Photoinduced switching of intramolecular hydrogen bond between amide NH and carboxyl oxygen

Article abstract of DOI:10.1039/b516049a

In this study, we synthesized two novel carboxylic acid and carboxylate compounds, both of which had an amide group linked with an azomethine moiety to introduce photoinduced switching of the intramolecular NH...O hydrogen bond. We suggest that the cis-ca

Full text of DOI:10.1039/b516049a

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PAPER
www.rsc.org/obc | Organic & Biomolecular Chemistry
Photoinduced switching of intramolecular hydrogen bond between amide NH
and carboxyl oxygen†
Takashi Matsuhira,a Hitoshi Yamamoto,*a Akira Onoda,b Taka-aki Okamuraa and Norikazu Ueyamac
Received 11th November 2005, Accepted 3rd February 2006
First published as an Advance Article on the web 22nd February 2006
DOI: 10.1039/b516049a
In this study, we synthesized two novel carboxylic acid and carboxylate compounds, both of which had
an amide group linked with an azomethine moiety to introduce photoinduced switching of the
intramolecular NH · · · O hydrogen bond. We suggest that the cis-carboxylate compound forms a
stronger intramolecular NH · · · O hydrogen bond than the cis-carboxylic acid compound.
Introduction
During enzymatic reactions, rearrangement of the hydrogen bond
network around the active site is believed to play a substantial
role in regulating reactivity.1 We have proposed that the hy-
drogen bond between the amide NH and the oxy-anion, such
as that in carboxylate and phenolate, stabilizes and decreases
the nucleophilicity of the anions.2 If the switching of the in-
tramolecular NH · · · O hydrogen bond by external stimulation
(e.g. photoirradiation) is achieved, new functional small molecules
are synthesized that can exhibit regulated reactivity like that of
native enzymes. Arai and coo-workers have investigated the effect
of intramolecular NH · · · N or NH · · · O hydrogen bonds in the
=
cis form toward cis–trans photoisomerization of the C C double
bond.3 Recently, photoisomerization has been investigated mainly
Scheme 1 Photoisomerization and thermal reversion of diarylazome-
thines E-1, E-2 and E-2ꢀ.
as a photoswitching device.4 In this study, we designed two novel
compounds: carboxylic acid E-1/Z-1 and carboxylate E-2/Z-2,
both of which have an amide group linked with a photoinduced
azomethine moiety. These compounds are expected to switch the
indicates N1 atom at equivalent position (1.5 − x, −0.5 + y,
intramolecular NH · · · O hydrogen bond, accompanied by trans-
1.5 − z)]. Both of the C · · · O distances in the carboxylate group,
to-cis photoisomerization and by cis-to-trans thermal isomeriza-
˚
˚
O11 · · · C01 (1.266[4] A) and O12 · · · C01 (1.253[3] A), are virtually
identical. Solid Fourier transform infrared (FT-IR) spectra of
E-1 and E-2 were measured to determine the existence of an
intermolecular hydrogen bond. E-1 exhibits an NH band at
3307 cm−1, whereas the m(NH) band appears at 3218 cm−1 in E-2,
indicating the presence of a stronger NH · · · O hydrogen bond.
These combined experimental results obtained by X-ray analysis
indicate that E-2 forms an intermolecular hydrogen bond in the
solid state.
tion of the azomethine double bond (Scheme 1).
Results and discussion
Molecular structure in solid state
The crystal structure of E-2 is shown in Fig. 1. The intermolecular
ꢀ
˚
N 1 · · · O11 distance (2.89 A) permitted sufficient hydrogen bond
formation; this interaction stabilizes the packing structure [Nꢀ1
aDepartment of Macromolecular Science, Graduate School of Science, Osaka
University, 1-1 Machikaneyama-cho, Toyonaka, Osaka 560-0043, Japan.
