Divergent dehydrogenative coupling of indolines with alcohols

Article abstract of DOI:10.1021/acscatal.6b03667

The dehydrogenative coupling of indolines with alcohols catalyzed by an iridium complex has been achieved to afford both N- and C3-alkylated indoles selectively, by simply changing the addition time of a base additive. The iridacycle catalyst plays multiple roles in these reactions, which dehydrogenates both amines and alcohols and catalyzes the coupling reactions. Mechanistic studies reveal that a borrowing hydrogen-dehydrogenation process and a dehydrogenationborrowing hydrogen process are involved in N-alkylation and C3-alkylation reactions, respectively. The C3-alkylation reaction involves the direct coupling of two sp3 carbon centers.

Full text of DOI:10.1021/acscatal.6b03667

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Letter
Divergent Dehydrogenative Coupling of Indolines with Alcohols
Xue Jiang, Weijun Tang, Dong Xue, Jianliang Xiao, and Chao Wang
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b03667 • Publication Date (Web): 31 Jan 2017
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ACS Catalysis
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Divergent Dehydrogenative Coupling of Indolines with Alcohols
Xue Jiang,† Weijun Tang,† Dong Xue,† Jianliang Xiao,†,‡ and Chao Wang,*†
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† Key Laboratory of Applied Surface and Colloid Chemistry, Ministry of Education, and School of Chemistry and
Chemical Engineering, Shaanxi Normal University, Xi’an, 710119, China
‡ Department of Chemistry, University of Liverpool, Liverpool, L69 7ZD, United Kingdom
ABSTRACT: The dehydrogenative coupling of indolines with alcohols catalyzed by an iridium complex has been achieved
to afford both N- and C3-alkylated indoles selectively, by simply changing the addition time of a base additive. The irida-
cycle catalyst plays multiple roles in these reactions, which dehydrogenates both amines and alcohols and catalyzes the
coupling reactions. Mechanistic studies reveal that a borrowing hydrogen-dehydrogenation process and a dehydrogena-
tion-borrowing hydrogen process are involved in N-alkylation and C3-alkylation reactions, respectively. The C3-alkylation
reaction involves the direct coupling of two sp3 carbon centers.
KEYWORDS: dehydrogenation, indole, alcohol, iridium, cross coupling
Indole moieties are found in many natural products,
pharmaceuticals, dyes, as well as fine chemicals.1 The se-
lective functionalization of indoles has attracted great
attention.2 The use of alcohol as alkylating reagents for
indole alkylation is appealing, with water as the only by-
product. However, the control of the N-3 or C3-alkylation4
selectivity is not easy, with most system preferring C3-
alkylation (Scheme 1). To the best of our knowledge, the
regioselective access to both N- and C3-alkylated indoles
with a single catalyst has not been reported yet.
Scheme 1. Divergent synthesis of N- and C3- alkylated
indoles
Dehydrogenation as a substrate activating strategy has
found broad application in organic synthesis.5 The dehy-
drogenation of amines and alcohols to imines/enamines
and carbonyl compounds has been utilized in a number of
green transformations, e.g. alkylation with alcohols5e,6 via
a borrowing hydrogen strategy,5b,7 and amide formation
from amines and alcohols.5d In most of the examples re-
ported, the catalyst dehydrogenates either the amine or
the alcohol substrates. For example, a Ru catalyst was able
to dehydrogenate cyclic amines, allowing for C3-
alkylation of amines with aldehydes;8 and catalysts were
reported to catalyse the functionalization of indoles with
alcohols via dehydrogenation.4c,4f-h A catalyst capable of
dehydrogenating both amines and alcohols is desirable, as
it would allow for a more diverse coupling patterns. How-
ever, such catalysts are rare,9 and to the best of our
knowledge, the catalytic dehydrogenation of both amines
and alcohols in a single dehydrogenative coupling reac-
tion is still elusive. Herein, we present a catalyst capable
of dehydrogenating both amines and alcohols, allowing
for the divergent dehydrogenative coupling of indolines
with alcohols (Scheme 1). Mechanistic studies suggest
that a borrowing hydrogen-dehydrogenation process for
N-alkylation and a dehydrogenation-borrowing hydrogen
process for C3-alkylation under aerobic conditions are
involved.
