Frustrated Lewis Pair Catalyzed Dehydrogenative Oxidation of Indolines and Other Heterocycles

Article abstract of DOI:10.1002/anie.201606426

An acceptorless dehydrogenation of heterocycles catalyzed by frustrated Lewis pairs (FLPs) was developed. Oxidation with concomitant liberation of molecular hydrogen proceeded in high to excellent yields for N-protected indolines as well as four other substrate classes. The mechanism of this unprecedented FLP-catalyzed reaction was investigated by mechanistic studies, characterization of reaction intermediates by NMR spectroscopy and X-ray crystal analysis, and by quantum-mechanical calculations. Hydrogen liberation from the ammonium hydridoborate intermediate is the rate-determining step of the oxidation. The addition of a weaker Lewis acid as a hydride shuttle increased the reaction rate by a factor of 2.28 through a second catalytic cycle.

Full text of DOI:10.1002/anie.201606426

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201606426
German Edition: DOI: 10.1002/ange.201606426
Dehydrogenation
Frustrated Lewis Pair Catalyzed Dehydrogenative Oxidation of
Indolines and Other Heterocycles
Alexander F. G. Maier, Sebastian Tussing, Tobias Schneider, Ulrich Flçrke, Zheng-Wang Qu,
Stefan Grimme,* and Jan Paradies*
Dedicated to Professor Gerhard Erker on the occasion of his 70th birthday
Abstract: An acceptorless dehydrogenation of heterocycles
catalyzed by frustrated Lewis pairs (FLPs) was developed.
Oxidation with concomitant liberation of molecular hydrogen
proceeded in high to excellent yields for N-protected indolines
as well as four other substrate classes. The mechanism of this
unprecedented FLP-catalyzed reaction was investigated by
mechanistic studies, characterization of reaction intermediates
by NMR spectroscopy and X-ray crystal analysis, and by
quantum-mechanical calculations. Hydrogen liberation from
the ammonium hydridoborate intermediate is the rate-deter-
mining step of the oxidation. The addition of a weaker Lewis
acid as a hydride shuttle increased the reaction rate by a factor
of 2.28 through a second catalytic cycle.
acceptorless catalytic dehydrogenations are rare and have
been reported for 1,4-cyclohexadienes[7] and ammonia–
borane.[8] Boron-derived frustrated Lewis pairs (FLPs)[9] are
recognized for their ability to activate hydrogen in the
absence of any transition metal, and significant break-
throughs have been accomplished in the catalytic hydro-
3
genation of organic molecules.[10] C(sp ) H bond activation
À
through hydride abstraction by organoboranes has been
observed as an undesired side reaction for selected
amines[11] or was deliberately exploited for catalytic transfer
hydrogenations using diisopropylamine[11a] or 1,4-cyclohexa-
dienes[7] as dihydrogen surrogates. However, the catalytic
oxidation of organic substrates with simultaneous release of
molecular hydrogen by FLPs as a synthetic method has not
been reported thus far.
T
he oxidation of organic compounds is a key reaction in
chemical synthesis. Immensely powerful and mild oxidation
methods have been developed based on the use of transition
metals,[1] hypervalent iodine reagents,[2] or organocatalysts.[3]
In most cases, stoichiometric byproducts are generated, which
need to be removed. Alternatively, the dehydrogenative
oxidation of organic compounds can be achieved in the
presence of transition metals and is globally applied for the
synthesis of commodity chemicals.[4] However, such reactions
usually require harsh conditions and sacrificial hydrogen
acceptors.[5] The realization of acceptorless processes as
desirable, environmentally benign transformations is highly
challenging even for transition-metal complexes.[6] Metal-free
We initiated our studies with stoichiometric and substoi-
chiometric reactions of N-methyl hexahydrocarbazole (1a)
with the strong Lewis acid B(C6F5)3 (2). The reaction of
1 equiv of indoline 1a with 1 equiv of borane 2 at room
temperature rapidly furnished 50% of the indole 3a, 50% of
À
À
the ammonium hydridoborate, [1a H][H 2], and 50% of the
free borane 2 within < 5 min (Scheme 1, left).
