Preparation of diazenecarboxamide-carboplatin conjugates by click chemistry

Article abstract of DOI:10.1016/j.ica.2010.07.031

An alternative, mild and highly efficient synthetic approach to platinum complexes with bioactive carrier ligands features a platinum-complex-tolerant copper(I)-catalyzed 1,3-dipolar cycloaddition. As demonstrated by the preparation of novel diazenecarbox

Full text of DOI:10.1016/j.ica.2010.07.031

Inorganica Chimica Acta 363 (2010) 3817–3822
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Preparation of diazenecarboxamide–carboplatin conjugates by click chemistry
*
Damijana Urankar, Janez Košmrlj
ˇ
Faculty of Chemistry and Chemical Technology, University of Ljubljana, Aškerceva 5, SI-1000 Ljubljana, Slovenia
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 6 May 2010
Received in revised form 6 July 2010
Accepted 12 July 2010
Available online 19 August 2010
An alternative, mild and highly efficient synthetic approach to platinum complexes with bioactive carrier
ligands features a platinum-complex-tolerant copper(I)-catalyzed 1,3-dipolar cycloaddition. As demon-
strated by the preparation of novel diazenecarboxamide–carboplatin conjugates, this approach is supe-
rior to other methodologies.
Ó 2010 Elsevier B.V. All rights reserved.
Keywords:
Platinum complex
Diazenecarboxamide
Cycloaddition
Click chemistry
1. Introduction
ganic scaffolds, which do not tolerate the potentially forcing reac-
tion conditions required in the final coordination step.
Hybrid molecules with a dual mode of action are receiving con-
siderable interest in the area of drug discovery [1]. An important
example in this context is the development of platinum complexes
that have bioactive carrier ligands. These ligands are capable of
modulating the resistances that tumour cells acquire during che-
motherapy. This effect has recently been illustrated by ethacrapla-
tin, where the glutathione-S-transferase (GST)-inhibitor, ethacrynic
acid, was attached to platinum [2] and the platinum complexes of
nitrofuran carbohydrazide, a human thioredoxin reductase inhibi-
tor [3]. In connection with this we have recently observed that dia-
zenecarboxamides [4] are powerful modulators of intracellular
glutathione (GSH) and show promise as bioactive ligands to plati-
num in anticancer treatments [5,6]. It should be noted that the tu-
mour resistances are multifunctional processes in which GSH and
GST often constitute the main cellular defence. Thus, a need for con-
venient, synthetic methodologies to access different conjugates
with platinum complexes has emerged.
A general scheme to access the above platinum hybrid molecule
involves the synthesis of a bioactive, organic scaffold, which is func-
tionalized with a mono- or polydentate coordination site through a
suitable linker. This then serves as an organic host for a guest metal
ion. In the final step of the reaction sequence, the latter is appended
to the metal (Scheme 1, Strategy a). However, this approach has sev-
eral disadvantages: first, it is linear and, second, it cannot be used
for the preparation of complexes that should consist of fragile or-
One of the promising alternatives to the above is a nonlinear ap-
proach in which both the organic scaffold and the desired metal
complex are preassembled and then, using an appropriate ‘click’
[7] reaction, subsequently combined into the final target molecules
(Scheme 1, Strategy b).
Cu(I)-catalyzed terminal alkyne–azide cycloaddition (CuAAC)
has recently emerged as a powerful ‘click’ reaction, known for its
exquisite selectivity and high yields [8,9]. There are, however, only
a handful examples demonstrating CuAAC reactions with com-
plexes of different transition metals as substrates [10,11]. To the
best of our knowledge, in platinum coordination spheres it has
only been exercised with organoplatinum compounds [11].
2. Experimental
The reagents were purchased from Aldrich, whereas the
K2[PtCl4] was purchased from Alfa Aesar, and used as received.
NMR spectra were recorded on a Bruker Avance DPX 300 spec-
trometer at 302 K. Chemical shifts are reported in parts per million
(ppm). 1H NMR spectra were referenced to internal Me4Si. 13C NMR
spectra are referenced to the residual DMSO-d6 (39.5 ppm) or
DMF-d7 peak (30.1 ppm), unless otherwise noted. 195Pt NMR spec-
tra are referenced to Na2[PtCl6] (325 mg/0.65 mL D2O), as an exter-
nal standard at 0 ppm. Coupling constants (J) are reported in hertz
(Hz). Mass spectra were recorded on a Q-TOF Premier instrument.
