Functionalization of BINOL and application in the homo- and heterogeneous enantioselective epoxidation of α,β-unsaturated ketones

Article abstract of DOI:10.1016/j.tetlet.2012.09.004

The selective functionalization of BINOL derivatives with 3-(dimethylamino)prop-1-yn-1-yl is described. The corresponding La and Yb complexes were evaluated toward the epoxidation of α,β-unsaturated ketones. The Yb-complexes display the highest catalytic

Full text of DOI:10.1016/j.tetlet.2012.09.004

Tetrahedron Letters 53 (2012) 6335–6338
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Functionalization of BINOL and application in the homo- and
heterogeneous enantioselective epoxidation of a,b-unsaturated ketones
⇑
Moulay Youness El Kadiri, Eric Framery, Bruno Andrioletti
Institut de Chimie et Biochimie Moléculaires et Supramoléculaires (Equipe CASYEN), UMR-CNRS 5246, Université Claude Bernard Lyon 1, Bâtiment Curien (CPE),
3 Boulevard du 11 novembre 1918, F-69622 Villeurbanne Cedex, France
4
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 25 June 2012
Revised 30 August 2012
Accepted 3 September 2012
Available online 18 September 2012
The selective functionalization of BINOL derivatives with 3-(dimethylamino)prop-1-yn-1-yl is described.
The corresponding La and Yb complexes were evaluated toward the epoxidation of
a,b-unsaturated
ketones. The Yb-complexes display the highest catalytic activity and selectivity, affording the expected
chiral epoxides in quantitative yields and up to 90% ee in homogeneous conditions, and 93% ee when
supported on silica gel.
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Enantioselective epoxidation
BINOL
Lanthanide
Homogeneous catalysis
Heterogenous catalysis
The asymmetric epoxidation of olefins is one of the most versa-
tile but challenging reactions in modern organic chemistry. Indeed,
it is very valuable for affording chiral intermediates commonly
used for the synthesis of a large variety of natural products and
enantioselective epoxidation of unsaturated ketones was greatly
improved, affording up to 98% ee, with the addition of a small
amount of phosphine- or arsine oxide (15 mol %) as additive.14
When ytterbium was used, the activity was further improved by
1
15
pharmaceuticals. In 1980, Sharpless et al. reported the stoichiom-
the addition of water (4.5 equiv relative to Yb). Consequently,
2
etric asymmetric epoxidation of allylic alcohols, further optimized
these lanthanide/BINOL catalysts appeared as very efficient
catalysts both in terms of reactivity and enantioselectivity for the
to a catalytic version thanks to the addition of molecular sieves
3
4
5
16
17
(
MS). About ten years later, the groups of Jacobsen, Katsuki,
epoxidation of
a,b-unsaturated N-acylimidazoles, amides, and
6
18
and Mukaiyama developed the asymmetric epoxidation of
unfunctionalized olefins using salen ligands. Ultimately, the more
challenging asymmetric epoxidation of electron-deficient olefins
was investigated. The first results in this field appeared at the
end of the 90s in the literature. During the course of our studies
N-acylpyrroles as carboxylic acid derivatives, and moderately
1
9
selective for ester derivatives.
BINOL is a robust and versatile chiral reagent, easily functional-
ized.20 However, it is worth reminding that the introduction of sub-
stituents on the BINOL skeleton might have a profound impact on
the activity and enantioselectivity of the catalyst due to the modi-
fied electronic and steric properties. As far as we know, few reports
have discussed this problem in the asymmetric epoxidation of
7
8
on epoxidation, we and others have been interested in the chiral
metal peroxide catalytic system pioneered by Weitz and Scheffer.
9
Considering the moderate yields and enantioselectivities obtained
1
0
when using the chiral platinum/diphosphine/peroxide complex,
a number of metals, including zinc associated to (1R,2R)-N-methyl-
a,b-unsaturated ketones. Noticeably, de Vries et al. have reported
0
0
on the advantages of the 6,6 -dibromo- or 6,6 -diphenyl-BINOL on
1
1
21
pseudoephedrine and magnesium combined with (+)-diethyl
the epoxidation of a,b-enones. Correlatively, several groups have
1
2
tartrate, were proposed, affording excellent results.
proposed to functionalize BINOL with ammonium salts in order to
By the same period of time, Shibasaki et al.13 disclosed an alter-
native based on lanthanides. The catalytic system involved lantha-
use the catalysts in asymmetric phase-transfer reactions. How-
22
ever, to the best of our knowledge, only one example of ammonium
salt derived from BINOL has been reported in the case of asymmetric
phase-transfer epoxidation of chalcone with alkaline hydrogen per-
0
0
num or ytterbium, (R)- or (S)-2,2 -dihydroxy-1,1 -binaphthalene
BINOL), tert-butyl hydroperoxide (TBHP) or cumene hydroperox-
ide (CMHP), and 4 Å MS in THF. Using the latter system, the
(
2
3
oxide. In addition, the phase-transfer nucleophilic epoxidation of
,b-enones is based mainly on the use of ammonium salts derived
a
2
4
⇑
Corresponding author. Tel.: +33 (0) 472 446 264; fax: +33 (0) 472 448 160.
