New chiral diamino ligands as sparteine analogues. Application to the palladium-catalyzed kinetic oxidative resolution of 1-phenyl ethanol

Article abstract of DOI:10.1016/j.tetasy.2008.05.006

Novel chiral 9-keto-bispidines were investigated as ligands in the palladium-catalyzed kinetic oxidative resolution (KOR) of 1-phenyl ethanol. The ligands were easily prepared by means of a two-step synthetic sequence starting from commercially available

Full text of DOI:10.1016/j.tetasy.2008.05.006

Tetrahedron: Asymmetry 19 (2008) 1363–1366
Contents lists available at ScienceDirect
Tetrahedron: Asymmetry
journal homepage: www.elsevier.com/locate/tetasy
New chiral diamino ligands as sparteine analogues. Application to the
palladium-catalyzed kinetic oxidative resolution of 1-phenyl ethanol
a
Giordano Lesma a, Tullio Pilati b, Alessandro Sacchetti a, , Alessandra Silvani
*
a Dipartimento di Chimica Organica e Industriale, Università degli Studi di Milano, Via G. Venezian 21, 20133 Milano, Italy
b Istituto di Scienze e Tecnologie Molecolari—CNR, Via Golgi 19, 20133 Milano, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
Novel chiral 9-keto-bispidines were investigated as ligands in the palladium-catalyzed kinetic oxidative
resolution (KOR) of 1-phenyl ethanol. The ligands were easily prepared by means of a two-step synthetic
sequence starting from commercially available products.
Received 17 April 2008
Accepted 13 May 2008
Available online 9 June 2008
Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction
1-phenyl ethanol. Bispidines 2b are structurally correlated to
(ꢀ)-sparteine and have been previously described as both achiral
The natural alkaloid (ꢀ)-sparteine 1 (Fig. 1) has been thor-
oughly studied as a chiral diamino ligand, mainly in asymmetric
deprotonation with alkyllithium bases.1 In addition to organolith-
ium reagents, (ꢀ)-sparteine has also been employed with other
metals such as Mg, Cu and Zn.2 In 2001, the use of a Pd-(ꢀ)-spar-
teine complex for the aerobic kinetic oxidative resolution (KOR)
of racemic alcohols was also reported.3
and chiral ligands in different metal-catalyzed reactions.6
2. Results and discussion
We planned the synthesis of the 9-keto-bispidine ligands 3a–d
(Fig. 2). These ligands could be easily prepared in enantiopure form
H
N
X
X
N
O
N
O
5
4
1
2
N
N
N
H
N
H
MeOOC
COOMe
MeOOC
COOMe
N
H
(-)-sparteine 1
2a X = O (bispidinone)
N
N
2b
X = H2 (bispidine)
O2N
NO2
Figure 1.
3a
3b
The main drawback in the (ꢀ)-sparteine methodologies is that
this alkaloid is readily available only as a single antipode from nat-
ural sources, and it is a difficult template to be optimized through
structural variations. In 2002, O’Brien4a reported the synthesis of a
(+)-sparteine-like tricyclic diamine, synthesized by starting from
the expensive natural occurring alkaloid (ꢀ)-cytisine, which cata-
lyzes different asymmetric transformations with good ee.4
In our ongoing studies on the preparation of sparteine ana-
logues,5 we herein report the synthesis of novel chiral C1-symmet-
ric diamino ligands, based on the 3,7-diaza-bicyclo[3.3.1]nonan-9-
one moiety (bispidinone 2a) and their preliminary application in
the palladium-catalyzed kinetic oxidative resolution of racemic
N
N
O
O
MeOOC
COOMe
MeOOC
COOMe
N
N
N
N
O2N
NO2
3c
3d
* Corresponding author. Tel.: +39 02 50314081; fax: +39 02 50314078.
E-mail address: alessandro.sacchetti@unimi.it (A. Sacchetti).
Figure 2. Ligands 3a–d.
