Efficient amine end-functionalization of living ring-opening metathesis polymers

Article abstract of DOI:10.1021/ma300602p

An efficient strategy for the synthesis of monoamine end-functionalized living polymers using ring-opening metathesis polymerization with ruthenium initiators is reported. A new end-capping agent for this purpose was synthesized, and its efficiency for end-functionalization was evaluated using two common ruthenium-based initiators. Finally, terminal cross-metathesis was also explored as another alternative toward the synthesis of amine end-functionalized polymers, and the comparison between the two techniques is presented.

Full text of DOI:10.1021/ma300602p

Article
pubs.acs.org/Macromolecules
Efficient Amine End-Functionalization of Living Ring-Opening
Metathesis Polymers
Amit A. Nagarkar, Aurelien Crochet, Katharina M. Fromm, and Andreas F. M. Kilbinger*
́
Department of Chemistry, University of Fribourg, Chemin du Musee 9, CH-1700 Fribourg, Switzerland
S
* Supporting Information
ABSTRACT: An efficient strategy for the synthesis of
monoamine end-functionalized living polymers using ring-
opening metathesis polymerization with ruthenium initiators is
reported. A new end-capping agent for this purpose was
synthesized, and its efficiency for end-functionalization was
evaluated using two common ruthenium-based initiators.
Finally, terminal cross-metathesis was also explored as another
alternative toward the synthesis of amine end-functionalized
polymers, and the comparison between the two techniques is presented.
Many strategies have been developed for the synthesis of
INTRODUCTION
■
monotelechelic functional polymers by ROMP. Prefunctional-
ized initiators have been used to synthesize such polymers,9 but
the synthesis of a new catalyst for every new functionality is
difficult and the yields are often low. End-capping agents based
on custom terminating agents,9d,10 acrylates,11 vinyl carbonates,
and lactones12 have also been used to synthesize mono-end-
functionalized polymers from ring-opening metathesis polymer-
ization. Recently, Hilf et al. reported the “sacrificial synthesis”
strategy for the synthesis of mono-end-functionalized living
ring-opening metathesis polymers.13 The strategy involves the
polymerization of a new monomer onto the end of a first
polymer block, thus leading to a diblock copolymer. This new
second block is then chemically cleaved to yield the desired
functionality at the end of the polymer chain. Via this method a
variety of mono-end-functional polymers carrying alcohol, thiol,
or carboxylic acid end groups are readily accessible. End-
functionalization strategies for a number of different functional
groups for living ROMP were recently reviewed.14,15
Ring-opening metathesis polymerization (ROMP) has very
quickly become one of the methods of choice of chemists to
synthesize highly functional low dispersity polymers with
control over molecular weight and architecture.1 The air and
moisture sensitive molybdenum carbene catalysts typically used
for olefin metathesis are sensitive to many protic and polar
functional groups such as for example aldehydes.2 Polymers
prepared with molybdenum and tungsten carbene initiators can
therefore be end-functionalized in a relatively straightforward
manner using suitably substituted aldehydes.3−7 However, due
to the restricted functional group tolerance of these initiators,
many functional groups cannot be present in the polymer
structure. In contrast, the Grubbs type ruthenium carbene
catalysts (Figure 1) are stable toward many functional groups.
Amine end-functionalization is more difficult due to the
ability of the free amino groups to coordinate to the ruthenium
catalyst.16 Protected amine chain transfer agents have been used
to overcome this difficulty.17 However, very long reaction times
are sometimes required which could potentially give rise to
chain transfer to the growing polymer chain itself depending on
the monomer structure polymerized.
A new cyclic olefin, 2-phenoxy-2,3,4,7-tetrahydro-1H-1,3,2-
diazaphosphepine 2-oxide (1), was synthesized to overcome
this difficulty. In this compound, the phosphorus bridge plays
two roles: it forms a closed ring, which allows its use as a
sacrificial monomer in ROMP, and simultaneously acts as a
protecting agent for the amino groups.18 Single crystal X-ray
Figure 1. Ruthenium initiators used for ring-opening metathesis
polymerization.
Polymers prepared with these ruthenium initiators can thus
tolerate the presence of many functional groups, but at the
same time methods for a functional termination are scarce for
the same reason.8 Ethyl vinyl ether termination is one of the
most widely used nonfunctional termination methods for
ROMP transferring a methylene unit onto the polymer chain
end.
