Enantioselective conjugate addition of aromatic amines to N-alkenoyloxazolidinones catalyzed by iodido(binaphtholato)samarium

Article abstract of DOI:10.1002/ejoc.200700861

Iodido(binaphtholato)samarium catalyzes the Michael addition of aromatic amines to N-alkenoyloxazolidinones affording β-amino acid derivatives with enantiomeric excesses of up to 88 %. A study of the effect of temperature on the asymmetric induction revea

Full text of DOI:10.1002/ejoc.200700861

FULL PAPER
DOI: 10.1002/ejoc.200700861
Enantioselective Conjugate Addition of Aromatic Amines to
N-Alkenoyloxazolidinones Catalyzed by Iodido(binaphtholato)samarium
Iréna Reboule,[a] Richard Gil,*[a] and Jacqueline Collin*[a]
Keywords: Asymmetric catalysis / Michael addition / Conjugate addition / Samarium / Lanthanides / Amines
Iodido(binaphtholato)samarium catalyzes the Michael ad-
dition of aromatic amines to N-alkenoyloxazolidinones af-
fording β-amino acid derivatives with enantiomeric excesses
of up to 88%. A study of the effect of temperature on the
asymmetric induction revealed an isoinversion in two reac-
tions. The observation of a non-linear effect suggests an
equilibrium between several active species including dimers.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2008)
TiCl2-BINOL,[13] magnesium salts or rare-earth triflates co-
ordinated by chiral bis-oxazoline ligands.[14] Rare-earth cat-
alysts coordinated by BINOL-type ligands such as yttrium-
lithiumtris(binaphthoxide) (YLB) and Sc(BNP)3 are highly
enantioselective in the 1,4-addition of O-alkylhydroxyl-
amine to enones.[15,16] Other nitrogen-containing substrates
such as hydroxylamine or carbamates have been employed
in enantioselective conjugate addition reactions using Cu-
(OTf)2 with bis-oxazoline ligands.[17,18] Aldoximes have
been employed in aza-Michael reactions catalyzed with
Zn(ClO4)2 coordinated by bis-oxazoline.[19] The addition of
N–H-containing heterocycles to α,β-unsaturated imides or
ketones catalyzed by Al–salen complexes gave very high en-
antiomeric excesses (up to 99%).[20]
Enantioselective catalyzed aza-Michael reactions involv-
ing amines have been less studied and only a few catalytic
systems have been described. Nickel perchlorate coordi-
nated by DBFOx-Ph catalyzes the addition of secondary
aromatic amines to α,β-unsaturated N-acyloxazolidinones
with moderate-to-high enantiomeric excesses.[21] Hii et al.
described the first use of cationic palladium complexes co-
ordinated by BINAP for the addition of aromatic amines
to N-alkenoyloxazolidinones or N-alkenoylcarbamates.[22]
High enantiomeric excesses were obtained with both sub-
strates but the enantioselectivity was lower with electron-
rich amines in the former case. In contrast, the enantio-
selectivity did not depend upon the electronegativity of the
amine in the case of α,β-unsaturated imides. Sodeoka and
co-workers improved the yield and enantioselectivity of this
catalytic system for reactions involving N-alkenoyloxazolid-
inones by using aromatic amine trifluoromethanesulfonate
salts as substrates and a cationic dimeric palladium com-
plex coordinated by BINAP as a precatalyst.[23] Several chi-
ral diphosphanes affording aza-Michael products with en-
antiomeric excesses of over 90% were selected by parallel
screening of various chiral ligands and recently a synthetic
application and mechanistic studies were reported.[24]
Introduction
The enantioselective formation of carbon–nitrogen
bonds by the addition of amines to double bonds is an
atom-economic reaction giving access to molecules of phar-
maceutical interest.[1] Several reviews concerning activated
or non-activated double bonds have been devoted to this
topic.[2] Most enantioselective hydroamination reactions in-
volving the addition of amines to unactivated double bonds
(1,2-addition) are intramolecular and allow the synthesis of
enantioenriched nitrogen heterocycles. The enantioselective
catalysts reported for these hydroamination/cyclization re-
actions are for the major part based on lanthanides,[3–6] al-
though some examples of chiral zirconium complexes and
a chiral lithium bis-amide have recently been described.[7–9]
With regard to intermolecular enantioselective hydroamin-
ation reactions, only the Ir-catalyzed addition of aniline to
norbornene and the Pd-BINAP-catalyzed addition of ani-
line to vinylstyrene or vinylnaphthalene have been re-
ported.[10] Such intermolecular additions of amines gen-
erally require the use of activated olefins and are referred
to as aza-Michael reactions. Nickel complexes coordinated
to chiral tridendate ferrocenylphosphanes catalyze the ad-
dition of various amines to α,β-unsaturated nitriles with en-
antiomeric excesses of up to 69%.[11] However, most of the
studies on enantioselective aza-Michael reactions concern
the reactions of various amino derivatives with α,β-unsatu-
rated acyl derivatives which are efficient methodologies for
the synthesis of β-amino acids.[12] Different catalytic sys-
tems have been successfully used for the conjugated ad-
dition of O-benzylhydroxylamine to N-alkenoyl substrates
(oxazolidinones, imides, pyrazolidinones), for example,
[a] Equipe de Catalyse Moléculaire, ICMMO, UMR 8182, Uni-
versité Paris-Sud,
91405 Orsay, France
Fax: +33-1-69154680
E-mail: jacollin@icmo.u-psud.fr
532
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Eur. J. Org. Chem. 2008, 532–539

Enantioselective Addition of Amines to Alkenoyloxazolidinones
During the course of our previous investigations we N-crotonoyloxazolidinone, comparison of the ratio 3ba/4ba
studied the use of SmI2(THF)2 in dichloromethane as an (80:20, entry 2) with that obtained in the similar samarium
efficient Lewis acid catalyst for carbon–carbon and carbon– diiodide catalyzed reaction (10:90) showed that the rate of
nitrogen bond-forming reactions.[25] Samarium diiodide cat- the amidation reaction is lower with iodido(binaphtholato)-
alyzes Mukaiyama–Michael reactions of α,β-unsaturated samarium. This asymmetric catalyst also exhibited higher
ketones,[25b,25c] as well as aza-Michael reactions involving activity than samarium diiodide for aza-Michael additions:
the addition of aromatic amines to α,β-unsaturated acylox- the reaction of substrate 1d with p-anisidine in the presence
azolidinones to form β-amino acid derivatives.[26] Aza- of a catalytic amount of SmI2(THF)2 did not provide the
Michael reactions can be followed by an amidation reaction product 3da.[32] At room temperature only moderate enan-
by cleavage of oxazolidinone to give the corresponding β- tiomeric excesses were observed for aza-Michael reactions
amino amides. With the aim of developing new enantiose- catalyzed by samarium complex 5.
