Redox-neutral functionalization of α-Csp3-H bonds of secondary cyclic amines: a highly atom-economical strategy forN-arylation/formal cross-dehydrogenative couplings

Article abstract of DOI:10.1039/d0gc04258g

An efficient redox-neutral method has been developed for α-Csp³-H functionalization of secondary cyclic aminesviaconcurrentN-arylation/formal cross dehydrogenation coupling (CDC) with sp²-C-H and sp³-C-H bonds of arenes an

Full text of DOI:10.1039/d0gc04258g

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Redox-neutral functionalization of α-Csp3–H
bonds of secondary cyclic amines: a highly
atom-economical strategy for N-arylation/formal
cross-dehydrogenative couplings†
a,b
Cite this: Green Chem., 2021, 23,
2950
Received 16th December 2020,
Accepted 24th March 2021
DOI: 10.1039/d0gc04258g
Priyankar Jha,a Saddam Husena,b and Ravindra Kumar
*
rsc.li/greenchem
An efficient redox-neutral method has been developed for (Scheme 1). With the emergence of the concepts of “green
α-Csp3–H functionalization of secondary cyclic amines via con- chemistry” and “atom economy”, the long standing problem
current N-arylation/formal cross dehydrogenation coupling (CDC) of how to avoid toxic waste needs to be addressed. In this
with sp2-C–H and sp3-C–H bonds of arenes and ketones, respect- context, the pioneering work of Seidel et al.7 reported the
ively. The developed protocol is operationally simple, highly atom redox-neutral functionalization of the amine α-C–H bond via
economical and environmentally benign (E-factor = ca. 0.5). The an iminium intermediate, formed in situ by the condensation
reaction mechanism has been explained based on the control of amine with an aromatic aldehyde or ketones.8 Herein, we
experiments and examination of reactive intermediates by mass envisioned p-quinol as a protective group and as an oxidant in
spectrometry.
one molecule that can be reduced to form an iminium inter-
mediate (IV) with the concurrent elimination of water
(Scheme 1). Such redox-neutral N-arylative functionalisations
of amines using quinols are still underdeveloped,9 which
could be attributed to the difficulty in controlling the selecti-
Functional cyclic amines are key constituents of numerous
natural products and drugs.1 Amidst the development of
several synthetic methods, functionalization of sp3 C–H bonds
in the α-position of the amine nitrogen atom and subsequent
C−C bond formations has attracted much attention in recent
years.2,3 The majority of methods have largely relied on pro-
tected or tertiary amines, followed by the oxidative activation
of C–H bonds. For instance, tetrahydroisoquinoline (THIQ) is
a key prevalent structure in natural products and bioactive
molecules.4 Not surprisingly, numerous efforts have been
devoted to the functionalization of THIQ with various coupling
partners.5,6 Conventionally, α-sp3 C–H bond functionalization
of THIQ required a two-step synthesis process; viz (i) protec-
tion on nitrogen6q or Cu/Pd-catalyzed N-arylation, where the
aryl group acts as an activating group and then (ii) oxidative
coupling in the presence of transition-metals (Ir,6a–c Ru,6c,u
Au,6d Mo,6e V,6f Pt,6g Fe,6h Co,6i Cu,6j–n etc.) and oxidants (DDQ,
TBHP, O2, etc.)6o–t under thermal or photocatalytic conditions
(Scheme 1). The oxidative coupling step involves the gene-
ration of a reactive intermediate, mostly iminium (I) followed
by nucleophilic addition.6u Most of these protocols generate
significant amounts of toxic metal-wastes in both steps
aDivision of Medicinal and Process Chemistry, CSIR-Central Drug Research Institute,
Lucknow-226031, U.P., India. E-mail: ravindra.kumar1@cdri.res.in
bAcademy of Scientific & Innovative Research (AcSIR), Ghaziabad 201002, India
†Electronic supplementary information (ESI) available: Experimental procedures
and spectroscopic characterization of all organic compounds. See DOI: 10.1039/
d0gc04258g
Scheme 1 Functionalization of the α-Sp3-C–H bond of secondary
cyclic amines.
