Palladium(II)-assisted [2+3] cycloaddition of nitrones to organonitriles: Synthesis of 2,3-dihydropyrrolo[1,2-a]quinazolin-5-one and Δ4- 1,2,4-oxadiazoline derivatives

Article abstract of DOI:10.1016/j.poly.2013.04.017

The [2+3] cycloadditions of different organonitriles NCR (R¹ = C₆F₅, R² = Pr, R³ = p-CHOC ₆H₄, R⁴ = Ph) with the cyclic nitrone or acyclic nitrone ^-O^+<

Full text of DOI:10.1016/j.poly.2013.04.017

Polyhedron 57 (2013) 20–23
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Polyhedron
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Palladium(II)-assisted [2+3] cycloaddition of nitrones to organonitriles: Synthesis
of 2,3-dihydropyrrolo[1,2-a]quinazolin-5-one and
D
4-1,2,4-oxadiazoline derivatives
⇑
Jamal Lasri
Centro de Química Estrutural, Complexo I, Instituto Superior Técnico, Technical University of Lisbon, Av. Rovisco Pais, 1049-001 Lisbon, Portugal
a r t i c l e i n f o
a b s t r a c t
The [2+3] cycloadditions of different organonitriles NCR (R1 = C6F5, R2 = Pr, R3 = p-CHOC6H4, R4 = Ph) with
Article history:
Received 11 February 2013
Accepted 4 April 2013
Available online 16 April 2013
the cyclic nitrone
or acyclic nitrone ÀO+N(Me)@C(H)(C6H2Me3-2,4,6), in the
-O+N=CHCH2CH2CMe2
presence of PdCl2, give the corresponding fused tricyclic fluorinated ketoimine trans-
(1),
fused
bicyclic
D
4-1,2,4-oxadiazoline
trans-
[PdCl2{NC(=O)C6F4NCCH2CH2CMe2}2] (1)
Keywords:
Palladium(II) chloride
Nitrile
(R2 = Pr (2), R3 = p-CHOC6H4 (3)) or monocyclic
D
4-1,2,4-oxadiaz-
[PdCl2{N=C(R)ONCHCH2CH2CMe2}2]
oline trans-
(4) palladium(II) complexes. The free tet-
[PdCl2{N=C(Ph)ON(Me)C(H)(C6H2Me3-2,4,6)}2] (4)
Nitrone
rafluoro-2,3-dihydropyrrolo[1,2-a]quinazolin-5-one (1a) and
D
4-1,2,4-oxadiazolines (2a–4a) are
[2+3]Cycloaddition
One-pot synthesis
Dppe
liberated upon reaction of complexes 1–4 with a diphosphine (dppe). All the compounds were character-
ized by elemental analyses, ESI+-MS, IR, 1H and 13C NMR spectroscopies.
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
types of nucleophiles [9,10] or 1,3-dipole reagents [11,12] when
coordinated to a metal centre has already been well demonstrated.
Oxadiazolines represent an important class of heterocycles
which, although known for over a century, are rather limited in
number. In recent years, these heterocycles have drawn greater
attention due to their promising biological activities [1]. Also fluo-
rine containing heterocycles have gained the attention of chemists
and biologists because many derivatives show a high biological po-
tency in their ability to inhibit specific enzymes, their good solubil-
ity in lipids and their ability to penetrate through cell membranes
[2].
In our current work, the main objectives were (i) to prepare pal-
ladium(II) complexes with N-heterocyclic ligands, and (ii) to inves-
tigate the liberation of those heterocycles from their respective
metal complexes.
