Cycloaddition Reactions of Alkene Radical Cations using Iron(III)-Phenanthroline Complex

Article abstract of DOI:10.1002/adsc.202000191

Single electron oxidation of electron-rich alkenes using the iron(III)-phenanthroline complex produced electrophilic alkene radical cations, which promoted efficient radical cation [2+1] cycloaddition reactions with diazo compounds. Subsequent chain propagation afforded tri- and tetra-substituted cyclopropanes. This methodology was also expanded to [3+2] cycloaddition reactions with vinyl diazoesters, validating this sustainable, first-row transition metal iron system for the single electron redox reactions. (Figure presented.).

Full text of DOI:10.1002/adsc.202000191

Title: Cycloaddition Reactions of Alkene Radical Cations using Iron(III)-
Phenanthroline Complex
Authors: Yong Hyun Cho, Jae Hyung Kim, Hyeju An, Kwang-Hyun
Ahn, and Eun Joo Kang
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.202000191
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10.1002/adsc.202000191
Advanced Synthesis & Catalysis
COMMUNICATION
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Cycloaddition Reactions of Alkene Radical Cations using
Iron(III)-Phenanthroline Complex
Yong Hyun Cho,a Jae Hyung Kim,a Hyeju An,a Kwang-Hyun Ahn,a and Eun Joo
Kanga*
a
Department of Applied Chemistry, Kyung Hee University, Yongin 17104, Korea
E-mail: ejkang24@khu.ac.kr
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.
+
Ru(bpy)3 via reductive quenching with amine,
excess reductants (iPr2EtN or Na2S2O3) were required
to perform the reductive Ru2+/Ru+ cycle and scavenge
the resulting I2.[5] Additionally, Li reported iodine
catalysis in diazo radical cyclopropanation: iodine
converted diazoacetate to diiodoacetate, which could
Abstract. Single electron oxidation of electron-rich
alkenes using the iron(III)-phenanthroline complex
produced electrophilic alkene radical cations, which
promoted efficient radical cation [2+1] cycloaddition
reactions with diazo compounds. Subsequent chain
propagation afforded tri- and tetra-substituted
cyclopropanes. This methodology was also expanded to
[3+2] cycloaddition reactions with vinyl diazoesters,
validating this sustainable, first-row transition metal iron
system for the single electron redox reactions.
2+
serve both as an oxidant for Ru(bpy)3 and be
transformed to an iodoacetate carbon radical
intermediate.[6]
Conversely, the 2-alkene skeleton that forms the
cyclopropane, can participate as an electrophilic
radical intermediate (Scheme 1, Method B). This
strategy was realized as a single electron oxidation
pathway forming an alkene radical cation from an
Keywords: Iron(III)-phenanthroline complex; Single
electron oxidation; Alkene radical cation; Cyclopropane;
Cyclopentene
electron-rich
alkene.
The
cationic
radical
cycloaddition reactions have been extensively
investigated by Bauld, and organic aminium salts
were employed as single electron oxidant. Stilbene
was oxidized to radical cation electrophile, and
further reacted with a diazoester as a nucleophilic C1
reagent to afford the cyclopropane.[7] Recently,
Ferreira reported various cycloaddition reactions
using Cr and Ru photooxidative catalysis. Potentially
well-matched alkenes were oxidized to synthesize
cyclopropanes via [2+1] cycloadditions using diazo
compounds, and cyclopentenes were synthesized via
[3+2] cycloadditions using vinyl diazoesters.[8]
Based on these reactions by Bauld and Ferreira, we
were interested in examining whether iron catalysts
could provide a similar reactivity affording these
products. In our previous report on the use of the
iron-phenanthroline complex, anethole-type radical
cations were generated and reacted with nucleophilic
alkenes or dienes in electronically mismatched [2+2]
and [4+2] cycloaddition reactions.[9] It was confirmed
that the iron complex exhibited enough oxidation
potential to oxidize alkenes as well as amines without
high-energy irradiation, providing a sustainable and
environmentally friendly redox catalysis system.[10]
Herein, Fe catalysis in radical-mediated cyclopropa-
nation reactions employing electron-rich alkenes
such as anethole, stilbene and alkyl-substituted dienes
is disclosed. While various iron carbenoid
cyclopropanations have been achieved using
Transition metal catalyzed cyclopropanation has
received much attention as a valuable synthetic
method for the production of bioactive and
pharmaceutically
important
cyclopropane
derivatives.[1] Traditional cyclopropanation reactions
have consisted of free carbenes or metal carbenoids
with alkenes,[2] however, these precursors of the
carbene transfer reaction, such as diazoacetate,
haloalkanes and alkenes, have recently been
investigated in single electron transfer or radical
reaction.[3] Electron transfer mediated radical cationic
or radical anionic approaches allow for the formation
of cyclopropanes with broader electronic and
stereochemical properties through completely
different reaction mechanisms.
