Synthesis and cytotoxicity studies of achiral azaindole-substituted titanocenes

Article abstract of DOI:10.1002/hc.20668

From the reaction of 1-methyl-1 H-pyr-rolo[2,3-b]pyridine (1a),1-(methoxymethyl)-1 H-pyrrolo[2,3-b]pyridine (1b), 1-isopropyl-1 H-pyrrolo[2,3-b]pyridine (1c), and 1-(4-methoxybenzyl)-1 H-pyrrolo[2,3-b] pyridine (1d) under Vilsmeier-Haak conditions, the co

Full text of DOI:10.1002/hc.20668

Heteroatom Chemistry
Volume 22, Number 2, 2011
Synthesis and Cytotoxicity Studies of Achiral
Azaindole-Substituted Titanocenes
Luis Miguel Mene´ndez Me´ndez, Anthony Deally, Donal F. O’Shea,
and Matthias Tacke
UCD School of Chemistry and Chemical Biology, Centre for Synthesis and Chemical Biology
(CSCB), Conway Institute of Biomolecular and Biomedical Research, University College Dublin,
Belfield, Dublin 4, Ireland
Received 25 August 2010; revised 26 November 2010
Chem 22:148–157, 2011; View this article online at
ABSTRACT: From the reaction of 1-methyl-1H-pyr-
wileyonlinelibrary.com. DOI 10.1002/hc.20668
rolo[2,3-b]pyridine (1a),1-(methoxymethyl)-1H-pyr-
rolo[2,3-b]pyridine (1b), 1-isopropyl-1H-pyrrolo[2,3-
b]pyridine (1c), and 1-(4-methoxybenzyl)-1H-pyrrolo
[2,3-b]pyridine (1d) under Vilsmeier–Haak con-
INTRODUCTION
ditions, the corresponding aldehydes in position 3
(2a–2d) were synthesized. These aldehydes were trans-
formed in the corresponding fulvenes (3a–3d) by the
Knoevenagel condensation and treated with Li[BEt3H]
to obtain the corresponding lithiated cyclopentadie-
nide intermediates (3ꢀa–3ꢀd). These intermediates
were, finally transmetallated to titanium with TiCl4
to yield the 7-azaindol-3-yl-substituted titanocenes bis
{[(1-methyl-1- H-pyrrolo[2,3-b]pyridin-3-yl)methyl]
cyclopentadienyl} titanium(IV) dichloride (4a),
bis{[(1-methoxymethyl-1-H - pyrrolo[2,3-b]pyridin-
3-yl)methyl]cyclopentadienyl} titanium(IV)dichloride
(4b), bis{[(1-Isopropyl-1-H-pyrrolo[2,3-b]pyridin-3-
yl)methyl]cyclopentadienyl} titanium(IV) dichloride
(4c), and bis{[(4-methoxybenzyl-1-H-pyrrolo[2,3-
b]pyridin-3-yl)methyl]cyclopentadienyl} titanium(IV)
dichloride (4d). All the titanocenes had their cytotox-
icity investigated through MTT-based preliminary in
vitro testing on the Caki-1 cell lines to determinate
their IC50 values. Titanocenes 4a–4c were found to
have IC50 values of 120 10, 83 13, and 54 12,
μM respectively, whereas 4d showed no cytotoxic
In the last few years, important results have been
reported about the cytotoxicity of titanium-based
reagents by showing their significant potential
against solid tumour, which makes them promising
anticancer drug candidates.
Titanocene Y (bis-[(p-methoxybenzyl)cyclopen-
tadienyl] titanium(IV) dichloride), which was syn-
thesized through hydridolithiation of p-anisyl ful-
vene with superhydride (Li[BEt3H]) followed by
transmetallation with TiCl4, is an example for these
compounds [1]. This titanocene showed an IC50
value of 21 μM when tested on the LLC-PK (epithelial
pig kidney) cell line, which has proven to be a good
mimic of a kidney carcinoma cell line and a reliable
tool for the optimization of titanocenes against this
type of cancer [1].
A further step in the field of these com-
pounds was the preparation and evaluation of new
anticancer drugs candidates including heteroaryl-
substituted and dimethylamino groups in the struc-
ture of titanocenes [3–5]. Two examples are Ti-
tanocene C [6] and Titanocene M [7], which showed
IC50 values of 5.5 and 5.4 μM, respectively, when
tested on the LLC-PK cell line, and their structures
are presented in Fig. 1.
ꢁ
C
activity.
2011 Wiley Periodicals, Inc. Heteroatom
Correspondence to: M. Tacke; e-mail: matthias.tacke@ucd.ie.
2011 Wiley Periodicals, Inc.
c
ꢀ
148

Synthesis and Cytotoxicity Studies of Achiral Azaindole-Substituted Titanocenes 149
N
Me2N
Me2N
OMe
OMe
N
N
N
N
Cl
Cl
Cl
Cl
Cl
Cl
NMe2
NMe2
Ti
Ti
Ti
N
Me2N
Me2N
Titanocene Y
Titanocene C
Titanocene M
FIGURE 1 Molecular structures of Titanocene Y, Titanocene C, and Titanocene M.
yl-substituted and (dimethylamino)-functionalized
titanocenes.
RESULTS AND DISCUSSION
The complete synthesis of titanocenes 4a–4d is
shown in Scheme 1. The syntheses of azaindoles 1a–
1d were made by the alkylation with the correspond-
ing alkyl halide [8,9]. Then, aldehydes 2a–2d were
prepared using the Vilsmeier–Haak reaction where
phosphoryl chloride and N,N-dimethylformamide
were used to introduce an aldehyde group in posi-
tion 3 of the corresponding 7-azaindole derivative.
The fulvenes 3a–3d were synthesized from these
aldehydes and freshly cracked cyclopentadiene un-
der Knoevenagel condensations using pyrrolidine as
the base of choice in methanol as a protic solvent.
Fulvenes were treated with LiBEt3H in dry ether
to give the lithiated cyclopentadienide intermedi-
ates (3ꢀa–3ꢀd); no specific synthetic problems were
encountered, when fulvenes derived from alkylated
7-azaindoles were used in the hydridolithiation re-
actions. And the last step was the transmetallation
reaction of these lithiated intermediates with TiCl4
(0.5 mol equivalents) under reflux for a day in THF
to give titanocenes 4a–4d as dark solids.
FIGURE
2 Structure and IC50 values of azaindole-
substituted titanocenes.