E-mail: jin@chem.sci.osaka-u.ac.jp; Fax: +81-6-6850-5474; Tel: +81-6-
6850-5451
bDepartment of Chemistry, Faculty of Science, Tokyo University of Science,
Kagurazaka 1-3, Shinjuku, Tokyo 162-8601, Japan; Fax: +81-3-5228-8251;
Tel: +81-3-5228-8251
cCenter for Advanced Science and Innovation, Osaka University, Yamadaoka
2-1, Suita, Osaka 565-0871, Japan; Fax: +81-6-6879-7796; Tel: +81-6-
6979-7795
† Electronic supplementary information (ESI) available: crystallographic
1
data, schematic illustration of low temperature H NMR measurement,
COSY NMR spectrum of E-2ꢀ/Z-2ꢀ and first-order kinetics of thermal
isomerization of Z-1 and Z-2ꢀ. See DOI: 10.1039/b516049a
Fig. 1 Molecular structure of E-2 (50% probability).
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Molecular structure in solution
The solubility of tetramethylammonium carboxylate (E-2) toward
tetrahydrofuran (THF) was too low for spectroscopic analysis.
Tetra-n-butylammonium carboxylate (E-2ꢀ), which had enhanced
solubility by the lipophilic cation, was subjected to measurement
by UV-vis and 1H NMR spectra. The solution structures in THF-
1
1
d8 were determined by H NMR spectra. H NMR spectra of
E-1 and E-2ꢀ in THF-d8 are shown in Fig. 2. A positive nuclear
Overhauser effect (NOE) between He and Haꢀ or He and Hcꢀ
coincided, indicating that the configurations of E-1 and E-2ꢀ are
in the trans form. The chemical shifts of the amide NH signal are
8.52 ppm in E-1 and 10.80 ppm in E-2ꢀ. The significant downfield
shift (Dd = 2.28 ppm) of the NH proton suggests that E-2ꢀ forms
a strong NH · · · O hydrogen bond in THF-d8 solution. However,
the temperature dependency of the amide NH chemical shift for
E-2ꢀ was −10.9 ppb K−1, whereas that of E-1 was −5.0 ppb K−1
in the range of 173–303 K. In addition, the upfield shift (Dd =
0.34 ppm) of the NH proton was observed when the solution was
diluted from 5 to 1 mM. The results show that E-2ꢀ forms an
intermolecular NH · · · O hydrogen bond in THF-d8 solution.
Fig. 3 UV-vis spectrum change of (a) E-1, and (b) E-2ꢀ, by UV irradiation
of 313 and 365 nm toward 0.08 mM in THF solution at 173K. Before
irradiation (solid lines) and after irradiation (dotted lines).
1
cis-Isomers were also observed in H NMR spectra. Using a
photoirradiation probe [designed for chemically induced dynamic
nuclear polarization (CIDNP)], UV light was directly introduced
into the NMR equipment. The spectra were taken before and
after UV irradiation in THF-d8 solution at 183 K, using a
200 W xenon-mercury arc passed through a liquid light guide.
Irradiation was continued until the establishment of a PSS (about
30 min of irradiation). 1H NMR spectra of trans-compounds
(E-1 and E-2ꢀ) and their PSS in THF-d8 solution at 183 K are
shown in Fig. 4. The signals that newly appeared after irradiation
were assigned by correlation of NMR measurements. Exchange
spectroscopy (EXSY) spectra under photoirradiation in THF-
d8 solution at 203 K are shown in Fig. 5. Because of the fast
rate of photoisomerization, equilibrium between E-1 and Z-1 was
achieved and positive cross-peaks based on the photochemical
exchange process between the two compounds were observed
in Hb, Hc, Hcꢀ, Hd, and He [Fig. 5(a)]. The homo-decoupling
measurement coincided with their expected coupling pattern, and
the negative NOE between NH and Ha coincided with Ha and
Haꢀ signals. As a consequence, all of the signals derived from Z-1
were assigned. In cis-form, the two rings of diarylazomethine are
in close proximity to each other. Significant upfield shifts of Hc,
Hcꢀ were observed in Z-1 because the protons of one ring are in
Fig. 2 1H NMR spectra of (a) E-1, and (b) E-2ꢀ, 5 mM in THF-d8 solution
at 303 K.
