Roof shape chiral alcohol: auxiliary for asymmetric synthesis of α-halo acid derivatives

Article abstract of DOI:10.1016/j.tetlet.2016.01.003

Roof shape chiral enantiopure alcohol, obtained by bio-catalytic separation of isomers, was used as a new auxiliary for asymmetric synthesis of α-halo acid derivatives. Esterification reaction of roof shape chiral enantiopure alcohol and racemic α-halo acids in the presence of DCC, DMAP furnished diastereomers of ester in non-racemic manner. Diastereoselectivity up to 90% was observed, the absolute configuration of newly generated chiral center was established by the single crystal X-ray diffraction analysis.

Full text of DOI:10.1016/j.tetlet.2016.01.003

Tetrahedron Letters 57 (2016) 692–695
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Roof shape chiral alcohol: auxiliary for asymmetric synthesis
of
a-halo acid derivatives
⇑
Nilesh Jain, Ashutosh V. Bedekar
Department of Chemistry, Faculty of Science, M. S. University of Baroda, Vadodara 390 002, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Roof shape chiral enantiopure alcohol, obtained by bio-catalytic separation of isomers, was used as a new
Received 4 November 2015
Revised 26 December 2015
Accepted 2 January 2016
Available online 6 January 2016
auxiliary for asymmetric synthesis of
a-halo acid derivatives. Esterification reaction of roof shape chiral
enantiopure alcohol and racemic -halo acids in the presence of DCC, DMAP furnished diastereomers of
a
ester in non-racemic manner. Diastereoselectivity up to 90% was observed, the absolute configuration of
newly generated chiral center was established by the single crystal X-ray diffraction analysis.
Ó 2016 Elsevier Ltd. All rights reserved.
Keywords:
Roof shape chiral alcohol
Chiral auxiliary
a-Halo acid derivative
Diastereoselective synthesis
Dynamic kinetic resolution
Asymmetric synthesis of optically active products using a vari-
ety of chiral auxiliaries, natural and artificial, is an established tool
in modern synthetic organic chemistry. For a number of years nat-
solvating agents for the discrimination of the signals of optically
active acids in NMR spectroscopy.
6
1
Optically pure a-substituted alkanoic acids and their number of
urally occurring chiral auxiliaries remained the preferred choice for
such studies. However, due to some limitations, such as unavail-
ability of both isomers, inability to obtain optically pure com-
pounds in large quantities, other structural limitations. There is a
continuous need to search new artificial chiral molecules to utilize
as auxiliaries for asymmetric synthesis.2 The design of molecules
suitable for the use as chiral auxiliary requires some specific struc-
tural arrangement. The shape, size, and arrangement of functional
groups in chiral molecules play a crucial role in the efficiency of
their use as auxiliary for asymmetric synthesis. Hence, molecules
with unique shape and arrangement of aromatic rings which offer
stereocontrol can be examined as auxiliaries. Synthetically pre-
pared chiral molecules have advantages over the naturally avail-
able compounds of chiral pool as their structures can be fine
tuned and their both enantiomers can be easily obtained. Weber
introduced a novel class of compounds resembling the shape of a
roof and studied their applications as clathrate hosts with inclusion
derivatives are an important class of natural and synthetic com-
pounds. Many strategies have been developed for their enantiose-
lective synthesis. The approach involving the selective alkylation of
7
a
the enolate of chiral esters is widely investigated. This includes
7
b,c
the use of chiral auxiliaries, utilizing hydroxy pentolactone,
7d
7e
carvone derives alcohols, 2-oxoimidazolidine 4-carboxylate,
cyclohexanol based auxiliaries, carbohydrate based alcohols,
and sultam based amides . Besides these, other strategies includ-
-alkylation by
phase-transfer catalysis, etc. have also been investigated. Separa-
tion of enantiomers of the easily available -substituted acids and
7
f
7g
7
h
8
ing asymmetric hydrogenation of acrylates,
a
9
a
their straightforward conversion to functionalized derivatives is
another attractive option to access such molecules. This may be
achieved by fractional crystallization of its salt with chiral resolv-
1
0
11
ing agents or by enzymatic kinetic resolution methods. Both
these classical approaches can furnish maximum 50% yield of the
desired isomer. To overcome this limitation dynamic kinetic reso-
lution (DKR) and dynamic thermodynamic resolution (DTR) have
been developed where the unreacted isomer of the starting mate-
rial is interconverted to the more reactive one or stable one, hence
3
properties. Such molecules also find many applications in differ-
4
ent areas ranging from material to medicinal chemistry. In our
continuing work, we have presented synthesis, resolution of roof
shape alcohols 1 and 2, and their derivatives.5 The roof shape
alcohols were also converted to amines to be scanned as chiral
,6
12
effectively increasing the yields to acceptable level. In this work
we explore the roof shape chiral alcohol 1 as a new auxiliary to
access optically enriched a-halo esters. These are important inter-
⇑
Corresponding author. Tel.: +91 0265 2795552.
