Asymmetric hydrolytic kinetic resolution with recyclable macrocyclic CoIII-salen complexes: A practical strategy in the preparation of (R)-mexiletine and (S)-propranolol

Article abstract of DOI:10.1002/chem.201103574

A chiral cobalt(III) complex (1 e) was synthesized by the interaction of cobalt(II) acetate and ferrocenium hexafluorophosphate with a chiral dinuclear macrocyclic salen ligand that was derived from 1R,2R-(-)-1,2-diaminocyclohexane with trigol bis-aldehyde. A variety of epoxides and glycidyl ethers were suitable substrates for the reaction with water in the presence of chiral macrocyclic salen complex 1 e at room temperature to afford chiral epoxides and diols by hydrolytic kinetic resolution (HKR). Excellent yields (47 % with respect to the epoxides, 53 % with respect to the diols) and high enantioselectivity (ee>99 % for the epoxides, up to 96 % for the diols) were achieved in 2.5-16 h. The Co^III macrocyclic salen complex (1 e) maintained its performance on a multigram scale and was expediently recycled a number of times. We further extended our study of chiral epoxides that were synthesized by using HKR to the synthesis of chiral drug molecules (R)-mexiletine and (S)-propranolol.

Full text of DOI:10.1002/chem.201103574

DOI: 10.1002/chem.201103574
Asymmetric Hydrolytic Kinetic Resolution with Recyclable Macrocyclic
CoIII–Salen Complexes: A Practical Strategy in the Preparation of
(R)-Mexiletine and (S)-Propranolol
Arghya Sadhukhan, Noor-ul H. Khan,* Tamal Roy, Rukhsana I. Kureshy,
Sayed H. R. Abdi, and Hari C. Bajaj[a]
Abstract: A chiral cobalt
(III) complex
room temperature to afford chiral ep-
oxides and diols by hydrolytic kinetic
resolution (HKR). Excellent yields
(47% with respect to the epoxides,
53% with respect to the diols) and
high enantioselectivity (ee>99% for
the epoxides, up to 96% for the diols)
were achieved in 2.5–16 h. The CoIII
macrocyclic salen complex (1e) main-
tained its performance on a multigram
scale and was expediently recycled
a number of times. We further extend-
ed our study of chiral epoxides that
were synthesized by using HKR to the
synthesis of chiral drug molecules (R)-
mexiletine and (S)-propranolol.
(1e) was synthesized by the interaction
of cobalt(II) acetate and ferrocenium
hexafluorophosphate with a chiral di-
nuclear macrocyclic salen ligand that
was derived from 1R,2R-(À)-1,2-diami-
nocyclohexane with trigol bis-aldehyde.
A variety of epoxides and glycidyl
ethers were suitable substrates for the
reaction with water in the presence of
chiral macrocyclic salen complex 1e at
Keywords: asymmetric synthesis ·
cobalt · kinetic resolution · macro-
cycles · salen ligands
Introduction
atom-efficient strategy of hydrolytic kinetic resolution
(HKR), which used a chiral CoIII–OAc salen complex as
a catalyst. This process provided direct access to unreacted
epoxides and 1,2-diols (also valuable) in high enantiomeric
excess and yield.[1b,4] Since then, many variants on the salen
catalyst have been developed to fine tune the performance
of this system and make this strategy more profitable. The
groups of Jacobsen,[1b,4a5] Weck and Jones,[6] and Kim,[7]
among others[8] have significantly contributed to the catalyst
design and optimization of the HKR reaction parameters,
including mechanistic investigation. Fluorous biphase sys-
tems[9] and ionic liquids[10] have also been used in these reac-
tions. Mechanistically, a bimetallic pathway for the HKR of
epoxides is the most-accepted reaction path and has led to
the design of bimetallic[5g,11] and poly-metallic catalysts,[5d,6b,-
d,e,h,7a,8a–c]oligomeric catalysts,[5a,b,6c] and dendrimeric cata-
lysts,[5c] which have also addressed the issue of catalyst recy-
clability. Building upon these discoveries and our own desire
to develop recyclable catalysts, herein, we report a the use
of a macrocyclic CoIII–salen catalyst for the asymmetric
HKR of terminal epoxides and its application in the synthe-
sis of chiral drug molecules, such as (R)-mexiletine, (S)-pro-
pranolol, on the gram scale.
