Metal-Free Domino Oligocyclization Reactions of Enynals and Enynones with Molecular Oxygen

Article abstract of DOI:10.1021/acs.orglett.0c04272

A novel metal-free direct addition of molecular oxygen to the C-C triple bond toward benzannulated oxygen-bridged seven-membered ring systems and aza[3.1.0]bicycle skeletons under 3O2 atmosphere has been described. The reaction proceeds through at least three intramolecular C-O and C-C bond forming steps via green, simple, and unprecedented domino radical processes with high selectivity and good yields.

Full text of DOI:10.1021/acs.orglett.0c04272

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Letter
Metal-Free Domino Oligocyclization Reactions of Enynals and
Enynones with Molecular Oxygen
Alireza Abbasi Kejani, Hormoz Khosravi, Frank Rominger, Saeed Balalaie,* and Bernhard Breit*
Cite This: Org. Lett. 2021, 23, 1291−1295
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ABSTRACT: A novel metal-free direct addition of molecular
oxygen to the C−C triple bond toward benzannulated oxygen-
bridged seven-membered ring systems and aza[3.1.0]bicycle
3
skeletons under O atmosphere has been described. The reaction
2
proceeds through at least three intramolecular C−O and C−C bond
forming steps via green, simple, and unprecedented domino radical
processes with high selectivity and good yields.
pplications of enynes and related π-precursors have
attracted much attention in the generation of cyclic
Scheme 1. Formation of C−O Double Bond with Different
Oxygen Sources via Cycloisomerization of Enynals/
Enynones and Enynes in Various Reports
A
bioactive and complex organic structures as a particularly
versatile building block for developing simple, practical, and
1
green synthetic methods in modern organic chemistry.
Among these methods, considerable attention has been
dedicated to the reaction of enynes with oxygen for regio-
and chemoselective synthesis of complex structures as a simple
2
and environmentally benign method. Despite all the
remarkable progress in the transition-metal-catalyzed oxygen
utilization, an efficient and metal-free addition of molecular
3
oxygen remains a great challenge. To the best of our
knowledge, molecular oxygen utilization on enyne scaffolds has
4
been addressed in only a few reports. Li et al. developed a
copper-catalyzed cyclization of 1,6-enynes incorporating two
5
oxygen atoms from O and H O (Scheme 1a). In another
2
2
work, Ma et al. presented aerobic coupling−ketooxygenation
reaction of enynes with alkynes in the presence of a Pd/Cu
6
catalyst system under atmospheric pressure of O2.
Molecular oxygen is an ideal and readily available green
7
oxidant that suffers from weak reactivity. Therefore, previous
reports have revealed applying various oxidizing reagents and
oxygen sources, such as tert-butyl hydroperoxide (TBHP),
8
H O, and acetate ion. Oh et al. disclosed that domino [3 + 2]
2
cycloaddition of o-(1,6-enynyl)benzaldehydes could be per-
formed by using Rh(I) as a catalyst and H O as an oxidant for
2
the synthesis of benzannulated oxygen-bridged seven-mem-
9
Received: December 28, 2020
Published: February 4, 2021
bered ring systems. The same cycloisomerization through two
10a
10b
different catalytic systems (iodine and gold ) enabled the
formation of the same product that has been reported by Liang
(Scheme 1b). Also, cyclopropanation of 1,6-enynes using
Pd(OAc) as a catalyst and acetate ion as an oxygen source has
2
11
been demonstrated by Sanford et al. Wang et al. developed a
©
2021 American Chemical Society
https://dx.doi.org/10.1021/acs.orglett.0c04272
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radical cascade cyclization reaction of 1,6-enynes that achieved
the same aza[3.1.0]bicycles using TBHP as an oxidant and
oxygen source (Scheme 1c). After careful deliberation, we
under a nitrogen atmosphere (Table 1, entry 3). This result
revealed that the reaction yield is dependent on oxygen
concentration. Surprisingly, the control reaction confirmed that
K S O does not participate in the reaction yield (Table 1,
entry 4). Screening of various solvents showed that DCE is the
best solvent provided the desired product in 70% yield (Table
1, entries 4−8). Reducing the reaction temperature from 80 to
60 °C leads to the desired product in 60% yield (Table 1, entry
9), and increasing the reaction temperature from 80 to 100 °C
toward the product in 69% yield (Table 1, entry 10).
