Rhodium(III)-catalyzed, C-H activated annulation to form isocoumarins and α-pyrones using the O-N bond as an internal oxidant

Article abstract of DOI:10.1002/adsc.201400200

A mild, efficient and regioselective C-H activation-based intermolecular redox-neutral annulation of O-benzoylhydroxylamines and internal alkynes has been achieved. The protocol employs an O-N bond as the internal oxidant and leads to isocoumarins and α-pyrones.

Full text of DOI:10.1002/adsc.201400200

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DOI: 10.1002/adsc.201400200
À
Rhodium(III)-Catalyzed, C H Activated Annulation to Form
N
À
Oxidant
Xing Guang Li,a Kai Liu,a Gang Zou,a and Pei Nian Liua,*
a
Shanghai Key Laboratory of Functional Materials Chemistry, Key Lab for Advanced Materials and Institute of Fine
Chemicals, East China University of Science and Technology, Meilong Road 130, Shanghai 200237, Peopleꢀs Republic of
China
Fax : (+86)-21-6425-0552; e-mail: liupn@ecust.edu.cn
Received: February 22, 2014; Revised: March 22, 2014; Published online: && &&, 0000
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201400200.
Abstract: A mild, efficient and regioselective C<
C->H activation-based intermolecular redox-neu-
tral annulation of O-benzoylhydroxylamines and in-
ternal alkynes has been achieved. The protocol em-
ploys an O N bond as the internal oxidant and
leads to isocoumarins and a-pyrones.
bial, antidiabetic and immunomodulatory activities,[10]
isocoumarin and a-pyrone motifs have been widely
used as the scaffolds to construct pharmaceutical
drugs.[11] However, their synthesis frequently requires
À
À
À
specific pre-activated C X or C M reagents as the
substrates.[12] Recently, Rh,[13] Ru[14] and Ir[15] cata-
lyzed oxidative annulations of arylcarboxylic acids
and alkynes have been developed, which require stoi-
chiometric external oxidants and high temperatures
above 1008C. Herein, we report a mild and efficient
À
À
Keywords: annulation; C H activation; O-hetero-
cycles; O N bond cleavage; redox neutral reaction;
rhodium
À
rhodiumACTHNUTRGENUG(N III)-catalyzed redox-neutral C H activation
annulation for the synthesis of isocoumarins and a-py-
À
rones by employing an O N bond as the internal oxi-
dant [Scheme 1, Eq. (4)].[16]
In recent years, transition metal-catalyzed chelation-
assisted annulative coupling reactions between aro-
À
ꢀ
matic or alkenyl C H bonds and C C or C=X (X=C
or N) bonds have attracted great interest as methods
to construct cyclic compounds.[1] Such transformations
generally require transition metal catalysts, stoichio-
metric oxidants and severe conditions above 1008C.
À
More recently, the redox-neutral C H activation an-
nulations[2] catalyzed by Pd,[3] Rh,[4] and Ru[5] have
emerged as a new strategy, which generally employed
À
an N O bond as the internal oxidant to construct N-
heterocycles [Scheme 1, Eq. (1)]. In 2013, the usage
À
of N N bond cleavage for redox process was reported
to construct N-heterocycles such as indoles[6] and qui-
nolones[7] [Scheme 1, Eq. (2)]. Nevertheless, the O N
À
bond as the internal oxidant was reported to form
benzofurans [Scheme 1, Eq. (3)].[8] As the facial route
to construct very useful O-heterocycles, new reactions
À
using the O N bond as the internal oxidant to realize
the redox-neutral annulation are in high demand.
Isocoumarin and a-pyrone are important structural
motifs of various bioactive natural products.[9] Be-
cause of their diverse biological activities including
À
Scheme 1. Redox-neutral C H activation annulations with
antitumor, anti-inflammatory, antiallergic, antimicro- different internal oxidants.
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Xing Guang Li et al.
Table 1. Optimization of the conditions for the reaction of
1a and 2a.[a]
2a could not proceed to afford 3a (entries 2 and 3).