The expansion of single electron redox systems
began with the development of photocatalysis, and
the first reported use of visible light induced
cyclopropanation was published by the Guo group
(Scheme 1, Method A).[4] Double single electron
2+
transfer of a Ru(bpy)3 photocatalyst system to
diethyl dibromomalonate resulted in the formation of
a malonate carbanion, and nucleophilic addition to an
electron-deficient alkene provided the desired
cyclopropane. The Suero group also reported a
stereoconvergent cyclopropanation reaction with
2+
Ru(bpy)3 by forming iodomethane radicals and
reacting with aryl-substituted electron-rich alkenes.
Since these reactions required the formation of
1
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10.1002/adsc.202000191
Advanced Synthesis & Catalysis
porphyrin, spirobisoxazoline, and bis(imino)-pyridine
iron complexes,[11] to the best of our knowledge, this
phenanthrolinyl iron catalyst represents the first
single electron transfer activity in cyclopropanation
reactions of alkenes and diazo compounds.
entry Fe complex (mol %) solvent Temp Time Yield
(oC) (h)
(%)[b]
1[c]
2
Fe(phen)3(PF6)3 (5) DCE
Fe(phen)3(PF6)3 (5) DCE
Fe(phen)3(PF6)3 (3) DCE
Fe(phen)3(PF6)3 (5) DCE
Fe(phen)3(PF6)3 (5) DCE
50
50
50
23
0
24
5
48
60
14
57
64
66
64
3
5
4
5
5
6
6
Fe(bpy)3(PF6)3 (5)
DCE
0
8
7
Fe(Me4phen)3(PF6)3 DCE
(5)
0
6
8
9
10
11
12
FeCl2 (5)
FeCl3 (5)
Fe(phen)3(PF6)3 (5) MeCN 0
Fe(phen)3(PF6)3 (5) DCM
Fe(phen)3(PF6)3 (5) DCE/T 0
FE(9:1)
DCE
DCE
0
0
24
24
24
4
-
-
32
56
10
0
24
13
Fe(phen)3(PF6)3 (10) DCE
Reaction conditions: 1a (0.2 mmol), 2a (0.6 mmol), Fe
0
4
83
[a]
[b]
complex (3-10 mol %), solvent (0.1 M). Determined by
1H NMR spectroscopy using 1,1,2,2-tetrachloroethane for
internal standard. [c] Reaction with 0.3 mmol of 2a. phen =
phenanthroline, bpy = 2,2’-bipyridine, Me4phen = 3,4,7,8-
tetramethyl-1,10-phenanthroline, DCE = 1,2-dichloroeth-
ane, TFE = 2,2,2-trifluoroethanol.
Scheme 1. Single Electron Transfer Mediated
Cyclopropanation.
We initially tested our strategy with the reaction of
anethole (1a) and ethyl diazoacetate (2a) using 5
mol % of Fe(phen)3(PF6)3 at 50 oC (Table 1, entry 1).