Received: March 25, 2012
Revised: May 10, 2012
Published: May 22, 2012
© 2012 American Chemical Society
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101.655(4) ; V = 2311.0(2) Å3, Z = 4, ρcalcd = 1.300 mg m−3, F(000) =
944, T = 200 K, λ = 1.541 86 Å, μ(Cu Kα), 4.09° < θ < 64.98°, 12 335
reflections of which 3544 unique and 3544 observed, 275 parameters
refined, GOOF (on F2) = 1.084, R1= ∑|Fo − Fc|/∑Fo = 0.0489, wR2
= 0.1301 for I > 2σ(I).
diffraction confirmed the synthesis of the new compound
(Figure 2, bottom).
Typical Polymerization Procedure in an NMR Tube. The
ruthenium initiator (either G1 or G3) was taken in an NMR tube and
purged by continuous flow of argon for 15 min. Degassed
dichloromethane-d2 was added to the tube under argon and shaken
until all the initiator was dissolved. The monomer MNI was purged
under argon in a separate vial. Degassed dichloromethane-d2 was
added to the monomer under a flow of argon via a syringe. This
solution was immediately transferred to the NMR tube containing the
initiator. The NMR tube was capped and inverted once to ensure
efficient mixing. The tube was kept standing for some time (typically
45 min) to ensure complete monomer consumption. Compound 1
was purged with argon for 30 min, dissolved in degassed dichloro-
methane-d2, and added to the NMR tube for end-functionalization.
The NMR tube was inverted to ensure complete mixing and
immediately transferred into the NMR spectrometer for subsequent
recording of NMR spectra.
Typical Procedure for Ring-Opening Metathesis Polymer-
ization. The catalyst (G1 or G3), monomer (MNI), and sacrificial
monomer 1 were taken in separate Schlenk flasks and purged free of
oxygen by three cycles of alternating high vacuum and argon
atmosphere. Dry dichloromethane was taken in a separate Schlenk
flask and degassed by three consecutive freeze−vacuum−thaw cycles.
This degassed dichloromethane was added to each Schlenk flask. The
catalyst solution was added quickly to the MNI solution using a
syringe, and the resulting solution was kept stirring at room
temperature for 45 min. An aliquot of this solution was quenched
with ethyl vinyl ether and analyzed as a reference sample. The
sacrificial monomer 1 was quickly added to the remaining solution via
a syringe, and the solution was kept stirring for 30 min. Ethyl vinyl
ether was subsequently added to quench the reaction, and the polymer
was precipitated in cold methanol. The polymer was redissolved in
dichloromethane and reprecipitated twice into methanol, filtered, and
dried under high vacuum.
Figure 2. Synthesis (top) and single crystal X-ray structure (bottom)
of 1. The phenyl rings are shown as single atoms (green) for clarity.
The hydrogen bond is represented as a blue dashed line.
EXPERIMENTAL SECTION
■
Materials. Grubbs initiators G1 and G3, phenyl phosphodichlor-
idate, ethyl vinyl ether, 4-(dimethylamino)pyridine (DMAP), di-tert-
butyl dicarbonate, and trifluoroacetic acid were purchased from Sigma-
Aldrich and used without further purification. (Z)-But-2-ene-1,4-
diammonium chloride19 (3) and exo-N-methylnorbornene imide20
(MNI) were synthesized as reported previously. Triethylamine was
purchased from Acros Chemicals, distilled from calcium hydride, and
stored over potassium hydroxide.
Instrumentation. Mass analysis of the polymers was carried out
on a Bruker FTMS 4.7T BioAPEX II using 2-[(2E)-3-(4-tert-
butylphenyl)-2-methylprop-2-enylidene]malononitrile (DCTB) as
the matrix and silver trifluoroacetate as the added salt. Relative
molecular weights and molecular weight distributions were measured
by gel permeation chromatography (GPC) equipped with a Viscotek
GPCmax VE2001 GPC Solvent/Sample Module, a Viscotek UV-
Detector 2600, a Viscotek VE3580 RI-Detector, and two Viscotek
T6000 M columns (7.8 × 300 mm, 103−107 Da). All measurements
were carried out at room temperature using THF as the eluent with a
flow rate of 1 mL/min. The system was calibrated with polystyrene
standards in a range from 103 to 3 × 106 Da. NMR spectra were
recorded on a Bruker Avance III 300 MHz NMR spectrometer (1H
NMR 300 MHz; 13C NMR 75 MHz). J-resolved 1H NMR
spectroscopy was done on a Bruker Avance III 500 MHz instrument
(1H NMR 500 MHz).
