Regioselection in C-bromo-N-phenylnitrilimine cycloaddition to (Z)-4-(arylmethylidene)azol-5-ones

Article abstract of DOI:10.1002/1522-2675(20011114)84:11<3313::AID-HLCA3313>3.0.CO;2-K

The results of a study of the cycloaddition reaction of bromonitrilimine with (Z)-4-(arylmethylidene)isoxazol-5-ones and pyrazol-5-ones are reported. Spectroscopic and X-ray crystallographic data are presented to support structural assignments of the reac

Full text of DOI:10.1002/1522-2675(20011114)84:11<3313::AID-HLCA3313>3.0.CO;2-K

Helvetica Chimica Acta Vol. 84 (2001)
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Regioselection in C-Bromo-N-phenylnitrilimine Cycloaddition
to (Z)-4-(Arylmethylidene)azol-5-ones
by Francesco Foti*1), Giovanni Grassi, and Francesco Risitano
¡
Dipartimento di Chimica Organica e Biologica, Universita, Vill. S. Agata, I-98166 Messina
and Francesco Nicolo¡ and Archimede Rotondo
¡
Dipartimento di Chimica Inorganica, Analitica e Struttura Molecolare, Universita, Vill. S. Agata,
I-98166 Messina
The results of a study of the cycloaddition reaction of bromonitrilimine with (Z)-4-(arylmethylidene)isox-
azol-5-ones and pyrazol-5-ones are reported. Spectroscopic and X-ray crystallographic data are presented to
support structural assignments of the reaction products.
The chemistry of pyrazoles has inspired intense research activity for some time,
since this class of heterocycles is associated with a large number of mainly synthetic
derivatives that exhibit a variety of valuable applications [1].
In line with a research program focusing on the synthesis and reactivity of N-
heterocyclic compounds of potential biopharmacological interest, we recently refined
the synthesis of C-bromo-N-phenylnitrilimine (1), a novel dipole, which enabled us to
easily obtain excellent yields of 3-bromopyrazoles by cycloaddition with dipolarophiles
containing CˆC and CꢀC bonds [2].
As an extension of this, we undertook a study of the reactivity of this dipole towards
some selected arylmethylidene derivatives. This topic does not appear to have been
satisfactorily studied, and only a few, in some ways conflicting, cases are reported in the
literature [3]. With this in mind, we examined the reactions of 1 with (Z)-4-
(arylmethylidene)-4,5-dihydroisoxazol-5-ones 2 and 4,5-dihydro-1H-pyrazol-5-ones 3,
and now report the results obtained.
The treatment of 2 or 3 with C-bromo-N-phenylnitrilimine (1), generated in situ,
leads to the formation of spirocycloadducts 4 and 5, or 6 and 7, respectively (Scheme).
The results are summarized in Table 1.
The structures of products 4 7 are consistent with analytical and spectroscopic
data. The mass spectra show the formation of monobrominated adducts, while IR and
1H-NMR data support regioselective cycloaddition to the arylmethylenic substrate. In
particular, the shift to higher frequencies of the isoxazolone carbonyl stretching in the
IR spectra and one new 1H-NMR resonance in the region between 5.01 and 6.06 ppm
demonstrate both the presence of the 4,4-disubstituted isoxazol-5-one ring and the
formation of the two possible 4',4-type (4 and 6) and 5',4-type (5 and 7) regioisomers.
These are easily differentiated on the basis of the relative d values of the benzylic H-
1
)
Phone: ‡39-90-676-5511, fax: ‡39-90-393897, e-mail: ffoti@isengard.unime.it

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Helvetica Chimica Acta Vol. 84 (2001)
Scheme
Table 1. Synthesis of Spiro Derivatives 4
Spiroisoxazolones 2
7
Spiropyrazolones 3
4',4 [%]a)
Ar
4',4 [%]a)
5',4 [%]a)
5',4 [%]a)
a
b
c
d
e
f
Ph
4a 54
7a 47
7b 57
7c 62
7d 55
7e 6
4-MeÀC6H4
4-MeOÀC6H4
4-ClÀC6H4
2-NO2ÀC6H4
2-ClÀC6H4
5b 51
5c 57
5d 43
4d 7
4e 50
6e 44
6f 44
a
1
) Yields determined by H-NMR of the crude mixture.
atom, which are shifted to higher frequencies in 4',4 spirans due to the close proximity
of the N-atoms.
The configurational features of spirans 4 7 were assigned on the basis of 2D-
NOESY-NMR experiments and X-ray crystallographic data.
Considering that the starting arylmethylidene substrates 2 [4] and 3 [5] are pure
(Z)-derivatives, it can be argued that the reactions with 1 proceed with complete
stereochemical control [6]. Indeed, in the corresponding spiro structures 4 7, the
concerted 1,3-dipole attack locates HÀC(9) syn with respect to PhÀC(4) and anti with
respect to the CO group of isoxazolone.
In agreement with the proposed structures, dipolar interactions between HÀC(9)
and Ho of PhÀC(4) were evidenced by 2D-NOESY-NMR spectra. In particular, for