Recently, we disclosed that cyclometalated iridium cat-
alysts (iridacyles)10 were able to dehydrogenate both alco-
hols and amines under the same mild conditions, allow-
ing for N-alkylation of amines with both alcohols and
amines.11 We envisioned that mixing alcohols or amines
that are capable of dehydrogenation in one pot, novel
coupling patterns might be expected. In our previous
work,11 the reaction of indolines with benzyl alcohol af-
forded N-benzylindole (5a) with 1 mol% of catalyst 1 in
2,2,2-trifluoroethanol (TFE) under argon at 100 oC in the
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Table 1. Dehydrogenative coupling of indoline with
Table 2. Substrate scope for dehydrogenative N-
benzyl alcohola
alkylation of indolines with alcoholsa
1
2
3
4
1 (0.5 mol%), K2CO3
R1
R1
+
R2
OH
N
CF3CH2OH,100 oC, 12 h
N
H
2
3
5a-s
R2
5
6
7
8
9
N
N
N
N
O
O
H3CO
H3C
5a, 82% yield
5b, 93% yield
5c, 93% yield
5d, 86% yield
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N
N
5eb, 65% yield
N
N
N
O2N
5fb, 86% yield
5gc, 76% yield
4b, 76% yield
aReaction conditions: 2a (1 mmol), 3a (1 mmol), 1 (0.005
mmol), K2CO3 (0.75 mmol), CF3CH2OH (2 mL), 100 C, 12 h,
N
N
o
N
isolated yield; PMP = para-methoxyphenyl; n.d. refers to not
5ic
, 77% yield
, 86% yield
5j, 76% yield
b
5hc
5kc
, 74% yield
detected. K2CO3 was added after 7 h and further reacted for
12 h. cWithout 1.
N
N
N
4
N
2
presence of K2CO3 for 12 h. Lowering the catalyst loading
to 0.5 mol%, a mixture of 4a, 5a and 6a were observed
(Table 1, entry 1). Performing the reaction under air, 5a
was formed preferentially in 12 h with 0.5 mol% of 1 (Ta-
ble 1, entry 2). Interestingly, when the base, K2CO3, was
added after 7 h, and allowing the reaction mixture to re-
act for a further 12 h under air, C3-alkylated product 6a
was isolated in 90% yield (Table 1, entry 3). The catalyst is
essential for these reactions. None of the products were
detected and the starting materials remained unreacted in
its absence (Table 1, entry 4). Other metal complexes
showed little or no activity for the coupling reactions un-
der the standard conditions (SI, Table S1). For example,
[Cp*IrCl2]2, which is an effective catalyst for dehydrogena-
tive C3-alkylation of indoles with alcohols, has low activi-
ty for the coupling of indolines with alcohols under our
conditions.4c
HO
HO
5lc, 46% yield
5mc, 71% yield
5nb, 60% yield
5ob, 61% yield
O
N
5
N
N
R
N
CH3
HO
R = Cl, 5sc, 98% yield
R = CH3, 5tc, 82% yield
5pc, 62% yield
5qc, 73% yield 5rc, 83% yield
aReaction conditions: 2 (1 mmol), 3 (1 mmol), 1 (0.005 mmol),
K2CO3 (0.75 mmol), CF3CH2OH (2 mL), 100 oC, 12 h, under air,
isolated yield. bWith 1.5 mmol alcohol. cWith 2 mmol alcohol.
two hydroxy groups were presented, only one could be
alkylated, allowing for further functionalization of the
free hydroxy group (5n-p). Satisfactory yields were ob-
tained for substituted indolines with ethanol as alkylating
reagent (5q-t).
The substrate scope for the dehydrogenative C3-
alkylation of indolines with alcohols was then studied
(Table 3). Differing from N-alkylation, both electron-rich
and electron-deficient benzyl alcohols reacted well to
afford the C3-alkylated indoles with good yields (6a-g).
Aliphatic alcohols, with differing chain length and steric
hindrance, are viable substrates as well (6h-p). Similarly,
relative low yield was obtained for the steric bulky isobu-
tanol (6n). While indoline bearing electron-donating
group at the 6-position afforded C3-aklylated product (6q)
in good yield with ethanol, electron-withdrawing group at
that position lead to a mixture C3- and N-alkylated prod-
ucts (6r and 5s).
Various indolines and alcohols were then tested to ex-
amine the versatility of the divergent synthetic protocols.
