Direct Oxygenation of C-H Bonds through Photoredox and Palladium Catalysis

Article abstract of DOI:10.1021/acs.joc.9b03197

This report presents the oxygenation of C-H bonds via the merger of photocatalysis and Pd catalysis. Herein, we describe the utilization of a photocatalyst to oxidize an organopalladium(II) intermediate to high-valent Pd^III or Pd^IV intermediates, which promotes the formation of C-O bonds. The demonstrated method works efficiently with various directing groups, such as oxime ether and benzothiazole. The applicability of this direct C-O bond formation method is shown by synthesizing several metal complexes of 2-(benzo[d]thiazol-2-yl)phenol that can be used in organic light-emitting diodes and pharmaceuticals.

Full text of DOI:10.1021/acs.joc.9b03197

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Article
Direct Oxygenation of C-H Bonds through
Photoredox and Palladium Catalysis
Sk. Sheriff Shah, Maniklal Shee, Amit Kumar Singh, Amrita Paul, and N. D. Pradeep Singh
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.9b03197 • Publication Date (Web): 20 Jan 2020
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Direct Oxygenation of C−H Bonds through Photoredox and
Palladium Catalysis
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Sk. Sheriff Shah, Maniklal Shee, Amit Kumar Singh, Amrita Paul, and N. D. Pradeep Singh*
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Department of Chemistry, Indian Institute of Technology Kharagpur, Kharagpur, West Bengal 721302, India
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ABSTRACT: This report presents the oxygenation of C−H bonds via the merger of
photocatalysis and Pd-catalysis. Herein, we describe the utilization of photocatalyst to
oxidize the organopalladium(II)-intermediate into high-valent PdIII or PdIV intermediates,
which promotes the formation of C−O bonds. This demonstrated method works efficiently
with various directing groups such as oxime ether, benzothiazole. The applicability of this
direct C−O bond formation method is shown by synthesizing several metal complexes of 2-
(benzo[d]thiazol-2-yl) phenol that can be used in organic LEDs and pharmaceuticals.
INTRODUCTION:
The activation of C−H bonds using transition metal catalysts to form new bonds has been
well documented.1 Pd-catalyzed C−H functionalization has been used extensively to construct
carbon-carbon (C−C) and carbon-heteroatom (C−X) bonds.2 In particular, the formation of
C−O bonds by Pd-catalyzed C−H functionalizations has gained significant attention in recent
years.3 These transformations proceed through the following steps (i) C−H bond activation at
PdII centre to form organopalladium PdII intermediate (A), (ii) the oxidation of PdII
intermediate into high-valent PdIII or PdIV intermediates (B or C) by oxidants such as
PhI(OAc)2, (iii) reductive elimination from the high valent PdIII or PdIV intermediates give
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C−O bond (Figure 1).4-6 Based on this strategy Crabtree showed the Pd-catalyzed
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acetoxylation of arenes using PhI(OAc)2 as an oxidant in 1996.7 Afterwards, Sanford et al.
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Figure 1. Oxidation of PdII to PdIII or PdIV by oxidants or electric currents.
pioneered Pd-catalyzed arene C−H acetoxylation employing PhI(OAc)2, K2S2O8, oxone, and
peroxides as stoichiometric oxidants (Scheme 1, entry a).4 Later on, transition metal catalyzed
C(sp2)−O bonds has been achieved successfully by the groups of Dong, Rao, Jiao,
Ackermann, and others.8 Simultaneously, several other research groups has shown Pd-
catalyzed C(sp3)−H oxygenation.9 Szabό et al. reported allylic C(sp3)−H bond oxygenation
using Pd catalyst.10
The oxidants used for C−H oxygenation reactions are reactive, expensive, toxic in nature, or
often generate by-products, which limits their widespread applicability. Alternatively, the use
of O2 as a terminal oxidant in Pd-catalyzed C−H oxygenations is promising. Stahl, Sigman,
and others contributed significantly to the Pd-catalyzed aerobic C−H activations. They have
developed PdII-catalyzed oxidation methods employing molecular oxygen as the sole
stoichiometric oxidant for the C–H oxygenation.11-13 Likewise, Sanford used NaNO3 as a
redox co-catalyst and O2 as a terminal oxidant in combination with Pd(OAc)2 for the
oxygenation of C(sp3)−H bonds.14 Very recently, Mei and others used electric currents
instead of oxidants to tune the oxidation state of PdII to high-valent PdIII or PdIV for the
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formation of C(sp2)−O and C(sp3)−O bonds (Scheme 1, entry b).15 These reports showed that
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the development of alternative way to oxidize PdII intermediate would be attractive.
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Photocatalysis has found a prominent place in synthetic chemistry due to its ability to
generate reactive intermediates under mild conditions.16 Recently, visible-light assisted
catalysis has been used to synthesize bioactive molecules.17 The use of photocatalysts in
place of oxidants for the PdII catalyzed C−H bond oxygenation would be an interesting
approach (Scheme 1, entry c). Over the last few years, the merger of transition metal catalysis
and photocatalysis, known as metallaphotocatalysis, has emerged as an efficient way to form
new bonds.18 MacMillan, Molander, and others have applied the metallaphotocatalysis
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Scheme 1. Pd-catalyzed oxygenation of C−H bonds under various conditions.
techniques to achieve cross-coupling reactions, where photoredox catalyst tunes the oxidation
state of the metal catalyst.19, 20 The merger of photocatalysis and palladium catalysis was first
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2+
reported by Osawa in 2007. This report showed that the employment of Ru(bpy)3 and
exposure to visible light accelerate Pd-catalysed Sonogashira reactions significantly.21 In
addition, Sanford's group developed a metallaphotoredox method combining photocatalyst
2+
[Ru(bpy)3 ] and palladium catalyst for the C(sp2)–H arylation with aryl diazonium salts.
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This dual catalytic system proceeded at room temperature without any oxidant.22 Rueping et
al. showed photocatalyst assisted Pd-catalyzed C–H activation, where the photocatalyst and
irradiation of visible light, instead of external oxidant, was employed to modulate the
oxidation state of Pd intermediate using O2 as the terminal oxidants.23 Concurrently, Tunge
and Wang investigated the combination of photoredox-induced decarboxylation method and
palladium catalysis to construct the C-C bonds.24, 25
Inspired by these literature reports on metallaphotocatalysis, we hypothesized that the PdII
intermediate can be oxidized to high-valent PdIII or PdIV intermediates by photocatalyst under
the irradiation of visible light in the presence of O2. Hence, this metallaphotocatalytic
procedure could be employed to construct C(sp2)−O and C(sp3)−O bonds.
RESULTS AND DISCUSSION:
To implement our hypothesized metallaphotoredox method for acetoxylation reactions, we
used 4CzIPN [1,2,3,5-tetrakis(carbazol-9-yl)-4,6 dicyanobenzene] as an organic
photocatalyst (PC) and oxime 1a as a substrate (the oxime moiety is easy to remove and post-
functionalize). 1a (0. 3 mmol), 4CzIPN (2 mol %), n-Bu4NOAc (3 equiv) and Pd(OAc)2 (10
mol %) were dissolved in a solvent mixture of CH3COOH (0.3 mL)/CH3CN (3 mL), then the
reaction mixture was irradiated under a CFL for 24 h in the presence of oxygen. To our
delight, acetoxylated product 2a was obtained in 47% yield (Table 1, entry 1). This result
encouraged us to screen the reaction conditions further.
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Table 1. Optimization of photocatalytic acetoxylation.a
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entry
conditions
solvent
yield(%)b
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2c
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as shown
as shown
as shown
EY as a PC
RB as a PC
CH3CN
DCM
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
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Ru(bpy)3 as a PC
air instead of O2
25 ºC instead of 70 ºC
45 ºC instead of 70 ºC
5 mol % of Pd(OAc)2
no photocatalyst
no Pd(OAc)2
no light
N2 in place of O2
no n-Bu4NOAc
0
0
0
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no CH3COOH
DCE
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aReaction conditions: 1a (0.3 mmol), Pd(OAc)2 (10 mol %), 4CzIPN (2 mol %), CH3COOH (0.3 mL), n-Bu4NOAc (3
equiv), O2, solvent (3 mL) at 70 ºC under the exposure from a 23 W CFL lamp for 24 h. bIsolated products. cReactions were
carried out at 35 ºC. EY = eosin Y, RB = rose bengal.
