Manganese dioxide-mediated oxidative coupling of 1,3-dicarbonyl compounds with α,β-unsaturated ketones: Direct access to 3,4-dicarbonyl substituted furans

Article abstract of DOI:10.1002/adsc.201400097

An efficient manganese dioxide-mediated and highly selective oxidative C-H/C-H functionalization of 1,3-dicarbonyl compounds with α,β- unsaturated ketones for the construction of tetrasubstituted furans in one step has been demonstrated. This catalytic system converts two C-H bonds and C￡ bonds to C￡C and C-O bonds. This reaction provides a facile and regio-defined method for the synthesis of 3,4-dicarbonyl substituted furans.

Full text of DOI:10.1002/adsc.201400097

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DOI: 10.1002/adsc.201400097
Manganese Dioxide-Mediated Oxidative Coupling of
1,3-Dicarbonyl Compounds with a,b-Unsaturated Ketones:
Direct Access to 3,4-Dicarbonyl Substituted Furans
Yuanyuan Yue,+a Yuanli Zhang,+a Weiwei Song,a Xin Zhang,a Jianming Liu,a,*
and Kelei Zhuoa,*
a
Key Laboratory of Green Chemical Media and Reactions, Ministry of Education, School of Chemistry and Chemical
Engineering, Collaborative Innovation Center of Henan Province for Green Manufacturing of Fine Chemicals, Henan
Normal University, Xinxiang, Henan 453007, Peopleꢀs Republic of China
Fax : (+86)-373-332-6336; e-mail: jmliu@htu.cn or klzhuo@263.net
+
Both authors contributed equally to this work
Received: January 25, 2014; Revised: May 15, 2014; Published online: July 31, 2014
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201400097.
À
Abstract: An efficient manganese dioxide-mediated
velop an efficient C H addition reaction to attack the
a-position of a,b-unsaturated carbonyl compounds.
Substituted furans and their derivatives are impor-
tant commodity chemicals, valuable synthetic building
blocks for agrochemicals, and active pharmaceutical
ingredients.[8] In spite of the various methods avail-
able for constructing these furan building blocks, the
direct regio-defined synthesis of polysubstituted
furans from basic chemical materials has always at-
tracted the attention of chemists. Many efforts have
been made by several research groups to develop ele-
gant transition metal-catalyzed processes to form
furan derivatives,[9–14] but the synthesis of 3,4-dicar-
bonyl substituted furans is rarely reported. Consider-
ing sustainability and environmental criteria, commer-
cially available and environmentally benign manga-
nese complexes are attractive because manganese is
À
À
and highly selective oxidative C H/C H functional-
ization of 1,3-dicarbonyl compounds with a,b-unsa-
turated ketones for the construction of tetrasubsti-
tuted furans in one step has been demonstrated.
À
This catalytic system converts two C H bonds and
C=O bonds to C=C and C O bonds. This reaction
À
provides a facile and regio-defined method for the
synthesis of 3,4-dicarbonyl substituted furans.
Keywords: 1,3-dicarbonyl compounds; 3,4-dicarbon-
yl substituted furans; manganese dioxide; oxidative
coupling; a,b-unsaturated carbonyl compounds
Direct functionalization of a,b-unsaturated carbonyl one of the most abundant elements in nature.[15] As
compounds through conjugate addition initiated we have known, manganese salts have been used for
domino reactions has emerged as a powerful synthetic many transformations, however, application of the
strategy with widespread applications in the synthesis easily available MnO2 as a sole oxidant to directly
of pharmaceuticals and natural products.[1–3] Therefore mediate oxidative coupling reactions has rarely been
it is still a great challenge to develop an efficient and reported. Herein, we have successfully demonstrated
highly selective conjugate addition of a,b-unsaturated an oxidative coupling of 1,3-dicarbonyl compounds
carbonyl compounds. Such reported examples were with a,b-unsaturated ketones mediated by MnO2,
often limited to activation of the b-position of a,b-un- which led to the efficient construction of polysubsti-
saturated carbonyl compounds.[4–6] However, to the tuted furans in one step. This reaction eliminates the
best of our knowledge, there have been only few ex- need for cumbersome preactivation and functional
amples, up to now, in which the radical is able to group manipulation, thereby minimizing the number
attach to the a-position of the a,b-unsaturated car- of synthetic steps and chemical waste (Scheme 1).
