Phosphine free Mn-complex catalysed dehydrogenative C-C and C-heteroatom bond formation: A sustainable approach to synthesize quinoxaline, pyrazine, benzothiazole and quinoline derivatives

Article abstract of DOI:10.1039/c8cc05877f

Herein the first sustainable synthesis of quinoxalines, pyrazines and benzothiazoles catalysed by a phosphine free Mn(i) complex via acceptorless dehydrogenative coupling (ADC) is reported. This method is also applied successfully to synthesize quinolines via the dehydrogenation (removal of H₂) and condensation (removal of H₂O) reaction between 2-aminobenzyl alcohols and secondary alcohols.

Full text of DOI:10.1039/c8cc05877f

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Phosphine Free Mn-complex catalysed dehydrogenative C-C and
C-Heteroatom bond formation: a sustainable approach to
synthesize quinoxaline, pyrazine, benzothiazole and quinoline
derivatives†
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Kalicharan Das, Avijit Mondal and Dipankar Srimani*
www.rsc.org/
Herein first sustainable synthesis of quinoxalines, pyrazines and Crabtree and co-workers.11 In 2013, methods to synthesize
benzothiazoles catalysed by phosphine free Mn(I) complex via diversely substituted pyrrole or pyridine derivatives catalysed
acceptorless dehydrogenative coupling (ADC) is reported. This by iridium or ruthenium complex were developed by the group
method is also applied successfully to synthesize quinolines via of Kempe,12 Milstein13 and Saito.14 In the same year, Beller and
dehydrogenation (removal of H2) and condensation (removal of co-workers also developed an efficient and selective synthesis
H2O) reaction between 2-aminobenzyl alcohols and secondary of pyrroles via three-component coupling catalysed by
alcohols.
ruthenium.15 Subsequently, noble metal catalysed synthesis of
a wide variety of heterocycles are reported.16 Despite the high
significance of these classical methods employing noble metal,
the development of environmentally benign, earth-abundant
3d transition metal catalyst is extremely desirable in terms of
sustainability and cost-effectiveness. Of late, cobalt pincer
complexes have been effectively applied to synthesize
different N-heterocycles.17
In the context of rapid depletion of fossil fuel and growing
awareness towards environmental safety as well as economic
benefits, the
development of
atom
economical,
environmentally benign catalytic reactions using alternative
raw material is presently an area of intense research. In this
perspective, acceptorless dehydrogenative coupling (ADC)
reaction is an extremely powerful approach to synthesize a
diverse range of useful building blocks. Abstraction of
hydrogen atoms from the adjacent atomic position of an
organic substrate is a thermodynamically uphill process. This is
usually attained by stoichiometric amount of oxidant or H2
acceptor and hence result in generation of copious waste.
Thus, ADC and hydrogen autotransfer (HA) reactions are well-
recognized sustainable process1-2 as it does not need any
external oxidant or prefunctionalization of substrates.
Although manganese is found to be the third most abundant
transition metal18 in the earth’s crust, the catalytic
de(hydrogenative) reactions with manganese is still in its
nascent stage.19 The seminal work on manganese catalysed
ADC reactions reported by the group of Milstein,20 Beller18,21
Kirchner22 and Kempe23 triggered an upswing in the
advancement of manganese catalysed sustainable synthesis of
complex organic molecules.24 However, the application of Mn-
complex towards the synthesis of heterocycles from biomass-
derived starting materials is rare. Very recently, manganese
catalysed synthesis of N-heterocycles such as pyrrole,25
Nitrogen-containing heterocycles have attracted significant
attention, as they are present in a large number of bioactive
molecules,3
natural
products,4
drugs,5
vitamins,6
pyrimidine and quinoline was described by the group of
22a
Kirchner
agrochemicals,7 dyes8 and flavors.9 Thus, there is a growing
interest to develop new sustainable, one-pot synthetic
strategies for the preparation of diversely functionalized
heterocyclic compounds. Very recently, ADC approach has
been largely applied to synthesize a wide range of heterocyclic
compounds.10 A ruthenium catalysed sustainable synthesis of
pyrrole directly from 1,4-diol and amines was developed by
and Kempe.23 Most of these de(hydrogenative)
reactions catalysed by manganese complexes use phosphine
based ligands. Due to the well-known air and moisture
sensitivities and the expensive nature of phosphine ligands,
the development of phosphine free Mn complex to catalyse
ADC reaction would be a significant advance.
