Asymmetric FeII-Catalyzed Thia-Michael Addition Reaction to α,β-Unsaturated Oxazolidin-2-one Derivatives

Article abstract of DOI:10.1021/acs.orglett.7b03118

A highly enantioselective Fe^II-catalyzed thia-Michael addition to α,β-unsaturated carbonyl derivatives was developed. The scope of the reaction was demonstrated with a selection of aromatic, heterocyclic and aliphatic thiols, and various Michael acceptors. The corresponding β-thioethers were obtained in good to excellent yields (up to 98%) and moderate to excellent enantioselectivities (up to 96:4 er). Unusual hepta-coordination of the metal and chelation to α,β-unsaturated oxazolidin-2-one derivatives allowed the construction of a coherent model rationalizing the enantioselective event. DFT calculations support the proposed model for observed stereoselectivities.

Full text of DOI:10.1021/acs.orglett.7b03118

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX-XXX
Asymmetric FeII-Catalyzed Thia-Michael Addition Reaction to
α,β‑Unsaturated Oxazolidin-2-one Derivatives
Samuel Lauzon,† Hoda Keipour,† Vincent Gandon,*,‡,§ and Thierry Ollevier*,†
†Dep
‡Institut de Chimie Molec
Cedex, France
́
artement de chimie, Universite
́
Laval, 1045 avenue de la Med
́ ́
ecine, Quebec, QC G1V 0A6, Canada
́
ulaire et des Mater
́
iaux d’Orsay, CNRS UMR 8182, Univ. Paris-Sud, Universite Paris-Saclay, Orsay 91405
́
§Institut de Chimie des Substances Naturelles, CNRS UPR 2301, Univ. Paris-Sud, Universite
Gif-sur-Yvette, France
́
Paris-Saclay, 1 av. de la Terrasse, 91198
S
* Supporting Information
ABSTRACT: A highly enantioselective FeII-catalyzed thia-Michael addition to α,β‑unsatu-
rated carbonyl derivatives was developed. The scope of the reaction was demonstrated with a
selection of aromatic, heterocyclic and aliphatic thiols, and various Michael acceptors. The
corresponding β-thioethers were obtained in good to excellent yields (up to 98%) and
moderate to excellent enantioselectivities (up to 96:4 er). Unusual hepta-coordination of the
metal and chelation to α,β-unsaturated oxazolidin-2-one derivatives allowed the construction
of a coherent model rationalizing the enantioselective event. DFT calculations support the
proposed model for observed stereoselectivities.
he use of iron complexes as catalysts has arisen from
highly asymmetric thia-Michael addition on acyclic α,β‑unsatu-
Ttraditional transition-metal catalysis in various synthetic rated derivatives using iron salts as environmentally benign chiral
transformations of modern organic chemistry.1 Many catalysts
catalysts is disclosed herein. Experimental insights into the
influence of the nucleophile on the stereoselectivity control,
together with DFT calculations, are also presented.
are originally derived from rare metals such as palladium,
ruthenium, platinum, and iridium. Their low availability, toxicity,
and high market prices encourage their replacement by
alternative metals.2 From a sustainable chemistry perspective,
developing new synthetic methods using iron, which is abundant,
inexpensive, environmentally benign, and relatively nontoxic in
comparison with other metals, is a major advantage.2b,3 The
ability to use iron in homogeneous asymmetric catalysis is
promising for the development of greener synthetic methods,4
contributing to major improvements for both academia and
pharmaceutical industries.5 The 1,4-addition to α,β-unsaturated
carbonyl derivatives is one of the most powerful and efficient
ways to create new C−C bonds and various heteroatom−carbon
bonds, such as N−C, O−C, and S−C bonds.6 Sulfur-containing
compounds, which are essential building blocks for biologically
active pharmaceutical agents, are important targets.7 Chiral
Brønsted acids and bases,8 such as thiourea and squaramide
cinchona alkaloid derivatives,9 and a noncovalent NHC10 have
been reported to be efficient catalysts for the thia-Michael
addition reaction. Previous work has been disclosed using chiral
Lewis acid catalysts derived from NiII,11 HfIV,12 ScIII,13 and CoII
salts14 for the asymmetric 1,4-addition of thiols to α,β-
unsaturated oxazolidin-2-ones. Chiral FeII and FeIII complexes
afforded β‑thioethers with excellent yields and enantioselectiv-
ities.15 However, most of the methods use costly transition
metals, high catalytic loadings, or chlorinated solvents.
