Photochemistry of β-(4-sydnonyl)-o-divinylbenzene: Competitive cis-trans isomerization and photolysis

Article abstract of DOI:10.1016/j.tetlet.2004.10.036

New cis- and trans-3-aryl-4-[2-(2-vinylphenyl)ethenyl]sydnones 2, aryl = phenyl, p-tolyl were prepared and transformed on irradiation in the presence of acrolein to a pyrazoline derivative that aromatized during isolation to trans-1-tolyl-3-[2-(2-vinylphenyl)ethenyl]pyrazole 7 and trans-5-formyl-1-tolyl- 3-[2-(2-vinylphenyl)ethenyl]pyrazole 8.

Full text of DOI:10.1016/j.tetlet.2004.10.036

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 9057–9060
Photochemistry of b-(4-sydnonyl)-o-divinylbenzene: competitive
cis–trans isomerization and photolysis
a
Kristina Butkovic, Nikola Basaric, Kristijan Lovrekovic, Zeljko Marinic,
a
a
b
ˇ
´
Aleksandar Visnjevac, Biserka Kojic-Prodic and Marija Sindler-Kulyk
´
´
´
c
c
a,
*
ˇ
ˇ
´
´
aDepartment of Organic Chemistry, Faculty of Chemical Engineering and Technology, University of Zagreb,
´
Marulicev trg 19, 10000 Zagreb, Croatia
´
ˇ ´
Department of Physical Chemistry, Rudjer Boskovic Institute, Bijenicka cesta 54, 10000 Zagreb, Croatia
b
ˇ
ˇ
Center for NMR, Rudjer Boskovic Institute, Bijenicka cesta 54, 10000 Zagreb, Croatia
ˇ
c
Received 3 September 2004; revised 27 September 2004; accepted 6 October 2004
Available online 22 October 2004
Abstract—New cis- and trans-3-aryl-4-[2-(2-vinylphenyl)ethenyl]sydnones 2, aryl = phenyl, p-tolyl were prepared and transformed
on irradiation in the presence of acrolein to a pyrazoline derivative that aromatized during isolation to trans-1-tolyl-3-[2-(2-vinyl-
phenyl)ethenyl]pyrazole 7 and trans-5-formyl-1-tolyl-3-[2-(2-vinylphenyl)ethenyl]pyrazole 8.
ꢀ 2004 Elsevier Ltd. All rights reserved.
Previous work within our group has illustrated the
potential of photochemical cycloaddition reactions of
furan 1a1 and pyrrole 1b2 o-divinylbenzene derivatives.
In developing the photochemical approach to the form-
ation of heteropolycyclic compounds we turned
our attention to sydnone derivatives of o-divinylbenz-
enes 2.
Photolysis of sydnones, a topic of extensive research by
several groups,6 appears to give a nitrile imine interme-
diate, proved by their intermolecular [3+2] cycloaddi-
tion reactions with alkene and alkyne derivatives,
leading to pyrazolines and pyrazoles, respectively. The
intramolecular trappings of photochemically formed
nitrile imines have been studied only on 4-phenyl-3-(2-
allylphenyl)sydnone6h and a few examples of 4-phenyl-
3-(2-alkenyloxyphenyl)sydnones.6q
Sydnones belong to five-membered heterocyclic com-
pounds known as ÔmesoionicÕ and can be represented
as hybrids of a number of mesomeric ionic structures.3
They undergo intermolecular 1,3-dipolar cycloaddition
in the presence of dipolarophiles followed by carbon
dioxide evolution and aromatization to pyrazoles.4 Syd-
nones5a as well as pyrazoles5b are interesting compounds
because of their biological activities.
In this work we describe the irradiation of 4-aryl-3-(o-
vinylstyryl)sydnones 2 that might yield pyrazolonaph-
thalene derivative 3 through the intramolecular [3+2]
cycloaddition of the vinyl group and the primarily
formed nitrile imine intermediate, or bicyclo[2.1.1]hex-
ene derivative 4 by intramolecular [2+2] cycloaddition.
R
N
N
O
Syd
R
C
O
N
N
R
2a R= phenyl
2b R= p-tolyl
3a R= phenyl
3b R= p-tolyl
4a R= phenyl
4b R= p-tolyl
1a R= 2-furyl
1b R= 2-pyrrolyl
Keywords: Sydnones; Pyrazoles; Photochemistry; Nitrile imines; Dipolar cycloaddition.