More recently, a variety of new dimethylamino-
functionalized
and
azaindole-substituted
ti-
tanocenes were synthesized and tested for their
cytotoxicity against LLC-PK cells [8]; their struc-
tures are presented in Fig. 2. Structurally, these
compounds present 7-azaindoles substituted at
position 1 linked to a Cp ring by a bridge carbon in
position 2. This bridge carbon is substituted with an
N,N-dimethylamino group.
These titanocenes showed an IC50 value from 8.8
to 87 μM when tested on the LLC-PK cell line [8].
This paper is presenting the synthesis and cyto-
toxicity of new 7-azaindolyl-substituted titanocenes
in comparison with the results obtained in the pre-
vious work. These titanocenes, which are various
achiral 7-azaindole-substituted at position 1, differ
from those previously reported by linking Cp rings at
position 3 (instead of position 2) and removing the
N,N-dimethylamino group at the bridging carbon.
The main idea of this article is to present a new se-
ries of 7-azaindol-3-yl-substituted titanocenes, their
synthesis, preliminary cytotoxicity studies, and to
compare with the results obtained for 7-azaindol-2-
Cytotoxicity Studies
Titanocenes 4a–4d were tested against the human
renal cell line, Caki-1 and exhibited IC50 values of
120
10, 83
13, and 54
12 μM, whereas 4d
showed no cytotoxicity as seen in Fig. 3.
These results shows that 4b has a similar value
comparing with its 7-azaindol-2-yl-substituted ana-
logue, which has a IC50 of 87 μM when tested on the
LLC-PK cell line [8], but in the case of 4a the value
has a 100-fold increase in magnitude in terms of IC50
value with respect to its 7-azaindol-2-yl-substituted
analogue.
Heteroatom Chemistry DOI 10.1002/hc

150 Mene´ndez Me´ndez et al.
SCHEME 1 Synthesis of titanocenes 4a-4d.
1,6
1,4
1,2
1,0
0,8
0,6
0,4
1,4
1,2
1,0
0,8
0,6
0,4
0,2
0,0
0,2
Titanocene 4c
Titanocene 4a
+
IC50
+
IC50= 120( )10E–6 (M)
0,0
–0,2
–0,4
–
–
Titanocene 4d
Titanocene 4b
IC50= cannot be calculated
IC50= 83(+)13E–6 (M)
–
1E–10
1E–9
1E–8
1E–7
1E–6
1E–5
1E–4
1E–3
log 10 Drug concentration (M)
log 10 Drug concentration (M)
FIGURE 3 Cytotoxicity studies of titanocenes 4a–4d against Caki-1 cells.
Heteroatom Chemistry DOI 10.1002/hc

Synthesis and Cytotoxicity Studies of Achiral Azaindole-Substituted Titanocenes 151
functionalized titanocenes that is believed to play
a role in the cytotoxicity of these compounds [8].
Structural DFT Studies of Dichloridobis{(1, 2, 3,
4,5-η)-1-[(1-methyl-1-H-pyrrolo[2,3-b]pyridin-3-
yl)methyl]cyclopenta - 2,4 - dien - 1 - yl} titanium
(IV) [TiCl2{η5-C5H4-CH(C7H4N2-CH3}2] (4a)
DFT calculations for titanocene 4a were made at
the B3LYP level, using the 6-31G(d, p) basis set (see
Fig. 4).
The length of the bond between Ti and the cy-
clopentadienyl C(19) and C(6) is 246.2 and 250.7
pm, respectively, whereas the C C bonds of the
cyclopentadienyl rings have bond lengths between
140.6 and 142.9 pm (see Table 1). The bond lengths
between C(19)–C(42) and C(6)–C(21) are very similar
150.4 and 150.2 pm, respectively. The optimization
shows a distance between C(42) and C(21) of 485.1
FIGURE 4 DFT-calculated structure of 4a.
On the other hand, the best cell testing result of
these compounds belongs to 4c that shows the lower
IC50 value of the group and its cytotoxicity is rela-
tively close to Titanocene Y (IC50 = 21 μM), whereas
4d showed no cytotoxicity for concentrations as high
as 500 μM.
One more time these results expose the possible
intramolecular stabilization of the mono or dication
in 7-azaindol-2-yl-substituted and dimethylamino-
pm, and the calculated bond length for C(42)–C(45)
and C(21)–C(23) are also very similar, 150.9 and
151.0 pm, respectively. Finally, the bond lengths be-
tween Ti and Cl (10) and Cl (11) are 235.0 and 234.0,
respectively.
The Cl(10)–Ti–Cl(11) angle is calculated to be
95.6◦. and the angles formed between C(19)–C(42)–
C(45) and C(6)–C(21)–C(23) are 113.2◦ and 112.8◦;
whereas C(19)–Ti–C(6) is 114.3◦.
TABLE 1 Selected Bond Lengths [pm] from the DFT-Calculated Structure of Titanocene 4a.
Bond
Length (pm)
Bond
Ti–C(6)
Ti–C(4)
Ti–C(3)
Ti–C(9)
Ti–C(7)
C(6)–C(4)
C(4)–C(3)
C(3)–C(9)
C(9)–C(7)
C(7)–C(6)
Ti–Cl(10)
Ti–Cl(11)
Length (pm)
Ti–C(19)
Ti–C(12)
Ti–C(13)
Ti–C(15)
246.2
241.3
236.8
243.1
245.8
141.4
142.2
141.9
140.6
142.9
150.4
150.2
485.1
150.9
250.7
240.0
237.8
241.0
245.8
142.0
142.6
140.9
142.1
141.6
235.0
234.0
Ti–C(17)
C(19)–C(12)
C(12)–C(13)
C(13)–C(15)
C(15)–C(17)
C(17)–C(19)
C(19)–C(42)
C(6)–C(21)
C(42)–C(21)
C(42)–C(45)
C(21)–C(23)
151.0
Heteroatom Chemistry DOI 10.1002/hc

152 Mene´ndez Me´ndez et al.
a percentage of the absorbance recorder for control
wells. The values used for the dose response curves
represent the values obtained from four consistent
MTT-based assays for each compound tested. The
data were analyzed using the programme package
Origin and delivered IC50 values and standard
deviations.