2. Results and discussion
2.1. Syntheses of palladium(II) complexes
The [2+3] cycloaddition of nitrones to organonitriles, NCR, is
one of the most important routes for the synthesis of 1,2,4-
oxadiazolines. However, there are some limitations for this meth-
od, as only organonitriles activated with a strong electron-with-
drawing group (e.g. R1 = CCl3, CO2R2) directly ligated to the triple
bond can react with the nitrones. Moreover, rather harsh condi-
tions and/or long reaction times are required [3]. Therefore, the
development of a new approach to accelerate the [2+3] cycloaddi-
tion of nitrones with nitriles under milder conditions is an impor-
tant aim of the research in this area. In this context, the
coordination of nitriles to a suitable metal centre became a wide-
spread synthetic strategy and facile route to the synthesis of a large
variety of compounds, inaccessible directly by pure organic chem-
istry [4–8]. The increased coupling ability of nitriles with different
Our work started with the [2+3] cycloadditions of different
organonitriles, NCR (R1 = C6F5, R2 = Pr, R3 = p-CHOC6H4, R4 = Ph),
with pyrroline N-oxide (cyclic nitrone)
or
acyclic nitrone ÀO+N(Me)@C(H)(C6H2Me3-2,4,6), in the presence
of PdCl2, to give the corresponding fused tricyclic fluorinated keto-
imine trans-
(1), fused
trans-
[PdCl2{NC(=O)C6F4NCCH2CH2CMe2}2] (1)
4-1,2,4-oxadiazoline
bicyclic
D
(R2 = Pr (2), R3 = p-CHOC
trans-[PdCl2{N=C(R)ONCHCH2CH2CMe2}2]
H4 (3)) or monocyclic
6-
D
4-1,2,4-oxadiazoline
(4) palladium(II)
[PdCl2{N=C(Ph)ON(Me)C(H)(C6H2Me3-2,4,6)}2] (4)
complexes (Scheme 1).
Complexes 1 and 2 have been reported previously by us [12a].
The new
D
4-1,2,4-oxadiazoline palladium(II) complexes 3 and 4
⇑
were characterized by elemental analyses, ESI+-MS, IR, 1H and 13C
Tel.: +351 965518973.
E-mail addresses: jamal.lasri@ist.utl.pt, jalas@alumni.uv.es
NMR spectroscopies (see Section 4). In the IR spectra of 3 and 4,
0277-5387/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2013.04.017

J. Lasri / Polyhedron 57 (2013) 20–23
21
NCR
PdCl2
-
R1 = C6F5
R3 = p-CHOC6H4
R2 = Pr
r.t., 1 d
R4 = Ph
O
+
N
acetone
reflux, 3d
acetone
r.t., 1 d
60 ºC, 1 d
+
N
+
N
+
N
-
-
-
O
O
O
O
F
F
F
O
O
F
O
N
Cl2Pd
N
Cl2Pd
N
Cl2Pd
N
N
O
N
Cl2Pd
N
N
2
2
2
2
1
2
3
4
dppe, CHCl3
dppe, CHCl3
dppe, CHCl3
dppe, CHCl3
40 ºC, 30 min
40 ºC, 30 min
40 ºC, 30 min
(c)
(a)
(b)
(d)
40 ºC, 30 min
-[Pd(dppe)2]Cl2
-[Pd(dppe)2]Cl2
-[Pd(dppe)2]Cl2
-[Pd(dppe)2]Cl2
O
F
F
F
O
N
O
N
F
N
O
O
N
N
N
N
N
2a
1a
4a
3a
Scheme 1.
m
(C@N) is detected as a strong band in the 1639–1657 cmÀ1 range,
which replaces the higher wavenumber (C„N) band of the start-
in the 13C NMR spectrum of 1a the resonances at 151.5 and
167.4 ppm are assigned to the N@CN and NC@O moieties (159.1
and 169.8 ppm for complex 1 [12a]), respectively.
m
ing organonitriles NCR. The NMR results confirm the formation of
the oxadiazoline ring [11a,12a,b]. Hence in the 1H NMR spectra
of 3 and 4, for example, the expected signals of the N–CH–N pro-
tons are detected in the 5.66–6.15 ppm range, whereas in the 13C
NMR spectra, the N–CH–N resonances appear in the 89.5–
91.3 ppm range and the C@N resonances are detected in the
163.6–163.7 ppm range.
