High yield preparation of a novel tetrakis[ruthenium tris(bipyridine)]calix[6]arene derivative with good diastereomeric purity

Article abstract of DOI:10.1016/S0957-4166(98)00433-9

In order to develop methodology for the selective preparation of complex molecular structures with potential photochemical or electron transfer functions, the diastereoselective synthesis of multiple ruthenium tris(bipyridine) complexes tethered to a cent

Full text of DOI:10.1016/S0957-4166(98)00433-9

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 9 (1998) 4089–4097
High yield preparation of a novel tetrakis[ruthenium
tris(bipyridine)]calix[6]arene derivative with good diastereomeric
purity
Dusan Hesek,a Yoshihisa Inoue,a,∗ Simon R. L. Everitt,a,† Mieko Kunieda,a Hitoshi Ishida a
and Michael G. B. Drew b
aInoue Photochirogenesis Project, ERATO, JST, 4-6-3 Kamishinden, Toyonaka 565-0085, Japan
bDepartment of Chemistry, The University of Reading, Whiteknights, Reading RG6 6AD, UK
Received 12 October 1998; accepted 26 October 1998
Abstract
In order to develop methodology for the selective preparation of complex molecular structures with potential
photochemical or electron transfer functions, the diastereoselective synthesis of multiple ruthenium tris(bipyridine)
complexes tethered to a central calix[6]arene core was investigated. Applying recently developed methodology, the
resolved precursor cis-∆-[Ru(bpy)2(DMSO)Cl]PF6 (98.6% ee) was efficiently reacted with a novel calix[6]arene
derivative, to give the tetrakis[Ru(bpy)2(bpy0)]calix[6]arene derivative (9) with almost complete retention of
absolute stereochemistry at each of the four metal centres, as seen by the unusually strong molar circular dichroism
(CD) spectrum. The synthesis of racemic 9 was also carried out, and demonstrated to have an inactive CD spectrum.
© 1998 Elsevier Science Ltd. All rights reserved.
1. Introduction
The redox and photophysical properties of ruthenium tris(bipyridine) complexes are well
established,1,2 and their use as photochemical molecular devices has been the subject of extensive
investigation.3–5 As the molecular architectures of these devices continue to increase in complexity,
the photochemist must seek out new procedures that allow the controlled preparation of more and
more complicated molecules in order to supply material for further study in this area, implying a need
for increased selectivity. One fundamental area of control which has been only partially addressed is
the low degree of stereocontrol inherent in the synthesis of ruthenium bis and tris(bipyridines). Von
Zelewsky et al.6,7 have made advances into this area through the use of the ‘chirogen’ ligand which
∗
†
Corresponding author.
E-mail: everitt@inoue.jst.go.jp
0957-4166/98/$ - see front matter © 1998 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(98)00433-9

