C8-H bond activation vs. C2-H bond activation: From naphthyl amines to lactams

Article abstract of DOI:10.1039/c6cc06358f

Pd-catalyzed selective amine-oriented C8-H bond functionalization/N-dealkylative carbonylation of naphthyl amines has been achieved. The amine group from dealkylation is proposed to be the directing group for promoting this process. It represents a straightforward and easy method to access various biologically important benzo[cd]indol-2(1H)-one derivatives.

Full text of DOI:10.1039/c6cc06358f

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Journal Name
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C8-H Bond Activation vs C2-H Bond Activation: From
Naphthyl Amines to Lactams
Cite this: DOI: 10.1039/x0xx00000x
Renyi Shi,a Lijun Lu,a Hangyu Xie,a Jingwen Yan,a Ting Xu,c Hua Zhang,a Xiaotian
Qi,c Yu Lan*c and Aiwen Lei*a, b
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
challenge to expand this novel N-dealkylative carbonylation protocol
to direct amine-oriented aromatic C-H bond functionalization of
naphthyl amines.
Pd-catalyzed
functionalization/
amines has been achieved. The amine group from
dealkylation is proposed to be the directing goup to promote
this process. It represents a straightforward and easy method
selective
amine-oriented
C8-H
bond
N-dealkylative carbonylation of naphthyl
to various biologically important benzo[cd]indol-2(1H)-one
derivatives. Moreover, O2 is utilized as the terminal oxidant
to promote the oxidative carbonylation process. Preliminary
mechanism studies were conducted indicating the possible
route of this transformation.
Scheme 1.
The chemical structure of benzo[cd]indol-2(1H)-ones constitutes
central core which widely exists in dyes, electronic typing materials
and DNA intercalative antitumors.9 Recently the extraordinary
biological and pharmaceutical properties of benzo[cd]indol-2(1H)-
ones have attracted more and more attention.10 Despite the great
importance of benzo[cd]indol-2(1H)-one derivatives, there are few
synthetic routes to access this structure. Among the known synthetic
During the past decade, Pd catalyzed aromatic C-H
functionalization to form C-C and C-X bonds has been intensively
investigated.1 While developing regioselective C-H bond
functionalization still remains a great challenge. To enhance the site-
selectivity of C-H bond activation, extensive efforts have been made
to develop C-H activation reactions of broadly useful substrates
using various directing groups.2 As one of the valuable commodity
chemicals and useful synthetic building blocks for agrochemicals,
active pharmaceutical ingredients and process chemicals, aromatic
amines have been widely utilized in chemical synthesis. However
the ortho-selective C-H bond functionalization of these amines still
remains less developed.3 Compared with anilines, Pd-catalyzed
directed C8-H functionalization of the commonly employed
naphthyl amines remains even more difficult for the site-selectivity
between C2-H and C8-H bond.
Carbonylation, the incorporation of CO into an organic molecule,
is now widely recognized as a very important tool in industrial and
organic chemistry, which meets the requirement of “atom economy”
and “green chemistry”.4 It allows the direct synthesis of important
carbonyl compounds starting from the simplest C-1 unit.3a, 5 Based
on our continuous interest in oxidative carbonylation reactions,3d, 6 it
might be a reasonable approach to synthesize benzo[cd]indol-2(1H)-
ones utilizing Pd-catalyzed intramolecular C-H oxidative
carbonylation. Yet, it remains difficult to use N-methylnaphthalen-1-
amines as the substrate in the direct oxidative carbonylation, which
urges us to solve the problem in other ways.7 In our previous work,
Pd/Cu-catalyzed aerobic N-dealkylative carbonylation of olefins and
tertiary amines has been demonstrated.3c, 8 But it remains a great
routes,
reaction
of
1,
8-naphthalic
anhydride
with
hydroxylammonium chloride is the most popular method, which
suffers from harsh conditions and narrow substrate scope. It still
remains a great challenge to develop an efficient and general
synthetic approach to construct various benzo[cd]indol-2(1H)-ones.
Herein, we reported the first Pd/Cu-catalyzed aerobic selective C-H
bond activation/N-dealkylative carbonylation of naphthalen-1-
amines to access biologically important benzo[cd]indol-2(1H)-ones.