Helvetica Chimica Acta Vol. 84 (2001)
3315
Fig. 1. Dipolar interactions of 4',4-type structures 4 and 6 observed by 2D-NOESY measurements
Fig. 2. ORTEP View of molecule 7a with atom-numbering scheme and thermal ellipsoids at 30% of probability,
while the H size is arbitrary. Selected bond lengths [ä] and angles [8]: Br(1)ÀC(23) 1.869(3), N(4)ÀN(3)
1.395(3), N(4)ÀC(24) 1.409(3), N(4)ÀC(2) 1.474(3), N(3)ÀC(23) 1.267(3), O(1)ÀC(1) 1.209(3), N(1)ÀC(1)
1.369(3), N(1)ÀN(2) 1.405(3), N(1)ÀC(10) 1.426(3), N(2)ÀC(3) 1.290(3), C(3)ÀC(4) 1.465(4);
N(3)ÀN(4)ÀC(2) 110.7(2), C(23)ÀN(3)ÀN(4) 107.8(2), C(23)ÀC(16)ÀC(2) 98.5(2), N(1)ÀC(1)ÀC(2)
105.2(2), C(1)ÀN(1)ÀN(2) 112.4(2), C(3)ÀN(2)ÀN(1) 109.1(2), N(2)ÀC(3)ÀC(2) 111.6(2),
N(2)ÀC(3)ÀC(4)ÀC(9) 26.9(4), N(2)ÀN(1)ÀC(10)ÀC(15) 5.4(4), N(3)ÀN(4)ÀC(24)ÀC(25) À14.3(4).