Typical Procedure for Terminal Cross-Metathesis. G1 was
purged under argon and dissolved in dry degassed dichloromethane,
and a degassed solution of MNI in dichloromethane was quickly
added. The solution was kept stirring for 45 min at rt. The chain
transfer agent 7 was separately purged under argon, dissolved in dry
degassed dichloromethane, and added to the G1-MNI solution.
Aliquots of this solution were taken every 30 min, quenched with 2
drops of ethyl vinyl ether, precipitated in cold methanol, and analyzed
1
by H NMR to monitor the completion of the reaction.
1H NMR of 3 from G1 (DCM-d2, 300 MHz): δ = 7.12−7.45 (m, 5
H), 6.49−6.61 (m, 1H), 6.25−6.38 (m, 0.75 H),5.86−6.04 (m, 1.4H),
5.85−6.04 (m, 28H), 5.45−5.62 (m, 5H), 5.05−5.25 (m, 2H), 2.95−
3.40 (m, 36H), 2.81−2.95 (m, 46H), 2.59−2.81 (m, 25H), 2.00−2.26
(m, 19H), 1.44−1.79 (m, 28H).
1H NMR of 6 from G1 (DCM-d2, 300 MHz): δ = 7.11−7.45 (m, 5
H), 6.48−6.61 (m, 0.9H), 6.25−6.38 (m, 0.71 H), 5.61−6.07 (m,
26H), 5.42−5.61 (m, 4H), 5.06−5.27 (m, 0.28H), 3.49−3.76 (m
1.6H), 2.95−3.49 (m, 34H), 2.81−2.95 (m, 42H), 2.60−2.80 (br s,
24H), 1.42−2.29 (m, 49H).
Single Crystal X-ray Diffraction. A crystal was mounted on a
loop, and all geometric and intensity data were taken from this crystal.
Data collection using Cu Kα radiation (λ = 1.541 86 Å) was performed
at 200 K on a STOE IPDS-IIT diffractometer equipped with an
Oxford Cryosystem open flow cryostat.21 Absorption correction was
partially integrated in the data reduction procedure.22 The structure
was solved and refined using full-matrix least-squares on F2 with the
SHELX-97 package.23 All heavy atoms could be refined anisotropically.
Hydrogen atoms were introduced as fixed contributors when a residual
electronic density was observed near their expected positions.
Crystallographic data (excluding structure factors) for the structures
in this paper have been deposited with the Cambridge Crystallo-
graphic Data Center, 12 Union Road, Cambridge CB21EZ, UK.
Copies of the data can be obtained on quoting the depositing numbers
CCDC- 856639.
1H NMR of 3 from G3 (DCM-d2, 300 MHz): δ = 7.15−7.47 (m, 5
H), 6.45−6.60 (m, 1H), 6.21−6.40 (m, 0.53 H), 5.60−5.88 (br s,
12H), 5.38−5.62 (br s, 11H), 5.02−5.25 (m, 2H), 2.88−3.35 (m,
68H), 2.50−2.76 (br s, 13H), 1.84−2.28 (m, 13H), 1.39−1.80 (m,
14H).
1H NMR of 6 from G3 (DCM-d2, 300 MHz): δ = 7.14−7.47 (m, 5
H), 6.47−6.62 (m, 1H), 6.23−6.40 (m, 0.5 H), 5.62−5.87 (br s, 13H),
5.35−5.62 (br s, 11H), 5.06−5.27 (m, 0.5H), 3.30−3.6 (br s, 2,2H),
2.85−3.35 (m, 70H), 2.55−2.79 (br s, 14H), 1.88−2.32 (m, 14H),
1.38−1.77 (m, 13H).
2-Phenoxy-2,3,4,7-tetrahydro-1H-1,3,2-diazaphosphepine
2-Oxide (1). Phenyl phosphodichloridate (1.20 g, 5.7 mmol, 1 equiv)
and dry dichloromethane (400 mL) were kept stirring in an ice bath.
