzene when ethyl 4-
nitrobenzimidate 1m was used as the substrate (4g−4l).
To gain deeper insight into the mechanism of this
transformation, a set of preliminary experiments were
conducted as shown in Scheme 3. We performed the reaction
with D5-1e in the optimal conditions and terminated the
reaction after 5 h (Scheme 3, eq 1). The H in corporation at
the dual ortho car-bon of substrate d-1e′ was obtained at a level
of 36%, suggesting that the cobalt-catalyzed C−H cleavage is
reversible. An intermolecular kinetic isotope value (kH/kD) of
4.2 using 1e and D5-1e with 1:1 ratio in optimized conditions at
a low conversion implicates that the cleavage of the C−H bond
is likely involved in the rate-determing step (Scheme 3, eq 2).
To detect the hydrogen gas possibly released from the reaction,
B
DOI: 10.1021/acs.orglett.7b02266
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Organic Letters
Letter
a b
,
favorable to the formation of key intermediate to enhance the
reactivity.14
Scheme 2. Scope of Maleimide
On the basis of the above results and literature reports,15,16
a
plausible reaction mechanism is proposed as shown in Scheme
4. The active catalyst Cp*Co(OTf)2 I is initially generated from
Scheme 4. Proposed Reaction Mechanism
ligand exchange between Cp*Co(CO)I2 and AgOTf. The
following directed C−H activation results in the formation of
the cobaltacyclic intermediate II, which undergoes the
migratory insertion into the maleimide to give intermediate
III. The subsequent abstraction of hydrogen by TfO− leads to
the formation of alkenylated product V and cobalt species
IV.16a The intramolecular aza-Michael addition of alkenylated
product V generates the spirocyclic compound. The cobalt
species IV reacts with HOTf to release H2 gas and regenerate
the active cobalt catalyst.16b A similar process of Cu(II)-
catalyzed reaction of benzamides with maleimides was reported
by Hirano and Miura.15a This work is different from latest work
by Ackermann and Li’s work,15g which only involves C−H
alkylation. The spiral-lactam can be easily obtained by the
hydrolysis of as-prepared spirocyclic compounds in high yield
(Scheme 5).
a
Reaction conditions was optimal conditions shown in Table 1.
Isolated yields. Without PhNO2.
b
c
Scheme 3. Mechanistic Investigations
Scheme 5. Synthetic Application
In summary, we have developed an unprecedented cobalt-
catalyzed spiro annulation of benzimidates with maleimides,
which provides a rapid approach to the five-membered nitrogen
containing spirocyclic molecules. For the first time, cobalt-
catalyzed oxidative C−H fuctionalization releasing the hydro-
gen gas without the use of oxidant was developed by utilization
of nitrobenzene as the promoter. The reaction could be readily
scaled up to gram scale, and a wide variety of functional groups
were well tolerated. Further efforts to expand the reaction
scope and detailed mechanistic studies are underway.
a larger scale experiment was conducted under the optimal
reaction conditions and a gas collecting bag was connected with
the reaction flask (Scheme 3, eq 3). To our delight, the result of
GC analysis clearly evidenced the generation of hydrogen from
the reaction (see the SI).
Dong and co-workers observed that the hydrogen gas
releasing from their rhodium-catalyzed alkenylation reaction
could reduce the double bond of acrylamide.11d However, no
hydrogenation products of maleimide were observed in our
reaction. To probe the role of nitrobenzene, which promotes
our reaction remarkably (Table 1, entry 13), the reaction
solution was analyzed carefully. However, any reduced
derivatives from nitrobenzene were not detected, indicating
that nitrobenzene did not act as oxidant or hydrogen scavenger
to accelerate the reaction. It is supposed that the coordination
of nitrobenzene with Cp*Co(III) catalytic species might be
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.7b02266.
C
DOI: 10.1021/acs.orglett.7b02266
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Supporting methods and data, crystal structure of 3e,
Letter
24768. (e) Gensch, T.; Klauck, F. J. R.; Glorius, F. Angew. Chem., Int.
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55, 4035. (g) Mei, R.; Wang, H.; Warratz, S.; Macgregor, S. A.;
Ackermann, L. Chem. - Eur. J. 2016, 22, 6759.
copies of NMR spectra (PDF)
Crystal data for 3e (CIF)
(9) (a) Sivakumar, G.; Vijeta, A.; Jeganmohan, M. Chem. - Eur. J.
2016, 22, 1. (b) Chavan, L. N.; Gollapelli, K. K.; Chegondi, R.; Pawar,
A. B. Org. Lett. 2017, 19, 2186. (c) Barsu, N.; Sen, M.; Premkumar, J.
R.; Sundararaju, B. Chem. Commun. 2016, 52, 1338. (d) Lerchen, A.;
Knecht, T.; Daniliuc, C. G.; Glorius, F. Angew. Chem., Int. Ed. 2016, 55,
15166.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: yhzhang@zju.edu.cn.
ORCID
(10) (a) Li, J.; Gao, X.; Liu, B.; Feng, Q.; Li, X.-B.; Huang, M.-Y.; Liu,
Z.; Zhang, J.; Tung, C.-H.; Wu, L.-Z. J. Am. Chem. Soc. 2016, 138,
3954.
Yuhong Zhang: 0000-0002-1033-3429
Notes
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6, 230. (d) Li, W. H.; Wu, L.; Li, S.-S.; Liu, C.-F.; Zhang, G.-T.; Dong,
L. Chem. - Eur. J. 2016, 22, 17926. (e) He, R.; Huang, Z.-T.; Zheng,
Q.-Y.; Wang, C. Angew. Chem., Int. Ed. 2014, 53, 4950. (f) He, K.-H.;
Zhang, W.-D.; Yang, M.-Y.; Tang, K.-L.; Qu, M.; Ding, Y.-S.; Li, Y.
Org. Lett. 2016, 18, 2840. (g) Yang, Y.; Xie, C.; Xie, Y.; Zhang, Y. Org.
Lett. 2012, 14, 957.
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(b) Zuo, Z.; Wang, H.; Fan, L.; Liu, J.; Wang, Y.; Luan, X. Angew.
Chem., Int. Ed. 2017, 56, 2767. (c) Arnone, A.; Bava, A.; Fronza, G.;
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Funding from Natural Science Foundation of China (No.
21472165, No. 21672186) and Important Project of Zhejiang
province (2017C03049) are highly acknowledged.
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