Benign catalysis with iron: Facile assembly of cyclobutanes and cyclohexenes: Via intermolecular radical cation cycloadditions

Article abstract of DOI:10.1039/c8gc00299a

We describe novel and facile iron-catalyzed crossed intermolecular radical cation cycloadditions of styrenes. This catalysis features high efficiency, atom economy, stereospecificity, scalability and very mild reaction conditions. Thus, these reactions re

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Benign catalysis with iron: facile assembly of cyclobutanes and
cyclohexenes via intermolecular radical cation cycloadditions
Received 00th January 20xx,
Accepted 00th January 20xx
Yushuang Yu, Yu Fu and Fangrui Zhong*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Abstract: We describe a novel and facile iron-catalyzed crossed comparison to photochemical approach5a,c-f to trigger the
intermolecular radical cation cycloadditions of styrenes. This radical cation cycloaddition reactions. In this regard, aminium
catalysis features high efficiency, atom-economy, stereospecificity, cationic radical salts have often been utilized,9 which, however,
scability and very mild reaction conditions, and thus represents a might get decomposed and induces side reactions.10
benign and sustainable strategy for the synthesis of valuable Moreover, as the most powerful oxidant, tris(4-
functionalized cyclobutanes and cyclohexenes.
bromophenyl)aminium hexachloroantimonate (TBPA+•) has
some adverse effect to environments and human health.11
Very recently, Donohoe and co-workers disclosed an elegant
study on hypervalent iodine reagents catalyzed radical cation
[2 + 2] dimerization of styrenes, but unfortunately with the
requisite use of relatively expensive fluorinated solvents.12
Concerning the limitations of these existing methods, the call
Cyclobutanes are highly prominent structural motifs
concerning both their important synthetic values and strong
bioactivity profiles.1 As a result, synthetic studies toward this
class of molecules have been pursued intensively. The most
straightforward and atom-economic strategy is alkene [2 + 2]
cycloaddition and studies in this regard has been centered on
photochemical methods under ultraviolet or visible light
conditions.2 Despite many advantages and significant advances
in the past decades, high-energy irradiation or expensive
photocatalysts are generally required. In addition,
electrochemical methods as a clean approach have also been
utilized.3 However, the reaction efficiency and standardized
and simplified instrumentation are yet to be addressed to
facilitate their practical application.4
for
a green and powerful method for radical cation
cycloadditions of alkenes is therefore urgent.
In this context, the radical cation cycloaddition of alkenes
provides an efficient and alternative strategy for constructing
cyclobutane rings.5 Radical cations constitute an important
class of intermediates due to their versatile reactivity in
Scheme 1 Catalytic intermolecular crossed [2+2] dimerization of styrenes
Iron is recognized as one of the most attractive metals,
given its natural abundance, low price, environmental
benignity and occurrence even in sophisticated biological
systems.13 Owning to their inherent multiple oxidation states,
iron complexes exhibit versatile catalytic reactivity and hold a
great promise to achieve sustainable synthetic protocols when
compared to other metals. In this context, iron compounds
forming C−C and C−
X bonds.6 A highly fascinating aspect
regarding these species is the reversal of electronic nature of
an inherently nucleophilic precursor into an electrophilic
counterpart upon one-electron oxidation. The synthetic value
of such an umpolung event has been shown in many
fundamentally important transformations such as pericyclic
reactions,7 rearrangement reactions.8 Regarding alkene
substrates, chemical oxidants are much less exploited in
have proven to be robust catalyst for C−
H oxidation,14 cross-
coupling reactions15 and other reactions. It has been disclosed
in some early studies that Fe(III) salts as oxidants could
catalyze
cation
radical
homodimerization
of
N-
vinylcarboazole.16 However, their effect in crossed
intermolecular cyclobutanation of alkenes remains surprisingly
unknown. These crossed coupling processes are generally
more challenging due to associated competitive
homodimerization, E/Z isomerization or cycloreversion.5a,e In
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this study, we report a benign and sustainable iron catalysis compounds were readily processed, providing various
toward the efficient assembly of unsymmetrical cyclobutanes unsymmetrically substituted cyclobutanes 3b in good to
(Scheme 1). excellent yields (60-88%). Despite potential steric effect, ortho-
−
g
Our initial aim was to identify a powerful iron complex for substituted styrenes were also proven to be suitable reaction
alkene radical cation dimerization. To this end, the effect of partners, and the corresponding four-membered rings were
different Fe(III) catalysts was first evaluated on the model constructed efficiently (3h
reaction between anethole 1a and styrene 2a under air (Table possessing different -alkyl chains or various substituents at
1). Considering its competitive homodimerization, styrene 1a their phenyl rings were all well tolerated, and 50–65% yields
was introduced to the reaction mixture via syringe pumper. were recorded for adducts 3k . In addition, substitution at
Pleasingly, the first instance with catalytic quantities of FeCl3 the -position of led to slightly decreased reactivity.
