Photochemical synthesis of s-triazolo[3,4-b]benzothiazole and mechanistic studies on benzothiazole formation

Article abstract of DOI:10.1021/jo962190v

A general photochemical synthesis of s-triazolo[3,4-b]benzothizoles from 4,5-disubstituted 1,2,4-triazole-3-thione is described. Steady photolysis, quantum yield determination, and laser flash photolysis experiments have been carried out. An intermolecular electron transfer mechanism has been proposed.

Full text of DOI:10.1021/jo962190v

5766
J . Org. Chem. 1997, 62, 5766-5770
P h otoch em ica l Syn th esis of s-Tr ia zolo[3,4-b]ben zoth ia zole a n d
Mech a n istic Stu d ies on Ben zoth ia zole F or m a tion
G. J ayanthi,† S. Muthusamy,† R. Paramasivam,† V. T. Ramakrishnan,*,†
N. K. Ramasamy,‡ and P. Ramamurthy*,‡
Departments of Organic Chemistry and Inorganic Chemistry, School of Chemistry, University of Madras,
Guindy Campus, Chennai - 600 025, India
Received November 25, 1996X
A general photochemical synthesis of s-triazolo[3,4-b]benzothiazoles from 4,5-disubstituted 1,2,4-
triazole-3-thione is described. Steady photolysis, quantum yield determination, and laser flash
photolysis experiments have been carried out. An intramolecular electron transfer mechanism
has been proposed.
In tr od u ction
medium.15 The studies were extended to get naphtho[1,2-
d]thiazole and naphtho[2,1-d]thiazole,16 2-(indol-1-yl)-
benzothiazole,17 benzothiazolo[2,3-b]quinazolin-12-one.18
The photochemical synthesis of heterocycles has been
of interest in achieving the synthesis of various hetero-
cyclic systems which in some cases proceed very ef-
ficiently. Oxidative photocyclization of cis-stilbenes to
phenanthrenes is well known.1 The closely related
cyclizations of systems containing heteroatoms, such as
Schiff bases,2 diphenylamines,3 and anilides,4 as well as
some nonoxidative processes5-8 have also been reported.
The photochemistry of compounds containing o-ha-
loaryl groups in the structure has given rise to many
heterocyclic systems. Irradiation of o-halobenzoic acid
anilides as well as N-(2-haloaryl)benzamides has been
reported to give phenanthridine derivatives.9,10 The
syntheses of condensed thienoquinoline ring systems
have been reported by the irradiation of suitable haloacid
amides.11,12
The formation of benzothiazole ring system has been
reported by oxidative photocyclization of thiobenzanilide2
as well as N-phenylthiourethane.13 We have first re-
ported the formation of 2-methylbenzothiazole by the
irradiation of o-halothioacetanilide.14 Subsequently the
irradiation studies were carried out with a variety of
substituted aniline derivatives in methanol or benzene
Resu lts a n d Discu ssion
In continuation of our earlier work,14-18 we report
herein the synthesis of s-triazolo[3,4-b]benzothiazole. The
required 4-(2′-haloaryl)-5-aryl-s-triazole-3-thione 3 was
prepared from the respective acid hydrazide 1 and
appropriate 2-halophenyl isothiocyanate 2. Irradiation
of 3 furnished the s-triazolo[3,4-b] benzothiazole 4 in 30-
60% yields. The products were characterized by IR,
NMR, MS, and elemental analysis (Table-1).
R2
R3
R1
NCS
CONHNH2
R1
R4
X 2
N
N
NH
8% NaOH
∆/5 h
R2
R3
S
R4
X
1
3
hν/254 nm
MeOH
R2
R3
R1
† Department of Organic Chemistry.
‡ Department of Inorganic Chemistry.
X Abstract published in Advance ACS Abstracts, August 1, 1997.
(1) (a) Blackburn, E. V.; Timmons, C. J . Quart. Rev. 1969, 23, 482-
503. (b) Beak, P.; Messer, W. R. In Organic Photochemistry; Chapman,
O. L., Ed.; Marcel Dekker: New York, 1969; Vol. 2, pp 117-167.
(2) Grellmann, K. H.; Tauer, E. Tetrahedron Lett. 1967, 1909-1910.
(3) Linschitz, H.; Grellmann, K. H. J . Am. Chem. Soc. 1964, 86,
303-304.