The attempt with [Cp*RhCl2]2 as the catalyst gave in-
creased 15% yield (entry 4), and the change of the ad-
ditive to AgOAc afforded a further enhanced 26%
yield, although NaOAc, KOAc affected the reaction
negatively (entries 5–7). The investigation of the sol-
vent effect showed that CH3CN could promote the re-
action of 1a and 2a to 36% yield, while other solvents
such as dioxane, THF, DME, ethanol were ineffective
for the annulation (entries 8–12). When TFE was
used as the solvent, a remarkably improved 61%
yield was obtained, which illustrates that the solvent
effect is crucial (entry 13). Moreover, the change of
the ratio of 1a to 2a from 1:2 to 2:1 produced 3a in an
excellent 94% yield (entry 14). If the temperature was
lowered to 408C, only a 50% yield was observed in
the reaction of 1a and 2a (entry 15). Moreover, de-
creasing the catalyst loading or the ratio of 1a to 2a
could lead to the decreased yields (entries 16–19). Fi-
nally, we were pleased to find that good yields could
also be obtained in the scaled-up experiments (en-
tries 20 and 21).
Under the optimized conditions, the reactions of
a series of O-benzoylhydroxylamine 1 with various in-
ternal alkynes 2 (1:2=2:1) were carried out at 608C
in TFE with 5 mol% [Cp*RhCl2]2 as the catalyst and
30 mol% AgOAc as the additive (Table 2). For vari-
ous N,N-diethyl-O-benzoylhydroxylamines 1 substitut-
ed with electron-donating or electron-withdrawing
substitutes on the aromatic ring, the reactions pro-
ceeded smoothly to afford the desired products 3b–3e
in excellent yields, while the CF3 substituted N,N-di-
ethyl-O-benzoylhydroxylamine gave the product 3f in
70% yield under the same conditions. The results
demonstrated that the electronic effect of the sub-
stituents on the benzene ring seems not to be remark-
able. The reaction of 2a with substrates 1 bearing the
methyl group on ortho or meta position of the phenyl
ring afforded the products 3g and 3h in 74% and 76%
yields, respectively. It is worthy of note that the reac-
tion of meta-methyl substituted 1 with 2a gave 3h as
the only product and no product of annulation adja-
cent to the methyl group was detected. Moreover, for
the substrate 1 substituted with two meta-chloro
groups, its reaction with 2a generated the product 3i
in remarkably decreased 51% yield. The decreased
yields of 3g and 3h compared with 3b, the high regio-
selectivity of 3h and the low reactivity to form 3i re-
Entry Catalyst
Additive Solvent Yield [%][b]
CsOAc MeOH 10
1[c]
Cp*Rh
Pd(OAc)2
[RuCl2A(p-cymene)]2 CsOAc MeOH trace
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
ACHTUNGTRENNUNG(OAc)2
2[c]
G
CsOAc MeOH
0
3[d]
4
G
CHTUNGTRENNUNG
CsOAc MeOH 15
NaOAc MeOH 13
5
ACHTUNGTRENNUNG
6
A
KOAc
MeOH 13
7
A
AgOAc MeOH 26
AgOAc dioxane 0
8
ACHTUNGTRENNUNG
9
A
AgOAc THF
AgOAc DME
AgOAc EtOH
0
9
9
10
ACHTUNGTRENNUNG
11
ACHTUNGTRENNUNG
12
A
AgOAc CH3CN 38
13
A
AgOAc TFE
AgOAc TFE
AgOAc TFE
AgOAc TFE
AgOAc TFE
AgOAc TFE
AgOAc TFE
AgOAc TFE
AgOAc TFE
61
94
50
60
82
56
72
85 (81)
68 (66)
14[e]
15[e,f]
16[e,g]
17[e,g,h]
18[e,h,i]
19[g,h,j]
20[k]
21[l]
N
[a]
Reaction conditions: 1a (0.2 mmol), 2a (0.4 mmol), cata-
lyst (0.01 mmol), additive (0.06 mmol), solvent (2 mL), at
608C, 16 h, under N2.
[b]
1
Determined by H NMR analysis using phenyltrimethyl-
silane as the internal standard; yields in the parentheses
are the isolated yields.