0957-4166/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2008.05.006

1364
G. Lesma et al. / Tetrahedron: Asymmetry 19 (2008) 1363–1366
by means of a two-step synthetic sequence relying on double Man-
nich reactions, starting from commercially available products on a
gram scale. Different substituents can be easily introduced onto
the diaza-bicyclo[3.3.1]nonan-9-one framework, thus allowing, in
principle, the preparation of a library of ligands. Ligands 3a–d are
characterized by the presence of two aromatic groups on C2 and
C4, which increase the steric hindrance at the coordinating nitro-
gen atom, and two carbomethoxy groups on C1 and C5. The source
of chirality is a chiral primary amine [(S)-1-phenyl-ethylamine or
(S)-1-naphthalen-1-yl-ethylamine] introduced in the last synthetic
step. Whilst ligands 3a–c can act as bidentate diamino ligands, for
ligand 3d, further coordination capabilities can be predicted due to
the presence of pyridine donor substituents.
Ligands 3a–d were synthesized according to Scheme 1.
Piperidine-4-ones 4 were prepared in a single step following a
reported procedure.7 A double Mannich reaction of 4 with either
(S)-1-phenyl-ethylamine or (S)-1-naphthalen-1-yl-ethylamine
and 2.5 equiv of formaldehyde in refluxing methanol gave the
desired products in high yields (73–92%).
at 80 °C under an oxygen atmosphere in the presence of 3 Å mole-
cular sieves. The catalytic system was realized using 20 mol % of
ligand and 5 mol % of Pd(nbd)Cl2. Chiral HPLC was used to deter-
mine both conversions and enantiomeric excesses. Reactions were
stopped after 72 h. In these conditions (ꢀ)-sparteine gave, in our
hands, 55% conversion and 96% ee [(S)-enantiomer] and a selecti-
vity factor (krel) of 31.9, completely in agreement with literature.3
Results are reported in Table 1.
Table 1
Palladium-catalyzed kinetic oxidative resolution of racemic 1-phenyl ethanol
OH
O
OH
*
Pd(nbd)Cl2 (5 mol%)
ligand (20 mol%)
+
MS3Å,O2,
toluene, 80 °C
b
Entry
Ligand
Conversiona (%)
eea (%)
krel
1
2
3
4
5
3a
3b
3c
3d
53
42
45
10
55
20 (R)
31 (R)
42 (R)
4 (R)
1.7
3.3
4.6
2.2
31.9
R*
(ꢀ)-Sparteine
96 (S)
N
a
O
Determined by HPLC analysis on a chiral column. See Section 4 for details.
krel = ln[(1 ꢀ C)(1 ꢀ ee)]/ln[(1 ꢀ C)(1 + ee)].
O
b
MeOOC
Ar
COOMe
CH2O,R*NH2
MeOOC
Ar
COOMe
Ar
For ligands 3a,c good conversions were achieved, as result of an
N
Ar
MeOH, reflux
N
efficient LAC (ligand accelerated catalysis) effect. Unfortunately the
ee proved to be modest. Comparisons between 3a and 3b indicated
that the presence of electron poor aromatic residues seems to be
favourable. By changing the phenyl group of 3b with the more ste-
rically hindered naphthalen-1-yl substituent in ligand 3c, we could
obtain the highest ee (42%) and selectivity (krel = 4.6). In contrast,
the presence of the two pyridine substituents in 3d caused a dra-
matic lowering of both yield and ee. A possible reason for the
low activity of ligand 3d could be the formation of stabilized highly
coordinated palladium complexes, in which 3d acts as a tetra-coor-
dinating agent.9 In this way, the palladium would be prevented
from coordinating the substrate, thus resulting in an inhibition of
the first step of the catalytic cycle.
In order to improve these results, we performed the reaction in
different solvents, such as dichloromethane, chloroform and tert-
butanol,10 without any success. The addition of exogenous inor-
ganic bases such as caesium carbonate or sodium carbonate did
not affect the results.11 Changing the palladium source from
Pd(nbd)Cl2 to PdCl2 only resulted in a slight shortening of the reac-
tion time, without any influence on the enantiomeric excess.
4
3
3a 92%; 3b 81%
3c 3d
73%;
83 %
Scheme 1. Synthesis of ligands 3a–d.
All bispidinones 3a–d could be easily purified by crystallization
from methanol and have been characterized by NMR spectroscopy
and MS. A 2-D NMR analysis allowed us to confirm a cis diequato-
rial relative disposition of the aryl substituents on the C2 and C4 of
the bispidinone framework. This observation is in agreement with
previous literature reports on the conformational and configura-
tional behaviour of substituted bispidinones.8 For ligand 3d, we
were able to perform an X-ray diffraction analysis on a single crys-
tal, isolated after crystallization from i-PrOH (Fig. 3). The structure
confirms the chair–chair conformation for the bispidine core, with
the two pyridyl residues in a cis diequatorial relative disposition.