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Table 2 Substrate scope for α-Csp3–H functionalization of secondary
vity among multiple reactive sites of the quinone intermediate
(III, Scheme 1).10
amines with indolesa
Recently, we reported that p-quinols can be activated by an
amine and subsequent site-selective functionalisation at sp2-C
with different nucleophiles.11 Here, we demonstrate the use of
p-quinol as an “aromatic to be” electrophilic surrogate as well
as for the selective functionalisation of amines at α-Csp3
carbon with arenes and ketones.
We began our study with the evaluation of reaction con-
ditions for a model reaction of p-quinol (1a) with THIQ (2a, 1.2
equiv.) and indole (3a, 1.2 equiv.) (Table 1).12 The desired
racemic CDC product 4a was formed in 62% and 60% yields in
acetonitrile and toluene, respectively (Table 1, entries 2 and 8).
However, the reaction was reasonably fast in toluene. The reac-
tion was equally effective with 1.0 equiv. of indole (Table 1,
entry 9). Furthermore, the rate of the reaction and chemical
yield were much improved in the presence of acetic acid
(10–20 mol%) (Table 1, entries 11–13; 94–98% yield). It is
worth mentioning that in an open air flask the desired
product was obtained in a slightly inferior yield (Table 1, entry
14; 83%).12 Under the optimum conditions (Table 1, entry 12),
various indoles were employed to react with THIQs and
p-quinols (Table 2). Irrespective of their electronic natures
and positions of substituents, it always resulted in the for-
mation of selective C3 position addition products (Table 2).
5-Methoxyindole gave the corresponding CDC product 4b in
78% yield. Halogen containing indoles gave the corresponding
products 4c−e in good yields (70–76%). Electron withdrawing
groups (nitrile, nitro and ester) on the indole at different posi-
Table 1 Evaluation of redox-neutral conditions for α-Csp3–H
functionalization of THIQ with indolesa
a Reaction was conducted on a 0.81 mmol scale (1), THIQ (1.2 equiv.),
indole (1.0 equiv.) and acetic acid (20 mol%). b p-Quinone dimethyl
monoketal was employed. c 1.2 equiv of pyrrole was used. Bis-addition
product (at C2 and C5) was also obtained in 3% yield.
Run
Solvent
Additive (mol%)
Time (h)
Yielda (4a, %)
1
2
3
4
5
6
7
8
Neat
CH3CN
THF
HFIP
DCE
MeOH
Dioxane
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
—
—
—
—
—
—
—
—
24
36
48
36
36
18
48
12
12
6
6
6
9
10
39
62
36
nd
nd
49
nd
60
tions (C5 and C6) were compatible to give the respective
desired products in high chemical yields (4f−h; 62–72%).
Notably, 5-nitroindole and 5-bromoindole required refluxing
temperature (120 °C) to complete the reaction. 7-Azaindole
also worked well to give the corresponding product 4i in 62%
yield. Differently substituted aryl surrogates i.e. p-quinol
(4-methyl, 4-ethyl, 4-isopropyl, 4-cyclohexyl, and 3,4-dimethyl)
were investigated to yield the corresponding N-aryl indonyl
THIQs (4a–m). The reaction with 5-hydroxyindole gave the
corresponding products 4n and 4o in 60% and 63% yields,
9b
10
11
12b
13b
14b,d
—
59
77
98
PhCO2H (20)
AcOH (20)
AcOH (20)
AcOH (10)
AcOH (20)
98 (81)c
94 (78)c
83 (72)c
a NMR yields were calculated using 4-hydroxy-2,4,6-trimethylcyclohexa-
2,5-dien-1-one as an internal standard. b 1.0 equiv. indole was used.
c Isolated yields are mentioned in parentheses. d Open air reaction. nd
= not determined; means a complex reaction mixture was observed.
