An atom-economical and regioselective metal-free C-5 chalcogenation of 8-aminoquinolines under mild conditions

Article abstract of DOI:10.1039/c9ob02235j

A general and simple metal-free protocol for expedient C-H functionalization leading to the regioselective generation of C-5 chalcogenated 8-aminoquinoline analogues in up to 90% yield at room temperature (25 °C) has been established. This methodology is an eco-friendly approach to the atom-economical utilization of diaryl/dialkyl chalcogenides for direct access to chalcogenated quinolines and is scalable to the gram scale without considerable decrease in the yield of the product. It represents a practical alternative to the existing metal-catalyzed functionalization of 8-aminoquinoline derivatives with broad functional group tolerance. The controlled experiments suggest that the reaction possibly proceeds through an ionic pathway at room temperature. Furthermore, the potentiality for the functionalization of free amines in chalcogenated-8-aminoquinolines provides an attractive perspective for further elaboration of the amine substituent through chemical manipulations. The applicability of the standardized method has been augmented through late-stage antimalarial drug diversification of primaquine analogues.

Full text of DOI:10.1039/c9ob02235j

Organic &
Biomolecular Chemistry
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An atom-economical and regioselective metal-free
C-5 chalcogenation of 8-aminoquinolines under
mild conditions†
Cite this: Org. Biomol. Chem., 2019,
17, 10245
c
Vipin Kumar, a Klaus Banert, b Devalina Ray *c and Biswajit Saha
*
A general and simple metal-free protocol for expedient C–H functionalization leading to the regio-
selective generation of C-5 chalcogenated 8-aminoquinoline analogues in up to 90% yield at room temp-
erature (25 °C) has been established. This methodology is an eco-friendly approach to the atom-econ-
omical utilization of diaryl/dialkyl chalcogenides for direct access to chalcogenated quinolines and is scal-
able to the gram scale without considerable decrease in the yield of the product. It represents a practical
alternative to the existing metal-catalyzed functionalization of 8-aminoquinoline derivatives with broad
functional group tolerance. The controlled experiments suggest that the reaction possibly proceeds
through an ionic pathway at room temperature. Furthermore, the potentiality for the functionalization of
free amines in chalcogenated-8-aminoquinolines provides an attractive perspective for further elaboration
of the amine substituent through chemical manipulations. The applicability of the standardized method has
been augmented through late-stage antimalarial drug diversification of primaquine analogues.
Received 16th October 2019,
Accepted 22nd November 2019
DOI: 10.1039/c9ob02235j
rsc.li/obc
this regard, seminal reports on the directed activation of distal
C–H bonds that are inaccessible for cyclometallation provide a
Introduction
Suitably functionalized 8-aminoquinolines are the centralized unique alternative to the traditional halide replacement reac-
building blocks for numerous pharmaceutical compounds, tions at desired targets.5 The preceding years have witnessed
motivating the generation of user friendly methods for their several C5–H functionalizations of 8-aminoquinolines using
direct access in the scientific community.1 Site selective transition metal catalysts mostly at elevated reaction tempera-
functionalization of inert C–H bonds can be considered as a tures while metal free C5–H activation at ambient temperature
leading approach for the step- and atom-economical gene- is still in its infancy.6 In this context, several reports have
ration of products with minimum waste accumulation.2 C–H shown the use of Cu, Pd, Fe, Ni, and Ru catalysts for the gene-
activation related to 8-aminoquinoline derivatives is either ration of C5 functionalized products obtained from the pro-
focused on the site selective C-5 and C-7 functionalization of tected 8-aminoquinolines in moderate to good yields
the aromatic counterpart within 8-aminoquinoline or utilizing (Scheme 2).7 However, a recent report has described these
it as a directing group to achieve β-Csp2–H activation in ali-
phatic or aromatic amides (Scheme 1). Elegant contributions
have been made in the establishment of geometrically accessi-
ble C–H bond functionalization of aromatics/heteroaromatics,
where directing group assisted chelation dictated the regio-
selectivity.3 In contrast, the regioselective functionalization of
a remote C–H bond represents a newly emerging ascendant
and appealing approach in the synthetic community.3b,4 In
aAmity Institute of Click Chemistry Research and Studies, Amity University,
Sector 125, Noida 201313, Uttar Pradesh, India
bChemnitz University of Technology, Organic Chemistry, Strasse der Nationen 62,
09111 Chemnitz, Germany
cAmity Institute of Biotechnology, Amity University, Sector 125, Noida 201313,
Uttar Pradesh, India. E-mail: dray@amity.edu, bsaha1@amity.edu
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
Scheme 1 Various modes of functionalization via C–H functionali-
c9ob02235j
zation in 8-aminoquinoline analogues.