[*] Dipl.-Chem. A. F. G. Maier, Dipl.-Chem. S. Tussing,
M. Sc. T. Schneider, Prof. Dr. J. Paradies
Institute of Organic Chemistry
University of Paderborn
Warburger Strasse 100, 33098 Paderborn (Germany)
E-mail: jan.paradies@uni-paderborn.de
Dr. U. Flçrke
Institute of Inorganic Chemistry
University of Paderborn
Warburger Strasse 100, 33098 Paderborn (Germany)
Scheme 1. Stoichiometric (left) and substoichiometric (right) reactions
of 1a and 2.
Dr. Z.-W. Qu, Prof. Dr. S. Grimme
Mulliken Center for Theoretical Chemistry
Institut fꢀr Physikalische und Theoretische Chemie
University of Bonn
The borane-induced hydride abstraction from indoline 1a
evidently proceeds at room temperature, and a second
equivalent of 1a is required to scavenge the proton from
the transiently generated 3H-indolium intermediate. Reac-
tions with selectively isotope-labeled [2-D]-1a and [3-D]-1a
Beringstrasse 4, 53115 Bonn (Germany)
E-mail: grimme@thch.uni-bonn.de
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
http://dx.doi.org/10.1002/anie.201606426.
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confirmed C2 as the hydride donor site and C3 as the proton
donor site in 1a (see the Supporting Information). The
ing required for H2 liberation strongly support the rate-
determining nature of this step.
À
À
ammonium salt [1a H][H 2] is not susceptible to hydride
abstraction, leaving 50% of free borane 2 unreacted. Accord-
ingly, the reaction of 1a with 0.5 equiv of B(C6F5)3 generated
equimolar amounts of the oxidation product 3a and indoli-
Next we expanded the scope of the acceptorless dehy-
drogenation to a catalytic version using 5 mol% of 2 and
a series of N-protected indolines (Table 1).[15] Twenty-one
indolines (1a–u) were subjected to the catalytic dehydrogen-
ation, providing access to the corresponding indoles 3a–u.
À
À
nium hydridoborate [1a H][H 2] as a precursor for hydro-
gen release through proton–hydride recombination
(Scheme 1, right). Indeed, subsequent heating of both reac-
tions to 908C for 30 min liberated molecular hydrogen and
furnished indole 3a in quantitative yield with simultaneous
Table 1: FLP-catalyzed dehydrogenation of N-protected indolines.
À
regeneration of borane 2. The important intermediate [1a
À
H][H 2] was characterized by 1D and 2D NMR spectroscopy.
The NOESY spectrum of the ammonium borate salt revealed
an intermolecular NH···HB dihydrogen bond,[12] which has
thus far only been reported for weaker Lewis acids[12] (see the
Supporting Information for details). This interaction supports
the relatively high acidity of the ammonium species so that
a dihydrogen bond is formed to the weak hydride donor
moiety.
R1
R2
R3
R4 R5
t [h]
Yield [%][a]
99 (93)
3a
3b
3c
3d
3e
3 f
3g
3h
3i
3j
3k
3l
3m
3n
3o
3p
3q
3r
-(CH2)4-
Me
H
Me
H
Ph
4-ClC6H4
H
H
Me
H
H
H
H
H
H
H
H
H
H
OMe
Br
Cl
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
Me
13.5
0.75 99 (94)
H
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
H
H
H
H
H
H
H
H
H
H
Me
H
H
H
Me
H
H
14.5
15.5
23.5
16.5
17
20
1.5
1.5
23.5
23.5
99 (97)
99 (94)
99 (94)
99 (99)[b]
85 (89)[b]
82 (74)
99 (82)
99 (99)
99 (99)
99 (59)
30
We succeeded in the crystallization of the 6-chloro
À
À
derivative [1j H][H 2], which was obtained from the
analogous reaction of 2 equiv 1j with 1 equiv 2 in toluene.