bonyl compounds facilitated by manganese com-
It is well-known that dihydrofuran can be generated
plexes.[7] Thus, it is both synthetically and mechanisti- via a single-electron-transfer (SET) process mediated
cally important to investigate the effect of electron by manganese dioxide. The reactivity of the free radi-
density on this type of transformation in order to de- cal depends on the energy level of the singly occupied
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Yuanyuan Yue et al.
Table 1. Optimization of the reaction conditions.[a]
Scheme 1. Approaches to 3,4-dicarbonyl substituted furans.
molecular orbital (SOMO). An electrophilic radical
having low potential energy behaves as a stable radi-
cal. The 1,3-dicarbonyl radical is electrophilic, and
then attacks the a-position of a,b-unsaturated carbon-
yl compounds. This step would be favored to form
compound A because the a-position of a,b-unsaturat-
ed carbonyl compounds has a higher electron density
than the b-position and it induces a 1,3-dicarbonyl
carbon-centered radical attacking at the a-position to
generate a key intermediate. Then, the active species
is transformed into the 3,4-dicarbonylfurans by an ox-
idative dehydrogenation reaction (Scheme 2).
Initially, we wanted to develop a simple and effi-
cient oxidative coupling reaction of 1,3-dicarbonyl
compounds and the a,b-unsaturated carbonyl com-
pounds to construct tetrasubstituted furans mediated
by oxidative metal salts. Thus we chose chalcone 1a
and acetylacetone 2a as our model substrates in order
to optimize the reaction conditions. When the reac-
tion was performed in DMF at 808C under an N2 at-
mosphere mediated by Ag2CO3 in the presence of
Entry Oxidant
Solvent Temp. Yield
A
[8C]
[%][b]
1
2
Ag2CO3, 1.0
Cu(OAc)2, 1.0
DMF
80
0
U
CH3CN 90
HOAc 120
HOAc 120
HOAc 130
HOAc 120
HOAc 120
0
3
4
Mn
Mn
N
trace
trace
26
30
20
33
27
36
38
57
71
71
5
6
7
MnO2, 4.0
MnO2, 6.0
BaMnO4, 4.0
8
MnO2, 4.0/FeCl3·6H2O, 0.05 HOAc 120
9
MnO2, 4.0/ZnCl2,0.5
MnO2, 5.0/ZnCl2, 1.0
MnO2 (a), 4.0
MnO2 (a), 4.0/ZnCl2, 3.0
MnO2 (a), 6.0/ZnCl2, 6.0
MnO2 (a), 7.0/ZnCl2, 7.0
HOAc 130
HOAc 130
HOAc 130
HOAc 130
HOAc 130
HOAc 130
10
11[c]
12[c]
13[c]
14[c]
[a]
Reaction conditions: 1a (0.50 mmol), 2a (2.5 mmol), sol-
vent (3.0 mL), in air.
Yield of the isolated product.
[b]
[c]
MnO2 (a) is activated MnO2
KOAc in a sealed reaction vessel according to the ally, dihydrofuran derivatives were generated from
conditions reported by Lei et al.,[14a] the anticipated radical cyclizations of carbonyl compounds with vari-
tetrasubstituted furan was not observed (Table 1, ous alkenes in the presence of Mn
entry 1). We proposed that the radical of the 1,3-di- reaction, a significant amount of dihydrofuran was
carbonyl compound might not be generated by indeed observed when Mn(OAc)3 was utilized as the
Ag2CO3. We tried to utilize Cu(OAc)2 as the catalyst, oxidant, while only a trace amount of the final furan
the desired product was not detected either (Table 1 was observed (Table 1, entry 4), indicating that
entry 2). Furthermore, we began to apply other metal Mn(OAc)3 might be less effective for converting a di-
(OAc)3.[15b] In our
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
catalysts that prefer a one-electron redox process to hydrofuran to the final furan. Moreover, it also shows
the oxidative coupling (Table 1, entries 3–12). Gener- that dihydrofuran might be a reaction intermediate
Scheme 2. Direct synthesis of 3,4-dicarbonyl substituted furans.