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enables good diastereoselectivity to be attained. However, the products that arise must contain the
chirogen ligand, which limits further modification of the structure. It is also of fundamental importance
to note that the resolution of multiple chiral centre compounds is not readily achieved (e.g. chiral
HPLC, co-crystallization with optically active compounds etc.), and for higher molecular weight
structures, enantiomeric purity is practically impossible. A generally applicable, highly stereoselective
methodology is therefore required. As part of our ongoing interest in the photochemical processes of
ruthenium poly(bipyridine) complexes, we have recently developed a new synthesis, starting from a
ruthenium bis(bipyridine) sulfoxide precursor.8 If the sulfoxide complex is resolved into its enantiomers
prior to reaction with a bipyridine nucleophile (using chiral HPLC techniques, see experimental section
for details), almost complete retention of the absolute stereochemistry at the metal centre is observed.
Thus, the reaction of cis-∆-[Ru(bpy)2(DMSO)Cl]PF6 (1) (>99% ee) with 4,40-dimethyl-2,20-bipyridine
proceeds in 97% yield, affording the ∆-isomer of the ruthenium tris(bipyridine) product (3) with an ee
of 96.8% (see Scheme 1).8
Scheme 1. Synthesis of an enantiopure ruthenium tris(bipyridine) complex (3), starting from resolved ruthenium bis(bipyridine)
sulfoxide (1). Complete retention of the absolute configuration at the metal centre was observed
The high yield and selectivity afforded by this reaction seemed to us ideally suited for use in the
preparation of more challenging structures which may act as photochemical devices, such as stereoregular
polymers, or homochiral dendrimers. To the best of our knowledge, systems of this type which contain
chiral ruthenium tris(bipyridine) building blocks have not yet been prepared. Racemic synthesis, followed
by a resolution procedure is also non-viable for multi-centre molecules, as a result of the very difficult
separation problems that would arise. In order to model the complexity of these systems, we chose
to prepare the calix[6]arene (9), selectively derivatized on the lower rim, which has similar structural
features to the above-mentioned classes of compound, i.e. a moderate level of flexibility and steric
hindrance, and multiple reaction sites. The products of this reaction would enable us to evaluate the
asymmetric formation of ruthenium tris(bipyridine) complexes in a multi-reaction process, and may also
afford a product which is capable of acting as a chiral host for small neutral organic compounds, prior
to carrying out photochemical processes on them. Indeed, simpler calix[4]arenes with bipyridine units
attached to the lower rim have already been used to incorporate lanthanide ions, and their luminescence
properties have been described,9 as well as a luminescent pH sensor based on a p-tert-butylcalix[4]arene
linked to one ruthenium tris(bipryridine).10
2. Results
The synthesis of the tetrakis[Ru(bpy)2(bpy0)]calix[6]arene derivative (9) is shown in Scheme 2,
which includes four novel calix[6]arene derivatives (5, 6, 8, and 9). The conversion of the previously
reported 39,42-bis(benzyloxy)calix[6]arene (4)11 to the tetracyano product (5) was carried out with
bromoacetonitrile in DMF at 80°C in 89% yield. This was subsequently reduced with diborane:THF

D. Hesek et al. / Tetrahedron: Asymmetry 9 (1998) 4089–4097
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complex, affording the tetraamine (6) in 68% yield, which was then quenched with four equivalents
of 2,20-bipyridine-4-carbonyl chloride (7) in the presence of triethylamine, affording the tetrabipyridine
calix[6]arene (8) in 55% yield. Reaction of this derivative with cis-∆-[Ru(bpy)2(DMSO)Cl]PF6 (98.6%
ee) (added in three equal portions in DMF at 0, 3 and 6 h) in EtOH:CH3CO2H at 75°C for 8 h in dark
conditions led to the formation of 9 in 55% yield. This reaction was assumed to proceed with almost
complete stereoretention at the metal centres, based on the subsequent observations.
Scheme 2. Synthesis of 9
1
The formation of the highly symmetrical tetraruthenium complex was confirmed by H NMR, and
assignment of the signals to the individual protons of 9 was achieved by analysis of the COSY spectra (see
Fig. 1). A singlet for the two benzylic CH2 of the lower rim benzyloxy group is observed (4.6 ppm), and

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1
Figure 1. 400 MHz H NMR (CD3CN) and H–H COSY spectra of 9. The numbers enclosed by square brackets indicate the
chemical shift (ppm) for the adjacent protons
a 4:2 ratio for the CH2 groups that link the arene groups in the ring (3.9 ppm and 3.8 ppm, respectively) is
also observed. The flexibility of the calixarene ring is apparent from the broadness of these two signals.
Indeed, variable temperature measurements for 9 showed a broadening of the peaks at 3.9 ppm and 3.8
ppm, although even at −40°C the conformation had not been completely ‘frozen’, demonstrating the
conformational freedom that the calix[6]arene ring system possesses. All of the expected resonances
were seen in the 13C NMR spectrum of 9, and the full assignments of these spectra are given in the
experimental section.
The enantiomeric purity of the starting material 1 was determined by chiral HPLC analysis and CD
spectroscopy, and the stereochemistry of the product assigned from the circular dichroism (CD) spectrum

