Regioselective ortho-hydroxylation of 2-arylbenzothiazole via substrate directed C-H activation

Article abstract of DOI:10.1016/j.tet.2012.12.067

An efficient protocol has been developed for the direct ortho-hydroxylation of 2-arylbenzothiazole using Pd(OAc)₂ as the catalyst and either of DIB/AcOH or Oxone/TFA combinations under air atmosphere. Under both these conditions, the methodology tolerates a diverse array of substituents giving good to excellent yields of corresponding ortho-hydroxylated products. Regioselective hydroxylation is observed at the less sterically hindered site of the phenyl ring possessing substituents at its meta position.

Full text of DOI:10.1016/j.tet.2012.12.067

Tetrahedron 69 (2013) 2175e2183
Contents lists available at SciVerse ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Regioselective ortho-hydroxylation of 2-arylbenzothiazole
via substrate directed CeH activation
*
Arghya Banerjee, Anupam Bera, Srimanta Guin, Saroj Kumar Rout, Bhisma K. Patel
Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati 781 039, India
a r t i c l e i n f o
a b s t r a c t
Article history:
An efficient protocol has been developed for the direct ortho-hydroxylation of 2-arylbenzothiazole using
Pd(OAc)2 as the catalyst and either of DIB/AcOH or OxoneÒ/TFA combinations under air atmosphere.
Under both these conditions, the methodology tolerates a diverse array of substituents giving good to
excellent yields of corresponding ortho-hydroxylated products. Regioselective hydroxylation is observed
at the less sterically hindered site of the phenyl ring possessing substituents at its meta position.
Ó 2013 Elsevier Ltd. All rights reserved.
Received 12 September 2012
Received in revised form 7 December 2012
Accepted 27 December 2012
Available online 3 January 2013
Keywords:
Pd-catalysis
ortho-Hydroxylation
2-Arylbenzothiazole
CeH activation
1. Introduction
number of pharmacophores, which are reported to exhibit diverse
biological activities.10 Hence, further derivatizing this important
Direct and selective functionalization of inert carbonehydrogen
(CeH) bonds is one of the most versatile approaches for the con-
struction of diverse complex molecules.1,2 These approaches, fa-
cilitated by various transition metals, have led to organic CeH
bonds viewed as dormant synthetic equivalents of many reactive
functional groups. In this context, particularly the Pd catalyzed CeH
functionalizations have attracted much attention to construct di-
verse array of CeC or CeX (X¼hetero atoms) bonds in the plethora
of synthetic transformations.3 Such functionalization reactions are
favored by the chelation of Pd catalyst to various N or O donor
atoms containing substrates,4 which assist the cyclometallation
step and thus the regioselective functionalizations.
Various directing group containing substrates including aryl-
pyrimidine have been well explored toward palladium catalyzed
oxidative CeO bond formation in particular o-hydroxylation or o-
acetoxylation.5 2-Arylbenzoxazole or benzothiazole possessing o-
chelating moiety could provide a similar ligand-directed cyclo-
metalated intermediate whereby the metal can be selectively de-
livered to a proximal CeH bond.6 However, they have rarely being
explored toward CeH functionalizations. There are only two in-
stances where the 2-arylbenzoxazole moiety been used, one to-
ward o-arylation7 and the other o-acetoxylation.8 Very recently
benzothiazoles have been o-arylated via proximal CeH function-
alization.9 Benzothiazoles are important heterocycle prevalent in
moiety would pave the path to the useful intermediates that may
find applications in medicinal chemistry11 and material science,12
particularly as sensors.13 With this objective we envisioned that
the 2-arylbenzothiazoles in the presence of transition metal, such
as Pd would undergo CeH functionalizations at the proximal site to
provide the desired functionality. Herein, we report our results on
the Pd(II) catalyzed o-hydroxylation/o-acetoxylation of various
benzothiazoles.
2. Results and discussion
To execute the above envisaged strategy, the initial experi-
mentation was performed with 2-phenylbenzothiazole (1) as the
model substrate. Treatment of substrate (1) with Pd(OAc)2
(2 mol %), OxoneÒ (1 equiv) in AcOH at 110 ꢀC provided the o-hy-
droxylated product (1a) rather than the expected o-acetoxylated
product but in a mere yield of 20% (Table 1, entry 1). A survey of the
literature on such directing group assisted hydroxylation/acetox-
ylation entail that Pd(OAc)2/DIB in acetic acid is one of the efficient
combination in producing either the o-acetoxylated or o-hydrox-
ylated product depending upon the reaction conditions.5 Thus, in
order to further improve the yield, the substrate (1) was subjected
to treatment under a similar reaction condition i.e., Pd(OAc)2
(2 mol %), DIB (1 equiv) in acetic acid (Table 1, entry 2). Encour-
agingly, an improvement in yield (58%) was observed under this
condition. Upon further increase in the catalyst quantity (3 mol %)
the yield improved up to 73% (Table 1, entry 3). It was gratifying to
* Corresponding author. E-mail address: patel@iitg.ernet.in (B.K. Patel).
0040-4020/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2012.12.067

2176
A. Banerjee et al. / Tetrahedron 69 (2013) 2175e2183
Table 1
Screening of reaction conditions
HO
HO
H
N
S
N
N
S
Catalyst, Solvent
+
Oxidant, 110 oC
S
(1)
AcO
Major (1a)
Minor (1b)
Entry
Catalyst (mol %)
Solvent
Oxidant
Time (h)
Yield (%)a,b (1a:1b)
1
2
3
4
5
6
7
8
Pd(OAc)2 (2.0)
Pd(OAc)2 (2.0)
Pd(OAc)2 (3.0)
Pd(OAc)2 (5.0)
Pd(OAc)2 (10.0)
Pd(OAc)2 (5.0)
Pd(OAc)2 (5.0)
Pd(OAc)2 (5.0)
PdCl2 (5.0)
AcOH
AcOH
AcOH
AcOH
AcOH
AcOH
AcOH
AcOH
AcOH
AcOH
AcOH
AcOH
TFA
OxoneÒ
DIB
DIB
DIB
DIB
PhIO
BTI
HTIB
DIB
DIB
DIB
5.5
5.0
5.0
5.0
6.0
5.0
6.0
6.0
6.0
6.0
6.5
6.5
6.0
5.0
6.0
5.0
5.0
5.0
20 (95:5)
58 (85:15)
73 (88:12)
90 (90:10)
91 (90:10)
65 (100:0)
70 (100:0)
80 (98:2)
70 (95:5)
85 (93:7)
45 (100:0)
50 (100:0)
92 (97:3)
85 (100:0)
10 (100:0)
00
9
10
11
12
13
14
15
16
17
18
PdBr2 (5.0)
Cu(OTf)2 (10.0)
Cu(OAc)2 (10.0)
Pd(OAc)2 (10.0)
Pd(OAc)2 (5.0)
Pd(OAc)2 (5.0)
Pd(OAc)2 (5.0)
Pd(OAc)2 (5.0)
Pd(OAc)2 (5.0)
DIB
DIB
TFA
OxoneÒ
DIB
Toluene
1,4-Dioxane
DMF
DIB
DIB
DIB
00
00
DMSO
Bold represents optimized condition which was used for all subsequent reactions.
a
Combined isolated yield of major hydroxylated (1a) and minor bifunctionalized product (1b).
All reactions were performed under air atmosphere.
b
observe that under the aforesaid reaction condition with a catalyst
p-OBu (5), 2,5-diOMe (6), 2,4-diOMe (7), and 3,4-diOMe (8) (Table
2) in the 2-aryl ring underwent smooth reaction to afford the cor-
responding hydroxylated products. The structure of the product
(2a) has been further confirmed by X-ray crystallography as shown
in Fig. 1.
load of 5 mol %, a high yield (90%) was obtained (Table 1, entry 4).