By the year 2015, the global market for chiral technology is
expected to reach $4.9 billion (Global Industry Analysts
Inc.; February 18, 2011) to meet the ever-increasing demand
from the pharmaceutical sector for optically pure molecules
that are utilized as the framework for enantiomerically pure
drugs.[1] With the increase in stress-related ailments, the
demand for cardiovascular drugs is growing rapidly, and has
fuelled the research into economically viable syntheses of
these drugs. To meet these demands, chiral epoxides are val-
uable intermediates in the syntheses of a large variety of
pharmaceutical compounds. Because racemic epoxides are
widely available at competitive prices, they have been used
in the synthesis of several chiral drugs, in particular (R)-
mexiletine[2] and (S)-propranolol.[3] Hence, the kinetic reso-
lution of racemic epoxides has become one of the most-pre-
vailing methods in the synthesis of chiral epoxides. Jacobsen
and co-workers introduced the very powerful and 100%
[a] A. Sadhukhan, Dr. N.-u. H. Khan, T. Roy, Dr. R. I. Kureshy,
Dr. S. H. R. Abdi, Dr. H. C. Bajaj
Discipline of Inorganic Materials and Catalysis
Central Salt and Marine Chemicals Research
Institute (CSMCRI)
Council of Scientific & Industrial Research (CSIR)
G. B. Marg, Bhavnagar- 364 021, Gujarat (India)
Fax : (+91)0278-2566970
Results and Discussion
CoIII-complex 1a was obtained in quantitative yield from
the reaction of chiral macrocyclic ligand 1[12a,b] with cobalt
acetate in MeOH/toluene (1:1). Catalysts 1b–1e were ob-
tained by the addition of solutions of p-toluene sulfonic
E-mail: khan251293@yahoo.in
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201103574.
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Chem. Eur. J. 2012, 18, 5256 – 5260

FULL PAPER
Table 1. HKR of epoxides catalyzed by macrocyclic CoIII catalysts.[a]
Substrate
Catayst Catalyst
t
Epoxide
Diol
Yield ee
AHCTUNGTREG[NNNU mol%] [h] Yield ee
[%][b] [%][c] [%][b] [%][c]
1
1a
0.008
6
44
>99 51
95
2
3
1b
1e
0.008
0.008
4
3
45
45
>99 50
>99 53
96
96
Scheme 1. Synthesis of the macrocyclic catalysts. a) Co
toluene, RT, 30 min. b) Counterion source, 3 h.
ACHTUNGRTENUN(NG OAc)2 in MeOH/
4
1a
0.008
5
4
45
45
96 47
94
5
6
1b
1e
0.008
0.008
98 48
>99 50
94
95
2.5 43
acid, ferrocenium tetrafluoroborate, ferrocenium hexafluor-
oantimonate, and ferrocenium hexafluorophosphate in
CH2Cl2, respectively, under aerobic conditions (Scheme 1;
also see the Supporting Information). The change in coun-
terion was necessary because of its decisive role in HKR re-
actions.[5f,7e] First, we screened the efficacy of catalysts 1a–
1e (0.008 mol%) in the HKR of racemic epichlorohydrin as
a model substrate under solvent-free conditions.[11] Catalysts
1a, 1b, and 1e were able to resolve the epoxide with ee>
99%, but with different activities (Figure 1). Macrocyclic
7
1a
0.075
24
46
91 41
90
8
9
1b
1e
0.075
0.075
18
16
46
47
94 48
99 48
92
92
10
1a
0.03
24
45
96 46
94
11
12
1b
1e
0.03
0.03
18
12
46
44
98 46
99 50
93
96
13
1a
0.03
24
45
97 47
94
14
15
1b
1e
0.03
0.03
18
12
46
45
98 51
99 51
94
95
16[d]
1a
0.03
24
45
96 50
94
17
18
1b
1e
0.03
0.03
24
18
45
46
97 48
99 50
95
95
[a] Reaction conditions: racemic epoxide=10 mmol, H2O=5 mmol, cata-
lyst=0.008–0.03 mol%; [b] yield of isolated product; [c] ee values were deter-
mined by GC and HPLC analysis of the reaction mixture; [d] reaction condi-
tions: racemic epoxide=10 mmol, H2O=5 mmol, catalyst=0.03 mol%,
THF=1 mL.
atively higher catalyst loadings (0.075 and 0.03 mol%, re-
spectively) for comparable results. Nevertheless, these cata-
lyst loadings are among the lowest reported so far for these
compounds. Furthermore, irrespective of which substrate
was used, catalyst 1e afforded higher activity and enantiose-
lectivity than catalysts 1a and 1b.
