Phosphine-Free Manganese(II)-Catalyst Enables Acceptorless Dehydrogenative Coupling of Alcohols with Indoles

Article abstract of DOI:10.1002/adsc.202100621

Herein, an air-stable, molecularly defined NNN?Mn(II) pincer complex catalyzed acceptorless dehydrogenative coupling of alcohols with indoles is reported. A wide variety of symmetrical and unsymmetrical bis(indolyl)methane derivatives as well as some structurally important products such as Vibrindole A, Turbomycin B alkaloid, Antileukemic, and Anticancer agents were synthesized. Mechanistic studies illustrate the importance of the NH moiety in the complex and the crucial role of metal-ligand cooperation during catalysis. (Figure presented.).

Full text of DOI:10.1002/adsc.202100621

RESEARCH ARTICLE
doi.org/10.1002/adsc.202100621
Phosphine-Free Manganese(II)-Catalyst Enables Acceptorless
Dehydrogenative Coupling of Alcohols with Indoles
Vinita Yadav,a, b Ekambaram Balaraman,c,* and Santosh B. Mhaskea, b,*
a
Division of Organic Chemistry, CSIR-National Chemical Laboratory (CSIR-NCL), Pune – 411008, India
E-mail: sb.mhaske@ncl.res.in
Academy of Scientific and Innovative Research (AcSIR), Ghaziabad – 201002, India
Department of Chemistry, Indian Institute of Science Education and Research (IISER) Tirupati, Tirupati – 517507, India
b
c
E-mail: eb.raman@iisertirupati.ac.in
Manuscript received: May 21, 2021; Revised manuscript received: July 20, 2021;
Version of record online: ■■■, ■■■■
Supporting information for this article is available on the WWW under https://doi.org/10.1002/adsc.202100621
Abstract: Herein, an air-stable, molecularly defined NNNÀ Mn(II) pincer complex catalyzed acceptorless
dehydrogenative coupling of alcohols with indoles is reported. A wide variety of symmetrical and
unsymmetrical bis(indolyl)methane derivatives as well as some structurally important products such as
Vibrindole A, Turbomycin B alkaloid, Antileukemic, and Anticancer agents were synthesized. Mechanistic
studies illustrate the importance of the NH moiety in the complex and the crucial role of metal-ligand
cooperation during catalysis.
Keywords: manganese; phosphine-free; acceptorless dehydrogenation; alcohol; indole
Transition-metal catalysis plays a vital role in the complexes have been effectively utilized for the
development of new sustainable and environmentally acceptorless dehydrogenation (AD) and borrowing
benign procedures for the green synthesis of a wide hydrogenation (BH) reactions.[6] Manganese is third
range of structurally important organic compounds, only to iron and titanium among the transition elements
pharmaceuticals, and fine chemicals.[1] They have in its abundance in Earth’s crust plus it is essential in
significantly revolutionized the key areas of organic various life forms. Hence, molecularly-defined man-
synthesis by providing efficient and sustainable alter- ganese complexes are the highly attractive candidates
natives to the so-called conventional organic for the design of new catalytic transformations.
transformations.[2] The noble-metal (Ru, Rh, Pd, or Ir)
N-heterocycles are prevalent in natural products,
based complexes have substantially dominated the pharmaceuticals, and materials science.[7] For instance,
field of homogenous catalysis over the past decades indole motifs play a vital role in many drug molecules,
due to their generality, improved catalytic activity, and bioactive alkaloids, agrochemicals, and advanced
selectivity in most of the organic transformations.[3] organic materials.[8] Particularly, bis(indolyl)methane
Undoubtedly, the attainments of such precious metal derivatives (BIMs) are of great significance in organic
complexes are enormous; however, because of the synthesis as well as in natural products and pharma-
increasing demand, high cost, and limited availability ceutical industries.[9] Several bioactive natural products
of noble metal precursors; the development of syn- such as Vibrindole A, Arsindoline A, Turbomycin B,
thetic procedures based on abundantly available, less and Arundine contain BIM scaffold (Scheme 1).[9a,d]
toxic, and inexpensive first-row transition metal com-
Besides, BIMs are key units in a wide range of
plexes is highly desirable.[4] There has been great bioactive compounds with widespread biological and
interest in the past few years in the development of pharmacological activities, for example, they have
earth-abundant-metal based complexes as catalysts for anticancer, antifungal, antimicrobial, antibacterial, and
various CÀ X (X=C, N, O) bond-forming reactions via antitumor activities.[9] Given the importance of BIMs,
the acceptorless dehydrogenative coupling (ADC) there is significant interest in the development of novel
strategy, which were largely the arena of precious and efficient synthetic protocols for their preparation.
metal catalysis.[5] Of late, manganese-based pincer Conventional methods for the synthesis of BIMs are
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Scheme 1. Selected examples of pharmaceutically active com-
pounds containing BIMs.[9a,d]
Scheme 2. Transition-metal catalyzed synthesis of BIMs.
largely based on the Friedel-craft alkylation of indoles
with aldehydes or ketones in the presence of Lewis/ example of NNNÀ Mn(II) pincer complex catalyzed
Brønsted acids, and transition metals.[10] However, due acceptorless dehydrogenative coupling of indoles and
to the use of stoichiometric amounts of acids, and alcohols to access a diverse range of bis(indolyl)
easily oxidizable aldehyde precursors, these methods methane derivatives.
