Iron(III) Chloride-Catalyzed Decarboxylative-Deaminative Functionalization of Phenylglycine: A Tandem Synthesis of Quinazolinones and Benzimidazoles

Article abstract of DOI:10.1002/adsc.201500335

The first iron(III) chloride-catalyzed decarboxylative-deaminative functionalization of phenylglycine with o-substituted nitroarenes was achieved for the synthesis of 4(3H)-quinazolinones and benzimidazoles. The reaction of 2-nitrobenzonitrile/2-nitro-N,N-diphenylamine with phenylglycine at 120 C in the presence of potassium carbonate as a base in toluene generated the products in 45-87% yields. Various functional groups like nitro, fluoride, chloride and trifluoromethyl were well tolerated under the present reaction conditions. In this tandem approach, involvement of transfer hydrogenation of the nitro functionality with in situ generated ammonia, imination, nitrile hydration to amide and oxidative cyclization sequences have been established. The process avoids the use of an external hydrogen source, costly catalysts as well as the isolation of amine and amide intermediates.

Full text of DOI:10.1002/adsc.201500335

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DOI: 10.1002/adsc.201500335
Iron(III) Chloride-Catalyzed Decarboxylative–Deaminative
Functionalization of Phenylglycine: A Tandem Synthesis of
Quinazolinones and Benzimidazoles
Manoranjan Kumar,a,b Richa,a Sushila Sharma,a,b Vinod Bhatt,a,b
and Neeraj Kumara,b,*
a
Natural Product Chemistry & Process Development Division, CSIR – Institute of Himalayan Bioresource Technology,
Palampur, Himachal Pradesh 176061, India
E-mail: neerajnpp@rediffmail.com or neeraj@ihbt.res.in
Academy of Scientific and Innovative Research, Anusandhan Bhawan, 2 Rafi Marg, New Delhi 110001, India
b
Received: April 3, 2015; Revised: June 9, 2015; Published online: September 11, 2015
IHBT Communication No. 3774.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201500335.
Abstract: The first iron(III) chloride-catalyzed decar- approach, involvement of transfer hydrogenation of
boxylative–deaminative functionalization of phenyl- the nitro functionality with in situ generated ammo-
glycine with o-substituted nitroarenes was achieved nia, imination, nitrile hydration to amide and oxida-
for the synthesis of 4(3H)-quinazolinones and benz- tive cyclization sequences have been established. The
imidazoles. The reaction of 2-nitrobenzonitrile/2- process avoids the use of an external hydrogen
nitro-N,N-diphenylamine with phenylglycine at source, costly catalysts as well as the isolation of
1208C in the presence of potassium carbonate as amine and amide intermediates.
a base in toluene generated the products in 45–87%
yields. Various functional groups like nitro, fluoride,
chloride and trifluoromethyl were well tolerated Keywords: benzimidazoles; iron catalysts; phenylgly-
under the present reaction conditions. In this tandem cine; quinazolinones; tandem synthesis
Introduction
Although the decarboxylative functionalization of
amino acids is known, simultaneous decarboxylative–
Tandem reactions lower the cost of production of deaminative functionalization (DDF) is new to organ-
target molecules by minimizing energy consumption, ic synthesis and offers an economic protocol to con-
waste, and purification of intermediates which in- vert the widely available amino acids into biologically
creases the sustainability of the whole process. In the active pharmacophores. Designing a catalytic transfor-
À
same way, tandem decarboxylation or deamination is mation that targets simultaneous C N bond cleavage
a fundamental metabolic process involved in bio- and carbon dioxide excision for the synthesis of bioac-
chemistry and peptide cleavage. For example, in tive pharmacophore reveals a great significance. In
plants, the aromatic amino acid decarboxylase this context, the Kalutharage group and Xiang et al.
À
(AAAD) enzyme catalyzes either decarboxylation or reported C C bond formation via decarboxylative
decarboxylation–deamination on various aromatic and deaminative functionalization of amino acids.[9a,b]
amino acids.[1] In non-biological system, oxidative de- To the best of our knowledge, C N bond formation
carboxylation of a-amino acids can be induced at via DDF is still unknown.
