Rh-Catalyzed Construction of Quinolin-2(1H)-ones via C-H Bond Activation of Simple Anilines with CO and Alkynes

Article abstract of DOI:10.1021/jacs.5b05843

A novel and efficient Rh-catalyzed carbonylation and annulation of simple anilines with CO and alkynes through N-H and C-H bond activation for the direct synthesis of quinolin-2(1H)-ones has been developed. Simple anilines without preactivation, broad substrate scope with hetero/polycycles, and high-value products make this protocol very practical and attractive. A key rhodacycle complex was isolated and well-characterized.

Full text of DOI:10.1021/jacs.5b05843

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Rh‐Catalyzed Quinolin‐2(1H)‐one Construction via C‐H Bond Activa‐
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tion of Simple Anilines with CO and Alkynes
Xinyao Li†, Xinwei Li†, and Ning Jiao*,†,‡
†State Key Laboratory of Natural and Biomimetic Drugs, Peking University, Xue Yuan Rd. 38, Beijing 100191, China
‡State Key Laboratory of Elemento-organic Chemistry, Nankai University, Weijin Rd. 94, Tianjin 300071, China
Supporting Information Placeholder
Scheme 1. Transition-metal-catalyzed Oxidative Annula-
tion
ABSTRACT: A novel and efficient Rh-catalyzed carbonylation
and annulation of simple anilines with CO and alkynes through N-
H and C-H bonds activation for the direct synthesis of quinolin-
2(1H)-ones has been developed. Simple anilines without pre-
activation, broad substrate scope with hetero/polycycles, as well
as high-value products, make this protocol very practical and
attractive. A key rhodacycle complex was isolated and well
characterized.
Quinolin-2(1H)-ones are ubiquitous fused heterocyclic motifs
found in many natural products and pharmaceutically active com-
pounds.1 Members of quinolin-2(1H)-one family have wide appli-
cations in the medicinal chemistry regarding their broad and re-
markable biological activities such as anticancer,2 antibiotic,3
antiviral,4 antihypertensive,4a antibacterial,5 and other activities.6
Moreover, quinolin-2(1H)-ones have been used as valuable syn-
thetic intermediates in organic synthesis.7 Given its importance,
many approaches to quinolin-2(1H)-ones including the annulation
of pre-functionalized anilines with alkynes,8 anilines with acry-
lates,9 and others9 have been developed. Despite the significances,
most of them suffer from the pre-activated reactants, limited sub-
strate scope, and low efficiency. Therefore, the direct and eco-
nomic strategies for the efficient synthesis of quinolin-2(1H)-ones
are still highly desired.
Transition-metal catalyzed C-H bond functionalization has
proven to be a versatile and powerful synthetic strategy.10 Among
them, the direct dehydrogenative annulation of simple substrates
with alkynes to construct polycyclic aromatic11 and heteroaro-
matic compounds12,13 has been significantly developed. The two
component annulation strategy to synthesize monosubstituted
quinolin-2(1H)-ones9 and highly-related coumarins14 from simple
anilines or phenols via transition-metal-catalyzed functionaliza-
tion of the ortho C-H bonds have also been significantly studied.
