The synthesis of ruthenium and rhodium complexes with functionalized N-heterocyclic carbenes and their use in solid phase peptide synthesis

Article abstract of DOI:10.1002/ejic.200800366

While N-heterocyclic carbenes (NHCs) are ubiquitous ligands in catalysts for organic or industrial synthesis, their potential to form stable transition metal complexes has hardly been exploited in metal bioconjugates. In this work, we describe a straightforward synthesis of carboxylato-functionalized imidazolium salts for covalent binding to peptides. Carbene complexes of Ru and Rh were prepared from these imidazolium salts using Ag₂O, followed by transmetallation. The neuropeptide [Leu⁵]-enkephalin (Tyr-Gly-Gly-Phe-Leu) was chosen as a model peptide. Exploratory NMR experiments identified the Ru(p-cymene)Cl₂ complex of the asymmetrically substituted imidazol-2-ylidene 3b as the most suitable metal carbene precursor for solid phase peptide synthesis (SPPS). After optimization of the conditions for SPPS, a ruthenium-NHC pseudoenkephalin (dichloro(η⁶-p-cymene) -[1-methyl-3-(methyl-p-benzoyl-Gly-Gly-Phe-Leu-OH)imidazol-2-ylidene] ruthenium(II), 12) was synthesized from 3b on solid phase using the 2-Cl-Trt resin and cleavage by 2% TFA to yield the free carboxylic acid. Peptide 12 was fully characterized by HPLC, ¹H and ¹³C NMR and ESI-MS. Characteristic NMR signals, as well as the isotope pattern of Ru in the ESI-MS, unequivocally confirm the formation of this metalcarbene peptide bioconjugate. Wiley-VCH Verlag GmbH & Co. KGaA, 2008.

Full text of DOI:10.1002/ejic.200800366

FULL PAPER
DOI: 10.1002/ejic.200800366
The Synthesis of Ruthenium and Rhodium Complexes with Functionalized
N-Heterocyclic Carbenes and Their Use in Solid Phase Peptide Synthesis
Jessica Lemke[a] and Nils Metzler-Nolte*[a]
Keywords: Bioorganometallic chemistry / Imidazolium peptides / Metal carbene complexes / NHC ligands / Solid-phase
synthesis
While N-heterocyclic carbenes (NHCs) are ubiquitous li-
gands in catalysts for organic or industrial synthesis, their
potential to form stable transition metal complexes has
hardly been exploited in metal bioconjugates. In this work,
we describe a straightforward synthesis of carboxylato-func-
tionalized imidazolium salts for covalent binding to peptides.
Carbene complexes of Ru and Rh were prepared from these
imidazolium salts using Ag2O, followed by transmetallation.
The neuropeptide [Leu5]-enkephalin (Tyr-Gly-Gly-Phe-Leu)
was chosen as a model peptide. Exploratory NMR experi-
ments identified the Ru(p-cymene)Cl2 complex of the asym-
metrically substituted imidazol-2-ylidene 3b as the most suit-
able metal carbene precursor for solid phase peptide synthe-
sis (SPPS). After optimization of the conditions for SPPS, a
ruthenium-NHC pseudoenkephalin (dichloro(η6-p-cymene)-
[1-methyl-3-(methyl-p-benzoyl-Gly-Gly-Phe-Leu-OH)imid-
azol-2-ylidene]ruthenium(II), 12) was synthesized from 3b on
solid phase using the 2-Cl-Trt resin and cleavage by 2% TFA
to yield the free carboxylic acid. Peptide 12 was fully charac-
terized by HPLC, 1H and 13C NMR and ESI-MS. Characteris-
tic NMR signals, as well as the isotope pattern of Ru in the
ESI-MS, unequivocally confirm the formation of this metal-
carbene peptide bioconjugate.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2008)
tibacterial activity.[7] In spite of the numerous successful ap-
plication of carbene complexes for organic synthesis, only
a few groups have combined small biomolecules with N-
heterocyclic carbenes, namely sugars[8] and peptides.[9]
In the work of Xu and Gilbertson,[9] the ligand precursor
was an imidazolinium salt which was prepared in an eight-
step synthesis and used for solid phase peptide synthesis
(SPPS) to form an imidazolinium peptide salt. Complex-
ation with Grubbs’ first-generation catalyst could be per-
formed in solution and the formation of a metal carbene
Introduction
Labelling of biomolecules with organometallic moieties
has been used in biochemical and medicinal applications,
ranging from biosensors to radioimaging and drugs.[1] One
interesting area of the bioconjugate field are organometallic
bioconjugates where the metal complex is covalently bound
to a biomolecule, for instance DNA, PNA and peptides.[2]
In this area, a number of covalent organometallic bioconju-
gates were synthesized in our group, covering ferrocene-
and cobaltocenium peptide and PNA conjugates,[3] as well
as cobaltcarbonyl-alkyne peptides[4] and scorpionate deriv-
atives.[5] For such applications, the metalorganic part has to
be stable in water and air. Consequently, there is a constant
need to explore new ligand systems.
N-Heterocyclic carbene (NHC) complexes have been
widely investigated in the last decades, mostly due to their
potential as catalysts for organic or industrial synthesis.[6]
NHCs are isolobal to phosphane ligands but are generally
more stable in the presence of oxygen. In view of this, we
decided to explore the labelling of bioconjugates with car-
bene complexes.
1
was verified by the appearance of characteristic H NMR
resonances. To the best of our knowledge, no further char-
acterisation data were provided and no other metal carbene
peptide conjugates have been reported.
In this article, the preparation of functionalized carbene
precursors, as well as the successful application of carbene
chemistry to SPPS, will be described. Moreover, the synthe-
sis of a ruthenium carbene pseudoenkephalin peptide on
solid support and its full characterisation will be presented.
Results and Discussion
Recently, asymmetric silver and gold imidazolium car-
bene complexes were discovered which show promising an-
In this work, we focussed on preparing peptide conju-
gates with a metal carbene moiety using SPPS. We chose
the neuropeptide enkephalin as our model peptide, which
has the primary sequence Tyr-Gly-Gly-Phe-Leu. Instead of
the amino acid tyrosine, we decided to introduce the car-
bene at the N-terminal position, which resulted in a
[a] Faculty for Chemistry and Biochemistry, Ruhr University
Bochum,
Universitätsstrasse 150, 44801 Bochum, Germany
Fax: +49-234-32-14378
E-mail: nils.metzler-nolte@rub.de
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J. Lemke, N. Metzler-Nolte
FULL PAPER
pseudoenkephalin conjugate. For coupling of NHC-type li- synthesis using imidazolium pseudoenkephalin salts as li-
gands to the N-terminal amino group of a peptide, the li- gand precursors. Despite several attempts, imidazolium salt
gand has to bear functional groups which can form a bond 1 could not be hydrolysed to the free carboxylic acid, which
with amino groups. We therefore concentrated on carboxyl- would be required for use in SPPS, because it decomposed
ate derivatives of the NHC ligand.
under the basic conditions necessary for ester hydrolysis.