Solvent studies showed that DCE (1,2-dichloroethane) as a solvent gave maximum product
yield (69%, Table 1, entry 3). Use of other organic photocatalysts (eosin Y or rose bengal) in
2+
place of 4CzIPN provided lower yield (Table 1, entries 4, 5), whereas Ru(bpy)3 as a
photocatalyst furnished good result (57%, Table 1, entry 6). Air instead of O2 afforded 2a
with decreased in yield (Table 1, entry 7). Lowering the temperature to 25 ºC or 45 ºC
furnished a diminished yield (Table 1, entries 8, 9). Reduction in the amount of Pd(OAc)2 to
5 mol % reduced the yield (43%, Table 1, entry 10). The formation of 2a was ceased in the
absence of photocatalyst, Pd(OAc)2, light, or O2 (0%, Table 1, entries 11-14). Exclusion of
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n-Bu4NOAc and CH3COOH gave 46% and 29% product yield respectively (Table 1, entries
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15, 16).
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Use of photocatalyst instead of potentially toxic oxidants has several environmental benefits,
(i) it is activated under the irradiation from a simple household CFL (compact fluorescent
lamp), (ii) catalytic amount (2-5 mol %) is required to accomplish reactions, (iii) and it
regenerates during the catalytic process.
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With these optimized conditions in hand, we applied the merging photocatalytic method of
acetoxylation to various oxime substrates (Scheme 2). Arene rings with halogen (F, Cl, Br)
substituents proceeded well under the standard metallaphotoredox conditions (Scheme 2,
entries 2b-f). Electronically varied oxime substrates produced the corresponding
acetoxylation products satisfactorily (Scheme 2, entries 2g-l). However, substrates containing
nitro groups showed less reactivity (Scheme 2, entries 2g, 2o). To our expectation, the
substrate with ortho-substituent provided a lower yield compared to meta, and para
substituted substrates due to the steric hindrance (Scheme 2, entries 2j-l and 2b vs 2c). The
formation of palladacyclic intermediate (formed via the activation of the ortho-C−H bond)
impeded by the ortho-substituent due to the steric crowding. In addition, acetoxylation of
diarylketone O-methyl oximes afforded good product yield (Scheme 2, entry 2n).
Valerophenone, α-tetralone O-methyl oximes underwent oxygenation efficiently (Scheme 2,
entries 2p, 2q). Interestingly, the acetoxylation of the C(sp3)–H bonds of 8-methylquinoline
and pinacolone O-1-phenylpropyloxime obtained successfully using the presented method
(Scheme 2, entries 2r-t). It's worth noting that trace quantities of di-acetoxylation product
was observed (in the case of 1a). Moreover, mono-acetoxylation hinders the site for a second
acetoxylation reactions. 2r and 2s were obtained as a mixture from the same reaction.
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Scheme 2. Substrate scope of the acetoxylation.a
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aReaction conditions: 1 (0.3 mmol), Pd(OAc)2 (10 mol %), 4CzIPN (2 mol %), CH3COOH (0.3 mL), n-Bu4NOAc (3
equiv), O2, DCE (3 mL) at 70 ºC under the exposure from a 23 W CFL lamp for 24 h.
To investigate the generality of this merging photocatalytic method of oxygenation, we were
interested to carry out the hydroxylation of aromatic systems. Recently, we have shown the
hydroxylation of C(sp2)−H bonds by combining 4CzIPN, Pd(OAc)2, BrCCl3, and toluene
under visible light irradiation in the presence of O2.26 So, we reasoned that the present
metallaphotoredox methods could be used to construct C(sp2)−O bonds. To serve our
purpose, we have selected 2-phenylbenzo[d]thiazole because the hydroxylated 2-
phenylbenzo[d]thiazole is known to exhibit ESIPT (excited-state intramolecular proton
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transfer) and photoluminescent properties.27 A reaction mixture of 4CzIPN (2 mol %),
Pd(OAc)2 (10 mol %), 2-phenylbenzo[d]thiazole (0.3 mmol), CF3COOH (0.3 mL), and
CF3COOK (3 equiv) in DCE (3 mL) was exposed to visible light in the presence of O2.
Gratifyingly, the hydroxylation product 4a was obtained in 49% yield (Table 2, entry 1).
DMF as a solvent gave maximum product yield (Table 2, entry 3).
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Table 2. Optimization of photocatalytic hydroxylation.a
entry
conditions
solvent
yield(%)b
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as shown
DCE
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trace
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as shown
CH3CN
DMF
3
as shown
4
as shown
toluene
MeOH
DMF
5
as shown
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5 mol % of Pd(OAc)2
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AcOH instead of TFA DMF
9
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DMF
DMF
DMF
DCE
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our previous report 27
0
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aReaction conditions: 3a (0.3 mmol), Pd(OAc)2 (10 mol %), 4CzIPN (2 mol %), CF3COOH (0.3 mL), CF3COOK (3
b
c
equiv), O2, solvent (3 mL) at 70 ºC under the exposure from a 23 W CFL lamp for 24 h. Isolated products. Reaction
conditions: 3a (0.3 mmol), Pd(OAc)2 (10 mol %), 4CzIPN (2 mol %), CH3COOH (0.3 mL), n-Bu4NOAc (3 equiv) O2, DMF
d
(3 mL), under the exposure from a 23 W CFL lamp for 24 h. Reaction conditions: 3a (0.3 mmol), Pd(OAc)2 (0.1 equiv),
4CzIPN (2 mol %), BrCCl3 (2 equiv), O2, cyclohexane (1.5 mL), DCE (1.5 mL), under the exposure from a 23 W CFL lamp
for 24 h.
Reducing the amount of Pd(OAc)2 to 5 mol % furnished lower yield (57%, Table 2, entry 6).
Lowering the temperature to 25 ºC dropped the yield to 47% (Table 2, entry 7). Use of
CH3COOH (0.3 mL) and t-Bu4NOAc (3 equiv) in place of CF3COOH (0.3 mL) and
CF3COOK (3 equiv) provided reduced in yield (Table 2, entry 8). This metallaphotocatalytic
method was unable to provide hydroxylation product 4a in the absence of photocatalyst,
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Pd(OAc)2, light, or O2 (Table 2, entries 9-12). The present method was found to be more
efficient than our previously reported method for the hydroxylation of arenes (Table 2, entry
13).27 In these reaction conditions, we have not observed any di-hydroxylated products.
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Having the optimized condition in hand, the hydroxylation of diverse substituted 2-
phenylbenzo[d]thiazoles was investigated (Scheme 3). Substrates containing electron-
donating and electron-withdrawing groups provided good yields. To our satisfaction, the
halogen group containing substrates proceeded efficiently (Scheme 3, entries 4f-j). The
influence of substituent's at the ortho, meta or para positions towards the hydroxylation was
significant (Scheme 3, entries 4b, 4c, 4m). The hydroxylation of ortho-substituted substrates
gave a reduction in yield due to steric hindrance (Scheme 3, entry 4m). However, meta-
substituted substrates were found to be hydroxylated at the sterically less crowded site
(Scheme 3, entry 4c, 4i). Ester and cyano groups showed well tolerance towards the C−H
functionalization (Scheme 3, entries 4k, 4l). The hydroxylation of naphthalene derivatives
were obtained in moderate yields (Scheme 3, entries 4n, 4o). Additionally, the hydroxylation
of arenes directed by benzoxazole, imidazopyridine, and quinoline were obtained
successfully utilizing the metallaphotoredox method (Scheme 3, entries 4p-s).
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Scheme 3. Substrate scope of the hydroxylation.a
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aReaction conditions: 3 (0.3 mmol), Pd(OAc)2 (10 mol %), 4CzIPN (2 mol %), CF3COOH (0.3 mL), CF3COOK (3
equiv), O2, DMF (3 mL), under the exposure from a 23 W CFL lamp for 24 h. bIsolated products.
To understand the mechanism, we have carried out competition kinetic isotope effect (KIE)
experiments, which showed a KIE in intermolecular experiments (kH/kD = 3.35) and parallel
experiments (kH/kD = 1.22). This revealed that the C−H bond cleavage may be the rate-
determining step (Supporting Information (SI), S5-S6). Further mechanistic studies were
investigated employing PdII complex (A) as the metal catalyst instead of Pd(OAc)2 under the
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above-mentioned metallaphotoredox methods (SI, S7). The product 2a was obtained in 65%
yield (Scheme S1, eq. 1). However, the reaction of A alone in the absence of 1a (or any other
substrates) produced 2a only 9% in yield (Scheme S1, eq. 2). Furthermore, the reaction of A
in the presence of 1c produced 2a and 2c in 29 and 23% yields, respectively (Scheme S1, eq.