E-mail address: avbedekar@yahoo.co.in (A.V. Bedekar).
mediates for the synthesis of other functionalities by suitable
substitution reactions (see Fig. 1).
http://dx.doi.org/10.1016/j.tetlet.2016.01.003
040-4039/Ó 2016 Elsevier Ltd. All rights reserved.
0

N. Jain, A. V. Bedekar / Tetrahedron Letters 57 (2016) 692–695
693
*
OH
*
OH
O
O
*
HO
Ph
Ph
Br
(
R)
(R)
O
O
NBS
CCl4, hυ
1
2
6
h, 81 %
3
4
Figure 1. Roof shape chiral alcohol and diol.
1
1 % d.e.
Scheme 2. Bromination of roof shape phenyl acetate 3.
In our earlier work we have reported resolution of racemic 1 by
6
enzymatic method. In our initial efforts to synthesize
a
-halo ester,
it was condensed with phenyl acetic acid by the standard ester for-
1
3
(R)-1
mation protocol to access 3 in a good yield.
The ester 3 was subjected to -bromination using NBS, product
was isolated and characterized. However the diastereomer ratio
de) was quite low, established by HPLC analysis (see Schemes 1
and 2).
Optical enrichment of
during substitution reactions has been studied. In this study we
also wish to examine the possibility to enrich one isomer of -halo
ester 5 prepared from -halo acid and roof shape chiral enantiop-
O
a
coupling with
R'
4
(
HO
(
R/S)
X
H
O
R'
H X
O
(R)
(R)
(R)
(S)
R'
a
-halo ester derivatives of chiral alcohols
H
O
O
14
X H
a
and
a
ure alcohol 1 (Scheme 3). This can take place either during the
ester formation step or during the interconversion of its isolated
sample under appropriate conditions.
(R,R)-5
(R,S)-5
(R,S)-5
Scheme 3. Synthesis of diastereo enriched
(
R,R)-5
Accordingly (R)-1 was subjected to ester formation with (±)-6
a
-halo esters.
(Scheme 4). The product has two possible diastereomers, (R,S)-4
and (R,R)-4, and their ratio was carefully determined by HPLC
analysis.
O
The coupling reaction was performed using DCC and DMAP at
room temperature in dichloromethane (Table 1). The ratio of the
two diastereomers was examined, assuming that the chiral center
on the roof shape alcohol portion remains unchanged. A low
diastereoselectivity was observed in the absence of DMAP in the
coupling reaction. However, the presence of a base such as DMAP
or DABCO may assist abstraction of acidic a-hydrogen in the ester
formed resulting in the enrichment of kinetically or thermodynam-
ically stable isomer.
(R)
Ph
(R)
H
H
Ph
OH
O
O
6
HOOC Br
Br
DCC, DMAP
CH2Cl2, r.t.
15 h, 80%
4
(
R)-1
O
O
(
R)
(S)
(R)
(R)
H
Ph
H
Ph
O
Br H
H Br
In the presence of catalytic quantity of DMAP (20 mol %) consid-
erable improvement in the de ratio was observed, longer reaction
time only improved yield, without much effect on the selectivity.
Different solvents were also screened to establish the optimum
conditions for selectivity and conversion (Table 2). Although
toluene was found to be slightly better for selectivity, the conver-
sion was slightly higher in dichloromethane.
(
R,S)-4
(R,R)-4
Crystallized from CH3CN
(Major)
Crystallized from EtOAc-hexane
(Minor)
Scheme 4. Diastereoselective ester formation with a-bromo acid.
