Predetermined helical chirality in octahedral complexes with a novel pentadentate C2-symmetrical chiral bis(oxazoline) ligand

Article abstract of DOI:10.1002/adsc.200404020

The first example of the pentadentate bis(oxazolines) has been synthesized using a modular approach that readily provides access to this ligand class. This ligand allows the controlled transfer of carbon-centered to octahedral, metal-centered chirality, a

Full text of DOI:10.1002/adsc.200404020

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Predetermined Helical Chirality in Octahedral Complexes with a
Novel Pentadentate C2-Symmetrical Chiral Bis(oxazoline) Ligand
Michael Seitz,a Anja Kaiser,a Douglas R. Powell,b Andrew S. Borovik,b
Oliver Reisera,*
a
Institut f¸r Organische Chemie, Universit‰t Regensburg, Universit‰tsstr. 31, 93053 Regensburg, Germany
Fax: (þ49)-941-943-4121, e-mail: oliver.reiser@chemie.uni-regensburg.de
b
Department of Chemistry, University of Kansas, Lawrence, Kansas 66045, USA
Received: January 20, 2004; Accepted: April 16, 2004
Dedicated to Dr. Joe P. Richmond on the occasion of his 60th birthday.
Supporting Information for this article is available on the WWW under http://asc.wiley-vch.de/.
Abstract: The first example of the pentadentate bis-
(oxazolines) has been synthesized using a modular
approach that readily provides access to this ligand
class. This ligand allows the controlled transfer of
carbon-centered to octahedral, metal-centered chir-
ality, as demonstrated both in the solid state as well
as in solution. The characteristic CD band has been
identified for dicationic, octahedral transition metal
complexes with the D2-configuration.
kome×s ligand 2 [C2-symmetry of Co(II) complexes][6]
and Bernauer×s ligand 3 (predetermination of the helical
chirality)[7] to the new model compound 1.
Ligand synthesis: A modular synthesis could be devel-
oped for 1 (Scheme 1) starting from (S)-serine methyl
ester hydrochloride (4) and commercially available eth-
yl benzimidate hydrochloride to yield the known oxazo-
line 5.[8] Reduction to the alcohol 6 on a 250 mmol-scale
proceeded smoothly without detectable racemization
using LiAlH4 in THFat À308C instead of the rather ex-
pensive DIBAL-H recommended in the literature.[8] To-
sylation was achieved according to a literature proce-
dure to yield 7.[9] Preparation of the air-sensitive dithiol
10 started from commercially available 2,6-bis(hydroxy-
methyl)pyridine (8), following a procedure reported by
Chiotellis et al.[10] The final combination of the two pre-
Keywords: chirality; circular dichroism; helical struc-
tures; structure elucidation; transition metals
Many molecules containing the oxazoline motive have cursors 7 and 10 by nucleophilic substitution yielded the
proved to be outstanding ligands for asymmetric cataly- new ligand 1. Since only two compounds, 1 and 6, have to
sis.[1] Metal complexes of C2-symmetrical bis(oxazo- be purified by column chromatography, this synthetic
lines) and pyridine-(bisoxazolines) excel in a great num- route can be conveniently carried out on a large scale, af-
ber of enantioselective transformations with unprece- fording 1 in multigram quantities.
dented selectivity.[2] Extending their scope on the basis
Metal complexes solid and solution state structures:
of our previous work,[3] we wanted to design pentaden- Given the significance of cobalt (II) for dioxygen activa-
tate bis(oxazoline) ligands that can provide a C2-sym- tion[11] and its preference to form octahedral complexes,
metrical octahedral environment for a metal, leaving we investigated the complex-formation of 1 with this
one coordination site open for substrate/reagents. This metal. Complexation of Co(II) at ambient temperature
approach appears to be especially attractive since a chi- and under anhydrous conditions required the use of
ral metal center can be created in this way by a helical non-coordinating counterions. The metal precursor of
fold of such ligands, transferring the chirality from the
chiral, non-racemic organic ligand to a metal center.[4]
Various pentadentate ligand systems have been devel-
oped as a model system for bleomycin,[5] known to acti-
vate dioxygen and catalyze oxidation reactions of non-
activated organic substrates. We sought to develop
new chiral variants of such catalysts, following the de-
sign concept described in this paper.
