Rhodium/olefin-catalyzed reaction of arylboronic acids with an α-acetamido acrylic ester: Mizoroki-Heck-type reaction versus asymmetric conjugate addition

Article abstract of DOI:10.1016/j.tetasy.2010.03.021

In this paper we present our results concerning the rhodium/olefin-catalyzed reaction of arylboronic acids with an α-acetamido acrylic ester. With a chiral norbornadiene ligand rather low enantioselectivities (up to 21% ee) were obtained. Besides the expected conjugate adduct, we also observed the formation of a significant amount of Mizoroki-Heck-type product. The ratio of the conjugate adduct/Mizoroki-Heck product could be adjusted by a proper choice of the olefin ligand.

Full text of DOI:10.1016/j.tetasy.2010.03.021

Tetrahedron: Asymmetry 21 (2010) 540–543
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Rhodium/olefin-catalyzed reaction of arylboronic acids with an
acrylic ester: Mizoroki–Heck-type reaction versus asymmetric
conjugate addition
a-acetamido
*
Timothy Noël, Ya ßs ar Gök, Johan Van der Eycken
Laboratory for Organic and Bioorganic Synthesis, Department of Organic Chemistry, Ghent University, Krijgslaan 281 (S.4), B-9000 Ghent, Belgium
a r t i c l e i n f o
a b s t r a c t
Article history:
In this paper we present our results concerning the rhodium/olefin-catalyzed reaction of arylboronic
acids with an -acetamido acrylic ester. With a chiral norbornadiene ligand rather low enantioselectiv-
ities (up to 21% ee) were obtained. Besides the expected conjugate adduct, we also observed the forma-
tion of a significant amount of Mizoroki–Heck-type product. The ratio of the conjugate adduct/Mizoroki–
Heck product could be adjusted by a proper choice of the olefin ligand.
Received 22 January 2010
Accepted 24 March 2010
Available online 13 April 2010
a
Ó 2010 Elsevier Ltd. All rights reserved.
1
. Introduction
[Rh]
Chiral Ligand
NHAc
Ph
NHAc
*
+
PhB(OH)2
PhBF3K
/
The use of olefin ligands in transition metal chemistry has be-
CO Me
CO Me
2
2
1
come very important since the synthesis of Zeise’s salt. Olefin li-
gands are known as labile ligands. Most of the time, they serve
as placeholders for vacant coordination sites for ligand exchange
reactions with stronger binding ligands, for example, phosphanes
and nitrogen ligands. Recently, the use of chiral dienes as spectator
ligands in asymmetric catalysis has been explored by various
1
2
Scheme 1. Rhodium(I)-catalyzed asymmetric tandem conjugate addition/enantio-
selective protonation.
2
. Results and discussion
2
groups. Some of these chiral dienes show very high reactivity
and selectivity in several rhodium- and iridium-catalyzed asym-
First, we tried to optimize the reaction conditions with achiral
metric reactions.3
,4
olefin ligands (Table 1). The reaction proceeded well but, to our
surprise, we found that besides the conjugate addition product 2,
a Mizoroki–Heck-type product 3 was also formed (Table 1, entry
An interesting reaction is the 1,4-addition of PhB(OH)
acetamido acrylic ester 1 giving phenylalanine derivatives 2
Scheme 1). The stereochemistry is in this case not determined at
2
to a-
(
1
). To the best of our knowledge, for this substrate this has not
the stage of the insertion step but at the hydrolysis step. The use
of a bisphosphonite resulted in quantitative yield and a selectivity
been reported before in the literature.
8
Heck reactions are typically catalyzed by Pd(0)-complexes.
