Iron-Catalyzed Minisci Type Acetylation of N-Heteroarenes Mediated by CH(OEt)3/TBHP

Article abstract of DOI:10.1002/ejoc.201900033

Iron-catalyzed acetylation of electron deficient N-heteroarenes has been reported using triethylorthoformate as robust and inexpensive acetyl source. This new method is successfully applied for the acetylation of quinolines, isoquinoline, quinoxalines, arylpyridines, bipyridines, and benzothiazole.

Full text of DOI:10.1002/ejoc.201900033

A Journal of
Accepted Article
Title: Iron catalyzed Minisci type Acetylation of N-Heteroarenes
Mediated by CH(OEt)3/TBHP
Authors: Srinivasulu A, Shantharjun B, Vani D, Chinna Ashalu K, Mohd
A, Wencel-Delord J, Colobert F, and Rajender Kallu Reddy
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.201900033
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10.1002/ejoc.201900033
European Journal of Organic Chemistry
COMMUNICATION
Iron Catalyzed Minisci Type Acetylation of N-Heteroarenes
Mediated by CH(OEt)3/TBHP
A. Srinivasulu,a,b,c B. Shantharjun,a,b,c D. Vani,a,b K. Chinna Ashalu,a A. Mohd,d J. Wencel-Delord,d F.
Colobertd and K. Rajender Reddya,b*
Considering importance of functionalized N-heterocycles, we
surmised that design of new routes towards a large panel of
acetylated products, using cheap and abundant metal-catalyst is
a challenging research field. Accordingly, herein we report an
alternative strategy for acetylation of N-heterocycles, using for
the first time triethylorthoformate in combination with TBHP as
robust and inexpensive acetyl radical source. Remarkably, the
desired reactivity is achieved with a cheap and abundant iron
catalyst and the coupling occurs without the need of external
solvent, since triethylorthoformate is used as self-solvent: This
work follows our previous contribution on the iron catalyzed
oxidative cross coupling reactions.8-11
Abstract:Ironcatalysed acetylation of electron deficient N-
heteroarenes has been reported using triethylorthoformate
robust and inexpensive acetyl source. This new method is
successfully applied for the acetylation of quinolines,
isoquinoline, quinoxalines, arylpyridines, bipyridines and
benzothiazole.
Introduction
N-heterocycles, such as quinoline and isoquinoline
derivatives, are key structural motifs of natural products and
biologically active scaffolds. In particular, quinoline alkaloids
have attracted great interest, due to their antitussive,
antipsychotic, antispasmodic, anxiolytic, antibacterial, antifugial
and anticancer activities.1 Consequently, design of synthetic
approaches allowing rapid and straightforward access to these
compounds from simple and non-prefunctionalized precursors is
an attractive research field. In this context, Minisci reaction,2
implying radical C-H functionalization of electron deficient N-
heteroarenes established itself as a method of choice to install
alkyl or acyl moieties. Following this seminal work, a variety of
protocols allowing radical acylation of electron-deficient N-
heteroarenes have been devised.2 Accordingly, α-keto acids,2b
aldehydes,2c benzylamines,2d aryl methanes,2e and etc. have
proven to be appealing coupling partners to rapidly introduce
aromatic and aliphatic acyl moieties. In clear contrast, direct
installation of the acetyl moiety is much more challenging.