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Helvetica Chimica Acta Vol. 84 (2001)
compounds 4 and 6, dipolar cross-peaks were also observed between HÀC(9) and Hio
of PhÀN(8), further supporting the assigned 4',4-type structures for these spirocy-
cloadducts (Fig. 1).
The assignments of 5',4-type regioisomers were again verified by an X-ray analysis
carried out on 7a (Fig. 2).
The observed regiochemistry was in agreement with that expected (FMO method)
[7], and the levels of selectivity depended upon the stereoelectronic nature of both
isoxazolone and pyrazolone systems, 2 and 3, respectively. Thus, on the basis of the
results listed in Table 1, the formation of the 5',4 regioisomers prevailed in cases b d
with electron-donating and/or para-substituents; in cases e and f, with electron-
withdrawing and ortho-substituents, 4',4-type regioisomers were predominant. Both
benzylidene derivatives, which showed opposite behaviors, constituted exceptions:
while 2a only afforded 4a, the analogous 3a gave the derivative 7a. This rather
ambiguous behavior is probably due to variations in frontier-orbital energy with
consequent inversion of HOMOdipole-LUMOdipolarophile interaction.
Experimental Part
General. All solvents and reagents were obtained from commercial sources and purified before use if
necessary. (Arylmethylidene)isoxazolones 2 and pyrazolones 3 were prepared according to literature procedures
[8][9]. Flash column chromatography (FC): 200 300 mesh silica gel at increased pressure. M.p.: Reichert-
Kofler hot stage apparatus; uncorrected. IR Spectra: Nicolet FT-IR Impact 400D spectrometer. 1H-NMR
Spectra: Bruker ARX-300 spectrometer, in the solvent indicated; chemical shifts (d) refer to TMS as an internal
reference. MS: Finnigan-Mat 90 spectrometer. Elemental analyses for C, H, and N on a Carlo-Erba 1102.
General Procedure for the Synthesis of Spiro Derivatives (2 7). To a stirred soln. of glyoxylic acid
phenylhydrazone [10] (3.3 g, 20 mmol) in DMF (40 ml) at À58 was added dropwise a soln. of N-
bromosuccinimide (NBS; 7.1 g, 40 mmol) in DMF (20 ml) under an atmosphere of N2. After additional stirring
(15 min) at r.t., a DMF soln. (100 ml) of the appropriate arylmethylidene derivative (40 mmol) and TEA (2.2 g,
10 mmol) were added dropwise. The mixture was left at r.t. for 5 h, then poured into cold H2O (200 ml), and
extracted with Et2O (3Â). The combined Et2O extracts were dried (Na2SO4), the solvent was removed at
reduced pressure, and the resultant oil was purified by FC (silica gel; Et2O/petroleum ether 1 :4).
4-Benzylidene-3-phenyl-4H-isoxazol-5-one (2a; 9.96 g, 40 mmol) and 1 gave 4.8 g (54%) of 6-bromo-4,8,9-
triphenyl-2-oxa-3,7,8-triazaspiro[4.4]nona-3,6-dien-1-one (4a). Colorless needles. M.p. 163 1648 (MeCN). IR
(nujol): 1798, 1597, 889, 693. 1H-NMR (300 MHz, r.t.; CDCl3): 5.55 (s, 1 H); 6.82 7.81 (m, 15 H). EI-MS: 445/
‡
447 (M ). Anal. calc. for C23H16BrN3O2: C 61.90, H 3.61, N 9.42; found: C 62.06, H 3.72, N 9.53.
4-(4-Methylbenzylidene)-3-phenyl-4H-isoxazol-5-one (2b; 10.52 g, 40 mmol) and 1 gave 4.6 g (50%) of 3-
bromo-4-(4-methylphenyl)-1,9-diphenyl-7-oxa-1,2,8-triazaspiro[4.4]nona-2,8-dien-6-one (5b). Colorless nee-
dles. M.p. 145 1468 (MeCN). IR (nujol): 1800, 1597, 752, 694. 1H-NMR (300 MHz, r.t.; CDCl3): 2.34
‡
(s, 3 H); 5.02 (s, 1 H); 7.00 8.05 (m, 14 H). EI-MS: 459/461 (M ). Anal. calc. for C24H18BrN3O2: C 62.62,
H 3.94, N 9.13; found: C 62.79, H 3.99, N 9.19.
4-(4-Methoxybenzylidene)-3-phenyl-4H-isoxazol-5-one (2c; 11.16 g, 40 mmol) and 1 gave 5.4 g (57%) of 3-
bromo-4-(4-methoxyphenyl)-1,9-diphenyl-7-oxa-1,2,8-triazaspiro[4.4]nona-2,8-dien-6-one (5c). Colorless nee-
dles. M.p. 142 1438 (MeCN). IR (nujol): 1798, 1597, 889, 693. 1H-NMR (300 MHz, r.t.; CDCl3): 3.81 (s, 3 H);
‡
5.01 (s, 1 H); 6.87 8.04 (m, 14 H). EI-MS: 475/477 (M ). Anal. calc. for C24H18BrN3O3: C 60.52, H 3.81, N 8.82;
found: C 60.70, H 3.88, N 8.96.
4-(4-Chlorobenzylidene)-3-phenyl-4H-isoxazol-5-one (2d; 11.32 g, 40 mmol) and 1 gave 0.66 g (7%) of 6-
bromo-9-(4-chlorophenyl)-4,8-diphenyl-2-oxa-3,7,8-triazaspiro[4.4]nona-3,6-dien-1-one (4d). Colorless oil. IR
(nujol): 1798, 1601, 835, 691. 1H-NMR (300 MHz, r.t.; CDCl3): 5.50 (s, 1 H); 6.83 8.03 (m, 14 H). EI-MS: 479/
‡
481 (M ). Anal. calc. for C23H15BrClN3O2: C 57.46, H 3.14, N 8.74; found: C 57.31, H 3.05, N 8.82) and 3-bromo-
4-(4-chlorophenyl)-1,9-diphenyl-7-oxa-1,2,8-triazaspiro[4.4]nona-2,8-dien-6-one (5d). Colorless needles. M.p.
165 1668 (MeOH). IR (nujol): 1798, 1596, 863, 765, 696. 1H-NMR (300 MHz, r.t.; CDCl3): 5.02 (s, 1 H); 7.06
‡
8.03 (m, 14 H). EI-MS: 479/481 (M ). Anal. calc. for C23H15BrClN3O2: C 57.46, H 3.14, N 8.74; found: C 57.60,
H 3.23, N 8.88).