Direct photoisomerization of E-1 and E-2ꢀ
Although UV irradiation of trans-diarylazomethines results in
extensive conversion into the corresponding cis-isomers, the cis-
isomers are thermally unstable and cause thermal reversion to
trans-isomers at high temperatures.5 The cis-isomers were stable
1
only at temperatures less than ca. 203 K.6 Using UV-vis and H
NMR measurements at low temperature, the photoisomerization
reactions of E-1 and E-2ꢀ were traced. UV-vis spectra of E-1 and
E-2ꢀ were measured by a static method below 203 K. According
to photoirradiation in THF solution at 173 K, changes occurred
in the UV-vis spectra of E-1 and E-2ꢀ, as shown in Fig. 3. E-1 and
E-2ꢀ were isomerized and reached a photostationary state (PSS) at
313 and 365 nm UV light irradiation in the UV-vis spectrometer.
Solid lines were those taken before irradiation, corresponding to
trans-isomers, and dotted lines were those observed in PSS. The
blue shift of kmax in cis-compounds indicates that two phenyl rings
are not in plane and that the conjugation is interrupted. These
changes in the spectra were reversible toward thermal reversion at
higher temperatures.
Fig. 4 1H NMR spectrum change of (a) E-1, and (b) E-2ꢀ, by UV
irradiation of 313 nm and 365 nm 5 mM in THF-d8 solution at
183 K. Before irradiation (upside) and after irradiation (downside).
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[Fig. 5(b)], positive NOE of NH–Ha and NH–Hd confirmed the
Ha and Hd signals. The COSY NMR spectrum coincided with
the Haꢀ, Hb, Hbꢀ, and Hc signals. Of two residual signals, an
upfield-shifted signal is assigned to Hcꢀ because Hcꢀ is closer to
the amide-side aromatic ring than Hdꢀ, and Hcꢀ is more affected
by the shielding effect of the aromatic ring. A positive NOE was
observed in Haꢀ–Hc and in Hc–Hcꢀ of Z-2ꢀ, which confirms that
the configuration of the Z-2ꢀ is in the cis-form.
When the temperature was raised, the new signals disappeared
and only the signals of the trans-isomer were observed. This
reaction cycle (i.e. photoisomerization and thermal reversion) is
completely reversible. The ratios of cis : trans isomers were 47 :
53 (E-1/Z-1) and 40 : 60 (E-2ꢀ/Z-2ꢀ) at PSS (173 K), respectively.
The chemical shifts of the amide NH signals of cis-isomers at
183 K were 9.07 ppm in Z-1 and 11.14 ppm in Z-2ꢀ. The downfield
shift of 2.07 ppm suggests that Z-2ꢀ forms an NH · · · O hydrogen
bond in the THF-d8 solution. The decrease in the temperature
coefficient during the trans-to-cis isomerization of E-2ꢀ (−10.9
ppb K−1) and Z-2ꢀ (−6.2 ppb K−1) indicates that the intramolecular
hydrogen bond is predominant in Z-2ꢀ, whereas the intermolecular
interaction is formed in E-2ꢀ.
In contrast to the sharp signals observed in carboxylic acids (E-
1, Z-1), the signals of carboxylates (E-2ꢀ, Z-2ꢀ) were broadened
at low temperature. Because of the intermolecular hydrogen
bond formed in THF-d8 solution, E-2ꢀ assumes an origomeric
structure easily because exchange reactions occur more slowly at
low temperature. It is thought that only signals of carboxylates
were broadened at low temperatures because of this equilibrium.
Decreasing the temperature also seemed to cause the signals of the
aromatic ring protons to shift to a low magnetic field; these signals
were observed as a result of the intermolecular deshielding effect
produced by the aromatic ring of the other molecule, which became
adjacent through the presence of the intermolecular NH · · · O
hydrogen bond. The intermolecular interaction of Z-2ꢀ is estimated
to be small because the signals those in E-2ꢀ are broader than those
in Z-2ꢀ.
Fig. 5 Photoirradiating NOESY NMR spectra of (a) carboxylic acid
E-1/Z-1, and (b) carboxylate E-2ꢀ/Z-2ꢀ, 5 mM in THF-d8 solution at
203 K. The cross-peaks in circles indicate the intramolecular NOE signals
in the Z-isomers, and those in the squares indicate the EXSY signals
between E- and Z-isomers.
