Potassium tert-Butoxide-Promoted Acceptorless Dehydrogenation of N-Heterocycles

Article abstract of DOI:10.1002/adsc.201900499

Potassium tert-butoxide-promoted acceptorless dehydrogenation of N-heterocycles was efficiently realized for the generation of N-heteroarenes and hydrogen gas under transition-metal-free conditions. In the presence of KOtBu base, a variety of six- and five-membered N-heterocyclic compounds efficiently underwent acceptorless dehydrogenation to afford the corresponding N-heteroarenes and H₂ gas in o-xylene at 140 °C. The present protocol provides a convenient route to aromatic nitrogen-containing compounds and H₂ gas. (Figure presented.).

Full text of DOI:10.1002/adsc.201900499

Title: Potassium <i>tert</i>-Butoxide-Promoted Acceptorless
Dehydrogenation of <i>N</i>-Heterocycles
Authors: Tingting Liu, Kaikai Wu, Liandi Wang, and Zhengkun Yu
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201900499
Link to VoR: http://dx.doi.org/10.1002/adsc.201900499

Advanced Synthesis & Catalysis
10.1002/adsc.201900499
COMMUNICATION
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Potassium tert-Butoxide-Promoted Acceptorless
Dehydrogenation of N-Heterocycles
Tingting Liu,a,b,c Kaikai Wu, Liandi Wang, and Zhengkun Yu *
a
a
a,d,
a
Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian, Liaoning 116023,
Peoples Republic of China
University of Chinese Academy of Sciences, Beijing 100049, Peoples Republic of China
Institute of Chemistry Henan Academy of Sciences, Zhengzhou 450002, Peoples Republic of China
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of
b
c
d
Sciences, 354 Fenglin Road, Shanghai 200032, Peoples Republic of China
Fax: +86-411-8437-9227
E-mail: zkyu@dicp.ac.cn
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
by Cp*Ir(III) complex catalysts.[5] Xiao and co-
workers applied cyclometalated iridium complex
catalysts and trifluoroethanol solvent in the
dehydrogenation reaction of N-heterocycles under
Abstract.
Potassium
tert-butoxide-promoted
acceptorless dehydrogenation of N-heterocycles was
efficiently realized for the generation of N-heteroarenes
and hydrogen gas under transition-metal-free conditions.
In the presence of KOtBu base, a variety of six- and
five-membered N-heterocyclic compounds efficiently
underwent acceptorless dehydrogenation to afford the
[6]
relatively mild conditions. Jones, et al. reached the
[7a]
[7b]
same goal by means of Fe(II) and Co(II)-PNP
complex catalysts. Crabtree group reported nickel(II)-
mediated electrochemical dehydrogenation of N-
corresponding N-heteroarenes and H
40 °C. The present protocol provides a convenient
route to aromatic nitrogen-containing compounds and
gas.
2
gas in o-xylene at
1
[
8]
[9]
heterocycles. Other structurally defined iridium,
[
10]
[11]
ruthenium, and osmium complex catalysts have
also been developed in this area. Paradies and
H
2
[12a]
[12b]
Keywords:
acceptorless
dehydrogenation;
N-
Grimme,
and Kanai,
independently reported
heterocycles; potassium tert-butoxide; transition-metal-
free; dihydrogen
Saturated N-heterocycles as liquid organic hydrogen
carriers (LOHCs) have recently been paid more and
more attention for acceptorless dehydrogenation (AD)
and releasing H
2
gas.[ Without energy loss,
1]
hydrogen-rich LOHCs can not only be stored for a
long time, but can also be transported over long
distances. Acceptorless dehydrogenation of saturated
N-heterocycles avoids the use of stoichiometric
oxidants or sacrificial hydrogen acceptors, and has
demonstrated potential applications in the field of
organic hydrogen storage materials.[ Hydrogen gas
2]
release
thermodynamically
endothermicity, but it is entropically favored.
from
saturated
N-heterocycles
due to
is
unfavored
its
Scheme 1. Acceptorless dehydrogenation of N-
[3]
heterocycles.
