Manganese(III) Acetate Catalyzed Aerobic Dehydrogenation of Tertiary Indolines, Tetrahydroquinolines and an N-Unsubstituted Indoline

Article abstract of DOI:10.1002/adsc.202100581

A Mn(OAc)₃ ? 2H₂O-catalyzed aerobic dehydrogenation of five and six-membered N-heterocycles for the synthesis of N-heteroarenes is reported. Of note, this protocol can be applied to the dehydrogenation of tertiary indolines with various electron-deficient N-substituents. Preliminary mechanistic investigations support that a single-electron transfer pathway might be involved. (Figure presented.).

Full text of DOI:10.1002/adsc.202100581

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doi.org/10.1002/adsc.202100581
Manganese(III) Acetate Catalyzed Aerobic Dehydrogenation of
Tertiary Indolines, Tetrahydroquinolines and an N-Unsubstituted
Indoline
a
a, b,
Xiaokang Niu and Lei Yang *
a
Key Laboratory of Organosilicon Chemistry and Material Technology of Ministry of Education, Collaborative Innovation Center
for the Manufacture of Fluorine and Silicone Fine Chemicals and Materials, Hangzhou Normal University, 311121 Hangzhou,
People’s Republic of China
E-mail: lyang@hznu.edu.cn; yangunibas@126.com
State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of
b
Sciences, 730000 Lanzhou, People’s Republic of China
Manuscript received: May 14, 2021; Revised manuscript received: July 12, 2021;
Version of record online: ■■■, ■■■■
Supporting information for this article is available on the WWW under https://doi.org/10.1002/adsc.202100581
[6]
[7]
photocatalysis and biocatalysis have also been
developed very recently. Among the methodologies
established, transition metal-catalyzed acceptorless
dehydrogenations of N-heterocycles are more promis-
ing since no stoichiometric oxidants are required, and
Abstract: A Mn(OAc) ·2H O-catalyzed aerobic
3
2
dehydrogenation of five and six-membered N-
heterocycles for the synthesis of N-heteroarenes is
reported. Of note, this protocol can be applied to the
dehydrogenation of tertiary indolines with various
electron-deficient N-substituents. Preliminary mech-
anistic investigations support that a single-electron
transfer pathway might be involved.
in addition, only H byproduct is produced during the
2
process (Scheme 1–1a). Important contributions have
[8]
been made in this field using heterogeneous or well-
defined homogeneous metal catalysts consisting of
[
9]
[10]
[11]
[12]
[13]
iridium, ruthenium, cobalt, 14]nickel, iron, and
[
osmium compounds as well.
More recently, an
Keywords: Manganese; dehydrogenation; tertiary in-
dolines; NH-heterocycles; single-electron transfer
acceptorless dehydrogenation of N-heterocycles using
a well-defined Mn-PNP complex under basic condi-
[15]
tions was also reported.
Despite these remarkable
achievements made, the introduction of multi-step
The dehydrogenation from saturated N-heterocycles, synthesized ligands, noble metal complexes, tedious
e.g., indolines and 1,2,3,4-tetrahydroquinolines, to catalysts preparation or harsh reaction conditions are
produce their corresponding unsaturated N-heterocyclic major concerns, thus restricting their potential applica-
aromatic compounds is a fundamental yet powerful tions.
transformation that has applications across synthetic
Alternatively, transition metal-catalyzed dehydro-
organic chemistry, material science and medicinal genations can also be achieved with the assistance of
[1]
[16]
chemistry. As a consequence, the development of sacrificial hydrogen acceptors (Scheme 1–1b).
efficient synthetic methods enabling such a trans- Among them, oxidative dehydrogenation using O as
2
formation has attracted continuous research interests in the hydrogen acceptor represents a more compelling
the past four decades. Conventional methods often route in terms of atom economy and environmental
require stoichiometric reagents or oxidants such as concerns (Scheme 1–1c). Although many elegant cata-
[2]
DDQ, iodates, peroxides, and metal-based oxidants.
lytic systems have been explored under relatively
However, the use of stoichiometric oxidants or milder conditions, most of these methods are based on
[
3d,17]
reagents is undesirable from both environmental and the use of noble metals.
economical points of view. more abundant, inexpensive and less toxic transition
In recent years, significant developments in the metal catalysts has emerged as attractive alternatives at
Meanwhile, the use of
[18–19]
direction of catalytic dehydrogenation of N-hetero- present.