Conclusions and Outlook
The Vilsmeier–Haak formylation reaction of
7-azaindoles in position 3 has been found very effec-
tive to obtain precursors to make suitable fulvene
groups at this position and prepare new cytotoxic
7-azaindole-substituted titanocene dichloride com-
plex. On the Caki-1 cell line, complexes 4a–4c
gave IC50 values of 120
10, 83
13, and 54
1
General
2 μM, whereas 4d showed no toxicity. These results
show that 7-azaindol-3-yl-N-substituted titanocenes
complex present, in general, lower cytotoxicity than
7-azaindol-2-yl-substituted and dimethylamino-
functionalized titanocenes. This shows that the
possible intramolecular stabilization of the mono-
or dication by the dimethylamino-substituent in
these titanocenes could play an important role in
the cytotoxicity of these compounds.
TiCl4 (1.0 M in toluene) and tBuLi (1.7 M in pentane)
were obtained commercially from Sigma Aldrich
(St. Louis, MO). Manipulations of air- and moisture
sensitive compounds were done by using standard
Schlenk techniques under Ar. UV/vis spectra: Uni-
cam UV4 spectrometer; λmax in nm (ε in dm3 mol−1
cm−1) in CH2Cl2. IR spectra: Perkin–Elmer Paragon-
1000 FT-IR spectrometer; in KBr disk. H- and 13C-
1
NMR spectra: Varian 300 or 400 spectrometer; δ in
ppm rel. to Me4Si as an internal standard, J in Hz.
MS: quadrupole tandem mass spectrometer (Quat-
tro Micro, Micromass/Water’s Corp., USA); Solu-
tions were made in acetonitrile/H2O 1:1; in m/z. Ele-
mental analyses: anal. Exeter-CE-440 elemental an-
alyzer for C, H, and N.
EXPERIMENTAL
MTT-Based Assay
Preliminary in vitro cell tests [10] were performed on
the cell line Caki-1 to compare the cytotoxicity of the
compounds presented in this work. These cell lines
were chosen based on their regular and long-lasting
growth behavior. They were obtained from the
ATCC (American Tissue Cell Culture Collection) and
maintained in Dulbecco’s modified eagle medium
containing 10% (v/v) FCS (fetal calf serum) 1% (v/v)
penicillin streptomycin and 1% (v/v) L-glutamine.
Cells were seeded in 96-well plates containing
Synthesis of 1-Methyl-1H-pyrrolo[2,3-b]pyridine
(1a). In a 250-mL round-bottom flask, 50 mL of
dry DMF was added dropwise over a suspension of
7-azaindole (4.00 g, 33.9 mmol, 1.00 equiv) and NaH
(60% in oil) (1.63 g, 40.6 mmol, 1.20 equiv), under
N2 at 0◦C during 5 min. The mixture was then stirred
for 15 min at 0◦C, and a solution of MeI (2.11 mL,
33.89 mmol, 1 equiv) in dry DMF (10 mL) was added
dropwise over 5 min. The mixture was stirred at RT
for 3 h. A brown solution was obtained, and DMF
was evaporated to yield a brown solid that was redis-
solved in 50 mL of water and extracted with CH2Cl2
(3 × 50 mL), organic layers were washed with brine
(50 mL) and water (30 mL).
200-μL microtiter wells at
a
density of
5000 cells/200 μL of medium and were incu-
bated at 37◦C for 24 h to allow for exponential
growth. Then, the compounds tested were dissolved
in 70 μL of DMSO and diluted with medium to
obtain stock solutions of 5 × 10−4 M in concen-
tration. The cells were then treated with various
concentrations of the compounds and incubated
for 48 h at 37◦C. Then the solutions were removed
from the wells; and the cells were washed with PBS
(phosphate buffer solution), and fresh medium was
added to the wells. Following a recovery period
of 24-h incubation at 37◦C, individual wells were
treated with a 200-μL of a solution of MTT (3-(4,5-
dimethylthiazol-2-yl)-2,5-diphenyl-2H-tetrazolium
bromide) in medium (solution of 20 mg of MTT in
40 mL of medium). The cells were incubated for 3
h at 37◦C. The medium was then removed, and the
purple formazan crystals were dissolved in 200 μL
of DMSO per well. Absorbance was then measured
at 540 nm by a Wallac–Victor (multilabel HTS
counter) plate reader. Cell viability was expressed as
The organic layer was dried over Na2SO4filtered
and concentrated to give brown oil with a very in-
tense smell (4.05 g, 30.6 mmol, 90%).
1H NMR (400 MHz, CDCl3) δ: 8.32 (d, J = 4.4,
1H), 7.89 (d, J = 7.8, 1H), 7.16 (d, J = 3.4, 1H), 7.03
(dd, J = 7.8, 4.7, 1H), 6.43 (d, J = 3.4, 1H), 3.88
(s, 3H). 13C NMR (75 MHz, CDCl3) δ: 148.0, 143.0,
129.2, 128.9, 120.7, 115.9, 99.5, 31.5 ES-MS (pos.):
m/z 133.1 ([M + H]+)
Synthesis of 1-Methyl-1H-pyrrolo[2,3-b]pyridine-
3-carbaldehyde (2a). Under N2, (4.69 mL, 50.4
mmol, 1.10 equiv) of POCl3 were added dropwise
over 30 mL of dry DMF under N2 for ten minutes
at RT and the mixture was stirred for five minutes.
Then 1-methyl-7-azaindole (6.05 g, 45.8 mmol, 1.00
Heteroatom Chemistry DOI 10.1002/hc

Synthesis and Cytotoxicity Studies of Achiral Azaindole-Substituted Titanocenes 153
equiv) in dry DMF was added dropwise at 0◦C for
remove most of the THF. The concentrated superhy-
dride was dissolved in 200 mL of dry diethyl ether to
give a cloudy yellow suspension. 2.20 g (10.5 mmol,
1.00 equiv) of the red solid fulvene was added to a
Schlenk flask and was dissolved in 90-mL dry diethyl
ether to give a red solution. Then this solution was
transferred to the superhydride solution via cannula.
The solution was left to stir for 16 h at RT to give a
light yellow precipitate. This solid (lithiated inter-
mediate) was isolated on a Schlenk frit under N2,
yielding a yellow solid (2.04 g., 9.40 mmol, 89.4%).
This solid was dissolved in dry THF (60 mL) under
N2, then 3.15 mL (3.15 mmol, 0.50 equiv) of a 1 M so-
lution of TiCl4 in toluene was added and the mixture
was stirred for 16 h in a Schlenk flask under reflux.
