Unsaturated 4,4′-bis-[5(4H)-oxazolones]: Synthesis and evaluation of their ortho-palladation through C-H bond activation

Article abstract of DOI:10.1016/j.ica.2010.12.054

Aromatic unsaturated 4,4′-bis-[5(4H)-oxazolones] have been prepared and their structural properties discussed. Metallation studies of these substrates towards palladium(II) acetate were also investigated.

Full text of DOI:10.1016/j.ica.2010.12.054

Inorganica Chimica Acta 368 (2011) 247–251
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Inorganica Chimica Acta
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Note
Unsaturated 4,40-bis-[5(4H)-oxazolones]: Synthesis and evaluation of their
ortho-palladation through C–H bond activation
Gheorghe-Doru Roiban a,c, , Tatiana Soler b, Ion Grosu a, Carlos Cativiela c, Esteban P. Urriolabeitia d
⇑
a Organic Chemistry Department, Babeßs-Bolyai University, Arany Janos 11, 400028 Cluj-Napoca, Romania
b Servicios Centrales de Investigación, Facultad Ciencias, 03690 San Vicente de Raspeig, Alicante, Spain
c Departamento de Química Orgánica, Instituto de Ciencia de Materiales de Aragón, CSIC-Universidad de Zaragoza, 50009 Zaragoza, Spain
d Departamento de Compuestos Organometálicos, Instituto de Ciencia de Materiales de Aragón, CSIC-Universidad de Zaragoza, 50009 Zaragoza, Spain
a r t i c l e i n f o
a b s t r a c t
Aromatic unsaturated 4,40-bis-[5(4H)-oxazolones] have been prepared and their structural properties
discussed. Metallation studies of these substrates towards palladium(II) acetate were also investigated.
Ó 2010 Elsevier B.V. All rights reserved.
Article history:
Received 15 October 2010
Received in revised form 16 December 2010
Accepted 20 December 2010
Available online 28 December 2010
Keywords:
4,40-Bis-[5(4H)-oxazolones]
Metallation
Ortho-palladation
C–H bond activation
Pincer compounds
1. Introduction
[12], the chemistry of these species, in particular unsaturated 4,40-
bis-[5(4H)-oxazolones], has been scarcely studied [13]. Moreover,
C–H bond activation is the most important method for the syn-
thesis of ortho-metallated complexes [1]. Nowadays, this reaction
is extensively studied due to its synthetic possibilities for the func-
tionalization of a large diversity of organic substrates [2]. Oxazol-
ones are very important organic precursors and appropriate
building blocks, which have attracted much attention due to their
use as intermediates in pharmacology and for the synthesis of amino
acids [3]. With numerous reactive sites, oxazolone scaffold allows
for a large diversity of transformations such as stereoselective cyc-
loadditions [4], opening of the heterocyclic ring [5], enantioselective
alkylation [6] and arylations [7]. The ortho-metallation of
5(4H)-oxazolones is still a scarcely represented process. Our recent
contributions have shown that unsaturated substrates can be regio-
selectively functionalized through ortho-palladation [8], and that
metallated oxazolones may evolve through [2 + 2] photocycloaddi-
tion to give unprecedented oxazolones [9].
no detailed structural information is available for 4,40-bis-[5(4H)-
oxazolones].
In addition, the reactivity of unsaturated bis-oxazolones towards
metallating agents is unknown. The only reported examples are lim-
ited to Pt and Pd coordination complexes of saturated 2,20-bis-
[5(4H)-oxazolones] [14]. However, as we have previously reported
[8,9], the palladation approach could constitute the first step of an
alternative synthetic strategy for reaching modified bis-oxazolones,
which in turn are precursors for bis-amino acids, targets which have
attracted significant interest in the last few years [15], but synthet-
ically limited to classic organic transformations [16].
