[3]Ferrocenophane ligands with an inserted methylene group

Article abstract of DOI:10.1002/ejoc.200800566

New planar chiral [3]ferrocenophane aminosulfane and aminophosphane ligands displayed interesting results in model Pd-catalyzed allylic substitution reactions. The phosphane derivative having a methylene group between the cyclopentadienyl ring and the phosphane group showed enantioselectivities up to 86%ee, whereas the ligand without the methylene group afforded almost racemic allylation products. Analogous sulfane ligands showed the opposite trend. Tentative catalytic complexes were studied by ³¹P NMR spectroscopy and DFT computational methods. Wiley-VCH Verlag GmbH & Co. KGaA, 2008.

Full text of DOI:10.1002/ejoc.200800566

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FULL PAPER
DOI: 10.1002/ejoc.200800566
[3]Ferrocenophane Ligands with an Inserted Methylene Group
[a]
[a]
[b]
ˇ
ˇ
Radovan Sebesta,* Filip Bilcík, and Branislav Horváth
Keywords: Ferrocene / Asymmetric catalysis / Allylic substitution / Phosphane ligands / Palladium
New planar chiral [3]ferrocenophane aminosulfane and ami-
nophosphane ligands displayed interesting results in model
Pd-catalyzed allylic substitution reactions. The phosphane
derivative having a methylene group between the cyclopen-
methylene group afforded almost racemic allylation prod-
ucts. Analogous sulfane ligands showed the opposite trend.
Tentative catalytic complexes were studied by 31P NMR
spectroscopy and DFT computational methods.
tadienyl ring and the phosphane group showed enantio- (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
selectivities up to 86%ee, whereas the ligand without the
Germany, 2008)
phosphanes were used more successfully in Rh-catalyzed
hydrogenations of olefins and ketones with medium-to-high
enantioselectivities.[9] The rigidness of the ligand caused by
the three-atom bridge was suggested as a possible reason
for the lower enantioselectivities relative to those obtained
with unbridged derivatives. These compounds were also
tested in Pd-catalyzed allylic substitution reactions:
enantioselectivities up to 71% were obtained. Interestingly,
P,N ligands of this type were unsuccessful in this allylation
reaction.[10] We recently demonstrated that [5]ferroceno-
phane derivatives are also useful ligands in Pd-catalyzed al-
lylic substitution, Rh-catalyzed hydrogenation, and Cu-cat-
alyzed conjugate addition reactions.[11] Erker described [3]
ferrocenophane diphosphanes with a slightly different
framework. By using these diphosphanes, dimethyl itacon-
ate was hydrogenated in up to 95%ee.[12]
Introduction
Asymmetric catalysis with the use of transition-metal
complexes with chiral ligands is an important strategy for
the preparation of enantiomerically pure or enriched com-
pounds.[1] At present, a great variety of potent catalytic sys-
tems is already known and the number of catalytic enantio-
selective processes is growing rapidly. Nevertheless, synthe-
ses of new and supposedly even more capable ligands con-
tinue to be an area of active research. Particularly, the ratio-
nal design of efficient catalysts still remains one of the chal-
lenges in this field.[2]
Ferrocene derivatives represent a prominent ligand class
due to their interesting stereochemical properties and the
availability of reliable methods for their synthesis and func-
tionalization. Notably, chiral ferrocenylphosphanes proved
to be valuable ligands for various enantioselective transfor-
mations.[3] Sulfur-containing ferrocene derivatives also
found use in asymmetric catalysis.[4] Heterobidentate li-
gands often positively influence catalyzed asymmetric reac-
tions because of additional electronic stereodifferentiation
in the coordination sphere of the transition metal.[5] Chiral
ferrocene heterobidentate P,N ligands proved useful, espe-
cially for Pd-catalyzed allylic substitution reactions.[6]
Among various structural scaffolds of ferrocene ligands,
carbon-bridged ferrocenophanes appear to be interesting
alternatives to unbridged compounds.[7] Weissensteiner
showed the utility of [3]ferrocenophane derivatives in sev-
eral asymmetric reactions. In the Pt-catalyzed carbonylation
of styrene, only small enantioselectivities were obtained.[8]
Several homo- and heteroannularly bridged ferrocenyldi-
Inspired by the work of Weissensteiner,[10] we decided to
prepare [3]ferrocenophane ligands with an additional meth-
ylene group between the Cp ring and the donor atoms. The
idea was that it could lead to a more flexible ligand, with
better catalytic performance, especially in terms of enantio-
selectivity. Here we present the synthesis of such derivatives
along with their testing in model Pd-catalyzed allylic substi-
tution reactions. For evaluation of the influence of the
methylene group on the ligand performance, we also per-
formed allylation reactions with the parent compounds
without the methylene group.
