Iron(II)-catalyzed enantioselective meso-epoxide-opening with anilines

Article abstract of DOI:10.1039/c2cc18032d

A highly enantioselective method for the catalytic opening of aromatic meso-epoxides with aniline derivatives was developed. The desired chiral β-amino alcohols were obtained in mostly good to very good yields with excellent enantioselectivities. Structur

Full text of DOI:10.1039/c2cc18032d

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Chemical Communications
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Volume 48 | Number 32 | 21 April 2012 | Pages 3789–3896
ISSN 1359-7345
COMMUNICATION
B. Plancq and T. Ollevier
Iron(II)-catalyzed enantioselective meso-epoxide-opening with anilines

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COMMUNICATION
Iron(II)-catalyzed enantioselective meso-epoxide-opening with anilinesw
Baptiste Plancq and Thierry Ollevier*
Received 22nd December 2011, Accepted 26th January 2012
DOI: 10.1039/c2cc18032d
1
4
A highly enantioselective method for the catalytic opening of
aromatic meso-epoxides with aniline derivatives was developed.
The desired chiral b-amino alcohols were obtained in mostly good
to very good yields with excellent enantioselectivities. Structural
evidence of the pre-catalyst revealed a scarcely disclosed heptadentate
Fe(II) complex with the chiral bipyridine ligand.
under aqueous conditions. The efficiency of this complex is
now demonstrated for the opening of aromatic epoxides.
First, we screened different solvents for the reaction of
cis-stilbene oxide 2a with aniline 3a catalyzed by complex
1ÁFe(ClO
4 2
) (Table 1). Among the various solvents tested, only
dichloromethane furnished the desired product with a moderate
yield at 0 1C. Acetonitrile, THF, 1,2-dimethoxyethane (DME)
or diethyl ether led only to trace amounts of amino alcohol 4a
after 24 h (entries 1–5). The high enantioselectivity obtained with
The opening of meso-epoxides is one of the most powerful and
1
atom-economical bond-forming reactions. By differentiating
2 2
CH Cl (entry 5) encouraged us to pursue our optimization
between the two enantiotopic carbons of the meso-epoxide
process in order to improve the yield of this transformation.
N
using a chiral Lewis acid, S 2 nucleophilic addition of achiral
Increasing the catalyst loading to 10 mol% of Fe(ClO ) only led
4 2
nucleophiles generates two neighboring stereogenic centers in
one straightforward operation. Catalytic asymmetric ring-
opening of meso-epoxides has been successfully performed
to a slight increase in enantioselectivity (entry 6) but still with a
moderate yield. Using 2 equivalents of aniline gave almost
exactly the same results as the reaction with equimolar amounts
of both reagents (entry 7 vs. entry 5). However, a significant
concentration effect was observed. Under more concentrated
reaction conditions (1.0 M vs. 0.3 M), a very good yield of 4a
was obtained, even with a slight increase in enantioselectivity.
Finally, the temperature did not have a strong influence on the
enantioselectivity, since the same high enantiomeric excess was
obtained at room temperature (entry 9). Under these conditions,
the product could be obtained in a much shorter reaction time,
with an improved chemical yield. Dichloroethane was also tested
2
3
4
with various nucleophiles such as azides, cyanides, alcohols,
5
6
7
8
9
water, thiols, selenols, indoles, or halides. The asymmetric
meso-epoxide-opening reaction with aniline derivatives is of
prime interest since it allows the formation of chiral b-amino
alcohol units, which are found in many biologically active
1
0
compounds and auxiliaries used in asymmetric reactions.
Few research groups have developed efficient asymmetric
11
ring-opening reactions of meso-epoxides with aniline derivatives.
Generally, these reactions need the use of expensive and/or toxic
catalysts that restrain their use. From an environmental point of
view, the development of new efficient benign chemical processes
is in high demand. Replacing expensive and toxic metals by
cheap and environmentally benign elements is an essential
concern nowadays. In this regard, we have developed an
efficient iron(II)-catalyzed enantioselective meso-epoxide-opening
reaction. Our method is on a par with the selectivities previously
reported by other groups, with the added advantage of using a
cheap and nontoxic catalyst in a low loading (5 mol%). Indeed,
iron is one of the most abundant metals on earth; it is inexpensive,
environmentally benign, and relatively nontoxic in comparison
a
Table 1 Optimisation of reaction conditions
b
Entry Solvent
T/1C Time/h Conc./M Yield 4a (%) ee (%)
1
2
with other metals.
1
2
3
4
5
6
7
8
9
1
MeCN
THF
DME
0
0
0
0
0
0
0
0
22
22
24
24
24
24
72
72
72
72
16
24
0.3
0.3
0.3
0.3
0.3
0.3
0.3
1.0
1.0
1.0
o5
o5
o5
o5
68
60
70
84
90
—
—
—
—
94
95
94
95
95
95
1
3
The combination of Bolm’s ligand 1 with iron(II) salts has
been recently demonstrated by our group to be a very efficient
catalyst for highly enantioselective Mukaiyama aldol reactions
Et
2
O
CH
CH
CH
CH
CH
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
2
2
2
2
c
d
De´partement de chimie, Universite´ Laval,
1
045 avenue de la Me´decine, Que´bec (Que´bec) G1V 0A6, Canada.
E-mail: thierry.ollevier@chm.ulaval.ca
w Electronic supplementary information (ESI) available: Experimental
procedures and full characterization data for all compounds are
provided. CCDC 850236 (1). For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c2cc18032d
0
(CH Cl)
2
2
88
a
b
Conditions: epoxide (1.0 equiv.), aniline (1.0 equiv.). Determined
c
by chiral HPLC analysis. Fe(ClO
d
4
)
2
(10 mol%), 1 (12 mol%).
Epoxide (1.0 equiv.), aniline (2.0 equiv.).
3
806 Chem. Commun., 2012, 48, 3806–3808
This journal is c The Royal Society of Chemistry 2012

