Naphthol synthesis: Annulation of nitrones with alkynes: Via rhodium(III)-catalyzed C-H activation

Article abstract of DOI:10.1039/c7cc05000c

An efficient and redox-neutral naphthol synthesis has been realized via rhodium(iii) catalyzed C-H activation of α-carbonyl nitrones and annulation with alkynes, where the nitrone group functioned as a traceless directing group.

Full text of DOI:10.1039/c7cc05000c

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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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Naphthol Synthesis: Annulation of Nitrones with Alkynes via
Rhodium(III)-Catalyzed C-H Activation
Qiang Wang,ab Youwei Xu,ab Xifa Yang,ab Yunyun Liab and Xingwei Li*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
An efficient and redox-neutral naphthol synthesis has been documented and has provided mild, atom- and step-
realized via rhodium(III) catalyzed C-H activation of α-carbonyl economical approaches to access complex molecules from
nitrones and annulation with alkynes, where the nitrone group simple arenes bearing a large array of directing groups.4 This
functioned as a traceless directing group.
has been realized owing to sufficient interactions between a
reactive Rh-C species and a reactive site for subsequent
cyclization. While a great deal of effort has been devoted to
the formation of various heterocycles via rhodium(III)-
catalyzed C-H activation,5 in which heteroatom directing
groups, in part or as a whole, have been incorporated into the
final product, construction of carbocyclic compounds still lags
Naphthols are an important structural motif in numerous
pharmaceuticals and natural products (Fig. 1).1 Consequently,
synthesis of naphthols has been an ongoing topic for decades.
Traditional methods of constructing naphthols generally suffer
from harsh conditions, low yield, and relatively low efficiency.2
Thus, the development of efficient synthetic methodologies
that allow assembly of naphthols from readily available
starting materials has attracted continuing interest. Transition
metal-catalyzed synthesis of naphthols via hydroxylation of
aryl halides has been well studied, providing mild and
synthetically viable access to naphthols.3 However, it is
desirable to resort to the abundant, simpler arenes as
substrates.
behind.6-9
(a) Zhu's work7
OH
O
CHO
Ph
[Rh], oxidant
+
Ph
Ph
NMe2
(b) Our previous work8,9
O
Ph
OH
O
Ph
Ph
[Rh]
+
+
120 o
C
N
tBu
Ph
Ph
O
OH
OH
HO
MeO
HO
O
O
Me
NH
OH Me
HO
H3
N2
[Rh]
Me
PPh3
O
H3
HO
C
C
OH
H
N
OMe
OMe
O
O
Me
Me
120 oC, - OPPh3
EWG
Ac
Me
H3
C
MeO
MeO
OMe
O
MeO
EWG
OH
(c) This Work
O
O
OH
OH OMe
Polycarcin V
Propranolol
Dioncophylline A
Hypocrellin A
[Rh]
R
+
R
Fig. 1 Representative Bioactive Compounds with Naphthol
redox-neutral
N
R
O
tBu
Skeletons
R
Scheme 1. Naphthol synthesis via C-H activation.
In the past decade, the versatility of Rhodium(III) catalyzed
C-H activation and annulation strategy has been well
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We resort to rhodium-catalyzed approaches to naphthol
synthesis (Scheme 1). Zhu recently described an oxidative
synthesis of 2-formyl-1-naphthols via C-H activation of
enaminones and coupling with alkynes (Scheme 1a).7 Our
group disclosed that certain arylnitrones could undergo C-H
activation and coupling with cyclopropenones, providing 2,3-
disubstituted 1-naphthols (Scheme 1b),8 where the arylnitrone
functions
as
a
benzylidene
synthon.
