Asymmetric synthesis of chiral Roche ester and its derivatives via Rh-catalyzed enantioselective hydrogenation with chiral phosphine-phosphoramidite ligands

Article abstract of DOI:10.1016/j.tetasy.2008.12.026

Methyl 3-hydroxy-2-methylpropionate, known as the Roche ester, was prepared with high enantioselectivity (up to 96.7% ee) via the Rh-catalyzed asymmetric hydrogenation of methyl 2-hydroxymethylacrylate with a chiral 1,2,3,4-tetrahydro-1-naphthylamine-deri

Full text of DOI:10.1016/j.tetasy.2008.12.026

Tetrahedron: Asymmetry 20 (2009) 210–213
Contents lists available at ScienceDirect
Tetrahedron: Asymmetry
journal homepage: www.elsevier.com/locate/tetasy
Asymmetric synthesis of chiral Roche ester and its derivatives
via Rh-catalyzed enantioselective hydrogenation with chiral
phosphine-phosphoramidite ligands
Min Qiu a,b, Dao-Yong Wang a,b, Xiang-Ping Hu *, Jia-Di Huang a,b, Sai-Bo Yu a,b
Jun Deng , Zheng-Chao Duan , Zhuo Zheng
a
a,
,
a,b
a,b
a,
*
Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China
Graduate School of Chinese Academy of Sciences, Beijing 100039, China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Methyl 3-hydroxy-2-methylpropionate, known as the Roche ester, was prepared with high enantioselec-
tivity (up to 96.7% ee) via the Rh-catalyzed asymmetric hydrogenation of methyl 2-hydroxymethylacry-
Received 19 November 2008
Accepted 17 December 2008
Available online 31 January 2009
late with
a chiral 1,2,3,4-tetrahydro-1-naphthylamine-derived phosphine-phosphoramidite ligand
(THNAPhos 5a) even at a low catalyst loading (0.1 mol %). An investigation on the substrate scope
revealed that the ester group present in the substrate has a significant effect on the enantioselectivity,
and the substrate with the bulkier ester group tended to give lower enantioselectivity.
Ó 2009 Elsevier Ltd. All rights reserved.
1
. Introduction
Chiral methyl 3-hydroxy-2-methylpropionate (Roche ester, 1a,
tion of methyl 2-hydroxymethylacrylate with BeePHOS and
DuPHOS, affording an ee-value of up to 90%. Genêt and Ratovelo-
6
manana–Vidal found that the Roche ester derivatives with bulky
ester groups could be obtained in high enantioselectivity (up to
94% ee) in the first Ru-catalyzed asymmetric hydrogenation of
the corresponding 2-hydroxymethylacrylates, although the parent
Fig. 1) is a very important building block for the enantioselective
synthesis of many natural products and pharmaceutical com-
pounds. The enantioselective access to this compound is therefore
1
7
of great interest.
Roche ester 1a was obtained in only 88% ee. Reek et al. reported
that the Roche ester can be obtained with an ee-value of up to
8
9
8% ee using a Rh-INDOLPHOS catalyst. However, the hydrogena-
1
1
1
a: R = Me, Roche ester
b: R = Et
tion needed to be performed at ꢀ40 °C. When the hydrogenation
temperature rose to room temperature, the enantioselectivity
dropped to 93% ee. Very recently, Holz and Börner showed that
the Rh-catalyzed asymmetric hydrogenation of 2-hydroxymethyl-
acrylates with catASium M ligands gave rise to the Roche ester
and its derivatives in excellent enantioselectivities (up to 99%
O
c: R = i-Pr
*
HO
OR 1d: R = n-Bu
1
1
e: R = t-Bu
f: R = Bn
Figure 1. Roche ester and its derivatives 1a–f.
9
ee), which represent the best results so far. Since the Roche ester
The present methods for the asymmetric synthesis of 1a include
is a liquid and the enantiomeric purity cannot be increased by crys-
tallization, the development of a new catalytic system with
2
8
the aldol reaction of formaldehyde; the oxidative degradation of a
3
chiral homoallylic acetate; diastereoselective addition of chiral
properties superior to its predecessors, for example, high enanti-
oselectivity (more than 95% ee) and mild reaction conditions for
the asymmetric synthesis of this building block, is therefore still
highly desirable.