Jacobsen and co-workers investigated the mechanistic as-
pects of the HKR and the related epoxide ring-opening re-
actions by using monomeric (salen) metal catalysts, and re-
ported second-order kinetic dependence on the catalyst con-
centration,[13] which led to a cooperative mechanism of the
catalysis.
Based on this report, a possible mechanism for the HKR
of terminal epoxide is proposed herein. Accordingly, the
two metal centers that were in close proximity to one anoth-
er (energy-minimized structures A and B),[6a,14k] owing to
cyclic nature of the ligand (Figure 2, top), worked in a coop-
erative manner (Scheme 2).[5a,b,7d,g,14] Jones et al. proposed
a cooperative interaction between two salen units that were
in close proximity to one another, based on density func-
Figure 1. Screening of catalysts 1 for the HKR of (Æ)-epichlorohydrin at
RT. Reaction conditions: racemic epoxide=10 mmol, H2O=5 mmol, cat-
alyst=0.008 mol%. Left-hand columns: ee of epoxide (%); right-hand
columns: time (h).
À
CoIII catalyst 1e with PF6 as the counterion was more-
active towards HKR, and afforded the desired enantiopure
epichlorohydrin in 3 h (see 1a; OAcÀ counterion) and 1b
(OTsÀ) afforded epichlorohydrin in 6 and 4 h, respectively).
À
Although it was unclear why PF6 was better for the HKR
of epichlorohydrin, a similar trend was also reported by
Kim et al.[7e] Catalysts, 1a, 1b, and 1e, were then applied to
the HKR of various other terminal epoxides (Table 1). A
catalyst loading of 0.008 mol% was found to be optimal for
the HKR of epichlorohydrin and propene oxide. However,
styrene oxide and aryloxy-substituted epoxides required rel-
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N. H. Khan et al.
tional theory calculations.[6a] Thus, one Co center coordinat-
ed with a nucleophile (ÀOH in this case), whilst the other
activated the epoxide through weak coordination. Conse-
quently, the diol was formed through an intermediate
(Figure 2, bottom). The best-performing catalyst (1e) was
then investigated for its recyclability in the HKR of racemic
epichlorohydrin. After completion of the catalytic reaction,
the catalyst was recovered in quantitative yield and success-
fully recycled five times without any apparent loss in activity
or enantioselectivity (Table 2). To ascertain the stability[6i] of
complex 1e, kinetic experiments for the HKR of epichloro-
hydrin (21.6 mmol scale) were performed with fresh catalyst
and catalyst that had been recovered from two consecutive
cycles (Figure 4). The kinetic data clearly showed that com-
plex 1e was stable under the HKR conditions. Furthermore,
the IR spectrum of the recovered catalyst was the same as
that of the fresh catalyst. After recovery of the catalyst, the
reaction mixture showed no trace of Co metal, which ruled
out leaching of Co that might happen as a consequence of
partial degradation of the catalyst during the HKR reaction.
This result led us to attribute the prolonging of the reaction
time (from the fourth cycle onward) to the loss of the cata-
lyst during the recovery process. Notably, catalyst 1e did not
require a reactivation step during the catalyst-reuse experi-
À
ments, which was attributed to the presence of the PF6
counterion[7e] (for the experimental details, see the Support-
ing Information), which also diminished the possibility of
Figure 2. Energy-minimized structure of complex 1 (top) and plausible in-
termediate with epichlorohydrin in the HKR reaction (bottom).
Scheme 2. Cooperative mechanism of the HKR of racemic epoxides.
Table 2. Recyclability of the macrocyclic CoIII catalyst 1e.[a]
Run
t [h]
Epoxide
Diol
ee[b] [%]
Yield [%]
ee[b] [%]
Yield [%]
1
2
3
4
5
6
3
3
4
6
6
6
>99
>99
99
99
99
45
43
43
42
42
42
96
96
96
95
95
95
53
53
52
52
52
52
99
Figure 3. The catalytic activity of CoIII catalysts 1e, 1b, and 1a in the
asymmetric HKR of (Æ)-epichlorohydrin at RT: a) ee value of the epox-
ide versus time; and b) yield versus time. Reaction conditions: racemic
epoxide=10 mmol, H2O=5 mmol, catalyst=0.008 mol%.