3). Surprisingly, the reaction of A with 1b given 2a and 2b with 21 and 9% in yields,
respectively (Scheme S1, eq. 4). The above results indicate that the free substrate 1a, 1b, and
1c may act as a ligand for unsaturated PdII centers.5d
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On the basis of the reaction conditions and a literature survey, we proposed the C−O bond
formation mechanism in Scheme 4.6, 15, 28 The catalytic cycle includes, (i) cyclopalladation
of compound 1 or 3 with Pd(OAc)2 forms the Pd(II) complex A, (ii) photoexcitation of PC
(4CzIPN) to its excited state PC* under the irradiation of visible light, (iii) single electron
transfer (SET) from PC* to O2 produced an O2•− radical (O2•− radicals were detected by EPR
experiments using DMPO as a trapping agent (SI, S9-S10) and PC+, (iv) oxidation of
complex A (E1/2ox = +0.83 V versus SCE in CH3CN29) by PC+ [(E1/2ox = +1.35 V versus SCE
•−
in CH3CN30), O2 or other peroxy species generated in situ could also act as an external
oxidants28b] generates complex B, and regenerates the PC, and finally, (v) C−O bond
formation (2 or 4') occurs via the reductive elimination from complex B, and subsequently
regenerates the PdII catalyst. Trifluoroacetoxylation products 4' upon hydrolysis (aqueous
workup) gave the hydroxylation products 4. Further mechanistic evidences are required to
support this tentative mechanism, which is under investigation.
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Scheme 4. A plausible mechanism for the oxygenation of C−H bonds.
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To explore the synthetic utility of this direct C−O bond formation, a variety of metal
complexes has been synthesized (Scheme 5). Bis(2-(2-hydroxyphenyl)benzothiazolate)zinc
(Zn(BTZ)2) is a known white electroluminescent material in organic LEDs (1, Scheme 5).31
Duarte et al. have studied the chelating properties of 2-(benzo[d]thiazol-2-yl)-4-iodophenol
and 2-(benzo[d]oxazol-2-yl)-4-iodophenol toward Cu(II) and Zn(II), and the use of these
complexes as potential Alzheimer’s disease therapeutics (2, Scheme 5).32 Moreover, removal
of the oxime group achieved easily by treating with HCl in Et2O to afford ortho-acylphenol
(3, Scheme 5).33 ortho-Acylphenols are valuable synthetic intermediates and features in many
bioactive molecules and natural products.8, 33 Notably, substrates shown in scheme 3 are
highly fluorescent due to the ESIPT phenomenon and useful for the photoresponsive drug
delivery system.27 Therefore, the present method of C−O bond formation can be applied in
the fields of organic synthesis, photoluminescent materials, and pharmaceuticals.
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Scheme 5. Synthetic utility of the metallaphotocatalysis oxygenation reactions.
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CONCLUSION:
In conclusion, we have developed a metallaphotoredox method for the construction of C−O
bonds. The oxygenation of C(sp2)−H and C(sp3)−H bonds obtain by integrating organic
photocatalyst, 4CzIPN and a metal catalyst, Pd(OAc)2. Mechanistic studies reveals that the
oxidation of PdII intermediate to high-valent PdIII or PdIV intermediate achieved by
photocatalyst. The present method of oxygenation is compatible with various substrates such
as 2-arylbenzothiazoles, oximes etc. Contrary to conventional methods, this alternative
method requires oxygen as the only terminal oxidant and offers a clean and simple technique
for the oxygenation reaction. The applicability of this direct C−O bonds formation reaction is
shown by synthesizing metal complexes of 2-(benzo[d]thiazol-2-yl)phenol and 2-
(benzo[d]oxazol-2-yl)phenol, which are known to be useful in organic LEDs and
pharmaceuticals.
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EXPERIMENTAL SECTION:
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General information. All chemicals were purchased from Aldrich or Spectrochem, and were
used without further purification. All glassware was washed properly and dried in an oven
prior to use. Commercial solvents were purified according to procedures described in Perrin’s
handbook.34 TLC (Thin Layer Chromatography) was performed on silica gel coated
aluminium plates (MERCK, 60F254), which were visualized by UV fluorescence.
Preparative Thin Layer Chromatography was performed on silica gel coated glass plates
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(silica gel GF-254). H and C{1H}NMR spectroscopy were performed on a Bruker FT-
NMR spectrometer (400 MHz, 500 MHz, and 600 MHz ). The coupling constants (J) are
reported in Hertz (Hz). Multiplicity, s = singlet, d = doublet, t = triplet, m = multiplet.
(i) Synthesis of 4CzIPN (1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene). 4CzIPN
was synthesized according to a literature procedure.30 NaH (60% in mineral oil, 0.30 g, 7.5
mmol) was added slowly to a stirred solution of carbazole (0.835 g, 5 mmol) in dry THF (25
mL) at room temperature under
a
nitrogen atmosphere. After 45 min,
tetrafluoroisophthalonitrile (0.20 g, 1.00 mmol) was added. The reaction mixture was stirred
at room temperature for 12 h, then water was added to the reaction mixture to quench the
excess NaH. The resulting mixture was then concentrated using rotavapor. The crude product
was washed by water and EtOH, which was further purified by recrystalization from
hexane/CH2Cl2 to yield 0.716 g (91%, 4CzIPN).
(ii) Synthesis of oxime ether (1a-q). The oxime ethers were synthesized according to the
literature procedure.35 The corresponding ketone (1 equiv) was mixed with NH2OMe•HCl
(1.5 equiv) in pyridine in a 20 mL sealed tube. The sealed tube was heated to 80 °C in a oil
bath for 45 minutes. The reaction mixture was then diluted with EtOAc and washed with 20%
aqueous AcOH to remove pyridine (3 times). The combined organic layer was then
neutralized with aqueous NaHCO3, washed with brine, and dried over Na2SO4. The
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corresponding oxime ether was purified by silica gel chromatography (eluent, EtOAc/hexane,
1:20).
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2-(1-Methoxyiminoethyl) benzene (1a). Compound 1a was prepared as per the reaction
condition to synthesize oxime ethers (ii). The ketone (240 mg, 2 mmol), NH2OMe•HCl (250
mg, 3 mmol) and pyridine (5 mL) were used to synthesize oxime ether. 1a was obtained as
colourless oil (271 mg, 91%). 1H NMR (400 MHz, CDCl3) δ 7.73 – 7.60 (m, 2H), 7.37 (dd, J
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= 5.0, 1.6 Hz, 3H), 4.01 (s, 3H), 2.24 (s, 3H); C{1H} NMR (126 MHz, CDCl3) δ 154.6,
136.6, 129.0, 128.3, 126.0, 61.8, 12.6. Spectral data matched with the literature report.15b
3-Chloro-2-(1-methoxyiminoethyl) benzene (1b). Compound 1b was prepared as per the
reaction condition to synthesize oxime ethers (ii). The ketone (308 mg, 2 mmol),
NH2OMe•HCl (250 mg, 3 mmol) and pyridine (5 mL) were used to synthesize oxime ethers.
1b was obtained as colourless oil (319 mg, 87%). 1H NMR (400 MHz, CDCl3) δ 7.39 (d, J =
8.3 Hz, 1H), 7.33 (dd, J = 6.8, 2.3 Hz, 1H), 7.28 (dd, J = 8.7, 4.8 Hz, 2H), 3.99 (s, 3H), 2.22
(s, 3H); 13C{1H} NMR (126 MHz, CDCl3) δ 155.9, 153.0, 136.9, 135.6, 132.6, 130.8, 130.2,
129.93, 129.8, 129.5, 129.4, 128.0, 126.8, 126.7, 61.8, 61.7, 21.0, 16.2. Spectral data matched
with the literature report.38
5-Chloro-2-(1-methoxyiminoethyl) benzene (1c). Compound 1c was prepared as per the
reaction condition to synthesize oxime ethers (ii). The ketone (308 mg, 2 mmol),
NH2OMe•HCl (250 mg, 3 mmol) and pyridine (5 mL) were used to synthesize oxime ether.
1c was obtained as colourless oil (326 mg, 89%). 1H NMR (400 MHz, CDCl3) δ 7.59 (d, J =
13
8.6 Hz, 2H), 7.33 (d, J = 6.8 Hz, 2H), 3.99 (s, 3H), 2.20 (s, 3H); C{1H} NMR (126 MHz,
CDCl3) δ 153.9, 141.9, 139.9, 135.2, 134.1, 134.0, 128.7, 128.5, 128.3, 127.3, 118.3, 62.5,
19.0. Spectral data matched with the literature report.15b
5-Bromo-2-(1-methoxyiminoethyl) benzene (1d). Compound 1d was prepared as per the
reaction condition to synthesize oxime ethers (ii). The ketone (398 mg, 2 mmol),
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NH2OMe•HCl (250 mg, 3 mmol) and pyridine (5 mL) were used to synthesize oxime ether.
