Rhodium(iii)-catalysed carboxylate-directed C-H functionalizations of isoxazoles with alkynes

Article abstract of DOI:10.1039/c9cc03283e

An efficient oxidative [4+2] annulation of isoxazolyl-4-carboxylic acids with internal alkynes proceeded in the presence of the [Cp?RhCl₂]₂ catalyst. Oxidants controlled the formation of pyranoisoxazolones and isoquinolines. A decarb

Full text of DOI:10.1039/c9cc03283e

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
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DOI: 10.1039/C9CC03283E
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Rhodium(III)-Catalysed Carboxylate-Directed C-H
Functionalizations of Isoxazoles with Alkynes
Somaraju Yugandar a and Hiroyuki Nakamura*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
An efficient oxidative [4+2] annulation of isoxazolyl-4-carboxylic biological activities and applications in pharmaceuticals and
acids with internal alkynes proceeded in the presence of agrochemicals, and also it acts as a masked 1,3-dicarbony
[Cp*RhCl2]2 catalyst. Oxidants control the formation of intermediates in organic synthesis.12,13 Conventional synthetic
pyranoisoxazolones and isoquinolines. Decarboxylative approach approaches of functionalized isoxazoles are based on ring
for hydroarylation of alkynes with isoxazolyl-4-carboxylic acids was construction with pre-functionalized linear components12 and
also achieved in the presence of [Cp*Rh(CH3CN)3][SbF6]2 catalyst.
the direct C-H coupling on isoxazole ring is a still challenging
task.14 Recently, we succeeded in the generation of an 4-
isoxazolyl anion species to introduce various functional groups
including a carboxylate into C-4 of isoxazoles.15 In this paper, we
Transition metal-catalyzed direct C-H functionalization of
arenes has gained much attention in organic synthesis for
making C-C and C-heteroatom bonds, without pre-
functionalization of the starting materials because these
reactions are highly step and atom economy.1 A directing group
(DG) is able to facilitate C-H activation by enhancing the
effective coordination with catalysts, resulting in both high
reactivity and selectivity. Although various DGs have been
reported for the direct C-H activation,2 removal of these DGs
from the products is not easy task and always requires
additional chemical transformations.3 In this regard, the
carboxylate has been focused as a simple DG.
demonstrated
the
first
carboxylate-directed
C-H
functionalizations of isoxazoles (Scheme 1b). Rh(III) catalysts
enabled the direct C-H activation of isoxazolyl-4-carboxylic acids
to react with alkynes for systematic synthesis of
pyranoisoxazolones ([4+2] annulation), isoquinolines (N-O
cleavaged decarboxylative [4+2] annulation), and 5-
alkenylisoxazoles (hydroarylation of alkynes).
Miura et al., reported the first oxidative [4+2] annulation of
benzoic acids with alkynes using Rh(III) catalysts for the
synthesis of isocoumarins,4 where the DG is involved as a
component in a newly formed ring system in the reaction. After
their report, other groups have also reported the synthesis of
isocoumarin derivatives using various transition metals such as
Pd(II) (acrylic acids),5 Ru(II),6 Ir(III)7 and recently Co(III)8
complexes for this annulation (Scheme 1a; right). In contrast, a
decarboxylative approach for hydroarylation of akynes with
benzoic acids also proceeded under the Ru(II)-catalyzed
conditions (Scheme 1a; left).9,10 A carboxylate acts as a DG to
induce hydroarylation to alkynes and then decarboxylation
takes place to afford ortho-alkenyl benzenes, although the
strategy of a carboxylate as a removable ortho-directing group
has been reported in tandem or separate manners.11
Scheme 1 (a) Carboxylate-direct functionalizations of arenes (known) vs. (b)
carboxylate-direct functionalizations of isoxazoles with alkynes (this work).