3. Conclusions
In this work we have shown that PdCl2 activates very effectively
organonitriles, NCR, towards coupling with nitrones (cyclic and
acyclic ones) to give, under mild conditions and via a single-pot
[2+3] cycloadditions, fused tricyclic fluorinated ketoimine, fused
bicyclic or monocyclic
D
4-1,2,4-oxadiazoline palladium(II) com-
2.2. Liberation of the heterocyclic compounds 1a–4a
plexes, in moderate to good yields. In addition, the N-heterocyclic
ligands can be liberated from the metal centre and the current
The liberation of the N-heterocyclic ligands 1a–4a was carried
out by the reaction of complexes 1–4 with 2 eq of 1,2-bis(diphen-
ylphosphino)ethane (dppe). When the precipitation of the colour-
less compound [Pd(dppe)2]Cl2 (identified by IR and 31P{1H} NMR
[13]) was complete, the formation of the free heterocycles tetra-
fluoro-2,3-dihydropyrrolo[1,2-a]quinazolin-5-one (1a) (Scheme 1,
method provides
dium(II)-mediated route to the syntheses of free tetrafluoro-2,3-
dihydropyrrolo[1,2-a]quinazolin-5-one and
4-1,2,4-oxadiazoline
a more facile and cheaper one-pot palla-
D
derivatives, in comparison with related platinum systems which
do not proceed via single-pot reactions [11c,f].
reaction a) and
D
4-1,2,4-oxadiazolines (2a–4a) (Scheme 1, reac-
tions b–d) was confirmed by elemental analyses, ESI+-MS, IR, 1H
4. Experimental
and 13C NMR spectroscopies (see Section 4). For example, the IR
spectrum of 1a exhibits strong
m(NC@O) and
m(C@N) vibrations
4.1. General
at 1676 and 1583 cmÀ1 (1691 and 1649 cmÀ1 for complex 1
[12a]), respectively. In the 1H NMR spectrum of 1a the resonances
of the methyl and methylene groups are observed at d 1.75, 1.76,
2.27 and 3.15 (d 1.78, 1.79, 2.46 and 4.61 for complex 1). Moreover,
Solvents and reagents were obtained from commercial sources
(Aldrich) and used as received. Acyclic nitrone was prepared
according to the published methods [11a,14]. Complexes 1 and 2

22
J. Lasri / Polyhedron 57 (2013) 20–23
have been reported previously by us [12a]. C, H and N elemental
analyses were carried out by the Microanalytical Service of the
Instituto Superior Técnico. 1H and 13C spectra (in CDCl3) were mea-
sured on a Bruker Avance II 400 MHz (UltraShield™ Magnet) spec-
trometer at ambient temperature. 1H and 13C chemical shifts (d)
are expressed in ppm relative to Si(Me)4. J values are in Hz. Infrared
spectra (4000–400 cmÀ1) were recorded on a Bio-Rad FTS 3000MX
and a Jasco FT/IR-430 instrument in KBr pellets and the wavenum-
bers are in cmÀ1. Electrospray mass spectra were carried out with
an ion-trap instrument (Varian 500-MS LC Ion Trap Mass Spec-
trometer) equipped with an electrospray (ESI) ion source. The solu-
tions in methanol were continuously introduced into the mass
whereupon the colourless [Pd(dppe)2]Cl2 precipitated. This precip-
itate was separated by filtration and identified by 31P{1H}(CDCl3)
NMR: d = 55.9 (56.7 ppm [13]). The solution was then dried in va-
cuo, washed with three 5 mL portions of hexane and dried under
air.
No reaction was observed using complex 1 and 1 eq of dppe,
and only the starting material was recovered.
