Palladium(0)-Catalyzed Intermolecular Arylative Dearomatization of β-Naphthols

Article abstract of DOI:10.1002/anie.201608724

The first Pd⁰-catalyzed intermolecular arylative dearomatization of β-naphthols with aryl halides is described. It was found that Q-Phos could facilitate the palladium-catalyzed cross-coupling-type dearomatization of β-naphthols, while avoiding

Full text of DOI:10.1002/anie.201608724

Angewandte
Communications
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International Edition: DOI: 10.1002/anie.201608724
German Edition: DOI: 10.1002/ange.201608724
Cross-Coupling Reactions
Palladium(0)-Catalyzed Intermolecular Arylative Dearomatization of
b-Naphthols
Ren-Qi Xu, Ping Yang, Hang-Fei Tu, Shou-Guo Wang, and Shu-Li You*
Abstract: The first Pd0-catalyzed intermolecular arylative
dearomatization of b-naphthols with aryl halides is described.
It was found that Q-Phos could facilitate the palladium-
catalyzed cross-coupling-type dearomatization of b-naphthols,
while avoiding O-arylation, to construct 2-naphthalenones in
excellent yields and with high chemoselectivity.
electrochemical method.[15] Herein, we report the first Pd0-
catalyzed intermolecular arylative dearomatization of
b-naphthols (1) with aryl halides, which constructs 2-naph-
thalenones bearing an all-carbon quaternary stereogenic
center at the a-position, in excellent yields and with high
chemoselectivity (Eq. (2)).
P
henol derivatives are important chemical starting materials
and widely used in organic synthesis.[1] Dearomatization of
phenol derivatives provides an efficient and straightforward
approach to cyclic enones, which are popular structural motifs
in functional molecules.[2,3] Palladium-catalyzed cross-
coupling reactions have been applied in the dearomatization
of phenol derivatives.[4–8] However, to date, the reported
works are mainly limited to intramolecular reactions, which
can avoid the competitive Friedel–Crafts-type reaction, and
coupling reaction with phenol oxygen nucleophile, by effi-
ciently forming favorable five or six-membered rings. How-
ever, the intramolecular reaction generally requires multi-
step synthesis of the designed substrates and thus limits the
structures of the products. Consequently, there is a great
demand to develop palladium-catalyzed intermolecular de-
aromatization of phenol derivatives by cross-coupling with
simple electrophiles such as aryl halides. The challenge for
such a reaction process is obvious, as phenols and aryl halides
The study was launched by utilizing 1,3-dimethyl-2-
naphthol (1a) and bromobenzene (2a) as the model
substrates to examine different ligands with [Pd(C3H5)Cl]2
as a palladium precursor. The results are summarized in
Table 1. None of the desired dearomatized product was
obtained when Buchwald-type ligand XPhos (L1)[16] and rac-
Feringa ligand (L2) were used (Table 1, entries 1 and 2), while
the utilization of SIPr·HBF4 (L3) and (di-tBu)XPhos (L4)[16]
gave trace amounts of product (3aa; Table 1, entries 3 and 4).
Fortunately, with ferrocenyl dialkylphosphine Q-Phos (L5)[17]
as the ligand, the arylative dearomatization proceeded
smoothly, with good conversion and excellent chemoselectiv-
ity (73% yield, 3aa/4aa = 20/1; Table 1, entry 5). Subse-
quently, several bases were examined (Table 1, entries 6–9).
To our delight, stronger alkali base, such as Cs2CO3, could
improve the conversion without affecting the chemoselectiv-
ity, and the desired dearomatized product (3aa) was obtained
in 92% isolated yield (Table 1, entry 7). Arene-type solvents,
such as fluorobenzene and o-xylene, could also give satisfac-
tory yields (Table 1, entries 10 and 11). Notably, temperature
plays an important role. The reaction at 608C was sluggish,
and the reaction at 1208C led to an increase of the O-arylation
side product (Table 1, entries 14 and 15). Therefore, the
optimized conditions were obtained as the following:
[Pd(C3H5)Cl]2 (2.5 mol%), Q-Phos (L5; 7.5 mol%), and
Cs2CO3 (1.5 equiv) in toluene at 808C (Table 1, entry 7).