D. Hesek et al. / Tetrahedron: Asymmetry 9 (1998) 4089–4097
4093
Figure 2. CD and UV/vis spectra for the resolved ruthenium bis(bipyridine) sulfoxide, 1, for 9 and for enantiopure 3. The CD
spectrum of racemic 9, as well as the tris- and bis-substituted products are also included for comparison purposes. Solutions of
concentration 9×10−5 mol l−1, 1×10−6 mol l−1 and 3×10−5 mol l−1 for 1, 9 and 3, respectively, were used when recording the
CD spectra
(see Fig. 2). The CD spectrum shows unusually high activity for 9, which has an extrema of ca. −900 dm3
mol−1 cm−1 for the LC band centred around 280 nm, compared to the LC band (280 nm) of the starting
sulfoxide complex 1. This CD spectrum was assigned on the basis of an unambiguous assignment of the
CD spectrum of 3, for which both the CD spectrum and single crystal X-ray analysis were available.8
Although the tetrasubstituted products could be isolated by normal HPLC techniques, attempts to resolve
the ∆,∆,∆,∆-product using chiral HPLC were completely unsuccessful. This may have been complicated
by the conformational freedom of 9 and the by-products, which caused excessive broadening of the peaks
under a variety of conditions. The synthesis of racemic 9 afforded a product which had similar 1H NMR
to the enantiopure material, but its CD spectrum was found to be completely inactive. The UV/vis of 9,
and the CD and UV/vis spectra of the bis- and tris-products are also shown in Fig. 2 for comparative
purposes.
It is noteworthy that the enantioselective preparation of this complex avoided the formation of a very
complex mixture of diastereomers that would prove very difficult, if not impossible, to separate. This
was demonstrated when the synthesis of racemic 9 was carried out, starting from 8. This precursor was
reacted with either racemic Ru(bpy)2Cl2, or racemic cis-[Ru(bpy)2(DMSO)Cl]PF6, and in both cases,
the CD spectra of the isolated tetraruthenium complexes showed no activity.
The evolution of 9 was also followed by non-chiral HPLC, and using this technique it was possible to
observe the formation of the di-, tri- and tetra-substituted products (see Table 1). From these data we can
see that the di-substituted product could be isolated as the major product during the initial stages of the
reaction.
3. Discussion and conclusions
The preparation of multi-chiral centre high molecular weight compounds (e.g. polymers, dendrimers,
etc.) affords special resolution problems, which are not easily overcome. Indeed, the simplest method
involves the preparation of an enantiomerically pure product through the use of a completely diastereo-
selective process, thus removing the need for any resolution processes! In reality, attempts to achieve
high enantiomeric purity products should rely on the highest possible level of stereocontrol that can

