Spirocyclic 2,5-dihydro-1H-imidazole 1-oxyl radicals with a mesogenic substituent on C4. Synthesis and crystal structure

Article abstract of DOI:10.1134/S107042801401014X

Stable spirocyclic 2,5-dihydro-1H-imidazole 1-oxyl radicals containing a mesogenic fragment were synthesized on the basis of 1-[4-hydroxy(alkoxy)phenyl]- 2-hydroxyamino-2-methylpropan-1-ones. The crystalline structure of two aminoxyl radicals was determin

Full text of DOI:10.1134/S107042801401014X

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2014, Vol. 50, No. 1, pp. 72–77. © Pleiades Publishing, Ltd., 2014.
Original Russian Text © E.V. Zaytseva, Yu.V. Gatilov, S.A. Amitina, R.Tamura, I.A. Grigor’ev, D.G. Mazhukin, 2014, published in Zhurnal Organicheskoi
Khimii, 2014, Vol. 50, No. 1, pp. 78–83.
Spirocyclic 2,5-Dihydro-1H-imidazole 1-Oxyl Radicals
with a Mesogenic Substituent on C4.
Synthesis and Crystal Structure
E. V. Zaytsevaa, Yu. V. Gatilova, S. A. Amitinaa, R. Tamurab,
I. A. Grigor’eva, and D. G. Mazhukina,c
a Vorozhtsov Novosibirsk Institute of Organic Chemistry, Siberian Branch, Russian Academy of Sciences,
pr. Akademika Lavrent’eva 9, Novosibirsk, 630090 Russia
e-mail: elena@nioch.nsc.ru; council@nioch.nsc.ru
b Graduate School of Human and Environmental Studies, Kyoto University, Kyoto, 606-8501 Japan
c Novosibirsk State University, ul. Pirogova 2, Novosibirsk, 630090 Russia
Received July 17, 2013
Abstract—Stable spirocyclic 2,5-dihydro-1H-imidazole 1-oxyl radicals containing a mesogenic fragment were
synthesized on the basis of 1-[4-hydroxy(alkoxy)phenyl]-2-hydroxyamino-2-methylpropan-1-ones. The crys-
talline structure of two aminoxyl radicals was determined by X-ray analysis.
DOI: 10.1134/S107042801401014X
Paramagnetic properties of liquid crystals attract
interest from the viewpoint of variation of magnetic,
electrical, and optical characteristics of these com-
pounds depending on their phase state [1]. Tamura
et al. [2] reported on the synthesis of all-organic rod-
shaped liquid crystals I containing a chiral five-mem-
bered cyclic aminoxyl unit. These liquid crystals were
shown to possess such extraordinary properties as
ferroelectricity [3], nonlinear paramagnetic suscepti-
bility induced by a weak permanent magnet [4], and
optical second harmonic generation [5] in different
liquid crystalline phases.
e.g., for controlling liquid crystalline state by varying
pH or for obtaining an organic magnetic ionic liquid
(cf. [6]).
Introduction of a spirocyclic fragment as building
block into a thermotropic liquid crystal molecule was
the subject of studies performed by several research
teams. Vögtle and co-workers [7] synthesized rod-
shaped liquid crystals on the basis of mono- and
dispiro derivatives of the cyclobutane, cyclopentane,
and cyclohexane series. Promising results have been
achieved in the synthesis of ferroelectric liquid crystals
from triangulane structures (3+3+3) [8] and gem-di-
fluorodispirocyclopropanes (3+6+3) [9], which formed
smectic SmA and SmC* phases under different
conditions.
One more attractive feature provided by introduc-
tion of a spiro entity into a paramagnetic molecule is
possible increase in the stability of the radical center
toward biogenic reducing agents, which was recently
demonstrated with cyclic aminoxyl radicals derived
In view of the above stated, it seems promising to
synthesize and explore new organic liquid crystalline
paramagnetics having an aminoxyl fragment as a com-
ponent of a different heterocycle, e.g., of the dihydro-
imidazole series. The presence of an additional imine-
type nitrogen atom was expected to extend the scope
of possible modification of their molecules due to
appearance of an additional functionalization center,
Me
O
CnH2n+1
O
N
O
OCmH2m+1
·
Me
O
I
72

SPIROCYCLIC 2,5-DIHYDRO-1H-IMIDAZOLE 1-OXYL RADICALS
73
·
Me
O
O
Me
Me
N
N
Me
O
N
N
O
CmH2m+1
O
CnH2n+1
O
II
III
from piperidine (TEMPO) [10] and pyrrolidine
(PROXYL) [11]. This may be important in the use of
paramagnetic liquid crystals in magnetically controlled
targeted drug delivery in vivo [1].
thermodynamic stability of aminoxyl radicals derived
from dihydroimidazoles [15] ensures their subsequent
chemical modification at an available functional group
in the benzene ring with retention of the radical center.