Over the past few years, we have developed a series of unsym-
metrical hybrid chiral phosphine-phosphoramidite ligands (Fig. 2),
which showed excellent enantioselectivities and catalytic activities
in the Rh-catalyzed asymmetric hydrogenation of a variety of
4
alcohols to arylketenes; and bacterial or microbial enzyme-medi-
5
ated transformations. To the best of our knowledge, there are only
a few reports on the catalytic asymmetric synthesis of the Roche
ester and its derivatives except for some recent examples on the
catalytic asymmetric hydrogenation of 2-hydroxymethylacrylates.
In 2003, Saito et al. reported the enantioselective synthesis of the
Roche ester 1a via the first Rh-catalyzed asymmetric hydrogena-
functionalized olefins including
a-dehydroamino acid esters,
b-dehydroamino acid esters, enamides, dimethyl itaconate,
*
Corresponding author. Tel.: +86 411 84379206; fax: +86 411 84684746 (X.-P.
Hu).
E-mail addresses: xiangping@dicp.ac.cn (X.-P. Hu), zhengz@dicp.ac.cn (Z. Zheng).
10
a-enamido phosphonates, and
a-enol ester phosphonates. Due
to the easy accessibility, modularity, and high efficiency in
0
957-4166/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2008.12.026

M. Qiu et al. / Tetrahedron: Asymmetry 20 (2009) 210–213
211
Table 1
Rh-catalyzed asymmetric hydrogenation of methyl 2-hydroxymethylacrylate 2aa
H
N
O
P
O
O
P
O
[
Rh(COD) ]BF4 / L*
2
N
HO
HO
*
CO Me
CO Me
2
PPh2
2
H , solvent
2
Fe
PPh2
rt, 24 h
2
a
1a
(
R ,S ,R )-PPFAPhos 3
(Sc,Sa)-PEAPhos 4
Entry
Ligand
(R ,S ,R
,S )-4
H
2
(bar)
Solvent
Conv.b (%)
eec (%)
c
p
a
1
2
3
4
5
6
7
8
9
c
p
a
)-3
10
10
10
10
10
10
10
10
10
20
5
CH
2
CH
2
CH
2
CH
2
CH
2
Cl
Cl
Cl
Cl
Cl
2
2
2
2
2
100
100
100
100
100
100
100
100
100
100
100
100
100
36.3 (S)
93.8 (S)
96.7 (R)
7.0 (S)
94.1 (S)
3.4
19.3
73.8
14.9
95.1
(S
(R
(R
c
a
c
c
,R
,S
a
)-5a
)-5b
H
N
O
P
O
H
N
O
P
O
a
a
(S )-6
(R
c
(R
c
(R
c
(R
c
(R
c
(R
c
(R
c
(R
c
,R
,R
,R
,R
,R
,R
,R
,R
a
)-5a
a
)-5a
a
)-5a
a
)-5a
a
)-5a
a
)-5a
a
)-5a
a
)-5a
THF
Toluene
MeOH
PPh2
PPh2
i-PrOH
(
R ,R )-THNAPhos 5a
c a
(S )-HY-Phos 6
c
a
a
10
11
CH
2
CH
2
CH
2
CH
2
Cl
Cl
Cl
Cl
2
2
2
2
(
R ,S )-THNAPhos 5b
95.5
d
12
10
10
86.2
Figure 2. Chiral phosphine-phosphoramidite ligands developed within our group.
13
90.9e
a
Reactions were performed in 2 mL of solvent, Rh/L* = 1: 1.1, Rh/2a = 1: 100,
0 bar of at room temperature for 24 h using 0.25 mmol of 2a and
catalytic hydrogenation, we therefore surmised that this class of li-
gand may be also effective for the catalytic asymmetric hydrogena-
tion of 2-hydroxymethylacrylates. As a result, we herein report our
studies on the catalytic asymmetric synthesis of the Roche ester
and its derivatives via the Rh-catalyzed asymmetric hydrogenation
using a readily accessible, chiral 1,2,3,4-tetrahydro-1-naphthyl-
amine-derived phosphine-phosphoramidite ligand, in which the
Roche ester was prepared in 96.7% ee under mild conditions.
1
2
H ,
[Rh(COD) ]BF as a metal precursor unless otherwise specified.