5
of 77% ee. A bisphosphite ligand formed the product in 91% yield
9
10
Also, iridium - and ruthenium -catalyzed Heck reactions are de-
scribed in the literature.
and a selectivity of 72% ee.6 The use of a BINAP/rhodium catalyst
3
and PhBF K resulted in 100% conversion. However, the product
was racemic when water was used as a proton source. The use of
Lautens et al. were the first to describe a rhodium-catalyzed
1
1
Heck-type coupling with styrenes. It was attributed to the fact
that styrenes are incapable of enolization. The same was also ob-
guaiacol, a more-hindered proton source, proved to be crucial
7
and a selectivity of 89.5% ee was achieved. With PhB(OH)
2
, a lower
1
2
served by Genêt et al. Almost at the same time, Mori et al. re-
ported that it was possible to produce the Heck product with a
yield and selectivity were obtained (42% yield, 42% ee). This was
explained by the fact that boronic acids can act as competitive pro-
ton sources.
We wondered whether chiral diene ligands would be effective
in the tandem conjugate addition/enantioselective protonation.
As we describe in this paper, olefin ligands were tested in the
1
3
rhodium catalyst in anhydrous THF. However by a simple sol-
vent-switch to THF/H O, the major product became the 1,4-adduct.
The reason for this is that the intermediate oxa- -allylrhodium is
2
p
readily protonated to form the conjugate adduct. It appeared that
the conjugate addition becomes more dominant when the sub-
strate has a more electron-deficient carbonyl. Methyl vinyl ketone
gives exclusively the 1,4-adduct, unsaturated amides give a mix-
ture and unsaturated esters give preferentially the Heck product.
Lautens et al. observed that the reaction of t-butyl acrylate with
1
2
,4-addition of PhB(OH) to a-acetamido acrylic ester 1.
*
Corresponding author. Tel.: +32 9 264 44 80; fax: +32 9 264 49 98.
E-mail address: Johan.Vandereycken@UGent.be (Johan Van der Eycken).
0
957-4166/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2010.03.021

T. Noël et al. / Tetrahedron: Asymmetry 21 (2010) 540–543
541
phenylboronic acids gave preferentially the Heck product.14
Remarkable was the fact that the solvent was toluene/H O. With
bulky boronic acids, the conjugate product was selectively formed.
The boronic acid was probably bulky enough to interfere in the Rh–
lower yield and a lower selectivity for the 1,4-adduct were ob-
tained (Table 1, entry 5). A remarkable result was observed with
norbornadiene and ethene as ligands: the selectivity was clearly
in favour of the Heck product (Table 1, entries 6–8). So, by a proper
choice of the ligand the selectivity could clearly be altered. Zou
et al. have already demonstrated that Heck-coupling was obtained
selectively in the presence of triphenylphosphane, while conjugate
addition was favoured in the presence of bisphosphanes.16
2
H elimination. In the presence of
late, the formation of the 1,4-adduct was selective. A possible
explanation is that through -bond inductive electron-donation,
the oxa- -allylrhodium intermediate is more basic which results
a- or b-substituents on the acry-
r
p
in a faster protonation. Recently, a base-free rhodium-catalyzed
Mizoroki–Heck reaction was discovered. Again, the presence of
water resulted in the formation of the conjugate adduct as a single
Although the selectivity for the conjugate addition was low in
the presence of norbornadiene (22% of 1,4-adduct), we decided
to perform the reaction in the presence of the chiral Hayashi ligand
1
5
3a,4
product.
4 (Table 2).