It was significant to observe interesting crystallization pattern
for the diastereomer of 4. The major isomer was crystallized from
acetonitrile while the minor was obtained from a mixture of ethyl
acetate and hexane. Single crystal X-ray diffraction analysis of pure
crystals of both isomers helped us to establish the absolute config-
Table 1
Diastereoselective coupling of (R)-1 with (±)-6
a
No DMAP
Reaction
time (h)
Yieldb
(%)
Diastereomer ratioc (R,
S)/(R,R)-4
[de]
(
mol %)
1
2
3
0
20
20
15
15
3
40
80
60
54:46
76:24
78:22
8
52
56
uration of newly generated chiral center at the
a-carbon of the
esters. The major isomer crystallized from acetonitrile clearly indi-
cated the configuration of the
the minor isomer (Fig. 2).
a-carbon to be ‘S’, while it was ‘R’ in
a
b
c
All reactions run in CH
Isolated.
Determined by HPLC.
2
Cl2, addition of DMAP at 0 °C, then at rt.
In the next set of experiments
a
-chloropropionic acid 7 was
condensed with (R)-1 in the presence of DCC-DMAP (Scheme 5).
Product 8 was isolated and the diastereomer ratio was established
1
by H NMR analysis, where the methyl doublet for both appeared
at measurable positions.
Table 2
Effect of solvent on the coupling of (R)-1 with (±)-6
a
No
Solvent
CH Cl
THF
CH CN
Toluene
Yieldb (%)
Diastereomer ratioc (R,S)/(R,R)-4
[de]
O
1
2
3
4
2
2
80
54
56
64
76:24
78:22
62:38
80:20
52
56
24
60
Ph
(
R)
(
R)
OH
O
3
PhCH2COOH
DCC, DMAP
CH2Cl2, r.t., 15 h
a
All reactions were run with DMAP (20 mol %), addition of DMAP at 0 °C, then at
1
3
95 %
rt for 15 h.
b
Isolated.
Determined by HPLC.
c
Scheme 1. Esterification of acid with roof shape alcohol.

6
94
N. Jain, A. V. Bedekar / Tetrahedron Letters 57 (2016) 692–695
Table 3
a
Diastereoselective coupling of (R)-1 with (±)-7
No DMAP
mol %)
Reaction
time (h)
Yieldb
(%)
Diastereomer ratioc (R,
S)/(R,R)-8
[de]
(
1
2
3
4
20
20
20
50
3
15
40
15
76
82
91
95
95:5
94:6
91:9
93:7
90
88
82
87
a
b
c
All reactions were run in CH
Isolated.
Determined by HPLC.
2
Cl2, addition of DMAP at 0 °C, then at rt.
DMAP, CH Cl
2
2
(R,S)-4
38 % d.e.
(
R,S)-4
2
7 % d.e.
0 oC, 3 h
DMAP, CH Cl
2
2
(R,S)-4
53 % d.e.
(
R,S)-4
-78 oC, 3 h
2
7 % d.e.
DABCO, CH Cl
2
2
(
R,S)-4
57 % d.e.
(
R,S)-4
-78 oC, 3 h
3
1 % d.e.
Scheme 6. Enrichment of diastereomeric ester (R,S)-4.
O
(
R)
(S)
H
Me
O
Cl
O
HCl, dioxane
Me
(
R)-1 +
HO
5
0 oC, 8 h
(
R,S)-8
91 %
Cl
(S)-7
6 %; 89 % ee
9
8 % ee
8
9 % d.e.
7
Scheme 7. Hydrolysis of diastereomeric ester of a-chloro acid.
Figure 2. ORTEP diagram of (R,S)-4 (top) [CCDC No. 1002050] and of (R,R)-4
bottom) [CCDC No. 1002049].
(
When partially chiral and partially racemic sample of 4, which
is initially formed, is exposed to base, the acidic proton can be
easily abstracted. The resulting E-enolate intermediate can selec-
tively recapture the proton to form more stable isomer of -halo
a
1
5
O
a
(
R)
(R)
H
Me
H
Me
OH
O
ester. The smaller size of methyl in case of the chloro derivative
8, probably favors the E-enolate even more, resulting in a better
selectivity. Such dynamic kinetic resolution for substitution reac-
HOOC Cl 7
Cl
DCC, DMAP
(Table 3)
tions is reported for a-halo esters attached to chiral axillaries for
selective reactions, including some roof shape ones.
(
R)-1
8
16
17
The sample of optically enriched (R,S)-8 was subjected to acid
Scheme 5. Diastereoselective ester formation with a-chloro acid.