With the optimized conditions in hand(see SI), the carbonylation
of a variety of naphthalen-1-amine compounds were tested (Scheme
2). In general, tertiary naphthalene-1-amine substituted with alkyl
group at the 4th position could go the standard reaction smoothly to
give 2d in 61% yield. While electron-rich group could be well
tolerated (2h). Substrate substituted with halogens including Cl and
Br furnished the corresponding carbonylation products in good
yields (2b and 2c), which can provide useful handles for further
synthetic transformations. Unfortunately, substrates substituted with
-NO2, -COOMe and other electron-withdrawing group gave less
products in this protocol. Both electron-donating and electron-
withdrawing substituents on benzene ring of 4-phenylnaphthalen-1-
amine derivatives were well tolerated under the current conditions
(2e, 2f and 2g). The same results were found in 5-phenylnaphthalen-
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DOI: 10.1039/C6CC06358F
1-amine derivatives (2j, 2k and 2l). 5-Bromo-N, N- yl)benzo[cd]indol-2(1H)-one
and
1-methyl-4-(naphthalen-2-
dimethylnaphthalen-1-amine could give the desired product in 37% yl)benzo[cd]indol-2(1H)-one could be obtained in moderate to good
yield under the standard conditions (2i).
yields (2m and 2n). The same phenomenon would be detected with
5-naphthyl-N, N-dimethylnaphthalen-1-amine (2o and 2p).
Furthermore, this protocol could also be applied to N, N-dialkyl
naphthalen-1-amines with different N-alkyl substituents (Table 1).
Different alkyl groups like -Et, -Bu, -Oct, were tolerated to obtain a
mixture of desired products in moderate yields.
To get some detailed information of the mechanism, control
experiments were carried out. Firstly, no desired product was
detected by utilizing N-methylnaphthalen-1-amine as the substrate
under the standard conditions (Scheme 3, Eq. 1). However, when 1a
was used to react with MeOH using 1.2 eq. PdCl2, no product of
oxidative C-H alkyloxycarbonylation was detected, yet 2a was
afforded (Eq. 2). It indicates that the N, N-dimethylamino group
might not be directly used as the directing group. Besides,
formaldehyde (CH2O) was detected by GC in this N-dealkylative
carbonylation reaction of 1a. Therefore, we assumed that C-N bond
cleavage occurs during the reaction, which leads to the selective C-H
bond activation and carbonylation.
HN
Standard condition
2a
+
CO
Eq. 1
Eq. 2
n. d.
MeOOC
NMe2
1.2 eq. PdCl2, AcOH
toluene/DMA, CO
2a
+
CO/MeOH
+
1a
40%
24 h, 110 o
C
n. d.
2 eq. PdCl2, AcOH
toluene/DMA, CO
+
+
CO
CO
2a
1a
1a
Eq. 3
Eq. 4
24 h, 110 o
C
42%
2 eq. Pd(OAc)2, AcOH
2a
toluene/DMA, CO
24 h, 110 o
C
65%
Scheme 2. Palladium/copper-catalyzed intramolecular oxidative N-
dealkylative carbonylation of naphthalen-1-amine derivatives 1.
Standard reaction conditions: 1 (0.2 mmol), PdCl2 (10 mol%),
Cu(OAc)2 •H2O (30 mol%), AcOH (40 mol%), CO/O2 = 2/1,
toluene/DMA =1.0/0.2, 110 oC, 24 h. Isolated yield.
HN
N
O
1 eq. PdCl2, AcOH
toluene/DMA, CO
+
+
CO
CO
2a
Eq. 5
Eq. 6
Eq. 7
+
24 h, 110 o
C
17%
Standard condition
Table 1. Palladium/copper-catalyzed intramolecular oxidative N-
dealkylative carbonylation of naphthalen-1-amine derivatives 1.
+
+
2a
d6-2a
1a
1a
d7-1a
kH/kD = 3.1
Standard condition
Standard condition
+
+
CO
CO
2a
kH/kD = 4.2
d7-1a
d6-2a
Scheme 3. Control experiments.
As shown in Eq. 3, stoichiometric reaction was also performed.
With 2 eq. PdCl2, 1a could undergo the C-N oxidative dealkylative
carbonylation smoothly to give the desire product in 42% yield,
which indicates that PdCl2 could promote the C-N bond cleavage in
the absence of Cu salt.7d, 7g, 7k, 7l, 11 When 2 eq. Pd(OAc)2 was used,
65% yield was obtained (Eq. 4). On the other hand, when
stoichiometric PdCl2 and N-methylnaphthalen-1-amine were used in
this process, N-methyl-N-(naphthalen-1-yl)formamide was detected
by GC-MS as shown in Eq. 5, suggesting the existence of the
insertion process of CO to the Pd-N bond to obtain the carbamoyl
palladium intermediate. kH/kD=3.1 and 4.2 were observed for the
intermolecular KIE experiment, suggesting that C-H bond cleavage
might be involved in the rate-determining step (Eq. 6 and Eq. 7). All
these results indicates that C-N bond activation occurs firstly to
Standard reaction conditions: 1 (0.2 mmol), PdCl2 (10 mol%),
Cu(OAc)2 •H2O (30 mol%), AcOH (40 mol%), CO/O2 = 2/1,
toluene/DMA =1.0/0.2, 110 oC, 24 h. Isolated yield.