The interest to synthesize 4,40-bis-[5(4H)-oxazolones] is
increased by the potential use of these structures as multidentate
ligands. For instance, some symmetrically substituted 4,40-bis-
[5(4H)-oxazolones] may be classified as pincer (NCN) ligands
[1,17]. In this communication we describe the improved synthesis
and structural characterization of 4,40-bis-[5(4H)-oxazolones] and
their reactivity towards metallating substrates as palladium(II)
acetate.
Following our current research on functionalization of unsatu-
rated oxazolones [8,9], we have focused now our attention on
bis-oxazolones. While the use of these substrates was limited to
polymers [10], chain coupling reagents [11], and in pharmacology
2. Results and discussion
⇑
Corresponding author at: Organic Chemistry Department, Babesß-Bolyai Uni-
The unsaturated 4,40-bis-[5(4H)-oxazolones] 2a, 2b [10a,18]
and 2c [18,19] were prepared according to literature procedures
versity, Arany Janos 11, 400028 Cluj-Napoca, Romania.
E-mail address: gdroiban@chem.ubbcluj.ro (G.-D. Roiban).
0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.12.054

248
G.-D. Roiban et al. / Inorganica Chimica Acta 368 (2011) 247–251
(Scheme 1). Benzenedicarboxaldehydes (1a–c) were reacted with
hippuric acid using Erlenmeyer conditions to afford ortho-, meta-
and para-substituted (Z,Z0)-2,20-diphenyl-4,40-phenylenedimethyl-
ene-bis-5[(4H,40H)-oxazolones] 2a–c in moderate to good yields.
Although two geometric isomers are possible for 2a–c, the
Erlenmeyer reaction favors the thermodynamically more stable Z
isomer, which is easily isolated by recrystallization [8]. The struc-
ture and the correct geometry for 2a–c were inferred from spectro-
scopic (IR, NMR), MS, X-ray and elemental analyses data. The
structures of the unsaturated 4,40-bis-[5(4H)-oxazolones] 2a–c
are drawn in Chart 1.
diffraction methods. Crystals of 2cꢁC6H5Me suitable for X-ray mea-
surements were obtained after recrystallization from toluene
(Fig. 1).
ꢀ
2c crystallizes in the triclinic system, space group P1, with two
centrosymmetric molecules in the unit cell, together with two tol-
uene molecules. The molecular structure displays a ZZ configura-
tion about the C9C10 and C17C18 double bonds, as expected. In
addition, if we look along the axis C17–C14–C11–C10 and perpen-
dicularly to the plane containing the phenylene unit we observe
the E orientation of the two azlactone rings. In comparison with
previously determined (Z)-2-phenyl-4-arylidene-5(4H)-oxazol-
ones, which are planar [20], the introduction of a second oxazolone
ring resulted in a partial loss of planarity. The value of the dihedral
angle between the plane of phenylene and the plane containing the
aromatic ring (C3–C8) is 16.24(4)° while that of the phenylene
with the plane containing the other terminal aromatic ring (C21–
C26) is 14.67(4)°. This fact induces a slight helicity of the system.
The molecular configuration of 2c is stabilized by several inter-
molecular hydrogen bonds. A closer check of the crystal structure
revealed C–Hꢁ ꢁ ꢁO intermolecular interactions involving O1 and
O3 atoms from the oxazolone rings [C17–H17ꢁ ꢁ ꢁO1 2.6702(19) Å,
C12–H12ꢁ ꢁ ꢁO1 2.7585(22) Å, C6–H6ꢁ ꢁ ꢁO3 2.7022(24) Å and
The IR spectra of 2a–c exhibit strong carbonyl stretching vibra-
tion bands around
m
= 1785 cmꢀ1, while the absence of any band
assignable for carboxylic OH or amidic NH function supports the
formation of heterocyclic system. The NMR data fully agree with
the assigned configurations. Only one set of signals is present on
each spectrum, showing the presence of only one isomer in solu-
tion. The pattern of signals shows that the two oxazolone units
are chemically equivalent, meaning that the molecule is symmetric
with respect to the central C6H4 ring. We assume that the configu-
ration around the C@C double bond is the thermodynamic Z-iso-
mer, as shown in Chart 1, by analogy of the synthetic method
here employed with previously reported syntheses of Z-oxazol-
ones. This assumption will be confirmed through the determina-
tion of the crystal structure of 2c.