Infrared spectra were recorded on a BIO-RAD Excalibur Series spec-
trophotometer using samples in potassium bromide discs. Elemen-
tal analyses (C, H, N) were performed with a Perkin–Elmer 2400
* Corresponding author. Tel.: +386 1 241 9256; fax: +386 1 241 9220.
E-mail address: janez.kosmrlj@fkkt.uni-lj.si (J. Košmrlj).
0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.07.031

3818
D. Urankar, J. Košmrlj / Inorganica Chimica Acta 363 (2010) 3817–3822
iodide was removed by filtration. To the filtrate was added a water
(8 mL) solution of 1,1-cyclobutanedicarboxylic acid (649 mg,
4.50 mmol) and NaOH (360 mg, 9.00 mmol). The reaction mixture
was stirred overnight, filtered and the solvent of the filtrate was re-
moved on a rotary evaporator. The residue was suspended in water
(5 mL) and chilled on an ice bath. A grey solid precipitated, which
was collected by filtration and dried to afford pure 2b (1.51 g,
3.34 mmol, 67% from K2[PtCl4]), mp >300 °C (H2O). dH (300 MHz,
DMSO-d6) 1.67 (quintet, J 7.6, 2H), 2.45–2.75 (m, 8H), 3.91 (m, 1H),
5.20–5.50 (m, 4H, D2O exchangeable). dC (75 MHz, D2O, 1,4-dioxane
as external standard at d 66.7 ppm) 15.4, 30.4, 31.9, 45.1, 56.4, 58.4,
181.9. dPt (64 MHz, D2O) À1954.
m
max(cmÀ1) 3175, 3100, 2101,
1602, 1380, 1255. m/z (ESI+) 475.1 ([M+Na]+, 100), 453.1 ([M+H]+,
28), 396.1 (62), 222.1 (94), 94.1 (50). HRMS (ESI+) Calc. for
C9H16N5O4195Pt+ [M+H]+: 453.0850. Found: 453.0863.
Scheme 1. Strategies to platinum complexes involving organic scaffold.
2.4. General procedure for the preparation of 3
Series II CHNS/O Analyzer. Melting points were determined on a
Kofler block and are uncorrected.
A mixture of the appropriate propargyl-appended diazenecarb-
oxamide 1 (0.250 mmol), cis-[Pt(2-azidopropane-1,3-diamine)Cl2]
(2a, 113 mg, 0.250 mmol), CuSO4Á5H2O (6.2 mg 0.025 mmol,
10 mol%) and granular copper (60.3 mg, 0.950 mmol, 3.8 equiv.) in
DMF (1 mL) was stirred at room temperature for the time indicated
in Table 1. Isolation procedure for 3a–c,e,f,h,i,k: Two drops of satu-
rated aqueous ammonium hydrogen carbonate solution were added
to the reaction mixture. The resulting mixture was stirred for a few
minutes and subjected to column chromatography on silica gel.
After an initial elution with a mixture of dichloromethane and meth-
anol (in a ratio of 10:1, followed by 1:1) to remove the DMF, pure
product was isolated by subsequent elution with methanol. Isolation
procedure for 3d,g,j: The reaction mixture was diluted with DMF
(10 mL) and ammonium hydrogen carbonate (70 mg) was added.
The mixture was heated at the boiling point until the product dis-
solved. After filtration through a pad of Celite the filtrate was con-
centrated on a rotary evaporator (to ca. 0.5 mL) and cooled down
to room temperature. The product was precipitated by the addition
of methanol (20 mL), collected by filtration and washed with meth-
anol (2 mL) and diethyl ether (2 mL). Pure products 3 were obtained
by these two methods in the yields indicated in Table 1.
2.1. 2-Azidopropane-1,3-diamine dihydrochloride (4)
Into a solution of tert-butyl 2-azidopropane-1,3-diyldicarba-
mate [12] (315 mg, 1.00 mmol) in ethyl acetate (3 mL), aqueous
hydrochloric acid (1.4 mL, 6 M) was added. The reaction mixture
was stirred for 8 h and then chilled at 5 °C overnight. The resulting
crystals were filtered off and washed several times with a few mil-
lilitres of ethyl acetate until the crystals were no longer sticky, to
give the pure product 4 (175 mg, 93%) as white needles, mp
227–231 °C. dH (300 MHz, DMSO-d6) 2.84–3.00 (m, 2H), 3.10–
3.24 (m, 2H), 4.22–4.33 (m, 1H), 8.50 (br s, 6H). dC (75 MHz,
DMSO-d6) 40.9, 57.6. m
max(cmÀ1) 3000, 2135, 1473, 1281, 959. Anal.