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Table 3 Substrate scope for α-Csp3–H functionalization of secondary
respectively. However, a minor regioisomer of addition at the
C4 position (ortho to the phenolic group) was also observed
(ca. 6%).12 6,7-Dimethoxy-THIQ gave the corresponding
desired product 4p in 80% yield. Tetrahydro-β-carboline or
tryptoline, another privileged scaffold frequently encountered
in bioactive molecules,13 was also examined for its functionali-
zation at α-Csp3–H. Free indole (NH) and N-methyl substituted
tryptoline gave the desired products (4q and 4r, 51% and 49%
yields, respectively). p-Quinone dimethyl monoketal was also
compatible to give the corresponding N-p-methoxyphenyl
(PMP) containing C1-indonyl THIQ (4s) in 79% yield. Reaction
with N-methylindole gave the corresponding C3-alkylated
product 4t in a moderate yield (47%). However, it was noticed
that there was no reaction in the absence of the acid (cf entry
9, Table 1). Other heterocycles (pyrrolidine and piperidine)
were also tested with an indole (3a) under the present as well
as slightly modified conditions, such as temperature, solvents
and change of dienones. However, a complex reaction mixture
was obtained in all of our attempts.12
amines with phenolsa
Other arene nucleophiles, such as phenols, were employed
for the CDC reaction to give amino-phenols, called Betti
bases.14 2-Naphthol was first treated with THIQ and p-quinol
under identical redox-conditions, giving 5a in a moderate yield
(48%), which was further increased to 65% after variation in
the stoichiometry of reactants (Table S1,† entry 6).12 The
present reaction conditions were then examined with a range
of phenols (Table 3). 1-Naphthol, sesamole, p-cresol, and other
phenols underwent CDC reactions selectively at the ortho-posi-
tion with THIQ and p-quinols (5b−i; 48–68% yields). Coupling
took place at the para-position, when 2,6-disubstituted phenol
was employed (5j). 6,7-Dimethoxy- and 7-bromo-substituted
THIQs gave the corresponding products with 1- and
2-naphthols in good yields (5k−5n). Considering the privileged
scaffolds in drug/drug-like molecules, quinolines are fre-
quently found in anti-bacterials.15 Under the present redox-
neutral conditions, 4-hydroxyquinoline was compatible to give
the corresponding ortho-substituted THIQ-quinolines 5o in
moderate yields (44%). Estrone gave the corresponding ortho-
substituted estrone-THIQ hybrid product 5p in 40% yield (1 : 1
dr). An essential oil extract, eugenol, underwent redox reaction
with THIQ to form 5q in a good yield (57%). Whereas, trypto-
line gave the corresponding product 5r with 2-naphthol in
51% yield. p-Quinone dimethyl monoketal gave the corres-
ponding PMP group containing product (5s) with 1-naphthol
and THIQ in 67% yield. 2-Methoxynaphthalene was examined
in place of free-phenol, however, no reaction took place even at
higher temperature (110 °C).
a Reaction was conducted on a 0.81 mmol scale (1), THIQ (1.5 equiv.),
phenol (1.2 equiv.) and acetic acid (20 mol%). b p-Quinone dimethyl
monoketal was employed.
As a proof of concept, the present redox-neutral conditions
To see the reactivity pattern of arenes in the present reac-
were also investigated with ketones and esters as C(sp3)–H eno- tion, we treated 6,7-dimethoxy-THIQ and 1a with sets of equi-
lizable coupling partners (Table 4). The alkylation product 6a molar mixtures of two nucleophiles (Table 5). For example,
was obtained in 57% yield with acetone, whereas ethyl methyl with 1- and 2-naphthols or 1-naphthols and indole, the reactiv-
ketone gave the alkylation product selectively on the methyl ity of 2-naphthol and indole was completely suppressed and
group (6b; 60% yield). The PMP-protected acetone addition the product was selectively formed with 1-naphthol (5l;
product (6c) was synthesized from a p-quinone dimethyl Table 5, entries 1 and 2). However, the selective formation of
monoketal in 73% yield. However, ethyl acetate failed to give indolyl-THIQ occurred when a mixture of 2-naphthol and
any product, probably due to its lower nucleophilicity.
indole was employed (4p; Table 5, entry 3). These experiments
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Table 4 Substrate scope of α-Csp3–H functionalization of secondary
amines with ketonesa
a Reaction was conducted on a 0.81 mmol scale (1a). THIQ (1.2 equiv.),
ketone/ester (10 equiv.) and acetic acid (20 mol%).