Helvetica Chimica Acta Vol. 84 (2001)
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4-(2-Nitrobenzylidene)-3-phenyl-4H-isoxazol-5-one (2e; 11.76 g, 40 mmol) and 1 gave 4.9 g (50%) of 6-
bromo-9-(2-nitrophenyl)-4,8-diphenyl-2-oxa-3,7,8-triazaspiro[4.4]nona-3,6-dien-1-one (4e). Yellow plates. M.p.
183 1848 (MeOH). IR (nujol): 1792, 1594, 749, 691. 1H-NMR (300 MHz, r.t.; CDCl3): 6.01 (s, 1 H); 6.70 8.15
‡
(m, 15 H). EI-MS: 445/447 (M ). Anal. calc. for C23H15BrN4O4: C 56.23, H 3.08, N 11.40; found: C 56.40,
H 3.16, N 11.52.
4-Benzylidene-4,5-dihydro-1,3-diphenyl-1H-pyrazol-5-one (3a; 12.96 g, 40 mmol) and 1 gave 3-Bromo-
1,4,7,9-tetraphenyl-1,2,7,8-tetraazaspiro[4.4]nona-2,8-dien-6-one (7a). Colorless needles. M.p. 186 1878
(MeCN). IR (nujol): 1716, 1597, 748, 697. 1H-NMR (300 MHz, r.t.; CDCl3): 5.11 (s, 1 H); 6.89 8.15
‡
(m, 20 H). EI-MS m/z 520/522 (M ). Anal. calc. for C29H21BrN4O: C 66.80, H 4.06, N 10.75; found: C 66.97,
H 4.11, N 10.87.
4,5-Dihydro-4-(4-methylbenzylidene)-1,3-diphenyl-1H-pyrazol-5-one (3b; 13.5 g, 40 mmol) and 1 gave 6.1 g
(57%) of 3-bromo-4-(4-methylphenyl)-1,7,9-triphenyl-1,2,7,8-tetraazaspiro[4.4]nona-2,8-dien-6-one (7b). Color-
1
less needles. M.p. 201 2028 (MeCN). IR (nujol): 1723, 1598, 753, 690. H-NMR (300 MHz, r.t.; CDCl3): 2.24
‡
(s, 3 H); 5.08 (s, 1 H); 6.89 8.15 (m, 19 H). EI-MS: 534/536 (M ). Anal. calc. for C30H23BrN4O: C 67.30, H 4.33,
N 10.46; found: C 67.47, H 4.39, N 10.57.
4,5-Dihydro-4-(4-methoxybenzylidene)-1,3-diphenyl-1H-pyrazol-5-one (3c; 14.16 g, 40 mmol) and 6.8 g
(62%) of 1 gave 3-bromo-4-(4-methoxyphenyl)-1,7,9-triphenyl-1,2,7,8-tetraazaspiro[4.4]nona-2,8-dien-6-one
(7c). Colorless needles. M.p. 192 1938 (MeCN). IR (nujol): 1717, 1597, 753, 691. 1H-NMR (300 MHz, r.t.;
‡
CDCl3): 3.71 (s, 1 H); 5.07 (s, 1 H); 6.78 8.14 (m, 19 H). EI-MS: 550/552 (M ). Anal. calc. for C30H23BrN4O2:
C 65.34, H 4.20, N 10.16; found: C 65.46, H 4.28, N 10.26.
4-(4-Chlorobenzylidene)-4,5-dihydro-1,3-diphenyl-1H-pyrazol-5-one (3d; 14.32 g, 40 mmol) and 1 gave
6.1 g (55%) of 3-bromo-4-(4-chlorophenyl)-1,7,9-triphenyl-1,2,7,8-tetraazaspiro[4.4]nona-2,8-dien-6-one (7d).
Colorless needles. M.p. 207 2088 (MeOH). IR (nujol): 1716, 1598, 747, 689. 1H-NMR (300 MHz, r.t.; CDCl3):
‡
5.08 (s, 1 H); 6.91 8.14 (m, 19 H). EI-MS: 554/556 (M ). Anal. calc. for C29H20BrClN4O: C 62.66, H 3.63,
N 10.08; found: C 62.78, H 3.69, N 10.21.
4,5-Dihydro-4-(2-nitrobenzylidene)-1,3-diphenyl-1H-pyrazol-5-one (3e; 14.76 g, 40 mmol) and 1 gave 5 g
(44%) of 6-bromo-9-(2-nitrophenyl)-2,4,8-triphenyl-2,3,7,8-tetraazaspiro[4.4]nona-3,6-dien-1-one (6e). (Yel-
low-orange needles. M.p. 184 1858 (MeOH). IR (nujol): 1716, 1598, 747, 688. 1H-NMR (300 MHz, r.t.; CDCl3):
‡
6.01 (s, 1 H); 6.72 8.05 (m, 19 H). EI-MS: 565/567 (M ). Anal. calc. for C29H20BrN5O3: C 61.49, H 3.56,
N 12.36; found: C 61.63, H 3.62, N 12.47) and 0.68 g (6%) of 3-bromo-4-(2-nitrophenyl)-1,7,9-triphenyl-1,2,7,8-
tetraazaspiro[4.4]nona-2,8-dien-6-one (7e). Yellow-orange needles. M.p. 204 2058 (MeOH). IR (nujol): 1730,
‡
1588, 758, 690. 1H-NMR (300 MHz, r.t.; CDCl3): 5.69 (s, 1 H); 6.86 7.96 (m, 15 H). EI-MS: 565/567 (M ).