E-mail address: bruno.andrioletti@univ-lyon1.fr (B. Andrioletti).
from cinchonine or quinidine derivatives, even if two examples
of ammonium salts derived from binaphthyl were also reported.25
0
040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.09.004

6
336
M. Y. El Kadiri et al. / Tetrahedron Letters 53 (2012) 6335–6338
Based on the limited number of precedents described in the lit-
N
26
Br
erature and on our experience in BINOL modification and catalyst
2
7
recycling, we decided to undertake the synthesis of the two new
ligands 1 and 2 (Fig. 1), designed for allowing an easy, non-cova-
lent grafting on a polar silica-gel support. 1 and 2 derive from BI-
NOL and display one or two N,N-dimethyl-2-propynylamine
OH
OPiv
i
OH
OH
ii
0
functions on the 6, or 6,6 positions of BINOL. The catalytic perfor-
mances of the two new ligands were evaluated toward the cata-
3
(Ref. 28b, 29)
1 (85%)
lytic asymmetric epoxidation of two
a,b-unsaturated ketones
i: Me2NCH2C CH, PdCl2(PPh3)2, CuI, Et3N, 75°C, 5 days
ii: KOH, MeOH - THF, r.t., 15 h
(
chalcone and benzalacetone) under homo- and heterogeneous
conditions, including phase-transfer catalysis.
0
The 6- and 6 -positions of BINOL are equally activated toward
Scheme 1. Synthesis of the (R)-6-monosubstituted-BINOL ligand 1.
electrophilic aromatic substitution.28 However, it has been
reported that the introduction of a bulky pivaloyl group on the
Br
Br
2
-position alters the electronic character of the naphthalene units
0
29
allowing a desymmetrization of the 6- and 6 -positions. Thus, we
OH
OH
OR
OR
decided to use this strategy to prepare the (R)-6-[3-(dimethyl-
0
0
amino)prop-1-yn-1yl]-2,2 -dihydroxy-1,1 -binaphthalene ligand 1
(
Scheme 1). Compound 3 was prepared according to literature pro-
2
9,30
tocols,
and was reacted with N,N-dimethyl-2-propynylamine
Br
Br
under classical Pd-catalyzed Sonogashira coupling conditions, to
give, after deprotection under basic condition, the expected ligand
4
(Ref.27)
5a: OR = OMe (Ref. 29)
5b: OR = OMOM (Ref. 31)
1
in 85% yield after purification.
In parallel, we investigated the synthesis of (R)-6,6 -bis[3-
0
N
0
0
(
dimethylamino)prop-1-yn-1yl]-2,2 -dihydroxy-1,1 -binaphtha-
lene ligand 2 (Scheme 2). Firstly, we tried the direct Pd-catalyzed
i
OR
OR
3
1
Sonogashira coupling between the dibromo-BINOL derivative 4,
1
and N,N-dimethyl-2-propynylamine. If the H NMR spectra of the
crude material confirmed that the expected coupling compound 2
had been formed as the major product, its isolation by column chro-
matography remained unsuccessful, because of the strong interac-
tion between 2 and silica-gel. To prevent this issue, we protected
N
N
6a: OR = OMe (50%)
6b: OR = OMOM (85%)
the hydroxyl groups as methoxy function after bromination of the
0
6
,6 -positions of the BINOL. Thus, a direct methylation of compound
with NaH and methyliodide afforded the corresponding protected
4
32
ii
derivative 5a in quantitative yield. Following, the Sonogashira-
coupling was performed, affording product 6a in 50% yield after
purification on silica gel. Unfortunately, the deprotection of the
OH
OH
methoxy groups using the classical BBr
3
method,33 remained elusive
N
because of the presence of the dimethylamines. For this reason, we
decided to protect the hydroxyl groups as –OMOM functions that
can be cleaved under mild acidic conditions. Thus, the protected
derivative 5b was isolated in quantitative yield.34 Interestingly, in
comparison with the corresponding methoxy derivative 5a, com-
pound 5b displayed a higher reactivity toward the Sonogashira cou-
pling as the bis-dimethylamino derivative 6b was isolated after
purification on silica gel in 85% yield. An acidic treatment followed
by a careful adjustment of the pH using a saturated aqueous solution
2
(100% from 6b)
no reaction from 6a)
(
i: Me
2
NCH
ii for 6a BBr
2
C
3
CH, PdCl
, r.t.
