Towards peptide-substituted titanocene anticancer drugs

Article abstract of DOI:10.1016/j.poly.2011.05.036

An alkyne-substituted fulvene was transformed via hydridolithiation followed by transmetallation with titanium tetrachloride into bis-[p-(prop-2-ynyloxy)-benzyl-cyclopentadienyl] titanium(IV) dichloride. Single crystals of this titanocene derivative could be obtained and the structure determined by X-ray diffraction. It showed that this compound crystallises in the space group C2/c with four molecules in the monoclinic cell. The alkyne-substituted titanocene dichloride derivative was then subject to a copper-catalysed azide-alkyne cycloaddition with its azide-functionalised methylester-protected phenylalanine reaction partner in order to form a linking triazole. This reaction was performed under anhydrous conditions employing a dichloromethane/acetonitrile solvent mixture with copper(I) iodide and 2,6-lutidine as the catalyst system. Under these conditions the adduct between the protein mimic and the titanocene was formed without hydrolysing the titanium dichloride moiety.

Full text of DOI:10.1016/j.poly.2011.05.036

Polyhedron 30 (2011) 2387–2390
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Towards peptide-substituted titanocene anticancer drugs
Johannes Zagermann a, Anthony Deally b, Nils Metzler-Nolte a, Helge Müller-Bunz b, Denise Wallis b,
Matthias Tacke b,
⇑
a Lehrstuhl für Anorganische Chemie I, Ruhr-Universität Bochum, Universitätsstrasse 150, D-44801 Bochum, Germany
b Conway Institute of Biomolecular and Biomedical Research, Centre for Synthesis and Chemical Biology (CSCB), UCD School of Chemistry and Chemical Biology,
University College Dublin, Belfield, Dublin 4, Ireland
a r t i c l e i n f o
a b s t r a c t
Article history:
An alkyne-substituted fulvene was transformed via hydridolithiation followed by transmetallation with
titanium tetrachloride into bis-[p-(prop-2-ynyloxy)-benzyl-cyclopentadienyl] titanium(IV) dichloride.
Single crystals of this titanocene derivative could be obtained and the structure determined by X-ray dif-
fraction. It showed that this compound crystallises in the space group C2/c with four molecules in the
monoclinic cell. The alkyne-substituted titanocene dichloride derivative was then subject to a copper-
catalysed azide–alkyne cycloaddition with its azide-functionalised methylester-protected phenylalanine
reaction partner in order to form a linking triazole. This reaction was performed under anhydrous condi-
tions employing a dichloromethane/acetonitrile solvent mixture with copper(I) iodide and 2,6-lutidine as
the catalyst system. Under these conditions the adduct between the protein mimic and the titanocene
was formed without hydrolysing the titanium dichloride moiety.
Received 25 February 2011
Accepted 31 May 2011
Available online 22 June 2011
Keywords:
Bioorganometallic chemistry
Metal-based anticancer drugs
Titanocene dichloride
[3+2] Cycloaddition
Targeting vector
Ó 2011 Elsevier Ltd. All rights reserved.
Enkephalin
1. Introduction
(CuAAC) or ‘‘click reaction’’ [12]. In this specific experiment, the
coupling of a alkyne-substituted titanocene is performed with a
In the last years significant progress was made with respect to
the development of titanocene anticancer reagents focusing on
difficult to treat advanced renal-cell cancer [1,2]. The lead
compound Titanocene Y (bis-[(p-methoxybenzyl)cyclopentadienyl]
titanium(IV) dichloride), which was synthesised through hydrido-
lithiation of p-anisyl fulvene with Super Hydride (LiBEt3H)
followed by transmetallation with TiCl4 [3], exhibits a unique
interaction with its target DNA [4] and is not cross-resistant with
respect to platinum-based anticancer drugs [5]. Titanocene Y
shows good results in vitro with respect to cytotoxicity [6] and
anti-angiogenesis [7] as well as impressive tumour volume growth
reductions in vivo [8,9]. Nevertheless, it would be highly attractive
to reduce the general toxicity of Titanocene Y by making it more
selective with respect to uptake into cancer cells. Therefore, the
introduction of a ‘‘targeting substituent’’, which is covalently
bonded to the titanocene dichloride moiety is a possible approach
to achieve this goal; the tumour-targeting oligopeptide enkephalin
would be an obvious choice [10].
low-molecular weight azide-functionalised protein mimic.