lective catalysts we prepared lanthanide complexes with bi-
naphthylamine or binaphthol-type ligands. A new family of
lanthanide ionic complexes derived from chiral substituted
(R)-binaphthylamine ligands, [Li(THF)4][Ln{(R)-C20H12-
(NR)2}2], proved indeed to be efficient enantioselective cat-
alysts for intramolecular hydroamination reactions.[27]
Iodido(binaphtholato)lanthanides have been employed as
efficient Lewis acid catalysts for Diels–Alder reactions with
moderate enantioselectivity,[28] while iodido(binaphthol-
ato)samarium afforded high activity and enantioselectivity
in iminoaldol reactions involving a glyoxylic imine.[29] This
Scheme 1. Aza-Michael reaction of aromatic amines with N-alk-
latter samarium complex catalyzes the ring-opening of
meso-epoxides by aromatic amines to give β-amino alcohols
with high enantiomeric excesses.[30] We wish now to report
the use of this catalyst in the enantioselective conjugate ad-
dition of aromatic amines to N-alkenoyloxazolidinones and
the effect of various parameters on these reactions. The re-
sults of aza-Michael reactions with N-fumaroyloxazolidin-
ones have already been reported.[31]
enoyloxazolidinones catalyzed by iodido(binaphtholato)samarium.
Table 1. Addition of p-anisidine (2a) to N-alkenoyloxazolidinones
catalyzed by iodido(binaphtholato)samarium (5).
Entry Substrate[a]
t [h]
Product
3/4[b]
% ee 3[c]
1
2
3
4
5
1a
1b
1c
1d
1e
16
48
48
48
48
3aa
100:0
80:20
63:37
87:13
100:0
25
22[d]
4[e]
3ba + 4ba
3ca + 4ca
3da + 4da
3ea
19[e]
18
Results and Discussion
[a] Reactions were performed at room temperature in CH2Cl2 using
10 mol-% 5 and a 2a/1 ratio of 1.2:1. [b] The ratio 3/4 was measured
1
We previously found that the reaction of aromatic amines
with N-alkenoyloxazolidinones 1 catalyzed by samarium di-
iodide in dichloromethane either selectively affords the aza-
Michael addition product 3 or the amide 4 formed by the
consecutive reaction of 3 with the aromatic amine.[26] The
ratio 3/4 depends not only on the quantity of aromatic
by H NMR of the crude product. [c] Enantiomeric excesses were
determined by HPLC. [d] –19% ee for 4ba. The minus sign indi-
cates a change in the major peak in the HPLC analysis. [e] The
enantiomeric excesses for 4ca and 4da could not be determined by
HPLC.
We previously found that the enantioselectivities of iodi-
amine, but also on the reagents (N-alkenoyloxazolidinone do(binaphtholato)samarium-catalyzed reactions are dra-
and aromatic amine). The rate of amidation is higher for matically influenced by temperature. For iminoaldolization
electron-enriched amines and a higher ratio of amide 4 has an increase in enantiomeric excess with temperature was
been found in reactions involving p-anisidine.