[a] Reaction conditions: racemic epichlorohydrin=10 mmol, H2O=
5 mmol, catalyst 1e=0.008 mol%; [b] ee values were determined by GC
and HPLC analysis of the reaction mixture.
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Hydrolytic Kinetic Resolution with CoIII–Salen Complexes
FULL PAPER
Scheme 4. Synthesis of (S)-propranolol.
tained by the 1e-mediated HKR of racemic 1-naphthyl gly-
cidyl (g).
Figure 4. Recycling of catalyst 1e in the HKR of (Æ)-epichlorohydrin
over 3 cycles. Catalyst=0.008 mol%.
Conclusion
A new class of chiral macrocyclic CoIII–salen complexes has
been used as efficient, recyclable, and scalable catalysts for
the HKR of terminal epoxides. In particular, the combina-
racemization[7e] during distillation of the product from the
reaction mixture (Figure 3a,b).Having established the HKR
parameters with catalyst 1e, this procedure was extended to
the synthesis of chiral drugs, (R)-mexiletine and (S)-propra-
nolol.
À
tion of catalyst 1e and PF6 ions afforded enantioenriched
epoxides (up to 47% yield, ee>99%) and diols (up to 53%
yields, up to 96% ee) at room temperature under solvent-
free conditions. The synthesis of the pre-catalysts (the corre-
sponding macrocyclic ligands) was very convenient and re-
producible, to afford the desired dimeric ligands in reasona-
bly high yield. Multigram-scale reactions showed no drop in
the performance of these catalysts, which suggested that this
procedure is scalable.
Mexiletine (commercially available as Mexitil) is an im-
portant I-B anti-arrhythmic agent that is largely used for ar-
rhythmia, allodynia, and myotonic syndromes, etc.[2] (R)-
Mexiletine is more-potent than (S)-mexiletine in experimen-
tal arrhythmias and in binding studies on cardiac sodium
channels. Moreover, the use of the mexiletine racemate in
the treatment of neuromuscular disorder is limited, owing to
its possible side-effects.[2] Typically, the synthesis of (R)-mex-
iletine consists of the resolution of racemic intermediates,
chemoenzymatic routes, or by using stereospecific proce-
dures. However, most of these methods have several disad-
vantages, such as tedious and time-consuming experiments,
unavailability or expensive chiral starting materials, low
yields, low enantiomeric purity, etc. Recently, Muthukrishn-
an and co-workers reported the synthesis of (R)-mexiletine
from epichlorohydrin in good ee.[15a] Herein, (S)-Propene
oxide, which that was obtained by the HKR racemic pro-
pene oxide using catalyst 1e, was used as a starting material
for the synthesis of (R)-mexiletine in 3 steps with high over-
all yield (80%) and ee (>98%; Scheme 3; for details of the
experimental procedure, see the Supporting Informa-
tion).[15b–d] The well-known b-blocker, (S)-propranolol, was
obtained directly in good yield (95%) and excellent ee
(99%; Scheme 4) from the ring-opening of chirally pure (S)-
1-naphthyl glycidyl ether with isopropyl amine by using Na-
zeolite as a catalyst.[16] (S)-1-Naphthyl glycidyl ether was ob-
Experimental Section
Microanalysis of the products was carried out on Perkin–Elmer 2400
CHNS analyzer, ElementarVario micro, and Perkin–Elmer Optical Emis-
sion Spectrometer Optima 2000 DV. The 1H NMR and 13C NMR spectra
were recorded on Bruker 200 MHz and 500 MHz instruments at ambient
temperature. FTIR spectra were recorded on Perkin–Elmer Spectrum
GX spectrophotometer in KBr window. The purity of the products were
determined by gas chromatography (GC) on a Shimadzu GC 14B instru-
ment with a stainless-steel column (2 m long, 3 mm inner diameter, 4 mm
outer diameter) that was packed with 5% SE30 (mesh size 60–80) and
equipped with an FID detector. Ultrapure nitrogen was used as the carri-
er gas (rate 30 mLminÀ1). The injection port and detector temperature
were kept at 2008C. The synthetic standards of the products were used to
determine the conversions by comparing the peak height and area. Enan-
tiomeric excesses were determined by HPLC (Shimadzu SCL-10AVP
and Shimadzu CBM-20 A) by using a Daicel Chiralpak OD column with
2-propanol/n-hexane as the eluent and by GC analysis by using a Shimad-
zu GC 2010 instrument with SupelcoAstec Chiral DEXTM G-TA or ATA
columns. Optical rotations of the chiral complexes and their ligand pre-
cursors were recorded on an automatic Polari meter (Digipol 781, Ru-
dolph) instrument. Energy-minimized structures were drawn by using
ChemDraw version 12.