To our delight, cyclopropane (3aa) was produced in
48% yield, suggesting that the Fe(III) complex could
act as a single electron oxidant to selectively oxidize
the anethole 1a to the electrophilic radical cation
species required for reaction with the nucleophilic
diazo compound. Increasing the equivalents of the
diazoacetate led to full conversion and improved
yield, however, reducing the amount of the iron
complex decreased the yield (entries 2-3). Lowering
the temperature resulted in a higher yield (entries 4-5),
and subsequent examinations of this reaction were
With the best conditions for cyclopropanation in
hand, the generality of this novel Fe catalysis was
investigated and compared with the existing reactions
by Bauld and Ferreira regarding the alkene scope
(Table 2).[7,8a] With electron-rich substituents on the
aryl group in anethole, the corresponding products
were accessed in good yield in approximately 1:1
diastereomeric mixtures (3ba-3ea). It is noteworthy
that allyl substituted anethole derivative 1c only
participated via the internal alkene radical cation
intermediate. A range of stilbenes were competent
substrates for cyclopropanation, and the yields were
good to excellent (3ia-3ka, 3ma). Stilbene
derivatives 1l and 1n were both unproductive in the
cycloaddition, which did not seem to be good
electrophiles due to stabilizing by charge
delocalization. Trisubstituted alkenes, such as α-
methylanethole (1o) and α-methylstilbene (1p), were
also amenable, while 2-methylprop-1-enylbenzene
derivatives (1q and 1r) produced a mixture of the
cyclopropane and a methyl-migrated byproduct.[12]
As the previous reports observed hydrogen, phenyl,
and methyl migration phenomena,[7a,8a] this byproduct
was also assumed to be produced by 1,2-migration of
the methyl group from the first C-C bond forming
intermediate, which supports the reaction roles of
alkenes as electrophiles and diazoacetates as
nucleophiles in this Fe-catalyzed cyclopropanation.
Additionally, cyclopropaindene 3sa was produced in
high yield for the first time by radical cation
cyclopropanation methodology. Benzofuranyl and
benzothiophenyl substituents were also suitable under
our conditions and 2,5-dimethyl-2,4-hexadiene (1v)
was converted to propenyl substituted cyclopropane
3va in 98% yield. For comparative purposes, reaction
yields of isolated compounds were similar to the ones
carried
out
at
0
oC.
Bipyridine
and
tetramethylphenanthroline ligands were applied to
test the oxidative reactivities of the Fe(III) complexes,
but the differences in reactivity were not significant,
except for iron chloride salts, which did not afford
any cyclopropane product (entries 6-9). Examination
of the reaction medium showed that 1,2-
dichloroethane was the optimal solvent for this
transformation. Particularly, fluorinated ethanol
known to stabilize a charged intermediate only
lowered the conversion yield (entries 10-12). Finally,
increasing the catalyst loading to 10 mol % achieved
83% yield and was determined to be the optimal
reaction conditions (entry 13).
Table 1. Condition Screening for Cyclopropanation.[a]
2
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10.1002/adsc.202000191
Advanced Synthesis & Catalysis
obtained by [Cr(Ph2phen)3](BF4)3, except for 3na in
62% yield under Cr system[8a] and unproductive
under Fe system.
catalyst, a 2:3 mixture of 3ag and 3ag’ was observed,
and resubjecting 3ag to the Fe conditions also
resulted in rearrangement to the dihydrofuran in high
yield. Finally, using 10 mol % Fe complex resulted in
the predominant formation of dihydrofuran 3ag’,
highlighting an efficient formation of this 5-alkoxy
dihydrofuran compound.