To the best of our knowledge, no other pentadentate
bis(oxazolines) have been reported to date. In order to
arrive at such ligands we combined the design of New- Figure 1. Design of the new ligand 1 based on known 2 and 3.
Adv. Synth. Catal. 2004, 346, 737 741
DOI: 10.1002/adsc.200404020
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Michael Seitz et al.
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Scheme 1. Synthesis of ligand 1. Conditions: a) Ethyl benzi-
midate (0.91 equivs.), 1,2-dichloroethane, 20 h, reflux, 90%;
b) LiAlH4 (0.55 equivs.), THF, À358C, 45 min, 08C 30 min,
64%; c) Ts-Cl (1.1 equivs.), NEt3 (2.2 equivs.), CHCl3, 08C
to rt, 20 h, 78%; d) (i) SOCl2 (2.2 equivs.), Et2O, 0 8C to rt,
18 h; (ii) NaHCO3 (aq.), CH2Cl2, 93% (2 steps); e) Ar atmos-
phere, thiourea (2.0 equivs.), EtOH, reflux, 1 h, (ii) NaOH
(4.0 equivs.), H2O, reflux, 4 h, 64% (2 steps); f) 7 (2.1 equivs.),
NaH (2.1 equivs.), 10 (1.0 equivs.), DMF, 08C to rt, 18 h,
72%.
Figure 2. Thermal ellipsoid plot for the cation of 12 (CCDC
No. 216870). The displacement ellipsoids are drawn at the
50% probability level. Except for the atoms attached to the
chiral centers of the oxazolines, all other hydrogens have
been omitted for clarity. Crystal data for 12 (for more de-
tailed information see the supporting information):
C33H35CoF6N3O9S4, M¼918.81, monoclinic space group P21,
a¼11.481(3) ä, b¼8.809(2) ä, c¼18.976(4) ä, ˚¼
105.688(5)8, V¼1847.7(7) ä3, T¼100(2) K, Z¼2, F(000)¼
942, m(Mo-Ka)¼0.779 mmÀ1, 10392 reflections measured,
5661 unique (Rint ¼0.0439), 5392 observed (I>2s(I)), 505
parameters refined, R1¼0.0710 (observed data), wR(F2)¼
0.1866 (all data), S¼1.047.
Scheme 2. Synthesis of complex 12.
Scheme 3. Synthesis of the Ru(II) complexes 14 and 15.
The structure of 12 clearly shows the validity of the li-
our choice was Co(OTf)2(CH3CN)2 (11), prepared in an
analogous manner compared to the known anhydrous
Fe(II) triflate.[12] Combining ligand 1 and the cobalt
salt 11 in THF yielded the pink solid 12, which precipi- gand design for the construction of a well-defined heli-
tated from the reaction mixture. cal-chiral environment, at least in the solid state. Never-
Single crystals suitable for X-ray analysis were ob- theless, in solution the behavior can be different. There-
tained by vapor diffusion of diethyl ether into a solution fore, it was desirable to investigate appropriate com-
of 12 in acetonitrile. The structure (Figure 2) shows a plexes by NMR spectroscopy. The best candidate in
pseudo-C2-symmetrical, distorted octahedral Co(II) our view was diamagnetic Ru(II), because of its high
complex. THF is coordinating as the sixth ligand in a tendency for octahedral coordination geometries and
trans-position to the pyridine ring, the triflate anions the great stability of the expected 18-electron species.