We tested ligands 3a–d in the kinetic oxidative resolution of
racemic 1-phenyl ethanol. Reactions were carried out in toluene
3. Conclusions
In conclusion, we have developed a new class of bispidinone-
based chiral ligands. All compounds could be easily obtained by a
two-step synthesis, starting from commercially available and low
cost products on gram scale. Ligands with electron poor aromatic
substituents catalyzed the kinetic oxidative resolution (KOR) with
good conversion and moderate enantioselectivity. These ligands
represent the first example of simple diamino chiral ligands
applied in the palladium-catalyzed kinetic oxidative resolution of
racemic alcohols.
4. Experimental
4.1. General
All solvents were distilled and properly dried, when necessary,
prior to use. All chemicals were purchased from commercial
sources and used directly, unless indicated otherwise. During the
Figure 3. Plot of the structure of ligand 3d as determined by X-ray crystallography
(hydrogen atoms omitted: see Section 4).

G. Lesma et al. / Tetrahedron: Asymmetry 19 (2008) 1363–1366
1365
usual workup, all organic extracts were dried (Na2SO4 or MgSO4)
and evaporated. All reactions were monitored by thin layer chro-
matography (TLC) on precoated silica gel 60 F254 (Merck); spots
were visualized with UV light or by treatment with 1% aqueous
KMnO4 solution. 1H and 13C NMR spectra were recorded with Bru-
ker AC 300 (1H, 300 MHz; 13C, 75.4 MHz) and 400 MHz Avance (1H,
400 MHz; 13C, 100 MHz) NMR spectrometers. Chemical shifts are
reported in parts per million downfield from SiMe4 (d = 0.0). HPLC
62.8, 58.5, 56.9, 53.5, 52.7, 52.6, 43.3, 15.1. HRMS-FAB m/z calcd
666.2326, found 666.2321. Anal. Calcd for C36H34N4O9: C, 64.86;
H, 5.14; N, 8.40; O, 21.60. Found: C, 64.80; H, 5.16; N, 8.37.
4.2.5. (1R,2S,4R,5S)-3-Methyl-9-oxo-7-((S)-1-phenyl-ethyl)-2,4-
di-pyridin-2-yl-3,7-diaza-bicyclo[3.3.1]nonane-1,5-dicarboxylic
acid dimethyl ester 3d
2.84 g, 83% yield. Mp 174 °C (decomp.). ½a D25
¼ þ8:0 (c 1, CHCl3).
ꢁ
analyses were performed on
a
Jasco LC2000 Series System
1H NMR (CDCl3, 400 MHz): 8.43 (t, 1H, J = 3.7 Hz), 7.92 (d, 1H,
J = 7.9 Hz), 7.84 (d, 1H, J = 7.9 Hz), 7.60 (td, 1H, J = 7.7, 1.8 Hz),
7.53 (td, 1H, J = 7.7, 1.8 Hz), 7.29 (m, 3H), 7.13 (m, 1H), 4.65 (s,
1H), 4.64 (s, 1H), 3.79 (s, 3H), 3.76 (s, 3H), 3.47 (q, 1H, J = 6.9 Hz),
3.05 (m, 2H), 2.58 (m, 2H), 1.96 (s, 3H), 1.31 (d, 3H, J = 6.9 Hz).
13C NMR (CDCl3, 100 MHz): d 204.4, 169.4, 169.3, 159.0, 158.9,
149.7 (2C), 141.7, 136.8, 136.7, 129.0 (2C), 128.9 (2C), 128.0,
124.2, 124.0, 123.5, 123.4, 72.0, 71.9, 64.7, 62.9, 62.8, 56.1, 53.8,
52.7, 52.6, 43.3, 15.9. HRMS-FAB m/z calcd 528.2373, found
528.2379. Anal. Calcd for C30H32N4O5: C, 68.17; H, 6.10; N, 10.60;
O, 15.13. Found: C, 68.22; H, 6.18; N, 10.55.
equipped with a UV detector and a Chiralcell OD HPLC column.
