"quick and click" assembly of functionalised indole rings via metal-promoted cyclative tandem reactions

Article abstract of DOI:10.1039/c4ra12000k

An efficient and convenient synthesis of a variety of decorated indoles using a three-component tandem metal-catalysed process is described. We propose here a new "synthetic kit" that allows for the "quick and click" assembly of indole rings using readily available, and inexpensive starting materials under environmentally friendly reaction conditions. This journal is

Full text of DOI:10.1039/c4ra12000k

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“Quick and click” assembly of functionalised indole
rings via metal-promoted cyclative tandem
reactions†
Cite this: RSC Adv., 2014, 4, 59297
Received 8th October 2014
Accepted 4th November 2014
*
Francesca Capitta, Lidia De Luca and Andrea Porcheddu
DOI: 10.1039/c4ra12000k
www.rsc.org/advances
An efficient and convenient synthesis of a variety of decorated indoles that is too oen neglected or is too weak is the access to a wide
using three-component tandem metal-catalysed process is range of safe and affordable reagents, which should provide the
a
described. We propose here a new “synthetic kit” that allows for the highest degree of molecular diversity in the designed molecular
“quick and click” assembly of indole rings using readily available, and target.
inexpensive starting materials under environmentally friendly reaction
In light of these observations, we intend to develop a tandem
procedure for preparing a library of indoles that have elicited
great interest from both the academic and industrial commu-
nities.4 As part of our continued interest in the preparation of
indole derivatives,5 we intend to focus our attention on the
Castro reaction,6 which employs iodoanilines and alkyne
derivatives as building blocks for preparing densely function-
alized indole rings (Scheme 1).7
conditions.
In recent years, many classic organic reactions have been
revisited and redesigned with a modern twist in order to
increase their efficiency and minimize their environmental
impact.1 The development of new reagents and/or catalysts has
allowed for interesting improvements in the assembly of
complex carbon frameworks that, in some ways, were unimag-
inable just a few decades ago using traditional organic tech-
niques.2 Although many of these processes have become highly
efficient, the tedious and time-consuming problems associated
with nal purication are still difficult to address. These issues
become increasingly complex in the diversity-oriented synthesis
of a molecular target in which a multi-step approach is required
to assemble different components and reagents. Moreover, each
additional step involves a loss of material, which signicantly
reduces the nal yields.
Unfortunately, the low commercial availability of alkynes
and/or their high cost represent some of the main drawbacks of
the Castro indole synthesis. Therefore, we plan to prepare a
library of terminal alkynes starting from readily available
reagents, such as aldehydes, and to subsequently use them
without further purications in the Sonogashira/Castro indole
synthesis via a tandem process (Scheme 1).8
The Bestmann–Ohira reaction,9 a valuable modication of
the Seyferth–Gilbert procedure,10 allows to smoothly access to
In this context, the domino (or tandem or cascade) reactions
attempt to answer all these questions.3 These tandem reactions
are not to be considered as being the sum of already known
individual reactions; rather, they should be viewed as powerful
tool to harmonise the best chemical processes in order to
construct complex molecular scaffolds. In comparison to
traditional single-step processes, the use of cascading reactions
represents a true advantage from an atom-economy point of
view, as these reactions drastically reduce the amount of waste
that needs to be disposed. In a synthetic project, another feature
`
Dipartimento di Chimica e Farmacia, Universita degli Studi di Sassari, via Vienna 2,
07100 Sassari, Italy. E-mail: anpo@uniss.it; Fax: +39 079229559
† Electronic supplementary information (ESI) available: Experimental procedure,
characterization data, copies of 1H NMR, and 13C NMR spectra. See DOI:
10.1039/c4ra12000k
Scheme 1 Revised Castro indole synthesis via a cascade process.
Bestmann–Ohira
Reagent
(BOR)
¼
dimethyl-1-diazo-2-
oxopropylphosphonate.
