Structural characterization of new 2-aryl-5-phenyl-1,3,4-oxadiazin-6-ones and their N-aroylhydrazone precursors

Article abstract of DOI:10.1016/j.molstruc.2013.11.005

A series of novel 2,5-disubstituted 1,3,4-oxadiazin-6-ones and their N-aroylhydrazone precursors were synthesized and characterized by NMR and UV-Vis spectroscopy. The electronic properties of 2-aryl-5-phenyl-1,3,4-oxadiazin-6- ones are mainly dependent o

Full text of DOI:10.1016/j.molstruc.2013.11.005

Journal of Molecular Structure 1058 (2014) 106–113
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Structural characterization of new 2-aryl-5-phenyl-1,3,
4-oxadiazin-6-ones and their N-aroylhydrazone precursors
⇑
Mihaela Liliana Tßînßtasß, Andreea Petronela Diac, Albert Soran, Anamaria Terec, Ion Grosu, Elena Bogdan
Babeßs-Bolyai University, Supramolecular Organic and Organometallic Chemistry Center (SOOMCC), Arany Janos 11, RO-400028 Cluj-Napoca, Romania
g r a p h i c a l a b s t r a c t
h i g h l i g h t s
ꢀ
New oxadiazinones by intramolecular
cyclization of corresponding N-
aroylhydrazones.
ꢀ
Electronic properties of
oxadiazinones are mainly influenced
by the 2-aryl substituent.
Isolation of Z,anti isomer of N-
aroylhydrazones.
Strong intermolecular interactions in
the solid state revealed for N-
aroylhydrazones.
ꢀ
ꢀ
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 25 August 2013
Received in revised form 30 October 2013
Accepted 4 November 2013
Available online 9 November 2013
A series of novel 2,5-disubstituted 1,3,4-oxadiazin-6-ones and their N-aroylhydrazone precursors were
synthesized and characterized by NMR and UV–Vis spectroscopy. The electronic properties of 2-aryl-5-
phenyl-1,3,4-oxadiazin-6-ones are mainly dependent on the 2-aryl substituent and their absorption
maxima exhibit a red shift in dichloromethane. Single crystal X-ray diffraction on four acylhydrazones
indicated the isolation of isomer with Z configuration of the C@N double bond. Intermolecular interac-
tions through strong H-bonding and CAHꢁ ꢁ ꢁ
p
contacts serve to link the molecules into a three-dimen-
Keywords:
4,5-Diaza-a-pyrones
N-acylhydrazones
sional supramolecular network.
Ó 2013 Elsevier B.V. All rights reserved.
UV–Vis spectra
Single crystal X-ray structure
Supramolecular interactions
1. Introduction
are known for their electrical properties. Thus, the Diels–Alder
reactions of some 2,5-diaryl-6-oxo-1,3,4-oxadiazine-6-one with
benzyne or naphthyne in order to obtain linear polyacenes were
reported [21,22]. When cycloaddition is performed under acidic
catalysis (trifluoroacetic acid), enol-lactone derivatives are
1,3,4-Oxadiazin-6-ones are known as valuable substrates for
access to a wide variety of bicyclic and heterocyclic compounds
by their [2 + 2] and [4 + 2] cycloaddition reactions with alkynes, al-
kenes and cycloalkenes [1–20]. The oxadiazinones act as electron
deficient 2,3-diaza-1,3-butadienes playing the role of diene in
obtained without
c-ketoketene intermediate formation [7]. An
exception was reported for 2,5-diphenyl-1,3,4-oxadiazin-6-one,
playing the role of dienophile component when reacted with 2,3-
dimethyl-1,3-butadiene [9].
The synthesis of the first member of this class of heterocycles,
2,5-diphenyl-1,3,4-oxadiazin-6-one, was reported by Steglich
[10], and the first oxadiazinones bearing an alkyl side chain in po-
sition 2 or 5 was obtained by Padwa [7]. Later, a considerable num-
ber of derivatives with aryl and alkyl substituents were reported by
Christl [2,3,6,9,11,12,18,19]. These works describe oxadiazinones
Diels–Alder reactions with reversed electron demand, when
c-
ketoketene are formed after loss of nitrogen from the initially
produced cycloadduct [6,7,19]. Recently, their reactivity in cyclo-
addition reaction has found a new application in obtaining organic
semiconductors such as polycyclic aromatic hydrocarbons, which
⇑
Corresponding author. Tel.: +40 745 356059; fax: +40 264 590 818.
E-mail address: ebogdan@chem.ubbcluj.ro (E. Bogdan).
0022-2860/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molstruc.2013.11.005

M.L. Tßînßtaßs et al. / Journal of Molecular Structure 1058 (2014) 106–113
107
as versatile substrates in cycloaddition reactions, revealing inter-
esting application in the synthesis of various compounds, such as
cyclopenta[c]pyrans [17]. An efficient four-step synthetic pathway
riding. Further details on the data collection and refinement meth-
ods can be found in Table 1. The drawings were created with the
Diamond program [34].
leading to oxadiazinones was recently developed starting from a-
aminoacid esters by a sequence of diazotation, reduction and
acylation reactions, followed by cyclization of the resulted N-acyl-
hydrazones [23]. A series of 1,3,4-oxadiazinylacetates were ob-
tained in rather low yields as side products along with
oxadiazepine derivatives by the condensation reaction of several
carbohydrazides with dimethyl but-2-ynediolate [24].
The acylhydrazones are of particular interest due to the various
applications, such as electrophilic reagents in many reactions, as
well as their biological activity [25–28]. Moreover, acylhydrazones
are known for their ability to bind transitional metals forming
complexes with antibacterial activity [29–31].