2 3 2 3
(PPh ) , CuI, Et N, 75°C; 5 days
for 6b HCl (37%), MeOH, r.t.; 15h, pH-neutralization with NaHCO
3
0
Scheme 2. Synthesis of the (R)-6,6 -disubstituted-BINOL ligand 2.
of NaHCO
In an attempt to evaluate the catalytic performance of ligands 1
and 2, we examined the asymmetric epoxidation of ,b-unsatu-
3
, afforded ligand 2 in quantitative yield.
(0.66 M), using 10 mol % of catalyst prepared in situ from La(OiPr)
3
or Yb(OiPr) and ligand 1 or 2 in a 1:1 ratio and by adding 60 mol %
3
of triphenylphosphine oxide as additive, 2 equiv of tert-butyl
hydroperoxide (TBHP) and dried 4 Å MS (800 mg per mmole of en-
one). The reactions were performed at 30 °C. The results obtained
are gathered in Table 1.
a
rated ketones such as chalcone and benzalacetone. Using the epox-
idation conditions pioneered by Shibasaki et al.,1 the asymmetric
epoxidation of the afore-mentioned olefins was performed in THF
4b
In order to test our catalytic reactions and compare our results
1
4b
with Shibasaki’s,
we decided to run a reference reaction using
N
N
La/(R)-BINOL and chalcone (R = Ph) as a substrate. After 2 days, a
full conversion was obtained, and a medium enantiomeric excess
of 57% was measured (Table 1, entry 3). The difference with results
reported in the literature (Y = 99%, ee = 96%) may be attributed to
the source of lanthanide salt or the water content (amount of
MS) that is known to be critical for this type of reactions. A refer-
ence reaction with the Yb/(R)-BINOL catalyst was also performed
(Table 1, entry 4). In the latter case, the enantiomeric excess
reached 81%. Replacing (R)-BINOL with ligand 1 or 2 also afforded
the best enantioselectivities when Yb was used at the expense of
OH
OH
OH
OH
N
1
2
Figure 1. Target BINOL-based ligands.

M. Y. El Kadiri et al. / Tetrahedron Letters 53 (2012) 6335–6338
6337
Table 1
obtained with (R)-BINOL (Table 1, entry 11) and our ligand re-
vealed a higher reactivity with our ligand at the expense of the
enantioselectivity. Indeed, La (R)-BINOL complex gave a 29% con-
version and 50% ee after 4 days of reaction. The best results were
Representative results for the catalytic asymmetric epoxidation of
a
,b-unsaturated
ketones36
3
Catalyst (10 mol%), Ph P(O) (60 mol%)
O
O
O
3
obtained with complexes prepared from Yb(OiPr) and ligand 1
TBHP in toluene (2 equiv)
or 2 (Table 1, entries 15 and 16). With the two Yb complexes, the
conversions of the benzalacetone to the corresponding epoxides
were complete after 5 days, with enantiomeric excesses reaching
R
Ph
R
Ph
MS 4Å (800 mg/mmol), THF(0.66M), 30°C
9
0%. When Yb (R)-BINOL complex was used as catalyst, after 7 days
N°
R
Catalysta
La(OiPr)
Yb(OiPr)
La/(R)-BINOL
Yb/(R)-BINOL
La/1
Time (d)
Yieldb (%)
eec (%)
of reaction a yield of 26% with an enantiomeric excess of 73% was
obtained (Table 1, entry 12).