require significant modification in terms of atom-
At the outset, we explored the reaction with 4-
economy and sustainability. Therefore, the develop- isopropylbenzyl alcohol (1c) and unsubstituted indole
ment of sustainable and green procedures for the 2a as the model substrates (Table 1). Thus, the reaction
efficient synthesis of BIMs is highly desirable. In this
context, the ADC reaction can be considered as a
highly atom-economical and environmentally benign
Table 1. Optimization of the reaction conditions.[a,b]
approach in which the intermediates resulted from the
initial dehydrogenation process participate in further
reactions and liberates molecular hydrogen (and water)
as the sole by-product. Furthermore, the possibility of
using alcohol as an alkylating agent increases the
interest for this sustainable reaction, since alcohols are
easy to handle, inexpensive, and abundantly available
by a variety of industrial processes or even from
renewable resources such as lignocellulose biomass.[11]
Utilizing alcohols as the alkylating agent, Yokoyama
and Hikawa reported a palladium-catalyzed cascade
process for the synthesis of BIMs.[12a] Of late, the
research group of Sekar reported an iron(II) chloride-
1,1’-binaphthyl-2,2’-diamine (FeCl2-BINAM) complex
catalyzed domino synthesis of BIMs directly from
indoles and alcohols.[12b] In this protocol, an earth-
abundant iron was used as the catalyst along with an
expensive BINAM ligand and an excess of dicumyl
peroxide (DCP) as an oxidant. Very recently, the
research group of Srimani reported the ruthenium
pincer complex catalyzed selective synthesis of C-3
alkylated indoles and BIMs directly from indoles and
alcohols.[12c] Langer et al. also reported a copper-
catalyzed alkylation of indoles with an excess of
alcohols using air as an oxidant for the synthesis of
BIMs (Scheme 2).[12d]
Entry Reaction condition
Yield of 3c (%)[b]
1
2
3
alcohol:indole (1:2)
alcohol:indole (1:1)
alcohol:indole (2:1)
65
(70)[c]
96
(99)[c]
96
(99)[c]
<5%
trace
20
4
5
6
7
no catalyst
no KOtBu
MnCl2 ·4H2O
[Mn] (3 mol%)
86
(90)[c]
85
8
9
30 mol% KOtBu
NaOtBu, Cs2CO3, K2CO3 <47%
as base
10
m-xylene, n-octane,
1,4-dioxane, CH3CN
as solvent
<61%
[a] Reaction conditions: alcohol 1c (0.5 mmol), indole 2a
(0.5 mmol), catalyst [Mn] (5 mol%), KOtBu (50 mol%) and
toluene solvent (1.0 mL) were heated at 120 C (oil-bath
°
Despite notable contributions for the synthesis of
BIMs from indoles and alcohols, an efficient and
robust method based on earth-abundant transition-
metal complexes remains scarce. Unlike previously
reported precious catalytic systems, this is the first
temperature) for 20 h under Ar atm.
[b] Yield of the isolated product.
[c] GC conversion of indole using mesitylene as an internal
standard.
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Table 2. Synthesis of bis(indolyl)methanes: Scope of alcohols.[a,b]
of 1c (0.5 mmol) and 2a (1.0 mmol) in the presence of
NNNÀ Mn(II) pincer complex, [Mn] (5 mol%) and
KOtBu (50 mol%) in toluene solvent in a closed
°
system at 120 C (oil-bath temperature) for 20 h
resulted in 65% yield of 3,3’-((4-isopropylphenyl)
methylene)bis(1H-indole) (3c; Table 1, entry 1). Nota-
bly, with 0.5 mmol of indole (i.e. alcohol:indole=1:1)
and keeping the remaining conditions same, the
reaction ensued in approximately complete conversion
of starting indole 2a with a selective formation of the
product 3c in 96% isolated yield (Table 1, entry 2).
When the reaction was performed using 2 eq. of
alcohol with respect to indole, the starting indole was
fully converted to the desired product (Table 1,
entry 3). Gratifyingly, despite the possibility of form-
ing the C3-benzylated and N-benzylated products, the
product 3c was obtained selectively in excellent yield.
The starting material indole 2a was recovered un-
changed when a control experiment was conducted in
the absence of a base, while in the absence of the
catalyst system, only a trace amount of product
formation (<5%) was observed (Table 1, entries 4 and
5). By lowering the mol% of Mn-catalyst (3 mol%)
and KOtBu (30 mol%), a reduced yield of product 3c
was observed under the standard reaction conditions
(Table 1, entries 7–8). Next, changing the base from
KOtBu to either of NaOtBu (47%), Cs2CO3 (trace), and
K2CO3 (0%) resulted in lower yield of the desired
product (Table 1, entry 9). A comparison of the Mn-
(II)-catalyzed reaction in different solvents such as m-
xylene, n-octane, 1,4-dioxane, CH3CN showed that
toluene was the best solvent (Table 1, entry 10).
[a] Reaction conditions: alcohol 1 (0.5 mmol), indole 2a
(0.5 mmol), catalyst [Mn] (5 mol%), KOtBu (50 mol%), and
°
toluene solvent (1.0 mL) were heated at 120 C (oil-bath
temperature) for 20 h under Ar atm.
°
Notably, shifting the temperature from 120 C in either
[b] Yield of the isolated product.
an upward or downward direction resulted in a lower
yield of the desired product. Accordingly, the optimal
condition for the reaction is alcohol (0.5 mmol), indole
(0.5 mmol), [Mn] (5 mol%) and KOtBu (0.25 mmol)
in 1.0 mL of toluene solvent in a close system at
[c] The reaction was carried out for 48 h.
[d] The reaction performed with alcohol (3.0 eq.), [Mn]
(10 mol%), and KOtBu (1.0 eq.) for 28 h.
°
120 C for 20 h.
With the optimal reaction conditions, next, we have tron-withdrawing group such as fluoro, chloro, bromo,
investigated the scope and generality of the present iodo and trifluoromethyl groups were well tolerated
NNNÀ Mn(II)-catalyzed dehydrogenative coupling of and the desired products 3i–m were obtained in 62À
alcohols with indoles to access diverse BIMs (Table 2). 94% yields. Surprisingly, 4-nitrobenzyl alcohol (1o)
Initially, the reaction of unsubstituted indole 2a with failed to give the desired product. In addition, reaction
benzyl alcohol (1a) selectively afforded 3,3’- with di- and tri-substituted benzyl alcohols (1g, 1h
(phenylmethylene)bis(1H-indole) product which acts and 1n) worked well and provided the desired
as an antibiotic and natural product named Turbomycin products in 65–91% yields. A polyaromatic compound,
B in 91% isolated yield.[9d] Notably, the reaction of 1-naphthalenemethanol (1p) reacted smoothly under
indole with primary benzylic alcohols bearing an the optimized reaction conditions and provided the
electron-donating substituent such as methyl, thiometh- desired product 3p in 88% yield. Interestingly,
yl, isopropyl, and methoxy groups afforded the heteroaromatic alcohol such as 3-pyridinemethanol
corresponding bis(indolyl)methane derivatives 3b–f in (1q) also reacted efficiently with 2a to afford the
60–96% isolated yields. It is noteworthy that the product 3q in 72% yield. When 1,1-diphenylmethanol
product 3e is an orphan nuclear receptor.[9a] Similarly, was reacted with indole under the standard reaction
electronically deactivated benzylic alcohols, which are conditions, only a trace amount of product was
extremely poor substrates in Lewis- or Brønsted acid- observed along with the complete conversion of a
catalyzed FriedelÀ Crafts reactions, bearing an elec- secondary alcohol to the corresponding ketone. Later,
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under the modified reaction conditions (prolonged reaction efficiency and the corresponding products 4d
reaction time), the product 3r was obtained in 15% and 4e were obtained in 25% and 30% yields,
isolated yield. Pleasingly, aliphatic alcohols such as respectively. Further, the effect of substituents on the
ethanol, 1-butanol, and 1-pentanol were also reactive pyrrole ring of indole was examined. The reaction of
and afforded the desired products 3s–3u in 15–66% 2-methyl and 2-phenyl substituted indole with benzyl
yields, however, the reaction was performed with alcohol under the standard reaction conditions afforded
10 mol% Mn-catalyst and 1.0 eq. of KOtBu. The a poor yield of the desired products. To our delight,
product 3s is a natural product known as Vibrindole with 1.0 eq. of base, the desired products 4f and 4g
A,[9a] which is used to treat fibromyalgia and bowel were isolated in 40% and 35% yields, respectively.
syndrome.