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Table 1
The relative ratios of the tetrakis-, tris- and bis-derivatives, monitored by HPLC. Retention times using
a Mightysil RP-18 column (33:67, 0.1 M NaPF6 aq.:CH3CN, with a flow rate of 0.5 ml min−1, at room
temperature). Products were detected by UV detector at 292 nm. Retention times are 11.4, 17.94 and
19.92 min, respectively
be achieved. The reaction of enantiopure ∆-[Ru(bpy)2(DMSO)Cl]PF6 with a bipyridine nucleophile
has been found to proceed in high yield, with ca. 97% retention of the absolute chirality at the metal
centre, making it a good candidate for the preparation of high enantiomeric purity poly chiral centre
products. If we assume that the bipyridine units in the tetrakis(bipyridine) compound 8 can be treated
independently, and that the formation of one ruthenium complex does not interfere with subsequent
chiral centres (which is possible, due to the large, flexible nature of 8), and using a stereoretention of 97%
(based on the single reaction, see Scheme 1), then using a simple probability calculation, we can predict
that the desired ∆,∆,∆,∆-isomer will form in 88.5% [(0.97)4]. Products containing only a single error
will account for 10.9% [4×{(0.97)3×(0.03)}] of the total yield of the tetra-substituted product. Thus,
the fully ∆-product and the ∆,∆,∆,Λ-product account for 99.4% of the total tetra-substituted product.
Although such a simple probability treatment cannot account for phenomenon such as cooperativity, etc.,
we can predict that our product contains ca. 90% of the desired product, and the only other contaminant
of any significance will contain one erroneous stereocentre. This is in agreement with the very large
extrema observed in the CD spectrum of 9. Of course, such a statistical treatment can be extended to
reactions which form multiple chiral centres. In general, an n-centre reaction, reacting with N% retention
(in a non-cooperative fashion) affords the n-∆ product in [100×{(N/100)n}]% yield and the (n−1)∆,Λ
product in [100×n×{(N/100)n−1×((100−N)/100)}]% yield (in both cases, as a percentage of the total
n-substituted product isolated). In order to achieve a good enantiomeric purity, the yield and N must
be high. In our synthesis of 9, the stereoretention upon the nucleophilic reaction of a bipyridine with
cis-∆-[Ru(bpy)2(DMSO)Cl]PF6 has been extended to encompass a ‘multicentre’ reaction, and has been
shown to afford the desired product in good yield, with an acceptable level of diastereoselectivity, without
resorting to difficult resolution procedures. A conventional synthesis was also applied to the preparation
of racemic 9, and this gave a complex mixture of diastereomers, highlighting the advantages of the direct
diastereoselective synthesis. The investigation of further reactions of other bipyridine nucleophiles with
enantiomerically enriched ruthenium bis(bipyridine) sulfoxide complexes will be the subject of further
reports from our laboratory.
4. Experimental
The reagents used in these studies were reagent grade or better, and were used without further
purification. Solvents were purified according to published methods. Dimethyl sulfoxide (DMSO) were
dried overnight over molecular sieves 4 Å, distilled and stored under argon.