20 h.
[c]
[d]
[e]
[f]
12 h.
1a (0.4 mmol) and 2a (0.2 mmol) were used.
At 408C.
0.005 mmol catalyst and 0.03 mmol additive were used.
24 h.
[g]
[h]
[i]
0.002 mmol catalyst and 0.012 mmol additive were used.
1a (0.3 mmol) and 2a (0.2 mmol) were used.
1a (20 mmol), 2a (10 mmol), catalyst (0.5 mmol) and ad-
ditive (3 mmol) were used, 24 h.
1a (15 mmol), 2a (10 mmol), catalyst (0.25 mmol) and ad-
ditive (1.5 mmol) were used, 24 h.
[j]
[k]
[l]
Initially, the cyclization of N,N-diethyl-O-benzoyl- vealed that the steric effect on the benzene ring in
hydroxylamine (1a) and 1,2-diphenylacetylene (2a) substrate 1 is distinct for the annulation of 1 and 2. If
was carried out as a model reaction to optimize the the benzene ring of 1a was changed to thiophene, the
reaction conditions (Table 1). Gratifyingly, the isocou- annulation product 3j was also obtained in excellent
marin product 3a (10% yield) was obtained in MeOH 92% yield. However, the substrate 1 incorporating
after 20 h at 608C with 5 mol% Cp*Rh
catalyst and 30 mol% CsOAc as the additive tion, the annulation reaction could be expanded to al-
(entry 1). When [RuCl2A(p-cymene)]2 and Pd(OAc)2 kenyl substrate 1l and the reaction with 2a produced
were applied as the catalysts, the reaction of 1a and a-pyrone 3l successfully in 80% yield.
ACHTUNGRTEN(NNUG OAc)2 as the naphthalene only gave a 52% yield for 3k. In addi-
C
ACHTUNGTRENNUNG
2
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RhodiumACTHNUTRGNEUG(N III)-Catalyzed, C H Activated Annulation
Table 2. Reactions of substrates 1 and 2 to afford the prod-
ucts 3.[a,b]
yields were obtained, which indicated that the elec-
tron-withdrawing substituents might affect the annula-
tion negatively. For symmetrical aliphatic alkynes
bearing two ethyl or butyl groups, the reactions with
1a also proceeded smoothly to afford the products 3q
and 3r in 72–74% yields. Finally, the reactivity and re-
gioselectivity of asymmetrical internal alkynes were
examined in the annulation reaction. Internal phenyl-
acetylenes substituted with methyl group, n-hexyl
group, tert-butyl group or trimethylsilyl group all re-
acted with 1a smoothly to give the corresponding
products 3s–3v in 66–81% yields with perfect regiose-
lectivity, and the structure of product 3v was unam-
biguously confirmed by single-crystal X-ray diffrac-
tion analysis.[17] However, terminal alkynes such as
phenylacetylene, 1-octyne and trimethylsilylacetylene
showed no reactivity towards 1a under the current
conditions.
To investigate the reaction mechanism further,
competition experiments between different alkynes
were performed under the optimal conditions
(Scheme 2).
N,N-Diethyl-O-benzoylhydroxylamine
(1a) was treated with 1,2-diphenylacetylene (2a) and
1,2-dibutylacetylene (2r), and 37% yield of product 3a
and 31% yield of product 3r were obtained, which
suggested that the difference of the reactivity between
diarylalkyne and dialkylalkyne is not remarkable in
this annulation reaction [Scheme 2, Eq. (1)]. For the
diarylalkyne substituted with an electron-donating
OMe group, its reaction with 1a gave a 46% yield,
which was much higher than the 25% yield obtained
from the reaction of 1a with the diarylalkyne substi-
tuted with an electron-withdrawing CF3 group
[Scheme 2, Eq. (2)]. The result demonstrated that the
electron-donating substituent of the internal alkyne is
beneficial for the reactivity of the annulation. More-
over, the reaction of isotopically-labeled [D5]-1a with
2a was carried out and more than 98% deuterium on
the C-6 site in [D4]-3a remained [Scheme 2, Eq. (3)],
À
which revealed the irreversible C H metalation in the
catalytic cycle. Finally, the kinetic isotope effect
(KIE) experiment was performed and a significant
KIE was observed (kH/kD =2.13), indicating that the
C H bond cleavage at the ortho position of 1a is
most likely involved in the rate-limiting step.