The imidazolium salt 1 (Scheme 1) could be obtained as Thus, we set out to synthesise an activated ester imid-
a white solid in good yields, by refluxing N-methylimidazole azolium precursor directly, namely with the pentafluo-
with methyl p-(bromomethyl)benzoate in THF.[10] The 1- rophenyl group (Pfp).[14]
{[4-(methoxycarbonyl)phenyl]methyl}-3-methylimida-
The carbene precursor 8 was synthesized in a two-step
zolium bromide (1) was fully characterised. As typical spec- procedure, first starting from p-(bromomethyl)benzoic acid
troscopic features, the two neighbouring protons of the (4) and pentafluorophenol (5) to give the Pfp-ester 6, then
imidazolium ring (N–CH=CH–N) show pseudo-triplets continuing with the same procedure as used for 1
3
4
due to similar J and J coupling at δ = 7.91 and 7.84 ppm (Scheme 2). A second imidazolium salt was synthesized by
in deuterated DMSO. The acidic proton (N–CH–N) gives refluxing 4 directly with N-methylimidazole to yield the
rise to a broad downfield-shifted singlet at δ = 9.31 ppm. imidazolium salt 9 with the free carboxylic acid used in
As direct deprotonation of the imidazolium salt failed, the Scheme 3.
imidazolium salt was first transformed into a silver carbene
by using silver(I) oxide (Ag2O), which acts both as base and
halide scavenger.[11] The silver NHC complex then easily
underwent exchange reactions with binuclear halide-
bridged metal complexes of rhodium and ruthenium
(Scheme 1).[12] The yellow rhodium dicyclooctadiene chlo-
ride carbene complex 2a and the orange ruthenium p-cy-
mene dichloride carbene complex 2b were thus obtained.
Both air-stable compounds were isolated in good yields and
fully characterised. A characteristic feature of complexes 2
1
and 3 is the absence of the downfield-shifted H NMR sig-
nal of the imidazolium ring (N–CH–N). The two remaining
Scheme 2. Synthesis of the Pfp-imidazolium salt 8. i) DCC, EtOAc/
imidazolium ring protons were now observed as doublets
DMF (30:1), 0 °C, 2 h, room temp., 4 h; ii) THF, reflux overnight,
3
around δ = 7 ppm with a J coupling constant of about
2 Hz, which is in accordance with literature values for sim-
ilar NHC-carbene complexes.[13]
N2.
Both imidazolium salts were then employed for solid-
phase synthesis. Following a standard SPPS protocol,[3a] the
imidazolium pseudoenkephalin salts 10 and 11 were synthe-
sized on two different resins (Scheme 3). Imidazolium salt
8 was coupled to the resin-bound peptide, using HOBt to
increase the yield. Imidazolium salt 9 was activated like an
amino acid by using HOBt and TBTU in the presence of
DIPEA as a base. The 2-chlorotrityl (2-Cl-Trt) and Sieber
amide resins were used to yield either the free carboxylic
acid or the C-terminal amide after cleavage. In both cases,
cleavage from the resin was performed under very mild con-
ditions, taking 1% trifluoroacetic acid (TFA) solution in
CH2Cl2 (v/v) for the Sieber amide or 1% TFA solution in
CH2Cl2 (v/v) for the 2-Cl-Trt resin, respectively. After pre-
cipitation with cold diethyl ether and dissolving the crude
product in a mixture of acetonitrile and water for lyophilis-
ation, the peptides were purified by preparative HPLC if
necessary and fully characterised spectroscopically. The
imidazolium peptides 10 and 11 were obtained as ligand
precursors, with the free carboxylic acid and amide at the
C-terminus.
Scheme 1. Synthesis of the rhodium and ruthenium carbene com-
plexes 2a, b and 3a, b.
With these compounds in hand, two different synthetic
routes towards metal-carbene peptides were tested. First,
the methyl imidazolium peptides were taken as NHC-ligand
With this easy-to-handle procedure for the generation of precursors to be complexed via reaction with Ag2O, fol-
carbene complexes from functionalized imidazolium salts in lowed by transmetallation. Unfortunately, it was impossible
hand, we next tried to transfer the conditions to peptide to obtain the desired rhodium- and ruthenium carbene
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Ru and Rh Complexes with NHC Ligands
Scheme 3. Synthesis of the 1-methylimidazolium pseudoenkephalins 10 and 11.
complexes from the NHC-ligand precursors 10 and 11 by TFA showed a degradation of approximately 30% after 4 h
this route. No rhodium carbene complexes could be de- which seemed tolerable. Such conditions are well compati-
tected, neither for silver carboxylate nor Ag2O in various ble with the 2-Cl-Trt resin. It follows that only the ruthe-
solvents at room temperature or under reflux. In the case of nium carbene complex in combination with the 2-Cl-Trt-
the ruthenium carbene pseudoenkephalin, traces of formed resin can be employed in SPPS.
carbene complex could be identified with ESI mass spec-
trometry.
Coupling of the ruthenium complex 3b to the resin-
bound peptide was performed according to the protocol for
Thus, we focussed on a second entry by direct coupling the imidazolium peptide salts (Scheme 4), using 2.5 equiv.
of Pfp-activated metal carbene complexes to a resin-bound of HOBt and 2.5 equiv. of 3b. The resin was left on a shaker
peptide. Thus, the complexes 3a and 3b were synthesized for 48 h to ensure full conversion. Then, double cleavage
following standard procedures (Scheme 1). Both Pfp com- was performed with the above-mentioned conditions (com-
plexes were fully characterised. The synthesis of metal-car- pare 2b) for 2 h each. The combined crude products were
bene pseudoenkephalins was persued by synthesizing the precipitated with cold diethyl ether/pentane and dissolved
tetrapeptides on the resin, followed by reaction with the in a mixture of acetonitrile and water for lyophilisation. The
rhodium- and ruthenium carbene Pfp-esters 3a and 3b purified ruthenium carbene pseudoenkephalin 12 was ob-
(Scheme 1). Cleavage of the peptide from the resin is usually tained as a light-orange powder in relatively low yield but
carried out on resins with acid-labile linkers, using concen- in ca. 95% purity as confirmed by analytical HPLC. Thus,
trated TFA up to 95% (v/v). Therefore, NMR studies in it was used without further purifications for spectroscopic
deuterated CH2Cl2 were performed with the complexes 2a measurements.
and 2b as test systems, increasing the amount of TFA every
In order to improve the yield of 12, NMR experiments
two hours by 1% (v/v), in order to find conditions suitable were carried out to optimise the cleavage mixture. By leav-
for the desired metal conjugates. The rhodium complex 2a ing the resin for 2 h with 10% TFA in methanol (v/v), fol-
already degraded at the lowest concentration of 1% TFA, lowed by a second cleavage with 20% TFA in methanol
giving the initial imidazolium salt precursor 1 as shown by (v/v), bioconjugate 12 could be obtained in a yield Ͼ 30%,
1H NMR spectroscopy. For the ruthenium complex 2b, 2% and again with Ͼ 95% purity. In our experience, this is a
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J. Lemke, N. Metzler-Nolte
FULL PAPER
Scheme 4. Solid-phase synthesis of the ruthenium carbene pseudoenkephalin 12.
fair yield for organometallic peptide conjugates. In differ- terns are in perfect accordance (Figure 1). These cumulated
ence to the imidazolium peptide 10, which has a retention data provide strong support for the formation of ruthenium
time (tR) of 13.5 min in RP-HPLC, the p-cymene ruthenium carbene pseudoenkephalin 12.