1d was obtained as colourless oil (415 mg, 91%). 1H NMR (400 MHz, CDCl3) δ 7.61 – 7.44
(m, 4H), 3.99 (s, 3H), 2.19 (s, 3H); 13C{1H} NMR (126 MHz, CDCl3) δ 153.3, 135.5, 131.5,
127.5, 123.2, 62.0, 12.2. Spectral data matched with the literature report.15b
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4-Bromo-2-(1-methoxyiminoethyl) benzene (1e). Compound 1e was prepared as per the
reaction condition to synthesize oxime ethers (ii). The ketone (398 mg, 2 mmol),
NH2OMe•HCl (250 mg, 3 mmol) and pyridine (5 mL) were used to synthesize oxime ether.
1e was obtained as colourless oil (406 mg, 89%). 1H NMR (400 MHz, CDCl3) δ 7.81 (t, J =
1.8 Hz, 1H), 7.59 – 7.54 (m, 1H), 7.50 – 7.45 (m, 1H), 7.23 (t, J = 7.9 Hz, 1H), 4.00 (s, 3H),
2.19 (s, 3H); 13C{1H} NMR (126 MHz, CDCl3) δ 148.3, 133.9, 127.1, 125.1, 124.2, 119.8,
117.9, 57.3, 7.6. Spectral data matched with the literature report.15b
5-Fluoro-2-(1-methoxyiminoethyl) benzene (1f). Compound 1f was prepared as per the
reaction condition to synthesize oxime ethers (ii). The ketone (276 mg, 2 mmol),
NH2OMe•HCl (250 mg, 3 mmol) and pyridine (5 mL) were used to synthesize oxime ether.
1
1f was obtained as colourless oil (284 mg, 85%). H NMR (400 MHz, CDCl3) δ 7.67 – 7.61
(dd, J = 5.6 Hz, 2.8 Hz, 2H), 7.05 (dd, J = 8.8 Hz, 8.4 Hz, 2H), 4.00 (s, 3H), 2.20 (s, 3H).
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[4JF-H = 2.8 Hz, JF-H = 8.4 Hz]; C{1H} NMR (126 MHz, CDCl3) δ 164.3, 162.3, 153.5,
132.7 (d, J = 3.2 Hz), 127.8 (d, J = 8.2 Hz), 115.4 (d, J = 21.5 Hz), 61.8, 12.5. Spectral data
matched with the literature report.15b
2-(1-Methoxyiminoethyl)-5-nitrobenzene (1g). Compound 1g was prepared as per the
reaction condition to synthesize oxime ethers (ii). The ketone (330 mg, 2 mmol),
NH2OMe•HCl (250 mg, 3 mmol) and pyridine (5 mL) were used to synthesize oxime ether.
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1g was obtained as yellow solid (357 mg, 92%). H NMR (400 MHz, CDCl3) δ 8.22 (d, J =
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8.9 Hz, 2H), 7.83 (d, J = 8.9 Hz, 2H), 4.05 (s, 3H), 2.25 (s, 3H); C{1H} NMR (126 MHz,
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CDCl3) δ 152.5, 147.9, 142.6, 126.7, 123.6, 62.4, 12.3. Spectral data matched with the
literature report.15b
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2-(1-Methoxyiminoethyl)-5-methyl benzene (1h). Compound 1h was prepared as per the
reaction condition to synthesize oxime ethers (ii). The ketone (268 mg, 2 mmol),
NH2OMe•HCl (250 mg, 3 mmol) and pyridine (5 mL) were used to synthesize oxime ether.
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1h was obtained as yellow solid (297 mg, 91%). H NMR (400 MHz, CDCl3) δ 7.55 (d, J =
8.2 Hz, 2H), 7.18 (d, J = 8.2 Hz, 2H), 4.00 (s, 3H), 2.37 (s, 3H), 2.22 (s, 3H); 13C{1H} NMR
(126 MHz, CDCl3) δ 154.5, 138.9, 133.8, 129.1, 125.9, 61.7, 21.2, 12.5. Spectral data
matched with the literature report.39
3-Methoxy-2-(1-methoxyiminoethyl) benzene (1j). Compound 1j was prepared as per the
reaction condition to synthesize oxime ethers (ii). The ketone (300 mg, 2 mmol),
NH2OMe•HCl (250 mg, 3 mmol) and pyridine (5 mL) were used to synthesize oxime ether.
1j was obtained as yellow solid (mixture of E/Z isomer, 300 mg, 84%). 1H NMR (400 MHz,
CDCl3) δ 7.32 (ddd, J = 14.8, 7.4, 1.7 Hz, 2H), 6.96 (ddd, J = 7.4, 4.0, 0.9 Hz, 1H), 6.93 –
13
6.88 (m, 1H), 3.98 (s, 3H), 3.83 (s, 3H), 2.18 (s, 3H); C{1H} NMR (126 MHz, CDCl3) δ
157.4, 156.6, 130.1, 129.6, 129.5, 128.3, 127.0, 120.6, 120.4, 111.2, 111.0, 61.6, 61.5, 55.6,
55.4, 21.2, 15.9. Spectral data matched with the literature report.40
2-(1-Methoxyiminoethyl)-4-methoxybenzene (1k). Compound 1k was prepared as per the
reaction condition to synthesize oxime ethers (ii). The ketone (300 mg, 2 mmol),
NH2OMe•HCl (250 mg, 3 mmol) and pyridine (5 mL) were used to synthesize oxime ether.
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1k was obtained as colourless oil (mixture of E/Z isomer, 322 mg, 90%). H NMR (600
MHz, CDCl3) δ 7.37 – 7.31 (m, 2H), 6.99 – 6.95 (m, 1H), 6.92 (d, J = 8.2 Hz, 1H), 4.00 (s,
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3H), 3.85 (s, 3H), 2.20 (s, 3H); C{1H} NMR (126 MHz, CDCl3) δ 157.4, 156.6, 130.1,
129.7, 129.5, 128.3, 127.0, 120.6, 120.4, 111.2, 111.0, 61.6, 61.5, 55.6, 55.4, 21.2, 15.9.
Spectral data matched with the literature report.41
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5-Methoxy-2-(1-methoxyiminoethyl) benzene (1l). Compound 1l was prepared as per the
reaction condition to synthesize oxime ethers (ii). The ketone (300 mg, 2 mmol),
NH2OMe•HCl (250 mg, 3 mmol) and pyridine (5 mL) were used to synthesize oxime ether.
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1l was obtained as colourless oil (333 mg, 93%). H NMR (400 MHz, CDCl3) δ 7.70 – 7.53
(dd, J = 6.8 Hz, J = 2.4 Hz, 2H), 7.00 – 6.83 (dd, J = 6.8 Hz, J = 2.0, 2H), 3.98 (s, 3H), 3.82
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(d, J = 1.3 Hz, 3H), 2.20 (d, J = 1.6 Hz, 3H); C{1H} NMR (126 MHz, CDCl3) δ 160.3,
154.2, 129.2, 127.3, 113.7, 61.7, 55.3, 12.5. Spectral data matched with the literature
report.15b
1-(1-Methoxyiminoethyl)naphthalene (1m). Compound 1m was prepared as per the
reaction condition to synthesize oxime ethers (ii). The ketone (340 mg, 2 mmol),
NH2OMe•HCl (250 mg, 3 mmol) and pyridine (5 mL) were used to synthesize oxime ether.
1m was obtained as colourless oil (362 mg, 91%). 1H NMR (400 MHz, CDCl3) δ 8.06 (d, J =
8.5 Hz, 1H), 7.87 (t, J = 7.0 Hz, 2H), 7.54 – 7.50 (m, 2H), 7.47 (d, J = 7.7 Hz, 2H), 4.06 (s,
3H), 2.36 (s, 3H); C{1H} (126 MHz, CDCl3) δ 156.4, 135.5, 133.90, 130.93, 129.0, 128.4,
13
126.4, 125.9, 125.3, 125.2, 125.2, 61.9, 17.2. Spectral data matched with the literature
report.42
2-(Methoxyimino(phenyl)methyl)benzene (1n). Compound 1n was prepared as per the
reaction condition to synthesize oxime ethers (ii). The ketone (364 mg, 2 mmol),
NH2OMe•HCl (250 mg, 3 mmol) and pyridine (5 mL) were used to synthesize oxime ether.
