Iron-catalyzed Minisci acylation of N-heteroarenes with α-keto acids

Article abstract of DOI:10.1016/j.tet.2019.01.060

An efficient and mild protocol has been developed for the Minisci acylation reactions of nitrogen-containing heteroarenes with α-keto acids. Distinct from the conventional Minisci acylation conditions, the chemistry was performed using non-noble metal Fe(II), instead of expensive Ag(I) salt, as catalyst. A wide range of substrates, including aliphatic or aromatic α-keto acids, as well as various N-heteroarenes, proved to be compatible with the protocol. Scale-up experiment also demonstrates the practicality of the approach.

Full text of DOI:10.1016/j.tet.2019.01.060

Accepted Manuscript
Iron-catalyzed Minisci acylation of N-heteroarenes with α-keto acids
Xiu-Zhi Wang, Cheng-Chu Zeng
PII:
S0040-4020(19)30114-0
https://doi.org/10.1016/j.tet.2019.01.060
TET 30112
DOI:
Reference:
To appear in: Tetrahedron
Received Date: 2 November 2018
Revised Date: 23 January 2019
Accepted Date: 25 January 2019
Please cite this article as: Wang X-Z, Zeng C-C, Iron-catalyzed Minisci acylation of N-heteroarenes with
α-keto acids, Tetrahedron (2019), doi: https://doi.org/10.1016/j.tet.2019.01.060.
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ACCEPTED MANUSCRIPT
Iron-catalyzed Minisci acylation of N-heteroarenes with α-keto acids
Xiu-Zhi Wang and Cheng-Chu Zeng*
Beijing Key Laboratory of Environmental and Viral Oncology, College of Life
Science & Bioengineering, Beijing University of Technology, Beijing 100124, China.
Corresponding author: zengcc@bjut.edu.cn
Graphical Abstract:
2
O
Fe(II)/DMSO/S2O8
+
Het
Het
R
R
COOH
N
N
Ag-free
O
36 examples
up to 93% yield
Abstract:
An efficient and mild protocol has been developed for the Minisci acylation
reactions of nitrogen-containing heteroarenes with α-keto acids. Distinct from the
conventional Minisci acylation conditions, the chemistry was performed using
non-noble metal Fe(II), instead of expensive Ag(I) salt, as catalyst. A wide range of
substrates, including aliphatic or aromatic α-keto acids, as well as various
N-heteroarenes, proved to be compatible with the protocol. Scale-up experiment also
demonstrates the practicality of the approach.
Key words: Minisci acylation reaction, α-keto acids, Fe(II) as catalyst,
nitrogen-containing heteroarenes
1. Introduction
The addition of
a
carbon-centered radical to an electron-deficient
N-heteroaromatic base is defined as Minisci reactions,1 which are distinct from the
common Friedel-Craft reactions that involve ion-type addition of electrophiles to
electron-rich arenes.2 Minisci acylation reactions of N-heteroarenes have particularly
important applications in medicinal chemistry and biochemistry since the acylated
compounds are widely found in natural products and bioactive organic molecules.3
Mechanically, a Minisci acylation reaction involves the initial formation of an acyl
1