Additionally, the reaction of 3-methylindole and N-
Next, we have explored the scope of indoles in the methylindole with benzyl alcohol didn’t lead to any
present Mn-catalyzed ADC reaction with alcohols product formation and only the unreacted starting
(Table 3). Initially, we have explored the effect of indoles were recovered. The inert nature of N-meth-
substituents on the six as well as a five-membered ring ylindole suggests that the indole NÀ H bond is involved
of indole, and it was found that the nature along with in a key interaction with the base. Next, we thought to
the position of the substituents on indole has a examine the scope of our protocol with respect to a 1:1
considerable effect on the desired product yields. The mixture of different indoles to synthesize unsym-
reaction of benzyl alcohol and 4-tert-butylbenzyl metrical bis(indolyl)methanes.
alcohol with electron-rich indole such as 5-meth-
Delightfully, the reaction of indole and 5-meth-
oxyindole provided the corresponding products 4a and oxyindole with benzyl alcohol underwent the desired
4b in 95% and 80% yields, respectively. It is important reaction with almost complete conversion of the
to note that product 4b acts as an Antileukemic starting indoles, and the desired unsymmetrical bis
agent.[9a] The electronically deactivated indoles bearing (indolyl)methane 4j was obtained in 45% yield.
a mild electron-withdrawing substituent on the six- Although the starting indole was fully consumed, the
membered benzene ring of indole such as 5-bromo desired product yield was low due to the formation of
afforded the desired product 4c in good yield while symmetrical products with respect to indole and 5-
strong electron-withdrawing substituent such as 5-nitro methoxyindole. Further, the reaction of 5-meth-
and 5-cyano groups have remarkable effect on the oxyindole and 6-bromoindole with benzyl alcohol also
delivered the corresponding product 4k in moderate
yield. A heteroaromatic indole, 7-azaindole, provided
the desired product 4l in 50% isolated yield. To
Table 3. Synthesis of bis(indolyl)methanes: Scope of indoles.[a,b]
demonstrate the practical utility of the present
phosphine-free Mn-catalysis, a large-scale reaction
with benzyl alcohol (1a) and indole (2a) was
performed, and the product 3a was obtained in 87%
yield which shows that the efficiency of the small-scale
reaction remains upon scale-up (see ESI).
Having developed an efficient method to access
substituted BIMs, the mechanism of this coupling
reaction was investigated. Firstly, the progress of the
reaction was monitored using gas chromatography
(GC) to gain detailed information about the catalytic
process (Figure 1). Monitoring the progress of the
reaction indicated that around 0.8 equivalent of 4-
methoxybenzyl alcohol was used during the reaction
and the consumption of indole follows an exponential
decay. The rate of product formation is faster as
approximately 27.6% bis(indolyl)methane 3e was
observed just after the completion of 2 h. The
intermediate 4-methoxybenzaldehyde was also de-
tected in GC which was utilized in the reaction with
indole.
Further, the reaction of 4-methoxybenzyl alcohol
under the standard reaction conditions in the absence
of indole provided the dehydrogenated product 4-
methoxybenzaldehyde in 90% yield (Scheme 3a). The
reaction failed to give the quantitative conversion of 4-
[a] Reaction conditions: alcohol 1 (0.5 mmol), indole 2a
(0.5 mmol), catalyst [Mn] (5 mol%), KOtBu (50 mol%) and
°
toluene solvent (1.0 mL) were heated at 120 C (oil-bath
temperature) for 20 h under Ar atm.
[b] Yield of the isolated product.
[c] The reaction was carried out with 1.0 eq. KOtBu.
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to give the corresponding aldehydes or ketones while
KOtBu catalyzes the condensation of indoles with
aldehydes or ketones to give the alkylideneindolenine
intermediates and further reaction with another indole
moiety to provide the desired BIMs. Notably, the
evolution of hydrogen gas during the reaction was
qualitatively analyzed by gas chromatography, which
suggests that the process proceeds via ADC strategy
(Scheme 3c). Further, the reaction of indole with
deuterium-labelled alcohol 1e-[D] was studied under
the standard reaction conditions and the analysis of the
product using 1H NMR revealed 91% deuterium
incorporation in the product (Scheme 3d). Next, we
performed parallel and competitive experiments of 1e
and 1e-[D] with indole to perceive the kinetic isotope
effect (KIE) (Scheme 3e and 3f). The KH/KD value
obtained in parallel and competitive experiments were
2.19 and 2.03, respectively. The KIE values obtained
are in close agreement with each other and provide
very strong support that the dehydrogenation of the
alcohol took place at or before the rate-determining
step.
Figure 1. Kinetic profile of the NNNÀ Mn(II) pincer complex
catalyzed ADC of 1e with indole (2a).
Based on the mechanistic studies and the previous
literature reports,[13] a plausible mechanism for the
acceptorless dehydrogenative coupling of alcohols
with indoles catalyzed by an NNNÀ Mn (II) pincer
complex is shown in Scheme 4. In the presence of a
base, the precatalyst generates the coordinatively
unsaturated reactive intermediate I followed by alcohol
activation via metal-ligand cooperation (MLC) results
in the formation of an alkoxy type intermediate II. At
this point, the complex II undergoes β-hydride elimi-
nation of the alkoxide to yield the dehydrogenated
product (aldehyde or ketone) with the formation of
MnÀ H species III which further releases a molecule of
hydrogen to generate the active catalyst I and the cycle
continues. Next, the in situ generated aldehyde or
ketone undergoes a base-catalyzed condensation reac-
tion with indole (2a) to yield alkylideneindolenine
intermediate which upon Michael-type nucleophilic
addition of another indole molecule results in the
formation of the BIM product.