Indeed, acetylation is not possible when toluene derivatives or
benzylamines substrates are used as aldehyde source and
when 2-oxopropanoic acid is selected as acetyl precursor, use
of a silver salt in combination with strong oxidants and additives
are needed.2 Zhu and co-workers discovered that gold is also
promising catalyst for such reaction, allowing a radical coupling
between a large panel of N-heterocycles and ethanol.3 Recently,
silver-catalyzed acetylation mediated by a mild Selectfluor
oxidant was disclosed by Baxter.4 In parallel, a metal-free
acetylation of isoquinoline was achieved using α-keto acids as
coupling partner, in combination with potassium persulfate
Results and Discussion
Table 1: Screening of Reaction Conditionsa
Oxidant
(equiv)
S.No
Catalyst (mol %)
Temp ( °C)
Yield (%)b
1
2
Fe(ClO4)3.XH2O (10)
Fe(ClO4)3.XH2O (10)
Fe(ClO4)3.XH2O (10)
Fe(ClO4)3.XH2O (10)
Fe(ClO4)3.XH2O (10)
Fe(ClO4)3.XH2O (20)
Fe(ClO4)2.XH2O (10)
FeCl3.6H2O (10)
TBHP (3)
100
90
74
61
65
78
78
77
62
58
54
42
20
TBHP (3)
TBHP (3)
TBHP (4)
TBHP (5)
TBHP (4)
TBHP (4)
TBHP (4)
TBHP (4)
TBHP (4)
TBHP (4)
3
110
100
100
100
100
100
100
100
100
4
5
6
7
8
9
FeCl3 (10)
10
11
FeCl2 (10)
Fe(acac)2 (10)
oxidant.5
A
rare example of mild protocol employing
TBHP in
Dec (3)
12
13
14
15
16
Fe(ClO4)3.XH2O (10)
Fe(ClO4)3.XH2O (10)
Fe(ClO4)3.XH2O (10)
Fe(ClO4)3.XH2O (10)
Fe(ClO4)3.XH2O (10)
-
100
100
100
100
100
100
100
56
formaldehyde as coupling partner was disclosed by Antonchick.6
However, although efficiency of this strategy and its generality
need for PIFA oxidant and TMSN3 might be limiting. Finally, an
original electrocatalytic approach has been conceived very
recently, delivering acetylated heteroarenes in a presence of
pyruvic acid.7
DTBP (3)
TBPB (3)
DCP (3)
H2O2 (3)
TBHP (4)
-
NR
NR
NR
NR
NR
NR
17
18
Fe(ClO4)3.XH2O (10)
[a]
[b]
Catalysis and Fine chemicals Division, CSIR-Indian Institute of
Chemical Technology, Tarnaka, Hyderabad, Telangana State -500
007, India.E-mail: rajender@iict.res.in; rajenderkallu@gmail.com
Academy of Scientific and Innovative Research, New Delhi-
110025.
aReaction conditions: 1 (1 mmol), 2 (2 mL), 3 (70% TBHP in water), time 12 h.
bIsolated yields after column chromatography.
We initiated our experimental work by establishing optimal
reaction conditions for acetylation of 2-methylquinoline 1a with
triethylorthoformate 2. Rapidly it was discovered that when 1a (1
mmol) was reacted with an excess amount of 2 in the presence
of 3 (3 equiv of TBHP 70% aq solution) at 100 °C, 4-acetyl-2-
methyl quinoline 4a was formed in 74% yield (Table 1, entry 1).
By varying the temperature to 90 °C or 110 °C, the efficiency of
[c]
[d]
Equal author contribution
Laboratoired’InnovationMoléculaire et Applications (UMR CNRS
7042), Université de Strasbourg / Université de haute Alsace,
ECPM, 25 Rue Becquerel, 67087, Strasbourg, France
Supporting information for this article is given via a link at the end
of the document.((Please delete this text if not appropriate))
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10.1002/ejoc.201900033
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acridine afforded 4j in moderate yield (50%). Notably, the
mixtures of acetylated products were obtained using “not biased”
quinoline congeners, with a comparable rate at both, C2 and C4
positions. When 8-methyl quinoline (1k) underwent the
acetylation, both regioisomers4ka (2-acetyl-8-methyl quinoline)
and 4kb (4-acetyl-8-methyl quinoline) were delivered in 44% and
40% yields respectively. When 6-methyl quinoline (1l) was
subjected to the standard reaction conditions, the mixture of 2-
acetylated, 4-acetylated and 2,4-diacetylated products (4la-4lc)
were formed. The regio-isomers4la (2-acetyl-6-methyl quinoline)
and 4lb (4-acetyl-6-methyl quinoline) were generated in 45%
and 34% yields respectively. The diacylated product 4lc was
isolated only as a minor product in 5% yield. In the case of
simple quinoline, two region isomers 4ma (2-acetyl quinoline)
and 4mb (4-acetyl quinoline) were delivered in 42% and 44%
yield, while the diacetylated product 4mc was separated as a
minor compound in 8% yield (Table 2, entries 4ma-4mc).