NHC-pseudoenkephalin 12 has an increased tR of 16 min.
This suggests a higher liphophilicity originating from the
organometallic moiety in 12.
Conclusions
1
Next, the H NMR spectra of the imidazolium pseudo-
In summary, the present work describes a way to synthe-
size ruthenium carbene peptide conjugates using solid-
enkephalin 10 and the ruthenium carbene pseudoenke-
phalin 12 were compared. The occurrence of peaks for the
phase synthesis without decomposition. The method is ex-
emplified for the p-cymene ruthenium NHC pseudoenke-
phaline derivative 12. The ESI-MS spectra and NMR spec-
troscopic data clearly confirm the formation of the ruthe-
nium bioconjugate 12. Further, the overall yield of the bio-
conjugate could be increased by careful optimization of the
cleavage conditions in preliminary NMR studies. Unfortu-
nately, the successful synthesis for ruthenium conjugate 12
could not be used for the rhodium analogue due to decom-
position of the latter complex even under relatively mild
cleavage conditions. Future work will concentrate on im-
proving these conditions to make more metal carbene com-
plexes amendable to bioconjugation under SPPS condi-
tions. Given the well-documented cytotoxic activity of p-
cymene ruthenium compounds such as the RAPTA com-
pounds from Dyson’s group,[15] it will be interesting to in-
vestigate the biological properties of our new Ru–NHC
peptide conjugates.
p-cymene ligand and the absence of the acidic proton of the
imidazolium ring (N–CH–N) clearly indicates the successful
coupling of the ruthenium carbene 3b to the peptide with-
out decomposition during work-up. Further, the integrals
for the iPr group of the first amino acid (Leu) and of the
p-cymene moiety at the other end of the molecule appear
in a 1:1 ratio, which is another proof of the successful syn-
thesis of conjugate 12.
ESI mass spectrometry (positive ion detection mode) was
also performed. For the imidazolium pseudoenkephalin 10,
[M – Br]+ was observed as the major peak at m/z = 591.21
(calculated value m/z = 591.29). The spectrum of conjugate
12 showed two peaks (Figure 1), namely the [M – Cl]+ peak
at m/z = 861.15 and the [M – 2Cl – H]+ at m/z = 825.20.
These results are in accordance with the calculated values
of 861.27 for the [M – Cl]+ and 825.29 for the [M – 2Cl –
H]+ peaks. Also, the observed and calculated isotope pat-
Experimental Section
Abbreviations: DIPEA = N,N-diisopropylethylamine, HOBt = 1-
hydroxybenzotriazole, Pfp = pentafluorophenyl, SPPS = solid
phase peptide synthesis, TBTU
= 2-(1H-benzotriazole-1-yl)-
1,1,3,3-tetramethyluronium tetrafluoroborate, TFA = trifluoro-
acetic acid, NHC = N-heterocyclic carbene, 2-Cl-Trt = 2-chloro-
trityl, MS = molecular sieves.
General Remarks: All reagents were purchased from commercial
sources and used as received. H-Leu-2-chlorotrityl-resin (H-Leu-
2-Cl-Trt), Sieber amide resin and all amino acid derivatives were
purchased from Novabiochem, enantiomerically pure -amino ac-
ids were used throughout. The Fmoc-Leu-Wang resin, DIPEA,
HOBtϫH2O and TBTU were purchased from Iris Biotech (Ger-
Figure 1. ESI-MS (positive) of the imidazolium pseudoenkephalin
salt 10 (left) and the ruthenium carbene pseudoenkephalin 12
(right). a, b): simulated spectra for the peak groups around m/z =
861 ([M – Cl]+) and m/z = 825 ([M – 2 Cl – H]+) of 12.
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Ru and Rh Complexes with NHC Ligands
3
many). Solvents were distilled from molecular sieves (CH2Cl2,
CH3CN) and over CaH2 (DMF) or taken from the solvent purifica-
tion system MBraun SPS (THF). Glassware was used oven dry.
NMR spectra were recorded on either Bruker DPX 250 (1H at
250.13 MHz) or on Bruker DRX 400 (1H at 400.13 MHz) spec-
trometers. The NMR chemical shifts (δ) are reported in ppm rela-
tive to the residual proton chemical shifts of the deuterated solvent
set relative to external TMS. Microanalyses were performed on a
H, HAr), 6.9 (br. s, 1 H, N-CH=CH-N), 6.72 (d, JH,H = 1.91 Hz,
N-CH=CH-N), 5.04 (br. s, N-CH2-Ar), 4.09 (s, 3 H, CO2CH3),
3.89 (s, 3 H, N-CH3), 3.44–3.20 (m, 2 H, COD), 2.49–1.82 (m, 8
H, COD) ppm. 13C NMR (CD2Cl2, 100 MHz): δ = 166.6 (COO),
142.1 (NCN), 130.1 (CAr), 129.9 (CqAr), 128.7 (CAr), 122.9 (N-
CH=CH-N), 120.5 (N-CH=CH-N), 98.7 (N-CH2-Ar), 69.2 (COD),
68.2 (COD), 52.1 (CO-OCH3), 37.7 (N-CH3), 33.2 (COD), 33.1
(COD), 29.3 (COD), 28.9 (COD) ppm. MS (FAB pos.): m/z = 476
Analytik Jena multi EA 3100 and on a CE-Instruments EA 1110. – [M]+, 441 [M – Cl]+. C21H26ClN2O2Rh (476.81): calcd. C 52.9, H
Electrospray ionisation mass spectra (ESI-MS) were recorded with
Bruker Esquire 6000. Fast-atom bombardment mass spectra (FAB-
MS) were recorded on a Finnigan VG Autospec [glycerol or 3-
nitrobenzyl alcohol (NBA) as matrix]. RP-HPLC was performed
on a customized HPLC instrument, consisting of Varian ProStar
detector (model 330), a Varian solvent delivery system (model 210)
and an analytical C-18 Microsorb (4.6 mmϫ250, 60 Å/8 µm;
Dynamax, Varian) or semipreparative C-18 Microsorb
(10 mmϫ250 mm, 60 Å/8 µm; Dynamax, Varian) column. Water
(A) and acetonitrile (B) were used as solvents, both containing
0.1% TFA. The flow rate was 1 mLmin–1 (analytical) or
4 mLmin–1 (semipreparative) and peaks were detected at 254 nm
and 220 nm. Typically, gradients were linear from 5% Ǟ 95% (B)
over 30 min. The test of stability in the presence of TFA was carried
out by making a solution of ruthenium and rhodium carbene com-
plex in 500 µL of CD2Cl2. TFA was then added to the solution
starting from 1% to a level of 5% v/v in TFA every 2 h and the 1H
spectrum was recorded immediately and subsequently at intervals
of 2 h. The ratio of carbene complex to degradation product imid-
azolium salt was determined by comparison of the integrals. For
the use of methanol as cleavage solvent, NMR spectra were mea-
sured in CD3OD, adding 1, 3, 5, 10 and 20% (v/v) of TFA every
2 h.