The present work continues our previous studies
directed toward the synthesis of mesogenic spirocyclic
mono- and diradicals of the azole series [12], which can
also acts as spin probes for studying liquid crystals by
ESR methods when dissolved in diamagnetic liquid
crystals [13]. We now describe the synthesis and struc-
tural features of stable 2,5-dihydro-1H-imidazole 1-oxyl
radicals II and III containing a spirocyclic moiety and
a mesogenic substituent in position 4 of the heteroring.
An effective synthetic approach to dihydroimid-
azole functional radicals is based on the condensation
of substituted 1-hydroxyaminoalkyl aryl ketones with
aliphatic ketones in the presence of ammonia or am-
monium acetate, followed by oxidation of intermediate
1-hydroxy-2,5-dihydro-1H-imidazoles [14]. High
By heating α-hydroxyamino ketones IV and Va–Vd
with cyclohexanone in boiling methanol in the pres-
ence of ammonium acetate we obtained 1-hydroxy-
2,5-dihydro-1H-imidazoles VI and VIIa–VIId which
were oxidized (without additional purification) with
manganese dioxide in methanol (VI) or with sodium
periodate in chloroform–water (VIIa–VIId) to the
corresponding stable aminoxyl radicals VIII and IIa–
IId (Scheme 1). Acyl derivatives IIIa–IIIg containing
a 4-[4-(alkoxy)benzoyloxy]phenyl group were synthe-
sized by treatment of VIII with potassium tert-butox-
ide to generate phenoxide ion which was involved in
reaction with 4-alkoxybenzoyl chlorides IXa–IXg. The
desired stable aminoxyl radicals IIIa–IIIg were
isolated in 25–73% yield (Scheme 1). The IR and ESR
Scheme 1.
Me
OH
O
O
Me
N
NHOH
NH4OAc, MeOH, Δ
+
N
Me Me
IV, Va–Vd
RO
RO
VI, VIIa–VIId
Me
Me
O
O
N
Me
Me
N
[O]
or
N
N
HO
RO
VIII
IIa–IId
Me
O
Me
N
O
(1) t-BuOK, THF
(2) 4-CmH2m+1OC6H4C(O)Cl (IXa–IXg)
N
VIII
O
CmH2m+1
O
IIIa–IIIg
IV, VI, VIII, R = H; II, V, VII, R = CnH2n+1; n = 8 (a), 10 (b), 12 (c), 16 (d);
III, IX, m = 8 (a), 9 (b), 10 (c), 11 (d), 12 (e), 14 (f), 16 (g).
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 50 No. 1 2014

ZAYTSEVA et al.
74
C39
(a)
(b)
C38
C26
C25
C18
O2
C25
O4
C17
C13
C24
C20
C22
C15
C19
O3
C12
N2
C3
C14
C11
23
C16
O2
C14
C
C5
C15
C16
C4
C13
C2
C9
C6
C12
C11
C8
C3
C7
N2
C10
C2
N1
C1
N1
C6
C1
C7
C8
O1
C5
C9
O1
C4
Molecular structures of aminoxyl radicals (a) IIb and (b) IIIg in crystal according to the X-ray diffraction data.
spectra and elemental analyses of the isolated radicals
were consistent with the assumed structures.
To conclude, we have developed a procedure for
the synthesis of stable spirocyclic aminoxyl radicals of
the 2,5-dihydroimidazole series containing mesogenic
substituents of two types.
The structure of IIb and IIIg was proved by X-ray
analysis (see figure). The dihydroimidazole rings in
molecules IIb and IIIg are planar within ±0.023(2)
and ±0.025(3) Å, respectively, and the bond lengths
therein are very similar and close to the corresponding
bond lengths in 2,2,5,5-tetramethyl-4-phenyl-2,5-dihy-
dro-1H-imidazole 1-oxyl [16]. The cyclohexane frag-
ments adopt a chair conformation. The decyl and
hexadecyl groups have linear structure typical of long-
chain alkyl groups such as those in N-[amino(phenyl)-
methylidene]-3,4-bis(decyloxy)benzamide [17] and
4-(benzyloxy)phenyl 4-(hexadecyloxy)-3-methoxyben-
zoate [18]. The crystal packing of IIb and IIIg is
characterized by association of alkyl groups with
formation of “alkyl layers.” In the crystalline structure
of IIIg such layers consist of parallel alkyl groups
[18], whereas the alkyl groups of IIb are oriented
crosswise [17].