Although dehydrogenation of N-heterocycles by
means of external oxidants or sacrificial hydrogen
acceptors has been well documented,[4] acceptorless
B(C
6
F ) -catalyzed acceptorless dehydrogenation of
5
3
N-heterocycles. Photoredox catalysis has also been
applied as an effective strategy to execute the same
processes. TEMPO (2,2,6,6-tetramethylpiperidine-
dehydrogenation is strongly desired because of the
atom-economy requirement and potential application
for hydrogen storage (Scheme 1a). In this regard,
Fujita and Yamaguchi, et al. reported the
dehydrogenation of tetrahydroquinoline derivatives
[13]
1
-oxyl) was used as an effective organo-
[14]
electrocatalyst for the same goal.
Heterogeneous
1
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Advanced Synthesis & Catalysis
10.1002/adsc.201900499
transition-metal catalysts have recently been used for
reversible dehydrogenation/hydrogenation of N-
were further optimized, leading to 3a in 92% isolated
yield (Table 1, entries 10-16). Due to the possible
coordination of KOtBu to the dehydrogenation
[15]
heterocycles.
There have been some recent
[18]
publications on acceptorless dehydrogenative
product, stoichiometric KOtBu was required.
[16]
coupling of N-heterocycles. However, due to the
involvement of transition metals and other
specialized catalysts and/or additives, the existing
dehydrogenation protocols are often expensive or not
very environmentally friendly.
Lowering temperature to 110 °C or conducting the
reaction in refluxing toluene dramatically diminished
the product yield (Table 1, entry 17). By comparison
Table 1. Screening of reaction conditions.[a]
It has been well known that strong bases such as
NaOH and KOH can promote Meerwein-Pondorf-
Verley-Oppenauer redox reactions of carbonyl
compounds and alcohols.[ Stoichiometric KOtBu
base can mediate radical arene C-H/C-I (Br) cross-
coupling in the presence of an amine or N-
17]
[
b]
Base Amount
Time
[h]
Yield
[%]
Entry
Base
[
equiv]
[18]
1
2
DBU
NaOH
KOH
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
2.5
2.0
0.5
0.1
2.5
3.0
3.0
3.0
3.0
48
48
48
48
48
48
48
48
48
48
48
48
48
48
36
36
48
36
36
10
23
31
41
69
66
19
72
0
heteroarene as the catalyst or additive. 20 mol %
KOtBu was documented to inhibit iron(II)-catalyzed
3
acceptorless
dehydrogenation
of
1,2,3,4-
tetrahydroquinaldine,[
7a]
while 10 mol% KOtBu
4
CsOH
exhibited a positive effect on iron nanoparticle-
catalyzed acceptorless dehydrogenation of N-
5
NaOMe
NaOEt
LiOtBu
NaOtBu
LDA
6
[15a]
heterocycles.
KOtBu (20 mol %) catalyzed the
7
hydrogenation of ketones, and the reaction exhibited
the first-order kinetics with respect to the ketone
substrate, hydrogen, and the base catalyst.[
8
18d]
9
During the ongoing investigation of acceptorless
dehydrogenation of N-heterocycles,[ we found that
a base could facilitate acceptorless dehydrogenation
of 1,2,3,4-tetrahydroquinoline to form quinoline and
10
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
99
99
76
23
5
10c]
1
1
1
1
2
3
H gas in the absence of a transition-metal catalyst.
2
Intrigued by such an unexpected base effect, we
explored base-mediated dehydrogenation of a variety
of N-heterocycles. Herein, we disclose KOtBu-
promoted acceptorless dehydrogenation of N-
heterocycles under transition-metal-free conditions
14
1
5
75
[
c]
1
6
99 (92)
16
[d]
1
7
[e]
[c]
(
Scheme 1b).