There was no further substantial improvement in the yield even
when a 10 mol % of the catalyst was used (Table 1, entry 5). With
this initial condition in hand other reaction parameters, such as
catalysts, oxidants, solvents, and temperature were varied to check
if there was any further improvement in the yield. The results are
summarized in Table 1.
Similar results were obtained with substrates bearing electron-
withdrawing groups, such as p-Cl (9), o-Cl (10), 2,4-diF (11), m-Br
(12), m-NO2 (13), and m-F (14) (Table 3). Notably, the electron-
donating groups on the 2-arylbenzothiazoles exhibited an accel-
erating effect on the reaction rate except on steric ground as in
substrates (6) and (7), while those with electron-withdrawing
substituents exerted a deactivating effect on the reaction rates.
However, irrespective of the nature of the substituents no major
differences in the yields of the corresponding products were ob-
served. Probably due to the chelation of Pd catalyst to the hydroxyl
group and the nitrogen, mostly monoselective o-hydroxylated
product was observed, which is consistent with the literature re-
port.15 However, an exception to this observation was the formation
of products bearing both o-hydroxyl and o-acetoxy groups probably
obtained through simultaneous CeO bond formation for certain
substrates (1, 4). Noteworthy to mention that for substrates (1) and
(4), the mono-hydroxylated product (1a and 4a) was obtained in
major amount while the hydroxyl-acetoxy product (1b and 4b) was
obtained in minor quantity. Similar o-diacetoxy products have been
reported in the presence of acetic acid using analogous substrate
directing moiety 2-phenyl pyridine.15 The formation of the partial
hydroxyl-acetoxy products (1b and 4b) suggest that the mono-
hydroxylated products (1ae14a) are obtained probably via hydro-
lysis of their corresponding mono acetoxylated products. Treat-
ment of separately synthesized acetoxylated product under the
reaction condition gave hydroxylated product supporting the hy-
drolytic path for hydroxylated product and very less amount of
hydroxyl-acetoxy product was also formed, which supports the
simultaneous two CeO bond formation for certain substrates (1, 4).
For substrates possessing substituent at their meta position (8,
12e14), exclusively a single regioisomer was obtained with the
hydroxylation occurring at the sterically less hindered ortho site.
As can be seen from Table 1 among the various oxidants tested,
such as iodoxybenzene (PhIO), bis-trifluoroacetoxyiodobenzene
(BTI), and hydroxytosyloxyiodobenzene (HTIB) (Table 1, entry
6e8), all gave inferior results as compared to DIB (diacetoxy-
iodobenzene) (Table 1, entry 4). The other palladium salts, such as
PdCl2 and PdBr2 were found to be less effective than Pd(OAc)2
(Table 1, entry 9e10). The use of Cu salts, such as Cu(OTf)2 and
Cu(OAc)2 in lieu of Pd(OAc)2 provided poorer yields even though
used in higher catalytic quantity (Table 1, entry 11e12). Notably, the
replacement of AcOH with trifluoroacetic acid (TFA) provided
a marginal improvement in the yield (92%) (Table 1, entry 13) but
the former was preferred due to cost consideration. Interestingly,
cheap oxidant OxoneÒ in combination with TFA (Table 1, entry 14)
gave a comparable yield (85%) as that of DIB/AcOH combination.
Other solvents, such as toluene, 1,4-dioxane, DMSO or DMF exam-
ined did not give satisfactory results (Table 1, entry 15e18) under
similar catalyst-oxidant combination. The reaction either did not
proceed or gave a poor yield of the desired product. The use of tert-
butanol as solvent gave only traces of product under the identical
reaction condition, although it was found to be an efficient solvent
during o-hydroxylation of 2-arylpyridine.5d Thus the combination
of Pd(OAc)2 as the catalyst, DIB as co-oxidant in acetic acid at 110 ꢀC
(Table 1, entry 4) gave most satisfactory yield, which was used for
subsequent reactions.
Having arrived at the optimized condition, we proceeded to
investigate the scope and generality of this methodology. The re-
sults illustrated in Table 2 attest that the methodology is compat-
ible to a wide array of functionalities. Substrates possessing
electron-donating groups, such as p-Me (2), p-OMe (3), p-CMe3 (4),

A. Banerjee et al. / Tetrahedron 69 (2013) 2175e2183
2177
Table 2
One-pot synthesis of 2-(20-hydroxyphenyl)benzothiazole
HO
HO
H
N
N
N
S
Pd(OAc)2, AcOH
+
DIB, 110 o
C
R1
R1
S
R1
S
AcO
(1-8)
(1-8a)
(1-8b)
R1 = EDG
Substrate
Product
HO
Time (h)
5
Yield (%)a, b
H
N
S
N
90 (9:1)
1a, X = H;
S
1b, X = OAc
1
X
H
H
H
HO
HO
N
S
N
S
4
85
Me
Me
2a
2
N
S
N
3
90
OMe
3
OMe
3a
S
HO
N
S
N
S
tBu
5
85 (4:1)
tBu
4a, X = H;
4
X
4b, X = OAc
H
H
HO
HO
N
S
N
S
4.5
80
OBu
5
OBu
5a
OMe
OMe
N
S
N
S
7
8
75
75
6a
6
MeO
H
MeO
HO
N
S
N
S
OMe
7
OMe
7a
MeO
H
MeO
HO
N
S
N
S
5
70
OMe
8
OMe
8a
OMe
OMe
a
Confirmed by IR, 1H NMR, and 13C NMR spectroscopy.
Yields after silica gel column chromatography.
b
The exclusive formation of single regioisomer is governed by the
steric effect of the substituent due to the preferential palladation at
the sterically less hindered position as has been demonstrated by
Sanford group.5g
This methodology was equally successful for substrates pos-
sessing electron-withdrawing as well as electron-donating sub-
stituents (15e18) in the benzothiazole moiety. Substrate (15)
having a eCl group in the benzothiazole moiety gave the expected
o-hydroxylated (15a) along with o-acetoxylated (15c) product in
the ratio of 2:3 after 5 h. This ratio remained unchanged even after
prolonged reaction time (12 h). An exception to the earlier trend,
substrate (16) having a eCF3 group in the benzothiazole moiety also
provided the hydroxy-acetoxy product (16b) exclusively rather
than the usual mono o-hydroxylated product. While substrate (17)
Fig. 1. ORTEP view of compound 2a.14

2178
A. Banerjee et al. / Tetrahedron 69 (2013) 2175e2183
Table 3
One-pot synthesis of 2-(20-hydroxyphenyl)benzothiazole
HO
HO
H
N
N
N
Pd(OAc)2, AcOH
+
DIB, 110 oC
R1
R1
S
R1
S
R2
S
(9-18)
R2
R2
AcO
(9-18a)
(9-18b)
R1 = EWG; R2 = EWG, EDG
Substrate
Product
Time (h)
Yield (%)a,b
80
H
H
HO
N
N
S
5
7
Cl
Cl
9
9a
S
HO
N
S
N
S
78
75
85
80
10
10a
Cl
Cl
H
HO
N
S
N
S
8
F
F
11
12
13
11a
12a
F
F
H
HO
N
S
N
S
4.5
Br
Br
H
H
HO
HO
N
S
N
S
8
13a
14a
NO2
NO2
N
S
N
S
6.5
85
14
F
F
X
H
Cl
N
S
Cl
N
S
5
5
85 (2:3)
15a, X = OH;
15c, X = OAc
15
HO
N
H
F3C
F3C
N
80
16b
S
16
17
S
AcO
H
HO
N
Me
Me
Me
Me
N
8
7
70
65
17a
18c
S
S
H
AcO
N
N
S
18
S
MeO
MeO
a
Confirmed by IR, 1H NMR, and 13C NMR spectroscopy.