are non-coordinating. The two phenyl rings provide a Refluxing ligand 1 and Ru(II) precursor 13[13] in anhy-
rather tight chiral pocket for the metal, effectively drous ethanol and filtration through Celite using CH2
shielding two opposite quadrants of the accessible front Cl2 yielded a brown solid with practically quantitative
space of the complex.
conversion (Scheme 3). This material consists of two
The configuration at the metal center is exclusively D2. symmetric, diamagnetic Ru(II) complexes as shown by
The corresponding L2-complex would be less favored, 1H NMR spectroscopy (Figure 3), which were assigned
since the phenyl rings attached to the oxazolines would to the species [Ru(1)(CH2Cl2)]Cl2 (14)[14] and
be placed directly above and below the ligand backbone [Ru(1)Cl]Cl (15) on the basis of proton-NMR and
(see Ru complexes, Figure 4).
mass spectrometry.
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Adv. Synth. Catal. 2004, 346, 737 741

Novel Pentadentate C2-Symmetrical Chiral Bis(oxazoline) Ligand
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Figure 3. 1H NMR (CDCl3, 600 MHz) spectra of the mixture
of Ru(II) complexes of 1 between 5.2 and 1.6 ppm. For the
full spectrum see the supporting information.
Figure 5. CD spectra of Co complex 12 and Ru complexes
14/15 were recorded in acetonitrile (Figure 5).
Figure 4. Molecular modelling of the favored D2- and disfa-
vored L2-configuration of 15 (PM3-TM, Titan 1.0.5, Schrˆ-
dinger Inc.).
Figure 6. Pyridine crown ether 16 and proposed sector rules
for n!p*transitions by Palmer et al. ( þ and correspond
to the sign of the CD signal for the presence of substituents
above the pyridine ring; below this plane the signs are invert-
ed).
The Ru(II) species show well resolved proton signals,
indicating a rather rigid structure. The most prominent
feature is that 14 and 15 have two groups of benzylic sig-
In order to further investigate the solution structures,
nals (doublets at d¼4.98, 3.76 ppm and 4.54, 3.08 ppm) CD spectra of 12 and 14/15 were recorded in acetonitrile
showing geminal coupling (Jꢀ18 Hz). In distinct con- (Figure 5).
trast, the spectrum of the free ligand 1 exhibits only
one singlet for all four benzylic protons.
Both spectra show strong CD bands (12: 232 nm, 14/
15: 229 nm/209 nm). Since ligand 1 alone exhibits no de-
For the symmetrical species 14 and 15, only two modes tectable CD signals, the observed bands can clearly be
of ligand complexation, D2 and L2, are possible (Fig- attributed to the corresponding complexes. The signals
ure 4). Several arguments point towards the assignment are assigned to the corresponding n!p*transitions of
to only one single configuration: a) The NMR spectra of the pyridine chromophore, following the analysis of
14 and 15 exhibit very similar, only shifted signal pat- the reported C2-symmetrical pyridine crown ether 16
terns, which indicates an analogous spatial arrangement (Figure 6) by Palmer et al.[15] Their sector rules for C2-
of the ligand around the metal. b) Mass spectrometry symmetrical pyridine derivatives predict a positive CD
(ESI, CH3CN) clearly shows two cationic species, band (n!p*) for D2-configurations and a negative sig-
[Ru(1)Cl]þ and [Ru(1)CH3CN]2þ, explaining the rather nal for L2.
large NMR shift of the signal patterns by the mono- and
Since all CD bands observed for 12 and 14/15 show
dicationic nature of the complexes. c) Molecular model- only positive signs, we conclude that these complexes
ling of the possible D2- and L2-species on a semi-empiri- should reside in the D2-configuration (vide supra).
cal level indicates the greater stability (DEꢀ3 kcal/ Moreover, the dicationic species 12 and 14 show very
mole) of the expected D2-configuration of 15 (Figure 4). similar maxima (12: 232 nm and 14: 229 nm[16]). This in-
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Michael Seitz et al.