(1) C20H26N4O4P2, M = 448.39 g mol−1, monoclinic, P21/c (Nr.
14), a = 11.0494(6); b = 9.2838(5); c = 23.0031(12) Å; β =
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Figure 3. 1H NMR spectra of (a) G1 and MNI reacted for 45 min. (b) After the addition of 1.2 equiv of 1 to (a). (c) G3 and MNI reacted for 45
min. (d) After addition of 5 equiv of 1 to (c).
Scheme 1. Schematic Synthesis of Amine End-Functionalized Living Ring-Opening Metathesis Polymers
DMAP (0.07 g, 0.57 mmol, 0.1 equiv) and triethylamine (4.34 g, 42.9
mmol, 7.7 equiv) were slowly added. The mixture was kept stirring for
15 min until the solution turned yellow. (Z)-But-2-ene-1,4-diaminium
chloride (1.00 g, 6.3 mmol, 1.1 equiv) was separately dissolved in dry
dichloromethane (30 mL) and triethylamine (2.17 g, 21.4 mmol, 3.8
equiv). This mixture was added to the phenyl phosphodichloridate
solution over 4 h. The resulting mixture was stirred at rt for 30 min
and refluxed under argon for 3 h. Water was added, and the mixture
was extracted thrice with dichloromethane, dried over MgSO4, and
concentrated in vacuum. Column chromatography (1:4 ethyl
acetate:hexane) of the yellow waxy solid over silica gave the colorless
product (0.78 g, 61.35% yield). The compound was crystallized from
DCM/n-hexane for single crystal X-ray structure determination.
1H NMR (chloroform-d, 300 MHz): δ = 7.12−7.39 (m, 5 H), 5.65
(t, J = 2.3 Hz, 2 H), 3.59−3.85 (m, 4 H), 3.17−3.49 (m, 2 H).
13C NMR (chloroform-d, 75 MHz): δ = 150.82, 129.59, 128.76,
124.55, 120.56, 39.02.
(Z)-Di-tert-butyl But-2-ene-1,4-diyldicarbamate (7). (Z)-But-
2-ene-1,4-diaminium chloride 3 (0.20 g, 1.25 mmol, 1 equiv) was
added slowly to an ice cold solution of di-tert-butyl dicarbonate (0.65
mg, 3 mmol, 2.4 equiv) and triethylamine (1.00 g, 9.9 mmol, 7.9
equiv) in 20 mL of dry dichloromethane. The solution was allowed to
warm up to room temperature and stirred for 6 h. Water was added,
and the dichloromethane layer was washed 3 times with water,
concentrated, dried over MgSO4, and evaporated under vacuum to
give 0.35 g of 7 in 98% yield. The compound was pure by 1H and 13
NMR analysis and was used without further purification.
C
1H NMR (chloroform-d, 300 MHz): δ = 5.56 (t, J = 4.5 Hz, 2 H),
4.85 (br. s., 1.7 H), 3.70−3.88 (m, 4 H), 1.45 ppm (s, 18 H).
13C NMR (chloroform-d, 75 MHz): δ = 155.84, 128.73, 79.24,
36.99, 28.29.
RESULTS AND DISCUSSION
■
In order to examine the polymerization capability of the new
sacrificial monomer 1, 5 equiv of 1 was added to the ruthenium
initiators G1 and G3 in dichloromethane-d2 in separate NMR
tubes. In the NMR tube containing catalyst G1, the initiator
successfully reacted with 1 but failed to propagate. The
1
ruthenium carbene peak in the H NMR spectrum shifted
partially from 20.01 to 18.93 ppm. The unreacted initiator peak
(20.01 ppm) is typically observed due to the slow initiation
profile of the less reactive G1 initiator. However, with G3,
compound 1 did propagate but yielded only very short
1
oligomers consisting of 2−3 repeat units, as analyzed by H
NMR spectroscopy. In this case, the ruthenium carbene peak
shifted completely from 19.04 to 14.28 ppm. Compound 1 was
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Figure 4. (top) FT-ICR mass spectra of polymer 3 synthesized from G1 (reference sample), polymer 6 synthesized from G1. (bottom) FT-ICR
mass spectra of polymer 3 synthesized from G3 (reference sample) and polymer 6 synthesized from G3. Insets show the isotopically resolved peaks
which are in good agreement with the calculated exact mass of the polymers.
designed as a sacrificial monomer and should ideally initiate fast
and propagate slowly. The initial experiments with initiators G1
and G3 showed that these prerequisites were fulfilled, which is
important for an efficient and atom economical polymer end-
functionalization with a sacrificial monomer.