EXPERIMENTAL
The spectral studies were performed at the Joint
Chemical Service Center, Siberian Branch, Russian
Academy of Sciences. Elemental analyses and DSC
measurements were carried out at the Microanalysis
Laboratory, Novosibirsk Institute of Organic
Chemistry (Siberian Branch, Russian Academy of
Sciences). The IR spectra were recorded in KBr on
a Bruker Vector-22 spectrometer. The ESR spectra
were measured on a Bruker ESP 300 instrument from
solutions in chloroform with a concentration of
10–4 M. The melting points were determined using
a Mettler Toledo FP81NT automatic hot stage. The
DSC curves were obtained on a Netzsch STA 409
PC/PG instrument in a helium atmosphere at a heating
rate of 5 deg/min. The progress of reactions was moni-
tored by TLC on Sorbfil UV-254 and Polygram Alox
N/UV254 plates using chloroform, chloroform–metha-
nol, and hexane–ethyl acetate as eluents. Aminoxyl
radicals II, III, and VIII were isolated by column
chromatography on silica gel (0.060–0.200 mm, Acros
Organics). The solvents used were not additionally
purified unless otherwise stated.
According to the differential scanning calorimetry
(DSC) data, aminoxyl radicals IIa–IId and IIIa–IIIg
are stable in the temperature range from 25 to 120°C.
Radicals IIa–IId, IIIb–IIIe, and IIIg displayed on the
DSC curves only peaks corresponding to transition
from the crystalline state to isotropic. Radicals IIIa
and IIIf (m = 8, 14) showed peaks in a narrow range
(2–6°C) preceding the melting point transition. Pre-
sumably, compounds IIIa and IIIf undergo phase
transition to mesophase, but further physical studies
are necessary to confirm this assumption and identify
the transition character.
α-Hydroxyamino ketones IV and Va–Vd were
synthesized by analogy with [19], and 4-alkoxybenzoic
acids were prepared according to [20].
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SPIROCYCLIC 2,5-DIHYDRO-1H-IMIDAZOLE 1-OXYL RADICALS
75
The X-ray diffraction data for compounds IIb and
IIIg were obtained on a Bruker Kappa Apex diffrac-
tometer (MoKα radiation, graphite monochromator).
Corrections for absorptions were applied empirically
using SADABS program. The structures were solved
by the direct method and were refined by the least-
squares procedure for all independent reflections using
SHELXTL-97 software package. The positions of non-
hydrogen atoms were refined in anisotropic approxi-
mation, and hydrogen atoms were refined according to
the riding model. The sets of crystallographic data for
IIb and IIIg were deposited to the Cambridge Crystal-
lographic Data Centre (entry nos. CCDC 938838 and
938839, respectively).
1597, 1568 (C=N). ESR spectrum: triplet, AN = 14.56 G,
giso = 2.0058. Found, %: C 76.43; H 10.23; N 6.20.
C28H45N2O2. Calculated, %: C 76.14; H 10.27; N 6.34.
{3-[4-(Hexadecyloxy)phenyl]-2,2-dimethyl-1,4-
diazaspiro[4.5]dec-3-en-1-yl}oxidanyl (IId). Yield
45%, mp 91°C (from EtOH). IR spectrum, ν, cm–1:
1597, 1568 (C=N). ESR spectrum: triplet, AN = 14.56 G,
giso = 2.0058. Found, %: C 77.07; H 10.70; N 5.62.
C32H53N2O2. Calculated, %: C 77.21; H 10.73; N 5.63.
X-Ray diffraction data for IIb. C26H41N2O2,
M 413.61, monoclinic crystal system, space group
C2/c; unit cell parameters (150 K): a = 44.254(3), b =
6.0278(4), c = 18.314(1) Å; β = 95.656(3)°; V =
4861.7(5) Å3; Z = 8; dcalc = 1.130 g/cm3; μ 0.071 mm–1.
Total of 23882 reflection intensities were measured,
4265 of which were independent (Rint = 0.0578); wR2 =
0.1431, S = 1.073 (all reflections); R = 0.0471
[2895 reflections with I > 2σ(I)].
