Asymmetric Diels-Alder Reaction of α,β-Unsaturated Oxazolidin-2-one Derivatives Catalyzed by a Chiral Fe(III)-Bipyridine Diol Complex

Article abstract of DOI:10.1021/acs.orglett.7b03939

An asymmetric Fe^III-bipyridine diol catalyzed Diels-Alder reaction of α,β-unsaturated oxazolidin-2-ones has been developed. Among various Fe^II/Fe^III salts, Fe(ClO₄)₃·6H₂O was selected as th

Full text of DOI:10.1021/acs.orglett.7b03939

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Asymmetric Diels−Alder Reaction of α,β-Unsaturated Oxazolidin-2-
one Derivatives Catalyzed by a Chiral Fe(III)-Bipyridine Diol Complex
Mao Li, Virginie Carreras, Angela Jalba, and Thierry Ollevier*
́ ́ ́ ́
Departement de Chimie, Universite Laval, 1045 avenue de la Medecine, Quebec, QC G1V 0A6, Canada
S
* Supporting Information
ABSTRACT: An asymmetric FeIII-bipyridine diol catalyzed Diels−Alder reaction
of α,β-unsaturated oxazolidin-2-ones has been developed. Among various FeII/
FeIII salts, Fe(ClO4)3·6H2O was selected as the Lewis acid of choice. The use of a
low catalyst loading (2 mol % of Fe(ClO4)3·6H2O and 2.4 mol % of Bolm’s
ligand) afforded high yields (up to 99%) and high enantiomeric excesses (up to
98%) of endo-cycloadducts for the Diels−Alder reaction between cyclopentadiene
and substituted acryloyloxazolidin-2-ones. Other noncyclic dienes led to decreased
enantioselectivities. A proposed model supports the observed stereoinduction.
he Diels−Alder reaction is one of the most straightfor-
and indoles,22 and thia-Michael addition reactions.23 The
T
ward and atom economical methods to construct chiral
present investigation provides attractive new conditions for
the asymmetric Diels−Alder reaction between various dienes
and oxazolidinone dienophiles, catalyzed by Bolm’s ligand (L*)
combined with various iron salts.
Initial studies were focused on screening the efficiency of
various solvents on the Diels−Alder reaction (Table 1).
six-membered rings in organic chemistry1 and represents a
great interest in total synthesis and for the preparation of
biologically active compounds.1b,2 This reaction is an attractive
synthetic transformation providing regio- and stereoselective
compounds. Much effort has been invested for better
stereocontrol of the process with the use of chiral Lewis
acids. The Diels−Alder reaction between oxazolidinones and
cyclopentadiene is one of the most successful reactions to
evaluate the catalytic activity of a metal Lewis acid catalyst.1c,d
Asymmetric Diels−Alder reactions are well-known involving
Cu,3 Mg,4 Zn,5 Al,6 Pd,7 B,8 Ti,9 Sc,10 Ni,11 and lanthanides.12
Pioneering work by Narasaka,9b Corey,13 Evans,14 Bolm,15 and
others16 involved oxazolidinone derivatives chosen as sub-
strates.
Table 1. FeIII-Catalyzed Reaction of Cyclopentadiene with 3-
a
Acryloyloxazolidin-2-one 2a: Solvent Study
b
c
d
entry
solvent
yield (%)
endo/exo
ee (%)
1
2
3
4
5
6
7
CH2Cl2
(CH2Cl)2
MeCN
THF
83
83
85
52
62
76
92
82:18
87:13
90:10
85:15
87:13
91:9
90
90
96
76
12
92
0
CHCl3
Iron complexes show major advantages due to their low
toxicity, environmental benignity, commercial availability, and
low cost.17 From a green chemistry point of view, the
development of novel iron-catalyzed reactions represents a
challenge.18 A few iron catalysts were used in the asymmetric
Diels−Alder reaction since the 1990s, the reaction of 3-
acryloyloxazolidin-2-one being usually chosen as benchmark
reaction.19 An efficient C2-symmetrical chiral bis-oxazoline-FeI3
e
DMC
H2O
90:10
a
Conditions: Fe(ClO4)3·6H2O (5 mol %), L* (6 mol %), 1 (3.5
b
mmol), 2a (0.5 mmol), and solvent. Yield of isolated products
c
d
(diastereomeric mixture). Determined by 1H NMR. Determined for
e
the endo isomer by chiral HPLC. Carried out at 2 °C.