ACCEPTED MANUSCRIPT
radical and its subsequent radical addition to a protonated N-heteroarene, followed by
deprotonation and further oxidation to give the final product. As a result, based on the
difference of acyl radical precursors, a number of processes have been developed to
achieve the Minisci acylation. Aldehydes are one of the widely employed acyl radical
precursors for the Minisci acylation reactions. In the presence of a transition metal
catalyst, peroxides decompose to generate various oxygen-centered radicals directly
or from which to induce generation of secondary radicals; these radicals abstract the
hydrogen of an aldehyde group to form the acyl radical (Scheme 1a, Pathway I). In
this area, significant progresses have been achieved by Anslyn, Li, Yu and
Antonchick.4 A combination of catalytic N-hydroxyphthalimide (NHPI) and
stoichiometric co-oxidants is also used for the formation of the acyl group (Scheme 1a,
Pathway II). For example, NHPI/Co(II)/O2, NHPI/Pd(II)/O2 and NHPI-Ce(IV) were
reported to accomplish the acylation reaction of heteroaromatic bases or arenes.5
Considering that aldehydes could be in situ generated from alcohol or activated alkane
under the Minisci conditions, Patel et al achieved the acylation of nitrogen-containing
heterocycles with toluene as the substrate and AlCl3 as catalyst in the presence of
t-butyl hydroperoxide (Scheme 1a, Pathway III).6 These approaches often require
noble transition metals (Ru or Pd) and stoichiometric oxidants. In addition, in most of
these examples, a mixture of mono- and di-acylated products is produced since the
initially generated mono-acylated intermediate easily undergoes further acylation. The
limited chemical selectivity leads to trouble in separation of products and further
application.
Decarboxylative cross-coupling of α-keto acids with N-heteroarenes provides an
alternative approach to the acylated heteroarenes (Scheme 1a, Pathway IV). Minisci
and co-workers firstly examined the reaction of α-keto acids with protonated pyrazine
or pyridine in a DCM/H2O two-phase solvent system using AgNO3 as catalyst.7 Later
on, Muthusubramanian’s group employed AgCO3 as catalyst to achieve acylation of
pyridine nitrogen oxides in a DCM/H2O two-phase system.8 Intramolecular
decarboxylation cross-coupling of α-keto acids under high temperature conditions was
also reported by Laha et al to give azafluorenones and fluorenones.9 Then, Zhang and
2

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co-workers presented a method for decarboxylative coupling of carboxylic acids with
N-heterocycles under visible-light irradiation.10 Right recently, Kyungsoo Oh et al
achieved the cross-dehydrogenation couple of 2H-carbazole and α-keto acids using
AgNO3 as the catalyst.11 As mentioned above, for the decarboxylative cross-coupling
of α-keto acids with N-heteroarenes, expensive transition metal Ag(I) was generally
used as the catalyst, which is not practicable. Therefore, the development of new
approach using cheap metal catalyst is highly desired.
It is well-known that methyl radical could be efficiently generated from DMSO
under Fenton conditions, and later on, this process is utilized to methylate aromatic
and heteroarenes, or undergoes radical substitution with alkyl iodide to form new
alkyl radical for further Minisci reactions.12,13 Based on this fact, we envisioned that
the Fe(II)/DMSO/peroxides system may also generate acyl radical from
decarboxylation of α-keto acids (Scheme 1b). Herein, we reported the first example of
the Minisci acylation of electron-deficient N-heteroarenes with α-keto acids mediated
by Fe(II), instead of the expensive Ag(I) salts.
a) Aldehyde or α-keto acids as the precursors of acyl radical
O
X
NHPI/oxidant
pathway II
O
R
H
pathway I
R
H
O
C
X = OH or C
R
O
Ag (I), oxidant
pathway IV
X
Ph CH3
R
COOH
via [ PhCHO ]
pathway III
b) Our work: α-keto acids as acyl radical precursors in Fe(II)/DMSO/peroxide catalytic
oxidation system
O
O
C
Fe(II)/DMSO/peroxide
R
COOH
Ag-free
R
Scheme 1. Formation of acyl radicals
2. Results and discussion
3