We first examined the reaction of isoxazolyl-4-carboxylic
acids 1a (1.0 equiv.) and diphenylacetylene 2a (1.2 equiv.),
[Cp*RhCl2]2 (5 mol %) as a catalyst and Ag2CO3 (0.5 equiv.) as an
oxidant in DMF at 100 °C for 16 h. As shown in Table S1 in the
supporting information, the expected pyranoisoxazolone 3aa
was obtained in 27% yield (entry 1). The molecular structure of
3aa was further confirmed by single-crystal X-ray analysis (CCDC
Isoxazole, as a heteroaromatic arene, is an important five-
membered heteroaromatic ring system exhibiting various
a. Laboratory for Chemistry and Life Science, Institute of Innovative Research,
Tokyo Institute of Technology, Yokohama, 226-8503, Japan. E-mail:
hiro@res.titech.ac.jp
Electronic Supplementary Information (ESI) available: See DOI: 10.1039/x0xx00000x
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1870838; also see Table 1). The increase of the amount of respectively, on aryl ring underwent the oxidative annulation
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DOI: 10.1039/C9CC03283E
oxidant Ag2CO3 afforeded the desired pyranoisoxazolone 3aa in with 1a to provide 3ab and 3ad in high yields. Similarly, electron
72% yield (entry 2). Although other oxidants, such as K2S2O8 and rich alkynes such as 2f-h also provided the corresponding
AgNTf2 were not effective, AgOAc gave 3aa in 53% yield (entries pyranoisoxazolones 3af-ah in good yields. When we used
3-5). Further, solvents and other additives have been examined unsymmetrical alkynes 2i-l, surprisingly single regioisomer of
to improve the yields, however 3aa was observed in moderate pyranoisoxazolones 3ai-al was obtained exclusively. The
to poor yields (eitries 6-11). The best result was observed when regioisomer of pyranoisoxazolone 3ai was confirmed by single-
the reaction carried out under open air and 3aa was obtained in crystal X-ray analysis (CCDC 1870840). However, unsymmetrical
76% yield (entry 12). In the absence of Ag2CO3, 3aa was aliphatic-disubstituted alkyne 2m, gave a mixture of inseparable
observed in 8% yield, revealing the importance of oxidant in this regioisomers of pyranoisoxazolone (3am and 3’am) (~1:1) in
transformation (entry 13). Interestingly, when Cu(OAc)2•H2O 62% yield. We also examined the scope of isoxazolyl-4-
was used as an oxidant instead of Ag2CO3, 3aa was not observed carboxylic acids 1. We have introduced different para-
and the product found to be isoquinoline 4aa (entry 14). The substituents such as electron donating (-OMe) and electron
structure of 4aa was further confirmed by single-crystal X-ray withdrawing (-NO2) groups on aryl ring at 3-position of
analysis (CCDC 1870841; also see Table 3). However, 4aa was isoxazolyl-4-carboxylic acids 1b and 1c, respectively.
not observed in presence of CuO or in the absence of Rh(III) Interestingly, these substrates underwent the oxidative
catalyst (entries 17 and 18, respectively).
annulation with both symmetrical and unsymmetrical alkynes
(2a, 2g and 2i) to provide the corresponding
pyranoisoxazolones, 3ba, 3bg, 3ca and 3ci, in high yields. The
synthetic utility of this reaction was further extended by
introducing alkyl (n-propyl) group at 3-position of 4-isoxazolyl
carboxylic acid 1d and the corresponding pyranoisoxazolones
3da and 3dh could also be obtained in 78% and 57% yields,
respectively.
As mentioned earlier, we have observed the isoquinoline
4aa exclusively instead of pyranoisoxazolone 3aa, when we
used Cu(OAc)2•H2O as an oxidant and [Cp*RhCl2]2 as a catalyst.
Thus, we next utilized for the synthesis of various substituted
isoquinolines 4 through the oxidative annulation of isoxazolyl-
4-carboxylic acids 1 and alkynes 2 as shown in Table 2. Various
3,4-substituted isoquinolines 4ab, 4af-ag, 4bd and 4cb were
obtained in moderate to good yields. Similarly, the reaction
proceeded highly regioselectively with unsymmetrical alkynes
2i-l to provide a single regioisomer of isoquinolines 4ai, 4aj, 4al
and 4bk respectively, in good yields. However, the reaction of
simple isoxazolyl-4-carboxylic acid with 2a resulted in
decomposition of the isoxazole ring.
Table 1 Scope of Alkynesa
R3
N
O
N
O
[Cp*RhCl2]2 (5 mol %)
R3
R2
R1
R1
+
Ag2CO3 (1.0 equiv.)