Comparable results were obtained starting from quinolines
having the electron donating and electron withdrawing groups at
C4 position of quinoline giving 4-methyl-2-acetyl quinoline (4n)
and 4-chloro-2-acetyl quinoline (4o) (Table 2, 4n, 74% and 4o,
73%). When isoquinoline was subjected to the reaction, only 1-
acetyl isoquinoline was afforded in good yield (Table 2, 4p). In
contrast, acetylation of quinoxaline and 2-methyl quinoxaline
turned out to be less efficient, furnishing the corresponding
functionalized products 4q and 4r in 29% and 26% yield
respectively. Challenging diacetylation of phenanthroline gave
C2-symmetric 4s isolated in low, 22% yield (Table 2, 4s).
the coupling was decreased to 61% and 65% respectively
(Table 1, entries 2 and 3). Subsequently, it was determined that
4 equivalents is an optimal amount of TBHP oxidant (Table 1,
entries 4-5). Increasing catalyst loading, 20 mol%, had slightly
harmful impact on the reaction (Table 1 entry 6). By changing
the oxidation state of Fe (III) to Fe (II) the yield of the product 4a
has decreased to 62% (Table 1, entry 7). Other Fe (III) catalysts
such as ferric chloride hexa hydrate or simple ferric chloride
delivered 4a in lowered yields of 58% and 54% respectively
(Table 1, entries 8 and 9). Altering the oxidation state of the
catalyst from Fe(III) to Fe (II) drastically hampered the reaction
(4a isolated in 42% with FeCl2 and 20% with Fe(acac)2
respectively; Table 1, entries 10 and 11). The absence of water
in TBHP (TBHP in Decane) did not improve the yield of the
product 4a (Table 1, entry 12). No enhancement was achieved
by changing the oxidant from TBHP to DTBP (Di tert-butyl
peroxide), TBPB (tert-Butyl peroxybenzoate), DCP (Dicumyl
peroxide) and H2O2 (Table 1, entries 13-16). The product 4a,
was not detected neither in the absence of Fe(III) perchlorate
catalyst nor TBHP (Table 1, entries 17 and 18).
The generality and substrate scope of the reaction was
evaluated using various N-heteroarenes as shown in the Table
2, 4a-4s. Particularly 2-methyl quinolines reacted well with
triethylorthoformate and TBHP in the presence of Fe(III)
perchlorate.xH2O and afforded the corresponding acetyl
quinolines from moderate to good yields (52-75% yields, Table
2, 4a-4h).
Table 2: Substrate scope of quinolines, isoquinoline, quinoxalinea
Table 3: Acetylation of Pyridines, bipyridines and benzothiazolea
The substrate scope was extended to pyridines, bipyridines and
benzothiazole (Table 3, 6a-6l). When 2-phenyl pyridine 5a was
subjected to the standard reaction conditions, 6a was obtained
regioselectively but in moderate 37 % yield.12 Rewardingly,
addition of 1 equiv of pivalic acid enabled drastic improvement in
the reaction efficiency, providing the desired product in 65%
yield. This modified protocol was further employed for
acetylation of an array of N-heterocycles (Table 3, entries 6a-6l).