In a second experiment, the norbornene monomer MNI was
added to the NMR tubes containing G1 and G3 initiated
compound 1. In both cases no initiation or propagation of MNI
could be observed. In both cases, the ruthenium catalysts when
reacted with 1 formed a very unreactive carbene. Unfortunately,
we did not succeed at isolating the new carbene complex as it
decomposed too rapidly in air.
To test the end-functionalization efficiency of 1, the NMR
experiments were repeated with progressively reduced amounts
of the sacrificial monomer. It was found that 1.2 equiv of 1
(with respect to the propagating carbene) added to an MNI
polymerization was the minimum amount necessary to
completely shift the peak of the initiated ruthenium carbene
of G1 from 19.46 to 18.93 ppm within 3 min. However, at least
5 equiv of 1 was necessary to completely shift the initiated
ruthenium carbene peak of G3 from 18.49 to 14.28 ppm within
3 min. This is in very good agreement with our previous
observation that G3 slowly polymerizes 1 to small oligomers
and that the catalyst G1 does not propagate 1 at all.
To investigate whether 1 could deactivate the MNI-initiated
ruthenium catalysts G1 and G3 to give end-functionalized
polymers, 20 equiv of MNI was added to the above-mentioned
initiators in two separate NMR tubes. The solutions were kept
at room temperature for 40 min, giving them enough time to
polymerize to completion. An aliquot of each solution was
quenched with ethyl vinyl ether as reference samples. A large
excess (10 equiv) of 1 was subsequently added to both
remaining solutions in order to end-functionalize the polymers.
After 3 min, the 1H NMR spectrum of both samples showed no
traces of the propagating MNI carbene and only the signals for
the carbene complex with 1, as observed in Figure 3.
Figure 3b shows an unexpected coupling pattern observed
for the carbene signal resulting from the reaction of G1 and 1.
A 2D J-resolved H NMR spectrum was therefore recorded to
1
investigate the coupling pattern in more detail. The signal at
18.93 ppm involves at least two different nuclei, each giving a
triplet coupling pattern (see Supporting Information). This
1
complex H NMR pattern for the ruthenium carbene peak can
currently not be explained. Addition of ethyl vinyl ether,
however, resulted in the formation of the Fischer carbene signal
at 14.51 ppm. Similarly, addition of ethyl vinyl ether to the G3
initiated polymer 4 resulted in the complete shift of the carbene
signal at 14.28 ppm to a Fischer carbene signal at 13.63 ppm
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Figure 5. GPC studies of reference polymer sample versus intermediate polymer and amine end-functionalized polymer (see Scheme 1).
Scheme 2. Synthesis and Terminal Cross-Metathesis of Symmetrical Protected Diamine
(see Supporting Information). Thus, in both cases, the
ruthenium alkylidene is not very reactive toward norbornene
derivatives but still forms a Fischer carbene with ethyl vinyl
ether.
In order to investigate whether amine end-functional
polymers could be obtained using sacrificial monomer 1,
polymerizations on a slightly larger scale (100 mg) were carried
out. Amine end-functionalized polymers were easily obtained
from the polymers quenched with 1 by simple acidic hydrolysis
using 10% HCl in acetone (Scheme 1).
The amine end-functionalization of the polymers was
confirmed by FT-ICR mass spectrometry (Figure 4), GPC
(Figure 5), and 1H NMR spectroscopy. A complete shift of the
molecular weight distribution was observed in the FTICR mass
spectra of the amine end-functionalized polymers compared to
the reference polymer samples quenched with ethyl vinyl ether
(Figure 4). Comparison of the mass spectra of the ethyl vinyl
ether terminated polymer and the amine end-functionalized
polymer showed a difference of exactly 29 mass units,
corresponding exactly to a terminal −CH2−NH2 unit. It should
be noted that incomplete end-functionalization with lesser
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unwanted metathesis between different olefinic bonds depend-
ing on the monomer structure.