In this context, the use of Mn-based
cycles have been achieved using homogeneous or catalysts is of great interest due to the redox properties
[
3]
[20]
heterogeneous metal catalysts
and metal-free of different oxidation states of Mn. Along this line, a
[4]
[5]
catalysts. Notably, examples of electrocatalysis , handful of Mn-based catalytic materials have been
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Scheme 2. Scope with respect to tertiary indolines with elec-
[a] [a]
tron-withdrawing N-substitutions.
Conditions: 1 (0.2 mmol),
Mn(OAc) ·2H O (20 mol%), AcOH (2.0 mL), under air
3
2
[b]
(
1 atm), 80°C, 12–24 h. Performed on a 25-fold scale. Yields
are given for isolated yields.
Scheme 1. State-of-the-art and current transition-metal-cata-
lyzed dehydrogenation of indolines.
commercially available Mn(OAc) ·2H O-catalyzed ox-
3
2
idative dehydrogenation of indolines and tetrahydro-
quinolines with air as the terminal oxidant. Applic-
prepared and exhibited good catalytic activities in ability toward indolines bearing both electron-
aerobic dehydrogenation of N-heterocycles withdrawing and electron-donating N-substituents is a
Scheme 1–2, top), thanks to an initial aerobic key feature of the current Mn(OAc) ·2H O/O cata-
dehydrogenation using mesoporous manganese oxide lytic system.
[19]
(
3
2
2
[19a]
catalyst developed by Suib in 2015.
Regardless of
We initially chose N,N-dimethylindoline-1-carbox-
these advances, reported protocols are limited to amide 1a as the model substrate to investigate the
heterogeneous Mn-based catalysts whose cumbersome aerobic dehydrogenation process. After extensive
[22]
preparation methods are normally required. In addition, screening, we found that heating 1a in acetic acid
these documented aerobic dehydrogenations are largely (AcOH) at 80°C under 1 atm of O , in the presence of
2
limited to NH-heterocycles, and methods applicable 20 mol% of Mn(OAc) ·2H O, provided the desired
3
2
for N-substituted heterocycles, by contrast, are far less indole product 2a in quantitative yield after 12 h
[
17,19e]
developed.
(Table 1, entry 1). Besides Mn(OAc) ·2H O, other
3
2
[21]
Considering the fact that Mn(OAc) ·2H O is a manganese salts, such as MnO , Mn(OAc) , Mn(acac) ,
well-known single-electron transfer (SET) reagent, we MnF , and MnCl , could also effectively catalyze the
3
2
2
2
3
2
2
reasoned that Mn(OAc) ·2H O might be used as a dehydrogenation reaction to furnish 2a in relatively
3
2
catalyst in aerobic dehydrogenation of indolines. The lower yields (Table 1, entries 2–6). The use of protic
dehydrogenation is supposed to initiate from SET from solvents other than AcOH, such as 1,1,1,3,3,3-hexa-
3
+
nitrogen to Mn . Then the formed aminium radical fluoro-2-propanol (HFIP) or 2,2,2-trifluoroethanol
cation intermediate underwent a direct CÀ H abstraction (TFE), clearly gave worse results (Table 1, entries 7
to afford the imine cation intermediate, followed by and 8). Only lower conversion or no reaction was
deprotonation and aromatization to afford indole observed when using MeOH, toluene or THF as the
product (Scheme 1–2, bottom). Herein, we describe a solvent (Table 1, entries 9–11). Alternative oxidants,
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[a]
Table 1. Optimization of Reaction Conditions.