Results and Discussion
Amine 1 was prepared from ferrocene according to de-
scribed methods.[13] Directed ortho-lithiation of amine 1
with BuLi proceeded highly diastereoselectively.[14] We used
(CH2O)n as the electrophile to introduce a hydroxymethyl
group on the cyclopentadienyl (Cp) ring. Column
chromatography afforded pure alcohol 2 in 65% yield with
no minor diastereoisomer detectable by NMR spectroscopy.
[a] Department of Organic Chemistry, Faculty of Natural Sciences,
Comenius University,
Mlynská dolina, 84215 Bratislava, Slovakia
Fax: +421-2-60296337
E-mail: sebesta@fns.uniba.sk
[b] Institute of Chemistry, Faculty of Natural Sciences, Comenius
University,
Mlynská dolina, 84215 Bratislava, Slovakia
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R. Sebesta, F. Bilcˇík, B. Horváth
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To transform the hydroxy group into a phosphanyl or sul-
fanyl group, we modified a method for the preparation of
iodo derivatives.[15] We found that it is possible to perform
the desired nucleophilic substitution reaction in one pot by
adding the appropriate phosphane or sulfane directly into
the reaction mixture together with NaI and Me3SiCl. This
leads, presumably, to the formation of an intermediate iodo
derivative, which then undergoes nucleophilic substitution.
Under these conditions, desired phosphane 3 and sulfane 4
were obtained only in rather moderate yields of 67 and
59%, respectively (Scheme 1). In contrast, this simple one-
pot procedure is very convenient from a practical point of
view.
Scheme 3.
With the use of ligand 5, allylic substitution afforded
product 8 only with 7%ee, which is in agreement with the
literature.[10] By using aminophosphane 3, which contains
an additional methylene group between the Cp and PPh2
groups, we obtained diester 8 under various conditions,
generally in good yields and enantioselectivities (up to
86%ee). Surprisingly, in the sulfur pair, allylation with the
use of ligand 6 proceeded with 68%ee but ligand 4 (with
an inserted methylene group) afforded racemic compound
8. An interesting detail is that with a Pd/ligand 3 ratio of
1:2, we isolated diester 8 with 65%ee (R enantiomer), but
with a Pd/3 ratio of 1:1, we obtained (S)-8. No such effect
was observed with sulfane 6. Results of the catalysis experi-
ments are summarized in Table 1.
Table 1. Pd-catalyzed allylic substitution.
L
Pd/L
Solvent
T
Time
[h]
Yield[a]
[%]
ee[b]
[%]
3
3
3
3
3
3
3
4
4
5
6
6
1:1
1:1
1:1
1:1
1:1
1:2
1:2
1:1
1:2
1:1
1:1
1:2
CH2Cl2
CH2Cl2
toluene
CHCl3
THF
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
r.t.
0
r.t.
r.t.
r.t.
r.t.
0
r.t.
r.t.
r.t.
r.t.
r.t.
3
5
92
94
85
82
87
84
88
–
82 (S)
84 (S)
86 (S)
85 (S)
71 (S)
43 (R)
65 (R)
–
Scheme 1.
22
18
20
18
21
48
48
18
48
48
For the sake of comparison, we also prepared ligands
with PPh2 and SPh groups attached directly to the Cp ring.
Lithiated amine 1 was quenched with Ph2PCl and PhSSPh
as electrophiles to produce ligands 5[14] and 6, respectively
(Scheme 2).
6
0
15
41
48
7 (S)
68 (R)
67 (R)
[a] Isolated yield of purified diester 8. [b] Determined by HPLC on
a Chiralcel AD-H column (hexane/iPrOH, 9:1; 0.75 mLmin–1).
An explanation for the reversal in the enantioselectivity
upon changing the ratio of Pd/3 could be that different cat-
alytically active complexes are formed. It is generally ac-
cepted that nitrogen is a weaker donor than phosphorus.