THF was evaporated in vacuo, and a dark black solid
was obtained. This solid was redissolved in CH2Cl2,
and it was filtered with Celite by low pressure. An
organic layer was collected (brown-red liquid), and
the solvent was eliminated to yield a red oil, which
was washed with pentane and a dark red solid was
obtained (2.08 g., 41.0%)
5 min. The formation of a brown solid was observed,
and the mixture was stirred at 40◦C for 2 h. After
this time, 10 g of ice was added and the reaction
mixture (pH = 3) was basified with 60 mL of 40%
NaOH. The mixture was refluxed for 5 min, and a
red solution was observed. DMF was evaporated in
vacuo, and the brown solid obtained was redissolved
in water (200 mL). This aqueous layer was extracted
with CH2Cl2 (4 × 50 mL); the organic layers were
combined, washed with brine (50 mL), dried over
Na2SO4, and concentrated to give (3.25 g, 20.2 mmol,
44.5%) a brown solid.
1H NMR (400 MHz, CDCl3) δ: 9.96 (s, 1H), 8.54
(dd, J = 7.8, 1.6, 1H), 8.43 (dd, J = 4.8, 1.6, 1H), 7.83
(s, 1H), 7.26 (dd, J = 4.2, 3.6, 1H), 3.96 (s, 3H). 13C
NMR (101 MHz, CDCl3) δ: 184.41, 148.68, 145.06,
138.88, 130.38, 118.81, 117.60, 116.25, 31.72. ES-
MS(pos.) m/z: 161.1 [M + H]+.
Synthesis of 3-(Cyclopenta-2,4-dienylideneme-
thyl)-1-methyl-1H-pyrrolo
[2,3-b]pyridine
(3a).
1-Methyl-7-azaindole-3-carboxaldehyde (3.25 g,
20.3 mmol, 1.00 equiv) was dissolved in dry Me(OH)
(40 mL) under N2, and freshly cracked cyclopenta-
diene (3.40 mL, 50.7 mmol, 2.50 equiv) was added
at 0◦C. The solution was stirred for 5 min, and
pyrrolidine (2.54 mL, 30.4 mmol, 1.5 equiv) in dry
MeOH (10 mL) was added, and mixture was stirred
for 90 min at RT. After this time, a red solution
was observed, AcOH (2.32 mL, 40.6 mmol, 2.00
equiv) was added, and solution was stirred at RT for
another 10 min.
At this point, 50 mL of Et2O and 50 mL of wa-
ter were added to the flask. An aqueous phase was
extracted with Et2O (3 × 50 mL), the organic lay-
ers were combined, washed with brine (2 × 50 mL),
dried over Na2SO4 filtered, and concentrated to yield
a dark red solid (3.40 g, 16.3 mmol, 80.4%)
1H NMR (400 MHz, CDCl3) δ: 8.41–7.07 (m,
8H C7H4N2), 6.35 (m, 8H C5H4), 4.31 (s, 4H), 4.11
(s, 6H). 13C NMR (101 MHz, CDCl3) δ: 138.35,
136.15,134.11,131.87,131.30,129.27,128.22,127.10,
125.25,123.37,114.91,114.39 (C5H4, C7H4N2); 33.06
(C5H4CH2); 26.25 (CH3–). Anal. calcd for C28H26
Cl2N4Ti (536.10): C 62.68, H 4.85, Cl 13.43, N 10.44;
found: C 59.15, H 5.31, Cl 16.28, N 8.65. ES-MS(neg)
m/z: 519.1 [M – H – Cl – H2O]−. IR absorptions
(KBr, cm−1): 3402, 3083, 2923, 2853, 2654, 2190,
1637, 1524, 1459, 1262, 1131, 1038, 798, 747. UV–
vis (CH2Cl2, nm): λ 290 (ε 19051), λ 256 (ε 18962), λ
233 (ε 28035).
Synthesis of 1-(methoxymethyl)-1H-pyrrolo[2,3-
b]pyridine (1b). 7-Azaindole (2.00 g, 16.9 mmol,
1.00 equiv) was mixed with NaH (60% dispersion
in mineral oil) (0.81 g, 20.3 mmol, 1.20 equiv) in
a 100-mL round-bottom flask under N2 at 0◦C, and
dry DMF (40 mL) was added dropwise within 10 min
and the mixture was then stirred for a further 30 min
at 0◦C. Then a solution of methoxy methyl chloride
(1.28 mL, 16.9 mmol, 1.00 equiv) in DMF (5 mL)
was added dropwise at 0◦C under N2for 5 min, and
the mixture was stirred at room temperature for 3
h. After this, a light yellow suspension was observed.
25 mL of water was added over the reaction mixture,
and the solvent was eliminated under reduced pres-
sure to yield a yellow oil. 30 mL of water was added to
the oil; and the mixture was extracted with CH2Cl2
(3 × 25 mL), the organic layer was combined and
dried over Na2SO4and concentrated to give 2.28 g
(14.0 mmol, 83.1% yield) of a brown oil.
1H NMR (400 MHz, CDCl3) δ; 8.39 (dd, J = 4.7,
1.4, 1H), 8.11 (dd, J = 7.9, 1.4, 1H), 7.64 (s, 1H), 7.34
(s, 1H), 7.17 (dd, J = 7.9, 4.7, 1H), 6.73 (d, J = 5.2,
1H), 6.64 (m, 1H), 6.47 (d, J = 5.0, 1H), 6.38 (m, 1H),
3.94 (s, 3H). 13C NMR (101 MHz, CDCl3) δ: 148.03,
144.03, 141.39, 133.84, 131.53, 129.16, 129.13,
127.53, 126.82, 120.29, 119.53, 116.93, 111.63, 31.74.
ES-MS(pos.) m/z: 209.43 [M + H]+.
Synthesis of bis{[(1-Methyl-1-H-pyrrolo[2, 3-b]
pyridin - 3 - yl)methyl]cyclopentadienyl }titanium(IV)
dichloride [TiCl2{η5-C5H4-CH(C7H4N2-CH3}2] (4a).