Asymmetric Michael addition of benzylthiol 2a to (E)-3-
crotonoyloxazolidin-2-one 1a, run in THF, was initially selected
as the model reaction for the screening of various iron salts with
(S,S)-Bolm’s ligand18 L* (Table 1). FeCl2 showed a moderate
chiral induction in a low conversion (entry 1). Fe(BF4)2·6H2O,
Fe(OTf)2, and Fe(ClO4)2·6H2O afforded (3R)-3a with good
enantioselectivities and moderate to good conversions (entries
2−4). FeIII salts, such as FeCl3 and Fe(acac)3, allowed the
formation of thioether (3R)-3a with low chiral inductions
(entries 5 and 6). Fe(OTf)3 and Fe(ClO4)3·6H2O led to good
er’s with low conversions (entries 7 and 8). Overall, both
conversions and enantioselectivities were higher when FeII vs
FeIII salts were used. An optimum 95:5 er was obtained, together
with high conversion, when Fe(ClO4)2·6H2O was used, and thus,
this system was used as a chiral catalyst in our study (entry 4). It is
noteworthy to mention that the addition of 4 Å molecular sieves,
known as attractive additives in asymmetric reactions,19
enhanced the enantioselectivity but extended the reaction
time.20 Since 4 Å MS were advantageous in terms of er, they
were used as an additive in the thia-Michael addition reaction.
The optimization study was first performed using various
solvents (Table 2). An increase of the substrate concentration
from 0.5 to 1 M in THF led to a higher conversion and shorter
reaction time but unchanged er (entry 1 vs Table 1, entry 4).
Et2O and CH2Cl2 led to good yields with lower enantioselectiv-
In our continuing studies in asymmetric catalysis using iron
salts, i.e., Mukaiyama aldol, meso-epoxide opening, and aromatic
sulfide oxidation reactions, FeII/chiral bipyridine complexes have
proven to be efficient enantioselective catalysts.16 A simple and
Received: October 6, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b03118
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Table 1. Iron-Catalyzed Thia-Michael Addition to (E)-3-
Crotonoyloxazolidin-2-one−Catalyst Optimization
Scheme 1. FeII-Catalyzed Conjugate Addition of Different
Thiols to (E)-3-Crotonoyloxazolidin-2-one
a
a c
−
b
c
entry
FeXn
yield (%)
er
1
2
FeCl2
14
50
42
77
20
64
19
28
63:37
91:9
Fe(BF4)2·6H2O
Fe(OTf)2
d
3
93:7
4
5
6
7
8
Fe(ClO4)2·6H2O
FeCl3
95:5
62:38
55:45
88:12
89:11
Fe(acac)3
Fe(OTf)3
Fe(ClO4)3·6H2O
a
Conditions: FeXn (5 mol %), L* (6 mol %), 4 Å MS, 1a (0.5 mmol),
and 2a (1 mmol), THF. Conversion by 1H NMR.17 Determined by
chiral HPLC (OD−H column). Reaction stopped after 72 h.
b
c
a
Conditions: Fe(ClO4)2·6H2O (5 mol %), L* (6 mol %), 4 Å MS, 1a
d
b
(0.5 mmol), 2b−o (2.5 mmol), MeCN. Determined by chiral HPLC.