Therefore, we supposed that by increasing the amount of
ligand to two equivalents relative to the amount of palla-
dium, the corresponding 2:1 complex will be formed. How-
Scheme 2.
With the ligands in hand, we performed the catalysis ex- ever, 31P NMR spectroscopic experiments suggest that the
periments. Asymmetric Pd-catalyzed reactions involving an situation is more complicated. In the 31P NMR spectrum
allylic moiety are important tools for creating new carbon– of the 1:1 Pd complex of ligand 3 (prepared from [Pd(allyl)
carbon bonds, as well as carbon–heteroatom bonds.[16] Cl]2 and free ligand 3 in CDCl3) one major signal at δ =
Therefore, we chose Pd-catalyzed allylic alkylation as a 18.2 ppm was observed, probably corresponding to 1:1 Pd/
benchmark reaction. As the allylic substrate we used 1,3- 3 complex having phosphorus and nitrogen as donor atoms.
diphenyl-1-acetoxypropene (7) and generated the nucleo- If more ligand 3 was added, up to a Pd/3 ratio of 1:2, the
phile in situ from dimethyl malonate with the use of KOAc/ 1:1 complex seemed to be still predominant, but it was in
BSA. The catalyst (2 mol-%) was formed by mixing the ap- equilibrium with other species. The dynamic nature of the
propriate free ligand with [Pd(allyl)Cl]2 before the reaction solution was also noticeable from peak broadening, which
(Scheme 3).
was even more evident upon heating to 40 °C. Certainly,
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[3]Ferrocenophane Ligands with an Inserted Methylene Group
different species were present when the Pd/3 ratio was 1:1 ene group) to 94.4°. This modification resulted in a con-
and 1:2, and a signal for the uncomplexed ligand (– siderably more-active and enantioselective catalyst, proba-
15.1 ppm) was absent in both cases (Figure 1).
bly because of the increased steric influence of the ligand
on the allylic cation. Calculations also predict a trans effect
of phosphorus leading to elongation of the Pd–C1 bond.
Table 2 shows selected geometrical parameters of DFT-op-
timized Pd complexes with ligands 3, 5, and 6. Orientation
of the 1,3-diphenylallyl cation was W-type for ligands 3 and
5, which would lead to the observed (S) enantiomer of com-
pound 8; for ligand 6, the cation was M-type.
Table 2. Geometrical parameters of the calculated Pd–allyl com-
plexes of type 9.
Ligand
N–Pd–P [°]
Pd–C1 [Å]
Pd–C3 [Å]
3
5
6
94.4
89.0
90.8
2.605
2.524
2.424
2.287
2.267
2.288
Figure 1. 31P NMR spectra of Pd/3 complexes.
Conclusions
The structure of the likely intermediate in the reaction,
the Pd complex of ligand 3 and the diphenylpropenylium
cation, was optimized in DGauss engine in the CaChe
package[17] with the DFT method by using the B88-LYP
functional and the DZVP basis set. Attack of the nucleo-
phile at the carbon atom trans to the phosphorus atom[18]
would lead to the observed (S) enantiomer of diester 8. Fig-
ure 2 depicts the DFT-optimized structure of this interme-
diate. In an attempt to understand the influence of the li-
gand structure on the catalyzed reaction, we examined also
similar intermediates with ligands 5 and 6. We were not able
to produce any optimized structure with ligand 4, which
suggests, as also supported by catalytic experiments, that
no productive, catalytically active complex is formed in this
case.
We prepared new [3]ferrocenophane aminophosphane
and aminosulfane derivatives, which showed promising re-
sults in Pd-catalyzed allylic substitution reactions. Our
work shows that subtle changes in the structure of the li-
gand can have a dramatic influence on its catalytic perform-
ance. In the case of aminophosphane 3, which has a methyl-
ene group inserted between the phosphorus atom and the
Cp ring, a highly positive effect was observed; however, in-
sertion of a methylene group into the corresponding amino-
sulfane derivative led to an inactive catalyst. Further work
in this area is in progress in our laboratory.
Experimental Section
General: Homochiral amine (R)-1 and ligand (R,SP)-5 were pre-
pared by following literature procedures.[12,13] All reactions were
carried out under an N2 atmosphere and in the case of work with
organometallic reagents by using standard Schlenk techniques.
Column chromatography was performed on 40/100 mesh silica gel
columns. NMR spectra were recorded with a Varian Gemini 2000
spectrometer at 300 MHz for 1H, 75 MHz for 13C and 121 MHz
for 31P nuclei. Tetramethylsilane was used as an internal standard.
