Enantioselective iron-catalyzed intramolecular cyclopropanation reactions

Article abstract of DOI:10.1002/anie.201406853

An iron-catalyzed asymmetric intramolecular cyclopropanation was realized in high yields and excellent enantioselectivity (up to 97% ee) by using the iron complexes of chiral spiro-bisoxazoline ligands as catalysts. The superiority of iron catalysts exhibited in this reaction demonstrated the potential abilities of this sustainable metal in asymmetric carbenoid transformation reactions.

Full text of DOI:10.1002/anie.201406853

Angewandte
Chemie
DOI: 10.1002/anie.201406853
Asymmetric Catalysis
Enantioselective Iron-Catalyzed Intramolecular Cyclopropanation
Reactions**
Jun-Jie Shen, Shou-Fei Zhu,* Yan Cai, Huan Xu, Xiu-Lan Xie, and Qi-Lin Zhou
Abstract: An iron-catalyzed asymmetric intramolecular cyclo-
propanation was realized in high yields and excellent enantio-
selectivity (up to 97% ee) by using the iron complexes of chiral
spiro-bisoxazoline ligands as catalysts. The superiority of iron
catalysts exhibited in this reaction demonstrated the potential
abilities of this sustainable metal in asymmetric carbenoid
transformation reactions.
styrenes, but these reactions are only moderately enantiose-
lective (up to 86% ee).[6]
Recently, a highly enantioselective (up to 97% ee)
intermolecular cyclopropanation was realized with an engi-
neered cytochrome P450 enzyme.[7] However, to the best of
our knowledge, iron-catalyzed asymmetric intramolecular
cyclopropanation remains unknown,[8] although intramolecu-
lar cyclopropanation is a powerful tool for the construction of
complicated multiple ring systems and has been widely used
in organic synthesis.[5] As part of our continuing studies of
iron-catalyzed carbenoid transformations,[9] herein we report
the first iron-catalyzed asymmetric intramolecular cyclopro-
panation. Specifically, we used iron catalysts with chiral spiro-
bisoxazoline ligands to carry out intramolecular cyclopropa-
nation of diazoesters in high yields and excellent enantiose-
lectivities (up to 97% ee), thus providing a convenient
method for generating synthetically versatile [3.1.0]bicycloal-
kanes.[10]
Initially, we performed the cyclopropanation reaction of
2-methylallyl 2-diazo-2-phenylacetate (1a) in chloroform at
608C in the presence of an iron catalyst generated in situ from
10 mol% FeCl3, 12 mol% of the chiral spiro-bisoxazoline
ligand (Ra,S,S)-3a, and 12 mol% NaBArF (Table 1). Under
these conditions, the cyclopropanation product (1S,5S)-5-
methyl-1-phenyl-3-oxabicyclo[3.1.0]hexan-2-one (2a) was
obtained in 67% yield and 71% ee (entry 1). The structure
and absolute configuration of 2a were determined by X-ray
diffraction of a single crystal.[11] Although there are four
possible isomers of 2a, which has two chiral centers, only the
enantiomers (1S,5S)-2a and (1R,5R)-2a, in which the phenyl
and methyl groups are cis to each other, were detected in the
reaction mixture. The trans isomers did not form, owing to the
highly strained nature of the fused ring system.
E
nantioselective catalysis by chiral transition-metal com-
plexes constitutes one of the most powerful tools for the
synthesis of optically active organic compounds on both
laboratory and industrial scales,[1] and many chiral catalysts
have been developed in recent decades. However, most of
these catalysts are based on precious metals, such as
palladium, rhodium, ruthenium, iridium, and osmium. The
scarcity of precious metals makes them expensive and
unsustainable over the long term. In addition, the biological
toxicity of heavy metals seriously limits their use in pharma-
ceutical production. Therefore, the development of sustain-
able catalysts in which scarce or toxic metals are replaced with
abundant and harmless metals is urgently required.[2]
Iron, which is readily available, inexpensive, and environ-
mentally benign, is an ideal alternative to precious metals.
However, compared to other transition metals, iron is less
developed as a catalyst for organic processes, particularly
asymmetric reactions.[3] Only a few iron-catalyzed reactions
show good enantioselectivity,[4] and iron catalysts generally
exhibit lower enantioselectivity and a narrower substrate
scope than precious metal catalysts. In addition, there are
a number of important reactions for which iron catalysts have
never been reported to show good enantioselectivity, includ-
ing the asymmetric cyclopropanation reaction between diazo
compounds and olefins, which has been extensively studied
and widely used in organic synthesis during the past several