On the other hand, we tested naphthyl substituted substrates to
construct conjugated molecules which might have great potential in
photoelectric material(Scheme 2). Both 1-methyl-4-(naphthalen-1-
2 | J. Name., 2012, 00, 1-3
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obtain carbamoyl palladium intermediate which is the key to C-H
bond activation.
Based on the above observations and related mechanistic studies,
we proposed the following catalytic cycle for this intramolecular
To step further in studying the mechanism of CO insertion step oxidative C-H bond activation/N-dealkylative carbonylation reaction
and C-H activation, DFT calculations were also carried out. Two (Scheme 4). Taking the reaction of N, N-dimethylnaphthalen-1-
possible pathways were proposed and calculated (Figure 1, see SI) In amine 1a as an example, the Pd-catalyzed aerobic C-N bond
Path-A, the intramolecular CO insertion first occurs before C-H
activation of 1a released the intermediate I. Then, Pd (II) complex I
underwent sequential CO insertion to generate the key intermediate
II, followed by C-H bond activation to achieve six-members
palladacycle III. Finally, reductive elimination occurs to form the
product 2a, and releases the Pd(0) species, which was re-oxidized by
O2 with the assistance of the copper co-catalyst to regenerate Pd(II)
complex.
Figure 1. Free energy profiles of Path-A and Path-B for Pd-
catalyzed carbonylation
bond activation. Through transition state 2-ts, intermediate 3 is
formed with a barrier of 14.9 kcal/mol. Subsequent concerted
metallation-deprotonation (CMD) takes place via transition state 4-
ts, generating complex 5; the activation free energy of this process is
23.3 kcal/mol. Finally, the C-N bond reductive elimination through
transition state 6-ts can afford lactam coordinated Pd0 complex 7
with an activation free energy of 16.2 kcal/mol. The relative free
energy of complex 7 is 12.8 kcal/mol exothermic compared with
complex 1. Alternatively, the C2-H bond activation might take place
through transition state 4b-ts, generating the complex 5b reversibly.
Although the corresponding activation free energy is 21.4 kcal/mol,
which is very close to that of transition state 4-ts, subsequent
reductive elimination is unfavored to occur because of the higher
energy barrier (32.8 kcal/mol). Thus, the C-H bond activation prefers
to occur on C8 site.
On the other hand, the deprotonation might first occur through
transition state 9-ts, generating complex 10 with a barrier of 26.6
kcal/mol (Path-B). Followed by CO insertion into N-Pd bond via
transition state 11-ts leads to the formation of complex 5. The
overall barrier reaches up to 52.3 kcal/mol, which is 29.0 kcal/mol
higher than that in Path-A. Besides, CO can also insert into the C-Pd
bond through transition state 12-ts, generating complex 13.
Subsequent reductive elimination via transition state 14-ts gives
complex 7 with a barrier of 16.6 kcal/mol. As the activation free
energy of CO insertion via 12-ts is 11.5 kcal/mol higher than that in
Path-A, Path-B can be safely ruled out. Consequently, Pd-catalyzed
carbonylation is most likely to proceed through CO insertion,
concerted metallation-deprotonation on C8 site, and C–N bond
reductive elimination to give the product.
Scheme 4. Proposed mechanism
In conclusion, we have developed a novel palladium-catalyzed
intramolecular aerobic oxidative amine-directed C-H bond
activation/N-dealkylative carbonylation reaction of tertiary
naphthalen-1-amines utilizing O2 as the terminal oxidant. This
transformation provides an effective and straightforward protocol
towards the synthesis of biologically and synthetically useful
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DOI: 10.1039/C6CC06358F
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Acknowledgement
This work was supported by the National Natural Science
Foundation of China (21390400, 21520102003, 21272180,
21302148), the Hubei Province Natural Science Foundation of China
(2013CFA081), the Research Fund for the Doctoral Program of
Higher Education of China (20120141130002), and the Ministry of
Science and Technology of China (2012YQ120060). The Program
of Introducing Talents of Discipline to Universities of China (111
Program) is also appreciated.
Notes and references
a
College of Chemistry and Molecular Sciences, Institute for Advanced
Studies (IAS), Wuhan University, Wuhan 430072, P. R. China; E-mail:
aiwenlei@whu.edu.cn
b
State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou
Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou
730000, P. R. China;
c
School of Chemistry and Chemical Engineering, Chongqing University,
Chongqing 400030, P. R. China; E-mail: lanyu@cqu.edu.cn
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