Ceric ammonium nitrate (CAN) promoted PdII-catalyzed substrate-directed o-benzoxylation and decarboxylative o-aroylation

Article abstract of DOI:10.1002/ejoc.201403367

Inexpensive ceric ammonium nitrate (CAN) is an efficient oxidant for the Pd-catalyzed substrate-directed o-benzoxylation and decarboxylative o-aroylation processes. In the presence of CAN, the reaction of directing arenes with carboxylic acids resulted in o-benzoxylated products, whereas a decarboxylative o-aroylation occurred by using innodataalpha-keto acids, which led to the formation of o-aroylation products.

Full text of DOI:10.1002/ejoc.201403367

Job/Unit: O43367
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Date: 26-11-14 13:53:18
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DOI: 10.1002/ejoc.201403367
Ceric Ammonium Nitrate (CAN) Promoted PdII-Catalyzed Substrate-Directed
o-Benzoxylation and Decarboxylative o-Aroylation
Sourav Kumar Santra,[a][‡] Arghya Banerjee,[a][‡] Nilufa Khatun,[a] and Bhisma K. Patel*[a]
Keywords: Synthetic methods / C–H activation / Cross-coupling / Oxidation / Palladium / Cerium / Regioselectivity
Inexpensive ceric ammonium nitrate (CAN) is an efficient
oxidant for the Pd-catalyzed substrate-directed o-benzoxyl-
ation and decarboxylative o-aroylation processes. In the
presence of CAN, the reaction of directing arenes with carb-
oxylic acids resulted in o-benzoxylated products, whereas a
decarboxylative o-aroylation occurred by using α-keto acids,
which led to the formation of o-aroylation products.
catalytic cycle, which is often done with the aid of stoichio-
metric amounts of sacrificial terminal oxidants or other ad-
ditives. Ceric ammonium nitrate (CAN), a one-electron oxi-
dant, has been employed for various functional group
transformations[4] and in the syntheses of heterocycles.[5] In
spite of its immense applications as an oxidizing agent, its
use as a terminal oxidant in palladium-catalyzed processes
is completely unexplored. In continuation of our efforts
with regard to transition-metal-catalyzed substrate-directed
C–H functionalizations,[1h,6,12a] we have explored the use of
CAN as a terminal oxidant. The reduction potentials of
well-known oxidants that are used for decarboxylative o-
aroylation and o-benzoxylation reactions are +0.80 V for
AgI/Ag0 and +2.01 V for S2O82–/SO42–. In comparison, the
reduction potential of CeIV/CeIII is +1.61 V, which is be-
tween that of AgI/Ag0 and S2O82–/SO42–. Thus, CeIV/CeIII
is moderately oxidizing in nature, and as a single-electron
oxidant, it may be capable of forming a carboxy radical
from a carboxylic acid and an aroyl radical through a carb-
oxy radical from an α-keto acid.
Introduction
The C–O bond formation reaction that proceeds through
a C–H bond functionalization to give an ester has led to a
resurgence of transition-metal-catalyzed reactions.[1] In this
context, directing[2] and nondirecting[3] cross-dehydro-
genative coupling (CDC) reactions are the most preferred
approaches because of their step and atom economy. In ad-
dition to carboxylic acids,[1a,1b,2a–2c,2e] aroyl peroxides,[1c]
acyl chlorides,[1g] acid anhydrides,[1k] aldehydes,[2d] alkyl-
benzenes,[2d] terminal alkenes,[1h] and terminal alkynes[1h]
have been employed as o-benzoxy surrogates. A carboxylic
acid has a propensity to form a metal complex, thereby ren-
dering the metal inactive and inhibiting the progress of a
reaction. To overcome this problem, a modified carboxylate
source such as PhI(OCOR)2 has been employed by
Sanford[1a] as well as others.[1b] The combinations of
AgSbF6/(NH4)2S2O8,[2a,2b] CuI/Ag2CO3,[2c] and P(Cy)3·
HBF4/CuI[2e] (Cy = cyclohexyl) along with metal catalysts
such as Ru, Pd, and Rh have, in part, obviated the problems
associated with their direct use. No doubt these modifica-
tions have improved the yields, but they are economically
unviable. Thus, the lack of a cost-effective and generalized
strategy for an o-benzoxylation that involves a range of di-
recting groups leaves an ample opportunity to devise an
alternative approach.