First, we carried out the reaction in analogy with the
Several reaction parameters were adjusted in order to optimize
the reaction outcome (Table 1, entries 2–8). A lower temperature
resulted in a lower yield and also a lower selectivity for the conju-
gate addition product (Table 1, entry 2). Addition of KOH gave a
experiment with the achiral norbornadiene ligand (Table 1, entry
6), resulting in a slightly higher total yield (Table 2, entry 1). More-
over, a higher selectivity for the conjugate adduct was observed,
but no chiral induction. The use of aq KOH as a cosolvent resulted
in both a lower yield of a 1,4-adduct and a lower total yield. A
noticeable enantioselectivity was observed in this case however,
albeit very low (Table 2, entry 2). Again, lowering the reaction
similar result (Table 1, entry 3). Rh(COD)(CH
3
CN)
2
ꢀBF
4
as a catalyst
resulted in a similar selectivity as the first experiment but a some-
ꢁ
what lower yield (Table 1, entry 4). With OH as counterion, both a
Table 1
a
Rhodium(I)-catalyzed conjugate addition versus Mizoroki–Heck reaction
NHAc
[Rh] / Ligand
NaF (3 equiv)
Ph
NHAc
CO Me
Ph
NHAc
CO Me
+
PhB(OH)
2
+
(
4 equiv)
CO Me
2
2
2
dioxane/H O (10:1)
2
1
2
3
Entry
Rhodium catalyst [Rh]
Additive
Temperature (°C)
Time (h)
Total yieldb (%)
77
% Conjugate adduct 2c
% Mizoroki–Heck product 3c
1
2
3
4
5
6
7
8
[Rh(COD)Cl]
[Rh(COD)Cl]
[Rh(COD)Cl]
2
2
2
/
/
100
50
50
100
100
100
100
100
24
24
24
48
24
24
48
48
82
67
64
83
74
22
25
21
18
33
36
17
26
78
75
79
d
21
23
54
56
36
39
d
1.5 M aq KOH
d
Rh(COD)(CH
[Rh(COD)OH]
[Rh(nbd)Cl]
[Rh(ethene)Cl]
Rh(acac)(ethene)
3
CN)
2
ꢀBF
4
/
/
/
/
/
d
2
d
2
d
2
d
2
34
a
b
c
Reagents and conditions: 1 (0.5 mmol), PhB(OH)
Isolated as a mixture of conjugate adduct 2 and Mizoroki–Heck product 3.
The ratio of conjugate adduct 2/Mizoroki–Heck product 3 was determined via GC on a Chirasil-Val column.
The reaction was not complete after the indicated time. Formation of side products was not observed.
2
(2.0 mmol), NaF (1.5 mmol), [Rh] (3 mol % Rh), dioxane/H
2
O (10:1).
d
Table 2
a
Chiral diene/rhodium(I)-catalyzed conjugate addition versus Mizoroki–Heck reaction
4
[
Rh(ethene)Cl]2
NHAc
Ph
NHAc
*
Ph
NHAc
CO Me
+
PhB(OH)
2
+
(
4 equiv)
NaF (3 equiv)
solvent
CO Me
CO Me
2
2
2
1
2
3
Entry
Solvent
Temperature (°C)
Time (h)
Total yieldb,c (%)
% Conjugate adductd,e (ee [%])
% Heck productc
1
2
3
4
5
Dioxane/H
Dioxane/ 1.5 M aq KOH (10:1)
2
O (10:1)
100
100
50
50
50
24
24
48
48
48
40
27
8
12
10
30 (rac)
24 (9)
27 (6)
60 (20)
46 (21)
70
76
73
40
54
Dioxane/H
Toluene/H
EtOH/H
2
O (10:1)
O (10:1)
2
2
O (10:1)
a
b
c
Reagents and conditions: 1 (0.25 mmol), PhB(OH)
Isolated as a mixture of 1,4-adduct and Heck product.
The reaction was not complete after the indicated time. Formation of side products was not observed.
The ratio of conjugate adduct 2/Heck product 3 and the enantiomeric excess were determined via GC on a Chirasil-Val column.
The absolute configuration of the major enantiomer was determined to be (S).