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Manganese Dioxide-Mediated Oxidative Coupling
Table 2. Oxidative coupling of the 1,3-dicarbonyl com-
pounds and chalcones.[a,b]
Figure 1. Single crystal X-ray structure of polysubstituted
furan 3a.
for this transformation. Unfortunately, the catalytical-
ly activated manganese dioxide was not effective for
this reaction (for more details, see the Supporting In-
formation). Extensive screening revealed that activat-
ed MnO2 represented the most effective catalyst
(Table 1, entry 11). Encouraged by this initial finding,
we then optimized the reaction conditions by chang-
ing the activated MnO2 and Lewis acids. When the
amount of ZnCl2 was increased from 4.0 equiv. to
6.0 equiv., the yield of 3a also increased from 38% to
71% (Table 1, entries 12 and 13). When the amount
of ZnCl2 was 7.0 equiv., the yield of the target prod-
uct did not change markedly (Table 1, entry 14). From
the above results, the optimum conditions were af-
firmed as follows: chalcone 1a (1.0 equiv.), acetylace-
tone 2a (5.0 equiv.), MnO2 (a) (6.0 equiv.), and ZnCl2
(6.0 equiv.) in HOAc at 1308C for 24 h. As a Lewis
acid, ZnCl2 is more likely to coordinate with the 1,3-
dicarbonyl compound to stabilize the C-radical and
increase the activity of the C-radical to attack to the
chalcone. The structure of 3a was unambiguously con-
firmed by asingle-crystal X-ray diffraction analysis
(Figure 1).[16]
[a]
Reaction conditions: 1 (0.50 mmol), 2 (2.5 mmol), MnO2
(a) (6.0 equiv.), ZnCl2 (6.0 equiv.), HOAc (3.0 mL), in
air.
[b]
Yield of the isolated product.
With the optimized conditions in hand, various a,b-
unsaturated carbonyl compounds 1 were reacted with
acetylacetone 2a to prepare the corresponding poly- fortunately, the direct oxidative coupling of methyl
substituted furans 3 (Table 2). a,b-Unsaturated car- cinnamate and coumarin with acetylacetone did not
bonyl compounds bearing either electron-donating or give the desired product (3k, 3m). To our delight,
electron-withdrawing substituents reacted successfully benzylidene acetone worked well with 2a, and gave
with acetylacetone to give products (3a–3j) in 48– the desired product (3l) in moderate yield. Moreover,
84% yield. It was noteworthy that electronic effect other 1,3-dicarbonyl compounds were also found to
had little influence on this oxidative coupling. Un- be suitable reaction partners with a,b-unsaturated car-
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Table 3. Oxidative coupling of the b-keto esters and a,b-un-
saturated ketones.[a,b]
Scheme 3. Preliminary mechanistic study.
other b-keto esters with different substituents
(Table 3) were well tolerated (5b, 5c). In addition, we
discovered that the standard procedure for the syn-
thesis of polysubstituted furans produced the corre-
sponding products in moderate yields in the presence
of benzylideneacetone (5d–5g). Moreover, for a,b-un-
saturated carbonyl compounds bearing electron-with-
drawing substituents, the oxidative coupling afforded
the target products in a slightly higher yield under the
same conditions (5h–5j). Unfortunately, benzyl ace-
toacetate, acetoacetanilide and methyl (E)-3-(phenyli-
mino)butanoate were not suitable for the reaction be-
cause they were decomposed by MnO2 under the cur-
rent conditions (5k–5m).