observed,[29] while an isoinversion effect was recorded for
In order to evaluate the activity and selectivity of iodido- the ring-opening of cyclohexene oxide by o-anisidine with
(binaphtholato)samarium (5) as a catalyst for aza-Michael the highest enantiomeric excess obtained at the inversion
reactions we performed the reactions of various N-alkenoyl- temperature of –40 °C.[30] To improve the enantioselectivity
oxazolidinones 1 with 1.2 equiv. of p-anisidine in dichloro- of this process, aza-Michael reactions were performed at
methane at room temperature in the presence of 10 mol-% various temperatures. The results are shown in Table 2. A
of complex 5 (Scheme 1). The results are shown in Table 1. high enantiomeric excess (88%) was obtained at –40 °C for
Under these conditions fumaroyloxazolidinone selectively the addition of p-anisidine to N-fumaroyloxazolidinone
yielded the Michael addition product (entry 1), as was ob- (entry 2). Other aza-Michael reactions were thus studied at
served with samarium diiodide. The substrates substituted this temperature under the conditions described above
by alkyl or phenyl groups on the terminal carbon of the [1.2 equiv. of amine 2 and 10 mol-% iodido(binaphtholato)-
double bond afforded mixtures of Michael addition product samarium (5)]. A decrease in temperature selectively pro-
3 and amide 4 (entries 2–4). Use of substrate 1e with a vided the Michael product 3ba in the reaction of p-anisidine
methyl substituent at the α-position to the carbonyl selec- with N-crotonoyloxazolidinone (1b) (entries 3–6), but with
tively led to the Michael adduct (entry 5). In the case of a lower enantiomeric excess (3ba is nearly racemic at 0 and
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533

I. Reboule, R. Gil, J. Collin
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–20 °C). At –40 °C the sense of the enantioselectivity is re- oxazolidinones. Aza-Michael adducts are selectively ob-
versed. Such unusual behaviour was observed by Sibi et al. tained with o-anisidine and aniline, and with p-anisidine at
for the enantioselective conjugate amine addition catalyzed a low temperature. Conjugate addition of amines to N-alk-
by magnesium bromide coordinated by bis-oxazolines.[14c] enoyloxazolidinones 1a and 1e provided new β-amino acid
The addition of o-anisidine to N-crotonoyloxazolidinone derivatives. Since reactions involving p-anisidine (2a) af-
(1b) selectively afforded the aza-Michael product 3bb in all forded higher enantiomeric excesses than those with o-anisi-
cases (entries 7–9). For aza-Michael reactions catalyzed by dine or aniline, further studies of the aza-Michael additions
samarium diiodide it has already been observed that amid- of this amine on substrates 1a and 1e have been performed.
ation does not occur with o-anisidine probably due to steric
In our first studies of the aza-Michael reaction catalyzed
hindrance.[26] The ee for 3bb was slightly higher at lower by iodido(binaphtholato)samarium we examined the effect
temperatures, but was still low. Similarly, the addition of of temperature on the addition of p-anisidine (2a) to sub-
aniline (2c) to N-crotonoyloxazolidinone (1b) selectively strate 1a.[31] We found that the enantiomeric excesses did
provided the Michael adduct 3bc with very low asymmetric not show a monotonous variation with temperature. The
induction even at low temperatures (entries 10–12). The re- enantiomeric excess first increased upon decreasing the
actions of N-hexenoyloxazolidinone (1c) with p-anisidine temperature to reach a maximum value of 88% at –40 °C.
(2a) and o-anisidine (2b) showed the same trends as the Reactions performed at still lower temperatures resulted in
reactions with 1b. At room temperature with 2a a mixture lower asymmetric inductions. Such variations in enantio-
of aza-Michael product 3ca and amidation product 4ca was selectivity with an isoinversion effect have already been re-
formed (entry 13), while at –40 °C 3ca was selectively ob- ported.[33] The corresponding Eyring plot is presented in
tained with 45% ee (entry 15). Reactions with 2b led to the Figure 1 and shows non-linear behaviour; it shows two lin-
selective formation of 3cb, but with low conversions and ear regions intersecting at the inversion temperature (Tinv).
enantioselectivities (entries 16 and 17). Addition of 2a to N- This intersection corresponds to a value of –39 °C in Fig-
cinnamoyloxazolidinone (1d) furnished mixtures of adducts ure 1. A study of the effect of temperature on the enantio-
at room temperature, but selectively gave adduct 3da at meric excess of the adduct 3ea obtained by the addition of
–40 °C with low ee values in both cases (entries 18 and 19). p-anisidine to substrate 1e showed a similar non-monoto-
In the selective aza-Michael addition of p-anisidine to the nous variation, as can be seen in Table 3. For this reaction
substrate 1e with two substituents in the α-position to the only small variations in the enantiomeric excess were ob-
carbonyl, a decrease in temperature led to an improvement served between –20 and –50 °C. The corresponding Eyring
in the enantiomeric excess (76%, entries 20 and 21). Iodido- plot in Figure 2 also exhibits two linear regions. The graphi-
(binaphtholato)samarium (5) appears to be an efficient cat- cally determined inversion temperature is identical to that
alyst for the reaction of aromatic amines with N-alkenoyl- observed for 3aa (–39 °C). Previously we described similar
Table 2. Effect of temperature on the enantiomeric excess of aza-Michael reactions catalyzed by iodido(binaphtholato)samarium (5).
Entry Substrate
Amine
Temp. [°C][a]
t [h]
Products
3/4
% Conv. (% isolated yield of 3) % ee of 3[b]
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
1a
1a
1b
1b
1b
1b
1b
1b
1b
1b
1b
1b
1c
1c
1c
1c
1c
1d
1d
1e
1e
2a
2a
2a
2a
2a
2a
2b
2b
2b
2c
2c
2c
2a
2a
2a
2b
2b
2a
2a
2a
2a
25
–40
25
0
–20
–40
25
0
–40
25
0
–40
25
0
–40
0
–40
25
–40
25
18
18
48
48
48
48
48
48
48
48
48
48
48
48
48
48
48
48
48
48
48
3aa
3aa
100:0
100:0
80:20
80:20
85:15
Ͼ95:5
100:0
100:0
100:0
100:0
100:0
100:0
63:37
93:7
100:0
100:0
100:0
87:13
100:0
100:0
100:0
100
100 (65)
100 (30)
100
25[c]
88
22[d]
5
3ba + 4ba
3ba + 4ba
3ba + 4ba
3ba
100
4
100 (66)
100 (64)
98 (75)
47 (20)
100 (67)
98 (55)
88 (60)
100
–8[e]
17
28
29
5
3bb
3bb
3bb
3bc
3bc
11[d]
12[d]
4
3bc
3ca + 4ca[e]
3ca + 4ca[f]
3ca
97
100
20
45
7
18
19
28
18
76
3cb
3cb
71 (38)
25 (10)
100
3da + 4da[g]
3da
100 (7)[h]
100 (80)
75 (50)
3ea
3ea
–40
[a] Reactions were performed in CH2Cl2 using 10 mol-% 5 and a 2/1 ratio of 1.2:1. [b] Enantiomeric excesses were determined by HPLC.
