Iron(III)-Catalyzed Domino Claisen Rearrangement/Regio- And Chemoselective Aerobic Dehydrogenative Cyclization of β-Naphthyl-Substituted-Allenylmethyl Ether

Article abstract of DOI:10.1021/acs.orglett.8b03941

An FeCl3-catalyzed allenic Claisen rearrangement/regio- and chemoselective aerobic dehydrogenative cyclization domino reaction is developed, providing a wide range of 2-aryl/alkyl, 3-(substituted-vinyl)naphtho[2,1-b]furans in high yields at 95-130 °C in a

Full text of DOI:10.1021/acs.orglett.8b03941

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Iron(III)-Catalyzed Domino Claisen Rearrangement/Regio- and
Chemoselective Aerobic Dehydrogenative Cyclization of
β‑Naphthyl-Substituted-Allenylmethyl Ether
Chenyun Zhang, Long Zhen, Zhi Yao, and Liqin Jiang*
School of Chemistry and Molecular Engineering, East China Normal University, 500 Dongchuan Road, Shanghai 200241, China
S
* Supporting Information
ABSTRACT: An FeCl3-catalyzed allenic Claisen rearrange-
ment/regio- and chemoselective aerobic dehydrogenative
cyclization domino reaction is developed, providing a wide
range of 2-aryl/alkyl, 3-(substituted-vinyl)naphtho[2,1-b]-
furans in high yields at 95−130 °C in an atom- and step-
economic fashion. Mechanistic studies suggest that the FeCl3
catalyst is responsible for the high regio- and chemoselectivity in reaction. A blue-emitting product shows quantum yield of 0.95.
The reaction proceeds readily on the gram scale, and synthetic applications of the products are also demonstrated.
he development of novel domino, tandem, or cascade1
Scheme 1. Regio- and Chemoselective Allenic Claisen
Rearrangement/Cyclization for Naphthopyran Derivatives
or Naphthofuran Derivatives
Tprocesses catalyzed by cheap and abundant first-row
transition metals, especially involving unstable intermediates,
has the potential to provide practical, atom- and step-economic
access to complex structures. In sharp contrast to large
amounts of reports on the Claisen rearrangements of
noncumulated double bonds,2 examples of Claisen rearrange-
ments of allenes are very limited and underdeveloped.3 High-
temperature thermal Claisen rearrangements of aryl unsub-
stituted-allenylmethyl ethers followed by intramolecular
oxocyclization have been shown to provide 4-methyl-2H-1-
benzo(naphtho)pyrans (Scheme 1, previous work).3a,c The
mechanism of this reaction involves formation of a 2-(o-
hydroxyaryl)buta-1,3-diene intermediate (I) via allenic Claisen
rearrangement. Aryl substituted-allenylmethyl ethers have
never been reported in Claisen rearrangement. The reason
might be the lack of stability of their Claisen rearrangement
products and their in situ rapid and efficient transformation to
stable products might develop a new avenue for unprecedented
complex structures.