5.5, N 5.9; found C 48.73, H 5.14, N 5.42.
Ruthenium(II) Complex 2b: To a dried Schlenk tube charged with
molecular sieves (4 Å, 50 mg) were added 1 (95 mg, 0.31 mmol) and
Ag2O (37 mg, 0.16 mmol). The mixture was backflashed three
times with N2 and then dry CH2Cl2 was added (30 mL). The flask
was closed and shaken for 4 h in the dark. A solution of dichloro(p-
cymene)ruthenium(II) dimer (95 mg, 0.16 mmol) in CH2Cl2 was
added (10 mL) and the solution was shaken overnight in the dark.
The solution was filtered through celite and concentrated in vacuo.
The orange residue was recrystallized from CH2Cl2/pentane
(10 mL/30 mL) at 4 °C. After filtration, an orange solid was ob-
1
tained (70 mg, 42% yield). H NMR (CD2Cl2, 250 MHz): δ = 8.10
(AAЈXXЈ, N = 8.46 Hz, 2 H, HAr), 7.36 (AAЈXXЈ, N = 8.46 Hz, 2
3
H, HAr), 7.08 (d, JH,H = 2.00 Hz, 1 H, N-CH=CH-N), 6.86 (d,
3JH,H = 2.00 Hz, 1 H, N-CH=CH-N), 5.35 (br. s, 2 H, Hp-cymAr),
4.97 (br. s, 2 H, Hp-cymAr), 4.03 (s, 3 H, CO2CH3), 3.88 (s, 3 H, N-
3
CH3), 2.91 [quint, JH,H = 6.90 Hz, 1 H, CH(CH3)2], 1.99 (s, 3 H,
3
Ar-CH3), 1.24 [d, JH,H = 6.93 Hz, 6 H, CH(CH3)2] ppm. 13C
NMR (CD2Cl2, 63 MHz): δ = 175.5 (CO2CH3), 166.5 (CqAr), 143.0
(NCN), 130.0 (CAr), 127.6 (CAr), 124.4 (N-CH=CH-N), 122.8 (N-
CH=CH-N), 109.2 (Cp-cymAr), 98.6 (Cp-cymAr), 81.8 (N-CH2-Ar),
54.5 (CO-OCH3), 52.06 (N-CH3), 39.68 (CH3-Ar), 30.79 [CH-
(CH3)2], 18.52 [CH(CH3)2] ppm. MS (ESI+, CH3CN): m/z = 501.01
[M – Cl]+. C23H28Cl2N2O2Ru (536.46): calcd. C 51.49, H 5.26, N
5.22; found C 50.22, H 6.19, N 5.11.
1-{[4-(Methoxycarbonyl)phenyl]methyl}-3-methylimidazolium Bro-
mide (1): To a solution of 1-methylimidazole (2.29 g, 10 mmol) in
dry THF (10 mL) was added dropwise a solution of methyl p-(bro-
momethyl)benzoate (0.82 g, 10 mmol) in THF (10 mL). After com-
plete addition, the mixture was refluxed for 24 h during which a
white precipitate was formed. The solvent was decanted from the
precipitate and the solid was washed with THF (3ϫ10 mL) and
then dried in vacuo to give a white solid (2.42 g, 78% yield). 1H
NMR ([D6]DMSO, 200 MHz): δ = 9.31 (s, 1 H, N-CH-N), 7.91 [t,
Rhodium(I) Complex 3a: To a dried Schlenk tube, charged with
molecular sieves (4 Å, 100 mg), were added 8 (283 mg, 0.61 mmol)
and Ag2O (74 mg, 0.32 mmol). The mixture was backflashed three
times with N2 and then dry CH2Cl2 was added (30 mL). The flask
was closed and shaken for 4 h in the dark. A solution of chloro(1,5-
cyclooctadiene)rhodium(I) dimer (150 mg, 0.31 mmol) in CH2Cl2
was added (10 mL) and the solution was shaken overnight in the
dark. The solution was filtered through celite and concentrated in
vacuo. The yellow residue was crystallized from CH2Cl2/pentane
(10 mL/30 mL) at 4 °C. After filtration, a yellow solid was obtained
(350 mg, 93% yield). 1H NMR (CD2Cl2, 250 MHz): δ = 8.19
(AAЈXXЈ, N = 8.36 Hz, 2 H, HAr), 7.60 (AAЈXXЈ, N = 8.36 Hz, 2
3
3JH,H = 1.62 Hz, 1 H, N-CH=CH-N], 7.84 (t, JH,H = 1.62 Hz, 1
H, N-CH=CH-N), 7.99 (AAЈXXЈ N = 8.33 Hz, 2 H, HAr), 7.55
(AAЈXXЈ N = 8.33 Hz, 2 H, HAr), 5.56 (s, 2 H, N-CH2-Ar), 3.88
(s, 3 H, CO2CH3), 3.85 (s, 3 H, N-CH3) ppm. 13C NMR ([D6]-
DMSO, 50 MHz): δ = 165.7 (C=O), 140.0 (CqAr), 136.9 (N-CH-
N), 129.7 (CqAr), 129.6 (CAr), 128.4 (CAr), 124.0 (N-CH=CH-N),
122.4 (N-CH=CH-N), 52.2 (CO2CH3), 51.2 (N-CH2-Ar), 35.9 (N-
CH3) ppm. MS (FAB pos.): m/z = 231 [M – Br]+. C13H15BrN2O2
(311.18): calcd. C 50.18, H 4.86, N 9.00; found C 49.29, H 5.86, N
8.86.