The structure of (Z,Z0)-2,20-diphenyl-4,40-p-phenylene-dimethyl-
ene-bis-5[(4H,40H)-oxazolone] (2c) was determined by X-ray
N
O
O
OH
O
CHO
Ac O
2
O
Fig. 1. ORTEP plot of 2c with thermal ellipsoids drawn at the 50% probability level.
+
N
H
O
NaOAc
CHO
N
O
1a, or tho
1b
2a, ortho
2b
, meta
1c, par a
, met a
2c, para
Scheme 1. Reagents and conditions: Ac2O, NaOAc, 100 °C, 2 h; 2a (42%), 2b (83%),
2c (78%).
O
O
O
O
N
N
O
O
2a
N
N
O
O
O
O
N
N
2b
O
O
2c
Chart 1.
Fig. 2. Diamond view of intermolecular interactions in 2c.

G.-D. Roiban et al. / Inorganica Chimica Acta 368 (2011) 247–251
249
C10–H10ꢁ ꢁ ꢁO3 2.6515(19) Å] (Fig. 2, top). Further stabilization of
2c in the solid state results from the formation of a dimeric struc-
NMR spectroscopy, the two broad singlet peaks at ꢀ75.78 and
ꢀ76.44 ppm indicating a dynamic structure at room temperature.
Therefore, the whole molecule shows two different behaviors on
the NMR time scale. The pincer ligand displays a static behavior,
giving sharp and well resolved signals, while the trifluoroacetate
is involved on a dynamic process, likely its coordination and deco-
ordination. This dynamic process seems to be closely related with
the high steric hindrance exerted by the two phenyl groups of the
oxazolone at 2,20 positions. To further confirm the formation of a
monomeric structure we have used MALDI to detect palladacycle
species. The spectrum of 3 exhibits only the ion corresponding to
the [M-TFA]+ fragment at m/z 524.8.
It is worthy of note that the formed palladacycle 3 has two six-
membered fused metallacycles. These pincer complexes are
formed to a lesser extent than five-membered ones [17], show in-
creased catalytic efficiency [24] and they are limited to few exam-
ples [25].
ture in which two bis-oxazolones are connected by
p–p interac-
tions: tilted edge-to-face and offset face-to-face. Tilted edge-to-
face interactions are given by the aromatic rings of toluene mole-
cules which are parallel and quasi perpendicular on the other aro-
matic rings (Fig. 2, top). Each aromatic ring belonging to toluene
has two edge tilted to face interactions (on both sides) with the ter-
minal aromatic rings of two bis-oxazolone molecules in edge tilted
to face arrangements. The distances between the centroid of the
toluene aromatic ring and the contact hydrogens are 2.9925 (3) Å
and 2.7213 (2) Å, respectively. The latter interactions are given
by the aromatic rings of two superposed bis-oxazolones, the dis-
tance between the ring centroids from phenyls centroids is
3.75396 (6) Å suggesting the existence of weak offset face-to-face
interactions (Fig 2, bottom). The same interactions were noticed
between the phenylene centroids 3.5578 (6) Å.
Following our aim to study the metallating behavior of bis-oxa-
zolones, ligands 2a–c were reacted with Pd(OAc)2 using our previ-
ously optimized conditions [9]. Under these conditions (CF3COOH,
70 °C), neither 2a nor 2c gave characterizable species, the products
separated after workup having very low solubility in deuterated
solvents. In the case of 2a the only evidence for the formation of
a palladated product was the MALDI mass spectrum, since peaks
of ions corresponding to [2aPd]+ were detected with a high abun-
dance at m/z 524.8 amu.