In conclusion, we have reported an example of base
metal-catalyzed alkylation of indoles using alcohols to
Scheme 3. Mechanistic experiments.
methoxybenzyl alcohol in the absence of a base or
Mn-catalyst. Additionally, the condensation of indole
with 4-methoxybenzaldehyde under the standard reac-
tion conditions was found to be fast and a similar rate
was observed when the reaction was performed with
base alone (Scheme 3b). Hence, the manganese cata-
lyst is playing a key role in the activation of alcohols
Scheme 4. Plausible mechanism.
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120.0, 119.9, 119.1, 111.0, 39.7, 33.6, 24.0. HRMS (ESI) m/z
calcd for C26H23N2 (MÀ H)+: 363.1856, found: 363.1862.
access a diverse range of value-added bis(indolyl)
methanes. An easy to handle, air- and moisture-stable
earth-abundant manganese catalyst is used for this
process. Mechanistic studies showed that the reaction
proceeds via acceptorless dehydrogenation of alcohols
followed by a catalyst-independent cascade process.
The protocol is scalable and suitable in the preparation
of important bioactive molecules.
3,3’-((4-(Methylthio)phenyl)methylene)bis(1H-
Indole) (3d)
1
Red solid, 60% (55.2 mg) isolated yield. H NMR (400 MHz,
CDCl3): δ ppm 7.85 (br s, 2 H), 7.39 (d, J=8.0 Hz, 2 H), 7.34
(d, J=8.4 Hz, 2 H), 7.26 (d, J=8.4 Hz, 2 H), 7.16–7.18 (m, 4
H), 7.01 (t, J=7.4 Hz, 2 H), 6.62 (br s, 2 H), 5.84 (s, 1 H), 2.46
(s, 3 H). 13C NMR (100.6 MHz, CDCl3): δ ppm 141.1, 136.7,
135.5, 129.2, 127.0, 126.7, 123.6, 121.9, 119.9, 119.5, 119.2,
111.0, 39.6, 16.0. HRMS (ESI) m/z calcd for C24H19N2S
(MÀ H)+: 367.1263, found: 367.1270.
Experimental Section
General Procedure for Synthesis of Bis(3-Indolyl)
methanes
To an oven dried schlenk tube, indole (0.5 mmol, 1 eq.), alcohol
(0.5 mmol, 1 eq.), [Mn] catalyst (5 mol%), KOtBu (0.25 mmol,
50 mol%) and toluene (1 mL) were added under a gentle stream
3,3’-((4-Methoxyphenyl)methylene)bis(1H-Indole)
(3e)
°
of argon. The reaction mixture was kept for stirring at 120 C
Orange solid, 85% (74.8 mg) isolated yield. 1H NMR
(500 MHz, CDCl3): δ ppm 7.90 (br s, 2 H), 7.40 (d, J=8.0 Hz,
2 H), 7.36 (d, J=8.4 Hz, 2 H), 7.25–7.27 (m, 2 H), 7.18 (t, J=
7.6 Hz, 2 H), 7.01 (t, J=7.6 Hz, 2 H), 6.82–6.84 (m, 2 H), 6.65
(d, J=1.9 Hz, 2 H), 5.85 (s, 1 H), 3.79 (s, 3 H). 13C NMR
(126 MHz, CDCl3): δ ppm 157.9, 136.7, 136.2, 129.6, 127.0,
123.5, 121.9, 120.0, 119.2, 113.5, 111.0, 55.2, 39.3. HRMS
(ESI) m/z calcd for C24H19N2O (MÀ H)+: 351.1492, found:
351.1496.
(oil-bath temperature) for 20 h. Then, the reaction was
quenched with water (2 mL) and extracted with dichloro-
methane/ethyl acetate (3×4 mL). The combined organic layer
was dried over anhydrous Na2SO4 and the solvent was
evaporated under reduced pressure. The crude mixture was
purified by silica gel column chromatography (230–400 mesh
size) using petroleum-ether/ethyl acetate as an eluting system to
yield the desired bis(3-indolyl)methane product 3 or 4.
3,3’-(Phenylmethylene)bis(1H-Indole) (3a)
3,3’-((2-Methoxyphenyl)methylene)bis(1H-Indole)
(3f)
1
Red solid, 91% (73.3 mg) isolated yield. H NMR (500 MHz,
CDCl3): δ ppm 7.86 (br s, 2 H), 7.42 (d, J=8.1 Hz, 2 H), 7.35–
Orange solid, 62% (54.6 mg) isolated yield. 1H NMR
(500 MHz, DMSO-d6): δ ppm 10.78 (br s, 2 H), 7.35 (d, J=
8.2 Hz, 2 H), 7.24 (d, J=7.8 Hz, 2 H), 7.13–7.19 (m, 2 H),
7.00–7.05 (m, 3 H), 6.86 (t, J=7.6 Hz, 2 H), 6.81 (t, J=
7.3 Hz, 1 H), 6.74 (br s, 2 H), 6.23 (s, 1 H), 3.80 (s, 3 H). 13C
NMR (126 MHz, DMSO-d6): δ ppm 156.3, 136.7, 132.7,
129.2, 127.1, 126.8, 123.6, 120.9, 120.1, 119.0, 118.2, 117.9,
111.5, 110.9, 55.6, 31.5. HRMS (ESI) m/z calcd for C24H19N2O
(MÀ H)+: 351.1492, found: 351.1496.
7.38 (m, 4 H), 7.28–7.32 (m, 2H), 7.17–7.26 (m, 3 H), 7.01–
7.05 (m, 2 H), 6.64 (dd, J=2.4, 0.9 Hz, 2 H), 5.91 (s, 1 H). 13
C
NMR (126 MHz, CDCl3): δ ppm 144.0, 136.6, 128.7, 128.2,
127.0, 126.1, 123.6, 121.9, 119.9, 119.6, 119.2, 111.0, 40.1.
HRMS (ESI) m/z calcd for C23H17N2 (MÀ H)+: 321.1386,
found: 321.1393.
3,3’-(p-Tolylmethylene)bis(1H-Indole) (3b)
1
Red solid, 93% (78.2 mg) isolated yield. H NMR (400 MHz,
CDCl3): δ ppm 7.89 (br s, 2 H), 7.41 (d, J=7.9 Hz, 2 H), 7.36
(d, J=8.1 Hz, 2 H), 7.25 (d, J=8.0 Hz, 2 H), 7.16–7.20 (m, 2
H), 7.10 (d, J=7.8 Hz, 2 H), 7.00–7.04 (m, 2 H), 6.66 (dd, J=
2.4, 0.9 Hz, 2 H), 5.87 (s, 1 H), 2.34 (s, 3 H). 13C NMR
(100.6 MHz, CDCl3): δ ppm 141.0, 136.7, 135.5, 128.9, 128.5,
127.1, 123.5, 121.8, 119.9, 119.2, 111.0, 39.7, 21.1. HRMS
(ESI) m/z calcd for C24H19N2 (MÀ H)+: 335.1543, found:
335.1548.