Although the crystal quality was rather mediocre, the
Cl Me
À
À
molecular structure of [1j H][H 2] could be unambiguously
F
Me
determined (Figure 1).[13] The analysis confirmed the struc-
H
H
H
H
H
H
H
H
H
neopentyl 17
Bn
Bn
Bn
Me
Me
H
20.75 99 (86)
18
15
17.25 95 (93)
24
30
99 (88)
99 (97)
Br
OMe
H
H
Br
OMe
H
Bn
Me
Me
H
PMB
PMB
PMB
PMB
69 (51)
51 (55)
3s
3t
3u
14.75 99 (79)
15.75 95 (94)
H
[a] Yields determined by 1H NMR analysis with ferrocene as the internal
standard. Reaction conditions: indoline (0.1 mmol), B(C6F5)3 (5 mmol,
5 mol%), ferrocene (30 mmol), [D8]toluene (0.2m). Yields of isolated
products are given in parentheses; these were obtained under the
following conditions: indoline (0.7 mmol), B(C6F5)3 (35 mmol, 5 mol%),
toluene (0.2m). [b] The yields of isolated 3 f and 3g contain 4 and 11% of
the respective indolines as impurities.
À
À
Figure 1. Molecular structure of [1j H][H 2]. Selected bond lengths
[ꢁ] and angles [8]: N1–C1 1.506(8), N1–C8 1.486(8), N1–C9 1.497(7),
C1–C2 1.555(8), C2–C3 1.507(9), B1–C11 1.636(9), B1–C21 1.608(9),
B1–C31 1.653(10), B1–H10 1.21(5), N1–H1 0.93; (B)H10···H1(N)
1.73; C1-N1-C8 106.8(5), C1-N1-C9 113.0(5), C8-N1-C9 114.9(6), C11-
B1-C21 110.8(6), C11-B1-C31 112.5(6), C21-B1-C31 116.1(6). Aniso-
tropic displacement ellipsoids set at 50% probability.[13]
The reaction time was first confirmed by performing the
reaction on 0.1 mmol scale. Most reactions provided quanti-
tative yields of the corresponding indoles 3 after heating to
1208C for 1.5 to 23.5 h. Experiments on 0.7 mmol scale
confirmed the excellent yields of the indoline oxidation on
NMR scale (82–99%, 1a–l). The slower formation of 3g in
comparison to 3 f (see also the Supporting Information)
supports the importance of positive-charge stabilization in the
hydride abstraction step.[16] The influence of steric bulk at the
N atom was probed in the oxidation of neopentyl-substituted
indoline 1m. Indole 3m was obtained in a low yield of 30%
after 17 h, indicating that either hydride abstraction or proton
transfer becomes unfavorable owing to steric congestion.
Furthermore, we expanded the scope of the dehydrogen-
ative oxidation to indolines with removable N-protecting
groups, for example, benzyl (Bn) and 4-methoxybenzyl
(PMB) moieties (1n–u). The two PMB-protected indolines
ture of an ammonium hydridoborate salt featuring a short
(B)H10···H1(N) distance of 1.73 ꢀ, indicating a dihydrogen
bond.[14] These experiments clearly support rapid borane-
induced hydride abstraction from C2 to generate the transient
3H-indolium ion (4; see also Figure 2) followed by intermo-
lecular proton transfer from H3 to the indoline nitrogen atom
to form the stable ammonium borate [1 H][H 2]. The
detection and identification of the stable ammonium borate
[1a H][H 2] by NMR spectroscopy together with the heat-
À
À
À
À
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1r and 1s were converted into the corresponding indoles in
mined to be the rate-limiting step, which is in acceptable
diminished yields. Pleasingly, for all Bn- as well as for the two
PMB-protected indolines 1t and 1u, the reaction proceeded
in excellent yields. Generally, the reaction time and the yield
of the dehydrogenative oxidation were only marginally
influenced by the reaction scale.