Herein, we have prepared the new NNS-Mn(I) complex by
refluxing the corresponding ligands with MnBr(CO)5 in THF26
(See ESI† for the characterizaꢀon data of the complexes). The
complexes have been applied for the synthesis of the diverse
range of heterocycles such as quinoxaline, pyrazine, quinoline
and benzothiazole via catalytic dehydrogenation and
condensation reaction. To the best of our knowledge, Mn-
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H
N
H
H
containing
3,4-diaminotoluene
(1.0
mmol),
1,2-
Br
Br
Br
N
N
diphenylethane-1,2-diol (1.3 mmol), cat
1 (0.04 mmol) and
N
S
N
SEt
CO
N
S
Mn
CO
Mn
CO
Mn
CO
OC
CO
OC
OC
CO
KOH (0.27 mmol) for 20 h gave 53% isolated yield of the
desired quinoxaline. Keeping the other conditions unaltered,
when xylene was used as a solvent, the yield was improved to
57% (Table 1, entry 2). Under the similar condition, using
Ph
2
1
3
xylene solvent, cat
2
gave 66% yield whereas cat
3
gave 50% of
was
the desired product (Table 1, entries 4 and 5). Thus, cat
2
found to be the best choice for this reaction. Next, the
reaction was studied under the neat condition in the presence
of cat 2, gratifyingly 79% desired quinoxaline was isolated. The
yield of the desired product was decreased with low catalyst
loading or with a lower amount of base (Table 1, entries 12
and 13). Under the optimized reaction condition, MnBr(CO)5
gave a significantly lower amount of 6b. We also investigated
the catalytic activity of the equimolar mixture of MnBr(CO)5 (4
mol%) and 2-(tert-butylthio)-N-(pyridin-2-ylmethyl)ethan-1-
amine ligand (4 mol%) under the same condition. Only 17% 6b
was obtained after 20 h via in situ protocol. After optimization,
the present protocol has been applied to synthesize diverse
range of quinoxaline and pyrazine from 1,2-diamine and 1,2-
diols. Initially, 1,2-disubstituted vicinal diols were reacted with
different 1,2-diaminobenzene and it has been observed that
1,2-diaminobenzene having electron donating group gave
slightly better yield compared to the electron withdrawing
group.
Fig 1. Tridentate ligand derived NNS-Mn(I) complexes and the molecular structure of 3
with thermal ellipsoid 30% probability level (all the hydrogens except N2 and the
counter ion are not shown for the clarity).
complex mediated sustainable synthesis of quinoxalines,
pyrazine and benzothiazole using dehydrogenative approach is
not yet reported.
At the outset, we have selected catalyst
1-3 to synthesize
quinoxaline and pyrazine from 1,2-diamine and 1,2-diol since
these are an important class of heterocyclic compounds and
have various applications. First, the reaction between 3,4-
diaminotoluene and 1,2-diphenylethane-1,2-diol was taken as
a model system to find out the optimum reaction condition for
the synthesis of quinoxaline. Thus, refluxing a toluene solution
Table 2 Synthesis of quinoxaline and pyrazine from 1,2-diamine and 1,2-diola,b
NH2
NH2
HO
HO
R3
R4
Cat 2 ( 4 mol % )
KOH ( 0.27equiv.