3,3’-(Benzo[d][1,3]dioxol-5-ylmethylene)bis(1H-
Indole) (3g)
1
Red solid, 65% (59.5 mg) isolated yield. H NMR (400 MHz,
CDCl3): δ ppm 7.76 (br s, 2 H), 7.45 (d, J=7.9 Hz, 2 H), 7.33
(d, J=8.0 Hz, 2 H), 7.22 (t, J=7.5 Hz, 2 H), 7.07 (t, J=
7.4 Hz, 2 H), 6.86–6.88 (m, 2 H), 6.77 (d, J=8.1 Hz, 1 H), 6.59
(br s, 2 H), 5.92 (s, 2 H), 5.84 (s, 1 H). 13C NMR (100.6 MHz,
CDCl3): δ ppm 147.4, 145.7, 138.1, 136.6, 126.9, 123.5, 121.9,
121.5, 119.8, 119.6, 119.2, 111.0, 109.3, 107.9, 100.7, 39.8.
HRMS (ESI) m/z calcd for C24H17N2O2 (MÀ H)+: 365.1285,
found: 365.1289.
3,3’-((4-Isopropylphenyl)methylene)bis(1H-Indole)
(3c)
1
Red solid, 96% (87.4 mg) isolated yield. H NMR (400 MHz,
CDCl3): δ ppm 7.88 (br s, 2 H), 7.41 (d, J=8.0 Hz, 2 H), 7.34
(d, J=8.1 Hz, 2 H), 7.25–7.27 (m, 2 H), 7.16 (t, J=7.8 Hz, 2
H), 7.13 (d, J=8.1 Hz, 2 H), 7.00 (t, J=7.5 Hz, 2 H), 6.66 (d,
J=1.9 Hz, 2 H), 5.86 (s, 1 H), 2.88 (quint, J=6.9 Hz, 1 H),
1.24 (d, J=6.9 Hz, 6 H). 13C NMR (100.6 MHz, CDCl3): δ
ppm 146.4, 141.2, 136.6, 128.5, 127.1, 126.2, 123.5, 121.8,
3,3’-((3,4,5-Trimethoxyphenyl)methylene)bis(1H-
Indole) (3h)
1
Red solid, 82% (84.5 mg) isolated yield. H NMR (500 MHz,
DMSO-d6): δ ppm 10.78 (br s, 2 H), 7.33–7.35 (m, 4 H), 7.03
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(t, J=7.4 Hz, 2 H), 6.86–6.89 (m, 4 H), 6.72 (s, 2 H), 5.78 (s, 1
H), 3.65 (s, 6 H), 3.63 (s, 3 H). 13C NMR (126 MHz, DMSO-
d6): δ ppm 152.5, 140.7, 136.5, 135.7, 126.6, 123.5, 120.8,
119.1, 118.1, 111.4, 105.8, 60.0, 55.8, 39.0. HRMS (ESI) m/z
calcd for C26H23O3N2 (MÀ H)+: 411.1703, found: 411.1708.
118.8, 111.1, 40.0. HRMS (ESI) m/z calcd for C24H16F3N2
(MÀ H)+: 389.1260, found: 389.1267.
3,3’-((3,5-Bis(trifluoromethyl)phenyl)methylene)bis
(1H-Indole) (3n)
1
Red solid, 91% (104.2 mg) isolated yield. H NMR (500 MHz,
3,3’-((4-Fluorophenyl)methylene)bis(1H-Indole) (3i)
CDCl3): δ ppm 7.96 (br s, 2 H), 7.85 (s, 2 H), 7.81 (s, 1 H),
7.39 (t, J=6.9 Hz, 4 H), 7.23–7.27 (m, 2 H), 7.08 (t, J=
7.6 Hz, 2 H), 6.62 (d, J=1.8 Hz, 2 H), 6.05 (s, 1 H). 13C NMR
(126 MHz, CDCl3): δ ppm 146.7, 136.7, 131.4 (q, J=33.2 Hz),
128.7, 126.5, 123.7, 123.4 (q, J=273.2 Hz), 122.4, 120.5
(apparent sept, J=3.4 Hz), 119.6, 119.4, 117.9, 111.3, 40.0.
HRMS (ESI) m/z calcd for C25H15F6N2 (MÀ H)+: 457.1134,
found: 457.1142.
1
Red solid, 72% (61.2 mg) isolated yield. H NMR (400 MHz,
CDCl3): δ ppm 7.81 (br s, 2 H), 7.25–7.29 (m, 4 H), 7.18–7.23
(m, 2 H), 7.07–7.12 (m, 2 H), 6.84–6.95 (m, 4 H), 6.54 (dd, J=
2.4, 1.0 Hz, 2 H), 5.79 (s, 1 H). 13C NMR (100.6 MHz, CDCl3):
δ ppm 161.4 (d, J=244.1 Hz), 139.6 (d, J=2.9 Hz), 136.7,
130.0 (d, J=8.0 Hz), 126.9, 123.5, 122.0, 119.8, 119.5, 119.3,
114.9 (d, J=21.1 Hz), 111.1, 39.4. HRMS (ESI) m/z calcd for
C23H16FN2 (MÀ H)+: 339.1292, found: 339.1296.
3,3’-(Naphthalen-1-ylmethylene)bis(1H-Indole) (3p)
3,3’-((4-Chlorophenyl)methylene)bis(1H-Indole)
1
Red solid, 88% (81.9 mg) isolated yield. H NMR (400 MHz,
(3j)
DMSO-d6): δ ppm 10.86 (br s, 2 H), 8.29 (d, J=8.0 Hz, 1 H),
7.92 (d, J=7.6 Hz, 1 H), 7.77 (d, J=8.4 Hz, 1 H), 7.43–7.47
(m, 2 H), 7.39 (d, J=8.0 Hz, 2 H), 7.36 (d, J=7.6 Hz, 1 H),
7.31 (d, J=7.6 Hz, 3 H), 7.05 (t, J=7.4 Hz, 2 H), 6.86 (t, J=
7.4 Hz, 2 H), 6.78 (br s, 2 H), 6.68 (s, 1 H). 13C NMR
(100.6 MHz, DMSO-d6): δ ppm 140.4, 136.7, 133.7, 131.4,
128.7, 126.7, 125.9, 125.6, 125.4, 124.4, 124.1, 121.0, 119.1,
118.4, 117.8, 111.6, 35.4. HRMS (ESI) m/z calcd for C27H19N2
(MÀ H)+: 371.1543, found: 371.1548.