decades.[5] Several chiral iron porphyrins and analogues with
complicated structures have been used in asymmetric inter-
molecular cyclopropanation reactions of a-diazoacetates with
Various other iron precursors were then evaluated, and all
gave the desired product in moderate to good yield (Table 1,
entries 1–8). FeCl2 exhibited higher catalytic activity than
FeCl3 (compare entries 1 and 2), and with FeCl2·4H2O, the
transformation was complete within 17 hours, and the product
was obtained in 77% ee (entry 3). Although water was
[*] J.-J. Shen, Prof. S.-F. Zhu, Dr. Y. Cai, H. Xu, X.-L. Xie, Prof. Q.-L. Zhou
State Key Laboratory and Institute of Elemento-Organic Chemistry
Collaborative Innovation Center of Chemical Science and
Engineering (Tianjin), Nankai University
À
present in this case, no O H insertion product was detec-
ted.[9a] Iron precursors with oxygen ligands, including [Fe-
(acac)2], Fe(OAc)2, and Fe(OTf)2, exhibited lower activity
and enantioselectivity than FeCl2·4H2O (entries 4–6). Ferrous
salts Fe(BF4)2·6H2O and Fe(ClO4)2·4H2O proved to be the
best catalyst precursors (entries 7 and 8). Both of these iron
sources accomplished the intramolecular cyclopropanation
reaction in a relatively short time (5 and 7 h, respectively) and
generated the desired product in high yields (83% and 94%,
respectively) and enantioselectivities (88% and 92% ee,
respectively). It is noteworthy that the enantioselectivity
Tianjin 300071 (China)
E-mail: sfzhu@nankai.edu.cn
[**] We thank the National Natural Science Foundation of China and the
National Basic Research Program of China (2011CB808600), the
“111” project (B06005) of the Ministry of Education of China, and
the National Program for Support of Top-notch Young Professionals
for financial support.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201406853.
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Table 1: Enantioselective iron-catalyzed intramolecular cyclopropanation
of 1a: Optimizing reaction conditions.[a]
Table 2: Enantioselective intramolecular cyclopropanation of 1a: Com-
parison of metal precursors.[a]
Entry
[M]
t [h]
Yield [%]
ee [%]
1
2
3
4
5
6
7
8
Fe(ClO4)2·4H2O
[Cu(CH3CN)4]ClO4
[{RhCl(CO)2}2]
[Pd(PhCN)2Cl2]
[{RuCl2(C6H6)}2]
CoCl2
7
0.1
3
94
90
82
66
39
64
62
59
92
48
20
4
3
52
9
3
0.5
30
7
AgClO4
AuCl
5
7
[a] Reaction conditions and analysis are identical to those in Table 1,
entry 8, but using the indicated metal precursor.
Entry [Fe]
Ligand
t [h]
Yield [%][b] ee [%][c]
1
2
3
4
5
6
7
FeCl3
FeCl2
FeCl2·4H2O
[Fe(acac)2]
Fe(OAc)2
Fe(OTf)2
(Ra,S,S)-3a 30
(Ra,S,S)-3a 24
(Ra,S,S)-3a 17
(Ra,S,S)-3a 30
(Ra,S,S)-3a 30
(Ra,S,S)-3a 30
67
92
82
83
64
57
83
71
73
77
59
69
62
88
92 (82)
55
57
36
44
14
58
4
a vital role in controlling the activity of the catalyst and the
enantioselectivity of the reaction.
We also tested various other transition metals, including
copper, palladium, rhodium, ruthenium, cobalt, silver, and
gold, in the intramolecular cyclopropanation reaction under
the standard reaction conditions (Table 2). Although all the
tested transition metals afforded the desired product 2a in
moderate to good yields, the enantioselectivities were much
lower than those obtained with the iron catalyst under the
standard reaction conditions.
Fe(BF4)2·6H2O
Fe(ClO4)2·4H2O (Ra,S,S)-3a 7 (17) 94 (83)
Fe(ClO4)2·4H2O (Ra,S,S)-3b
Fe(ClO4)2·4H2O (Ra,S,S)-3c 10
Fe(ClO4)2·4H2O (Ra,S,S)-3d 30
Fe(ClO4)2·4H2O (Sa,S,S)-3a 30
Fe(ClO4)2·4H2O (Ra,S,S)-4
Fe(ClO4)2·4H2O (Sa,S,S)-4
Fe(ClO4)2·4H2O (S,S)-5
Fe(ClO4)2·4H2O (S,S)-6
(Ra,S,S)-3a
5
8[d]
9
7
85
87
59
51
55
78
66
82
10
11
12
13
14
15
16
24
24
30
7
Under the optimized reaction conditions, the a-diazoester
substrates 1a–m, having various substituents on the phenyl
ring were subjected to the asymmetric intramolecular cyclo-
propanation (Table 3, entries 1–13). All the tested substrates
produced the desired cyclopropanation products in high
yields (86–96%). Diazoesters with an electron-withdrawing
72
[a] Reaction conditions: [Fe]/ligand/NaBArF/1a=0.030:0.036:0.036:0.3
(mmol), in 4 mL CHCl3 at 608C. [b] Yield of isolated product. [c] Deter-
mined by HPLC using a Chiralpak AS column. [d] The data within
parentheses were obtained with 5 mol% catalyst. acac=acetylaceto-
nate, NaBArF =sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate,
Tf =trifluoromethanesulfonate.
Table 3: Enantioselective iron-catalyzed intramolecular cyclopropana-
tion of a-diazoesters 1a–n.[a]
(92% ee) obtained with the iron catalysts in this study is
comparable to the best enantioselectivity previously obtained