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Results and discussion
A decent number of reports on metal-catalyzed as well as
metal-free Csp2–H chalcogenation of aromatic and heteroaro-
matic compounds have been published to date.12 In this
regard, I2/DMSO catalyzed C–H functionalization observed at
elevated temperatures encouraged us to initiate the coupling
reaction of 8-aminoquinolines at room temperature under
similar conditions.13 Gratifyingly, the preliminary investi-
gations that commenced by reacting 8-aminoquinoline (1
equiv.) with diphenyl diselenide (0.5 equiv.) in the presence of
30 mol% of I2 as the catalyst and DMSO (3 equiv.) as the
oxidant under solvent free conditions afforded the products in
58% yield as a mixture of 5-selanyl (3a, 50%) and 5,7-diselanyl-
8-aminoquinolines (4a, 8%) (entry 1, Table 1). Keeping the
reaction conditions unaltered, experiments were performed
with DCE and DCM as the solvents, which led to decreased
reactivity as well as selectivity (entries 2 and 3, Table 1).
In an attempt to tune the reaction conditions for the
enhancement of the yield as well as the selectivity of C-5 sub-
stituted 8-aminoquinoline, the amount of DMSO was
increased to 6 equivalents in the absence of a solvent at 80 °C,
keeping the rest of the reaction conditions unaltered (entry 4,
Scheme 2 Remote
derivatives.
C-5 chalcogenation of
8-aminoquinoline
transformations with unprotected 8-aminoquinolines, albeit Table 1). The variation didn’t lead to a noticeable change in
using a Cu-catalyst (Scheme 2).8 These reactions are generally the yield or selectivity of the desired product 3a. The incorpor-
known to proceed through the formation of cation-radical ation of DMF as the solvent instead of DMSO at 80 °C resulted
intermediates.9
in a decrease in the overall yield of the reaction to 50% at the
The focus on metal free, site-selective C–H activation has cost of reduced selectivity (entry 5, Table 1). When DMSO was
enormously streamlined the essence of organic transform-
ations. This addresses the perpetual requirement of an envir-
Table 1 Optimization of the reaction conditionsa
onmentally benign approach by eliminating the detrimental
aspects in process development for the generation of new
molecular architecture.
Moreover, addressing the regioselectivity in C–H
functionalization and sustaining the greener approach simul-
taneously has been a persistent longstanding challenge due to
the subtle differences in the reactivity of the C–H bonds in the
scaffold. In this regard, iodine-catalyzed C–H functionalization
reactions have apparently broken new ground as an alternative
metal-free version.10 Although there is no direct information
on the metal-free regioselective C-5 chalcogenation of
8-amido/aminoquinolines, a single instance from a report on
C-4 functionalization of anilines has shown the formation of a
5,7-dichalcogenated derivative as the sole product in the case
of 8-aminoquinolines.11 It clearly indicates the underlying
challenges in controlling the regioselectivity in the formation
of monosubstituted C-5 chalcogenated 8-aminoquinolines
versus the generation of 5,7- or 7-substituted analogues. The
necessity for the establishment of an eco-friendly, step and
atom-economical, and convenient method impelled us to
develop a regioselective, metal-free C-5 chalcogenation of
8-aminoquinolines at ambient temperature, which has
remained unexplored to date. Moreover, the availability of free
amines in chalcogenated 8-aminoquinoline derivatives pro-
vides a wide scope for further synthetic manipulation and
elaboration.
Yieldb (%)
Iodine
(mol%)
Oxidant
(equiv.)