2
4
b
c
Conversion was determined by GC.
Enantiomeric excesses were determined by GC using a capillary chiral column
[
cyclodex-b,2,3,6-methylated, 30 m ꢁ 0.25 mm (i.d.)]. The absolute configuration
was determined by comparing the specific rotations with the reported data or the
GC retention times with GC data in the literature.
d
Using [Rh(COD) ]SbF6 as a metal precursor.
Rh/2a = 1:1000.
2
e
2
. Results and discussion
employed in this hydrogenation, displaying very low enantioselec-
tivity and favoring a hydrogenation product with a configuration
Substrates 2a–f for the hydrogenation reaction can be easily
c a
opposite to that obtained with (R ,R )-THNAPhos 5a (entry 4). Hav-
prepared from acrylates and paraformaldehyde via the Baylis–Hill-
man reaction via the reported procedures (Scheme 1).11
ing found that (R ,R )-THNAPhos 5a is the best ligand, we set out to
c
a
determine the optimal conditions for this asymmetric hydrogena-
tion. The effect of solvents on this hydrogenation was first investi-
gated, and in all cases, full conversions were achieved. However,
the results disclosed that the enantioselectivity of this hydrogena-
tion was strongly dependent on the nature of the solvent used. The
paraformaldehyde
DABCO
OR
HO
OR
THF/H2O (1/1)
O
O
2 2
best result was obtained in aprotic CH Cl , whereas all other apro-
2
tic solvents, such as THF and toluene led to lower selectivities
(entries 3, 6, and 7). Alcoholic solvents proved to be less efficient
for this hydrogenation, and gave lower enantioselectivities in com-
Scheme 1. Synthesis of substrates 2.
With these substrates in hand, we first investigated the ligand
effect in the Rh-catalyzed asymmetric hydrogenation of methyl
parison with those obtained in CH
2 2
Cl (entries 8 and 9). The hydro-
gen pressure has slight impact on the enantioselectivity.
a
2
-hydroxymethylacrylates 2a. Hydrogenation was performed at
room temperature under a H pressure of 10 bar in the presence
of 1 mol % of catalyst prepared in situ from [Rh(COD) ]BF and
.1 equiv of phosphine-phosphoramidite ligand. The results are
summarized in Table 1.
Initially, ferrocene-based phosphine-phosphoramidite ligand,
Lowering the hydrogen pressure to 5 bar or elevating the hydrogen
pressure to 20 bar led to a slight decrease in enantioselectivity
2
2 6 2
(entries 10 and 11). Using [Rh(COD) ]SbF instead of [Rh(COD) ]
2
4
1
BF as the catalyst precursor also resulted in decreased enantiose-
4
lectivity (entry 12). Reducing the catalyst loadings from 1 mol % to
0.1 mol % also gave rise to the Roche ester 1a with a good enanti-
oselectivity of 90.9% ee, thus demonstrating the high efficiency of
the present catalytic system in this hydrogenation (entry 13). This
result represents the highest substrate/catalyst ratio successfully
c p a
(R ,S ,R
)-PPFAPhos 3,1 was examined, and full conversion was
observed. However, the Roche ester 1a was obtained in only low
enantioselectivity (entry 1). Gratifyingly, the hydrogenation with
a
0c
-phenylethylamine-derived (S
,S
c a
)-PEAPhos 41
0d
afforded the
used in this hydrogenation reported so far.