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(S)-2-Phenyl-4-tosyloxymethyl-oxazoline (7; 8.60 g, 26.0 mmol,
2.1 equivs.) was added as a solid and the ice bath was re-
dicates that the solid state structure of 12 is retained in
solution, a finding which is not trivial.
[9]
moved. Stirring was continued at ambient temperature over-
night. Then 100 mL water and 200 mL CH2Cl2 were added
and the phases were separated. The organic layer was extracted
with 4ꢁ100 mL water and dried (MgSO4). After removal of
the solvent under vacuum, the residue was purified by column
chromatography (SiO2, hexanes/EtOAc 1:1 to hexanes/
EtOAc 3:7) to afford the slightly yellow oil 1 that solidified af-
ter several days to give a waxy slightly brown solid; yield: 4.36 g
In conclusion, we have shown that the design of ligand
1 is appropriate to obtain a well-defined helical-chiral
environment with exclusive D2-configuration. For the
cobalt complex 12, the solid and the solution state struc-
ture must be very similar. Furthermore, it was possible
to identify the characteristic CD bands for an octahedral
D2-arrangement in dicationic transition-metal com-
plexes. On the basis of this finding, the prediction of sol-
ution-state structures of other dicationic transition-met-
al complexes (Mn, Fe, Ni, Cu, Zn...) should be possible.
The elucidation of the structures of the complexes
should allow a rational approach for the testing of their
catalytic activity in various transformations. These in-
vestigations are currently under way.
1
(72%); mp 59 618C; [a]2D0: þ21.6 (c 1.16, CH2Cl2); H NMR
(300 MHz, CDCl3): d¼7.98 7.85 (m, 4H), 7.60 (t, J¼7.7 Hz,
1H), 7.53 7.32 (m, 6H), 7.25 (d, J¼7.7 Hz, 2H), 4.57 4.39
(m, 4H), 4.29 4.15 (m, 2H), 3.89 (s, 4H), 2.94 (dd, J¼13.3,
4.8 Hz, 2H), 2.65 (dd, J¼13.3, 7.8 Hz, 2H); 13C NMR
(75.5 MHz, CDCl3): d¼164.5, 158.2, 137.4, 131.5, 128.3 (2
peaks), 127.6, 121.4, 72.0, 66.5, 38.3, 36.6; IR (film): n¼3074,
2978, 2930, 2910, 1970, 1912, 1820, 1731, 1646, 1589, 1573,
1495, 1453, 1418, 1360, 1306, 1264, 1177, 1084, 1063, 1029,
967, 909, 818, 782, 751, 694 cmÀ1; MS (ESI): m/z (%)¼491.2
(34), 490.1 (100); HRMS (EI): calcd. for C27H27N3O2S2 [M]þ:
489.1545; found: 489.1541.
Experimental Section
General Remarks
Co(OTf)2(CH3CN)2 (11)
Only proceduresfor new compounds are mentioned here. For a
more detailed experimental description of the full synthetic
route, NMR spectra and crystal structure data for 12 see the
Supporting Information.
Cobalt (935 mg, 15.9 mmol, 1.0 equiv.; Aldrich,>99.9%, <100
mesh) was suspended in 16 mL anhydrous CH3CN. The mix-
ture was cooled to 08C and 5.0 g (33.3 mmol, 2.1 equivs.) triflic
acid (Merck,>98%) were added dropwise in the course of
3 min. The ice-bath was removed and stirring was continued
for 30 min. After heating the mixture for 2 h under reflux,
the solvent and excess triflic acid were removed under reduced
pressure. The solid was dissolved in dry CH3CN and residual
cobalt was removed by filtration through a pad of Celite 535
(Fluka). The resulting red solution was concentrated under re-
duced pressure until crystallization began. The solid was redis-
solved by adding a minimum amount of dry CH3CN. This sol-
ution was layered with approximately twice its volume of dry
Et2O and allowed to stand at room temperature for two days.