N
N
N
S
R4
4
(4) Thyagarajan, B. S.; Kharasch, N.; Lewis H. B.; Wolf, W.; J . Chem.
Soc., Chem. Commun. 1967, 614-615.
(5) (a) Cleveland, P. G.; Chapman, O. L. J . Chem. Soc., Chem.
Commun. 1967, 1064-1065. (b) Chapman, O. L.; Eian, G. L.; Bloom,
A.; Clardy, J . J . Am. Chem. Soc. 1971, 93, 2918-2928.
(6) Ninomiya, I.; Naito, T.; Kiguchi, T. Tetrahedron Lett. 1970,
4451-4453.
Mech a n istic Stu d ies. Benzothiazoles have been
prepared19,20 under basic conditions from o-halothioac-
etanilides; N-(3-iodophenyl)benzamide also gave 2-phen-
(7) Kanaoka, Y.; Itoh, K. J . Chem. Soc., Chem. Commun. 1973, 647-
648.
(15) Paramasivam, R.; Ramakrishnan, V. T. Indian J . Chem. Sect.
B 1987, 26B, 930-934.
(8) Yang, N. C.; Shani, A.; Lenz, G. R. J . Am. Chem. Soc. 1966, 88,
5369.
(16) Paramasivam, R.; Muthusamy, S.; Ramakrishnan, V. T. Indian
J . Chem. Sect. B 1989, 28B, 597-598.
(9) Kessar, S. V.; Singh, G.; Balakrishnan, P. Tetrahedron Lett. 1974,
2269-2270.
(17) Muthusamy, S.; Ramakrishnan, V. T. J . Heterocycl. Chem.
1991, 28, 759-763.
(10) Grimshaw, J .; Prasanna de Silva, A. J . Chem. Soc., Chem.
Commun. 1980, 302-303.
(18) Muthusamy, S.; Ramakrishnan, V. T. Synth. Commun. 1992,
22, 519-533.
(11) J ayachandran, T.; Paramasivam, R.; Ramakrishnan, V. T. Ind.
J . Chem. Sect. B 1986, 25B, 89-91.
(19) Bunnett, J . F.; Scamehorn, R. G.; Traber, R. P. J . Org. Chem.
1976, 41, 3677-3682.
(12) Luo, J . K.; Castle, R. N. J . Heterocycl. Chem. 1991, 28, 1825-
1830.
(20) (a) Bunnett, J . F.; Kato, T.; Flynn, R. R.; Skorcz, J . A. J . Org.
Chem. 1963, 28, 1-6. (b) Bunnett, J . F.; Hrutfiord, B. F. J . Am. Chem.
Soc. 1961, 83, 1691-1697. (c) Stanetty, P.; Krumpak, B. J . Org. Chem.
1996, 61, 5130-5133.
(13) Bellus, D.; Schaffner, K.; Helv. Chim. Acta. 1968, 51, 221-224.
(14) Paramasivam, R.; Palaniappan, R.; Ramakrishnan, V. T. J .
Chem. Soc., Chem. Commun. 1979, 260-261.
S0022-3263(96)02190-1 CCC: $14.00 © 1997 American Chemical Society

Photochemical Synthesis of s-Triazolo[3,4-b]benzothiazole
J . Org. Chem., Vol. 62, No. 17, 1997 5767
Ta ble I. Su bstitu ted s-Tr ia zolo [3,4-b]ben zoth ia zoles
irradiation
time (h)
yield
(%)
3 f 4
R1
R2
R3
R4
X
a
b
c
d
e
f
H
H
CH3
H
CH3
H
H
H
H
H
H
H
H
CH3
H
OCH3
H
CH3
H
H
H
H
H
CH3
CH3
H
Cl
Cl
Cl
Cl
Br
Br
Cl
15
15
25
20
24
18
5
41
40
38
56
48
47
34
g
-CHdCHCHdCH-
Ta ble 2. 2-Su bstitu ted Ben zoth ia zoles
irradiation
time (h)
5 f 6
R
X1
X2
Cl
Cl
Cl
CH3
Cl
Cl
Cl
Cl
X3
X4
a
b
c
d
e
f
g
h
i
CH3
CH3
CH3
CH3
CH2C6H5
C6H5
Indol-1-yl
Cl
Cl
Br
Br
Cl
Cl
Cl
Cl
Cl
-
-
-
42
24
18
8
42
72
35
40
42
Br
Br
Br
-
CH3
CH3
-
-
-
-
-
1,3-Dioxocyclohexan-2-yl
5,5-Dimethyl-1,3-dioxocyclohexan-2-yl
-
-
Cl
-
-
ylbenzothiazole; hence a benzyne intermediate has been
proposed. Under photochemical conditions, photoinduced
elimination of HX resulting in a benzyne intermediate
followed by cyclization can be contemplated.