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1n was obtained as white solid (367 mg, 87%). H NMR (400 MHz, CDCl3) δ 7.49 (dd, J =
7.9, 1.5 Hz, 2H), 7.43 (t, J = 4.8 Hz, 3H), 7.40 – 7.31 (m, 5H), 3.99 (s, 3H); C{1H} NMR
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(126 MHz, CDCl3) δ 156.8, 136.7, 133.6, 129.4, 129.4, 128.9, 128.4, 128.3, 128.0, 62.5.
Spectral data matched with the literature report.15b
2-(Methoxyimino(4-nitrophenyl)methyl) benzene (1o). Compound 1o was prepared as per
the reaction condition to synthesize oxime ethers (ii). The ketone (454 mg, 2 mmol),
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MHz, CDCl3) δ 8.30 (d, J = 8.7 Hz, 1H), 8.17 (d, J = 8.8 Hz, 1H), 7.66 (d, J = 8.9 Hz, 1H),
7.52 (d, J = 8.6 Hz, 1H), 7.46 (dd, J = 6.7, 5.3 Hz, 1H), 7.43 (s, 1H), 7.41 – 7.37 (m, 1H),
7.36 (d, J = 7.6 Hz, 1H), 7.34 – 7.30 (m, 1H), 4.01 (d, J = 16.1 Hz, 3H); 13C{1H} NMR (126
MHz, CDCl3) δ 154.8, 148.1, 147.8, 142.4, 139.9, 135.1, 132.1, 130.3, 129.8, 129.4, 129.0,
128.5, 128.5, 128.4, 127.5, 123.4, 62.9, 62.6. Spectral data matched with the literature
report.43
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2-(1-Methoxyiminopentyl) benzene (1p). Compound 1p was prepared as per the reaction
condition to synthesize oxime ethers (ii). The ketone (324 mg, 2 mmol), NH2OMe•HCl (250
mg, 3 mmol) and pyridine (5 mL) were used to synthesize oxime ether. 1p was obtained as
colourless oil (351 mg, 92%). 1H NMR (400 MHz, CDCl3) δ 7.66 – 7.60 (m, 2H), 7.39 – 7.33
(m, 3H), 3.97 (s, 3H), 2.74 (t, J = 7.7 Hz, 2H), 1.54 – 1.47 (m, 2H), 1.43 – 1.34 (m, 2H),
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0.91 (t, J = 7.2 Hz, 3H); C{1H} NMR (101 MHz, CDCl3) δ 158.7, 136.0, 128.9, 128.4,
126.3, 61.8, 28.7, 26.3, 22.9, 13.8. Spectral data matched with the literature report.15b
3,4-Dihydronaphthalen-1(2H)-one-O-1-methyl oxime (1q). Compound 1q was prepared as
per the reaction condition to synthesize oxime ethers (ii). The ketone (292 mg, 2 mmol),
NH2OMe•HCl (250 mg, 3 mmol) and pyridine (5 mL) were used to synthesize oxime ether.
1q was obtained as colourless oil (311 mg, 89%). 1H NMR (400 MHz, CDCl3) δ 7.98 (d, J =
7.7 Hz, 1H), 7.26 – 7.22 (m, 1H), 7.19 (dt, J = 7.5, 3.8 Hz, 1H), 7.13 (dd, J = 4.3, 3.3 Hz,
13
1H), 3.99 (s, 3H), 2.74 (dd, J = 6.4, 3.2 Hz, 4H), 1.88 – 1.81 (m, 2H); C{1H} NMR (126
MHz, CDCl3) δ 153.9, 139.5, 130.8, 128.9, 128.6, 126.4, 124.3, 61.9, 29.8, 24.3, 21.5.
Spectral data matched with the literature report.44
(iii) Synthesis of 3,3-dimethylbutan-2-one O-benzyl oxime (1r). The oxime ether 1r was
synthesized according to literature procedure.15a To an oven-dried 250 mL round bottom flask
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was added pinacolone 10 mmol (1.25 mL, 1 equiv), hydroxylamine hydrochloride 15 mmol
(1.04 g, 1.5 equiv), NaOAc 25 mmol (2.05 g, 2.5 equiv), and solvent (100 mL, H2O:EtOH =
2:3). The reaction mixture was heated at 100 °C for overnight. After completion of the
reaction, the mixture was cooled to room temperature and then diluted with H2O (25 mL) and
extracted with ethyl acetate (100 mL, ×3). The combined organic layer was dried by Na2SO4
then concentrated under reduced pressure. The crude oxime was used without further
purification for the next step.
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To an oven-dried 100 mL round bottom flask was added the crude oxime 5.0 mmol (0.575 g,
1.0 equiv), benzyl bromide 7.5 mmol (1.28 g, 1.5 equiv), NaI 0.75 mmol. (0.112 g, 0.15
equiv), tetrabutylammonium iodide 0.25 mmol ( 0.09 g, 0.05 equiv), KOH 6.5 mmol (0.364
g, 1.3 equiv), and THF (35 mL). The reaction mixture was stirred at room temperature for 12
h. After completion of the reaction, the mixture was concentrated under reduced pressure. To
the mixture ethyl acetate was added and washed with saturated NaHCO3, water, and brine.
The aqueous part was further washed with ethyl acetate (100 mL ×2) then the combined
organic layer was dried over Na2SO4 then concentrated in a rotavapor. The crude product was
purified by silica gel chromatography (eluent, EtOAc/hexane, 1:20) to afford 3,3-
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dimethylbutan-2-one O-benzyl oxime 1r was obtained as colourless oil (850 mg, 83%). H
NMR (400 MHz, CDCl3) δ 7.42 – 7.28 (m, 5H), 5.08 (s, 2H), 1.83 (s, 3H), 1.11 (s, 9H);
13C{1H} NMR (126 MHz, CDCl3) δ 163.7, 138.5, 128.4, 128.1, 128.0, 127.7, 127.4, 75.3,
37.0, 31.4, 30.2, 27.6, 10.6. Spectral data matched with the literature report.15a
(iv) Synthesis of 2-arylbenzothiazoles. 2-Arylbenzothiazoles were synthesized according to
the literature procedure.28 To an oven-dried 20 mL sealed tube was charged with a magnetic
stir bar then added corresponding aldehyde (1 equiv), 2-aminothiophenol (1.3 equiv), and
º
dimethyl sulfoxide (DMSO, 5 mL) then heated it to 110 C for 1hr. After cooling to room
temperature the reaction mixture was quenched with water and washed with ethyl acetate.
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The organic layer was separated, then concentrated under reduced pressure. The crude
product was purified by silica gel chromatography (eluent, EtOAc/hexane, 1:20) to afford 2-
arylbenzothiazoles. Compound 3a-d, 3f-h, 3l were synthesized according to our previously
published paper.26
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2-(2,5-Dimethoxyphenyl)benzo[d]thiazole (3e). Compound 3e was prepared as per the
reaction condition to synthesize 2-arylbenzothiazoles (iv). 2,5-Dimethoxybenzaldehyde (498
mg, 3 mmol), 2-aminothiophenol (487 mg, 3.9 mmol), and dimethyl sulfoxide (DMSO, 5
mL) were used to synthesize 2-arylbenzothiazole. 3e was obtained as white solid (667 mg,
82%). 1H NMR (400 MHz, CDCl3) δ 8.12 – 8.00 (m, 2H), 7.93 (d, J = 7.9 Hz, 1H), 7.49 (t, J
= 7.7 Hz, 1H), 7.37 (t, J = 7.5 Hz, 1H), 7.03 (d, J = 3.0 Hz, 2H), 4.02 (s, 3H), 3.91 (s, 3H);
13C{1H} NMR (126 MHz, CDCl3) δ 165.3, 162.9, 153.99, 151.92, 150.6, 149.0, 143.2, 136.2,
133.5, 126.3, 125.9, 124.9, 124.6, 122.8, 121.5, 121.2, 121.0, 118.8, 115.1, 113.5, 112.7,
99.5, 56.4, 56.0. Spectral data matched with the literature report.45
2-(3-Iodophenyl)benzo[d]thiazole (3i). Compound 3i was prepared as per the reaction
condition to synthesize 2-arylbenzothiazoles (iv). 3-Iodobenzaldehyde (696 mg, 3 mmol), 2-
aminothiophenol (487 mg, 3.9 mmol), and dimethyl sulfoxide (DMSO, 5 mL) were used to
synthesize 2-arylbenzothiazole. 3i was obtained as white solid (880 mg, 87%). 1H NMR (400
MHz, CDCl3) δ 8.27 (d, J = 1.5 Hz, 1H), 8.08 (d, J = 8.1 Hz, 1H), 7.96 (d, J = 7.6 Hz, 1H),
7.89 (d, J = 7.8 Hz, 1H), 7.59 (d, J = 7.8 Hz, 1H), 7.50 (t, J = 7.6 Hz, 1H), 7.36 (dt, J = 25.2,
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7.7 Hz, 2H); C{1H} NMR (126 MHz, CDCl3) δ 166.0, 153.9, 135.4, 135.1, 133.7, 130.4,
130.2, 126.5, 126.1, 125.5, 123.4, 123.2, 121.6. Spectral data matched with the literature
report.46
2-(2,4-Dichlorophenyl)benzo[d]thiazole (3j). The compound 3j was prepared as per the
reaction condition to synthesize 2-arylbenzothiazoles (iv). 2, 4-Dicholorobenzaldehyde (525
mg, 3 mmol), 2-aminothiophenol (487 mg, 3.9 mmol), and dimethyl sulfoxide (DMSO, 5
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mL) were used to synthesize 2-arylbenzothiazole. 3j was obtained as white solid (655 mg,
78%). 1H NMR (400 MHz, CDCl3) δ 8.23 (d, J = 8.6 Hz, 1H), 8.13 (d, J = 8.2 Hz, 1H), 7.96
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CDCl3) δ 162.9, 152.3, 136.6, 136.0, 133.3, 132.5, 130.8, 130.5, 127.5, 126.4, 125.6, 123.5,
121.4. Spectral data matched with the literature report.47
Methyl 4-(benzo[d]thiazol-2-yl)benzoate (3k). Compound 3k was prepared as per the
reaction condition to synthesize 2-arylbenzothiazoles (iv). Methyl 4-formylbenzoate (492 mg,
3 mmol), 2-aminothiophenol (487 mg, 3.9 mmol), and dimethyl sulfoxide (DMSO, 5 mL)
were used to synthesize 2-arylbenzothiazole. 3k was obtained as white solid (589 mg, 73%).