We questioned whether the use of a β-naphthyl substituted-
allenylmethyl ether 14 under suitable reaction conditions could
reverse the regioselectivity of intramolecular addition of
naphthol to the diene in the intermediate I, leading to the 5-
membered naphthofuran rather than the 6-membered
naphthopyran (Scheme 1, this work). The naphthofuran5
skeleton was found in natural products and biological active
compounds6 which show a wide range of biological activity.7
Atom- and step-economic methods for functionalized novel
naphthofuran derivatives are still highly demanded. Intra-
molecular oxidative coupling of 2-vinylphenols is a straightfor-
ward way to build benzo(naphtho)furans.8 However, this
approach to benzofurans often requires nonideal reaction
conditions, including superstoichiometric amounts of I2,8a or
specialized first-row catalysts (Cp*CoI2(CO))8e as well as
Cu(OAc)2/1,10-phenanthroline requiring 10 equiv of
NaOH.8f In this report, we demonstrate that the first use of
β-naphthyl substituted-allenylmethyl ethers under cheap FeCl3
catalyst and O2 conditions reverses the regio- and chemo-
selectivity to provide a wide range of 2-aryl/alkyl, 3-
(substituted-vinyl)naphtho[2,1-b]furans in high yields at 95−
130 °C without naphthopyran products. This atom- and step-
8b
stoichiometric PhI(OAc)2 or Ag2O.8c Other approaches
Received: December 10, 2018
include the use of noble metal catalysts8d or expensive
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b03941
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Table 1. Optimization of the Reaction Conditions
b
entry
1
catalyst
oxidant
solvent
T (°C)/t (h)
2
yield (%)
c
d
1
2
3
4
5
6
7
8
1a
1a
1a
1a
1a
1a
1a
1a
1a
1b
1b
1b
1b
1a
PhI(OAc)2
CH3CN
CH3CN
CH3CN
82/48
82/48
82/48
reflux/36
reflux/36
reflux/36
reflux/36
95/20
95/20
95/36
130/36
130/36
130/36
120/5
2a
2a
2a
2a
2a
2a
2a
2a
2a
2b
2b
2b
2b
2a
4
33
34
65
50
59
67
72
<5
0
c
d
d
DDQ
FeCl3
FeCl3
c
c
d
d
d
CH3CN and DMF
CH3CN and DMF
CH3CN and DMF
CH3CN and DMF
toluene and DMF
toluene and DMF
toluene and DMF
xylene and DMF
xylene and DMF
xylene and DMF
xylene and DMF
FeCl3
FeCl3
FeCl3
FeCl3
air
O2
O2
O2
O2
O2
O2
O2
O2
O2
9
10
11
12
13
14
FeCl3
FeCl3
78
7
48
77
e
Cu(OAc)2/1,10 phenanthroline
InCl3
FeCl3
a
Conditions: 0.15 mmol of 1a/1b, 0.045 mmol of FeCl3, and 40 mg of 4 Å M.S. in 4.2 mL of solvent (mixing solvent ratio = 2.5:1) reacted under
b
c
d
e
O2. Isolated yield. 2.0 equiv of oxidant. Without 4 Å M.S. 20 mol %.
economic process was proposed to proceed through a
trisubstituted allenic Claisen rearrangement followed by a
domino aerobic iron-catalyzed oxidative cyclization to form
new C−C and C−O bonds (Scheme 1, this work). To the best
of our knowledge, this is the first example of an iron-catalyzed
intramolecular aerobic oxidative coupling of O−H with
C(sp2)−H to form benzofuran-like structures. Mechanistic
study suggests that FeCl3 plays the crucial role on the high
regio- and chemoselectivity and the reactivity of the domino
reaction.
solvent at 130 °C under 30 mol % of FeCl3 and O2 afforded 2b
in 78% yield (entry 11). Decreasing the amount of FeCl3 (SI)
or replacing FeCl3 with Pd(OAc)2, Cu(OAc)2/1,10-phenan-
throline (entry 12), or other iron salts including FeBr3,
Fe(acac)3, Fe(OTf)3, FeF3, Fe(ClO4)3·9H2O, FeCl3·6H2O, as
well as InCl3 (entry 13) all led to the decreased yield of 2b
(SI). 1a under the later optimized conditions (entry 11) but at
120 °C afforded 2a in 77% yield after only 5 h (entry 14).
With the optimized conditions in hand, we first investigated
the scope of the substitution R1 at the 3-position of
trisubstituted allenes 1 (Scheme 2, 2c−2y). 1 bearing ortho,
meta, or para electro-withdrawing (1c−1g, 1i−1k), electro-
neutral (1h, 1l, 1o), or electro-donating (2m, 2n, 2p, 2q)
substituted aryls at the 3-position (R1) with R2 as alkyl gave
good yields of the desired naphthofuran derivatives 2c−2q. It
is important to note that chloro, bromo, fluoro, trifluor-
omethyl, and ester are compatible in the present trans-
formation. Chloro, bromo, and fluoro could be further
elaborated for functionalization. Heterocycles such as 3-thienyl
(1r) and 2-thienyl (1s) are also tolerated. Next, 1 bearing alkyl
substitution at the 3-position was examined and consistently
gave good results (2t−2y). It is noteworthy that cyclopropyl as
R1 was preserved and delivered 2x in 73% yield. When R1 was
bulky tert-butyl (1w) or bulky cyclohexyl (1y), the product 2w
and 2y were also smoothly produced in 78% and 76% yields,
respectively.