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terminal alkynes directly from aldehydes or their synthetic steps. Therefore, we performed out a set of experiments
surrogates at room temperature under very mild conditions, designed to determine the best ratio between these two solvents
with a readily available and easy to handle reagent (BOR, in order to further improve the yield of the nal indole. The best
dimethyl-1-diazo-2-oxopropylphosphonate).11 In addition, the results were achieved by preparing the terminal alkynes from
reaction conditions and by-products of this efficient one-carbon the corresponding aldehydes in the minimum volume of MeOH
homologation of aldehydes into alkynes should not interfere (0.4 mL) and by adding Pd(PPh3)2Cl2 (3 mol%), and CuI (5
with the subsequent chemical processes needed for the mol%) to this mixture, closely followed by a solution of N-tosyl-
construction of an indole ring.
2-iodoaniline and NEt3 in CH3CN (2.8 mL) (Scheme 3).
While the conversion of aldehyde into alkynes took place
The alkynylation reaction was conveniently carried out in
methanol by treating benzaldehyde 1 (0.5 mmol) with the smoothly at room temperature (overnight), the following
Bestmann–Ohira reagent (0.6 mmol) in the presence of K2CO3 Sonogashira coupling and metal-catalysed heteroannulation
(1 mmol) overnight at rt. Moreover, 2-iodoaniline 3 (0.5 mmol), were best performed at reux, for at least 16 hours, in order to
NEt3 (0.5 mmol), Pd(PPh3)2Cl2 (3 mol%) and CuI (5 mol%) were ensure full conversion of the reagents. Under such optimised
subsequently added to the crude reaction containing the in situ condition, benzaldehyde 1 and N-tosyl-2-iodoaniline 6 were
generated phenylacetile 2, and the resulting mixture was uneventfully converted to the desired indole product 9 in a 72%
reuxed overnight. The desired indole 5 was recovered only in overall yield. A lower reaction temperature caused an appre-
trace amounts, while the main reaction product was the 2- ciable reduction in the reaction rate and a signicant decrease
ethynylaniline 4 alongside the unreacted 2-iodoanilina 3. We in the chemical yields. In the absence of either a Pd- or Cu-
have also screened other metal catalytic systems with unfortu- based catalyst, the reaction failed to provide the expected
nately no success results.
indole product.
These preliminary results suggested that cyclization was the
With a clearer picture of the reaction parameters, we then
critical stage of the entire process and that it was necessary to tested the ratio of aldehyde/aniline with the aim of further
match the reaction conditions used in the rst step and the last improving the effectiveness of this reaction. Interestingly, a
step. We rst tried to perform the indole synthesis by changing maximum indole yield of 84% was obtained when the aldehyde
either K2CO3 or NEt3, but also by using different bases; however, proportion was switched from 1 to 1.5 equivalents.
the nal result did not improve. In addition, the use of other
The reaction time is a key parameter of the process as pro-
solvents, or a combination thereof, was examined, but they were tected 2-ethynylaniline and indole 9 have a very close Rf and
found to be ineffective in promoting the cyclisation step. Only complete conversion of the alkyne intermediate 2 into the cor-
upon switching from 2-iodoaniline to the N-tosyl-2-iodoaniline responding indole 9 is essential for a simplied work-up
6 did the indole yield improve dramatically, up to 61% procedure.
(Scheme 2, pathway 2).12 The use of different N-protecting
It is interesting to note that the exposition of this one-pot/
groups (Scheme 2, iodoanilines 7 and 8) resulted in either low alkynylation/coupling/cyclization process to microwave irradi-
indole yields or recovered starting material yields (Scheme 2, ation (MWI) allowed the reactions to be completed in less than 2
products 10 and 11).
hours, in yields comparable to those that were thermally heated
Interestingly, when this one-pot three-steps construction of (Scheme 4).13
an indole ring was split into two different and consecutive
With these optimum conditions in hand, we have extended
reactions, we observed complete reagent conversion by using this synthetic protocol to other aldehyde derivatives in order to
MeOH in the rst step and acetonitrile in the remaining two investigate the substrate scope of the reaction. The results are
summarized in Scheme 4. Under standard conditions, the
coupling reactions of 6 with various terminal alkynes, generated
in situ from both aromatic and aliphatic aldehydes, provided a
diverse array of densely functionalised indoles in good to
excellent yields (Scheme 4, products 9–35).
Scheme 2 Metal-catalysed tandem indole synthesis from o-iodoa-
niline derivatives and benzaldehyde. Reagents and conditions: (a) BOR,
K2CO3, MeOH, rt, overnight. (b) Pd(PPh3)2Cl2 (3 mol%), CuI (5 mol%),
NEt3, reflux, overnight.