1
8
catalyzed hydrolysis reaction (Scheme 7). The recovered alcohol
was obtained in optically pure form and the cleaved -chloropro-
1
a
The result of the above experiment was analogous to coupling
pionic acid was confirmed to be of ‘S’ configuration by comparison
with (±)-6. The diastereomer ratio of the product was also found
to be much higher compared to 4 (Table 3). Longer reaction time
resulted in a small drop in the selectivity.
of its specific rotation.
Thus we present the use of new chiral roof shape auxiliary to
achieve efficient stereocontrol in the base mediated dynamic
These coupling reactions were conducted with equimolar ratio
of alcohol (R)-1 and racemic acids 6 or 7 and the yield and the
diastereomer ratios were more than 50%. This reduces the possibil-
ity of only one isomer of acid participating selectively in the reac-
tion with chiral alcohol. The other possibility for the enrichment of
diastereomer may involve interconversion of isomers of the ester
to achieve the equilibrium in favor of stereochemically and ther-
modynamically more stable product. To test this possibility, a sam-
ple of 4 with moderate de was subjected to basic condition for
some time and quenched. The product was carefully and com-
pletely recovered and analyzed to indicate some enrichment in
the ratio (Scheme 6). At a slightly elevated temperature we
observed almost racemization. Reaction at room temperature
resulted in marginal enrichment, while at a low temperature con-
siderable enhancement was found. The organic base DMAP and
DABCO showed similar effect in this experiment.
kinetic resolution of a-halo esters.
Acknowledgments
We thank Council of Scientific and Industrial Research (CSIR),
New Delhi for Senior Research Fellowship to N.J. We are also grate-
ful to PURSE Program of Department of Science and Technology
(DST), New Delhi for the grant received by the Faculty of Science
to acquire single crystal X-ray diffraction facility.
Supplementary data
Supplementary data (Experimental procedures, characteriza-
tion data and copies of the spectra are included in this section.)
associated with this article can be found, in the online version, at
http://dx.doi.org/10.1016/j.tetlet.2016.01.003.

N. Jain, A. V. Bedekar / Tetrahedron Letters 57 (2016) 692–695
695
I.; Veige, A. S. Polyhedron 2010, 29, 553; (n) Zhang, F.; Song, H.; Zi, G. J.
Organomet. Chem. 1993, 2010, 695.
References and notes
5
6
7
.
.
.
Jain, N.; Bedekar, A. V. Tetrahedron: Asymmetry 2011, 22, 1176.
1
.
(a) Seyden-Penne, J. Chiral Auxiliaries and Ligands in Asymmetric Synthesis;
Wiley: New York, 1995; (b) Gnas, Y.; Glorius, F. Synthesis 1899, 2006, 12; (c)Key
Chiral Auxiliary Applications; Roos, G., Ed.; Academic Press: Boston, 2014.
Selected landmark examples: (a) Corey, E. J.; Ensley, H. E. J. Am. Chem. Soc. 1975,
Jain, N.; Mandal, M. B.; Bedekar, A. V. Tetrahedron 2014, 70, 4343.
(a) Arya, P.; Qin, H. Tetrahedron 2000, 56, 917; (b) Durst, T.; Koh, K. Tetrahedron
Lett. 1992, 33, 6799; (c) Ammazzalorso, A.; Amoroso, R.; Bettoni, G.; De Filippis,
B. Chirality 2001, 13, 102; (d) Amongero, M.; Visnovezky, D.; Kaufman, T. S. J.
Braz. Chem. Soc. 2010, 21, 1017; (e) Kubota, H.; Kubo, A.; Takahashi, M.;
Shimizu, R.; Date, T.; Okamura, K.; Numami, K.-I. J. Org. Chem. 1995, 60, 6776;
2
.
97, 6908; (b) Evans, D. A.; Ennis, M. D.; Mathre, D. J. J. Am. Chem. Soc. 1982, 104,
1
737; (c) Hanessian, S.; Delorme, D.; Beaudoin, S.; Leblanc, Y. J. Am. Chem. Soc.
1
984, 106, 5754; (d) Poll, T.; Sobczak, A.; Hartmann, H.; Helmchen, G.
(
f) Sarakinos, G.; Corey, E. J. Org. Lett. 1999, 1, 1741; (g) Angibaud, P.;
Tetrahedron Lett. 1985, 26, 3095; (e) Whitesell, J. K.; Chen, H. H.; Lawrence, R.