*
Corresponding author. Tel.: +385 1 4597 246; fax: +385 1 4597 250; e-mail addresses: marija.sindler@fkit.hr; marija.sindler@pierre.fkit.hr
0040-4039/$ - see front matter ꢀ 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2004.10.036

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K. Butkovic et al. / Tetrahedron Letters 45 (2004) 9057–9060
9058
The starting compounds 2 were prepared by a Wittig
reaction following the described procedure1c from a
diphosphonium salt and the corresponding sydnone
aldehydes.7
small to be isolated. No formation of a major product
was found. When the ethanol solution of cis-2b
(10À5 M) was irradiated and the process followed by
UV spectral change the first measurements showed a
bathochromic shift and increase of the absorption maxi-
mum (Fig. 1). As can be seen, after 30s the absorption
maximum decreases. When the irradiation of trans-2b
was followed by UV the absorption maximum gradually
decreases (see Fig. 2). From these experiments it can be
presumed that the cis–trans isomerization is competing
with photolysis.
The new compounds, cis- and trans-2 were separated by
column chromatography and completely analysed and
identified spectroscopically.
The irradiation experiments on cis- and trans-2 were
performed in 10À3 M solutions of benzene, acetonitrile
or methanol, respectively, at 350nm. Besides some tarry
materials and a trans-starting compound the photomix-
ture contained various products but in quantities too
To investigate the irradiation process, and to prove the
formation of the nitrile imine as the intermediate, the
irradiation of 2b in the presence of the dipolarophile,
acrolein was performed. The alkene was chosen as dipo-
larophile to trap the nitrile imine intermediate and give
cis-3-Phenyl-4-[2-(2-vinylphenyl)ethenyl]sydnone (cis-2a): yellow
crystals; mp 114–116ꢁC; UV (EtOH) kmax/nm (e/dm3 molÀ1 cmÀ1):
221 (15503), 246 (14,375), 287 (7047), 343 (6765); IR (KBr) mmax
/
cmÀ1: 1733 (CO); 1H NMR (CDCl3) d/ppm (300MHz): 7.52–7.36 (m,
3H), 7.31 (d, 1H, J = 8.1Hz), 7.23–7.30 (m, 2H), 7.19 (d, 1H,
J = 7.8Hz), 7.10 (d, 1H J = 7.8Hz), 6.96 (d, 1H, J = 7.8Hz), 6.82 (d,
1H, J = 12.0Hz), 6.56 (dd, 1H, J = 10.8 and J = 17.4Hz), 6.23 (d, 1H,
J = 12.0Hz), 5.48 (dd, 1H, J = 17.4 and 1.2Hz), 5.19 (dd, 1H,
J = 10.8 and 1.2Hz); 13C NMR (CDCl3) d/ppm (75MHz): 166.64 (s),
136.43 (s), 134.99 (d), 134.88 (d), 134.88 (s), 134.47 (s), 131.81 (d),
129.90 (d), 128.65 (d), 128.55 (d), 127.78 (d), 126.28 (d), 123.82 (d),
116.26 (t), 112.76 (d), 106.50 (s); MS m/z (%, fragment): 292 (10,
M+ + 2H), 290 (15, M+), 231 (100), 230 (88), 217 (57), 129 (58), 128
(98).