indoline 1c were all suitable substrates. Moreover,
phosphonate or phosphoryl-protected indoline 1d or
1
e could be compatible with the aerobic dehydrogen-
ation reaction, smoothly affording 2d and 2e in
moderate to high yields. N-heteroaryl substituted indo-
line 1f or 1g could smoothly transform to generate 2f
and 2g in 86% and 92% yields, respectively. In
addition, phenyl group on the N atom was also
tolerated well, delivering the desired product 2h in
91% yield. Next, an investigation into different
substituted N-C(O)NMe2 indolines showed that a
number of functional groups could be introduced into
the phenyl ring of indoline at different positions to
give the corresponding products in good to excellent
yields. For examples, indolines bearing methyl or
methoxyl groups at different positions of the phenyl
ring underwent facile dehydrogenation, affording the
corresponding products 2i, 2j, 2l, 2q, and 2r in good
to excellent yields. The halogen-containing indolines
possessing chloro or bromo substituents delivered 2k
and 2m in 75 and 66% yields, respectively. The
present catalytic system also proved to be tolerant of
indoles with valuable electrophilic ester groups. This
tolerance of electron-withdrawing functional groups
provides a possibility to use this process in combina-
tion with further conventional cross-coupling trans-
formations. However, a substituent at the 3-position of
the indoline strongly diminished the efficiency of the
reaction (2s), which may be due to steric inhibition
during the reaction. In addition, we were pleased to
find that the dehydrogenation of 1t bearing a phenyl
[b]
Entry Variations from above conditions
Yield [%]
1
2
3
4
5
6
7
8
9
1
None
>99
81
MnO instead of Mn(OAc) ·2H O
2
3
2
Mn(OAc) instead of Mn(OAc) ·2H O 37
Mn(acac) instead of Mn(OAc) ·2H O 82
2
3
2
3
3
2
MnF instead of Mn(OAc) ·2H O
68
64
85
44
trace
trace
8
83
63
20
65
26
78
2
3
2
MnCl instead of Mn(OAc) ·2H O
HFIP as solvent
TFE as solvent
MeOH as solvent
Toluene as solvent
THF as solvent
2
3
2
0
11
12
13
14
15
16
17
18
K S O (2.0 equiv.) as oxidant, N
2
TBHP (2.0 equiv.) as oxidant, N2
Under N2
2
2
8
Performed at 60°C
Performed at 30°C
Using 10 mol% of Mn(OAc) ·2H O
3
2
Under air
>99
[a]
Conditions: 1a (0.2 mmol), catalyst (20 mol%), oxidant,
solvent (2.0 mL), atmosphere (1 atm), 12 h.
H NMR yield by using 1,3,5-trimethoxybenzene as an
internal standard.
[b] 1
such as K S O or TBHP, led to diminished yields of group at the C-7 position was also carried out smoothly
2
2
8
2
a (Table 1, entries 12 and 13). In contrast, only 20% to produce 2t in 72% yield.
yield of 2a was obtained when the reaction was Encouraged by these results obtained, we then
performed in the absence of an oxidant, suggesting that investigated the compatibility of indolines with N-alkyl
the oxidant is vital for the successful catalytic cycle of substituents (Scheme 3). We first explored the reac-
this transformation (Table 1, entry 14). Carrying out tivity of 1u under standard Mn(OAc) ·2H O/AcOH/
3
2
the reaction at lower temperatures (Table 1, entries 15 air conditions. However, decomposition of 1u leading
and 16) or in the presence of a decreased amount of to benzaldehyde and N-acetyl indole was observed.