2. Experimental
2.1. General conditions
Titanium tetrachloride, Super Hydride (LiBEt3H, 1.0 M solution
in THF) and all organic starting materials were obtained from
Aldrich Chemical Company and used without further purification.
All solvents were dried using standard methods, distilled and col-
lected under an atmosphere of nitrogen prior to use. Manipulations
of air and moisture sensitive compounds were done using standard
Schlenk techniques, under a nitrogen atmosphere or in a Braun
glovebox. NMR spectra were measured on either a Varian 300,
400 or 500 MHz or a Bruker DPX250 spectrometer. Chemical shifts
are reported in ppm and are referenced to TMS. Coupling constants
(J) are quoted in Hertz. 13C{1H} assignments were obtained from
standard attached proton test (APT) experiments. IR spectra were
recorded on a Varian 3100 FT-IR Spectrometer employing KBr or
NaCl disks. UV–Vis spectra were recorded on a Varian Cary 50
UV4 Spectrometer. Electrospray ionisation mass spectra (ESI-MS)
were recorded on a Bruker Esquire 6000 spectrometer. In the Dub-
lin laboratory CHN analysis was done with an Exeter Analytical
CE-440 Elemental Analyser, while Cl was determined in mercuri-
metric titrations. At the Ruhr-University Bochum elemental analy-
ses were carried out at the RUBiospek department on a Elementar
The aim of the research is to develop a route to alkyne-
substituted titanocene derivatives, which could be easily reacted
with azide-substituted biological molecules, such as proteins using
Huisgen’s well known [3+2] cycloaddition reaction [11], which was
rediscovered as the copper catalysed azide–alkyne cycloaddition
⇑
Corresponding author.
E-mail address: matthias.tacke@ucd.ie (M. Tacke).
0277-5387/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2011.05.036

2388
J. Zagermann et al. / Polyhedron 30 (2011) 2387–2390
Table 1
Hanau vario EL. X-ray diffraction data for the compounds were col-
lected using a Bruker SMART APEX CCD area detector diffractome-
ter. A full sphere of reciprocal space was scanned by phi–omega
scans. Pseudo-empirical absorption correction based on redundant
reflections was performed by the program SADABS [13]. The struc-
tures were solved by direct methods using SHELXS-97 [14] and
Crystallographic refinement data for titanocene 2.
Identification code
2
Empirical formula
Formula weight
T (K)
Wavelength (Å)
Crystal system
Space group
Unit cell dimensions
a (Å)
C30 H26 Cl2 O2 Ti
537.31
100(2)
0.71073
Monoclinic
C2/c (#15)
refined by full matrix least-squares on F2 for all data using SHELXL
-
97 [14]. All hydrogen atoms were located in the difference fourier
map and allowed to refine freely for titanocene 2. The data collec-
tion details as well as reliability factors are listed in Table 1. Suit-
able crystals were grown in saturated dichloromethane solution
with slow infusion of pentane.
