Isolation of the half-condensed pyrazoline intermediates en route to N-(1,10-phenanthroline)-pyrazole derivatives from the Knorr reaction

Article abstract of DOI:10.1080/00397910802439209

During the synthesis of 2-pyrazole-1,10-phenanthroline derivatives by the Knorr reaction from benzoylacetone and 2-hydrazine-1,10-phenanthroline, three half-condensed intermediates, the 5-hydroxyl pyrazolines (2b-2d), were isolated and characterized by NM

Full text of DOI:10.1080/00397910802439209

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Isolation of the Half-Condensed
Pyrazoline Intermediates
en Route to N-(1,10-
Phenanthroline)-pyrazole
Derivatives from the Knorr
Reaction
Peiju Yang a , Yue Yang a , Cui Zhang a , Xiao-Juan
Yang a & Biao Wu a
a State Key Laboratory for Oxo Synthesis and
Selective Oxidation, Lanzhou Institute of Chemical
Physics, Chinese Academy of Sciences , Lanzhou,
China
Published online: 29 Jan 2009.
To cite this article: Peiju Yang , Yue Yang , Cui Zhang , Xiao-Juan Yang & Biao Wu
(2009) Isolation of the Half-Condensed Pyrazoline Intermediates en Route to N-
(1,10-Phenanthroline)-pyrazole Derivatives from the Knorr Reaction, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic
Organic Chemistry, 39:5, 907-916, DOI: 10.1080/00397910802439209
To link to this article: http://dx.doi.org/10.1080/00397910802439209
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Synthetic Communications1, 39: 907–916, 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910802439209
Isolation of the Half-Condensed Pyrazoline
Intermediates en Route to N-(1,10-
Phenanthroline)-pyrazole Derivatives
from the Knorr Reaction
Peiju Yang, Yue Yang, Cui Zhang, Xiao-Juan Yang, and Biao Wu
State Key Laboratory for Oxo Synthesis and Selective Oxidation,
Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences,
Lanzhou, China
Abstract: During the synthesis of 2-pyrazole-1,10-phenanthroline derivatives by
the Knorr reaction from benzoylacetone and 2-hydrazine-1,10-phenanthroline,
three half-condensed intermediates, the 5-hydroxyl pyrazolines (2b–2d), were
isolated and characterized by NMR spectroscopy and x-ray diffraction (for 2b
and 2d). These pyrazoline intermediates could be readily dehydrated under
the catalysis of acetic acid to afford the corresponding pyrazoles with high
regioselectivity.
Keywords: b-Diketone, 2-hydrazine-1,10-phenanthroline, Knorr reaction,
pyrazoline
INTRODUCTION
Pyrazole and pyrazoline derivatives have attracted considerable attention
as a result of their various promising pharmacological properties.[1–4]
A
number of procedures have been developed for the synthesis of pyrazole
derivatives, including Knorr synthesis (condensation of hydrazines with
1,3-dicarbonyl compounds) to provide the pyrazoles.[5] The mechanism
Received June 25, 2008.
Address correspondence to Biao Wu, State Key Laboratory for Oxo Synthesis
and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese
Academy of Sciences, Lanzhou 730000, China. E-mail: wubiao@lzb.ac.cn
907

908
P. Yang et al.
of Knorr synthesis shows that the 5-hydroxyl-substituted pyrazoline
intermediates can be derived in the reaction process, and the correspond-
ing pyrazoles can be obtained by treating the intermediates under some
special conditions.[6,7] However, studies on such intermediates are
relatively rare.[8] Recently we have utilized the Knorr reaction to synthe-
size a series of pyrazole-substituted phenanthroline compounds for
transition-metal ethylene polymerization=oligomerization catalysts.[9]
We also isolated three 3-methyl-5-hydroxyl-5-phenyl-substituted pyrazo-
line intermediates (Scheme 1) and herein report the synthetic procedures
and characterization of the pyrazolines (2b–2d).
RESULTS AND DISCUSSION
The 2-hydrazine-1,10-phenanthrolines (1a–1d) were prepared by
an established method.[10] The pyrazole compounds 3a–3d were syn-
thesized following the procedure for 2-[N-(3,5-dimethylpyrazole)]-1,
10-phenanthroline, which was obtained by reacting acetylacetone and
2-hydrazine-1,10-phenanthroline (1a) in dry methanol for 24 h at rt
and 1 h under reflux.[11] However, when the same conditions were
Scheme 1. Synthesis of the 5-hydroxyl pyrazoline intermediates (2a–2d).