2.2. Synthesis of hydrazones 3
2.2.1. General procedure
A solution of 1 (5.2 mmol) in water (60 mL) was added drop-
wise (over 1–2 h) under vigorous stirring to a solution of hydrazide
2 (5.2 mmol) in water (60 mL) heated to 50–60 °C. The mixture
was stirred for additional 2 h, and then cooled on an ice bath until
the complete precipitation of the white solid. After filtration, the
solid was successively washed with cold water and cold diethyl
ether (or 2-propanol). The hydrazones have been used in the next
step without further purification.
In this context, we focused in this work on the synthesis and
investigation of the absorption properties of some new 2,5-dia-
ryl-oxadiazinone derivatives and their corresponding precursors,
N-acylhydrazones. Single crystal X-ray diffraction on four N-acyl-
hydrazones revealed the isolation of the Z,anti isomer, stabilized
by intramolecular NAHꢁ ꢁ ꢁO hydrogen bonds.
2.2.2. 2-(2-(3-Bromobenzoyl)hydrazono)-2-phenylacetic acid (3a)
Yield 93%, white solid, m.p. 177–178 °C. 1H NMR (300 MHz,
DMSO-d6) d ppm: 7.39–7.49 (overlapped peaks, 3H, H5, H6, H7),
7.53 (t from overlapped dd, 1H, J = 7.8 Hz, H14), 7.62–7.76 (m, 2H,
H4, H8), 7.84 (overlapped peaks, 2H, H13, H15), 8.02 (d, 1H,
J = 0.6 Hz, H11), 12.70 (s, 1H, NH). 13C NMR (75 MHz, DMSO-d6) d
ppm: 121.4, 128.1, 128.3, 128.7, 129.6, 131.0, 134.5, 135.2, 135.5,
2. Experimental
164.0, 165.4. UV/Vis (1,4-dioxane) kmax (loge) = 236 (4.16), 315
(4.21) nm. EI–MS m/z (rel. int.) = 44 (31), 50 (46), 51 (32), 65
(10), 76 (53), 89 (18), 103 (34), 119 (6), 139 (12), 155 (34), 157
(31), 183 (100), 185 (99), 199 (39), 201 (37), 301 (9), 303 (9),
345 (2), 347 (0.2) [M+]. MS (APCI+): 347.0 [M+H]+, 303.0 [M–
CO2]+. HRMS (APCI+): calcd for C15H12BrN2O3 [M+H]+: 347.0026;
found: 347.0023.
2.1. General remarks
Chemicals and solvents of commercial grade were used without
further purification. 1H and 13C NMR spectra were recorded in
CDCl3 or DMSO-d6 at room temperature on Brucker Avance 300
and Brucker Avance 500 spectrometers (d in ppm, J in Hz); spectra
were referenced using the solvent signal as internal standard. The
numbering of the compounds used for assigning the NMR spectra
signals was done arbitrary (for acylhydrazones 3 the same num-
bering as in the ORTEP diagrams was used, for oxadiazinones 4
see Chart 1). The EI mass spectra were recorded on a GC–MS Shi-
madzu QP 2010 spectrometer. HRMS in APCI mode ionization were
recorded with an LTQ XL ThermoScientific mass spectrometer.
UV–Vis absorption spectra were measured on a Cecil 9500 spectro-
photometer using quart cuvettes (1 cm); the solutions for
acylhydrazones 3 were 3.0 ꢂ 10ꢃ5 M and all solutions for oxadiaz-
inones 4 were 4.0 ꢂ 10ꢃ5 M. Melting points were measured with a
Kleinfeld melting point apparatus and are uncorrected. The single
crystals were obtained from DMSO:methanol = 1:1 by gel method
using agar-agar gel [32]. Data were collected at room temperature
on a Bruker SMART APEX diffractometer, using graphite monochro-
2.2.3. 2-(2-(3-Chlorobenzoyl)hydrazono)-2-phenylacetic acid (3b)
Yield 96%, white solid, m.p. 178–179 °C. 1H NMR (300 MHz,
DMSO-d6) d ppm: 7.41–7.48 (overlapped peaks, 3H, H5, H6, H7),
7.60 (t from overlapped dd, 1H, J = 7.8 Hz, H14), 7.66–7.73 (over-
lapped peaks, 3H, H4, H8, H13), 7.80 (d, 1H, J = 7.8 Hz, H15), 7.86–
7.88 (br. s, 1H, H11), 12.75 (s, 1H, NH). 13C NMR (75 MHz, DMSO-
d6) d ppm: 127.4, 128.2, 128.3, 128.8, 129.7, 130.8, 132.1, 134.5,
135.1, 135.3, 164.0, 165.4. UV/Vis (1,4-dioxane) kmax (loge) = 236
(4.14), 316 (4.23) nm. EI–MS m/z (rel. int.) = 39 (3), 44 (10), 50
(12), 51 (10), 65 (6), 75 (23), 76 (23), 89 (4), 103 (60), 111 (45),
113 (15), 139 (100), 141 (34), 155 (53), 157 (18), 257 (10), 302
(0.2) [M+]. MS (APCI+): 303.1 [M+H]+, 259.1 [M–CO2]+. HRMS
(APCI+): calcd for
303.0525.