Recyclability of the catalyst for the HKR of epichlorohydrin: A 10 mL
flask equipped with a stirrer bar was charged with epichlorohydrin (2 g,
21.6 mmol) and catalysts 1e (0.00173 mmol) at RT. The reaction mixture
was cooled to 08C, and H2O (11.8 mmol) was added dropwise over
30 min. The reaction was allowed to warm to RT and was stirred for
2.5 h. Subsequently, (S)-epichlorohydrin was recovered from the reaction
mixture by vacuum distillation. To the remaining solution, was added
a mixture of n-hexane/Et2O mixture (5:1). The precipitate was recovered
by filtration, washed with n-hexane (3ꢁ5 mL), dried under reduced pres-
sure for 6 h, and was then used in the recycling experiments.
Scheme 3. Synthesis of (R)-mexiletine: a) bismuth triflate, 2,5-dimethyl-
phenol, CH2Cl2, 6 h, RT, 86%; b) PPh3, phthalimide, DIAD, THF, RT,
4 h, 80%; c) N2H4, H2O, EtOH, reflux, 3 h, 85%. DIAD=diisopropyl
azodicarboxylate.
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N. H. Khan et al.
Koo, G. J. Kim, Synth. Commun. 2006, 36, 2371–2383; d) Y. S. Kim,
X. Guo, G. J. Kim, Catal. Today 2010, 150, 91–99; e) G. J. Kim, H.
Lee, S. J. Kim, Tetrahedron Lett. 2003, 44, 5005–5008; f) C. K. Shin,
S. J. Kim, G. J. Kim, Tetrahedron Lett. 2004, 45, 7429–7433; g) Y. S.
Kim, X. F. Guo, G. J. Kim, Chem. Commun. 2009, 4296–4298.
[8] a) Y. Song, X. Yao, H. Chen, C. Bai, X. Hu, Z. Zheng, Tetrahedron
Lett. 2002, 43, 6625–6627; b) Y. Song, H. Chen, X. Hu, C. Bai, Z.
Zheng, Tetrahedron Lett. 2003, 44, 7081–7085; c) B. M. Rossbach,
K. Leopold, R. Weberskirch, Angew. Chem. 2006, 118, 1331–1335;
Angew. Chem. Int. Ed. 2006, 45, 1309–1312; d) H. Q. Yang, L.
Zhang, L. Zhong, C. Li, Angew. Chem. 2007, 119, 6985–6989;
Angew. Chem. Int. Ed. 2007, 46, 6861–6865; e) H. Q. Yang, L.
Zhang, W. G. Su, C. Li, J. Catal. 2007, 248, 204–212; f) S. J. Wezen-
berg, A. W. Kleij, Adv. Synth. Catal. 2010, 352, 85–91; g) R. M.
Haak, B. M. Martinez, E. C. Escudero-Adan, J. Bene tBuchholz,
A. W. Kleij, Dalton Trans. 2010, 39, 593–602; h) M. Beigi, S. Roller,
R. Haag, A. Lieses, Eur. J. Org. Chem. 2008, 2135–2141; i) J. Park,
K. Lang, K. A. Abboud, S. Hong, Chem. Eur. J. 2011, 17, 2236–
2245.
[9] a) I. Shepperson, M. Cavazzini, G. Pozzi, S. Quici, J. Fluorine Chem.
2004, 125, 175–180; b) M. Cavazzini, S. Quici, G. Pozzi, Tetrahedron
2002, 58, 3943–3949.
[10] C. R. Oh, D. J. Choo, W. H. Shim, D. H. Lee, E. J. Roh, S. Lee, C. E.
Song, Chem. Commun. 2003, 1100–1101.