Mn(OAc) ·2H O (Table 1, entry 17) had negative Gratifyingly, this undesired decomposition could be
3
2
effect on the efficiency of the reaction. Finally, we avoided upon a simple solvent replacement. After a
[22]
were pleased to find that the reaction could be quick reoptimization,
conducted under ambient air without any loss of MeOH-TFE (2,2,2-trifluoromethane) mixed solvent
reactivity (Table 1, entry 18). and a lower catalyst loading (5 mol%) could allow the
we found that the use of
With the optimized reaction conditions in hand, we formation of 2u in good yield. Of note, the reaction
then turned our attention to the scope of the reaction. could be extended to indolines bearing other simple or
The scope of the reaction with respect to indolines ester-, alkoxyl-, or siloxy-functionalized N-alkyl
with different electron-withdrawing or easily remov- groups, affording their corresponding indoles 2v–y in
able N-substitutions was first studied and the results moderate to very good yields. Importantly, indolines
are summarized in Scheme 2. The indole product 2a bearing N-3-bromopropyl (1z), N-ethanol (1a’), and N-
was readily isolated in 99% yield under the optimal allyl (1b’) were also tolerated and furnished the desired
reaction conditions (20 mol% of Mn(OAc) ·2H O, indole products, albeit in lower yields. Furthermore,
3
2
AcOH, 80°C, air, 12 h). The reaction also proceeded we found that cyclin-dependent kinase 4 (CDK4)
[23]
satisfyingly on a 25-fold (5 mmol) scale, giving rise to selective inhibitor 2aa
or indole–indolinyl com-
2
a in 88% yield. Diversified other N-substituents in pound 2ab could be prepared from diindoline 1aa via
the indoline scaffold were also tolerated. Methyl 1H- dehydrogenation under slightly different reaction con-
indoline-1-carboxylate 1b or N-pivaloyl-substituted ditions, and dehydrogenation of 2ab proceeded
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lines bearing an alkyl or an aryl group at the C-2
position, affording corresponding 2-substituted quino-
lines in moderate to good yields. Moreover, NH-
indolines, e.g., 5-methoxyindoline 3d, also partici-
pated in this transformation. Although only moderate
to good catalytic efficiency was shown for the
dehydrogenation of NH-heterocycles tested, the current
Mn(OAc) ·2H O/air system stands as one of the few
3
2
effective catalytic systems which are applicable for
aerobic dehydrogenation of both N-substituted and N-
[17,19e]
unsubstituted heterocycles.
To gain some mechanistic insights into this aerobic
dehydrogenation process, we first carried out several
radical inhibition experiments under standard reaction
conditions (Scheme 5). When butylated hydroxyto-
luene (BHT), 1,1-diphenylethylene, or an electron
[24]
transfer (ET) scavenger 1,4-dinitrobenz-ene
was
added in the dehydrogenation of 1a, significant
decreases in the yields of 2b highlighted that a radical
process via SET was possibly involved in the catalytic
cycle. Surprisingly, when radical trapping reagent
TEMPO was subjected into the dehydrogenation, a
yield of 57% was still obtained (Scheme 5a). In
addition, a comparative yield of 2b was achieved even
Scheme 3. Scope of tertiary indolines with alkyl N-substitution-
[a,b] [a]
s.
Conditions: 1 (0.2 mmol), Mn(OAc) ·2H O (5 mol%),
3
2
TFE/MeOH (v/v=1:1, 2.0 mL), under air (1 atm), 80°C, 10 h. performing the reaction under N (Scheme 5b). These
2
[b]
[c]
Yields are given for isolated yields. 1aa, (0.2 mmol), interesting results together with a recent elegant report
[d]
[25]
Mn(OAc) ·2H O (40 mol%).
1aa, (0.2 mmol), Mn- on iron-catalyzed
dehydrogenation of N-Hetero-
3
2
[e]
(
OAc) ·2H O (10 mol%). 2ab, (0.1 mmol), Mn(OAc) ·2H O cycles in the presence of TEMPO drive us to explore
3
2
3
2
(10 mol%).
the potential multiple effects of TEMPO in this
transformation, and thereby two more control experi-
ments were then conducted. Interestingly, we found
smoothly to furnish 2aa in 78% yield under similar that TEMPO, which has been successfully used as a
conditions. catalyst for the electro-chemical acceptorless dehydro-
Impressively, we found that our current Mn- genation of N-heterocycles,
[5a]
could be used as an
(
OAc) ·2H O/O system was amenable for dehydro- effective promoter for the current thermodynamic
3
2
2
genation of NH-heterocycles (Scheme 4). For instance, dehydrogenation under air or even N atmosphere
2
the dehydrogenation of 1,2,3,4-tetrahydroquinoline 3a (Scheme 5b). Subsequently, experiments employing
proceeded well upon the Mn(OAc) ·2H O/air/TFE- 1,2,3,4-tetrahydroquinoline-type substrates were con-
3
2
MeOH system to furnish quinoline 4a in 64% yield. ducted. As expected, no dehydrogenation product was
The reaction is also compatible with tetrahydroquino- detected when using 2,2,4,7-tetramethyl-1,2,3,4-tetra-
hydroquinoline 3e (Scheme 5c) or 1-(pyrimidin-2-yl)-
1,2,3,4-tetrahydroquinoline 3f (Scheme 5d) as the
substrate, highlighting the importance of the α CÀ H
proton and the NÀ H motif in 1,2,3,4-tetrahydroquino-
line-type substrate for the successful dehydrogenation.