HR FAB mass spectra in the positive mode were measured on VG
70–70 EQ-HF instrument equipped with its standard sources. Opti-
cal rotations were measured with a Perkin–Elmer 241 polarimeter.
4.2. Synthesis
4.2.1. Typical procedure for the synthesis of the bispidinones
3a–d
To a suspension of paraformaldehyde (7.5 mmol) in 20 ml of
methanol, the piperidinone 4 (3 mmol) and (S)-1-phenyl-ethyl-
amine or (S)-1-naphthalen-1-yl-ethylamine (3 mmol) were added.
The mixture was heated at reflux for 24 h. On cooling to room tem-
perature the product precipitated and was collected by filtration.
The crude was purified by crystallization with methanol, affording
the pure bispidinones as a solid.
4.3. Procedure for the kinetic oxidative resolution of 1-phenyl
ethanol
At first 0.25 g of 3 Å molecular sieves was placed in a 25 mL
reaction flask and flame-dried under vacuum. After cooling under
nitrogen, 5 mL of dry toluene was added followed by 7 mg of
Pd(nbd)Cl2 (0.025 mmol, 0.05 equiv) and by the ligand (0.10 mmol,
0.2 equiv). Nitrogen was removed under reduced pressure and the
flask filled with O2. The reaction mixture was heated to 80 °C for
4.2.2. (1R,2R,4S,5S)-3-Methyl-9-oxo-2,4-diphenyl-7-((S)-1-
phenyl-ethyl)-3,7-diaza-bicyclo[3.3.1]nonane-1,5-dicarboxylic
acid dimethyl ester 3a
2.53 g, 92% yield. Mp 134 °C. ½a D25
ꢁ
¼ þ6:5 (c 1, CHCl3). 1H NMR
30 min, then 60 lL of 1-phenyl ethanol (0.5 mmol, 1 equiv) was
(CDCl3, 400 MHz): d 8.04 (m, 2H), 7.48–7.04 (m, 13H), 4.42 (s,
1H), 4.38 (s, 1H), 3.75 (s, 3H), 3.70 (s, 3H), 3.61 (q, J = 6.7 Hz, 1H),
3.40 (dd, J = 11.6, 1.8 Hz, 1H), 3.20 (d, J = 12.1, 1.8 Hz, 1H), 2.70
(m, 2H), 1.82 (s, 3H), 1.53 (d, J = 6.7 Hz, 3H). 13C NMR (CDCl3,
100 MHz): d 202.3, 169.3, 169.1, 142.0, 139.1, 138.9, 130.1–128.2
(15C), 73.7, 73.6, 65.4, 64.2, 64.1, 56.4, 55.7, 52.8, 52.7, 43.8, 17.7.
HRMS-FAB m/z calcd 526.2468, found 526.2471. Anal. Calcd for
added. The reaction was monitored by HPLC analysis for 72 h
(HPLC analysis conditions: column, Chiralcel OD; eluant 95:5
n-hexane/i-PrOH; flow rate, 0.8 mL/min; UV detector, 230 nm.
Commercially available (R)-1-phenyl ethanol and (S)-1-phenyl
ethanol were used as an analytical standard. Retention time of
(R)-1-phenyl ethanol: 11 min. Retention time of (S)-1-phenyl
ethanol: 13 min). Aliquots of the reaction mixture were collected,
filtered through a small plug of silica gel (EtOAc eluant), evapo-
rated and analyzed.
C32H34N2O5: C, 72.98; H, 6.51; N, 5.32; O, 15.19. Found: C, 73.00;
H, 6.38; N, 4.86.