CONCLUSION
■
A fast and efficient way for the synthesis of amine end-
functionalized ROMP polymers has been developed. The newly
synthesized cyclic phosphoramide sacrificial monomer reacts
extremely fast with the Grubbs first and third generation
initiators but propagates very slowly. This kinetic behavior is
the ideal case for a sacrificial monomer which almost resembles
the kinetics of a terminating agent. Mass spectrometry analysis
of the amine end-functional polymers only showed functional
polymer and no traces of nonfunctionalized material. We
compared this new sacrificial monomer to a Boc protected
amine chain transfer agent which was used in a terminal cross-
metathesis. The terminal cross-metathesis reaction was not as
fast as the phosphoramide sacrificial monomer, but end-
functionality efficiency determined by mass spectrometry also
showed the amine functional polymer only. The amine end-
functionalized polymers were derivatized further with excellent
degree of functionalization (>95%, see Supporting Informa-
tion).
Both end-functionalization agents can be synthetized in a
straightforward manner. In cases where secondary metathesis
reactions need to be minimized after the polymer end-
functionalization, the new sacrificial phosphoramide monomer
has the advantage of a fast macroinitiation forming a relatively
unreactive stable carbene.
The polymers obtained have narrow polydispersities typical
of the Grubbs-type ruthenium initiators of the first and third
generation. The new amine end-group functionalization
method reported here opens a synthetic path to many possible
complex macromolecular architectures and polymer conjugates.
Figure 6. Mass spectrum of amine mono-end-functionalized polymer
9 resulting from terminal cross-metathesis with transfer agent 7.
equivalents of 1 could easily be detected in the mass spectrum
(see Supporting Information).
Figure 5 shows the GPC traces of the ethyl vinyl ether
terminated reference samples (3), the polymers reacted with 1
(5), and the amine end-functionalized polymers after hydrolysis
of the sacrificial block (6). The shifts in hydrodynamic volume
between the individual samples are relatively small as expected
for a slowly or nonpropagating sacrificial monomer 1. While
only very small changes in GPC retention time can be observed
for the polymers initiated with G1, larger shifts in retention
time are observed for the polymerizations initiated with G3.
This further supports the assumption that initiator G1 only
reacts once with the sacrificial monomer 1, whereas G3 slowly
propagates monomer 1.
ASSOCIATED CONTENT
■
S
In all the polymerization experiments, the degree of
polymerization was kept very low (DPn ca. 10) due to the
mass limitation of the MALDI FT-ICR mass spectrometer,
which is only sensitive up to a molecular weight of ca. 4000 g
* Supporting Information
Details of the crystal structure, 2D J-resolved 1H NMR
1
spectrum, H NMR spectra of the Fischer carbenes, MALDI
FT-ICR spectrum of the incomplete end-functionalized
1
mol−1 and in order to be able to detect the end group in H
1
polymer, and H NMR spectrum of further functionalization
1
NMR studies. H NMR studies did, however, allow us to
of the amino end-functionalized polymer. This material is
available free of charge via the Internet at http://pubs.acs.org.
observe successful end-functionalization of even larger
polymers having a molecular weight of up to 20 kDa. For
these larger polymers the ruthenium carbene peak was found to
undergo similar deactivation upon the addition of 1, proving
that 1 can be effective for the end-capping of much larger
polymers as well.
AUTHOR INFORMATION
Corresponding Author
*E-mail: andreas.kilbinger@unifr.ch.
■
Notes
In order to compare this new sacrificial synthesis method to
the terminal cross-metathesis end-functionalization method, a
new chain transfer agent 7 was synthesized (Scheme 2, top).
MNI was reacted with G1 for 45 min, allowing complete
consumption of the monomer, and then 10 equiv of 7 was
added. The terminal cross-metathesis required ca. 4 h for the
complete conversion to the protected amine end- function-
alized polymer which was monitored by the change in
integration value of the boc protons as compared to the
terminal phenyl group in 1H NMR spectroscopy of the
precipitated polymers. The amine functionality at the polymer
end was confirmed by FT-ICR MS, and the mass of each peak
was 29 units greater than that of the reference sample polymer
10 terminated by ethyl vinyl ether. The potential disadvantage
of this method is that the ruthenium alkylidene is active even
after the addition of the transfer agent which could give rise to
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank Fredy Nydegger for FT-ICR mass
spectroscopy, Felix Fehr for NMR analysis, and the Swiss
National Science Foundation (SNF) for financial support.
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