Initially, the generality of dehydrogenative N-alkylation
protocol was investigated (Table 2). Aromatic alcohols
with electron-donating substituents on their phenyl rings
reacted well with indolines affording yields ranging from
76-93 % in 12 h (Table 2, 5a-e). However, the reaction of
electron-deficient aromatic alcohols tended to produce a
mixture of 4 and 5. The very electron-deficient alcohol, 4-
nitrobenzyl alcohol gave the N-alkylated indolines prod-
uct 4b in 73% yield. Aliphatic alcohols are suitable sub-
strates as well, affording moderated to good yields for
alcohols with varying chain length (Table 2, 5e-j). Notably,
methanol could be used, allowing for N-methylation of
indolines (5e). The cyclopropyl group was tolerated under
the reaction conditions (5k). The steric hindrance of sub-
strates seems to affect their activity. Thus, while 71% yield
was obtained for 1-isopentyl-1H-indole (5m), only 46%
yield was observed for 1-isobutyl-1H-indole (5l). When
The mechanism of these coupling reactions was then
considered. Aldehydes were believed to be intermediates
for alkylation reactions via borrowing hydrogen.5b,7 React-
ing indoline with benzaldehyde under standard reaction
conditions, both N-alkylated product 5a and C3-alkylated
product 6a were obtained (Scheme 2), suggesting that
aldehydes might serve as intermediate for these coupling
reactions. Furthermore, the reaction of indole with benzyl
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ACS Catalysis
Table 3. Substrate scope for dehydrogenative C3-
alkylation of indoline with alcoholsa
1
2
3
4
5
R2
(1) 1 (0.5 mol%),100 oC,
CF3CH2OH, 7 h
R1
R2 OH
R1
+
N
(2) K2CO3, 12 h
N
H
H
2
3
6a-p
6
7
O
OCH3
CH3
O
8
9
N
N
N
H
H
H
N
H
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6a
6d
, 90% yield
, 71% yield
6b, 81% yield
6c, 87% yield
Cl
NO2
CF3
2
N
H
N
H
N
N
H
H
Figure 1. Kinetic profile for the reaction of 2a and 3a in the
6g
, 83% yield
6e
6f
6h
6n
, 80% yield
, 90% yield
, 91% yield
presence of K2CO3. Reaction conditions: 2a (1 mmol), 3a (1
mmol), 1 (0.005 mmol), K2CO3 (0.75 mmol), CF3CH2OH (2
n
3
o
1
mL), 100 C, under air. Yields were determined by H NMR
with 1,3,5-trimethoxybenzene as internal standard.
N
H
N
H
N
H
N
H
n = 2, 6k, 87% yield
6i
6j
6l
, 70% yield
, 78% yield n = 3, , 83% yield
, 62% yield
even in the presence of alcohols. Further experiments by
subjecting benzyl alcohol or indoline alone to the stand-
ard conditions with or without K2CO3 revealed that K2CO3
promoted the oxidation of benzyl alcohol to aldehyde but
decreased the rate for dehydrogenation of indoline to
indole (eq. 2 and 3). In the catalytic reaction in the pres-
ence of K2CO3, the produced aldehyde might rapidly react
with indoline to form an iminium intermediate, which
further accelerates the alcohol dehydrogenation reaction
and decreases the rate of indoline dehydrogenation, lead-
ing to the formation of N-alkylated indoles as the major
product. If K2CO3 was not added initially, the catalyst only
dehydrogenated indoline to indole. And when K2CO3 was
introduced, the alcohol dehydrogenation began and the
resulting aldehyde reacted with indole to give the C3-
alkylated product.
6m
, 87% yield
n= 4,
2
N
H
Cl
6rb
N
H
H3C
N
H
N
H
, 36% yield
, 57% yield
6qb
, 88% yield
6p
6o
, 88% yield
, 89% yield
5s
aReaction conditions: 2 (1 mmol), 3 (1 mmol), 1 (0.005 mmol),
and CF3CH2OH (2 mL) were stirred at 100 oC for 7 h under air;
after cooling to r.t., K2CO3 was added and further stirred for
b
12 h, isolated yield. See SI for more details. With 2 mmol
alcohol.
Scheme 2. Mechanistic studies for possible interme-
diates
O
1
(0.5 mol%), K2CO3
N
+
+
H
CF3CH2OH, 100 oC,
12 h, Air
N
H
1 (0.5 mol%)
N
H
(2)
CF3CH2OH, 100 oC,
N
N
H
5a
6a
, 31% yield
, 53% yield
H
3 h, Air
without K2CO3: 61% yield
with K2CO3: 34% yield
1
(0.5 mol%), K2CO3
OH
1 (0.5 mol%)
+
OH
O
(3)
CF3CH2OH, 100 oC,
12 h, Air
N
H
N
H
CF3CH2OH, 100 oC,
3 h, Air
6a
, 93% yield
without K2CO3: no reaction
with K2CO3: 14% yield
alcohol afforded the C3-alkylated product exclusively,
suggesting indole as an intermediate for the C3-alkylation
but not for the N-alkylation reaction.