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O
cat. ( 8 mol%)
N
N
N
oxidant (3 equiv)
O
CH3COCOOH
+
+
O
N
additive
N
N
3a
1a
2a
4a
Scheme 2. Minisci acylation of pyrazine with pyruvic acid
We commenced our studies by choosing pyrazine (1a) and pyruvic acid (2a) as
the model substrates to optimize the reaction conditions (Scheme 2); the results were
summarized in Table 1. The initial Minisci acylation reaction of 1a with 2a was
carried out in a mixed solvents of DCM and H2O using FeSO4·7H2O (0.08 equiv.) as
catalyst, (NH4)2S2O8 as external oxidant and H2SO4 as additive at 40 oC for 12 hours,
and the desired mono-acylated product 3a, was isolated exclusively in a 26% yield
(Entry1). Notably, a possible Minisci methylation reaction of pyrazine (1a) with the
methyl radical, generated from further decarbonylation of the acyl radical
intermediate, was not observed. Encouraged by this result, other iron salts, such as
ferrocene and Fe(NO)3·9H2O were then tested. However, the yields of 3a decreased
sharply to 3% and 11%, respectively (Entries 2 and 3). Further catalyst screening
disclosed that MnCl2·4H2O, Mn(NO3)2, NiCl2·6H2O, CoCl2·6H2O, CuBr2 and
CuCl2·2H2O all afforded trace 3a, and most of the starting materials were recovered
(Entries 4-9). In addition, it was observed that 3a was afforded in 17% yield or in
trace amount when K2S2O8 and H2O2, instead of (NH4)2S2O8, were used as the
external oxidants (Entries 10 and 11).
We next examined the effect of solvents on the Minisci acylation of pyrazine. It
was observed that the yield of 3a did not improve when the reaction was carried out in
aqueous methanol or a mixed solution of water and 1,2-dichloroethane (DCE) (Entries
12 and 13). To our delight, when the Minisci acylation reaction was performed in
aqueous DMSO (V:V = 1:1), the reaction proceeded very fast and the starting 1a was
consumed in 2 hours. Moreover, more than 47% total yield of 3a (21% yield) and
diacylated product, 4a (26% yield), were afforded, which indicates that DMSO may
act synergistically with Fe(II) ions to promote the reaction. Considering the
problematic work-up in using high boiling point of DMSO as solvent, the effect of
volume ratio of water to DMSO was examined and found that the reaction could
4

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complete in 1 hour, meanwhile, the yield of 3a increased to 26%, along with 33%
yield of 4a when the reaction was carried out in water with DMSO additive (Entries
15-17).
It was reported that two-phase solution system considerably improves the
chemical selectivity of Minisci acylation (monoacylation vs diacylation), 7 therefore,
the mixed solvent of DCM and water (V:V = 3:1) in the presence of DMSO was used
as the solvent. As expected, the yield of 3a increased to 47% and the yield of 4a
decreased to 21% (Entry 18). For Minisci-type radical addition reactions, the presence
of acidic additives is very important to activate the heterocyclic base. As a result, the
effect of protic acids on the reaction was investigated. It was found that among the
acids tested (H2SO4, AcOH, CF3COOH, HCOOH), formic acid gave the best result
and 58% yield of 3a was isolated, meanwhile, the yield of 4a decreased to 10% yield
(Entries 18-21). Notably, without DMSO, only 5% yield of 3a was isolated (Entry 22),
which demonstrates that the synergistic role of Fe(II) with DMSO is essential for the
Minisci acylation, although the exact mechanism is not clear at this stage.
Table 1. Optimization of reaction conditions a
Catalyst
Oxiditive
(3 equiv.)
Additive
(1 equiv.)
H2SO4
H2SO4
H2SO4
H2SO4
H2SO4
H2SO4
H2SO4
H2SO4
H2SO4
H2SO4
H2SO4
3a
(%)
26
4a
(%)
0
Entry
(mol%)
Solvent
.
1
2
FeSO4 7H2O (NH4)2S2O8
DCM: H2O = 1: 1
DCM: H2O = 1: 1
DCM: H2O = 1: 1
DCM: H2O = 1: 1
DCM: H2O = 1: 1
DCM: H2O = 1: 1
DCM: H2O = 1: 1
DCM: H2O = 1: 1
DCM: H2O = 1: 1
DCM: H2O = 1: 1
DCM: H2O = 1: 1
Ferrocene
(NH4)2S2O8
3
0
.
3
Fe(NO)3 9H2O (NH4)2S2O8
11
0
.
4
MnCl2 4H2O (NH4)2S2O8
trace
trace
trace
trace
trace
trace
17
0
5
Mn(NO3)2
(NH4)2S2O8
(NH4)2S2O8
(NH4)2S2O8
(NH4)2S2O8
(NH4)2S2O8
K2S2O8
0
.
6
NiCl2 6H2O
0
.
7
CoCl2 6H2O
0
8
CuBr2
0
.
9
CuCl2 2H2O
0
.
10
11
FeSO4 7H2O
0
.
FeSO4 7H2O
H2O2
trace
0
5

