Rh(III)-Catalyzed [4 + 2] Self-Annulation of N-Vinylarylamides

Article abstract of DOI:10.1021/acs.orglett.8b02872

An efficient rhodium(III)-catalyzed self-annulation of N-vinylarylamide has been developed. This reaction features a simple system and good reactivity with complete regioselectivity. The protocol provides easy access to an aminal incorporated dihydroisoqu

Full text of DOI:10.1021/acs.orglett.8b02872

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Rh(III)-Catalyzed [4 + 2] Self-Annulation of N‑Vinylarylamides
Rui Sun,† Xiao Yang,† Xue Chen,† Chunchun Zhang,‡ Xiaoyu Zhao,† Xin Wang,† Xueli Zheng,†
Maolin Yuan,† Haiyan Fu,*,† Ruixiang Li,† and Hua Chen*,†
†Key Laboratory of Green Chemistry & Technology, Ministry of Education College of Chemistry, Sichuan University Chengdu
610064, P. R. China
‡Analytical & Testing Center, Sichuan University, Chengdu, Sichuan 610064, P. R. China
S
* Supporting Information
ABSTRACT: An efficient rhodium(III)-catalyzed self-annu-
lation of N-vinylarylamide has been developed. This reaction
features a simple system and good reactivity with complete
regioselectivity. The protocol provides easy access to an
aminal incorporated dihydroisoquinolinone, which proved to
be a versatile synthetic synthon. The kinetic isotope effect
experiments showed that C−H activation is the rate-limiting
step, and competition studies revealed the annulation exhibits
a strong self-recognition mode. In addition, a seven-
membered rhodacycle species was isolated and established
as the key reaction intermediate.
minal incorporated dihydroisoquinolinones are important
structural motifs in bioactive molecules and drug
Scheme 1. Strategies for Synthesis of Aminal Isoquinolinone
A
candidates.1 For example, the polycyclic compounds contain-
ing a substructure core as shown in Figure 1 are agents for the
Figure 1. Representive agents for the treatment of respiratory
syncytical virus infections.
treatment of respiratory syncytical virus infections.2 In
addition, due to the presence of a synthetically important
aminal functionality,3 the aminal dihydroisoquinolinone would
be easily transformed into structurally diverse isoquinolinones.
As a result, numerous strategies for preparation of such a
compound have appeared in the literature (Scheme 1).4 So far,
the most common approaches rely on the reaction of delicately
designed substrates with amine, diamine, or vinylamine, etc.
For example, the condensation of keto/aldehyde−acid/acyl
chloride compound with diamine,5 the reaction of isocoumarin
with amine,6 or palladium-catalyzed cyclization−amination of
N-allene benzamides7 could all construct such a skeleton.
However, these established methods are limited to specific and
not easily available substrates.
alkyl,10 aryl,11 or 8-AQ (aminoquinoline)12 have been
investigated to effect this type of reaction. These protocols
either require an external oxidant or redox-neutral conditions,
depending on whether an NH−O bond exists in the directing
groups. The coupling partners have been extended from
internal alkyne12a,13 to terminal alkynes,14 alkenes,12b,15
arenes,16 benzyne,17 allenes,18 ketenimines,19 and α-halo- or
pseudohaloketones.20 Several examples of intramolecular
reactions have also been reported by Rovis,21 Glorius,22 Van
der Eyken,23 and others.24 These intramolecular reactions
allowed for simultaneously building of multiple rings and
provided an efficient method to synthesize more complex
scaffolds.25 While the utility of this process for synthesis of
various isoquinolinone cores has been well documented, it has
In 2010, Fagnou and co-workers first reported the Cp*Rh-
catalyzed 4 + 2 annulation of benzamides with internal alkynes
for construction of the isoquinolinone derivatives by using
CONHOMe as the directing group.8 Since this seminal work,
various amide directing groups modified by pivaloyloxy,9
Received: September 8, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b02872
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Organic Letters
Letter
t
rarely been used to prepare the aminal incorporated
isoquinolinone.26 In considering the development of a more
direct and practical route for the synthesis of such a
compound, we recognized that a Rh(III)-catalyzed 4 + 2
reaction using N-vinylbenzamide as substrate could provide a
solution. In this strategy, a rhodacycle is initially formed via a
sequential N−H/C−H activation, followed by a regioselective
CC insertion to the Rh−C bond, giving the intermediate
with the nitrogen atom closely oriented to the rhodium center,
wherein a reductive elimination could afford the dihydroiso-
quinolinone with an aminal group.27 Guided by this rationale,
we anticipate that the N-vinylarylamide would be a promising
candidate for the synthesis of such a compound. However, it is
surprising to note that no N-vinylarylamides have thus far been
explored in this kind of 4 + 2 reaction, possibly in large part
due to its ease of polymerization.28 In this report, we disclose
an unprecedented Rh (III)-catalyzed 4 + 2 annulation reaction
between two molecules of N-vinylarylamides, the annulation
products of which prove to be versatile synthetic intermediates.