N
N
R3
R4
)
+
+ 2H2 + 2H2O
Neat, 140 oC, 20 h
5
4
6
R
3, R4 = Aryl, Alkyl
Table 1 Optimization of the reaction conditions for the synthesis of quinoxalinea
Me
N
N
Me
Me
N
N
Cl
N
N
Cat (4 mol%)
Base
Cl
N
N
H3C
NH2
NH2
H3C
N
Ph
Ph
HO
HO
Ph
Ph
6d (69%)
+ 2H2 + 2H2O
6b (79%)
+
6c (81%)
6a (72%)
Solvent, 140 oC, 20 h
Me
Me
Me
N
O2
N
N
N
N
Me
N
6b
4b
5a
Br
N
N
N
N
Base
(mmol)
Solvent
(ml)
Entry
Yield (%)b
6f (58%)
6h (82%)
Cat.
Me
6g (79%)
6e (62%)
Cl
Cl
Me
Me
Br
Me
Me
1
2
3
4
Cat-1
Cat-1
Cat-2
Cat-2
Toluene (3)
Xylene (3)
Toluene (3)
KOH (0.27)
KOH (0.27)
KOH (0.27)
KOH (0.27)
53
57
62
66
N
Me
Me
N
N
Cl
Cl
N
N
N
N
N
Xylene (3)
Xylene (3)
Neat
6j (74%)
6k (85%)
6i (77%)
Me
Me
6l (72%)
Cat-3
Cat-2
KOH (0.27)
KOH (0.27)
5
6
7c
8
50
79
Cl
Cl
N
Me
Me
N
N
Cl
N
Cat-2
Cat-2
Cat-2
Cat-2
Neat
Neat
Neat
KOH (0.27)
60
52
19
N
NaOH (0.27)
NaHCO3 (0.27)
N
Cl
N
Me
N
9
6o (66%)
6p (65%)
6n (78%)
6m (78%)
10
Neat
Na2CO3 (0.27)
23
11
12d
Cat-2
Cat-2
Cat-2
Neat
Neat
Neat
K2CO3 (0.27)
KOH (0.27)
KOH (0.13)
34
46
52
N
Me
Me
Me
N
Me
Me
N
N
+
Me
N
Me
Me
13
N
N
6s (56%)c
14e
15
KOH (0.27)
KOH (0.27)
N
Cat-2
Neat
Neat
51
14
6r (52%)c
Mn(CO)5Br
6q + 6q' (74%)
N
N
Cl
N
Me
N
N
Me
Cl
N
6t (53%)c
6u (72%)
a
6v (82%)
Conditions: 3,4-diaminotoluene (1 mmol), 1,2-diphenylethane-1,2-diol (1.3
b
c
mmol), base (0.27 mmol), cat (0.04 mmol), 20 h, under argon. Isolated yield.
1:1 ratio of 4b and 5a. d 2 mol% cat 2. e 10 h.
a Conditions: 1,2-Diamine (1 mmol), 1,2-diol (1.3 mmol), KOH (0.27 mmol), Cat 2 (0.04
mmol), 20 h under argon. b Isolated yield. c 36 h.
2 | J. Name., 2012, 00, 1-3
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Table 3 Synthesis of benzothiazole through acceptorless dehydrogenative coupling of
Table 4 Synthesis of quinoline from 2-aminobenzyl alcohol and secondary alcohola,b
2-aminothiophenol with primary alcohola,b
a Conditions: 2-aminothiophenol (1 mmol), alcohol (1.3 mmol), KOH (0.27 mmol),
Cat 2 (0.04 mmol), 20 h under argon. b Isolated yield.
a
Conditions: 2-aminobenzyl alcohol (1 mmol), secondary alcohol (1.3 mmol),
tBuOK (1.2 mmol), Cat 2 (0.05 mmol), under argon. Isolated yield. 48 h. d 1.5
b
c
Under the optimized reaction condition, monosubstituted
vicinal diols reacted smoothly with o- phenylenediamine, 4,5-
mmol secondary alcohol.