2 2 4 2 2
(1.0 mmol), NaF (0.75 mmol), [RhCl(C H ) ] (3 mol % Rh), chiral diene ligand 4 (4/Rh = 1.1/1.0), solvent.
d
e

5
42
T. Noël et al. / Tetrahedron: Asymmetry 21 (2010) 540–543
NHAc
CO Me
OH
B
2
Ph
OH
transmetalation
LnRh(OH)
NHAc
I
A
B(OH)3
H O
+
2
CO2Me
H2
Ph
NHAc
1
B
H2O
PhH
LnRh(OH)
H O
CO Me
2
2
Rh Ln
II
H2O
Ph
1
,4-adduct
LnRh(H)
IV
2
I
NHAc
CO Me
Ph
NHAc
β-H-Elimination
2
1
NHAc
CO Me
Insertion
2
Ph
LnRh
III
CO Me
Heck product
2
3
Scheme 2. Proposed catalytic cycle for olefin/rhodium-catalyzed conjugate addition versus Mizoroki–Heck-coupling.
temperature resulted in
enantioselectivity for the 1,4-adduct (Table 2, entry 3). Using tolu-
ene/H O (10:1) as a solvent system gave a higher selectivity in fa-
vour of the 1,4-adduct. The enantioselectivity was higher although
still unsatisfactory (Table 2, entry 4). The use of EtOH/H O resulted
in an almost equimolar mixture of the 1,4-adduct 2 and Heck prod-
uct 3 and a comparable enantioselectivity as for entry 4 (Table 2,
entry 5).
A mechanism for the observed transformations is proposed in
Scheme 2. The catalytically active hydroxorhodium I transmeta-
lates with phenylboronic acid to form the phenylrhodium species
II. Next, insertion of the alkene 1 causes the formation of a Rh–C
bond III. Hydrolysis results in the formation of the 1,4-adduct 2
and regenerates the hydroxorhodium I catalyst. A second possibil-
ity is the b-hydride elimination which results in the formation of
the Heck product 3 and a rhodium hydride species IV. Subse-
quently the rhodium hydride IV should be converted into the
hydroxorhodium I. This can be done if we consider that the alkene
a
lower total yield and
a
lower
reagents were purchased and used without purification, unless
otherwise noted. Analytical TLC was performed using Macherey-
Nagel SIL G-25 UV254 plates. Flash chromatography was carried
2
1
13
out with Rocc silicagel (0.040–0.063 mm). H NMR and C NMR
were recorded on a Bruker Avance 300 or on a Bruker AM 500 spec-
trometer as indicated, with chemical shifts reported in parts per
million (ppm) relative to TMS, using the residual solvent signal
2
1
3
as a standard. C NMR spectra were recorded using the attached
proton test (APT). IR-spectra were recorded on a Perkin–Elmer
spectrum 1000 FT-IR spectrometer with a Pike Miracle HATR mod-
ule. EI Mass spectra were recorded with a Hewlett–Packard 5988A
mass spectrometer. LC–MS analysis was performed on an Agilent
1100 series HPLC with quaternary pump, DAD and single quadru-
pole MS detector type VL with an API-ES source, using a Phenom-
enex Luna C18(2) column (250 ꢂ 4.6 mm, particle size 5
lm). Exact
molecular masses were measured on a Kratos MS50TC mass spec-
trometer. Melting points were measured with a Kofler melting
point apparatus.
1
is used as a hydride acceptor (path A). Although we did not
immediately discover traces of this product, it would explain the
low yields obtained when the Heck product was the major product.
It can also explain the lower ee’s in the asymmetric reactions when
the Heck product 3 is used as a hydride acceptor. A second possi-
4
.2. Synthesis of (S,S)-Bn-nbd* 4
solution of bis-triflate4 (104.7 mg, 269.7
(dppf). CH Cl (4.7 mg, 5.79 mol) in Et O (1.0 mL) was
A
lmol) and
PdCl
2
2
2
l
2
bility is the reaction of Rh–H (IV) with H
2
O to give Rh–OH (path
B). The excess of boronic acid is partly consumed by reaction of
the phenylrhodium species II with H O.