Entry
Solvent
Time (h)
3a
4a
1
2
30
30
30
30
30
30
30
30
30
30
30
10
10
5
DMSO (3)
DMSO (3)
DMSO (3)
DMSO (6)
—
Neat
DCE
DCM
Neat
24
24
24
24
24
24
24
1.5
1.0
1.0
1.5
1.5
1.5
1.5
1.5
1.5
50
45
35
45
35
55
40
40
0
8
5
3
15
15
15
5
4c
5c
6
DMF
—
DMSO
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
7
8
9
10
11
12
13d
14
15
16
AIBN (0.2)
K2S2O8 (1)
H2O2 (1.1)
TBHP (1.1)
TBHP (0.5)
TBHP (0.5)
TBHP (0.5)
TBHP (0.5)
TBHP (0.5)
—
5
15
0
40
70
85
59
75
0
0
5
Trace
32
Trace
0
—
10
15
Trace
a Reaction conditions: 1a (28.84 mg, 0.2 mmol, 1 equiv.), 2a (31.21 mg,
0.1 mmol, 0.5 equiv.), iodine, oxidant in solvent (1 mL) at rt. b Isolated
yields. c Reactions were performed at 80 °C in an oil bath. d 2a was
used in 1 equiv. (62.0 mg, 0.2 mmol) instead of 0.5 equiv.
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Table 2 C-5 chalcogenation of 8-aminoquinoline scaffoldsa,b
used as the solvent in the reaction mixture at rt, the overall
yield was relatively higher (60%, entry 6, Table 1). All further
reactions for optimization were performed with acetonitrile as
the solvent at rt with a catalytic amount of iodine. The incor-
poration of 20 mol% of AIBN as the oxidant resulted in a
reduction in the overall yield of the products (entry 7, Table 1).
Extrapolating the choice of oxidant to inorganic reagents such
as K2S2O8 didn’t lead to a significant variation from the pre-
vious outcome (entry 8, Table 1). Unfortunately, the use of the
oxidant H2O2 (1.1 equiv.) did not afford any product; instead it
caused decomposition of the reaction mixture in 1 h (entry 9,
Table 1). Interestingly, the incorporation of tert-butyl hydroper-
oxide (tBuOOH) as the oxidant (1.1 equiv.) led to the formation
of monosubstituted derivative 3a as the only product in 40%
yield (entry 10, Table 1). However, reduction in the quantity of
tBuOOH from 1 to 0.5 equiv. resulted in a dramatic enhance-
ment in the overall yield of 5-(phenylselenyl)-quinolin-8-amine
(3a, 70%) (entry 11, Table 1). Altering the catalyst amount from
30 to 10 mol% led to a remarkable increase in the regio-
selectivity and in the yield of product 3a to 85%, affording the
disubstituted side product in a trace amount (entry 12, Table 1).
An increase in the proportion of diphenyl diselenide (2a) from
0.5 equiv. to 1 equiv. led to a decrease in the selectivity and
yield of 3a (59%) along with increased production of 4a (32%)
(entry 13, Table 1). Further reduction in the amount of catalyst
to 5 mol% did not enhance the yield of 3a (entry 14, Table 1).
In order to assess the role of individual reagents in the reac-
tion mixture, iodine was eliminated while keeping the other
conditions unaltered. The reaction did not proceed at all in
the absence of iodine (entry 15, Table 1). In contrast, the
absence of TBHP in the reaction mixture in the presence of
10 mol% of iodine resulted in a minor yield of product 3a,
which proves that the presence of iodine is mandatory in the
reaction for the formation of the C5-selenylated product (entry
16, Table 1). The optimized reaction conditions, which
afforded the desired product 3a in 85% yield (entry 12,
Table 1), were further applied to various substrates for the gen-
a Reaction conditions: 1 (1.0 mmol, 1.0 equiv.), 2 (0.5 mmol, 0.5
equiv.), iodine (0.1 mmol, 0.1 equiv.), oxidant (0.5 mmol, 0.5 equiv.) in
CH3CN (1 mL) as a solvent at rt. b Isolated yield of the product.
c Reaction was carried out at 50 °C for 30 min.