D. Hesek et al. / Tetrahedron: Asymmetry 9 (1998) 4089–4097
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Circular dichroism spectroscopy was performed on a JASCO J-720WI spectropolarimeter at 25°C in
acetonitrile, or in the HPLC eluent (NaPF6 aq.:CH3CN mixed solvent). Concentrations of the solutions
were determined by UV/vis measurements. Resolution of 1 was performed on a preparative scale using a
recycling liquid chromatograph JAI LC-908 equipped with a preparative chiral column, Daicel Chiralcel
OD-R (20 mm×250 mm). An aqueous solution of NaPF6 (0.1 M) and acetonitrile was used as eluent, with
a flow rate of 3 ml min−1. The chromatograph was monitored at 292 nm with a UV detector. Monitoring
the products of the synthetic reactions, and the degree of substitution of the separated fractions was
performed using an analytical HPLC system (JASCO GULLIVER series) equipped with a Mightysil RP-
18 column, with an HPLC pump PU-980, a 3-line Degasser DG-980-50, a UV/vis detector UV-970, and
a column oven 860-CO, with a flow rate of 0.5 ml min−1, at room temperature. Products were detected
by UV detector at 292 nm. The eluent flow rate was 0.5 ml min−1 (33:67, 0.1 M NaPF6 aq.:CH3CN) and
the chromatograph was monitored at 292 nm and recorded with a JASCO integrator 807-IT. Analytical
thin-layer chromatography was performed with plastic backed silica sheets (Merck Kieselgel 60 F254).
1H and 13C NMR spectra were recorded on a JEOL JNM-EX 400 spectrometer, operating at 399.65 and
100.40 MHz, respectively. Chemical shifts are reported relative to either the solvent reference or internal
1
standard tetramethylsilane (TMS, 0.00 ppm) for H NMR and solvent reference for 13C NMR. Pulsed
field gradient experiments were used for H–H correlation experiments, applying the VCOSYNH pulse
sequence.
4.1. 39,42-Dibenzyloxy-37,38,40,41-tetrakis(cyanomethyloxy)calix[6]arene, 5
Potassium carbonate (1.2 g) and 411 (1.6 g, 2 mmol) were stirred in a round bottom flask equipped
with a reflux condenser in DMF (50 ml) under nitrogen for 10 min at 80°C. Bromoacetonitrile (0.56 ml,
8.8 mmol) was then slowly added dropwise to this solution over 10 min, and the reaction mixture was
stirred for a further 2 h at 80°C. The DMF was removed under reduced pressure to give a yellow solid.
This was dissolved in CH2Cl2 (80.0 ml), which was washed with 1 M HCl and then with water (2×20
1
ml), before being concentrated and dried in vacuo. Yield of 5: 1.7 g, 89%. H NMR (CDCl3) 7.36 (bs,
10H, Ar–H), 6.9–6.7 (m, 18H, Ar–H), 4.82 (s, 4H, CH2), 4.06 (s, 8H, CH2), 3.96 (bs, 12H, CH2). 13C
NMR (CDCl3) 154.8, 153.6, 137.1, 134.2, 133.6, 129.9, 129.3, 128.6, 128.4, 128.1, 125.3, 124.1, 115.5,
75.6, 57.3, 31.1, 30.9. Anal. calcd C65H56N4O6: C, 78.92; H, 5.71; N, 5.66. Found: C, 78.71; H, 5.55; N,
5.65.
4.2. 39,42-Dibenzyloxy-37,38,40,41-tetrakis(aminoethyloxy)calix[6]arene, 6
A mixture of 5 (0.97 g, 1 mmol) and BH3–THF (12 ml; 1 M solution) in dehydrated THF (50 ml)
was stirred overnight at room temperature under a nitrogen atmosphere. The reaction mixture was then
quenched with 1 M HCl and the solution evaporated to dryness under reduced pressure. The residue was
dissolved in water, and 1 M aq. NaOH was used to raise the solution to pH 8. This was then extracted with
CH2Cl2, which was separated, washed three times with water and then dried over anhydrous MgSO4.
The CH2Cl2 was removed under reduced pressure, and the residue was used in the next reaction step
1
without further purification. Yield 68%. H NMR (CD3CO2D) 7.8–6.8 (m, 28H, Ar–H), 5.18 (bs, 4H,
CH2), 4.4–3.2 (bm, 28H, CH2). 13C NMR (CD3CO2D) 155.7, 138.9, 136.6, 136.4, 136.3, 136.1, 135.9,
131.5–130.0, 129.9–126.5, 70.7, 41.7, 32.6–32.1. Anal. calcd C65H72N4O6·DMF: C, 75.74; H, 7.38; N,
6.49. Found: C, 75.06; H, 6.44; N, 5.11.