Calc. for C3H11Cl2N5: C, 19.16; H, 5.90; N, 37.24. Found: C, 19.12; H,
5.98; N, 36.91%.
2.2. cis-[Pt(2-azidopropane-1,3-diamine)Cl2] (2a)
DBU (152 mg, 1.00 mmol) was added into a solution of 4
(94 mg, 0.50 mmol) in DMF (1 mL), followed by cis-[Pt(DMSO)2Cl2]
[13] (224 mg, 0.50 mmol). The reaction mixture was stirred for
2 days. After the addition of water (2 mL) and cooling at 5 °C for
a few hours, the precipitate was collected by filtration and washed
with water (2 Â 2 mL) to give 2a (90 mg, 0.24 mmol) as a grey so-
lid, mp >300 °C. dH (300 MHz, DMF-d7) 2.80–3.10 (m, 4H), 4.18–
4.40 (m, 1H), 5.00–5.48 (m, 4H). dC (75 MHz, DMF-d7) 54.9, 59.4.
2.4.1. Complex 3a
Orange solid, mp >300 °C. dH (300 MHz, DMSO-d6) 1.67 (quintet,
J 7.6, 2H), 2.57–2.78 (m, 6H), 2.89–3.05 (m, 2H), 4.66 (br t, J 9.0,
1H), 5.16 (s, 2H), 5.40–5.60 (m, 2H), 5.78–5.93 (m, 2H), 6.85–6.96
(m, 1H), 7.28–7.38 (m, 2H), 7.47 (br s, 1H), 8.05 (d, J 8.6, 2H),
8.12 (d, J 8.6, 2H), 8.39 (s, 1H), 11.06 (br s, 1H). dC (75 MHz,
DMSO-d6) 15.0, 30.3, 30.5, 47.4, 55.6, 58.0, 61.1, 106.4, 110.5,
112.3, 123.4 (q, J 273), 123.5, 123.7, 126.9 (q, J 3.8), 130.0, 132.5
(q, J 31.7), 138.9, 142.9, 153.2, 158.4, 159.3, 177.4. dPt (64 MHz,
dPt (64 MHz, DMF-d7) À2286.
m
max(cmÀ1) 3234, 3123, 2108, 1587,
1259. m/z (ESI+) 403.7 ([M+Na]+, 100), 346.7 (50). HRMS (ESIÀ)
Calc. for C3H835Cl2N5194PtÀ [MÀH]À: 377.9784. Found: 377.9781.
Anal. Calc. for C3H9Cl2N5Pt: C, 9.45; H, 2.38; N, 18.38. Found: C,
9.83; H, 2.76; N, 17.99.
DMSO-d6) À1958.
m
max(cmÀ1) 3140, 1735, 1613, 1335, 1122, 852.
m/z (ESIÀ) 798.2 ([MÀH]À, 100). HRMS (ESIÀ) Calc. for
C
26H26F3N8O6194PtÀ ([MÀH]À): 797.1554. Found: 797.1564. Anal.
2.3. cis-[Pt(2-azidopropane-1,3-diamine)(CBDCA)] (2b)
Calc. for C26H27F3N8O6Pt: C, 39.05; H, 3.40; N, 14.01. Found: C,
38.71; H, 3.51; N, 13.62.