19b
catalyst,19a and pybox-FeBr3 catalyst with AgSbF6 have been
Cyclopentadiene (1) and 3-acryloyloxazolidin-2-one (2a)
were selected as model substrates to optimize the reaction
conditions. It was noted that the conversion and enantio-
selectivity are strongly influenced by the solvents used. High
ee’s (90%) were first obtained by using polar, aprotic, and
noncoordinating solvents, such as CH2Cl2 and (CH2Cl)2
(entries 1 and 2), but long reaction times (48 h) were needed
to obtain complete conversions. By using a coordinating
solvent, such as MeCN, the reaction time was decreased to
1.5 h with a higher ee (96%) and a good isolated yield (85%)
(entry 3). When the reaction was run in another polar, aprotic,
disclosed by Corey and Shibasaki, respectively, providing high
enantioselectivities. Good ee’s have also been reported using
chiral bidentate C2-symmetrical bis(sulfoxide) ligands with
FeI3,19c diphosphine with FeII and FeIII,19d,e and a 2,2′-
binaphthyl-diimine ligand with FeCl2.11b However, some of
the methods suffer from a number of drawbacks, such as limited
scope and lack of generality. Chiral 2,2′-bipyridyl ligands have
been often used in asymmetric reactions due to their stability
and excellent coordinating ability to a wide range of metal ions.
As part of an ongoing interest in iron catalysis, our laboratory
studied the use of a chiral 2,2′-bipyridyl diol ligand developed
by Bolm,20 coordinated with FeII, capable of catalyzing
asymmetric Mukaiyama aldol,21 epoxide-opening with anilines
Received: December 18, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b03939
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
and coordinating solvent, such as THF, a longer reaction time
was needed (15 h), and a low yield and ee were obtained (entry
4). Chloroform was also tested but afforded a low yield and
enantioselectivity (entry 5). Then, a greener solvent, dimethyl
carbonate (DMC), was tested (entry 6), and an ee of 92% was
obtained after 5 h.24 Water, known as often being beneficial in
the Diels−Alder reaction,25 was then tested as a solvent, and
the reaction was complete after 15 h (entry 7). A longer
reaction time was needed, and no enantioselectivity was
observed. However, none of these solvents gave results superior
to MeCN (entry 3), which was consequently chosen in further
studies.
Table 3. Optimization of the Iron Salt Used in the Model
Reaction
a
b
c
entry
FeXn
yield (%)
endo/exo
ee (%)
d
1
2
3
4
5
6
7
8
Fe(ClO4)2·6H2O
Fe(OTf)2
Fe(OTf)3
FeCl2
97
90:10
94:6
78
40
96
0
d
95
d
96
96:4
e
22
88:12
90:10
76:24
83:17
87:13
e
FeCl3
7
2
e
FeI3
14
2
e
FeBr3
11
2
e
FeBr3 + AgSbF6
50
36
a
Conditions: FeXn (2 mol %), L* (2.4 mol %), 1 (3.5 mmol), 2a (0.5
To improve the efficiency of the reaction, various catalyst
loadings and temperatures were screened. A temperature
decrease led to an increase in ee (Table 2, entries 1−3).
b
c
1
mmol), and MeCN. Determined by H NMR. Determined for the
endo isomer by chiral HPLC. Yield of isolated product (diastereo-
meric mixture). Yield determined by H NMR.
d
e
1
Table 2. FeIII-Catalyzed Reaction of Cyclopentadiene with
Having identified optimal conditions, we next sought to
demonstrate the scope of this procedure. Under the optimized
reaction conditions,27 the range of dienophiles was broadened,
and the results are listed in Scheme 1. First, dienophile
3‑Acryloyloxazolidin-2-one 2a: Screening of the Catalyst
a
Loading
b
c
d
entry
x
y
temp (°C) yield (%)
endo/exo
ee (%)
1
2
3
4
5
6
7
8
5
6
rt
85
84
96
91
93
99
82
89
92:8
91:9
93:7
88:12
90:10
92:8
92:8
92:8
96
98
98
96
98
98
92
70
Scheme 1. Influence of the Dienophile on the Reaction of
Cyclopentadiene and Various Acryloyloxazolidin-2-ones
5
6
0
a
5
6
−30
−30
−30
−30
−30
−40
5
10
5
2.5
2
2.4
1.2
2.4
1
2
a
Conditions: Fe(ClO4)3·6H2O (x mol %), L* (y mol %), 1 (3.5
b
mmol), 2a (0.5 mmol), and MeCN. Yield of isolated products
c
d
(diastereomeric mixture). Determined by 1H NMR. Determined for
the endo isomer by chiral HPLC.