C
15H12ClN2O3 [M+H]+: 303.0531; found:
mated Mo Ka radiation (k = 0.71073 Å). For this purpose the crystal
2.2.4. 2-(2-(3-Methoxybenzoyl)hydrazono)-2-phenylacetic acid (3c)
Yield 94%, white solid, m.p. 173–174 °C. 1H NMR (300 MHz,
DMSO-d6) d ppm: 3.83 (s, 3H, OCH3), 7.22 (d, 1H, J = 8.1 Hz, H13),
7.40–7.52 (overlapped peaks, 6H, H5, H6, H7, H11, H14, H15), 7.64–
7.74 (m, 2H, H4, H8), 12.87 (s, 1H, NH). 13C NMR (75 MHz, DMSO-
d6) d ppm: 55.4, 112.8, 118.2, 119.5, 128.2, 128.4, 129.4, 130.2,
134.3, 134.9, 159.5, 164.1, 165.4. UV/Vis (1,4-dioxane) kmax
was mounted on a cryo-loop with Paratone-N oil. The structure
was solved by direct methods (SHELXS-97) [33] and refined by full
matrix least-squares procedures based on F2 with all measured
reflections (SHELXL-97) [33]. All non-hydrogen atoms were refined
anisotropically. All hydrogen atoms were introduced in their ideal-
ized positions, except for the hydrogen atom of the carboxyl group
which was identified from the electron density map, and refined as
(loge) = 235 (4.19), 315 (4.21) nm. EI–MS m/z (rel. int.) = 39 (4),
44 (8), 63 (12), 64 (14), 76 (13), 77 (25), 92 (21), 103 (50), 107
(45), 121 (7), 135 (100), 151 (74), 253 (11), 298 (0.3) [M+]. MS
(APCI+): 299.1 [M+H]+, 255.1 [M–CO2]+. HRMS (APCI+): calcd for
C
16H15N2O4 [M+H]+: 299.1026; found: 299.1022.
2.2.5. 2-(2-(4-tert-Butylbenzoyl)hydrazono)-2-phenylacetic acid (3d)
Yield 92%, white solid, m.p. 175–176 °C. 1H NMR (300 MHz,
DMSO-d6) d ppm: 1.32 (s, 9H, C(CH3)3), 7.40–7.47 (overlapped
peaks, 3H, H5, H6, H7), 7.59 (d, 2H, J = 8.4 Hz, H12, H14), 7.65–7.72
(m, 2H, H4, H8), 7.80 (d, 2H, J = 8.4 Hz, H11, H15), 13.02 (s, 1H,
NH). 13C NMR (75 MHz, DMSO-d6) d ppm: 30.9, 34.8, 125.8,
127.4, 128.1, 128.4, 129.3, 130.1, 135.0, 155.4, 164.1. UV/Vis
Chart 1. Numbering of oxadiazinones 4a–f.

108
M.L. Tßînßtaßs et al. / Journal of Molecular Structure 1058 (2014) 106–113
Table 1
Summary of crystal data and structure refinements for compounds 3a, 3b, 3k, 3l.
Compound
3a
3b
3k
3l
Empirical formula
Formula weight
Crystal size (mm)
Crystal habit
Wavelength (Å)
Temperature (K)
Crystal system
Space group
a (Å)
C
15H11BrN2O3
C15H11ClN2O3
302.71
0.46 ꢂ 0.18 ꢂ 0.17
Colorless block
0.71073
297(2)
Orthorhombic
Pbca
16.160(4)
10.317(3)
16.666(4)
90.00
C16H14N2O3
282.29
0.41 ꢂ 0.23 ꢂ 0.23
Colorless block
0.71073
297(2)
Monoclinic
P2(1)/c
7.8312(18)
22.132(5)
8.750(2)
90.00
C15H12N2O3
268.27
0.60 ꢂ 0.33 ꢂ 0.29
Colorless block
0.71073
297(2)
Orthorhombic
Pbca
12.308(3)
13.392(3)
16.241(4)
90.00
347.17
0.60 ꢂ 0.45 ꢂ 0.21
Yellow block
0.71073
297(2)
Orthorhombic
Pbca
16.052(3)
10.3456(18)
17.054(3)
90.00
b (Å)
c (Å)
a
(°)
b (°)
90.00
90.00
2832.2(8)
8
1.628
90.00
90.00
2778.6(12)
8
1.447
111.392(4)
90.00
1412.2(6)
4
1.328
0.093
90.00
90.00
2677.1(10)
8
1.331
c
(°)
Volume (Å3)
Z
Density (calculated) (g cmꢃ1
)
Absorption coefficient (mmꢃ1
)
2.914
0.286
0.095
F (000)
1392
1248
592
1120
h range for data collections (°)
Tmax/Tmin
Reflections collected
2.39–25.00
0.5797/0.2738
19,085
2.44–25.00
0.9530/0.8796
18,795
1.84–25.00
0.9789/0.9627
13,411
2.51–25.00
0.9731/0.9454
23,881
Independent reflections, Rint
Completeness to h = 25.00°
Refinement method
2495, 0.0570
100%
2448, 0.0656
100%
2495, 0.0430
100%
2350, 0.0513
99.9%
Full-matrix least-squares on F2 Full-matrix least-squares on F2 Full-matrix least-squares on F2 Full-matrix least-squares on F2
Data/restraints/parameters
2495/0/195
1.055
2448/0/194
1.202
2495/0/199
1.230
2350/0/186
1.335
Goodness-of-fit on F2
Final R indicies [I > 2
R indices (all data)
r
(I)]
R1 = 0.0408
wR2 = 0.0956
R1 = 0.0552
wR2 = 0.1005
R1 = 0.0683
wR2 = 0.1214
R1 = 0.0821
wR2 = 0.1270
0.226, ꢃ0.227
921308
R1 = 0.0813
wR2 = 0.1599
R1 = 0.1010
wR2 = 0.1691
0.210, ꢃ0.237
921309
R1 = 0.0799
wR2 = 0.1456
R1 = 0.0887
wR2 = 0.1497
0.176, ꢃ0.173
921310
Largest diff. peak and hole, e Aꢃ3 0.470, ꢃ0.310
CCDC No.
