Iron-Catalyzed Oxidative C-H Functionalization of Internal Olefins for the Synthesis of Tetrasubstituted Furans

Article abstract of DOI:10.1021/acs.orglett.7b01431

Tetrasubstituted furans were efficiently synthesized from Fe(OAc)₂-catalyzed C-H/C-H cross-dehydrogenative-coupling (CDC) reactions of activated carbonyl methylenes with S,S-functionalized internal olefins, that is, α-oxo ketene dithioacetals and analogues, under oxidative conditions. β-Ketoesters, 1,3-dicarbonyls, β-keto nitrile, and amide derivatives were used as the coupling partners. The resultant alkylthio- and carbonyl-functionalized furans could be further transformed to diverse arylated furan derivatives and furan-fused N-heterocycles, respectively. The control experiments have revealed a radical reaction pathway.

Full text of DOI:10.1021/acs.orglett.7b01431

Letter
pubs.acs.org/OrgLett
Iron-Catalyzed Oxidative C−H Functionalization of Internal Olefins
for the Synthesis of Tetrasubstituted Furans
Jiang Lou,†,‡ Quannan Wang,†,‡ Kaikai Wu,† Ping Wu,†,‡ and Zhengkun Yu*,†,§
†Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, P. R. China
‡University of Chinese Academy of Sciences, Beijing 100049, P. R. China
§State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 354
Fenglin Road, Shanghai 200032, P. R. China
S
* Supporting Information
ABSTRACT: Tetrasubstituted furans were efficiently synthesized
from Fe(OAc)2-catalyzed C−H/C−H cross-dehydrogenative-coupling
(CDC) reactions of activated carbonyl methylenes with S,S-function-
alized internal olefins, that is, α-oxo ketene dithioacetals and analogues,
under oxidative conditions. β-Ketoesters, 1,3-dicarbonyls, β-keto
nitrile, and amide derivatives were used as the coupling partners.
The resultant alkylthio- and carbonyl-functionalized furans could be further transformed to diverse arylated furan derivatives and
furan-fused N-heterocycles, respectively. The control experiments have revealed a radical reaction pathway.
he furan motif is an important structural unit abundant in
copper/P(tBu)3-mediated oxidative radical [3 + 2] cyclization
Tmany biologically active natural products, pharmaceut- between olefins or alkynes and cyclic ketones,15a and the same
icals, and agrochemicals.1 Continuous efforts have been
devoted to the synthesis of functionalized furans and furan-
type of reactions occurred between styrenes and aryl alkyl
ketones.15b Photocatalytic reactions of styrenes and α-
chloroalkyl ketones afforded polysubstituted furans.15c Man-
ganese dioxide promoted the oxidative cyclization of enones
with 1,3-dicarbonyl compounds to give 3,4-dicarbonyl-sub-
stituted furans.15d In order to avoid formation of dihydrofurans,
functionalized olefins were usually reacted with carbonyl
compounds. Using such a synthetic strategy, 2-siloxy-1-
olefins,16a gem-difluoro olefins,16b β-nitrostyrenes,16c enami-
nes,16d 2,3-dibromo-1-propenes,16e and α,β-unsaturated carbox-
ylic acids16f have been reported for substituted furan synthesis.