N-(1,10-Phenanthroline)-pyrazole Derivatives
909
employed for the reaction of 1a and benzoylacetone, a mixture of 3a and
the 5-hydroxyl-pyrazoline intermediate 2a was obtained. When the
reaction was continued at reflux for another 5 h, the half-condensed 2a
disappeared gradually until only 3a was left. Moreover, when the reac-
tion was carried out in an acidic environment, the final pyrazole 3a
was isolated as the only product in a few hours. Similarly, the 5-hydroxyl-
pyrazoline intermediates 2b, 2c, and 2d were isolated from the reaction of
1b–1d and benzoylacetone and were dehydrated to the corresponding
pyrazoles under acidic conditions.
To isolate the 5-hydroxyl-pyrazoline intermediates (2a–2d), the reac-
tion time should be carefully controlled to avoid the conversion of pyra-
zolines to pyrazoles. However, 2a could not be separated completely from
the trace of pyrazole 3a (less than 10% in 1H NMR spectrum) even when
the reaction was terminated very soon. The pyrazoline compounds 2b, 2c,
and 2d were purified by recrystallization from dichlorometha-
1
ne=petroleum ether and were identified by H and 13C NMR spectro-
1
scopy. As an example, the H NMR spectrum of 2b displays a singlet
at 3.30 ppm corresponding to the 4-CH2 group of the pyrazoline, while
the 13C NMR spectrum shows signals at 54.8 and 95.8 ppm for the pyr-
azoline 4- and 5-C, respectively. Similar results have been observed for
3-aryl-5-hydroxyl-5-trifluoromethyl-4,5-dihydropyrazole.[4] In contrast,
the NMR spectra of the pyrazole product 3b showed 4-CH proton
(6.43 ppm) and the carbon atoms on the 4,5-positions of pyrazole
(110.3 and 145.3 ppm) in the aromatic region.[9] The assignment of the
intermediates to pyrazolines was further confirmed by x-ray diffraction,
which reveals the single bond between the 4- and 5-C atoms (same
˚
C–C bond length of 1.532(3) A for 2b and 2d).
The intermediate compounds 2a–2d were dehydrated to form the cor-
responding pyrazoles 3a–3d under different conditions. Water was easily
eliminated from 2a by prolonging the reaction time or by heating
its methanol solution to afford 3a in good yield, which is similar to
5-hydroxyl-3-(pyridin-2-yl)-5-trifluoromethyl-4,5-dihydropyrazole repor-
ted previously.[3] However, these conditions failed to promote efficient
dehydration of compounds 2b–2d. In such cases, it was necessary to
add acetic acid as catalyst to yield 3b, 3c, and 3d. It is evident that the
dehydration of the 5-hydroxyl compound is more difficult with increasing
bulk of the aryl substituent on the 9-position of 1,10-phenanthroline
(2a < 2b < 2c < 2d).
Normally, when a hydrazine RNH-NH2 is reacted with an
asymmetric diketone, two isomers of hydroxyl-pyrazoline intermedi-
ate and the pyrazole should appear with different substituents on the
3- and 5-positions.[8] It is surprising that we have obtained only one
5-hydroxyl-pyrazoline intermediate and pyrazole isomer from the