Scheme 3 Optimisation of the reaction conditions.
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signicantly hamper the reaction, which proceeded smoothly
leading to the expected products in good yields. Conversely, the
reaction of a strongly electron-decient aldehyde, such as 4-
nitrobenzaldehyde with 6, failed to give the corresponding
product (Scheme 4, product 25) despite increasing catalyst
loading (up to 10 mol%) and/or prolonging the reaction time.
A wide range of other sensitive functionalities were tolerated,
including uoro- (Scheme 4, indole 19), chloro- (Scheme 4,
indoles 20–22), carbonyl- (Scheme 4, indole 23), cyano- (Scheme
4, indole 24), olen (Scheme 4, indole 26), which are amenable
to further manipulations.
Additionally, the procedure allowed for a quick and easy
assembly of indole derivatives having heteroaryl residues such
as the 2-thienyl, and 4-pyridyl rings (Scheme 4, products 27–28).
This methodology was not only restricted to aromatic alde-
hydes; good indole yields were also obtained for aliphatic eno-
lisable linear aldehydes as well as for cyclic ones (Scheme 4,
indoles 29–31). These reaction conditions were also applicable
to sterically hindered aldehydes such as pivalaldehyde, which
led to indoles 32 in a 71% isolated yield.
To broaden the substrate scope further, the reaction between
4-methylbenzaldehyde and a representative set of diversely
substituted 2-iodoanilines was next successfully scrutinised and
the results are compiled in Scheme 4 (indoles 33–35). The
successful preparation of 2-substituted indoles highlighted that
the previous Sonogashira coupling step selectively occurred at
the ortho-position, triggering the subsequent indole formation.
The generality of this method was also demonstrated by
applying the conditions optimised for 2-substituted indole
substrates to a set of representative internal alkynes and the
results are summarized in Scheme 5. Upon the completion of
the reaction between aldehyde 36–39 and BOR, the resulting
terminal alkynes 40–43 were then subjected to a cascade reac-
tion, rst with iodoarenes 44–47 in the presence of Pd(PPh3)2Cl2
(3 mol%) and CuI (5 mol%) and then with N-tosyl-2-iodoaniline
6, using Pd(OAc)2 (5 mol%) and LiCl (0.33 mmol) as catalyst
system. A careful study of the reaction parameters revealed that
the presence of a source of chloride anions (either LiCl or n-
Bu4NCl) was benecial to the reaction, as it maximised the
indole yields.4i A variety of in situ prepared internal alkynes 48–
51 proved to be efficient partners for this tandem process,
thereby leading to the expected indoles 52–55 in various iso-
lated yields (44–77%).
Scheme 4 Substrate scope of the tandem metal-catalysed synthesis
of indole derivatives. Reaction conditions were as follow: (a) aldehyde
(0.5 mmol), BOR (0.6 mmol), K2CO3 (1 mmol), MeOH (0.4 mL), 60 ꢀ
C
(MWI), 45 min. (b) N-tosyl-2-iodoaniline derivative (0.33 mmol), NEt3
(0.5 mmol), Pd(PPh3)2Cl2 (3 mol%), CuI (5 mol%), CH3CN (2.8 mL) 100
ꢀC for 1 h. (c) Yield of isolated pure products.
Conclusions
In summary, N-protected 2-iodoanilines were smoothly trans-
formed into indoles by a sequential Castro reaction, employing
aldehydes instead of the commonly used alkynes.
Bestmann–Ohira reagent was fruitfully used to convert (in
situ) a variety of aldehydes into their corresponding homolo-
gated terminal alkynes. This tandem approach allows for the
“quick and easy” assembly of an array of multi-substituted
indole derivatives by taking advantage of the structural diver-
sity of aldehydes. The entire process is promoted by a Pd- and
Cu-based catalyst, is dramatically accelerated by microwave
irradiation heating.
Scheme 5 Tandem heteroannulation reaction with in situ generated
internal alkynes. (a) Yield of isolated pure products.
In general, the presence of electron-donating, and electron-
withdrawing substituents on the aromatic aldehydes, with the
exception of the carbonyl group (Scheme 4, product 23), did not
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Financial support from Regione Autonoma della Sardegna
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