To a solution of K2[PtCl4] (2.08 g, 5.00 mmol) in water (35 mL) was
added KI (8.30 g, 50.0 mmol) and the reaction mixture was stirred for
1 h. A solution of 4 (940 mg, 5.00 mmol) in aqueous NaOH (400 mg,
10.0 mmol in 7.5 mL of water) was added dropwise to the reaction
mixture, which was then stirred at 60 °C for 5 min. The reaction mix-
ture was chilled to 0 °C and the precipitated brown solid was filtered
off and washed successively with small amounts of ice-cold water,
methanol and diethyl ether to give cis-[Pt(2-azidopropane-1,3-dia-
mine)I2] (2.74 g, 4.86 mmol, 97% yield from K2[PtCl4]). This com-
pound was suspended in water (100 mL) and a water solution of
AgNO3 (1.53 g, 9.00 mmol in 15 mL of water) was added dropwise,
under stirring. The stirring was continued for 3 h. The produced silver
2.4.2. Complex 3b
Orange solid, mp >300 °C. dH (300 MHz, DMSO-d6) 1.70 (quintet,
J 7.7, 2H), 2.60–2.75 (m, 6H), 2.86–3.05 (m, 2H), 4.65 (br t, J 10.5,
1H), 5.15 (s, 2H), 5.40–5.60 (m, 2H), 5.75–6.92 (m, 2H), 6.88–6.98
(m, 1H), 7.30–7.40 (m, 2H), 7.48 (s, 1H), 8.48 (s, 1H), 11.36 (br s,
1H). dC (75 MHz, DMSO-d6) 15.0, 30.3, 30.5, 47.4, 55.6, 58.0, 61.2,
106.5, 110.9, 112.5, 123.5, 130.0, 138.7, 142.9, 158.1, 158.4,
177.4. dPt (64 MHz, DMSO-d6) À1957.
m
max(cmÀ1) 3430, 3205,
1648, 1611, 1518, 1030. m/z (ESIÀ) 820.1 ([MÀH]À, 100), 624.1
(96), 409.1 (90). HRMS (ESIÀ) Calc. for
C
25H25F5N8O6194PtÀ
[MÀH]À: 819.1209. Found: 819.1215.

D. Urankar, J. Košmrlj / Inorganica Chimica Acta 363 (2010) 3817–3822
3819
Table 1
Diazenecarboxamide–platinum conjugates 3a–k (Scheme 2).
Entry
Reaction time (h)
Product 3
R
Yielda (%)
N-Aryl analogues
1
2
3
4
3
3.5
5
3a
3b
3c
3d
4-CF3–C6H4
C6F5
4-Cl–C6H4
4-NO2–C6H4
65
49
65
41
3.5
N-Alkyl analogues
5
6
7
8
9
3
2
12
3
6
3e
3f
3g
3h
3i
4-CF3–C6H4
4-Cl–C6H4
4-NO2–C6H4
4-CH3–C6H4
4-CH3O–C6H4
C6H5
81
84
47
75
93
44
73
10
11
12
3
3j
3k
4-F–C6H4
a
Pure product.
2.4.3. Complex 3c
(d, J 8.7, 2H), 7.85 (d, J 8.7, 2H), 8.20 (s, 1H), 9.11 (br t, J 5.8, 1H).
dC (75 MHz, DMSO-d6) 15.0, 30.3, 30.5, 35.4, 47.4, 55.6, 58.0,
122.1, 124.6, 129.8, 137.9, 144.2, 149.6, 162.4, 177.4. dPt
Orange solid, mp >300 °C. dH (300 MHz, DMSO-d6) 1.67 (quintet,
J 7.6, 2H), 2.57–2.78 (m, 6H), 2.89–3.05 (m, 2H), 4.66 (br t, J 10.3,
1H), 5.15 (s, 2H), 5.40–5.60 (m, 2H), 5.78–5.92 (m, 2H), 6.85–6.96
(m, 1H), 7.28–7.38 (m, 2H), 7.46 (br s, 1H), 7.75 (d, J 7.9, 2H),
7.97 (d, J 7.9, 2H), 8.39 (s, 1H), 10.96 (br s, 1H). dC (75 MHz,
DMSO-d6) 15.0, 30.3, 30.5, 47.4, 55.6, 58.0, 61.1, 106.3, 110.4,
112.3, 123.4, 124.8, 129.9, 138.2, 139.0, 143.0, 149.7, 158.4,
(64 MHz, DMSO-d6) À1957.
m
max(cmÀ1) 3164, 1721, 1659, 1490,
1366, 845. m/z (ESI+) 697.1 ([M+Na]+, 100), 675.1 ([M+H]+, 13),
638.6 (23). HRMS (ESI+) Calc. for C19H23ClN8O5194PtNa+ [M+Na]+:
696.1025. Found: 696.1044.
159.6, 177.4. dPt (64 MHz, DMSO-d6) À1959.