for this reaction using chiral dirhodium catalysts.[12]
The substituents on the oxazoline rings of chiral spiro-
bisoxazoline ligands 3 markedly affected the outcome of the
cyclopropanation reaction (Table 1, entries 9–11). Specifi-
cally, an alkyl substituent lowered both the activity and the
enantioselectivity. The ligand (Sa,S,S)-3a yielded inferior
results to those obtained with the diastereomer (Ra,S,S)-3a
(entry 12), and clearly indicates that the chirality of the ligand
(Ra,S,S)-3a is appropriately matched for this reaction. For
comparison, we also evaluated several widely used chiral
bisoxazoline ligands with different scaffolds, such as 2,2’-
bis(4,5-dihydro-4-phenyloxazol-2-yl)-1,1’-binaphthyl (4), 2,6-
bis(4,5-dihydro-4-phenyloxazol-2-yl)pyridine (5), and 2,2’-
bis(4,5-dihydro-4-phenyloxazol-2-yl)propane (6), under the
standard reaction conditions (entries 13–16) and found that
poorer yields and enantioselectivities were obtained. These
results demonstrate that the bulky, rigid scaffold of 3 plays
Entry
Substrate
Product
t [h]
Yield [%]
ee [%]
1
2
3
4
5
6
7
8
C6H5 (1a)
2-FC6H4 (1b)
2a
2b
2c
2d
2e
2 f
2g
2h
2i
2j
2k
2l
2m
2n
7
7
8
10
7
10
20
6
7
10
7
7
12
7
94
93
96
88
89
90
86
90
94
91
90
93
94
92
92
93
94
4
2-ClC6H4 (1c)
2-MeC6H4 (1d)
3-CF3C6H4 (1e)
3-ClC6H4 (1 f)
3-MeC6H4 (1g)
3-MeOC6H4 (1h)
4-FC6H4 (1i)
4-ClC6H4 (1j)
4-MeC6H4 (1k)
4-MeOC6H4 (1l)
3,4-Cl2C6H3 (1m)
2-Naphthyl (1n)
96
94
90
93
91
90
85
44
94
93
9
10
11
12
13
14
[a] Reaction conditions and analysis are identical to those in Table 1,
entry 8.
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group, such as fluoro (1b and 1i), chloro (1c, 1 f, 1j, and 1m),
or trifluoromethyl (1e), on the phenyl ring exhibited higher
enantioselectivity (entries 2, 3, 5, 6, 9, 10, and 13), with 3-
(trifluoromethyl)phenyl-a-diazoacetate (1e) giving the high-
est enantioselectivity (entry 5). In contrast, a strongly elec-
tron-donating methoxy group at the para position of the a-
diazoester (1l) lowered the enantioselectivity to 44% ee
(entry 12). The enantioselectivity of the reaction was strongly
affected by steric hindrance in the a-diazoester: reaction of an
a-diazoester having an ortho-methyl group on the phenyl ring
(1d) gave extremely low enantioselectivity (entry 4). In
addition, 2-naphthyl diazoester (1n) also showed very high
enantioselectivity (entry 14).
carbenoids in this reaction rather than just serve as Lewis acid
catalysts. The studies of the detailed mechanism of this
reaction are still ongoing.
In summary, we realized an efficient iron-catalyzed
asymmetric intramolecular cyclopropanation reaction using
iron complexes of chiral spiro-bisoxazoline ligands. This
method provides an efficient protocol for the preparation of
synthetically useful chiral [3.1.0]bicycloalkanes. The superi-
ority of iron catalysts in this reaction demonstrates the
possibility of using environmentally benign iron to replace
precious metals in important asymmetric carbenoid transfer
reactions through developing suitable chiral ligands.
Received: July 3, 2014
Revised: August 15, 2014
Published online: && &&, &&&&
The effect of the allyl moiety in the substrate was also
investigated (Scheme 1). The R1 group strongly affected the
Keywords: asymmetric catalysis · diazo compounds · iron ·
.
small ring systems · spiro compounds
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Scheme 1. The effects of olefin moiety.
enantioselectivity of the reaction. The substrate 1o, in which
R1 is a phenyl group, gave the highest enantioselectivity (97%
ee), whereas when R1 was ethyl (1p) or isopropyl (1q), the
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H, was dramatically lower (6% ee). The substrates 1s (R1 =
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2011, 33, 111 – 138.
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ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 1 – 5
These are not the final page numbers!

Angewandte
Chemie
Communications
Asymmetric Catalysis
J.-J. Shen, S.-F. Zhu,* Y. Cai, H. Xu,
X.-L. Xie, Q.-L. Zhou
&&&& — &&&&
Enantioselective Iron-Catalyzed
Intramolecular Cyclopropanation
Reactions
Ironed out: An iron-catalyzed asymmetric
intramolecular cyclopropanation was
realized in high yields and excellent
enantioselectivity by using the iron com-
plexes of chiral spiro-bisoxazoline ligands
as catalysts. NaBArF =sodium tetrakis-
[3,5-bis(trifluoromethyl)phenyl]borate.
Angew. Chem. Int. Ed. 2014, 53, 1 – 5
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
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These are not the final page numbers!