These results are consistent with other research
[18c,19b–e]
studies,
which suggest that a similar reaction
mechanism via a SET initiated dehydrogenation to
form an imine intermediate, tautomeric cyclic imine,
and second dehydrogenation process might be involved
for the aerobic dehydrogenation of 1,2,3,4-tetrahydro-
quinolines under current Mn(OAc) ·2H O/O catalytic
3
2
2
system.
Scheme 4. Scope of NH-heterocycles. Conditions:
3
Subsequently, in order to get some information
(
1
0.2 mmol), Mn(OAc) ·2H O (30 mol%), TFE/MeOH (v/v= about possible reaction intermediates, we followed the
3
2
:1, 2.0 mL), under air (1 atm), 100°C, 48 h. Yields are given
oxidative dehydrogenation of 1a by TLC. Indeed,
several unknown products were observed at the early
for isolated yields.
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[22]
18
was detected by HRMS . The O-labeled experiment
indicates the incorporation of oxygen from molecular
oxygen.
Though the mechanism for the present transforma-
tion is as yet unclear according to these preliminary
[
4e,18c,19b,26]
mechanistic studies and previous results
plausible reaction mechanism has been postulated
Scheme 5f, path a). The dehydrogenation is supposed
, a
(
3
+
to initiate from SET from nitrogen to Mn along with
*
À
the generation of superoxide radical anion O2 . Then
the formed aminium radical cation intermediate I react
*
À
with O2 to give intermediate III and a peroxide
radical. The reaction between the peroxide radical and
III results in the formation of IV (confirmed by
[22]
HRMS) , followed by reduction or homolysis to
afford V. Finally, the expected product 2a is obtained
by elimination of a water molecule. However, a
mechanism involving the formation of an iminium
cation intermediate II could not be ruled out at this
stage.
In summary, we have established
a
Mn-
(
OAc) ·2H O/O catalytic system for the oxidative
3
2
2
dehydrogenation of N-heterocycles under mild reaction
conditions. The generality of this protocol was demon-
strated on a broad range of N-alkyl, aryl, acyl, and
removable protecting groups substituted indolines, and
its applicability was further explored with dehydrogen-
ation of N-unsubstituted 1,2,3,4-tetrahydroquinolines
and an NH-indoline. Further applications of this Mn-
(
OAc) ·2H O/O catalytic system in oxidative trans-
3
2
2
formations are currently underway in our laboratory.
Experimental Section
General procedure for the synthesis of aerobic dehydrogen-
ation products 2a–t (Scheme 2). Indoline (0.2 mmol, 1 equiv.),
Mn(OAc) ·2H O (0.2 equiv.) and anhydrous acetic acid (2 mL)
3
2
was added to an oven-dried 25 mL J-Young tube under air
atmosphere. The reaction mixture was stirred at 80°C for 12 h
or 24 h. Upon completion, it was cooled to room temperature
and extracted with EtOAc and H O. The combined organic
2
phases were dried over anhydrous Na SO , filtered and
2
4
evaporated in vacuo. The crude residue was purified by flash
column chromatography on silica gel using EtOAc–petroleum
ether mixture as an eluent to afford the desired compound.
Acknowledgements
Scheme 5. Mechanistic considerations.
This activity was financially supported by a Start-up Grant from
Hangzhou Normal University (No. 2019DL018). Financial
stage of the reaction. After several trials, we were support from Hangzhou City, Xihu Scholar program (to L. Y.) is
pleased to isolate one of the possible intermediate 1a’’ also acknowledged. The authors would like to thank the
(
Scheme 5e). In addition, when 1a’’ was used as the generous support from Prof. Li-Wen Xu during the research.
substrate, a high isolated yield of 2a was obtained The technical staff of the analysis and test center of the key
even at a lower temperature (60°C). These results
laboratory of MOC is thanked for their assistance. We also
thank Jing Xu and Yi Cheng in this group for reproducing the
results for 2n, 4c, 2v, and 2a’.
clearly support that 1a’’ might be present as a reaction
intermediate involved in this dehydrogenation. Further-
18
18
more, under O atmosphere, O-labeled product 1a’’
2
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