Finally, we decided to take advantage of the high polarity of the
amino-groups for supporting 2 on silica gel. Indeed, not only graft-
ing on SiO2 would prevent the deleterious coordination of the lan-
thanides by the amine, but release of the ligand could be achieved
1
2
3
4
5
6
7
8
9
Ph
3
2
2
2
2
2
2
2
2
2
2
4
7
5
5
5
5
7
100
65
100
100
41
84
80
75
100
10
0
0
57
81
0
3
La/2
La/2
0
0
+
+d
3
by simple addition of NEt . Accordingly, we immobilized the Yb/li-
Yb/1
Yb/2
Yb/2
49
57
0
50
73
0
gand 2 complex on silica and evaluated the resulting heteroge-
neous catalyst in the epoxidation of benzalacetone (Table 1,
entry 17). Using the optimized conditions determined for the
homogeneous catalysis, after 7 days of reaction, a low conversion
of ca 10%. was measured. Despite the poor conversion possibly
attributable to diffusion limitations, we did not observe any notice-
able loss of enantioselectivity, as an ee of 86% was measured. After
filtration, the supported Yb catalyst was reused in second epoxida-
tion without any addition of ligand and Yb metal (Table 1, entry
+
+d
10
11
12
13
14
15
16
17
18
e
Me
La/(R)-BINOL
Yb/(R)-BINOL
La/1
La/2
Yb/1
29
26
e
100
38
100
100
10
0
90
90
86
93
Yb/2
f
Yb/2/SiO
Yb/2/SiO
2
g
2
10
40
1
8). Interestingly, after 10 days, a conversion of 40% and a similar
a
Catalytic system.
Determined by HPLC.
Determined by HPLC by comparison with the corresponding racemate.
Dicationic ammonium salts after quaternization with MeI.
b
c
d
e
f
enantioselectivity of 93% were measured (difference in the exper-
imental error range), confirming the absence of leaching of the cat-
alyst during the reaction and the treatment.
5
mol % of catalyst.
In this Letter, we have demonstrated that the selective intro-
duction of one or two 3-(dimethylamino)prop-1-yn-1yl functions
on the BINOL skeleton constitutes a promising approach for the
development of a recyclable version of the catalytic system pio-
neered by Shibasaki et al. In particular, we have demonstrated that,
in solution, the presence of the amino-function(s) does not hamper
noticeably the catalytic performances of BINOL for the enantiose-
lective epoxidation of electron poor olefins. Interestingly, we have
proven that the amino-functionalized BINOL displays a higher cat-
alytic activity when Yb was used at the expense of La. At last, we
have taken advantage of the basicity of the BINOL-derived catalyst
for immobilizing the Yb-complex on silica gel without post-func-
tionalization. The latter complex could be recycled without loss
of (enantio)selectivity, highlighting the potentiality of the ap-
proach. Work is underway in our Laboratory for improving the cat-
alytic efficacy (conversion) of the supported catalyst using a silica
gel with a larger surface area.
Heterogeneous catalysis with Yb complex immobilized on silica.
Second run without any addition of Yb complex.
g
La. Indeed, in the case of La complexes prepared from ligand 1 or 2,
the yields reached 41 and 84% respectively, but no enantiomeric
excess was measured (Table 1, entries 5 and 6). The rationale for
this observation relies on the presence of the remote dimethylam-
3
5
ines that offer a pincer-type system able to coordinate the metal.
Interestingly, even if 1 can not bind La as tightly as 2, the presence
of a single dimethylamino-group is sufficient for preventing the re-
quired O-alkylation (Table 1, entry 5). To prevent this coordination,
we decided to quaternarize the amino-functions by reacting meth-
yliodide with compound 2. Under these conditions, the corre-
sponding dicationic ligand, noted 2 was isolated in quantitative
yield. Using the catalyst prepared from La(OiPr) and dicationic li-
gand 2++ in Shibasaki’s conditions (Table 1, entry 7), a similar yield
+
+
3
(
80%) was obtained but the enantiomeric excess remained insignif-
+
+
icant. This result was ascribed to the poor solubility of ligand 2
Acknowledgments
and the corresponding La complex in THF, the conversion resulting
from the presence of a small amount of uncoordinated La(OiPr) in
3
M.Y.E.K. thanks the French Ministry of Superior Education and
Research for a fellowship.
solution. To confirm this hypothesis, a ligand-free reaction was
done (Table 1, entry 1), and after the same reaction time, full con-
version in epoxide was obtained. Interestingly, as Yb is more oxo-
philic than La, the amine functions do not compete with the O-
coordination. Hence, using Yb complexes prepared with ligands 1
or 2, asymmetric epoxidations of chalcone proceeded with med-
ium enantiomeric excesses of 49% and 57%, respectively (Table 1,
entries 8 and 9). Noticeably, the Yb complex based on the dication-
ic ligand 2++ only gave a low conversion in epoxide with no enan-
tiomeric excess (Table 1, entry 10) because of the insolubility
reasons mentioned above.
Supplementary data
Supplementary data (experimental details and complete spec-
troscopic characterizations of new products) associated with this
article can be found, in the online version, at http://dx.doi.org/
1
0.1016/j.tetlet.2012.09.004.
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3
no enantiomeric excess was measured. A full conversion was ob-
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lated in 38% yield with ligand 2. A comparison of the results
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36. For the details of the experimental conditions, see Supplementary data.
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