[c] Catalyst 5 prepared from (R)-binaphthol afforded (S)-3aa; see ref.[31] for the determination of the configuration. [d] Catalyst 5 prepared
from (R)-binaphthol afforded (R)-3ba and (R)-3bc: the configurations were determined by comparison of the sign of the optical rotations
with those described in the literature (see ref.[23]). [e] The minus sign indicates a change in the major peak on HPLC analysis. [f] The
enantiomeric excess of 4ca could not be determined by HPLC. [g] The enantiomeric excess of 4da could not be determined by HPLC.
[h] Compound 3da decomposed on silica.
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Enantioselective Addition of Amines to Alkenoyloxazolidinones
behaviour for the ring-opening of cyclohexene oxide by o- X-ray diffraction were unsuccessful. The preparation of cat-
anisidine catalyzed by iodido(binaphtholato)samarium alyst 5 using Ph2CHK to generate the BINOL potassium
which also has an inversion temperature of –39 °C.[30]
salt involves treatment with hexane to eliminate diphenyl-
methane.[30] The 1H NMR spectra of iodido(binaphtholato)-
samarium (5) thus prepared show only one molecule of
THF coordinated to samarium which has a formal coordi-
nation number of four.[26,34] The formation of a dimer or
oligomers can be anticipated and these could also be the
origin of the isoinversion phenomena. The observation of a
non-linear correlation between the enantiomeric excesses of
products and catalysts has been recognised as a tool for
detecting association in catalytic active species.[35] Examples
of asymmetric amplification in various chiral lanthanide-
catalyzed reactions have been the topic of a review.[36] We
thus investigated the possible involvement of a non-linear
effect in the reaction leading to 3ea catalyzed by iodido-
(binaphtholato)samarium (5).
The study of the effect of enantiomeric purity of binaph-
thol on the enantiomeric excess of the Michael product 3ea
requires the preparation of several iodido(binaphtholato)-
samarium complexes (5) with different enantiomeric puri-
ties. Two methods can be envisaged for the synthesis of such
catalysts and both of them were examined. In the first series
of experiments different complexes were prepared by the
usual method with binaphthol samples having different en-
antiomeric purities (8, 24, 40, 67% ee). The aza-Michael
reaction involving 1e and p-anisidine catalyzed by these
complexes was studied at three temperatures: room tem-
perature and 0 and –40 °C (the temperature affording the
highest enantiomeric excess). The results indicated in Fig-
ure 3 show a non-linear relationship between the enantio-
meric excess of the Michael adduct 3ea and the enantio-
meric excess of binaphthol. A negative non-linear effect is
observed for the three different temperatures. As an alterna-
tive, two scalemic catalysts were prepared by mixing iodido-
(binaphtholato)samarium complexes prepared separately
with enantiomerically pure (R)- and (S)-binaphthol (giving
catalysts with 32 and 47% ee, respectively). The Michael
addition reaction involving 1e and p-anisidine was per-
formed with both catalysts at the inversion temperature of
–40 °C and the enantiomeric excesses were compared with
Figure 1. Eyring plot for the aza-Michael addition of p-anisidine
(2a) to N-acyloxazolidinone (1a) catalyzed by samarium complex
5.
Table 3. Effect of temperature on the enantiomeric excess of prod-
uct 3ea.
Temp. [°C]
% Conversion[a] 26 68 75 80 89 90 89 100 100
% ee 58 69 76 68 71 68 74 49 18
–60 –50 –40 –35 –30 –25 –20
0
25
[a] After a reaction time of 48 h.
Figure 2. Eyring plot for the aza-Michael addition of p-anisidine
(2a) to N-acyloxazolidinone (1e) catalyzed by samarium complex
5.
The inversion temperatures can be explained by a reac- those calculated for a linear correlation between the enan-
tion pathway with at least two enantioselective steps tiomeric excesses of the catalyst and the product
weighted differently according to the temperature. Observa- (Scheme 2). The values of the enantiomeric excesses of 3ea
tion of the same values for the inversion temperatures of produced by reactions catalyzed by the mixtures of (R)- and
two different reactions, aza-Michael reaction and aminoly- (S)-5 are higher than the calculated values. These results
sis of epoxides, suggests the involvement of different cata- indicate asymmetric amplification in these cases.
