Regiocontrolled direct C4 and C2-methyl thiolation of indoles under rhodium-catalyzed mild conditions

Article abstract of DOI:10.1039/c7cc07086a

A straightforward Rh(iii)-catalyzed general strategy was developed for the site-selective remote C4 (sp²) and C2 (sp³)-methyl thiolation of an indole core, keeping the oxime directing group at the C3 position. The transformation was accomplished under mild conditions with a wide scope and functional group tolerance. The directing group can easily be removed after operation. Methyl substitution at the C2 position of the indole core led to C2 (sp³)-methyl thiolation.

Full text of DOI:10.1039/c7cc07086a

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DOI: 10.1039/C7CC07086A
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Regiocontrolled Direct C4 and C2‐Methyl Thiolation of Indoles
under Rhodium Catalyzed Mild Conditions
Saurabh Maity†, Ujjwal Karmakar† and Rajarshi Samanta*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
A straightforward Rh(III)catalyzed general strategy was developed established Pd catalyzed C4 arylation and fluoroalkylation of indole
for the site selective remote C4 (sp2) and C2 (sp3)‐methyl derivatives with the aid of keto directing group at the C3
thiolation of indole core keeping the oxime directing group at the position.4,10
C3 position. The transformation was accomplished under mild
conditions with wide scope and functional group tolerance. The
directing group can easily be removed after operation. Methyl
substitution at the C2 position of indole core led to C2 (sp3)‐
methyl thiolation.
Among many nitrogen containing heterocyclic scaffolds, indole
motif is one of the most ubiquitous structure in the natural
products and fourth most prevalent heteroatomic in marketed
pharmaceuticals.1 Thus, in current years, significant efforts
Scheme 1: Direct C4 functionalization of indole derivatives.
have been devoted to the transition metal catalyzed direct
functionalizations at the C2 and C3 positions due to its
inherent reactivity.2 In sharp contrast, site‐selective
A
biomimetic Fridel‐Crafts type C4 alkyaltion of 7‐
functionalization of remote less activated C‐H bonds in the methoxyindole derivatives was achieved by Dethe’s group.11
benzene core of indole derivatives continue to be Very recently, Prabhu’s group and You’s group independently
challenging.3‐6 Hence, selective C‐H functionalization at the reported the Ir catalyzed C4 amination of indole derivatives
poor nucleophilic C4 position of indole has been explored with the assist of C3‐aldehyde group.12, 13
recently due to the limitations of earlier two successful
approaches (Scheme 1).7, 8 After the major breakthrough by Jia
and co‐workers in Pd‐catalyzed C4 olefination of tryptophan
derivatives,8a Prabhu and co‐workers published Ru catalyzed
C4 olefination of indole derivatives using the aldehyde and
ketone directing group.8b,c Recently, Jia’s group extended this
concept with Rh catalyzed conditions on NH free indole
derivatives.8d After the development of Rh catalyzed
regioselective C‐H bond activation/annulation of indolyl
aldehydes at the C4 position by You’s group,9a Wang and co‐
workers also developed Rh catalyzed oxidative annulation
reaction of 3‐(1H‐indol‐3‐yl)‐3‐oxopropanenitriles with internal
alkynes.9b In recent significant advancements, Shi’s group
Fig. 1: Biologically relevant scaffold containing C4 and 2‐methylthio‐substituted indoles.
The thioether moiety is prevalent in bioactive compounds.14
Transition metal catalyzed direct thiolation of inactivated C‐H
bonds has emerged as an attractive strategy due to the known
limitations of sulfur compounds like catalyst deactivation
properties and its over oxidation in the presence of oxidant.15
Interestingly, 4‐thioindoles and 2‐thiomethyl indoles are
common in pharmaceuticals, natural products and organic
Department of Chemistry, Indian Institute of Technology Kharagpur, Kharagpur
721302, India.
†These authors contributed equally.
materials (Figure 1).
