Isolation and Magnetic Properties of Heterocycle-Carrying N-Alkoxyarylaminyl Radicals

Article abstract of DOI:10.1021/jo0302105

Two N-tert-butoxy-2,6-diaryl-4-(4-pyridyl)phenylaminyls (1) and three N-tert-butoxy-2,6-diaryl-4-(1H-imidazol-1-yl)phenylaminyls (2) were prepared by the reaction of the lithium salts of the corresponding anilines with tertbutyl peroxybenzoate. Although 1

Full text of DOI:10.1021/jo0302105

SCHEME 1
Isola tion a n d Ma gn etic P r op er ties of
Heter ocycle-Ca r r yin g N-Alk oxya r yla m in yl
Ra d ica ls1
Yozo Miura,*,† Toshiyuki Nishi,† and Yoshio Teki‡
Department of Applied Chemistry, Graduate School of
Engineering, and Department of Material Science and
Chemistry, Graduate School of Science, Osaka City
University, Sumiyoshi-ku, Osaka 558-8585, J apan
miura@a-chem.eng.osaka-cu.ac.jp
Received J une 30, 2003
Abstr a ct: Two N-tert-butoxy-2,6-diaryl-4-(4-pyridyl)phen-
ylaminyls (1) and three N-tert-butoxy-2,6-diaryl-4-(1H-im-
idazol-1-yl)phenylaminyls (2) were prepared by the reaction
of the lithium salts of the corresponding anilines with tert-
butyl peroxybenzoate. Although 1 could not be isolated as
radical crystals, 2 was successfully obtained as red crystals.
The X-ray crystallographic analysis and magnetic suscep-
tibility measurements were performed for one isolated
radical.
SCHEME 2
Although a variety of thioaminyl radicals (RN˙ SR′) have
been isolated as radical crystals,2 N-alkoxyaminyls
(RN˙ OR′), the isoelectronic radicals of thioaminyls, were
unsuccessful in isolation for a long time despite a number
of ESR studies on N-alkoxyaminyls.3 However, quite
recently, the first isolation of N-alkoxylaminyls was
accomplished by our group. The isolated N-alkoxyaminyls
are N-tert-butoxy-2,4,6-triaryl-, N-tert-butoxy-2-tert-bu-
tyl-4,6-diaryl-, and N-tert-butoxy-4-tert-butyl-2,6-dia-
rylphenylaminyls4 and N-tert-butoxy-2-tert-butyl- and
N-tert-butoxy-2,6-di-tert-butyl-1-pyrenylaminyls, which
are sterically protedted.5 Interestingly, the isolated N-
alkoxyaminyls were shown to be oxygen-insensitive, even
in solution. Isolable and oxygen-insensitive free radicals
have attracted increasing much attention as a spin source
or building blocks of the magnetic materials.6 In the
present study, we prepared two kinds of heterocycle-
carrying N-tert-butoxyarylaminyls, N-tert-butoxy-2,6-dia-
ryl-4-(4-pyridyl)phenylaminyls (1) and N-tert-butoxy-2,6-
diaryl-4-(1H-imidazol-1-yl)phenylaminyls (2). Although
1 could not be obtained as crystals, 2 was successfully
isolated as red crystals. Herein, we report the isolation,
the X-ray crystallographic analysis, and the magnetic
measurements of 2.
* To whom correspondence should be addressed. Phone: +81-6-
6605-2798. Fax: +81-6-6605-2769.
† Graduate School of Engineering.
‡ Graduate School of Science.
(1) ESR Studies of Nitrogen-centered Free Radicals. 58. Part 57:
Miura, Y.; Nakamura, S.; Teki, Y. J . Org. Chem. 2003, 68, 8244.
(2) Miura, Y. Trends Org. Chem. 1997, 6, 197-217; Miura, Y. Rec.
Res. Dev. Org. Chem. 1998, 2, 251-268.