Another question is what the role of the iridacycle cata-
lyst is in these transformations and how the products are
formed. Control experiments showed that in the absence
of 1, neither indolines nor benzyl alcohols underwent de-
hydrogenation, indicating that 1 catalysed the dehydro-
genation of both amines and alcohols. The N-alkylated
product 5a could not be formed from indoline and ben-
zaldehyde under standard conditions without 1, suggest-
ing the involvement of 1 in the steps leading to 5a. Kinetic
profile of the reaction between 2a and 3a in the presence
of K2CO3 indicate that 4a is an intermediate during the
reaction (Figure 1). Subjecting 4a to the standard condi-
tions of N-alkylation, full conversion of 4a to 5a was ob-
served (eq. 4). These observations suggest that the reac-
An interesting question is how the addition time of the
base, K2CO3, switched the coupling reaction from N-
alkylation to C3-alkylation. Removing K2CO3 from the
standard reaction conditions for N-alkylation of indoline
with benzyl alcohol, none of the coupling product was
observed. Instead, indole was isolated in 95% yield and
benzyl alcohol remained unreacted (eq. 1). This result
1
(0.5 mol%)
OH
OH
+
(1)
+
N
H
N
CF3CH2OH, 100 oC,
12 h, Air
H
95% yield
>95% unreacted
suggests that, in the absence of K2CO3, the iridacycle cata-
lyst dehydrogenates indolines to indoles preferentially
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tion first produces 4a via a borrowing hydrogen process, iridium hydride. The switch of selectivity from 6a to 7 in
1
2
3
4
5
6
7
8
which was then dehydrogenated to form 5a.
the reaction of equation 5 could be due to the lack of hy-
drogen source to reduce the intermediate 9, which in-
stead is attacked by indole. The N-alkylation reaction
could also proceed under argon (Table 1), and experi-
ments showed that the C3-alkylation reaction went equal-
ly well under both argon and air (SI, Table S2). These re-
sults suggest that the iridium hydride could either react
with oxygen or be protonated to release hydrogen gas to
regenerate the active catalyst. Indeed, hydrogen gas could
be detected in the head gas of both reactions under air (SI,
section 8). It is interesting to note that cycle A involves a
borrowing hydrogen–dehydrogenation process and cycle
B involves a dehydrogenation–borrowing hydrogen pro-
cess. And this iridacycle catalyst allowed the borrowing
hydrogen process to be carried out under air.
Experiments were also designed to probe the role of 1 in
the C3-alkyaltion reaction. 6a was believed to be formed
via indole and benzaldehyde.4c No reaction took place
between indole and benzaldehyde in the absence of 1 un-
der the C3-alkylation conditions, suggesting that 1 is again
essential for the C-C coupling event, probably acting as a
Lewis acid to activate the aldehyde. Interestingly, the re-
action of indole and benzaldehyde under the standard C3-
alkylation conditions afforded only a small amount 6a
and a major new product 7 (eq. 5). The formation of 7 was
also observed by Grigg4c and Beller12 and their co-workers.
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In conclusion, by the use of an iridacycle catalyst, the
divergent synthesis of indoles through selective coupling
of indolines and alcohols has been developed. The irida-
cycle catalyst plays multiple roles in these transfor-
mations, which catalysed the dehydrogenation of both
amines and alcohols and the subsequent coupling reac-
tions. The C3-alkylation reaction realized the coupling of
two sp3 carbon centers with air as oxidant or under oxi-
dant free conditions.
ASSOCIATED CONTENT
Based on the above analysis and literature,3,4 catalytic
cycles for both N- and C3-alkylation were proposed
(Scheme 3). Cycle A describes the formation of 5 from
indoline and alcohol via aldehyde and iminium interme-
diates. In this cycle, the iridium hydride, formed from
dehydrogenation of alcohol in the presence of base, could
reduce the iminium intermediate 8, generated from in-
dolines and aldehyde, to afford 4, which is then dehydro-
genated by the iridium catalyst to give 5. Cycle B depicts
the formation of 6 via the intermediacy of indole and al-
dehyde. In the absence of base, indoline is dehydrogenat-
ed to indole; and with the addition of base, alcohol dehy-
drogenation takes place to give aldehyde. Indole then
reacts with the aldehyde by iridium catalysis to afford
intermediate 9, which is then transformed to 6 by the
Supporting Information. The Supporting Information is
available free of charge on the ACS Publication website at
DOI: xxx-xxx
Additional optimization studies, experimental procedures
and characterization data, 1H and 13C NMR spectra
AUTHOR INFORMATION
Corresponding Author
* E-mail: c.wang@snnu.edu.cn
ORCID
Chao Wang: 0000-0003-4812-6000
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
Scheme 3. Proposed mechanism for the divergent
formation of indoles
This research was supported by the National Natural Science
Foundation of China (21473109), Science and Technology
Program of Shaanxi Province (2016KJXX-26), the Program
for Changjiang Scholars and Innovative Research Team in
University (IRT_14R33), and the 111 project (B14041).
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