Straightforward access to chiral diol bidentate ligands: Their efficient enantioselective induction in the addition of diethylzinc to aromatic aldehydes

Article abstract of DOI:10.1002/aoc.3223

Enantiopure C₂-symmetric diol bidentate ligands have been synthesized in a straightforward manner through a three-step reaction with good yields. The synthesized C₂-symmetric diol bidentate ligands were used in the addition of diethylzinc to various aromatic aldehydes, a general catalytic benchmark reaction, in order to assess their enantioselective induction properties. The enantioselective addition of diethylzinc to 1-naphthaldehyde and 3-chlorobenzaldehyde was achieved with an enantiomeric excess (ee) of up to 98%. All synthesized ligands were also evaluated in the addition of diethyzinc to aromatic aldehydes including an extra metal such as Ti(IV) (up to 99% ee).

Full text of DOI:10.1002/aoc.3223

Full Paper
Received: 29 July 2014
Revised: 25 August 2014
Accepted: 27 August 2014
Published online in Wiley Online Library: 2 October 2014
(wileyonlinelibrary.com) DOI 10.1002/aoc.3223
Straightforward access to chiral diol bidentate
ligands: their efficient enantioselective
induction in the addition of diethylzinc to
aromatic aldehydes
Yaşar Gök*, Soner Küloğlu, Halil Zeki Gök and Levent Kekeç
Enantiopure C2-symmetric diol bidentate ligands have been synthesized in a straightforward manner through a three-step reac-
tion with good yields. The synthesized C2-symmetric diol bidentate ligands were used in the addition of diethylzinc to various ar-
omatic aldehydes, a general catalytic benchmark reaction, in order to assess their enantioselective induction properties. The
enantioselective addition of diethylzinc to 1-naphthaldehyde and 3-chlorobenzaldehyde was achieved with an enantiomeric ex-
cess (ee) of up to 98%. All synthesized ligands were also evaluated in the addition of diethyzinc to aromatic aldehydes including
an extra metal such as Ti(IV) (up to 99% ee). Copyright © 2014 John Wiley & Sons, Ltd.
Additional supporting information may be found in the online version of this article at the publisher’s web-site.
Keywords: enantioselective addition; C2-symmetry; chiral diol; alkylation
Introduction
Experimental
General Methods
The enantioselective addition of organozinc to aldehydes is an
important method of carbon–carbon bond formation in order
to obtain optically active secondary alcohols, which are impor-
tant pharmaceutical intermediates and biologically active
compounds.[1] In the past few years, enantioselective alkylation
of aldehydes with organometallic reagents in the presence of
catalytic amounts of chiral molecules has reached an impressive
level of complexity. Since the initial work by Oguni, Noyori and
Soai, many chiral amino alcohol-type ligands have attracted
great interest.[2] Nowadays, this enantioselective method is used
to screen various C2-symmetric novel chiral ligands such as
amino thiols,[3] amino alcohols,[4] BINOL,[5] titanium complexes[6]
and diols.[7] This kind of symmetry gives an advantage to the
chiral ligand for reducing the number of possible diastereomeric
transition states.[8] Among the C2-symmetric chiral ligands,
oxygen ligands such as hydrobenzoin (1,2-diphenylethane-1,2-
diol) are frequently used with alkali metals and early transition
metals because of the affinity of oxygen atoms to these metals.
Chiral hydrobenzoin and its derivatives take a prominent place
in various areas of chiral synthesis. Several synthetic methods
have been developed in order to obtain hydrobenzoin and its
derivatives.[9]
All reactions were carried out under an argon atmosphere in dry
solvents under anhydrous conditions, unless otherwise stated. All
reagents were purchased and used without purification, unless oth-
erwise noted. Analytical TLC was performed using Macherey-Nagel
SIL G-25 UV254 plates. Flash chromatography was carried out with
Rocc silica gel (0.040–0.063 mm). H NMR, 13C NMR and 31P NMR
1
spectra were recorded with a Varian Mercury 400 MHz spectrome-
ter, with chemical shifts reported relative to tetramethylsilane
(for 1H and 13C) and relative to 85% aqueous phosphoric acid
(for 31P), using the residual solvent signal as a standard. 13C
NMR spectra were recorded using the attached proton test. IR
spectra were recorded with a PerkinElmer Spectrum 65 FT-IR
spectrometer. Analytical chiral HPLC separations were performed
using a VWR Hitachi series or Dionex 3000 UHPLC system with
diode array detection. Optical rotations were measured with a
Krüss P3000 series polarimeter. Melting points were measured
with a Thermo Scientific 9200 melting point apparatus.