Taken together these results demonstrate that some unsatu-
rated 4,40-bis-[5(4H)-oxazolones] can be metallated. We believe
that the simple case presented herein will open the way to a whole
new series of metallated compounds which may be used in cata-
lytic studies and in preparation of bis-amino acids. The synthesis
of more complex bis-oxazolonic substituted substrates as well as
use of different transition metals is currently under investigation
in our laboratories.
However, when 2b was treated in the same conditions as 2a or
2c, the C–H bond activation occurs easily and the pincer compound
3 was isolated in 86% yield after crystallization from dichlorometh-
ane/hexane (Scheme 2). It is to be noted that due to its symmetri-
cal structure, with two donor nitrogen atoms and a central ligating
C atom 2b classifies as a NCN pincer ligand [17]. The increase inter-
est to synthesize pincer complexes refers not only to their numer-
ous applications in asymmetric synthesis [21], catalysis [22] but
also to the big variety of transformations in which these com-
pounds are involved [23].
3. Experimental
3.1. Materials
The unsaturated oxazolones 2b and 2c, were already synthe-
sized but an entirely spectroscopic characterization is missing
and therefore a fully characterization is given here.
3.2. Physical methods
The 1H and 13C NMR spectra are consistent with the symmetri-
cal structure, and show sharp signals due to the presence of a sin-
Chemicals of commercial grade were used without further puri-
fication. 1H and 13C{1H} NMR spectra were recorded in CDCl3, at
room temperature, on a Bruker Avance 400 spectrometer (d in
ppm, J in Hz) at 1H operating frequency of 400.13 MHz. 1H and
13C NMR spectra were referenced using the solvent signal as inter-
nal standard. MALDI mass spectra were recorded on a MALDI-TOF
Microflex (Bruker) spectrometer from CHCl3 solutions (DCTB as
matrix). Infrared spectra (4000–380 cmꢀ1) were recorded on a Per-
kin–Elmer Spectrum One IR spectrophotometer, using Nujol mulls
between polyethylene sheets. Elemental analyses were carried out
on a Perkin–Elmer 2400-B microanalyser.
00
gle species. The deshielded signal attributed to H2 in 2b is not
longer seen on the 1H NMR spectrum of 3, giving unambiguous
proof of the metallation. The vinylic protons of the benzylidene
00
fragment H7 , which are equivalent and appear as a singlet, shift
downfield from 7.32 ppm in 2b to 7.66 ppm in 3. Full assignments
of all resonances in 3 were made using NOESY-1D experiments.
The selective saturation of the signal at 7.66 ppm (assigned to
00
H7 ) induces a clear NOE effect on the signal at 7.48 ppm (assigned
00
to the H6 proton of the ortho-palladated ring). On the other hand,
00
the selective saturation of the signal at 7.32 ppm (assigned to H5
)
00
gives a clear NOE with the doublet at 7.48 ppm (H6 ), showing
unambiguously the formation of the ortho-palladated unit. The
presence of trifluoroacetate anion has been confirmed by 19F
3.3. Synthesis of compounds
3.3.1. Preparation of (Z,Z0)-2,20-diphenyl-4,40-o-
phenylenedimethylene-bis-[5(4H,40H)-oxazolone] (2a)
Benzene-1,2-dicarboxaldehyde (0.595 g, 4.435 mmol), N-ben-
zoylglycine (1.590 g, 8.871 mmol), anhydrous sodium acetate
(0.728 g,
8.871 mmol)
and
acetic
anhydride
(2.720 g,
O
O
O
O
26.615 mmol) were added in a 100 mL round-bottomed flask.