X4
X4
H
N
C
S
R
N
S
hν
R
X3
X3
X1
X2
X2
6
5
The enol nature of the thioacetanilides was supported15
1
by the H-NMR signals in the region δ 2.2-2.5 due to
the SH proton in some cases. However, the benzyne
mechanism can be ruled out, because N-(2,3-dihaloaryl)-
thioamides 5 (Table 2) as well as the naphthyl system 7
undergo facile photocyclization.
R
Cl
S
N
H
C
S
R
N
hν
8
7
a, R = phenyl
b, R = indol-1-yl
c, R = 1,3-dioxocyclohexan-2-yl
d, R = 5,5-dimethyl-1,3-dioxocyclohexan-2-yl
e, R = C6H5CONH-
f, R = methyl
F igu r e 1. Absorption spectral changes during the photolysis
of N-(2-bromo-4-methylphenyl)thioacetamide in nitrogen-
saturated methanol. Each spectrum recorded after 25 irradia-
tion.
A number of reports of photocyclization accompanied
by elimination of HCl or HBr have appeared in the
literature, although the details of the mechanism have
not been discussed clearly.21 The actual mechanism of
the photocyclization reaction, thus, cannot be unequivo-
cally putforth on the basis of the products present in the
irradiated solution and the various reactions with dif-
ferent substrates. In order to elucidate the mechanism
for the above photocyclization, studies involving quantum
yield of the products and flash photolysis identification
of intermediates were carried out in the present work.
Stea d y P h otolysis. Steady photolyses were carried
out for selected substrates (qualitatively) using a low-
pressure mercury pen-ray lamp which emits light of
wavelength 254 nm. Absorption spectra of the com-
pounds in the ultraviolet region were taken after each
specified duration of irradiation. Absorption spectral
changes accompanying the 254 nm photolysis of N-(2-
bromo-4-methylphenyl)thioacetamide in methanol showed
the isosbestic points at 255 and 316 nm during conversion
to the product (Figure 1); N-(2-chlorophenyl)thiobenz-
amide showed the isosbestic points at 239 and 267 nm;
(21) Bryce-Smith, D.; Gilbert, A. (Senior reporters). A specialist
periodical report: Photochemistry; Royal Society of Chemistry: Great
Britain, 1989; Vol. 20, pp 442-444.

5768 J . Org. Chem., Vol. 62, No. 17, 1997
J ayanthi et al.
N-(2,5-dichlorophenyl)indolethiocarboxamide showed the
isosbestic point at 242 nm, which disappeared, and the
reappearance of isosbestic points has been observed at
275 and 300 nm until the product spectrum was obtained;
3-(2-chlorophenyl)-2-thioxo-1,2-dihydro-4(3H)quinazoli-
none showed an isosbestic point initially at 285 nm which
disappeared and reappeared again at 270 and 316 nm,
until the product spectrum was obtained. The changes
indicate that the reaction is a multi-step process in
nature. 2-(Indol-1-yl)benzothiazole, 7-chloro-2-(indol-1-
yl)benzothiazole, and 12H-benzothiazolo[2,3-b]quinazo-
lin-12-one were separately irradiated using pen-ray lamp;
recording of the UV spectrum at frequent intervals
showed that all the above benzothiazoles undergo sec-
ondary photolysis on further irradiation.
The quantum yield determination was carried out for
the formation of 2-phenylbenzothiazole from N-(2-chlo-
rophenyl)thiobenzamide. Light intensity was measured
by ferrioxalate actinometry.22 The quantum yield (φ) for
the formation of product was found to be 0.037 ( 0.003.
La ser F la sh P h otolysis. A set of experiments were
designed to establish the cleavage of aryl-halogen bond.
The compounds chosen for this study were (a) N-(2-
chlorophenyl)thioacetamide (9), (b) N-(3-chlorophenyl)-
thioacetamide (10), and (c) N-(2-chlorophenyl)acetamide
(11).