Results and Discussion
To the best of our knowledge, 2-arylbenzothiazole has
never been o-benzoxylated through a C–H bond function-
alization, however, it has been ortho-functionalized by using
Pd catalysts.[6a,6b] To test the efficacy of CAN as an oxidant,
A typical substrate-directed transition-metal-catalyzed o-
benzoxylation proceeds through a cyclometallation, oxidat-
ive addition, or ligand exchange followed by a reductive
elimination. For a ligand exchange path, the metal is in its
reduced state and needs to be reoxidized to maintain the
a
CDC reaction between 2-phenylbenzothiazole (1,
1 equiv.) and benzoic acid (a, 1.2 equiv.) was carried out by
using Pd(OAc)2 (5 mol-%) and CAN (1 equiv.) in toluene
(2.5 mL) at 110 °C. The expected mono-o-benzoxylated
product 1a was obtained in 38% isolated yield. In addition
to spectroscopic analysis for the characterization of the
product, the structure of 1a was further confirmed by using
XRD analysis (see Figure 1). Fascinated by this preliminary
success, we assessed other reaction parameters such as sol-
vent, oxidant, catalyst, and amounts to achieve the best
[a] Department of Chemistry, Indian Institute of Technology
Guwahati,
Guwahati 781 039, Assam, India
E-mail: patel@iitg.ernet.in
http://www.iitg.ac.in/patel/
[‡] These authors contributed equally.
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201403367.
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FULL PAPER
possible yield. Of the optimization results (see Table 1), the Table 1. Screening of reaction conditions for o-benzoxylation.
conversion of 2-phenylbenzothiazole (1, 1 equiv.) into o-
benzoxylated product 1a was best achieved by using ben-
zoic acid (1.2 equiv.) and CAN (1.5 equiv.) in the presence
of Pd(OAc)2 (5 mol-%) in a mixture of 1,2-dichloroethane
(DCE)/CH3CN (5:1, 3 mL) at 110 °C. The addition of
CH3CN was necessary to make the medium homogeneous,
thereby improving the yield (71%, see Table 1). When the
reaction was carried out with 2 equiv. of benzoic acid for a
longer period of time (17 h), trace amounts (11%) of di-
benzoxylated product 1aa were observed.
Entry Catalyst [mol-%]
Solvent
Oxidant
% Yield 1a/1aa[a,b]
1
2
3
4
5
6
7
8
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)
Pd(OAc)2 (5.0)
Pd(OAc)2 (5.0)
Pd(OAc)2 (10.0)
Pd(TFA)2 (5.0)
PdCl2 (5.0)
toluene
o-xylene
CH3CN
1,4-dioxane
DMF[d]
DMSO[d]
DCE
DCE
DCE
DCE
DCE
CAN
CAN
CAN
CAN
CAN
CAN
CAN
CAN
CAN
38
10
0[c]
0[c]
0[c]
0[c]
56
62
41
43
9
10
11
12
13
12
13
14
15
CAN
CAN
CAN
PdBr2 (5.0)
48
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)
Pd(OAc)2 (5.0)
DCE
DCE
DCE
DCE
67[e]
0[c]
oxone
Ag2CO3/CuI
K2S2O8
CAN
0[c]
0[c]
Figure 1. ORTEP view of 1a.[16]
DCE/CH3CN
DCE/CH3CN
71[e,f]
62:11[g]
CAN
Encouraged by the efficacy of CAN as an oxidant, we
further implemented the esterification strategy to the cou-
pling between 2-phenylbenzothiazole and a series of carb-
oxylic acids. This strategy was well-suited for a variety of
aromatic acids that contained electron-donating and -with-
drawing substituents. Aromatic acids that have electron-do-
nating groups such as p-Me, p-OMe, and o-OMe (i.e., b, c,
and d, respectively) provided the corresponding o-benzoxyl-
ated products 1b, 1c, and 1d in good to moderate yields (see
[a] Reagents and conditions: 2-arylbenzothiazole (0.5 mmol),
benzoic acid (0.60 mmol), and oxidant (0.5 mmol) at 110 °C for
12 h. [b] Isolated yield. [c] Complete recovery of starting materials.