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D. Hesek et al. / Tetrahedron: Asymmetry 9 (1998) 4089–4097
4.3. 39,42-Dibenzyloxy-37,38,40,41-tetrakis(2,20 -bypyridine-5-carboxamidoethyloxy)calix[6]arene, 8
2,20-Bipyridine-5-carbonyl chloride 7 (1.0 g, 4.6 mmol) and 6 (1.0 g, 1 mmol) were added to a stirred
solution of triethylamine (1.5 ml) in CH2Cl2 (60 ml) under a nitrogen atmosphere, and the mixture was
then heated to reflux. After 3 h, TLC analysis (silica gel; CHCl3:MeOH, 8:1, v/v) revealed that all of
6 had been consumed. The mixture was cooled to room temperature, and all the volatile components
removed in vacuo. The resulting material was then dissolved in CH2Cl2 which was washed with water.
The organic layer was then isolated, and the solvent evaporated. The crude sample was purified by column
chromatography (silica gel; CHCl3:MeOH, 8:1, v/v) which afforded 8 as a white powder (55% yield).
1H NMR (CD3CN) 8.66 (d, J=0.8 Hz, 4H, PyH-3), 8.40 (m, 12H, PyH-6, PyH-60, NH), 8.13 (d, J=8
Hz, 4H, PyH-30), 7.72 (t, 4H, PyH-40), 7.58 (d, J=5.6 Hz, 4H, PyH-5), 7.40 (m, 4H, PyH-50), 7.20–6.60
(m, 28H, Ar–H,), 4.86 (s, 4H, CH2), 3.90–3.80 (m, 12H, CH2), 3.60 (s, 8H, CH2), 2.02 (bs, 4H, CH2).
13C NMR (CD3CN) 167.8, 157.2, 155.6, 154.2, 150.3, 150.2, 149.7, 149.7, 143.2, 138.4, 137.5, 135.4,
135.2, 134.8, 129.2, 129.1, 128.8, 128.6, 128.4, 125.2, 125.0, 124.7, 121.9, 121.8, 121.4, 118.9, 75.6,
70.8, 40.7, 31.1, 30.7. Anal. calcd C108H92N12O10·H2O: C, 74.72; H, 5.46; N, 9.86. Found: C, 74.75; H,
5.57; N, 9.00.
4.4. 39,42-Dibenzyloxy-37,38,40,41-tetrakis(∆-[Ru(bpy)2(2,20-bypridine-5-carboxamidoethyloxy)]-
calix[6]arene octakis(hexafluorophosphate), 9
The calix[6]arene derivative 8 (0.17 g, 0.1 mmol) was suspended in a mixture of ethanol (40
ml) and glacial acetic acid (10 ml), and this mixture was heated to reflux under nitrogen. cis-∆-
[Ru(bpy)2(DMSO)Cl]PF6, 1 (225 mg, 0.36 mmol), dissolved in a minimum amount of DMF, was added
in three separate portions at 0, 3 and 6 h. The reaction was allowed to proceed for a total of 8 h, in dark
conditions. After cooling to room temperature, the dark red solution was filtered through Celite® and
the solvents removed under reduced pressure. The dark red residue was then dissolved in CH3CN and
chromatography was carried out on a Sephadex® LH-20 column (with CH3CN as eluent), affording 9 as
a glassy red solid in 0.22 g (50% yield), contaminated with a small amount of incompletely derivitized
products. The tetrakis isomer was separated by preparative HPLC to give pure 9 for analytical purposes.
1H NMR (CD3CN) 8.70 (s, 4H, PyH-3), 8.53 (dd, J=8.4 Hz, 8H, PyH-3), 8.46 (dd, J=8.4 Hz, 8H, PyH-
3), 8.28 (d, J=8.4 Hz, 4H, PyH-3), 8.11–7.83 (m, J=1.6 Hz, J=7.6 Hz, 24H, PyH-4), 7.84 (d, J=5.6 Hz,
4H, PyH-5), 7.76–7.63 (m, 28H, PyH-5, PyH-6, NH), 7.46–7.25 (m, 20H, PyH-6), 7.20 (m, 4H, Ar–H),
6.90–6.60 (m, 24H, Ar–H), 4.59 (s, 4H, CH2), 3.94 (s, 4H, CH2), 3.83 (s, 8H, CH2), 3.60 (s, 8H, CH2),
3.20 (s, 8H, CH2). 13C NMR (CD3CN) 164.8, 158.7, 157.8, 157.7, 157.2, 153.3, 152.9, 152.6, 152.5,
143.1, 138.9, 138.8, 138.4, 135.6, 135.5, 129.7, 129.6, 129.4, 129.1, 129.0, 128.9, 128.7, 128.6, 128.5,
125.8, 125.5, 125.3, 125.2, 125.1, 122.4, 72.0, 41.3, 30.9, 30.8. Racemic 9 was prepared according to the
same procedure, using either racemic 1, or Ru(bpy)2Cl2 in the place of cis-∆-[Ru(bpy)2(DMSO)Cl]PF6.
Anal. calcd C188H160F48N28O10P8Ru4: C, 49.79; H, 3.56; N, 8.65. Found: C, 48.91; H, 3.50; N, 8.25.
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