[11] a) R. I. Kureshy, S. Singh, N. U. H. Khan, S. H. R. Abdi, I. Ahmad,
A. Bhatt, R. V. Jasra, Chirality 2005, 17, 590–594; b) R. I. Kureshy,
N. H. Khan, S. H. R. Abdi, S. T. Patel, R. V. Jasra, J. Mol. Catal. A
2002, 179, 73.
[12] a) N. H. Khan, A. Sadhukhan, N. C. Maity, R. I. Kureshy, S. H. R.
Abdi, S. Saravanan, H. C. Bajaj, Tetrahedron 2011, 67, 7073–7080;
b) R. I. Kureshy, A. Das, N. H. Khan, S. H. R. Abdi, H. C. Bajaj,
ACS Catal. 2011, 1, 1529–1535.
Acknowledgements
A.S. and N.H.K. thank the CSIR Network project on catalysis for finan-
cial assistance, the Analytical Division of the Institute for providing the
instrumentation facility, and are grateful for a CSIR-JRF fellowship. The
authors are thankful to the referees for their valuable suggestions that
helped to improve this manuscript.
[1] a) P. Kumar, V. Naidu, P. Gupta, Tetrahedron 2007, 63, 2745–2785;
b) S. E. Schaus, B. D. Brandes, J. F. Larrow, M. Tokunaga, K. B.
Hansen, A. E. Gould, M. E. Furrow, E. N. Jacobsen, J. Am. Chem.
Soc. 2002, 124, 1307–1315.
[2] a) A. Kleemann, J. Engel, B. Kutscher, D. Reichert, Pharmaceutical
Substances: Syntheses, Patents, Applications; Georg Thieme: Stutt-
gart, 2001; b) A. Carocci, C. Franchini, G. Lentini, F. Loiodice, V.
Tortorella, Chirality 2000, 12, 103–106; c) P. E. Fenster, K. A.
Comess, Pharmacotherapy 1986, 6, 1–9; d) R. Rudel, F. Lehmann-
Horn, Physiol. Rev. 1985, 65, 310–356; e) E. Kalso, M. R. Tramꢂr,
H. J. McQuay, R. A. Moore, Eur. J. Pain 1998, 2, 3–14; f) C. Fran-
chini, A. Carocci, A. A. Catalano, M. M. Cavalluzi, F. Corbo, G.
Lentini, A. Scilimati, P. Tortorella, D. C. Camerino, A. De Luca, J.
Med. Chem. 2003, 46, 5238; g) K. Kun Huang, M. Ortiz-Marciales,
W. Correa, E. Pomales, X. Y. Lopez, J. Org. Chem. 2009, 74, 4195.
[3] a) H. S. Bevinakatti, A. A. Banerji, J. Org. Chem. 1991, 56, 5372–
5375; b) H. Sasai, N. Ltoh, T. Suzuki, M. Shibasaki, Tetrahedron
Lett. 1993, 34, 855–858; c) S. P. Panchgalle, R. G. Gore, S. P.
Chavan, U. R. Kalkote, Tetrahedron: Asymmetry 2009, 20, 1767–
1770.
[4] M. Tokunaga, J. F. Larrow, F. Kakiuchi, E. N. Jacobsen, Science
1997, 277, 936–938.
[5] a) J. M. Ready, E. N. Jacobsen, J. Am. Chem. Soc. 2001, 123, 2687–
2688; b) J. M. Ready, E. N. Jacobsen, Angew. Chem. 2002, 114,
1432–1435; Angew. Chem. Int. Ed. 2002, 41, 1374–1377; c) R.
Breinbauer, E. N. Jacobsen, Angew. Chem. 2000, 112, 3750–3753;
Angew. Chem. Int. Ed. 2000, 39, 3604–3607; d) D. A. Annis, E. N.
Jacobsen, J. Am. Chem. Soc. 1999, 121, 4147–4154; e) T. Belser,
E. N. Jacobsen, Adv. Synth. Catal. 2008, 350, 967–971; f) D. E.
White, E. N. Jacobsen, Tetrahedron: Asymmetry 2003, 14, 3633–
3638; g) R. G. Konsler, J. Karl, E. N. Jacobsen, J. Am. Chem. Soc.
1998, 120, 10780–10781.
[6] a) K. Venkatasubbaiah, C. S. Gill, T. Takatani, C. D. Sherrill, C. W.