Yields after silica gel column chromatography.
b
possessing two eMe groups in the benzothiazole ring gave hy-
droxylated product (17a) exclusively, the other substrate (18) pos-
sessing a eOMe group in the benzothiazole moiety gave o-
acetoxylated as the major product (18c). The reason behind the
resistant to hydrolysis to yield corresponding hydroxylated product
in the later case is not clear at this moment. Surprisingly no general
trend in their reactivity could be observed for substituents present
on the benzothiazole rings (15e18).

A. Banerjee et al. / Tetrahedron 69 (2013) 2175e2183
2179
However, 2-heteroarylbenzothiazoles having heterocycles, such
1.0
0.8
0.6
0.4
0.2
0.0
as pyridine, thiophene or furan were completely inert under the
present reaction condition providing no hydroxylated product. This
is probably due to the chelation of suitably placed hetero atoms N,
O, and S of pyridine, thiophene and furan, respectively, to Pd, which
renders the catalyst inactive.
3a
4a
14a
~336 nm
An insight into the mechanism could be had from some of the
basic observations of this methodology. This methodology worked
better for substrates containing electron-donating substituents in
the aryl ring compared to the substrates having electron-
withdrawing ones, suggesting the possible occurrence of an oxi-
dative addition of Pd to the aryl ring. The fact that the reaction
worked well with other non acetate bearing terminal oxidants,
such as PhIO, BTI, HTIB, and OxoneÒ in acetic acid there by sug-
gesting that DIB does not suffice as the major source of acetate
(hydroxyl functionality) and acetic acid is probably the source of
hydroxy functionality. The use of toluene as the solvent in lieu of
acetic acid with DIB as the terminal oxidant did not give satisfactory
result, which further supports our assumption. These observations
imply that acetic acid is possibly the major source of the hydroxyl
functionality and DIB acts merely as an oxidant. Keeping in view the
above observations and earlier literature reports5deg a plausible
mechanism has been proposed (Scheme 1). This reaction is ex-
pected to occur via a Pd(II)/Pd(IV) catalytic cycle involving the
following steps: (i) the Pd(II) precursor coordinates to the substrate
followed by a cyclopalladation step to afford the intermediate (I);
(ii) two electron oxidation employing DIB to generate the Pd(IV)
species (II); (iii) reductive elimination of the metal resulting the
CeO bond formation and concomitant release of the o-acetoxy
product; (iv) a ligand exchange step releases the catalyst for next
catalytic cycle. The acetoxylated product thus produced undergoes
an in situ hydrolysis to give the hydroxyl functionality (Scheme 1).
~289 nm
200
300
400
500
Wavelength (nm)
Fig. 2. UV-visible spectra of compounds (3a, 4a, 14a).
400
350
300
250
200
150
100
50
3a
4a
14a
~ 378 nm
H
N
S
Pd(OAc)2
Pd(OAc)2
0
AcO
350
400
450
500
550
N
S
Wavelength (nm)
Reductive
Cyclopalladation
elimination
AcOH
Fig. 3. Photoluminescence spectra of compounds (3a, 4a, 14a).
Hydrolysis
S
N
S
N
benzothiazoles form the basis of many pharmacophores and there
have not been many reports on the exploitation of this moiety.
Besides, the performance under ambient air and moisture, a broad
scope and wide array of functional group tolerance and exhibition
of high levels of regioselectivity pertaining to the meta-
substituted substrates are the remarkable features of this meth-
odology. Further applications of this synthetic methodology are
currently underway.
HO
IV
N
II
AcO Pd OAc
Oxidative
addition
Pd
O
O
O
S
O
(II)
(I)
PhI(OAc)2, AcOH
Scheme 1. Pd (II) catalyzed formation 2-(benzothiazol-2-yl)-phenol (1a) from 2-
phenylbenzothiazole (1).
These hydroxylated derivatives are sufficiently fluorescent
active. Photoluminescent and UV data of some representative
compounds have also been recorded. Substrates possessing
electron-donating groups, such as p-OMe (3), p-CMe3 (4), and
electron-withdrawing group m-F (14) in the 2-aryl ring show their
absorption bands at around 289 nm and 336 nm in ethanol solvent
(Fig. 2). Upon excitation at 336 nm all three compounds show their
excitation band at around 378 nm (Fig. 3). These bidentate fluo-
rescent compounds may find potential application as various sen-
sors and in material science as LED.12,13
4. Experimental section
4.1. General remarks
Unless otherwise stated, all reagents were purchased from com-
mercial sources and used without further purification. Reaction
progress wasmonitored by TLC using silicagel 60F254 (0.25 mm)with
detection by UV or iodine. Chromatography was performed using
silica gel (60e120) mesh size with freshly distilled solvents. Columns
were typically packed as slurry and equilibrated with the appropriate
solvent system prior to use.1H NMR (400 MHz) and 13C NMR
(100 MHz) spectra were recorded on FT-400 MHz instrument
using TMS as an internal standard. Data are presented as follows:
chemical shift (ppm), multiplicity (s¼singlet, d¼doublet, t¼triplet,
quin¼quintet, m¼multiplet, b¼broad, br s¼broad singlet, br
3. Conclusions
In conclusion we have developed an efficient protocol to access
the hydroxylated benzothiazoles via a palladium catalyzed CeH
functionalization/CeO bond formation. The functionalized

2180
A. Banerjee et al. / Tetrahedron 69 (2013) 2175e2183
m¼broad multiplet), coupling constant J (Hertz). Elemental analyses
were carried out on a PerkineElmer 2400 elemental analyzer. Melt-
ing points were recorded and are uncorrected. IR spectra were
recorded in KBr or neat. UV-visible spectra were recorded by dis-
solving calculated amount of the sample in an appropriate solvent on
a PerkineElmer Lambda UV-visible spectrophotometer. Photo-
luminescence measurements were done using a Varian Cary Eclipse
fluorescence spectrophotometer. For both UV-visible and florescence
studies, solutions of hydroxylated derivatives (3a, 4a, and 14a) were
made in 1:1 ethanol and water mixed solvent. Concentration of
monomer was maintained at 20ꢁ10ꢂ6 M for UV-visible studies, and it
was maintained at around 20ꢁ10ꢂ6 M for photoluminescence stud-
ies. The 2-arylbenzothiazoles were prepared from corresponding N-
phenylbenzothioamides using hypervalent iodine mediated oxida-
tive cyclization following the reported procedure.16
3H, eOCOCH3) ppm; 13C NMR (CDCl3, 100 MHz):
160.3, 149.5, 148.9, 135.5, 130.5, 125.6, 123.9, 122.2, 121.5, 116.0,
115.7, 113.5, 21.9 ppm; Elemental analysis: found C, 63.20; H, 3.95;
N, 4.86. C15H11NO3S requires C, 63.14; H, 3.89; N, 4.91%.
d¼169.4, 166.0,
4.2.3. 2-(Benzo[d]thiazole-2-yl)-5-methyl-phenol (2a). The above
general procedure was followed. The product was purified by col-
umn chromatography (5% EtOAc/hexane) to give the title com-
pound 2a (205 mg, 85%) as white solid; Rf (5% EtOAc/hexane) 0.65;
mp 140e141 ꢀC; nmax (KBr): 3435, 2919,1628,1577,1479,1434,1377,
1313, 1256, 1221, 1108, 960, 935, 878, 781, 758, 728 cmꢂ1; 1H NMR
(CDCl3, 400 MHz):
d
¼12.45 (1H, s, Ar. CeOH), 7.95 (1H, d, J¼8.0 Hz,
Ar. CeH), 7.88 (1H, d, J¼8.0 Hz, Ar. CeH), 7.56 (1H, d, J¼8.0 Hz, Ar.