The mixture was heated for 2 h at 100 °C and then allowed to cool
at room temperature. The precipitate was washed with ethanol,
cooled down for 30 min and then filtered. 2a was obtained as an
Pd(OAc)2
CF3CO2H
Pd
N
N
N
N
O
O
O
O
F3CCO2
yellow solid after recrystallization from ethanol (Yield 0.783 g,
3
42%). 1H NMR (CDCl3) d ppm: 7.54 (t, J = 7.6 Hz, 4H, 2H3 , 2H5 ),
0
0
0
00
00
00
7.59–7.66 (m, 4H, 2H4 , H4 , H5 ), 7.70 (s, 2H, 2H7 ), 8.19 (d,
J = 7.2 Hz, 4H, 2H2 , 2H6 ), 8.79 (m, 2H, H3 , H6 ). 13C{1H} NMR
3
0
0
00
00
2b
3
0
00
0
(CDCl3) d ppm: 125.32 (2C, C1 ), 126.59 (2C, C7 ), 128.59 (4C, 2C2 ,
Scheme 2. Reagents and conditions: Pd(OAc)2, CF3CO2H, 70 °C, 2 h; (86%).
0
0
0
0
00
00
2C6 ), 129.03 (4C, 2C3 , 2C5 ), 130.83 (2C, C4 ), 132.65 (2C, C3 , C6 ),

250
G.-D. Roiban et al. / Inorganica Chimica Acta 368 (2011) 247–251
00
00
00
00
133.74 (2C, C4 , C5 ), 134.01 (2C, C1 , C2 ), 134.91 (2C, C2), 161.25
(C@N). Anal. Calc. for C28H15F3N2O6Pd (637.99): C, 52.64; H, 2.37;
(2C, C1), 167.04 (2C, C3). MS (MALDI+) m/z, (rel. int.%): 420.1
N, 4.39. Found: C, 52.87; H, 2.43; N, 4.56%.
(98.5%) [M]+. IR:
m
= 1792 cmꢀ1 (C@O), 1639 cmꢀ1 (C@N). Anal.
Calc. for C26H16N2O4 (420.11): C, 74.28; H, 3.84; N, 6.66. Found:
3.4. Crystal structure determination
C, 74.55; H, 3.67; N, 6.74%.
Data collection was performed at room temperature on an Ox-
ford Diffraction Xcalibur2 diffractometer using graphite-monocro-
3.3.2. Preparation of (Z,Z0)-2,20-diphenyl-4,40-m-
phenylenedimethylene-bis-5[(4H,40H)-oxazolone] (2b)
mated Mo K
collected based on three
a
radiation (k = 0.71073 Å). An hemisphere of data was
-scan or -scan runs. The diffraction
x
u
Benzene-1,3-dicarboxaldehyde (1.650 g, 12.301 mmol), N-ben-
zoylglycine (4.410 g, 24.602 mmol), anhydrous sodium acetate
(2.018 g, 24.602 mmol) and acetic anhydride (7.530 g,
73.808 mmol) were added in a 100 mL round-bottomed flask.
The mixture was heated for 2 h at 100 °C and then allowed to cool
at room temperature. The precipitate was washed with ethanol,
cooled down for 30 min and then filtered. 2b was obtained as an
yellow solid after recrystallization from EtOH (Yield 4.290 g,
frames were integrated using the program CRYSALIS RED [26] and
the integrated intensities were corrected for absorption with SADABS
[27]. The structure was solved and developed by Patterson and
Fourier methods [28]. All non-hydrogen atoms were refined with
anisotropic displacement parameters. The hydrogen atoms were
placed at idealized positions and treated as riding atoms. Each
hydrogen atom was assigned an isotropic displacement parameter
equal to 1.2 times the equivalent isotropic displacement parameter
of its parent atom. The structure was refined to F2o, and all reflec-
tions were used in the least-squares calculations [29].
83%). 1H NMR (CDCl3) d ppm: 7.32 (s, 2H, H7 ), 7.49 (t, 3J = 7.8 Hz,
00
4H, 2H3 , 2H5 ), 7.52–7.70 (m, 3H, 2H4 , H5 ), 8.19 (d, 3J = 7.2 Hz,
0
0
0
00
4H, 2H2 , 2H6 ), 8.33 (d, 3J = 8.1 Hz, 2H, H4 , H6 ), 9.00 (s, 1H, H2 ).
