Regioselective synthesis of multisubstituted furans via metalloradical cyclization of alkynes with α-diazocarbonyls: Construction of functionalized α-oligofurans

Article abstract of DOI:10.1021/ja309446n

Co(III)-carbene radicals generated from activation of α- diazocarbonyls by Co(II)-porphyrin complexes have been shown to undergo a new type of tandem radical addition reaction with alkynes that affords five-membered furan structures. The Co(II) complex of

Full text of DOI:10.1021/ja309446n

Communication
pubs.acs.org/JACS
Regioselective Synthesis of Multisubstituted Furans via
Metalloradical Cyclization of Alkynes with α‑Diazocarbonyls:
Construction of Functionalized α‑Oligofurans
Xin Cui, Xue Xu, Lukasz Wojtas, Martin M. Kim, and X. Peter Zhang*
Department of Chemistry, University of South Florida, Tampa, Florida 33620-5250, United States
S
* Supporting Information
fundamentally different catalytic system that operates in a
nonionic manner.
ABSTRACT: Co(III)−carbene radicals generated from
activation of α-diazocarbonyls by Co(II)−porphyrin
complexes have been shown to undergo a new type of
tandem radical addition reaction with alkynes that affords
five-membered furan structures. The Co(II) complex of
3,5-DitBu-IbuPhyrin, [Co(P1)], is effective in catalyzing
the metalloradical cyclization reaction under neutral and
mild conditions. The [Co(P1)]-catalyzed process tolerates
a wide range of α-diazocarbonyls and terminal alkynes with
varied steric and electronic properties, producing poly-
functionalized furans with complete regioselectivity. The
catalytic synthesis features a high degree of functional
group tolerance and can be applied iteratively to construct
functionalized α-oligofurans.
Co(II)−porphyrin complexes [Co(Por)] with well-defined
structures have emerged as unique metalloradical catalysts for
olefin cyclopropanation.9 Mechanistic studies have confirmed
that Co(II)-based metalloradical cyclopropanation proceeds
through an unusual Co(III)−carbene radical intermediate that
undergoes stepwise radical addition−substitution.10 A highly
enantioselective cyclopropenation process was recently devel-
oped by applying the metalloradical catalysis to alkyne substrates
(Scheme 1, cycle I).11 To explore the use of metalloradical
Scheme 1. Competitive Pathways for Co(II)-Based
Metalloradical Cyclization of Alkynes with α-Diazocarbonyl
Reagents: Cyclopropenation versus Furan Formation
olyfunctionalized furans constitute an important class of
P
five-membered O-heterocycles with widespread applica-
tions.1 Their oligomers have also attracted considerable interest
due to their great potential in material science.2 Transition-
metal-catalyzed cyclization reactions, especially intermolecular
cyclizations of structurally simple starting materials, are among
the most direct and practical methods for the construction of
substituted furans.3 Because of the synthetic ease of the
substrates and the mild reaction conditions, such transformations
have received increasing attention and have been recognized as
some of the most appealing routes to polyfunctionalized
furans.3b−f,h−j The Rh2-catalyzed [3 + 2] cycloaddition of
arylacetylenes with α-diazocarbonyl compounds represents one
of the most direct approaches for the construction of
polysubstituted furans.4 Electron-rich arylacetylenes have been
shown to form furans efficiently with different acceptor/acceptor
(A/A)-substituted diazo reagents, as they tend to stabilize the
key zwitterionic intermediate.4b Because they cannot provide this
stabilization, other types of alkyne substrates, such as aliphatic
and electron-deficient alkynes,4b,5 generally perform poorly in
the Rh2-based furan synthesis, except when cyclic α-diazo-
carbonyls are used6 or the reaction is carried out in an
intramolecular fashion.7 Moreover, direct access to furans with
sensitive functionalities remains challenging in Rh2- and other
metal-based cycloaddition systems,3,4b,8 which share common
reaction mechanisms that are intrinsically ionic in nature.