cooled in an ice bath. To the resulting red suspension was added
BnMgCl (1.35 mL, 1.77 mmol, 20 w/w% in THF) under argon. The
reaction mixture was stirred for 1 h at room temperature; the reac-
tion was quenched with brine (25 mL) and the mixture was ex-
tracted with EtOAc (4 ꢂ 50 mL). The combined organic phases
1
7
2
3
. Conclusion
4
were dried over MgSO and concentrated in vacuo. The crude prod-
In conclusion, the asymmetric conjugate addition of phenylbo-
ronic acid to -acetamido acrylic ester catalyzed by a rhodium(I)-
catalyst in the presence of C -symmetrical diene ligand 4 was
uct was purified by flash chromatography over silicagel (isooctane/
a
CHCl , 96/4) resulting in a white solid which contained a signifi-
3
2
cant amount of 1,2-diphenylethane which was removed under re-
investigated, but resulted in rather low enantioselectivities. More-
over, we have described for the first time a Mizoroki–Heck-type
duced pressure (<1 mm Hg, 1 night) resulting in pure (S,S)-Bn-nbd*
1
4 as a white solid, 53.0 mg (72%). H NMR (500 MHz, CDCl ): d 1.94
3
coupling with an
a
-acetamido acrylic ester in the presence of an
(t, J = 1.7 Hz, 2H), 3.14 (dt, J = 3.8, 1.7 Hz, 2H), 3.49 (s, 4H), 6.02 (dt,
J = 3.8, 1.6 Hz, 2H), 7.09 (d, J = 7.2 Hz, 4H), 7.18 (tt, J = 7.2, 1.2 Hz,
olefin/rhodium(I)-catalyst. The ratio of the conjugate adduct 2/
Mizoroki–Heck product 3 could be adjusted by the proper choice
of the ligand.
1
3
2H), 7.26 (t, J = 7.2 Hz, 4H) ppm. C NMR (75.4 MHz, CDCl ): d
3
38.0 (CH ), 53.1 (CH), 71.4 (CH ), 125.9 (CH), 128.1 (CH), 129.0
2
2
(
CH), 134.3 (CH), 139.1 (C), 156.9 (C) ppm. IR (HATR): 2970,
4
4
. Experimental
2950, 2929, 2878, 1600, 1492, 1451, 1306, 1183, 1069, 1026,
ꢁ1
9
48, 856, 780, 752, 737, 708 cm . EI-MS m/z (rel. intensity%):
+
.1. General
272 (M , 9), 181 (34), 165 (21), 156 (34), 141 (17), 128 (22), 115
(
39), 103 (8), 91 (88), 84 (29), 65 (30), 49 (71), 40 (100). ES-MS:
+
20
D
All reactions were carried out under an argon atmosphere in dry
solvents under anhydrous conditions, unless otherwise stated. All
273 [M+H] . ½
a
ꢃ
3
¼ ꢁ180 (c 1.07, CHCl ). Mp: 67 °C. HRMS (EI)
calcd for C21
H
20: 272.1565; found 272.1569.

T. Noël et al. / Tetrahedron: Asymmetry 21 (2010) 540–543
543
4
.3. Typical procedure for the racemic conjugate addition of
2. For reviews regarding chiral diene ligands: (a) Shintani, R.; Hayashi, T.
Aldrichim. Acta 2009, 42, 31–38; (b) Defieber, C.; Grützmacher, H.; Carreira, E.
M. Angew. Chem., Int. Ed. 2008, 47, 4482–4502; (c) Johnson, J. B.; Rovis, T.
Angew. Chem., Int. Ed. 2008, 47, 840–871; (d) Glorius, F. Angew. Chem., Int. Ed.
2004, 43, 3364–3366.
phenylboronic acid to -acetamido acrylic ester
a
Methyl 2-acetamidoacrylate 1 (71.6 mg, 0.5 mmol), [Rh(COD)
(7.4 mg, 0.015 mmol), phenylboronic acid (243.9 mg, 2 mmol)
and NaF (63.0 mg, 1.5 mmol) were dissolved in dioxane (1.5 mL)
and stirred for 30 min at room temperature. Then H O (150 L)
was added and the resulting reaction mixture was heated to
00 °C under argon in a sealed tube. After 26 h, full conversion
3.