We began our investigation with N-vinylbenzamide 1a as the
substrate. As shown in Table 1, under the optimal conditions,
and solvents are inferior to the effects of BuOLi and toluene
(Table 1, entries 6, 7, and 9). Interestingly, at a higher
temperature of 130 °C, the absence of oxidant and base led to
an unexpected hydro-olefinated product (Z)-N,N′-(but-1-ene-
1,3-diyl)dibenzamide in 35% yield without any 2a being
detected (Table 1, entry 8). Finally, the reaction yield is
slightly reduced without the protection of N2 balloons (Table
1, entry 10).
With the optimal conditions in hand, we next examined the
scope of the substrate. As can be seen in Scheme 2, the
a,b
Scheme 2. Scope of the N-Vinylarylamides
a,b
Table 1. Optimization of the Reaction Conditions
yield of 2a
entry
deviation from standard conditions
no change
(%)
1
2
81
0
RhCl3, [Cp*IrCl2]2, PdCl2 or Co(OAc)2 instead of
[Cp*RhCl2]2
d
3
4
5
6
7
8
9
10
at 70 °C
55
trace
36
e
in absence of oxidant
cd
Ag2CO3 instead of Ag2O
c,d
t
KOAc instead of BuOLi
38
c,d
in absence of base
in absence of both the oxidant and the base
9
f
0
c,d
DCE instead of toluene
no N2 balloon protection
40
72
a
Standard conditions: N-vinylbenzamide (0.2 mmol), [Cp*RhCl2]2
(3 mol %), oxidant (0.2 mmol), base (0.5 mmol), solvent (0.2 M), 50
b
c
d
°C, 24 h. Isolated yield. Reaction run at 70 °C. Without N2
e
f
balloon protection. Reaction run at 130 °C. (Z)-N,N′-(But-1-ene-
1,3-diyl)dibenzamide was obtained.
a
Reaction conditions: N-vinylarylamides (0.2 mmol), [Cp*RhCl2]2
t
(3 mol %), Ag2O (0.2 mmol), and BuOLi (0.5 mmol), toluene (0.2
M), 50 °C, 24 h, N2 balloon atmosphere. Isolated yield. 1.2 mmol of
1l was used. Reaction run at 60 °C. KOAc instead of BuOLi.
b
c
t
d
e
[Cp*RhCl2]2 (3 mol %), Ag2O (1 equiv), and BuOLi (2.5
t
equiv) in toluene (0.2 M) at 50 °C for 24 h, the desired aminal
incorporated isoquinolinone 2a was obtained in 81% isolated
yield without the isomer 2aa being detected (Table 1, entry 1).
Other commonly used catalyst precursors in this kind of
reaction, such as [Cp*IrCl2]2, PdCl2, RhCl3·3H2O, Co(OAc)2,
were all ineffective (Table 1, entry 2). Although the substrate
was completely converted by elevating the temperature to 70
°C, the yield of product decreased rapidly accompanied by a
large amount of unidentified byproducts (Table 1, entry 3).
The oxidant proved to be the prerequisite for this reaction, and
in comparison with Ag2O, other oxidants such as Ag2CO3,
AgNO3, and CF3COOAg led to a decreased yield, while
AgSbF6 completely inhibited the reaction (Table 1, entries 4
and 5 and Table S3). Moreover, the effects of the other bases
annulation reaction generally occurs smoothly with a variety of
substituents in 23−80% yields with complete regioselectivity.
The introduction of alkyl and CF3 groups to the para position
has a marginal effect on the reaction, delivering the desired
products in 73−80% yields (2b, 2c, 2h, and 2l). In contrast,
the para-halogen, chloromethyl, and OMe substitution only led
to moderate yields, ranging from 41 to 58% (2e−g, 2i),
allowing for further transformation if needed. Unfortunately,
the strong electron-withdrawing substituents, such as CN and
NO2 groups, led to no reactivity (2j, 2k), with the majority of
the starting materials recovered. In the case of the meta-
substituted substrates (2p−r), low site-selectivity was observed
B
DOI: 10.1021/acs.orglett.8b02872
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Organic Letters
Letter
because two ortho-C−H bonds are available. An inseparable
mixture of two annulation isomers was formed in 3:2−5:1
ratios, with the less crowded one as the major product. Similar
phenomena were also observed for N-vinyl-2-naphthamide
(2s). Interestingly, the reaction displayed excellent selectivity
for the substrates bearing methyl or chloromethyl groups at the
meta position, with the annulation occurring exclusively at the
sterically less hindered position in moderate yields (2n, 2o).
