Iron(III)-Mediated Bicyclization of 1,2-Allenyl Aryl Ketones: Assembly of Indanone-Fused Polycyclic Scaffolds and Dibenzo[ a, e]pentalene Derivatives

Article abstract of DOI:10.1021/acs.orglett.9b02079

The rapid construction of three-dimensional fused carbocycles is a key challenge in synthetic chemistry. Herein, an unprecedented and practical tandem Nazarov/oxidative umpolung 4π-ring closure of readily available 1,2-allenyl aryl ketones mediated by iro

Full text of DOI:10.1021/acs.orglett.9b02079

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Iron(III)-Mediated Bicyclization of 1,2-Allenyl Aryl Ketones: Assembly
of Indanone-Fused Polycyclic Scaffolds and Dibenzo[a,e]pentalene
Derivatives
Maozhong Miao,* Mengchao Jin, Panpan Chen, Lei Wang, Shouzhi Zhang, and Hongjun Ren*
Department of Chemistry, Zhejiang Sci-Tech University, Hangzhou, Zhejiang 310018, P. R. China
S
* Supporting Information
ABSTRACT: The rapid construction of three-dimensional fused carbocycles
is a key challenge in synthetic chemistry. Herein, an unprecedented and
practical tandem Nazarov/oxidative umpolung 4π-ring closure of readily
available 1,2-allenyl aryl ketones mediated by iron(III) chloride has been
developed, furnishing a new family of indanone-fused molecular architectures
in moderate to excellent yields. The indanone-containing blocks can be
efficiently converted to unsymmetrical dibenzo[a,e]pentalenes. Significantly,
divergent synthetic applications have been achieved to provide densely
functionalized polycyclic arrays.
hree-dimensional carbocyclic scaffolds belong to intrigu-
the indanone core consisting of polycyclic compounds and
their analogues, the existing methods generally required time-
consuming multistep procedures, non-easily accessible pre-
cursors, or harsh conditions, leading to a limited range of
products.9 As a consequence, the development of novel
strategies for the expedient and concise synthesis of
indanone-fused cyclic entities from readily accessible starting
materials in a selective and benign fashion is of great interest.
Iron salts have received a considerable amount of attention
as promising alternatives over precious metal catalysts because
of their abundance, environmental friendliness, and long-term
expediency. Because they can adopt multiple oxidation states,
iron complexes are highly versatile catalysts for diverse organic
transformations.10 However, a synthetic strategy that is based
on reversing the usual reactivity by iron salt via a single-
electron transfer (SET) process remains a critical challenge.
On the other hand, the Nazarov cyclization has emerged as a
substantial synthetic utility for pentannulation. Major studies
have focused on expansions of alternative variants11 or
approaches to generate the requisite pentadienyl cation.12 In
this scenario, suitably functionalized allenes13 with unique
reactivity have proven to be effective substrates for the Nazarov
reaction. Unfortunately, the Nazarov reaction involving the use
of 1,2-allenyl aryl ketones has been studied less, because
competitive activation of an electron-poor allenic double bond
in the presence of suitable acid or transition metal catalysts
toward cycloisomerization to give polyfunctionalized furans
was usually preferred, and this approach was intensely studied
T
ing classes of organic compounds due to their diverse
biological and physical properties.1 In particular, indanone-
fused polycyclic frameworks and related derivatives are
privileged structural motifs that occur widely in a myriad of
bioactive and functional molecules. Figure 1 highlights some
Figure 1. Bioactive and material molecules bearing indanone-fused
units.
typical examples, including structurally intricate natural
products such as pterocaryquinone (I),2 nakiterpiosin (II),3
cyanosporasides A/B (V),4 and pallidol (VII);5 potent
medicines such as MMP-9 inhibitor (III) against human
prostate cancer6 and DDI (IV) as a binding agent toward DNA
bulges;7 and organic material molecules such as CPV-dye
(VI)8 for dye-sensitized solar cells. Despite the significance of
Received: June 17, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b02079
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Organic Letters
Letter
by Gevorgyan14 and co-workers. To avoid the furan products,
we15 presented a catalyst-free Nazarov cyclization of 1,2-allenyl
aryl ketones under thermal conditions to produce 2,3-
dihydroindanones. Notably, Plietker16 elegantly developed a
formal olefin hydroarylation of aryl allenyl ketones to afford 3-
arylidene-indan-1-ones upon using a cationic Fe(0) complex
[(Ph3P)2Fe(CO)(NO)]BF4 as a catalyst (Scheme 1, eq A).
