Access to 2-naphthols: Via Ru(ii)-catalyzed C-H annulation of nitrones with α-diazo sulfonyl ketones

Article abstract of DOI:10.1039/c9cc02949d

Efficient synthesis of 2-naphthols was realized by Ru(ii)-catalyzed C-H activation of aryl nitrones and intermolecular [3+3] annulation with α-diazo sulfonyl ketones under redox-neutral conditions. Easily available α-diazo sulfonyl ketones act as a three-carbon component in the reaction.

Full text of DOI:10.1039/c9cc02949d

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc3N@C2n (2n 68 and 80). Synthesis of the
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DOI: 10.1039/C9CC02949D
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Access to 2-Naphthols via Ru(II)-Catalyzed C-H Annulation of
Nitrones with -Diazo Sulfonyl Ketones
Lingheng Kong, Xi Han, Xingwei Li *
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Efficient synthesis of 2-naphthols was realized by Ru(II)-catalyzed compounds for lactone synthesis, with the amide being an
C-H activation of aryl nitrones and intermolecular [3+3] annulation electrophilic directing group (Scheme 1c).12 Despite the
with -diazo sulfonyl ketones under redox-neutral conditions. progress, diazo reagents have been rarely applied as C3
Easily available -diazo sulfonyl ketones act as a three-carbon coupling reagents, especially when catalyzed by cost-effective
component in the reaction
Ru(II) complexes. Therefore, it is necessary to explore [n+3]
annulation using easily available new three-carbon reagents.
We reasoned that -diazo -sulfonylacetone might serve as
a C3 synthon in catalytic C-H activation assisted by an
electrophilic directing group, and the subsequent annulation
reaction can be chemoselective owing to the strong electron-
withdrawing nature of the sulfonyl group that increases the ‘-
carbon nucleophilicity while weakens the O-nucleophilicity.
What’s more, examples of Ru(II)-catalyzed arene C-H
activation and intermolecular coupling with diazo compounds
remain limited.13 We now report Ru(II)-catalyzed [3+3]
annulation of nitrones with -diazo sulfonyl ketones, leading
to efficient synthesis of 2-naphthols (Scheme 1d). Of note,
although synthesis of 1-naphthols has been well-studied by
following C-H activation strategy,14 2-naphthols have been
rarely accessed via this method.2e
2-Naphthols are synthetically important carbocycles and are
ubiquitously embedded in a large number of bioactive natural
products, pharmaceuticals, and agrochemicals.1 As a result,
numerous synthetic methods have been developed to access
2-naphthols over the past decades. 2-Naphthols are typically
synthesized via intramolecular cyclization (such as oxidative
cyclization, electrophilic cyclization, photo- or thermal-
promoted cyclization, and intramolecular Aldol or Dieckmann
condensation) and intermolecular cyclization reactions.2 These
cyclization systems generally suffer from lengthy synthetic
steps, harsh reaction conditions, and necessity of highly
functionalized starting materials. Therefore, more efficient
synthesis of 2-naphthols using readily available starting
materials is under great demand.