1H NMR (400 MHz, CDCl3) δ 8.17 (s, 4H), 8.11 (d, J = 8.0 Hz, 1H), 7.94 (d, J = 8.0 Hz, 1H),
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7.53 (t, J = 7.7 Hz, 1H), 7.43 (t, J = 7.7 Hz, 1H), 3.96 (s, 3H); C{1H} NMR (126 MHz,
CDCl3) δ 166.5, 166.4, 154.1, 137.4, 135.2, 132.0, 130.2, 127.4, 126.6, 125.7, 123.6, 121.7,
52.3. Spectral data matched with the literature report.48
2-(2-Bromophenyl)benzo[d]thiazole (3m). Compound 3m was prepared as per the reaction
condition to synthesize 2-arylbenzothiazoles (iv). 2-Bromobenzaldehyde (555 mg, 3 mmol),
2-aminothiophenol (487 mg, 3.9 mmol), and dimethyl sulfoxide (DMSO, 5 mL) were used to
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synthesize 2-arylbenzothiazole. 3m was obtained as white solid (687 mg, 79%). H NMR
(400 MHz, CDCl3) δ 8.15 (d, J = 8.2 Hz, 1H), 8.00 (dt, J = 7.8, 1.5 Hz, 1H), 7.96 (dd, J = 8.2,
1.2 Hz, 1H), 7.75 (dd, J = 7.9, 1.3 Hz, 1H), 7.54 (tt, J = 7.2, 1.2 Hz, 1H), 7.48 – 7.43 (m, 2H),
7.34 (td, J = 7.7, 1.8 Hz, 1H); 13C{1H} NMR (126 MHz, CDCl3) δ 165.6, 152.7, 136.1, 134.5,
134.1, 132.1, 131.2, 127.5, 126.3, 125.5, 123.6, 122.1, 121.4. Spectral data matched with the
literature report.48
2-(Naphthalen-2-yl)benzo[d]thiazole (3n). Compound 3n was prepared as per the reaction
condition to synthesize 2-arylbenzothiazoles (iv). 2-Naphthaldehyde (468 mg, 3 mmol), 2-
aminothiophenol (487 mg, 3.9 mmol), and dimethyl sulfoxide (DMSO, 5 mL) were used to
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synthesize 2-arylbenzothiazole. 3n was obtained as yellow solid (630 mg, 80%). H NMR
(400 MHz, CDCl3) δ 8.58 (d, J = 1.8 Hz, 1H), 8.22 (dd, J = 8.5, 1.8 Hz, 1H), 8.12 (d, J = 8.2
Hz, 1H), 8.01 – 7.93 (m, 3H), 7.89 (dd, J = 7.2, 2.8 Hz, 1H), 7.59 – 7.49 (m, 3H), 7.45 – 7.38
(m, 1H); 13C{1H} NMR (126 MHz, CDCl3) δ 168.1, 154.2, 135.1, 134.6, 133.2, 131.0,
128.86, 128.84, 127.9, 127.6, 127.4, 126.9, 126.4, 125.2, 124.4, 123.2, 121.6. Spectral data
matched with the literature report.49
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2-(Naphthalen-1-yl)benzo[d]thiazole (3o). Compound 3o was prepared as per the reaction
condition to synthesize 2-arylbenzothiazoles (iv). 1-Naphthaldehyde (468 mg, 3 mmol), 2-
aminothiophenol (487 mg, 3.9 mmol), and dimethyl sulfoxide (DMSO, 5 mL) were used to
synthesize 2-arylbenzothiazole. 3o was obtained as reddish oil (650 mg, 83%). 1H NMR (400
MHz, CDCl3) δ 8.93 (d, J = 8.4 Hz, 1H), 8.20 (d, J = 8.1 Hz, 1H), 7.99 (t, J = 7.7 Hz, 2H),
7.94 (d, J = 7.5 Hz, 2H), 7.64 (d, J = 7.0 Hz, 1H), 7.60 – 7.54 (m, 3H), 7.46 (t, J = 7.4 Hz,
1H); C{1H} NMR (126 MHz, CDCl3) δ 167.6, 154.2, 135.5, 134.0, 131.1, 130.8, 130.7,
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129.4, 128.4, 127.6, 126.5, 126.3, 125.9, 125.3, 125.0, 123.6, 121.4. Spectral data matched
with the literature report.50
(v) Synthesis of 2-arylbenzoxazole (3p, q). 2-Arylbenzoxazole was synthesized according to
the literature procedure.36 To an oven-dried 25 mL round bottom flask was added
corresponding aldehyde (1 equiv), 2-aminophenol (1.2 equiv), and diacetoxyiodobenzene
(PIDA, 1.2 equiv) in ethanol. The mixture was stirred at 60 °C for 6 h. After completion of
the reaction, the mixture was cooled to room temperature and the solvent was removed under
reduced pressure. The reaction mixture was diluted with ethyl acetate and washed with water,
brine, and then dried over Na2SO4. The product was purified by silica gel chromatography
(eluent, EtOAc/hexane, 1:20).
2-Phenylbenzo[d]oxazole (3p). Compound 3p was prepared as per the reaction condition to
synthesize 2-arylbenzoxazole (v). Benzaldehyde (318 mg, 3 mmol), 2-aminophenol (391 mg,
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to synthesize 2-arylbenzoxazole. 3p was obtained as white solid (503 mg, 86%). H NMR
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(600 MHz, CDCl3) δ 8.12 (dd, J = 12.7, 6.2 Hz, 3H), 7.94 (d, J = 8.0 Hz, 1H), 7.52 (t, J = 5.6
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Hz, 4H), 7.42 (t, J = 7.6 Hz, 1H); C{1H} NMR (126 MHz, CDCl3) δ 168.0, 154.2, 135.1,
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133.6, 131.0, 129.0, 127.6, 126.3, 125.2, 123.3, 121.6. Spectral data matched with the
literature report.48
2-(3-Iodophenyl)benzo[d]oxazole (3q). Compound 3q was prepared as per the reaction
condition to synthesize 2-arylbenzoxazole (v). 3-Iodobenzaldehyde (3 mmol, 696 mg), 2-
aminophenol (3.6 mmol, 391 mg), diacetoxyiodobenzene (PIDA, 3.6 mmol, 1.16 g), and
ethanol (20 mL) were used to synthesize 2-arylbenzoxazole. 3q was obtained as white solid
(800 mg, 84%). 1H NMR (600 MHz, CDCl3) δ 8.31 (s, 1H), 8.11 (d, J = 8.2 Hz, 1H), 8.02 (d,
J = 7.8 Hz, 1H), 7.94 (d, J = 8.0 Hz, 1H), 7.64 (dd, J = 4.4, 3.6 Hz, 1H), 7.54 (t, J = 7.7 Hz,
1H), 7.43 (t, J = 7.6 Hz, 1H), 7.39 (t, J = 7.9 Hz, 1H); C{1H} NMR (126 MHz, CDCl3) δ
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166.0, 153.9, 135.4, 135.1, 133.7, 130.4, 130.2, 126.5, 126.1, 125.5, 123.4, 123.2, 121.6.