40000
30000
t [s]
20000
trans-3-Phenyl-4-[2-(2-vinylphenyl)ethenyl]sydnone (trans-2a): yellow
crystals; mp 121–123ꢁC; UV (EtOH) kmax/nm (e/dm3 molÀ1 cmÀ1):
246 (14,642), 289 (8331), 359 (20,196); IR (KBr) mmax/cmÀ1: 1747
(CO); 1H NMR (CDCl3) d/ppm (600MHz): 8.07 (d, 1H, J = 15.9Hz),
7.73 (t, 1H, J = 8.4Hz), 7.69 (t, 2H, J = 8.4Hz), 7.61 (d, 2H,
J = 8.4Hz), 7.47 (d, 1H, J = 7.6Hz), 7.30 (d, 1H, J = 7.6Hz), 7.25 (t,
1H, J = 7.6Hz), 7.20 (t, 1H, J = 7.6Hz), 7.08 (dd, 1H, J = 10.8 and
17.4Hz), 6.49 (d, 1H, J = 15.9Hz), 5.64 (dd, 1H, J = 17.4 and 1.2Hz),
5.37 (dd, 1H, J = 10.8 and 1.2Hz); 13C NMR (CDCl3) d/ppm
(75MHz): 166.08 (s), 136.86 (s), 134.34 (d), 134.34 (s), 133.40 (s),
132.26 (d), 130.22 (d), 128.85 (d), 128.43 (d), 127.71 (d), 126.65 (d),
125.57 (d), 124.93 (d), 117.07 (t), 111.92 (d),108.72 (s); Elemental
analysis of the isomer mixture calcd for C18H14N2O2 (Mr = 290.324):
C, 74.47; H, 4.86; N, 9.65%. Found: C, 74.24; H, 5.00%.
30
15
0
60
120
240
480
960
10000
0
200
300
400
500
λ [nm]
Figure 1. UV spectra after irradiation of cis-2b (ethanol, 366nm).
cis-3-(p-tolyl)-4-[2-(2-vinylphenyl)ethenyl]sydnone (cis-2b): yellow
crystals; mp 68–70ꢁC; UV (EtOH), kmax/nm (e/dm3 molÀ1 cmÀ1):
337.6 (8350); 1H NMR (CDCl3) d/ppm (300MHz): 7.32 (d, 1H,
J = 7.5Hz), 7.21–7.06 (m, 6H), 6.97 (d, 1H, J = 7.5Hz), 6.80 (d, 1H,
J = 11.7Hz), 6.57 (dd, 1H, J = 10.5 and 17.4Hz), 6.20 (d, 1H,
J = 11.7Hz), 5.50 (dd, 1H, J = 17.4 and 1.0Hz), 5.18 (dd, 1H,
J = 10.5 and 1.0Hz), 2.38 (s, 3H); 13C NMR (CDCl3) d/ppm
(75MHz): 166.00 (s), 142.88 (s), 136.75 (s), 134.38 (s), 134.23 (d),
130.80 (s), 130.63 (d), 128.52 (d), 128.29 (d), 127.60 (d), 126.54 (d),
125.43 (d), 124.57 (d), 117.00 (t), 112.01 (d), 108.60 (s), 21.37 (q); MS
m/z (%, fragment): 304 (100, M+).
40000
30000
t [s]
0
15
30
60
120
240
480
960
1920
20000
trans-3-(p-tolyl)-4-[2-(2-vinylphenyl)ethenyl]sydnone (trans-2b): yel-
low crystals; mp 163–165 ꢁC; UV (EtOH),
kmax/nm (e/
dm3 molÀ1 cmÀ1): 359.0 (18,770); IR (KBr), mmax/cmÀ1: 1728 (CO);
1H NMR (CDCl3) d/ppm (300MHz): 7.97 (d, 1H, J = 16.0Hz), 7.46–
7.34 (m, 4H), 7.27–7.08 (m, 4H), 7.01 (dd, 1H, J = 10.8 and 17.1Hz),
6.42 (d, 1H, J = 16.0Hz), 5.57 (dd, 1H, J = 17.1 and 1.2Hz), 5.29 (dd,
1H, J = 10.8 and 1.2Hz), 2.44 (s, 3H); 13C NMR (CDCl3) d/ppm
(75MHz): 166.17 (s), 142.00 (s), 135.93 (s), 134.63 (s), 134.49 (d),
134.25 (d), 131.53 (s), 130.05 (d), 128.30 (d), 128.13 (d), 127.34 (d),
125.64 (d), 123.22 (d), 115.54 (t), 112.52 (d), 105.00 (s), 21.18 (q); MS
m/z (%; fragment): 304 (100, M+); Elemental analysis of the isomer
mixture calcd for C19H16N2O2 (Mr = 304.33): C, 74.98; H, 5.30; N,
9.20%. Found: C, 75.29, H, 5.47, N 9.12%.
10000
0
200
300
400
500
λ [nm]
Figure 2. UV spectra after irradiation of trans-2b (ethanol, 366nm).