: yield, 90%. IR (cmÀ1): 1676
NC(=O)C6F4NCCH2CH2CMe2 1a:
m
(NC@O), 1583 m
(NC@N). 1H NMR (CDCl3), d: 1.75 and 1.76 (two
s, 6H, two CH3), 2.27 (t, JHH 8 Hz, 2H, CH2), 3.15 (t, JHH 8 Hz, 2H,
CH2). 13C{1H} NMR (CDCl3), d: 27.1 and 27.3 (CH3), 31.3 and 38.3
(CH2), 71.6 (C(Me)2), 126.9–134.4 (Caromatic), 151.5 (NC@N), 167.4
(NC@O). ESI+-MS, m/z: 287 [M + H]+. Anal. Calc. for C13H10N2OF4:
C, 54.55; H, 3.52; N, 9.79. Found: C, 54.73; H, 3.34; N, 9.54%.
spectrometer source with a syringe pump at a flow rate of 10 lL/
min. The drying gas temperature was maintained at 350 °C and
dinitrogen was used as the nebulizer gas at a pressure of 35 psi.
Scanning was performed from m/z = 50 to 1500.
: yield, 87%. IR (cmÀ1): 1691
N=C(Pr)ONCHCH2CH2CMe2 2a:
m
(C@N). 1H NMR (CDCl3), d: 0.99 (t, JHH 7.2 Hz, 3H, CH3CH2), 1.31
4.2. Synthesis of complex 3
and 1.39 (two s, 6H, two CH3), 1.62–1.72 (m, 2H, CH2), 2.20–2.25
(m, 2H, CH2), 2.90–2.92 (m, 4H, CH2), 5.15 (m, 1H, N–CH–
N).13C{1H} NMR (CDCl3), d: 14.2 (CH3CH2), 19.5 (CH2CH3), 22.5
and 27.9 ((CH3)2C), 29.7 and 31.1 (CH2CH2), 32.1 (CH2CH2CH3),
61.7 (Me2C–N), 87.1 (N–CH–N), 167.7 (C(O)@N). ESI+-MS, m/z:
183 [M+H]+. Anal. Calc. for C10H18N2O: C, 65.90; H, 9.95; N,
15.37. Found: C, 65.50; H, 9.57; N, 15.55%.
A solution of 4-cyanobenzaldehyde (29.6 mg, 0.226 mmol) in
acetone (4 mL) was added at room temperature to cyclic nitrone
(25.6 mg, 0.226 mmol) and palladium(II) chloride (20.0 mg,
0.113 mmol), and the mixture was stirred at room temperature
for 1 d. Along the course of the reaction, the brown powder of PdCl2
dissolved, forming an homogeneous light yellow solution. The
reaction mixture was then dried in vacuo, washed with three
5 mL portions of diethyl ether and dried under air. The final com-
plex 3 was recrystallized from acetone.
: yield, 85%. IR (cmÀ1):
N=C(p-CHOC6H5)ONCHCH2CH2CMe2 3a.
1635 m(C@N) and 1681 m
(C@O). 1H NMR (CDCl3), d: 1.27 and 1.31
(two s, 6H, two CH3), 1.96 (t, JHH 7.5 Hz, 2H, CH2), 2.44 (t, JHH
7.5 Hz, 2H, CH2), 5.17 (m, 1H, N–CH–N), 7.48 (d, JHH 7.8 Hz, 2H,
CHaromatic), 7.89 (d, JHH 7.8 Hz, 2H, CHaromatic), 10.09 (s, 1H, CHO).
13C{1H} NMR (CDCl3), d: 22.1 and 28.2 (CH3), 28.7 and 31.7 (CH2),
68.2 (Me2C–N), 90.5 (N–CH–N), 128.9, 130.7, 134.3 and 138.5 (Caro-
matic), 159.9 (C(O)@N), 191.5 (HC@O). ESI+-MS, m/z: 245 [M + H]+
and 277 [M + Na]+. Anal. Calc. for C14H16N2O2: C, 68.83; H, 6.60;
N, 11.47. Found: C, 68.49; H, 6.54; N, 11.65%.