921311
(1,4-dioxane) kmax (log
e
) = 235 (4.23), 316 (4.34) nm. EI–MS m/z
2.3. Synthesis of 1,3,4-oxadiazin-6-ones (4)
(rel. int.) = 44 (26), 50 (18), 51 (24), 65 (6), 76 (22), 91 (53), 103
(44), 118 (24), 134 (17), 146 (20), 161 (100), 162 (92), 177 (31),
279 (7), 325 (0.3) [M+]. MS (APCI+): 325.2 [M+H]+, 281.2 [M–
CO2]+. HRMS (APCI+): calcd for C19H21N2O3 [M+H]+: 325.1547;
found: 325.1544.
2.3.1. General procedure (for compounds 4a–d, 4f)
A solution of dicyclohexylcarbodiimide (DCC) (3.48 mmol) in
absolute THF (5 mL) was added, under argon, to a stirred solution
of N-aroylhydrazone carboxylic acid 3 (3.48 mmol) in absolute
THF (30 mL), and the mixture was stirred at rt for 24 h. The white
solid, consisting of N,N0-dicyclohexylurea, was filtered off and the
solvent was removed under vacuum. The obtained yellow solid
was triturated with dry diethyl ether to give pure 4.
2.2.6. 2-(2-(4-Hydroxybenzoyl)hydrazono)-2-phenylacetic acid (3e)
Yield 95%, white solid, m.p. 203–204 °C. 1H NMR (500 MHz,
DMSO-d6) d ppm: 6.92 (d, 2H, J = 8.8 Hz, H12, H14), 7.43–7.45 (over-
lapped peaks, 3H, H5, H6, H7), 7.66–7.68 (m, 2H, H4, H8), 7.75 (d, 2H,
J = 8.8 Hz, H11, H15), 10.32 (s, 1H, NH). 13C NMR (125 MHz, DMSO-
d6) d ppm: 115.6, 123.1, 128.1, 128.4, 128.9, 129.2, 130.2, 135.2,
2.3.2. 2-(3-Bromophenyl)-5-phenyl-6H-1,3,4-oxadiazin-6-one (4a)
Yield 87%, yellow solid, m.p. 154–155 °C. 1H NMR (300 MHz,
CDCl3) d ppm: 7.44 (t from overlapped dd, 1H, J = 8.0 Hz, H5 ),
0
161.4, 164.2, 165.4. UV/Vis (1,4-dioxane) kmax (log
e
) = 235 (4.20),
00
00
00
296 (4.35) nm. EI–MS m/z (rel. int.) = 39 (13), 44 (8), 65 (40), 76
(9), 93 (48), 103 (42), 121 (100), 137 (63), 239 (13), 284 (0.2)
[M+]. MS (APCI+): 285.1 [M+H]+, 241.1 [M–CO2]+. HRMS (APCI+):
calcd for C15H13N2O4 [M+H]+: 285.0870; found: 285.0867.
7.48–7.63 (overlapped peaks, 3H, H3 , H4 , H5 ), 7.76 (ddd, 1H,
0
0
J = 8.0, 1.9, 1.0 Hz, H4 ), 8.22 (dt, 1H, J = 8.0, 1.0 Hz, H6 ), 8.31–8.35
00
00
(m, 2H, J = 7.4 Hz, H2 , H6 ), 8.44 (t from overlapped dd, 1H,
J = 1.9 Hz, H2 ). 13C NMR (75 MHz, CDCl3) d ppm: 123.2, 126.6,
0
128.6, 129.1, 129.4, 130.5, 130.7, 130.9, 132.2, 136.5, 147.7,
153.1, 156.2. UV/Vis (acetonitrile) kmax (log
333 (4.30) nm; (1,4-dioxane) kmax (log ) = 238 (3.88), 336
(4.28) nm; (ethyl acetate) kmax (log ) = 335 (4.29) nm; (dichloro-
methane) kmax (log ) = 227 (3.98), 250 (3.80), 343 (4.29) nm. EI–
MS m/z (rel. int.%) = 50 (12), 63 (30), 76 (28), 89 (29), 155 (41),
157 (39), 183 (100), 185 (95), 328 (5), 330 (5) [M+]. MS (APCI+):
329.0 [M+H]+, 301.0 [M–N2]+. HRMS (APCI+): calcd for C15H10BrN2-
O2 [M+H]+: 328.9920; found: 328.9922.
e) = 248 (sh, 3.84),
2.2.7. 2-(2-Nicotinoylhydrazono)-2-phenylacetic acid (3f)
e
Yield 91%, white solid, m.p. 199–120 °C. 1H NMR (300 MHz,
DMSO-d6) d ppm: 7.35–7.53 (overlapped peaks, 3H, H5, H6, H7),
7.55–7.80 (overlapped peaks, 3H, H4, H8, H14), 8.23 (d, 1H,
J = 7.8 Hz, H15), 8.80 (d, 1H, J = 3.3 Hz, H13), 9.01 (br. s, 1H, H11),
12.82 (s, 1H, NH). 13C NMR (75 MHz, DMSO-d6) d ppm: 123.9,
128.3, 128.8, 129.7, 130.4, 134.5, 135.3, 148.4, 151.9, 164.1,
e
e
165.4. UV/Vis (1,4-dioxane) kmax (loge) = 235 (4.12), 311
(4.27) nm. EI–MS m/z (rel. int.) = 44 (32), 50 (41), 51 (98), 52 (29)
65 (7), 76 (28), 78 (100), 89 (12), 103 (58), 106 (98), 122 (70),
148 (9), 224 (25), 270 (0.2) [M+]. MS (APCI+): 270.1 [M+H]+,
226.1 [M–CO2]+. HRMS (APCI+): calcd for C14H12N3O3 [M+H]+:
270.0873; found: 270.0873.