[d] DMF = N,N-dimethylformamide, DMSO = dimethyl sulfoxide.
[e] CAN (0.75 mmol) was used. [f] DCE/CH3CN (5:1, total 3 mL)
was used. [g] Benzoic acid (1 mmol) and CAN (1 mmol) were used
at 110 °C for 17 h.
This strategy was also successful with 2-aryl-substituted
Scheme 1). Aromatic acids that contained moderately and benzothiazoles that contained the electron-donating substit-
strongly electron-withdrawing substituents such as p-Cl, p- uent p-Me (i.e., 2) and moderately electron-withdrawing
NO2, and o-NO2 (i.e., e, f, and g) afforded the correspond- groups m-Cl and o-Cl (i.e., 3 and 4). These substituted sub-
ing o-benzoxylated products 1e, 1f, and 1g in 74, 80, and strates provided the expected o-benzoxylated products 2a,
83% yields respectively (see Scheme 1). A similar trend in 3a, and 4a, respectively, as shown in Scheme 2. Substituted
reactivity was observed for di- and trisubstituted aromatic benzothiazoles that contained the electron-withdrawing –
acids that contained electron-withdrawing substituents such CF3 group and electron-donating –Me group (i.e., 5 and 6,
as 2,6-dichloro and 2,4,5-trifluoro (i.e., h and i), and these respectively) gave o-benzoxylated products 5a and 6a in
carboxylic acids afforded their corresponding o-benzoxyl- good yields (see Scheme 2). The versatility of CAN as an
ated products 1h and 1i in decent yields (see Scheme 1). oxidizing agent was successfully demonstrated with other
Fused aromatic acid j also afforded a good yield of o-carb- well-investigated directing arenes such as acetophenone O-
oxylated product 1j. The oxidant CAN was also amenable methyl oxime, 2-phenylpyridine, and 2,3-diphenylquinoxal-
to the α,β-unsaturated system trans-cinnamic acid (k) and ine. Under the present optimized conditions, acetophenone
provided a moderate yield (57%) of o-functionalized prod- O-methyl oxime (7) provided the desired o-benzoxylated
uct 1k. The efficiency of the coupling reaction was poor product 7a in good yield (80%) in a shorter reaction time
for aliphatic carboxylic acids, which may be a result of the (6 h). Acetophenone O-methyl oxime that contained the
instability of the in situ generated aliphatic carboxy radi- electron-deficient p-Cl substituent (i.e., 8) afforded a lower
cals. Phenylacetic acid (l), propanoic acid (m), and pivalic yield (71%) of o-benzoxylated product 8a, whereas elec-
acid (n) underwent the CDC reaction to afford o-carboxyl- tron-rich m-methylacetophenone O-methyl oxime (9) gave
ated products 1l, 1m, and 1n in 48, 45, and 34% yield, the regioselective o-benzoxylated product 9a in 84% yield.
respectively.
The most employed directing ligand moiety, 2-phenyl pyr-
2
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CAN-Promoted o-Benzoxylation and o-Aroylation
Scheme 1. Scope of acids for o-benzoxylation of 1.[a,b] [a] Reagents and conditions: 1 (0.5 mmol), carboxylic acid (0.6 mmol), and CAN
(0.75 mmol) at 110 °C for 12 h. [b] Isolated yield. [c] The remainder was unreacted starting materials.
idine (10), provided o-benzoxylated product 10a in 69% thesis. Thus, we questioned whether CAN could act as an
yield by using CAN. This yield is comparable with those alternative for the silver salts, the persulfate (S2O82–), or
reported by other groups that employed PdII/CuI/ combination thereof during a decarboxylative o-aroylation.