m
max(cmÀ1) 3142,
2.4.7. Complex 3g
1614, 1595, 1386. m/z (ESIÀ) 365.1 ([M]À, 100), 582.7 (30). HRMS
(ESIÀ) Calc. for C25H2735ClN8O6194PtÀ [MÀH]À: 763.1291. Found:
763.1300. Anal. Calc. for C25H27ClN8O6Pt: C, 39.20; H, 3.55; N,
14.63. Found: C, 38.81; H, 3.55; N, 14.33.
Dark brown solid, mp >300 °C. dH (300 MHz, DMSO-d6) 1.67
(quintet, J 7.2, 2H), 2.55–2.80 (m, 6H), 2.85–3.05 (m, 2H), 4.53 (d,
J 5.4, 2H), 4.66 (br s, 1H), 5.35–5.60 (m, 2H), 5.75–5.95 (m, 2H),
8.03 (d, J 8.7, 2H), 8.22 (s, 1H), 8.45 (d, J 8.7, 2H), 9.25 (br s, 1H).
dC (75 MHz, DMSO-d6) 15.0, 30.3, 30.5, 35.4, 47.4, 55.6, 58.0,
122.2, 123.8, 125.2, 144.0, 149.6, 154.2, 162.1, 177.4. dPt (64 MHz,
2.4.4. Complex 3d
Orange solid, mp >300 °C. dH (300 MHz, DMSO-d6) 1.69 (quintet,
J 7.5, 2H), 2.60–2.77 (m, 6H), 2.90–3.06 (m, 2H), 4.66 (br t, J 10.2,
1H), 5.15 (s, 2H), 5.40–5.60 (m, 2H), 5.77–5.92 (m, 2H), 6.90 (br
s, 1H), 7.30–7.40 (m, 2H), 7.46 (br s, 1H), 8.15 (br s, 2H), 8.38 (s,
1H), 8.50 (br s, 2H), 11.14 (br s, 1H). dC (75 MHz, DMSO-d6) 15.0,
30.3, 30.5, 47.4, 55.6, 58.0, 61.2, 123.4, 142.9, 177.3. dPt (64 MHz,
DMSO-d6) À1958.
m
max(cmÀ1) 3181, 1717, 1651, 1621, 1527,
1346, 864. m/z (ESI+) 707.1 ([M+Na]+, 30), 385.1 ([M+H]+, 8),
325.2 (66), 234.2 (74), 231.1 (65), 114.0 (100). HRMS (ESI+) Calc.
for C19H23N9O7Na195Pt+ [M+Na]+: 707.1266. Found: 707.1288.
2.4.8. Complex 3h
DMSO-d6) À1957.
m
max(cmÀ1) 3441, 3096, 1737, 1632, 1611,
Orange solid, mp >300 °C. dH (300 MHz, DMSO-d6) 1.67 (quintet,
J 9.0, 2H), 2.42 (s, 3H), 2.54–2.77 (m, 6H), 2.85–3.05 (m, 2H), 4.50
(d, J 6.0, 2H), 4.63 (br t, J 9.0, 1H), 5.35–5.51 (m, 2H), 5.75–5.95
(m, 2H), 7.73 (d, J 8.0, 2H), 7.75 (d, J 8.0, 2H), 8.19 (s, 1H), 9.02
(br t, J 6.0, 1H). dC (75 MHz, DMSO-d6) 15.0, 21.1, 30.3, 30.6, 38.7,
47.5, 55.6, 58.0, 122.1, 123.0, 130.1, 143.9, 144.4, 149.2, 162.7,
1375, 1347. m/z (ESIÀ) 776.2 ([M]À, 100), 624.1 (74), 461.1 (40),
341.1 (71). HRMS (ESIÀ) Calc. for C25H26N9O8194PtÀ [MÀH]À:
774.1531. Found: 774.1557.
2.4.5. Complex 3e
Ochre solid, mp >300 °C. dH (300 MHz, DMSO-d6) 1.67 (quintet, J
9.0, 2H), 2.54–2.75 (m, 6H), 2.85–3.05 (m, 2H), 4.53 (d, J 6.0, 2H),
4.60 (br t, J 9.0, 1H), 5.40–5.60 (m, 2H), 5.77–5.92 (m, 2H), 8.01
(s, 4H), 8.22 (s, 1H), 9.21 (br t, J 6.0, 1H). dC (75 MHz, DMSO-d6)
15.0, 30.2, 30.5, 35.3, 47.4, 55.5, 57.9, 122.1, 123.4, 123.6 (q, J
273), 126.8 (q, J 3.9), 132.2 (q, J 31.1), 144.0, 153.2, 162.2, 177.3.