Nevertheless, despite significant
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
progresses in transition metal catalyzed direct C2 and C3
selective thiolation in indole core,16, 17 there is no direct sp2‐C4
This journal is © The Royal Society of Chemistry 20xx
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thiolation and sp3‐2‐methylthiolation report to the best of our Interestingly, in the absence of Cu(OAc)2 the reaVciteiwoAnrtidcliedOnnlionet
knowledge. Intrigued by the recent direct remote proceed well in HFIP solvent (see SI, TaDbOleI:S110.,10e3n9t/rCy7C1C90).70T8h6iAs
functionalizations of indole core3 and our studies on C‐H result indicates that more likely Cu(OAc)2 is highly necessary to
functionalizations18, we assumed
a
suitably placed obtain this mild optimized reaction conditions. Other
appropriate, easily removable directing group in indole core transition metal catalysts like [Cp*IrCl2]2 or [Ru(p‐cymene)Cl2]2
would serve the desired site selective thiolation. The challenge either provided no yield or poor yield of desired product (see
of C4‐selectivity for this strategy comes from the possibility of SI, Table S1, entries 20‐21).
C2 selective five membered metallacycle formation over C4 With the optimized conditions in hand, a wide array of indoles
selective six membered metallacycle.8,19 Herein, we report a with various protecting groups and functionalities were
direct Rh(III) catalyzed mild, site selective C4 (sp2)‐thiolation explored (Scheme 2) for C4‐thiolation. The reaction worked
and C2 (sp3)‐methyl thiolation of indole core using removable smoothly with indoles having various electronically and
oxime directing group at the C3 position.
Initially, we explored the thiolation of methyl oxime derivative (Scheme 2, 3a
of N‐benzyl‐3‐acetylindole (1a) with diphenyldisulfide (2a obtained during the exchange of C3‐acetyl oxime with the
sterically variable functional groups at C5, C6 or C7 positions
‐
3e). Further, satisfying product’s yield was
)
using catalytic [Cp*RhCl2]2/AgSbF6 and stoichiometric copper more sterically crowded oximes (Scheme 2, 3f‐3h). To make
acetate in 1,2‐DCE at 80 °C. This resulted in the formation of this reaction more general, various N‐protecting groups were
the 4‐phenylthioindole derivative 3a in 36% isolated yield (see explored. We were also pleased to see that the other N‐
SI, Table S1, entry 1). The efficiency of the reaction was protected indole derivatives offered very good yield with
immediately increased up to 65% isolated yield of the desired admirable degree of selectivity (Scheme 2, 3i‐3m). When the
product by the choice of Ag2CO3 as oxidant (see SI, Table S1, methyl oxime was changed with benzyl oxime, the desired
entry 2). Further, screening of other solvents with different product was obtained in 71% yield (Scheme 2, 3n). Moreover,
polarity, led to lower yield or even no desired product (see SI, the robustness of this C4‐thiolation method was further
Table S1, entries 3‐8). When the other silver salts were tested, extended by the C4‐selenylation of indole derivatives
the yield of the desired product 3a was increased upto 72% for was obtained under otherwise optimized reaction conditions
Ag2O (see SI, Table S1, entries 9‐12). To improve the yield (Scheme 2, 3o 3r). Notably, no desired product was obtained
1 that
‐
further, the reactions were carried out in presence of various when the reaction was carried out on indole substrate with
acetate additives (see SI, Table S1, entries 13‐15). Satisfyingly, free NH group. To observe the scalability of the optimized
the yield of the desired product 3a was improved to 77% in method, 1a was C4‐ thiolated in 1 gram scale with 65%
presence of Cu(OAc)2 (see SI, Table S1, entry 15). Desired isolated yield using only 1 mol% Rh(III) catalyst (Scheme 2, 3a).
product was obtained with comparable yield in Next, the reaction was extended with wide range of disulfides
trifluoroethanol solvent (see SI, Table S1, entry 16). (Scheme 3).