Subsequently, various b-naphthols (1a–1l) were reacted
with bromobenzene to examine the generality of the novel
dearomatization under the optimized reaction conditions
(Table 2). It was found that the substituents at the 1- and
3-positions of 2-naphthol are of great importance. When the
À
are known to undergo a C O bond-forming reaction in the
presence of a palladium catalyst (Eq. (1)).[9,10] Therefore, to
avoid the O-arylation process will be key for a highly efficient
palladium-catalyzed intermolecular cross-coupling-type de-
aromatization of phenol derivatives. As naphthols display
relatively weak aromaticity compared to phenols, we envis-
aged that the utilization of naphthols might facilitate the
proposed palladium-catalyzed cross-coupling-type dearoma-
tization reaction. Such an intermolecular reaction would
provide easy access to the naphthalenone scaffold. Previous
limited reports on intermolecular dearomative arylation of
naphthol or phenol derivatives include the utilization of
stoichiometric aryl lead or aryl bismuth reagents,[11,12] hyper-
valent-iodine-mediated dearomatizing phenylation,[13] and
the oxidation of phenols by stoichiometric oxidant[14] or an
[*] R.-Q. Xu, P. Yang, H.-F. Tu, S.-G. Wang, Prof. Dr. S.-L. You
State Key Laboratory of Organometallic Chemistry
Shanghai Institute of Organic Chemistry
Chinese Academy of Sciences
345 Lingling Lu, Shanghai 200032 (China)
E-mail: slyou@sioc.ac.cn
Homepage: http://shuliyou.sioc.ac.cn/
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201608724.
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Table 1: Optimization of the reaction conditions.[a]
substituents were both alkyl groups, the corresponding
2-naphthols underwent dearomatization smoothly, in good
to excellent yields and chemoselectivity (Table 2, entries 2–5).
However, 1-methyl-2-naphthol (1j) was less reactive; only
48% yield of the desired product was obtained and some
starting material remained. Additionally, the presence of
a phenyl group (1k) or an electron-withdrawing group such as
CO2Me (1l) at the 1-position could inhibit the dearomatiza-
tion (Table 2, entries 11 and 12). Overall, electron-rich
b-naphthols facilitate the dearomatization, as 1,3-dimethyl-
2-naphthol derivatives bearing 6-Ph (1 f), 6-Me (1g), 7-Ph
(1h), and 7-Me (1i) groups all led to their corresponding
dearomatized products in excellent yields and with high
chemoselectivity (91–94% yields, 3/4 > 20/1; Table 2,
entries 6–9).
Subsequently, the reactions of naphthol 1a with various
aryl halides (2) were examined. As shown in Table 3, when
different halides such as Cl (2b) and I (2c) were tested, the
reactions occurred smoothly with excellent chemoselectivity
(> 20:1), but in slightly decreased yields (Table 3, entries 2
and 3). After that, the substituent effect of aryl bromides was
carefully examined. Aryl bromides bearing an electron-
Entry
Ligand
Base
Solvent
t
3aa Yield
3aa:4aa
[h]
[%][b]
1
2
3
4
5
6
7
8
L1
L2
L3
L4
L5
L5
L5
L5
L5
L5
L5
L5
L5
L5
L5
K2CO3
K2CO3
K2CO3
K2CO3
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
PhF
o-xylene
DCE
dioxane
toluene
toluene
10
10
10
10
10
10
6
10
10
15
6
0
ND
ND
ND
0
trace
trace
80 (73[d])
trace
>99 (92[d])
73
<1:20[c]
20:1
K2CO3
Na2CO3
Cs2CO3
K3PO4
ND
>20:1
>20:1
ND
>20:1
>20:1
ND
>20:1
>20:1
13:1
9
tBuOK
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
24
95
99
12
14
44
80
10
11
12
13
14[e]
15[f]
2
15
22
2
Table 3: The reaction substrate scope: Aryl halides.[a]
[a] Reaction conditions: 1a (0.2 mmol), [Pd(C3H5)Cl]2 (0.005 mmol),
ligand (0.015 mmol), 2a (0.3 mmol), base (0.3 mmol) in solvent
(1.0 mL), 808C. [b] Determined by 1H NMR using CH2Br2 (0.2 mmol) as
an internal standard. [c] Only etherified product was obtained in 25%
NMR yield. [d] Isolated yield. [e] Temperature was 608C. [f] Temperature
was 1208C.