2.3.3. 2-(3-Chlorophenyl)-5-phenyl-6H-1,3,4-oxadiazin-6-one (4b)
Yield 99%, yellow solid, m.p. 137–138 °C. 1H NMR (300 MHz,
0
0
00
00
CDCl3) d ppm: 7.47–7.63 (overlapped peaks, 5H, H5 , H4 , H3 , H4
,
00
0
H5 ), 8.18 (dt, 1H, J = 7.8, 1.4 Hz, H6 ), 8.24 (t from overlapped dd,

M.L. Tßînßtaßs et al. / Journal of Molecular Structure 1058 (2014) 106–113
109
1H, J = 1.8 Hz, H2 ), 8.32–8.35 (m, 2H, H2 , H6 ). 13C NMR (75 MHz,
CDCl3) d ppm: 126.4, 128.2, 128.8, 129.3, 129.4, 130.5, 130.9,
132.4, 133.8, 135.5, 147.9, 153.3, 156.5. UV/Vis (acetonitrile) kmax
(log
e
) = 238 (3.78), 333 (4.18) nm; (ethyl acetate) kmax (loge) = 332
0
00
00
(4.38) nm; (dichloromethane) kmax (log
e
) = 242 (3.83), 339
(4.30) nm. EI–MS m/z (rel. int.) = 65 (8), 76 (32), 78 (88), 89 (22),
103 (66), 106 (100), 251 (0.3) [M+]. MS (APCI+): 252.1 [M+H]+,
224.1 [M–N2]+. HRMS (APCI+): calcd for C14H10N3O2 [M+H]+:
252.0768; found: 252.0769.
(log
(3.86), 334 (4.33) nm; (ethyl acetate) kmax (log
(dichloromethane) kmax (log ) = 225 (3.86), 249 (3.86), 342
e
) = 243 (3.89), 332 (4.32) nm; (1,4-dioxane) kmax (log
e) = 242
e
) = 333 (4.33) nm;
e
(4.33) nm. EI–MS m/z (rel. int.) = 41 (6), 55 (10), 57 (13), 63 (18),
75 (19), 89 (17), 111 (47), 113 (16), 139 (100), 141 (33), 284 (5)
[M]+. MS (APCI+): 285.0 [M+H]+; 257.0 [M–N2]+. HRMS (APCI+):
calcd for C15H10ClN2O2 [M+H]+: 285.0425; found: 285.0428.
2.3.7. Procedure for 4e
Acetic anhydride (53.02 mmol) was added to a suspension of 3e
(3.52 mmol) in absolute THF (30 mL), and the mixture was stirred
for 4 h at 50 °C. The solvent and excess of acetic anhydride were re-
moved under vacuum. Anhydrous diethyl ether was added to the
oily residue and a light yellow solid precipitated. The solid was
washed with cold anhydrous diethyl ether (4 ꢂ 30 mL) and dried
under vacuum (3–4 h).
2.3.4. 2-(3-Methoxyphenyl)-5-phenyl-6H-1,3,4-oxadiazin-6-one (4c)
Yield 92%, yellow solid, m.p. 99–100 °C. 1H NMR (300 MHz,
CDCl3) d ppm: 3.91 (s, 3H, OCH3), 7.18 (ddd, 1H, J = 8.1, 2.3,
0
0
1.1 Hz, H4 ), 7.46 (t from overlapped dd, 1H, J = 8.1 Hz, H5 ), 7.48–
00
00
00
7.61 (overlapped peaks, 3H, H3 , H4 , H5 ), 7.81 (t from overlapped
0
0
dd, 1H, J = 2.3 Hz, H2 ), 7.87 (dt, 1H, J = 8.1, 1.1 Hz, H6 ), 8.32–8.36
2.3.8. 2-(4-Hydroxyphenyl)-5-phenyl-6H-1,3,4-oxadiazin-6-one (4e)
Yield 80%, pale-yellow solid, m.p. 202–206 °C. 1H NMR
(300 MHz, CDCl3) d ppm: 7.45 (d, 2H, J = 9.0 Hz, H3 , H5 ), 7.49–
(m, 2H, H2 , H6 ). 13C NMR (75 MHz, CDCl3) d ppm: 55.6, 112.1,
00
00
120.6, 120.7, 128.6, 128.7, 129.0, 130.1, 131.0, 132.1, 148.1,
0
0
152.7, 157.4, 159.9. UV/Vis (acetonitrile) kmax (log
4.22), 252 (sh, 3.85), 338 (4.33) nm; (1,4-dioxane) kmax (log
(3.80), 340 (4.28) nm; (ethyl acetate) kmax (log ) = 339 (4.25) nm;
(dichloromethane) kmax (log ) = 228 (4.05), 256 (3.81), 347
e
) = 220 (sh,
00
00
00
00
7.62 (overlapped peaks, 3H, H3 , H4 , H5 ), 8.31–8.35 (m, 2H, H2 ,
e
) = 254
H6 ), 8.41 (d, 2H, J = 9.0 Hz, H2 , H6 ). 13C NMR (75 MHz, CDCl3) d
00
0
0
e
ppm: 121.5, 126.8, 128.7, 129.1, 130.1, 130.8, 132.3, 147.7, 153.0,
e
153.1, 156.3. UV/Vis (acetonitrile) kmax (log
(4.29) nm; (1,4-dioxane) kmax (log ) = 254 (3.91), 353 (4.25) nm;
(ethyl acetate) kmax (log ) = 341 (4.40) nm; (dichloromethane) kmax
(log ) = 256 (3.89), 358 (4.24) nm. EI–MS m/z (rel. int.) = 39 (11),
e) = 256 (3.96), 352
(4.29) nm. EI–MS m/z (rel. int.) = 63 (16), 77 (21), 92 (16), 107
(17), 135 (100), 280 (5) [M+]. MS (APCI+): 281.1 [M+H]+, 253.1
[M–N2]+. HRMS (APCI+): calcd for C16H13N2O3 [M+H]+: 281.0921;
found: 281.0922.