Orange solid, 75% (66.8 mg) isolated yield. 1H NMR
(400 MHz, CDCl3): δ ppm 7.86 (br s, 2 H), 7.28 (d, J=8.6 Hz,
4 H), 7.15–7.20 (m, 4 H), 7.10 (t, J=7.6 Hz, 2 H), 6.94 (t, J=
7.5 Hz, 2 H), 6.56 (br s, 2 H), 5.78 (s, 1 H). 13C NMR
(100.6 MHz, CDCl3): δ ppm 142.5, 136.7, 131.8, 130.0, 128.3,
126.9, 123.6, 122.1, 119.8, 119.3, 119.2, 111.1, 39.6. HRMS
(ESI) m/z calcd for C23H16ClN2 (MÀ H)+: 355.0997, found:
355.1002.
3,3’-((4-Bromophenyl)methylene)bis(1H-Indole)
3,3’-(Pyridin-3-ylmethylene)bis(1H-Indole) (3q)
(3k)
1
Pink solid, 72% (58.2 mg) isolated yield. H NMR (500 MHz,
1
DMSO-d6): δ ppm 10.90 (br s, 2 H), 8.61 (s, 1 H), 8.39 (d, J=
3.8 Hz, 1 H), 7.70 (d, J=7.6 Hz, 1 H), 7.36 (d, J=8.0 Hz, 2
H), 7.28–7.30 (m, 3 H), 7.05 (t, J=7.4 Hz, 2 H), 6.87–6.89 (m,
4 H), 5.91 (s, 1 H). 13C NMR (126 MHz, DMSO-d6): δ ppm
149.6, 147.2, 140.3, 136.6, 135.7, 126.4, 123.7, 123.3, 121.1,
119.0, 118.4, 117.2, 111.6, 37.1. HRMS (ESI) m/z calcd for
C22H18N3 (M+H)+: 324.1495, found: 324.1494.
Red solid, 62% (61.4 mg) isolated yield. H NMR (400 MHz,
CDCl3): δ ppm 7.89 (br s, 2 H), 7.35–7.42 (m, 6 H), 7.18–7.24
(m, 4 H), 7.02–7.06 (m, 2 H), 6.62 (dd, J=2.3, 0.8 Hz, 2 H),
5.86 (s, 1 H). 13C NMR (100.6 MHz, CDCl3): δ ppm 143.1,
136.6, 131.3, 130.4, 126.8, 123.6, 122.0, 119.9, 119.8, 119.3,
119.0, 111.1, 39.6. LCMS (ESI) m/z for C23H18BrN2 (M+H)+:
401.0.
Di(1H-Indol-3-yl)diphenylmethane (3r)
3,3’-((4-Iodophenyl)methylene)bis(1H-Indole) (3l)
Colourless solid, 15% (14.9 mg) isolated yield. 1H NMR
(500 MHz, CDCl3): δ ppm 7.87 (br s, 2 H), 7.27–7.31 (m, 6 H),
7.17–7.22 (m, 6 H), 7.04–7.08 (m, 2 H), 6.74–6.80 (m, 6 H).
13C NMR (126 MHz, CDCl3): δ ppm 145.9, 136.8, 130.1,
127.7, 127.4, 125.9, 124.9, 123.8, 122.5, 121.5, 119.0, 110.9,
53.8. HRMS (ESI) m/z calcd for C29H21N2 (MÀ H)+: 397.1699,
found: 397.1763.
1
Red solid, 64% (71.7 mg) isolated yield. H NMR (400 MHz,
CDCl3): δ ppm 7.90 (br s, 2 H), 7.61 (d, J=8.4 Hz, 2 H), 7.36–
7.39 (m, 4 H), 7.18–7.22 (m, 2 H), 7.11 (d, J=8.1 Hz, 2 H),
7.01–7.05 (m, 2 H), 6.64 (dd, J=2.4, 0.9 Hz, 2 H) 5.84 (s, 1
H). 13C NMR (100.6 MHz, CDCl3): δ ppm 143.8, 137.3, 136.6,
130.8, 126.8, 123.6, 122.1, 119.8, 119.3, 119.0, 111.1, 91.4,
39.8. LCMS (ESI) m/z for C23H17IN2 (M)+: 448.0.
3,3’-(Ethane-1,1-diyl)bis(1H-Indole) (3s)
3,3’-((3-(Trifluoromethyl)phenyl)methylene)bis(1H-
Indole) (3m)
White solid, 15% (10.0 mg) isolated yield. 1H NMR (400 MHz,
CDCl3): δ ppm 7.89 (br s, 2 H), 7.60 (d, J=8.0 Hz, 2 H), 7.36
(d, J=8.1 Hz, 2 H), 7.16–7.20 (m, 2 H), 7.04–7.08 (m, 2 H),
6.93 (d, J=1.1 Hz, 2 H), 4.70 (q, J=7.1 Hz, 1 H), 1.83 (d, J=
7.1 Hz, 3 H). 13C NMR (100.6 MHz, CDCl3): δ ppm 136.6,
126.9, 121.7, 121.6, 121.2, 119.7, 119.0, 111.0, 28.1, 21.7.
LCMS (ESI) m/z for C18H17N2 (M+H)+: 261.0.
1
Pink solid, 94% (91.7 mg) isolated yield. H NMR (500 MHz,
CDCl3): δ ppm 7.93 (br s, 2 H), 7.67 (s, 1 H), 7.52 (t, J=
7.8 Hz, 2 H), 7.37–7.41 (m, 5 H), 7.19–7.23 (m, 2 H), 7.03–
7.07 (m, 2 H), 6.62 (br s, 2H), 5.97 (s, 1 H). 13C NMR
(126 MHz, CDCl3): δ ppm 145.0, 136.6, 132.0, 130.4 (q, J=
31.9 Hz), 128.7, 126.7, 125.4 (q, J=3.8 Hz), 124.3 (q, J=
272.2 Hz), 123.6, 123.1 (q, J=3.8 Hz), 122.1, 119.7, 119.4,
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3,3’-(Butane-1,1-diyl)bis(1H-Indole) (3t)
3,3’-(Phenylmethylene)bis(5-Nitro-1H-Indole) (4d)
Brown solid, 47% (33.9 mg) isolated yield. 1H NMR
(500 MHz, CDCl3): δ ppm 7.83 (br s, 2 H), 7.65 (d, J=8.1 Hz,
2 H), 7.32 (d, J=8.1 Hz, 2 H), 7.17–7.21 (m, 2 H), 7.06–7.10
(m, 2 H), 6.96 (d, J=2.0 Hz, 2 H), 4.53 (t, J=7.4 Hz, 1 H),
2.21–2.27 (m, 2 H), 1.45–1.50 (m, 2 H), 1.00 (t, J=7.4 Hz, 3
H). 13C NMR (126 MHz, CDCl3): δ ppm 136.5, 127.1, 121.6,
121.4, 120.5, 119.6, 118.9, 111.0, 38.1, 33.6, 21.4, 14.2. HRMS
(ESI) m/z calcd for C20H19N2 (MÀ H)+: 287.1543, found:
287.1547.