:
yield,
Trans-[PdCl2{N=C(p-CHOC6H4)ONCHCH2CH2CMe2}2] 3
79%. IR (cmÀ1): 1639
m(C@N) and 1705
m
(C@O). 1H NMR (CDCl3),
d: 1.30 and 1.32 (two s, 6H, two CH3), 2.07 (t, JHH 7.5 Hz, 2H,
CH2), 3.39 (t, JHH 7.5 Hz, 2H, CH2), 5.66 (dd, JHH 7.2 Hz, JHH 3.3 Hz,
1H, N–CH–N), 8.07 (d, JHH 7.8 Hz, 2H, CHaromatic), 8.93 (d, JHH
7.8 Hz, 2H, CHaromatic), 10.11 (s, 1H, CHO). 13C{1H} NMR (CDCl3),
d: 21.9 and 27.6 (CH3), 29.3 and 34.3 (CH2), 69.9 (Me2C–N), 89.5
(N–CH–N), 128.3, 130.6, 137.2 and 139.3 (Caromatic), 163.7
(C(O)@N), 191.1 (HC@O). ESI+-MS, m/z: 667 [M+H]+. Anal. Calc.
for C28H32N4O4Cl2Pd: C, 50.50; H, 4.84; N, 8.41. Found: C, 50.39;
H, 4.54; N, 8.45%.
: yield, 80%. IR (cmÀ1):
N=C(Ph)ON(Me)C(H)(C6H2Me3-2,4,6) 4a.
1675 (C@N). 1H NMR (CDCl3), d: 2.18, 2.26 and 2.42 (three s, 9H,
CH3Ph), 2.52 (s, 3H, CH3N), 5.45 (s, 1H, N–CH–N), 7.43–7.94 (m,
7H, CHaromatic). 13C{1H} NMR (CDCl3), d: 20.3, 20.9 and 21.7 (CH3-
Ph), 47.1 (CH3N), 90.7 (N–CH–N), 126.9, 127.4, 128.6, 131.8,
131.9, 133.9, 136.9 and 140.1 (Caromatic), 161.7 (C(O)@N). ESI+-MS,
m/z: 281 [M+H]+. Anal. Calc. for C18H20N2O: C, 77.11; H, 7.19; N,
9.99. Found: C, 77.46; H, 7.34; N, 9.55%.
4.3. Synthesis of complex 4
A solution of acyclic nitrone (39.6 mg, 0.224 mmol) in benzoni-
trile (4 mL) was added at room temperature to palladium(II) chlo-
ride (20.0 mg, 0.113 mmol), and the mixture was heated at 60 °C
under stirring for 1 d. The solvent was then removed in vacuo
and the resulting oily residue was treated in a manner similar to
that described above to obtain the final complex 4.
Acknowledgments
This work has been supported by the Fundação para a Ciência e a
Tecnologia (FCT), Portugal. The author expresses gratitude to the FCT
and the Instituto Superior Técnico (IST) for his research contract
(Ciência 2007 program). J.L. gratefully acknowledges the Portuguese
NMR Network (IST-UTL Center) for access to the NMR facility.
Thanks are also due to Ms. Ana Dias for the analytical service.
:
yield, 67%. IR
[PdCl2{N=C(Ph)ON(Me)C(H)(C6H2Me3-2,4,6)}2] 4.
(cmÀ1): 1657 (C@N). 1H NMR (CDCl3), d: 2.32, 2.49 and 2.91 (three
s, 9H, CH3Ph), 3.17 (s, 3H, CH3N), 6.15 (s, 1H, N–CH–N), 6.90–7.97
(m, 7H, CHaromatic). 13C{1H} NMR (CDCl3), d: 20.1, 20.7 and 21.4
(CH3Ph), 47.5 (CH3N), 91.3 (N–CH–N), 128.5, 129.4, 131.7, 133.1,
133.8, 136.3, 137.9 and 140.3 (Caromatic), 163.6 (C(O)@N). ESI+-MS,
m/z: 703 [MÀCl]+. Anal. Calc. for C36H40N4O2Cl2Pd: C, 58.58; H,
5.46; N, 7.59. Found: C, 58.63; H, 5.69; N, 7.66%.
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