RuII-catalyzed vinylative dearomatization of naphthols via a C(sp2)-H bond activation approach

Article abstract of DOI:10.1021/ja410060e

Intermolecular annulation reactions of 1-aryl-2-naphthols with internal alkynes proceed efficiently in the presence of a Ru catalyst and a Cu oxidant to generate spirocyclic compounds by sequential cleavage of the C(sp ²)-H bond, migratory insertion of the alkyne, and dearomatization of the naphthyl ring. Various spirocyclic molecules bearing an all-carbon quaternary stereocenter could be obtained by this novel method with good yields and excellent regioselectivity, and the current process tolerates a variety of synthetically important functional groups.

Full text of DOI:10.1021/ja410060e

Communication
pubs.acs.org/JACS
RuII-Catalyzed Vinylative Dearomatization of Naphthols via a
C(sp2)−H Bond Activation Approach
Jiang Nan,† Zhijun Zuo,† Lei Luo, Lu Bai, Huayu Zheng, Yini Yuan, Jingjing Liu, Xinjun Luan,*
and Yaoyu Wang
Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of the Ministry of Education, College of Chemistry &
Materials Science, Northwest University, Xi’an 710069, China
S
* Supporting Information
metal-catalyzed phenol-related reactions to generate diverse
benzofurans,8 dibenzofurans,9 and dibenzopyranones10 were
ABSTRACT: Intermolecular annulation reactions of 1-
aryl-2-naphthols with internal alkynes proceed efficiently
in the presence of a Ru catalyst and a Cu oxidant to
generate spirocyclic compounds by sequential cleavage of
the C(sp2)-H bond, migratory insertion of the alkyne, and
dearomatization of the naphthyl ring. Various spirocyclic
molecules bearing an all-carbon quaternary stereocenter
could be obtained by this novel method with good yields
and excellent regioselectivity, and the current process
tolerates a variety of synthetically important functional
groups.
also realized by using the C-H activation tactics. Notwithstanding
these pioneering examples, further development of versatile
phenol functionalization processes is needed to enrich the scope
and utility of this category of synthetically valuable trans-
formations.
In this context we will focus on a Ru-catalyzed oxidative
annulation reaction by coupling of phenol derivatives with
internal alkynes (Scheme 1). This catalytic method provides a
Scheme 1. RuII-Catalyzed Dearomatization of Naphthols
he ubiquitous phenols are a class of privileged chemical
T
feedstock being widely utilized in organic synthesis, and a
plethora of natural products and valuable materials have been
prepared from these readily available starting materials.1 To
streamline substantial advances in this arena, more efficient and
economical methods for phenol functionalization are highly
desirable.
straightforward avenue to access 3D spirocyclic compounds
bearing the spiro[indene-1,1′-naphthalene] skeleton, which is an
important but synthetically challenging structural motif in
various biologically active natural products11 and optoelectronic
materials.12 The significance of this transformation is further
highlighted by the evolution of dearomatization of naphthyl rings
in the substrates compelled by a C-H activation strategy.
Dearomatization of phenols often serves as a key step in the rapid
construction of highly functionalized alicyclic molecules.1c,d
However, progress of metal-catalyzed direct dearomatization of
phenol derivatives to form C-C bonds has been relatively slow,
and the majority of the few protocols established in this area were
mainly limited by employing preactivated agents such as aryl
halides as coupling partners.13 Thereby, metal-catalyzed C-H
activation would be a complementary approach for phenol
dearomatization. Until now, a handful of phenol-involved, metal-
catalyzed C-H activation reactions have been described,6−10 but
the envisioned dearomatization of phenol rings has not yet been
disclosed. Herein we report the first successful example of
transition-metal-catalyzed vinylative dearomatization of naph-
thols through an unprecedented C-H activation/dearomatiza-
tion tandem process.