Additionally, variation to vinyl diazoacetate 2h
was examined with anethole 1a whether the
corresponding [3+2] cyclopentene annulation could
occur as a benchmark reaction developed with Cr and
Ru photocatalysis.[8b] With 10 mol
%
of
Fe(phen)3(PF6)3, anethole 1a and vinyl diazoacetate
2h afforded the formation of cyclopentene 4ah as a
single diastereomer in 92% yield (Table 3). Butyl or
benzyl diazoacetate and other variants efficiently
underwent cyclopentene cyclization (4ai-j, 4fh and
4hh). Mechanistically, the vinyl group of the diazo
reagent can attack the alkene radical cation
intermediate, and benzylic radical stabilization would
offer a rationale for the observed regioselectivity.[14]
A 5-endo or 4-exo radical cyclization generates a
cyclopentyl or cyclobutyl radical intermediate, which
is reduced to produce the final cyclopentenoate
product, similar with the mechanism reported by
Ferreira.[8b]
Table 2. Substrate Scope of Cyclopropanation.[a]
Table 3. Substrate Scope of Cyclopentene Annulation.[a]
[a]
Reaction conditions: 1 (0.2 mmol), 2a (0.6 mmol),
Fe(phen)3(PF6)3 (0.02 mmol), DCE (0.1 M), 0 oC. The ratio
of 1,2-anti-3 and 1,2-syn-3. Reaction scale of 1a with 5
mmol.
[a]
Reaction conditions: 1 (0.1 mmol), 2 (0.8 mmol),
[b]
[b]
Fe(phen)3(PF6)3 (0.01 mmol), CH3NO2 (0.1 M), r.t.
[c]
[c]
Obtained as a mixture of cyclopropane and
Reaction scale of 1a with 1 mmol.
mmol of 2, CH3NO2/CH2Cl2 (1:5), r.t.
Reaction with 0.5
[d]
methyl-migrated byproduct.
Fe(phen)3(PF6)3.
Reaction with 3 mol % of
The methyl-migrated byproducts confirmed the
presence of the cationic intermediate under the
optimized reaction conditions. In addition,
experiments were performed with cyclopropyl
substituted anethole as a radical probe substrate. The
Next, we investigated the diazo scope with respect
to the carbonyl functional group and α-substituents.
While diazo ketones are known to be less
nucleophilic than diazo esters, α-diazo aryl ketones
successfully participated in the reaction, providing
cyclopropanes (3ab, 3jb-3je). Notably, the reaction
involving α-methyl diazoacetate (2g) selectively
controlled the formation of cyclopropane 3ag with 3
mol % of the Fe(phen)3(PF6)3 catalyst without any
formation of the dihydrofuran byproduct (3ag’),
while the 1-alkyl substituted cyclopropane such as
3ag was known to form the ring-opened radical
cation by further single electron oxidation.[8a,13] When
the reaction was performed under 5 mol % of Fe
major product was bicyclopropane
6
as
a
diastereomeric mixture, and the potential product of
cyclopropyl-ring opening was not observed (Scheme
2 (1)).[15] This result indicates that the key benzyl
radical intermediate undergoes rapid cyclization
rather than β-cleaved ring opening. When H2O was
introduced as an alternative nucleophile, TEMPO
adduct 7 was generated providing evidence of the
cyclopropyl methyl radical species (Scheme 2 (2)).
3
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10.1002/adsc.202000191
Advanced Synthesis & Catalysis
Scheme 2. Reaction Result of Cyclopropyl Substituted
Anethole.
Figure 1. (a) Product Distribution from (Z)-Anethole 1a. (b) Calculated Energy Profile for the Formation of 3aa
Diastereomers. (c) Proposed Reaction Pathway.
As for the characteristic stereoconvergent results of
the radical-mediated the
reactions,[16,17]
was possible through a stepwise reaction pathway.
Interestingly, C-C rotation of the (Z)- to (E)-anethole
radical cation intermediate is thermodynamically
disfavored and requires a step barrier of 23.96
kcal/mol associated with the transition state TS-Z/E
(Figure 1(b)). In addition, the ratio of syn,anti-3aa
and anti,anti-3aa (1.0:1.2) obtained from the (E)-
anethole was different from the ratio of the identical
products (2.2:1.6) obtained in the reaction of (Z)-
stereochemical outcome of the cyclopropanation
reaction with (Z)-anethole was investigated (Figure
1(a)). Had this reaction been a concerted reaction
with the Fe carbenoid species, only products with a
2,3-syn relationship would have been formed.