177.4. dPt (64 MHz, DMSO-d6) À1957.
m
max(cmÀ1) 3211, 1714,
1635, 1368, 1154. m/z (ESI+) 676.2 ([M+Na]+, 100), 654.2
([M+H]+, 61), 216.1 (51). HRMS (ESI+) Calc. for C20H27N8O5194Pt+
[M+H]+: 654.1752. Found: 654.1774. Anal. Calc. for C20H26N8O5Pt:
C, 36.76; H, 4.01; N, 17.15. Found: C, 36.71; H, 3.98; N, 16.88.
dPt (64 MHz, DMSO-d6) À1957.
m
max(cmÀ1) 3221, 1710, 1629,
2.4.9. Complex 3i
1325, 1129, 1065, 854. m/z (ESI+) 730.1 ([M+Na]+, 100), 708.1
([M+H]+, 17). HRMS (ESI+) Calc. for C20H23F3N8O5194PtNa+
[M+Na]+: 730.1289. Found: 730.1296.
Ochre solid, mp >300 °C. dH (300 MHz, DMSO-d6) 1.67 (quintet, J
7.3, 2H), 2.54–2.77 (m, 6H), 2.85–3.05 (m, 2H), 3.88 (s, 3H), 4.49 (d,
J 5.4, 2H), 4.62 (br s, 1H), 5.35–5.66 (m, 2H), 5.75–5.96 (m, 2H),
7.15 (d, J 8.9, 2H), 7.86 (d, J 8.9, 2H), 8.18 (s, 1H), 8.97 (br t, J 5.0,
1H). dC (75 MHz, DMSO-d6) 15.0, 30.3, 30.6, 35.4, 47.4, 55.6, 55.8,
57.9, 114.8, 122.1, 125.3, 144.5, 145.2, 162.6, 163.4, 177.4. dPt
2.4.6. Complex 3f
Ochre solid, mp >300 °C. dH (300 MHz, DMSO-d6) 1.67 (quintet, J
7.9, 2H), 2.54–2.77 (m, 6H), 2.85–3.05 (m, 2H), 4.50 (d, J 6.0, 2H),
4.66 (br t, J 9.0, 1H), 5.35–5.51 (m, 2H), 5.75–5.95 (m, 2H), 7.70
(64 MHz, DMSO-d6) À1958.
m
max(cmÀ1) 3220, 3154, 1656, 1626,
1502, 1258, 1145. m/z (ESI+) 692.2 ([M+Na]+, 100), 670.2

3820
D. Urankar, J. Košmrlj / Inorganica Chimica Acta 363 (2010) 3817–3822
([M+H]+, 19), 216.1 (33), 94.1 (32). HRMS (ESI+) Calc. for
20H27N8O6194Pt+ [M+H]+: 670.1701. Found: 670.1716.
C
2.4.10. Complex 3j
Dark brown solid, mp >300 °C. dH (300 MHz, DMSO-d6) 1.67
(quintet, J 8.0, 2H), 2.40–2.66 (m, 6H), 2.88–3.08 (m, 2H), 4.50 (d,
J 5.8, 2H), 4.62 (br t, J 10.8, 1H), 5.40–5.55 (m, 2H), 5.75–5.94 (m,
2H), 7.58–7.70 (m, 3H), 7.82–7.89 (m, 2H), 8.20 (s, 1H), 9.07 (br
t, J 5.6, 1H). dC (75 MHz, DMSO-d6) 15.0, 30.3, 30.6, 35.4, 47.4,
55.6, 58.0, 122.1, 122.8, 129.6, 133.2, 144.3, 151.1, 162.7, 177.4.
Scheme 3. Synthesis of cisplatin analogue 2a.
dPt (64 MHz, DMSO-d6) À1957.
m
max(cmÀ1) 3442, 3204, 2950,
1715, 1661, 1632, 1362. m/z (ESI+) 662.1 ([M+Na]+, 12), 344.1
(100). HRMS (ESI+) Calc. for C19H25N8O5195Pt+ [M+H]+: 640.1596.
Found: 460.1600.