CeH), 7.49 (1H, t, J¼7.2 Hz, Ar. CeH), 7.38 (1H, t, J¼8.0 Hz, Ar. CeH),
6.92 (1H, s, Ar. CeH), 6.77 (1H, d, J¼8.0 Hz, Ar. CeH), 2.37 (3H, s,
AreCH3) ppm; 13C NMR (CDCl3, 100 MHz):
d¼169.6, 158.1, 152.1,
4.2. General procedure for the synthesis of substituted 2-
(benzo[d]thiazole-2-yl)-phenol from substituted 2-
phenylbenzo[d]thiazole (1e18)
144.0, 132.7, 128.5, 126.8, 125.5, 122.2, 121.7, 120.9, 118.3, 114.6,
21.9 ppm; HRMS (ESI) MHþ, found 242.0630. C14H11NOS requires
242.0634. Elemental analysis: found C, 69.74; H, 4.65; N, 5.75.
C14H11NOS requires C, 69.68; H, 4.59; N, 5.80%.
An oven-dried flask was charged with Pd(OAc)2 (0.05 mmol),
diacetoxyiodobenzene (DIB) (1.0 mmol), substituted 2-phenylbenzo
[d]thiazole (1e18) (1.0 mmol), and acetic acid (2 mL). Then the re-
action vessel was kept in an oil bath preheated to 110 ꢀC and allowed
to stir for the stipulated period of time. The progress of the reaction
was monitored by TLC. During this period complete disappearance of
substituted 2-phenylbenzo[d]thiazole was observed with the ap-
pearance of one major product having higher Rf [in some cases (1b,
4b,16b,15c,18c)] another product was also observed having lower Rf
with respect to the substrate. The reaction mixture was then cooled
and admixed withwater (5 mL). The product wasextractedwithethyl
acetate (2ꢁ10 mL) and the combined organic layer was washed with
saturated sodium bicarbonate (NaHCO3) solution. Organic layer was
dried over anhydrous sodium sulfate (Na2SO4) and evaporated under
reduced pressure. The crude product so obtained was further purified
through silica gel column chromatography (EtOAc/hexane) to yield
the pure product. The identity and purity of the products were con-
firmed by spectroscopic analysis.
4.2.4. 2-(Benzo[d]thiazole-2-yl)-5-methoxy-phenol (3a). The general
procedure was followed. The product was purified by column
chromatography (5% EtOAc/hexane) to give the title compound 3a
(231 mg, 90%) as white solid; Rf (5% EtOAc/hexane) 0.61; mp 130 ꢀC;
nmax (KBr): 3775, 3435, 3058, 2957, 2929, 2842, 1632, 1585, 1530,
1480, 1439, 1393, 1338, 1315, 1282, 1222, 1186, 1164, 1146, 1125, 1029,
976, 951, 852, 755, 727 cmꢂ1
;
1H NMR (CDCl3, 400 MHz):
d
¼12.72
(1H, s, Ar. CeOH), 7.89 (1H, d, J¼8.4 Hz, Ar. CeH), 7.81 (1H, d,
J¼8.0 Hz, Ar. CeH), 7.52 (1H, d, J¼8.8 Hz, Ar. CeH), 7.44 (1H, t,
J¼7.4 Hz, Ar. CeH), 7.33 (1H, t, J¼7.2 Hz, Ar. CeH), 6.56 (1H, s, Ar.
CeH), 6.49 (1H, d, J¼8.8 Hz, Ar. CeH), 3.82 (3H, s, AreCH3) ppm; 13
C
NMR (CDCl3, 100 MHz):
d
¼169.4, 163.6, 160.1, 152.0, 132.3, 129.8,
126.7, 125.2, 121.8, 121.5, 110.6, 107.8, 101.5, 55.6 ppm; Elemental
analysis: found C, 65.42; H, 4.38; N, 5.39. C14H11NO2S requires C,
65.35; H, 4.31; N, 5.44%.
4.2.5. 2-(Benzo[d]thiazole-2-yl)-5-tert-butyl-phenol (4a). The above
general procedure was followed. The product was purified by col-
umn chromatography (5% EtOAc/hexane) to give the title compound
4a (192 mg, 68%) as white solid; Rf (5% EtOAc/hexane) 0.64; mp
126 ꢀC; nmax (KBr): 3439, 3049, 2958, 2862, 2697, 1629, 1568, 1437,
1367, 1314, 1281, 1219, 1199, 1098, 1012, 976, 925, 872, 856, 810, 774,
4.2.1. 2-(Benzo[d]thiazole-2-yl)-phenol (1a). The above general
procedure was followed. The product was purified by column
chromatography (5% EtOAc/hexane) to give the title compound 1a
(184 mg, 81%) as white solid; Rf (5% EtOAc/hexane) 0.66; mp
120e122 ꢀC; nmax (KBr): 3781, 3433, 3053, 2952, 2923, 2852, 1893,
1781, 1616, 1589, 1479, 1434, 1401, 1314, 1257, 1219, 1095, 1032, 972,
728 cmꢂ1
;
1H NMR (CDCl3, 400 MHz):
d
¼12.32 (1H, s, Ar. CeOH),
7.85 (1H, d, J¼8.0 Hz, Ar. CeH), 7.74 (1H, d, J¼8.0 Hz, Ar. CeH), 7.49
(1H, d, J¼8.4 Hz, Ar. CeH), 7.37 (1H, t, J¼7.2 Hz, Ar. CeH), 7.26 (1H, t,
J¼7.2 Hz, Ar. CeH), 7.05 (1H, s, Ar. CeH), 6.89 (1H, d, J¼8.4 Hz, Ar.
CeH), 1.26 (9H, s, 3ꢁ AreCH3) ppm; 13C NMR (CDCl3, 100 MHz):
817, 742 cmꢂ1
;
1H NMR (CDCl3, 400 MHz):
d
¼12.51 (1H, s, Ar.
CeOH), 7.98 (1H, d, J¼8.0 Hz, Ar. CeH), 7.88 (1H, d, J¼8.0 Hz, Ar.
CeH), 7.70e7.67 (1H, m, Ar. CeH), 7.51e7.47 (1H, m, Ar. CeH),
7.41e7.37 (2H, m, Ar. CeH), 7.10 (1H, d, J¼8.0 Hz, Ar. CeH), 6.95 (1H,
d
¼169.4, 157.9, 157.1, 152.1, 132.6, 128.2, 126.7, 125.4, 122.1, 121.6,
t, J¼7.6 Hz, Ar. CeH) ppm; 13C NMR (CDCl3, 100 MHz):
d¼169.6,
117.3, 114.8, 114.4, 35.2, 31.2 ppm; HRMS (ESI) MHþ, found 284.1103.
C17H17NOS requires 284.1104. Elemental analysis: found C, 72.08; H,
6.11; N, 4.90. C17H17NOS requires C, 72.05; H, 6.05; N, 4.94%.
158.2, 152.1, 137.7, 132.9, 130.4, 128.6, 126.9, 125.7, 122.4, 121.7, 119.7,
118.1 ppm; HRMS (ESI): MHþ, found 228.0475. C13H9NOS requires
228.0478. Elemental analysis: found C, 68.76; H, 4.04; N, 6.11.
C13H9NOS requires C, 68.70; H, 3.99; N, 6.16%.