14.0 mL (13.7 mmol, 1.30 equiv) of a solution of
superhydride (LiBEt3H) in THF (1 M) was concen-
trated by the removal of the solvent by heating it to
60˚C under reduced pressure of 10−2 mbar for 40 min
and then to 90˚C for 20 min in a Schlenk flask to
Heteroatom Chemistry DOI 10.1002/hc

154 Mene´ndez Me´ndez et al.
1H NMR (300 MHz, CDCl3) δ: 8.32 (dd, J = 4.7,
1.5, 1H), 7.89 (dd, J = 7.8, 1.6, 1H), 7.30 (d, J = 3.6,
1H), 7.07 (dd, J = 7.8, 4.7, 1H), 6.49 (d, J = 3.6, 1H),
5.62 (s, 2H), 3.28 (s, 3H). 13C NMR (75 MHz, CDCl3)
δ: 148.1, 143.2, 128.8, 127.8, 120.6, 116.4, 101.1, 74.7,
56.2. ES-MS(pos.) m/z 163.1 [M + H]+.
CDCl3) δ: 148.39, 144.31, 142.26, 134.25, 129.93,
129.73, 128.58, 127.65, 126.76, 120.28, 119.72,
117.54, 113.11, 75.05, 56.72. ES-MS(pos.) m/z: 239.5
[M + H]+
Synthesis of bis{[(1-Methoxymethyl-1-H-pyr-
rolo[2,3-b]pyridin-3-yl)methyl]cyclopentadienyl} tita-
nium(IV) Dichloride [TiCl2{η5-C5H4-CH(C7H4N2-
CH2-O-CH3}2] (4b). In a 250-mL Schlenk flask
under reduced pressure, 10 mL of a 1 M solution of
superhydride (LiBEt3H) in THF was stirred at 60◦C
for 20 min and at 90◦C for 30 min at a pressure of
10−2 mbar to remove most of the THF.
In another Schlenk flask under N2, 3-(cyclo-
penta-2,4-dienylidenemethyl)-1-(methoxymethyl)-
1H-pyrrolo[2,3-b]pyridine (1.80 g., 7.50 mmol, 1.00
equiv) was dissolved in dry Et2O (200 mL) and the
red solution formed added to the LiBEt3H solution.
While stirring overnight at RT, an orange solution
was obtained. It contained precipitated yellow lithi-
ated intermediate. This solid was isolated using a
Schlenk frit under N2, yielding a yellow solid (1.47
g, 5.95 mmol, 79.3%)
Synthesis of 1-(Methoxymethyl)-1H-pyrrolo[2,3-
b]pyridine-3-carbaldehyde (2b). In a 100-mL round-
bottom flask, under N2, 15 mL of dry DMF
was stirred with POCl3 (1.31 mL, 14.0 mmol,
1.00 equiv) at 0◦C for
5
min. After this,
a solution of 1-(methoxymethyl)-1H-pyrrolo[2,3-
b]pyridine (2.28 g., 14.0 mmol, 1.00 equiv) in DMF
(10 mL) was added dropwise at 0◦C and then the mix-
ture was stirred for 2 h at 40◦C. A yellow precipitate
was observed in the flask. The solvent was eliminated
in vacuo, and a yellow solid was obtained. The solid
was redissolved in water (150 mL), and the solution
was extracted with CH2Cl2 (4 × 35 mL). Organic lay-
ers were combined and washed with brine (1 × 30
mL), dried over Na2SO4, filtered, and concentrated
to yield a yellow solid (2,13 g., 11.1 mmol, 79.5%)
1H NMR (400 MHz, CDCl3) δ: 10.00 (s, 1H), 8.55
(dd, J = 7.9, 1.4, 1H), 8.42 (dd, J = 4.7, 1.4, 1H), 7.97
(s, 1H), 7.27 (dd, J = 7.9, 4.7, 1H), 5.68 (s, 2H), 3.35
(s, 3H). 13C NMR (101 MHz, CDCl3) δ: 185.01, 148.62,
145.46, 137.18, 130.75, 119.32, 117.61, 117.48, 75.44,
56.87. ES-MS(pos.) m/z: 160.3 [M – MeOH + H]+
This intermediate was redissolved under N2, in
40 mL of dry THF, and 3 mL (3 mmol, 0.5 equiv) of
a 1 M solution of TiCl4 in toluene was added and the
mixture was stirred overnight under reflux, which
resulted in a black solution. THF was removed in
vacuo at RT, and a dark green solid was obtained.
The solid was redissolved in CH2Cl2, and the solution
®
3-(Cyclopenta-2,4-dienylidenemethyl)-1-(metho-
was filtered through a plug of Celite . The organic
xymethyl)-1H-pyrrolo[2,3-b]pyridine (3b). In
a
layer was collected, the solvent evaporated in vacuo
to yield a red solid, which was washed with pentane
(1.20 g, 2.01 mmol, 34.0%).
100-mL round-bottom flask, 3-(cyclopenta-2,4-
dienylidenemethyl)-1-(methoxymethyl)-1H-pyrrolo
[2,3-b]pyridine (2.13 g., 11.2 mmol, 1.00 equiv) was
dissolved in dry MeOH (30 mL) under N2 at 0◦C.
Then freshly cracked cyclopentadiene (1.88 mL,
27.9 mmol, 2.50 equiv) was added dropwise to the
mixture and the solution was stirred for 5 min.
Then 1.40 mL of pyrrolidine in 10.0 mL of MeOH
was added dropwise and the mixture was stirred for
90 min at RT.
1H NMR (400 MHz, CDCl3) δ: 8.40- 7.10 (m, 4H,
C7H4N2), 6.34–6.44 (m, 8H, C5H4), 5.65 (s, 4H), 4.26
(s, 4H), 3.32 (s, 6H). 13C NMR (101 MHz, CDCl3) δ:
129.80,129.00,128.14,126.51,126.45,126.36,125.26,
122.93,116.14,114.95,113.30 (C5H4,C7H4N2); 75.17,
56.48 (MeOCH2) 34.07 (C5H4CH2). Anal. calcd for
C30H30Cl2N4O2Ti (596.12): C 60.32, H 5.06, Cl 11.74,
N 9.39; found: C 56.05, H 5.26, Cl 11.74, N 7.56.
ES-MS(neg) m/z: 577.3 [M – H – Cl + H2O]−. IR
absorptions (KBr, cm−1): 3397, 3092, 2929, 1635,
1434, 1384, 1338, 1183, 1125, 1038, 800, 777. UV–
vis (CH2Cl2, nm): λ 290 (ε 17416), λ 260 (ε 15794), λ
232 (ε 27636), λ 229 (ε 25633)
After that, 1.30 mL of AcOH was added to the
mixture and a gas formation was observed, while
the mixture was stirred for a further 10 min.