3
H, HAr), 6.92 (d, JH,H = 1.85 Hz, 1 H, N-CH=CH-N), 6.73 (d,
3JH,H = 1.85 Hz, 1 H, N-CH=CH-N), 4.96 (s, 2 H, N-CH2-Ar),
4.11 (s, 3 H, N-CH3), 3.43–3.43 (m, 2 H, COD), 2.54–2.23 (m, 4
H, COD), 1.96–1.80 (m, 6 H, COD) ppm. 13C NMR (CD2Cl2, 63
MHz): δ = 162.3 (COO), 144.2 (NCN), 131.1 (CAr), 128.6 (CAr),
126.6 (Cq,Ar), 123.1 (N-CH=CH-N), 120.4 (N-CH=CH-N), 98.8
(N-CH2-Ar), 68.47 (COD), 68.24 (COD), 37.71 (N-CH3), 33.11
(COD), 32.71 (COD), 29.01 (COD), 28.69 (COD) ppm. 19F NMR
Rhodium(I) Complex 2a: To a dried Schlenk tube charged with mo-
lecular sieves (4 Å, 100 mg) were added 1 (190 mg, 0.61 mmol) and
Ag2O (74 mg, 0.32 mmol). The mixture was backflashed three
times with N2 and then dry CH2Cl2 was added (30 mL). The flask
was closed and shaken for 4 h in the dark. A solution of chloro(1,5-
cyclooctadiene)rhodium(I) dimer (150 mg, 0.31 mmol) in CH2Cl2
was added (10 mL) and the solution was shaken in the dark over-
night. The solution was filtered through celite and concentrated in
vacuo. The yellow residue was recrystallized from CH2Cl2/pentane
3
(CD2Cl2, 235 MHz): δ = –153.1 (d, JF,F = 17.1 Hz, 2 F, ArF),
3
3
–158.9 (t, JF,F = 21.6 Hz, 1 F, ArF), –163.2 (dd, JF,F = 16.9,
21.6 Hz, 2 F, ArF) ppm. MS (ESI+, CH3CN): m/z = 593.04 [M –
Cl]+. C26H23ClF5N2O2Rh (628.83): calcd. C 49.66, H 3.69, N 4.45;
found C 50.13, H 4.42, N 5.11.
(10 mL/30 mL) at 4 °C. After filtration, a yellow solid was obtained Ruthenium(II) Complex 3b: To a dried Schlenk tube charged with
(130 mg, 88% yield). 1H NMR (CD2Cl2, 400 MHz): δ = 8.01 molecular sieves (4 Å, 100 mg) were added 8 (283 mg, 0.61 mmol)
(AAЈXXЈ, N = 8.29 Hz, 2 H, HAr), 7.47 (AAЈXXЈ, N = 8.18 Hz, 2 and Ag2O (74 mg, 0.32 mmol). The mixture was backflashed three
Eur. J. Inorg. Chem. 2008, 3359–3366
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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3363

J. Lemke, N. Metzler-Nolte
FULL PAPER
times with N2 and then dry CH2Cl2 was added (30 mL). The flask
was closed and shaken for 4 h in the dark. A solution of dichloro(p-
cymene)ruthenium(II) dimer (190 mg, 0.31 mmol) in CH2Cl2 was
added (10 mL) and the solution was shaken overnight in the dark.
The solution was filtered through celite and concentrated in vacuo.
The orange residue was crystallized from CH2Cl2/pentane (10 mL/
during which a white precipitate was formed. The solvent was de-
canted from the precipitate and the solid was washed with THF
(3ϫ10 mL) and then dried in vacuo to give a white solid (1.96 g,
66% yield). 1H NMR ([D6]DMSO, 200 MHz): δ = 9.39 (s, 1 H, N-
3
CH-N), 7.96 (AAЈXXЈ N = 8.33 Hz, 2 H, HAr), 7.87 (t, JH,H
=
3
1.62 Hz, 1 H, N-CH=CH-N), 7.79 (t, JH,H = 1.62 Hz, 1 H, N-
30 mL) at 4 °C. After filtration, an orange solid was obtained CH=CH-N), 7.54 (AAЈXXЈ N = 8.33 Hz, 2 H, HAr), 5.58 (s, 2 H,
(256 mg, 61% yield). 1H NMR (CD2Cl2, 250 MHz): δ = 8.18 N-CH2-Ar), 3.85 (s, 3 H, N-CH3) ppm. 13C NMR ([D6]DMSO,
(AAЈXXЈ, N = 8.51 Hz, 2 H, HAr), 7.48 (AAЈXXЈ, N = 8.51 Hz, 2 50 MHz): δ = 168.8 (COO), 139.4 (CqAr), 136.8 (N-CH-N), 131.0
3
H, HAr), 7.01 (d, JH,H = 2.01 Hz, 1 H, N-CH=CH-N), 6.86 (d, (CqAr), 129.7, (CAr), 128.3 (CAr), 124.0 (N-CH=CH-N), 122.4 (N-
3JH,H = 2.01 Hz, 1 H, N-CH=CH-N), 5.39 (AAЈXXЈ, N = 4.40 Hz, CH=CH-N), 51.2 (N-CH2-Ar), 35.9 (N-CH3) ppm. MS (FAB
2 H, Hp-cymAr), 5.03 (AAЈXXЈ, N = 4.40 Hz, 2 H, Hp-cymAr), 4.04 pos.): m/z = 217 [M – Br]+. C12H13BrN2O2 (297.15): calcd. C 48.50,
3
(s, 2 H, N-CH2-Ar), 2.93 [quint, JH,H = 6.92 Hz, 1 H, CH(CH3)2], H 4.41, N 9.43; found C 48.02, H 4.92, N 10.85.
3
2.02 (s, 3 H, Ar-CH3), 1.26 [d, JH,H = 6.93 Hz, 6 H, CH(CH3)2]
Imidazolium Bromide 10: The peptide, 2-chlorotrityl-resin-bound
ppm. 13C NMR (CD2Cl2, 63 MHz): δ = 175.8 (COO), 162.3 (CqAr),
GlyGlyPheLeu, was synthesized by standard SPPS methods.[3a] The
145.1 (NCN), 131.0 (CAr), 128.5 (CAr), 126.5 (CqAr), 124.6
imidazolium salt 8 was then coupled to the resin-bound peptide
(N-CH=CH-N), 122.6 (N-CH=CH-N), 109.7 (Cp-cymAr), 98.8
as described here. Imidazolium salt 8 (5 equiv.) and HOBtϫH2O
(Cp-cymAr), 82.0 (N-CH2-Ar), 54.5 (N-CH3), 39.7 (CH3-Ar), 30.9
(5 equiv.) were combined in 2 mL of DMF, mixed vigorously, and
left to stand for 5 min. The activated imidazolium salt solution was
[CH(CH3)2], 18.6 [CH(CH3)2] ppm. 19F NMR (CD2Cl2, 235 MHz):
3
3
δ = –153.1 (d, JF,F = 17.1 Hz, 2 F, ArF), –158.9 (t, JF,F = 21.7
shaken with the peptide-loaded 2-Cl-Trt resin for ca. 24 h. After
coupling of the salt to the oligopeptide, the reaction mixture was
filtered, the resin was washed with DMF (3ϫ2 mL), CH2Cl2
(3ϫ2 mL), and methanol (3ϫ2 mL), shaken for 30 min in meth-
anol, and dried under reduced pressure for ca. 1 h. The product
was cleaved from the resin by treatment with a solution of TFA
(2 mL, 2% v/v in dichloromethane) for ca. 4 h. The reaction mix-
ture containing the crude imidazolium salt enkephalin was col-
lected, the resin was extracted again with dichloromethane
(2ϫ2 mL) and the combined filtrates were concentrated under re-
duced pressure to a volume of ca. 1 mL, at which point cold diethyl
ether (ca. 10 mL, –70 °C) and pentane (10 mL) were added to pre-
cipitate the peptide conjugate. The mixture was centrifuged and the
solvents decanted to collect a white solid. The solid was taken up
in cold ether again and centrifuged (2 times). The supernatant was
treated again with pentane and the precipitate collected. The col-
lected solids were combined, dissolved in a minimum volume of
acetonitrile and water and lyophilised. The crude product was puri-
fied by reversed-phase HPLC giving 10 as a white powder (120 mg,
70% based upon the resin loading of 0.86 mmol/g); C31H39BrN6O6
3
Hz,1 F, ArF), –163.2 (dd, JF,F = 16.3, 21.7 Hz, 2 F, ArF) ppm.