4.2.3. (1R,2R,4S,5S)-3-Methyl-2,4-bis-(4-nitro-phenyl)-9-oxo-7-
((S)-1-phenyl-ethyl)-3,7-diaza-bicyclo[3.3.1]nonane-1,5-
dicarboxylic acid dimethyl ester 3b
4.4. Crystal data for 3d
Mr = 528.60, trigonal, P31, a = 14.3904(12), c = 11.5091(10) Å,
1.86 g, 81% yield. Mp 196 °C. ½a D25
ꢁ
¼ þ4:9 (c 1, CHCl3). 1H NMR
V = 2064.0(2) Å3, Z = 3, T = 123 K, Dc = 1.276 g cmꢀ3
, l(Mo Ka) =
0.088 cmꢀ1, F(000) = 840; block, 0.48 ꢂ 0.32 ꢂ 0.22 mm, Bruker
APEX2000 diffractometer; 28,606 data collected, 3982 unique,
(CDCl3, 300 MHz): d 8.25–8.00 (m, 4H), 7.55–7.15 (m, 9H), 4.60
(s, 1H), 4.55 (s, 1H), 3.75 (s, 3H), 3.70 (s, 3H), 3.60 (q, J = 7.1 Hz,
1H), 3.25 (d, J = 13.1 Hz, 1H), 3.20 (d, J = 13.1, Hz, 1H), 2.75 (m,
2H), 2.20 (s, 3H), 1.45 (d, J = 7.1 Hz, 3H). 13C NMR (CDCl3,
75 MHz): d 202.8, 167.7, 167.5, 147.8 (2C), 145.3 (2C), 140.9,
129.7–123.5 (13C), 72.0, 71.9, 62.9, 62.8, 58.5, 56.9, 53.5, 52.7,
52.6, 43.3, 15.1. HRMS-FAB m/z calcd 616.2169, found 616.2165.
Anal. Calcd for C32H32N4O9: C, 62.33; H, 5.23; N, 9.09; O, 23.35.
Found: C, 62.28; H, 5.30; N, 9.17.
Rint = 0.0459, 3386 with Io > 2r(Io). The structure was solved by
direct method,12 and refined anisotropically by matrix least-
squares based on F2,13 to give R1 = 0.0405, wR2 = 0.0641 for all
3982 reflections and 480 parameters and 1 restraint. The absolute
configuration was chosen on the known chirality of the (S)-1-phe-
nyl-ethylamine group, unchanged by the chemical synthesis.
Supplementary crystallographic data were deposited as CCDC
635829 with the Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK.
4.2.4. (1R,2R,4S,5S)-3-Methyl-7-((S)-1-naphthalen-1-yl-ethyl)-
2,4-bis-(4-nitro-phenyl)-9-oxo-3,7-diaza-bicyclo[3.3.1]nonane-
1,5-dicarboxylic acid dimethyl ester 3c
References
1.12 g, 73% yield. Mp 115 °C. ½a D25
ꢁ
¼ þ12:9 (c 0.5, CHCl3). 1H
NMR (CDCl3, 300 MHz):
d 8.45 (d, J = 8.5 Hz, 1H), 8.25 (d,
1. (a) Chuzel, O.; Riant, O. In Topics in Organometallic Chemistry. In (Chiral
Diazaligands for Asymmetric Synthesis); Springer: Berlin/Heidelberg, 2005; Vol.
15, pp 59–92; (b) Hoppe, D.; Christoph, G. In Chemistry of Organolithium
Compounds; Rappoport, Z., Marek, I., Eds.; John Wiley & Sons Ltd.: Chichester,
UK, 2004; pp 1055–1164; (c) Schuetz, T. Synlett 2003, 901–902; (d) Hoppe, D.;
Hense, T. Angew. Chem., Int. Ed. 1997, 36, 2282–2316.
2. For Mg: Shintani, R.; Fu, G. C. Angew. Chem., Int. Ed. 2002, 41, 1057; For Cu:
Maheswaran, H.; Prasanth, K. L.; Krishna, G. G.; Ravikumar, K.; Sridhar, B.;
Kantam, M. L. Chem. Commun. 2006, 39, 4066–4068; For Zn: (a) Jana, S.;
Sherrington, D. C. Angew. Chem., Int. Ed. 2005, 44, 4804–4808; (b) Mimoun, H.;
J = 8.5 Hz, 1H), 8.15–7.95 (m, 3H), 7.80–7.45 (m, 8H), 7.25 (m,
2H), 4.65 (q, J = 7 Hz, 1H ), 4.55 (s, 1H), 4.50 (s, 1H), 3.80 (s, 3H),
3.70 (s, 3H), 3.35 (d, J = 13 Hz, 1H), 3.25 (d, J = 13 Hz, 1H), 3.15
(d, J = 13 Hz, 1H), 3.00 (d, J = 13 Hz, 1H), 1.70 (s, 3H), 1.55 (d,
J = 7 Hz, 3H). 13C NMR (CDCl3, 75 MHz): d 202.9, 167.8, 167.5,
147.9, 147.7, 145.1, 145.0, 137.6, 134.3, 131.7, 130.4, 130.1,
129.9, 129.4, 128.6, 128.2, 126.2–123.2 (9C), 72.0, 71.9, 62.9,

1366
G. Lesma et al. / Tetrahedron: Asymmetry 19 (2008) 1363–1366
de Laumer, J. Y.; Giannini, L.; Scopelliti, R.; Floriani, C. J. Am. Chem. Soc. 1999,
121, 6158–6166.