lytic species rather than different reaction mechanisms as
The positive and negative non-linear effects can be ex-
an explanation of the isoinversion. Cainelli and co-workers plained by the formation of dimers. The use of scalemic
proposed Tinv to be the temperature for the interconversion binaphthol for the preparation of the catalysts would lead
between two different solvation clusters that behave like two to the formation of heterochiral [(R)-5–(S)-5] and homochi-
different molecules.[33e,33f] For the aza-Michael reaction as ral dimers [(R)-5–(R)-5] and [(S)-5–(S)-5]. The negative
well as for the aminolysis of meso-epoxides catalyzed by non-linear effect suggests that the homochiral dimers are
iodido(binaphtholato)samarium, the inversion temperature less active than the heterochiral dimer. The positive non-
could be explained similarly. Several catalytic species with linear effect obtained with the mixtures of complexes indi-
variable numbers of coordinated THF molecules and/or cates that the homochiral dimers [(R)-5–(R)-5] and [(S)-5–
amines can be envisaged. Numerous attempts to prepare (S)-5] are not completely dissociated to monomers under
crystals of iodido(binaphtholato)samarium (5) suitable for the conditions of the reaction. Complete dissociation would
Eur. J. Org. Chem. 2008, 532–539
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535

I. Reboule, R. Gil, J. Collin
FULL PAPER
chiral dimers we examined the effect of concentration on
the reaction affording 3ea catalyzed by (R)-5 at –40 °C. The
reaction was performed by using 20 mL of CH2Cl2 instead
of 5 mL which afforded a lower conversion (38% instead of
75%) and a lower enantiomeric excess (64% instead of
76%). Since dilution should favour the dissociation of di-
mers and increase the monomer ratio, this result suggests
that the homochiral dimer [(R)-5–(R)-5] is more active and
enantioselective than the monomer (R)-5. As a possible ex-
planation we propose that iodido(binaphtholato)samarium
(5) is isolated as a dimeric species that can dissociate to
a small extent under the conditions of the reaction. Both
monomer and dimer species should be active for the aza-
Michael reaction with the heterochiral dimer more active
than the homochiral complex and the monomer species.
Kinetic constants for the aza-Michael reaction are
expected to decrease in the following order: k[(R)-5–(S)-5] Ͼ
k
[(R)-5–(R)-5]Ͼk[(R)-5]
.
Non-linear effects have previously been observed in dif-
ferent asymmetric reactions involving bidentate binaphthol
ligands and different metals, for example, titanium[35e,35f] or
lanthanum.[36,37] In several cases the non-linear correlation
was explained by the formation of dimers with metal–oxy-
gen bonds. For the iodido(binaphtholato)samarium-cata-
lyzed reactions species such as dimer A (Scheme 3) with sa-
marium–iodine bridges and THF and/or amines as ligands
can be proposed. Dimeric structures with metal–halogen
bridges are frequently found in lanthanide complexes and
X-ray structures of dimeric samarium aryloxide/halide com-
plexes have been reported.[38] As an alternative, a dimeric
structure such as B (Scheme 3) with samarium atoms coor-
dinated to two different binaphthol ligands can be sug-
gested. Such structures were found for alkyl(biphenolato)-
lanthanums employed as intramolecular hydroamination
catalysts.[39] An equilibrium between the monomeric and di-
meric forms of yttrium alkoxides has been described, the
dimer being favoured by a non-coordinating solvent.[40]
Thus, iodido(binaphtholato)samarium could be present in
the catalytic medium as a dimer or as an equilibrium be-
tween monomer, dimer and oligomers. The coexistence of
Figure 3. Correlation between the enantiomeric excess of binaph-
thol and the enantiomeric excess of aza-Michael adduct 3ea at a)
20, b) 0 and c) –40 °C.
Scheme 2. Aza-Michael reaction catalyzed by mixtures of (R)- and
(S)-5.
either give a linear correlation or lead to the formation of
the heterochiral dimer together with a negative non-linear
Scheme 3. Proposed dimeric structures for the catalytic species in
effect. To study the possibility of the dissociation of homo- the aza-Michael reactions.
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Enantioselective Addition of Amines to Alkenoyloxazolidinones
room temperature. The reaction mixture was then cooled to the
required temperature outside the glove-box and a solution of N-
alkenoyloxazolidinone 1 (0.5 mmol) in dichloromethane (1 mL)
was introduced by syringe. The reaction mixture was stirred for 16
or 48 h at the same temperature, quenched with a solution of 0.1 
HCl (10 mL) and extracted with CH2Cl2. The crude product was
purified by preparative chromatography (heptane/AcOEt, 75:25) to
afford compound 3 (3aa, 3ba, 3bb, 3bc, 3ca, 3cb, 3da and 3ea). The
enantiomeric excesses were determined by chiral HPLC as de-
scribed below.
several catalytic species may explain the non-linear behav-
iour as well as the isoinversion observed for the aza-
Michael reactions. However, further studies are needed to
determine the structure of the active species involved in the
catalytic process.
Conclusions
Iodido(binaphtholato)samarium (5) is an efficient enan-
tioselective catalyst for aza-Michael additions involving N-
alkenoyloxazolidinones and aromatic amines. Interestingly,
higher enantiomeric excesses were obtained with p-anisidine
than with o-anisidine or aniline. These reactions afforded
β-amino acid derivatives with up to 88% enantiomeric ex-
cess. An isoinversion phenomenon was evidenced for two
reactions with the highest enantioselectivities observed at
the same temperature. A non-linear relationship between
the enantiomeric excesses of the catalyst and the aza-
Michael adducts led to the proposal that the catalyst is
composed of dimeric structures. Further work in this area
is ongoing to gain an insight into the structure of this fam-
ily of catalysts and to extend their application to other
enantioselective catalytic reactions.
