Single-Electron-Transfer Strategy for Reductive Radical Cyclization: Fe(CO)5 and Phenanthroline System

Article abstract of DOI:10.1021/acs.orglett.6b02375

An electron-transfer strategy using low-valent iron pentacarbonyl [Fe(CO)₅] to generate radical species from alkyl iodides was achieved. A range of pyrrolidines, tetrahydrofurans, and carbocycles were synthesized via 5-exo cyclization reactions of alkyl radical intermediates generated by electron transfer from a system involving Fe(CO)₅, 1,10-phenanthroline, and diisopropylamine. Moreover, tandem addition reactions with Michael acceptors were also explored. Photophysical and electrochemical studies support a mechanism that involves electron transfer from the low-valent Fe reductant to alkyl iodide.

Full text of DOI:10.1021/acs.orglett.6b02375

Letter
pubs.acs.org/OrgLett
Single-Electron-Transfer Strategy for Reductive Radical Cyclization:
Fe(CO)5 and Phenanthroline System
Joon Young Hwang,† Jong Hwa Baek,† Tae Il Shin,† Jung Ha Shin,† Jae Won Oh,† Kwang Pyo Kim,†
Youngmin You,*,‡ and Eun Joo Kang*,†
†Department of Applied Chemistry, Kyung Hee University, Yongin 17104, Korea
‡Division of Chemical Engineering and Materials Science, Ewha Womans University, Seoul 03760, Korea
S
* Supporting Information
ABSTRACT: An electron-transfer strategy using low-valent
iron pentacarbonyl [Fe(CO)5] to generate radical species from
alkyl iodides was achieved. A range of pyrrolidines,
tetrahydrofurans, and carbocycles were synthesized via 5-exo
cyclization reactions of alkyl radical intermediates generated by
electron transfer from a system involving Fe(CO)5, 1,10-
phenanthroline, and diisopropylamine. Moreover, tandem
addition reactions with Michael acceptors were also explored. Photophysical and electrochemical studies support a mechanism
that involves electron transfer from the low-valent Fe reductant to alkyl iodide.
ron compounds are considered to be green chemicals that are
hydride was shown to participate in an electron transfer process
Ieconomical and sustainable and have been intensively via anionic iron(I) hydrido species ([HFeCl(dppe)2]−; dppe
explored in various kinds of organic reactions as oxygen carriers,1
=1,2-bis(diphenylphosphino)ethane), which was generated from
iron(II) catalyst and stoichiometric NaBH4 reductant.15
Lewis acids,2 and cross-coupling catalysts.3 As an alternative
coupling catalyst, iron has received substantial attention due to
its unprecedented characteristics toward alkyl halide substrates
and single-electron-transfer processes in catalytic cycles. These
features are not surprising since iron complexes possess rich
multivalent states that are stable and easily accessible. For
instance, the action of tris(1,10-phenanthroline)iron(III) ([Fe-
(phen)3]3+) as a single-electron oxidant can be found in oxidative
substitutions of methylarenes,4 α−β cleavage of diarylpropanes,5
and SOMO cycloaddition reactions.6 Additionally, Fe(acac)3 and
FeCl3 were applied to an oxidative enolate heterocoupling7a and
sulfur-directed C−H arene/alkene coupling reactions.7b,c
Conversely, reductive single-electron transfer by the Fe(II/III)
couple proceeded with the in situ generated iron(II) complex by
external reductants, such as Grignard reagents or silanes, in
pinacol coupling and nitroarene addition reactions.8 Low-valent
iron species have rarely been directly employed as single-electron
reductants, while they have been generated in situ in organic
reactions. We questioned whether low-valent iron might mediate
radical reactions of alkyl halides via a single-electron-transfer
process.
Nonchain reactions include the single-electron-transfer (SET)
process from a low-valent transition metal, which could be the
metal itself,16 metal salts,17 or reduced metal species generated by
RMgX, RLi, or RSiH3.18 These low-valent transition metals
transfer one electron to substrates to afford radical anion
intermediates, which are easily transformed into reactive alkyl
radical species. Photoactivation methods of Ru, Ir, and Au are
usually performed to achieve this type of electron transfer in
catalytic systems.19 However, the methods mentioned above
require additives (AIBN or alkylborane) to prepare active metal
radical intermediates, hydride reductants to prepare low-valent
metal species, and the use of scarce or expensive chemicals. For
these reasons, concerted efforts on SET and radical generation
have been expended. We questioned whether the commercial
low-valent iron might mediate radical reactions, and we describe
the first reductive cyclization reactions mediated by iron−
phenanthroline complexes.