a
Table 1. Optimization of the Reaction Conditions
b
b
additive
(equiv)
Int-a
(%)
2a
entry
catalyst (equiv)
solvent
(%)
1
2
3
4
5
6
7
8
9
FeCl3 (0.2)
FeCl3 (1)
FeCl3 (2)
FeBr3 (2)
AlCl3 (2)
InCl3 (2)
Fe(acac)3 (2)
Co(acac)3 (2)
Mn(OAc)3·2H2O
(2)
−
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
67
39
0
0
89
81
NR
NR
NR
19
50
87
60
0
0
NR
NR
NR
Scheme 1. Previous Work and Working Hypothesis
−
−
−
−
−
−
−
−
e
e
e
e
e
e
10
11
12
13
14
15
16
17
FeCl3 (2)
FeCl3 (2)
FeCl3 (2)
FeCl3 (2)
FeCl3 (0.05)
FeCl3 (0.05)
FeCl3 (0.05)
FeCl3 (0.05)
FeCl3 (2)
−
−
−
−
toluene
THF
0
NR
NR
NR
0
0
CM
CM
0
72
NR
NR
NR
51
71
CM
CM
75
e
e
e
e
MeOH
DMSO
DCM
DCM
DCM
DCM
DCM
DCM
e
e
TBHP (2)
DTBP (2)
BPO (2)
DDQ (2)
−
f
f
f
f
c
18
d
19
FeCl3 (2)
−
0
77
a
The reaction was carried out using 1a (0.1 mmol), metal catalysts,
Apparently, activation of the carbonyl functionality was greatly
favored over the double bond of allene during iron catalysis.
Along this line, we hypothesized that a classical Nazarov
cyclization occurred to afford Int-a through the carbonyl
activation by Lewis acid FeCl3. Then, Int-a might be easily
oxidized at position α with the promotion of excess FeCl3 to
give the corresponding radical Int-b via a SET process. The
1,3-allylic radical migration provided radical intermediate Int-
c, which could be further oxidized by iron(III) into key
pentadienyl cation Int-d, followed by the loss of a proton.
Finally, int-d underwent 4π-ring closure (Nazarov-type
cyclization) to furnish the desired products 2 (eq B). Herein,
we present the first example of a cascade Nazarov/oxidative
umpolung 4π-ring closure of 1,2-allenyl aryl ketones for
directly building polycyclic ring-fused indanone structures by
employing a cheap iron catalyst and its application for the
synthesis of highly functionalized dibenzo[a,e]pentalenes 4 via
reduction and 1,2-migration.
Initially, 1,2,4,4-tetraphenylbuta-2,3-dien-1-one (1a) was
subjected to 0.2 equiv of FeCl3 in DCM at 50 °C for 12 h,
and hydroarylation product Int-a was formed in a 67% yield
together with a new product, which was identified as the
proposed bicyclization compound 2a, in 19% yield (Table 1,
entry 1). Gratifyingly, increasing the amount of FeCl3
improved the yield of 2a significantly (entries 2 and 3), and
2a was obtained exclusively in 87% yield without observing any
furan byproduct when 2 equiv of FeCl3 was employed. Of the
catalysts that were screened, FeBr3 led to a diminished yield of
2a, other metal catalysts such as AlCl3 and InCl3 delivered Int-
a cleanly, and Fe(acac)3, Co(acac)3, and Mn(OAc)3·2H2O
failed to generate any product (entries 4−9, respectively).
Subsequently, the solvent effect was surveyed. Nonpolar
solvent toluene offered the desired 2a in a decreased yield
(entry 10), and polar solvents like THF, MeOH, and DMSO
were completely ineffective (entries 11−13, respectively). In
and additives (2 equiv), if necessary, in 1 mL of solvent at 50 °C in a
b
c
sealed tube for 12 h. Isolated yield. The reaction was conducted at
d
room temperature for 24 h. The reaction was conducted under a N2
e
f
atmosphere. No reaction. Complex mixture.
addition, this transformation could be switched in a catalytic
fashion. The reaction proceeded smoothly in the presence of
FeCl3 (5 mol %) with additional oxidants such as tert-butyl
peroxybenzoate (TBHP) or di-tert-butyl peroxide (DTBP) to
produce 2a efficiently, albeit in slightly lower yields (entry 14
or 15, respectively). Unfortunately, the use of dibenzoyl
peroxide (BPO) and 2,3-dicyano-5,6-dichlorobenzoquinone
(DDQ) led to a complex mixture (entries 16 and 17,
respectively). Finally, the reaction was performed at room
temperature or under a N2 atmosphere (entry 18 or 19,
respectively), slightly decreasing the observed yield of 2a.