DMF, 100 °C, 2 h, in air
R2
O
O
OH
O
1
2
3
N
O
N
O
R2
R3
Ph
Ph
Cl
Cl
O
O
O
O
3ab, R2 = R3 = 4-C6H4OMe (81%)
3ad, R2 = R3 = 4-C6H4Br (62%)
3ae, R2 = R3 = 4-C6H4CF3 (8%)
3aa (76%, 72%b)
X-ray of 3aa
N
O
N
O
N
O
n-Pr
Et
Me
Cl
Cl
Cl
n-Pr
Et
Me
3ah, 66%
O
O
O
O
O
O
3af, 70%
3ag, 73%
N
O
N
O
Et
Me
Cl
Cl
Ph
Ph
O
O
O
O
N
3ai, 68%
3aj, 65%
X-ray of 3ai
O
N
O
N
O
n-Pr
Ph
n-Bu (Et)
Et (n-Bu)
n-Bu
Cl
Cl
Cl
In the substrate scope of pyranoisoxazolens 3, we observed
the formation of 5ae i.e., hydroarylation of alkynes as a major
product over 3ae (Table 1). Intrigued by these results, we were
further interested in the investigation of hydroarylation of
alkynes using Rh(III)catalyst. We have tried various reaction
conditions for the formation of 5 using [Cp*RhCl2]2, and other
catalysts, additives, bases and solvents (Tables S2-S4). Finally,
we found that 2.5 mol% of Rh(III) catalyst with the combination
of AdCO2H (1.0 equiv.) and 0.3 equiv. of K2CO3 in mesitylene at
120 °C gave the best yield of 5aa in 78 % (Table 3). The E-
regioselectivity of 5aa and 5ae was further confirmed by single-
crystal X-ray analysis (CCDC 1899700 and 1899699)
respectively. With this optimized reaction conditions, we
observed various hydroarylations of alkynes with isoxazolyl-4-
carboxylic acids 1 in good yields. Unsymmetrical alkynes also
provided excellent regioselectivity 5ai-ak and 5cl. Surprisingly,
with aliphatic alkynes we have not observed the formation of
5af and 5ag, instead only annulated products were observed
(3af and 3ag). However, terminal alkynes did not provide 5
under these reaction conditions. The plausible mechanism for
Ph
O
O
O
O
O
O
3ak, 56%
3al, 60%
3am/3'am, 62%(~50:50)
N
O
N
O
N
O
Et
Ph
Ph
MeO
O2N
MeO
Et
Ph
3ca, 64%
Ph
O
O
O
O
N
O
O
3bg, 84%
3ba, 80%
O
N
O
N
O
Me
Me
Ph
O2N
Me
3dh, 57%
Ph
Ph
O
O
O
O
O
O
3ci, 60%
3da, 78%
a Reaction conditions: 1 (0.2 mmol), 2 (0.24 mmol), [Cp*RhCl2]2 (5 mol %), Ag2CO3
(1 equiv.), DMF (1 mL) under open air atmosphere. b Isolated yield in 1 mmol scale.
We next evaluated the generality and scope of these
protocols for the introduction of various substituents in 3 and 4
(Tables 1 and 2). We first examined the reaction of 1a with
different symmetrical and unsymmetrical alkynes 2 for the
synthesis of pyranoisoxazolones 3 (Table 1). Symmetrical
alkynes 2b and 2d, having different para-substituents such as
electron donating (-OMe) and electron withdrawing (-Br),
2 | J. Name., 2012, 00, 1-3
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formation of pyranoisoxazolones
COMMUNICATION
carboxylic acid group of 1 is required or not for the
3
and decarboxylative
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transformation. The reaction proceeded very slowly (48 h) to
DOI: 10.1039/C9CC03283E
hydroarylation products 5 is shown in Scheme S1 in the
afford 4aa in 18% yield along with 44% of starting material
recovered, suggesting that the carboxylic acid group is
necessary to accelerate the reaction rate to provide the product
4aa in good yields. Further, the reaction of 1a and 2a in the
presence of MeOH (10 equiv.) under the isoquinoline formation
condition affored the ethyl ester 6 in 21% yield in addition to
4aa (Scheme 2; eq. 1). Finally, the rhodacyle intermediate 7 was
able to be isolated by treating of stoichiometric amount of
[Cp*RhCl2]2 with 1a as a possible reactive intermediate and its
structure was confirmed by single-crystal X-ray analysis (CCDC
1870842; eq. 2). The reaction of the intermediate 6 with 2a in
the presence of Cu(OAc)2•H2O provided the isoquinoline 4aa in
supporting information.