Yellow solid, 25% (25.8 mg) isolated yield. 1H NMR
(400 MHz, DMSO-d6): δ ppm 11.66 (br s, 2 H), 8.32 (d, J=
2.3 Hz, 2 H), 7.97 (dd, J=9.0, 2.3 Hz, 2 H), 7.55 (d, J=
9.1 Hz, 2 H), 7.39–7.41 (m, 2 H), 7.30–7.34 (m, 2 H), 7.20–
7.24 (m, 1 H), 7.14 (d, J=1.8 Hz, 2 H), 6.20 (s, 1 H). 13C
NMR (100.6 MHz, DMSO-d6): δ ppm 143.8, 140.2, 139.8,
128.5, 128.2, 127.6, 126.4, 125.8, 120.5, 116.6, 116.2, 112.1,
38.5. HRMS (ESI) m/z calcd for C23H15O4N4 (MÀ H)+:
411.1088, found: 411.1103.
3,3’-(Pentane-1,1-diyl)bis(1H-Indole) (3u)
3,3’-(Phenylmethylene)bis(1H-Indole-
5-Carbonitrile) (4e)
Brown solid, 66% (49.9 mg) isolated yield. 1H NMR
(400 MHz, CDCl3): δ ppm 7.68 (br s, 2 H), 7.51 (d, J=8.1 Hz,
2 H), 7.19–7.21 (m, 2 H), 7.03–7.07 (m, 2 H), 6.93–6.97 (m, 2
H), 6.84 (d, J=2.4 Hz, 2 H), 4.38 (t, J=7.4 Hz, 1 H), 2.10–
Colourless solid, 30% (27.9 mg) isolated yield. 1H NMR
(400 MHz, DMSO-d6): δ ppm 11.50 (br s, 2 H), 7.80 (br s, 2
H), 7.53–7.55 (m, 2 H), 7.37–7.41 (m, 4 H), 7.30 (t, J=7.5 Hz,
2 H), 7.19–7.22 (m, 1 H), 7.14 (d, J=1.9 Hz, 2 H), 6.03 (s, 1
H). 13C NMR (100.6 MHz, DMSO-d6): δ ppm 144.0, 138.2,
128.4, 128.2, 126.3, 126.2, 124.6, 123.8, 120.8, 118.8, 112.9,
100.4, 38.5. HRMS (ESI) m/z calcd for C25H15N4 (MÀ H)+:
371.1291, found: 371.1298.
2.15 (m, 2 H), 1.26–1.33 (m, 4 H), 0.78 (t, J=7.1 Hz, 3 H). 13
C
NMR (100.6 MHz, CDCl3): δ ppm 136.5, 127.1, 121.7, 121.4,
120.5, 119.6, 118.9, 111.0, 35.6, 33.9, 30.5, 22.8, 14.1. HRMS
(ESI) m/z calcd for C21H21N2 (MÀ H)+: 301.1699, found:
301.1705.
3,3’-(Phenylmethylene)bis(5-Methoxy-1H-Indole)
3,3’-(Phenylmethylene)bis(2-Methyl-1H-Indole) (4f)
(4a)
1
Pink solid, 40% (35 mg) isolated yield. H NMR (500 MHz,
1
Red solid, 95% (90.8 mg) isolated yield. H NMR (400 MHz,
DMSO-d6): δ ppm 10.76 (br s, 2 H), 7.18–7.27 (m, 7 H), 6.87–
6.91 (m, 2 H), 6.81 (d, J=7.8 Hz, 2 H), 6.65–6.69 (m, 2 H),
5.93 (s, 1 H), 2.07 (s, 6 H). 13C NMR (126 MHz, DMSO-d6): δ
ppm 144.3, 135.1, 132.1, 128.7, 128.3, 127.9, 125.8, 119.5,
118.5, 117.9, 112.2, 110.3, 38.6, 11.9. HRMS (ESI) m/z calcd
for C25H21N2 (MÀ H)+: 349.1699, found: 349.1704.
CDCl3 + DMSO-d6): δ ppm 10.45 (br s, 2 H), 7.32 (d, J=
7.3 Hz, 2 H), 7.19–7.24 (m, 4 H), 7.13 (t, J=7.3 Hz, 1 H),
6.64–6.72 (m, 6 H), 5.69 (s, 1 H), 3.59 (s, 6 H). 13C NMR
(100.6 MHz, CDCl3 +DMSO-d6): δ ppm 152.6, 144.6, 131.7,
128.2, 127.7, 126.8, 125.5, 124.2, 117.5, 111.7, 110.4, 101.2,
55.1, 39.8. HRMS (ESI) m/z calcd for C25H21O2N2 (MÀ H)+:
381.1598, found: 381.1603.
3,3’-(Phenylmethylene)bis(2-Phenyl-1H-Indole) (4g)
Brown solid, 35% (41.5 mg) isolated yield. 1H NMR
(400 MHz, DMSO-d6): δ ppm 11.33 (br s, 2 H), 7.38 (d, J=
8.1 Hz, 2 H), 7.29–7.32 (m, 4 H), 7.27 (d, J=7.4 Hz, 2 H),
7.21–7.24 (m, 7 H), 7.16 (d, J=7.1 Hz, 2 H), 6.99–7.03 (m, 2
H), 6.90 (d, J=8.1 Hz, 2 H), 6.65–6.69 (m, 2 H), 5.98 (s, 1 H).
13C NMR (100.6 MHz, DMSO-d6): δ ppm 145.5, 136.3, 135.3,
132.8, 128.7, 128.3, 128.2, 128.0, 127.2, 126.0, 120.9, 120.8,
118.5, 114.2, 111.3, 38.9. HRMS (ESI) m/z calcd for C35H25N2
(MÀ H)+: 473.2012, found: 473.2019.