1-Bromo-2-(diethylphosphonyl)methylbenzene (2)
In this paper, we report the synthesis of optically pure C2-sym-
metric hydrobenzoin derivatives in an inexpensive and easily ac-
cessible way with good overall yields. Various types of synthesis
of these hydrobenzoin derivatives have been reported.[9] How-
ever, our synthetic route includes mild conditions and short
and cheap synthetic steps. It is noteworthy that there are not
many examples of hydrobenzoin derivatives used in the
enantioselective diethylzinc addition to aldehydes so far to the
best of our knowledge.
A mixture of 2-bromomethyl-1-bromobenzene (1; 8.5 g, 34 mmol)
and triethyl phosphite (11.5 g, 68 mmol) was refluxed at 140 °C for
*
Correspondence to: Yaşar Gök, Department of Chemistry, Faculty of Arts and
Sciences, Osmaniye Korkut Ata University, 80000 Osmaniye, Turkey. E-mail:
yasargok@osmaniye.edu.tr
Department of Chemistry, Faculty of Arts and Sciences, Osmaniye Korkut Ata
University, 80000, Osmaniye, Turkey
Appl. Organometal. Chem. 2014, 28, 835–838
Copyright © 2014 John Wiley & Sons, Ltd.

Y. Gök et al.
5 h. The excess reagent was removed in vacuo. The crude product
was purified by flash chromatography over silica gel (pentane–
EtOAc, 50:50) resulting in 10.12 g (97%) of pure 2 as a yellow oil.
1H NMR (400 MHz, CDCl3, δ, ppm): 1.27 (t, J = 7.1Hz, 6H9), 3.42
(d, J = 22.0 Hz, 2H7), 4.01–4.12 (m, 4H8), 7.07–7.17 (m, 1H3), 7.25–
7.37 (m, 1H5), 7.45–7.51 (m, 1H4), 7.55–7.62 (m, 1H6). 13C NMR
(100 MHz, CDCl3, δ, ppm): 16.3 (d, JC–P = 6.1Hz, (C9)), 33.4 (d, JC–
P = 138.0 Hz, (C7)), 62.2 (d, JC–P =6.7 Hz, (C8)), 124.9 (d, JC–P = 4.9Hz,
(C1)), 127.4 (d, JC–P = 3.4Hz, (C4)), 128.5 (d, JC–P = 3.5 Hz, (C5)), 131.6
(d, JC–P = 5.0Hz, (C2)), 132.9 (d, JC–P =3.3 Hz, (C3)), 136.1 (d, JC–
P = 4.0 Hz, (C6)). 31P NMR (162 MHz, CDCl3, δ, ppm): +25.29. IR (KBr,
cmÀ1): 2981, 2929, 2906, 1810, 1634, 1590, 1567, 1474, 1439,
1391, 1253, 1162, 1024, 963, 846, 793, 752, 701, 658, 593, 539.
by flash chromatography over silica gel (hexane–EtOAc, 90:10)
resulting in 6.8g (>99% enantiomeric excess (ee), 65% yield) of
pure diol 4 as a white solid.
(1R,2R)-1,2-Bis(biphenyl-2-yl)ethane-1,2-diol (5)
To a solution of 4 (0.1 g, 0.268 mmol) and Pd(PPh3)4 (0.019 g,
0.016mmol) in toluene (11 ml) were added phenylboronic acid
(0.067 g, 0.52 mmol), EtOH (4ml), and aqueous K2CO3 (2M, 8 ml).
The resulting reaction mixture was refluxed for 26h. To the reaction
mixture was added water (100 ml). The aqueous layer was further
extracted with CH2Cl2 (3× 100 ml) and the combined organic
phases were washed with brine (20 ml), dried over MgSO4 and con-
centrated in vacuo. The crude product was purified by flash chro-
matography over silica gel (hexane–EtOAc, 90:10) resulting in
0.085g (>99% ee, 87% yield) of pure diol 5 as a white solid.