Jones, Chem. Eur. J. 2009, 15, 3951–3955; b) X. Zheng, C. W. Jones,
M. Weck, Adv. Synth. Catal. 2008, 350, 255–261; c) X. Zheng, C. W.
Jones, M. Weck, J. Am. Chem. Soc. 2007, 129, 1105–1112; d) M.
Holbach, M. Weck, J. Org. Chem. 2006, 71, 1825–1836; e) X.
Zheng, C. W. Jones, M. Weck, Chem. Eur. J. 2006, 12, 576–583;
f) C. S. Gill, K. Venkatasubbaiah, N. T. S. Phan, M. Weck, C. W.
Jones, Chem. Eur. J. 2008, 14, 7306–7313; g) S. Jain, K. Venkatasub-
baiah, C. W. Jones, R. J. Davis, J. Mol. Catal. A 2010, 316, 8–15;
h) C. S. Gill, W. Long, C. W. Jones, Catal. Lett. 2009, 131, 425–431;
i) K. Venkatasubbaiah, X. Zhu, C. W. Jones, Top. Catal. 2010, 53,
1063–1065; j) X. Zhu, K. Venkatasubbaiah, M. Weck, C. W. Jones,
ChemCatChem 2010, 2, 1252–1259; k) N. Madhavan, J. W. Jones, M.
Weck, Acc. Chem. Res. 2008, 41, 1153–1165; l) C. W. Jones, Top
Catal. 2010, 53, 942–952.
[13] L. P. C. Nielsen, C. P. Stevenson, D. G. Blackmond, E. N. Jacobsen,
J. Am. Chem. Soc. 2004, 126, 1360–1362.
[14] a) J. Park, K. Lang, K. A. Abboud, S. Hong, J. Am. Chem. Soc.
2008, 130, 16484–16485; b) S. Peukert, E. N. Jacobsen, Org. Lett.
1999, 1, 1245–1248; c) P. Goyal, X. Zheng, M. Weck, Adv. Synth.
Catal. 2008, 350, 1816–1822; d) Y. S. Kim, C. Y. Lee, G. J. Kim, Bull.
Korean Chem. Soc. 2009, 30, 1771–1777; e) X. Guo, Y. S. Kim, G. J.
Kim, Top. Catal. 2009, 52, 153–160; f) Y. S. Kim, X. Guo, G. J. Kim,
Top. Catal. 2009, 52, 197–204; g) R. B. Kawthekar, Y. H. Lee, G. J.
Kim, J. Porous Mater. 2009, 16, 367–378; h) K. Y. Lee, C. Y. Lee,
G. J. Kim, Bull. Korean Chem. Soc. 2009, 30, 389–396; i) K. Y. Lee,
C. Y. Lee, G. J. Kim, React. Kinet. Catal. Lett. 2008, 93, 75–83;
j) R. B. Kawthekar, W. Bi, G. J. Kim, Appl. Organomet. Chem. 2008,
22, 583–591; k) Y. Liu, J. Rawlston, A. T. Swann, T. Takatani, C. D.
Sherrill, P. J. Ludovice, M. Weck, Chem. Sci. 2011, 2, 429–438.
[15] a) M. Sasikumar, M. D. Nikalje, M. Muthukrishnan, Tetrahedron:
Asymmetry 2009, 20, 2814–2817; b) A. Kamal, S. K. Ahmed, M.
Sandbhor, M. Naseer, A. Khan, M. Arifuddin, Chem. lett. 2005, 34,
1142–1143; c) A. Venkat Narsaiah, B. V. S. Reddy, K. Premalatha,
S. S. Reddy, J. S. Yadav, Catal. Lett. 2009, 131, 480–484; d) J. M.
Bothwell, S. W. Krabbe, R. S. Mohan, Chem. Soc. Rev. 2011, 40,
4649–4707.
[16] R. I. Kureshy, S. Singh, N. H. Khan, S. H. R. Abdi, E. Suresh, R. V.
Jasra, J. Mol. Catal. A 2007, 264, 162–169.
[7] a) M. A. Kwon, G. J. Kim, Catal. Today 2003, 87, 145–151; b) S. S.
Thakur, W. Li, S. J. Kim, G. J. Kim, Tetrahedron Lett. 2005, 46,
2263–2266; c) S. Singh Thakur, S. W. Chen, W. Li, C. K. Shin, Y. M.
Received: November 14, 2011
Published online: March 15, 2012
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