À
high temperature or achieved by the use of various
Quinazolinones are an important class of nitrogen
oxidants to generate Strecker aldehydes.[2] In recent heterocycles present in a large number of natural
years, various biologically important molecules have products and have a broad range of biological activi-
been synthesized by decarboxylative functionalization ties including anti-HIV, anticancer, antitubercular,
of abundantly available amino acids by using various anti-inflammatory, antimalarial, antitumor, anticon-
catalyst and oxidants[3–9] (Scheme 1).
vulsant, fungicidal, and antimicrobial.[10] In view of
their importance, a number of approaches has been
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Iron(III) Chloride-Catalyzed Decarboxylative–Deaminative Functionalization
an attractive approach in terms of economy and re-
ducing the amount of waste generated.
In continuation of our previous efforts on the uti-
lization of natural products as reagent/catalyst in or-
ganic synthesis,[14] we screened various amino acids
and found that phenylglycine, although a non-natural
À
amino acid, is a good coupling partner for C N bond
formation with various o-nitrobenzonitriles and o-
nitro-N,N-diphenylamines. Herein, we disclose an
FeCl3-catalyzed decarboxylative–deaminative oxida-
tive cyclization of phenylglycine with o-nitrobenzoni-
triles/o-nitro-N,N-diphenylamines in a toluene/K2CO3
system to afford various 2-aryl-4(3H)-quinazolinone/
1,2-diphenylbenzimidazole derivatives.
Results and Discussion
The preliminary experiments were carried out with 2-
nitrobenzonitrile (1a) and phenylglycine (2a) as
a model substrates for the optimization of reaction
conditions. After initial screening of different iron
salts, bases, solvents and reaction temperature it was
found that 10 mol% of FeCl3 in toluene with two
equivalents of K2CO3 at 1208C after 16 hours afford-
ed the desired product (3a) in 87% isolated yield
(Table 1, entry 3). As expected, 3a was not observed
in the absence of FeCl3 (Table 1, entry 10). With other
iron salts a relatively lower yield of 3a was observed
(Table 1, entries 11–15). A very good yield of 3a was
observed with two equivalents of K2CO3, whereas
three equivalents lowered the yield (Table 1, entries 3
and 4). In the absence of base, 12% of 3a was ob-
served (Table 1, entry 9) while other selected bases re-
sulted in traces or lower yields (Table 1, entries 5–8).
The use of other solvents like PEG-400, DMF, and
DMSO gave lower yields of 3a while in water the cor-
responding amide was observed as a major product
(Table 1, entries 16–19).
With the optimized reaction conditions in hand, we
examined the scope and limitations of the method for
structurally diverse o-nitrobenzonitriles (1) and phe-
nylglycines (2).
As illustrated in Table 2, various 2-nitrobenzoni-
triles, regardless of the presence of electron-donating
and electron-withdrawing groups reacted smoothly to
give the corresponding 4(3H)-quinazolinones in good
to excellent yields. Electron-withdrawing groups like
Scheme 1. Approaches for amino acid functionalizations.
developed for the synthesis of 4(3H)-quinazoli- chloro, nitro, and trifluoromethyl were well tolerated
nones.[11–13] These methods mainly rely on the conden- and afforded the desired products (3f–3n, 3u–3x) in
sation of o-aminobenzamide with aldehydes,[11e] alco- 45–72% yields. The unsubstituted substrate 1a and
hols,[13d] or carboxylic acids[11a] and their deriva- those with electron-donating groups like methyl and
tives[11b,c] as coupling partners. Usually amines are pre- methoxy on 1 afforded the desired products (3a–3e,
pared by the reduction of nitro compounds for which 3o–3t) in 55–87% yields which is comparably higher
a stoichiometric amount of reagents is required. than those substrates bearing electron-withdrawing
Therefore, designing a one-pot multistep synthetic groups. When 1a was treated with other a-amino
route starting from less expensive nitroarenes will be acids, such as phenylalanine, tryptophan and glycine
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Manoranjan Kumar et al.