0
0
00
00
00
13C{1H} NMR (CDCl3) d ppm: 125.28, 134.12, 134.17 (3C, C1 , C1 ,
00
0
Acknowledgements
0
0
0
0
C2), 128.54 (4C, 2C2 , 2C6 ), 128.96 (4C, 2C3 , 2C5 ), 129.49 (1C,
00
00
0
00
00
C5 ), 130.61 (2C, 2C7 ), 133.57 (2C, C4 ), 134.49 (2C, C4 , C6 ),
The financial support of this work by CNCSIS–UEFISCSU PNII–
IDEI 570/2007, TD87/2007 (Romania), the Ministerio de Ciencia e
Innovacion (Projects CTQ2008–01784, CTQ2010-17436 and
CTQ2007–62245, Spain), and Gobierno de Aragon (PI071–09) is
gratefully acknowledged.
00
136.00 (1C, C2 ), 164.12 (1C, C1), 167.45 (1C, C3). MS (MALDI+) m/
z, (rel. int.%): 420.1 (46.8%) [M]+. IR:
m
= 1789 cmꢀ1 (C@O),
1647 cmꢀ1 (C@N). Anal. Calc. for C26H16N2O4 (420.11): C, 74.28;
H, 3.84; N, 6.66. Found: C, 74.22; H, 3.62; N, 6.93%.
3.3.3. Preparation of (Z,Z0)-2,20-diphenyl-4,40-p-
phenylenedimethylene-bis-5[(4H,40H)-oxazolone] (2c)
Appendix A. Supplementary material
Benzene-1,4-dicarboxaldehyde (0.963 g, 7.179 mmol), N-ben-
zoylglycine (2.570 g, 14.359 mmol), anhydrous sodium acetate
(1.178 g, 14.359 mmol) and acetic anhydride (1.178 g,
14.359 mmol) were added in a 100 mL round-bottomed flask.
The mixture was heated for 2 h at 100 °C and then allowed to cool
at room temperature. The precipitate was washed with ethanol,
cooled down for 30 min and then filtered. 2c was obtained as a yel-
low solid after recrystallization from ethanol (Yield 2.354 g, 78%).
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.ica.2010.12.054.
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3.3.4. Preparation of (3)
To a stirred solution of 2b (0.295 g, 0.701 mmol) in TFA (5 mL),
palladium acetate (0.315 g, 1.403 mmol) was added. The solution
was refluxed at 70 °C for 2 h and the mixture was then treated with
water. The precipitate formed was filtered and washed several
times with water to remove the acid. After complete dryness the
solid was dissolved in CH2Cl2 and the precipitated with hexane
to afford 3 as a yellow solid (Yield 0.386 g, 86%). 1H NMR
(400 MHz, CDCl3) d ppm: 7.32 (t, 3J = 7.4 Hz, 1H, H5 ), 7.42–7.51
00
3
0
0
00
00
0
(m, 6H, 2H3 , 2H5 , H4 , H6 ), 7.59 (t, J = 7.6 Hz, 2H, 2H4 ), 7.66 (s,
2H, 2H7 ), 8.21 (d, J = 7.5 Hz, 4H, 2H2 , 2H6 ). 13C{1H} NMR (CDCl3)
3
00
0
0
0
00
00
d ppm: 122.10, 126.09, 135.05, 139.10 (4C, C1 , C1 , C2, C2 ), 126.59
00
0
0
0
0
(1C, C5 ), 128.81 (4C, 2C3 , 2C5 ), 130.61 (4C, 2C2 , 2C6 ), 135.09 (2C,
00
00
0
00
C4 , C6 ), 135.57 (2C, 2C4 ), 138.14 (2C, C7 ), 160.34 (2C, C1), 169.25
(2C, C3). 19F NMR (376 MHz, CDCl3) d ppm: ꢀ76.44, ꢀ75.78 (broad
s, 3F). MS (MALDI+) m/z, (rel. int.%): 524.8 (100%) [M-CF3COOꢀ]+.
IR:
m
= 1804, 1791 cmꢀ1 versus (C@O), 1650, 1635 cmꢀ1 versus

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