(R)-3-[3-(2-Methoxyphenylamino)butyryl]oxazolidin-2-one
(3bb):
Oil, yield 104 mg, 75%. [α]2D0 = –6.12 (c = 0.67, CHCl3) for 28%
ee. 1H NMR (250 MHz, CDCl3): δ = 6.60–6.90 (m, 4 H), 4.20–4.29
(m, 2 H), 4.10–4.13 (m, 1 H), 3.81–3.88 (m, 5 H), 3.31 (dd, J1
=
15.7, J2 = 6.2 Hz, 1 H), 3.03 (dd, J1 = 15.7, J2 = 6.2 Hz, 1 H), 1.30
(d, J = 5.0 Hz, 3 H) ppm. 13C NMR (62.5 MHz, CDCl3): δ = 177.6,
153.5, 146.9, 136.7, 121.1, 116.5, 110.5, 109.6, 61.9, 55.3, 45.4, 42.4,
41.5, 21.1 ppm. IR (CHCl ): ν = 3394, 1781, 1697, 1602, 1387,
˜
3
1364 cm–1. HRMS (EI): calcd. for C14H18N2NaO4 301.1159; found
301.1169. HPLC: Chiralcel OD-H, hexane/ethanol, 85:15, 0.7 mL/
min, λ = 254 nm, t1,minor = 30.83 and t2,major = 33.73 min.
Preparation of Racemic 3bb Catalyzed by Cu(OTf)2: Cu(OTf)2
(72 mg, 0.2 mmol) was weighed in the glove-box and dichlorometh-
ane (4 mL) was added followed by a solution of o-anisidine (2b)
(615 mg, 5 mmol) and N-acyloxazolidinone 1b (144 mg, 1 mmol) in
dichloromethane (1 mL). The reaction mixture was stirred at 35 °C
during 16 h and treated as described above. The crude product was
purified by preparative chromatography (hexane/AcOEt, 75:25) to
afford 3bb (conversion 81%, yield 166 mg, 60%).
Experimental Section
General Remarks: All manipulations were carried out under argon
using standard Schlenk or glove-box techniques. THF was distilled
from sodium benzophenone ketyl and degassed immediately prior
to use. Hexane, benzene and dichloromethane were distilled from
CaH2 and degassed immediately prior to use. Iodido(binaphthol-
ato)samarium (5) was prepared according to the procedure pre-
viously described.[30] α,β-Unsaturated N-acyloxazolidinones were
prepared according to reported procedures.[41] All aromatic amines
were recrystallized or distilled and degassed immediately prior to
use. Thin-layer chromatographic plates were prepared from silica
gel 60 PF254 for preparative thin-layer chromatography.
3-[3-(4-Methoxyphenylamino)hexanoyl]oxazolidin-2-one (3ca): Oil,
yield 84 mg, 55%. [α]2D0 = +24.5 (c = 2.18, CHCl3) for 45% ee
{ref.[22c] [α]2D0 = +0.69 (c = 0.026, CHCl3) for 10% ee}. H NMR
1
(250 MHz, CDCl3): δ = 6.76 (d, J = 8.7 Hz, 2 H), 6.60 (d, J =
8.7 Hz, 2 H), 4.22–4.37 (m, 2 H), 3.81–3.98 (m, 3 H), 3.75 (s, 3 H),
3.45 (br. s, 1 H), 3.31 (dd, J1 = 15.0, J2 = 7.9 Hz, 1 H), 3.01 (dd,
J1 = 15.0, J2 = 4.6 Hz, 1 H), 1.37–1.66 (m, 4 H), 0.93 (t, J = 7.1 Hz,
3 H) ppm. 13C NMR (62.5 MHz, CDCl3): δ = 172.2, 153.6, 152.1,
141.5, 114.9, 114.8, 61.8, 55.7, 51.7, 42.4, 39.9, 38.0, 19.3,
13.9 ppm. IR (CHCl ): ν = 3744, 1777, 1685, 1600, 1384 cm–1.
˜
3
HRMS: calcd. for C16H22N2NaO4 329.1472; found 329.1480.
HPLC: Whelk O1, hexane/2-propanol, 90:10, 0.5 mL/min, λ =
254 nm, 15 °C, t1,major = 70.4 and t2,minor = 77.2 min.
Bruker AM 250 and 400 spectrometers operating at 250 and
1
400 MHz for H and 62.5 and 100 MHz for 13C were used for re-
cording the NMR spectra. Chemical shifts for the 1H and 13C
NMR spectra were referenced internally according to the residual
solvent resonances and are reported relative to tetramethylsilane.
Infrared spectra were recorded in CHCl3 or in KBr pellets with a
Perkin–Elmer 1000 FT-IR spectrometer and are reported in cm–1.
HRMS were recorded with a Thermo-Finnigan-Mat 95 spectrome-
ter. Optical rotations were measured with a Perkin–Elmer 341 Pola-
rimeter and are reported as follows: [α]rD.t. (c in g per 100 mL, sol-
vent). The enantiomeric excesses of the products were determined
by chiral stationary phase HPLC with Chiralcel OD-H, Whelk O1
or Chiralpak AD columns and compared with racemic 3bb pre-
pared with SmI2(THF)2 or Cu(OTf)2. The method for the determi-
nation of enantiomeric excesses has been already reported for
3aa,[31] 3ba[23] and 3bc.[23]
Preparation of Racemic 3ca Using Cu(OTf)2: In the glove-box, Cu-
(OTf)2 (13 mg, 0.05 mmol) was suspended in dichloromethane
(4 mL). Then a solution of p-anisidine (2a) (74 mg, 0.6 mmol) and
N-acyloxazolidinone 1c (92 mg, 0.5 mmol) in dichloromethane
(1 mL) was added. The reaction mixture was stirred at room tem-
perature during 48 h and treated as described previously. The crude
product was purified by preparative thin-layer chromatography
(hexane/AcOEt, 75:25). Yield 9 mg, 6%.