Radicals 1 and 2 were prepared by the reaction of the
lithium salts (6 and 10) of 2,6-diaryl-4-(4-pyridyl)phen-
ylanilines (5) or 2,6-diaryl-4-(1H-imidazol-1-yl)phenylani-
lines (9) with tert-butyl peroxybenzoate in THF at -78
°C.4,5,7 The preparation of 5 and 9 is shown in Schemes
1 and 2. When a THF solution of tert-butyl peroxyben-
zoate was added to a solution of 6 or 10 in THF, the
resultant mixtures immediately showed a characteristic
red color. The appearance of the red color clearly indi-
cates the generation of N-alkoxyarylaminyls, and the
radicals were separated by column chromatography.
Although the pyridine ring-carrying N-tert-butoxyaryl-
(3) Danen, W. C.; Neugebauer, F. A. Angew. Chem., Int. Ed. Engl.
1975, 14, 783. Woynan, H.; Ingold, K. U. J . Am. Chem. Soc. 1980, 102,
3813. Ahrtens, W.; Wieser, K.; Berndt, A. Tetrahedron Lett. 1973, 3141.
Balaban, A. T.; Frangopol, P. T.; Frangopol, M.; Negoita, N. Tetrahe-
dron 1967, 23, 4661. Terabe, S.; Konaka, R. J . Chem. Soc., Perkin
Trans. 2 1973, 369. Negoita, N.; Baican, R.; Balaban, A. T. Tetrahedron
Lett. 1973, 1877. Ahrens, W.; Berndt, A. Tetrahedron Lett. 1973, 4281.
Kaba, R. A.; Ingold, K. U. J . Am. Chem. Soc. 1976, 98, 7375. Negareche,
M.; Boyer, M.; Tordo, P. Tetrahedron Lett. 1981, 22, 2879.
(4) Miura, Y.; Tomimura, T.; Matsuba, N.; Tanaka, R.; Nakatsuji,
M.; Teki, Y. J . Org. Chem. 2001, 66, 7456 and the references therein.
(5) Miura, Y.; Matsuba, N.; Tanaka, R.; Teki, Y.; Takui, T. J . Org.
Chem. 2002, 67, 8764.
(6) Magnetic Properties of Organic Materials; Lahti, P. M., Ed.;
Marcel Dekker: New York, Basel, 1999.
(7) Meesters, A. C. M.; Benn, M. H. Synthesis 1978, 679.
10.1021/jo0302105 CCC: $25.00 © 2003 American Chemical Society
Published on Web 12/03/2003
10158
J . Org. Chem. 2003, 68, 10158-10161

F IGURE 2. ESR spectra of 2a and 2a -d10 in benzene: (a) an
observed spectrum of 2a (upper) and computer simulation
(bottom); (b) an observed spectrum of 2a -d10 (upper) and
computer simulation (bottom).
TABLE 1. ESR P a r a m eter s of 1 a n d 2 in Ben zen e a t
Room Tem p er a tu r e
F IGURE 1. ORTEP drawing of 2a at 50% probability.
a,b
a
radical
aN
aH
g
1a
1b
2a
2a -d10
2b
0.992
0.992
1.016
1.016
1.020
2.0042
2.0042
2.0042
2.0042
2.0042
aminyls could not be obtained as crystals despite much
effort, the imidazole ring-carrying N-tert-butoxyarylami-
nyls 2 were successfully isolated as red needles (2a ) or
red microcrystals (2b) in 13-14% yield. In the case of 1,
the radicals were obtained as a viscous red oil, which
never solidified. HPLC analysis of the red oil indicated
that it contained considerable amounts of impurities
which could not be removed by column chromatography.
Since recrystallization of 2a from hexane gave good
single crystals suitable for the X-ray crystallographic
analysis, the X-ray crystallographic analysis could be
successfully carried out. The ORTEP drawing is depicted
in Figure 1. The small torsion angle of 3.7° for C2-C1-
N1-O1 indicates that N1 and O1 are planar to ring A
defined by C1-C6. On the other hand, the dihedral angle
between ring A and the imidazole ring is 37°, exhibiting
that the unpaired electron spin can be delocalized to the
imidazole ring to some extent. In contrast, the large
dihedral angles between ring A and ring B defined by
C7-C12 (50°) and between ring A and ring C defined by
C16-C21 (71°) strongly suggest that the delocalization
of the unpaired electron spin onto rings B and C are very
small or negligibly small.