In the past decades, metal-catalyzed C-H activation has been
established as an effective strategy for C-C bond formation.3 In
particular, metal-catalyzed carbenoid functionalization has
emerged as a straightforward and powerful strategy to
construct C-C bonds under PdII-, RhIII-, IrIII-, CoIII-, RuII-catalysis.4
A series of elegantly works have been reported by the groups
of Yu,5 Glorius,6 Ackermann,7 Wang,8 Chang,9 and others
(Scheme 1a), where the carbene reagents often function as C1
or C2 synthons.10 Occasionally, diazo reagents can act as three-
atom synthons. Thus, Cui and co-workers reported Rh(III)-
catalyzed [4+3] cycloaddition of amides and vinyl diazo
reagents under mild conditions (Scheme 1b).11 On the other
hand, Liu and others developed Rh(III)-catalyzed O-
nucleophillic [3+3] annulation between benzamides and diazo
Previous reports
Pd, Rh, Ir,
Co, or Ru
N2
DG
R4
DG
+
+
+
(a)
(b)
R1 CO2R2
(rare for Ru)
R3
O
O
NH
Rh(III)
N2
OPiv
Ar
R1O2C
N
Ar
R1O2C
H
C3 synthon
O
O
N2
Rh(III)
R2
O
N
R1
(c)
R2
O-nucleophilic
O
R1
This work
(d)
O
Ru(II)
Ts
N
O
+
OH
C-nucleophilic
N2
Ts
Scheme 1. Transition Metal-Catalyzed C-H Activation of Arenes with Diazo
Key Laboratory of Applied Surface and Colloid Chemistry of MOE, School of
Chemistry and Chemical Engineering, Shaanxi Normal University, Xi’an 710062,
China
Compounds
E-mail: lixw@snnu.edu.cn
†Electronic Supplementary Information (ESI) available: Experimental procedures
and characterization data of all compounds. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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Table 1. Optimization of the Reaction Conditionsa
proved to be the optimal silver salt; switching to other silver
salts such as AgOAc, AgOTf , AgBF4 or AgPF6 led to a slightly or
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DOI: 10.1039/C9CC02949D
O
significantly lower coupling efficiency (entries 6-10). A yield of
69% was secured when the diazo compound 2a was used as
the limiting reagent (entry 11). The yields slightly decreased
[Ru(p-cymene)Cl2]2 (5.0 mol %)
additive, solvent, temp, 12 h
Ts
+
N
O
OH
N2
Ts
1a
2a
3aa
o
o
using DCE as a solvent at 130 C or 120 C (entries 12-13). To
our delight, switching the silver salt to an equimolar mixture of
AgNTf2 and AgPF6 (DCE, 120 oC) gave rise to 79% isolated yield
(entry 15). The Lewis acidic mixed silver salts AgNTf2/AgPF6
might facilitate intramolecular cyclization step. Moderate
yields were obtained when [Cp*RhCl2]2 or [Cp*IrCl2]2 was used
as a catalyst, while trace of product was observed when
Cp*Co(CO)I2 was used (entries 16-18).
With the optimized reaction conditions in hand, the scope
and generality of aryl nitrones in this process were next
employed (Scheme 2). A range of aryl nitrones bearing
electron-donating -withdrawing, and halogen substituents at
the para position reacted smoothly with 2a, affording the
desired products 3aa-3ia in 51-79% yields. A para-hydroxyl
group, which can often be problematic in the C-H activation
process, was also compatible (3ia). The reaction also worked
well for meta methyl-, chloro-, and ester- substituted nitrones
(3ja, 3ka, and 3la) in moderate to high yields and excellent
regioselectivity (>25:1). Exception was found for a meta
methoxy-substituted nitrone (3ma, 20:1 rr). To our delight, a
few ortho-substituted nitrones (3na-3ra) coupled with
enhanced efficiency, suggesting that the reaction was
insensitive to steric effect. Furthermore, heterocycle-
containing nitrones (3sa and 3ta), 2-naphthyl nitrone (3ua),
and fused ring nitrone (3va) all coupled efficiently to provide
the annulated products in moderate yields.