4.2.6. 5-tert-Butyl-2-(benzo[d]thiazol-2-yl)-3-hydroxyphenyl
ace-
tate (4b). The above general procedure was followed. The product
was purified by column chromatography (30% EtOAc/hexane) to
give the title compound 4b (62 mg, 17%) as yellow solid; Rf (30%
EtOAc/hexane) 0.25; mp 94e95 ꢀC; nmax (KBr): 3434, 3060, 2954,
2923, 2857, 1776, 1635, 1571, 1454, 1434, 1384, 1316, 1286, 1234,
1223, 1183, 1111, 1075, 1057, 1020, 974, 921, 891, 876, 847, 822, 752,
4.2.2. 2-(Benzo[d]thiazol-2-yl)-3-hydroxyphenyl acetate (1b). The
above general procedure was followed. The product was purified by
column chromatography (30% EtOAc/hexane) to give the title
compound 1b (26 mg, 9%) as yellow solid; Rf (30% EtOAc/hexane)
0.26; mp 132e133 ꢀC; nmax (KBr): 3384, 3054, 2921, 2846, 1773,
1635,1577,1473,1434,1369,1314,1278, 1248,1186,1102,1056,1031,
1012, 968, 907, 886, 853, 816, 796, 757, 728 cmꢂ1; 1H NMR (CDCl3,
725 cmꢂ1
;
1H NMR (CDCl3, 400 MHz):
d
¼13.63 (1H, s, Ar. CeOH),
8.02 (1H, d, J¼8.4 Hz, Ar. CeH), 7.92 (1H, d, J¼8.0 Hz, Ar. CeH), 7.53
(1H, t, J¼7.6 Hz, Ar. CeH), 7.43 (1H, t, J¼7.6 Hz, Ar. CeH), 7.05 (1H, s,
Ar. CeH), 6.76 (1H, s, Ar. CeH), 2.56 (3H, s, eOCOCH3), 1.33 (9H, s,
400 MHz):
d
¼13.82 (1H, s, Ar. CeOH), 8.04 (1H, d, J¼8.8 Hz, Ar.
CeH), 7.95 (1H, d, J¼8.4 Hz, Ar. CeH), 7.55 (1H, t, J¼8.4 Hz, Ar. CeH),
7.46 (1H, t, J¼8.0 Hz, Ar. CeH), 7.37 (1H, t, J¼7.2 Hz, Ar. CeH), 7.03
(1H, d, J¼8.0 Hz, Ar. CeH), 6.76 (1H, d, J¼8.4 Hz, Ar. CeH), 2.57 (s,
3ꢁ AreCH3) ppm; 13C NMR (CDCl3,100 MHz):
¼168.9,163.9, 159.8,
d
156.7, 149.7, 148.7, 132.9, 126.9, 125.7, 122.1, 121.3, 112.7, 111.3, 108.2,

A. Banerjee et al. / Tetrahedron 69 (2013) 2175e2183
2181
35.4, 31.0, 22.4 ppm; HRMS (ESI) MHþ, found 342.1156. C19H19NO3S
requires 342.1158. Elemental analysis: found: C, 66.92; H, 5.65; N,
4.04. C19H19NO3S requires C, 66.84; H, 5.61; N, 4.10%.
154.4, 153.7, 152.2, 142.8, 132.3, 126.8, 125.1, 121.8, 121.6, 110.4, 108.3,
101.2, 56.9, 56.2 ppm; HRMS (ESI) MHþ, found 288.0687. C15H13NO3S
requires 288.0689. Elemental analysis: found: C, 62.74; H, 4.62; N,
4.84. C15H13NO3S requires C, 62.70; H, 4.56; N, 4.87%.
4.2.7. 2-(Benzo[d]thiazole-2-yl)-5-butoxy-phenol (5a). The above
general procedure was followed. The product was purified by col-
umn chromatography (5% EtOAc/hexane) to give the title com-
pound 5a (239 mg, 80%) as white solid; Rf (5% EtOAc/hexane) 0.60;
mp 107e108 ꢀC; nmax (KBr): 3434, 3054, 2954, 2938, 2869, 1633,
1583, 1483, 1472, 1439, 1388, 1334, 1317, 1282, 1257, 1241, 1218, 1197,
1188, 1120, 1072, 1038, 1011, 980, 961, 933, 902, 837, 807, 755, 726,
4.2.11. 2-(Benzo[d]thiazol-2-yl)-5-chlorophenol (9a). The above
general procedure was followed. The product was purified by col-
umn chromatography (5% EtOAc/hexane) to give the title compound
9a (209 mg, 80%) as white solid; Rf (5% EtOAc/hexane) 0.65; mp
107e108 ꢀC; nmax (KBr): 3775, 3435, 3047, 2922, 2851, 1907, 1616,
1573,1504,1474,1454,1434,1399,1370,1315,1249,1204,1088,1012,
700, 648 cmꢂ1
;
1H NMR (CDCl3, 400 MHz):
d
¼12.67 (1H, s, Ar.
973, 903, 829, 800, 756, 730 cmꢂ1 1H NMR (CDCl3, 400 MHz):
;
CeOH), 7.90 (1H, d, J¼8.0 Hz, Ar. CeH), 7.83 (1H, d, J¼8.0 Hz, Ar.
CeH), 7.53 (1H, d, J¼8.8 Hz, Ar. CeH), 7.45 (1H, t, J¼7.6 Hz, Ar. CeH),
7.33 (1H, t, J¼7.6 Hz, Ar. CeH), 6.57 (1H, s, Ar. CeH), 6.50 (1H, d,
J¼8.8 Hz, Ar. CeH), 3.99 (2H, t, J¼6.4 Hz, aliphaticeOCH2e),
1.79e1.74 (2H, m, aliphaticeCH2e), 1.52e1.47 (2H, m,
aliphaticeCH2e), 0.98 (3H, t, J¼7.2 Hz, aliphaticeCH3) ppm; 13C
d
¼12.68 (1H, s, Ar. CeOH), 8.04 (1H, d, J¼8.0 Hz, Ar. CeH), 7.97 (1H,
d, J¼8.8 Hz, Ar. CeH), 7.84 (1H, t, J¼8.8 Hz, Ar. CeH), 7.51e7.43 (1H,
m, Ar. CeH), 7.42 (1H, d, J¼8.8 Hz, Ar. CeH), 7.38e7.34 (1H, m, Ar.
CeH), 7.07 (1H, s, Ar. CeH) ppm; 13C NMR (CDCl3, 100 MHz):
d
¼168.5, 158.8, 154.2, 137.2, 135.2, 132.2, 128.8, 126.6, 125.5, 123.4,
121.6, 120.1, 118.1 ppm; Elemental analysis: found: C, 59.70; H, 3.12;
N, 5.30. C13H8ClNOS requires C, 59.66; H, 3.08; N, 5.35%.
NMR (100 MHz, CDCl3):
d
¼169.5, 163.2, 160.1, 152.0, 132.3, 129.7,
126.7, 125.1, 121.8, 121.5, 110.4, 108.2, 101.9, 68.1, 31.3, 19.4,
14.0 ppm; HRMS (ESI) MHþ, found 300.1051 C17H17NO2S requires
300.1053. Elemental analysis: found: C, 68.25; H, 5.76; N, 4.63.
C17H17NO2S requires C, 68.20; H, 5.72; N, 4.68%.