Elemental analyses were performed with a Carlo Erba Instru-
mentazione analyzer. Melting points were measured with a Kofler
hotplate and are uncorrected. Mass spectra were recorded with a
Waters Premium QTOF instrument.
(R,SP)-2-Hydroxymethyl-[1,1Ј-(1-dimethylamino)propanediyl]-
ferrocene (2): To a solution of amine 1 (413 mg, 1.60 mmol) in an-
hydrous Et2O (5 mL) was dropwise added nBuLi (1.6  in hexane,
2.0 mL, 2.40 mmol) at 0 °C. The reaction mixture was stirred for
4 h at room temperature, and an orange precipitate was formed.
Then, solid (CH2O)n (144 mg, 4.80 mmol) was added. The precipi-
tate dissolved, and the reaction mixture was stirred for an ad-
ditional 18 h at room temperature. The reaction was quenched with
saturated NaHCO3 (3 mL). The layers were separated, and the
aqueous phase was extracted with Et2O (3ϫ15 mL). The combined
organic extract was washed with H2O (2 ϫ 15 mL) and dried
Figure 2. DFT optimized structure of 9.
One of the most important parameters for ligands in
transition-metal catalysts is the ligand bite angle.[19] The
additional methylene group in ligand 3 increased the bite
angle (N–Pd–P) from 89.0° in ligand 5 (without the methyl- (Na2SO4). The solvent was evaporated, and the residue was puri-
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R. Sebesta, F. Bilcˇík, B. Horváth
FULL PAPER
fied by column chromatography (SiO2; hexane/EtOAc/Et3N,
1:1:0.1) and crystallized from hexane to give pure alcohol 2
(300 mg, 65 %) as orange crystals. M.p. 78–80 °C. 1H NMR
83.6 (qC), 88.4 (qC), 125.8 (CH-Ph), 128.8 (CH-Ph), 129.4 (CH-
Ph), 138.0 (qC-Ph) ppm. [α]D = +158 (c = 0.5, EtOH). HRMS
(ESI): calcd. for C21H23FeNNaS [M + Na]+ 400.0767; found
(300 MHz, CDCl3): δ = 1.87–2.00 (m, 1 H), 2.16–2.30 (m, 1 H), 400.0625.
2.30–2.35 (s, 6 H, NMe2), 2.42–2.57 (m, 2 H), 2.57–2.66 (ddd, J =
(R,SP)-2-Phenylsulfanyl-[1,1Ј-(1-dimethylamino)propanediyl]ferro-
2.3, 4.3, 14.5 Hz, 1 H), 3.69–3.73 (m, 1 H, Cp), 3.92–3.97 (m, 2 H,
Cp), 3.97–4.00 (m, 1 H, Cp), 4.03–4.07 (m, 2 H, Cp), 4.11 (d, J =
12.7 Hz, 1 H, CH2OH), 4.12–4.16 (m, 1 H, Cp), 4.83 (d, J =
12.7 Hz, 1 H, CH2OH) ppm. 13C NMR (75 MHz, CDCl3): δ = 25.2
(CH2), 37.5 (CH2), 44.9 (CH3), 61.1 (CH2OH), 66.1 (CH), 66.2
(CH), 67.1 (CH), 69.7 (CH), 71.19 (CH), 71.2 (CH), 71.4 (CH),
71.5 (CH), 85.5 (qC), 86.4 (qC), 87.33 (qC) ppm. [α]D = –125 (c =
0.2, MeOH). C16H21FeNO (299.19): calcd. C 64.23, H 7.07, N 4.68;
found C 64.20, H 6.50, N 4.79.
cene (6): To a solution of amine 1 (470 mg, 1.82 mmol) in anhy-
drous Et2O (10 mL) was dropwise added nBuLi (1.6  in hexane,
1.70 mL, 2.73 mmol) at 0 °C. The reaction mixture was stirred for
4 h at room temperature and an orange precipitate was formed.
