Rhodium-catalyzed oxidative benzannulation of n-adamantyl-1-naphthylamines with internal alkynes via dual C-H bond activation: Synthesis of substituted anthracenes

Article abstract of DOI:10.1021/acs.orglett.6b01991

Rhodium-catalyzed oxidative benzannulation of N-adamantyl-1-naphthylamines with internal alkynes to produce highly substituted anthracenes in satisfactory to good yields was developed. The annulation reaction proceeded smoothly under mild conditions in th

Full text of DOI:10.1021/acs.orglett.6b01991

Letter
pubs.acs.org/OrgLett
Rhodium-Catalyzed Oxidative Benzannulation of N‑Adamantyl-1-
naphthylamines with Internal Alkynes via Dual C−H Bond Activation:
Synthesis of Substituted Anthracenes
Xuan Zhang, Xiaoqiang Yu,* Dingwei Ji, Yoshinori Yamamoto, Abdulrahman I. Almansour,§
†
,∥
,†
†
†,‡
§
§
,†
Natarajan Arumugam, Raju Suresh Kumar, and Ming Bao*
†
State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116023, China
WPI-AIMR (WPI-Advanced Institute for Materials Research), Tohoku University, Sendai 980-8577, Japan
Department of Chemistry, College of Sciences, King Saud University, P.O. Box 2455, Riyadh 11451, Saudi Arabia
‡
§
*
S Supporting Information
ABSTRACT: Rhodium-catalyzed oxidative benzannulation of
N-adamantyl-1-naphthylamines with internal alkynes to
produce highly substituted anthracenes in satisfactory to
good yields was developed. The annulation reaction proceeded
smoothly under mild conditions in the presence of
[
Cp*RhCl ] as the precatalyst and Cu(OAc) as the oxidant.
2
2
2
he development of convenient and efficient methods for
the synthesis of polycyclic aromatic compounds has
T
attracted considerable attention. Polycyclic aromatic com-
pounds can be utilized as versatile and key synthetic
intermediates for the preparation of organic semiconductors
1
and luminescent materials. Numerous methods, including the
2
Diels−Alder reaction of benzynes with cyclopentadienones,
aromatic homologation of organometallic reagents with
alkynes, and transition-metal-catalyzed aromatic homologation
of appropriate aromatic substrates with alkynes, have been
3
4
developed over the past years. Transition-metal-catalyzed
aromatic homologation is the most economical and practical
among these methods. In particular, this type of aromatic
homologation that proceeds through double C−H bond
cleavage has been used as extremely powerful tools for the
synthesis of highly substituted polycyclic aromatic compounds.
The above-mentioned synthetic method usually requires a
suitable directing group linked on the aromatic substrate.
activation of naphthalene substrates with acylamino directing
groups, we succeeded in the benzannulation by using a
sterically hindered directing group (adamantoylamino) for the
first time (eq 3). The results are reported in the current work.
In the initial study, we examined the reaction of N-
(naphthalen-1-yl)acetamide (1a) with 1,2-diphenylethyne
7
4
j
4t
Heterocycles (such as pyrazole, benzoimidazole, and
4
d
4h,i
pyridine ), aminocarbonyl groups,
groups
and acetylamino
4
n,r
have been employed as effective directing groups
(4a) in the presence of [Cp*RhCl
2 2
] as the precatalyst and
Cu(OAc) as the oxidant in N,N-dimethylformamide (DMF) at
for this purpose. An acylamino group could be easily installed
into aromatic substrates and converted to other functional
groups. Thus, acylamino groups were frequently utilized as
2
1
10 °C for 12 h. A similar result was observed as reported by
4r
Fagnou and co-workers. Benzoindole formation reaction
5
exclusively occurred via ortho-C−H bond cleavage to produce
directing groups for C−H bond activation.
5
a′ in 21% yield (Scheme 1). We then considered that a
We have recently found that reaction regioselectivity in the
oxidative annulation of ethyl naphthalen-1-ylcarbamates with
internal alkynes could be easily controlled by using different
sterically hindered directing group may inhibit the occurrence
of an undesired benzoindole forming reaction to take place.
The undesired reaction was completely inhibited when the
methyl in the N-acyl group was replaced with tert-butyl. The
benzannulation product 6a was isolated in 39% yield (Scheme
6
rhodium catalyst systems. The benzoquinoline formation
reaction proceeded via peri-C−H bond cleavage in the presence
of a neutral rhodium catalyst (eq 1), whereas the benzoindole
formation reaction proceeded via ortho-C−H bond cleavage in
the presence of a cationic rhodium catalyst (eq 2). During the
continuing research on the rhodium-catalyzed C−H bond
Received: July 8, 2016
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b01991
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 1. Effect of Steric Hindrance of Directing Group on
precatalyst and DMF as the solvent. Among the tested oxidants,
Reaction Regioselectivity
the Cu(OAc) proved to be the best oxidant (entry 12 versus
2
entries 13−15). Therefore, the subsequent benzannulation
reactions of 1-adamantoyl-1-naphthylamines 3a−3e with
internal alkynes 4a−4q were performed in the presence of
[
Cp*RhCl ] as the precatalyst and Cu(OAc) as the oxidant in
2
2
2
DMF at 110 °C for 12 h.