50 mL of water and 50 mL of Et2O were added
to the flask, and the aqueous layer was extracted in
Et2O (3 × 50 mL). Organic layers were combined,
washed with brine (2 × 35 mL), dried over Na2SO4,
filtered, and concentrated to yield a red solid (2.05 g,
8.61 mmol, 77.0%)
Synthesis of 1-Isopropyl-1H-pyrrolo[2,3-b]pyri-
dine (1c). In a 250-mL round-bottom flask, 60 mL
of dry DMF was added dropwise over a suspension
of 7-azaindole (4.00 g, 33.9 mmol, 1.00 equiv) and
NaH (60% in oil) (1.63 g, 40.6 mmol, 1.20 equiv),
under N2 at 0◦C for 5 min. During this addition,
H2gas evolved and a pale suspension was formed;
1H NMR (400 MHz, CDCl3) δ: 8.39 (dd, J = 4.7,
1.5, 1H), 8.11 (dd, J = 7.9, 1.5, 1H), 7.75 (s, 1H),
7.33 (s, 1H), 7.20 (dd, J = 7.9, 4.7, 1H), 6.75–6.36 (m,
4H), 5.68 (s, 2H), 3.34 (s, 3H). 13C NMR (101 MHz,
Heteroatom Chemistry DOI 10.1002/hc

Synthesis and Cytotoxicity Studies of Achiral Azaindole-Substituted Titanocenes 155
the mixture was stirred at RT for another 10 min.
100 mL of H2O and 100 mL of AcOEt were added
to the solution, and the aqueous layer was extracted
in AcOEt (3 × 50 mL), then organic phases were
combined, washed with brine (2 × 50 mL), dried
over Na2SO4, filtered, and concentrated to yield a
dark red oil. (3.48 g, 14.7 mmol, 55.0%).
Then isopropyl bromide (3.80 mL, 40.6 mmol, 1.20
equiv) was added, and the solution was stirred at RT
overnight (after 1 h, the solution turned yellow). In
the workup, the reaction mixture was dissolved in
400 mL of ice-water and then extracted with AcOEt
(3 × 30 mL), the organic layers were combined,
washed with brine (2×30 mL), filtered, and dried
in vacuo to yield a yellow oil (5.36 g, 98%).
1H NMR (400 MHz, CDCl3) δ: 8.37 (dd, J = 4.7,
1.5, 1H), 8.11 (dd, J = 7.9, 1.5, 1H), 7.76 (s, 1H), 7.38
(s, 1H), 7.16 (dd, J = 7.9, 4.7, 1H), 6.77 – 6.36 (m,
4H), 5.23 (m, 1H), 1.57 (d, J = 6.8, 6H). 13C NMR
(101 MHz, CDCl3) δ: 147.49, 143.73, 141.11, 133.68,
129.61, 128.96, 127.45, 127.28, 126.83, 120.48,
119.55, 117.10, 111.89, 46.06, 22.77. ES-MS(pos.)
m/z: 237.46 [M + H]+.
1H NMR (400 MHz, CDCl3) δ: 8.30 (dd, J = 4.7,
1.5, 1H), 7.87 (dd, J = 7.8, 1.5, 1H), 7.30 (d, J =
3.6, 1H), 7.02 (dd, J = 7.8, 4.7, 1H), 6.44 (d, J = 3.6,
1H), 5.20 (m, 1H), 1.50 (d, J = 6.8, 6H). 13C NMR
(101 MHz, CDCl3) δ: 146.96, 142.41, 128.60, 124.05,
120.69, 115.57, 99.43, 45.19, 22.89. ES-MS(pos.) m/z:
161.38 [M + H]+.
Synthesis of bis{[(1-Isopropyl-1-H-pyrrolo[2,3-
b]pyridin-3-yl)methyl] cyclopentadienyl} titanium
(IV)
dichloride
[TiCl2{η5-C5H4-CH(C7H4N2-CH-
Synthesis of 1-Isopropyl-1H-pyrrolo[2,3-b]pyri-
(CH3)2}2] (4c). In a 250-mL Schlenk flask, a 1 M
solution of LiBEt3H in THF (18.15 mL, 18.15 mmol,
1.30 equiv) was heated at 60◦C for 20 min and at 90◦C
for 30 min under vacuum of 10−2 mbar to remove
most of the THF. In another 250-mL Schlenk flask,
3-(cyclopenta-2,4-dienylidenemethyl)-1-(isopropyl)-
1H-pyrrolo[2,3-b]pyridine (3.30 g., 13.9 mmol, 1.00
equiv) was dissolved in dry Et2O (200 mL) under
N2 and the solution was added to the concentrated
LiBEt3H solution via cannula. Then the mixture was
stirred overnight at RT and a light yellow precipitate
of the lithiated intermediate was obtained. The
yellow precipitate was isolated using a Schlenk frit
under N2, yielding (1.54 g, 6.30 mmol, 45.0%) of a
light yellow solid.
dine-3-carbaldehyde (2c). In
a 250-mL round-
bottom flask, POCl3 (5.01 mL, 54.0 mmol, 1.50 equiv)
was dissolved in dry DMF (40 mL) and the mix-
ture was stirred for 10 min under N2 at RT. Then
a solution of 1-isopropyl-1H-pyrrolo[2,3-b]pyridine
(5.76 g., 36.0 mmol, 1.00 equiv) in DMF (10.0 mL)
was added dropwise under N2 at 0◦C for 3 min and
the solution was stirred at 40◦C overnight during
which the solution turned brown and a white pre-
cipitate was observed. The mixture was basified with
a solution of 40% NaOH and poured into 300 mL
of water-ice. The white solid was filtered off, and
the brown solution was extracted with AcOEt (3 ×
40 mL). The organic layers were combined, washed
with brine (2 × 40 mL), filtered, and concentrated to
yield a dark brown solid (5.15 g, 27.3 mmol, 76.0%)
1H NMR (400 MHz, CDCl3) δ: 9.94 (s, 1H), 8.50
(dd, J = 7.8, 1.6, 1H), 8.37 (dd, J = 4.7, 1.6, 1H),
7.93 (s, 1H), 7.21 (dd, J = 7.8, 4.7, 1H), 5.19 (m,
1H), 1.55 (d, J = 6.8, 6H). 13C NMR (75 MHz,
CDCl3) δ: 184.48, 148.12, 144.76, 134.79, 130.36,
118.91, 117.93, 116.44, 46.55, 22.65. ES-MS(pos.)
m/z: 189.41 [M + H]+.
In a 250-mL Schlenk flask, the lithiated interme-
diate (1.54 g., 6.30 mmol, 1.00 equiv) was dissolved
in dry THF (100 mL) under N2.At this point, (3.15
mL, 3.15 mmol, 0.50 equiv) of a 1 M solution of
TiCl4 in toluene was added and the mixture was kept
under refluxed for 16 h. A red solution was obtained,
and THF was eliminated in vacuo at RT using to
yield a red oil, which was redissolved in CH2Cl2 and
®
filtered through a plug of Celite . The solvent was
evaporated in vacuo, and a dark red solid was ob-
tained (1.55 g., 2.60 mmol, 42.0%).