yield
(%)b
entry
additive
acid
solvent
T (oC)
1
2
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgNTf2
AgOAc
AgOTf
AgBF4
MesCOOH
Ph-CF3
Ph-CF3
Ph-CF3
Ph-CF3
Ph-CF3
Ph-CF3
Ph-CF3
Ph-CF3
Ph-CF3
Ph-CF3
Ph-CF3
DCE
100
100
100
115
130
130
130
130
130
130
130
130
120
120
120
120
120
120
45
48
PivOH
AcOH
PivOH
PivOH
PivOH
PivOH
PivOH
PivOH
PivOH
PivOH
PivOH
PivOH
PivOH
PivOH
PivOH
PivOH
PivOH
3
40
4
54
5
60
6
66
7
62
8
30
9
63
10
11c
12c
13c
14c
15c
16c,e
17c,f
18c,g
AgPF6
54
AgNTf2
AgNTf2
AgNTf2
69
64
[Ru(p-cymene)Cl2]2 (5.0 mol %)
AgNTf2 (20.0 mol %), AgPF6 (20.0 mol %)
DCE
63
O
Ar
Ts
+
N
O
Ar
PivOH (2.0 equiv), DCE, 120 o
C
O
H
N2
2a
AgNTf2+AgBF4
AgNTf2+AgPF6
AgNTf2+AgPF6
AgNTf2+AgPF6
AgNTf2+AgPF6
DCE
78
Ts
1
3
3aa
R = H,
, 79%
DCE
82 (79)d
73
3ba
R = OMe,
, 66%
R = tBu,
R = Ph,
, 74%
3ca
R
3ja
3ka
R = Me,
R = Cl,
, 64%
, 66%
3da
, 67%
DCE
3ea
R = F,
, 51%
3la
, 81%
R
OH
R = COOMe,
OH
3fa
R = Br,
, 60%
Ts
Ts
3ga
R = CF3,
, 61%
DCE
55
3ha
, 69%
R = COOMe,
3ia
R = OH,
, 56%
R
3na
, 74%
R = Me,
MeO
DCE
<5
3oa
, 90%
R = OMe,
R = F,
+
OH
3pa
, 53%
, 76%
OH
3qa
R = Br,
OH
OMe Ts
Ts
3ma
a Reaction conditions: 1a (0.2 mmol), 2a (0.3 mmol), [Ru(p-cymen)Cl2]2 (5.0 mol
%), additive (20.0 mol %), acid (2.0 equiv), solvent (2.0 mL), 16 h under N2 in a
sealed tube. b NMR yield using 1,3,5-trimethoxybenzene as an internal standard. c
1a (0.3 mmol) and 2a (0.2 mmol) were used.
chromatography.
[Cp*Co(CO)I2] (10.0 mol %).
3ra
R = CF3,
, 63%
Ts
, 80% (20:1)
S
O
d
Isolated yield after
[Cp*IrCl2]2 (4.0 mol %).
OH
OH
OH
OH
e
f
g
Ts
, 61%
Ts
3sa
Ts
[Cp*RhCl2]2 (4.0 mol %).
Ts
3ta
3ua
3va
, 62%
, 22%
, 23%
Scheme 2. Scope of Arylnitrons. Reaction conditions: nitrone 1 (0.3 mmol), 2a (0.2
mmol), [Ru(p-cymene)Cl2]2 (5.0 mol %), AgNTf2 (20.0 mol %), AgPF6 (20.0 mol %),
and PivOH (2.0 equiv) in DCE (2.0 mL ) at 120 oC for 16 h in a sealed tube under
N2. Isolated yield.
We initiated our studies with the coupling of aryl nitrone 1a
with 1-diazo-1-tosylpropan-2-one (2a) using [Ru(p-
cymene)Cl2]2 as a catalyst in trifluorotoluene at 100 °C. The
desired 2-naphthol 3aa was obtained in 45% yield in the
presence of AgSbF6 and MesCOOH additives (Table 1, entry 1).
The yield of 3aa was slightly improved when PivOH was used
(entries 2, and 3). The reaction turned out to be temperature
sensitive, and the isolated yield was improved to 60% when
the reaction was performed at 130°C (entries 4 and 5). AgNTf2
The coupling of aryl nitrone 1a with various -
diazocarbonyl--sulfonyl compounds was next examined
(Scheme 3). para-Substituted benzenesulfonyl -diazos
bearing halogen or CF3 group coupled to afford the desired
products 3ab-3ad in good yields (69-76%). Meanwhile, the
2 | J. Name., 2012, 00, 1-3
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sulfonyl substituents were also extended to phenyl (3ae) and source in the presence of diazo 2a. Significant levels of
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2-naphthyl (3af) with moderate to high yields. In the case of deuterium incorporation (43% and 38%D) OaIt: 1t0h.e103o9r/tCh9oC-pCo02s9it4io9Dn
3,4-disubstituted (3ag), alkyl-substituted (3ah, and 3ai), and of the recovered 1a and the product 3aa were observed,
heterocycle-containing (3aj) sulfonyl diazos, the coupling also indicating reversibility of the C-H bond cleavage (Scheme 4a).
proceeded smoothly to give the target products in 65-74% To further probe the C-H activation event, KIE studies from
yields. To our delight, 1-diazo-1-(phenylsulfonyl)butan-2-one parallel reactions using 1a and 1a-d5 gave kH/kD = 0.9, and a
and 1-diazo-4-phenyl-1-tosylbutan-2-one were also applicable value of kH/kD = 2.0 was obtained under intermolecular
(3ak and 3al), indicating that steric effect at the alkyl position competetion conditions, indicating that the C-H bond cleavage
was tolerated.