6–9
Roche ester with (S)-configuration and in up to 93.8% ee (entry
Having established the optimized conditions {1 mol % of
2
). When (S
c
,S
a
)-PEAPhos 4 was replaced with 1,2,3,4-tetrahydro-
catalyst loading prepared in situ from [Rh(COD)
2
]BF
4
and ligand,
)-THNAPhos 5a,1 the enantiose-
0e
1
-naphthylamine-derived (R
,R
c a
performed under 10 bar of H pressure in CH Cl
2
2
2
at room temper-
lectivity was further increased (up to 96.7% ee, entry 3). The im-
proved enantioselectivity is presumably due to the increased
ature for 24 h}, we subsequently evaluated the effect of the ester
function of 2-hydroxymethylacrylate substrates on this hydroge-
nation. A series of 2-hydroxymethylacrylate substrates 2a–f with
different ester groups were submitted to this hydrogenation by
rigidity conferred by a cyclohexyl fragment in (R
c
,R
a
)-THNAPhos
5
1
5
a. However, further increasing the rigidity by replacing the
,2,3,4-tetrahydro-1-naphthylamino moiety in (R
,R
c a
)-THNAPhos
using Rh/(S
the optimized conditions, and the results are shown in Table 2. In
all cases, full conversions were obtained, and (S ,S )-PEAPhos
displayed the opposite stereo-induction to (R ,R )-THNAPhos. The
c a c a
,S )-PEAPhos and Rh/(R ,R )-THNAPhos catalysts under
a with a 1-naphthylamino fragment10g resulted in a decreased
enantioselectivity to 94.1% ee (entry 5). For the sake of comparison,
ligand (R ,S )-THNAPhos 5b with an (S )-binaphthyl fragment was
c a
c
a
a
c
a

2
12
M. Qiu et al. / Tetrahedron: Asymmetry 20 (2009) 210–213
Table 2
Hydrogenations were carried out in a stainless steel autoclave. Sol-
vents were reagent grade, dried, and distilled before use following
the standard procedures. 2-Hydroxymethylacrylates 2a–f are
Catalytic asymmetric hydrogenation of substrates 2 with Rh-(S
c
,S
a
)-PEAPhos and
Rh-(R
c
,R
a
)-THNAPhos catalysta
11
known compounds, which were prepared according to the litera-
ture methods. All other chemicals were obtained commercially.
[
Rh(COD) ]BF (1 mol%)
2 4
ligand 4 or 5a (1.1 mol%)
1
HO
HO
*
H NMR spectra were recorded on BRUKER DEX-400 spectrome-
CO R
CO R
2
2
H2 (10 bar), CH Cl
ter. Chemical shift values (d) are denoted in ppm, and are referenced
2
2
rt, 24 h
1
2
1
to residue protons in deuterated solvents for H NMR (CDCl
3
:
7
5
.27 ppm). Mass spectra (MS) were measured on a Agilent 6890-
973N instrument. Optical rotation was recorded using a JASCO P-
Entry
Substrate
Ligand
Conv.b (%)
eec (%)
1
2
3
4
5
6
7
8
9
2a: R = Me
2a: R = Me
2b: R = Et
4
5a
4
5a
4
5a
4
5a
4
5a
4
100
100
100
100
100
100
100
100
100
100
100
100
93.8
96.7
87.8
95.7
86.0
90.5
91.6
95.3
49.8
79.1
90.1
90.1
1020 high sensitive polarimeter. Enantiomeric excesses were deter-
mined by capillary GC analysis with a chiral b-120 column
(
0.25 mm ꢁ 30 m) for 1a–b and 1e, and by HPLC analysis with a chi-
2b: R = Et
ral column (Chiralcel OD-H, 0.46 mm ꢁ 25 cm) for 1c–d and 1f.
4.2. General procedure for asymmetric hydrogenation
In a nitrogen-filled glovebox, a stainless steel autoclave was
charged with [Rh(COD)
(R ,R
)-THNAPhos 5a (1.8 mg, 0.28 ꢁ 10 mmol) in 1 mL of a de-
gassed CH Cl . After stirring at room temperature for 10 min, a
2c: R = i-Pr
2c: R = i-Pr
2d: R = n-Bu
2d: R = n-Bu
2e: R = t-Bu
2e: R = t-Bu
2f: R = Bn
10
11
12
ꢀ
2
2
]BF
4
(1.0 mg, 0.25 ꢁ 10 mmol) and
ꢀ2
2f: R = Bn
5a
c
a
a
2
2
Reactions were performed in 2 mL of CH
of H , at room temperature for 24 h using 0.25 mmol of 2 and [Rh(COD)
metal precursor.
2
Cl
2
, Rh/L* = 1: 1.1, Rh/2 = 1:100, 10 bar
substrate (0.25 mmol) in 1 mL of the same solvents was added to
the reaction mixture. The hydrogenation was performed at room
temperature under an H pressure of 10 bar for 24 h. The reaction
2
2
2
4
]BF as a
b
Conversion was determined by GC or 1H NMR.