c
Absolute configurations assigned from the literature, except for
Table 2. FeII-Catalyzed Thia-Michael Addition to (E)-3-
Crotonoyloxazolidin-2-one−Solvent Optimization
optically active 3b, 3c, 3f, and 3l (unknown absolute configurations).
a
Scheme 2. FeII-Catalyzed Conjugate Addition of Benzylthiol
a c
−
to α,β-Unsaturated Carbonyl Compounds
b
d
entry
solvent
T (°C)
time (h)
yield (%)
er
1
2
3
4
5
6
THF
25
25
24
24
88
82
77
89
94
95:5
Et2O
84:16
82:18
89:11
96:4
CH2Cl2
PhMe
MeCN
MeCN
MeCN
MeCN
MeCN
25
24
25
72
25
24
c
−10
−20
25
48
67
98:2
e
c
7
144
144
24
36
97:3
f
c
8
g
51
94:6
9
25
94
51:49
a
Conditions: Fe(ClO4)2·6H2O (5 mol %), L* (6 mol %), 4 Å MS, 1a
b
(0.5 mmol), 2a (1 mmol), solvent. Yield of isolated product.
a
c
d
Conversion by 1H NMR. Determined by chiral HPLC (OD−H
Conditions: Fe(ClO4)2·6H2O (5 mol %), L* (6 mol %), 4 Å MS,
b
e
1a−k (0.5 mmol), 2a (2.5 mmol), MeCN. Determined by chiral
HPLC. Absolute configurations assigned from the literature, except
column). With 7.5 mol % of Fe(ClO4)2·6H2O and 9 mol % of L*.
c
f
g
With 1 equiv of 2a. 6 mol % of L* used in the absence of Fe(ClO4)2·
for optically active 4c, 4d, 4f, 4g, and 4i (unknown absolute
configurations).
6H2O.
ities (entries 2 and 3). Using PhMe as a solvent afforded a good
er, albeit in an extended reaction time (entry 4). A coordinating
and polar solvent, such as MeCN, afforded (3R)-3a in high yield
and excellent enantioselectivity (entry 5). Considering the
azaphilicity of iron,21a,b the binding of MeCN in the coordination
sphere of FeII increased the transition-state stability, which
affected both the rate and selectivity.21c These optimized
catalytic conditions involving Fe(ClO4)2·6H2O/L* in MeCN
were chosen for further studies.22 The temperature was then
decreased from +25 to −10 °C, and then −20 °C, and up to 98:2
er’s were obtained, whereas conversions were low (entries 6 and
7). However, pursuing this study at 25 °C appeared to be the
most practical alternative inducing minor er variations. When a
stoichiometric quantity of thiol 2a was used, a similar er was
obtained in a prolonged reaction time (entry 8). Virtually no
enantioselectivity was obtained when (S,S)-Bolm’s ligand was
used in the absence of an FeII salt (entry 9).
Figure 1. Postulated mechanism.
B
DOI: 10.1021/acs.orglett.7b03118
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Figure 2. Geometries of the computed complexes IIa.
To demonstrate the scope of the Fe(ClO4)2·6H2O-catalyzed
asymmetric Michael addition, a range of thiols was studied.
Substituted aromatic, heterocyclic, and aliphatic thiols were used.
As previously shown, the stereoselectivity does not depend on
thiol concentration. Consequently, 5 equiv of thiol was used to
promote the conversion.23 Michael addition reactions of various
substituted thiols (2b−o) on (E)-3-crotonoyloxazolidin-2-one
1a were run under the optimal reaction conditions (Scheme 1).
The FeII-catalyzed Michael addition of various thiols to (E)-3-
crotonoyloxazolidin-2-one 1a surprisingly led to a wide range of
enantioselectivities. Up to 96:4 er’s were obtained when para-
and ortho-substituted benzylthiols were used (3b−f). In
comparison, lower stereoselectivity control was observed using
para- and ortho-substituted phenylthiols ((3R)-3h−(3R)-3k).