eralization of the methodology. A broad range of chalco- the aryl substituent in arylselenium iodide lowers the overall
genides successfully participated in this mild and versatile C-5 reactivity of the electrophilic aromatic substitution with 8-ami-
chalcogenation of aminoquinolines with high efficiency noquinoline resulting in reduced yields of 3c and 3d. The
(Table 2). A wide variety of functionalities were tolerated with strong electron withdrawing p-Cl and m–NO2 functionalities in
different electronic and positional effects, under the present diphenyl diselenide led to the C5-selective products (3e and
reaction conditions, demonstrating the high efficiency of the 3f) in comparatively better yields (70% and 72%). The C-5 chal-
method. The C-5 functionalization of 8-aminoquinoline with cogenation of 6-methoxy and methyl substituted 8-aminoqui-
phenyl and benzyl chalcogenides afforded regioselective pro- noline led to the generation of C-5 selective selenylated pro-
ducts 3a and 3b in good yields. The structures of C-5 chalcoge- ducts 3g, 3h and 3i in moderate to good yields without any di-
nated products were confirmed by the 1D and 2D NMR spec- substituted products being formed in trace amounts. In order
troscopic data (1H, 13C, 77Se, DEPT 135, COSY, HMBC, HSQC to enhance the scope of the reaction, N-methyl-8-aminoquino-
and nOe experiments) of product 3b. Mild and strong electron line was subjected to C-5 selective chalcogenation conditions.
donating groups at the para- and ortho-positions of diphenyl The reaction failed to proceed at room temperature even after
diselenide relatively reduced the yield of C-5 products 3c (62%) 24 h; however, the reaction was completed in 30 min at 50 °C
and 3d (65%). The probable reason for this electronic effect leading to the exclusive formation of the C-5 substituted
might be associated with the increase in the electron density product 3j in 50% yield. The enhanced yield of 3j as compared
at the selenium centre of the in situ generated aryl/alkyl sel- to the reported 23% yield obtained under the Cu-catalyzed
enium iodide (Scheme 5). The presence of mild and strong conditions shows the efficacy of the present protocol for C-5
electron donating groups at the para- and ortho-positions of functionalization.8
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The use of dimethyl diselenide as a substrate however
decreased the yield drastically to 45%. A reduced yield of ali-
phatic selenylated product 3k is speculated due to the chal-
lenges associated with the handling of highly volatile dimethyl
diselenide even at room temperature. The yields of C5-selective
sulfide products (3l–3u) were comparable to those of selenides.
The regioselective formation of diaryl sulfides was not affected
significantly in the presence of the electron donating methoxy,
methyl or amine group at the para position of the phenyl sul-
fides 3m–o. However, the hydroxyl substituent at the meta-
position reduced the yield of product 3p to 58%. There was no
noticeable change in the yield of the products 3q and 3r (72%
and 75%) due to the presence of an electron withdrawing sub-
stituent such as the nitro or chloro group at the para position
of diphenyl disulfides as compared to an electron donating
group. The applicability of this method was further illustrated
with direct C5–H sulfenylation of 6-substituted-8-aminoquino-
lines. The reaction of diphenyl disulfide with 6-methyl and
6-methoxy-8-aminoquinoline as substrates successfully
resulted in the formation of the corresponding
C-5 monosubstituted products 3s and 3t, respectively, in good
to excellent yields, demonstrating the efficiency of the present
method. Furthermore, the regioselective introduction of ali-
phatic sulfides such as n-propyl sulphide at the C-5 position of
8-aminoquinoline successfully generated 3u in 46% yield due
to lower reactivity which was comparable to the yield of ali-
phatic selenylated product 3k. The reaction was scalable to the
Scheme 4 Controlled and isotope effect experiments for chalcogena-
tion reactions.
gram scale, as was demonstrated by carrying out the C-5 chal- 1 equiv. of both the radical quenchers without any side pro-
cogenation with 2 grams of 8-aminoquinoline (1a) under stan- ducts being formed in trace amounts in the reaction mixture.
dard conditions. The yield of the product was found to be 78% This observation rules out the possibility of a radical pathway
which was slightly less with respect to the corresponding small enforcing the chances to proceed through an ionic mecha-
scale reaction (1a, 1 mmol).