[2c]
Ag2CO3 and RhI/P(Cy)3·HBF4/CuI[2e] combinations and To check the efficacy of CAN as a terminal oxidant, a CDC
is slightly lower than that obtained by using the PdII/PhI- reaction between 2-phenylbenzothiazole (1, 1 equiv.) and
[1a]
(OCOR)2
catalytic system. However, the 2,3-diphenyl- phenylglyoxylic acid (aЈ) (1.2 equiv.) was carried out in the
quinoxaline (11) directing system afforded o-benzoxylated presence of Pd(OAc)2 (5 mol-%) and CAN (1.5 equiv.) in
product 11a in a mere yield of 37% (see Scheme 2). 1,2-dichloroethane. The desired o-aroylated product 1aЈ was
Palladium-catalyzed decarboxylative cross-coupling reac- obtained in a paltry yield of 26%. With this preliminary
tions of benzoic acids are best carried out in the presence success, to achieve the best possible conversion, other reac-
of silver or copper salts.[7] On the other hand, directed[8] or tion parameters such as solvent, catalyst, oxidant, and their
nondirected[9] decarboxylative cross-coupling reactions of quantities were varied (see Table 2). Other oxidants were
α-keto acids are effective in the presence of either silver, a also examined for comparison. The optimization results
persulfate (S2O82–) source, or combination thereof, which indicate that the decarboxylative coupling between 2-phen-
makes the process economically unviable for large scale syn- ylbenzothiazole (1, 1 equiv.) and phenylglyoxylic acid (aЈ,
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Scheme 2. Scope of directing groups for o-benzoxylation.[a,b] [a] Reagents and conditions: directing arene (0.5 mmol), benzoic acid
(0.6 mmol), and CAN (0.75 mmol) at 110 °C for 6–12 h. [b] Isolated yield.
1.2 equiv.) was best achieved by using Pd(OAc)2 (10 mol-%) Br, and p-Cl (i.e., 4, 14, and 15) substituents on the 2-phenyl
and CAN (1.5 equiv.) in DMF at 110 °C (see Table 2). ring coupled efficiently with aЈ to provide the desired o-
These standardized conditions were used to examine the aroylated products in moderate to good yields (see
decarboxylative o-aroylation reactions of a range of direct- Scheme 3). However, no correlation could be ascertained
ing arenes.
between the effect of substituent on the 2-phenyl ring of
Direct o-aroylation reactions have primarily been ac- the 2-arylbenzothiazole and the yield of product. m-Bromo-
complished by using three different approaches. In sub- substituted 2-arylbenzothiazole 14 provided o-aroylated
strate-directed CDC approaches, aldehydes[6a,10] and benzyl product 14aЈ with the reaction occurring at the less steri-
alcohols[11] are employed as aroyl surrogates. By using alk- cally hindered ortho site as a result of a favorable cyclopal-
ylbenzenes as the synthetic equivalent of the aroyl moiety, ladation step. The presence of the strong electron-with-
our group and others have recently demonstrated their in- drawing –CF3 substituent on the benzothiazole ring of 5
stallation at the ortho site of various directing substrates.[12] provided a comparable yield of 5aЈ to that of electroneutral
However, the most striking o-aroylation strategy is through substrate 1 (see Scheme 3).
the decarboxylation of α-ketocarboxylic acids. Thus, using
2-Phenylbenzoxazole (16), a structural analogue of 2-
above standardized conditions, we scrutinized the scope phenylbenzothiazole (1), provided o-aroylated product 16aЈ
and generality of a decarboxylative coupling reaction with in low yield (44%), even by using an excess amount of
various 2-aryl-substituted benzothiazoles and phenylglyox- Pd(OAc)2 (15 mol-%) and after a prolonged reaction time
ylic acid (aЈ). 2-Phenylbenzothiazole that contained electro- (48 h). Similarly, p-Me-substituted 2-phenylbenzoxazole 17
neutral –H (i.e., 1), electron-donating p-Me, p-OMe, and p- gave a meager yield of 40% under the identical conditions.