25.1349(14)
6.5066(4)
18.7714(11)
90
b (Å)
c (Å)
a
(°)
b (°)
124.060(1)
90
2543.3(3)
2.2. Synthesis
c
(°)
V (Å3)
Z
4
2.2.1. 6-[p-(Prop-2-ynyloxy)-phenyl] fulvene (1)
Dcalc (mg/m3)
1.403
0.573
1112
The synthesis of p-(prop-2-ynyloxy) benzaldehyde followed the
procedure outlined by Frixa and coworkers [15]. To a Schlenk flask
with p-(prop-2-ynyloxy) benzaldehyde (2.48 g, 15 mmol) dis-
solved in 40 mL MeOH, 2.1 mL (31 mmol) freshly cracked cyclo-
pentadiene and 1.9 mL (23 mmol) pyrrolidine were added at
room temperature under a nitrogen atmosphere. The solution
was stirred at ambient temperature for 20 h to give a dark orange
solution. The reaction was quenched with 2.7 mL (46 mmol) acetic
acid and 40 mL of water. The product was extracted into CHCl3
(3 Â 20 mL), dried over MgSO4, filtered, and the solvent was
removed under reduced pressure to give a dark red oil. This oil
was purified using column chromatography (silica gel, dichloro-
methane) to give a dark red solid (Yield: 2.12 g, 10.0 mmol, 66%).
1H NMR (CDCl3, 500 MHz): d 2.46 (t, 1H, OCH2CCH), 4.65 (d, 2H,
OCH2CCH), 6.69 (m, 2H, C5H4), 6.49 (d, 1H, C5H4), 6.32 (d, 1H, C5H4),
7.02 (d, 2H, J = 7.0, part of C6H4), 7.58 (d, 2H, J = 7.5, part of C6H4)
ppm. 13C NMR (CDCl3, 126 MHz, proton decoupled): 157.4 (Cq–O
of C6H4), 142.7 (C1 of fulvene), 136.9 (C3, C4 of fulvene), 134.1
(C2, C5 of fulvene), 131.3 (Cq–CH@C), 129.4 (Cq–CH of C6H4),
129.0 (C3, C5 of C6H4), 114.6 (C2, C6 of C6H4), 74.9 (OCH2CCH)),
77.1 (OCH2CCH)) 54.8 (OCH2CCH) ppm. IR absorptions (solid,
KBr): 3280, 2956, 2923, 1852, 1756, 1720, 1693, 1671, 1648,
1594, 1469, 1558, 1539, 1507, 1465, 1457, 1438, 1418, 1383,
1369, 1344, 1300, 1262, 1224, 1177, 1152 1139, 1089, 1026, 970,
928, 915, 903, 876, 850, 825, 808, 759, 730, 707, 677, 668, 658,
Absorption coefficient (mmÀ1
F(0 0 0)
Crystal size (mm)
h range for data collection (°)
Index ranges
)
0.50 Â 0.40 Â 0.15
1.96–32.05
À37 6 h 6 36, À9 6 k 6 9,
À27 6 l 6 27
Reflections collected
Independent reflections
Completeness to hmax
Absorption correction
Maximum and minimum transmission 0.9190 and 0.7687
Refinement method
29,656
4243 [Rint = 0.0205]
95.7%
Semi-empirical from equivalents
Full-matrix least-squares on F2
4243/0/211
1.044
R1 = 0.0338, wR2 = 0.0893
R1 = 0.0360, wR2 = 0.0912
0.511 and À0.262
Data/restraints/parameters
Goodness-of-fit (GOF) on F2
Final R indices [I >2
R indices (all data)
Largest difference in peak and hole
(e ÅÀ3
r(I)]
)
(3.7 mmol) of a 1.0 M titanium tetrachloride solution was added
directly to the Schlenk flask containing the dissolved intermediate.
After 24 h of reflux the solution became dark red, was then cooled
and the solvent was removed under reduced pressure. The remain-
ing dark red residue was extracted with 60 mL of dichloromethane
and filtered through Celite to remove the remaining LiCl. The red
filtrate was filtered twice more by gravity filtration. The solvent
was removed under reduced pressure to give a dark brown solid
(Yield: 1.39 g, 2.59 mmol, 71%).