910
P. Yang et al.
reaction of 2-hydrazine-1,10-phenanthroline with benzoylacetone. This
high regioselectivity is likely because of the steric hindrance of the substi-
tuents on the diketone.
X-Ray Crystal Structures
Yellow crystals of compounds 2b and 2d were obtained by slow evapora-
tion of a solution in dichloromethane=petroleum ether (50:50). The mole-
cular structures of the two compounds are depicted in Figs. 1 and 2,
respectively. The structural studies show an intramolecular O–H ꢀ ꢀ ꢀ N
hydrogen bond between one of the phenanthroline N atoms and the
OH donor on the pyrazoline moiety in both compounds. The O–H ꢀ ꢀ ꢀ N
contact is incorporated in a six-membered ring with the S(6) motif,[12]
which might be the cause of the stability of the pyrazolines.
In the two compounds, the pyrazoline ring is roughly coplanar with
the phenanthroline moiety (dihedral angle of 7.3ꢁ and 19.2ꢁ for 2b and
2d) and is nearly perpendicular to the attached phenyl ring (72.2 and
Figure 1. Molecular structure of 2b. Displacement ellipsoids are shown at the 50%
˚
probability level. Selected bond lengths (A): N(3)–N(4), 1.397(2); N(3)–C(22),
1.474(3); C(20)–C(21), 1.476(3); C(21)–C(22), 1.532(3); N(4)–C(20), 1.285(2).
˚
Hydrogen bond parameters: O(1) ꢀ ꢀ ꢀ N(2), 2.744(2) A; H(1B) ꢀ ꢀ ꢀ N(2), 2.21
ꢁ
˚
(2) A; ;O(1)–H(1B) ꢀ ꢀ ꢀ N(2), 122.9 .

N-(1,10-Phenanthroline)-pyrazole Derivatives
911
Figure 2. Molecular structure of 2d. Displacement ellipsoids are shown at the
˚
50% probability level. Selected bond lengths (A): N(3)–N(4), 1.404(2); N(3)–
C(25), 1.491(2); C(23)–C(24), 1.478(3); C(24)–C(25), 1.532(3); N(4)–C(23),
˚
1.281(2). Hydrogen bond parameters: O(1) ꢀ ꢀ ꢀ N(2), 2.739(2) A; H(1B) ꢀ ꢀ ꢀ N(2),
ꢁ
˚
2.21 A; ;O(1)–H(1B) ꢀ ꢀ ꢀ N(2), 122.2 .
81.6ꢁ for 2b and 2d, respectively). However, there is significant difference
between 2b and 2d: whereas the 9-phenyl group tends to be coplanar with
the central phenanthroline plane in 2b (dihedral angle 18.3ꢁ), the more
steric demanding mesityl substituent is twisted away from the phenan-
throline fragment by 74.75ꢁ in 2d.
In summary, three 5-hydroxyl substituted pyrazoline intermediates
have been isolated from the condensation of 2-hydrazine-1,10-phenan-
throline derivatives with benzoylacetone. These intermediates can be
further dehydrated to give the corresponding pyrazoles with high regios-
electivity. The substituents on the phenanthrolines have significant effects
on the formation and dehydration of the pyrazoline intermediates. With
increasing steric hindrance, the pyrazolines gain more stability and it is
more difficult to convert them to the pyrazoles.

912
P. Yang et al.
EXPERIMENTAL
Benzoylacetone[13]
and
2-hydrazine-1,10-phenanthroline starting
materials (1a–1d)[10] were prepared using literature methods. 1H and
13C NMR spectra were obtained in CDCl3 on a Mercury plus-400
spectrometer with TMS as the internal standard. Infrared (IR) spectra
were recorded as KBr pellets with an HP5890 II GC=Nexus-870 spec-
trometer. Electrospray ionization mass spectrometry (ESI-MS) spectra
were measured on a Waters ZQ 4000 instrument. Elemental analyses
were performed with an Elementar VarioEL instrument.
2-[N-(3-Methyl-5-hydroxyl-5-phenylpyrazoline)]-9-aryl-1,10-
phenanthroline (2b–2d)
A mixture of 2-hydrazine-9-aryl-1,10-phenanthroline (3.0 mmol) and
benzoylacetone (0.58 g, 3.6 mmol) was stirred in dry methanol (15 mL)
until a mass suspension appeared, which was filtered immediately
and washed with cold methanol. The product was recrystallized from
chloroform=activated charcoal.
2-[N-(3-Methyl-5-hydroxyl-5-phenylpyrazole)]-9-phenyl-1,10-
phenanthroline (2b)
Pale yellow solid (0.81 g, 63%). Mp: 194–195 ꢁC. ESI-MS: m=z 431.6,
1
[M þ H]þ. H NMR (400 MHz, CDCl3, d=ppm): 8.47 (s, 1H, OH), 8.28
(d, J ¼ 7.2 Hz, 2H, Ph-H), 8.07 (d, J ¼ 8.8 Hz, 1H, H-4), 8.02 (d,