18
KOtBu
KOtBu
99 (93)
99
[
f]
Initially, the reaction conditions were screened by
19
using 1,2,3,4-tetrahydroquinoline (1a) as the model
substrate. It is noteworthy that the glasswares and
magnetos were new for these experiments, and the
solvents were dried, distilled, and degassed prior to
use. Under a nitrogen atmosphere, treatment of 1a
with 3.0 equiv of organic base DBU (1,8-
diazabicyclo[5.4.0]undec-7-ene) in o-xylene at
[a]
[b]
Conditions: 1a (0.5 mmol), base, o-xylene (2 mL), 140 °C, 0.1 MPa N
LDA = lithium diisopropylamide.
Determined by GC analysis.
2
.
[c]
[d]
[e]
[f]
Isolated yield given in parentheses.
110 °C.
Using 99.99% KOtBu.
The reaction was conducted under an air atmosphere.
1
(
40 °C for 48 h afforded quinoline (2a) in 10% yield
Table 1, entry 1). DABCO (1,4-
diazabicyclo[2.2.2]octane), CO and Cs CO
to 99% KOtBu, high purity (99.99%) KOtBu was
applied in the same reaction, resulting in a similar
result (Table 1, entry 18). The reaction also smoothly
proceeded under an air atmosphere (Table 1, entry
K
2
3
,
2
3
could not further improve the reaction efficiency.
Alkali metal hydroxides NaOH, KOH, and CsOH
enhanced the yield to 23-41% (Table 1, entries 2-4).
Altering the base to sodium methoxide or ethoxide
remarkably increased the yield of 3a to 66-69%.
Sodium tert-butoxide further improved the reaction to
form 3a (72%), while the yield was sharply dropped
to 19% in the case of using LiOtBu base (Table 1,
entries 5-8). LDA (lithium diisopropylamide) showed
a remarkable negative lithium ion effect, which
completely inhibited the dehydrogenation reaction of
1
9), but for safety it was carried out for screening of
conditions under a nitrogen atmosphere. It is
noteworthy that H gas was formed as the only
2
Star
byproduct which was detected by a THERMO gas
analyzer (see the ESI for details).
Under the optimal conditions, the scope of six-
membered N-heterocycles (1) was explored (Table 2).
The reactivities of methyl-substituted N-heterocyclic
substrates, that is, 2-, 3-, 4-, 6-, 7-, and 8-methyl-
1
a (Table 1, entry 9). The highest yield (99%) was
1
,2,3,4-tetrahydroquinolines (1b-g), varied to afford
achieved in the presence of KOtBu base (Table 1,
the corresponding dehydrogenation products 2b-g
entry 10). Both the base loading and reaction time
2
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Advanced Synthesis & Catalysis
10.1002/adsc.201900499
(
64-95%), and the 2- and 8-positioned methyl groups
exhibited an obvious steric effect on the yields of 2b
66%), 2g (64%), 2h (70%), and 2i (72%). However,
-phenyl-1,2,3,4-tetrahydroquinoline (1j) reacted
Next, the protocol generality was extended to the
dehydrogenation of indolines. The reaction
conditions were simply modified by using 2 equiv of
KOtBu base (see the ESI for details). Under the
optimal conditions, indole derivatives were
(
2
well to give 2j (92%), and the 2-phenyl group did not
exhibit a steric effect. This result is presumably
attributed to the coordination interaction between
efficiently obtained and H gas was formed as the
2
only byproduct (Table 3). The dehydrogenation
reaction of indoline (3a) afforded indole (4a) in 92%
yield. It should be noted that Jiao, et al. reported that
trace amount of dioxygen facilitated the
dehydrogenation of 2-cyclopropylindoline in the
presence of large excess of KOtBu base (12 equiv),
[18b]
potassium cation and the 2-aryl ring.