Spectral data matched with the literature report.51
(vi) Synthesis of 2-phenylimidazo[1,2-a]pyridine (3r). 2-Phenylimidazo[1,2-a]pyridine was
synthesized according to the literature procedure.38 A mixture of 2-aminopyridine (0.28 g, 3
mmol), phenacyl bromide (0.896 g, 4.5 mmol), and NaHCO3 (0.504 g, 6 mmol) in ethanol (4
mL) was stirred for 12 h at room temperature. After completion of the reaction, the mixture
was cooled to room temperature and the solvent was removed under reduced pressure. The
reaction mixture was diluted with ethyl acetate and washed with water, brine, and then dried
over Na2SO4. The product was purified by silica gel chromatography (eluent, EtOAc/hexane,
1:20). 3r was obtained as white solid (471 mg, 81%). 1H NMR (400 MHz, CDCl3) δ 8.11 (dd,
J = 6.8, 0.9 Hz, 1H), 8.01 – 7.93 (m, 2H), 7.85 (s, 1H), 7.64 (d, J = 9.1 Hz, 1H), 7.44 (t, J =
7.7 Hz, 2H), 7.37 – 7.30 (m, 1H), 7.21 – 7.13 (m, 1H), 6.77 (t, J = 6.7 Hz, 1H); C{1H}
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NMR (126 MHz, CDCl3) δ 145.5, 133.7, 128.6, 127.9, 125.9, 125.6, 124.6, 117.1, 112.2,
108.2. Spectral data matched with the literature report.52
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(vii) Synthesis of 2-methylbenzo[h]quinoline (3s). 2-Methylbenzo[h]quinoline was
synthesized according to our previously published paper.26
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(viii) Photocatalytic acetoxylation of C−H bonds. To a oven dried 10 mL round bottom
flask was added corresponding oxime ether (0.3 mmol), Pd(OAc)2 (10 mol %), 4CzIPN (2
mol %), CH3COOH (0.3 mL), n-Bu4NOAc (3 equiv), and dichloroethane (DCE, 3 mL). The
round bottom flask was fitted with O2 balloon and stirred at 70 ºC on a hotplate stirrer under
the exposure from a 23 W CFL lamp for 24 h. After completion of the reaction, ethyl acetate
was added to the reaction mixture and the organic layer was washed with water and brine.
The combined reaction mixture was dried over Na2SO4. The reaction mixture was
concentrated under reduced pressure. The crude product was purified by preparative thin
layer chromatography (eluent, EtOAc/hexane, 1:20) to afford the desired acetoxylation
products. All the reactions were carried out twice and the average yield is reported.
[CFL specifications- Brand- Havells; Model No.- T3 B-22; power- 23 watts.]
The vials (borosilicate) containing the reactions mixture were kept at 6 cm away from the
CFL.
Source of heat was a hotplate.
Large scale acetoxylation of C−H bond (1.5 mmol). To an oven dried 50 mL round bottom
flask was added 1-phenylethan-1-one O-methyl oxime 1a (1.5 mmol, 225 mg), Pd(OAc)2 (10
mol %, 34 mg), 4CzIPN (2 mol %, 23.5 mg), CH3COOH (1.5 mL), n-Bu4NOAc (3 equiv, 4.5
mmol, 1.35 g), and dichloroethane (1,2-DCE, 20 mL). The round bottom flask was fitted with
O2 balloon and stirred at 70 ºC on a hotplate stirrer under the exposure from a 23 W CFL
lamp for 24 h. After completion of the reaction, EtOAc was added to the reaction mixture and
the organic layer was washed with water and brine. The combined reaction mixture was dried
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over Na2SO4. The reaction mixture was concentrated under reduced pressure. The crude
product was purified by silica gel chromatography (eluent, EtOAc/hexane, 1:20) to afford 2-
(1-methoxyiminoethyl)phenyl acetate 2a (143 mg, 46%).
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2-(1-Methoxyiminoethyl)phenyl acetate (2a). Compound 2a was prepared according to the
general photocatalytic reaction condition (viii). 1a (0.3 mmol, 45 mg), Pd(OAc)2 (10 mol %,
6.7 mg), 4CzIPN (2 mol %, 4.7 mg), CH3COOH (0.3 mL), n-Bu4NOAc (3 equiv, 0.9 mmol,
270 mg), and dichloroethane (DCE, 3 mL) were used for photocatalytic acetoxylation
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reaction. 2a was obtained as colourless liquid (43 mg, 69%). H NMR (400 MHz, CDCl3) δ
7.43 (d, J = 7.6 Hz, 1H), 7.36 (t, J = 7.7 Hz, 1H), 7.25 (t, J = 7.5 Hz, 1H), 7.10 (d, J = 8.0 Hz,
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1H), 3.96 (s, 3H), 2.28 (s, 3H), 2.16 (s, 3H); C{1H} NMR (126 MHz, CDCl3) δ 169.1,
153.3, 148.0, 130.3, 129.6, 129.4, 126.0, 123.1, 61.8, 21.0, 15.0. Spectral data matched with
the literature report.15b
3-Chloro-2-(1-methoxyiminoethyl)phenyl acetate (2b). Compound 2b was prepared
according to the general reaction condition (viii). 1b (0.3 mmol, 55 mg), Pd(OAc)2 (10 mol
%, 6.7 mg), 4CzIPN (2 mol %, 4.7 mg), CH3COOH (0.3 mL), n-Bu4NOAc (3 equiv, 0.9
mmol, 270 mg), and dichloroethane (DCE, 3 mL) were used for photocatalytic acetoxylation
reaction. 2b was obtained as colourless oil (28 mg, 39%). 1H NMR (400 MHz, CDCl3) δ 7.34
– 7.27 (m, 2H), 7.05 (dd, J = 6.0, 2.8 Hz, 1H), 3.96 (s, 3H), 2.25 (s, 3H), 2.11 (s, 3H);
13C{1H} NMR (151 MHz, CDCl3) δ 168.9, 151.5, 149.2, 134.1, 130.3, 129.7, 127.3, 121.4,
62.0, 20.7, 15.7; HRMS (ESI-TOF) m/z: [M + H]+ Calcd for C11H13NO3Cl 242.0584; Found
242.0589.