The ESR spectra of 1 showed a broad 1:1:1 triplet, and
no hyperfine splittings due to aromatic protons were
observed. On the other hand, the ESR spectra of 2 were
split by the interaction with some aromatic protons
although the splittings were quite poor. A typical ESR
spectrum of 2a is illustrated in Figure 2, and the hfc
constants and g values for 1 and 2 are summarized in
Table 1.
No or poor splittings due to the aromatic protons
observed for 1 and 2 can be in part ascribed to the
presence of many protons with unresolved small hyper-
fine splittings. To improve the broad spectrum to a
sharper one, the 2- and 6-phenyl groups were deuterated.
The ESR spectrum of the 2- and-6-phenyl-deuterated
radical, 2a -d10, is shown in Figure 2. Computer simula-
tion of the ESR spectrum of 2a -d10 gave aN ) 1.020, aH
(2H) ) 0.160, aH (1H) ) 0.100 mT. The protons with a
hfc constant of 0.160 mT are assigned to the anilino meta
protons, H3 and H5, and the proton with a hfc constant
c
0.160 (2H),d 0.100 (1H)e
a
b
Hyperfine coupling constants are given in mT. Due to the
NO nitrogen. c The hyperfine coupling constants are determined
by computer simulation. Due to the anilino meta protons. e Due
d
to the proton attached to the imidazole 2-position.
TABLE 2. Com p a r ison of th e Obser ved a n d Ca lcu la ted
Hfc Con sta n ts for 2a
position
calcd hfc consta,b
obsd hfc constb,c
Nd
H3
H5
1.364
0.240
0.238
1.016
0.160e
0.160e
0.100e
Him-2
-0.042
a
The MO calculations were performed with the DFT UBecke
b
3LYP hybrid method using the 6-31G basis set. Given in mT.
c The hfc constants are given in the absolute value. The value
d
for the NO nitrogen. e The values for 2a -d10 are shown.
of 0.100 mT is assigned to Him-2 attached to C2 of the
imidazole ring (see Table 2).8 The assignment of Him-2 is
based on the MO calculations by the DFT UBecke 3LYP
method using the 6-31G basis set.9 The highest spin-
density position in the imidazole ring predicted by the
MO calculations is the 2-position. The calculated spin
density distribution for 2a is depicted in Figure 3, and
comparison of the observed hfc constants with the
calculated ones is made in Table 2. As found in Table 2,
the calculated hfc constants are in satisfactory agreement
with the observed values, suggesting that the present
DFT calculations are reliable.
(8) The numberings, H2 and C2, do not coincide with the number-
ings in the ORTEP drawing shown in Figure 1.
J . Org. Chem, Vol. 68, No. 26, 2003 10159

F IGURE 3. Spin density distribution of 2a calculated by the
DFT UBecke 3LYP hybrid method using the 6-31G basis set.
F IGURE 5. Dimer structure determined by the X-ray crystal-
lography and the packing mode of the dimers. The distances
between N2- - -C14′ and C14- - -N2′ are both 3.47 Å.
C14- - -N2′ with a distance of 3.47 Å to form a dimer,
which is stacked along the c-axis. The nearest distance
between the dimers is 5.99 Å, which is too long to
interact. The calculated spin densities at N2 and C14 are
-0.114 and +0.009. According to the McConnell rule,10
the interaction within the dimer is ferromagnetic, in
agreement with the observation. However, since the spin
density on C14 is very low, the observed weak interaction
can be explained by the very low spin density on C14.
In conclusion, two pyridine ring-carrying N-alkoxyaryl-
aminyls 1 and three imidazole ring-carrying N-alkoxy-
arylaminyls 2 were prepared by the reaction of the
lithium salts of the corresponding anilines with tert-butyl
peroxybenzoate, and the imidazole ring-carrying N-tert-
butoxyarylaminyls could be isolated as radical crystals.
The magnetic susceptibility measurements showed that
2a interacts ferromagnetically. Analysis of the ømolT vs
T plots using the dimer model gave 2J /kB ) 3.4 K as the
ferromagnetic interaction within the dimers.
F IGURE 4.
ømolT vs T plot for 2a . The theoretical line is
calculated by the dimer model with 2J /kB ) 3.4 K.