,2,3,4-tetrahydroquinoline (1k) also reacted well to
form the target product 5-methoxyquinoline (2k) in
9% yield. 9,10-Dihydroacridine and 1,2,3,4-
5-Methoxy-
1
8
tetrahydrobenzo[h]quinoline underwent the reaction
to give the target products acridine (2l, 74%) and
benzo[h]quinoline (2m, 78%) in good yields. The
dehydrogenation reaction of 1,2,3,4,7,8,9,10-
octahydro-1,10-phenanthroline (1n) led to 1,2,3,4-
tetrahydro-1,10-phenanthroline (2n) in 68% yield, but
the corresponding perdehydrogenated product 1,10-
phenanthroline (2o) was not detected in the reaction
mixture. In a separate dehydrogenation reaction of 2n,
compound 2o was only obtained in 15% yield
Table 2. Dehydrogenation of six-membered N-
[
a]
heterocycles (1).
because it was unstable under the strong basic
conditions,[
18e]
implicating that the possible strong
coordination of the quinolinyl nitrogen atom of the
substrate to potassium cation diminishes the
+
interaction between the K ion and the aliphatic
anionic nitrogen in the reaction intermediate, which
thus reduces the reaction efficiency of 2n.
Tetrahydroisoquinoline (1p) also efficiently reacted
to form the corresponding product isoquinoline (2p,
8
6%), whereas the -methyl groups exhibited a steric
effect on the yields of 1- and 3-methylisoquinolines
q (55%) and 2r (60%), respectively. 6-Methoxy-
isoquinoline (2s) was obtained in a good yield (78%).
The dehydrogenation reactions of
2
tetrahydroquinoxalines 1t-x smoothly proceeded to
give the corresponding quinoxaline products (2t-x) in
8
4-94% yields, and only in the case of using 5-
methyl-tetrahydroquinoxaline (1v) a slight steric
effect affected the formation of 2v (84%).
Unfortunately,
2,6-dimethylpiperidine
(1y),
piperazine (1z), and 2,2,4,7-tetramethyl-1,2,3,4-
tetrahydroquinoline (1z1) could not react to yield the
corresponding dehydrogenation products 2,6-
dimethylpyridine (2y), pyrazine (2z), and 2,2,4,7-
tetramethyl-1,2-dihydroquinoline (2z1), suggesting
the crucial role of both the benzo moiety and the
NH-CH functionality in the N-heterocycle substrates.
It is noteworthy that electron-withdrawing groups
[1a]
[a]
Conditions: 1 (0.5 mmol), 99% KOtBu (1.5 mmol), o-xylene (2 mL),
.1 MPa N , 140 °C, 36 h. Yields refer to the isolated products.
0
2
[b]
Using 99.99% KOtBu.
[c]
[d]
[e]
[f]
such as CF
3
, CN, and CO
2
Et, etc. on the benzo
Using 2,6-dimethylpiperidine (1y).
Using piperazine (1z).
moiety of the substrates dramatically diminished the
reaction efficiency. Halogen substituent (F, Cl, or Br)
bearing N-heterocycles underwent both tert-
butoxylation and dehydrogenation under the standard
conditions, but no tert-butoxylation/dehydrogenation
product could be successfully isolated. As a
comparison, 1,2,3,4-tetrahydronaphthalene (1z2) did
not react under the stated conditions, further
suggesting the crucial role of an NH-CH functionality
in the substrates.
Using 2,2,4,7-tetramethyl-1,2,3,4-tetrahydroquinoline (1z1).
Using 1,2,3,4-tetrahydronaphthalene (1z2).
and under the dioxygen-free conditions by means of a
small N flow with a positive pressure the indoline
2
[19]
dehydrogenation reaction could be excluded.
However, when indoline (3a) was used as the
substrate under a slight N
2
flow with a positive
3
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Advanced Synthesis & Catalysis
10.1002/adsc.201900499
pressure, the efficient formation of indole (4a, 91%),
was not affected, excluding the effect of air or O2. All
the 2-, 3-, 4-, 5-, 6-, and 7-methylindolines (3b-g)
of an N-H functionality in the N-heterocycle
substrates.
The
reaction
of
2-phenyl-2,3-
dihydrobenzothiazole (3p) was complicate that no
target product could be isolated, presumably due to
its decomposition under the strong basic conditions.