634, 620, 605, 553, 526 cmÀ1. UV–Vis (CH2Cl2, nm): 228 (
e 8122),
1H NMR (CDCl3, 500 MHz): d 2.49 (s, 2H, OCH2CCH), 4.03 (s, 4H,
C5H4–CH2), 4.66 (s, 4H, OCH2CCH), 6.30 (m, 8H, C5H4), 6.91 (d, 2H,
J = 6.9 Hz, C6H4), 7.14 (d, 2H, J = 7.1 Hz, C6H4) ppm. 13C NMR (CDCl3,
126 MHz, proton decoupled): d 156.3 (Cq–O of C6H4), 137.5 (C2, C5
of C5H4), 132.5 (C3, C4 of C5H4), 131.3 (Cq–CH2 of C6H4), 130.1 (C1
of C5H4), 115.1 (C2, C6 of C6H4), 75.5 (OCH2CCH), 78.60 (OCH2CCH),
55.9 (OCH2CCH), 36.1 (C5H4–CH2) ppm. IR absorptions (solid, KBr):
3297, 3115, 2962, 2956, 2923, 2852, 1608, 1578, 1509, 1454, 1432,
261 (e 9510), 330 (e 1735), kmax 394 (e 451). ESI-MS (pos. mode):
m/z 209.37 ([M+H]+) (exact mass for C15H12O = 208.09). Anal. Calc.
for C15H12O: C, 86.5; H, 5.8. Found: C, 85.6; H, 5.9%.
2.2.2. Bis-[p-(prop-2-ynyloxy)-benzyl-cyclopentadienyl] titanium
dichloride (2)
10.0 mL (10.0 mmol) of 1 M solution of Super Hydride (LiBEt3H)
in THF was concentrated by removal of the solvent by heating it to
60 °C under reduced pressure of 10À2 mbar for 40 min and then to
90 °C for 20 min in a Schlenk flask. The concentrated Super Hydride
was dissolved in 30 mL of dry diethyl ether to give a cloudy white
suspension. 1.91 g (9.14 mmol) of the dark red solid 6-(p-(prop-2-
ynyloxy)-phenyl) fulvene was added to a Schlenk flask and was
dissolved in 60 mL dry diethyl ether to give a orange solution.
The fulvene solution was transferred to the Super Hydride solution
via syringe. The solution was left to stir for 16 h in which time a
pale yellow precipitate of the lithium cyclopentadienide interme-
diate formed and the solution had changed its colour from orange
to clear. The precipitate was filtered onto a frit and was washed
with diethyl ether. The pale yellow precipitate was dried briefly
under reduced pressure and was transferred to a Schlenk flask un-
der nitrogen. 1.58 g (7.31 mmol, 80% yield) of the lithiated cyclo-
pentadienide intermediate was obtained and dissolved in 60 mL
of dry THF to give a clear solution in a Schlenk flask. 3.7 mL
1383, 1259, 1240, 1213, 1174, 1097, 1018, 801, 667, 648, 475 cmÀ1
UV–Vis (CH2Cl2, nm): 288 ( 36338), 263 ( 29823), 297 ( 17061),
324 ( 10,075), 423 ( 2998), kmax 555 ( 443). Anal. Calc. for
TiC30H26O2Cl2: C, 67.0; H, 4.9; Cl, 13.2. Found: C, 69.0; H, 5.8; Cl,
10.9%.
.
e
e
e
e
e
e
2.2.3. N3–CH2–C(O)–Phe–OMe (3)
Open to air, azidoacetic acid (404 mg, 4 mmol) and TBTU
(1.28 g, 4 mmol) were stirred in dichloromethane (12 mL) for
15 min. N,N-diisopropylethylamine (0.68 mL, 28 mmol) was added
and the resultant homogenous solution stirred for 10 min. H–
Phe–OMe–HCl (0.86 g, 4.0 mmol) was added and the mixture stir-
red in the dark at room temperature over night. After removal of
the volatiles, the remaining oil was dissolved in ethyl acetate
(40 mL) and the solution washed with aqueous KHSO4 (1 N,
2 Â 40 mL), NaHSO4 (5%, 2 Â 50 mL) and brine (2 Â 50 mL). The

J. Zagermann et al. / Polyhedron 30 (2011) 2387–2390
2389
Table 2
Selected bond lengths and angles of titanocene 2.