J.; Huttenloch, O.; Waldmann, H. Eur. J. Org. Chem. 2000, 391–399; (e)
Huttenloch, O.; Spieler, J.; Waldmann, H. Chem. Eur. J. 2001, 7, 671–675; (f)
Huttenloch, O.; Laxman, E.; Waldmann, H. J. Chem. Soc., Chem. Commun. 2002,
673–675; (g) Huttenloch, O.; Laxman, E.; Waldmann, H. Chem. Eur. J. 2002, 8,
4767–4780.
3. (a) Jensen, D. R.; Pugsley, J. S.; Sigman, M. S. J. Am. Chem. Soc. 2001, 123, 7475–
7476; (b) Stoltz, B. M.; Ferreira, E. M. J. Am. Chem. Soc. 2001, 123, 7725–7726.
4. (a) Dearden, M. J.; Firkin, C. R.; Hermet, J. P. R.; O’Brien, P. J. Am. Chem. Soc. 2002,
124, 11870–11871; (b) Dearden, M. J.; McGrath, M. J.; O’Brien, P. J. Org. Chem.
2004, 69, 5789–5792; (c) O’Brien, P. Chem. Commun. 2008, 655–667.
5. (a) Lesma, G.; Cattenati, C.; Pilati, T.; Sacchetti, A.; Silvani, A. Tetrahedron:
Asymmetry 2007, 18, 659–663; (b) Lesma, G.; Danieli, B.; Passerella, D.;
Sacchetti, A.; Silvani, A. Lett. Org. Chem. 2006, 3, 430–436; (c) Danieli, B.; Lesma,
G.; Passarella, D.; Sacchetti, A.; Silvani, A. Tetrahedron Lett. 2005, 46, 7121–
7123; (d) Danieli, B.; Lesma, G.; Passarella, D.; Sacchetti, A.; Silvani, A.
Tetrahedron: Asymmetry 2003, 14, 2453–2458; (e) Danieli, B.; Lesma, G.;
Passarella, D.; Piacenti, P.; Sacchetti, A.; Silvani, A. Tetrahedron Lett. 2002, 43,
7155–7158.
7. Kuhl, U.; Englberger, W.; Haurand, M.; Holzgrabe, U. Arch. Pharm. Pharm. Med.
Chem. 2000, 333, 226–230.
8. (a) Siener, T.; Holzgrabe, U.; Droshin, S.; Brandt, W. J. Chem. Soc., Perkin Trans. 2
1999, 1827–1834; (b) Muhl, U.; von Korff, M.; Baumann, K.; Burschka, C.;
Holzgrabe, U. J. Chem. Soc., Perkin Trans. 2 2001, 2037–2042.
9. For a comprehensive review on bispidine coordination chemistry see: Comba,
P.; Kerscher, M.; Schiek, W. Prog. Inorg. Chem. 2007, 55, 613–704.
10. Bagdanoff, J. T.; Ferriera, E. M.; Stoltz, B. M. Org. Lett. 2003, 5, 835–837.
11. Mandl, S. K.; Sigman, M. S. J. Org. Chem. 2003, 68, 7535–7537.
12. Burla, M. C.; Camalli, M.; Carrozzini, B.; Cascarano, G. L.; Giacovazzo, C.;
Polidori, G.; Spagna, R. J. Appl. Cryst. 2003, 36, 1103.
6. (a) Hodgson, D. M.; Norsikian, S. L. Org. Lett. 2001, 3, 461–463; (b) Comba, P.;
Merz, M.; Pritzkow, H. Eur. J. Inorg. Chem. 2003, 9, 1711–1718; (c) Gallagher, D.
J.; Wu, S.; Nikolic, N. A.; Beak, P. J. Org. Chem. 1995, 60, 8148–8154; (d) Spieler,
13. Sheldrick, G. M. Acta Cryst. 2008, A64, 112–122.