Indeed, by lowering the temperature, the reaction proceeded
smoothly to afford the desired product. Using −30 °C as the
optimized reaction temperature, various catalyst loadings and
ratios of Fe(ClO4)3·6H2O to L* were tested (entries 4−7). A
low catalyst loading, i.e. 2 mol % of Fe(ClO4)3·6H2O, together
with 2.4 mol % of L*, turned out to be efficient enough to
afford an excellent yield (99%), good diastereoselectivity (endo/
exo = 92:8), and a high ee of 98% (entry 6). A lower catalyst
loading of 1 mol % of Fe(ClO4)3·6H2O, together with 1.2 mol
% of L*, led to a decrease of the catalytic efficiency and the ee
(entry 7). A control experiment was performed at a lower
temperature (−40 °C, entry 8). Surprisingly, the level of
enantioselectivity was neither increased nor maintained.
Other FeII/FeIII salts, i.e. Fe(ClO4)2·6H2O, Fe(OTf)2, and
Fe(OTf)3, provided excellent yields (Table 3, entries 1−3, 95−
97%), although the highest ee was obtained with the FeIII salt.
No enantioselectivity was observed by using FeCl2 and FeCl3
(entries 4−5). FeI3, previously used by Corey with a bis-
oxazoline ligand,19a appeared to be inefficient in terms of
conversion and enantioselectivity (entry 6). Using FeBr3, a low
yield and no enantioselectivity were obtained (entry 7). As
previously disclosed by Shibasaki,19b the use of AgSbF6 as an
additive with FeBr3 had a positive impact on both yield and
enantioselectivity of the Diels−Alder reaction (entry 8).
However, Fe(ClO4)3·6H2O and Fe(OTf)3 were still far more
superior. The optimized reaction conditions (2 mol % of
Fe(ClO4)3·6H2O and 2.4 mol % of L* in MeCN) were selected
for further studies.26
a
Conditions: Fe(ClO4)3·6H2O (2 mol %), L* (2.4 mol %), 1 (3.5
b
mmol), 2a (0.5 mmol), and MeCN. Yield of isolated products
c
d
(diastereomeric mixture). Determined by 1H NMR. Determined for
e
the endo isomer by chiral HPLC. Fe(ClO4)3·6H2O (4 mol %), L*
(4.8 mol %), CH2Cl2.
(E)-3-(but-2-enoyl)oxazolidin-2-one 2b was employed. After 1
day, this dienophile efficiently reacted with the cyclopentadiene
and afforded compound 3b with an excellent yield (95%) and a
high ee (98%). Cycloadduct 3c was obtained with a moderate
yield (70%) and 80% ee even when using a catalyst loading of 4
mol % of Fe(ClO4)3·6H2O and 4.8 mol % of L* in CH2Cl2.28
This decrease could be attributed to the steric effect of the Ph
ring on dienophile 2c. Using dienophile (E)-3-(4,4,4-trifluor-
obut-2-enoyl)oxazolidin-2-one 2d, the reaction proceeded
B
DOI: 10.1021/acs.orglett.7b03939
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
smoothly at −10 °C and was complete after 8 h. However, a
dramatic decrease of ee (14%) was observed. Analogous
dienophiles 2e and 2f were also tested to get further insight
on the role of the structure of the dienophile. A high ee (96%)
was achieved by employing dienophile 2e after a short reaction
time (8 h). This means that the absence of an oxygen atom in
the oxazolidinone ring has no impact on the selectivity. Using
dienophile 2f, no enantiomeric excess was observed. This result
suggests that the coordination of the two carbonyl oxygens of
the dienophile to the complex is essential to afford
enantioselectivity. To extend the scope of the Diels−Alder
reaction, other electron-poor dienophiles incorporating 2-
alkenoylpyridine and N-oxide substitution were also tested
(2g−j). These similar substrates have been used previously by
others in a CuII catalyzed Diels−Alder reaction3d and are
known to coordinate to the metal center by the pyridine and
the carbonyl group lone pairs. Unfortunately, no ee was
observed by using 2-alkenoylpyridine N-oxide 2g, suggesting
that the chelation of 2g to the FeIII complex by the carbonyl
group and the negatively charged oxygen coming from the
oxide does not occur. Using 2-alkenoylpyridine dienophiles 2h
and 2i, possessing a Ph or an electron-withdrawing group, such
as p-ClC6H4, 84% ee and 70% ee were respectively obtained.