3,3’-((4-(Tert-Butyl)phenyl)methylene)bis
(5-Methoxy-1H-Indole) (4b)
1
Red solid, 80% (87.6 mg) isolated yield. H NMR (400 MHz,
CDCl3): δ ppm 7.81 (br s, 2 H), 7.27–7.32 (m, 4 H), 7.23 (d,
J=8.5 Hz, 2 H), 6.82–6.85 (m, 4 H), 6.69 (br s, 2 H), 5.76 (s, 1
H), 3.70 (s, 6 H), 1.32 (s, 9 H). 13C NMR (100.6 MHz, CDCl3):
δ ppm 153.6, 148.7, 140.8, 131.8, 128.3, 127.6, 125.0, 124.3,
119.5, 111.8, 111.6, 102.0, 55.8, 39.8, 34.4, 31.4. HRMS (ESI)
m/z calcd for C29H29O2N2 (MÀ H)+: 437.2224, found: 437.2229.
(R)-3-((1H-Indol-3-yl)(phenyl)methyl)-5-Methoxy-
3,3’-(Phenylmethylene)bis(5-Bromo-1H-Indole) (4c) 1H-Indole (4j)
1
Orange solid, 75% (89.6 mg) isolated yield. 1H NMR
(400 MHz, DMSO-d6): δ ppm 11.07 (br s, 2 H), 7.43 (d, J=
2.0 Hz, 2 H), 7.33–7.35 (m, 4 H), 7.27–7.31 (m, 2 H), 7.17–
7.21 (m, 1 H), 7.16 (dd, J=8.6, 1.9 Hz, 2 H), 6.89 (d, J=
2.0 Hz, 2 H), 5.86 (s, 1 H). 13C NMR (100.6 MHz, DMSO-d6):
δ ppm 144.3, 135.3, 128.4, 128.2, 126.1, 125.3, 123.5, 121.2,
117.7, 113.6, 110.9, 38.9. HRMS (ESI) m/z calcd for
C23H15Br2N2 (MÀ H)+: 476.9597, found: 476.9604.
Red solid, 45% (39.6 mg) isolated yield. H NMR (500 MHz,
CDCl3): δ ppm 7.87 (br s, 1 H), 7.77 (br s, 1 H), 7.41 (d, J=
8.0 Hz, 1 H), 7.37 (d, J=7.3 Hz, 2 H), 7.35 (d, J=8.4 Hz, 1
H), 7.30 (t, J=7.8 Hz, 2 H), 7.22–7.25 (m, 2 H), 7.19 (t, J=
8.0 Hz, 1 H), 7.03 (t, J=7.8 Hz, 1 H), 6.85–6.87 (m, 2 H), 6.64
(d, J=1.5 Hz, 1 H), 6.61 (d, J=1.9 Hz, 1 H), 5.85 (s, 1 H),
3.72 (s, 3 H). 13C NMR (126 MHz, CDCl3): δ ppm 153.6,
144.0, 136.7, 131.8, 128.7, 128.2, 127.4, 127.0, 126.1, 124.4,
123.6, 121.8, 119.9, 119.5, 119.3, 119.1, 111.9, 111.7, 111.0,
101.8, 55.8, 40.2. HRMS (ESI) m/z calcd for C24H19N2O
(MÀ H)+: 351.1492, found: 351.1498.
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2018, 57, 46–60; Angew. Chem. 2018, 130, 48–63;
c) L. M. Azofra, A. Poater, Dalton Trans. 2019, 48,
14122–14127; d) J. A. Luque-Urrutia, M. Solà, D.
Milstein, A. Poater, J. Am. Chem. Soc. 2019, 141, 2398–
2403; e) J. Masdemont, J. A. Luque-Urrutia, M. Gimfer-
rer, D. Milstein, A. Poater, ACS Catal. 2019, 9, 1662–
1
Red solid, 50% (53.8 mg) isolated yield. H NMR (400 MHz,
CDCl3): δ ppm 7.88 (br s, 1 H), 7.83 (br s, 1 H), 7.47 (d, J=
1.5 Hz, 1 H), 7.28–7.35 (m, 4 H), 7.21–7.25 (m, 3 H), 7.10 (dd,
J=8.4 Hz, 1.7 Hz, 1 H), 6.86 (dd, J=8.7 Hz, 2.4 Hz, 1 H),
6.82 (d, J=2.4 Hz, 1 H), 6.61–6.62 (m, 2 H), 5.80 (s, 1 H),
3.72 (s, 3 H). 13C NMR (100.6 MHz, CDCl3): δ ppm 153.7,
143.6, 137.4, 131.8, 128.6, 128.3, 127.3, 126.3, 125.9, 124.3,
124.2, 122.5, 121.2, 119.7, 119.0, 115.4, 113.9, 111.9, 111.8,
101.8, 55.8, 40.1. HRMS (ESI) m/z calcd for C24H18BrN2O
(MÀ H)+: 429.0597, found: 429.0603.
3,3’-(Phenylmethylene)bis(1H-Pyrrolo[2,3-b]
pyridine) (4l)
Colourless solid, 50% (40.5 mg) isolated yield. 1H NMR
(400 MHz, DMSO-d6): δ ppm 11.42 (br s, 2 H), 8.16 (d, J=
3.9 Hz, 2 H), 7.60 (dd, J=7.8, 1.3 Hz, 2 H), 7.36 (d, J=
7.3 Hz, 2 H), 7.28 (t, J=7.5 Hz, 2 H), 7.17–7.21 (m, 1 H), 6.98
(d, J=2.1 Hz, 2 H), 6.92 (dd, J=7.9, 4.6 Hz, 2 H), 5.86 (s, 1
H). 13C NMR (100.6 MHz, DMSO-d6): δ ppm 148.9, 144.1,
142.5, 128.3, 127.2, 126.2, 123.9, 118.8, 116.5, 114.9, 38.9.
HRMS (ESI) m/z calcd for C21H17N4 (M+H)+: 325.1448,
found: 325.1444.
Acknowledgements
S.B.M. acknowledges financial support from SERB, India. EB
acknowledges funding from Swarnajayanti Fellowship (DST/
SJF/CSA-04/2019-2020 & SERB/F/5892/2020-2021), SERB,
India (Grant No: CRG/2018/002480/OC), and IISER-Tirupati.
S.B.M. and E. B. acknowledges CSIR-NCL, Pune. VY acknowl-
edge UGC, India for fellowship.
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Phosphine-Free Manganese(II)-Catalyst Enables Acceptorless
Dehydrogenative Coupling of Alcohols with Indoles
Adv. Synth. Catal. 2021, 363, 1–11
V. Yadav, E. Balaraman*, S. B. Mhaske*