[2,2-Dimethyl-3-(4-alkoxyphenyl)-1,4-diazaspiro-
[4.5]dec-3-en-1-yl]oxidanyls IIa–IId (general proce-
dure). A mixture of 2 mmol of 1-(4-alkoxyphenyl)-2-
hydroxyamino-2-methylpropan-1-one Va–Vd, 2 mmol
(0.196 g) of cyclohexanone, and 8 mmol (0.624 g) of
ammonium acetate in 10 mL of methanol was heated
for 5 h under reflux. The mixture was cooled in an ice
bath, and the precipitate was filtered off, washed with
methanol, and dried. Intermediate 1-hydroxy-2,5-dihy-
dro-1H-imidazole VIIa–VIId thus isolated was
oxidized without purification. A two-phase mixture
consisting of a solution of VIIa–VIId in 10 mL of
chloroform and a solution of 6.92 mmol (1.48 g) of
sodium periodate in 10 mL of water was stirred for 1 h
at room temperature. The organic phase was separated,
washed with 10 mL of water, dried over MgSO4, and
concentrated, and the residue was purified by column
chromatography on silica gel using chloroform as
eluent. Samples of IIa–IId for elemental analysis and
single crystals for X-ray analysis were obtained by
crystallization from ethanol.
[3-(4-Hydroxyphenyl)-2,2-dimethyl-1,4-diaza-
spiro[4.5]dec-3-en-1-yl]oxidanyl (VIII). A mixture of
3 mmol (0.591 g) of 2-(hydroxyamino)-1-(4-hydroxy-
phenyl)-2-methylpropan-1-one (IV), 3.6 mmol
(0.353 g) of cyclohexanone, and 12 mmol (0.936 g) of
ammonium acetate in 15 mL of methanol was heated
for 4 h under reflux. The mixture was concentrated, the
residue was treated with 10 mL of acetone, the precip-
itate was filtered off, and the filtrate was evaporated.
The oily residue was dissolved in 25 mL of methanol,
15 mmol (1.305 g) of manganese(IV) oxide was added,
the mixture was stirred for 2 h at 20°C and filtered, the
filtrate was concentrated, and the residue was subject-
ed to column chromatography on silica gel using
chloroform as eluent. Yield 0.424 g (52%), mp 190°C
(decomp., from hexane–EtOAc). IR spectrum, ν, cm–1:
3258 (OH), 1599, 1582 (C=N). ESR spectrum: triplet,
AN = 14.56 G, g = 2.0058. Found, %: C 70.44; H 7.53;
N 10.22. C16H21N2O2. Calculated, %: C 70.30; H 7.74;
N 10.25.
{2,2-Dimethyl-3-[4-(octyloxy)phenyl]-1,4-diaza-
spiro[4.5]dec-3-en-1-yl}oxidanyl (IIa). Yield 61%,
mp 61°C (from EtOH). IR spectrum, ν, cm–1: 1599,
1568 (C=N). ESR spectrum: triplet, AN = 14.56 G,
giso = 2.0058. Found, %: C 75.17; H 10.04; N 7.35.
C24H37N2O2. Calculated, %: C 74.76; H 9.67; N 7.27.
{3-[4-(4-Alkoxybenzoyloxy)phenyl]-2,2-dimethyl-
1,4-diazaspiro[4.5]dec-3-en-1-yl}oxidanyls IIIa–IIIg
(general procedure). A solution of 0.9 mmol of
4-alkoxybenzoic acid in 2 mL of thionyl chloride was
heated for 90 min under reflux. Excess thionyl chloride
was distilled off under reduced pressure (water-jet
pump), the residue was dissolved in 10 mL of anhy-
drous THF, and the solution was added to a suspension
of 0.082 g (0.3 mmol) of compound VIII and 0.101 g
(0.9 mmol) of potassium tert-butoxide in 10 mL of
anhydrous THF. The mixture was stirred for 1 h and
concentrated under reduced pressure, the residue was
{3-[4-(Decyloxy)phenyl]-2,2-dimethyl-1,4-diaza-
spiro[4.5]dec-3-en-1-yl}oxidanyl (IIb). Yield 43%,
mp 88°C (from EtOH). IR spectrum, ν, cm–1: 1599,
1568 (C=N). ESR spectrum: triplet, AN = 14.56 G,
giso = 2.0058. Found, %: C 75.69; H 9.93; N 6.80.