12
noticed that all previous examples involved metal-catalyzed
2
2
8
13
condition or oxidizing reagents. Herein, we present an
unprecedented direct addition of molecular oxygen to enynals/
enynones scaffolds through a novel metal-free radical domino
reaction to synthesize benzannulated oxygen-bridged seven-
membered ring systems and aza[3.1.0]bicycle structures
(
Scheme 1d).
The benzannulated oxygen-bridged seven-membered ring
With optimized conditions in hand, the substrate scope was
examined (Scheme 2). The reaction was well tolerated by
systems and aza[3.1.0]bicycle scaffolds are prevalent in natural
1
4
15
16
products such as Bruguierol-C, Brussonol, Amitifadine,
17
and Indolizomycin (Figure 1); consequently, the develop-
ment of new green strategies that enable the rapid construction
of such structures are in high demand.
Scheme 2. Scope of Benzannulated Oxygen-Bridged Seven-
Membered Ring Systems Using Domino Radical Cyclization
Figure 1. Structure of bioactive benzannulated oxygen-bridged seven-
membered ring systems and aza[3.1.0]bicycle skeletons.
The initial reaction was carried out with enynal 1a in the
presence of potassium persulfate (K S O ) (2.0 equiv) as an
2
2
8
inexpensive and convenient oxidant in DCE at 80 °C under
aerobic conditions. The desired product 2a was obtained in
3
(
2% yield and with perfect regio- and diastereoselectivity
Table 1, entry 1). Interestingly, by using an oxygen
atmosphere, the reaction yields dramatically increased to
70% (Table 1, entry 2). While the reaction could not occur
a
Table 1. Optimization of the Reaction Conditions
b
c
entry
oxidant
atm.
solvent
DCE
DCE
DCE
DCE
temp. (°C)
yield (%)
diverse functional groups and provided various benzannulated
oxygen-bridged seven-membered ring systems with good to
high yields (Scheme 2). The effect of different linker atoms
such as nitrogen, carbon, and oxygen (1a−o) undergoing the
cycloisomerization reaction was first investigated. The reaction
of nitrogen linker atom-substituted with sulfonyl and
carbamate groups (1a−d) proceeded in 69−81% yields. In
addition, carbon and oxygen linkers (2e and 2f, respectively)
proceeded in good yield. Using higher homologues in
substrates (1g−h), a different series of products (2g−h)
1
2
3
4
5
6
7
8
9
K S O
Air
O2
N2
O2
O2
O2
O2
O2
O2
O2
80
80
80
80
80
80
80
80
60
100
32
70
NR
70
68
38
35
NR
60
69
2
2
8
8
8
K S O
2
2
d
K S O
2
2
−
−
−
−
−
−
−
1,4-dioxane
EtOH
H O
2
DMF
DCE
DCE
1
0
1
were obtained. Different R substituents on the benzene ring
a
were found to be tolerated. Thus, both electron-donating
groups (−OMe, −OBn), as well as the electron-withdrawing
group (−Cl), provided the cyclization products in 78−85%
Reaction conditions: 1a (71.0 mg, 0.2 mmol), DCE (1.0 mL), 1 atm
b
c
d
of O , 80 °C and 14 h. Oxidant (2.0 equiv). Isolated yields, NR =
No reaction.
2
1
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yields (2h−k, 2o). Also, precursors with differently substituted
alkene functions (1k−n) reacted to the corresponding
products in high yields (59−80%) and perfect diastereose-
lectivities. The configuration of product 2m was confirmed by
X-ray crystallographic analysis.