Table 1. Optimization of reaction conditions.[a]
Table 2. Decarboxylative–deaminative coupling of phenyl-
glycines with 2-nitrobenzonitriles.[a]
Entry Base
Solvent
Catalyst
Yield [%][b]
1
2
3
4
5
6
K2CO3
toluene
toluene
toluene
toluene
toluene
toluene
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
15[c]
35[d]
87
K2CO3
K2CO3
K2CO3
Na2CO3
Cs2CO3
80[e]
trace
10
7
8
9
NaHCO3 toluene
FeCl3
FeCl3
FeCl3
–
trace
8
12
NR
54
5
10
trace
8
35
10
KtOBu
no base
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
10
11
12
13
14
15
16
17
18
19
FeCl2
ferrocene
iron powder
Fe(OAc)2
FeSO4·7H2O
PEG-400 FeCl3
DMF
DMSO
H2O
FeCl3
FeCl3
FeCl3
12
0[f]
[a]
All the reactions were carried out using 1a (1 mmol), 2a
(1.5 mmol), catalyst (10 mol%), base (2 mmol), solvent
(5 mL) at 1208C for 16 h unless otherwise indicated.
Isolated yields.
1 mmol of K2CO3 used.
1.5 mmol of K2CO3 used.
[b]
[c]
[d]
[e]
[f]
3 mmol of K2CO3 used.
Corresponding amide was isolated as the major product.
under the standard reaction conditions, the corre-
sponding Strecker aldehyde was formed and no fur-
ther reaction was observed.
The scope of the optimized method was further ex-
tended towards the synthesis of 1,2-diphenylbenzimi-
dazoles (Table 3, 5a–5d), where o-nitrobenzonitrile
was replaced with 2-nitro-N,N-diphenylamine (4).
Previously, for the synthesis of benzimidazoles various
methods have been developed starting either from
1,2-phenylenediamine, or 2-nitroaniline via condensa-
tion with various carboxylic acids,[15] alcohols,[16] and
aldehydes.[17] Including this, Buchwald[18] and Ma
et al.[19] also reported a benzimidazole synthesis via
condensation of 2-haloacetanilide with various ani-
lines. The present method represents a new and direct
approach for the synthesis of benzimidazoles.
[a]
All the reactions were carried out using 1 (1 mmol), 2
(1.5 mmol), K2CO3 (2 mmol) and FeCl3 (10 mol%) in tol-
uene (5 mL) at 1208C for 16 h. All the yields indicated
after purification are averages of two runs.
The mechanism of the product formation was es-
tablished
by
different
control
experiments.
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Iron(III) Chloride-Catalyzed Decarboxylative–Deaminative Functionalization
Table 3. Decarboxylative–deaminative coupling of phenyl-
glycine with 2-nitro-N,N-diphenylamine.[a]
[a]
All the reactions were carried out using 4 (1 mmol), 2
(1.5 mmol), K2CO3 (2 mmol) and FeCl3 (10 mol%) in tol-
uene (5 mL) at 1208C for 16 h.
(Scheme 2). First to establish the fate of phenylglycine
(2a), it was heated in the absence of 2-nitrobenzoni-
trile (1a) under the optimized reaction conditions.
GC-MS analysis of the reaction mixture after 12 h
showed the formation of benzaldehyde (A, m/z=
106), benzylamine (B, m/z=107) and Schiff base of
(A+B), i.e, N-benzylidenebenzylamine (C, m/z=195)
Scheme 2. Mechanistic studies.
which supported the iron(III)-induced oxidative de- sponding aniline in the presence of ammonia
carboxylation of amino acids (Scheme 2a). The forma- [Scheme 2b (2)].
tion of benzaldehyde (dH =9.75 for CHO) was further
The formation of imine (G), the expected key inter-
confirmed by 1H NMR analysis of phenylglycine mediate for product formation, was confirmed by the
under standard reaction conditions in the absence of reaction of F with A [Scheme 2c (1)]. Heating of sep-
1a in deuterated benzene at different time intervals arately prepared imine (G) under the standard reac-
(see the Supporting Information).