(E)-2,2′-Dibromostilbene (3)[10]
Compound 2 (10.12g, 33.0mmol) dissolved in dry THF (200 ml) was
added to a solution of 2-bromobenzaldehyde (12.46 g, 66.0mmol)
in dry THF (500 ml) under an argon atmosphere at room tempera-
ture. Next, t-BuOK (5.72g, 49.5 mmol) was added to the reaction
mixture. The resulting reaction mixture was stirred at room temper-
ature for an additional 40 min. After hydrolysis with water, the mix-
ture was stirred for a further 30 min and the precipitated solid was
filtered off, resulting in 9.8g (80%) of 3 as a pure white solid.
General Procedure for Enantioselective Diethylzinc Addition to
Aromatic Aldehydes
Under an argon atmosphere, chiral ligand 4 or 5 (0.025 mmol) was
dissolved in dry solvent (Ti(OiPr)4 (0.25 mmol) was added and stirred
for 1 h for the titanium-mediated reactions). Et2Zn (0.5 mmol, 1 M in
hexane) was added and the resulting yellow solution was stirred
for 20 min at room temperature. Then, the reaction was cooled to
0 °C and aromatic aldehyde (0.25 mmol) was added. The reaction
was stirred for another 24 h. After quenching with 1 ml of saturated
NH4Cl solution, 25 ml of water was added and then extracted with
EtOAc (3 × 25 ml). The combined organic phases were dried over
Na2SO4 and evaporated in vacuo. The crude product was purified
by flash chromatography to give the corresponding alcohol. Enantio-
meric excesses were determined using HPLC analysis with a chiral
stationary phase column (Chiracel OD-H or Chiracel OB).
(1R,2R)-1,2-Bis(2-bromophenyl)ethane-1,2-diol (4)[10]
First, AD-mix-β (75 g) was dissolved in a mixture of tert-butyl alcohol
(200 ml) and water (200 ml). Next, methanesulfonamide (3.5g,
35 mmol) was added and the reaction mixture was cooled to 0 °C,
whereupon the inorganic salts were partially precipitated. Com-
pound 3 (9.4g, 28mmol) was added in one portion and the hetero-
geneous slurry was stirred vigorously at 0 °C for 5 days. The mixture
was allowed to warm to room temperature and Na2SO3 (48 g,
384.0 mmol) was added. The reaction mixture was stirred at room
temperature for another 24h. Next, EtOAc (300 ml) was added
and the organic phase was separated, after which the aqueous
phase was further extracted with EtOAc (2× 300ml). The combined
organic phases were washed with 2 M KOH, dried over MgSO4, fil-
tered and concentrated in vacuo. The crude product was purified
Results and Discussion
The C2-symmetric diol bidentate ligands were synthesized accord-
ing to a three-step synthetic route outlined in Scheme 1. The key
compound, dibromo-substituted (E)-stilbene (3), is not commer-
cially available, and therefore needs to be synthesized. Compound
β
Scheme 1. Synthesis of optically pure C2-symmetric diol bidentate ligands 4 and 5.
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Copyright © 2014 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2014, 28, 835–838

Enantioselective addition of diethylzinc to aromatic aldehydes
3 has been synthesized using various methods such as McMurry
coupling[11] or ruthenium-catalysed olefin metathesis.[12] However,
harsh conditions, expensive synthetic steps or small differences in
E:Z ratio reported in the literature prompted us to devise a different
route involving mild conditions and more economical synthetic
steps. Thus, 3 was synthesized in two steps in good overall yields.
The first step is an Arbuzov reaction to afford phosphonate 2
starting from commercially available 1. The second step gives the
key compound 3 with a Horner–Wadsworth–Emmons reaction.
This route can be easily scaled up. Next, 3 is reacted in a Sharpless
asymmetric dihydroxylation reaction to obtain 4 (65%, >99% ee).
Compound 4 is treated with phenylboronic acid and K2CO3 in the
presence of [Pd(PPh3)4] in EtOH–H2O–toluene (1:2:4) at reflux tem-
perature, affording diphenyl-substituted 5 in good yield (87%,
>99% ee) in 26 h via a Suzuki cross coupling.
Although hydrobenzoin and its derivatives are used as chiral li-
gands in various enantioselective reactions, there are not many ex-
amples of the enantioselective addition of diethylzinc to aldehydes.