Table 5 CDC reaction towards different arenesa
Entry Nu1 (1.0 equiv.) Nu2 (1.0 equiv.) Selective product (% yield)
Scheme 3 Mechanistic study. The reaction was conducted on
a
0.5 mmol scale of 1a and reaction aliquots were analysed at different
intervals of time by mass spectrometry.
1
2
3
5l = 63%
5l = 61%b
4p = 68%
in the absence or presence of AcOH (Scheme 3, eqn (1)).12 The
desired product 5m was obtained in 60% yield. Another
control experiment was performed in the absence of any
nucleophile and the progress of the reaction was monitored by
HRMS analysis (eqn (2)).12 A mass peak corresponding to
N-arylimine (C) was distinctly observed and there was no mole-
cular ion peak corresponding to the cyclic amide product (8;
expected via radical pathway).16 Another set of reactions was
performed in the presence of indole and mass spectrometric
analysis showed the formation of an hemiaminal (A) and
N-aryliminium (C) intermediates along with indole addition
product 4a (ca. 20%) even at room temperature.12
Furthermore, it was evident that the reaction proceeds faster
and provides a high yield in the presence of acids (Table 1,
entries 10–13). Here, we presume that acid might facilitate OH-
elimination and in turn tautomerization to the iminium ion (A
to C). These controlled experiments strongly support that the
reaction proceeds via an iminium ion intermediate (C) which
undergoes the Betti reaction or a Mannich-type reaction to give
the C1-functionalized amines (4–6).
a Reaction was conducted at 0.5 mmol scale (1a), THIQ (1.2 equiv.) and
AcOH (20 mol%), toluene (1.2 mL). b 4p was also formed in ca < 5%
yield.
revealed the reactivity pattern of arenes under the present con-
ditions as follows; 1-naphthol > indole > 2-naphthol.
As THIQ-addition products have cleavable PMP groups (4s, 5s
and 6c), we attempted to deprotect it under oxidative conditions.
A complex reaction mixture was obtained in all of our attempts
with 4s and 5s (or with N-methylindole and O-methylnaphthol
adducts).12 This is probably due to oxidisable indole or naphthol
units. However, PMP group was successfully cleaved from 6c to
afford free amine 7 in 85% yield under oxidation conditions
using ceric ammonium nitrate (CAN) (Scheme 2). The greener
aspect of the present reactions was evaluated with green chem-
istry metrics and they showed an excellent E-factor (0.5–1.1).12
To gain mechanistic insight into the present reaction, a
control experiment was conducted with 1a, 6,7-dimethoxy-
THIQ, 2-naphthol and stable free radical TEMPO (1.5 equiv.)
Conclusions
In conclusion, we developed a redox-neutral approach for the
synthesis N-aryl-C1-functionalized THIQs and tryptolines in a
single step. The developed method is highly compatible with a
wide substrate scope. The reaction is highly atom economical,
operationally simple and environmentally benign (E-factor =
0.5). The green features of the present method are the absence
of any transition-metal and H2O being the only by-product.
Further exploration of this reaction and the development of
asymmetric variants are in progress.
Scheme 2 Cleavage of the PMP group.
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
Financial support from SERB, New Delhi, grant no-CRG/2018/
002147 (GAP321) is highly acknowledged. PJ and SH thank UGC
and CSIR, respectively, for providing fellowship. The authors
acknowledge SAIF, CSIR-CDRI, for providing analytical support.
This manuscript bears CDRI communication number 10219.
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