Incidentally, lower yield was obtained when the reaction was
performed in DCE at 40 °C (see SI, Table S1, entry 17).
Furthermore, the efficacy of the reaction was drastically
improved as the reaction proceeded at 40 °C with 83% isolated
yield under HFIP (see SI, Table S1, entry 18) and decreased
amount of oxidant Ag2O.
Scheme 3 Scope of C4‐thiolation with various disulfides. aReaction conditions: 1a (0.1
mmol), 2 (0.15 mmol), [Cp*RhCl2]2 (2.5 mol%), AgSbF6 (10 mol%), Ag2O (0.05 mmol),
Cu(OAc)2 (0.1 mmol), HFIP (0.1 M), 40 °C, 14‐16 h.
Electron dontating groups at meta and para positions of aryl
ring in diaryldisulfides provided moderate to good yield
(Scheme 3, 4a
were obtained for para‐halogen substituted diarylsulfides
(Scheme 3, 4e 4g). Gladly, halogens at the ortho and meta‐
positions of the aryl ring in diaryldisulfides provided excellent
yields (Scheme 3, 4h 4j). Incidentally, the thiolation could be
extended to obtain sterically more crowded
‐4d). Very good yields of the desired products
‐
Scheme 2 Scope using various indoles: aReaction conditions: 1 (0.1 mmol), 2a (0.15
mmol), [Cp*RhCl2]2 (2.5 mol%), AgSbF6 (10 mol%), Ag2O (0.05 mmol), Cu(OAc)2 (0.1
mmol), HFIP (0.1 M), 40 °C, 8‐16 h. Mes = mesityl. breaction was done in 1 gm scale
using 1 mol% Rh(III) catalyst. cdiphenyldiselenide was used in spite of 2a.
‐
naphthylthioindole derivative in good yield (Scheme 3, 4k).
When dibenzyldisulfide was explored under our developed
2 | J. Name., 2012, 00, 1‐3
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conditions, desired thiolated product was isolated in very poor strict inert conditions (Scheme 5iii).This reflectVsiewthAartitclemOnolirnee
DOI: 10.1039/C7CC07086A
yield (Scheme 3, 4l). Unfortunately, no desired or trace likely oxygen has no role in the catalytic cycle of the reaction.
amounts of desired products were obtained from the other
aliphatic disulfides under the optimized conditions.
Scheme 5 Product modifications and control experiments.
Next, to know the radical intermediates involvement, the
reaction was carried out in the presence of stoichiometric
amount of radical trapping reagent 2,2,6,6‐tetramethyl‐1‐
piperidinoxyl (TEMPO) (Scheme 5iv). The reaction proceeded
smoothly and the desired product was isolated in 80% yield.
This result suggests that most likely the reaction does not
Scheme 4 Scope of sp3‐thiolation of 2‐methylindoles: aReaction conditions: 5 (0.1
mmol), 2 (0.15 mmol), [Cp*RhCl2]2 (2.5 mol%), AgSbF6 (10 mol%), Ag2O (0.05 mmol),
Cu(OAc)2 (0.1 mmol), HFIP (0.1 M), 40 °C, 14‐16 h.
follow
a single electron transfer pathway. Moreover,
treatment of indole oxime 1a with CD3OD as the co‐solvent in
the presence or absence of disulfide 2a offered 37‐44%
deuterium incorporation at the C4 position exclusively
(Scheme 5v‐vi). This outcome reveals that the C‐H activation at
the C4 position is reversible. Furthermore, there was no
deuterium incorporation observed at the C2‐methyl of 5a in
the presence or absence of disulfide 2a. However, no desired
product formation was observed during the use of thiophenol
as the thiolating partner.