´
K. Butkovic et al. / Tetrahedron Letters 45 (2004) 9057–9060
9059
O
O
R
N
C
N
R
N
N
OHC
N
R
N
C N N R
C
H2C CHCHO
cis-2b
hν
hν
C N N R
C N N R
6
R
N
N
N R
O
O
5
CHO
N R
N
C
N
trans-2b
7
8
Scheme 1.
1
an adduct which would not absorb at the same wave-
length as the starting compound.
respectively.§ In the H NMR, compound 7 had dou-
blets due to pyrazole protons at 7.86 and 6.68ppm
with a characteristic coupling constant of 2.4Hz and
M+ 286 in its MS spectrum. Compound 8 has a singlet
The irradiation of an acetonitrile solution of either cis-
or trans-2b in the presence of acrolein gave, regioselec-
tively, the trans-styrylpyrazoline derivative 6 in more
1
due to an aldehyde proton at 9.86ppm in its H NMR
spectrum and M+ 314 in the MS spectrum.
1
than 90% yield based on the H NMR spectrum of the
crude mixture (Scheme 1). The formation of cycloadduct
6 was obvious from the presence of vinyl protons and
ethylenic protons with a trans coupling constant in the
aromatic region. In the aliphatic region of the 1H
NMR spectrum, an ABX system of three protons was
found: two doublets of doublets at 3.48 and 3.24ppm
that were assigned to geminal protons, and a multiplet,
due to the coupling with geminal and aldehyde protons,
at 4.65ppm. The doublet at 9.63ppm indicated the pres-
ence of an aldehyde group on a saturated carbon
atom.The corresponding signals of the aliphatic carbons
in the 13C NMR spectrum are at 67.86 (d) and 34.10 (t).à
During the purification by column chromatography on
silica gel, two products were isolated, neither of which
showed the characteristic ABX signals. From the analy-
sis of the NMR spectra and mass spectra it was
concluded that the pyrazoline 6 aromatizes to trans-1-
tolyl-3-[2-(2-vinylphenyl)ethenyl]pyrazole (7, 30%) and
trans-5-formyl-1-tolyl-3-[2-(2-vinylphenyl)ethenyl]pyr-
azole (8, 10%) by loss of formaldehyde or hydrogen,
The structure of pyrazole derivative 8 was also proved
by X-ray structure analysis (Fig. 3).
Based on these experiments it can be concluded that the
3-aryl-4-[2-(2-vinylphenyl)ethenyl]sydnones (2, aryl =
phenyl, p-tolyl) undergo fast isomerization to the
trans-isomer and competitive photolysis of the sydnone
moiety giving the nitrile imine 5 which cannot react
intramolecularly. The existence of the nitrile imine 5
was confirmed on irradiation in the presence of acrolein
and by regioselective formation of the trans-styrylpyraz-
oline derivative 6 that during isolation aromatizes to the
pyrazole derivatives 7 and 8.
§ trans-1-Tolyl-3-[2-(2-vinylphenyl)ethenyl]pyrazole 7: yellow crystals;
mp 79–81ꢁC; 1H NMR (CDCl3) d/ppm (300MHz): 7.86 (d, 1H,
J = 2.4Hz), 7.63–7.56 (m, 3H), 7.51–7.44 (m, 2H), 7.33–7.23 (m, 4H),
7.14 (dd, 1H, J = 10.8 and 17.1Hz), 7.12 (d, 1H, J = 16.5Hz), 6.68 (d,
1H, J = 2.4Hz), 5.67 (dd, 1H, J = 17.1 and 1.0Hz), 5.39 (dd, 1H,
J = 10.8 and 1.0Hz), 2.39 (s, 3H); 13C NMR (CDCl3) d/ppm
(75MHz): 151.92 (s), 137.61 (s), 136.30 (s), 136.14 (s), 135.06 (s),
134.83 (d), 129.84 (d), 128.19 (d), 127.80 (d), 127.75 (d), 127.66 (d),
126.48 (d), 126.10 (d), 122.54 (d), 118.86 (d), 116.55 (t), 104.64 (d),
20.83 (q); MS m/z (%, fragment): 286 (100, M+).