2.4.11. Complex 3k
Orange solid, mp >300 °C. dH (300 MHz, DMSO-d6) 1.67 (quintet,
J 7.6, 2H), 2.55–2.77 (m, 6H), 2.88–3.06 (m, 2H), 4.50 (s, 2H), 4.64
(br t, J 9.0, 1H), 5.40–5.62 (m, 2H), 5.75–5.95 (m, 2H), 7.47 (d, J 8.8,
2H), 7.88–7.99 (m, 2H), 8.20 (s, 1H), 9.09 (br s, 1H). dC (75 MHz,
DMSO-d6) 15.0, 30.3, 30.6, 35.4, 47.5, 55.6, 57.9, 116.6, 116.9,
122.1, 125.4, 125.6, 144.3, 147.8, 162.5, 177.4. dPt (64 MHz,
Scheme 4. Synthesis of carboplatin analogue 2b.
DMSO-d6) À1958.
m
max(cmÀ1) 3460, 3159, 1717, 1655, 1628,
3.2. Azido-appended platinum(II) complexes
1501, 1364, 1226, 1138, 850. m/z (ESIÀ) 656.1 ([MÀH]À, 100).
HRMS (ESIÀ) Calc. for C19H22FN8O5194PtÀ [MÀH]À: 655.1324.
Found: 655.1340.
For the azido functionalized diamine bidentate ligand we se-
lected 2-azidopropane-1,3-diamine dihydrochloride (4), which
was obtained from tert-butyl 2-azidopropane-1,3-diyldicarbamate
[12] by standard BOC deprotection chemistry using hydrochloric
acid. Through this ligand, amine platinum(II) complexes of formu-
lae [Pt(diamine)Cl2] (2a) and [Pt(diamine)(CBDCA)] (2b, CBDCA =
1,1-cyclobutanedicarboxylate) were prepared.
The synthesis of 2a was accomplished by the reaction of 4 with
cis-[PtCl2(DMSO)2] [13] (DMSO = dimethyl sulfoxide) in the pres-
ence of 1,8-diazabiciclo[5.4.0]undek-7-en (DBU), albeit in a moder-
ate 48% yield (Scheme 3). The 195Pt NMR chemical shift for 2a of d
À2286 ppm is consistent with the cis-diamino-dichloridoplati-
num(II) complex [16]. With the exception of the DMSO and the
N,N-dimethylformamide (DMF), the complex 2a turned out to be
virtually insoluble in water and in most common organic solvents,
which diminishes its value in the synthesis of potential biologically
active compounds. Thus, no attempts were made to improve its
yield.
The preparation of a more soluble carboplatin analogue 2b
turned out to be less straightforward. In general, several methods
for the synthesis of amine platinum(II) dicarboxylate complexes
have been developed. As reported by Lin and co-workers [17],
the most general protocols include (i) the reaction of the dimethyl
sulfoxide platinum complex [Pt(DMSO)2(dicarboxylate)] with dia-
mines, (ii) the reaction of the dichloroplatinum complex [Pt(diami-
ne)Cl2] with the disilver salt of dicarboxylic acid and (iii) the
transformation of the dichloridoplatinum complex [Pt(diami-
ne)Cl2] with silver nitrate into the dinitratoplatinum complex
[Pt(diamine)(NO3)2], followed by a reaction with the disodium salt
of dicarboxylic acid. Water was used as the reaction solvent in the
above approaches.
3. Results and discussion
Our plan for the preparation of diazenecarboxamide–platinum
conjugates, as introduced in Scheme 1 Strategy b, is depicted in
Scheme 2. Relatively inert terminal alkyne and azido moieties
should be easily introduced into the organic molecules to afford
the prerequisite diazenecarboxamides 1 and 2-azidopropane-1,3-
diamine. The latter is coordinated to platinum(II) into 2. In the
key step both ‘click’ partners are combined by the CuAAC reaction
into the target molecule 3.
3.1. Propargyl-appended diazenecarboxamides
Two types of propargyl-appended diazenecarboxamides 1 were
selected: N-aryl and N-alkyl. Their rational and synthesis have
been described in [14,15].
We unsuccessfully employed different reaction conditions to
react 4 with cis-[Pt(DMSO)2(CBDCA)]. As subsequently reported
by Lin and Pierpont [18], and more recently by the group of Navar-
ro-Ranninger and co-workers [19], the reaction is strongly temper-
ature and concentration dependent. Unfortunately, conducting the
reaction between 4 and cis-[Pt(DMSO)2(CBDCA)]1 at different tem-
peratures and concentrations, as well as by employing different
Scheme 2. Synthesis of diazenecarboxamide (shortly ‘‘diazene”)–platinum conju-
gates 3 by platinum tolerant CuAAC.