[a]
Reaction conditions:
1
(0.4 mmol),
2
(0.2 mmol),
[Cp*RhCl2]2 (5 mol%), AgOAc (30 mol%), TFE (2 mL),
at 608C, 16 h, N2, unless otherwise noted.
Isolated yield.
À
[b]
[c]
808C, 24 h, and 1.0 mmol of 1 was used.
As depicted in Scheme 3, a plausible mechanism
for this annulation reaction is proposed.[3–8] Firstly,
Subsequently, the scope of internal alkynes was substrate 1 coordinates to Cp*Rh
(III)X2, and a rhoda-
À
À
also examined in the redox-neutral C H activation cycle intermediate A is formed via the C H activa-
annulation. Internal diarylalkynes bearing electron- tion of the C-2 site of 1.[18] Regioselective coordina-
donating Me and OMe substituents on the para posi- tion and insertion take place to produce a seven-
tion of the phenyl ring showed good reactivity in the membered rhodacycle intermediate C. Complex C
reaction with 1a, and the products 3m and 3n were might go through a reductive elimination step to give
generated in 90% and 87% yields, respectively. How- cation D and Rh(I) species (path I).[4b,6b,7] With the
ever, when diarylalkynes bearing electron-withdraw- aid of the Rh(I) species, intermediate D might be re-
ing Cl or CF3 groups were used as the substrates to duced to form the product 3 and regenerate the
react with 1a, obviously decreased 58% and 72% RhACTHNUTRGNE(UNG III) catalyst. As an alternative to path I, complex
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Scheme 3. Proposed reaction mechanism.
Experimental Section
Typical Experimental Procedure for Product 3a
To an oven-dried sealed tube charged with [Cp*RhCl2]2
(6.2 mg, 0.01 mmol, 5 mol%), and AgOAc (10.0 mg,
0.06 mmol, 30 mol%) in TFE (2 mL) were added 1,2-diphe-
nylacetylene (2a) (35.6 mg, 0.2 mmol, 1 equiv.) and N,N-di-
ethyl-O-benzoylhydroxylamine (1a) (38.6 mg, 0.4 mmol,
2 equiv.). The reaction mixture was allowed to stir for 16 h
at 608C. The reaction mixture was diluted with EtOAc
(10 mL) and washed with water. The aqueous layer was ex-
tracted with EtOAc (3ꢂ10 mL). The combined organic
layer was dried over anhydrous sodium sulfate and concen-
trated under reduced pressure. The residue was purified by
flash column chromatography (n-hexane/EtOAc=25:1) to
afford isocoumarin 3a; yield: 53.5 mg (90%).
Scheme 2. Competition and isotope experiments.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (Project Nos. 21172069, 21190033 and
21372072), the Innovation Program of Shanghai Municipal
Education Commission (Project No. 12ZZ050), the Basic Re-
search Program of the Shanghai Committee of Sci. & Tech.
(Project No. 13M1400802), NCET (NCET-13-0798) and the
Fundamental Research Funds for the Central Universities.
C might also undergo an intramolecular nucleophilic
substitution, which leads to the breakage of the O N
bond and gives the desired product (path II).
À
In summary, we have developed a mild, efficient
À
and regioselective redox-neutral C H activation an-
nulation protocol for the synthesis of isocoumarins
À
and a-pyrones. The O N bond was employed as the
internal oxidant in the Rh
A
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6 RhodiumACHTUNGTRENNUNG(III)-Catalyzed, C H Activated Annulation to
À
Form Isocoumarins and a-Pyrones using the O N Bond as
an Internal Oxidant
Adv. Synth. Catal. 2014, 356, 1 – 6
Xing Guang Li, Kai Liu, Gang Zou, Pei Nian Liu*
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ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 0000, 000, 0 – 0
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