Cooperative Indium(III)/Silver(I) System for Oxidative Coupling/Annulation of 1,3-Dicarbonyls and Styrenes: Construction of Five-Membered Heterocycles

Article abstract of DOI:10.1002/adsc.201600280

A cooperative indium(III)/silver(I) system for the synthesis of various five-membered heterocycles, including dihydrofurans, pyrroles, spirolactones, and spiroiminolactones, through the sequential oxidative coupling/annulation reaction of 1,3-dicarbonyl compounds with styrenes has been developed. Four different heterocyclic systems were successfully synthesized depending on the substitution pattern of the substrates using readily available starting materials. This system has the advantages of a broad substrate scope, moderate to good chemical yields, an operationally easy and simple procedure, and short reaction times. (Figure presented.) .

Full text of DOI:10.1002/adsc.201600280

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DOI: 10.1002/adsc.201600280
Cooperative Indium(III)/Silver(I) System for Oxidative Coupling/
Annulation of 1,3-Dicarbonyls and Styrenes: Construction of
Five-Membered Heterocycles
Tae Yun Koa and So Won Youna,*
a
Center for New Directions in Organic Synthesis, Department of Chemistry and Research Institute for Natural Sciences,
Hanyang University, Seoul 04763, Korea
Fax: (+82)-2-2298-0319; e-mail: sowony73@hanyang.ac.kr
Received: March 13, 2016; Revised: April 11, 2016; Published online: June 2, 2016
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201600289.
Abstract:
A
cooperative indium(III)/silver(I)
ples of the use of aryl ketone derivatives. Potential
side reactions of aryl ketone-containing 1,3-dicarbon-
yls would be the formation of a-tetralones or 1-naph-
system for the synthesis of various five-membered
heterocycles, including dihydrofurans, pyrroles, spi-
rolactones, and spiroiminolactones, through the se-
quential oxidative coupling/annulation reaction of
1,3-dicarbonyl compounds with styrenes has been
developed. Four different heterocyclic systems were
successfully synthesized depending on the substitu-
tion pattern of the substrates using readily available
starting materials. This system has the advantages
of a broad substrate scope, moderate to good chem-
ical yields, an operationally easy and simple proce-
dure, and short reaction times.
À
thols (via overoxidation) resulting from C C bond
formation through the addition of a cationic (A) or
radical (A’) intermediate to the phenyl ring
(Scheme 1a, path b). Indeed, the Flowers group re-
ported a solvent-dependent, Ce(IV)-mediated oxida-
tive coupling/annulation of 1-aryl-1,3-dicarbonyls with
styrene to afford a mixture of a-tetralones and dihy-
drofurans with moderate to good selectivities
(Scheme 1b).[1e] a-Tetralones have been also synthe-
sized by a related Mn(III)-mediated reaction of aryl
methyl ketone with olefins (Scheme 1c).[3] In addition,
the Yu group reported that, compared to the use of
the corresponding alkyl ketone derivatives to give the
furans via the incorporation of a carbonyl group, the
aryl ketone-containing 1,3-dicarbonyls utilized an aryl
moiety in the intramolecular cyclization to generate
the 1-naphthol framework (Scheme 1d).[4]
Keywords: cyclization; heterocycles; indium; oxida-
tion; silver
1,3-Dicarbonyl compounds have been used as highly
Spirocyclic compounds, especially spirolactones, are
reactive and versatile building blocks in organic syn- of importance and broad interest because of their
thesis for the construction of a diverse range of carbo- ubiquity in a number of natural products and bioac-
cycles and heterocycles. Oxidative coupling/annula- tive compounds and their unique chemical and struc-
tion of 1,3-dicarbonyls with olefins[1] is a well-known tural features.[5] Consequently, they have attracted
reaction in the literature as an efficient method for considerable attention from the synthetic community,
the synthesis of dihydrofurans, which are commonly but the development of new synthetic methods for
found in numerous bioactive natural products and their synthesis from simple and readily available pre-
pharmaceuticals[2] and thus are of great importance. cursors has been highly challenging.[6,7] Kurosawa and
A wide range of one-electron oxidants has been uti- Fristad reported independently an Mn(III)-mediated
lized for this process, and the known methods com- spirodilactonization of malonic acid with olefins
monly require strongly acidic conditions and/or (Scheme 2a).[7a,b] While an ester group was inert in
a large excess amount of transition metal salts.[1a–g] this process, a free carboxylic acid moiety was re-
Recently, some progress has been achieved by the use quired for the effective lactonization. Recently, two
of catalytic amounts of metal or organic oxidants, im- elegant examples of asymmetric synthesis of spirolac-
proving the synthetic utility of this process.[1j,k] How- tones have been disclosed using Cu catalysis and or-
ever, our detailed literature survey reveals that the re- ganocatalysis (Scheme 2b and c).[7c,d]
ported methods generally utilize alkyl ketone-contain-
As part of our continuing interest in the reactions
ing 1,3-dicarbonyls as a substrate with very few exam- of 1,3-dicarbonyls with alkenes,[8] we became interest-
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Scheme 1. Oxidative coupling/annulation of 1,3-dicarbonyls and multiple bonds.