Z-to-E Thermal isomerization
The intramolecular hydrogen bond can influence the ground-state
equilibrium between configurational and conformational isomers
as well as photophysical and photochemical behavior.3,4 If the
intramolecular hydrogen bond is formed in a cis-compound, it may
decrease the cis-to-trans thermal reversion rate. Kinetic studies
of thermal reversion of diarylazomethine derivatives from cis-to-
trans isomers were reported in a wide temperature range and found
to follow first-order kinetics.5,6 Z-1 and Z-2ꢀ also produce cis-to-
trans thermal reversion in accordance with first-order kinetics.
Using the time-course experiments of UV-visible spectra in THF
solution, the thermal reversion rates of Z-1 and Z-2ꢀ in the
temperature range of 213–243 K were measured.
The thermal reversion of Z-1 is faster than that of Z-2ꢀ in
the temperature range 213–243 K. This result also suggests that
the NH · · · O intramolecular hydrogen bond forming at Z-2ꢀ is
stronger than the one formed at Z-1. Activation energies of cis-to-
trans thermal isomerization were determined using the Arrhenius
equation. Arrhenius plots of Z-1/E-1 and Z-2ꢀ/E-2ꢀ are shown
in Fig. 6. The activation energies of thermal reversion of Z-1
and Z-2ꢀ are 15.1 and 16.9 kcal mol−1, respectively. Z-2ꢀ has an
the shielding cone of another ring. The solution structure of Z-
1 was estimated based on NOE correlations. NOE correlations
of He–Hcꢀ or He–Haꢀ, which were observed characteristically
in E-1, disappeared in Z-1. Instead, the NOE correlation was
observed between Haꢀ and Hc in Z-1. These results confirm that
the configuration of the photoproduct Z-1 is exactly in cis-form.
In Z-1, NOE correlations of He–Ha and He–Hc are observed at
the same time, which indicates that the amide-side aromatic ring
of Z-1 rotates easily.
In contrast to carboxylic acid, the correlation between the two
compounds is not clearly observed in the carboxylate [Fig. 5(b)]. It
was also difficult to assign the signals using the homo-decoupling
method because of broadening of signals at low temperatures. To
assign the signals of Z-2ꢀ, the correlation spectroscopy (COSY)
NMR spectrum in PSS was measured (ESI†). Two singlets, NH
and He, do not have a COSY. One broad signal at 11.10 ppm, which
has a large temperature coefficient, is assigned to the NH signal.
Another sharp signal at 8.31 ppm is assigned to the He signal. In
the nuclear Overhauser effect spectroscopy (NOESY) spectrum
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referenced to the residual solvent protons at d 2.49, and to the
solvent carbons at d 39.5. Elemental analysis was performed
at the Elemental Analysis Center, Faculty of Science, Osaka
University. All melting points of the compounds were measured on
a micro melting point apparatus of YANAGIMOTO Co. ESI-MS
measurements were performed on a Finnigan MAT LCQ ion trap
mass spectrometer. FT-IR spectra were measured on a JASCO
FT/IR-8300 spectrometer.
Preparation of 3-(2,2-dimethylpropionylamino)benzaldehyde
To a concentrated HCl (60 mL) suspension of 3-nitrobenzaldehyde
(10.0 g, 66 mmol) was added stannous chloride (45.1 g, 240 mmol).
The temperature rose rapidly, 3-nitrobenzaldehyde dissolved, and
a clear, orange solution was obtained. The solution was refluxed
for 2 h, and the solution color turned to red. The solution was
cooled in an ice bath, and a pasty orange-red suspension resulted.
The precipitate was collected and washed with the diethyl ether,
dried overnight in an NaOH desiccator under reduced pressure,
yielding an orange powder (17.3 g). All of the orange powder
was suspended in CH2Cl2 (70 mL) and cooled in an ice bath (A).