tion conditions afforded the expected product 3a,
Further, we assumed that ammonia generated in hence confirming the involvement of the imine path-
situ by the degradation of phenylglycine reduces the way [Scheme 2c (2)]. The nitrile hydration to amide is
nitro functionality of 1a to furnish 2-aminobenzoni- a base-promoted process, hence we assumed that G
trile (F), since during the optimization of reaction was converted into its corresponding amide by FeCl3/
conditions F was also observed as a minor product. In K2CO3 (however, prior to imine formation, hydration
order to confirm the involvement of ammonia as a re- of 2-aminobenzonitrile to 2-aminobenzamide may
ducing agent, it (generated by heating urea) was also be possible) followed by oxidative cyclization to
passed through the reaction mixture of 1a and benzal- form quinazolinone. Earlier Wu et al. reported the ox-
dehyde (A) which resulted in the formation of prod- idative synthesis of quinazolinones from 2-aminobenz-
uct 3a [Scheme 2b (1)], while no product was ob- amide and benzaldehyde.[13d] To confirm the involve-
served in the absence of ammonia under the same re- ment of the amide pathway, the reaction of 2-nitro-
action conditions. It was further supported by the suc- benzamide (H) with phenylglycine (2a) was carrried
cessful reduction of 4-iodonitrobenzene to the corre- out under the standard conditions and 3a was isolated
as a major product in 78% yield (Scheme 2d).
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Manoranjan Kumar et al.
imine (G) after base-induced controlled hydration is
converted to X, which after cyclization is converted
into 2,3-dihydroquinazolin-4(1H)-one (I). The inter-
mediate I may help in the conversion of 1a to F via
transfer hydrogenation as established earlier,[20] and
itself is converted into the desired product (3a).
Conclusions
In conclusion, we have developed a direct and effi-
cient method for the synthesis of 4(3H)-quinazoli-
nones/benzimidazoles via functionalization of phenyl-
glycine using o-nitrobenzonitriles/o-nitro-N,N-diphe-
nylamine as easily available substrates with cheap and
non-toxic FeCl3 in a tandem manner. The beauty of
the method lies in the reduction of the nitro function-
ality with in situ generated ammonia which then cou-
ples with benzaldehyde followed by oxidative cycliza-
tion. The developed method involves a series of
chemical transformations controlled in a one-pot op-
eration thus minimizing the waste, energy and cost.
This method offers a simple and direct synthesis of
various biologically active pharmacophores/natural
products having quinazolinone and benzimidazole
frameworks.
Scheme 3. Isolation of 2,3-dihydroquinazolin-4(1H)-one in-
termediates.
Experimental Section
General Information
All reagents, high grade solvents and materials were used as
received from commercial sources unless otherwise stated.
Toluene was distilled from sodium and benzophenone.
Column chromatography was carried out over 60–120 mesh
silica gel (Merck India). TLC silica gel 60 F254 plates were
purchased from Merck India Ltd. Nitro compounds, amino
acids and NMR solvents were purchased from Sigma Al-
drich and Spectrochem. FeCl3 anhydrous (98%) was pur-
Scheme 4. Probable mechanism for 4(3H)-quinazolinone
synthesis.
1
chased from Loba chemie India. H NMR and 13C NMR ex-
The respective 2,3-dihydroquinazolin-4(1H)-one in-
termediates (I) were also isolated after 12 h from the
reaction mixture (Scheme 3). The isolated intermedi-
ate I was oxidized under the standard reaction condi-
tions [Scheme 2 (e)] to afford the desired product 3a
after 4 h which supported the involvement of dihydro-
quinazolinone in the proposed mechanistic cycle
(Scheme 4).