Dimethyl-1,2-phenylenediamine,
4,5-dichloro-o-
isolated. Here the excess base is required probably to assist
the condensation reaction. It is worth mentioning that the
amount of the base used in our protocol is substantially lower
than the previously reported PNP-Mn(I)complex.22a When the
reaction was performed in the toluene solvent or with the
lower amount of tBuOK (0.56 equiv), the yield dropped to 72%
and 56% respectively. To study the scope of the reaction,
different 1-aryl ethanol having both electron-withdrawing as
well as electron-donating group has been tested. In all the
cases good to excellent yield was observed. Next, to synthesize
2,3,4- trisubstituted quinoline derivative, 1-Phenyl-1-propanol
was treated with 2-Aminobenzhydrol, gratifyingly 85% 3-
methyl-2,4-diphenylquinoline was obtained. The performance
phenylenediamine, and 3,4-diaminotoluene and led to the
desired product 6n-6p in good yield. Notably, 3,4-
diaminotoluene gave a mixture of two isomeric quinoxaline
derivatives 6q and 6q′. The aliphatic 1,2-diol gave a moderate
yield of 6r-6t. Furthermore, employing 1,2-dicyclohexyl amine
as substrate led to the formation of pyrazine derivatives in
excellent yield (72%-82%).
Encouraged by this result we wanted to apply our
methodology to synthesize 2-substituted benzothiazole via
dehydrogenative coupling of alcohol and 2-aminothiophenol.
The reaction proceeds well with the benzyl alcohols possessing
both electron-donating and electron-withdrawing group in the
aromatic nucleus gave an excellent yield of the desired
benzothiazole. Not only m- and p- substituted benzyl alcohols,
but also o-substituted benzyl alcohol gave an excellent yield of
9l. It is interesting to note that halo substituted benzyl alcohol
were well tolerated and the desired halo-substituted
benzothiazoles 9g-9j were obtained in an excellent yield, which
could be further used for functionalization. Heteroaryl alcohols
such as 2-thiophenemethanol, furfuryl alcohol and 2-
pyridinemethanol worked successfully under the optimized
reaction condition and afforded good yield of expected
products. Next, we were interested in exploring the synthesis
of quinoline derivatives via simultaneous C-C and C-N bond
formation through dehydrogenative condensation reaction of
secondary alcohol with 2-aminobenzyl alcohol. Thus, when 2-
aminobenzyl alcohol (1.0 mmol) and 1-phenylethanol (1.3
of cat
2 was further investigated towards more challenging
aliphatic alcohols. Thus, 2-aminobenzyl alcohol reacted with 2-
octanol to give 61% of the desired quinoline after 48 h. The
reactions with aliphatic alcohols were found to be slower
compared to the benzyl alcohols and a moderate to good yield
of the corresponding quinoline derivatives were obtained. In
all the cases, the C-C condensation occurred at the less
hindered side of the in situ formed carbonyl compound.
In summary, we report the first example of an environment-
friendly sustainable protocol to synthesize quinoxalines,
pyrazine and benzothiazole catalysed by molecularly defined
air stable Mn(I) complex. A wide range of functional groups
survives well under these optimized reaction conditions. This
expedient protocol has been successfully extended to
synthesize a variety of quinoline derivatives via concurrent
formation of C-C and C-N bond. The usage of earth-abundant
mmol) was heated at 140 °C in the presence 5 mol% cat
2 and
tBuOK (1.2 equiv.) for 36 h, 81% desired quinoline was
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15 (a) M. Zhang, X. Fang, H. Neumann and M. Beller, J. Am.
Chem. Soc., 2013, 135, 11384-11388; (b) M. Zhang, H.
biocompatible manganese metal and non-phosphine ligand
system makes this protocol attractive.
Neumann and M. Beller, Angew. Chem., Int. Ed., 2013, 52
597-601.
,
We are thankful to SERB, DST (ECR/2016/000108) and DST-
INSPIRE (IFA-14-CH-143) for the financial assistance. We would
also like to acknowledge the Department of Chemistry and the
CIF, IIT-Guwahati, for the instrumental support and startup
grant. K.D and A.M would like to thank IITG for their
fellowship.
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Conflicts of interest
There are no conflicts to declare.
Ganguli, K. Jana and S. Kundu, Chem. Commun., 2018, 54
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DOI: 10.1039/C8CC05877F
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Sustainable synthesis of quinoxalines, pyrazines, benzothiazoles and quinolines catalysed by
phosphine free Mn(I) complex.
.