However,
cyclopropenones are not readily accessible, and the scope of
the nitrone is also rather limited. We also realized naphthol
synthesis via a C-H alkylation-Wittig reaction tandem between
phenacyl triphenylphosphonium salts and diazo compounds,9
where the diazo substrates are limited to acceptor, acceptor
ones. On the other hand, we reported in 2013 the first
catalytic C-H activation of benzaldehyde-derived nitrone and
annulation with alkyne for indenone synthesis.6b We
envisioned that coupling of alkyne with a phenylglyoxal-
derived nitrone (1a, Table 1) that contains a carbonyl directing
group and an EWG-activated imine may allow for 1-naphthol
synthesis with elimination of a nitroso co-product. This calls
for sufficient interactions between the nucleophilic Rh-C bond
and the electrophilic imine group (Scheme 1c). However,
challenges remain because ketone carbonyl is generally weak
in directing effect and its electrophilicity might induce
undesired [3+2] annulation.10
Table 1. Screening of Reaction Conditions for the Synthesis of
Naphthola
additive, and 4 Å MS (100 mg) in a solvent (5 mL) at 80 oC. C1 =
[Cp*RhCl2]2; C2 = [Cp*Rh(MeCN)3](SbF6)2. b Isolated yield. c CA = citric acid.
d At 90 oC. e Without 4 Å MS.
We commenced our studies by examining the reaction
parameters of the coupling of α-carbonyl nitrone 1a with
diphenylacetylene 2a in the presence of a [Cp*RhCl2]2 catalyst.
To our delight, the desire product 3aa was isolated in 20%
yield in the presence of HOAc (entry 1). Replacing the HOAc
with HOPiv resulted in a higher yield (entry 2). Screening of
solvents revealed DCE as the best choice (entries 3-4). Moving
to [Cp*Rh(MeCN)3](SbF6)2 as catalyst, we were delighted to
find that the yield was improved to 57% (entry 6). Further
investigation of the additives revealed that the reactive
efficiency was improved by introducing Ni(OTf)2 (entries 7-9).
It is likely that Ni(OTf)2 acted as a mild Lewis acid that activated
the nitrone toward nucleophilic attack by the Rh-vinyl bond. A
higher yield of 3aa was obtained when the reaction was
conducted at 90 oC (entry 9). Moreover, the yield was
improved to 82% when citric acid (CA) was further introduced
as an acid additive to further facilitate the C-H activation
process (entry 10). Finally, control experiments confirmed that
the Rh(III) catalyst was necessary for this reaction (entry 11).
Entry
Catalyst
(mol%)
Additive (mol%)
Acid (equiv)
Solvent
Yield (%) b
1
C1 (4)
C1 (4)
C1 (4)
C1 (4)
C1 (4)
C2 (5)
C2 (5)
C2 (5)
C2 (5)
C2 (8)
AgSbF6 (16)
AgSbF6 (16)
AgSbF6 (16)
AgSbF6 (16)
AgNTf2 (16)
-
HOAc (1.2)
HOPiv (1.2)
HOPiv (1.2)
HOPiv (1.2)
HOPiv (1.2)
HOPiv (1.2)
HOPiv (1.2)
HOPiv (1.2)
HOPiv (1.2)
DCE
DCE
EtOAc
TFE
20
39
35
13
27
57
63
72
76
82
2
3
4
5
DCE
DCE
DCE
DCE
DCE
DCE
6
7
Ni(OTf)2 (30)
Ni(OTf)2 (30)
Ni(OTf)2 (30)
Ni(OTf)2 (30)
8
9d
10 d
HOPiv (1.2)
and CAc (0.3)
Scheme 2. Scope of Nitrones in Naphthol Synthesis. a Reaction
11 d
-
Ni(OTf)2 (30)
Ni(OTf)2 (30)
HOPiv (1.2)
and CA (0.3)
DCE
DCE
-
conditions: nitrone
1 (0.2 mmol), alkyne 2a (0.21 mmol),
[Cp*Rh(MeCN)3](SbF6)2 (8 mol %), Ni(OTf)2 (30 mol %), citric
acid (30 mol %), HOPiv (0.24 mmol) and 4 Å MS (100 mg) in
DCE (5 mL) at 90 °C for 12 h under N2. b Isolated yield.