Synthesis of 3 - (Cyclopenta - 2, 4 - dienylideneme-
thyl)-1-(isopropyl)-1H-pyrrolo[2,3-b]pyridine(3c). 1-
Isopropyl - 1H - pyrrolo[2,3 - b]pyridine - 3 - carbald-
ehyde (5.15 g, 27.3 mmol, 1 equiv.) and freshly
cracked cyclopentadiene (4.58 mL, 68.4 mmol,
2.5 equiv) were dissolved in MeOH (110 mL) in a
250-mL round-bottom flask under N2 at 0◦C. Then
pyrrolidine (3.43 mL, 41.0 mmol, 1.50 equiv) was
added dropwise during 2 min and the resulting
solution was stirred overnight. As a result, a dark
red solution was obtained, which was neutralized
with acetic acid (3.12 mL, 54.7 mmol, 2.00 equiv).
1H NMR (400 MHz, CDCl3) δ: 8.57–6.84 (m, 8H,
C7H4N2), 6.44–6.06 (m, 8H, C5H4), 5.31–5.24 (m, 2H),
4.24 (s, 4H), 1.51 (m, 12H). 13C NMR (101 MHz,
CDCl3) δ: 137.24, 134.41, 134.21, 132.29, 131.34,
129.01, 128.21, 127.30, 123.05, 122.42, 115.53,
115.17 (C5H4, C7H4N2); 45.61 26.66 (Me2CH); 22.88
(C5H4CH2). Anal. calcd for C32H34Cl2N4Ti (592.16): C
64.86, H 5.74, Cl 11.86, N 9.49; found: C 68.00, H
6.25, Cl 8.53., N 9.29. ES-MS(neg) m/z: 573.66 [M –
H – Cl + H2O]−. IR absorptions (KBr, cm−1): 3427,
3092, 3047, 2971, 2927, 2872, 1595, 1436, 1277, 1230,
Heteroatom Chemistry DOI 10.1002/hc

156 Mene´ndez Me´ndez et al.
129.64, 127.79, 119.03, 117.70, 116.67, 114.43, 55.31,
1199, 1124, 801, 770. UV–vis (CH2Cl2, nm): λ 290
(ε 21262), λ 259 (ε 20779), λ 233 (ε 43100).
48.11. ES-MS(pos.) m/z: 267.14 [M + H]+.
Synthesis of 3-(Cyclopenta-2,4-dienylideneme-
thyl)-1-(4-methoxybenzyl)-1H-pyrrolo[2,3-b]pyridine
(3d). In a 500-mL round-bottom flask, 1-(4-metho-
xybenzyl) - 1H - pyrrolo[2,3 - b]pyridine - 3 - carbalde-
hyde. (2d) (6.72 g., 25.2 mmol, 1.00 equiv) was
dissolved in MeOH and freshly cracked CpH
(4.23 mL, 63.1 mmol, 2.50 equiv) was added at RT.
Then pyrrolidine (3.16 mL, 37.9 mmol, 1.50 equiv)
was added dropwise and the mixture was stirred
overnight at RT. After that time, a dark solution was
obtained, AcOH (3.00 mL) was added, and the mix-
ture was stirred for 5 min at RT. Then the solution
was poured over 200 mL of water and it was ex-
tracted with AcOEt (3 × 200 mL) and organic layers
were washed with brine (3 × 100 mL). After that,
it was dried over Na2SO4 and filtered. The solvent
was evaporated in vacuo to yield a dark red oil that
was purified using a flash chromatography column
(SiO2/CH2Cl2/AcOEt 10%) to yield a red oil (5.73 g.,
18.2 mmol, 72.2%).
Synthesis of 1-(4-Methoxybenzyl)-1H-pyrrolo[2,3-
b]pyridine (1d). In a 250-mL round-bottom flask,
a mixture of 7-azaindole (3.90 g, 33.0 mmol, 1.00
equiv) and NaH (60% in mineral oil) (1.98 g,
49.5 mmol, 1.50 equiv) was stirred in DMF (50 mL)
under N2 at 0◦C for 10 min and then the mixture was
allowed to warm up to RT during another 15 min. To
this light yellow solution, 4-methoxybenzylbromide
(5.80 mL, 59.4 mmol, 1.20 equiv) was added drop-
wise over 5 min and the resulting yellow solution
was stirred for 16 h at RT.
The crude reaction mixture was poured into ice-
water (250 mL) and was extracted with AcOEt (3 ×
50 mL); the organic layers were combined, washed
with brine (1 × 50 mL), filtered, and concentrated to
yield a yellow oil. The oil was purified by chromatog-
raphy on a column (SiO2) with AcOEt as the eluent
to give 1d (7.40 g, 31.0 mmol, 91.4%) as a yellow oil.
1H NMR (300 MHz, CDCl3)δ: 8.39 (dd, J = 4.7,
1.5, 1H), 7.93 (dd, J = 7.8, 1.5, 1H), 7.20–7.15 (m,
3H), 7.09 (dd, J = 7.8, 4.7, 1H), 6.85–6.82 (m, 1H),
6.48 (d, J = 3.5, 1H), 5.46 (s, 2H), 3.77 (s, 3H). 13C
NMR (101 MHz, CDCl3) δ: 159.11, 147.66,142.95,
129.89, 128.96, 128.78, 127.76, 120.56, 115.80,
114.08, 99.97, 55.22, 47.28. ES-MS(pos.) m/z: 239.43
[M + H]+.
1H NMR (400 MHz, CDCl3) δ: 8.41 (dd, J = 4.7,
1.5, 1H), 8.11 (dd, J = 7.9, 1.5, 1H), 7.60 (s, 1H),
7.33 (s, 1H), 7.25–7.14 (m, 3H), 6.87–6.81 (m, 2H),
6.67–6.33 (m, 4H), 5.45 (s, 2H), 3.78 (s, 3H). 13C NMR
(101 MHz, CDCl3) δ: 159.39, 148.00, 144.19,141.53,
133.83, 130.35, 129.25, 129.21, 129.19, 128.78,
127.57, 126.82, 120.27, 119.61, 117.20, 114.29,
112.19, 55.29, 47.78. ES-MS(pos.) m/z: 315.51
[M + H]+.