It is noteworthy that 99.99% KOtBu also promoted
the dehydrogenation of indolines, giving the results
comparable to those using 99% KOtBu.
efficiently
dehydrogenation reaction to give the target products
b-g in 78-90% yields. The 2- and 3-methyl groups
underwent
the
acceptorless
4
on the five-membered N-heterocyclic ring exhibited a
steric effect on the yields of 4b (79%) and 4c (78%),
respectively, while the methyl substituents on the aryl
ring did not show an obvious substituent effect. 2-
Phenylindoline (3h) also underwent the reaction
efficiently,
Dehydrogenation of 4-methoxyindoline (3i) produced
-methoxy-1H-indole (4i) in 55% yield,
giving
compound
4h
(84%).
4
Then, the synthetic protocol was applied for the
synthesis of potentially useful N-heteroarenes. Drug
development-relevant β-carboline is an important
motif in many synthetic compounds and natural
Table 3. Dehydrogenation of five-membered N-
[
a]
heterocycles (3).
[20]
products. Thus, 1,2,3,4-tetrahydro-β-carbolines 5
were subjected to the standard dehydrogenation
conditions, affording the corresponding 1-substituted-
9
H-pyrido[3,4-b]indoles 6 in 50-58% yields [Eq. (2)],
which provides a concise and green route to β-
carboline derivatives by comparison to the transition-
[21]
metal-catalyzed procedures. This methodology was
further utilized for the one-pot synthesis of
quinazolin-4(3H)-ones 7 in 51-56% yields (Scheme
2
). 2-Phenylquinazolin-4(3H)-one (7a) is a useful β-
[14]
glucuronidase inhibitor, and compounds 7b and 7c
exist in many natural products and synthetic
[
22]
molecules with diverse biological activities.
It
[
a]
Conditions: 3 (0.5 mmol), 99% KOtBu (1.0 mmol), o-xylene (2 mL),
.1 MPa N , 140 °C, 36 h. Yields refer to the isolated products.
should be noted that the desired dehydrogenation
products, that is, quinazolines A, could not be
successfully isolated due to their easy oxidation in air
under the strong basic conditions. In our hands, the
resultant reaction mixtures were allowed to be stirred
in air at room temperature for 1 h before they were
subjected to work-up, and quinazolin-4(3H)-one
derivatives 7 were then isolated by silica gel column
chromatography.
0
2
[
[
b]
c]
Using 99.99% KOtBu.
Using 6-chloroindoline (3m).
while 5-methoxy promoted the reaction to form 5-
methoxy-1H-indole (4j, 88%). However, 6-
methoxyindoline (3k) reacted much less efficiently
than 3j, affording compound 4k (70%). 5-
Cyanoindoline (3l) was sluggish under the stated
conditions, and its reaction only resulted in 5-
cyanoindole (4l) in a low yield (10%). Unexpectedly,
6
-chloroindoline (3m) underwent a tert-butoxylation/
dehydrogenation cascade to form 4m in 40% yield
Eq. (1)]. It was noticed that all the fluoro-, chloro-,
and bromo-substituted tetrahydroquinolines
[
Scheme 2. Synthesis of quinazolin-4(3H)-one derivatives.