The laser flash photolysis of 9 with pulses (266 nm)
from the fourth harmonic of Nd/YAG laser was carried
out. Chlorine atoms are not readily detectable in experi-
ments of this type. In order to make chlorine atoms
detectable, tetraethylammonium chloride was added to
generate Cl2•- which is known to absorb strongly at λmax
∼ 345 nm;23 significant absorption from Cl2•- was noted.
The laser flash photolysis experiments carried out with
10 and 11 showed very weak transient absorption spectra
for Cl2•-, thus supporting the absence of product forma-
tion during the irradiation (Figures 2 and 3). The signal
observed in the case of 10 may be due to a similar
electron transfer and elimination of Cl•, but to a small
extent. In addition, loss or shift of hydrogen from the
ortho position must also occur to give benzothiazole,
which was not observed. In the case of 11, the electron
transfer from oxygen can be expected to be weaker than
from sulfur, resulting in a weak signal for Cl2•-. Thus
the above experiment suggests the elimination of chlorine
radical.
F igu r e 2. Transient absorption spectra recorded 2 µs after a
266 nm laser pulse in argon-saturated acetonitrile solution for
N-(2-chlorophenyl)thioacetamide (9) and N-(3-chlorophenyl)-
thioacetamide (10). Insert left: Transient decay of 10 at the
absorption maximum. Inset right: Transient decay of 9 at the
absorption maximum.
H
N
H
N
CH3
•
9
•
–
CH3
–
+
+
Cl
S
•
S
Cl•
12′
12′′
F igu r e 3. Transient absorption spectrum of N-(2-chlorophen-
yl)acetamide (11) in argon-saturated acetonitrile solution.
Inset: Transient decay at the absorption maximum.
H
N
N
S
CH3
CH3
reaction might be expected to give 12′. The expected
initial homolysis of the Ar-Cl bond probably does not
occur since no dehalogenated thioamide or amine was
detected. The driving force for the homolysis may
emanate from the favorable formation of aryl-sulfur
bond. The detection of HCl in some of the experiments15
+
S
Cl–
12
12′′′
As suggested in the SRN1 mechanism,24 the formation
of radical anion by intramolecular electron transfer
(22) Hatchard, C. G.; Parker, C. A. Proc. R. Soc., (London) 1956,
A235, 518.
(23) Horvath, O.; Vogler, A. Inorg. Chem. 1993, 32, 5485-5489.
(24) Lowry, T. H.; Richardson, K. S. In Mechanism and Theory in
Organic Chemistry; 3rd ed.; Harper & Row Publishers: New York,
1987; p 645.

Photochemical Synthesis of s-Triazolo[3,4-b]benzothiazole
J . Org. Chem., Vol. 62, No. 17, 1997 5769
4-(2-Ch lor op h en yl)-5-(2-m eth ylp h en yl)-1,2,4-tr ia zole-
3-th ion e (3c). From o-toluic hydrazide (1.3 g, 0.009 mol) and
2-chlorophenyl isothiocyanate (1.5 g, 0.009 mol) was obtained
3c: yield 45%; mp 244-246 °C; IR (KBr, cm-1) 3090 (NH),
1605 (CdN); 1H NMR (60 MHz; acetone-d6) δ 2.45 (s, 3H,
ArMe), 7.20-7.80 (m, 8H, ArH); MS m/ z (relative intensity)
301 (M+, 14), [303, M + 2], 266 (100), 149 (73). Anal. Calcd
for C15H12 ClN3S: C, 59.69; H, 4.00; N, 13.92. Found: C, 59.80;
H, 3.98; N, 13.95.
is also experimental evidence for the above mechanism.
A mechanism involving intramolecular SRN1 substitu-
tion has been proposed earlier for the formation of
benzothiazole from o-halothioacetanilide, which was,
however, based on the irradiation studies on the thio-
amide anion.25 In the electron transfer reactions, amines
are known to participate as inter- and intramolecular
electron donors.26-28 So far, there appear to be no report
of any electron transfer from an amide nitrogen. In the
present work, the thioamides being the substrates, the
electron transfer may be expected to occur from sulfur
rather than from nitrogen. A mechanism involving
nucleophilic displacement29 by the thiolate anion has
been proposed for the base catalyzed cyclization of
triazole-3-thiols to yield s-triazolo[3,4-b]benzothiazole
ruling out the benzyne mechanism.