Palladium-catalyzed intramolecular oxidative cycloisomerization
of 2-cinnamyl-1,3-dicarbonyls was also applied to synthesize
furan derivatives.17
Although a variety of methods have been developed for furan
synthesis, more diverse and environmentally benign procedures
are strongly desired to access highly functionalized furans. In
this context, C−H/C−H cross-dehydrogenative-coupling
(CDC) reactions18 are attractive for the establishment of a
furan backbone. Gold-catalyzed alkynylation of 1,3-dicarbonyl
compounds with terminal alkynes efficiently proceeded to
afford 3-alkynylfurans.19a Stoichiometric Ag2CO3 mediated the
same reactions to give 1,2,4-trisubstituted furans.19b Molecular
iodine effected the oxidative cross-coupling of β-ketoesters and
terminal alkynes to generate furan derivatives.19c During the
ongoing investigation of internal olefinic C−H activation,20 we
were encouraged by FeCl3·H2O-catalyzed benzofuran synthesis
from the arene C(sp2)−H/C(sp3)−H CDC reactions of
phenols and β-keto esters21 and the merits of iron catalysis22
based complex molecules.2 In this regard, substituted furans
have usually been synthesized by means of the cyclization of
alkynyl- or allenyl-bearing carbonyl compounds or through
reactions between alkynes and carbonyl compounds. Thus,
transition-metal-catalyzed dehydrogenative heterocylization of
2- and 3-alkynyl enones was employed to access furan-fused
carbocycles or polysubstituted furans.3,4 Iodocyclization was
also applied for this purpose.5 Intramolecular cyclization of
propargylic alcohols afforded polysubstituted furans.6a,b Alkynyl
epoxides,7 allenyl ketones,8a allenyl or homopropargylic
alcohols,8b and allene-1,3-dicarboxylic esters8c can be used for
the same purpose. Copper-catalyzed heterocyclization of
alkynyl ketones and imines8d and phosphine-mediated
reductive condensation of γ-acyloxybutynoates8e readily yielded
furan derivatives.4a Brønsted acid catalyzed cyclization of 1,4-
diketones also gave furans.9 Transition-metal-catalyzed vinylic
C−H activation/[4 + 2] O-annulation of α-aryl enones10a and
oxidative cross-coupling of 1,3-dicarbonyl compounds or β-
ketoesters10b with internal alkynes formed functionalized
furans. α-Diazocarbonyls11a and N-tosylhydrazones11b were
used for metalloradical cyclization with alkynes to construct
polysubstituted furans. The combination of N-arylimines and
alkynylbenziodoxolones was utilized for the synthesis of
polysubstituted furans.12 A two-step reaction procedure of
aldehydes with propargylic alcohols was developed to prepare
highly substituted furans.13
Due to the ready C−H addition to olefinic CC bonds to
form dihydrofurans,14 only a limited number of olefins have
been documented for the synthesis of polysubstituted furans.
Fused furans and naphthofurans were synthesized through a
Received: May 11, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b01431
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Organic Letters
Letter
and reasonably envisioned that α-oxo ketene dithioacetals,23
a
a
Scheme 1. Scope of Ketene Dithioacetals 1
class of functionalized internal olefins, might be used for the
synthesis of highly functionalized furans through direct olefinic
C(sp2)−H functionalization with the C(sp3)−H bond of a
suitable carbonyl compound. Herein, we disclose Fe(OAc)2-
catalyzed oxidative annulations of α-oxo ketene dithioacetals
with β-ketoesters and related compounds for the synthesis of
tetrasubstituted furans.
Initially, the reaction of α-benzoyl ketene di(methylthio)-
acetal (1a) with ethyl acetoacetate (2a) was conducted to
optimize the reaction conditions (eq 1) (see the Supporting
Information for details). With 2 mol % of FeCl3 as the catalyst
and 3 equiv of tert-butyl peroxybenzoate (TBPB) as the
oxidant, the reaction of 1a and 2a in a 1:3 molar ratio
proceeded in N,N-dimethylacetamide (DMA) at 120 °C for 15
h under an argon atmosphere, forming the target product, that
is, tetrasubstituted furan 3a, in 40% yield. Use of Fe(OAc)2 as
the catalyst remarkably enhanced the yield to 83%. Increasing
the loading of TBPB to 4 equiv or using 5 mol % of the catalyst
did not improve the reaction efficiency, and use of a smaller
amount of the oxidant or catalyst deteriorated the product
yield. Both di-tert-butylperoxide (DTBP) and tert-butyl hydro-
peroxide (TBHP) were not effective oxidants. Elevating the
reaction temperature to 130 °C did not enhance the yield
either. Extending the reaction time to 20 h improved the
formation of 3a (87%), which was thus isolated in 79% yield.