The preliminary studies on the mechanism were
carried out subsequently. The desired furan product
3a was not obtained when 6.0 equiv. of 2,2,6,6-tetra-
methylpiperidine-N-oxyl, a radical inhibitor, were em-
ployed in the system, suggesting that a radical initia-
tion pathway may be involved in the reaction
(Scheme 3). In order to further investigate the regio-
selectivity, the structures of S1 and S2 from the 1,3-di-
carbonyl radical attacking the a- and b-positions of
chalcone, respectively, were modeled and optimized
at the same level (Figure 2). The calculated results
demonstrated that the energy of S1 was 12.28 kcal
molÀ1 lower than that of S2, indicating that the benzyl
radical S1 was more stable than S2. As a result, the
carbon-centered radical compound I was induced to
attack the a-position to generate benzyl radical S1.
On the basis of the results described above,[7b,11g] a ten-
tative reaction mechanism for the direct oxidative
coupling mediated by MnO2 (a) was postulated as
shown in Figure 3. In the presence of activated MnO2
[a]
Reaction conditions: 1 (0.50 mmol), 2 (2.5 mmol), MnO2
(a) (6.0 equiv.), ZnCl2 (6.0 equiv.), HOAc (3.0 mL), in
air.
[b]
Yield of the isolated product.
bonyl compounds. Various 1,3-dicarbonyl compounds, and ZnCl2, 1,3-dicarbonyl compounds would produce
including cyclohexane-1,3-dione, 5,5-dimethylcyclo- the carbon-centered radical compound I which is also
hexane-1,3-dione, and 5-phenylcyclohexane-1,3-dione
were effective substrates and gave the target products
(3n–3s) in yields of 15–86%. Overall, no adjustments
to the reaction conditions are necessary to afford
good yields of specially substituent furans from the
different 1,3-dicarbonyl compounds and chalcones.
With the successful demonstration of this new strat-
egy for the construction of diverse furan derivatives,
we turned our attention to more functionalized b-
keto esters and a,b-unsaturated ketones. Delightfully,
when we employed ethyl acetoacetate 4a as a sub-
strate, the oxidative coupling with chalcone afforded
the desired product (5a) in 60% yield. Furthermore, 31+G(d) level in HOAc solvent.
Figure 2. Optimized structures S1 and S2 at the B3LYP/6-
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Manganese Dioxide-Mediated Oxidative Coupling
Figure 3. Proposed reaction mechanism.
graphed by TLC on silica gel using hexane/ethyl acetate as
eluent to give the desired 3,4-dicarbonyl substituted furans.
stabilized by ZnCl2. Since the a-position of a,b-unsa-
turated ketones has a higher electron density than the
b position, it induces the carbon-centered radical
compound I to attack the a-position to generate
benzyl radical II. The benzyl radical II then converts
to the intermediate III by a further single oxidation.
The intermediate III undergoes intramolecular cycli-
zation to form the dihydrofuran product IV and af-
fords the desired product VI after deprotonation and
oxidation.
In summary, we have successfully developed an
MnO2-mediated regioselective oxidative coupling of
a,b-unsaturated ketones to prepare the 3,4-dicarbonyl
substituted furans. A variety of 1,3-dicarbonyl com-
pounds and a,b-unsaturated carbonyl compounds
were used to afford the corresponding polysubstituted
furans in moderate to excellent yields. This procedure
provides an atom-economic, environmentally benign,
and practical approach to complement the oxidative
coupling reaction of 1,3-dicarbonyl compounds for
the construction of carbon-carbon and carbon-heter-
oatom bonds with complete regioselectivity. Further
investigations on synthetic applications of this reac-
tion are now in progress.
Acknowledgements
We are grateful for financial support from National Natural
Science Foundation of China (21103044, 21173070,
21205029), Ph.D. Programs Foundation of Ministry of Edu-
cation of China (No. 20114104120003, 20124104120004),
Plan for Scientific Innovation Talent of Henan Province (No.
124200510014), Project funded by China Postdoctoral Sci-
ence Foundation, and the Program for Innovative Research
Team in University of Henan Province (2012IRTSTHN006).
Thanks are due to professor Mingsheng Tang to calculating
the SOMO/HOMO energy matching studies.
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