is probably not turnover-limiting (Scheme 4b). Moreover,
intermolecular competition experiment between equimolar
amounts of 1b and 1g was performed, and the electron-rich
aryl nitrone reacted at a higher rate (Scheme 4c). Significantly,
an alkylation intermediate 6 was isolated in 49% yield from a
coupling reaction performed at 80 oC (Scheme 4d). Control
experiments confirmed that the Ru(II) catalyst was not
[Ru(p-cymene)Cl2]2 (5.0 mol %)
AgNTf2 (20.0 mol %)/AgPF6 (20.0 mol %)
O
R1
SO2R
+
N
O
OH
PivOH (2.0 equiv), DCE, 120 o
C
R1 N2
SO2R
1a
3
2
OH
OH
OH
OH
OH
S
F
S
S
S
S
O
O
O
O
O
O
O
O
O
O
necessary for the subsequent cyclization (Scheme 4e).
Ph
O
Ts
2a
Br
CF3
N2
H/D (38%D)
H/D (43%D)
3ab
3ac
3ad
3ae
3af
, 76%
, 72%
, 69%
, 65%
, 76%
[Ru(p-cymene)Cl2]2 (5.0 mol %)
AgNTf2 (20.0 mol %), AgPF6 (20.0 mol %)
tBu
tBu
N
O
+
N
O
(a)
R
CD3COOD (2.0 equiv), DCE, 100 o
C
OH
H/D (43%D)
Ts
OH
OH
OH
OH
OH
1a
[Dn]-
[Dn]-3aa
1a
2a
S
S
S
S
S
O
O
O
O
O
O
O
O
O
O
2a
Cy
Ph
tBu
standard conditions
S
N
O
Ph
with CH3COOH (2.0 equiv)
100 oC, 15 min
3ak
3al
R = Me,
R = Bn,
, 66%
, 45%
(b)
KIE = 0.9
3ag
3ah
3ai
3aj
2a
, 74%
, 72%
, 65%
, 73%
tBu
tBu
tBu
standard conditions
N
O
D5
with CD3COOD (2.0 equiv)
100 oC, 15 min
Scheme 3. The Scope of -Diazocarbonyl Compounds. Reaction conditions:
nitrone 1a (0.3 mmol), diazo compound 2 (0.2 mmol), [Ru(p-cymene)Cl2]2 (5.0
mol %), AgNTf2 (20.0 mol %), AgPF6 (20.0 mol %), and PivOH (2.0 equiv) in DCE
(2.0 mL) at 120 oC for 16 h in a sealed tube under N2. Isolated yield.
2a
standard conditions
tBu
N
O
N
O
KIE = 2.0
+
D5
100 oC, 15 min
[Ru(p-cymene)Cl2]2 (5.0 mol %)
AgNTf2 (20.0 mol %), AgPF6 (20.0 mol %)
N
O
PivOH (2.0 equiv), DCE, 120 oC, 16 h
(c)
R
OH
Additional experiments have been carried out to further
define the scope and limitations of this reaction. Traces of
product were observed using ethyl 2-diazo-3-oxobutanoate
(2l) as coupling partner under the standard conditions (eq 1),
indicating the significance of the sulfonyl group. What’s more,
the arene substrate was not limited to aryl nitrones, and the 2-
naphthol product 3ga was obtained in 56% yield with
azomethine imine 4 as an arene (eq 2). To demonstrate the
synthetic utility of this method, a derivatization reaction was
carried out for a 2-naphthol product 3qa, and a dearomative
halogenation product 5 was obtained with excellent yield
when treated with NBS (eq 3).15
R
Ts
3ba
3ga
R = OMe (
)/CF3
(
) = 11:1
1b
1g
) = 1:1
R = OMe ( ) /CF3
(
tBu
Ac
Ts
O
N
tBu
N
N2
6
(d)
(e)
O
standard conditions
80 oC, 49%
Ac
1a
Ts
standard conditions
3aa
6
without [Ru(p-cymene)Cl2
]
2
69%
Scheme 4. Mechanistic Studies.