ACCEPTED MANUSCRIPT
.
12
13
FeSO4 7H2O (NH4)2S2O8
MeOH: H2O =1: 1
DCE: H2O =1: 1
DMSO: H2O =1: 1
DMSO: H2O =3: 1
DMSO: H2O =1: 3
20 mL H2O+
H2SO4
H2SO4
H2SO4
H2SO4
H2SO4
0
0
0
.
FeSO4 7H2O (NH4)2S2O8
18
21
13
21
14b
15b
16b
FeSO4 7H2O (NH4)2S2O8
26
7
.
.
FeSO4 7H2O (NH4)2S2O8
.
FeSO4 7H2O (NH4)2S2O8
18
.
17b
18
19
20
FeSO4 7H2O (NH4)2S2O8
H2SO4
H2SO4
AcOH
26
47
30
33
21
14
16
0.1 mL DMSO
DCM: H2O = 3: 1
0.1 mL DMSO
.
FeSO4 7H2O (NH4)2S2O8
DCM: H2O = 3: 1
0.1 mL DMSO
.
FeSO4 7H2O (NH4)2S2O8
DCM: H2O = 3: 1
0.1 mL DMSO
.
FeSO4 7H2O (NH4)2S2O8
CF3COOH 49
DCM: H2O = 3: 1
0.1 mL DMSO
.
21
22
FeSO4 7H2O (NH4)2S2O8
HCOOH
HCOOH
58
5
10
0
.
FeSO4 7H2O (NH4)2S2O8
DCM: H2O = 3: 1
a
Reaction conditions: pyrazine (1 mmol), pyruvic acid (3 mmol), acid (1 mmol),
ammonium persulfate (3 mmol), catalyst (0.08 equiv.), temperature: 40 oC
b Temperature: 50 oC
With the optimized reaction conditions established, we then turned to examine
the scope of substrates and generality of the protocol. First of all, diverse
N-heteroarenes were subjected to the acylation reaction with pyruvic acid 2a under
the standard conditions. As shown in Table 2, it was observed that electronic
properties have a bit influence on the reaction. For example, 2-cyanopyrazines (1b),
bearing an electron-withdrawing CN group on the pyrazine ring, gave corresponding
3b in 15% yield ( 31% yield based on the recovery starting material), whereas 57%
yield of 3c was afforded when an electron-donating methoxy was introduced. Notably,
the substitution pattern of products 3b and 3c could be determined by measuring the
coupling constant of the pyrazine ring protons in the NMR spectra of the compound.
6