Bond length (Å)
Ti-Cent
Ti–Cl
2.062(1)
2.367(3)
1.185(2)
C(14)–C(15)
Bond angle (o)
Cl–Ti–Cl#1
Scheme 1. Synthesis of the alkyne-substituted fulvene 1.
94.84(2)
Cent-Ti-Cent#1
132.31(1)
organic layer was dried over MgSO4 and evaporated to dryness to
give N3–CH2–C(O)–Phe–OMe as a colourless solid (Yield: 0.84 g,
3.2 mmol, 80%). An analytically pure sample was obtained by crys-
tallisation from ethylacetate.
Symmetry transformations used to generate equiva-
lent atoms: #1 Àx, y,Àz + 1/2.
1H NMR (250 MHz, CDCl3): d 7.33–7.10 (m, 5H, C6H5 of Phe), 6.65
outlined by Stone and Little [16] employing pyrrolidine as a cata-
lyst in methanol. The synthesis, which is shown in Scheme 1, gave
1 in 66% yield after purification. The fulvene was characterised and
purified by column chromatography, then carried onto the titano-
cene synthesis.
(d, J = 7.7, 1H, NHCO), 4.86 (m, 1H,
3.70 (s, 3H, CO2CH3) 3.19–3.01 (m, 2H, b-CH2 of Phe) ppm. 13C NMR
(62.5 MHz, CDCl3, proton decoupled): 171.6 (CO2CH3), 166.5
(CONH), 135.7 (Cq2 of C6H5), 129.4 (C3, C5 of C6H5), 128.9 (C2, C6 of
C6H5), 127.5 (C4 of C6H5), 53.2 ( -CH of Phe), 52.8 (COCH3), 52.6
a-CH of Phe), 3.90 (s, 2H, N3–CH2),
d
a
Titanocene 2 was synthesised via the hydridolithiation proce-
dure developed in the Tacke group [3]. This route uses LiBEt3H in
diethyl ether to transfer a hydride to a fulvene through nucleo-
philic addition to the exocyclic double bond of the fulvene 1 to
form an insoluble lithium cyclopentadienide intermediate. This
precipitated intermediate can then be washed on a glass frit, dried
and weighed to be used stoichiometrically in the next reaction
step. Two molar equivalents of the lithiated intermediate under-
went a transmetallation reaction in each case when reacted with
one molar equivalent of TiCl4 in THF under reflux, to give the
alkyne-substituted titanocene 2. This reaction proceeded smoothly
with the alkyne-substituted fulvene 1 to give an overall yield of
57% for both steps and is shown in Scheme 2.
(N3–CH2), 38.1 (b-CH2 of Phe) ppm. Anal. Calc. for C12H14N4O3: C,
54.96; H, 5.38; N, 21.36. Found: C, 55.70; H, 5.29; N, 21.75%.
2.2.4. Bis{[2-(4-((1-(2-(1-methoxy-1-oxo-3-phenylpropan-2-ylamino)
-2-oxoethyl)-1H-1,2,3-triazol-4-yl)methoxy)-benzyl)]-cyclopentadien
yl} titanium(IV) dichloride (4)
Bis-[p-(prop-2-ynyloxy)-benzyl-cyclopentadienyl] titanium(IV)
dichloride
2 (83 mg, 0.16 mmol), N3–CH2–C(O)–Phe–OMe 3
(89 mg, 0.34 mmol) and CuI (6 mg, 0.03 mmol) were mixed and
dissolved in CH2Cl2/acetonitrile (2:1, 12 mL). After addition of
2,6-lutidine (40 lL, 0.34 mmol) the mixture was degassed, purged
with argon and stirred for 12 h in the dark. Removal of the volatiles
gave a light brown solid, that was extracted with chloroform
(2 Â 8 mL). The volume of the orange extract was reduced to ca.