Synthesis of 1-(4-Methoxybenzyl)-1H-pyrrolo[2,3-
b]pyridine-3-carbaldehyde (2d). In a 250-mL round-
bottom flask, POCl3 (4.37 mL, 47.0 mmol, 1.50 equiv)
was dissolved in anhydrous DMF (40 mL) under N2
at 0◦C and then a solution of 1-(4-methoxybenzyl)-
1H-pyrrolo[2,3-b]pyridine (1d) (7.40 g., 31.1 mmol,
1.00 equiv) in 10 mL of dry DMF was added drop-
wise. A dark brown solution was observed, and the
mixture was stirred at 45◦C overnight. A brown so-
lution was obtained.
The solution was basified with a solution of
NaOH (40%) in water till pH 14, and it was poured
onto 400 mL of ice-water. Then it was extracted with
AcOEt (3 × 100 mL), and organic layers were dried
over Na2SO4 and filtered. and the solvent was evap-
orated to yield a yellow-brown solid (7.12 g., 27.1,
87.3%).
Synthesis of bis{[(4-Methoxybenzyl-1-H-pyrrolo
[2,3-b]pyridin-3-yl)methyl]cyclopentadienyl}
tita-
nium(IV) dichloride [TiCl2{η5-C5H4-CH(C7H4N2-
CH2-(C6H4-OCH3)2}2] (4d). In a 250-mL Schlenk
flask, a 1 M solution of LiBEt3H in THF (12.4 mL,
12.4 mmol, 1.30 equiv) was heated at 60◦C for 20 min
and at 90◦C for 30 min under vacuum of 10−2 mbar
to remove THF. In another 250-mL Schlenk flask, 3-
(cyclopenta-2,4-dienylidenemethyl)-1-(4-methoxy-
benzyl)-1H-pyrrolo[2,3-b]pyridine (3d) (3.00 g,
9.55 mmol, 1.00 equiv) was dissolved in dry Et2O
(200 mL) under N2 and the solution was added to
the concentrated solution of LiBEt3H via cannula;
the orange reaction mixture was stirred overnight
at RT and resulted in a yellow suspension of the
lithiated intermediate. The yellow solid was filtered
using a Schlenk frit under N2, yielding (2.24 g, 6.95
mmol, 72.7%) of a yellow powder.
1H NMR (400 MHz, CDCl3) δ: 9.91 (s, 1H), 8.56
(dd, J = 7.8, 1.3, 1H), 8.46 (dd, J = 4.7, 1.3, 1H), 7.76
(s, 1H), 7.29–7.25 (m, 3H), 6.90–6.87 (m, 2H), 5.46
(s, 2H), 3.80 (s, 3H). 13C NMR (101 MHz, CDCl3)
δ: 184.63, 159.66, 148.57, 145.19, 137.63, 130.57,
This litiated intermediate was dissolved in dry
THF (100 mL) in a 250-mL Schlenk flask under N2
and a dark red solution was observed. It was stirred
for a couple of minutes at RT and, then, a 1 M
Heteroatom Chemistry DOI 10.1002/hc

Synthesis and Cytotoxicity Studies of Achiral Azaindole-Substituted Titanocenes 157
solution of TiCl4 in toluene (3.47 mL, 3.47 mmol,
REFERENCES
0.50 equiv) was added dropwise under N2. The mix-
ture was refluxed overnight, which resulted in a dark
red solution. THF was removed in vacuo, and a red
solid oil was obtained. This was dissolved in CH2Cl2
and filtered through a plug of Celite . After the sol-
vent was evaporated in vacuo, a dark red solid was
obtained (1.40 g, 1.87 mmol, 53.8%).
[1] Sweeney N.; Mendoza O.; Mu¨ller-Bunz H.; Pampillo´n
C.; Rehmann F. J. K.; Strohfeldt K.; Tacke M.
J Organomet Chem 2005, 690, 4537–4544.
[2] Kro¨ger N.; Kleeberg U. R.; Mross K. B.; Edler L.; Sass
G.; Hossfeld D. K. Onkologie 2000, 23, 60–62.
[3] Pampillo´n C.; Claffey J.; Hogan M.; Strohfeldt K.;
Tacke M. Trans Met Chem 2007, 32, 434–441.
[4] Hickey T.; Claffey J.; Strohfeldt K.; Hogan M.;
Pampillo´n C.; Tacke M. Invest New Drugs 2007, 25,
425–433.
®
1H NMR (400 MHz, CD2Cl2) δ: 8.18–8.14 (m, 2H),
7.98–7.72 (m, 2H), 7.16–6.65 (m, 12H), 6.36–5.92 (m,
8H, C5H4), 5.26 (s, 4H), 3.76–3.53 (m, 10H). 13C NMR
(101 MHz, CD2Cl2) δ: 158.33, 147.13, 144.78, 141.92,
133.09, 131.40, 130.33, 129.63 129.61 127.94, 127.92,
126.65, 126.47, 126.22, 124.78, 114.28, 113.13, 46.26,
40.47, 25.20. Anal. calcd for C42H38N4O2TiCl2 (749):
C 67,30, H 5.11, Cl 9.46, N 7.47; found: C 67.48, H
5.30, Cl 5.27, N 6.85. UV–vis (CH2Cl2, nm): λ 229 (ε:
76,700), λ 232 (ε 78,600), λ 285 (ε 38,600). ES-MS
(pos.) m/z: 750.12 [M + H]+. IR absorptions (KBr,
cm−1): 3416, 3241, 3104, 3003, 2956, 2927, 2834,
2356, 1870, 1711, 1611, 1512, 1247, 1174, 1028, 799.
[5] Pampillo´n C.; Claffey J.; Strohfeldt K.; Tacke M. Eur
J Med Chem 2008, 43, 122–128.
[6] Pampillo´n C.; Sweeney N.; Strohfeldt K.; Tacke M.
J Organomet Chem 2007, 692, 2153–2159.
[7] Hogan M.; Claffey J.; Pampillo´n C.; Watson R. W. G.;
Tacke M. Organometallics 2007, 26, 2501–2506.
[8] Hogan M; Cotter J; Claffey J; Gleeson B; Wallis D;
O’Shea D. F.; Tacke M. Helv Chim Acta 2008, 91,
1787–1797.
[9] Quevauviller P.; Ritsema R.; Morabit R.; Dirkx W. M.
R. Appl Organomet Chem 1994, 8, 541–549.
[10] Mosmann T. J Immunol Methods 1983, 65, 55–
63.
Heteroatom Chemistry DOI 10.1002/hc