Then, PhSSPh (720 mg, 3.28 mmol) was added. The precipitate dis-
solved, and the reaction mixture was stirred for an additional 18 h
at room temperature. The reaction was quenched with saturated
NaHCO3 (7 mL). The layers were separated, and aqueous phase
was extracted with Et2O (3ϫ10 mL). The combined organic ex-
tracts were dried with Na2SO4. The solvent was evaporated, and
the residue was purified by column chromatography (SiO2; hexane/
EtOAc/Et3N, 1:1:0.1) and crystallized from EtOH to give com-
pound 6 (320 mg, 46%) as orange crystals. M.p. 140–142 °C. 1H
NMR (300 MHz, CDCl3): δ = 1.90–2.05 (m, 1 H), 2.15–2.20 (s, 6
H, NMe2), 2.20–2.30 (m, 1 H), 2.55–2.65 (ddd, J = 2.7, 3.8,
14.4 Hz, 1 H), 2.80–3.00 (m, 2 H), 3.94–3.98 (m, 1 H, Cp), 4.00–
4.05 (m, 1 H, Cp), 4.10–4.14 (m, 1 H, Cp), 4.17–4.21 (m, 1 H, Cp),
4.21–4.25 (dd, J = 2.5 Hz, 1 H, Cp), 4.28–4.34 (m, 2 H, Cp), 7.03–
7.12 (m, 1 H, Ph), 7.15–7.23 (m, 4 H, Ph) ppm. 13C NMR
(75 MHz, CDCl3): δ = 26.1 (CH2), 38.4 (CH2), 43.9 (CH3), 67.5
(CH), 67.7 (CH), 69.7 (CH), 70.6 (CH), 71.6 (CH), 73.5 (CH), 74.6
(CH), 77.3 (qC), 78.1 (CH), 82.6 (qC), 89.7 (qC), 125.0 (CH-Ph),
127.0 (CH-Ph), 128.4 (CH-Ph), 140.1 (qC-Ph) ppm. [α]D = +219 (c
= 0.5, MeOH). HRMS (ESI): calcd. for C20H19FeS (molecular ion
was not observed because of rapid elimination of the NMe2 group
leading to a stable α-carbocation) 347.0526; found 347.0341.
(R,SP)-2-Diphenylphosphanomethyl-[1,1Ј-(1-dimethylamino)pro-
panediyl]ferrocene (3): To a solution of hydroxymethyl derivative 2
(272 mg, 0.94 mmol) and NaI (282 mg, 1.88 mmol) in anhydrous
CH3CN (5 mL) was dropwise added Me3SiCl (324 µL, 2.55 mmol).
A yellow precipitate was formed, the mixture was stirred for 5 min
at room temperature, and then HPPh2 (166 µL, 1.04 mmol) was
added. The precipitate dissolved and the solution became orange.
The reaction mixture was stirred for an additional 2 h at room tem-
perature. The resulting solution was diluted with CH2Cl2 (10 mL)
and extracted with H2O (3ϫ8 mL) and dried (Na2SO4). The sol-
vent was evaporated, and the residue was purified by column
chromatography under a N2 atmosphere (SiO2; hexane/EtOAc/
Et3N, 1:1:0.1) to give pure phosphane 3 (293 mg, 67 %) as an
1
orange oil. H NMR (300 MHz, CDCl3): δ = 1.84–1.98 (m, 1 H),
2.18–2.34 (m, 1 H), 2.23–2.27 (s, 6 H, NMe2), 2.38–2.52 (m, 2 H),
2.52–2.62 (m, 1 H), 3.26 (dd, J = 2.0, 14.7 Hz, 1 H), 3.48 (dd, J =
1.7, 14.7 Hz, 1 H), 3.68–3.74 (m, 2 H, Cp), 3.80–3.88 (m, 3 H, Cp),
3.98–4.02 (m, 1 H, Cp), 4.08–4.13 (m, 1 H, Cp) ppm. 13C NMR
(75 MHz, CDCl3): δ = 25.7 (CH2), 29.05 (d, J = 15.1 Hz, CH2P),
38.8 (CH2), 46.0 (CH3), 66.6 (CH), 66.8 (CH), 68.2 (CH), 69.9
(CH), 70.9 (CH), 71.2 (d, J = 10.0 Hz, CH), 71.3 (CH), 73.4 (d, J
= 3.2 Hz, CH), 83.6 (qC), 84.4 (d, J = 17.3 Hz, qC), 88.7 (qC),
128.3 (CH-Ph), 128.5 (d, J = 6.0 Hz, CH-Ph), 128.6 (d, J = 6.9 Hz,
CH-Ph), 128.9 (CH-Ph), 132.6 (d, J = 18.0 Hz, CH-Ph), 133.8 (d,
J = 20.0 Hz, CH-Ph), 139.6 (d, J = 15.8 Hz, qC-Ph), 140.1 (d, J =
16.3 Hz, qC-Ph) ppm. 31P NMR (121.5 MHz, CDCl3): δ =
–15.1 ppm. [α]D = +77 (c = 0.5, MeOH). HRMS (ESI): calcd. for
C28H30NNaPFe [M + Na]+ 490.1331; found 490.1061.