For some selected examples: (a) Hayashi, T.; Ueyama, K.; Tokunaga, N.;
Yoshida, K. J. Am. Chem. Soc. 2003, 125, 11508–11509; (b) Fischer, C.; Defieber,
C.; Suzuki, T.; Carreira, E. M. J. Am. Chem. Soc. 2004, 126, 1628–1629; (c)
Tokunaga, N.; Otomaru, Y.; Okamoto, K.; Ueyama, K.; Shintani, R.; Hayashi, T. J.
Am. Chem. Soc. 2004, 126, 13584–13585; (d) Otomaru, Y.; Okamoto, K.;
Shintani, R.; Hayashi, T. J. Org. Chem. 2005, 70, 2503–2508; (e) Paquin, J.-F.;
Defieber, C.; Stephenson, C. R. J.; Carreira, E. M. J. Am. Chem. Soc. 2005, 127,
10850–10851; (f) Läng, F.; Breher, F.; Stein, D.; Grützmacher, H. Organometallics
Cl]
2
2
l
1
was observed via TLC. The reaction mixture was passed through
a short pad of silicagel and eluted with EtOAc. Evaporation in vacuo
and purification by flash chromatography over silicagel (hexane/
EtOAc, 50/50) resulted in a mixture of 2 and 3, 84.5 mg (77%;
2005, 24, 2997–3007; (g) Helbig, S.; Sauer, S.; Cramer, N.; Laschat, S.; Baro, A.;
Frey, W. Adv. Synth. Catal. 2007, 349, 2331–2337; (h) Wang, Z.-Q.; Feng, C.-G.;
Xu, M.-H.; Lin, G.-Q. J. Am. Chem. Soc. 2007, 129, 5336–5337; (i) Okamoto, K.;
Hayashi, T.; Rawal, V. H. Org. Lett. 2008, 10, 4387–4389; (j) Nishimura, T.;
Nagaosa, M.; Hayashi, T. Chem. Lett. 2008, 37, 860–861; (k) Sörgel, S.; Tokunaga,
N.; Sasaki, K.; Okamoto, K.; Hayashi, T. Org. Lett. 2008, 10, 589–592; (l)
Gendrineau, T.; Chuzel, O.; Eijsberg, H.; Genet, J.-P.; Darses, S. Angew. Chem., Int.
Ed. 2008, 47, 7669–7672.
8
2% of 2 and 18% of 3).
4
.4. Typical procedure for the asymmetric conjugate addition of
phenylboronic acid to
a-acetamido acrylic ester
4. (a) Noël, T.; Vandyck, K.; Van der Eycken, J. Tetrahedron 2007, 63, 12961–
12967; (b) Noël, T. Ph.D. Thesis Ghent University, 2009.
5
6
.
.
Reetz, M. T.; Moulin, D.; Gosberg, A. Org. Lett. 2001, 3, 4083–4085.
Chapman, C. J.; Wadsworth, K. J.; Frost, C. G. J. Organomet. Chem. 2003, 680,
206–211.
At first, [Rh(C
4.5 mg, 16.5 mol) were dissolved in a mixture of degassed diox-
ane (0.75 mL) and degassed H O (75 L) and stirred for 30 min at
2 4 2 2
H ) Cl] (2.9 mg, 7.5 lmol) and (S,S)-Bn-nbd* 4
(
l
7
.