2 mL and Et2O (20 mL) was added to obtain an orange precipitate,
that was separated by filtration, and washed with Et2O (2 Â 4 mL)
and hexane (2 Â 4 mL). Drying under reduced pressure gave 4 as an
orange powder (Yield: 80 mg, 0.075 mmol, 48%).
3.1. Structural discussion
For the purpose of X-ray diffraction, suitable single crystals of 2
were grown by vapour diffusion of pentane into a saturated solu-
tion of the compound in dichloromethane. The collection and
refinement data for the crystal is shown in Table 1. The determined
structure for the alkyne-substituted titanocene dichloride deriva-
tive showed rod-like molecules packed in the solid state. Selected
bond lengths and angles are shown in Table 2.
The titanocene dichloride derivative 2, which is shown in Fig. 1,
crystallises in the space group C2/c with the titanium centre placed
on the twofold axis. Titanocene 2 has a distorted tetrahedral shape
with respect to its two halogenido ligands and two cyclopentadie-
nide groups. The titanium–cyclopentadienide centroid distance
was determined as 2.062(1) Å, while the titanium–chlorine bond
length was found to be 2.367(3) Å. The centroid-to-titanium-to-
centroid angle was measured to be 132.31(1)°, which is a signifi-
cant deviation from a normal tetrahedral angle due to the relative
bulk of the substituted cyclopentadienide groups. The Cl–Ti–Cl
bond angle on the other side was squeezed to 94.84(2)°, which
again is a reflection on the bulky Cp ligands. All these values com-
pare very well to the corresponding ones in Titanocene Y [3].
Titanocene 2 is slightly more rod shaped than Titanocene Y, which
1H NMR (250 MHz, CDCl3,): d 7.67 (s, 2H, CH of triazole), 7.44
(m, 2H, C6H5 of Phe), 7.23–6.87 (m, 16H, C6H5 of Phe, C6H4), 6.50
(d, 2H, J = 7.7, NH), 6.25 (m, 8H, C5H4–CH2), 5.19 (s, 4H, C6H4–
OCH2), 4.99 (s, 4H, NCH2–CONH), 4.80 (dd, 2H, J = 7.7; 9.2,
a-CH
of Phe), 3.98 (s, 4H, C5H4–CH2), 3.70 (s, 6H, CO2CH3), 3.15–3.00
(m, 4H, b-CH2 of Phe) ppm. 13C NMR (62.5 MHz, CDCl3, proton
decoupled): Note: Signal for O–CH2–triazole is obscured by the sol-
vent signal. d 171.5 (CO2CH3), 164.8 (CONH), 157.0 (Cq–O of C6H4),
144.9 (Cq of triazole), 137.9 (C2, C5 of C5H4), 135.4 (C3, C4 of C5H4),
131.7 (C1 of C5H4), 135.6 (Cq of C6H5), 130.4 (C3, C5 of C6H4), 129.3
(C3, C5 of C6H5), 128.9 (C2, C6 of C6H5), 127.5 (CH2-Cq of C6H4,
C6H5), 115.1 (C2, C6 of C6H4), 122.2 (CH of triazole), 62.1 (N–
CH2–CO), 53.6 (a-CH of Phe), 52.8 (OCH3), 37.8 (b-CH2 of Phe),
36.2 (C5H4-CH2) ppm. Anal. Calc. for C54H54Cl2N8O8Ti: C, 61.08; H,
5.13; N, 10.55. Found: C, 60.98; H, 5.45; N, 7.92%. ESI-MS (pos.
mode): m/z 1021.24 [M–2Cl+CH3OÀ]+, 495.28 ([CpR+H+Na]+),
473.32 ([CpR+2H]+) (exact Mass for C54H54Cl2N8O8Ti = 1060.29).