Under an air or oxygen atmosphere, the product yield was
lessened to 70−75%. Without the catalyst or oxidant, the
desired reaction could not efficiently proceed to form 3a (35%)
or did not occur.
a
Conditions: 1 (0.5 mmol), 2a (1.5 mmol), Fe(OAc)2 (0.01 mmol),
TBPB (1.5 mmol), DMA (2.0 mL), 120 °C, 0.1 MPa argon, 20 h.
b
Yields refer to the isolated products. 1h (2.0 mmol), 2a (6.0 mmol),
Fe(OAc)2 (0.04 mmol), TBPB (6.0 mmol), DMA (5.0 mL). 48 h.
c
Under the optimized conditions, the scope of α-oxo ketene
dithioacetals (1) was explored (Scheme 1). The analogues of
1a, that is, substituted α-benzoyl ketene dithioacetals, exhibited
various reactivities to form the target furan products of type 3
in good to excellent yields. No obvious steric effects were
observed for the methyl and methoxy-substituted α-benzoyl
ketene dithioacetal substrates, and their reactions with 2a
afforded products 3b−f (75−84%). The steric/electronic
effects were obvious among the halo-substituted α-benzoyl-
bearing substrates. m-Cl (F)- and p-Br (F)-substituted
substrates reacted with 2a to give the corresponding products
3g and 3i−k in 60−61% yields, while p-Cl-benzoyl-bearing
substrate reacted more efficiently to yield furan 3h (78%), even
reaching 75% yield from a 2 mmol scale reaction (see the SI).
4-Trifluoromethyl demonstrated an obvious negative electronic
impact on the yield of 3l (62%). The bulky α-naphthoyl moiety
exhibited a steric effect to render the formation of 3m in 65%
yield. α-Heteroaroyl ketene dithioacetals smoothly underwent
the reaction to generate 3n−p (50−70%), exhibiting various
reactivities due to the different aromaticities of the O-, S-, and
N-heteroaryl functionalities. α-Acetyl ketene di(methylthio)-
acetal exhibited a good reactivity to afford 3q (75%), whereas a
steric effect was observed in the case of using α-cyclo-
propylcarbonyl substrate, leading to the target product 3r in
50% yield. Treatment of α-ester, amide, and cyano ketene
dithioacetals with 2a under the standard conditions could give
the target products 3s−u (40−66%), respectively, demonstrat-
ing a good diversity of the present synthetic methodology. α-
Oxo ketene di(ethylthio)acetals also efficiently reacted with 2a
to form the target products 3v (70%) and 3w (74%). It is clear
that the internal olefin substrates are widely substituent
tolerant.
Next, the protocol generality was investigated by performing
the reactions of α-oxo ketene dithioacetals 1 with a variety of β-
ketoesters 2 (Scheme 2). Under the standard conditions, the
reaction of α-(4-methoxybenzoyl) ketene di(methylthio)acetal
(1f) reacted with methyl acetoacetate (2b) to yield the target
furan product 4a (82%), exhibiting a reactivity similar to that of
ethyl acetoacetate (2a) as compared to the formation of 3f
(84%) (Scheme 1). Variation of the alkyl moieties to isopropyl,
tert-butyl, and benzyl in acetoacetates 2c−e did not obviously
alter the reaction efficiency, leading to furans 4b−d (77−78%).
However, when the the bulkiness of the β-ketoester substrates
was increased, the product yields were dropped from 84% for 3f
to 70−76% for 4e−h. It should be noted that 4,4,4-
trifluoroacetoacetate (2j) also exhibited a decent reactivity to
produce 4i (73%). In the case of using unsubstituted ethyl (or
methyl) 3-oxo-3-phenylpropanoates, the steric effect from the
substituted α-benzoyl moieties was not obvious, resulting in
4j−l in 67−70% yields. However, ethyl 3-oxo-3-(2′-
chlorophenyl)propanoate (2l) showed an obvious steric effect
to deteriorate the product yield to 60% for 4m, and m- or p-
halo-substituted phenyl groups did not exhibit obvious steric/
B
DOI: 10.1021/acs.orglett.7b01431
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
with hydrazine (Scheme 3). With benzeneboronic acid and its
4-chloro, 4- and 2-methoxy, and 3-methyl-substituted ana-
Scheme 2. Scope of β-Ketoesters 2
a
Scheme 3. Derivation of Furans 3 and 4
a
Conditions A: 3 or 4 (0.15 mmol), dioxane (2.0 mL). Conditions B:
3 or 4 (0.3 mmol), MeCN (2.0 mL). Yields refer to the isolated
b
products. Seven days.
logues, 2-arylated tetrasubstituted furans 5a−h were efficiently
obtained (80−93%). The condensation reactions afforded
potentially bioactive furan-fused pyridazinone derivatives24
6a−d in 50−86% yields.
a
Conditions: 1 (0.5 mmol), 2 (1.5 mmol), Fe(OAc)2 (0.01 mmol),
TBPB (1.5 mmol), DMA (2.0 mL), 120 °C, 0.1 MPa argon, 20 h.