In comparison to C(sp2)‐H, direct thiolation of C(sp3)‐H remains
a great challenge.20 Significantly, a large number of bioactive
compounds are available with 2‐methylthiolated indole core
(Figure 1). Arguably, one of the easiest methods to construct
such scaffold is direct thiolation at the C2‐methyl group of 2‐
methylindole via C(sp3)‐H activation. To achieve this in
regioselective fashion, we envisaged that the steric interaction
between R1 and C2‐methyl group in indole derivative
5 might
bring the necessary conformation to activate C2‐methyl group
via oxime directed Rh(III) catalyzed C(sp3)‐H bond
functionalization. Gratifyingly, 2‐methylthiolated indole
derivative 6a was obtained from the 2‐methylindole derivative
5a in 65% yields under the optimized reaction conditions
(Scheme 4). Substrate with benzyloxime in the place of
methyloxime also worked smoothly (Scheme 4, 6b) to provide
the desired product. Good reactivity was seen when steric bulk
in keto group was increased (Scheme 4, 6c). To our pleasure,
various diaryldisulfides demonstrated good reactivities,
irrespective of electronic and steric properties of the
Scheme 6 Plausible mechanisms for (sp2) C4 and (sp3) C2‐ methyl thiolation of indoles:
substituents on the phenyl ring (Scheme 4, 6d‐6j). Notably, we
found no systematic influence of substituents in the ortho‐,
meta‐, or para‐positions. Other indole NH‐protecting groups
like para‐methoxybenzyl or allyl group were also found to be
On the basis of the control experiments and previous literature
reports15b,20 a tentative mechanism is proposed (Scheme 6).
The active catalyst
A is generated from its precursor by halide
useful in this thiolation reaction (Scheme 4, 6k‐6l).
Furthermore, removal of protecting group was carried out to
increase the synthetic utility of the developed protocol. The
abstraction with AgSbF6 followed by acetate transfer. Now for
catalytic cycle I, the subsequent formation of 6‐membered
rhodacycle through oxime directed C4‐H bond cleavage
debenzylated product
7 was obtained from compound 3a in
provides intermediate
salt the disulfide facilitates to afford rhodium thiolate
intermediate through the generation of anionic copper
complex.16b,21 Finally, the reductive elimination provides
product 3a with Rh(I) species which is regenerated to Rh(III)
B. Presumably, in presence of copper
very good yield (Scheme 5i). Next, the directing ketoxime
group was removed from 3a in two consecutive steps like
removal of oxime group followed by reverse Friedel‐Crafts
C
acylation to obtain indole
8 (Scheme 5ii).
4,10 Alternatively, the
D
ketoxime group was directly eliminated in single step (Scheme
5ii). Presumably, this reaction does also proceed in similar
fashion. Further, the reaction was not much influenced under
active catalyst via reoxidation with silver salt. Now, indole
oxime 5a can be present in equilibrium of two different
conformers 5A and 5B. Presumably, steric interaction between
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6262; (c) V. Lanke, K. R. Bettadapur and K. R.VPierwaAbrhticule, OOnrlinge.
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C2‐methyl and C3‐ketomethyl (Scheme 6i) group facilitates the
higher stability of the conformation 5B. Evidently for catalytic
cycle II, the more stable conformer 5B provides rhodacycle
intermediate
transfers to rhodacycle
elimination affords compound 6a with Rh(I) species
E
through C(sp3)‐H activation. Next, the thiolate
E
and subsequent reductive
. Finally,
D
9
(a) X. Liu, G. Li, F. Song and J. You, Nat. Commun., 2014, 5,
reoxidation of Rh(I) species to Rh(III) active catalyst occurs by
silver salt.
5030; (b) T. Zhou, B. Lia and B. Wang, Chem. Commun., 2017,
53, 6343.
In summary, we have developed the direct Rh(III) catalyzed
regioselective C4 (sp2) C‐H and C2 (sp3)‐methyl thiolation of
indoles at mild conditions. The easily available and removable
oxime group has been used at the C3 position of indole for the
selectivity. Current studies are focused on the synthesis of
some relevant natural products using this methodology.
This work is supported by the SERB, India (YSS/2014/000383
and PDF/2016/000242). Fellowship received from IIT
Kharagpur (U.K.) is gratefully acknowledged.
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