12 d,e
C2 (8)
HOPiv (1.2)
and CA (0.3)
65
aReaction conditions: 1a (0.2 mmol), 2a (0.21 mmol), Rh(III) catalyst,
2 | J. Name., 2012, 00, 1-3
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With the establishment of the optimal conditions, the 3aa and alkyne 2a (Scheme 4b). Naphthol 3aa was also
reaction scope was then evaluated. A variety of nitrones converted to sulfamate , which then underwent a nickel-
underwent smooth coupling with 2a (Scheme 2). Electron- catalyzed cross-coupling reaction with 4-biphenylboric acid to
withdrawing and -donating substituents at the aryl ring were afford
in 91% yield (Scheme 4c).12
tolerated and the naphthols 3ba 3ja were isolated in moderate
5
6
-
to good yields. The less sterically congested C-H bond was
preferably functionalized for the meta-substituted nitrones
with good to high yields (3ka
nitrones also showed good reactivity despite the increased
steric hindrance (3qa 3ta). Halogen groups are compatible
-3pa). Notably, ortho-substituted
-
with this reaction, which should provide handles for further
functionalization.
Having explored the scope of the nitrone, we next examined
the scope of alkynes (Scheme 3). Symmetrical diarylalkynes
with electron-donating and -withdrawing substituents in the
aryl ring all reacted smoothly under the optimal conditions.
Furthermore, a sterically hindered alkyne was also found to be
suitable for this transformation (3ag). A heteroaryl alkyne also
coupled but with low efficiency under the standard conditions.
High regioselectivity (>20:1) and reactivity were observed for
prop-1-ynylbenzene (3ai), while a longer chain alkyne such as
but-1-ynylbenzene exhibited high activity but with lower
regioselectivity (3aj1 and 3aj2). In contrast, no reaction was
observed when a terminal alkyne phenylacetylene or an ester
functionalized alkyne (ethyl 3-phenylpropoiate) was used as a
coupling partner under the standard conditions, indicative of
the limitation in electronic effect and in the type of alkyne.
Scheme 4. Larger-Scale Synthesis and Synthetic Applications
We next conducted preliminary mechanistic studies to gain
insight into the reaction mechanism (Scheme 5). First, a H/D
exchange experiment between 1a and CD3COOD was conducted. 1H
NMR analysis revealed partial deuteration (16% D) at both ortho
positions (Scheme 5a). H/D exchange in the presence of
diphenylacetylene also led to H/D exchange in the recovered
starting material and the product was also partially deuterated
(Scheme 5b), indicating reversible C-H activation. Second, kinetic
isotope effect (KIE) experiments have been conducted. KIE obtained
from both parallel reactions (kH/kD = 3.0) and intermolecular
competition (kH/kD = 3.0) using 1a and 1a-d5 suggested that
cleavage of the C-H bond is likely involved in the turnover-limiting
step (Scheme 5c,d). Finally, 1, 3-cyclohexadiene was added as a
diene to a reaction mixture of 1a and 2a to trap the nitroso co-
product, and HRMS analysis suggested formation of a [4+2] adduct
7, which confirmed the formation of tBuNO co-product (Scheme
5e).8
a
Scheme 3. Scope of Nitrones in Naphthol Synthesis. Reaction
conditions: nitrone
1 (0.2 mmol), alkyne 2a (0.21 mmol)
[Cp*Rh(MeCN)3](SbF6)2 (8 mol%), Ni(OTf)2 (30 mol%), citric acid (30
mol%), HOPiv (0.24 mmol) and 4 Å MS (100 mg) in DCE (5 mL) at
90 °C for 12 h under N2. b Isolated yield.
To demonstrate the synthetic utility of this method, a larger-
scale reaction of 1a and 2a has been performed, which
afforded the naphthol 3aa in moderate yield (Scheme 4a).
Furthermore, the usefulness of an annulated product 3aa was
demonstrated in the following synthetic transformations. On
the basis of a literature report,11 an oxidative 4+2 coupling was
achieved by rhodium-catalyzed oxidative annulation between
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H/D
H/D
O
tBu
[Cp*Rh(MeCN)3](SbF6)2 (8 mol%),
CA (30 mol%), Ni(OTf)2 (30 mol%),
O
tBu
which provided a facile method to access 3,4-disubstituted
naphthols. This catalytic system features the application of a
weak, traceless directing group linked to an active imine site.