4.2.12. 2-(Benzo[d]thiazole-2-yl)-3-chloro-phenol (10a). The above
general procedure was followed. The product was purified by col-
umn chromatography (5% EtOAc/hexane) to give the title com-
pound 10a (203 mg, 78%) as yellowish white solid; Rf (5% EtOAc/
hexane) 0.63; mp 141e142 ꢀC; nmax (KBr): 3439, 3053, 2923, 2851,
2666, 1910, 1832, 1567, 1465, 1432, 1370, 1311, 1269, 1244, 1215,
1180, 1155, 1060, 966, 900, 827, 783, 757, 725 cmꢂ1; 1H NMR (CDCl3,
4.2.8. 2-(Benzo[d]thiazole-2-yl)-3,6-dimethoxy-phenol (6a). The above
general procedure was followed. The product was purified by column
chromatography (5% EtOAc/hexane) to give the title compound 6a
(215 mg, 75%) as brown solid; Rf (5% EtOAc/hexane) 0.67; mp
157e159 ꢀC; nmax (KBr): 3432, 3081, 3047, 2997, 2924, 2845, 2672,
2571, 2044, 1904, 1787, 1756, 1605, 1591, 1502, 1471, 1438, 1418, 1337,
400 MHz):
d
¼7.94 (1H, d, J¼8.0 Hz, Ar. CeH), 7.85 (1H, d, J¼8.0 Hz,
Ar. CeH), 7.45 (1H, t, J¼7.6 Hz, Ar. CeH), 7.36 (1H, t, J¼8.8 Hz, Ar.
CeH), 7.19 (1H, t, J¼8.2 Hz, Ar. CeH), 7.02 (1H, d, J¼8.4 Hz, Ar. CeH),
6.97 (1H, d, J¼8.0 Hz, Ar. CeH) ppm; 13C NMR (100 MHz, CDCl3):
1314, 1247, 1237, 1163, 1118, 1093, 1009, 853, 757 cmꢂ1
;
1H NMR
(CDCl3, 400 MHz):
d
¼7.98 (1H, d, J¼8.0 Hz, Ar. CeH), 7.90 (1H, d,
d
¼166.1, 160.9, 148.6, 133.4, 132.9, 131.7, 126.8, 125.9, 121.8, 121.5,
J¼8.0 Hz, Ar. CeH), 7.49 (1H, t, J¼7.6 Hz, Ar. CeH), 7.38 (1H, t, J¼7.2 Hz,
Ar. CeH), 6.92 (1H, d, J¼8.8 Hz, Ar. CeH), 6.39 (1H, d, J¼9.2 Hz, Ar.
CeH), 3.98 (3H, s, AreCH3), 3.90 (3H, s, AreCH3) ppm; 13C NMR
121.40, 121.1, 117.3 ppm; Elemental analysis: found: C, 59.72; H,
3.13; N, 5.31. C13H8ClNSO requires C, 59.66; H, 3.08; N, 5.35%.
(100 MHz, CDCl3):
d
¼165.5, 152.1, 150.7, 149.1, 143.3, 133.6, 126.5, 125.1,
4.2.13. 2-(Benzo[d]thiazol-2-yl)-3,5-difluorophenol
(11a). The
121.6, 121.1, 115.1, 107.9, 99.6, 57.1, 56.0 ppm; Elemental analysis:
found: C, 62.74; H, 4.61; N, 4.86. C15H13NO3S requires C, 62.70; H, 4.56;
N, 4.87%.
above general procedure was followed. The product was purified by
column chromatography (5% EtOAc/hexane) to give the title com-
pound 11a (197 mg, 75%) as yellow solid; Rf (5% EtOAc/hexane)
0.68; mp 100ꢂ101 ꢀC; nmax (KBr): 3417, 3054, 2917, 2846, 2521,
1901, 1777, 1648, 1598, 1593, 1508, 1472, 1435, 1392, 1315, 1263,
1225, 1175, 1125, 1103, 1053, 1001, 979, 815, 754, 724, 701,
4.2.9. 2-(Benzo[d]thiazole-2-yl)-3,5-dimethoxy-phenol
(7a). The
above general procedure was followed. The product was purified by
column chromatography (5% EtOAc/hexane) to give the title com-
pound 7a (215 mg, 75%) as yellow solid; Rf (5% EtOAc/hexane) 0.64;
mp 119e120 ꢀC; nmax (KBr): 3433, 2961, 2924, 2851, 2472, 1610,
1594,1466,1437, 1389, 1345, 1316, 1286, 1262, 1231, 1216, 1198, 1151,
628 cmꢂ1
;
1H NMR (CDCl3, 400 MHz):
d
¼7.97 (1H, d, J¼8.0 Hz, Ar.
CeH), 7.90 (1H, d, J¼8.0 Hz, Ar. CeH), 7.53e7.48 (1H, m, Ar. CeH),
7.43e7.39 (1H, m, Ar. CeH), 6.64e6.60 (1H, m, Ar. CeH),
6.50e6.44(1H, m, Ar. CeH) ppm; 13C NMR (CDCl3, 100 MHz):
1121, 1048, 964, 932, 825, 800, 752, 724 cmꢂ1
400 MHz):
;
1H NMR (CDCl3,
d 165.9, 163.5, 163.3, 163.2, 163.0, 162.9, 162.2, 161.6, 161.5, 161.4,
d
¼7.93 (1H, d, J¼8.4 Hz, Ar. CeH), 7.89 (1H, d, J¼7.6 Hz,
160.6, 160.5, 160.3, 160.2, 152.6, 149.3, 135.6, 135.5, 133.0, 132.9,
131.3,131.2,126.9,126.5,125.8,125.5,123.4,121.9,121.6,121.4,112.6,
112.4, 105.0, 104.7, 104.5, 101.3, 101.0, 95.7, 95.4, 95.1 ppm; HRMS
(ESI) MHþ, found 264.0285. C13H7F2NOS requires 264.0289. Ele-
mental analysis: found: C, 59.36; H, 2.75; N, 5.27. C13H7F2NOS re-
quires C, 59.31; H, 2.68; N, 5.32%.
Ar. CeH), 7.47 (1H, t, J¼7.6 Hz, Ar. CeH), 7.35 (1H, t, J¼7.2 Hz, Ar.
CeH), 6.30 (1H, s, Ar. CeH), 6.11 (1H, s, Ar. CeH), 4.02 (3H, s,
AreOCH3), 3.86 (3H, s, AreOCH3) ppm; 13C NMR (100 MHz, CDCl3):
d
¼165.5, 163.6, 162.2, 159.4, 149.4, 133.2, 126.3, 124.5, 122.5, 121.1,
101.7, 94.2, 90.7, 55.7 ppm; Elemental analysis: found: C, 62.72; H,
4.59; N, 4.84. C15H13NO3S requires C, 62.70; H, 4.56; N, 4.87%.
4.2.14. 2-(Benzo[d]thiazole-2-yl)-4-bromo-phenol (12a). The above
general procedure was followed. The product was purified by col-
umn chromatography (5% EtOAc/hexane) to give the title com-
pound 12a (259 mg, 85%) as yellowish white solid; Rf (5% EtOAc/
hexane) 0.61; mp 153e155 ꢀC; nmax (KBr): 3848, 3445, 2913, 1618,
1572, 1477, 1434, 1370, 1297, 1267, 1239, 1207, 1082, 979, 756,
4.2.10. 2-(Benzo[d]thiazole-2-yl)-4,5-dimethoxy-phenol
(8a). The
above general procedure was followed. The product was purified by
column chromatography (5% EtOAc/hexane) to give the title com-
pound 8a (201 mg, 70%) as yellow solid; Rf (5% EtOAc/hexane) 0.60;
mp 148e149 ꢀC; nmax (KBr): 3431, 2919, 2849, 2649,1709,1633,1601,
1528, 1505, 1468, 1434, 1358, 1351, 1314, 1272, 1245, 1203, 1185, 1154,
1130, 1037, 1012, 956, 882, 827, 807, 752, 721 cmꢂ1; 1H NMR (CDCl3,
721 cmꢂ1
;
1H NMR (CDCl3, 400 MHz):
d
¼12.55 (1H, s, Ar. CeOH),
7.99 (1H, d, J¼8.0 Hz, Ar. CeH), 7.91 (1H, d, J¼8.0 Hz, Ar. CeH), 7.78
(1H, s, Ar. CeH), 7.52 (1H, t, J¼7.2 Hz, Ar. CeH), 7.45e7.41 (2H, m, Ar.