e
e
e
57 (21), 63 (11), 77 (13), 93 (40), 103 (39), 104 (30), 105 (12),
121 (100), 137 (11), 239 (48), 266 (6) [M+]. MS (APCI+): 267.1
[M+H]+, 239.1 [M–N2]+. HRMS (APCI+): calcd for C15H11N2O3
[M+H]+: 267.0764; found: 267.0769.
2.3.5. 2-(4-tert-Butylphenyl)-5-phenyl-6H-1,3,4-oxadiazin-6-one (4d)
Yield 85%, yellow solid, m.p. 114–115 °C. 1H NMR (300 MHz,
CDCl3) d ppm: 1.38 (s, 9H, C(CH3)3), 7.48–7.58 (overlapped peaks,
0
0
00
00
00
0
0
5H, H3 , H5 , H3 , H4 , H5 ), 8.22 (d, 2H, J = 8.8 Hz, H2 , H6 ), 8.31–
8.35 (m, 2H, H2 , H6 ). 13C NMR (75 MHz, CDCl3) d ppm: 31.2,
00
00
3. Results and discussion
35.1, 124.7, 126.3, 128.3, 128.8, 129.1, 131.3, 132.1, 148.5, 152.6,
158.0. UV/Vis (acetonitrile) kmax (log
nm; (1,4-dioxane) kmax (log ) = 249 (3.93), 340 (4.33) nm; (ethyl
acetate) kmax (log ) = 253 (3.82), 340 (4.25) nm; (dichlorometh-
ane) kmax (log ) = 254 (3.96), 348 (4.33) nm. EI–MS m/z (rel.
int.%) = 41 (3), 56 (10), 77 (5), 91 (7), 118 (12), 146 (12), 161
(100), 306 (3) [M+]. MS (APCI+): 307.1 [M+H]+, 279.1 [M–N2]+.
HRMS (APCI+): calcd for C19H19N2O2 [M+H]+: 307.1441; found:
307.1440.
e) = 249 (3.98), 340 (4.37)
A
series of new 2,5-diaryl-1,3,4-oxadiazin-6-ones 4a–f
e
(Scheme 1, Table 2) was synthesized in very good yields by using
the literature procedure [10,35]. The two-steps method started
with the obtaining of aroylhydrazones 3a–f by the condensation
reaction of phenylglyoxylic acid (1) with various commercially
available aroylhydrazines 2a–f (Scheme 1, Table 2). Next, the intra-
molecular esterification of 3a–f, in the presence of DCC (except for
compound 4e, where acetic anhydride was used instead of DCC),
e
e
led to the target 4,5-diaza-a-pyrones 4. After purification by sev-
2.3.6. 5-Phenyl-2-(pyridin-3-yl)-6H-1,3,4-oxadiazin-6-one (4f)
Yield 88%, yellow solid, m.p. 138–139 °C. 1H NMR (300 MHz,
CDCl3) d ppm: 7.49–7.62 (overlapped peaks, 4H, H5 , H3 , H4 , H5 ),
eral crystallizations from diethyl ether in order to remove the
residual byproduct N,N0-dicyclohexylurea, the compounds 4a–f
were fully characterized.
0
00
00
00
00
00
0
8.31–8.35 (m, 2H, H2 , H6 ), 8.53 (dt, 1H, J = 8.1, 2.1 Hz, H6 ), 8.86
In order to study the photophysical properties of acylhydra-
zones 3 and oxadiazinones 4, as well as the influence of the 2-aryl
substituent, some previously reported p-substituted 2-aryl deriva-
tives were also synthesized. As for derivatives 3g–l and 4g–l, these
were obtained according to the literature [2,3,9,36].
(dd, 1H, J = 4.9, 2.1 Hz, H4 ), 9.49 (dd, 1H, J = 2.1, 0.8 Hz, H2 ). 13C
NMR (75 MHz, CDCl3) d ppm: 123.7, 123.8, 128.6, 129.0, 130.5,
132.3, 134.2, 135.2, 147.4, 149.2, 153.4, 153.7. UV/Vis (acetonitrile)
0
0
kmax (loge) = 238 (sh, 3.96), 330 (4.35) nm; (1,4-dioxane) kmax
Scheme 1. Two-steps synthesis of 2,5-diaryl-1,3,4-oxadiazin-6-ones 4a–f via N-aroylhydrazones 3a–f.

110
M.L. Tßînßtaßs et al. / Journal of Molecular Structure 1058 (2014) 106–113
Table 2
Yields of N-aroylhydrazones 3a–f and 2,5-diaryl-oxadiazinones 4a–f.
R group
Compnd.
Yield (%)
Compnd.
Yield (%)
m-Br-C6H4
m-Cl-C6H4
m-OMe-C6H4
p-tBu-C6H4
p-HO-C6H4
3-Pyridyl
3a
3b
3c
3d
3e
3f
93
96
94
92
95
91
4a
4b
4c
4d
4e
4f
87
99
92
85
80
88
Scheme 2. Possible isomers of hydrazones 3.