MS (ESI+, CH3CN): m/z = 653.02 [M – Cl]+. C28H25Cl2F5N2O2Ru
(688.49): calcd. C 48.85, H 3.66, N 4.07; found C 49.24, H 4.76, N
4.29.
Pentafluorophenyl 4-(Bromomethyl)benzoate (6): To an ice-cooled
solution (0 °C) of 4-(bromomethyl) benzoic acid (2.15 g, 10 mmol)
and pentafluorophenol (1.84 g, 10 mmol) in a mixture of ethyl ace-
tate/DMF (30 mL/1 mL) was added N,NЈ-dicyclohexylcarbodi-
imide (2.06 g, 10 mmol). The mixture was stirred at 0 °C for 1 h
and then warmed to room temperature. After 2 h, the formed dicy-
clohexylurea was filtered off and the solvent was removed in vacuo
to give 6 as a white solid (1.57 g, 41% yield). 1H NMR ([D6]-
DMSO, 250 MHz): δ = 8.17 (AAЈXXЈ, N = 8.11 Hz, 2 H, HAr),
7.72 (AAЈXXЈ, N = 8.11 Hz, 2 H, HAr), 4.82 (s, 2 H, Br-CH2-Ar)
ppm. 13C NMR ([D6]DMSO, 63 MHz): δ = 145.7 (COO), 130.8
(CAr), 125.4 (CAr), 32.6 (Br-CH2-Ar) ppm. 19F NMR ([D6]DMSO,
3
235 MHz): δ = –153.5 (d, JF,F = 19.10 Hz, 2 F, ArF), –157.5 (t,
3
3JF,F = 23.2 Hz,1 F, ArF), –163.2 (dd, JF,F = 19.1, 23.2 Hz, 2 F,
ArF) ppm. MS (FAB pos.): m/z = 383 [M – Br]+. C14H6BrF5O2
1
(670.21). H NMR (CD3CN, 400 MHz): δ = 8.66 (s, 1 H, N-CH-
(381.10): calcd. C 44.12, H 1.59; found C 45.83, H 2.38.
3
N), 8.29 (t, JH,H = 5.12 Hz, 1 H, NHArC=O), 7.92 (AAЈXXЈ, N =
3
Imidazolium Bromide 8: To a solution of 1-methylimidazole (0.25 g,
3 mmol) in dry THF (10 mL) was added dropwise a solution of 6
(1.14 g, 3 mmol) in THF (10 mL). After complete addition, the
mixture was refluxed for 24 h during which a white precipitate
formed. The solvent was decanted from the precipitate and the so-
lid was washed with THF (3ϫ10 mL) and then dried in vacuo
to give a white solid (1.09 g, 78% yield). 1H NMR ([D6]DMSO,
250 MHz): δ = 9.31 (s, 1 H, N-CH-N), 8.23 (AAЈXXЈ, N = 8.54 Hz,
2 H, HAr), 7.85 (s, 1 H, N-CH=CH-N), 7.79 (s, 1 H, N-CH=CH-
N), 7.69 (AAЈXXЈ, N = 8.54 Hz, 2 H, HAr), 5.64 (s, 2 H, N-CH2-
Ar), 3.89 (s, 3 H, N-CH3) ppm. 13C NMR ([D6]DMSO, 63 MHz):
δ = 142.3 (COO), 137.0 (N-CH-N), 130.9 (CAr), 129.1 (CAr), 125.9
(CqAr), 124.1 (N-CH=CH-N), 122.4 (N-CH=CH-N), 51.2 (N-CH2-
Ar), 35.9 (N-CH3) ppm. 19F NMR (CD2Cl2, 235 MHz): δ = –153.6
8.27 Hz, 2 H, HAr), 7.70 (t, JH,H = 5.55 Hz, 1 H, NHα,Gly), 7.53
(d, 3JH,H = 8.36 Hz, 1 H, NH-CαPhe), 7.43 (AAЈXXЈ, N = 8.27 Hz,
3
2 H, HAr), 7.39 (t, JH,H = 1.73 Hz, 1 H, N-CH=CH-N), 7.35 (t,
3
3JH,H = 1.73 Hz, 1 H, N-CH=CH-N), 7.31 (d, JH,H = 7.76 Hz, 1
H, NH-Cα,Leu), 7.26–7.14 (m, 5 H, HAr,Phe), 5.37 (s, 2 H, N-CH2-
Ar), 4.51 (m, 1 H, Cα,PheH), 4.25 (m, 1 H, Cα,LeuH), 3.94 (dq, 3JH,H
2
= 5.60, JH,H = 16.35 Hz, 2 H, ArCO-NHCα,GlyH2), 3.81 (s, 3 H,
3
3
N-CH3), 3.65 (d, JH,H = 5.90 Hz, 2 H, Cα,GlyH2), 3.20 (dd, JH,H
2
3
= 4.34, JH,H = 14.02 Hz, 1 H, Cβ,PheH2), 2.97 (dd, JH,H = 10.06,
2JH,H
14.02 Hz, H, β,PheH2), 1.63–1.48 (m, H,
γ,LeuHCβ,LeuH2), 0.80 [dd, JH,H = 6.18, JH,H = 15.95 Hz, 6 H,
=
1
C
3
3
2
C
CH(Cδ,LeuH3)2] ppm. 13C NMR (CD3CN, 100 MHz): δ = 174.5
(COO), 172.4, 171.6, 170.3, 168.4 (CPhe,Gly,Gly,ArON), 138.9
(Cq,p-Ar), 138.4 (Cq,p-Ar), 137.5 (N-CH-N), 135.2 (Cq,Ar,Phe), 130.2
(Cp-Ar), 129.53 (Cp-Ar), 129.2 (CAr,Phe), 127.4 (CAr,Phe), 125.0 (N-
CH=CH-N), 123.4 (N-CH=CH-N), 55.9 (Cα,Phe), 53.2 (N-CH2-
Ar), 52.2 (Cα,Leu), 44.8 (ArCO=NHCα,Gly), 43.8 (Cα,Gly), 40.9
(Cβ,Leu), 38.0 (Cβ,Phe), 36.9 (N-CH3), 25.4 (Cγ,Leu), 23.2, 21.7
(Cδ,Leu) ppm. RP-HPLC: tR = 13.52 min. MS (ESI+, CH3CN): m/z
= 591.22 [M – Br]+.