We started our studies with trisubstituted allene 1a as model
substrate. Selected results are shown in Table 1 [details in the
Supporting Information (SI)]. Initially, a series of stoichio-
metric oxidants were screened in refluxing CH3CN. PhI-
(OAc)2, Ag2O, K2S2O8, t-butyl hydroperoxide (TBHP), and
di-t-butyl-peroxide (DTBP) gave poor results (entry 1 and the
SI). However, 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ)
and FeCl3 indeed afforded the desired product 2a in 33% and
34% yields, respectively (entries 2 and 3). The structure of 2a
was unambiguously confirmed by single-crystal X-ray analysis
(shown in Table 1). Then, solvents were extensively screened
at 82 °C under 2.0 equiv of FeCl3 (SI), and the mixing solvent
of CH3CN and DMF (2.5:1) was found to be the best (entry
4). Next, the use of 30 mol % of FeCl3 under air was examined;
50% yield of 2a was obtained (entry 5). Replacing air with O2
and further adding 4 Å molecular sieves (M.S.) improved the
yield to 67% (entries 6 and 7). Then, the use of the mixing
solvent of toluene and DMF (2.5:1) instead of CH3CN and
DMF (2.5:1) was examined at 90, 95, and 100 °C (SI), and 95
°C afforded the best yield of 2a up to 72% (entry 8). The
control reaction of O2 and 4 Å M.S. in the absence of 30 mol %
FeCl3 gave almost no 2a, and other complex products were
observed, demonstrating the crucial role of FeCl3 on the
success of the desired reaction (entry 9). When we applied the
optimized conditions (entry 8) to 1b with donating
substitution at R2, no reaction occurred (entry 10). After
further screening (SI), the use of xylene and DMF (2.5:1) as
Next, we turned our attention to the substitution R2 at the 1-
position of 1 (Scheme 2, 2z−2aj). In addition to alkyl
(Scheme 2, 2c−2y), R2 as aryls and bromo are also tolerated
(Scheme 2, 2z−2ac). Moreover, when R2 was thiocyanate, a
series of 2-aryl, 3-(1-thiocyanate-substituted-vinyl) naphtho-
furans 2ad−2ai were obtained in moderate to good yields at
120 or 95 °C. Trifluoromethylthio, a “privileged” group in
medicinal chemistry, served as R2 and performed well,
delivering 2aj in 77% yield. Notably, the double allenic Claisen
rearrangement/intramolecular oxidative cyclization of 1ak, 1al,
and 1am also proceeded smoothly to give the corresponding
B
DOI: 10.1021/acs.orglett.8b03941
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Organic Letters
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Scheme 2. Scope of Substitutions R1 and R2 of Naphthyl-
1,3-Disubstituted Allenylmethyl Ethers 1
Scheme 3. Scope of Aryl and Substitutions R4 and R5 of
Aryl-1,3-Disubstituted Allenylmethyl Ethers 1
a
a
a
Reaction conditions: 0.15 mmol of 1, 0.045 mmol of FeCl3, and 40
mg of 4 Å M.S. in 3 mL of xylene and 1.2 mL of DMF reacted under
b
c
O2 at 130 °C for 36 h. Isolated yield. 45 h. 145 °C in DMF for 48 h.
d
120 °C for 9 h.