The magnetic susceptibility measurements were car-
ried out for 2a with a superconducting quantum interfer-
ence device (SQUID) in the temperature range 1.8-300
K. The diamagnetic components were estimated by the
Pascal sum rule. In Figure 4, the ømolT vs T plot is
depicted (ømol: the molar magnetic susceptibility). The
radical purity estimated from the ømolT value (0.350 emu
K mol-1) in the high-temperature region was 93% (the
Curie constant for the paramagnetic 1/2 spin system is
0.376 emu K mol-1). Figure 4 shows that the ømolT value
is constant in the temperature range 30-300 K and
increases slightly below 30 K. Since the X-ray crystal
structure of 2a shows the formation of dimer, the
observed magnetic behavior was analyzed using the
dimer model, and a good agreement of the experimental
data with the theoretical curve was obtained when 2J /
kB was 4.6 K (38 J mol-1). As described in Figure 5, the
two radical molecules contact between N2- - -C14′ and
Exp er im en ta l Section
Gen er a l Meth od s. 4-(1H-Imidazol-1-yl)aniline (7) was ob-
tained by reduction of 4-(1H-imidazol-1-yl)-1-nitrobenzene11 with
Pd/C in THF (yield 99%).12
2,6-Dibr om o-4-(1H-im id a zol-1-yl)a n ilin e (8). A mixture of
4.73 g (30 mmol) of 7, 26.6 g (67 mmol) of BTMA Br3, and 7.60
g of CaCO3 in 150 mL of CH2Cl2 and 60 mL of MeOH was stirred
at room temperature for 2 h. After filtration, the filtrate was
washed with 10% NaHSO3 and extracted with CH2Cl2. The
combined CH2Cl2 extracts were then washed with brine, dried
(MgSO4), evaporated, and chromatographed on silica gel with
ethyl acetate. Crystallization from hexane-ethyl acetate gave
8 as colorless prisms in 52% yield: mp 172-173 °C; 1H NMR
(CDCl3) δ 4.69 (s, 2H), 7.14 (s, 1H), 7.17 (s, 1H), 7.45 (s, 2H),
7.70 (s, 1H). Anal. Calcd for C9H7Br2N3: C, 34.10; H, 2.23; N,
13.26. Found: C, 34.22; H, 2.35; N, 13.47.
(9) All calculations were carried out on Gaussian 98 (Version A.9):
Frisch, M. J .; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M.
A.; Cheeseman, J . R.; Zakrzewski, V. G.; Montgomery, J . A., J r.;
Stratmann, R. E.; Burant, J . C.; Dapprich, S.; Millam, J . M.; Daniels,
A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J .; Barone, V.;
Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford,
S.; Ochterski, J .; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma,
K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J . B.;
Cioslowski, J .; Ortiz, J . V.; Baboul, A. G.; Stefanov, B. B.; Liu, G.;
Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.;
Fox, D. J .; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.;
Challacombe, M.; Gill, P. M. W.; J ohnson, B.; Chen, W.; Wong, M. W.;
Andres, J . L.; Gonzalez, C.; Head-Gordon, M.; Replogle, E. S.; Pople,
J . A. Gaussian, Inc.: Pittsburgh, PA, 1998.
Gen er a l P r oced u r e for P r ep a r a tion of 9. A mixture of
1.00 g (3.2 mmol) of 8, 8.2 mmol of arylboronic acid, 3.5 g of
K2CO3 in 40 mL of benzene, 20 mL of water, and 5 mL of EtOH
was stirred for 30 min, and 0.29 g (0.25 mmol) of Pd(PPh3)4 was
(10) McConnell, H. M. J . Chem. Phys. 1963, 39, 1910.
(11) J ohnson, A. L.; Kauer, J . C.; Sharma, D. C.; Dorfman, R. I. J .
Med. Chem. 1969, 12, 1024.
(12) Venuti, M. C.; Stephenson, R. A.; Alvavez, R.; Bruno, J . J .;
Strosberg, A. M. J . Med. Chem. 1988, 31, 2136.