We then investigated the mechanism of the dehydrogen-
ative indoline oxidation by kinetic and quantum-mechanical
experiments. DFT calculations at the PW6B95-D3//PBEh-3c
+ COSMO-RS level of theory[17] provided detailed mecha-
nistic insight into the borane-catalyzed dehydrogenation of
indoline 1a (Figure 2). Owing to steric hindrance at the
agreement with the determined Eyring activation energy of
25.7 Æ 6 kcalmolÀ1. This proposed mechanism implies that the
activation barrier of H2 liberation may be reduced when the
Brønsted acidity of the ammonium ion or the hydridic
character of the hydridoborate is enhanced. The Lewis acidity
of boranes in active FLPs has an immense impact on the
reversibility of H2 activation[9a,12b,19] and on the reaction
rates.[12a,20] The Lewis acidity of partially fluorinated borane 5
is 15% lower than that of 2,[12a,21] which in turn may result in
a lower energy barrier for the H2 release from the corre-
sponding ammonium borate owing to the increased hydride
donor ability.[21a]
Disappointingly, borane 5 was entirely inactive in the
dehydrogenative oxidation of 1a as a consequence of the
diminished Lewis acidity (Scheme 2). However, borane 5 may
Scheme 2. Rate enhancement of the indoline dehydrogenation by the
weak Lewis acid 5 acting as a hydride shuttle.
Figure 2. DFT-computed reaction free energies (in kcalmolÀ1) for the
B(C6F6)3 (2) catalyzed dehydrogenation of indoline 1a..
act as hydride shuttle as boranes can undergo hydride
exchange[22] in equilibrium. Indeed, when a 1:1 ratio of
catalytically active 2 and catalytically inactive 5 (5 mol%
each) was used for the dehydrogenation of 1a, a substantial
rate enhancement of krel = 2.28 was observed (Scheme 2).
This boost in reactivity can be rationalized by the hydride
N center of indoline 1a, Lewis basic indoline 1a and B(C6F6)3
(2) can only form an unstable frustrated Lewis pair (3.6 kcal
molÀ1 higher in free energy than separated 1a and 2; see the
Supporting Information) rather than a tight acid–base com-
À
exchange equilibrium between [H 2] and 5. The hydride is
preferably located at the stronger Lewis acid 2 but small
À
À
plex through formation of a B N donor bond. In solution,
quantities of the stronger hydride donor [H 5] may still be
accessible (Scheme 3). This leads to the transient formation of
borane 2 can selectively abstract a hydride from the C2
position of 1a to form the separated ion pair 4 over a low
barrier of 11.1 kcalmolÀ1 (TS1). From this intermediate, two
reactions are conceivable. The direct formation of indole 3a
and molecular H2 from intermediate 4 was considered as
kinetically incompetent[18] (not shown, see the Information
Supporting) in comparison to a lower-barrier proton transfer
(15.5 kcalmolÀ1; TS2) to the N atom of a second indoline 1a,
which is in accord with the results of the isotope-labeling
experiments (see the Supporting Information). The formation
the ion pair [1a H][H 5],[23] which is now capable to liberate
H2 more readily than the comparatively weaker hydride
donor [H 2] (right cycle). Furthermore, free borane 2
liberated in the equilibrium (left cycle) is again reactive in
hydride abstraction from 1a. In summary, the substantial rate
increase by a factor of 2.28 is due to direct hydrogen release
from the initially formed hydridoborate salt [1a H][H 2] as
well as to 5 acting as a hydride shuttle.