N-(1,10-Phenanthroline)-pyrazole Derivatives
913
J ¼ 8.0 Hz, 1H, H-7), 7.92 (d, J ¼ 8.0 Hz, 1H, H-3), 7.76 (d, J ¼ 7.6 Hz,
2H, H-5, H-6), 7.72 (d, J ¼ 8.8 Hz, 1H, H-8), 7.60 (t, J ¼ 7.4 Hz, 2H,
Ph-H), 7.54–7.48 (m, 2H, Ph-H), 7.43 (d, J ¼ 8.8 Hz, 1H, Ph-H), 7.27–
7.24 (m, 2H, Ph-H), 7.14 (t, J ¼ 7.2 Hz, 1H, Ph-H), 3.30 (s, 2H, H-40),
2.16 (s, 3H, CH3). 13C NMR (100 MHz, CDCl3, d=ppm): 155.4 (C-2),
154.3 (C-9), 151.3 (C-12), 145.6 (C-30), 144.5 (Ph-C), 144.2 (Ph-C),
139.7 (C-7), 137.8 (C-4), 136.2 (C-11), 129.1–123.1 (Ph-C), 127.7 (C-6),
122.0 (C-5), 119.2 (C-8), 112.4 (C-3), 95.2 (C-50), 54.8 (C-40), 16.18
(CH3). IR (KBr, n=cmꢂ1): 3284 (OH), 3042, 1608 (C¼C), 1589 (C¼N),
1500, 1512, 1486, 1425. Anal. calcd. for C28H22N4O: C, 78.12; H, 5.15;
N, 13.01. Found: C, 78.33; H, 5.02; N, 12.89%.
2-[N-(3-Methyl-5-hydroxyl-5-phenylpyrazoline)]-9-naphthyl-1,10-
phenanthroline (2c)
Yellow solid (0.75 g, 52%). Mp: 175–177 ꢁC. ESI-MS: m=z 481.7,
1
[M þ H]þ. H NMR: 8.26 (d, J ¼ 8.4 Hz, 1H, H-4), 8.18 (d, J ¼ 8.0 Hz,
1H, H-7), 8.02 (dd, J ¼ 6.8, 8.0 Hz, 4H, Nap-H, H-8, H-6), 7.82–7.40
(m, 8H, Nap-H, Ph-H, H-5), 6.99 (t, J ¼ 7.2 Hz, 1H, H-3), 6.91 (t,
J ¼ 7.6 Hz, 2H, Ph-H), 3.20 (s, 2H, H-40), 2.16 (s, 3H, CH3). 13C NMR:
157.9 (C-2), 154.2 (C-9), 151.2 (C-12), 145.1 (C-30), 144.9 (Nap-C),
144.3 (Ph-C), 139.4, 137.6 (C-7), 135.4 (C-4), 134.0, 131.4 (C-11),
128.8–124.1 (Nap, Ph-C), 127.0 (C-6), 123.1 (C-5), 121.9 (C-8), 112.4
(C-3), 94.8 (C-50), 54.7 (C-40), 16.1 (CH3). IR (KBr, n=cmꢂ1): 3198
(OH), 3046, 1608 (C¼C), 1586 (C¼N), 1511, 1487, 1426, 1311, 849,
770. Anal. calcd. for C32H24N4O: C, 79.98; H, 5.03; N, 11.66. Found:
C, 80.12; H, 4.97; N, 11.79%.
2-[N-(3-Methyl-5-hydroxyl-5-phenylpyrazoline)]-9-mesityl-1,10-
phenanthroline (2d)
Yellow solid (0.68 g, 48%). Mp: 186–188 ꢁC. ESI-MS: m=z 473.9,
1
[M þ H]þ. H NMR: 8.34 (s, 1H, OH), 8.13 (d, J ¼ 8.4 Hz, 1H, H-4),
8.03 (d, J ¼ 8.8 Hz, 1H, H-7), 7.64 (m, 2H, H-8, H-6), 7.51 (dd, J ¼ 6.0,
2.4 Hz, 3H, H-5, Ph-H), 7.42 (d, J ¼ 8.0 Hz, 1H, H-3), 7.06 (m, 5H,
Ph-H, Mes-H), 3.23 (s, 2H, H-40), 2.42 (s, 3H, CH3), 2.15 (s, 3H, CH3),
2.00 (s, 6H, CH3). 13C NMR: 159.3 (C-2), 154.2 (C-9), 151.2 (C-12),
145.1 (C-30), 144.7 (Ph-C), 144.1, 138.8 (C-7), 137.1 (C-4), 137.6–124.0
(Ph-C), 128.2 (C-6), 122.9 (C-5), 122.4 (C-3), 122.1 (C-8), 95.0 (C-50),
54.7 (C-40), 21.2 (CH3), 20.4 (CH3), 16.2 (CH3). IR (KBr, n=cmꢂ1):
3282 (OH), 3027, 1610 (C¼C), 1587 (C¼N), 1512, 1494, 1370, 1312,