C26H41N2O2. Calculated, %: C 75.50; H 9.99; N 6.77.
{3-[4-(Dodecyloxy)phenyl]-2,2-dimethyl-1,4-di-
azaspiro[4.5]dec-3-en-1-yl}oxidanyl (IIc). Yield
44%, mp 70°C (from EtOH). IR spectrum, ν, cm–1:
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ZAYTSEVA et al.
76
treated with 15 mL of diethyl ether, and the organic
extract was washed with a saturated aqueous solution
of NaHCO3 (2×10 mL) and with water (2×10 mL),
dried over MgSO4, and evaporated. The residue was
subjected to column chromatography on silica gel
using hexane–ethyl acetate (4:1) as eluent. Samples of
IIIa–IIIg for elemental analysis and single crystals of
IIIg for X-ray analysis were obtained by crystalliza-
tion from ethanol.
ESR spectrum: triplet, AN = 14.51 G, giso = 2.0058.
Found, %: C 75.49; H 9.05; N 4.67. C37H53N2O4. Cal-
culated, %: C 75.34; H 9.06; N 4.75.
3-{4-[4-(Hexadecyloxy)benzoyloxy]phenyl}-2,2-
dimethyl-1,4-diazaspiro[4.5]dec-3-en-1-yl)oxidanyl
(IIIg). Yield 55%, mp 64°C (from EtOH). IR spec-
trum, ν, cm–1: 1728 (C=O); 1604, 1571 (C=N). ESR
spectrum: triplet, AN = 14.51 G, giso = 2.0058. Found,
%: C 76.01; H 9.30; N 4.49. C39H57N2O4. Calculated,
%: C 75.81; H 9.30; N 4.53.
(2,2-Dimethyl-3-{4-[4-(octyloxy)benzoyloxy]-
phenyl}-1,4-diazaspiro[4.5]dec-3-en-1-yl)oxidanyl
(IIIa). Yield 43%. The DSC curve (heating) showed
two peaks at 82.2 and 84.9°C (EtOH). IR spectrum, ν,
cm–1: 1734 (C=O); 1603, 1578 (C=N). ESR spectrum:
triplet, AN = 14.51 G, giso = 2.0058. Found, %: C 73.85;
H 8.01; N 5.46. C31H41N2O4. Calculated, %: C 73.63;
H 8.17; N 5.54.
X-Ray diffraction data for IIIg. C39H57N2O4,
M 617.87; triclinic crystal system, space group P-1;
unit cell parameters (200 K): a = 5.7726(15), b =
11.567(4), c = 27.462(9) Å; α = 92.98(1), β = 94.34(1),
γ = 102.69(1)°; V = 1779.4(9) Å3; Z = 2; dcalc
=
1.153 g/cm3; μ = 0.073 mm–1. Total of 22016 reflec-
tion intensities were measured, 5999 of which were
independent (Rint = 0.0513); wR2 = 0.2056, S = 1.041
(all reflections); R = 0.0615 [3351 reflections with
I > 2σ(I)].
(2,2-Dimethyl-3-{4-[4-(nonyloxy)benzoyloxy]-
phenyl}-1,4-diazaspiro[4.5]dec-3-en-1-yl)oxidanyl
(IIIb). Yield 48%, mp 57°C (from EtOH). IR spec-
trum, ν, cm–1: 1734 (C=O); 1603, 1576 (C=N). ESR
spectrum: triplet, AN = 14.51 G, giso = 2.0058. Found,
%: C 73.93; H 8.21; N 5.34. C32H43N2O4. Calculated,
%: C 73.96; H 8.34; N 5.39.
This study was performed under financial support
by the joint program of the Russian Foundation for
Basic Research and Japan Society for the Promotion of
Science (project no. 11-03-92107-YaF-a) and by the
Ministry of Education and Science of the Russian
Federation (contract no. 8456).
(3-{4-[4-(Decyloxy)benzoyloxy]phenyl}-2,2-di-
methyl-1,4-diazaspiro[4.5]dec-3-en-1-yl)oxidanyl
(IIIc). Yield 25%, mp 71°C (from EtOH). IR spec-
trum, ν, cm–1: 1730 (C=O); 1603, 1578 (C=N). ESR
spectrum: triplet, AN = 14.51 G, giso = 2.0058. Found,
%: C 74.10; H 8.26; N 5.23. C33H45N2O4. Calculated,
%: C 74.26; H 8.50; N 5.25.
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