led to the formation of the desired cyclobutane 3a as a major Nevertheless, tetrasubstituted cyclobutanes 3q were
−3j, 72−85%). Anethole analogues
β
−
p
β
2
−
s
product (46%, entry 1). Apart from it, unreacted starting obtained in higher yields (55% and 42%, respectively) than the
materials and homodimer of 1a were also detected. The only known protocol.12 Notably, the reaction of para-
reaction also proceeded with ferric nitrate and perchlorate, hydroxystyrene also proceeded smoothly to give product 3t in
but iron(III) acetylacetonate was ineffective, implying the comparably good yield (60%). This case represents the only
significant influence from the organic ligands (entries 2
best performance came from Iron(III) perchlorate hydrate, cyclobutane containing
furnishing 3a in 52% yield. While other metals such as AgNO3 success in the isolation of symmetrical dimers 3u
−
5). The methodology yet available for one-step synthesis of
phenol moiety. Moreover, the
showcased
a
−
x
and MnO2 failed to deliver the desired product, copper(II) the applicability of this catalytic system also for homo [2 + 2]
perchlorate hexahydrate was an effective catalyst (entries cycloaddition of styrenes. In all cases described above, the
6−
9). Notably, despite a slightly superior result with anhydrous cyclobutane products were isolated in diastereomerically pure
iron salt (entries 1−2), Iron(III) perchlorate hydrate was taken form. Pleasingly, a gram-scale experiment (3a, 2.16 g, 85%)
for further optimization for better practicality. Subsequent with 5 mol% of catalyst demonstrated the scalability of this
screening of solvents identified ethyl acetate as the best protocol.
reaction medium (entry 10 & table S1). Further improvement
Table 2 Substrate scope of iron-mediated cyclobutanationsa,b
was attained by employing excess of styrene 2a (entry 11) and
o
a slightly elevated reaction temperature (40 C), whereby the
yield of cyclobutane 3a was improved to 88% (entry 12).
Table 1 Reaction condition optimizationa
entry
1
cat.
solvent
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
EtOAc
EtOAc
EtOAc
T/ oC
25
25
25
25
25
25
25
25
25
25
25
40
yieldb
46
39
20
52
0
FeCl3
2
FeCl3•6H2O
Fe(NO3)3•9H2O
Fe(ClO4)3•6H2O
Fe(acac)3
3
4
5
6
AgNO3
0
7
MnO2
0
8
NiCl2
0
9
Cu(ClO4)2•6H2O
Fe(ClO4)3•6H2O
Fe(ClO4)3•6H2O
Fe(ClO4)3•6H2O
34
62
82
88
10
11c
12c
a reaction conditions: 1a (1.0 mmol), 2a (2.0 mmol), catalyst (10 mol%) in solvent
(4.0 mL) under air. b Isolated yield. c 5.0 mmol of 2a was used.
The optimal reaction conditions were then applied to
cycloaddition for other substrates (Table 2). Styrenes bearing
electronically diverse substituents on phenyl rings were firstly
employed to couple with anethole 1a. Delightfully, all these
a reaction conditions refer to table 1, entry 12. b Isolated yield.
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The success of the above cyclobutanation reactions
Some further studies were performed to gain more
encouraged us to attempt an analogues radical cation [4 + 2] mechanistic insight. First, control experiments showed that
cycloaddition17 of styrenes with dienes for the synthesis of TEMPO and BHT as radical scavengers interrupted both [2 + 2]
functionalized cyclohexenes, an important class of six- and [4+2] cycloadditions (see SI), suggesting a radical process
membered carbocycles.18 To our delight, under slightly is likely involved. Competition experiments revealed that
modified conditions, a variety of styrenes
1
as dieneophile radical cation of 1a was preferentially captured by diene 4a
3u 5a
efficiently participated in the anticipated Diels-Alder reaction over 2a, while homodimerization was disfavored (3a
:
:
=
in combination with different dienes, furnishing cyclohexenes 34 : 1 : 200, scheme 2a). Principally, these different reactivities
in excellent yields and with perfect regio- and could be interpreted by oxidative potentials of involved
stereoselectivities (Table 3). A gram-scale synthesis of 5a could substances. Indeed, we measured an oxidation potential of
be achieved quantitatively with 5 mol% of catalyst. Ester +1.62 V vs. SCE for Fe(ClO4)3•6H2O, which would be able to
substrate and N-vinylcarbazole were also tolerated, delivering oxidize 1a (+ 1.1 V) but not styrene 2a (+ 2.05 V).19 However,
adducts 5d and 5n, albeit with lower yields. In addition, no oxidation potentials for 3a (+ 1.59 V), 3u (+ 1.27 V) and 5a (+
cycloaddition (5o) occurred when β-methylstyrene was utilized 1.74 V) render them susceptible to be oxidized as well. Thus
In lieu of 1a; however, the presence of two methoxy groups we performed crossover reactions using these three adducts
induced decomposition of the resulting cyclohexene 5h (60%). as reactants. The mixture of 3a and isoprene 4a under the
These results imply that the electronic nature of both catalytic conditions resulted in almost full recovery of starting
reactants and adducts has impact on the isolated yields of materials, and no [4 + 2] adduct 5a was detected, ruling out
cycloaddition products.