In the past decades, transition-metal catalyzed carbonylation with
CO has been widely applied as a powerful protocol in both indus-
try and laboratory.15,16 However, in contrast to the well-known
transition-metal catalyzed indoles synthesis from anilines or their
derivatives with alkynes (a, Scheme 1),13 straightforward method-
ology to construct six-membered N-heterocycles through the de-
hydrogenative annulation of functionalized anilines with alkynes
and CO has been barely achieved. Larock et al.8a developed the
Pd-catalyzed carbonylative annulation of 2-iodoanilines with
alkynes and CO. Beller and Wu8b reported this transformation via
Pd-catalyzed C-H activation of N-pyridylanilines (the pyridyl is
required as directing group) with Mo(CO)6 (b, Scheme 1). Recent-
ly, Dong and coworkers reported a significant directed Rh-
catalyzed decarbonylative coupling of alkynes and isatins (c,
Scheme 1),17 which represents an distinct method for quinolin-
2(1H)-ones synthesis with the release of CO from isatins (c,
Scheme 1). However, the annulation of amines and alkynes with
CO incorporation for the construction of six-membered N-
heterocycles still encounters chellanges.18
Herein, we report a novel and efficient Rh-catalyzed three
component carbonylation and annulation strategy to construct 3,4-
disubstituted quinolin-2(1H)-ones from very simple anilines, CO
and alkynes through C-H activation (d, Scheme 1). Although the
Pd-catalyzed carbonylation of C-H bond of anilines for the syn-
thesis of isatoic anhydrides, isatins, and β-lactams has been signif-
icantly realized by the groups of Guan19a and Lei19b,c, respectively,
to the best of our knowledge, the carbonylation and annulation
with CO incorporation through C−H activation, particularly from
readily available substrates to high-value products has remained
undeveloped.20
Initially, the carbonylation and annulation reaction between N-
methylaniline (1a) and dec-5-yne (2a) in xylene under 1 atm CO
was chosen as the model reaction (Table 1). The reactions were
totally ineffective when Pd-catalysts that are efficient in C-H acti-
vation and carbonylation of anilines,19 were employed as the cata-
lyst (entries 1-2). To our delight, the reaction catalyzed by
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[Cp*RhCl2]2 afforded the designed quinolin-2(1H)-one 3aa in
32% yield (entry 3). The screening of other Rh(I) catalysts indi-
cated that the readily available Wilkinson’s catalyst Rh(PPh3)3Cl
performed efficient catalytic activity (80% yield, entry 6). Other
Cu salt oxidants gave low yields (see SI). After the screening on
different parameters with Ag salt as an additive and in the pres-
ence of a base (entries 7-10, and SI), it is noted that the reaction in
the presence of 0.5 equiv Li2CO3 produced 3aa in 95% yield (en-
try 8). Even the loading of Wilkinson’s catalyst was reduced to 2
mol%, the efficiency did not decrease (92% isolated yield, entry
11).
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Table 1. Optimization of the Conditionsa
c
a Reaction conditions: see entry 11, Table 1. b Isolated yields.
e
Without Li2CO3. d Rh(PPh3)3Cl (4 mol%) was used. Regioselec-
tivity ratio (the major isomer is substituted at 7-position as drawn).
a Reaction conditions: 1a (0.3 mmol), 2a (0.36 mmol), catalyst (x
mol%), Cu(OAc)2 (0.6 mmol), additive (y mol%) in xylene (1 mL)
under 1 atm CO at 130 oC for 36 h. b Determined by GC. c Isolated
yields.
To explore the effect of substituents on alkyne, a variety of in-
ternal alkynes were employed. Various aliphatic and aromatic
internal alkynes were compatible under these reaction conditions,
giving the expected products in moderate to good yields (Scheme
3). Dipropylalkyne and bis(benzyloxy)but-2-yne participated in
the present annulation to afford quinolin-2(1H)-ones 3ab and 3ac
in 80% and 50% yields, respectively. Diarylalkynes containing
electron-rich/deficient functional groups reacted smoothly to give
the corresponding products in moderate to good yields (3ad−ag).
In addition, di(thiophen-2-yl)ethyne as heteroaromatic alkyne was
also compatible, giving the expected product 3ah in 54% yield.
The unsymmetrical aryl alkyl alkynes such as 2i and 2j were also
suitable for this reaction, affording the corresponding products 3ai
and 3aj in high yields with moderate regioselectivity. Notably,
macrocyclic alkyne 2k was still tolerated, leading to the poly-
macrocyclic product 3ak in 55% yield.
With the optimum conditions in hand, we next explored the
scope of anilines 1 with 2a (Scheme 2). A series of N-
methylanilines bearing electron-donating groups (R = OMe,
t
NHAc, Me, Bu) furnished the annulation successfully to produce
quinolin-2(1H)-ones in high efficiencies (3ba-ea). Substrates with
weak electron-withdrawing groups such as Ph and Cl also per-
formed well, giving the corresponding products in good yields
(3fa, 3ga). While strong electron-withdrawing groups (R = F,
CO2Me, CN, NO2) were relatively sluggish and provided the
moderate yields (3ha-ka). o-, m-Methyl substituents on the phe-
nyl ring were well tolerated (3la, 3ma). N-alkyl substituted ani-
lines even with cyclopropyl were fully compatible (3na-qa). Sev-
eral multisubstituted anilines smoothly led to the desired products
in 55-86% yields (3ra-ta). In addition, N-aryl substituted aniline
proceeded successfully to deliver the quinolin-2(1H)-one 3ua in
50% yield. Furthermore, N-methylnaphthalen-1-amine and poly-
cyclic anilines such as 9H-fluoren-2-amine, was smoothly con-
verted into the polycyclic heteroaromatic compound 3va and 3wa,
respectively.