914
P. Yang et al.
850. Anal. calcd. for C31H28N4O: C, 78.79; H, 5.97; N, 11.86. Found: C,
78.98; H, 5.88; N, 11.93%.
X-Ray Crystal Structure Determination
Diffraction data were collected with a Bruker Smart CCD Apex II
diffractometer at 293 K with graphite-monochromated Mo Ka radiation
[14]
˚
(k ¼ 0.71073 A). An empirical absorption correction using SADABS
was applied for all data. The structures were solved by direct methods
using the SHELXS program. All nonhydrogen atoms were refined aniso-
tropically by full-matrix least-squares on F2 by the use of the program
SHELXL.[15] The hydrogen atoms were included in idealized positions
with thermal parameters equivalent to 1.2 times those of the atom to
which they were attached. It should be noted that the pyrazolines are
racemic, and the S configuration is given in Figs. 1 and 2, whereas the
enantiomer can be retrieved through symmetry operation of inversion
center. Crystallographic data for 2b and 2d are listed in Table 1.
Table 1. Crystallographic data for 2b and 2d
Compound
2b
2d
Formula
Fw
C28H22N4O
430.50
C31H28N4O
472.57
Crystal system
Space group
Monoclinic
P2(1)=n
Monoclinic
P2(1)=c
19.5147(9)
8.0629(4)
16.3432
92.685
2568.7(2)
4
1.222
1000
0.08
2.09–28.51
15670 (0.034)
6113
˚
a (A)
9.8966(6)
19.1756(11)
12.2694(7)
108.485(3)
2208.3(2)
4
1.295
904
0.08
˚
b (A)
˚
c (A)
b (ꢁ)
3
˚
V (A )
Z
Dcalc (g cmꢂ3
F(000)
)
l (mmꢂ1
)
h range (ꢁC)
Reflns collected (Rint
Independent reflns
2.05–26.77
13024 (0.059)
4699
)
Observed reflns [I > 2r(I)]
R1; wR2 [I > 2r(I)]
R1; wR2 (all data)
GOF (F2)
1685
3317
0.0418; 0.0752
0.1447; 0.0975
0.756
0.0547; 0.1382
0.1085; 0.1692
1.034

N-(1,10-Phenanthroline)-pyrazole Derivatives
915
CCDC-662909 and 662910 contain the supplementary crystallo-
graphic data for this paper. Copies of the data can be obtained free of
charge from the Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
ACKNOWLEDGMENT
We thank the Bairen Jihua program of the Chinese Academy of Sciences
for funding.
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