With the optimized reaction conditions in hand, we explored
the scope and the limitation of this type of benzannulation, and
the results are summarized in Scheme 2. As described in Table
1
). The yield of the benzannulation product could be further
improved by installing the more sterically hindered adamantan-
-yl into N-acyl group (Scheme 1, 7a: 60%). Based on these
1
, the desired product 7a was obtained in 75% yield. Reactions
1
Scheme 2. Rh(III)-Catalyzed Oxidative Benzannulation of 1-
findings, benzannulation reaction conditions were subsequently
optimized.
,
a b
Adamantoyl-1-naphthylamines with Internal Alkynes
The benzannulation reaction of 1-adamantoyl-1-naphthyl-
amine (3a) with 4a was selected as a model to optimize the
reaction conditions. Results are shown in Table 1. The use of
a
Table 1. Optimization of Reaction Conditions
b
entry
catalyst
oxidant
solvent
yield (%)
60
1
2
3
4
5
6
7
8
9
[Cp*RhCl2]2
RhCl ·3H O
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OTf)2
Ag CO
DMF
c
DMF
NR
3
2
c
RhCl(PPh3)3
[Ru(p-cymene)Cl2]2
Pd(OAc)2
DMF
NR
c
DMF
NR
c
DMF
NR
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
toluene
dioxane
acetone
DCE
21
33
55
40
32
71
75
t
10
11
12
13
14
15
AmOH
d
d
d
d
d
DMF
DMF
DMF
DMF
DMF
,
,
,
,
e
e
e
e
c
NR
c
NR
2
3
AgOAc
trace
a
Reaction conditions: 3a (0.25 mmol), 4a (0.5 mmol), catalyst (2.5
mol %), oxidant (0.5 mmol) in solvent (2.0 mL) at 110 °C for 12 h
b
c
under a N atmosphere. Isolated yield. No reaction was observed,
2
d
and the starting materials were recovered. 5.0 mol % of [Cp*RhCl ]
was used. 0.75 mmol of 4a was used.
2
2
e
RhCl ·3H O and RhCl(PPh ) as rhodium precatalysts instead
3
2
3 3
of [Cp*RhCl2]2 led to no reaction (entries 2 and 3).
Dichloro(p-cymene)ruthenium dimer {[Ru(p-cymene)Cl ] }
2
2
and Pd(OAc) were demonstrated to be also totally ineffective
2
(
entries 4 and 5). The solvents were then screened using
nonpolar [toluene and 1,2-dichloroethane (DCE)] and polar
t
[
DMF, dioxane, acetone, and tert-amyl alcohol ( AmOH)]
solvents (entries 1 and 6−10). DMF proved to be the best
solvent (entry 1). The yield of benzannulation product 7a was
found to increase with increased [Cp*RhCl ] loading (entry
2
2
1
1, 71%). The yield of 7a was found to be further increased
when the amount of 4a was increased to 3.0 equiv (entry 12,
5%). The oxidants, including Cu(OAc) , Cu(OTf) , Ag CO ,
a
Reaction conditions: 3 (0.25 mmol), 4 (0.75 mmol), [Cp*RhCl ] (5
2
2
mol %, 7.8 mg), Cu(OAc) (0.5 mmol, 91.0 mg) in DMF (2.0 mL) at
2
b
c
7
110 °C for 12 h under N atm. Isolated yields. No reaction; starting
2
2
2
3
2
d
and AgOAc, were finally screened using [Cp*RhCl ] as the
materials were recovered. 10 mol % of [Cp*RhCl ] was used.