à trans-5-Formyl-1-tolyl-3,4-dihydro-3-[2-(2-vinylphenyl)ethenyl]pyr-
azole 6: 1H NMR (CDCl3) d/ppm (600MHz): 9.63 (d, 1H,
J = 3.6Hz), 7.55 (d, 1H, J = 9.0Hz), 7.46 (d, 1H, J = 9.0Hz), 7.30–
7.27 (m, 2H), 7.11 (d, 2H, J = 8.4Hz), 7.07 (d, 1H, J = 16.5Hz), 7.02
(dd, 1H, J = 10.8 and 17.4Hz), 6.98 (d, 2H, J = 8.4Hz), 6.93 (d, 1H,
J = 16.5Hz), 5.64 (dd, 1H, J = 17.4 and 0.8Hz), 5.39 (dd, 1H,
J = 10.8 and 0.8Hz), 4.65 (m, 1H), 3.48 (dd, 1H, J = 16.8 and
13.2Hz), 3.24 (dd, 1H, J = 16.8 and 7.8Hz), 2.29 (s, 3H); 13C NMR
(CDCl3) d/ppm (75MHz): 199.39 (d), 149.02 (s), 142.27 (s), 136.68 (s),
136.67 (s), 134.57 (d), 134.22 (s), 131.11 (d), 130.02 (d), 128.43 (d),
128.05 (d), 126.97 (d), 126.07 (d), 122.64 (d), 117.41 (t), 113.27 (d),
67.86 (d), 34.10 (t), 20.54 (q).
trans-5-Formyl-1-tolyl-3-[2-(2-vinylphenyl)ethenyl]pyrazole 8: yellow
crystals; mp 56–59ꢁC; 1H NMR (CDCl3) d/ppm (300MHz): 9.86 (s,
1H), 7.61–7.56 (m, 2H), 7.51–7.45 (m, 2H), 7.41–7.34 (m, 4H), 7.31–
7.24 (m, 2H), 7.11 (dd, 1H, J = 10.9 and 17.3Hz), 7.06 (d, 1H,
J = 16.3Hz), 5.65 (dd, 1H, J = 17.3 and 1.2Hz), 5.39 (dd, 1H,
J = 10.9 and 1.2Hz), 2.45 (s, 3H); 13C NMR (CDCl3) d/ppm
(75MHz): 180.05 (d), 151.21 (s), 140.80 (s), 139.19 (s), 136.52 (s),
135.96 (s), 134.62 (d), 134.51 (s), 129.82 (d), 129.53 (d), 128.01 (d),
127.79 (d), 126.60 (d), 126.08 (d), 125.19 (d), 121.10 (d), 116.90 (t),
108.36 (d), 21.07 (q); MS m/z (%, fragment): 314 (100, M+).

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K. Butkovic et al. / Tetrahedron Letters 45 (2004) 9057–9060
9060
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Acknowledgements
This work was supported by grants from the Ministry
of Science, Education and Sports of the Republic of
Croatia (grant no. 0125004, 0098036 and 0098059).
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2000, 56, 1587–1593; (c) Basaric, N.; Marinic, Z.; Sindler-
Kulyk, M. Tetrahedron Lett. 2001, 42, 3641–3643; (d)
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Visnjevac, A.; Basaric, N.; Kojic-Prodic, B.; Sindler-Kulyk,
M. Acta Crystallogr. E 2001, 57, 01252–01254; (e) Rade-
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Soc. 1983, 30, 29–37; (b) Sindler-Kulyk, M.; Jakopcˇic´, K.;
Mance, A. D. J. Heterocycl. Chem. 1992, 29, 1013–1015; (c)
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Hergold-Brundic, A.; Nagl, A.; Spoljaric-Lukacic, L.;
Sindler-Kulyk, M. Z. Kristallogr.—New Cryst. Struct.
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J. Org. Chem. 2002, 551–556; (f) Basaric, N.; Visnjevac, A.;
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1997, 212, 407–407.
– Crystal data for 8 - C21H18N2O · 1/2 (CH2Cl2), Mr = 355.83, triclinic,
˚
a = 8.516 (2), b = 9.445(3), c = 12.309(5) A, a = 103.06(1),
3
˚
b = 90.93(1), c = 103.95(2)ꢁ, V = 933.4(5) A , T = 293(2) K, space
group: P-1, Z = 2, l(Mo-Ka) = 0.22mmÀ1, R1[Fo > 4r(Fo)] = 0.0621,
final wR(F2) = 0.1522; CCDC 249339.