This [4+2] annulation approach exhibited good functional
group tolerability and regioselectivity, obviating the need of
any oxidant. Further investigations on the synthetic application
of this transformation are in progress.
N
N
O
(a)
O
CD3COOD (1.2 equiv), 4 Å MS,
DCE, 90 oC, 12 h
H/D 16% D
1a-
dn
1a
8% D
H/D OH
O
tBu
11% D
H/D
O
tBu
Ph
[Cp*Rh(MeCN)3](SbF6)2 (8 mol%),
CA (30 mol%), Ni(OTf)2 (30 mol%),
N
N
(b)
O
O
+
+
CD3COOD (5 equiv), 4 Å MS,
DCE, 90 oC, 2 h
Ph
H/D
9% D
Ph
Ph
3aa-dn
1a
2a
1a-dn
[Cp*Rh(MeCN)3](SbF6)2 (8 mol%),
CA (30 mol%), Ni(OTf)2 (30 mol%),
tBu
O
tBu
O
3aa
+ 3aa-
d4
N
N
(c)
(d)
(e)
PivOH (1.2 equiv), 4 Å MS,
2a, DCE, 90 oC, 12 h
C6H5
O
+
C6D5
O
Acknowledgements
3:1
OH
1a
1a-
d5
Financial support from National Natural Science Foundation of
China (21525208 and 21472186) is acknowledged.
There are no conflicts of interest to declare.
[Cp*Rh(MeCN)3](SbF6)2 (8 mol%),
CA (30 mol%), Ni(OTf)2 (30 mol%),
O
tBu
N
+
Ph
Ph
O
PivOH (1.2 equiv), 4 Å MS,
DCE, 90 oC, 12 h
Ph
Ph
1a
O
2a
3aa
KIE = 3.0
Notes and references
OH
tBu
[Cp*Rh(MeCN)3](SbF6)2 (8 mol%),
CA (30 mol%), Ni(OTf)2 (30 mol%),
N
+
‡Footnotes relating to the main text should appear here. These
might include comments relevant to but not central to the
matter under discussion, limited experimental and spectral data,
and crystallographic data.
Ph
Ph
Ph
O
PivOH (1.2 equiv), 4 Å MS,
DCE, 90 oC, 12 h
D5
Ph
D4
Ph
3aa-d4
2a
1a-
d5
tBu
standard
conditions
O
N
O
N
+
Ph
O
1
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C
tBu
7 (HRMS)
1a
2a
Scheme 5. Mechanistic Studies
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OH
O
N(O)tBu
Ph
Cp*RhX2
1a
Ph
3aa
HX
HX
2
O
N
ORhXCp*
tBu
O
Rh
Ph
X
*Cp
A
Ph
D
tBuNO
2a
O
ORhXCp*
O
N
tBu
N
tBu
O
Ph
RhXCp*
C
Ph
Ph
Ph
B
3
Scheme 6. Proposed Catalytic Cycle
Based on the above experimental results and related
rhodium(III)-catalyzed annulation of arenes and alkyens,13
a
plausible mechanism is proposed for this naphthol synthesis
(Scheme 6).14 Oxygen-directed C-H activation of nitrone 1a
gives a rhodacylic intermediate
migratory insertion produced an alkenyl intermediate
migratory insertion of the Rh-C(alkenyl) bond into the imine
moiety of the N-tert-butylnitrone gives rhodium(III)
aminoxide which is proposed to undergo -carbon
elimination to release tBuNO together with formation of a
rhodium(III) phenoxide D 8
Subsequent protonolysis of
furnishes the naphthol 3aa and closes the catalytic cycle.
In summary, we have realized Rh(III)-catalyzed C-H
activation of -carbonyl nitrones and annulation with alkynes,
A
. Alkyne coordination and
B
. Then
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14 At this stage, we cannot rule out the possibility of carbon-
coordination initiated cyclometallation of the nitrone,
followed by
lead to formation of a rhodium
ESI for a detail description of this alternative pathway.
α
-elimination of the tBuNO co-product. This will
α
-oxo carbene species. See
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