Table 2 Synthesis of Isoquinolinesa
Me
R3
O
N
[Cp*RhCl2]2 (5 mol %)
N
+
R1
R1
R2
Cu(OAc)2•H2O (1.0 equiv.)
DMF, 100 °C, 2 h, Ar
R2
O
OH
R3
4
1a-c
2
R1 = OMe, Cl, NO2
Me
Me
N
N
Cl
Ph
Cl
R2
56% yield (eq. 3).
Ph
R3
O
4aa, (60%, 55%b)
4ab, R2 = R3 = 4-C6H4OMe (35%)
4af, R2 = R3 = n-Pr (42%)
4ag, R2 = R3 = Et (38%)
X-ray of 4aa
Me
OMe
Ph
N
O
[Cp*RhCl2]2 (5 mol %)
N
N
+
+
eq. 1
Cu(OAc)2•H2O
(1.0 equiv.)
MeOH (10 equiv.)
DMF, 100 °C, 2 h
Cl
Ph
Me
N
Me
Me
N
Cl
Ph
Cl
CO2H
Ph
Ph
Ph
N
1a
2a
4aa, 35%
6, 21%
Cl
Ph
MeO
R2
O2N
R2
Me
NH
R3
R3
R3
N
O
[Cp*RhCl2]2 (0.5 equiv.)
eq. 2
4ai, R3 = Me (52%)
4bd, R2 = R3 = 4-C6H4Br (63%)
4bk, R2 = Ph, R3 = n-Pr (55%)
4cb, R2 = R3 = 4-C6H4OMe (40%)
Rh
Cl
NaOAc ( 1 equiv.)
DCE, 100 °C, 4 h
Cl
Cl
CO2H
4aj, R3 = Et (46%)
*Cp
4al, R3 = n-Bu (49%)
1a
7, 20%
X-ray of 7
Me
Me
a
Reaction conditions: 1 (0.2 mmol), 2 (0.24 mmol), [Cp*RhCl2]2 (5 mol %),
Ph
Cu(OAc)2•H2O
( 1 equiv.)
Cu(OAc)2•H2O (1 equiv.), DMF (1 mL) under Ar atmosphere. b Isolated yield in 1
mmol scale.
N
NH
+
eq. 3
Rh
Cl
Ph
Cl
DMF, 100 °C, 2 h
Cl
*Cp
Ph
Ph
7
2a
4aa, 56%
Table 3 Hydroarylation of alkynes from isoxaolyl-4-carboxylic acids 1 and alkynes 2a
[Cp*Rh(CH3CN)3][SbF6]2
Scheme 2 Mechanistic Studies for the formation of 4
R3
N
O
N
O
(2.5 mol%)
R2
R1
R
+
AdCO2H (1.0 equiv.)
K2CO3 (0.3 equiv.)
mesitylene, 120 °C, 8-12 h
R2
R3
O
O
N
O
N
H
O
N
O
OH
OAc
H
H
1
2
1a
Cl
-CO2
Cl
N
Cl
N
O
AcOH
O
O
H
R2
8
8’
Ph
Me
reductive
R3
Cl
Cl
Ph
5aa, (78%, 72%b)
elimination
i. N-O bond
cleavage
ii. [Cp*RhCl2]2
C-H activation
N
[Cp*Rh(III)]
5ac, 52% (R2 = R3 = 4-C6H4Me)
5ad, 79 % (R2 = R3 = 4-C6H4Br)
X-ray of 5aa
Cl
Ph
2CuX2
2CuX
Ph
4aa
N
O
N
O
X = Cl, OAc
Me
Ph
Cl
*Cp
R2
R2
RhIII
N
NH
O
R3
Cl
Cl
R3
RhIII
Ph
Cp*
C
OH
Cl
5af, 0% (R2 = R3 = n-Pr)
5ag, 0% (R2 = R3 = Et)
Cl
X-ray of 5ae
5ae, 67%
R2 = R3 = 4-C6H4CF3
9
12
H2O
O
N
O
N
O
N
O
R2
Ph
Ph
R2
Ph(R3)
alkyne insertion
Me
Me
R3
O2N
Cl
R3
R3(Ph)
MeO
5ba, 73% (R2 = R3 = Ph)
5be, 82% (R2 = R3 = 4-C6H4CF3)
O
5ai, 68% (R3 = Me) (E/Z = 62:1)
5aj, 71% (R3 = Et) (E/Z = 72:1)
5ak, 76% (R3 = n-Pr) (E/Z = 99:1)
5ca, 70% (R2 = R3 = Ph)
5cd, 65% (R2 = R3 = 4-C6H4Br)
NH
RhIII
N
AcOH
-CO2
NH
RhIII
Cp*
Cl
Cl
Cl
RhIII
Cl
10
Cl
Cp*
HCl
Cp*
N
O
11
N
O
7
(X-ray analysis)
R2
Ph(n-Bu)
n-Bu(Ph)
n-Pr
R3
O2N
5da, 66% (R2 = R3 = Ph)
5dd, 60% (R2 = R3 = 4-C6H4Br)
5de, 52% (R2 = R3 = 4-C6H4CF3)
Scheme 3 Plausible mechanism for the formation of 4
5cl, 72% (E/Z = 99:1)
Based on these observations, the plausible mechanism for
the formation of isoquinolines 4 is shown in Scheme 3.