To examine the possibility of low-valent iron species as single-
electron reductants, various iron compounds were tested
together with 1,10-phenanthroline monohydrate (phen, 2.0
equiv), triethylamine (2.0 equiv), and ω-iodoalkene 1a in
acetonitrile (0.10 M) (Table 1). In the reaction with iron
dichloride, no conversion was observed at room temperature,
and the halogen-exchanged product ω-chloroalkene 3a was
detected at 60 °C (entry 1). Other Fe(II) compounds, including
FeF2, Fe(OAc)2, and FeO, did not exhibit noticeable reactivities
(entries 2−4). The commercially available Fe(0) compound iron
Radical cyclization is a well-developed method for C−C bond
formation and the efficient elaboration of cyclic compounds.
There are two representative reductive radical cyclization
methods: chain reaction and nonchain reaction. Radical chain
reactions progress using stoichiometric amounts of various alkyl
metal/semimetal hydrides, such as tin,9 germanium,10 and silane
hydrides11 (RxM−H). Other types of metal hydrides have also
13
been reported including HGaCl2, HInCl2,12 HIn(OAc)2 and
Cp2Zr(H)Cl,14 which were prepared from metal salts (MX3)
with silane or aluminum hydride reductant. Recently, iron
Received: August 9, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b02375
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Organic Letters
Letter
pyrrolidines 2a−f in good yields, and no 6-endo products were
observed even with the sterically encumbered environment of 1f
(Table 2). Cyclization of a secondary iodide gave the desired
Table 1. Reductive Cyclization with Fe Complex
Table 2. Substrate Variations
b
entry
[Fe]
FeCl2
ligand
phen
amine
Et3N
yield (%)
c
1
(68)
(38)
c
2
FeF2
phen
phen
phen
phen
phen
bpy
Et3N
c
3
Fe(OAc)2
FeO
Et3N
c
4
Et3N
c
5
Fe(CO)5
Fe(CO)5
Fe(CO)5
Fe(CO)5
Fe(CO)5
Fe(CO)5
Fe(CO)5
Fe(CO)5
Fe(CO)5
Fe(CO)5
Fe(CO)5
Fe(CO)5
Fe(CO)5
Fe2(CO)9
Fe3(CO)12
Fe(CO)5
Fe(CO)5
Et3N
74
90
83
56
72
89
92
83
92
87
72
95
43
38
46
8
6
Et3N
7
Et3N
8
terpy
Me2phen
phen
phen
phen
phen
phen
phen
phen
phen
phen
phen
Et3N
9
Et3N
10
11
12
13
14
15
TMEDA
i-Pr2NH
cHex2NH
EtNH2
AmylNH2
EDA
d
16
e
i-Pr2NH
i-Pr2NH
i-Pr2NH
i-Pr2NH
i-Pr2NH
17
18
19
f
20
g
21
phen
14
a
Reaction conditions: 1a (0.2 mmol), [Fe] (0.2 mmol), ligand (0.4
mmol, 2 equiv), amine (0.4 mmol, 2 equiv), CH3CN (0.1 M), rt for 6
b
h under N2, unless otherwise specified. Isolated yield (yield of 3a (X
= Cl) in parentheses in entry 1; yield of 3b (X = OAc) in parentheses
c
d
of entry 3). Reaction at 60 °C. Reaction with phen (0.6 mmol, 3
e
equiv). Reaction with Fe(CO)5 (0.1 mmol, 0.5 equiv) and phen (0.3
mmol, 1.5 equiv) resulted in 1a (25%) and 4 (17%). Reaction resulted
in 1a (90%). Reaction with phen (0.3 mmol) resulted in 1a (54%)
f
g
and 4 (18%). phen = 1,10-phenanthroline monohydrate, bpy = 2,2′-
bipyridine, terpy = 2,2′:6′,2″-terpyridine, Me2phen = 2,9-dimethyl-
1,10-phenanthroline, TMEDA = tetramethylethylene diamine, EDA =
ethylenediamine.
a
General conditions: substrate (0.2 mmol), 1,10-phenanthroline
monohydrate (0.6 mmol), diisopropylamine (0.4 mmol), Fe(CO)5
b
(0.2 mmol) in CH3CN (0.1 M). Isolated yield. The diastereomeric
ratio was determined by H NMR analysis. Reaction with 2 equiv of
phen.
c
pentacarbonyl [Fe(CO)5] was found to serve as an effective
reductant, resulting in 5-exo product 2a in 74% yield (entry 5).
Surprisingly, the reaction conditions at ambient temperature
improved the product formation to 90% yield. Among a variety of
diimine ligands, including bpy, terpy, and Me2phen, the phen
ligand was found to give the best result (entries 6−9).