(a) Navarre, L.; Darses, S.; Genêt, J.-P. Angew. Chem., Int. Ed. 2004, 43, 719–723;
2
l
(
6
b) Navarre, L.; Martinez, R.; Genêt, J.-P.; Darses, S. J. Am. Chem. Soc. 2008, 130,
159–6169.
room temperature. After the addition of methyl 2-acetamidoacry-
late 1 (35.8 mg, 0.25 mmol), phenylboronic acid (122 mg, 1 mmol)
and NaF (31.5 mg, 0.75 mmol), the resulting reaction mixture was
stirred for 24 h at 100 °C in a sealed tube. The reaction mixture was
passed through a short pad of silicagel and eluted with EtOAc.
Evaporation in vacuo and purification by flash chromatography
over silicagel (hexane/EtOAc, 50/50) resulted in a mixture of 2
and 3, 22.0 mg (40%; 30% of 2 (rac) and 70% of 3).
8. For the palladium-catalyzed Heck-type reaction of boronic acids see: (a) Ruan,
J.; Li, X.; Saidi, O.; Xiao, J. J. Am. Chem. Soc. 2008, 130, 2424–2425; (b) Enquist,
P.-A.; Lindh, J.; Nilsson, P.; Larhed, M. Green Chem. 2006, 8, 338–343; (c)
Andappan, M. M. S.; Nilsson, P.; von Schenck, H.; Larhed, M. J. Org. Chem. 2004,
69, 5212–5218; (d) Andappan, M. M. S.; Nilsson, P.; Larhed, M. Chem. Commun.
2004, 218–219; (e) Yoon, C. H.; Yoo, K. S.; Yi, S. W.; Mishra, R. K.; Jung, K. W.
Org. Lett. 2004, 6, 4037–4039; (f) Jung, Y. C.; Mishra, R. K.; Yoon, C. H.; Jung, K.
W. Org. Lett. 2003, 5, 2231–2234; (g) Du, X.; Suguro, M.; Hirabayashi, K.; Mori,
A.; Nishikata, T.; Hagiwara, N.; Kawata, K.; Okeda, T.; Wang, H. F.; Fugami, K.;
Kosugi, M. Org. Lett. 2001, 3, 3313–3316.
The adducts were fully characterized by making a comparison
of their spectral data with those reported in the literature. The
enantiomeric excess of the product is determined by GC analysis
9
.
Koike, T.; Du, X.; Sanada, T.; Danda, Y.; Mori, A. Angew. Chem., Int. Ed. 2003, 42,
89–92.
10. Farrington, E. J.; Brown, J. M.; Barnard, C. F. J.; Rowsell, E. Angew. Chem., Int. Ed.
2002, 41, 169–171.
with
30 m ꢂ 0.25 mm ꢂ 0.25
for 11 min, increasing to 190 °C (40 °C/min); retention times:
.11 min for (R)-2, 9.38 min for (S)-2 and 14.60 min for 3.
a
chiral stationary phase column: Chirasil-Val column
11. Lautens, M.; Roy, A.; Fukuoka, K.; Fagnou, K.; Martin-Matute, B. J. Am. Chem.
(
lm); temperature programme: 150 °C
Soc. 2001, 123, 5358–5359.
1
2. Amengual, R.; Michelet, V.; Genêt, J.-P. Tetrahedron Lett. 2002, 43, 5905–5908.
9
13. Mori, A.; Danda, Y.; Fujii, T.; Hirabayashi, K.; Osakad, K. J. Am. Chem. Soc. 2001,
23, 10774–10775.
1
1
1
1
4. Lautens, M.; Mancuso, J.; Grover, H. Synthesis 2004, 2006–2014.
5. Martinez, R.; Voica, F.; Genêt, J.-P.; Darses, S. Org. Lett. 2007, 9, 3213–3216.
6. Zou, G.; Guo, J.; Wang, Z.; Huang, W.; Tang, J. Dalton Trans. 2007, 28, 3055–
Acknowledgements
3
064.
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T.N., Y.G. and J.V.D.E. wish to thank Ghent University and COST-
Chemistry (Action D.40—‘Innovative Catalysis’) for financial
support.
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