However, all the relational products were observed,
suggesting that C-C bond rotation of the intermediate
4
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10.1002/adsc.202000191
Advanced Synthesis & Catalysis
1.93 (dd, J = 5.0, 9.1 Hz, 1 H), 1.68-1.58 (m, 1 H), 1.33 (d,
J = 6.2 Hz, 3 H), 1.27 (t, J = 7.1 Hz, 3 H) ppm; 1,2-syn-
3aa: δ 7.01 (d, J = 8.8 Hz, 2 H), 6.79 (d, J = 8.6 Hz, 2 H),
3.89 (q, J = 7.1 Hz, 2 H), 3.77 (s, 3 H), 2.29 (dd, J = 7.0,
9.0 Hz, 1 H), 2.02 (dd, J = 6.1, 11.9 Hz, 1 H), 1.77 (dd, J =
5.1, 9.2 Hz, 1 H), 1.26 (d, J = 6.0 Hz, 3 H), 1.03 (t, J = 7.0
Hz, 3 H) ppm. 13C-NMR (75 MHz, CDCl3), 1,2-anti-3aa: δ
171.9, 158.2, 132.7, 127.2, 113.9, 60.4, 55.3, 31.7, 29.0,
25.1, 14.3, 11.9 ppm; 1,2-syn-3aa: δ 171.2, 158.3, 130.1,
128.8, 113.3, 60.0, 55.1, 33.5, 29.8, 19.7, 17.6, 14.0 ppm.
anethole. Therefore, it is believed that this bond
rotation does not occur in the initially generated
anethole radical species, but rather occurs in the
intermediates formed upon reaction with the diazo
species. Depending on the face of the diazo
compound reacting with the (Z)-anethole radical
cation, Int-AZ and Int-BZ intermediates were formed
containing a benzyl radical and a diazonium group.[18]
These intermediates can follow two competitive
pathways, one for direct cyclopropanation and the
other for cyclopropanation after rotation of the benzyl
radical bond via Int-AE and Int-BE intermediates. In
the case of Int-AZ, although energies of the two
diastereomers anti,syn-3•+ and syn,anti-3•+ are almost
identical at -52.4 and -52.8 kcal/mol, respectively, the
transition state TS-AZ leading to anti,syn-3•+ is 3.2
kcal/mol lower in energy than the TS-AZ/E alternative
leading to syn,anti-3•+, in good agreement with the
experimentally observed selectivity. In contrast,
another intermediate Int-BZ prefers to afford the
putative bond-rotating intermediate Int-BE with a
barrier of only 1.1 kcal/mol, which is energetically
downhill by 6.1 kcal/mol, and is cyclized to the
thermodynamically favored anti,anti-3•+. The
initiation step of the radical cation cycloaddition is
proposed to be an electron transfer between the
Fe(III) complex and the anethole to produce the
anethole radical cation intermediate (Figure 1(c)).
This proposal is compatible with the potentials of the
Fe(III)/Fe(II) (1.10 V vs SCE) and anethole/radical
cation redox pairs (1.11 V vs SCE).[9] Single electron
transfer between the cyclized radical cation
intermediate and another equivalent of the anethole
affords the cyclopropane product 3a and propagates
the process.
+
HRMS (ESI): calcd for C14H19O3 [M+H]+ 235.1329,
found 235.1326.
Acknowledgements
This study was supported by the Ministry of Education, Science
and Technology, National Research Foundation (grant numbers
NRF-2018R1A2B6008824) and the GRRC program of Gyeonggi
province [GRRC-kyunghee2017(A01)].
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In conclusion, this work represents the first use of
an Fe(III)-phenanthroline complex to form alkene
radical cations in cycloaddition reactions with diazo
compounds.
Various
highly
substituted
[3] a) N. L. Bauld, Tetrahedron 1989, 45, 5307-5363; b) L.