Searching for a determining factor in the mechanism and to
better evaluating the generality of the reaction, we prepared the
proceeds via radical intermediates (Scheme 4, eq 1). Next, the
reaction of 1a was carried out in the presence of O-labeled
1
8
water (under standard conditions), but did not furnish any
1
8
obvious O-labeled products, which ruled out water as an
oxygen source (Scheme 4, eq 2). The reaction was tested in
the presence of di-tert-butyl peroxide (DTBP) as a source of
oxyradical, but the conversion has not happened (Scheme 4, eq
3). For investigation of the carbonyl group’s role, substrate 3g
has been designed and tested in the optimal conditions, but it
did not afford the desired product 4g (Scheme 4, eq 4), which
proved a vital role of the carbonyl group for the reaction to
proceed. Finally, we analyzed the reaction in the absence of
light, which showed that light has no effect on reaction
progress.
enynone 3a and its reaction with O under standard conditions
2
was investigated. Surprisingly, the reaction of enynone under
standard conditions led to a cyclopropanation process with the
formation of the bicyclic product 4a in 68% yield (Scheme 3).
Scheme 3. Scope of Aza[3.1.0]bicycles Skeletons
As shown in a plausible mechanism (Scheme 5), initial
cation radical INT-I was formed by one-electron oxidation
Scheme 5. Plausible Reaction Mechanism
Then, the scope of substrates 3a−f was tested in the optimal
conditions to achieve a library of diverse aza[3.1.0]bicycles
skeletons 4a−f (Scheme 3). The desired reaction was
performed in the presence of different linkers in 55−71%
yields. But when dimethylmalonate was used as a linker, the
trace amount of the 4f product was separated. Obtaining the
mentioned products would facilitate the achievement of a
plausible reaction mechanism.
To elucidate the reaction mechanism, we performed
preliminary control experiments, as shown in Scheme 4. On
exposure of 1a to 1.0 equiv of TEMPO as a radical scavenger
under the standard conditions, only trace amounts of 2a were
formed. This result suggested that the reaction presumably
with molecular oxygen, which followed subsequent 5-exo-dig
cyclization to generate intermediate INT-II. Intramolecular [2
+
1] cyclization of the vinyl radical provides cation radical
Scheme 4. Control Experiments
INT-III. The isobenzofuran cation radical INT-III is trapped
by triplet dioxygen to obtain the intermediate INT-IV.
Next, the intermediate INT-V is obtained via cleavage of the
O−O bond by the reaction with another radical cation INT-
IV. This intermediate plays a crucial role in the mechanistic
pathways to the two different products depending on whether
R is a hydrogen or a phenyl substituent. In the case of R =
phenyl, the product 4b is obtained via electron-accepting from
another substrate molecule, and consequently, this electron
transfer regenerates another INT-I. As for path II, the 6-exo-tet
cyclization of INT-V affords intermediate INT-VI.
Finally, the desired product 2b is formed through a
subsequent 5-endo-trig cyclization and one-electron reduction.
This mechanism is proposed based on radical cation chain
18
oxidation reactions.
To reveal the detailed mechanism, we conducted the density
functional theory (DFT) calculation. Precursors 1b and 3b
were chosen as the substrates model for aldehydes and
ketones. We expanded two possible mechanisms, as shown in
Figure 2 and Figure S1. The first mechanism starts with direct
1
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Figure 2. (a) Potential energy surfaces of the radical cation chain oxidation mechanism (for mechanistic details, see the Supporting Information).
−
1
All energies are in kcal mol . (b) HOMO orbitals of starting materials 1b, 3b, and related crucial intermediates. All energies are in eV.
dioxygen addition to the alkyne moiety, which the activation
barrier is 31.9 kcal/mol, and the overall energy barrier
bicyclic skeletons through a domino radical cyclization with
triplet oxygen. The carbonyl group could act as a vital group to
construct the desired compounds. The present method affords
a wide diversity of benzannulated oxygen-bridged seven-
membered ring systems and novel aza[3.1.0]bicycles skeletons
in high chemo- and diastereoselectivity and in good yields.