3-[3-(2-Methoxyphenylamino)hexanoyl]oxazolidin-2-one (3cb): Oil,
1
yield 58 mg, 38%. H NMR (250 MHz, CDCl3): δ = 6.85–6.60 (m,
4 H), 4.50–4.06 (m, 4 H), 3.80–3.78 (m, 4 H), 3.22 (dd, J1 = 15.2,
J2 = 7.6 Hz, 1 H), 3.03 (dd, J1 = 15.2, J2 = 5.7 Hz, 1 H), 1.60–1.30
(m, 4 H), 0.89 (t, J = 6.9 Hz, 3 H) ppm. 13C NMR (62.5 MHz,
CDCl3): δ = 171.9, 153.6, 146.8, 137.3, 121.2, 116.2, 110.2, 109.7,
General Method for the Preparation of Enantiomeric-Enriched Aza-
Michael Products 3: In an argon-filled glove-box, iodido(binaph-
tholato)samarium (35 mg, 0.05 mmol) and 4-Å molecular sieves
(50 mg) were suspended in dichloromethane (4 mL) in a Schlenk
flask equipped with a magnetic stirring bar. Amine 2 (0.6 mmol)
was then added and the reaction mixture was stirred for 10 min at
61.9, 55.4, 49.7, 42.4, 40.2, 37.9, 19.2, 14.1 ppm. IR (CHCl ): ν =
˜
3
3748, 1777, 1697, 1600, 1387, 1363 cm–1. HRMS (EI): calcd. for
C16H22N2NaO4 329.1472; found 329.1485. HPLC: Whelk O1, hex-
ane/2-propanol, 90:10, 0.5 mL/min, λ = 254 nm, 15 °C, t1,minor
26.5 and t2,major = 32.1 min.
=
Eur. J. Org. Chem. 2008, 532–539
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
537

I. Reboule, R. Gil, J. Collin
FULL PAPER
3-[3-(4-Methoxyphenylamino)-3-phenylpropionyl]oxazolidin-2-one Preparation of Racemic 4ca Catalyzed by SmI2(THF)2: Samarium
(3da): Solid, m.p. 137–140 °C, yield 12 mg, 7%. [α]2D0 = –1.21 (c =
diiodide (54 mg, 0.1 mmol) was weighed in the glove-box and
0.6, CHCl3) for 28% ee. 1H NMR (250 MHz, CDCl3): δ = 7.44– dichloromethane (8 mL) was added followed by p-anisidine
7.23 (m, 5 H), 6.69 (d, J = 5.0 Hz, 2 H), 6.52 (d, J = 5.0 Hz, 2 H),
4.90 (t, J = 5.0 Hz, 1 H), 4.35 (t, J = 7.50 Hz, 2 H), 3.96 (t, J =
7.50 Hz, 2 H), 3.69 (s, 3 H), 3.44 (m, 2 H) ppm. 13C NMR
(62.5 MHz, CDCl3): δ = 171.0, 153.6, 152.2, 142.3, 140.8, 128.6,
127.3, 126.3, 115.1, 114.6, 62.0, 55.7, 55.6, 42.85, 42.4 ppm. IR
(615 mg, 5 mmol) and N-acyloxazolidinone 1c (184 mg, 1 mmol).
The reaction mixture was stirred at room temperature during
48 hours and treated as described previously. The crude product
was purified by preparative thin-layer chromatography (hexane/Ac-
OEt, 75:25) to afford 4ca (yield 284 mg, 83%).
(CaF , CHCl ): ν = 3739, 1779, 1689, 1597, 1387, 1360 cm–1
.
˜
2
3
Study of Non-Linear Effects
HRMS (ESI): calcd. for C19H20N2NaO4 363.1315; found 329.1311.
HPLC: Chiracel OD-H, hexane/2-propanol, 90:10, 0.5 mL/min, λ
= 254 nm, t1,minor = 57.7 and t2,major = 59.4 min.
Preparation of Catalyst 5 with Mixtures of (R)- and (S)-Binaphthol:
In an argon-filled glove-box (R)- (32 mg, 0.11 mmol) and (S)-bi-
naphthol (11 mg, 0.04 mmol) were dissolved in THF (3 mL). Potas-
sium diphenylmethide (62 mg, 0.3 mmol), prepared as reported in
the literature,[42] was added in portions and the reaction mixture
was stirred for 2 hours. The white suspension of the potassium bi-
naphtholate salt was then added to a suspension of SmI3(THF)3
(43 mg, 0.15 mmol) in THF (3 mL) and the reaction mixture was
stirred for 18 h. The potassium iodide was separated by centrifuga-
tion and the solution concentrated in vacuo to afford a yellow so-
lid. This product, which is a mixture of (R)- and (S)-5 and di-
phenylmethane, was used after washing with hexane and dissolved
in dichloromethane (3 mL) before performing the catalytic reac-
tions as described above.