tBu (i.e., 2, 12, and 13), and electron-withdrawing o-Cl , m- However, for substrate 16, well-established oxidants[8] such
4
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CAN-Promoted o-Benzoxylation and o-Aroylation
Table 2. Screening of reaction conditions for o-aroylation.
Entry Catalyst [mol-%] Solvent
Oxidant
Yield [%][a,b]
1
2
3
4
5
6
7
8
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)
Pd(OAc)2 (5.0)
Pd(OAc)2 (10.0)
Pd(OAc)2 (2.0)
DCE
CAN
CAN
CAN
CAN
CAN
CAN
CAN
CAN
CAN
26
8
cyclohexane
dioxane
toluene
DMF
DMSO
DMF
34
39
47
0[c]
68
32
44
53
35
0[c]
60
41
DMF
9
Pd(TFA)2[d] (10.0) DMF
10
11
12
13
14
PdCl2 (10.0)
PdBr2 (10.0)
Pd(OAc)2 (10.0)
Pd(OAc)2 (10.0)
Pd(OAc)2 (10.0)
DMF
DMF
DMF
DMF
DMF
CAN
CAN
Ag2CO3
K2S2O8
(NH4)4Ce(SO4)4·2H2O
[a] Reagents and conditions: 2-arylbenzothiazole (0.25 mmol),
phenylglyoxylic acid (0.30 mmol), and oxidant (0.38 mmol) at
110 °C for 8 h. [b] Isolated yield. [c] Complete recovery of starting
materials. [d] Palladium(II) trifluoroacetate is Pd(TFA)2.
as AgI failed to give the desired product, whereas the use
of persulfate (S2O82–) gave a comparable yield to that by
using CAN.
The usefulness of CAN as an oxidizing agent was then
successfully applied to directing arene 3,5-diphenylisoxazole
(18). 3,5-Diphenylisoxazole (18) with N and O-chelating
atoms provided the expected o-aroylated product 18aЈ but
in a lower yield (37%). The product yields for these oxygen-
containing N-directed substrates 16, 17, and 18 are less than
those of the sulfur-containing ones (see Scheme 3). Because
of its oxophilic nature, the cerium atom possibly remains
bound to the oxygen atom of the oxygen-containing direct-
ing arene, thereby making it unavailable for the oxidative
addition and giving lower yields. However, the directing ar-
ene that contains two nitrogen atoms, as in the case of 2,3-
Scheme 3. Substrate scope for decarboxylative o-aroylation[a,b]
[a] Reagents and conditions: directing arene (0.25 mmol), α-keto
acid (0.30 mmol), and CAN (0.38 mmol) at 110 °C for 8 h. [b] Iso-
lated yield. [c] The remainder was recovered starting materials.
[d] Reactions were performed for 48 h by using 15 mol-% Pd(OAc)2.
[e] DCE was used in lieu of DMF.
To demonstrate the synthetic utility of CAN as a ter-
diphenylquinoxaline (11), provided o-aroylated product minal oxidant, both o-benzoxylation and o-aroylation stra-
11aЈ in good yield (71%) under the optimized reaction con- tegies were successfully applied in sequence (as a one-pot
ditions. It is worth noting that none of the above-mentioned reaction) for the construction of the o-bifunctionalized
directing groups have been previously employed in a de- compound 2-(benzo[d]thiazol-2-yl)-3-benzoylphenyl benzo-
carboxylative o-aroylation reaction with phenylglyoxylic ate (1aaЈ see Scheme 4). For this, 2-phenylbenzothiazole (1)
acid. Further o-aroylations of directing arenes 1 and 11 was chosen as the directing arene, and product 1aaЈ was
were carried out with substituted phenylglyoxylic acids. p- obtained in 30% overall yield.