Evidently, there is a need to develop new catalytic systems for
general and effective syntheses of polyfunctionalized furans from
different alkynes and α-diazocarbonyls. To meet the remaining
challenges in the field, it seems desirable to establish a
catalysis in other radical cyclization reactions, we envisioned the
possibility of an alternative catalytic pathway leading to the
construction of furans (Scheme 1, cycle II). In this process, the
vinyl radical intermediate B generated from radical addition of
Co(III)−carbene radical A to the alkyne would undergo a
consecutive radical addition to the carbonyl group to give a new
intermediate, C, which would release the furan product via
radical β-scission. In addition to its fundamental importance, this
catalytic tandem radical addition process would be synthetically
attractive, as it would enable the direct synthesis of multi-
substituted furans from α-diazocarbonyls and simple acetylenes.
More importantly, its nonionic mechanism would enable a
general furan synthesis with a high degree of functional group
tolerance.
Received: September 24, 2012
Published: December 3, 2012
© 2012 American Chemical Society
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Herein we report a general and regioselective synthesis of
polyfunctionalized furans involving intermolecular radical
cyclization of acetylenes with A/A-substituted diazo reagents
via Co(II)-based metalloradical catalysis. This unprecedented
tandem addition pathway of Co(III)−carbene radicals has a
broad substrate scope and can be applied to various
combinations of α-diazocarbonyls and terminal acetylenes,
including challenging aliphatic and electron-deficient alkynes.
Furthermore, the metalloradical cyclization process can tolerate
various functionalities (e.g., −NR2, −CHO, and −OH).
Table 2. [Co(P1)]-Catalyzed Formation of Multisubstituted
Furans from Cyclization of Phenylacetylene with Various
Acceptor/Acceptor-Substituted Diazo Reagents
a
Initial efforts investigated the possibility of [Co(Por)]-
catalyzed furan formation from the reaction of phenylacetylene
(1a) with α-cyanodiazoacetate 2a (Table 1), which was
Table 1. Influence of the Diazo Reagent and Temperature on
the Formation of Furans versus Cyclopropenes via [Co(P1)]-
Catalyzed Cyclization with Phenylacetylene
a
a
Reactions were carried out under N2 with 1.0 equiv of diazo reagent
and 2.0 equiv of 1a in one-time fashion without slow addition of the
diazo reagent. Isolated yields.
b
the ester groups (entries 1 and 2), ketones could also effectively
participate in the catalytic furan formation process, as
demonstrated by diazoketone 2c, which produced tosyl-
substitued furan 3ac in 87% yield (entry 3). More interestingly,
α-ketodiazoacetates 2d and 2e selectively cyclized with 1a to
form furyl esters 3ad15c and 3ae, respectively (entries 4 and 5),
indicating preferential involvement of the ketone carbonyl over
the ester carbonyl. A similar preference for aldehyde carbonyls
over ester carbonyls was also observed. For example, when
formyldiazoacetate 2f was employed, 2,4-disubstituted furan 3af
was isolated as the sole furan product in 80% yield through
regioselective cyclization involving the aldehyde group (entry 6).
After the successful cyclization of 1a with diazo reagents to
give furans, we investigated the alkyne scope of the [Co(P1)]-
catalyzed radical cyclization in combination with different A/A-
substituted diazo reagents.16 The Co(II)-based metalloradical
system was not only effective with acetylenes having different
electronic properties but also tolerated many functional groups
that would lead to side reactions in ionic processes (Table 3). For
example, 4-(N,N-dimethylamino)phenylacetylene was fruitfully
converted to 2,3,5-trisubstituted and 2,4-disubstituted furans
through cyclization with different diazocarbonyls [entries 1−4;
for the single-crystal structure of 3bh, see the Supporting
Information (SI)] without affecting the basic, electron-rich
NMe2 group. Arylacetylenes containing halogen and unpro-
tected hydroxyl groups were also regioselectively cyclized by
[Co(P1)] to form furans in good yields (entries 5 and 6). The
functional group tolerance of the Co(II)-based radical process
was further demonstrated by reactions of substrates containing
unprotected aldehyde groups. For example, cyclizations of 2-
formylphenylacetylene with various diazo reagents, including 2f,
resulted in the formation of aldehyde-containing furans in up to
96% yield (entries 7−10). Moreover, the Co(II)-catalyzed
system could be extended to heteroaromatic and vinyl acetylenes,
which formed the corresponding 2,3,5-trisubstituted furans in
high yields (entries 11−14).
a
Reactions were carried out under N2 with 1.0 equiv of diazo reagent
and 2.0 equiv of 1a in one-time fashion without slow addition of the
diazo reagent. Isolated yields.