9
phenyldiazoacetate under AgSbF6 gave 4a in 48% yield as a
single diastereoisomer (Scheme 4a). When 2b was oxidized
Scheme 4. Gram-Scale Experiment and Synthetic
Applications of the Products
a
Reaction conditions: 0.15 mmol of 1, 0.045 mmol of FeCl3, and 40
with m-CPBA, the ring-opening and dearomatized product 5a
was produced, and the structure of 5a was unambiguously
confirmed by single-crystal X-ray analysis (Scheme 4b). In
addition, 2k undergoes a Heck reaction or Heck addition/
Miyaura borylation, furnishing 6a in 72% yield and 7a in 47%
yield, respectively (Scheme 4c). 7a contains both Bpin
functionality and a dihydro-dibenzofuran skeleton which
widely exists in biological compounds.10
For insight into the mechanism, a series of other control
reactions were conducted. 1b under O2 and 4 Å M.S. in the
absence of FeCl3 gave 23% yield of b-I derived from Claisen
rearrangement of 1b in addition to the 27% yield of 2b
(Scheme 5A1). b-I (unstable for preservation) was immedi-
ately subjected to the optimized conditions (Table 1, entry
11), affording 77% yield of 2b after only 35 min (Scheme
5A2). To know the state of iron, the solid of the model
reaction (solvent was distillation) after the reaction was
studied by X-ray photoelectron spectroscopy (XPS). XPS
analysis has shown that the iron salt existed as FeCl3 and
FeOOH (see details in the SI). Moreover, 1b in the presence
of 3.0 equiv of 2,6-di-tert-butyl-4-methylphenol (BHT) under
the optimized conditions leads to the dramatically decreased
mg of 4 Å M.S. in 3 mL of xylene and 1.2 mL of DMF reacted under
O2 at 130 °C for 36 h (2c−2y, 2ak−2am) or 120 °C for 5−24 h (2z−
b
c
2aj). Isolated yield. 60 h. 95 °C in 3 mL of toluene and 1.2 mL of
d
DMF for 20−26 or 40 h (2ag, 2ai). 60 mol % of FeCl3.
2ak, 2al, and 2am in moderate to good yields. In particular,
blue-emitting 2ak shows an excellent quantum yield of as high
as 0.95 in CHCl3 solution and could be employed in the
organic light-emitting material field. 2an bearing 6-ester on the
naphthofuran was obtained in 70% yield (Scheme 3).
Interestingly, when R4 and/or R5 are methyl, the correspond-
ing products 2ao (E:Z = 1:1) and 2ap were smoothly
furnished (Scheme 3). Moreover, phenyl allenylmethyl ether
(1aq) was very inert, and 11% yield of 2aq was obtained at 145
°C in DMF (Schemed 3). α-Naphthyl 1-ethyl-3-phenyl-
allenylmethyl ether (1ar) was also examined, and it reacted
sooner than 1b, and naphthopyran 3a (E:Z = 1.6:1) as the
main product was obtained in 64% yield along with 22% yield
of the naphtho[1,2-b]furan 2ar (Scheme 3).
Gram-scale (1.1 g) 2b was produced under the optimized
conditions in 72% yield (SI). 2b reacted with methyl
C
DOI: 10.1021/acs.orglett.8b03941
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 5. Mechanism Study
25% yield of 2b (Scheme 5A3). This supports the idea that the
domino process might involve a radical mechanism. In
addition, 1b under O2 and 4 Å M.S. in the absence of FeCl3
with saturated hydrogen chloride xylene and DMF (4:1)
solution as solvent at 130 °C for 36 h gave 2b in 32% yield
(SI), ruling out the possibility of HCl to facilitate/catalyze the
transformation.
In summary, we have developed an FeCl3-catalyzed
trisubstituted allenic Claisen rearrangement/chemo-, and
regioselective aerobic dehydrogenative cyclization domino
reaction, affording a wide range of 2-aryl/alkyl, 3-(substi-
tuted-vinyl)naphtho[2,1-b]furans in moderate to good yields at
95−130 °C. The domino reaction highlights a cheap and green
catalytic system, high regio- and chemoselectivity, very broad
substrate scope, good functional group tolerance, and high
atom- and step-economy. Naphthyl substituted-allenylmethyl
ethers were first used in Claisen rearrangement. Mechanistic
study suggests that FeCl3 serves as catalyst which plays the key
role on the high regio- and chemoselectivity and the reactivity
of this reaction. This represents the first example of iron-
catalyzed intramolecular aerobic oxidative coupling of 2-
vinylphenols to form benzofuran-like structures. This also
represents the first method for synthesizing naphthofuran
derivatives in which electro-withdrawing, electro-neutral, and
electro-donating substituted aryls and various alkyls are all
tolerated at the 2-position. The substituted vinyl at the 3-
position of the products has rich chemistry for elaboration.