The same observation was made when unsubstituted thiophenol
2g was used ((3R)-3g vs (3R)-3a). Electron-rich and -deficient
groups in the para position of the thiol aromatic ring had
negligible influence on the enantioselectivity in both the benzyl
and phenyl series (3b−3e and (3R)-3h−(3R)-3j). These groups
changed the thiol reactivity, but with no influence on the
stereoselectivity control. o-Methyl substitution of the aryl thiol
induced an increased er, which was not observed using thiol 2f
((3R)-3k vs 3f). No enantioselectivity was obtained using
2‑mercaptopyridine (3l). Furfuryl thiol, as a heterocyclic
nucleophile, led to an excellent er ((3R)-3m). Butyl and
isopropyl thiols led to good and moderate enantioselectivities,
respectively ((3R)-3n and (3R)-3o). Finally, all β-thioethers in
this study were obtained in good to excellent yields (up to 98%).
A variety of α,β-unsaturated carbonyl compounds (1a−i) were
examined as substrates for the 1,4-addition of benzylthiol 2a,
under the optimal reaction conditions (Scheme 2). The
electronic and steric effects of various R1 groups on the
enantioselectivity were studied. Among a small chosen set of
α,β-unsaturated oxazolidin-2-ones, comprising electron-donat-
ing (1a, 1b) and electron-withdrawing (1c, 1d) groups, the
highest er was obtained with R1 = Me ((3R)-3a). A major
decrease of the enantioselectivity was observed when increasing
the electron-withdrawing ability of R1 from CH3 to CF3 ((3R)-3a
vs 4d), albeit a high level of er was maintained with R1 = CO2Et
(4c). Introducing a Me group at the α-position (R2) instead of
the β-position (R1) caused the enantioselectivity to no longer be
dependent on the Re addition face, but rather on the later
protonation step. A low enantioselective α-protonation step by
the chiral catalyst was highlighted by the 64:36 er obtained with
(2R)-4e. A slightly reduced er was afforded using a 2-
pyrrolidinone 5-membered cycle (4f). The nature of the
chelating properties of R3 was also examined. Pyridyl, phenyl,
and pyridyl N-oxide as R3 substituents were studied in regard to
their different chelating structures; the enantioselectivity
dropped dramatically in comparison with the oxazolidi-2-one
analogue (4g−4i vs (3S)-4b). The absence of a dicarbonyl
chelating system did not lead to any reaction using
α,β‑unsaturated amide 1j and ester 1k (0% of 4j and 4k,
recovered starting materials). Indeed, the dicarbonyl core of the
Michael acceptor appeared to be essential for maintaining high
levels of stereocontrol in the asymmetric 1,4-addition of thiols
using FeII/L* system.
A catalytic cycle was postulated based on precedents gained on
the coordination of FeII salt with L* (Figure 1).16a−c The
bipyridine ligand is coordinated to the metal center in a
tetradentate fashion via the coordination of two O and N atoms
in the four equatorial sites of the octahedral d6 FeII center. MeCN
molecules (noted as S) are coordinated to the axial sites as shown
in I. As observed from previous crystallographic studies using FeII
and L*,16a the appearance of a fifth labile equatorial substituent
arisen from electrostatic interactions, reveals the pentagonal
bipyramidal geometry adopted by the chiral catalyst. After ligand
exchange with substrate 1a, both carbonyl groups chelated FeII to
form complex II. This hypothesized structure of complex II is
similar to the previously reported C2-symmetrical FeIII/bis-
(oxazoline) complex by Corey.24 Then, the thiol would attack
the more accessible Re face of the β-carbon of the
α,β‑unsaturated carbonyl compound to give III. Enantioselectiv-
ities were induced by the steric hindrance of the tBu group of the
ligand, which blocks the nucleophilic attack on the Si face of the
substrate 1a. Finally, a low enantioselective protonation on III
(as noted with 4e) generates the expected thioether adducts. The
high enantioselectivities arising from the chiral induction in the
β-position are in agreement with this speculated mechanism.