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For example, all 2,3-disubstituted pyrazines have J ꢀ 2.5 Hz, whereas 2,6- and
2,5-disubstituted pyrazine have J ≤ 0.1 Hz and ~ 1.5 Hz, respectively. This
regio-selectivity results from the combination of the inductive and resonance effects
of the substituents.14
The Minisci acetylation reaction of quinoxalines with 2a also proceeded
smoothly under the standard conditions. For example, quinoxaline 1d afforded the
corresponding acetylated product 3d in 67% isolated yield. Similarly, the electronic
properties of the substituents on the quinoxaline ring have a great influence on the
reaction, and the electron-donating substituent benefits the reaction. As shown in
Table 2, quinoxalines 1e and 1f, bearing one or two electron-donating methyl groups
on the pyrazine or benzene subunit, respectively, afforded good yields of acetylated
product 3e (72%) and 3f (63%). In contrast, electron-withdrawing chloro and fluoro
groups led to the formation of 3g, 3h and 3i in very low yields, and the remaining
material were recovered. Notably, when quinoxaline derivatives with one substituent
on the benzene ring were subjected to the Minisci reaction with 2a, a mixture of
regioisomers was afforded with moderate to good total yields. For example, 1j, 1k, 1l
and 1m gave corresponding 3j, 3k, 3l and 3m, respectively.
Other N-heteroarenes, such as quinolones, isoquinoline, phthalazine and
benzthiazoles, were also amenable to the protocol. For 4-methylquinoline, 1n, the
corresponding 3n was isolated in 30% yield. In the case of isoquinoline 1o, the
corresponding acylated product 3o was afforded in 64% isolated yield after column
chromatograph. While phthalazine 1p was used as the substrate, a mixture of 3p and
4p was obtained with 55% total yield. In the case of benzthiazole, the adduct 3q was
afforded in 20% yield and 68% yield based on the recovery starting material. Notably,
the acylated reaction of 1q occurred in the 2-position of benzthiazole cycle. For more
complicated substrates, such as caffeine 1r, the corresponding product 3r was
obtained in 10% yield or 83% yield based on the recovered 1r. In the case of quinine
1s, a decomposed compound 3s was unexpected afforded in 6% yield although the
exact reason is not clear. Triazoles, such as 1,2,3-1H-triazole and 1,2,4-triazole, were
also detected, but the corresponding products 3t and 3u were not detected.
7

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Table 2. Scope of N-heteroarenes a
(NH4)2S2O8 (3 equiv.)
HCOOH (1 equiv.)
FeSO4 7H2O (0.08 equiv.)
X
.
R
CH3COCOOH
N-heteroarenes
N
0.1 mL DMSO
5 mL H2O+15 mL DCM
40 oC
O
1a-1s
2a
3a - 3s
O
O
O
N
O
N
N
N
O
N
CN
O
N
N
N
3a, 58%
4a, 10%
3b,15% (31%)b
3c, 57%
N
Cl
N
N
N
N
N
N
N
O
O
O
O
3g, 3% (51%)b
3f, 63%
3e, 72%
3d, 67%
O
O
Cl
Cl
N
N
F
F
N
N
N
O
N
N
O
N
3h, 34% (66%)b
3i, 7% (43%)b
3k, 64%
3j, 63%
O
O
N
O
N
Cl
N
O
O
N
N
N
3l , 37%
3n, 30%
3m, 55%
3o, 64%
O
N
O
N
S
N
N
N
O
O
O
O
O
N
N
N N
3p, 30% (43%)b
4p,25% (35%)b
3q, 20% (68%)b
3r, 10% (83%)b
O
O
H
O
N
O
N
N
N
HN
N
N
O
3t, 0%
3s, 6%(16%)b
3u, 0%
a
Reaction conditions: 1 (1 mmol), 2a (3 mmol), HCOOH (1 mmol), (NH4)2S2O8 (3
mmol), FeSO4·7H2O (0.08 equiv.), 0.1 mL DMSO, 5 mL H2O, 15 mL DCM, 40 oC.
b Recovery yield.
8

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To further explore the feasibility of our protocol, a number of α-keto acids were
evaluated for the Minisci acylation reaction with 1a. The aliphatic α-keto acids were
suitable acyl radical precursor. As shown in Table 3, 2b-2e gave the corresponding
acylated products in satisfactory yields (5b-5e), in particular, 5d was afforded in 93%
yield. It was observed that aromatic α-keto acids proceeded smoothly with 1a under
the standard conditions. For example, ortho-, meta- and para-monosubstituted, as
well as disubstituted aromatic α-keto acids provided the desired cross-coupling
products in moderate to good yields. In addition, the reaction tolerated a variety of
electron-donating or electron-withdrawing substituents such as methyl (5g), isopropyl
(5h), methoxy (5i, 5j), chloro (5k), bromo (5l), trifluoromethyl (5m) and fluoro
(5n-5p) groups. For substrate 2q, where three methyl groups were attached to the
benzene ring, the yield of the corresponding 5q reduced to 6%, likely due to the steric
hindrance. To our surprise, when 2-thiophenoglyoxylic acid, a heteroaromatic α-keto
acid, was subjected to react with 1a, the corresponding 5r was isolated in up to 92%
yield.
Table 3. Scope of α-keto acids a
9