Electron withdrawing halogen substitution at positions 6, 7 and 8
of 2-methyl quinolines furnished the corresponding acetylated
products 4b-4f in high yields. Slightly lower efficiency is
observed for electron-rich substrates bearing methyl or methoxy
motifs at 6 position of 2-methyl quinoline (55% and 52% Table 2,
4g and 4h). Acetylation of 2-phenyl quinoline furnished the
corresponding product 4i in 72% yield and the fused aromatic
Pyridines bearing para-halogen substituted phenyl moieties
underwent the reaction smoothly; 4-F-C6H4, 4-Cl-C6H4, 4-Br-
C6H4 pyridines furnished the corresponding products 6b-6d in
better yields than electron rich substrates containing motifs such
as 4-OMe-C6H4 and 3-Me-4-OMe-C6H3 groups (Table 3, 6e and
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10.1002/ejoc.201900033
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COMMUNICATION
6f). When 2-OMe-5-F-C6H3 was reacted under standard
acetylation conditions, the expected product 6g was accessed in
42% yield. 2-naphthyl pyridine was converted into the
functionalized product 4-acetyl-2-naphthyl pyridine (6h) in 48%
yield (Table 3, 6h). Acetylation of the disubstituted substrate 2,6-
diphenyl pyridine delivered 4-acetyl-2,6-diphenyl pyridine 6i in
41% yield and 2,2-bipyridine and 4,4-bipyridines were converted
into the expected products 6j and 6k sluggishly. Under the
standard reaction conditions, benzothiazole underwent the
acetylation and afforded the 2-acetyl benzothiazole in moderate
yield 42% (Table 3, 6l). However, acetylation of 4-nitrophenyl
pyridine, 2-methyl pyridine and simple pyridine failed. Up on
successful acetylation on benzothiazole, then benzoxazole was
also subjected to the standard reaction conditions but
deceivingly no desired product was formed.
and forms more electron deficient intermediate B. The acetyl
radical present in the reaction medium attacks at the more
electrophilic C1 position of the heteroarene intermediate B to
provide the corresponding radical cation C. The hydrogen
t
abstraction from the radical cation C by the BuO radical which
t
releases from TBHP followed by elimination of BuOH leads to
the rearomatized intermediate D.14 Furthermore, the final
product E is furnished together with the regeneration of catalyst
D (Scheme 2).
Scheme 2: Proposed mechanism
In conclusion, we have developed an iron-catalyzed acetylation
of
quinolines,
isoquinolines
and
quinoxalines
using
triethylorthoformate/TBHP system as acetyl source. This novel
acetylation method is extended to pyridines, bipyridines and
benzothiazole. We have demonstrated an operationally simple
method using commercially available and cheaper catalyst
Fe(ClO4)3 as well as robust triethylorthoformate and TBHP.
Scheme 1: Control experiments
In order to find out acetyl source as well as mechanism, several
control experiments were carried out (Scheme 1). Reactions
with
triethylorthoacetate
(Scheme
1a)
and
triethylorthopropionate(Scheme 1b) under optimized reaction
conditions with 4-methyl quinoline, provided the same 2-acetyl-
4-methyl quinoline (4n). Similalrytripropylorthoformate (Scheme
1c) and tributylorthoformate(Scheme 1d) under similar
conditions resulted in 2-propionylated and 2-butanoylated
products respectively. The above reactions suggest that acyl
functionality is generated from the respective alkoxy groups of
the orthoformates. For example we assume that HC(OEt)3
generates the acetaldehyde in situ, and further converts into
acetyl radical in presence of TBHP (Scheme 2). This was further
established with the reaction with acetaldehyde where the
desired product (4n) in 30% yield was observed (Scheme 1e).
The formation of desired product was not observedwith Ethanol
(Scheme 1f). Radical mechanism was established by radical
scavenger (TEMPO) where the desired product (4n) was
isolated only 10% yield(Scheme 1g).
Experimental Section
General procedure for iron catalyzed acetylation of 4a-4s with
triethyl orthoformate and tert-butyl hydroperoxide:
In
a
reaction vessel, Fe(ClO4)3. XH2O (0.1 mmol) andN-
mL of triethyl
heteroarenes (1a-s) (1 mmol) were dissolvedin
2
orthoforrmate (2). To the above mixture tert-butyl hydroperoxide 70% aq
solution (4 equiv.) was added slowly over a period of 2 minutes. The
reaction mixture was stirred at 100 C for 12 hours. The progress of the
°
reaction was monitored by TLC. After completion of the reaction, the
reaction mixture was allowed to come to room temperature and extracted
with ethyl acetate. The organic layer was washed with brine, water and
dried over anhydrous Na2SO4. The organic layer was concentrated under
reduced pressure and afforded the crude product. The crude product was
purified by silica gel column chromatography using petroleum ether/ethyl
acetate (4:1 ratio) mixture as eluent and the pure product was analyzed
by 1H NMR, 13C NMR and ESI-HRMS.