2,2-Dimethylpropionyl chloride and triethylamine were added to
CH2Cl2 (40 mL) and cooled in an ice bath (B). (A) was slowly added
to (B). After the mixture had been stirred for 90 min, the reactant
was washed with 2% HCl aq., 4% NaHCO3 aq., water, and with
saturated NaCl aq. in a dropping funnel. The organic layer was
dried over anhydrous Na2SO4. Removal of solvents under reduced
pressure followed by recrystallization from ethyl acetate–diethyl
ether (1 : 3) gave colorless crystals (4.9 g, 36%). Mp 104–105 ◦C;
Found: C, 70.07; H, 7.34; N, 6.73. Calc. for C12H15NO2: C, 70.22;
H, 7.37; N, 6.82%; dH(270 MHz; DMSO-d6) 1.23 (9H, s, tert-butyl),
7.52 (1H, t, J 7.6 Hz, aryl), 7.59 (1H, dt, J 7.6 and 1.4, aryl), 7.95
(1H, dt, J 7.6 and 1.4, aryl), 8.22 (1H, t, J 1.4, aryl), 9.45 (1H,
br s, NH) and 9.96 (1H, s, CHO); dC(125 MHz; DMSO-d6) 27.06,
39.18, 120.14, 124.80, 125.96, 129.25, 136.52, 140.15, 176.81 and
192.97; m/z (ESI) 204.4 ([M − H]+ requires 204.1).
Fig. 6 Arrhenius plots of Z-to-E thermal isomerization for Z-1 (ꢁ) and
Z-2ꢀ (ꢀ).
activation energy that is 1.8 kcal mol−1 higher for this reaction.
This difference in the level of activation energy is thought to arise
from the formation of an intramolecular NH · · · O hydrogen bond
in Z-2ꢀ.
Conclusion
In conclusion, carboxylic acid E-1 and carboxylate E-2ꢀ, both
of which have an amide group linked to an azomethine moiety
that switches the intramolecular NH · · · O hydrogen bond through
photoirradiation, were synthesized and characterized. E-2ꢀ was
determined to form intermolecular, rather than intramolecular,
NH · · · O hydrogen bonds in the solid state and in THF solution.
Using UV light irradiation, the generation of a cis-isomer was
1
observed by UV-visible and H NMR spectra. The temperature
coefficient for the chemical shift of amide NH signals and cis-to-
trans thermal reversion rates of their compounds show that the
intramolecular NH · · · O hydrogen bond of Z-2ꢀ is stronger than
that of Z-1. Switching of the intramolecular NH · · · O hydrogen
bond toward the carboxylate group by external stimulations
(e.g. photoirradiation and heating) will achieve the regulation
of nucleophilicity of carboxylate-like native enzymes by low
molecular weight compounds.
Preparation of (E)-3-{[3-(2,2-dimethylpropionylamino)-
benzylidene]amino}benzoic acid (E-1)
Experimental
General procedures
3-(2,2-Dimethylpropionylamino)benzaldehyde
(1.0266
g,
All manipulations involving air- and moisture-sensitive com-
pounds were carried out by the use of standard Schlenk tech-
niques under an argon atmosphere. 3-Nitrobenzaldehyde, 2,2-
dimethylpropionyl chloride were purchased from Tokyo-Kasei Co.
3-Aminobenzaldehyde, stannous chloride were purchased from
Nacalai tesque Co. Inc. Dichloromethane was distilled over CaH2.
Tetrahydrofuran was distilled over CaH2 and dried over Na.