nism. The reaction was carried out with the amide derivatives
The utility of this approach was demonstrated by late-stage (N-acetyl and N-pivolyl) of 8-aminoquinoline as an alternative
derivatization of the N-Boc protected antimalarial drug prima- substrate in order to justify the requirement of an amine
quine in good yields. Selenylation and sulfenylation of N-Boc group for regioselective C-5 chalcogenation (Scheme 4). In
primaquine under the standard conditions afforded the both the cases, the reactions completely failed to proceed even
corresponding products 3v and 3w in 50 and 90% yields with after 48 h at rt and the starting materials were recovered from
95% purity (detected from HPLC data in the ESI†) (Scheme 3).
the reaction mixture elucidating the role of the amino group in
The mechanistic interpretation was made by conducting the reaction. Furthermore, the chalcogenation reaction was
controlled experiments with substrate 1a under standard con- carried out with thiophenol as the alternative source of substi-
ditions with the incorporation of TEMPO as well as BHT as the tuted sulfide instead of phenyl disulfide (Scheme 4). The
radical quencher (Scheme 4). The isolated yields of the desired product 3l was generated in 45% yield probably via the
expected product (3a) remained unaltered with 0.2 equiv. and in situ formation of PhSI in the reaction.13a In order to deter-
mine the kinetics of the reaction, an intermolecular competi-
tive experiment was performed with a 1 : 1 mixture of H- and
D-labeled 8-aminoquinoline substrates under standard con-
ditions (Scheme 4). Analysis of the H/D content at the 7-posi-
tion in 5-chalcogenated-8-aminoquinoline products (3m, 3mD)
revealed a nearly equal mixture of isotopes, corresponding to a
kinetic isotope effect (KIE) ≈ 1. These results indicate that the
abstraction of the C-5 proton may not have occurred in the rate
determining step, and thus there is no significant difference
in the rate of reaction with the deuterated substrate.
A plausible mechanistic interpretation has been outlined
based on the controlled experiments. The reaction mechanism
has been speculated to follow an ionic pathway (Scheme 5). In
Scheme 3 Late-stage selenylation and sulfenylation of N-Boc
primaquine.
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Humboldt foundation. Dr Devalina Ray is grateful to
DST-SERB (DST-SERB-CS-058/2014) for the financial support.
Mr Vipin Kumar is thankful to the CSIR-UGC for the fellow-
ship. We are also thankful to Amity Institute of Click
Chemistry Research and Studies for providing lab space.
Notes and references
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Scheme 5 Plausible mechanistic approach.
previous reports, it was predicted that the reaction of diaryl
dichalcogenide with catalytic iodine in the presence of DMSO
as an oxidant leads to the in situ generation of a reactive inter-
mediate RZI (A) at ≥40 °C.13
Under our reaction conditions, however, the transformation
presumably proceeds through the generation of a similar reac-
tive intermediate A at room temperature (Scheme 5). The
nucleophilic attack of 2 equiv. of 8-aminoquinoline with a
reactive intermediate releases the cationic intermediate B
which after deprotonation with the help of iodide liberates the
C-5 chalcogenated product 3a. The hydroiodic acid generated
as a result of deprotonation with iodide is oxidized back to
iodine with tBuOOH, which in turn is reduced to tBuOH in the
reaction mixture.
Conclusion
In conclusion, an atom economical, metal-free, Csp2–H
functionalization of 8-aminoquinolines at ambient tempera-
ture has been developed demonstrating the exclusive regio-
selectivity for C-5 chalcogenated substitution products. This
protocol is an eco-friendly and simplified synthesis method
with diversely functionalized substrates resulting in the gene-
ration of products in moderate to excellent yields. Controlled
experiments indicate that the reaction possibly proceeds
through an ionic pathway at room temperature. The reaction
conditions are scalable without a considerable decrease in
product yield. The generalized method was successfully
implemented for late-stage diversifications of pharmaceutically
important primaquine analogues.
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Acknowledgements
We are grateful for the financial support provided by the
DST-SERB (DST-SERB-CS-192/2011) and Alexander von
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