Enantiomeric excesses were determined by capillary GC analysis with a chiral
c
mixture was then passed through a short silica gel column to re-
move the catalyst. After evaporating the solvent, the crude product
b-120 column (0.25 mm ꢁ 30 m) for 1a–b, 1e, and by HPLC analysis with a chiral
column (Chiralcel OD-H, 0.46 mm ꢁ 25 cm) for 1c–d and 1f.
1
was subjected to determine the conversion by GC or H NMR and
the enantiomeric excesses by GC or HPLC.
results in Table 2 indicated that the introduction of a bulky ester
function into the substrates has a deleterious influence on the
enantioselectivity. With the increase of the steric demand of the
ester moiety from methyl 2a to the bulkier ester groups 2b–f, a sig-
nificant decrease in enantioselectivity was observed. Thus, ethyl
ester 2b led to a slight drop in enantioselectivity when compared
to the methyl ester 2a (entry 4 vs entry 2), while substrate 2e with
a bulky tert-butyl group led to a drastic drop in the enantioselectiv-
4
.2.1. (R)-3-Hydroxy-2-methylpropionic acid methyl ester 1a
20
D
20
D
½
aꢂ
¼ ꢀ22:8 (c 0.85, MeOH) {lit.
½a
ꢂ
¼ ꢀ28:2 (c 1.34,
1
MeOH); ee >99% commercially available from Aldrich}. H NMR
400 MHz, CDCl ): d = 1.18 (d, J = 7.2 Hz, 3H), 2.65–2.72 (m+br,
H), 3.67–3.76 (m, 5H). GC analysis: chiral b-120 column
0.25 mm ꢁ 30 m); t = 6.32 min, (R)-isomer; t = 6.59 min, (S)-
isomer; ee = 96.7%.
(
2
(
3
R
R
ity [49.8% ee with (S
THNAPhos 5a] (entries 9 and 10). The results also disclosed that
in most cases, the Rh/(R ,R )-THNAPhos catalyst showed a better
enantioselectivity than Rh/(S ,S )-PEAPhos. One exception is in
c a c a
,S )-PEAPhos 4 and 79.1% ee with (R ,R )-
4
.2.2. (R)-3-Hydroxy-2-methylpropionic acid ethyl ester 1b
20
D
1
c
a
½aꢂ
¼ ꢀ22:2 (c 1.0, MeOH). H NMR (400 MHz, CDCl
3
): d = 1.18
c
a
(
d, J = 7.2 Hz, 3H), 1.28 (t, J = 7.2 Hz, 3H), 2.50 (br, 1H), 2.66 (ddq,
the hydrogenation of substrate 2f with a benzyl ester function, in
which similar enantioselectivities were achieved by the use of PEA-
Phos 4 and THNAPhos 5a (entries 11 and 12).
J = 4.8, 6.8, 7.2 Hz, 1H), 3.68 (dd, J = 4.8, 11.2 Hz, 1H), 3.74 (dd,
J = 6.8, 11.2 Hz, 1H), 4.18 (q, J = 7.2 Hz, 2H). GC analysis: chiral
b-120 column (0.25 mm ꢁ 30 m); t
R
= 8.08 min, (R)-isomer; t
R
=
8
.37 min, (S)-isomer; ee = 95.7%.
3
. Conclusion
4
.2.3. (R)-3-Hydroxy-2-methylpropionic acid i-propyl ester 1c
2
0
1
In conclusion, we have developed an efficient Rh-catalyzed
asymmetric hydrogenation of 2-hydroxymethylacrylate ester com-
pounds 2 with a chiral 1,2,3,4-tetrahydro-1-naphthylamine-de-
c a
rived phosphine-phosphoramidite ligand [(R ,R )-THNAPhos 5a],
½aꢂ
¼ ꢀ16:0 (c 1.2, MeOH). H NMR (400 MHz, CDCl
3
): d = 1.16
D
(
d, J = 7.2 Hz, 3H), 1.23–1.26 (m, 6H), 2.60–2.66 (m, 1H), 2.70 (br,
1
H), 3.65–3.75 (m, 2H), 5.01–5.08 (m, 1H). HPLC analysis: chiralcel
OD-H column; flow rate: 1.0 mL/min; eluent: n-hexane/propan-2-
which provides a direct way to synthetically important Roche ester
derivatives in high enantioselectivities (up to 96.7% ee). In particu-
lar, even at low catalyst loadings (0.1 mol %), the hydrogenation also
gave good results (full conversion and 90.9% ee), which represents
the highest substrate/catalyst ratio successfully used in this hydro-
genation reported so far. Further investigations on the substrate
scope revealed that the ester function present on the substrate has
a significant effect in the enantioselectivity, while the substrate with
a bulkier ester group tended to give lower enantioselectivity.
ol (99/1); detection at 215 nm; t
R
= 10.8 min, (R)-isomer; t
R
=
1
1.7 min, (S)-isomer; ee = 90.5%.