Moreover, 3,5-dimethyl- and -difluorine-substituted substrates
which can avoid the site-selectivity problem also gave
synthetically useful yields (2t, 2u). Finally, the ortho-
substituted N-vinylbenzamides could also undergo this type
of reaction, albeit in a low yield (2v). However, no product was
obtained with the ortho-halogenated N-vinylbenzamides. This
could be attributed to a conformational restriction exerted by
hydrogen-bonding between the halogen atom and the amide
NH hydrogen, which makes the ortho C−H bond an
inaccessible site for Rh(III) to form the important rhodacycle
intermediate.
Scheme 4. Competition Experiments
Scheme 5. Mechanism Study
Product 2a is a synthetically useful intermediate and can be
transformed into various useful molecules (Scheme 3). By
Scheme 3. Useful Transformation
employing different conditions, the benzamido and vinyl
groups can be removed simultaneously (3) or individually (4,
5). They can also be further functionalized to install another
functionality (6). Importantly, during the formation of
compounds 3, 5, and 6, an equal amount of benzamide was
isolated as a byproduct, which can be reused by conversion
into the N-vinylbenzamide.29
A series of intermolecular competition experiments were
conducted to gain insight into the mechanism. The 2:1 molar
ratio of 1a and a commonly used annulation partner, such as
styrene, vinyl benzoate, N-vinylformamide, or N-methoxyben-
zamide, were subjected to the standard conditions. The 4 + 2
annulation only occurs between two molecules of 1a without a
cross-annulation product being detected (Scheme 4). These
data reveal a strong self-recognition mode in the formal 4 + 2
reaction of N-vinylbenzamide.
into the expected product 2h under the otherwise identical
standard conditions, whereas B′ was fully recovered and could
not catalyze the reaction. It is clearly shown that the
intermediate D is most likely the reaction catalytic species,
while B′ is catalytically inactive (Scheme 5b). More
importantly, these results provide direct evidence for the
five- and seven-membered rhodacycle species30 involved in the
reaction process. The failure in isolation of the mononuclear
five-membered rhodacyle species is most likely due to the rapid
double bond insertion to the Rh−C bond, which might be the
reason why the annulation exhibited a strong self-recognition
mode in competition studies.
On the basis of the above results and the related literature,31
a plausible reaction mechanism is proposed in Scheme 6. First,
t
To determine the turnover-limited step of the catalytic cycle,
the kinetic isotope effect (KIE) experiment was carried out by
treating a 1:1 molar ratio of 1a and D2-1a with [Cp*RhCl2]2,
Ag2O, and tBuOLi under the standard conditions for 5 h. Then
in the presence of BuOLi, catalyst precursor [Cp*RhCl2]2 is
activated to species A, which subsequently cleaves the N−H
and ortho C−H bonds of 1h to give the key five-membered
rhodacycle complex B. Thereafter, rapid coordination of
another molecule 1h via the double bond gives rise to
intermediate C, which undergoes double bond insertion into
the Rh−C bond in two ways to furnish the seven-membered
rhodacycle D or D′. The amide carbonyl group coordinates to
rhodium(III) in intermediate D, forming a five-membered ring,
which provides an extra stabilization of about 21.1 kcalmol−1 in
comparison with D′ (see the Supporting Information). A
possible benefit from this extra stabilization is that the
completely regioselective insertion of the double bond occurs
to afford intermediate D. Then reductive elimination from D
1
the separated targeted products were subjected to H NMR
measurement, which gave a KIE value of 2.6, indicating that
the C−H bond cleavage was the rate-determining step
(Scheme 5a). Subsequently, we reacted 1h with 25 mol % of
t
[Cp*RhCl2]2 in the presence of BuOLi without Ag2O and
successfully obtained the important intermediate D in 32%
isolated yield, accompanied by a trace amount of dirhodium
complex B′; their structures were confirmed by NMR
spectroscopy, HRMS, and X-ray structural analysis, respec-
tively. The catalytic amount of D could successfully convert 1h
C
DOI: 10.1021/acs.orglett.8b02872
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Organic Letters
Letter
for their financial support. We also thank the Centre of Testing
& Analysis, Sichuan University, for NMR measurements. Some
of the computational results described in this work were
obtained with the help of Prof. Dr. De-Yin Wu at the College
of Chemistry and Chemical Engineering in Xiamen University.
Scheme 6. Proposed Mechanism
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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.8b02872.
■
S
Experimental details and full spectroscopic data for all
new compounds (PDF)
Accession Codes
CCDC 1857356 and 1863830 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.
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AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: scufhy@scu.edu.cn.
*E-mail: scuhchen@scu.edu.cn.
ORCID
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Haiyan Fu: 0000-0002-7185-2062
Ruixiang Li: 0000-0003-0756-3722
Hua Chen: 0000-0003-4248-8019
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
We thank the National Natural Science Foundation of China
(Nos. 21572137 and 21871187) and the Key Program of
Sichuan Science and Technology Project (No. 2018GZ0312)
D
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