Entry
2, R1, X
3:4[b]
Yield of 3 [%][c]
1
2
3
4
5
6
7
8
2a, H, Br
2b, H, Cl
2c, H, I
2d, 4-Me, Br
2e, 4-OMe, Br
2 f, 4-tBu, Br
2g, 4-nBu, Br
2h, 4-Ph, Br
2i, 4-Cl, Br
>20:1
>20:1
>20:1
>20:1
>20:1
>20:1
>20:1
18:1
>20:1
>20:1
4:1
2.1:1
1:2.1
3aa, 92
3ab, 81
3ac, 84
3ad, 93
3ae, 95
3af, 99
3ag, 91
3ah, 93
3ai, 90
3aj, 86
3ak, 75
3al, 62
3am, 31
3an, 91
3ao, 99
3ap, 92
3aq, trace
Table 2: The reaction substrate scope: Naphthols.[a]
9
10
11
12
13
14
15
16
17
2j, 4-F, Br
2k, 4-CF3, Br
2l, 4-CO2Me, Br
2m, 4-NO2, Br
2n, 3-Me, Br
2o, 3,5-(tBu)2, Br
2p, 3-F, Br
Entry
1, R1, R2, R3
3:4[b]
Yield of 3 [%][c]
1
2
3
4
5
6
7
8
1a, Me, Me, H
1b, Et, Me, H
1c, CH2CH2Ph, Me, H
1d, Me, Et, H
>20:1
>20:1
>20:1
>20:1
10:1
>20:1
>20:1
>20:1
>20:1
4:1
3aa, 92
3ba, 93
3ca, 99
3da, 91
3ea, 73
3 fa, 92
3ga, 94
3ha, 92
3ia, 91
>20:1
>20:1
>20:1
<1:20[d]
1e, Me, Bn, H
2q, 2-Me, Br
1 f, Me, Me, 6-Ph
1g, Me, Me, 6-Me
1h, Me, Me, 7-Ph
1i, Me, Me, 7-Me
1j, Me, H, H
9
10
11
12
3ja, 48[d]
3ka, 9[e]
3la, trace
1k, Ph, Me, H
1l, CO2Me, Me, H
ND
ND
[a] Reaction conditions: 1 (0.2 mmol), [Pd(C3H5)Cl]2 (0.005 mmol),
Q-Phos (0.015 mmol), 2a (0.3 mmol), Cs2CO3 (0.3 mmol) in toluene
(1.0 mL), 808C. [b] Determined by H NMR. [c] Isolated yield. [d] 1j
[a] Reaction conditions: 1a (0.2 mmol), [Pd(C3H5)Cl]2 (0.005 mmol),
Q-Phos (0.015 mmol), 2 (0.3 mmol), Cs2CO3 (0.3 mmol) in toluene
(1.0 mL), 808C. [b] Determined by H NMR. [c] Isolated yield. [d] Only
etherified product was obtained in 20% NMR yield. [e] 1a (0.1 mmol),
[Pd(C3H5)Cl]2 (0.005 mmol), Q-Phos (0.015 mmol), 2t (0.1 mmol),
Cs2CO3 (0.15 mmol) in toluene (1.0 mL), 808C.
1
1
(0.2 mmol), [Pd(C3H5)Cl]2 (0.005 mmol), Q-Phos (0.015 mmol), 2a
(0.3 mmol), K2CO3 (0.3 mmol) in toluene (1.0 mL), 808C. [e] NMR yield,
1
determined by H NMR using CH2Br2 (0.2 mmol) as an internal
standard.
2
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donating substituent or halide (Me, OMe, tBu, nBu, Ph, Cl, F)
at the para-position could be tolerated to give their desired
products in excellent yields and with high chemoselectivity
(86–99% yields, 3/4: 18/1– > 20/1; Table 3, entries 4–10).