Anal. calc. for C29H20BrN5O3: C 61.49, H 3.56, N 12.36; found: C 61.58, H 3.63, N 12.49).
4-(2-Chlorobenzylidene)-4,5-dihydro-1,3-diphenyl-1H-pyrazol-5-one (3f; 9.96 g, 40 mmol) and 1 gave 4.9 g
(44%) of 6-bromo-9-(2-chlorophenyl)-2,4,8-triphenyl-2,3,7,8-tetraazaspiro[4.4]nona-3,6-dien-1-one (6f). Color-
1
less oil. IR (nujol): 1720, 1595, 754, 689. H-NMR (300 MHz, r.t.; CDCl3): 6.06 (s, 1 H); 6.74 8.02 (m, 15 H).
‡
EI-MS: 554/556 (M ). Anal. calc. for C29H20BrClN4O: C 62.66, H 3.63, N 10.08; found: C 68.81, H 3.74, N 10.29.
X-Ray Crystal-Structure Determination of 7a2) (see Table 2 and Fig. 2). Crystals suitable for X-ray analysis
were obtained by recrystallization from MeCN solns. Diffraction data were collected at r.t. from a colorless
0.18 Â 0.35 Â 0.37 mm3 prismatic crystal sample with a Siemens P4 automated four-circle single-crystal
diffractometer with graphite-monochromated MoKa radiation (l ˆ 0.71073 ä). No crystal decay was evidenced
by the check reflections monitored after every 197 measurements. Intensities were evaluated by profile fitting of
a 96-steps peak scan among 2q shells procedure [11] and then corrected for Lorentz polarization effects.
Absorption effects were not taken into account. Data collection and reduction was performed by XSCANS [12]
and SHELXTL packages [13]. Structure was solved by a combination of standard direct methods [14] and
Fourier synthesis, and refined by minimizing the function Sw(Fo2 À Fc2)2 with the full-matrix least-squares
technique based on all independent F2 values with SHELXL97 [15]. H-Atoms were located on the difference
Fourier maps and included in the model as isotropic atoms, while all the non-H-atoms were anisotropic. An
empirical extinction parameter was included in the last refinement cycles. Final geometric calculations and
drawings were carried out with the PARST program [16] and the XPWutility of the Siemens package, respectively.
2
)
Further details of crystal structure of compound 7a can be obtained, free of charge, on application to
Cambridge Crystallographic Data Centre (CCDC), 12 Union Road, Cambridge CB21EZUK (fax:
‡44(1223)336033; e-mail: deposit@ccdc.cam.ac.uk), quoting the deposition No. CCDC-163175.

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Helvetica Chimica Acta Vol. 84 (2001)
Table 2. Crystal Data and Structure Refinement for Compound 7a
Empirical formula
Formula weight
Crystal system
Space group
C29H21BrN4O
521.41
Monoclinic
P21/n (ITC No. 14)
Lattice parameters
a, b, c [ä]
b [8]
10.095(2), 13.676(4), 17.859(3)
103.76(1).
V
Z
2394.8(9)
4
Density (calculated)
Absorption coefficient
F(000)
1.446 mg/m3
1.747 mmÀ1
1064
Reflections collected
Independent reflections
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2s(I)]
R Indices (all data)
Extinction coefficient
Largest diff. peak and hole
5339 [q range ˆ 1.90 25.028]
4167 [R(int) ˆ 0.0185]
4167/0/401
1.035
R1 ˆ 0.0366, wR2 ˆ 0.0796
R1 ˆ 0.0572, wR2 ˆ 0.0900
0.0035(4)
0.459 and À0.506 e ¥ äÀ3
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Received May 11, 2001