3. Results and discussion
appears more Z-shaped in its packing. There are no
p–p interac-
The synthesis of the alkyne-substituted fulvene 1 from the
tions to be seen between the substituted phenyl groups in any of
corresponding benzaldehyde derivative followed the conditions
the compounds and no solvent molecules are needed to stabilise
Scheme 2. Synthesis of the alkyne-substituted titanocene dichloride derivative 2.

2390
J. Zagermann et al. / Polyhedron 30 (2011) 2387–2390
ClI
C5
C12
C11
C9
C6
O
C1
C4
C13
C7
C10
C15
C14
C1I
Ti
Cl
C8
C3
C2
I
Fig. 1. Molecular structure of 2; thermal ellipsoids are drawn on the 50% probability level; symmetry operation: Àx, y, Àz + 1/2.
4. Conclusions and outlook
A first step towards peptide-substituted and therefore targeted
titanocene dichloride derivatives was performed. It is principally
possible to react an alkyne-substituted metallocene dihalogenide
compound with a low molecular weight azide-functionalised pro-
tein mimic under CuAAC conditions employing a dichloromethane/
acetonitrile 2:1 solvent mixture that prevents hydrolysis of the
dihalogenide ligands. Unfortunately but not unexpectedly, this
experimental procedure could not be extended to the oligopeptide
enkephalin, which made the synthesis of a peptide-substituted
titanocene dichloride derivative starting from 2 impossible. In
the future, a new titanocene derivative carrying the alkynyl-substi-
tuent as an anion will be synthesised and reacted with azide-func-
tionalised enkephalin to reach the goal of a peptide-substituted
titanocene anticancer drug.
Acknowledgements
The authors thank the Higher Education Authority (HEA), the
Centre for Synthesis and Chemical Biology (CSCB), the Irish
Research Council for Science, Engineering and Technology (IRC-
SET), University College Dublin (UCD) and COST D39 for funding.
J.Z. is a fellow of RUB research school, while D.W. received a post-
graduate studentship from IRCSET.
Scheme 3. Synthesis of triazole-conjugate 4 by CuAAC.
the lattice of 2. The carbon–carbon triple bond between C14 and
C15 was found to be 1.185(2) Å in length and delivers the basis
for the reactivity of the metallocene in the copper-catalysed
azide–alkyne cycloaddition.
Appendix A. Supplementary data
CCDC 814189 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge via http://
www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
UK; fax: (+44) 1223 336 033; or e-mail: deposit@ccdc.cam.ac.uk.
3.2. Cu-catalysed [3+2] cycloaddition
In order to test the suitability of alkyne-substituted titanocene
dichloride derivatives in Cu-catalysed [3+2] cycloadditions, 2 was
coupled to azide-functionalised, methylester-protected phenylala-
nine 3 as outlined in Scheme 3. To avoid the use of water, the reac-
tion was performed in a dichloromethane/acetonitrile mixture
using copper iodide in the presence of 2,6-lutidine [12]. Formation
of the triazole compound 4 was monitored by decrease of the 1H
NMR signal of the alkynyl-function and appearance of a singlet
for the triazole hydrogen (7.67 ppm in CDCl3). Although character-
isation of 4 by 1H as well as 13C NMR indicated no impurities,
elemental analysis repeatedly gave low values for nitrogen,
presumably due to formation of titanium nitride during sample
combustion. Analysis of 4 by ESI-MS (pos. mode) showed one
major signal with a titanium isotope pattern (m/z = 1021.24),
corresponding to loss of two chloride ligands and addition of a
methoxide ion. Two additional signals at m/z = 495.28 ([CpR+H
+Na]+) and 473.32 ([CpR+2H]+) indicate cleavage of the functional-
ised Cp ligands.
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