Thus, we established the standard conditions as 1,2-allenyl aryl
ketone (1 equiv) and FeCl3 (2 equiv) in a solution of DCM
(0.1 M) at 50 °C to explore the substrate scope.
The scope of FeCl3-mediated bicyclization is summarized in
Scheme 2. The aromatic substituents tethered to the carbonyl
group were varied first. Various electron-donating and
electron-withdrawing groups were well tolerated on the
aromatic ring to afford the corresponding products 2a−2h in
47−94% yields. Among them, 7,9-diMeO-substituted com-
pound 2d was obtained in a rather modest yield. Additionally,
substrates containing thiophenyl or naphthyl functionality
could furnish the desired products 2i and 2j in 57% and 72%
yields, respectively. It should also be noted that m-Cl-C6H4-
and β-naphthyl-substituted precursors could be cyclized to give
2f and 2j with exclusive regioselectivity. The substituents
attaching at the α position of 1,2-allenyl aryl ketones were next
varied. A range of substituted aromatic rings (2k−2n and 2p)
and an alkyl group (2o) were proven to be compatible in the
present reaction to produce targeted compounds in high yields.
B
DOI: 10.1021/acs.orglett.9b02079
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 2. Substrate Scope of the FeCl3-Mediated
Bicyclization
Scheme 3. Control Experiments
a,b
be thought to form in the presence of excess FeCl3 and (2) in
this case, proton elimination favored over 4π-ring closure.
Thus, initial Fe(III)-catalyzed classical Nazarov cyclization to
provide Int-a, following a SET process to generate the
pentadienyl cation Int-d intermediate promoted by excess
FeCl3, and final 4π-electrocyclization, constitutes a significant
component of the mechanism of the reaction (see Scheme 1,
eq B).
Dibenzo[a,e]pentalene derivatives make up a class of planar
compounds possessing expanded π-conjugation systems, which
have attracted increasing amounts of interest with regard to
their distinct electrical and optical properties.17 Several
methods for the preparation of dibenzo[a,e]pentalenes have
been developed, including traditional group transformation
processes,18 Lewis acid-induced cyclizations,19 and recent
transition metal-catalyzed reactions.20 However, there are
relatively few reports about general approaches to the
unsymmetrical dibenzo[a,e]pentalene structures with accurate
assembly of functional groups. With bicyclization product 2 in
hand, we envisaged that the unsymmetrical dibenzo[a,e]-
pentalenes 4 might be formed via carbonyl reduction and acid-
promoted 1,2-migration processes. After some trials, the 5,10-
diphenylindeno[2,1-a]indene 4a was obtained in 75% yield
over two steps, including reduction by DIBAL-H in DCM at
−78 °C (96% yield) and 1,2-phenyl migration mediated by
TsOH·H2O in DCE at 80 °C (78% yield) (Scheme 4). The
scope of the reaction was then briefly checked. Substituents
such as 3-Cl (4b), 3-CF3 (4c), 3-Me (4e), 3-Br (4g), and 3-
OMe (4h) could be suitably arranged in a dibenzo[a,e]-
pentalene framework to afford the desired products in
moderate yields (50−72%). Notably, this strategy was useful
for the construction of dibenzo[a,e]pentalene with extended
fused-ring 4d and 4f in 66% and 80% yields, respectively.
To show the utility of compounds 2 and 4, we assessed the
fluorescent spectroscopic properties21 of some selected
compounds as Figures S1−S6 show. Moreover, three-dimen-
sional indanone-fused polycycles 2 are versatile synthetic
building blocks that exhibit a strong ability to access other 3D-
fused structures. Therefore, we carried out some representative
transformations (Scheme 5), which were all accomplished with
remarkable regio- and stereoselectivity. Compounds 2a, 2c-
OMe, and 2h-CF3 were transformed to O-heterocycle
a
The reaction was carried out using 1 (0.2 mmol) and FeCl3 (0.4
b
mmol) in 2 mL of DCM at 50 °C in a sealed tube for 12 h. Isolated
yield. A 5.580 g (15 mmol) portion of 1a was employed.
c
Finally, the substituent effect of diaryl groups binding to the
end of the allenic moiety was evaluated. In the case of
symmetrical diaryl substrates, the reaction proceeded smoothly
to give the expected products 2q−2u in reasonable yields.
Notably, when 1,2-allenyl aryl ketones with nonsymmetrical
aryl rings were involved, this bicyclization afforded 2v−2y in
high yields with well-defined regioselectivity.
To gain insights into the reaction, several control experi-
ments were performed. The addition of nitroxyl radical 2,2,6,6-
tetramethyl-1-piperidinyloxy (TEMPO) led to no reaction.