the cycloreversion path of 3a. However, we indeed isolated 5a
in 25% yield when 3u was treated in the same protocol,
confirming the oxidation of cyclobutane 3u back to 1a (Scheme
2b). Not any cycloreversion of 5a was detected (see SI), which
was in line with the interpretation based on their oxidation
potentials. Moreover, careful examining reaction mixture
Table 3 Substrate scope of [4+2] cycloadditiona,b
comprising 1a, 2a and iron salt at appropriately 50%
MeO
MeO
conversion proved that E/Z isomerization of anethole did not
occur (Scheme 2c), thus underlining a potential advantage of
this chemical method over irradiation conditions. On the other
hand, the reaction of a mixture of cis- and trans-anethole with
diene 4a gave diastereomerically pure cyclohexene 5a (see SI).
This outcome implies that the respective radical cations adopt
more stable anti conformation by C-C bond rotation.
MeO
5a (97%, 1.26 gram)
5b (95%)
5c (96%)
MeO
MeO
MeO
Br
Fe(ClO4)3—6H2O
(10 mol%)
(a)
1a
+
2a
+
3a : 3u : 5a = 17 : 0.5 : 100
CH3CN, RT, 24 h
OAc
1 eq
2 eq
4a (2.0 eq)
4a (2.0 eq)
5d (61%)
5e (95%)
5f (95%)
(b)
(c)
Fe(ClO4)3—6H2O
(10 mol%)
MeO
F
MeO
MeO
MeO
3a: R = C6H5
3u: R = 4-OMe-C6H4
for 3a: 5a (0%) + recovered 3a (94%)
for 3u: 5a (25%) + recovered 3u (67%)
+
CH3CN, RT, 24 h
5g (90%)
5h (60%)
5i (98%)
Fe(ClO4)3—6H2O
(10 mol%)
+
1a
Ph
3a + 3u
+
+
MeO
MeO
MeO
MeO
MeO
CH3CN, RT, 24 h
2a (2.0 eq)
1a (~50% conversion) 1a' (NOT detected)
Scheme 2 Mechanistic studies
5g(92%)
5k (90%)
5l (97%)
MeO
N
5m (72%, endo:exo = 5:1)
5n (62%)
5o (0%)
a reaction conditions: 1 (1.0 mmol), 4 (2.0 mmol), iron catalyst (10 mol%) in solvent (3.0
mL) under air. b Isolated yield.
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Figure 1 Progress of iron-catalyzed [2 + 2] cycloaddition of 1a with 2a under
controlled conditions
The authors thank financial supports from the National Natural
Science Foundation of China (21602067). The authors are also
grateful to the Analytical and Testing Centre of HUST,
Analytical and Testing Centre of School of Chemistry and
Chemical Engineering (HUST) as well as 100 Talents Program of
the Hubei Provincial Government. Prof. Yuefa Gong and
Dengfu Lu (HUST) are acknowledged for helping with cyclic
voltammetry studies.
Additional experiments were conducted by introducing
1,10-phenanthroline as a ligand (10 mol%) to the [2 + 2]
reaction mixture (t = 1 h, ca. 45% conversion). 1H NMR studies
showed that the cyclobutanation was still progressing to reach
full conversion but at a slightly dropped rate as compared to
standard conditions (Figure 1). In sharp contrast, pre-formed
iron complex of this ligand completely interrupted the
formation of 3a. The results suggest that chain propagation is
likely involved in this radical cation [2 + 2] reactions.18 On the
other hand, although a beneficial influence of oxygen was
observed, performing the cycloaddition in degassed
acetonitrile under argon also led to full translation of 1a at a
slightly dropped rate. We thus conclude a possible role of
aerobic atmosphere to turn over the iron(II) to iron(III), which
accordingly contributes to the overall efficiency of the
reaction. Taken together, a plausible mechanism is proposed
(Scheme 3, shown in [2 + 2] dimerization). Anethole 1a is
Conflicts of interest
There are no conflicts to declare.
Notes and references
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initially oxidized by iron(III) to radical cation species
coupled with styrene 2a to generate radical cation
intermediate regains an electron dominantly from the neutral
substrate 1a and concurrently releases product 3a
consequently affording a new radical cation to continue the
A
, which is
B
. This
2
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Scheme 3 A plausible reaction mechanism
In summary, we have developed the first intermolecular
radical cation cycloadditions of styrenes using catalytic amount
of ferric salts. The method reported represents a novel,
efficient and green approach to selectively assemble various
functionalized cyclobutanes and cyclohexenes under very mild
reaction conditions. Given both synthetic and biological values
of these classes of skeletons, as well advantages of iron
catalysis in many aspects, we believe our methodology
disclosed herein will find widespread applications in synthetic
chemistry. Further investigations to apply radical cation
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TOC:
Benign catalysis with iron: facile assembly of cyclobutanes and
cyclohexenes via intermolecular radical cation cycloadditions
Yushuang Yu, Yu Fu and Fangrui Zhong*
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