CeH), 6.99 (1H, d, J¼8.8 Hz, Ar. CeH) ppm; 13C NMR (CDCl3,
400 MHz):
d
¼7.93 (1H, d, J¼8.4 Hz, Ar. CeH), 7.87 (1H, d, J¼8.0 Hz, Ar.
CeH), 7.48 (1H, t, J¼7.2 Hz, Ar. CeH), 7.37 (1H, t, J¼8.4 Hz, Ar. CeH),
7.04 (1H, s, Ar. CeH), 6.63 (1H, s, Ar. CeH), 3.94 (3H, s, AreOCH3),
100 MHz):
d
¼168.0, 157.2, 151.8, 135.5, 132.8, 130.7, 127.1, 126.2,
3.92 (3H, s, AreOCH3) ppm; 13C NMR (100 MHz, CDCl3):
d¼169.2,
122.6, 121.8, 120.0, 118.5, 111.2 ppm; Elemental analysis: found: C,

2182
A. Banerjee et al. / Tetrahedron 69 (2013) 2175e2183
51.08; H, 2.69; N, 4.50. C13H8BrNOS requires C, 51.00; H, 2.63; N,
4.57%.
3423, 3345, 3224, 2925, 2851, 2402, 1761, 1686, 1630, 1589, 1466,
1371, 1322, 1277, 1179, 1022, 965, 887, 789, 721, 701 cmꢂ1; 1H NMR
(CDCl3, 400 MHz):
d
¼13.41 (1H, s, Ar. CeOH), 8.29(1H, s, Ar. CeH),
4.2.15. 2-(Benzo[d]thiazole-2-yl)-4-nitro-phenol (13a). The above
general procedure was followed. The product was purified by col-
umn chromatography (5% EtOAc/hexane) to give the title com-
pound 13a (218 mg, 80%) as white solid; Rf (5% EtOAc/hexane) 0.66;
mp 197e199 ꢀC; nmax (KBr): 3434, 3065, 2924, 2853, 2428, 1736,
1626, 1578, 1525, 1485, 1382, 1331, 1278, 1237, 1215, 1104, 984, 902,
8.04 (1H, d, J¼8.4 Hz, Ar. CeH), 7.67 (1H, d, J¼8.8 Hz, Ar. CeH), 7.39
(1H, t, J¼8.2 Hz, Ar. CeH), 7.02 (1H, d, J¼8.4 Hz, Ar. CeH), 6.77 (1H,
d, J¼8.0 Hz, Ar. CeH), 2.56 (3H, s, eOCOCH3) ppm; 13C NMR (CDCl3,
100 MHz):
d
¼168.5, 166.0, 160.3, 149.3, 149.1, 136.4, 132.8, 130.5,
122.2, 122.0, 119.4, 115.8, 113.6, 110.2, 22.3 ppm; HRMS (ESI) MHþ,
found 354.0405. C16H10F3NO3S requires 354.0406. Elemental
analysis: found: C, 54.45; H, 2.92; N, 3.90. C16H10F3NO3S requires C,
54.39; H, 2.85; N, 3.96%.
886, 808, 747 cmꢂ1; 1H NMR (CDCl3, 400 MHz):
d¼13.52 (1H s, Ar.
CeOH), 8.64 (1H, s, Ar. CeH), 8.28e8.25 (1H, m, Ar. CeH), 8.04 (1H,
d, J¼8.0 Hz, Ar. CeH), 7.97 (1H, d, J¼8.0 Hz, Ar. CeH), 7.60e7.56 (1H,
m, Ar. CeH), 7.52e7.48 (1H, m, Ar. CeH), 7.19 (1H, d, J¼9.2 Hz, Ar.
4.2.20. 2-(5,6-Dimethylbenzo[d]thiazol-2-yl)phenol
(17a). The
CeH) ppm; 13C NMR (100 MHz, CDCl3):
132.8, 127.9, 127.5, 126.7, 124.7, 122.7, 122.0, 188.9, 116.7 ppm; Ele-
mental analysis: found: C, 57.41; H, 2.99; N, 10.23. C13H8N2O3S re-
quires C, 57.35; H, 2.96; N, 10.29%.
d
¼167.5, 163.4, 151.4, 140.5,
above general procedure was followed. The product was purified by
column chromatography (5% EtOAc/hexane) to give the title com-
pound 17a (179 mg, 70%) as white solid; Rf (5% EtOAc/hexane) 0.65;
mp 135e136 ꢀC; nmax (KBr): 3436, 2924, 2846, 1742, 1621, 1585,
1484, 1454, 1404, 1311, 1270, 1251, 1222, 1196, 1155, 1138, 1034, 960,
4.2.16. 2-(Benzo[d]thiazole-2-yl)-4-fluoro-phenol (14a). The above
general procedure was followed. The product was purified by col-
umn chromatography (5% EtOAc/hexane) to give the title com-
pound 14a (208 mg, 85%) as white solid; Rf (5% EtOAc/hexane) 0.64;
mp 118e119 ꢀC; nmax (KBr): 3423, 2923, 1610, 1587, 1555, 1488,
1456, 1436, 1385, 1314, 1267, 1242, 1214, 1172, 1124, 1012, 998, 877,
858, 806, 753 cmꢂ1; 1H NMR (CDCl3, 400 MHz):
d
¼12.60 (1H, s, Ar.
CeOH), 7.75 (1H, s, Ar. CeH), 7.64 (1H, d, J¼7.6 Hz, Ar. CeH), 7.62
(1H, s, Ar. CeH), 7.34 (1H, t, J¼7.8 Hz, Ar. CeH), 7.07 (1H, d, J¼8.2 Hz,
Ar. CeH), 6.92 (1H, t, J¼7.5 Hz, Ar. CeH), 2.39 (s, 3H, AreCH3), 2.38
(s, 3 Hz, AreCH3) ppm; 13C NMR (CDCl3, 100 MHz):
d
¼168.2, 157.8,
150.5, 136.0, 135.2, 132.3, 129.9, 129.2, 122.4, 121.5, 119.4, 117.7, 117.0,
20.3 ppm; Elemental analysis: found: C, 70.58; H, 5.16; N, 5.47.
C15H13NSO requires C, 70.56; H, 5.13; N, 5.49%.