The first report was by Salvadori and co-workers who used (S,S)-
hydrobenzoin as chiral ligand with moderate results.[13] Then, Prasad
and Joshi changed the reaction conditions and obtained the corre-
sponding chiral alcohol with an ee of 89%.[14] Taking into account
these data, C2-symmetric diol bidentate ligands which are derivatives
of hydrobenzoin were synthesized in a straightforward manner and
their enantioselective induction was investigated in the addition of
diethylzinc to aromatic aldehydes (Table 1). This reaction is a known
method for the formation of carbon–carbon bonds.[2] Reactions were
carried out using 2 equiv. of diethylzinc (1 M solution in hexane) at
room temperature for 24 h in the presence of 0.1 equiv. of the
developed ligands in various solvents.[7c,15] We started with
1-naphthaldehyde as test substrate in order to examine the effi-
ciency of the chiral ligands. 1-Naphthaldehyde gives the correspond-
ing secondary alcohol with excellent enantioselectivities and
moderate yields in both dichloromethane and toluene catalysed by
the diol 5 (Table 1, entries 3 and 4). When the reaction is performed
with the diol 4 in toluene, we observe a decrease in enantioselectivity
(Table 1, entry 2). However, changing the solvent to dichloromethane
results in a higher enantioselectivity (Table 1, entry 1). We next inves-
tigated the influence of these two solvents on the addition of
diethylzinc to various aromatic aldehydes catalysed by both diols 4
and 5. Using benzaldehyde gives unsatisfactory results (Table 1, en-
tries 5, 6 and 8). Remarkably, good enantioselectivity is observed
when using diol 5 in dichloromethane (Table 1, entry 7). Also, excel-
lent enantioselectivities with moderate yields are obtained in the
ethylation of 3-chlorobenzaldehyde using diol 5 in both solvents
(Table 1, entries 11 and 12). Changing the catalyst to diol 4 affords ex-
cellent enantioselectivity in dichloromethane (Table 1, entry 9) but a
sharp decrease in toluene (Table 1, entry 10). The reactions were then
carried out with test substrates having an electron-releasing substitu-
ent on the phenyl ring, namely 2- and 3-methoxybenzaldehyde
(Table 1, entries 13–20). It is noted that with 2-methoxybenzaldehyde,
good enantioselectivity in dichloromethane is obtained using diol 4
(Table 1, entry 13). Changing the solvent to toluene leads to moderate
results (Table 1, entry 14). We observe decreases in enantioselectivity
when using diol 5 in both solvents (Table 1, entries 15 and 16). On
changing the position of the methoxy moiety on the phenyl ring to
the meta-position, no selectivity is obtained using either diol 4(Table 1,
entries 17 and 18) or diol 5 (Table 1, entries 19 and 20). In addition,
changing the solvent does not alter the selectivity.
Table 1. Enantioselective addition of diethylzinc to aromatic aldehydes
catalysed by chiral diol 4 or 5
Yield (%)a ee (%)bc
The second part of our study is the investigation of the efficiencies
of the chiral diols 4 and 5 on adding an extra metal, Ti(IV), in the
Entry
Ar
Ligand
Solvent
1
1-Naph
4
4
5
5
4
4
5
5
4
4
5
5
4
4
5
5
4
4
5
5
Dichloromethane
Toluene
53
57
42
49
45
42
32
38
40
36
51
40
53
49
26
46
47
42
43
49
94
2
1-Naph
67
Table 2. Enantioselective addition of diethylzinc to aromatic aldehydes
in the presence of Ti(OiPr)4 and chiral diol 4 or 5a
3
1-Naph
Dichloromethane
Toluene
91
97d
4
1-Naph
5
Ph
Dichloromethane
Toluene
57
6
Ph
Racemic
79d
44d
7
Ph
Dichloromethane
Toluene
8
Ph
Entry
Ar
Ligand
Yield (%)b
ee (%)cd
9
3-ClC6H4
3-ClC6H4
3-ClC6H4
3-ClC6H4
2-MeOC6H4
2-MeOC6H4
2-MeOC6H4
2-MeOC6H4
3-MeOC6H4
3-MeOC6H4
3-MeOC6H4
3-MeOC6H4
Dichloromethane
Toluene
93
10
11
12
13
14
15
16
17
18
19
20
17
1
2
3
4
5
6
7
8
9
10
1-Naph
4
5
4
5
4
5
4
5
4
5
7
79
86
93e
94
28
rac
97
99
39
32
Dichloromethane
Toluene
95
1-Naph
7
98
Ph
12
15
19
17
36
42
98
99
Dichloromethane
Toluene
86
Ph
67
3-ClC6H4
3-ClC6H4
2-MeOC6H4
2-MeOC6H4
3-MeOC6H4
3-MeOC6H4
Dichloromethane
Toluene
50
15
Dichloromethane
Toluene
Racemic
Racemic
Racemic
Racemic
Dichloromethane
Toluene
aReactions were carried out with 1 equiv. of Ti(OiPr)4.
bIsolated yield.
cDetermined by HPLC analysis with a chiral stationary phase column
aIsolated yield.
bDetermined by HPLC analysis with a chiral stationary phase column
(Chiracel OD-H or Chiracel OB).