The formed slightly sticky crystals were collected , washed
with Et2O and dried under vacuum to constant weight to give
the pink amorphous powder 11 that was stored under N2; yield:
5.67 g (81%). IR (nujol): n¼3172, 2726, 2675, 2319, 2292, 1310,
1213, 1185, 1039, 722, 642, 516 cmÀ1; MS (ESI, CH3CN):
m/z (%)¼330.8 (100, [Co(CH3CN)3(OTf)]þ), 289.7 (74,
[Co(CH3CN)2(OTf)]þ); anal. calcd. for C6H6CoF6N2O6S2
(439.18): C 16.41, H 1.38, N 6.38, S 14.60; found: C 16.63,
H 1.47, N 6.61, S 14.46.
All reactions were carried out under a dry, oxygen-free at-
mosphere of N2 using Schlenk techniques. Commercially avail-
able reagents were used as received. DMF, CH3CN and CH2Cl2
were distilled over P4O10 and stored under N2 over molecular
sieves 3 ä. EtOH was dried over Mg and stored under N2.
THF and Et2O were dried with Na/benzophenone and stored
over Na wire under N2. EtOAc and hexanes for chromato-
graphic separations were distilled before use. For column chro-
matography silica gel Geduran 60 (Merck, 0.063 0.200 mm)
was used. TLC analysis was done on silica gel 60 F254 (Merck)
coated on aluminum sheets.
NMR spectra were recorded on Bruker Avance 300 (1H:
300 MHz, 13C: 75.5 MHz) and Bruker Avance 600 (1H:
600 MHz) spectrometers with TMS as internal standard. IR
spectroscopy was done on a Mattson Genesis Series FT-IR
(sample preparation as indicated). X-Ray analysis was per-
formed by the Crystallography Laboratory (University of Kan-
sas, D. R. Powell) on a Bruker APEX ccd area detector(1)
mounted on a Bruker D8 goniometer using graphite-mono-
chromated Mo-Ka radiation (l¼0.71073 ä). Elemental analy-
sis (Heraeus elementar vario EL III) and mass spectrometry
(Finnigan ThermoQuest TSQ 7000) were done by the Central
Analytical Laboratory (Universit‰t Regensburg). CD spectra
were recorded on a Jasco J-710 spectropolarimeter using CH3
CN solutions (HPLC grade) in 1 mm cuvettes (cylindrical).
[Co(1)(THF)](OTf)2 (12)
Compounds 1 (113 mg, 231 mmol, 1.0 equiv.) and 11 (101 mg,
231 mmol, 1.0 equiv.) were dissolved separately in 2.5 mL dry
THF each. The cobalt solution was added to the ligand solu-
tion. A pink solid formed almost immediately. The mixture
was stirred for additional 3 h. The solid was collected and wash-
ed with dry THF. Drying the complex under reduced pressure
yieldedthe pink solid 12 that wasstored under N2;yield: 145 mg
(68%). Single-crystals suitable for X-ray analysis were ob-
tained by vapor diffusion of Et2O into a CH3CN-solution of
Ligand 1
2,6-Bis(mercaptomethyl)pyridine (10; 2.12 g, 12.4 mmol, 1.0
equiv.)[10] was dissolved in 100 mL dry DMF under N2 and
cooled to 08C. An NaH suspension (60% in mineral oil;
1.04 g, 26.0 mmol, 2.1 equivs.) was added in portions and the
mixture was stirred until the evolution of hydrogen had ceased.
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Novel Pentadentate C2-Symmetrical Chiral Bis(oxazoline) Ligand
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the title compound. For crystal data (CCDC 216870) see Fig-
ure 2 and for a more detailed description the supporting infor-
mation. IR (nujol): n¼3162, 2725, 2672, 1304, 1154, 1122, 1028,
967, 722, 636 cmÀ1; MS (ESI, CH3CN): m/z (%)¼274.1 (100,
[M THF]2þ): anal. calcd. for C33H35CoF6N3O9S4 (918.81): C
43.14, H 3.84, N 4.57, S 13.96; found: C 43.04, H 3.80, N 4.60,
S 13.98.