M. J. Org. Chem. 1985, 50, 4663; (f) Enders, D.; Kipphardt, H.; Fey, P. Org. Synth.
Chaumette, J. L.; Desmurs, J. R.; Duhamel, L.; Plé, G.; Valnot, J. Y.; Duhamel, P.
Tetrahedron: Asymmetry 1919, 1995, 6; (h) Ward, R. S.; Pelter, A.; Goubet, D.;
Pritchard, M. C. Tetrahedron: Asymmetry 1995, 6, 469.
(a) Knowles, W. S. Angew. Chem., Int. Ed. 2002, 41, 1998; (b) Noyori, R. Angew.
Chem., Int. Ed. 2002, 41, 2008.
1
987, 65, 183; (g) Oppolzer, W.; Moretti, R.; Thomi, S. Tetrahedron Lett. 1989,
30, 5603; (h) Gras, J.-L.; Pellissier, H. Tetrahedron Lett. 1991, 32, 7043; (i) Fuji,
8
.
.
K.; Tanaka, F.; Node, M. Tetrahedron Lett. 1991, 32, 7281; (j) Studer, A.;
Hintermann, T.; Seebach, D. Helv. Chim. Acta 1995, 78, 1185; (k) Boezio, A. A.;
Solberghe, G.; Lauzon, C.; Charette, A. B. J. Org. Chem. 2003, 68, 3241; (l) Tessier,
A.; Lahmar, N.; Pytkowicz, J.; Brigaud, T. J. Org. Chem. 2008, 73, 3970; (m)
Binder, C. M.; Bautista, A.; Zaidlewicz, M.; Krzemi n´ ski, M. P.; Oliver, A.;
Singaram, B. J. Org. Chem. 2009, 74, 2337; (n) Luo, Y.-C.; Zhang, H.-H.; Wang, Y.;
Xu, P.-F. Acc. Chem. Res. 2010, 43, 1317; (o) Lin, L.; Fu, X.; Ma, X.; Zhang, J.;
Wang, R. Synlett 2012, 2559; (p) Zhao, G.; Yang, C.; Sun, H.; Lin, R.; Xia, W. Org.
Lett. 2012, 14, 776; (q) Chen, G.; Shigenari, T.; Jain, P.; Zhang, Z.; Jin, Z.; He, J.; Li,
S.; Mapelli, C.; Miller, M. M.; Poss, M. A.; Scola, P. M.; Yeung, K.-S.; Yu, J.-Q. J.
Am. Chem. Soc. 2015, 137, 3338.
9
Maruoka, K.; Ooi, T. Chem. Rev. 2003, 103, 3013.
1
1
1
0. Faigl, F.; Fogassy, E.; Nógrádi, M.; Pálovics, E.; Schindler, J. Tetrahedron:
Asymmetry 2008, 19, 519.
1. (a) Kagan, H. B.; Fiaud, J. C. Top. Stereochem. 1988, 18, 249; (b) Vedejs, E.; Jure,
M. Angew. Chem., Int. Ed. 2005, 44, 3974.
2. (a) Beak, P.; Anderson, D. R.; Curtis, M. D.; Laumer, J. M.; Pippel, D. J.;
Weisenburger, G. A. Acc. Chem. Res. 2000, 33, 715; (b) Pellissier, H. Tetrahedron
2008, 64, 1563; (c) Lee, W. K.; Park, Y. S.; Beak, P. Acc. Chem. Res. 2009, 42, 224;
(
d) Turner, N. J. Curr. Opin. Chem. Biol. 2010, 14, 115; (e) Pellissier, H. Adv. Synth.
Catal. 2011, 353, 659; (f) Lee, S. Y.; Murphy, J. M.; Ukai, A.; Fu, G. C. J. Am. Chem.
Soc. 2012, 134, 15149; (g) Mansueto, R.; Perna, F. M.; Salomone, A.; Florio, S.;
Capriati, V. Chem. Commun. 2013, 4911; (h) Wang, S.; Zhou, S.; Wang, J.; Nian,
Y.; Kawashima, A.; Moriwaki, H.; Aceña, J. L.; Soloshonok, V. A.; Liu, H. J. Org.
Chem. 2015, 80, 9817; (i) Gooch, L. M.; Rossington, S. B.; Wilkinson, J. A.