Examination of various tertiary, secondary, and primary amines
revealed that diisopropylamine was the optimal choice as a
sacrificial electron and hydrogen donor (entries 10−15). Almost
50% of cyclized product 2a was observed when the reaction was
carried out with 0.5 equiv of Fe(CO)5 in a glovebox (entry 17),
indicating a 1:1 stoichiometry of Fe(CO)5 and 1a. Changing the
Fe(0) complex to Fe2(CO)9 and Fe3(CO)12 reduced the yield to
38−46% (entries 18 and 19). Control experiments performed in
the absence of the phen ligand or i-Pr2NH showed recovery of
starting material or formation of the atom-transferred iodoalkane
byproduct 4 (entries 20 and 21). Additionally, experiments
conducted with bromo- and chloroalkenes fail to generate any
product.
1
pyrrolidine 2g in 85% yield. Various iodoacetals were reacted to
afford 2-ethoxytetrahydrofurans (2h−j) and hexahydro-2H-
furo[2,3-b]pyrans (2k−n). Oxacycles 2h and 2l−m reported
previously under other types of radical process also resulted in
similar diastereomeric ratios, implying that this strategy afforded
a radical process under milder conditions or shorter reaction
times. The cis ring junction protons in 2k−n also provide
evidence for the radical mechanism, while starting allyloxyiodo-
tetrahydropyrans 1k−n had trans stereochemistry. The cycliza-
tion of allyl 2-iodoethyl malonate and 2-iodoethyl phenyl acrylate
also gave the carbocycles 2o and 2p in high yield.21
To apply the cyclized methyl radical intermediate to further
C−C bond formation, we accomplished tandem radical
cyclization reactions with various electrophilic Michael accept-
ors, such as methyl vinyl ketone (MVK), phenyl vinyl ketone,
and acrolein (Scheme 1). When i-Pr2NH was added in the
tandem reaction even with the reduced amount (1 equiv), a
tandem product was produced as a minor product (30%)
accompanied by the cyclized one 2a (61%), which indicates that
The scope of the reductive cyclization reaction with respect to
ω-iodoalkene substrate was explored using the optimized
conditions (entry 16, Table 1).20 Diversely substituted alkene
substrates were clearly transformed into the corresponding
B
DOI: 10.1021/acs.orglett.6b02375
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Organic Letters
Letter
Scheme 1. Tandem Reactions with Michael Acceptors
Figure 1. (a) UV−vis absorption spectra of an CH3CN solution
containing 1.0 mM 1a, 3.0 mM phen, 1.0 mM Fe(CO)5, and varied
concentrations of i-Pr2NH (0, 1.0, 10 mM). (b) Low-temperature EPR
spectrum of an CH3CN solution containing 10 mM 1a, 100 mM phen,
and 10 mM Fe(CO)5.
the hydrogen abstraction of the cyclized methyl radical
intermediate is faster than addition to the β-carbon in the α,β-
unsaturated carbonyl compound for α-carbonyl radical inter-
mediate formation. According to the previous reports,22 the α-
carbonyl radical adducts (E1/2 = −0.59 to −0.73 V vs SCE) could
be reduced with the available electron donor species, such as low-
valent iron species, and the resulting enolates would be
protonated to afford the tandem addition products. Similarly,
in the absence of i-Pr2NH, the tandem reactions of vinyl ketones
with 1a resulted in the formation of methyl ketone 5a and phenyl
ketone 5b in 68−69% yield, indicating the feasibility of reduction
by Fe species and proton transfer by hydrated phen ligand.23 The
reaction also works well for the preparation of tetrahydrofuranyl
ketone 6a and aldehyde 6b. Tandem addition reactions were very
specific to α,β-unsaturated carbonyl compounds, while acryloni-
trile, acrylate, and styrene derivatives rarely participated as the
radical acceptor.22
was independently prepared.24 When the reaction was
performed in the absence of i-Pr2NH, UV−vis absorption
spectra showed broad and weak absorptions at 527 and 586 nm
that were reminiscent of [Fe(phen)3]3+. Reaction mixtures in
CH3CN or toluene are EPR-silent, irrespective of the presence of
i-Pr2NH. However, a strong EPR signal was observed when the
concentration of phen was increased (10 equiv). The rhombic
EPR signature with a g value of 4.2194 is characteristic of a
Fe(III) species (Figure 1b).25a An additional weak EPR signal
with a g value of 2.0083 was observed, which can be assigned to
be a radical anion of phen.25b These results support the
mechanism involving electron transfer of Fe(CO)5 and
formation of Fe(III) species.26
To gain more insight into the redox process, we collected