Therefore, in accordance with additional experi-
ments and literature findings, a probable mechanism
has been outlined for this FeCl3-catalyzed decarboxy-
lative–deaminative oxidative coupling of phenylgly-
cine with o-substituted nitroarenes (Scheme 4). Ini-
tially phenylglycine (2a) after decarboxylation and de-
amination is converted into benzaldehyde. The am-
monia evolved from 2a reduces the nitro group of 1a
in the presence of FeCl3 into F. The intermediate F
periments were performed on Bruker Avance 300 and
600 MHz spectrometers. NMR spectra were recorded in
CDCl3, CD3OD, CD3SOCD3 and C6D6. Chemical shifts are
reported in parts per million (ppm) downfield from an inter-
nal standard. Splitting patterns are given as follows: s, sin-
glet; d, doublet; t, triplet; q, quartet; m, multiplet; br,
broad; brs, broad singlet; dd, double doublet; dp, doublet of
triplet. All coupling constants are given in Hertz (Hz). GC-
MS analysis was carried out using DB-5 MS capillary
column, (30 m”0.25 mm i.d., 0.25 mm) on a Shimadzu (QP
2010) series Gas Chromatograph-Mass Spectrometer
(Tokyo, Japan) coupled with AOC-20i auto-sampler. The in-
itial temperature of column was 708C held for 4 min and
was programmed upto 2308C at 48CminÀ1, then held for
15 min at 2308C; the sample injection volume was 2 mL in
GC grade dichloromethane. Helium was used as carrier gas
at a flow rate of 1.1 mLminÀ1 in split mode (1: 50). Mass
spectra were recorded on a QTOF-Micro from Waters Mi-
condenses with benzaldehyde to form imine G. The cromass.
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Iron(III) Chloride-Catalyzed Decarboxylative–Deaminative Functionalization
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General Procedure for the Synthesis of
Quinazolinones
In a 12-mL test tube, the 2-nitrobenzonitrile (1 mmol), phe-
nylglycine (1.5 mmol), K2CO3 (2 mmol), and anhydrous
FeCl3 (10 mol%) were dissolved in toluene (5 mL) and
stirred at 1208C for 16 h in an oil bath. After completion of
the reaction, the reaction tube was cooled to room tempera-
ture and the reaction mixture was extracted with ethyl ace-
tate (3”5 mL). The combined organic layer was washed
with saturated brine and distilled water (3”5 mL each),
dried over Na2SO4 and the solvent was evaporated under
vacuum. The product was purified using column chromatog-
raphy (silica gel 60–120 mesh, n-hexane/ethyl acetate). Puri-
fied products were characterized by NMR, ESI-MS, and
GC-MS techniques.
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General Procedure for the Synthesis of
Benzimidazoles
In a 12-mL tube, 2-nitro-N,N-diphenylamine (1 mmol), phe-
nylglycine (1.5 mmol), K2CO3 (2 mmol), anhydrous FeCl3
(10 mol%) were dissolved in toluene (5 mL) and stirred at
1208C for 16 h in an oil bath. After completion of reaction,
the reaction tube was cooled to room temperature and the
reaction mixture was extracted with ethyl acetate (3”5 mL).
The combined organic layer was washed with saturated
brine and distilled water (3”5 mL each), dried over Na2SO4
and the solvent was evaporated under vacuum. Products
were purified using column chromatography (silica gel 60–
120 mesh, n-hexane/ethyl acetate). Purified products were
characterized by NMR, ESI-MS, and GC-MS techniques.
Procedure for the Synthesis of 2-[(Benzylidene)-
amino]benzonitrile (G)
Benzaldehyde (1 mmol), 2-aminobenzonitrile (1.5 mmol),
and sodium metabisulfite (catalytic) was mixed and irradiat-
ed under multimode microwave (900 W) for 10 min. After
completion of the reaction, the reaction mixture was cooled
to room temperature and extracted with ethyl acetate. The
ethyl acetate layer was washed with saturated brine, dried
over Na2SO4 and evaporated under reduced pressure. The
product was purified using column chromatography (silica
gel 60–120 mesh, n-hexane/ethyl acetate). The purified prod-
uct was characterized by NMR and ESI-MS techniques.
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Acknowledgements
Authors are grateful to the Director of the Institute, for en-
couragement and support. Mr. MK, VB and Ms. SS thank
UGC for granting research fellowships. Financial support
from CSIR (ORIGIN, CSC-0108) is also acknowledged.
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