À
À
À
À
À
À
À
of this contact ion pair, [1a H][H 2], and 3a is exergonic by
À3.3 kcalmolÀ1, which enabled the NMR spectroscopic
characterization of this species at room temperature, includ-
ing the detection of the ion pairing by dihydrogen bonding.
Upon heating to 908C, proton–hydride recombination may
occur to release indoline 1a, molecular H2, and borane
catalyst 2, which is almost energy-neutral and involves
a moderate barrier of 18.5 kcalmolÀ1 at room temperature.
This reaction should be favored at higher temperatures owing
to favorable entropy effects. The overall catalyzed dehydro-
genation is thus exergonic by À3.8 kcalmolÀ1 with a barrier of
18.5 kcalmolÀ1; proton–hydride recombination was deter-
Scheme 3. Proposed function of borane 5 in the rate enhancement of
the dehydrogenation through a hydride shuttle mechanism.
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Scheme 4. Oxidation of other heterocyclic compound classes. Condi-
tions A: 0.2m, B(C6F5)3 (10 mol%), mesitylene, 1658C; conditions B:
0.2m, B(C6F5)3 (5 mol%), toluene, 1208C.
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investigated the viability of this approach for other compound
classes (Scheme 4). All tested substrates were susceptible to
FLP-catalyzed acceptorless dehydrogenation. The pyridine
derivatives 7a and 7b as well as thiazole 8 were obtained from
the corresponding 1,4-dihydropyridines and thiazoline in 48–
64% yield. For the oxidation of an isoindoline and two 1,2-
dihydroquinolines, elevated temperatures were required to
obtain N-tert-butyl-isoindole (6) in 53% yield and the
2-substituted quinolines 9a and 9b in 93% and 94% yield.
In summary, we have developed an FLP-catalyzed accept-
orless dehydrogenation of N-protected indolines that pro-
vides the corresponding products in excellent yields. This
approach also proved successful for the oxidation of isoindo-
lines, 1,4-dihydropyridines, thiazoles, and 1,2-dihydroquino-
lines. The dehydrogenation proceeds by borane-induced
hydride abstraction from the C2 atom of the indoline. The
generated 3H-indolium intermediate is rapidly deprotonated
by a second indoline, generating an ammonium hydridobo-
rate. This intermediate was unambiguously characterized by
NMR spectroscopy and X-ray structure analysis. The release
of H2 through proton–hydride recombination is the rate-
determining step. This step was accelerated by the addition of
catalytic amounts of the weak Lewis acid 5, which acts as
a hydride shuttle and transiently generates a stronger hydride
donor. This interplay between stronger and weaker Lewis
acids might inspire the development of innovative FLP-
catalyzed reactions.
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Keywords: 1,2-dihydroquinolines · 1,4-dihydropyridines ·
acceptorless dehydrogenation · frustrated Lewis pairs · indolines
À
À
CCDC 1489310 ([1j H][H 2]) contains the supplementary
crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data
Centre.
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[18] The reaction between the 3H-indolium and borate to furnish
Received: July 2, 2016
Revised: August 2, 2016
Published online: && &&, &&&&
indole 1l and H2 is exergonic by À4.3 kcalmolÀ1 with a moderate
barrier of 18.1 kcalmolÀ1
.
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Dehydrogenation
A. F. G. Maier, S. Tussing, T. Schneider,
U. Flçrke, Z.-W. Qu, S. Grimme,*
J. Paradies*
&&&& — &&&&
Frustrated Lewis Pair Catalyzed
Dehydrogenative Oxidation of Indolines
and Other Heterocycles
The acceptorless dehydrogenation of N-
protected indolines and other heterocy-
cles is catalyzed by frustrated Lewis pairs.
Mechanistic as well as quantum-
tion of molecular hydrogen to be the rate-
determining step. The addition of
a weaker Lewis acid as a hydride shuttle
increased the reaction rate by a factor of
2.28.
mechanical studies revealed the libera-
6
www.angewandte.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
These are not the final page numbers!