Methyl-substituted phenylglyoxylic acid bЈ coupled ef-
Taking cues from the above experimental observations,
ficiently with both directing arenes 1 and 11 to provide their plausible mechanisms can be proposed for both the o-
desired products 1bЈ and 11bЈ in almost identical yields (see benzoxylation and o-aroylation reactions (see Scheme 5).
Scheme 3). The coupling reaction of p-Cl-substituted phen- When a typical o-benzoxylation or o-aroylation was carried
ylglyoxylic acid cЈ with 1 and 11 proceeded better than out in the presence of the radical scavenger 2,2,6,6-tetra-
those of p-Me-substituted phenylglyoxylic acid bЈ and pro- methylpiperidine-1-oxyl (TEMPO, 1 equiv.), significant re-
vided o-aroylated products 1cЈ and 11cЈ.
ductions in the product yields (5–7%) were observed,
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PdIII[13] intermediate II, as was detected by mass spectral
analysis of the reaction mixture (see Supporting Infor-
mation, Figure S1). Furthermore, the detection of a mono-
meric PdIV[14] species in the reaction aliquot[15] (see Sup-
porting Information, Figure S1) may be the result of a Pd–
Pd cleavage in dimeric PdIII intermediate II to give mono-
meric PdIV[14d] and PdII species. A reductive elimination
leads to the o-benzoxylated product and forms the active
dimeric species III. Intermediate III further releases another
o-benzoxylated product by C–O bond formation and regen-
erates dinuclear PdII active species II for the next catalytic
cycle. For the decarboxylative o-aroylation process, the de-
tection of TEMPO ester (E) along with quenching the reac-
tion supports the formation of an aroyl radical (see Sup-
porting Information). A similar mechanism could be envis-
aged for the o-aroylation process.
Scheme 4. One-pot heterobifunctionalization of 2-phenylbenzo-
thiazole (1).
thereby supporting a radical pathway for these processes.
Because of the formation of relatively stable benzoxy radi-
cals from aromatic acids that have an electron-withdrawing
group, these substrates afforded better yields of product
than those obtained by using aromatic acids that have elec-
tron-donating groups. When the benzoxylation of 1 was
carried out in the presence of 1, 1.5, and 2 equiv. of oxidant
(CAN) under otherwise identical conditions, the isolated
yields of the product after 12 h were 66, 71, and 72%
respectively. These experiments suggest a minimum require-
ment of 1 equiv. of oxidant is needed for this transforma-
tion, thereby indicating a PdII/PdIII catalytic cycle.[13]
Conclusions
In summary, we have demonstrated the use of the inex-
pensive terminal oxidant CAN as an efficient substitute for
a set of expensive oxidants/additives in the Pd-catalyzed
substrate-directed o-benzoxylation and decarboxylative o-
aroylation processes that proceed through a CDC reaction.
Mechanistic investigations reveal a radical pathway for both
of these strategies.
Experimental Section
Synthesis of 2-(Benzo[d]thiazol-2-yl)phenyl Benzoate (1a) from 2-
Phenylbenzothiazole (1) and Benzoic Acid (a): To an oven-dried
round-bottomed flask (25 mL) fitted with a reflux condenser were
added 2-phenylbenzothiazole (1, 0.105 g, 0.5 mmol), benzoic acid
(0.073 g, 0.6 mmol), Pd(OAc)2 (0.006 g, 0.025 mmol), ceric ammo-
nium nitrate (0.411 g, 0.75 mmol), 1,2-dichloroethane (2.5 mL),
and acetonitrile (0.5 mL). The reaction mixture was heated at re-
flux in an oil bath that was preheated to 110 °C. Upon completion
of the reaction (12 h), the solvent was evaporated under reduced
pressure, and the reaction mixture was combined with ethyl acetate
(30 mL). The ethyl acetate layer was carefully washed with satu-
rated sodium hydrogen carbonate solution (2ϫ 5 mL), dried with
anhydrous sodium sulfate, and evaporated under reduced pressure.