(Chiracel OD-H or Chiracel OB).
cDetermined by comparison of optical rotations with the literature.
dDetermined the opposite configurations by comparison of optical
rotations with the literature.
dDetermined by comparison of optical rotations with the literature.
eDetermined the opposite configuration by comparison of optical
rotations with the literature.
Appl. Organometal. Chem. 2014, 28, 835–838
Copyright © 2014 John Wiley & Sons, Ltd.
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Y. Gök et al.
K. Muñiz-Fernández, A. Seger, G. Raabe, K. Günther, J. Org. Chem. 1998,
63, 7860; e) L. Pu, H.-B. Yu, Chem. Rev. 2001, 101, 757.
addition of diethylzinc to aromatic aldehydes (Table 2). Titanium-
mediated reactions are reported to give more efficient and selective
results.[16] Considering these literature data, the efficiencies of the chi-
ral diols in combination with Ti(OiPr)4 was explored. Dichloromethane
was chosen as reaction solvent as it gave a better enantioselectivity in
the absence of the additive. In our reaction, an aldehyde:Et2Zn (1 M so-
lution in hexane): i(OiPr)4:ligand ratio of 1:2:1:0.1 was used at room tem-
perature for 24 h. Significantly, excellent enantioselectivities are
recorded with 2-methoxybenzaldehyde, 1-naphthaldehyde and benz-
aldehyde using both diols 4 and 5 (Table 2, entries 1–4, 7 and 8). We
also observe some enantioselectivities with 3-methoxybenzaldehyde;
however, racemic mixtures are obtained without titanium (Table 2,
entries 9 and 10). Adding an extra metal gives satisfactory results for
all test substrates used except 3-chlorobenzaldehyde (Table 2, entries
5 and 6).
[2] a) N. Oguni, T. Omi, Tetrahedron Lett. 1984, 25, 2823; b) K. Soai,
T. Shibata, in Comprehensive Asymmetric Catalysis, Vol. 2 (Eds.: E. N.
Jacobsen, A. Pfaltz, H. Yamamoto), Springer Verlag, Berlin, 1999,
pp. 911–922; c) G. Zhao, X.-G. Li, R. Wang, Tetrahedron: Asymmetry
2001, 12, 399; d) J. Lu, X. Xu, S. Wang, C. Wang, Y. Hu, H. Hu, J. Chem.
Soc. Perkin Trans. 1 2002, 24, 2900; e) S. W. Kang, D. H. Ko, K. H. Kim,
D.-C. Ha, Org. Lett. 2003, 5, 4517; f) A. Bisai, P. K. Singh, V. K. Singh,
Tetrahedron 2007, 63, 598; g) J. E. D. Martins, M. Wills, Tetrahedron:
Asymmetry 2008, 19, 1250; h) J. Mao, Synth. Commun. 2009, 39, 3710;
i) J. T. Zacharia, T. Tanaka, Y. Uesaka, M. Hayashi, Synthesis 2012, 44, 1625.
[3] a) J. C. Anderson, M. Harding, Chem. Commun. 1998, 3, 393; b)
E. Rijnberg, M. D. Janssen, J. Boersma, G. Van Koten, Tetrahedron Lett.
1994, 35, 6521.
[4] a) W. M. Dai, H. J. Zhu, Tetrahedron: Asymmetry 1996, 7, 1245; b)
G. Bringmann, M. Breuning, Tetrahedron: Asymmetry 1998, 9, 667.
[5] a) X. W. Yang, J. H. Shen, C. S. Da, H. S. Wang, J. Org. Chem. 2000, 65, 295;
b) H. Kodama, J. Ito, A. Nagaki, T. Ohta, I. Furukawa, Appl. Organometal.
Chem. 2000, 14, 709.
[6] a) S. Pritchett, D. H. Woodmansee, P. Gantzel, P. J. Walsh, J. Am. Chem.