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a
Reaction conditions: 1a (1 mmol), 2 (3 mmol), HCOOH (1 mmol), (NH4)2S2O8 (3
mmol), FeSO4·7H2O (0.08 equiv.), 5 mL H2O, 15 mL DCM and 0.1 mL DMSO, 40
oC.
With the established method, the gram-scale reaction was performed. As
illustrated in Scheme 3, when 20 mmol of pyrazine, 1a, was allowed to react with
pyruvic acid under the standard condition, the mono-acetylated adduct 3a and
diacetylated product 4a were isolated in 50% and 9% yields, respectively. The result
indicates that the protocol is practicable and easily scaled up.
10

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O
.
FeSO4 7H2O (8 mol%)
N
N
N
N
(NH4)2S2O8 (3 equiv)
O
CH3COCOOH
+
+
O
N
DCM : H2O = 3 : 1
DMSO (0.1 mL)
HCOOH (1 equiv), 40 oC
N
3a
50%
1a
2a
4a
20 mmol
60 mmol
9%
Scheme 3. Gram-scale reaction
To better understand the reaction mechanism, a series of control experiments
were conducted. As shown in Scheme 4, when the acylation reaction of 1a and 2a was
performed under the standard conditions, but without (NH4)2S2O8, the desired
products 3a and 4a were not produced (Scheme 4a). In addition, the yield of 3a
dramatically decreased to 24% from 58% when the reaction was repeated in the
.
absence of FeSO4 7H2O (Scheme 4b). These results confirm the necessity of
.
FeSO4 7H2O and (NH4)2S2O8. Also, the acylation reaction of 1a is proved to proceed
via a radical pathway since the reaction was suppressed in the presence of TEMPO,
and only 14% yield of 3a was isolated (Scheme 4c).
Scheme 4. Control experiments
Based on these studies and literature reports,7-8, 11 possible mechanism for our
Minisci acylation reactions is proposed. As shown in Scheme 5, the reaction starts
from the oxidation of Fe(II) by persulfate to form Fe(III), which enable the
decarboxylation of α-keto acid, generating acyl radical 4. Alternatively,
Fe(II)/DMSO/S2O82- oxidation system generates a methyl radical, which may abstract
11

ACCEPTED MANUSCRIPT
a hydrogen atom to afford acyl radical 4. Once the acyl radical 4 is formed, it soon
undergoes radical addition with protonated heteroarenes 5 to afford intermediate 6.
Further oxidation by Fe(III) or persulfate, followed by deprotonation to give the final
acylated products 3.
2
S2O8
2+ + CO2
Fe3+
Fe
Het
O
+
Het
+
N
H
H
O
O
R C
Het
Fe (III)
R
5
R
R
COOH
N
N
2
or S2O8
1
O
H3C
CH4 + CO2
3
6
4
DMSO/Fe2+/S2O8
(Fenton reaction)
2
Scheme 5. Possible pathways for the Minisci acylation reaction using
Fe(II)/DMSO/S2O82- oxidation system
In summary, we have developed an efficient protocol for the intermolecular
decarboxylative acylation of N-heteroarenes with α-keto acids. Instead of the classical
and expensive Ag(I) catalyst, FeSO4 was used as the catalyst to accelerate the reaction
rate and to improves reaction efficiency. The protocol proves to be have a broad
substrate scope. Some of the products are afforded in more than 93% yield. The
operationally simple protocol may find application in industrial production of some
spices.
Acknowledgements
This work was supported by grants from the National Key Technology R&D Program
(2016YFD0400801) and the National Natural Science Foundation of China (No.
21871019).
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16190.
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