3
3
(d, JF,F = 19.1 Hz, 2 F, ArF), –157.3 (t, JF,F = 23.2 Hz, 1 F, ArF),
3
–162.2 (dd, JF,F = 19.1, 23.2 Hz, 2 F, ArF) ppm. MS (FAB pos.):
m/z = 847 [2M – Br]+, 383[M – Br]+. C18H12BrF5N2O2 (463.20):
calcd. C 46.67, H 2.61, N 6.05; found C 46.22, H 3.78, N 6.08.
Imidazolium Bromide 9: To
a solution of 1-methylimidazole
(1.67 mL, 21 mmol) in dry THF (20 mL) was added dropwise a
solution of p-(bromomethyl)benzoic acid (2.15 g, 10 mmol) in THF
(10 mL). After complete addition, the mixture was refluxed for 24 h
Imidazolium Bromide 11: Sieber amide resin-bound GlyGlyPheLeu
was synthesized by standard SPPS methods.[3a] The imidazolium
3364
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© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2008, 3359–3366

Ru and Rh Complexes with NHC Ligands
salt 9 was coupled to the resin-bound peptide as described here.
Imidazolium salt 9 and HOBtϫH2O were combined in 2 mL of
DMF, mixed vigorously, and left to stand for 5 min. The activated
imidazolium salt solution was shaken with the peptide-loaded
Sieber amide resin for ca. 24 h. After coupling of the salt to the
oligopeptide, the reaction mixture was filtered, the resin was
washed with DMF (3ϫ2 mL), CH2Cl2 (3ϫ2 mL), and methanol
(3ϫ2 mL) then shaken for 30 min in methanol and dried under
reduced pressure for ca. 1 h. The product was cleaved from the
resin by treatment with a solution of TFA (2 mL, 1% v/v in dichlo-
romethane) for ca. 4 h. The reaction mixture containing the crude
imidazolium salt peptide product was collected, the resin was ex-
tracted again with dichloromethane (2ϫ2 mL) and the combined
filtrates were concentrated under reduced pressure to a volume of
ca. 1 mL at which point cold diethyl ether (ca. 10 mL, –70 °C) and
pentane (10 mL) were added to precipitate the peptide conjugate.
The mixture was then centrifuged and the solvents decanted to col-
lect a white solid. The solid was taken up in cold ether again and
centrifuged (2 times). The supernatant was treated again with pen-
tane and the precipitate collected. The collected solids were com-
bined, dissolved in a minimum volume of acetonitrile and water
and lyophilised. The crude product was purified by reversed-phase
HPLC giving 11 as a white powder (100 mg, 75% based upon the
CH2Cl2 (2ϫ2 mL) and the combined filtrates were concentrated
under reduced pressure to a volume of ca. 1 mL at which point
cold diethyl ether (ca. 10 mL, –70 °C) and pentane (10 mL) were
added to precipitate the peptide conjugate. The mixture was centri-
fuged and the solvents decanted to collect an orange solid. The
solid was re-dissolved in cold ether again and centrifuged (2 times).
The supernatant was treated again with pentane and the precipitate
collected. The collected solids were combined, dissolved in a mini-
mum volume of acetonitrile and water and lyophilised. The crude
product was purified by reversed-phase HPLC yielding the ruthe-
nium carbene pseudoenkephalin as an orange powder (3 mg,
Method A, 8 mg, Method B); C41H52Cl2N6O6Ru (896.86). 1H
NMR ([D6]DMSO, 400 MHz): δ = 12.52 (br. s, COOH), 8.76 (t,
3
3JH,H = 5.74 Hz, 1 H, NHArC=O), 8.16 (d, JH,H = 7.81 Hz, 1 H,
3
3
NH-CαPhe), 8.12 (t, JH,H = 5.62 Hz, 1 H, NHα,Gly), 7.99 (d, JH,H
= 8.50 Hz, 1 H, NH-Cα,Leu), 7.86 (AAЈXXЈ, N = 8.35 Hz, 2 H,
3
3
HAr), 7.66 (d, JH,H = 2.00 Hz, 1 H, N-CH=CH-N), 7.33 (d, JH,H
= 2.00 Hz, 1 H, N-CH=CH-N), 7.28–7.14 (m, 2 H, HAr, 5 H,
HAr,Phe), 6.23 (d, JH,H = 5.72 Hz, 1 H, HAr,p-cymene), 6.10 (d, JH,H
= 10.17 Hz, 1 H, HAr,p-cymene), 6.10 + 5.87 (2 s, 2 H, N-CH2-Ar),
3
3
3
3
5.84 (d, JH,H = 9.48 Hz, 1 H, HAr,p-cymene), 5.45 (d, JH,H
=
9.48 Hz, 1 H, HAr,p-cymene), 4.51 (dt, 1 H, Cα,PheH), 4.23–4.17 (m,
1 H, Cα,LeuH), 3.91 (s, 3 H, N-CH3), 3.88 (m, 2 H, Cα,GlyH2), 3.66
(dq, 3JH,H = 5.65, 2JH,H = 16.78 Hz, 2 H, ArCO-NHCα,GlyH2), 3.06
1
resin loading of 0.68 mmol/g); C31H40BrN7O5 (669.23). H NMR
3
3
2
(CD3CN, 400 MHz): δ = 8.92 (s, 1 H, N-CH-N), 8.71 (t, JH,H
=
(dd, JH,H = 4.34, JH,H = 14.02 Hz, 1 H, Cβ,PheH2), 2.81–2.66 [m,
5.36 Hz, 1 H, NHArC=O), 8.17 (t, 3JH,H = 5.55 Hz, 1 H, NHα,Gly), 1 H, Cβ,PheH2, 1 H, CH(CH3)2], 2.06 (s, 3 H, Ar-CH3), 1.67–1.50
8.01 (AAЈXXЈ, N = 8.36 Hz, 2 H, HAr), 7.78 (d, JH,H = 7.76 Hz, (m, 3 H, Cγ,LeuHCβ,LeuH2), 1.13 [dd, JH,H = 6.90, JH,H
3
3
2
=
=
3
2
1 H, NH-CαPhe), 7.48–7.46 (m, 4 H, HAr, NHα,Gly, N-CH=CH-N), 10.56 Hz, 6 H, CH(CH3)2], 0.84 [dd, JH,H = 6.45, JH,H