Transition-metal-catalyzed C-H functionalization has unim-
peachably emerged as a powerful tool to render shorter synthetic
routes for complex molecules by eliminating the need for prior
preparation of activated substrates.2 Notably, Rh-,3 Pd-,4 and Ru-
catalyzed5 C-H activation followed by an annulation reaction
with alkynes has been frequently used as a rapid approach to
construct various heterocycles and carbocycles. In particular,
incorporating phenol derivatives in these reactions has been of
very recent interest, and several elegant transformations
involving 1-naphthols,6 simple phenols,7 and 2-arylphenols6a
have been successfully implemented to give rise to planar
annulation products (eqs 1−3). Meanwhile, some other facile
In an initial attempt, we investigated the envisioned oxidative
coupling of 2-phenylphenol with 1-phenyl-1-propyne (2a) under
various reaction conditions. Unfortunately, the desired dear-
Received: September 30, 2013
Published: November 6, 2013
© 2013 American Chemical Society
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Communication
omatization product could only be obtained at extremely low
yield (<5%), although the most frequently used [(Cp*RhCl2)2],
Pd(OAc)2, and [RuCl2(p-cymene)2] catalysts combined with
different weak oxidants [Cu(OAc)2, Ag2CO3, Ag2O, AgOAc]
have been tested. To this end, we turned our attention to other
substrates with substituted phenol rings. Gratifyingly, 1-phenyl-
2-naphthol (1a) was found to be very effective for this
unprecedented transformation (Table 1). At the outset,
Table 2. Survey the Scope of Naphthol Coupling Partner
Table 1. Optimization of the Reaction Conditions
a
entry
[M], amount (mol%)
solvent
yield (%)
1
2
3
4
5
6
[RuCl2(p-cymene)2], 2.5
[RuCl2(p-cymene)2], 2.5
[RuCl2(p-cymene)2], 2.5
[RuCl2(p-cymene)2], 2.5
[RuCl2(p-cymene)2], 2.5
[RuCl2(p-cymene)2], 2.5
[RuCl2(p-cymene)2], 2.5
[RuCl2(p-cymene)2], 2.5
Pd(OAc)2, 5.0
tAmOH
48
36
62
18
19
85
38
42
<5
26
toluene
DME
DCE
DMF
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
b
a
Ar1 = p-CF3-C6H4.
7
c
8
9
trifluoromethyl (3j,k), and nitro (3m) groups. Satisfactorily, the
reactions of substrates which possess a m-substituted phenyl
group (3k−m) occurred regioselectively at the less sterically
hindered 6-postion, whereas the other regioisomer was not
observed. However, the existence of a substituent on the ortho
position dramatically hampered the cyclization (<5% yield),
presumably because of steric hindrance. As expected, sub-
stitutions at the naphthyl ring were tolerated as well (3n,o). It is
noteworthy that potentially labile halogen functionalities
remained intact in the reactions (3e,l,n), thus offering excellent
handles for further synthetic manipulations. More importantly,
the value of this method was further amplified by the successful
replacement of upper phenyl group with heterocycles such as
thiophene system (3p,q). Most strikingly, 1q underwent highly
regioselective C-H functionalization at the α-position of thienyl
ring to generate 3q as the major product (>19:1:1:1).15
10
[(Cp*RhCl2)2], 2.5
b
a
c
Isolated yield. K2CO3 was not added. 10 mol% K2CO3 was used.
[RuCl2(p-cymene)2] was chosen as the catalyst for the reactions
being carried out at 90 °C with stoichiometric Cu(OAc)2 as the
oxidant. After surveying a variety of solvents in the presence of
K2CO3, the best result was obtained by using 1,4-dioxane (entry
6). Further control experiment proved that 2.0 equiv of K2CO3
was necessary for promoting higher product yield (entries 7 and
8). Pd(OAc)2 showed inferior result (entry 9), although it has
been extensively employed in a great number of oxidative
annulation reactions with alkynes. Next, complex
[(Cp*RhCl2)2] was evaluated, and it resulted in 26% yield of
3a entry 10). Overall, the best result was achieved using 2.5 mol%
of [RuCl2(p-cymene)2], 2.1 equiv of Cu(OAc)2, and 2.0 equiv of
K2CO3 in 1,4-dioxane at 90 °C for 48 h, providing 3a in 85% yield
with excellent regioselectivity (only one regioisomer was
observed). The structure of 3a was unambiguously assigned by
NMR spectroscopy and X-ray studies (SI).14 With respect to the
regioselectivity, the methyl-phenyl alkyne 2a was installed in a
manner that phenyl group is adjacent to the spirocyclic carbon
center, which is consistent with literature precedent.5 Compared
to previous reports,13e−g which have been limited to the
dearomatization of 1-alkyl-2-naphthols by using metal catalysis,
this new catalytic approach allows the dearomatization of larger
aromatic frameworks (1-aryl-2-naphthols).
As shown in Table 2, the steric factor of aryl ring in 1b−m has
been thoroughly studied. To reveal a more instructive principle
of the electronic effect of substrate 1 on this transformation,
illustrative competition experiments between 1c and 1j were
conducted (Scheme 2). The results highlighted EWGs on the
aryl moiety to be beneficial.