However, aryl bromides bearing a strong electron-withdraw-
ing substituent (CF3, CO2Me, NO2) at the para-position led to
an increase of the O-arylation side product (Table 3,
entries 11–13). More O-arylation product is formed when
the substrate bearing a stronger electron-withdrawing sub-
stituent is employed. Aryl bromides bearing a meta-substitu-
ent also participated smoothly in the dearomative cross-
coupling reaction in excellent yields and with high chemo-
selectivity (Table 3, entries 14–16). Probably as a result of the
steric effect, the reaction of ortho-substituted aryl bromide
(2q) only gave O-arylation product in 20% yield (determined
by NMR spectroscopy; Table 3, entry 17). Pleasingly, when
more complicated aryl bromides were tested, such as
2-bromonaphthalene and 5-bromoindole, these reactions
afforded dearomatized products 3ar and 3as in 95% and
71% yield, respectively, with excellent chemoselectivity. The
structure of 3ar was confirmed by an X-ray crystallographic
analysis. It is worth noting that natural product flustramine
B[18] could be employed as a coupling partner to give
dearomatized product 3at in 71% yield.
To our delight, 4-methyl-1-naphthol (1m) underwent
dearomatization to afford benzoenone (3ma) in 81% yield
[Eq. (7)], while 2-methyl-1-naphthol underwent a Friedel–
Crafts-type reaction at the para-position under standard
reaction conditions (for details, see the Supporting Informa-
tion). Moreover, substituted phenol (1n) is also a suitable
substrate to afford dienone (3na) in 60% yield under slightly
forced conditions [Eq. (8)], while 2,4,6-trimethylphenol
underwent an O-arylation process to afford the ether product
(for details, see the Supporting Information).
To further demonstrate the utility of this method, a gram-
scale reaction and transformations of the 2-naphthalenone
products have been carried out. The intermolecular arylative
dearomatization of 1a with 3,5-di-tert-butylbromobenzene
(2o) in a 3.5 mmol scale gave the desired product 3ao in 96%
yield and excellent chemoselectivity (3ao:4ao > 20:1) while
the catalyst loading could be further reduced to 1.25 mol%
[Eq. (3)]. Dearomatized product 3ai could undergo the
Sonogashira coupling reaction with ethynyltriisopropylsilane
to afford 5ai in 85% yield [Eq. (4)]. Additionally, the ketone
group of product (3aa) could be selectively transformed to an
alcohol (6aa) in 99% yield (d.r. > 20:1; Eq. (5)) under Luche
reduction conditions, and the double bond of 3ja could be
hydrogenated to form 7ja in 81% yield [Eq. (6)].
Preliminary mechanistic investigation revealed that the
dearomatization product and the O-arylation product cannot
be interconverted with each other under standard conditions
(for details, see the Supporting Information). A catalytic cycle
was proposed as depicted in Scheme 1. The in situ formed Pd0
À
species undergoes oxidative addition across the C Br bond in
bromobenzene, affording
a
phenyl palladium species.
Assisted by base, the 1,3-dimethyl-2-naphthol proceeds
ligand exchange to form oxo-p-allylic palladium intermediate
I, which then undergoes reductive elimination to afford
product 3aa and finish the catalytic cycle.
In summary, we have developed the first palladium-
catalyzed intermolecular arylative dearomatization of
Scheme 1. Proposed catalytic cycle for palladium-catalyzed arylative
dearomatization of naphthols.
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b-naphthols with aryl halides to construct 2-naphthalenones
bearing an all-carbon quaternary stereogenic center at the
a-position, in excellent yields and with high chemoselectivity.
It was found that Q-Phos could facilitate the dearomatization
of b-naphthols while avoiding an O-arylation process. Further
studies on the reaction mechanism and development of
catalytic asymmetric reactions are currently under investiga-
tion.
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We thank the National Basic Research Program of China
from MOST (2015CB856600, 2016YFA0202900), National
Natural Science Foundation of China (21332009, 21421091),
and Chinese Academy of Sciences for generous financial
support.
Keywords: cross-coupling · dearomatization ·
homogeneous catalysis · naphthol · palladium
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Received: September 6, 2016
Published online: && &&, &&&&
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5
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Communications
Cross-Coupling Reactions
R.-Q. Xu, P. Yang, H.-F. Tu, S.-G. Wang,
S.-L. You*
&&&& — &&&&
Palladium(0)-Catalyzed Intermolecular
Arylative Dearomatization of b-Naphthols
2-Napthalenones made easy: Pd0-cata-
lyzed intermolecular arylative dearomati-
zation of b-naphthols with aryl halides is
described. It was found that Q-Phos
could facilitate the palladium-catalyzed
cross-coupling dearomatization of b-
naphthols while avoiding O-arylation, to
construct 2-naphthalenones in excellent
yields and with high chemoselectivity.
6
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These are not the final page numbers!