3
7.42 (t, JH,H = 1.76 Hz, 1 H, N-CH=CH-N), 7.32–7.17 (m, 5 H,
HAr,Phe), 6.61 (s, 1 H, CONH2), 6.10 (s, 1 H, CONH2), 5.42 (s, 2 100 MHz):
H, N-CH2-Ar), 4.45–4.40 (m, 1 H, Cα,PheH), 4.16–4.10 (m, 1 H, (CPhe,Gly,Gly,ArON), 138.9 (Cq,p-Ar), 138.4 (Cq,p-Ar), 137.5 (NCN),
22.06 Hz,
6
H, CH(Cδ,LeuH3)2] ppm. 13C NMR (CD3CN,
δ
= 174.5 (COO), 172.4, 171.6, 170.3, 168.3
3
2
C
α,LeuH), 3.95 (dq, JH,H = 5.5, JH,H = 16.16 Hz, 2 H, ArCo- 135.2 (Cq,Ar,Phe), 130.2 (Cp-Ar), 129.5 (Cp-Ar), 129.2 (CAr,Phe), 127.4
3
NHCα,GlyH2), 3.67 (d, JH,H = 5.68 Hz, 2 H, Cα,GlyH2), 3.63 (s,
2 H, N-CH3), 3.22–3.06 (m, 2 H, Cβ,PheH2), 1.60–1.45 (m, 3 H,
(CAr,Phe), 125.0 (N-CH=CH-N), 123.4 (N-CH=CH-N), 55.9
(Cα,Phe), 53.2 (N-CH2-Ar), 52.2 (Cα,Leu), 44.8 (ArCO=NHCα,Gly),
43.8 (Cα,Gly), 40.9 (Cβ,Leu), 38.0 (Cβ,Phe), 36.9 (N-CH3), 25.4
3
2
C
γ,LeuHCβ,LeuH2), 0.74 [dd, JH,H = 6.05, JH,H = 21.21 Hz, 6 H,
CH(Cδ,LeuH3)2] ppm. 13C NMR (CD3CN, 100 MHz): δ = 176.3 (Cγ,Leu), 23.2, 21.7 (Cδ,Leu), 21.4 (Ar-CH3), 21.2 [CH(CH3)2], 17.3
(COO), 172.9, 172.7, 171.8, 168.7 (CPhe,Gly,Gly,ArON), 139.7
[CH(CH3)2] ppm. RP-HPLC: tR = 15.97 min. MS (ESI+, CH3CN):
(Cq,p-Ar), 139.2 (Cq,p-Ar), 138.4 (N-CH-N), 135.2 (Cq,Ar,Phe), 130.8 m/z = 861.17 [M – Cl]+, 825.21 [M – 2Cl – H]+.
(Cp-Ar), 130.0 (Cp-Ar), 129.9 (CAr,Phe), 128.1 (CAr,Phe), 125.7 (N-
CH=CH-N), 124.0 (N-CH=CH-N), 57.5 (Cα,Phe), 53.7 (N-CH2-
Ar), 53.5 (Cα,Leu), 45.3 (ArCO=NHCα,Gly), 44.8 (Cα,Gly), 38.2
Acknowledgments
(Cβ,Phe), 37.6 (N-CH3), 26.0 (Cβ,Leu), 25.4 (Cγ,Leu), 23.2, 21.7
We thank the Ruhr University Research School for funding and
Dr. Stephanie Cronje (Stellenbosch University, fellow of the Alex-
ander von Humboldt Foundation) for helpful discussions.
(Cδ,Leu) ppm. tR = 12.91 min. MS (ESI+, CH3CN): m/z = 590.27
[M – Br]+.
Ruthenium(II) Complex 12: 2-Chlorotrityl-resin-bound GlyGly-
PheLeu was synthesized by standard SPPS methods.[3a] Carbene
complex 3b (2.5 equiv.) and HOBtϫH2O (2.5 equiv.) were com-
bined in 2 mL of DMF, mixed vigorously, and left to stand for
5 min. The activated carbene complex solution was shaken with the
resin-bound peptide for ca. 48 h. After coupling of the complex to
the oligopeptide, the reaction mixture was filtered, the resin was
washed with DMF (3ϫ2 mL), CH2Cl2 (3ϫ2 mL), and methanol
(3ϫ2 mL) then shaken for 30 min in methanol and dried under
reduced pressure for ca. 1 h.
[1] a) U. Schatzschneider, N. Metzler-Nolte, Angew. Chem. Int. Ed.
2006, 45, 1504–1507; b) D. R. van Stavaren, N. Metzler-Nolte,
Chem. Rev. 2004, 104, 5931–5985; c) N. Metzler-Nolte, Angew.
Chem. Int. Ed. 2001, 40, 1040–1044.
[2] The field of Bioorganometallic Chemistry was covered recently
in a monograph and a review article: a) G. Jaouen (Ed.), Bioor-
ganometallics, Wiley-VCH, Weinheim, 2006; b) N. Metzler-
Nolte, in Comprehensive Organometallic Chemistry III, Vol. 1
(Ed.: G. Parkin), Elsevier, Amsterdam, 2006, pp. 883–920.
[3] a) S. I. Kirin, F. Noor, N. Metzler-Nolte, J. Chem. Educ. 2007,
84, 108–111; b) F. Noor, A. Wüstholz, R. Kinscherf, N.
Metzler-Nolte, Angew. Chem. Int. Ed. 2005, 44, 2429–2432; c)
N. Metzler-Nolte in Bioorganometallics (Ed.: G. Jaouen),
Wiley-VCH, Weinheim, 2006, pp. 125–179; d) N. Metzler-
Nolte, Chimia 2007, 61, 736–741; e) A. Maurer, H.-B. Kraatz,
N. Metzler-Nolte, Eur. J. Inorg. Chem. 2005, 3207–3210.
[4] M. A. Neukamm, A. Pinto, N. Metzler-Nolte, Chem. Commun.
2008, 232–234.
Method A: The product was cleaved twice from the resin by treat-
ment with a solution of TFA in CH2Cl2 (2 mL, 2% v/v) for ca. 2 h
each.
Method B: The product was cleaved from the resin by treatment
with a solution of TFA in methanol (2 mL, 5% v/v) for 2 h, fol-
lowed by a second cleavage cycle for 2 h with a solution of TFA in
methanol (2 mL, 10% v/v).
The reaction mixtures containing the crude ruthenium enkephalin
carbene product were collected, the resin was extracted again with
[5] a) M. C. Kuchta, A. Gross, A. Pinto, N. Metzler-Nolte, Inorg.
Chem. 2007, 46, 9400–9404. For related work and a recent re-
Eur. J. Inorg. Chem. 2008, 3359–3366
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
3365

J. Lemke, N. Metzler-Nolte
FULL PAPER
view of the field of metal-peptide conjugates by SPPS see: b)
G. Dirscherl, R. Knape, P. Hanson, B. König, Tetrahedron
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