Gram-scale and various applications of the products have also
been demonstrated. As high as 0.95 quantum yield of one
product also demonstrates these products’ potential utility in
material fields. Developing other new reactions/strategies
catalyzed by cheap and abundant first-row transition metals
as well as developing coupling reactions involving the cleavage
On the basis of the control reactions and XPS study, a
plausible reaction mechanism was proposed (Scheme 5B).
FeCl3 might coordinate with the oxygen of 1 to give A, or
when R2 was SCN/SCF3 R2 might facilitate/direct FeCl3 to
coordinate with the oxygen of 1 to give A′.11 Then, A or A′
undergoes Claisen rearrangement to form B. B converts to C
through keto−enol tautomerization along with the loss of HCl.
C transformed to radical intermediate D and FeCl2.
Subsequently, intramolecular radical cyclization of D gives E.
E is further oxidized by FeCl3 to afford cationic intermediate F
with FeCl2. Then, F loses hydrogen to generate the desired
product 2. FeCl2 in the presence of O2 and HCl is oxidized to
regenerate FeCl3. Part of FeCl2 in the presence of O2 and H2O
is oxidized to FeOOH. Adding molecular sieves could facilitate
the removal of water. Thus, accordingly the less FeOOH was
then formed, and the more effective catalyst FeCl3 could be
regenerated; the yield was increased. Thus, 1 only loses two
hydrogens to transform to 2 through this domino process
(Scheme 5B).
D
DOI: 10.1021/acs.orglett.8b03941
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Organic Letters
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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.8b03941.
Experimental procedures, XPS spectra, and compound
characterization data (PDF)
Accession Codes
CCDC 1865807−1865808 contain the supplementary crys-
tallographic 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
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
(7) (a) Shashikala Devi, K.; Ramaiah, M.; Vanita, G. K.; Veena, K.;
Vaidya, V. P. J. Chem. Pharm. Res. 2011, 3 (1), 445−451.
(b) Chandrashekhar, C. H.; Latha, K. P.; Vagdevi, H. M.; Vaidya,
V. P. Der. Pharma Chem. 2011, 3, 329. (c) Kirilmis, C.; Koca, M.;
AUTHOR INFORMATION
Servi, S.; Gur, S. Turk. J. Chem. 2009, 33, 375. (d) Lardic, M.; Patry,
̈
■
C.; Duflos, M.; Guillon, J.; Massip, S.; Cruzalegui, F.; Edmonds, T.;
Giraudet, S.; Marini, L.; Leonce, S. J. J. Enzyme Inhib. Med. Chem.
2006, 21, 313. (e) Hranjec, M.; Grdisa, M.; Pavelic, K.; Boykin, D.
W.; Karminski-Zamola, G. Farmaco 2003, 58, 1319.
Corresponding Author
*E-mail: lqjiang@sat.ecnu.edu.cn.
ORCID
(8) (a) Pan, C.; Yu, J.; Zhou, Y.; Wang, Z.; Zhou, M.-M. Synlett
2006, 2006, 1657. (b) Singh, F. V.; Wirth, T. Synthesis 2012, 44,
1171. (c) Huang, J.; Wang, W.; He, H.-Y.; Jian, L.; Fu, H.-Y.; Zheng,
X.-L.; Chen, H.; Li, R.-X. J. Org. Chem. 2017, 82, 2523. (d) Guo, L.;
Zhang, F.; Hu, W.; Lia, L.; Jia, Y. Chem. Commun. 2014, 50, 3299.
(e) Ghorai, J.; Reddy, A. C. S.; Anbarasan, P. Chem. - Eur. J. 2016, 22,
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Chem. 2015, 80, 4313.
Liqin Jiang: 0000-0002-9751-7510
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful for financial support from the Science and
Technology Commission of Shanghai Municipality
(15ZR1410500). We acknowledge insightful discussions and
suggestions from Prof. David J. Michaelis and Prof. Yian Shi.
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