Another radical scavenger, 2,6-di-tert-butyl-4-methylphenol
(BHT), also inhibited the formation of 2a (only 47% NMR
yield), thus supporting the intervention of radicals in these
processes (Scheme 3, eq 1). Moreover, the experiment using
pure Int-a under standard conditions could cleanly deliver the
corresponding 2a in 93% NMR yield, while other oxidants
such as TBHP and DDQ led to the recovery of Int-a (eq 2),
suggesting that the classical Nazarov product Int-a is an
important intermediate and FeCl3 plays critical role in the
second step transformation. The functionalized indenone 2z
was obtained in 70% yield rather than the desired product
when substrate 1z was employed (eq 3). This result indicated
that (1) the key intermediate pentadienyl cation Int-d might
C
DOI: 10.1021/acs.orglett.9b02079
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Organic Letters
Letter
In summary, we have disclosed an efficient tandem
cyclization of 1,2-allenyl aryl ketones 1 involving a classical
Nazarov cyclization and a SET process of 4π-ring closure to
furnish a series of three-dimensional polycyclic compounds
containing indanone and indene moieties 2 in moderate to
good yields in the presence of FeCl3. The 3D products 2 can
be transformed to the unsymmetrically planar dibenzo[a,e]-
pentalenes 4 through carbonyl reduction and acid-mediated
cationotropic rearrangement. Both compounds 2 and 4 are
useful fluorophores, and their photophysical properties were
intensively investigated. Moreover, the obtained 2 can serve as
a versatile building block for the construction of several types
of new 3D polycycles 5−10 in high yields. Further
investigations of the synthetic application and examination of
the physical properties are currently ongoing.
Scheme 4. Conversion of 2 to Unsymmetrical
Dibenzo[a,e]pentalenes 4
a,b
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b02079.
a
Experimental procedures and copies of 1H and 13C
NMR spectra for all new compounds (PDF)
In step 1, the reaction was carried out using 2 (1 mmol) and DIBAL-
H (1.05 mmol, 1.05 equiv) in 5 mL of DCM at −78 °C under N2 for
1 h. In step 2, the reaction was carried out using 3 (0.3 mmol) and
TsOH·H2O (0.6 mmol, 2 equiv) in 3 mL of DCE at 80 °C under air
Accession Codes
b
c
for 12 h. Isolated yield. BF3·Et2O (0.6 mmol, 2 equiv) was used
instead of TsOH·H2O.
CCDC 1903886, 1903888, 1903898, 1903902, and 1903931
contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge via www.ccdc.ca-
m.ac.uk/data_request/cif, or by emailing data_request@ccdc.
cam.ac.uk, or by contacting The Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax:
+44 1223 336033.
indeno[1,2-c]isochromen-5(6aH)-one derivatives 5a−5c, re-
spectively, in 52−89% yields with exclusive regioselectivity
through a FeCl3-mediated Baeyer−Villiger oxidation. Likewise,
the N-heterocycle 6,6a-dihydro-5H-indeno[1,2-c]isoquinolin-
5-one 6 could be furnished in 63% yield via Beckmann
rearrangement. The C−C σ bond cleavage of 2a-oxime
catalyzed by CoCp*(CO)I2 (10 mol %) and AgSbF6 (20
mol %) occurred regioselectively to provide highly function-
alized indene 7 in a fair yield. In the presence of 4 equiv of
PCC, oxidative CC bond scission of 2a took place to yield
the substituted 1,3-indandione 8. To our delight, the
epoxidation of 2a with m-chloroperbenzoic acid (m-CPBA)
in DCM provided polycycle 9 in 70% yield with excellent cis-
diastereoselectivity, derived from the ring opening of the in
situ-generated epoxide. Compound 9 was further converted to
the dihydroxylation derivative by hydrolysis (see the
Supporting Information). Furthermore, substrate 2a was
selectively cis-hydrogenated using Pd/C as the catalyst to
deliver 10 in almost quantitative yield.
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: mmzok@zstu.edu.cn.
*E-mail: renhj@zstu.edu.cn; renhj@tzc.edu.cn.
ORCID
Maozhong Miao: 0000-0001-6777-7189
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
The authors are grateful to the Natural Science Foundation of
Zhejiang Province (Grant LY19B020017) and the Zhejiang
■
Scheme 5. Diverse Transformations of 2 to Three-Dimensional Fused Polycycles 5−10
D
DOI: 10.1021/acs.orglett.9b02079
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Organic Letters
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Provincial Top Key Academic Discipline of Chemical
Engineering and Technology of Zhejiang Sci-Tech University
for financial support.
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DOI: 10.1021/acs.orglett.9b02079
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