Scheme 2. Spirolactonization of 1,3-dicarbonyls and olefins.
À
À
ed in developing a new, efficient protocol for the for- new protocol involves sequential C C and C O (or
mation of both dihydrofuran and spirolactone skele- C N) bond formations with high efficiency, leading to
À
tons. Despite the progress that has been made in each the construction of various five-membered heterocy-
field, the synthesis of dihydrofurans and spirolactones cles, including dihydrofurans, pyrroles, spirolactones,
from 1,3-dicarbonyls and olefins, new and more effi- and spiroiminolactones (Scheme 3). The operationally
cient synthetic methods with broad substrate scope simple and straightforward procedure under nearly
still continue to be vigorously pursued. Moreover, the neutral reaction conditions and the very short reac-
development of a new single protocol that can effec- tion times are particularly noteworthy advantages of
tively lead to both compounds from the readily avail- this system.
able starting materials is highly desirable and valua-
ble.
We began our investigations using 1a and styrene
2a as the test substrates to identify the optimal condi-
Herein, we report a cooperative In(III)/Ag(I) tions for the efficient formation of dihydrofurans
system for the sequential oxidative coupling/annula- (Table 1, for complete data, see the Supporting Infor-
tion reaction of 1,3-dicarbonyls with styrenes. This mation). In light of our recent results in the Pd(II)-
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Scheme 3. Oxidative coupling/annulation of 1,3-dicarbonyls and styrenes: synthesis of various five-membered heterocycles.
Table 1. Optimization studies.
the proposed reaction using a slight modification of
our previously reported conditions [Pd(OAc)2, CuCl2,
1208C instead of Pd(O2CCF3)2, Cu(OAc)2, 1008C].
Only dihydrofuran 3aa was obtained in 17% yield
without the formation of either the corresponding a-
tetralone or 1-naphthol (Table 1, entry 1), presumably
because of the incorporation of a carbonyl moiety
rather than a phenyl group in the cyclization process.
A variety of oxidants was first examined using
Pd(OAc)2 as a catalyst (entries 1–5). Ag(I) salts
Entry
Catalyst
Oxidant
Time [h]
Yield [%][a]
proved to be generally superior to other common oxi-
dants, and among the various Ag(I) salts examined,
Ag2O was the most effective oxidant for this reaction.
When a control experiment employing only Ag2O
was performed, 3aa was still formed, albeit in a moder-
ately low yield (entry 15). Thus, we speculated that
this oxidative annulation might proceed through a rad-
ical mechanism[1] facilitated by Pd(OAc)2 as a Lewis
acid for the activation of the b-keto ester 1a. To gain
additional mechanistic insights, using the conditions
listed for entry 5 in Table 1, this reaction was per-
formed in the presence of a radical scavenger, such as
TEMPO (2,2,6,6-tetramethylpiperidine N-oxyl) or
BHT (3,5-di-tert-butyl-4-hydroxytoluene) [Eq. (1)].
Both reactions were shut down. Instead, the corre-
sponding trapping products were obtained as a by-
product, suggesting that a radical intermediate was in-
volved in the reaction.