The crude product was purified over a silica gel column (hexane/
ethyl acetate, 10:0.2) to give pure 2-(benzo[d]thiazol-2-yl)phenyl
benzoate (1a, 0.117 g, 71% yield). The identity and purity of the
product was confirmed by spectroscopic analysis.
Synthesis of (2-Benzothiazol-2-ylphenyl)-phenyl-methanone (1aЈ)
from 2-Phenylbenzothiazole (1) and Phenylglyoxylic Acid (aЈ): An
oven-dried flask was charged with 2-phenylbenzothiazole (1,
0.053 g, 0.25 mmol), phenylglyoxylic acid (aЈ, 0.045 g, 0.30 mmol),
ceric ammonium nitrate (0.206 g, 0.38 mmol), and Pd(OAc)2
(0.006 g, 0.025 mmol) in DMF (2 mL). The reaction mixture was
then stirred in a preheated oil bath at 110 °C for 8 h. After the
stipulated time, the reaction mixture was cooled to room tempera-
ture and diluted with ethyl acetate (30 mL). The organic layer was
subsequently washed with a saturated solution of sodium hydrogen
carbonate solution (2ϫ 5 mL). The ethyl acetate layer was then
dried with anhydrous Na2SO4 and evaporated under reduced pres-
Scheme 5. Proposed mechanism for o-benzoxylation.
For the o-benzoxylation process, the initial cyclopallad-
ation of 2-phenylbenzothiazole (1) leads to the formation
of acetate bridged binuclear PdII intermediate I (see
Scheme 5). This dimeric PdII complex further undergoes a
bimetallic oxidative addition with the in situ generated
benzoxy radical that is obtained by the action of CAN with
benzoic acid. The proximity of the two Pd centers might
facilitate cooperative redox chemistry, in which both metals
participate synergistically to lower the barrier of the redox
transformation. The oxidative addition product is dimeric sure. The crude product was purified by chromatography on a silica
6
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Date: 26-11-14 13:53:18
Pages: 8
CAN-Promoted o-Benzoxylation and o-Aroylation
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thiazol-2-ylphenyl)-phenyl-methanone (1aЈ, 0.054 g, yield 68%).
Supporting Information (see footnote on the first page of this arti-
cle): General methods, details of crystal structure analysis of 1a,
mechanistic investigation, and spectra for all compounds.
Acknowledgments
B. K. P. acknowledges the support of this research by the Depart-
ment of Science and Technology (DST), New Delhi (SB/S1/OC-53/
2013) and the Council of Scientific and Industrial Research
(CSIR), New Delhi [02(0096)/12/EMR-II]. A. B., S. K. S., and
N. K. thank CSIR for support.
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Received: October 18, 2014
Published Online: ᭿
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7

Job/Unit: O43367
/KAP1
Date: 26-11-14 13:53:18
Pages: 8
S. K. Santra, A. Banerjee, N. Khatun, B. K. Patel
FULL PAPER
C–H Functionalization
S. K. Santra, A. Banerjee, N. Khatun,
B. K. Patel* ...................................... 1–8
Ceric Ammonium Nitrate (CAN) Pro-
moted PdII-Catalyzed Substrate-Directed
o-Benzoxylation and Decarboxylative o-
Aroylation
CAN can activate: Inexpensive ceric am-
monium nitrate (CAN) is an efficient
oxidant for the Pd-catalyzed substrate-di-
rected o-benzoxylation and decarboxylative
o-aroylation processes. In the presence of
CAN, the reaction of directing arenes with
carboxylic acids resulted in o-benzoxylated
products, and that with α-keto acids led to
the formation of o-aroylation products.
Keywords: Synthetic methods / C–H acti-
vation / Cross-coupling / Oxidation / Pal-
ladium / Cerium / Regioselectivity
8
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Eur. J. Org. Chem. 0000, 0–0