Otherwise, an electron-donating group such as p-MeOC6H4
(2j) was not beneficial to the enantioselectivity (12%).
Figure 1. Proposed transition states involving the FeIII chiral catalyst.
solvent molecule (noted as S) is coordinated to the axial site
and a fifth labile equatorial position is occupied by the
reversible chelation of an oxygen of the cis-oxazolidinone,3h as
supported by DFT.23 Consequently, the favored transition state
(A) shows the endo approach of the diene from the back side,
with the oxazolidin-2-one carbonyl occupying the equatorial
position. The endo approach arises from the steric hindrance of
the tert-butyl group of L*, which shields the diene approach
from the front side.
In summary, we have successfully demonstrated an efficient
catalytic system capable of catalyzing the Diels−Alder reaction
between various dienophiles and dienes to give high isolated
yields (up to 99%) with high ee’s (up to 98%) of the desired
products. This method includes using 2.4 mol % of L* and
2 mol % of Fe(ClO4)3·6H2O to generate the FeIII complex in
situ. To the best of our knowledge, this is the first example of an
iron-catalyzed Diels−Alder reaction using a chiral bipyridine
diol ligand. Work is currently focused on further applications of
the method toward the synthesis of biologically active targets
and will be reported in due course.
Various dienes were then tested in the Diels−Alder reaction
using the same model dienophile, i.e. 3-acryloyloxazolidinone
2a. All dienes afforded good yields and moderate to high ee’s of
products (Scheme 2). However, a higher catalyst loading and
Scheme 2. Influence of the Diene on the Enantioselectivity
a
of the Reaction with 3-Acryloyloxazolidinone
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.7b03939.
Experimental details, characterization date, NMR spectra,
and chromatograms (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: thierry.ollevier@chm.ulaval.ca.
ORCID
a
Conditions: Fe(ClO4)3·6H2O (4 mol %), L* (4.8 mol %), 1k−n (3.5
b
mmol), 2a (0.5 mmol), and MeCN. Carried out at −10 °C for 72 h.
c
d
Yield of isolated products (diastereomeric mixture). Determined by
e
f
chiral HPLC. ee of the major isomer. Determined by 1H NMR.
Thierry Ollevier: 0000-0002-6084-7954
Notes
g
Fe(ClO4)3·6H2O (8 mol %) and L* (9.6 mol %) were used.
The authors declare no competing financial interest.
longer reaction time were needed to complete the reactions.
The Diels−Alder reaction between less reactive 1,3-cyclo-
hexadiene 1k and 3-acryloyloxazolidinone 2a afforded a
moderate yield of product 3k (70%), a high diastereoselectivity
(endo/exo = 94:6), and a 92% ee for the endo adduct. By
employing unsubstituted butadiene 1l, an 88% ee with a 73%
yield of 3l was obtained. With methyl-substituted dienes (1m
and 1n), the enantioselectivities were decreased (50% ee and
60% ee, respectively).
ACKNOWLEDGMENTS
We gratefully acknowledge Natural Sciences and Engineering
Research Council of Canada (NSERC), the Center in Green
■
́
Chemistry and Catalysis (CGCC), and Universite Laval for
financial support of our program. M.L. thanks China Scholar-
ship Council (CSC) for a scholarship.
REFERENCES
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DOI: 10.1021/acs.orglett.7b03939
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DOI: 10.1021/acs.orglett.7b03939
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