5-Chloro-2-(1-methoxyiminoethyl)phenyl acetate (2c). Compound 2c was prepared
according to the general reaction condition (viii). 1c (0.3 mmol, 55 mg), Pd(OAc)2 (10 mol
%, 6.7 mg), 4CzIPN (2 mol %, 4.7 mg), CH3COOH (0.3 mL), n-Bu4NOAc (3 equiv, 0.9
mmol, 270 mg), and dichloroethane (DCE, 3 mL) were used for photocatalytic acetoxylation
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reaction. 2c was obtained as colourless oil (46 mg, 64%). 1H NMR (400 MHz, CDCl3) δ 7.37
(d, J = 7.3 Hz, 1H), 7.29 – 7.23 (m, 1H), 7.13 (s, 1H), 3.96 (s, 3H), 2.28 (s, 3H), 2.13 (s, 3H);
13C{1H} NMR (151 MHz, CDCl3) δ 168.8, 152.5, 148.3, 134.8, 130.2, 128.9, 126.3, 123.7,
62.0, 20.9, 14.9. Spectral data matched with the literature report.15b
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5-Bromo-2-(1-methoxyiminoethyl)phenyl acetate (2d). Compound 2d was prepared
according to the general reaction condition (viii). 1d (0.3 mmol, 68 mg), Pd(OAc)2 (10 mol
%, 6.7 mg), 4CzIPN (2 mol %, 4.7 mg), CH3COOH (0.3 mL), n-Bu4NOAc (3 equiv, 0.9
mmol, 270 mg), and dichloroethane (DCE, 3 mL) were used for photocatalytic acetoxylation
reaction. 2d was obtained as colourless oil (53 mg, 62%). 1H NMR (400 MHz, CDCl3) δ 7.38
(d, J = 8.3 Hz, 1H), 7.30 (d, J = 8.6 Hz, 2H), 3.95 (s, 3H), 2.27 (s, 3H), 2.13 (s, 3H); 13C{1H}
NMR (101 MHz, CDCl3) δ 168.7, 152.5, 148.4, 130.4, 129.4, 129.2, 126.5, 122.5, 62.0, 20.9,
14.8. Spectral data matched with the literature report.15b
4-Bromo-2-(1-methoxyiminoethyl)phenyl acetate (2e). Compound 2e was prepared
according to the general reaction condition (viii). 1e (0.3 mmol, 68 mg), Pd(OAc)2 (10 mol
%, 6.7 mg), 4CzIPN (2 mol %, 4.7 mg), CH3COOH (0.3 mL), n-Bu4NOAc (3 equiv, 0.9
mmol, 270 mg), and dichloroethane (DCE, 3 mL) were used for photocatalytic acetoxylation
reaction. 2e was obtained as colourless liquid (52 mg, 61%).1H NMR (400 MHz, CDCl3) δ
7.56 (s, 1H), 7.47 (d, J = 8.6 Hz, 1H), 6.98 (d, J = 8.6 Hz, 1H), 3.96 (s, 3H), 2.27 (s, 3H),
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2.12 (s, 3H); C{1H} NMR (151 MHz, CDCl3) δ 168.8, 152.2, 147.0, 132.5, 132.27, 132.1,
124.9, 119.1, 62.1, 21.0, 14.9. Spectral data matched with the literature report.15b
5-Fluoro-2-(1-methoxyiminoethyl)phenyl acetate (2f). Compound 2f was prepared
according to the general reaction condition (viii). 1f (0.3 mmol, 50 mg), Pd(OAc)2 (10 mol
%, 6.7 mg), 4CzIPN (2 mol %, 4.7 mg), CH3COOH (0.3 mL), n-Bu4NOAc (3 equiv, 0.9
mmol, 270 mg), and dichloroethane (DCE, 3 mL) were used for photocatalytic acetoxylation
reaction. 2f was obtained as yellow oil (45 mg, 67%). 1H NMR (400 MHz, CDCl3) δ 7.40 (t,
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2.13 (s, 3H), [4JF-H = 8.2 Hz, JF-H = 9.2 Hz]; C{1H} NMR (151 MHz, CDCl3) δ 168.7,
163.4, 161.8, 152.7, 148.8 (d, J = 10.9 Hz), 130.5 (d, J = 9.4 Hz), 126.6 (d, J = 3.9 Hz), 113.2
(d, J = 21.2 Hz, 1H), 111.1 (d, J = 24.1 Hz), 61.9, 21.0, 15.1. Spectral data matched with the
literature report.15b
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2-(1-Methoxyiminoethyl)-5-nitrophenyl acetate (2g). Compound 2g was prepared
according to the general reaction condition (viii). 1g (0.3 mmol, 58 mg), Pd(OAc)2 (10 mol
%, 6.7 mg), 4CzIPN (2 mol %, 4.7 mg), CH3COOH (0.3 mL), n-Bu4NOAc (3 equiv, 0.9
mmol, 270 mg), and dichloroethane (DCE, 3 mL) were used for photocatalytic acetoxylation
reaction. 2g was obtained as yellow oil (33 mg, 44%). 1H NMR (400 MHz, CDCl3) δ 8.11 (d,
J = 8.7 Hz, 1H), 8.00 (s, 1H), 7.62 (d, J = 8.4 Hz, 1H), 4.00 (s, 3H), 2.33 (s, 3H), 2.18 (s,
3H); C{1H} NMR (101 MHz, CDCl3) δ 168.4, 151.8, 148.2, 148.0, 136.6, 130.2, 120.8,
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119.0, 62.3, 20.9, 14.8. Spectral data matched with the literature report.15b
2-(1-Methoxyiminoethyl)-5-methylphenyl acetate (2h). Compound 2h was prepared
according to the general reaction condition (viii). 1h (0.3 mmol, 49 mg), Pd(OAc)2 (10 mol
%, 6.7 mg), 4CzIPN (2 mol %, 4.7 mg), CH3COOH (0.3 mL), n-Bu4NOAc (3 equiv, 0.9
mmol, 270 mg), and dichloroethane (DCE, 3 mL) were used for photocatalytic acetoxylation
reaction. 2h was obtained as yellow oil (43 mg, 65%).1H NMR (400 MHz, CDCl3) δ 7.31 (d,
J = 7.9 Hz, 1H), 7.06 (d, J = 7.9 Hz, 1H), 6.91 (s, 1H), 3.95 (s, 3H), 2.35 (s, 3H), 2.27 (s,
3H), 2.14 (s, 3H); C{1H} NMR (151 MHz, CDCl3) δ 169.3, 153.3, 147.7, 140.1, 129.1,
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127.3, 126.8, 123.7, 61.8, 21.0, 14.9. Spectral data matched with the literature report.53
2-(1-Methoxyiminoethyl)-5-methyl-1, 3-phenylene diacetate (2i). Compound 2i was
prepared according to the general reaction condition (viii). 1i (0.3 mmol, 66 mg), Pd(OAc)2
(10 mol %, 6.7 mg), 4CzIPN (2 mol %, 4.7 mg), CH3COOH (0.3 mL), n-Bu4NOAc (3 equiv,
0.9 mmol, 270 mg), and dichloroethane (DCE, 3 mL) were used for photocatalytic
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The Journal of Organic Chemistry
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acetoxylation reaction. 2i was obtained as yellow oil (28 mg, 34%). H NMR (400 MHz,
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CDCl3) δ 6.84 (s, 2H), 3.92 (s, 3H), 2.34 (s, 3H), 2.23 (s, 6H), 2.01 (s, 3H); C{1H} NMR
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(101 MHz, CDCl3) δ 168.8, 150.1, 148.7, 140.1, 121.5, 120.9, 61.9, 21.2, 20.8, 15.5; HRMS
(ESI-TOF) m/z: [M + H]+ Calcd for C14H18NO5 280.1185; Found 280.1193.
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3-Methoxy-2-(1-methoxyiminoethyl)phenyl acetate (2j). Compound 2j was prepared
according to the general reaction condition (viii). 1j (0.3 mmol, 53 mg), Pd(OAc)2 (10 mol
%, 6.7 mg), 4CzIPN (2 mol %, 4.7 mg), CH3COOH (0.3 mL), n-Bu4NOAc (3 equiv, 0.9
mmol, 270 mg), and dichloroethane (DCE, 3 mL) were used for photocatalytic acetoxylation
reaction. 2j was obtained as yellow oil (mixture of E/Z isomers, 23 mg, 32%). 1H NMR (400
MHz, CDCl3) δ 7.29 (t, J = 8.1 Hz, 1H), 6.81 – 6.77 (m, 1H), 6.71 (d, J = 8.2 Hz, 1H), 3.94
(s, 2H), 3.82 (s, 3H), 3.79 (s, 1H), 2.23 (s, 3H), 2.10 (s, 1H), 2.07 (s, 2H); 13C{1H} NMR (151
MHz, CDCl3) δ 169.2, 168.8, 158.4, 156.4, 151.7, 149.9, 148.8, 147.3, 129.6, 129.6, 120.3,
118.2, 115.0, 114.8, 108.6, 108.5, 61.7, 61.6, 55.9, 20.7, 20.1, 15.7; HRMS (ESI-TOF) m/z:
[M + H]+ Calcd for C12H16NO4 238.1079; Found 238.1083.
4-Methoxy-2-(1-methoxyiminoethyl)phenyl acetate (2k). Compound 2k was prepared
according to the general reaction condition (viii). 1k (0.3 mmol, 53 mg), Pd(OAc)2 (10 mol
%, 6.7 mg), 4CzIPN (2 mol %, 4.7 mg), CH3COOH (0.3 mL), n-Bu4NOAc (3 equiv, 0.9
mmol, 270 mg), and dichloroethane (DCE, 3 mL) were used for photocatalytic acetoxylation
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reaction. 2k was obtained as yellow liquid (mixture of E/Z isomers, 45 mg, 63%). H NMR
(400 MHz, CDCl3) δ 7.29 (t, J = 8.2 Hz, 1H), 6.79 (d, J = 7.9 Hz, 1H), 6.71 (d, J = 8.1 Hz,
1H), 3.93 (s, 2H), 3.82 (s, 3H), 3.79 (s, 1H), 2.23 (s, 3H), 2.10 (s, 1H), 2.07 (s, 2H); 13C{1H}
NMR (151 MHz, CDCl3) δ 169.3, 168.9, 158.4, 156.4, 151.7, 149.9, 148.7, 147.3, 129.7,
120.2, 118.1, 114.9, 114.8, 108.6, 108.5, 61.8, 61.6, 56.0, 55.9, 20.9, 20.8, 20.1, 15.7.
Spectral data matched with the literature report.54
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