1832
Synthesis and Photophysical Characterization of Proton Transfer-Based Thiourea Derivatives
J. Braz. Chem. Soc.
infrared spectroscopy (FTIR) spectra were recorded on a
Mattson Galaxy series FTIR 3000 model 3020 (KBr disc).
1H and 13C nuclear magnetic resonance (NMR) spectra
(APT, attached proton test) were recorded on a Varian
Inova 300 MHz spectrometer or Varian VNMRs 300 MHz
spectrometer. The chemical shifts were expressed as d (ppm)
relative to tetramethylsilane (TMS) as the internal standard
and using dimethyl sulfoxide (DMSO-d6) as the solvent
at room temperature. Data for 1H NMR were reported as
follows: multiplicity (s = singlet, d = doublet, t = triplet,
m = multiplet), integration, coupling constant (Hz) and
assignment. UV-Vis absorption spectra were performed
on a Shimadzu UV-2450 spectrophotometer. A slit width
of 2.0 was applied in the UV-Vis measurements. Steady
state fluorescence spectra were measured with a Shimadzu
spectrofluorometer model RF-5301PC, using emission
and excitation monochromator slit widths of 3.0/5.0. In
all photophysical experiments, the absorption maximum
was used as the excitation wavelength for fluorescence
measurements. All experiments were performed at 25 °C
in a concentration of 10-5 mol L-1. High resolution mass
spectrometry was recorded with electrospray ionization
(HRMS-ESI) data in the positive mode using a Q-TOF-MS
Bruker Impact II.
poured onto crushed ice and neutralized with NaHCO3
(20%). The precipitate was filtered and dried at 60 C.
The obtained compounds were purified by column
chromatography eluted with dichloromethane.
o
2-(5’-Amino-2’-hydroxyphenyl)benzoxazol (6)37
Yield: 2.35 g (52%); mp 173-175 ºC; FTIR (KBr)
ν / cm-1 3405 (νasNH2), 3250 (νsNH2), 1630 and 1548
(νC=C); 1H NMR (300 MHz, DMSO-d6) d 10.42 (s, 1H,
OH), 7.81 (m, 2H, H1 and H4), 7.44 (m, 2H, H2 and H3),
7.26 (d, 1H, HX, J 2.3 Hz), 6.84 (m, 2H, HA and HB), 4.96
13
(s, 2H, NH2); C NMR (75.5 MHz, DMSO-d6) d 162.8,
149.3, 148.6, 141.7, 139.6, 125.6, 125.2, 121.3, 119.0,
117.6, 110.9, 110.4, 109.6.
2-(5’-Amino-2’-hydroxyphenyl)benzothiazol (7)38
Yield: 1.53 g (53%); mp 196-198 ºC; FTIR (KBr)
ν / cm-1 3497 (νasNH2), 3325 (νsNH2), 3000 (νaromC–H),
1620 and 1470 (νC=Carom); 1H NMR (300 MHz, DMSO-d6)
d 10.72 (s, 1H, OH), 8.12-7.40 (m, 4H, H1, H2, H3, H4),
7.37-7.36 (d, 1H, H, Jm 2.7 Hz), 6.85-6.83 (d, 1H, H,
Jo 8.7 Hz), 6.77-6.76 and 6.74-6.73 (2d, 2H, H, Jm 2.7 Hz
13
and Jo 8.78 Hz), 4.8 (s, 2H, NH2); C NMR (75.5 MHz,
DMSO-d6) d 166.3, 151.6, 147.8, 141.5, 134.0, 126.4,
125.0, 121.9, 121.8, 119.9, 117.8, 117.6, 112.0.
Synthesis of p-isothiocyanate benzoic acid (2)
General synthesis procedure for the preparation of thiourea
A mixture of p-aminobenzoic acid (1) (4.0 g,
29.16 mmol) in dry acetone (30 mL) was added dropwise
to a solution of thiophosgene (2.24 mL, 29.16 mmol)
derivatives 8-9
Compound 2 (0.200 g, 1.1 mmol or 0.100 g, 0.56 mmol)
in CH3CN (15 mL) was added dropwise to a solution of 6
(0.252 g, 1.1 mmol) or 7 (0.135 g, 0.56 mmol) in CH3CN
(15 mL), respectively. The mixture was stirred for 6 h at 70oC.
The precipitate was filtered and dried at ambient temperature.
The obtained compounds were purified by column
chromatography eluted with acetone:MeOH (80:20 v/v).
o
in dry acetone (30 mL) at 0 C under vigorous stirring.
The reaction mixture was stirred for 3 h. The precipitate
was filtered, washed with cold acetone and dried at room
temperature giving the compound 2.
p-Isothiocyanate benzoic acid (2)36
Yield: 4.2 g (87%); mp > 310 oC; FTIR (KBr) ν / cm-1
1
3000 (νC–Harom), 2125 (νN=C=S); H NMR (300 MHz,
4-(3-(3-(Benzoxazol-2-yl)-4-hydroxyphenyl)thiourea)
benzoic acid (8)
Yield: 170 mg (56%); mp > 310 oC; FTIR (KBr) ν / cm-1
3480 (νOH), 3410-3217 (νNH) 3086-3029 (νaromC–H),
DMSO-d6) d 9.96 (d, 2H, J 9 Hz), 7.49 (d, 2H, J 9 Hz);
13C NMR (75.5 MHz, DMSO-d6) d 166.2, 135.6, 134.1,
130.9, 129.8, 126.1.
1
1688 (νC=O), 1605 (νaromC=C); H NMR (300 MHz,
Synthesis of aminobenzazoles 6-7
DMSO-d6) d 11.14 (s, 1H, OH), 10.27 (s, 1H, NH), 10.13
(s, 1H, NH), 8.16 (d, 1H, J 2.7 Hz), 7.93 (d, 2H, J 9 Hz),
7.87 (m, 2H, J 9.0 Hz), 7.71 (d, 2H, J 9.0 Hz), 7.60 (d,
1H, J 2.7 Hz), 7.57 (d, 1H, J 2.7 Hz), 7.48 (m, 2H), 7.15
(d, 1H, J 8.7 Hz); 13C NMR (APT) (75.5 MHz, DMSO-d6)
d 179.7, 167.7, 162.0, 154.9, 148.8, 143.6, 139.5, 131.7,
130.7, 129.8, 125.9, 125.4, 123.1, 121.7, 119.0, 117.1,
110.9, 109.8; HRMS (ESI-qTOF) m/z [M + H]+ calcd. for
C21H16N3O4S 406.0861; found 406.0856.
In a general way, the precursors were obtained by
reaction of an equimolar amount of 5-aminosalicylic
acid (3) (4.0 g, 26.12 mmol) and 2-aminophenol (4)
(2.8 g, 26.12 mmol) or 2-aminothiophenol (5) (2.76 mL,
26.12 mmol) in polyphosphoric acid (PPA) (60 mL) at
170 oC for 5 h. The reaction was followed by TLC using
dichloromethane as the eluent. The reaction mixture was