5.00 mmol) and 3-aminobenzoic acid (1.3720 g, 10.0 mmol)
was dissolved in methanol (70 mL) and stirred overnight at
room temperature. Removal of solvents under reduced pressure
followed by recrystallization from ethyl acetate–hexane (1 : 3)
gave a white powder (0.5471 g, 33.7%). Mp 185 ◦C; Found: C,
70.35; H, 6.10; N, 8.62. Calc. for C19H20N2O3: C, 70.35; H, 6.21;
N, 8.64%; mmax(KBr pellet)/cm−1 3307 (NH), 1700 (C O), 1655
=
=
=
Methanol, ethanol, and acetonitrile were distilled over CaH2 and
(C O) and 1565 (C N); dH(400 MHz; DMSO-d6) 1.24 (9H, s,
tert-butyl), 7.44 (1H, t, J 7.8 Hz, aryl), 7.49 (1H, dt, J 7.8 and
1.8, aryl), 7.53 (1H, t, J 7.8, aryl), 7.59 (1H, dt, J 7.8 and 1.8,
aryl), 7.76 (1H, t, J 1.8, aryl), 7.81 (1H, dt, J 7.8 and 1.8, aryl),
7.84 (1H, dt, J 7.8 and 1.8, aryl), 8.29 (1H, t, J 1.8, aryl), 8.63
1
˚
dried over molecular sieves (3A). H (400 and 270 MHz) and
13C (100 MHz) NMR spectra were measured on a JEOL JNM-
GSX400 or a JEOL JNM-GSX270 spectrometer. 1H (500 MHz)
and 13C (125 MHz) NMR spectra were measured on a JEOL
LA500 spectrometer. When THF-d8, CD3CN, and CDCl3 were
used as solvent, the 1H NMR and 13C NMR spectra were
referenced to the tetramethylsilane protons at d 0.00, and to the
tetramethylsilane carbons at d 0.00, respectively. When DMSO-
=
(1H, s, –CH N–), 9.39 (1H, s, NH) and 13.02 (1H, br s, COOH);
dC(150 MHz; THF-d8) 27.74, 40.23, 120.08, 122.67, 123.88,
125.15, 126.02, 127.74, 129.53, 129.75, 133.04, 137.80, 141.31,
153.27, 161.72, 167.38 and 176.79; m/z (ESI) 324.3 ([M + H]+
requires 325.2), 346.3 ([M + Na]+ requires 347.1).
1
d6 was used as solvent, H NMR and 13C NMR spectra were
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7.42 (1H, t, J 7.8, aryl), 7.56 (1H, dt, J 7.8 and 1.6, aryl), 7.67
(1H, t, J 1.6, aryl), 7.68 (1H, dt, J 7.8 and 1.6, aryl), 7.87 (1H,
dt, J 7.8 and 1.6, aryl), 8.22 (1H, t, J 1.6, aryl), 8.55 (1H, s,
Preparation of (tetramethylammonium) (E)-3-{[3-(2,2-
dimethylpropionylamino)benzylidene]amino}benzoate (E-2)
To a solution of (E)-3-{[3-(2,2-dimethylpropionylamino)ben-
zylidene]amino}benzoic acid in CH3CN was added a solution
of an equivalent amount of tetramethylammonium acetate in
CH3CN. The solution was stirred and concentrated to give an
orange oil. The oil was washed with diethyl ether to give a pale
orange powder. The powder was recrystallized from hot CH3CN to
give colorless crystals (yield was not certain). Mp 245 ◦C; Found:
C, 68.47; H, 7.89; N, 10.40. Calc. for C23H31N3O3: C, 69.49; H,
7.86; N, 10.57%; mmax(KBr)/cm−1 3218 (br, NH) and 1668 (CO);
dH(400 MHz; DMSO-d6) 1.24 (9H, s, tert-butyl), 3.09 (12H, s,
NMe4), 7.12 (1H, dt, J 7.8 and 1.6 Hz, aryl), 7.23 (1H, t, J 7.8,
aryl), 7.42 (1H, t, J 7.8, aryl), 7.56 (1H, dt, J 7.8 and 1.6, aryl),
7.65 (1H, t, J 1.6, aryl), 7.66 (1H, dt, J 7.8 and 1.6, aryl), 7.86
(1H, dt, J 7.8 and 1.6, aryl), 8.22 (1H, t, J 1.6, aryl), 8.55 (1H, s,
=
–CH N–) and 9.38 (1H, s, NH); dC(150 MHz; THF-d8) 14.01,
20.54, 24.75, 59.19, 120.43, 120.65, 122.46, 125.14, 125.46, 127.81,
128.34, 128.71, 138.23, 142.94, 144.41, 151.64, 160.98, 170.09 and
177.67; m/z (ESI) 313.2 ([M − N(n-butyl)4]− requires 323.1) and
564.4([M − H]− requires 564.4).