This domino reaction could facilitate the generation of
complex skeletons by forming several C−O and C−C bonds
in one step.
(through TS2-a) is 32.5 kcal/mol. In the second proposed
mechanism, as illustrated in Scheme 5 and Figure 2, the
oxidized 1b (INT-I) undergoes 5-exo-dig cyclization through
TS-I (4.9 kcal/mol) to cation radical INT-II, which is
immediately cyclized through [2 + 1] cycloaddition reaction
to yield INT-III barrierlessly (Figure S2). The activation
3
barrier of O addition to oxidized 1b (INT-III) is 10.6 kcal/
2
mol (through TS-II). In other words, the energy barrier of
molecular oxygen addition was reduced to 10.6 kcal/mol by
starting materials oxidation. The overall energy barrier of this
ASSOCIATED CONTENT
sı Supporting Information
■
*
mechanism is 28.1 kcal/mol through TS-III, which releases
1
O via a bimolecular transition state to generate INT-V. Then,
2
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c04272.
a stepwise [2 + 1] cycloaddition reaction of cyclopropyl with
aldehyde segment leads to INT-VII. On the basis of Marcus
theory, INT-V and INT-VII can be reduced to 2b and 4b′ (is
the same as 4b but R = H) by 1b (Table 2, and Figure 2b).
General experimental procedures, computational, char-
1
13
acterization details, H NMR, C NMR, and HR-MS
spectra of all compounds (PDF)
Table 2. Internal Reorganization, Activation, and Reaction
Free Energies for the Electron Transfer Reactions
Accession Codes
a
CCDC 2042834 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Authors
Saeed Balalaie − Peptide Chemistry Research Center, K. N.
Toosi University of Technology, Tehran 15875-4416, Iran;
Medical Biology Research Center, Kermanshah University of
Medical Sciences, Kermanshah 6715847141, Iran;
orcid.org/0000-0002-5764-0442; Email: balalaie@
kntu.ac.ir
Bernhard Breit − Institut für Organische Chemie, Albert-
Ludwigs-Universität Freiburg, D-79104 Freiburg im
Breisgau, Germany; orcid.org/0000-0002-2514-3898;
Email: bernhard.breit@chemie.uni-freiburg.de
aAll energies are in kcal mol−1.
Products 2 are thermodynamic products in the reaction of
aldehyde precursors. In the case of ketone 3b, although the
energy barriers of electron transfer of 3b to related INT-V and
INT-VII are 3.5 and 1.6 kcal/mol, respectively (Table 2), it
seems that orbital energies are not appropriated for reduction
of INT-VII (Figure 2b) by 3b, and this reason probably
controlled the chemoselectivity of reaction.
Authors
In conclusion, we have established an environmentally
benign, novel, and facile method to synthesize benzannulated
oxygen-bridged seven-membered ring systems and aza[3.1.0]-
Alireza Abbasi Kejani − Peptide Chemistry Research Center,
K. N. Toosi University of Technology, Tehran 15875-4416,
Iran
1
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Letter
Hormoz Khosravi − Peptide Chemistry Research Center, K. N.
Toosi University of Technology, Tehran 15875-4416, Iran;
orcid.org/0000-0002-6588-448X
Frank Rominger − Organisch-Chemisches Institut der
Universität Heidelberg, D-69120 Heidelberg, Germany
2017, 7, 7830−7834. (d) Liang, Y.-F.; Jiao, N. Acc. Chem. Res. 2017,
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Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c04272
(
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Notes
(
The authors declare no competing financial interest.
(
ACKNOWLEDGMENTS
■
(
We thank Alexander von Humboldt foundation for the Linkage
Research Group Program.
(
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