underwent the dehydrogenation reaction inefficiently
due to the side tert-butoxylation reaction. 6-Phenyl-
To gain insights into the dehydrogenation process,
mechanistic studies were conducted. When radical
scavenger TEMPO was added in the reaction mixture
of 1,2,3,4-tetrahydroquinoline (1a) or indoline (3a)
under the standard conditions, the product yield was
6
,7-dihydro-5H-[1,3]dioxolo[4,5-f]indole
(3n)
exhibited an excellent reactivity to generate the
dehydrogenation product 4n in 86% yield. 1-
Methylindoline (3o) stayed unchanged under the
standard conditions, suggesting the indispensable role
4
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Advanced Synthesis & Catalysis
10.1002/adsc.201900499
[
1d,14]
slightly decreased, suggesting that a radical pathway
can be excluded [Eq. (3)].[14] In the presence of 2~3
equiv of BHT (2,6-di-tert-butyl-4-methylphenol), the
same dehydrogenation reactions could not occur
because BHT readily reacted with KOtBu to form
tBuOH and potassium phenoxide B which could not
promote the dehydrogenation of 1a or 3a under the
stated conditions. It has been well known that imines
are usually formed or considered as the reaction
intermediates in a typical dehydrogenation process of
N-heterocycles. Thus, 3,4-dihydroisoquinoline (8a),
the imine generated from tetrahydroisoquinoline (1p),
was treated under the similar basic conditions, giving
isoquinoline (2p) in 90% yield [Eq. (4)]. In a similar
fashion, imine 2,3-diphenyl-1,2-dihydroquinoxaline
Subsequently, the resultant aromatic amine
species reacts with KOtBu base again to form the
potassium amide which undergoes the second
dehydrogenation process through transition state,
affording entropically favored aromatic N-
heterocyclic compound 2a and H
2
gas. It is
noteworthy that potassium cation may execute
coordination to the benzo functionality of the
[18a,18b]
substrate
to enhance its dehydrogenation
reactivity. It is noteworthy that 1,2,3,4-
tetrahydronaphthalene could not undergo the
dehydrogenation reaction under the standard
conditions (Table 2), suggesting the necessity of a
nitrogen atom in the substrate to coordinate
potassium tert-butoxide.
[1]
In conclusion, we have developed a highly
efficient transition-metal-free protocol to access
diverse N-heteroarenes through base-promoted
acceptorless dehydrogenation of N-heterocycles. The
present methodology has also demonstrated a
potential application for the synthesis of drug
development-relevant β-carboline and quinazolin-
(
9
8b) reacted to yield 2,3-diphenylquinoxaline (2x,
2%) as the product [Eq. (5)]. These results have
suggested that in the whole dehydrogenation process
partially dehydrogenated imine species may be the
key reaction intermediates.
4
(3H)-one derivatives. Due to easy manipulations,
readily available reactants, and transition-metal-free
conditions, the present work offers a simple and
applicable method for acceptorless dehydrogenation
of N-heterocycles to access N-heteroarenes and H
gas.
2
Experimental Section
Synthesis of quinoline (2a)
Under a nitrogen atmosphere, a mixture of 1,2,3,4-
tetrahydroquinoline (1a) (67 mg, 0.50 mmol) and
KOtBu (168 mg, 1.5 mmol) in o-xylene (2 mL) was
stirred at 140 °C for 36 h. After cooled to ambient
temperature, 30 mL saturated aqueous NH Cl was
4
added and the mixture was extracted with ethyl
acetate (3×10 mL). The combined extracts were dried
A plausible mechanism is proposed in Scheme
over anhydrous Na
2
SO , filtered, and evaporated all
4
3
.[
7,18]
Initially, KOtBu base deprotonates 1,2,3,4-
the volatiles under reduced pressure. The resultant
residue was subjected to purification by silica gel
column chromatography (eluent: petroleum ether (60-
tetrahydroquinoline (1a) to form the corresponding
potassium amide which interacts with in-situ
generated tert-butyl alcohol by coordination and
hydrogen bonding, establishing transition state C.
Imine intermediate 3,4-dihydroquinoline (D) is then
produced with regeneration of KOtBu base and
9
0 °C)/ethyl acetate = 7:1, v/v) to afford quinoline
(
2a) as a yellow oil (60 mg, 92%).
release of H
,2-dihydroquinoline (F) via enamine 1,4-
dihydroquinoline (E) is thermodynamically favored.
2
gas. Isomerization of intermediate D to
Acknowledgements
1
We are grateful to the National Natural Science
Foundation of China (21871253 and 21672209) and the
National
Basic
Research
Program
of
China
(2015CB856600) for support of this research.
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10.1002/adsc.201900499
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