Acylhydrazones of type 3 may exhibit geometric isomerism
with respect to the C@N bond (Scheme 2), while Z and E isomers
can also appear as syn and anti isomers due to a hindered rotation
around the NAN single bond. The Z isomer was obtained as the ma-
jor product, while the corresponding E diastereoisomer, although
observable in some cases was present in the reaction mixture in
less than 10%. This is in agreement with literature data, the isomer
exhibiting the bulkiest groups in trans disposition being predomi-
nant, while the anti conformation is favored when polar solvents,
such as DMSO, are used [37–40].
Compounds 3a–f showed quite low solubility in most common
organic solvents, still they are fairly soluble in dimethylsulfoxide or
dimethylformamide and slightly soluble in 1,4-dioxane. The 1H
NMR spectra of 3a–f, recorded in DMSO-d6, display the character-
istic signals for the aromatic protons along with a singlet in the
range 10.32–13.02 ppm corresponding to the NH proton (see
Fig. S1 in SI). The APCI and EI mass spectra of acylhydrazones
3a–f exhibit the molecular peak [M]+ albeit of low abundance
(<1%) due to rapid loss of CO2, and the peak corresponding to the
[M–CO2]+ fragment (7–25%). The higher abundance peaks present
in the EI mass spectra were assigned to further cleavage fragments
such as [PhCO]+, [PhCN]+, [RCO]+ or [RCN]+.
The investigations of the molecular structure of hydrazones 3a,
3b, 3k and 3l (Fig. 1) by single-crystal X-ray diffraction revealed
the isolation in the solid state of the isomer with Z,anti configura-
tion, which seems to be the preferred spatial arrangement for these
compounds [41–44]. The asymmetric unit of compound 3b consists
of a single molecule, in which a strong intramolecular NAHꢁ ꢁ ꢁO
hydrogen bond ensures the anti disposition [O(1)ꢁ ꢁꢁH(17) = 1.95 Å,
Fig. 1. View of the asymmetric unit of N-aroylhydrazones 3a (a), 3b (b), 3k (c) and
3l (d) with 40% probability ellipsoids.
ated through edge-to-face Hꢁ ꢁ ꢁ
troid (C3AC8) = 2.80(1) Å, lateral shift 0.25 Å and H6---centroid
(C10AC15) = 2.92(1) Å, lateral shift 0.28 Å, rvdW(C,H) = 3.05 Å]
interaction
p contacts [e.g. for 3b: H14---cen-
R
Rrcov(O,H) = 1.40 Å and
RrvdW(O,H) = 2.60 Å, and the angle O1---
(Table 3), except for 3k revealing a face-to-face Hꢁ ꢁ ꢁ
p
H17AN2 = 132.68°] [45]. In case of the other three aroylhydrazones
3a, 3k and 3l, the intramolecular NAHꢁ ꢁ ꢁO hydrogen bonds are
about the same length, except for 3a measuring 2.01 Å.
In the lattice, the supramolecular arrangement of the molecules
is ensured by a number of non-covalent interactions (Fig. 2,
Table 3). Intermolecular interactions through strong H-bonding
(Fig. 2, Table 3). In addition, in case of halogenated derivatives 3a
and 3b, notable halogen bonding interactions X---O@C (Fig. 2, Ta-
ble 3) contribute to the three-dimensional network. It is worthy
to mention the shorter X---O distance determined for 3a
[Br(1)---O(1) = 3.15(1) Å, Table 3] as compared to 3b [Cl(1)---
O(1) = 3.25(1) Å,
RrvdW(O,Cl) = 3.30 Å] [45,46]. The additional
[e.g.
rvdW(H,O) = 2.60 Å, O(3)---H(18)AO(2) = 154.2(2)°] result in a
ribbon-like supramolecular association, which are further associ-
for
3b:
O(3)---H(18) = 1.88 Å,
R
rcov(H,O) = 0.96 Å,
halogen bond interaction is probably responsible for the orthogo-
nal disposition of the benzene units in compounds 3a and 3b,
which displays an dihedral angle of 90.0(1)° and 88.7(1)° (Table 3),
R

M.L. Tßînßtaßs et al. / Journal of Molecular Structure 1058 (2014) 106–113
111
Fig. 2. Crystal packing view of N-aroylhydrazones (only hydrogen atoms involved in interactions are shown): unit cell view along b axis, showing the supramolecular
associations through H-bonding, Hꢁ ꢁ ꢁ contacts and CAOꢁ ꢁ ꢁX interactions in crystalline 3a (a) and 3b (b), respectively; unit cell view along a axis, showing the supramolecular
associations through hydrogen bonds and Hꢁ ꢁ ꢁ contacts for 3k (c) and 3l (d), respectively.
p
p
respectively, while in 3k and 3l the benzene rings lies almost copla-
nar, in a bisectional orientation (dihedral angle of 33.1(1)° and
19.5(1)°, Table 3).
319 nm for 3a–l, except for 3e presenting kmax at 296 nm. The
intramolecular cyclization of hydrazones 3 to the corresponding
oxadiazinones 4 causes a red shift of the longest wavelength
absorption band of about 18–56 nm for the later (Fig. 3). The elec-
tronic spectra of oxadiazinones 4 shows absorption maxima in the
optical range 236–254 nm and intense broad absorption bands in
The structure of oxadiazinones 4a–f has been determined by 1H
and 13C NMR spectra based on one- and two-dimensional NMR
experiments. The protons of the 5-phenyl group exhibit the same
pattern for all oxadiazinone derivatives, displaying three
overlapped signals for H-300, H-400 and H-500 in the range 7.50–
7.60 ppm, and more deshielded overlapped signals corresponding
to H-200 and H-600 (Chart 1). Considering the 2-aryl substituents,
four distinct signals were assigned accordingly to meta- or para-
substituted benzene, respectively (see Table S1 in SI).