Isoxazolyl-4-carboxylic acid 1a undergoes decarboxylation in
the presence of an acetate anion generated from Cu(OAc)2•H2O
to generate the isoxazole anion 8. There is an equilibrium
between 8 and 8‘ which undergoes N-O bond cleavage17
followed by C-H activation by Rh(III)-catalyst to provide the
intermediate 9. The hydroxide anion attacks ketene in
intermediate 9 to afford the corresponding product 10 which
a
Reaction conditions: 1 (0.1 mmol), 2 (0.15 mmol), [Cp*Rh(CH3CN)3][SbF6]2 (2.5
mol %), AdCO2H (1 equiv.), K2CO3 (0.3 equiv.), mesitylene (0.5 mL). b Isolated yield
in 1 mmol scale.
To clarify the mechanism for the formation of isoquinolines
4 in presence of Rh(III) catalyst, further experiments were
conducted. Since the decarboxylation was observed in the
reaction,
we
performed
a
reaction
of
3-(4-
chloropheny)isoxazole with 2a to confirm whether the
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J. Name., 2013, 00, 1-3 | 3
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Liu, Y. Shen, Z. Zhou and X, Lu, Angew. Chem. Int. Ed., 2013, 52, 6033;
undergoes decarboxylation and protonation to give isolable
rhodacycle 7. The generation of 10 was supported by the
formation of 6 in the presence of MeOH (Scheme 2; eq. 1).
Finally, HCl elimination of 7 forms five membered rhodacyle 11,
and then alkyne insertion followed by reductive elimination of
12 provides 4aa (Scheme 2). Under the [Cp*RhCl2]2 catalysed
conditions, the decarboxylation of isoxazolyl-4-carboxylic acid
1a (in the absence of alkyne 2a) was observed in the presence
of Ag2CO3, whereas the decomposition of 1a was observed in
the presence of Cu(OAc)2•H2O. These observations suggest that
Cu(OAc)2•H2O not only acts as an oxidant but also assists N-O
cleavage to generate intermediate 9 in the catalytic cycle.
In conclusion, we have developed an efficient Rh(III)-
catalyzed oxidative couplings of isoxazolyl-4-carboxylic acids 1
with internal alkynes 2 for the synthesis of pyranoisoxazolones
3 and isoquinolines 4 in good to excellent yields. Choice of
oxidizing agents is essential to control both transformations of
pyranoisoxazolones and isoquinolines. We have also developed
decarboxylative approach for hydroarylation of alkynes 5 using
Rh(III) catalyst, where carboxylic group acts as deciduous
group/traceless directing group after C-H functionalization. This
is the first to report for the synthesis of pyranoisoxazolones 3
and hydroarylation of alkynes 5 from isoxazolyl-4-carboxylic
acids and alkynes. The broad substrate scope, high efficiency
and good regioselectivity make these reactions more useful in
the drug discovery process of isoxazole containing fused
heterocycles and isoquinolines.
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This work was partially supported by Scientific Research (B)
(No.18H02685) from the Ministry of Education, Culture, Sports,
Science and Technology, Japan. We thank Mr. Yoshihisa Sei
(Technical Department, Tokyo Institute of Technology) for technical
assistance with X-ray crystallographic analysis.
Conflicts of interest
The authors declare no conflict of interest.
Notes and references
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4 | J. Name., 2012, 00, 1-3
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