858, 815, 785, 759, 728 cmꢂ1; 1H NMR (CDCl3, 400 MHz):
d
¼12.23
(1H, s, Ar. CeOH), 8.02 (1H, d, J¼8.0 Hz, Ar. CeH), 7.79e7.76 (1H, m,
Ar. CeH), 7.74e7.71 (1H, m, Ar. CeH), 7.44e7.38 (1H, m, Ar. CeH),
7.36e7.28 (2H, m, Ar. CeH), 7.12e7.07 (1H, m, Ar. CeH) ppm; 13C
4.2.21. 2-(6-Methoxybenzo[d]thiazol-2-yl)phenyl acetate (18c). The
above general procedure was followed. The product was purified by
column chromatography (30% EtOAc/hexane) to give the title
compound 18c (194 mg, 65%) as Gummy; Rf (30% EtOAc/hexane)
0.62; nmax (KBr): 3435, 2926, 2846, 1769, 1744, 1626, 1604, 1476,
NMR (100 MHz, CDCl3):
d
¼166.4, 164.3, 161.8, 154.0, 135.7, 135.6,
135.1, 130.7, 130.6, 126.5, 125.6, 123.5, 122.3, 121.7, 117.9, 117.7, 114.4,
114.2 ppm; Elemental analysis: found: C, 63.71; H, 3.34; N, 5.66.
C13H8FNOS requires C, 63.66; H, 3.29; N, 5.71%.
1371, 1234, 1185, 1025, 968, 817 cmꢂ1 1H NMR (CDCl3, 400 MHz):
;
4.2.17. 2-(6-Chlorobenzo[d]thiazol-2-yl)phenol (15a). The above
general procedure was followed. The product was purified by col-
umn chromatography (5% EtOAc/hexane) to give the title com-
pound 15a (88 mg, 34%) as white solid; Rf (5% EtOAc/hexane) 0.62;
mp 133e134 ꢀC; nmax (KBr): 3439, 2924, 2855, 1624, 1581, 1471,
d
¼7.88 (1H, d, J¼8.8 Hz, Ar. CeH), 7.35 (1H, d, J¼2.4 Hz, Ar. CeH),
7.32 (2H, t, J¼8.4 Hz, Ar. CeH), 7.11 (1H, dd, J¼9.0 Hz, Ar. CeH), 6.99
(1H, dd, J¼8.8 Hz, Ar. CeH), 6.73 (1H, dd, J¼8.8 Hz, Ar. CeH), 3.89
(3H, s, AreOCH3), 2.53 (3H, s, eOCOCH3) ppm; 13C NMR (CDCl3,
100 MHz):
d
¼168.6, 161.1, 159.7, 158.1, 148.4, 143.8, 134.4, 131.0,
1433, 1384, 1256, 1210, 1062, 963, 884, 807, 756 cmꢂ1
(CDCl3, 400 MHz):
;
1H NMR
122.7, 116.5, 115.5, 113.2, 110.6, 103.4, 55.7, 22.5 ppm; Elemental
analysis: found: C, 64.24; H, 4.40; N, 4.65. C16H13NSO3 requires C,
64.20; H, 4.38; N, 4.68%.
d
¼12.21 (1H, s, Ar. CeOH), 8.04e8.02 (2H, m, Ar.
CeH), 7.74 (1H, d, J¼8.4 Hz, Ar. CeH), 7.48e7.46 (3H, m, Ar. CeH),
7.33e7.30 (1H, m, Ar. CeH) ppm; 13C NMR (CDCl3, 100 MHz):
d
¼170.0, 158.1, 155.1, 133.4, 133.3, 132.4, 131.5, 129.2, 127.7, 125.8,
4.3. Crystallographic description
123.1, 122.4, 118.1 ppm; Elemental analysis: found: C, 59.69; H, 3.13;
N, 5.30. C13H8ClNSO requires C, 59.66; H, 3.08; N, 5.35%.
Crystal data were collected with a Bruker Smart Apex-II CCD dif-
fractometer by using graphite monochromated MoK
a
radiation
ꢀ
4.2.18. 2-(6-Chlorobenzo[d]thiazol-2-yl)phenyl acetate (15c). The
above general procedure was followed. The product was purified by
column chromatography (30% EtOAc/hexane) to give the title
compound 15c (155 mg, 51%) as brown solid; Rf (30% EtOAc/hex-
ane) 0.25; mp 181e182 ꢀC; nmax (KBr): 3425, 3079, 2923, 2857,1888,
1757, 1624, 1578, 1461, 1434, 1371, 1313, 1066, 1026, 963, 879, 804,
(l¼0.71073 A) at 298 K. Cell parameters were retrieved using
SMART17 software and refined with SAINT17 on all observed re-
flections. Data reductionwas performed with the SAINTsoftware and
corrected for Lorentz and polarization effects. Absorption corrections
were applied with the program SADABS.18 The structure was solved
by direct methods implemented in SHELX-9719 program and refined
by full-matrix least-squares methods on F2. All non-hydrogen atomic
positions were located in difference Fourier maps and refined an-
isotropically. The hydrogen atoms were placed in their geometrically
generated positions. The crystal was isolated in rectangular shape
from ethyl acetate and hexane mixture at room temperature.
CCDC numbers for compounds 2a: CCDC-846544. This data can
be obtained from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
727 cmꢂ1; 1H NMR (CDCl3, 400 MHz):
d
¼8.02 (1H, s, Ar. CeH), 7.85
(1H, d, J¼8.4 Hz, Ar. CeH), 7.43e7.37 (3H, m, Ar. CeH), 7.03 (1H, d,
J¼8.4 Hz, Ar. CeH), 6.77 (1H, d, J¼8.0 Hz, Ar. CeH), 2.56 (3H, s,
eOCOCH3) ppm; 13C NMR (CDCl3, 100 MHz):
d
¼168.6, 165.7, 160.3,
150.4, 148.9, 133.2, 132.6, 131.3, 126.5, 122.1, 121.9, 115.8, 113.6, 110.4,
22.4 ppm; HRMS (ESI) MHþ, found 304.0198. C15H10ClNO2S re-
quires 304.0194. Elemental analysis: found: C, 59.36; H, 3.37; N,
4.55. C15H10ClNO2S requires C, 59.31; H, 3.32; N, 4.61%.
4.2.19. 2-(6-(Trifluoromethyl)benzo[d]thiazol-2-yl)-3-hydroxyphenyl
acetate (16b). The above general procedure was followed. The
product was purified by column chromatography (30% EtOAc/
hexane) to give the title compound 16b (282 mg, 80%) as yellow
solid; Rf (30% EtOAc/hexane) 0.26; mp 121e122 ꢀC; nmax (KBr):
4.4. UV and photoluminescence spectra of some representa-
tive compounds
These hydroxylated derivatives are sufficiently fluorescent ac-
tive. Photoluminescent and UV data of some representative

A. Banerjee et al. / Tetrahedron 69 (2013) 2175e2183
2183
Lett. 2011, 13, 4466; (j) Neufeldt, S. R.; Sanford, M. S. Acc. Chem. Res. 2012, 45,
936.
compounds have been recorded. Substrates possessing electron-
donating groups, such as p-OMe (3), p-CMe3 (4), and electron-
withdrawing group m-F (14) in the 2-aryl ring show their absorp-
tion bands at around 289 nm and 336 nm (lmax) in a 1:1 ethanol
water mixture (Fig. 1). Upon excitation at 336 nm all three com-
pounds show their excitation band at around 378 nm (Fig. 2).
Throughout all these UV and photoluminescent measurements 1:1
ethanol/water mixture was used as solvent system maintaining
10ꢂ6 M concentration.
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Acknowledgements
We are grateful to the DST (SR/S1/OC-79/2009) and CSIR
[01(2270)/08/EMR-II] for funding. A.B., S.G. and S.K.R. thank the
CSIR for fellowships. Thanks are also due to CIF, IIT Guwahati for
Mass and NMR spectra.
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Supplementary data
X-ray crystallographic data (CIF file) of 2a as well as copies of 1H
and 13C NMR and HRMS spectra of products are available. Supple-
mentary data related to this article can be found online at http://
dx.doi.org/10.1016/j.tet.2012.12.067.
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