the area 333–356 nm, which are typical for
transitions (Fig. 3b).
p ?
p⁄ and n ? p⁄
The analysis of the absorption properties showed basically no
significant influence of the R substituent in the acylhydrazone ser-
ies 3. On the other hand, the comparison of UV–Vis spectra of par-
ent oxadiazinone 4l (334 nm) with 4a–k revealed a bathochromic
shift, especially for the para-substituted-2-aryl derivatives 4e and
4g–j (Fig. 3b). The most notable influence of the substituent was
noticed in case of 4e (351 nm) and 4i (356 nm), bearing the
p-hydroxy and p-methoxy-phenylene group, respectively. This
The UV–Vis spectra of acylhydrazones 3 (Fig. 3a) recorded in
1,4-dioxane (dissolved after ultrasonation for 30 min. at 50 °C, then
cooled to room temperature) consist of two different bands and
exhibit the longest absorption maxima in the optical range 311–

112
M.L. Tßînßtaßs et al. / Journal of Molecular Structure 1058 (2014) 106–113
Table 3
Selected intermolecular interactions through H-bonding, Hꢁ ꢁ ꢁ
p contacts and halogen bonds for 3a, 3b, 3k and 3l (bonds in Å, angles in °).
3a
3b
3k
3l
O3---H18
1.88
1.88
1.83
1.92
O3---H18AO2
154.1(2)
154.2(2)
162.1(1)
139.5(1)
H---O
–
–
H7---O1: 2.62(1)
H7---O3: 2.68(2)
CAH---O
–
–
C7AH7---O1: 133.7(1)
C7AH7---O3: 146.9(1)
H5---O1
–
–
–
2.52(2)
C5AH5---O1
–
–
–
146.5(1)
H14---centroid (C3AC8)
H14 (lateral shift)
H6---centroid (C10AC15)
H6 (lateral shift)
H16c---centroid (C10AC15)
H16c (lateral shift)
H13---centroid (C3AC8)
H13 (lateral shift)
2.85(1)
0.29
2.97(1)
0.32
–
–
–
–
2.80(1)
0.25
2.92(1)
0.28
–
–
–
–
–
–
–
–
–
–
2.85(1)
0.26
–
–
2.96(1)
0.38
2.87(1)
0.53
–
–
H---
X1---O1
p
Edge-to-face
3.15(1)
167.0(2)
90.0(1)
Edge-to-face
3.25(2)
165.4(1)
88.7(1)
Face-to-face
–
–
Edge-to-face
–
–
X1---O1AC1
Dihedral angle between benzene rings
33.1(1)
19.5(1)
(356 nm, loge = 4.40) bearing the p-methoxy-phenylene group
with m-methoxy derivative 4c (339 nm, loge = 4.29), where only
the –I effect occurs.
The impact of the substituent on the electronic spectra deter-
mined in 1,4-dioxane generally shows the same trend when re-
corded in dichloromethane, ethyl acetate and acetonitrile (see
Fig. S2 in SI). However, a red shift could be observed for the p-nitro
derivative 4j (354 nm) and a blue shift for the p-hydroxy derivative
4e (340 nm), when ethyl acetate was used. Besides, oxadiazinone
4i, showed a constant bathochromic (15–20 nm) along with hyper-
chromic effect in all solvents. The influence of the solvent has been
observed as generally causing a larger bathochromic shift in
dichloromethane in contrast to the hypsocromic shift in more polar
acetonitrile.
4. Conclusions
In summary, we report herein the good yield synthesis of some
new 2-aryl-5-phenyl-1,3,4-oxadiazin-6-ones through DCC-assisted
intramolecular cyclization of the corresponding N-aroylhydraz-
ones. The analysis of absorption spectra of oxadiazinones 4 and
their acylhydrazones 3 precursors, has revealed a bathochromic
effect at the conversion of 3 to the corresponding 4,5-diaza-a-pyr-
ones 4. To the best of our knowledge, this is the first report on the
electronic properties of oxadiazinone derivatives. It was noticed
that the absorption spectra were mainly dependent on the 2-aryl
substituent of oxadiazinones 4. By comparing the results obtained
for compounds 4 in dichloromethane, ethyl acetate, 1,4-dioxane
and acetonitrile, the longest wavelength absorption maxima gen-
erally exhibit a red shift in dichloromethane. The solid state molec-
ular structure investigations of N-aroylhydrazones 3a, 3b, 3k and
3l revealed the isolation of Z,anti isomer, stabilized by strong intra-
molecular H-bonding. As inferred in the lattice, the molecules pack
in a ribbon-like formation as a result of strong intermolecular H-
bonding, CAHꢁ ꢁ ꢁ
p contacts and also halogen bonds in case of 3a
and 3b, leading to a three-dimensional supramolecular network.
Fig. 3. Absorption spectra of N-aroylhydrazones 3a–l in 1,4-dioxane (3 ꢂ 10ꢃ5 M)
(a) and of oxadiazinones 4a–l (b) in 1,4-dioxane (4 ꢂ 10ꢃ5 M).
behavior could be explained by the enhancement of electron delo-
calization in conjugation system, which is also observable to a less
extent for the p-nitro derivative 4j (344 nm). The influence of the
electron delocalization effect is more obvious when compared
the absorption maxima and molecular extinction coefficient of 4i
Acknowledgements
This work was supported by CNCS-UEFISCDI, Project number
PN II_IDEI 2278/2008. A. Diac thanks to Babes-Bolyai University
for a one-year scholarship.

M.L. Tßînßtaßs et al. / Journal of Molecular Structure 1058 (2014) 106–113
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