b
previously shown to undergo cyclopropenation.11 Using the
Co(II) complex of the D2h-symmetric amidoporphyrin 3,5-
DitBu-IbuPhyrin, [Co(P1)], as the catalyst,12 we observed a
significant dependence of the product distribution on the
reaction temperature. While the reaction at room temperature
produced cyclopropene 4aa predominantly, 2,3,5-trisubstituted
furan 3aa was detected as the minor product and isolated in 31%
yield (entry 1). To our delight, increasing the reaction
temperature to 80 °C afforded 3aa as the only regioisomer in
68% yield (entry 2). 2-Diazomalonates have commonly been
used to prepare cyclopropene derivatives through Rh2-catalyzed
cyclopropenation of 1a.4b,13 Interestingly, although the reaction
was slow at room temperature, 2-diazomalonate 2b underwent
effective [Co(P1)]-catalyzed cyclization with 1a at 80 °C to form
2,3,5-trisubstituted furan 3ab regioselectively in excellent yield
with almost no cyclopropenation product (entries 3 and 4).14
Further experiments showed that in the presence of [Co(P1)],
2a/2b, and 1a at 80 °C, the previously prepared cyclopropene
4aa/4ab did not undergo effective isomerization to give the
corresponding furan 3, indicating that the furan is formed directly
from cyclization of 1a with the diazo reagent rather than from
ring expansion of the initially formed cyclopropene.15 These
results clearly showed that the Co(II)-catalyzed cyclopropena-
tion pathway can be completely switched to furan formation
through judicious use of the diazo reagent at a higher reaction
temperature (Scheme 1).
In addition to conjugated acetylenes, the Co(II)-based
metalloradical cyclization is applicable to alkyl- and silylacety-
lenes (Table 3). For example, 1-octyne was effectively cyclized
with different A/A-substituted diazo reagents to form the
corresponding furans in good yields (entries 15 and 16).
Under similar conditions, trimethylsilylacetylene was success-
fully cyclized to produce furans bearing a −TMS group (entries
[Co(P1)] was shown to be effective in catalyzing the
cyclization of 1a with various A/A-substituted diazo reagents at
80 °C, producing 2,3,5-trisubstituted furans with complete
regioselectivity (Table 2). In addition to diazoacetates 2a and 2b,
which formed the corresponding furans through cyclization with
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Table 3. Synthesis of Multisubstituted Furans by [Co(P1)]-
Scheme 2. Direct Construction of 6H-Furo[2,3-b]pyran-6-
one Structures from Metalloradical Biscyclization of Alkynes
with 5-Diazo-Meldrum’s Acid Catalyzed by [Co(P1)]
Catalyzed Metalloradical Cyclizations of Different
a b
,
Combinations of Alkynes and Diazo Reagents
product 5.18 The nature of 5a was further established by X-ray
structure analysis of its derivative 6a (see the SI), which was
readily obtained from 5a via hydrolysis and isomerization
(Scheme 2).
Because of their great potential as organic electronic materials,
oligofurans have attracted increasing interest.2 While α-
oligofurans have recently been prepared from presynthesized
furan monomers through coupling reactions,2d their direct
synthesis from acyclic precursors, an approach that would be
highly attractive, has remained largely underdeveloped. As an
application of the Co(II)-based metalloradical cyclization, we
developed an iterative radical cyclization process to construct
functionalized α-oligofurans with different numbers of furan
units (Scheme 3). The key to its success is the design and
a
Reactions were carried out under N2 with 1.0 equiv of diazo reagent
and 2.0 equiv of acetylene in one-time fashion without slow addition of
b
c
the diazo reagent. Isolated yields are shown. The structure was
determined by anomalous-dispersion effects in X-ray diffraction
measurements on the crystal. At 100 °C for 24 h.
d
17−19), which could potentially be deprotected to generate 2,3-
substituted furans or replaced with other groups to form new
furan derivatives. More significantly, ethynyl ester and ketone
derivatives underwent the Co(II)-catalyzed furan cyclization in
good to excellent yields (entries 20−22). This represents the first
catalytic system that cyclizes electron-deficient alkynes with both
cyclic and acyclic diazo reagents to form furan derivatives.