To probe into the reaction mechanism addition of 3 equiv of
a radical scavenger such as 2,2,6,6-tetramethyl-1-piperidinyloxy
or 2,6-di-tert-butyl-4-methylphenol to the reaction mixture of
1a with 2a completely inhibited the reaction, suggesting a
radical pathway involved in the reaction.22 The kinetic isotope
effect experiments were explored by conducting the reactions of
1a and its deuterated form 1a[D]20e with 2a under the standard
conditions, respectively. A secondary isotope effect25 was
observed with kH/kD = 1.1, which indicates that cleavage of
the internal olefinic C−H bond in 1a is not involved in the rate-
determining step of the overall catalytic cycle. On the basis of
these results and the literature reports,26 a plausible radical
oxidative reaction mechanism is proposed (see the SI for
details).
In summary, efficient iron-catalyzed direct regioselective
radical oxidative annulation of S,S-functionalized internal
olefins with β-ketoesters and analogues has been realized to
synthesize tetrasubstituted furans. The highly functionalized
furan products can be readily transformed to 2-arylfurans and
2,3-furan-fused pyridazinones through catalytic C−S cleavage
and condensation with hydrazine, respectively. The present
protocol provides a concise and environmentally benign route
to highly functionalized furan derivatives.
b
Yields refer to the isolated products. Fe(OAc)2 (0.05 mmol),
acetylacetone (2.0 mmol), TBPB (2.0 mmol).
electronic effects on the reaction efficiency to render the
formation of 4n−s (70−74%). 3- and 4-methoxy substituents
on the aryl moiety of β-ketoesters and a 4-bromo substituent
on the benzoyl moiety of an α-oxo ketene dithioacetal lessened
the substrate reactivity to form 4t−v (60−61%). α-Acetyl
ketene di(methylthio)acetal also exhibited a decent reactivity to
interact with various β-ketoesters to give 4w−z (68−80%). α-
(Thiophene-2-carbonyl) ketene di(methylthio)acetal reacted
with 4,4,4-trifluoroacetoacetate to form 4z1 in 65% yield. It is
noteworthy that internal olefin 1f reacted with the 1,3-
dicarbonyl compound, that is, acetoacetone, under the standard
conditions to afford 3,4-dicarbonyl-tetrasubstituted furan 4z2
(67%). The molecular structures of compounds 3 and 4 were
further confirmed by the X-ray single-crystal structural
determination of compound 4j (see the SI).
In contrast to most of the known substituted furans, the
present tetrasubstituted furans 3 and 4 bear three readily
convertible functional groups, i.e., alkythio, carbonyl, and ester,
at the 2-, 3-, and 4-positions of the furan backbone. This
structural feature is highly desired for furans to be used as
organic synthons. Thus, derivation of furans 3 and 4 was
conducted by palladium-catalyzed Liebeskind−Srogl cross-
coupling reactions with arylboronic acids and condensation
C
DOI: 10.1021/acs.orglett.7b01431
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.7b01431.
■
S
Experimental materials and procedures, NMR of
compounds, and X-ray crystallographic analysis for
compound 4j (PDF)
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: zkyu@dicp.ac.cn.
ORCID
Jiang Lou: 0000-0002-5653-1359
Quannan Wang: 0000-0002-4136-0016
Kaikai Wu: 0000-0003-1950-5343
Ping Wu: 0000-0003-2650-9043
Zhengkun Yu: 0000-0002-9908-0017
Notes
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to the National Natural Science Foundation of
China (21472185) and the National Basic Research Program of
China (2015CB856600) for support of this research.
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