1
2
3
4
5
6
7
8
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Sc(OTf)3
Zn(OTf)2
Mg(OTf)2
InBr3
In(OTf)3
CF3CO2H
Ag2O
In(OTf)3
In(OTf)3
–
CuCl2
K2S2O8
AgOAc
Ag2CO3
Ag2O
Ag2O
Ag2O
Ag2O
Ag2O
Ag2O
Ag2O
K2S2O8
Ag2O
–
24
24
24
24
1.5
0.5
1.5
1.5
1
1
4
24
1.5
24
3.5
17
0
(11)
(32)
(57)
(50)
63
(64)
(39)
(80)
60
9
10
11
12
13[b]
14
15
0
(30)
0
Ag2O
29
[a]
1
Yields were determined by H NMR using trichloroethy-
lene as an internal standard. Values in parentheses indi-
cate isolated yields.
[b]
Using 1 equiv. Ag2O.
Next, we extended our investigations to a variety of
Lewis and Brønsted acids in the presence of Ag2O
(entries 6–11). Among them, In(OTf)3 gave the best
catalyzed reaction of 2-alkenylphenyl b-keto esters to results, generating 3aa in 80% yield (entry 10). Con-
give 1-naphthol derivatives,[8c] we initially attempted sidering the recent reports of various Ag(I)-catalyzed
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oxidative reactions,[9] we attempted to use a catalytic
amount of Ag2O in the presence of K2S2O8; however,
no reaction occurred (entry 12). Reducing the amount
of Ag2O decreased the yield of 3aa (entry 13). More-
over, a control experiment in the absence of Ag2O
confirmed that Ag2O is essential for this transforma-
tion (entry 14). It should be noted that neither the
possible by-product a-tetralone (or 1-naphthol) deriv-
ative nor the corresponding fully unsaturated furan
was observed in all cases.[1e] Optimal reaction condi-
tions were finally established as entry 10 in Table 1
and consisted of 5 mol% In(OTf)3 and 2 equiv. Ag2O
in 1,2-dichloroethane at 1208C. This protocol could
be complementary to other known methods for the
efficient synthesis of dihydrofurans.
Table 2. Substrate scope: dihydrofurans.
Using the optimized reaction conditions, we then
explored the substrate scope of this process. First, we
examined the reaction of various 1,3-dicarbonyls with
styrene 2a (Table 2a). Both the aryl ketone- (1a–f, 1l–
o) and alkyl ketone-containing (1g–k) 1,3-dicarbonyls,
such as 1,3-diketones, b-keto esters, and b-keto ni-
triles, underwent the sequential oxidative coupling/an-
nulation reaction smoothly to afford the correspond-
ing dihydrofurans in good yields, regardless of the
electronic and steric properties of the aryl (1l–o),
alkyl (1g–k), and ester (1a–d) groups. In contrast,
these reactions failed to afford the desired products
when diethyl malonate and 2-nitro-1-phenylethanone
were used (results not shown).
Next, we proceeded to investigate the scope of the
terminal alkenes for this reaction (Table 2b). Elec-
tron-neutral, moderately electron-rich and electron-
deficient aryl groups were well tolerated (2c–f). The
1-naphthyl moiety could also be incorporated as a sub-
stituent at the alkene terminus to give 3af in good
yield (84%). In contrast, strongly electron-withdraw-
ing aryl substituents, such as NO2, resulted in a low
yield of the desired product (e.g., 3ae, 28%), whereas
a substrate with a strongly electron-donating substitu-
ent, such as MeO, afforded the corresponding furan
in a moderate yield (e.g., 3ab’, 40%). Interestingly,
the reaction of 4-methylstyrene (2c) with 1a for
a longer reaction time (24 h) provided only the furan
3ac’ in 32% yield (vs. after 2 h only 3ac in 61% yield).
We speculated that the furan formation might result
from the easy oxidation of a benzylic a position bear-
ing electron-rich aryl substituents (e.g., 4-MeOC6H4),
which could effectively stabilize the resulting carbo-
cation (via radical) together with an O atom.