4-(2-Ch lor op h en yl)-5-(4-m eth oxyp h en yl)-1,2,4-tr ia zole-
3-th ion e (3d ). Thione 3d was prepared using p-anisic hy-
drazide (1.0 g, 0.006 mol) and 2-chlorophenyl isothiocyanate
(1.0 g, 0.006 mol): yield 41%; mp 240-242 °C; IR (KBr, cm-1
)
3090 (NH), 1610 (CdN), 1020, 1250 (dCOC); 1H NMR (60
MHz, acetone-d6) δ 3.90 (s, 3H, OMe), 7.10-8.00 (m, 8H, ArH);
MS m/ z (relative intensity) 317 (M+, 21), [319, M + 2], 282
(100), 149 (88). Anal. Calcd for C15H12ClN3OS: C, 56.69; H,
3.80; N, 13.22. Found: C, 56.78; H, 3.78; N, 13.24.
4-(2-Br om o-4-m eth ylp h en yl)-5-(2-m eth ylp h en yl)-1,2,4-
tr ia zole-3-th ion e (3e). Thione 3e was prepared from o-toluic
hydrazide (1.5 g, 0.01 mol) and 2-bromo-4-methylphenyl
isothiocyanate (2.3 g, 0.01 mol): yield 42%; mp 221-223 °C;
IR (KBr, cm-1) 3080 (NH), 1590 (CdN); 1H NMR (60 MHz,
acetone-d6) δ 2.35 (s, 3H, ArMe), 2.50 (s, 3H, ArMe), 7.15-
7.70 (m, 7H, ArH); MS m/ z (relative intensity) 359 (M+, 8),
Con clu sion
Substituted s-triazolo[3,4-b]benzothiazoles were pre-
pared from the respective triazole-3-thiones by photo-
chemical method. In order to establish the mechanism,
steady photolysis experiments were carried out. The
changes in the isosbestic points indicate a multistep
nature and secondary photolysis. Laser flash photolysis
experiments indicate the expulsion of halogen as radical.
An intramolecular electron transfer mechanism has been
proposed.
[361, M + 2], 280 (100), 163 (75). Anal. Calcd for C16H14
-
BrN3S: C, 53.34; H, 3.91; N, 11.66. Found: C, 53.48; H, 3.89;
N, 11.69.
4-(2-Br om o-4-m eth ylp h en yl)-5-(4-m eth ylp h en yl)-1,2,4-
tr ia zole-3-th ion e (3f). Reaction of p-toluic hydrazide (1.5 g,
0.01 mol) with 2-bromo-4-methylphenyl isothiocyanate (2.3g,
0.01 mol) gave 3f: yield 86%; mp 270-272 °C; IR (KBr, cm-1
)
3080 (NH), 1600 (CdN); 1H NMR (400 MHz, acetone-d6) δ 2.30
(s, 3H, ArMe), 2.40 (s, 3H, ArMe), 7.15-7.60 (m, 7H, ArH);
MS m/ z (relative intensity) 359 (M+, 31), [361, M + 2], 280
(100), 163(77).
Exp er im en ta l Section
All the melting points are uncorrected. The reactions were
monitored by TLC. IR spectra were recorded for the samples
as KBr pellets. 1H NMR spectra were recorded on 60, 90, and
400 MHz instruments, and the shifts are reported in ppm
downfield from TMS. Mass spectra were determined at an
ionizing voltage of 70 eV. The photochemical reactions were
carried out in a quartz vessel in an Applied Photophysics
reactor (254 nm) and a Hanovia medium-pressure immersion
reactor.
4-(2-Ch lor op h en yl)-5-(1-n a p h th yl)-1,2,4-tr ia zole-3-th i-
on e (3g). Reaction of 1-naphthoic hydrazide (2.8 g, 0.015 mol)
with 2-chlorophenyl isothiocyanate (2.55 g, 0.015 mol) fur-
nished 3g: yield 69%; mp 262-264 °C; IR (KBr, cm-1) 3060
(NH), 1580 (CdN). 1H NMR (60 MHz, acetone-d6) δ 7.20-
8.50 (m, 11H, ArH); MS m/ z (relative intensity) 337 (M+, 14),
[339, M + 2], 302 (100), 149 (98).