10160 J . Org. Chem., Vol. 68, No. 26, 2003

added. The resultant mixture was then purged with N2 and
refluxed for 24 h with stirring. The mixture was washed with
brine, dried (MgSO4), evaporated, and chromatographed on
alumina with CH2Cl2. Crystallization from hexanes-ethyl ac-
etate gave 9 as colorless needles.
temperature, a solution of 0.36 g (1.92 mmol) of tert-butyl
peroxybenzoate in 5 mL of dry THF was added dropwise, and
the resultant mixture was stirred for 1 h at the same temper-
ature. The mixture was then gradually warmed to room tem-
perature, washed with 10% Na2S2O3, extracted with benzene,
and dried (MgSO4). After evaporation, the residue was chro-
matographed on silica gel cooled to 0 °C with 2:3 hexanes-ethyl
acetate and crystallized from hexane.
N-ter t-Bu toxy-4-(1H-im id a zol-1-yl)-2,6-d ip h en ylp h en yl-
a m in yl (2a ): red needles; yield 14%; mp 118-119 °C; UV-vis
(benzene) λmax 322 (ꢀ/L cm-1 mol-1 17100), 371 (7080), 514 nm
(1410). Anal. Calcd for C25H24N3O: C, 78.51; H, 6.32; N, 10.99.
Found: C, 78.50; H, 6.46; N, 10.81.
2,6-Dip h en yl-4-(1H-im id a zol-1-yl)a n ilin e (9a ): yield 70%;
mp 159-160 °C; 1H NMR (CDCl3) δ 3.96 (s, 2H), 7.15 (s, 2H),
7.16 (t, J ) 1.2 Hz, 1H), 7.23 (t, J ) 1.2 Hz, 1H), 7.39-7.54 (m,
10H), 7.78 (t, J ) 1.2 Hz, 1H). Anal. Calcd for C21H17N3: C,
81.00; H, 5.50; N, 13.49. Found: C, 81.21; H, 5.74; N, 13.43.
2,6-Di(ph en yl-d5)-4-(1H-im idazol-1-yl)an ilin e (9a-d10): yield
1
94%; mp 161-162 °C; H NMR (CDCl3) δ 3.96 (s, 2H), 7.15 (s,
2H), 7.17 (t, J ) 1.2 Hz, 1H), 7.23 (t, J ) 1.2 Hz, 1H), 7.79 (t,
J ) 1.2 Hz, 1H). Anal. Calcd for C21H7D10N3: C, 78.47; H, 5.33;
N, 13.07. Found: C, 78.55; H, 5.64; N, 12.99.
N-ter t-Bu toxy-4-(1H-im id a zol-1-yl)-2,6-bis(4-ch lor op h en -
yl)p h en yla m in yl (2b): red microcrystals; yield 13%; mp 151-
152 °C; UV-vis (benzene) λmax 316 (ꢀ/L cm-1 mol-1 15 900), 375
(7390), 512 nm (1430). Anal. Calcd for C25H22Cl2N3O: C, 66.52;
H, 4.91; N, 9.31. Found: C, 66.77; H, 5.07; N, 9.06.
2,6-Bis(4-ch lor op h en yl)-4-(1H-im id a zol-1-yl)a n ilin e (9b):
yield 88%; mp 197-199 °C; 1H NMR (CDCl3) δ 3.87 (s, 2H), 7.12
(s, 2H), 7.17 (s, 1H), 7.21 (s, 1H), 7.45 and 7.48 (each d, J ) 8.8
Hz, 8H), 7.77 (s, 1H). Anal. Calcd for C21H15Cl2N3: C, 66.33; H,
3.98; N, 11.05. Found: C, 66.41; H, 4.21; N, 10.97.
Gen er a l P r oced u r e for Isola tion of 2. A solution of 300
mg of 9 in 40 mL of anhyd THF was cooled to 78 °C under N2,
and 0.78 mL of a hexane solution of butyllithium (1.56 M) was
added. After the solution was stirred for 15 min at the same
Su p p or tin g In for m a tion Ava ila ble: Preparation of 4 and
5 and X-ray crystallographic data of 2a . This material is
available free of charge via the Internet at http://pubs.acs.org.
J O0302105
J . Org. Chem, Vol. 68, No. 26, 2003 10161