3-[3-(4-Methoxyphenylamino)-2-methylpropionyl]oxazolidin-2-one
(3ea): Oil, yield 70 mg, 50%. [α]2D0 = –25.28 (c = 1.24, CHCl3) for
1
76% ee. H NMR (250 MHz, CDCl3): δ = 6.79 (d, J = 8.8 Hz, 2
H), 6.61 (d, J = 8.8 Hz, 2 H), 4.40–4.26 (m, 2 H), 4.19–4.88 (m, 3
H), 3.76 (s, 3 H), 3.50 (dd, J1 = 12.6, J2 = 8.8 Hz, 1 H), 3.22 (dd,
J1 = 12.6, J2 = 5.4 Hz, 1 H), 1.26 (d, J = 6.6 Hz, 3 H) ppm. 13C
NMR (100 MHz, CDCl3): δ = 176.1, 153.4, 152.2, 141.9, 114.8,
114.2, 61.9, 55.7, 48.4, 42.7, 32.7, 15.1 ppm. IR (CHCl ): ν = 3432,
˜
3
1773, 1673, 1388 cm–1. HRMS (EI): calcd. C14H18N2NaO4
301.1159; found 301.1169. HPLC: Chiralcel OD-H, hexane/2-pro-
panol, 75:25, 0.7 mL/min, λ = 254 nm, t1,major = 35.8 and t2,minor
=
55.0 min.
For reactions catalyzed by mixtures of complexes, (R)- and (S)-
iodido(binaphtholato)samarium were weighed in the glove-box,
dissolved in dichloromethane (3 mL) and the catalytic reactions
were performed as described above.
General Method for the Preparation of Mixtures of Enantiomeric-
Enriched Aza-Michael and Amidation Products 3 and 4: The reac-
tions were performed as described above. The crude product was
purified by preparative thin-layer chromatography (hexane/AcOEt,
75:25) to afford a mixture of 3 and 4.
Acknowledgments
N-(4-Methoxyphenyl)-3-(4-methoxyphenylamino)butanamide (4ba):
Amide 4ba was separated from the mixture 3ba + 4ba by a second
preparative thin-layer chromatography (hexane/AcOEt, 75:25).
Amide 4ba was alternatively prepared by the method described for
the synthesis of compounds 3 using 2 equiv. of p-anisidine at room
temperature. Amide 4ba was isolated as a white solid (yield 79 mg,
50%). Solid, m.p. 95 °C. [α]2D0 = –61.9 (c = 1.9, CHCl3) for 27% ee.
1H NMR (250 MHz, CDCl3): δ = 8.83 (br. s, 1 H), 7.39 (d, J =
9.2 Hz, 2 H), 6.84 (d, J = 8.8 Hz, 4 H), 6.74 (d, J = 9.2 Hz, 2 H),
3.75–3.91 (m, 1 H), 3.79 (s, 6 H), 2.53 (d, J = 6.3 Hz, 2 H), 1.28
(d, J = 6.0 Hz, 3 H) ppm. 13C NMR (62.5 MHz, CDCl3): δ = 169.5,
156.2, 153.4, 140.2, 131.1, 121.6, 116.9, 114.9, 114.0, 55.6, 55.4,
The Centre National de la Recherche Scientifique (CNRS) is ac-
knowledged for financial support and the Ministère de l’Enseigne-
ment Supérieur et de la Recherche (MESR) for a Ph. D. grant
(I. R.).
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48.9, 43.4, 21.0 ppm. IR (CHCl ): ν = 1779, 1671, 1600, 1335 cm–1.
˜
3
HRMS (EI): calcd. for C18H22N2NaO3 337.1523; found 337.1529.
HPLC: Chiralpak AD, hexane/2-propanol, 75:25, 1.0 mL/min, λ =
254 nm, t1,major = 11.9 and t2,minor = 25.3 min.
N-(4-Methoxyphenyl)-3-(4-methoxyphenylamino)hexanamide (4ca):
Amide 4ca was separated from the mixture 3ca + 4ca by a second
preparative thin-layer chromatography (hexane/AcOEt, 75:25). It
was also prepared by the method described for the synthesis of
compounds 3 using 2 equiv. of p-anisidine at room temperature.
Amide 4ca was isolated as an oil. The enantiomeric excess of 4ca
could not be measured by HPLC. Oil, yield 86 mg, 50%. 1H NMR
(250 MHz, CDCl3): δ = 8.59 (br. s, 1 H), 7.32–7.29 (m, 2 H), 6.80–
6.67 (m, 6 H), 3.74–3.63 (m, 7 H), 2.55 (dd, J1 = 15.1, J2 = 3.9 Hz,
1 H), 2.42 (dd, J1 = 15.1, J2 = 7.6 Hz, 1 H), 1.63–1.47 (m, 2 H),
1.43–1.32 (m, 2 H), 0.88 (t, J = 7.2 Hz, 3 H) ppm. 13C NMR
(62.5 MHz, CDCl3): δ = 169.8, 156.1, 153.0, 140.8, 131.2, 121.6,
116.2, 114.9, 113.9, 55.5, 55.3, 52.7, 41.4, 37.3, 19.2, 13.9 ppm. IR
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(CHCl3):
ν
˜
=
3434, 1674, 1387 cm–1. HRMS: calcd. for
C20H26N2O3Na 365.1836; found 365.1835.
538
www.eurjoc.org
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2008, 532–539

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Received: September 12, 2007
Published Online: November 28, 2007
Eur. J. Org. Chem. 2008, 532–539
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
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