Ir r a d ia tion . A solution of 3a (0.7 g, 0.0024 mol) in absolute
methanol (150 mL) was flushed with nitrogen for 1 h and
irradiated for 15 h, monitoring by TLC. After removal of
solvent, the residue was chromatographed over a column of
silica gel. Elution with ethyl acetate-petroleum ether mixture
(1:6) furnished 3-phenyl-s-triazolo[3,4-b]benzothiazole 4a: yield
4-(2-Ch lor oph en yl)-5-ph en yl-1,2,4-tr iazole-3-th ion e (3a).
A mixture of benzhydrazide (1.4 g, 0.01 mol) and 2-chlorophen-
yl isothiocyanate (1.7 g, 0.01 mol) was refluxed in NaOH
solution (30 mL; 8%) for 5 h, cooled, and filtered and the filtrate
washed with ether. The aqueous layer was acidified with cold,
dilute HCl. The separated solid was filtered and washed with
water to give 3a : yield 59%; mp 222-224 °C (EtOH); IR (KBr,
41%; mp 144-146 °C (EtOH); (lit.30 mp 153 °C); IR (KBr, cm-1
)
1610 (CdN); 1H NMR (90 MHz, CDCl3) δ 7.4-8.0 (m, 9H,
ArH); MS m/ z (relative intensity) 251 (M+, 14), 148 (92). Anal.
Calcd for C14H9N3S: C, 66.91; H, 3.60; N, 16.72. Found: C,
66.73; H, 3.42; N, 16.70.
1
cm-1) 3090 (NH), 1620 (CdN); H NMR (90 MHz, DMSO-d6)
δ 7.40-8.25 (m, 9H, ArH); MS m/ z (relative intensity) 287
(M+, 18), [289, M + 2], 252 (100), 149 (85). Anal. Calcd for
C14H10ClN3S: C, 58.43; H, 3.50; N, 14.60. Found: C, 58.53;
H, 3.48; N, 14.63.
3-(4-Me t h ylp h e n yl)-s-t r ia zolo[3,4-b]b e n zot h ia zole
(4b): yield 40% from 0.6 g of 3b; mp 154-156 °C (EtOH); IR
(KBr, cm-1) 1590 (CdN); 1H NMR (400 MHz, CDCl3) δ 2.3 (s,
3H, ArMe), 7.0-7.7 (m, 8H, ArH); MS m/ z (relative intensity)
265 (M+, 85), 148 (100). Anal. Calcd for C15H11N3S: C, 67.90;
H, 4.17; N, 15.83. Found: C, 67.84; H, 4.02; N, 15.81.
3-(2-Meth ylp h en yl)-s-tr ia zolo[3,4-b]ben zoth ia zole (4c):
yield 38% from 0.3 g of 3c; mp 110-112 °C (EtOH); IR (KBr,
4-(2-Ch lor op h en yl)-5-(4-m eth ylp h en yl)-1,2,4-tr ia zole-
3-th ion e (3b). Thione 3b was prepared using p-toluic hy-
drazide (4.0 g, 0.027 mol) and 2-chlorophenyl isothiocyanate
(4.5 g, 0.027 mol): yield 87%; mp 238-240 °C; IR (KBr, cm-1
)
3070 (NH), 1606 (CdN); 1H NMR (400 MHz, DMSO-d6) δ 2.30
(s, 3H, ArMe), 7.05-7.55 (m, 8H, ArH); MS m/ z (relative
intensity) 301 (M+, 17), [303, M + 2], 266 (100), 149 (80). Anal.
Calcd for C15H12ClN3S: C, 59.69; H, 4.00; N, 13.92. Found:
C, 59.51; H, 3.94; N, 13.71.
1
cm-1); 1585 (CdN); H NMR (400 MHz, CDCl3) δ 2.3 (s, 3H,
ArMe), 7.0-7.7 (m, 8H, ArH); MS m/ z (relative intensity) 265
(M+, 82), 148 (45). Anal. Calcd for C15H11N3S: C, 67.90; H,
4.17; N, 15.83. Found: C, 67.84; H, 4.43; N, 15.56.
3-(4-Meth oxyph en yl)-s-tr iazolo[3,4-b]ben zoth iazole (4d):
yield 56% from 0.2 g of 3d ; mp 140-142 °C (EtOH); (lit.31 mp
145-146 °C); IR (KBr, cm-1) 1610 (CdN), 1020, 1250 (dCOC);
1H NMR (400 MHz, CDCl3) δ 3.9 (s, 3H, OMe), 7.1-7.8 (m,
8H, ArH); MS m/ z (relative intensity) 281 (M+, 100), 148 (84).