The use of disubstituted olefins was also examined
(Table 2c). Both 1,1- (2g and h) and 1,2-disubstituted
alkenes (2i–k) and dienes (2l) proved to be suitable
substrates for the formation of the corresponding di-
hydrofurans stereo- and regiospecifically. It should be
noted that most reactions were complete after a very
short reaction time (typically 0.5–2 h) and were quite
sensitive to the reaction time, presumably because of
[a]
Using 1 equiv. Ag2O.
[b]
Two separable regioisomers were obtained in 43% (R1 =
Me, R2 =COPh) and 24% (R1 =Ph, R2 =COMe) yields,
respectively.
In ClCH2CH2Cl (0.05M).
Using 1.5 equiv. Ag2O.
[c]
[d]
[e]
Using 10 mol% In(OTf)3.
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Table 3. Substrate scope: spiro(imino)lactones.
methyl b-keto ester 4a underwent oxidative cycliza-
tion under the standard reaction conditions to afford
a spirolactone 5aa and an alkenylation product 6aa.
The use of Cu(OAc)2 as an additive increased the
yield of 5aa, but a small amount of 6aa was still pro-
duced.[10] The reactions with the tetralone- and 1-ben-
zosuberone-derived methyl b-keto esters 4b and c pro-
ceeded smoothly in the absence of Cu(OAc)2 to give
the corresponding spirolactones 5ba and ca in excel-
lent yields. Except for 4-methoxystyrene, a wide
range of styrenes was found to be good substrates for
this reaction (5bc–bf). In sharp contrast to the dihy-
drofuran-forming reaction, the reaction of electron-
deficient styrenes proceeded uneventfully (5be). In
contrast, the oxidative cyclization reaction failed
when electron-rich styrenes (i.e., 4-methoxystyrene)
were used, leading to only alkenylation product 6bb
through oxidative coupling, regardless of whether
Cu(OAc)2 was present. When the b-keto esters were
replaced with b-keto amides, the desired spiroimino-
lactones could be obtained in moderate to good yields
À
as the result of the formation of a C O bond rather
than a C N bond during the oxidative cyclization
À
(5da–fa, 5eh). Both a-substituted styrenes and b-keto
amides required the use of Cu(OAc)2 as a co-oxidant
to facilitate oxidative cyclization, resulting in the cor-
responding spiro(imino)lactones. The method present-
ed herein represents an attractive route for the facile,
straightforward, and fast (15–30 min) construction of
spiro(imino)lactone systems from easily accessible
simple substrates: 1,3-dicarbonyls and styrenes.
Considering the previous report on the In-catalyzed
enamination of 1,3-dicarbonyls[11] and a few prece-
dents for its successful application in one-pot reac-
tions,[12] we explored a rapid one-pot construction of
five-membered nitrogen-heterocycles from three
simple and commercially available starting materials
[Eq. (2)]. With a slight modification of the original
conditions reported by the Wang group[11] [In(OTf)3
instead of InBr3], the b-enamino ester and ketone
were readily formed within minutes by the condensa-
tion of benzylamine with 1g or 1i, respectively. Then,
the in situ-generated enamine intermediates success-
[a]
In the presence of 1 equiv. Cu(OAc)2.
Using 10 mol% In(OTf)3.
In the presence of 2 equiv. Cu(OAc)2.
[b]
[c]
overoxidation (e.g., 3ac’). In general, a longer reac- fully underwent a subsequent oxidative coupling/an-
tion time decreased the chemical yields of the desired nulation with styrene under the standard conditions
products because of significant decomposition.
in the presence of Cu(OAc)2, affording the corre-
Subsequently, we investigated the reaction of styr- sponding pyrroles in moderate yields.[13]
enes with 2-substituted 1,3-dicarbonyl compounds
Based on the related mechanisms established for
(Table 3). Very intriguingly, the indanone-derived the oxidative coupling and cyclization of 1,3-dicarbon-
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Scheme 4. Proposed mechanism.