General Procedure for Allylic Substitution: Ligand (0.02 mmol) and
[Pd(allyl)Cl]2 (3.7 mg, 0.01 mmol) were dissolved in CH2Cl2
(3 mL), and the resulting solution was stirred for 10 min at room
temperature. This solution was added to acetate 7 (252 mg,
1.0 mmol) in CH2Cl2 (2 mL). Then, bis(trimethylsilyl)acetamide
(0.49 mL, 407 mg, 2.0 mmol), dimethyl malonate (230 µL, 264 mg,
2.0 mmol) and KOAc (5 mg, 0.05 mmol) were added, and the re-
sulting mixture was stirred. The reaction was monitored by TLC
and stopped when no starting material was detected or after 48 h.
The mixture was then diluted with CH2Cl2 (10 mL) and extracted
with saturated aqueous NH4Cl (3ϫ10 mL). The aqueous phase
was extracted with CH2Cl2 (2ϫ10 mL), and the combined organic
extracts were dried (Na2SO4) and concentrated. The crude product
was purified by column chromatography (SiO2; hexane/EtOAc,
9:1). Enantiomeric excesses were determined by HPLC on a Chi-
ralcel AD-H column. HPLC (hexane/iPrOH, 9:1; 0.75 mLmin–1):
tR = 16.6 [(R)-8], 23.7 [(S)-8] min.
(R,SP)-2-(Phenylsulfanyl)methyl-[1,1Ј-(1-dimethylamino)propane-
diyl]ferrocene (4): To a solution of hydroxymethyl derivative 2
(150 mg, 0.52 mmol) and NaI (156 mg, 1.04 mmol) in anhydrous
CH3CN (7 mL) was dropwise added Me3SiCl (179 µL, 1.40 mmol).
After the addition, a yellow precipitate was formed. The mixture
was stirred for 5 min at room temperature and then HSPh (166 µL,
1.04 mmol) was added. The precipitate dissolved, and solution be-
came darker. The reaction mixture was stirred for 4 h at room tem-
perature. The solution was diluted with CH2Cl2 (10 mL) and ex-
tracted with H2O (3ϫ8 mL) and dried (Na2SO4). The solvent was
evaporated, and the residue was purified by column chromatog-
raphy under a N2 atmosphere (SiO2; hexane/EtOAc/Et3N, 1:1:0.1)
to give sulfane 4 (120 mg, 59 %) as an orange oil. 1H NMR
(300 MHz, CDCl3): δ = 1.84–1.96 (m, 1 H), 2.08–2.24 (m, 1 H),
2.24–2.29 (s, 6 H, NMe2), 2.40–2.51 (m, 2 H), 2.51–2.61 (m, 1 H),
3.88–3.94 (m, 3 H, Cp), 3.98 (t, J = 2.5 Hz, 1 H, Cp), 4.03–4.07
(m, 1 H, Cp), 4.11 (d, J = 1.3 Hz, 2 H), 4.14–4.19 (m, 2 H, Cp),
7.13–7.21 (m, 1 H, Ph), 7.23–7.32 (m, 2 H, Ph), 7.37–7.43 (m, 2 H,
Ph) ppm. 13C NMR (75 MHz, CDCl3): δ = 25.3 (CH2), 34.0 (CH2),
38.9 (CH2), 45.6 (CH3), 67.08 (CH), 67.15 (CH), 67.47 (CH), 70.0
(CH), 70.7 (CH), 71.1 (CH), 71.51 (CH), 71.52 (CH), 83.3 (qC),
Acknowledgments
ˇ
We thank Prof. Stefan Toma for valuable discussions. Financial
support from the Ministry of Education of Slovak Republic, Grant
No. MVTS-COST/UK/07, is gratefully acknowledged. This work
was performed under auspices of COST D40 programme. NMR
measurements were provided by the Slovak State Programme Pro-
ject No. 2003SP200280203.
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