4
.2.4. (R)-3-Hydroxy-2-methylpropionic acid n-butyl ester 1d
2
0
1
½
aꢂ
¼ ꢀ14:8 (c 0.9, MeOH). H NMR (400 MHz, CDCl
3
): d = 0.94
D
(
t, J = 7.6 Hz, 3H), 1.18 (d, J = 7.2 Hz, 3H), 1.36–1.42 (m, 2H), 1.60–
1
4
.65 (m, 2H), 2.64–2.69 (m, 1H), 2.80 (br, 1H), 3.67–3.76 (m, 2H),
.10–4.13 (t, J = 6.6 Hz, 2H). HPLC analysis: chiralcel OD-H column;
flow rate: 1.0 mL/min; eluent: n-hexane/propan-2-ol (99/1);
R R
detection at 215 nm; t = 11.8 min, (R)-isomer; t = 13.3 min, (S)-
4
4
. Experimental
isomer; ee = 95.3%.
.1. General
4.2.5. (R)-3-Hydroxy-2-methylpropionic acid t-butyl ester 1e
2
D
0
1
½aꢂ
¼ ꢀ11:6 (c 1.15, MeOH). H NMR (400 MHz, CDCl
3
):
All synthetic reactions and manipulations were performed in a
d = 1.13 (dd, J = 2.0, 7.2 Hz, 3H), 1.46 (s, 9H), 2.53–2.58 (m, 1H),
nitrogen or argon atmosphere using standard Schlenk techniques.
2.73 (br, 1H), 3.64–3.71 (m, 2H). GC analysis: chiral b-120 column

M. Qiu et al. / Tetrahedron: Asymmetry 20 (2009) 210–213
213
(
0.25 mm ꢁ 30 m); t
R
= 9.57 min, (S)-isomer; t
R
= 9.78 min, (R)-iso-
G. H.; Niederer, D.; Daeffler, R.; Osmani, A.; Schreiner, K.; Seeger-Weibel, M.;
Bérod, B.; Schaer, K.; Gamboni, R. Org. Process Res. Dev. 2004, 8, 92–100; (i) Shin,
Y.; Fournier, J.-H.; Fukui, Y.; Brückner, A. M.; Curren, D. P. Angew. Chem., Int. Ed.
mer; ee = 79.1%.
2004, 43, 4634–4637; (j) Paterson, I.; Delgado, O.; Florence, G. J.; Lyothier, I.;
4
.2.6. (R)-3-Hydroxy-2-methylpropionic acid benzyl ester 1f
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2
0
1
½aꢂ
¼ ꢀ24:2 (c 0.85, MeOH). H NMR (400 MHz, CDCl
3
):
D
2
3
.
.
d = 1.17 (d, J = 7.2 Hz, 3H), 2.68–2.73 (m, 1H), 2.87 (br, 1H), 3.65–
3
.76 (ddd, J = 2.4, 4.8, 6.6 Hz, 2H), 5.13 (s, 2H), 7.30–7.37 (m, 5H).
+
4. Czeskis, B. A.; Moissenkov, A. M. J. Chem. Soc., Perkin Trans. I 1989, 1353–1354.
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rate: 1.0 mL/min; eluent: n-hexane/propan-2-ol (98/2); detection
at 215 nm; t = 23.0 min, (R)-isomer; t = 25.8 min, (S)-isomer;
R R
5
.
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6
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1
Acknowledgments
7
2
Financial support of this work by the National Natural Science
Foundation of China (20802076, 20873145) and the Knowledge
Innovation Program of the Chinese Academy of Sciences (DICP-
S200802) is gratefully acknowledged.
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Wassenaar, J.; Kuil, M.; Reek, J. N. H. Adv. Synth. Catal. 2008, 350, 1610–1614.
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