In the endo complex II depicted above, the CC bond of 1a is
oriented toward the bipyridine backbone, making its Re face the
most accessible one for the thiol. Exchanging the equatorial and
apical carbonyls would place the CC bond in the opposite
direction, with the Si face being more accessible. Thus, the endo/
exo orientation of the substrate is a critical feature, as it
determines the absolute configuration of the product. To validate
the endo configuration as the preferred one, DFT computations
were carried out at the B3LYP/SDD(Fe)-cc-pVTZ (other
elements)//B3LYP/SDD(Fe)-6-31+G(d) (other elements), as
recently described for the mechanism of FeII-catalyzed
asymmetric Mukaiyama aldol reaction.25 Using MeCN as axial
ligand, three isomers IIa−c could be optimized (Figure 2). In the
exo complexes IIa and IIb, the oxazolidin-2-one carbonyl
occupies the apical position. The second carbonyl is located at
the equatorial position in IIa. In IIb, the second carbonyl is
actually H-bonded to a hydroxyl group. In the endo series, no H-
bonded isomers could be modeled, as they irremediably collapse
to IIc exhibiting the oxazolidin-2-one carbonyl at the equatorial
position and the second carbonyl at the apical position. In
agreement with the observed stereoselectivity of the title
C
DOI: 10.1021/acs.orglett.7b03118
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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e, B. Nat. Prod. Rep. 2015, 32, 1541.
reaction, the endo complex IIc is the most stable of the three
(ΔG298 IIc (0.0); IIa (3.0); IIb (3.5)).
(b) Roschangar, F.; Colberg, J.; Dunn, P. J.; Gallou, F.; Hayler, J. D.;
Koenig, S. G.; Kopach, M. E.; Leahy, D. K.; Mergelsberg, I.; Tucker, J. L.;
Sheldon, R. A.; Senanayake, C. H. Green Chem. 2017, 19, 281.
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Zhao, Y.; Yan, L.; Tan, C. H.; Jiang, Z. Y. Adv. Synth. Catal. 2014, 356,
1292.
In summary, we have successfully developed an efficient
asymmetric Michael addition of thiols. An Fe(ClO4)2·6H2O
conjointly used with Bolm’s ligand has been shown to be an
effective catalyst for the asymmetric Michael addition of thiols to
α,β-unsaturated oxazolidin-2-ones. The FeII/(S,S)-L* catalytic
system was suitable for catalyzing asymmetric thia-Michael
addition giving yields up to 98% and er’s up to 96:4. The method
is practical and cost-effective, and high enantioselectivities were
obtained at room temperature. The chiral bipyridine ligand can
be easily prepared and recycled in the purification process. A
model has been proposed to support the high levels of
stereoinduction observed at the β-position. DFT calculations
are in agreement with the postulated model. Further develop-
ments will be reported in due course.
ASSOCIATED CONTENT
* Supporting Information
■
S
(10) (a) Chen, J.; Meng, S.; Wang, L.; Tang, H.; Huang, Y. Chem. Sci.
2015, 6, 4184. (b) Yuan, P. F.; Meng, S. X.; Chen, J. A.; Huang, Y. Synlett
2016, 27, 1068.
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.7b03118.
(11) Kanemasa, S.; Oderaotoshi, Y.; Wada, E. J. Am. Chem. Soc. 1999,
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Experimental details, characterization date, NMR spectra,
and chromatograms (PDF)
AUTHOR INFORMATION
■
(13) (a) Abe, A. M. M.; Sauerland, S. J. K.; Koskinen, A. M. P. J. Org.
Chem. 2007, 72, 5411. (b) Kitanosono, T.; Sakai, M.; Ueno, M.;
Kobayashi, S. Org. Biomol. Chem. 2012, 10, 7134.