Tetrahedron Lett. 2015, 56, 4025.
3
.
.
(a) Weber, E.; Csöregh, I.; Ahrendt, J.; Finge, S.; Czugler, M. J. Org. Chem. 1988,
53, 5831; (b) Weber, E.; Hens, T.; Gallardo, O.; Csöregh, I. J. Chem. Soc., Perkin
Trans. 2 1996, 737.
4
(a) Waldmann, H.; Weigerding, M.; Dreisbach, C.; Wandrey, C. Helv. Chim. Acta
1
994, 77, 2111; (b) Scheffer, J. R.; Ihmels, H. Liebigs Ann. Recl. 1925, 1997; (c)
Rabjohns, M. A.; Hodge, P.; Lovell, P. A. Polymer 1997, 38, 3395; (d) Doherty, S.;
Robins, E. G.; Knight, J. G.; Newman, C. R.; Rhodes, B.; Chapkin, P. A.; Clegg, W. J.
Organomet. Chem. 2001, 640, 182; (e) Wang, B.; Feng, X.; Huang, Y.; Liu, H.; Cui,
X.; Jiang, Y. J. Org. Chem. 2002, 67, 2175; (f) Alibert, S.; Santelli-Rouvier, C.;
Pradines, B.; Houdoin, C.; Parzy, D.; Karolak-Wojciechowska, J.; Barbe, J. J. Med.
Chem. 2002, 45, 3195; (g) Alibert, S.; Santelli-Rouvier, C.; Castaing, M.;
Berthelot, M.; Spengler, G.; Molnar, J.; Barbe, J. Eur. J. Med. Chem. 2003, 38,
1
3. Ammazzalorso, A.; Amoroso, R.; Bettoni, G.; De Filippis, B.; Fantacuzzi, M.;
Giampietro, L.; Maccallini, C.; Tricca, M. L. Eur. J. Org. Chem. 2006, 4088.
4. Park, Y. S. Tetrahedron: Asymmetry 2009, 20, 2421.
5. (a) Corey, E. J.; Gross, A. W. Tetrahedron Lett. 1984, 25, 495; (b) Angibaud, P.;
Chaumette, J. L.; Desmurs, J. R.; Duhamel, L.; Pié, G.; Valnot, J. Y.; Duhamel, P.
Tetrahedron: Asymmetry 1919, 1995, 6.
1
1
1
6. (a) Camps, P.; Pérez, F.; Soldevilla, N. Tetrahedron: Asymmetry 1998, 9, 2065; (b)
Calmes, M.; Glot, C.; Michel, T.; Rolland, M.; Martinez, J. Tetrahedron:
Asymmetry 2000, 11, 737; (c) Nam, J.; Lee, S.-K.; Park, Y. S. Tetrahedron 2003,
2
53; (h) Millet, J.; Torrentino-Madamet, M.; Alibert, S.; Rogier, C.; Santelli-
Rouvier, C.; Mosnier, J.; Baret, E.; Barbe, J.; Parzy, D.; Pradines, B. Antimicrob.
Agents Chemother. 2004, 48, 2753; (i) Sasaoka, A.; Imam Uddin, M.; Shimamoto,
A.; Ichikawa, Y.; Shiro, M.; Kotsuki, H. Tetrahedron: Asymmetry 2006, 17, 2963;
5
6
9, 2397; (d) Jang, I. J.; Kang, S. Y.; Kang, K. H.; Park, Y. S. Tetrahedron 2011, 67,
221.
(
j) Zeng, Z.; Zhao, G.; Gao, P.; Tang, H.; Chen, B.; Zhou, Z.; Tang, C. Catal.
1
1
7. Hashimoto, S.; Matsunaga, H.; Kunieda, T. Chem. Pharm. Bull. 2000, 48, 1541.
8. Veit, A.; Lenz, R.; Seiler, M. E.; Neuburger, M.; Zehnder, M.; Giese, B. Helv. Chim.
Acta 1993, 76, 441.
Commun. 2007, 8, 1443; (k) Jeletic, M. S.; Jan, M. T.; Ghiviriga, I.; Abboud, K. A.;
Veige, A. S. Dalton Trans. 2009, 2764; (l) Naidu, A. B.; Jaseer, E. A.; Sekar, G. J.
Org. Chem. 2009, 74, 3675; (m) Lowry, R. J.; Jan, M. T.; Abboud, K. A.; Ghiviriga,