To gain insight into the mechanism, several experiments were
conducted. Upon treatment of the model reaction of 1a and 2a
under standard condition with isotopically labeled
Scheme 3. Substrate Scope of Alkynesa,b
It is noteworthy that tetrahydroquinoline, tetrahydro-1H-
benzo[b]azepine, and dihydrodibenzooxazepines performed well
to give the complex polycyclic heteroaromatic compounds in
moderate to good yields, which are usually the core structural
motif in some nature products and bioactive compounds (4-8,
Scheme 2).
Scheme 2. Substrate Scope of Anilinesa,b
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Scheme 4. Proposed Mechanism
a Reaction conditions: see entry 11, Table 1. b Isolated yields. c
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Regioselectivity ratio (the major isomer: phenyl substituent at 3-
position as drawn).
experiment, a remarkable H/D exchange of 20% in product 3aa
was found with the addition of deuterated water, thereby implicat-
ing a reversible cyclometalation mode (eq 1). In addition, an in-
termolecular KIE for the annulation reaction was determined to be
kH/kD = 1.25, indicating that the C(sp2)−H bond cleavage should
not be involved in the rate-determining step of the catalytic cycle
(eq 2).
insertion forms Rh(III) complex C. Then C undergoes concerted
metalation-deprotonation (CMD) process to give key rhodacycle
complex D. The subsequent ligand exchange of D with alkyne 2
provides Rh(III)-alkyne complex E, which proceeds insertion to
generate seven-membered cyclic Rh(III) complex F. Finally, re-
ductive elimination of F delivers product 3, while the Rh(I) spe-
cies is reoxidized to Rh(III) A in the presence of Cu(OAc)2.
Control experiments showed that substrates keep retained in
the absence of alkyne 2a with the formation of trace amount of N-
methyl-N-phenylformamide 9 (eq 3). Furthermore, there was no
desired product observed in the absence or presence of CO, when
9 was treated with 2a (eq 4). These results indicate that the for-
mation of formylated aniline is not involved in this process. In
addition, no indole product 10 was detected in the absence of CO
(eq 5), which means that the hydroamination might not be the
initial step in this transformation.13 Notably, when stoichiometric
reaction between Rh(PPh3)3Cl and 1a was carried out with
Cu(OAc)2 under CO, Rh(III) metallacyclic complex 11 was iso-
lated in 65% yield, and confirmed by X-ray diffraction (eq 6).
Rhodacycle 11 provides 3aa both catalytically and in a stoichio-
metric reaction with 2a in good yields (eq 7).
In conclusion, we have developed a novel and efficient Rh-
catalyzed carbonylation and annulation of simple anilines with
CO and alkynes for the direct synthesis of quinolin-2(1H)-ones
through N-H and C-H bonds activation. Readily available anilines
without pre-activation, broad substrate scope with het-
ero/polycyclic rings, as well as the high-value products, make this
protocol very practical and attractive. A rhodacycle species was
isolated and well characterized, which is likely to be a key inter-
mediate in the catalytic reaction. Experimental and calculational
mechanistic studies of this transformation and further applications
are ongoing in our laboratory.
ASSOCIATED CONTENT
Supporting Information
Experimental details and NMR spectra. This material is available
free of charge via the Internet at http://pubs.acs.org.
On the basis of the above results and precedent reports, a
plausible mechanism is proposed (Scheme 4). Upon the formation
of Rh(III) A from Rh(I) oxidized by Cu(OAc)2, the ligand ex-
change with CO affords Rh(III)-CO species B. Subsequently,
aniline 1 coordinates to B, followed by CO
AUTHOR INFORMATION
Corresponding Author
jiaoning@pku.edu.cn
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
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Bhanuchandra, M.; and Sahoo, A. K. Angew. Chem., Int. Ed. 2013,
Financial support from National Basic Research Program of Chi-
na (973 Program) (grant No. 2015CB856600) and National Natu-
ral Science Foundation of China (Nos. 21325206, 21172006), and
National Young Top-notch Talent Support Program are greatly
appreciated. We thank Kai Wu in this group for reproducing the
results of 3ta and 3ac.
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