(14) Kawatsura, M.; Komatsu, Y.; Yamamoto, M.; Hayase, S.; Itoh, T.
Tetrahedron 2008, 64, 3488.
(15) (a) Kawatsura, M.; Komatsu, Y.; Yamamoto, M.; Hayase, S.; Itoh,
T. Tetrahedron Lett. 2007, 48, 6480. (b) White, J. D.; Shaw, S. Chem. Sci.
2014, 5, 2200.
(16) (a) Ollevier, T.; Plancq, B. Chem. Commun. 2012, 48, 2289.
(b) Plancq, B.; Ollevier, T. Chem. Commun. 2012, 48, 3806.
(c) Kitanosono, T.; Ollevier, T.; Kobayashi, S. Chem. - Asian J. 2013,
8, 3051. (d) Lafantaisie, M.; Mirabaud, A.; Plancq, B.; Ollevier, T.
ChemCatChem 2014, 6, 2244. (e) Plancq, B.; Lafantaisie, M.;
Companys, S.; Maroun, C.; Ollevier, T. Org. Biomol. Chem. 2013, 11,
7463. (f) Jalba, A.; Regnier, N.; Ollevier, T. Eur. J. Org. Chem. 2017,
1628.
(17) Based on the integration of the Me groups of 1a and 3a.
(18) (a) Bolm, C.; Zehnder, M.; Bur, D. Angew. Chem., Int. Ed. Engl.
1990, 29, 205. (b) Bolm, C.; Ewald, M.; Felder, M.; Schlingloff, G. Chem.
Ber. 1992, 125, 1169. (c) Ishikawa, S.; Hamada, T.; Manabe, K.;
Kobayashi, S. Synthesis 2005, 2176.
Corresponding Authors
*E-mail: vincent.gandon@u-psud.fr.
*E-mail: thierry.ollevier@chm.ulaval.ca.
ORCID
Samuel Lauzon: 0000-0002-3672-3601
Vincent Gandon: 0000-0003-1108-9410
Thierry Ollevier: 0000-0002-6084-7954
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to the Natural Sciences and Engineering
Research Council of Canada (NSERC), the Centre in Green
́
Chemistry and Catalysis (CGCC), and Universite Laval for
financial support of our program. S.L. thanks NSERC and
FRQNT for scholarships. V.G. thanks UPS, CNRS, and IUF. We
thank Prof. P. Deslongchamps (Universite
discussions.
́
Laval) for helpful
(19) Hong, L.; Sun, W.; Yang, D.; Li, G.; Wang, R. Chem. Rev. 2016,
116, 4006.
(20) Fe(OTf)2: 93:7 er, 42% conv, 72 h vs 89:11 er, 40% conv, 48 h.
(21) (a) Kobayashi, S.; Busujima, T.; Nagayama, S. Chem. - Eur. J. 2000,
6, 3491. (b) Huber, R.; Bigler, R.; Mezzetti, A. Organometallics 2015, 34,
3374. (c) Dyson, P. J.; Jessop, P. G. Catal. Sci. Technol. 2016, 6, 3302.
(22) MeCN was also more effective using Fe(OTf)2: 95:5 er, 72% conv
of 3a. 12 mol % L*/10 mol % Fe(ClO4)2·6H2O: 91:9 er, 96% of 3a. The
er decreased when 20 mol % Pyr (34% conv, 75:25 er, 48 h) or 200 mol
% K2HPO4 (95% conv, 51:49 er, 24 h) was used.
(23) Reaction times using 2o: 2 equiv, >130 h vs 5 equiv, 43 h.
(24) (a) Corey, E. J.; Imai, N.; Zhang, H. Y. J. Am. Chem. Soc. 1991, 113,
728. (b) Corey, E. J.; Ishihara, K. Tetrahedron Lett. 1992, 33, 6807.
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DOI: 10.1021/acs.orglett.7b03118
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