Soc. 1998, 120, 6423; b) M. G. Baker-Salisbury, B. S. Starkman,
G. M. Frisenda, L. A. Roteta, J. M. Tanski, Inorg. Chim. Acta 2013, 409,
394; c) T. Harada, K. Handa, Y. Hiraoka, Y. Marutani, M. Nakatsugawa,
Tetrahedron: Asymmetry 2004, 15, 3879.
[7] a) D. Seebach, A. K. Beck, B. Schmidt, Y. M. Wang, Tetrahedron 1994,
50, 4363; b) J. Balsells, J. T. Davis, P. Carroll, P. J. Walsh, J. Am. Chem.
Soc. 2002, 124, 10336; c) Y. Gök, L. Kekeç, Tetrahedron Lett. 2014,
55, 2227.
Conclusions
C2-symmetric diol bidentate ligands were prepared in good yields
using short synthetic sequences. The synthesized ligands were
evaluated in the alkylation of various aromatic aldehydes with
diethylzinc. The best results were obtained with 1-naphthaldehyde
and 3-chlorobenzaldehyde. When the aromatic ring bears electron-
releasing substituents, different results were obtained related to the
position of the substituents on the aromatic ring. While 2-methoxy-
substituted benzaldehyde afforded good to moderate selectivities,
3-methoxy-substituted benzaldehyde showed no selectivity. Mod-
erate selectivities were obtained with benzaldehyde, a standard
test substrate for this kind of reaction. Furthermore, with the excep-
tion of 1-naphthaldehyde and 3-chlorobenzaldehyde, treatment
using titanium isopropoxide under the same conditions as in the
absence of Ti(IV) increased all the enantioselectivities.
[8] a) J. Whitesell, Chem. Rev. 1989, 89, 1581; b) C. Halm, M. J. Kurth, Angew.
Chem. Int. Ed. 1998, 37, 510.
[9] K. Okano, Tetrahedron 2011, 67, 2483.
[10] For an unrelated synthesis of this compound, see: a) V. Raniyar, H. Zhai,
D. G. Hall, J. Am. Chem. Soc. 2008, 130, 8481; b) I. Cho, L. Meimetis,
R. Britton, Org. Lett. 2009, 11, 1903.
[11] a) J. E. McMurry, Chem. Rev. 1989, 89, 1513; b) P. Wyatt, S. Warren,
M. McPartlin, T. Woodroffe, J. Chem. Soc. Perkin Trans. 1 2001, 3, 279.
[12] a) H. H. Fox, R. R. Schrock, R. O’Dell, Organometallics 1994, 13, 635; b)
S. Chang, Y. Na, H. J. Shin, E. Choi, L. S. Jeong, Tetrahedron Lett. 2002,
43, 7445.
[13] C. Rosini, L. Franzini, D. Pini, P. Salvadori, Tetrahedron: Asymmetry 1990,
1, 587.
[14] K. R. K. Prasad, N. N. Joshi, Tetrahedron: Asymmetry 1996, 7, 1957.
[15] A. Zhang, L. Yang, N. Yang, Y. Liu, Tetrahedron: Asymmetry 2014,
25, 389.
Acknowledgements
This study was supported by the Scientific & Technological Re-
search Council of Turkey (TUBITAK; project no. TBAG-112T017)
and the Research Fund of Osmaniye Korkut Ata University (project
no. OKUBAP-2013-PT3-004).
[16] a) F.-Y. Zhang, C.-W. Yip, R. Cao, A. S. C. Chan, Tetrahedron: Asymmetry
1997, 8, 585; b) F. Lake, C. Moberg, Tetrahedron: Asymmetry 2001, 12,
755; c) J. Balsells, J. T. Davis, P. Carroll, P. J. Walsh, J. Am. Chem. Soc.
2002, 124, 10336.
References
Supporting Information
[1] a) M. Kitamura, S. Suga, K. Kawai, R. Noyori, J. Am. Chem. Soc. 1986, 108,
6071; b) K. Soai, S. Yokoyama, T. Hayasaka, J. Org. Chem. 1991, 56, 4264;
c) R. Noyori, M. Kitamura, Angew. Chem. Int. Ed. 1991, 30, 49; d) C. Bolm,
Additional supporting information may be found in the online ver-
sion of this article at the publisher’s web-site.
wileyonlinelibrary.com/journal/aoc
Copyright © 2014 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2014, 28, 835–838