(25) Bowman, W. R.; Heaney, H.; Smith, P. H. G. Tetrahedron Lett.
1982, 23, 5093-5096.
(26) Turro, N. J. In Modern Molecular Photochemistry; The Benjamin/
Cummings Publishing Co.: California, 1978; pp 375-591. (b) Grim-
shaw, J .; Prasanna de Silva, A. Chem. Soc. Rev. 1981, 10, 181-203.
(27) Barltrop, J . A.; Coyle, J . D. In Excited states in Organic
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5770 J . Org. Chem., Vol. 62, No. 17, 1997
J ayanthi et al.
Anal. Calcd for C15H11N3OS: C, 64.04; H, 3.94; N, 14.93.
Found: C, 64.35; H, 4.12; N, 15.00.
N-(2-Chlorophenyl)thiobenzamide (4.1 × 10-4 mol/L) in
methanol was used for the quantum yield determination,
which had an absorbance of more than 4.0 at 254 nm to absorb
the entire light; the solution was irradiated using a pen-ray
lamp for a specified time (2 min), and the absorbance was
monitored at 290 and 300 nm. Three separate irradiation
experiments were carried out for each run, and the absorbance
was determined with three independent solutions.
Laser flash photolysis experiments of the samples were
carried out in argon-saturated acetonitrile solutions using a
Quanta Ray GCR-2(10) Nd-YAG Laser using the fourth
harmonics output of 266 nm with a pulse width of 8 ns. The
signals were detected using a 250 W pulsed Xenon lamp,
Czerny Turner monochromator, and R-928 PMT and captured
using a Hewlett-Packard 54201A digital storage oscilloscope.
3-(2-Meth ylp h en yl)-7-m eth yl-s-tr ia zolo[3,4-b]ben zoth i-
a zole (4e): yield 48% from 0.4 g of 3e; mp 120-122 °C (EtOH);
IR (KBr, cm-1) 1600 (CdN); 1H NMR (400 MHz, CDCl3) δ 2.3
(s, 3H, ArMe), 2.45 (s, 3H, ArMe), 6.8-7.5 (m, 7H, ArH); MS
m/ z (relative intensity) 279 (M+, 85), 162 (50). Anal. Calcd
for C16H13N3S: C, 68.79; H, 4.69; N, 15.04. Found: C, 68.50;
H, 4.83; N, 14.79.
3-(4-Meth ylph en yl)-7-m eth yl-s-tr iazolo[3,4--b]ben zoth i-
a zole (4f): yield 57% from 0.25 g of 3f: mp 182-184 °C
(EtOH); IR (KBr, cm-1) 1610 (CdN); 1H NMR (400 MHz,
CDCl3) δ 2.45 (s, 3H, ArMe), 2.5 (s, 3H, ArMe), 7.1-7.7 (m,
7H, ArH); MS m/ z (relative intensity) 279 (M+, 78), 162 (100).
Anal. Calcd for C16H13N3S: C, 68.79; H, 4.69; N, 15.04.
Found: C, 68.61; H, 4.72; N, 15.04.
Ack n ow led gm en t. We wish to thank the University
Grants Commission, New Delhi, for the Special As-
sistance Programme to the Department of Organic
Chemistry, CSIR, New Delhi, for providing a research
fellowship (G.J .), IIT - Madras, IIT - Bombay, and IISc,
Bangalore, for spectral and analytical data. One of the
authors (P.R.) is thankful to University Grants Com-
mission for the support under COSIST programme.
3-(1-Na p h th yl)-s-tr ia zolo[3,4-b]ben zoth ia zole (4g): yield
34% obtained by Immersion type photolysis from 0.5 g of 3g:
mp 154-156 °C (EtOH); IR (KBr, cm-1) 1580 (CdN); 1H NMR
(400 MHz, CDCl3) δ 6.6-8.2 (m, 11H, ArH); MS m/ z (relative
intensity) 301 (M+, 62), 148 (57). Anal. Calcd for C18H11N3S:
C, 71.74; H, 4.01; N, 13.94. Found: C, 71.81; H, 3.86; N, 13.84.
In steady photolysis experiments, a low pressure mercury
pen-ray quartz lamp (254 nm) was used. Absorption spectra
of samples were recorded initially and after each specified
duration of irradiation in a standard 1 cm quartz cell.
J O962190V