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Table 2 Catalytic asymmetric cis-stilbene oxide-opening reaction
with various aniline derivatives—substrate scope
Table 3 Catalytic asymmetric aromatic meso-epoxide-opening reac-
tion with aniline
a
a
b
ee (%)
Entry
1
Epoxide
Time/h
72
Yield 5 (%)
b
ee (%)
Entry
Aniline
R
Time/h
Yield 4 (%)
1
2
3
4
5
6
7
8
H
16
72
72
16
48
16
24
24
90
78
70
90
78
87
90
90
95
92
82
94
90
94
96
96
2-Me
2-OMe
4-Cl
2-Cl
4-Br
79
90
2
36
80
95
4-CF
4-CN
3
a
b
Conditions: epoxide (1.0 equiv.), aniline (1.0 equiv.). Determined
by chiral HPLC analysis.
9
72
70
93
1
1
0
1
24
16
81
87
95
94
1
2
3
H
16
16
95
88
95
96
1
Br
a
b
Conditions: epoxide (1.0 equiv.), aniline (1.0 equiv.). Determined
by chiral HPLC analysis.
and comparable results were obtained (entry 10). CH Cl
2
2
was then
O]2+Á2ClO4À
Fig. 1 ORTEP (50% ellipsoid) of a [1ÁFeÁ2MeCNÁH
2
À
kept as the solvent of choice under our optimized conditions.
Optimal conditions established for the enantioselective epoxide-
opening reaction with aniline 3a were applied to differently
substituted anilines (Table 2). In all cases, the desired amino
alcohol was obtained in high enantioselectivity. For mono-
substituted anilines, electron-donating and -withdrawing sub-
stituents, in ortho- or para-positions, had negligible influence
on the enantioselectivity (entries 1–8). Except for o-anisidine that
complex. 2ClO
4
and hydrogens are omitted for clarity.
The bipyridine ligand is coordinated to the metal center in a
tetradentate manner. An additional water molecule in the
equatorial position affords a slightly distorted pentagonal
basis. Two acetonitrile molecules in apical positions furnished
II
a heptacoordinated chiral Fe complex. Additional HRMS
measurements (ESI-TOF) corroborated the formation of the
complex in solution. Both molecular peaks corresponding to
the mono- and the bis-cationic complexes were detected at
1
5
gave a slightly lower enantioselectivity (entry 3), other substi-
tuted anilines furnished the desired amino alcohol in Z90% ee.
Sterically hindered anilines, such as 2,6-dimethylaniline and
N-methylaniline, maintained good yields and led to high
enantioselectivities (entries 9 and 10). a- and b-Naphthylamines
also reacted smoothly to provide the product in very good yields
with high enantioselectivities (entries 11 and 12). Functionalized
naphthylamine, such as 1-amino-4-bromonaphthalene, also led
to a highly enantio-enriched amino alcohol, which could be
further transformed to introduce other functional groups
+
2+
483.0978 [M + ClO ] and 192.0746 [M] respectively (Fig. 2).
4
In summary, the catalytic enantioselective opening of aromatic
meso-epoxides with various aniline derivatives has been achieved
4 2
with Fe(ClO ) and Bolm’s bipyridine ligand. The desired chiral
b-amino alcohols were obtained in mostly good to very good
yields with excellent enantioselectivities (generally 90–96% ee).
The chiral pre-catalyst structure was characterized both in solid
state and in solution. Mono- and bis-cationic complexes were
detected by high resolution ESI-MS and a crystal structure
analysis revealed a scarcely disclosed heptacoordinated chiral
(
entry 13). Two other aromatic meso-epoxides were tested in
16
the opening-reaction with aniline (Table 3). The corresponding
amino alcohols were obtained in good yields and high enantio-
selectivities (ee Z90%).
II
Fe complex. Our method has the advantage of using a low
catalyst loading of an environmentally benign Lewis acid and an
equimolar amount of both reagents, allowing atom-economical
synthesis. In addition, the chiral ligand can be easily recycled at
the end of the reaction. Further studies to clarify the precise
mechanism are now in progress.
To obtain some information on the chiral Fe pre-catalyst,
we performed an X-ray structure analysis of single crystals
17
4 2
obtained from the 1–Fe(ClO ) complex (Fig. 1). The complex
adopts a pentagonal bipyramidal geometry. The structure of
this complex is similar to those previously reported by Kobayashi
This work was financially supported by the Natural Sciences
and Engineering Research Council of Canada (NSERC),
18
19
et al. with ScBr3 and BiBr3 or by Schneider et al. with InBr3.
6c
This journal is c The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 3806–3808 3807

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6
16 Decreased enantioselectivities were obtained using non-aromatic
meso-epoxides (cyclohexene oxide, ee o 10%).
2
+
À
4
).
17 CCDC 850236 ([1ÁFeÁ2MeCNÁH
2
O] Á2ClO
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3
808 Chem. Commun., 2012, 48, 3806–3808
This journal is c The Royal Society of Chemistry 2012