yl compounds with olefins,[1] a plausible mechanistic nylation product 6b generated from the oxidative
proposal is outlined in Scheme 4. The activation of elimination of III’’ is an unlikely intermediate in the
1,3-dicarbonyl compound (e.g., 1a) by coordination to spirolactone-forming process. While the reason for
In(OTf)3 is followed by single-electron transfer (SET) the formation of only alkenylation product 6bb from
from the initially formed intermediate I to Ag(I) to 4-methoxystyrene (2b) remains unclear at this stage,
produce intermediate II. Then, II undergoes addition we surmised that a lactonization of the benzylic car-
to styrene 2 to afford intermediate III. Subsequently, bocation IV’’ derived from 2b, which is stabilized by
further oxidation of the enol form III’ resulting from an electron-rich aryl substituent (i.e., 4-MeOC6H4),
tautomerization of III to the benzylic carbocation IV might be slower than its competitive oxidative elimi-
followed by the intramolecular cyclization of IV’ nation.
forms the desired dihydrofuran product 3a. Alterna-
In summary, we have developed a simple and
tively, a radical 5-endo-trig cyclization of III followed straightforward method for the synthesis of various
by either oxidation/deprotonation or hydrogen ab- five-membered heterocycles, i.e., dihydrofurans, pyr-
straction of the as-generated radical intermediate to roles, spirolactones, and spiroiminolactones, through
give dihydrofuran 3a could also be invoked.
the sequential oxidative coupling/annulation reaction
A similar reaction of 2-substituted 1,3-dicarbonyl of two types of readily available, simple chemical
compounds (e.g., 4b) with styrene 2 proceeds to form feedstocks: 1,3-dicarbonyls and olefins. A cooperative
the radical intermediate III’’, which is further oxidized In(III)/Ag(I) system enables the sequential formation
À
À
À
to the benzylic carbocation IV’’ by Ag(I) alone or, of a new C C and C O (or C N) bonds with good
more favorably, with the aid of
a co-oxidant scope and high efficiency and is thus an effective al-
Cu(OAc)2.[10] Subsequent lactonization of IV’’ affords ternative or complementary to the known methods
the desired spirolactone product 5b via hydrolysis of for related transformations. The salient features of
the resulting V. Subjecting the alkenylation product this protocol are the synthesis of four different five-
6b (e.g., 6bb or 6bh) to the standard reaction condi- membered heterocycles depending on the substitution
tions in the presence or absence of Cu(OAc)2 did not pattern of the substrates, the use of easily available
give any spirolactone product 5b, suggesting that alke- starting materials, broad substrate scope, moderate to
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Experimental Section
General Procedure for the Synthesis of Dihydro-
furans 3
To
a solution of 1 (0.1~0.2 mmol, 1 equiv.), In(OTf)3
(5 mol%), and Ag2O (2.0 equiv.) in ClCH2CH2Cl (0.1M) in
a pressure tube was added styrene 2 (2 equiv.). After being
stirred at 1208C for the reported time, the reaction mixture
was concentrated under vacuum. The residue was purified
by column chromatography on silica gel to afford the corre-
sponding product 3. All reactions were carried out 3–4 times
repetitively and the average values of yields are given.
[2] For selected recent examples, see: a) T. Koike, T.
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General Procedure for the Synthesis of Spiro(imino)-
lactones 5
To
a solution of 4 (0.1~0.2 mmol, 1 equiv.), In(OTf)3
(5 mol%), Ag2O (2.0 equiv.), and Cu(OAc)2 (if needed,
1.0 equiv.) in ClCH2CH2Cl (0.1M) in a pressure tube was
added styrene (2 equiv.). After being stirred at 1208C for
the reported time (15 or 30 min), the reaction mixture was
concentrated under vacuum. The residue was purified by
column chromatography on silica gel to afford the corre-
sponding product 5 (and 6). All reactions were carried out
3–4 times repetitively and the average values of yields are
given.
CCDC 1473500 (5bf-A) contains the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data
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Acknowledgements
This work was supported by both Basic Science Research
Program and Nano·Material Technology Department Pro-
gram through the National Research Foundation of Korea
(NRF) funded by the Ministry of Education and MSIP (Nos.
2012R1A1A2041471, 2012M3A7B4049644, 2014-011165, and
2015R1A2A2A01002559). We thank Dr. Ji-Eun Lee (Central
Instrument Facility, Gyeongsang National University) for X-
ray crystallographic analysis and Mr. Sung Hong Kim (Anal-
ysis Research Division Daegu Center, Korea Basic Science
Institute) for HR-MS analysis.
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