Obermann, Chem. Ber. 1989, 112, 499 508; H. Nishiya-
ma, H. Sakaguchi, T. Nakamura, M. Horihata, M. Kon-
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Ruthenium Complexes 14 and 15
Under N2 a Schlenk flask was charged with 55.7 mg (111 mmol,
1.0 equiv.) [RuCl2(benzene)]2 (13)[13] and 15 mL anhydrous
EtOH were added. The suspension was treated with 109.1 mg
(222.7 mmol, 2.0 equivs.) of 1 and the mixture was heated to re-
flux for 10 h. After cooling down to ambient temperature the
solvent was removed under vacuum (4 h) and the residue was
taken up in 5 mL dry CH2Cl2. The resulting yellow-brown sol-
ution was passed through a pad of Celite 535 (Fluka) and the
solvent was removed under reduced pressure again to yield
161 mg of a dark-yellow solid, that consisted of two symmetric
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[6] G. R. Newkome, V. K. Gupta, F. R. Fronczek, S. Pappa-
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1
diamagnetic RuII complexes as shown by H NMR. The two
species are presumably [Ru(1)(CH2Cl2)]Cl2 (14)[14] and
[Ru(1)Cl]Cl (15) (ratio 7:3), supported also by ESI-MS: MS
(ESI, CH3CN): m/z (%)¼315.6 (8, [Ru(1)(CH3CN)]2þ),
626.2 (100, [Ru(1)Cl]þ).
[Ru(1)(CH2Cl2)]Cl2 (14): 1H NMR (600 MHz, CDCl3): d¼
8.27 (d, J¼7.3 Hz, 4H), 7.72 7.34 (m, 9H), 5.30 (s, 2H), 4.98
(d, J¼18.0 Hz, 2H), 4.82 4.69 (m, 2H), 4.06 3.94 (m, 2H),
3.76 (d, J¼18.0 Hz, 2H), 3.59 3.37 (m, 4H), 2.51 2.41 (m,
2H).
[Ru(1)Cl]Cl (15): d¼8.00 (d, J¼7.5 Hz, 4 H), 7.72 7.34 (m,
9H), 4.82 4.69 (m, 2H), 4.54 (d, J¼17.0 Hz, 2 H), 4.30 4.20
(m, 2 H), 4.06 3.94 (m, 2 H), 3.28 3.20 (m, 2 H), 3.08 (d, J¼
17.0 Hz, 2H), 1.82 1.72 (m, 2H).
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Acknowledgements
This work was supported by the German Research Foundation
(DFG) as part of the program SP1118 and the Fonds der Chem-
ischen Industrie (Ph.D. fellowship A. K. and Forschungsbei-
hilfe) and through generous chemical gifts from Degussa AG.
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References and Notes
[1] Reviews on the general role of oxazolines: M. Glos, O.
Reiser, Organic Synthesis Highlights, Vol. IV, (Eds.:
H. G. Schmaltz, H. Waldmann), VCH, Weinheim 2000,
[14] In solution the exchange CH2Cl2 solvent is probably fast
(1H NMR: CDCl3, MS: CH3CN).
[15] R. B. Dyer, R. A. Palmer, R. G. Ghirardelli, J. S. Brad-
shaw, B. A. Jones, J. Am. Chem. Soc. 1987, 109, 4780
4786.
[16] Unfortunately we cannot assign the maximum at 229 nm
to 14 and at 209 nm to 15 with certainty, it could be re-
versed. However, we have prepared other dicationic
metal complexes of 1 (e. g., Zn2þ, Cu2þ), which all
show a strong positive CD band around 230 nm.
¡
pp. 17 29; M. Gomez, G. Muller, M. Rocamora, Coord.
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Adv. Synth. Catal. 2004, 346, 737 741
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