With appropriate diazo reagents, the Co(II)-based metal-
loradical cyclization can be carried out in tandem with 2 equiv of
alkyne, leading to the construction of O-biheterocycles with
interesting structures. For example, with 5-diazo-Meldrum’s acid
2j, a cyclic α-diazoacetate that has not been previously shown to
cyclize with acetylenes to form heterocycles, 2 equiv of
arylacetylenes 1a and 1l were successfully cyclized by [Co(P1)]
to form bicyclic 6H-furo[2,3-b]pyran-6-ones 5a and 5l,
respectively (Scheme 2). The formation of O-biheterocycles 5
can be properly rationalized on the basis of the proposed
metalloradical cyclization mechanism (Scheme 1, cycle II). As
detailed in Scheme 2, the tertiary radical intermediate C
generated from the initial sequence of consecutive radical
additions (A → B → C) undergoes radical fragmentation via
consecutive radical scission of two β-C−O bonds (C → D → E)
to give the key acyl radical intermediate E.17 The release of
acetone, which was confirmed experimentally (see the SI),
presumably drives the radical β-scission of the two C−O bonds
rather than the Co−C bond in C. Further radical addition−
substitution of E with another alkyne molecule affords spiro
compound G, which rearranges to the thermally more stable
Scheme 3. Construction of Functionalized α-Oligofurans via
Iterative Metalloradical Cyclization of 7
a
a
Conditions: (a) 7, [Co(P1)] (5 mol %), 1,2-C6H4Cl2, 80 °C, 48 h;
(b) K2CO3, MeOH, 15 min.
synthesis of bifunctional ketodiazoacetate 7 bearing a TMS-
protected internal alkyne unit, which was effectively cyclized with
1a by [Co(P1)] to afford furan monomer F1 without affecting
the internal triple bond. Monomer F1 was readily desilylated
with K2CO3 to provide F1H, whose terminal alkyne could be
cyclized again with 7 to afford α-furan dimer F2. Iteration of the
desilylation and cyclization steps should afford functionalized α-
oligofurans with specific numbers of repeating units and well-
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Figure 1. Optical spectra of α-oligofurans F1−4 in THF.
In summary, we have developed a new Co(II) metalloradical-
catalyzed system for the regioselective synthesis of furans by
cyclization of alkynes with diazocarbonyls. This metalloradical
cyclization has a wide substrate scope and an exceptionally high
degree of functional group tolerance, permitting effective access
to multisubstituted furans with diverse functionalities, and it was
successfully applied to the construction of O-biheterocycles and
α-oligofurans through double and iterative radical cyclization
processes, respectively. This first demonstration of Co(II)-based
metalloradical catalysis for five-membered heterocyclization
involving tandem radical addition to CC and CO bonds
may stimulate the development of new catalytic radical
cyclization processes for selective syntheses of more diverse
carbo- and heterocycles.
ASSOCIATED CONTENT
* Supporting Information
Experimental details and analytical data. This material is available
free of charge via the Internet at http://pubs.acs.org.
■
S
(10) (a) Lu, H. J.; Dzik, W. I.; Xu, X.; Wojtas, L.; de Bruin, B.; Zhang, X.
P. J. Am. Chem. Soc. 2011, 133, 8518. (b) Dzik, W. I.; Xu, X.; Zhang, X.
P.; Reek, J. N. H.; de Bruin, B. J. Am. Chem. Soc. 2010, 132, 10891.
(11) Cui, X.; Xu, X.; Lu, H. J.; Zhu, S. F.; Wojtas, L.; Zhang, X. P. J. Am.
Chem. Soc. 2011, 133, 3304.
AUTHOR INFORMATION
Corresponding Author
xpzhang@usf.edu
Notes
■
(12) Ruppel, J. V.; Jones, J. E.; Huff, C. A.; Kamble, R. M.; Chen, Y.;
Zhang, X. P. Org. Lett. 2008, 10, 1995.
The authors declare no competing financial interest.
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2005, 127, 3714. (b) Liao, L. A.; Zhang, F.; Yan, N.; Golen, J. A.; Fox, J.
M. Tetrahedron 2004, 60, 1803.
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reactions under the same conditions and gave poor product yields.
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ACKNOWLEDGMENTS
We are grateful for financial support by NSF (CHE-1152767)
and NIH (R01-GM098777).
■
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