UV light irradiation technique for UV-vis spectrum measurement
at 173 K
The temperature was controlled in the UV cell using the DN1704
liquid nitrogen cryostat. A Xe/Hg lamp (MUV-202U, Moritex
Co.) was used for UV light irradiation. The UV light was directed
using a liquid light guide. The sample was prepared under Ar
atmosphere and dissolved in the degassed solvent. The sample
in the UV cell was irradiated after the temperature was lowered.
After irradiation, the spectrum was measured. During irradiation
and spectrum measurements, the sample was always kept at the
desired temperature.
=
–CH N–) and 9.39 (1H, s, NH).
X-Ray crystallography. A suitable, single, colorless crystal
of E-2 was mounted on a fine nylon loop with Nujol and
immediately frozen at 200 K. All measurements were performed
on a Rigaku RAXIS-RAPID Imaging Plate diffractometer with
graphite monochromated MoKa radiation. The structures were
solved by direct methods (SIR 92) and the following refine-
ments were performed using SHELXL-97 and teXsan crystallo-
graphic software packages. All non-hydrogen atoms were refined
anisotropically. H1 was placed by reflection and hydrogen atoms
without H1 were placed in the calculated position and including
least-squares refinement.
UV light irradiation technique for 1H NMR spectrum
measurement at low temperatures
A Xe/Hg lamp was used for UV light irradiation. The UV light
was directed using a liquid light guide. The sample was dissolved
in the degassed solvent and sealed in an NMR tube under an
Ar atmosphere. After the temperature was lowered, UV light was
irradiated in the NMR spectrometer using a CIDNP probe. After
irradiation, the spectrum was measured. During irradiation and
spectrum measurements, the sample was always kept at the desired
Crystal data for E-2. C23H31N3O3, Mr = 397.52, monoclinic, a
◦
˚
= 16.08(2), b = 8.501(8), c = 17.00(3) A, b = 104.5(1) , V =
1
3
˚
temperature. A schematic illustration of the low temperature H
2249(16) A , T = 200
1 K, space group P21/n, Z = 4,
l(MoKa) = 0.8 cm−1, total number of reflections measured 21 943,
unique reflections 5009 (Rint = 0.161), final R indices: R1 = 0.059,
wR2 = 0.103 for all data. CCDC reference number 267875. For
crystallographic data in CIF or other electronic format see DOI:
10.1039/b516049a.
NMR spectrum measurement is given in the ESI†.
Acknowledgements
One of the authors (T. M.) expresses his special thanks for the
center of excellence (21COE) program “Creation of Integrated
EcoChemistry” of Osaka University.
Preparation of (tetra-n-butylammonium) 3-{[3-(2,2-
dimethylpropionylamino)benzylidene]amino}benzoate (E-2ꢀ)
References
To a solution of (E)-3-{[3-(2,2-dimethylpropionylamino)ben-
zylidene]amino}benzoic acid (0.0909 g, 2.8 × 10−4 mol) in ethanol
(4.0 mL) was added a 2.8 M ethanol solution of sodium ethoxide
(0.10 mL, 2.8 × 10−4 mol). The solution was stirred overnight
and concentrated to give a white-yellow powder. The powder was
washed with diethyl ether, and then dissolved in ethanol again.
A solution of tetra-n-butylammonium chloride (0.0778 g, 2.8 ×
10−4 mol) in EtOH was added. After stirring for a few minutes,
the solvent was removed under reduced pressure to give a pale
white-orange powder. The powder was washed by diethyl ether.
Recrystallization from THF–hexane (1 : 2) gave pale white-orange
powder (yield was not certain). Found: C, 67.23; H, 9.72; N, 6.54.
Calc. for C35H55N3O3 + (H2O)0.3: C, 67.23; H, 9.93; N, 6.72%;
dH(400 MHz; DMSO-d6) 0.91 (12H, t, J, –CH2CH3 × 4), 1.23
(9H, s, tert-butyl), 1.30 (8H, multiplet, –CH2CH3 × 4), 1.56 (8H,
multiplet, –CH2CH2CH3 × 4), 3.15 (8H, multiplet, N–CH2– ×
4), 7.12 (1H, dt, J 7.8 and 1.6, aryl), 7.23 (1H, t, J 7.8, aryl),
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