2
2
2 2
B
DOI: 10.1021/acs.orglett.6b01991
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
of 3a with internal alkynes 4b−4f bearing halogen atoms on
para- or meta-positions of the benzene rings proceeded
smoothly to produce the corresponding benzannulation
products 7b−7f in satisfactory yields (50%−72%). The reaction
of 3a with internal alkynes 4g bearing an electron-withdrawing
Scheme 4. A Plausible Mechanism for the Rh-Catalyzed
Benzannulation
group CF on the para-positions of the benzene rings provided
3
the benzannulation product 7g in 53% yield. Internal alkynes
4
h−4m bearing an electron-donating group methyl (Me) or
methoxy (OMe) on benzene rings were subsequently
examined, and the corresponding benzannulation products
7
h, 7i, and 7k−7m were obtained in moderate-to-satisfactory
yields (43%−63%), except 7j. The reason for the nonreaction
observed in the treatment of 3a with 4j may be due to steric
hindrance caused by the ortho-methyl group in 4j. These results
mentioned above indicate that the electron property of the
substituent linked on the benzene rings of alkynes did not
influence alkyne reactivity. The applicability of an internal
alkyne with a heterocycle such as thiophene was also
investigated, and 21% of benzannulation product 7n was
obtained. Surprisingly, the use of unsymmetrical alkynes 4o−
4
q, which bear a phenyl group and an alkyl group (Me, Et, and
n
Pr), led to the formation of 7o−7q as sole products,
respectively (41%−56% yields). The reactivities of 1-
8
would then be reoxidized to active catalytic species Cp*Rh-
adamantoyl-1-naphthylamines 3b−3e, with MeO, Br, and Cl
on the naphthalene ring, respectively, were finally investigated
by using 4a as a reaction partner. The corresponding
benzannulation products 7r−7u were obtained in moderate-
to-good yields (48%−85%). All new products, 7a−7i and 7k−
(OAc)
2
by Cu(OAc)
2
. The formation of intermediate C′ is
considered to be inhibited by the steric hindrance of the
adamantan-1-yl group. Consequently, the formation of the
benzoindole product was inhibited.
In summary, we developed a rhodium-catalyzed regioselec-
tive oxidative benzannulation by using a sterically hindered
adamantoylamino group as the directing group for the synthesis
of highly substituted anthracenes. A series of highly substituted
anthracenes were obtained in moderate-to-high yields from the
reactions of 1-adamantoyl-1-naphthylamines with internal
alkynes via dual C−H bond cleavages. To the best of our
knowledge, this paper presents the first successful example of
the rhodium-catalyzed benzannulation of N-acyl 1-naphthyl-
amines.
7
u, were identified through their NMR and HRMS data, as well
as IR spectra. Product 7a was further identified by determining
9
its X-ray structure.
To elucidate the mechanism of this type of benzannulation
reaction, we conducted a H−D exchange experiment to
investigate whether these reactions initially proceeded via an
ortho-C−H bond cleavage pathway (Scheme 3). We found that
Scheme 3. H−D Exchange Experiment on Amide Substrate
3
a in the Presence of DOAc
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.6b01991.
Experimental procedures and characterization data
2
0% of ortho-H in 3a was replaced by D. This result reveals that
(PDF)
metalation is directed by the amide group and that cyclo-
rhodation is a reversible process in the absence of an alkyne
under the protonic conditions.
AUTHOR INFORMATION
Corresponding Authors
■
On the basis of our experimental outcomes and previous
4
h−j
reports,
a plausible catalytic cycle is proposed to account for
*E-mail: yuxiaoqiang@dlut.edu.cn.
*E-mail: mingbao@dlut.edu.cn.
the present catalytic benzannulation reaction (Scheme 4). The
catalytic cycle starts from Cp*Rh(OAc) , which is generated in
Present Address
2
situ from the ligand exchange reaction between [Cp*RhCl2]2
∥
Department of Chemistry, Colorado State University, Fort
Collins, Colorado 80523, United States.
and Cu(OAc) . Coordination of the oxygen atom of 3a to the
2
rhodium catalyst species and subsequent ortho C−H bond
activation would generate a six-membered rhodacyclic inter-
mediate A with the liberation of an acetic acid molecule. Then,
the insertion of internal alkyne 4a into the Rh−C bond would
occur to produce intermediate B, which would subsequently
undergo a second C−H bond activation to afford intermediate
C. A second insertion of alkyne 4a and subsequent reductive
elimination would occur to generate highly substituted
anthracene product 7a and a Rh(I) species. The Rh(I) species
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to the National Natural Science Foundation of
China (Nos. 21372035 and 21361140375) for their financial
support. This work was also supported by the Fundamental
Research Funds for the Central Universities (DUT15LK37).
C
DOI: 10.1021/acs.orglett.6b01991
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
The authors acknowledge the Deanship of Scientific Research
at King Saud University for the Research Grant, RGP-VPP-026.
(7) Only a trace amount of benzannulation product was observed
when a palladium catalyst was used under literature conditions (see
4
(
n).
8) The structure of 7o was surmised according to the nuclear
Overhauser effect (NOE) determination and the literature. (a) See 4j.
b) See 4t. (c) Kim, Y.; Hong, S. Chem. Commun. 2015, 51, 11202.
9) The structure of 7a was confirmed by X-ray single crystal
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DOI: 10.1021/acs.orglett.6b01991
Org. Lett. XXXX, XXX, XXX−XXX