cyclic and differential pulse voltammograms of [Fe(phen)3]2+
and [Fe(phen)3]3+. A reversible one-electron redox process of
Fe(II/III) was found at 1.10 V vs SCE in deaerated CH3CN
solutions (Figure S2). An oxidation potential (Eox) of the
sacrificial electron donor was less positive [e.g., Eox(TEA) = 0.70
V vs SCE],27 which supports the notion that Fe(II) was the final
oxidation state in the presence of amines. The electron transfer
from amines to the Fe(III) species is crucial because the resulting
radical cation of amines can provide a hydrogen atom to the
radical intermediate of the cyclized products. Further reduction
to low-valent Fe species was observed at more negative potentials
at −1.33 and −1.46 V vs SCE. Since reduction of 1a occurred at
−1.15 V vs SCE (Figure S2), the low-valent species derived from
Fe(CO)5 would be capable of promoting reductive cleavage of 1a
through exoergic one-electron transfer with a free energy change
greater than −0.18 eV. The lack of reactivities by [Fe(phen)3]-
(PF6)2, [Fe(phen)3](PF6)3, and their combined system
corroborated this hypothesis. Although the electron stoichiom-
etry and an action of the phen ligands in the redox processes
require further resolution, the mechanistic studies provided
evidence that the Fe(CO)5−phen system is an effective
reductant for radical generation from alkyl iodide.
The selective 5-exo mode cyclization suggested that the
Fe(CO)5−phen system promoted one-electron-reductive cleav-
age of the C−I bond, generating a radical intermediate. To
examine this hypothesis, we ran the reactions in the presence of a
radical clock or a radical scavenger 2,2,6,6-tetramethyl-1-
piperidinyloxy, TEMPO (Scheme 2). The substrate 7 underwent
Scheme 2. Experiments for Mechanistic Investigations
radical cyclization, followed by a cyclopropane ring-opening
reaction, affording the allyl group in product 8 with high yield
(82%). TEMPO was found to inhibit cyclization of the radical
intermediate, as TEMPO-trapped derivatives 9 and 10 were
observed in the product mixture.
To investigate the redox processes mediated by the Fe(CO)5−
phen system, we performed mechanistic studies employing
photophysical and electrochemical techniques. The UV−vis
absorption spectrum obtained for the reaction solution (1.0 mM
1a, 1.0 mM Fe(CO)5, 3.0 mM phen, and 1.0 mM i-Pr2NH)
exhibited a strong absorption band with a peak wavelength at 508
nm along with shoulders at 476 and 437 nm (Figure 1a). The
Taken together, we propose a mechanism outlined in Figure 2.
The Fe−phenanthroline complex generated from Fe(CO)5 and
phenanthroline transfers an electron to alkyl iodide substrates to
achieve alkyl radical intermediates (II), which undergo the 5-exo
radical cyclization. The formation of 2a is completed by a
hydrogen atom abstraction from the aminium radical cation
species, which was formed by oxidation by FeIII(phen)n.
In summary, we have described the Fe(CO)5-mediated
reductive cyclization of organohalides. Phenanthroline ligand is
crucial for a single-electron-transfer system, and amine is
spectral signatures were consistent with MLCT transition (λobs
=
508 nm; ε = 1.15 × 104 M−1 cm−1) of [Fe(phen)3](PF6)2 that
C
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(20) This condition also proved to be efficient in the hydro-
dehalogenation reaction of alkyl iodides without a pendent olefin,
affording the reduced products in good yield (see Scheme S1).
(21) Iodoalkyne (S3) and iodoallene (S5) also participated as radical
acceptors to provide pyrolidines with side products. See the Supporting
Information.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.6b02375.
Experimental procedures and characterization data for
new compounds (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: odds2@ewha.ac.kr.
*E-mail: ejkang24@khu.ac.kr.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
(22) (a) Kohls, P.; Jadhav, D.; Pandey, G.; Reiser, O. Org. Lett. 2012,
14, 672. (b) Ruiz Espelt, L.; Wiensch, E. M.; Yoon, T. P. J. Org. Chem.
2013, 78, 4107. (c) Terrett, J. A.; Clift, M. D.; MacMillan, D. W. C. J. Am.
Chem. Soc. 2014, 136, 6858.
This study was supported by the Ministry of Education, Science
and Technology, National Research Foundation (Grant Nos.
NRF-2012R1A1A1044770 and NRF-2015R1D1A1A01060349)
(23) Deuterium experiments were tested with deuterium oxide in
tandem reactions (Table S2).
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