Scheme 2. Intermolecular Competition between Naphthols 1
With the optimized reaction conditions in hand, the reaction
scope was first examined by varying the substitution patterns on
the naphthol coupling partner (Table 2). Overall, an important
number of desired spirocyclic products were successfully
prepared, affording good yields (up to 96%) with excellent
regioselectivity for unsymmetrical alkyne 2a (>19:1). Various
substituents on the phenyl ring were found to be tolerable in this
process, including electron-neutral or electron-donating groups
(EDGs) such as methyl (3b) and methoxy (3c) groups, and
electron-withdrawing groups (EWGs) such as fluoro (3d),
chloro (3e,l), cyano (3f), formyl (3g), acetyl (3h), ester (3i),
Next, we sought to investigate the scope of alkynes. The results
demonstrated that the oxidative annulation process occurred
smoothly with various symmetrical and unsymmetrical alkynes
(Table 3). Regarding the alkynes containing aromatic sub-
stituents, both EDGs (methoxy) and EWGs (fluoride,
trifluoromethyl) were tolerated (3b′−d′). Moreover, the
reactions of alkynes with heterocyclic substitutent proceeded
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Communication
Table 3. Survey the Scope of Alkyne Coupling Partner 2
Scheme 5. Deuteration Experiments
properly to give corresponding product in moderate yield (3e′).
Notably, a dialkylacetylene could undergo the envisioned
cyclization with a naphthol derivative as well (3f′). However,
terminal alkynes are not applicable under the current reaction
conditions. To further study the regioselectivity of this
transformation, several representative unsymmetrical alkynes
including aryl, heteroaryl and alkyl substituents were subjected to
the reaction conditions (3g′−i′). The results indicated that the
initial C-C bond formation in the alkyne insertion step favorably
occurred at the alkyne carbon bearing the alkyl substituent, and
the desired products were obtained with excellent regioselectivity
(>19:1). Remarkably, the regioselectivity of the formation of 3g′
is superior to the recently reported Ru-catalyzed oxidative
annulations with cyclopropyl-substituted alkynes.5q
the reaction conditions. Moreover, the kinetic isotope effects
were observed in both intramolecular (kH/kD = 3.8) and
intermolecular (kH/kD = 5.0) competition experiments, which
suggests that the C-H bond cleavage is most likely involved in the
rate-limiting step and might follow the concerted metalation/
deprotonation mechanism.16
Based on the above results and previous reports,3−5 a possible
mechanism is proposed (Scheme 6). First, deprotonation and an
Scheme 6. Proposed Mechanism
To compare the activity difference of alkynes, we carried out a
set of competition experiments (Scheme 3), which indicated the
favored product 3a being derived from electron-rich alkyne 2a.
Scheme 3. Intermolecular Competition between Alkynes 2
With the aim of evaluating the efficiency and practicality of this
catalytic process, a scale-up experiment (3 mmol of 1j) was
carried out (Scheme 4). As a result, gram-scale preparation of 3j
(1.08 g) was achieved in 90% yield with >19:1 regioselectivity.
irreversible C-H bond cleavage occurs to give a six-membered
ruthenacycle 5. This would be then followed by coordination
with alkyne 2 and subsequent regioselective migratory insertion
to form a rather strained eight-membered intermediate 6. The
most important step of this reaction should be the unique enol-
keto tautomerization of the phenol ring to deliver the key
intermediate 7, which is a C-bound Ru enolate. To our
knowledge, there is one similar intermediate to ruthenacycle 7
being proposed in a Ru-catalyzed reaction.4c,5n Finally, reductive
elimination takes place to release product 3 and concomitantly
regenerates RuII catalyst 4 to furnish the catalytic cycle.
Scheme 4. Gram-Scale Preparation of 3j
To shed more light on the reaction mechanism, a series of
deuterium-labeling experiments were performed (Scheme 5).
With D2O as solvent, no deuterium was detected in the recovered
1a. Repeating this experiment in the presence of 0.5 equiv of 2a,
no deuterium was found in either product 3a or recovered 1a.
These results indicates that cycloruthenation is irreversible under
In summary, we have developed the first Ru-catalyzed
vinylative dearomatization reaction of 1-aryl-2-naphthols relying
on a C-H activation strategy. This transformation provides a
facile route to access a class of highly functionalized spirocyclic
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Journal of the American Chemical Society
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compounds with good yields and excellent regioselectivity.
Further investigations into the scope and mechanism of this
reaction, as well as extensive explorations of related enantiose-
lective transformations, are in progress.
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental procedures, spectral data for all new compounds,
and crystallographic data in cif format. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
xluan@nwu.edu.cn
Author Contributions
†J.N. and Z.Z. contributed equally to this work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
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We thank the National Science Foundation of China (21222201,
21271147, 51202192, J1210057), the “Thousand Plan” Youth
program, the Ministry of Education of China
(20126101120011), and the Northwest University for generous
financial support. We thank Prof. Biao Wu, Prof. Huaiming Hu,
and Ms. Rui Zhang for X-ray crystallographic assistance.
(8) Lee, D.-H.; Kwon, K.-H.; Yi, C. S. J. Am. Chem. Soc. 2012, 134,
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