1
cis-[Pt(DMSO)2(CBDCA)] was prepared as reported in Ref. [18].

D. Urankar, J. Košmrlj / Inorganica Chimica Acta 363 (2010) 3817–3822
3821
bases to free-base the 2-azidopropane-1,3-diamine from its dihydro-
chloride salt 4, only led to the formation of complex mixtures of
unidentified products. Also unsuccessful were attempts to react 2a
with either the disilver salt of CBDCA or with silver nitrate. No prod-
uct formation could be observed, which could be accounted for by
the complete insolubility of 2a in water.
Scheme 5. Attempts to coordinate diazenecarboxamide-appended propane-1,3-
Thus, we surmised that employing the more soluble iodine
complex, cis-[Pt(2-azidopropane-1,3-diamine)I2], instead of 2a
should be advantageous (Scheme 4). This diiodidoplatinum com-
plex was prepared from 4 and potassium tetraiodoplatinate [20].
A subsequent reaction with silver nitrate generated the dinitrato
complex, which upon the addition of the disodium salt of CBDCA
afforded pure 2b in an appreciable 67% overall yield from 4. Com-
pound 2b exhibits the 195Pt NMR chemical shift of d À1954 ppm,
typical for the [Pt(diamine)CBDCA] coordination [19].
diamine to platinum(II) failed.
copy, complex mixtures of unidentified products were obtained.
Under more forcing conditions, i.e., conducting the reactions at ele-
vated temperatures, the diazenecarboxamide part of the molecule
rapidly decomposed.
4. Conclusion
3.3. CuAAC reaction
A mild and highly efficient synthetic route to diazenecarboxa-
mide–carboplatin conjugates by copper-catalyzed 1,3-dipolar
cycloaddition has been developed. The method complements the
existing methodologies and can be extended to the preparation
of other platinum complexes with potentially bioactive carrier li-
gands, especially for sensitive ligands. A screening of the subset
of compounds 3 has been currently initiated against tumours,
showing an activity similar to carboplatin.
When attempting the CuAAC reaction in the presence of other
metal complexes, several complications may arise, including redox
reactions with redox-sensitive copper(I), and ligand-exchange
reactions displacing the metal from the substrate complex by cop-
per(I) or copper(II) species. These may lead to inhibition of the
CuAAC reaction and product contamination, respectively.
In a classical CuAAC reaction the copper(I) catalyst is generated
by the in situ reduction of copper(II) ions with ascorbic acid. Dia-
zenecarboxamides, however, are sensitive to ascorbic acid and
the problem of their interference in the redox process has previ-
ously been addressed by the use of a copper(0)/copper(II) couple.
In addition, reaction solvents such as DMSO and DMF proved ben-
eficial in the CuAAC reaction involving alkyne functionalized dia-
zenecarboxamides as the reaction partners [14,15].
Acknowledgements
Financial support from the Ministry of Higher Education, Sci-
ence and Technology of the Republic of Slovenia and the Slove-
nian Research Agency (P1-0230-0103) is acknowledged. We
ˇ
thank Dr. Bogdan Kralj and Dr. Dušan Zigon (Mass Spectrometry
Adopting the above findings, in a typical procedure a mixture of
alkyne 1, azide 2b, CuSO4Á5H2O and granular copper in DMF was
stirred in the presence of air for 3–12 h (Scheme 2). The crude reac-
tion mixture was subjected to column chromatography on silica
gel, which allowed facile removal of the DMF and isolation of the
pure products 3 in good-to-excellent yields (Table 1). Moderate
isolated yields were seen for the products 3d,g,j (entries 4,7,10)
with sparing solubility. An alternative chromatography-free isola-
tion procedure in these instances was more appropriate, as de-
ˇ
Center, Jozef Stefan Institute, Ljubljana, Slovenia) for mass spec-
tral measurements.
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a saturated aqueous
ammonium hydrogen carbonate solution to the reaction mixture,
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reactions to remove any adventitious copper ions from the prod-
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ˇ
ˇ
ˇ
The 1H and 13C NMR spectra of the compounds 3a–k featured
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It is important to note that for the preparation of the diazene-
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