General procedure for iron catalyzed acetylation of 6a-6l with
triethyl orthoformate and tert-butyl hydroperoxide:
In a reaction vessel, Fe(ClO4)3. XH2O (0.1 mmol), Pivalic acid (1.1
mmol) and2-aryl pyridines (5a-i) or bipyridines 5j and 5k or benzothiazole
5l (1 mmol) were dissolvedin 2 mL of triethyl orthoforrmate (2). To the
Based on the control experiments and literature precedents,13the
following mechanism is proposed. Triethylorthoformate
generates the acetyl radical in presents of TBHP (Scheme 2). In
parallel, iron perchlorate co-ordinates N-atom of heteroareneA
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[12] Data of 6a is matching with the reported compound (a) Y. Huang., D.
Guan, L. Wang, Chin. J. Chem.2014, 32, 1294; (b) J. Wang, S. Wang, G.
Wang, J. Zhang, X. -Q. Yu, Chem. Commun.2012, 48, 11769.
[13] I. I. Khatuntsev, E. V. Pastushenko, A. B. Terent'ev, Russ. Chem.
Bull.1984, 33, 2109.
[14] a) M. O. Ratnikov, M. P. Doyle, J. Am. Chem. Soc. 2013, 135, 1549; b) A.
Bravo, H. –R. Bjørsvik, F. Fontana, L. Liguori, F. Minisci, J. Org. Chem.
1997, 62, 3849; c) M. S. Kharasch, A. Fono, J. Org. Chem. 1959, 24, 72;
d) E. Boess, L. M. Wolf, S. Malakar, M. Salamone, M. Bietti, W. Thiel, M.
Klussmann, ACS Catal. 2016, 6, 3253.
above mixture tert-butyl hydroperoxide 70% aq solution (4 equiv.) was
added slowly over a period of 2 minutes. The mixture was stirred at
100 °C for 12 hours. The progress of the reaction was monitored by TLC.
After completion of the reaction, the reaction mixture was allowed to
come to room temperature and extracted with ethyl acetate. The organic
layer was washed with brine, water and dried over anhydrous Na2SO4.
The organic layer was concentrated under reduced pressure and
afforded the crude product. The crude product was purified by silica gel
column chromatography using petroleum ether/ethyl acetate (4:1 ratio)
mixture as eluent. The pure products were analyzed by 1H NMR, 13C
NMR and ESI-HRMS.
Acknowledgements
A.S. thanks to CSIR-India, B.S. thanks UGC-India and D.V.
thanks to DST-India for the financial support in the form of SRFs
and JRF. Dr. KCA, Dr. KRR and Dr. FC thanks to IFCPAR –
India (CEFIPRA) for financial support (5305-1). KRR also thanks
the reviewers for their critical comments and suggestions.
CSIR-IICT Commun. No: IICT/Pubs./2018/187.
Keywords:
Acetylation.
Iron
catalyst.
N-heteroarens;
triethylorthoformate. TBHP
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Entry for the Table of Contents (Please choose one layout)
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Key Topic*
A. Srinivasulu,a,b,c B. Shantharjun,a,b,c D.
Vani,a,b K. Chinna Ashalu,a A. Mohd,d J.
Wencel-Delord,d F. Colobertd and K.
Rajender Reddya,b
*
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IronCatalyzedMinisci Type Acetylation of
N-Heteroarenes Mediated by
CH(OEt)3/TBHP
Iron catalysed acetylation of electron deficient N-heteroarenes has been
reported using triethylorthoformateas robust and inexpensive acetyl source.
This new method is successfully applied for the acetylation of quinolines,
isoquinoline, quinoxalines, arylpyridines, bipyridines and benzothiazole.
This article is protected by copyright. All rights reserved.