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stilbene derivatives with α-hydrogen diazo
compounds, and cyclopentenes were also produced
with vinyl diazoesters. Mechanistic investigations
and DFT calculation supported the proposed radical
cation cycloaddition reactions, and the sustainable
Fe(III)-catalyzed single electron oxidation is expected
to be applicable in a wide range of redox reactions.
[4] Y. Zhang, R. Qian, X. Zheng, Y. Zeng, J. Sun, Y. Chen,
A. Ding, H. Guo, Chem. Commun. 2015, 51, 54-57.
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Experimental Section
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General procedure for radical cation cyclopropanation:
To a solution of electron-rich alkene 1 (0.2 mmol, 1.0
equiv.) and diazoacetate 2 (0.60 mmol, 3.0 equiv.) in DCE
o
(2.0 mL, 0.1 M) at 0 C, 10 mol % of Fe(phen)3(PF6)3
(20.6 mg, 0.020 mmol) was added. After being stirred at 0
oC for the proper reaction time, the crude 1,2-anti/syn
diastereo-meric mixture was concentrated and purified
using silica gel column chromatography to afford the
corresponding cyclopropane product (3).
[8] a) F. J. Sarabia, E. M. Ferreira, Org. Lett. 2017, 19,
2865-2868; b) F. J. Sarabia, Q. Li, E. M. Ferreira,
Angew. Chem. Int. Ed. 2018, 57, 11015-11019.
Ethyl 2-(4-methoxyphenyl)-3-methylcyclopropanecarb-
[9] J. H. Shin, E. Y. Seong, H. J. Mun, Y. J. Jang, E. J.
1
oxylate 3aa: H NMR (300 MHz, CDCl3), 1,2-anti-3aa: δ
Kang, Org. Lett. 2018, 20, 5872-5876.
7.01 (d, J = 8.8 Hz, 2 H), 6.81 (d, J = 8.6 Hz, 2 H), 4.16 (q,
J = 7.2 Hz, 2 H), 3.78 (s, 3 H), 2.36 (t, J = 5.8 Hz, 1 H),
5
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10.1002/adsc.202000191
Advanced Synthesis & Catalysis
[10] J. Y. Hwang, A. Y. Ji. S. H. Lee, E. J. Kang, Org. Lett.
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[11] Since the development of iron porphyrin-catalyzed
cyclopropanation, various carbene transfer processes
have been achieved by heme-protein enzyme exhibiting
high catalytic activities and selectivities. a) S.-F. Zhu,
Q.-L. Zhou, Natl. Sci. Rev. 2014, 1, 580-603; b) Y. Wei,
A. Tinoco, V. Steck, R. Fasan, Y. Zhang, J. Am. Chem.
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[14] The mechanistic investigation with (Z)-anethole and
cyclobutyl diazo compound is described in Scheme S3,
and the plausible mechanism for cyclopentene
annulation is proposed in Scheme S4.
[15] a) L. Legnani, B. Morandi, Angew. Chem. Int. Ed.
2016, 55, 2248-2251; b) Y.-Y. Liu, X.-H. Yang, R.-J.
Song, S. Luo, J.-H. Li, Nat. Commun. 2017, 8, 14720.
[16] The Bauld group reported that the cyclopropanation of
(Z)-stilbene afforded the stereoconvergent product in
ref 7a, and Ferreira also observed the stereoconvergent
cyclopentene formation of (Z)-anethole in ref 8b.
[17] a) N. L. Bauld, J. Yang, Tetrahedron Lett. 1999, 40,
8519-8522; b) N. L. Bauld, D. Gao, J. Chem. Soc.,
Perkin Trans. 2 2000, 931-934.
[12] The structure of methyl-migrated byproduct is as
follows, and a detailed information
on the structure identification is
described in Scheme S1.
[18] Among the three staggered conformations, the lowest-
energy arrangement is the one in which the benzyl
radical and ester groups are as far apart as possible in
anti conformation. However, the structure most similar
to the transition state driven to the product appeared to
have the benzyl radical and diazonium to be anti-
periplanar. See the detailed energy profile in Figure S1.
[13] a) J. Luis-Barrera, V. Laina-
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