O
Ts
-
tBuNHOH
O
N
O
OH
N
O
COOEt
Ts
3aa
RuLX2
N2 2l
6
N
O
1a
(1)
L = p-cymene
X = SbF6 or PF6
OH
COOEt
3al
standard conditions
< 5%
HX
N
HX
1a
O
Ts
N
2a
N2
N
N
O
(2)
(3)
O
Ru
L
Ru
X
F3C
OH
standard conditions
56%
L
X
F3C
O
Ts
Ts
Ac
4
3ga
A
C
Br
Br
NBS (1.2 equiv)
KOAc (1.5 equiv)
N2
Ts
Ac
H2O, rt
98%
2a
OH
O
N
Ts Br
Ts
3qa
O
X
N2
Ru
5
L
Ac
Ts
Experimental studies have been performed to probe the
reaction mechanism. H/D exchange reactions have been
carried out for aryl nitrone 1a with CD3COOD as a deuterium
B
Scheme 5. Proposed Catalytic Cycle.
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2012, 45, 788-802; (e) P. B. Arockiam, C. Bruneau and P. H.
On the basis of the mechanistic experiments and previous
reports,12 a plausible catalytic cycle is proposed in Scheme 5.
Cyclometalation of aryl nitrone 1a affords a cyclometalated Ru(II)
complex A. Subsequent diazo 2a coordination and denitrogenation
gives a ruthenium carbene species B. Facile migratory insertion into
the carbene then occurs to give a seven-membered ruthenacyclic
intermediate C, which is protonolyzed to give the isolable alkylation
View Article Online
Dixneuf, Chem. Rev., 2012, 112, 5879-5918; (f) T. Brückl, R.
D. Baxter, Y. Ishihara and P. S. Baran,DAOcIc: .10C.1h0e3m9/.CR9eCsC.,022904192D,
45, 826-839; (g) S. R. Neufeldt and M. S. Sanford, Acc. Chem.
Res., 2012, 45, 936-946; (h) L. Ackermann, Acc. Chem. Res.,
2014, 47, 281-295; (i) F. Wang, S. Yu and X. Li, Chem. Soc.
Rev., 2016, 45, 6462-6477; (j) Y. Yang, J. Lan and J. You,
Chem. Rev., 2017, 117, 8787-8863; (k) J. Hummel, J. Boerth
and Ellman, J. A. Chem. Rev., 2017, 117, 9163-9227; (l) D.-W.
Gao, Q. Gu, C. Zheng and S.-L. You, Acc. Chem. Res., 2017, 50,
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Kim, ACS Catal., 2017, 7, 2821-2847.
intermediate
6 and regenerate the Ru(II) catalyst. Finally,
intermediate 6 undergoes the intramolecular nucleophilic addition
and subsequent elimination of N-(tert-butyl)hydroxylamine to
furnish the product 3aa. Given the isolation of intermediate 6, it is
likely that the subsequent cyclization process is turnover-limiting,
and this proposal is in agreement with our measured small value of
KIE.
4
(a) A. Ford, H. Miel, A. Ring, C. N. Slattery, A. R. Maguire and
M. A. McKervey, Chem. Rev., 2015, 115, 9981-10080; (b) Y.
Xia, D. Qiu and J. Wang, Chem. Rev., 2017, 117, 13810-13889;
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In summary, we have demonstrated Ru(II)-catalyzed
intermolecular [3+3] annulation of nitrones with -diazo
sulfonyl ketones for the synthesis of 2-naphthols via a C-H
activation pathway. Stable and easily available diazo
compounds react as C3 synthons under redox-neutral
conditions. The reactions are generally efficient and proceeded
with high functional group compatibility. Further C-H
functionalization-annulation systems and other novel
transformations of -diazo sulfonyl ketones are underway in
our laboratory.
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We acknowledge the financial support for this work from the
National NSF of China (21525208) and the Shaanxi Normal
University.
Conflicts of interest
There are no conflicts of interest to declare.
11 S. Cui, Y. Zhang, D. Wang and Q. Wu, Chem. Sci., 2013, 4,
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Notes and references
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