Tris(trimethylsilyl)silane and visible-light irradiation: A new metal- and additive-free photochemical process for the synthesis of indoles and oxindoles

Article abstract of DOI:10.1039/c5cc06329a

A combined tris(trimethylsilyl)silane and visible-light-promoted intramolecular reductive cyclization protocol for the synthesis of indoles and oxindoles has been developed. This straightforward and efficient method shows tolerance towards a broad spectru

Full text of DOI:10.1039/c5cc06329a

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Tris(trimethylsilyl)silane and Visible-Light Irradiation: A New
Metal- and Additive-Free Photochemical Process for the Synthesis
of Indoles and Oxindoles
Received 00th January 20xx,
Accepted 00th January 20xx
Gustavo Piva da Silva, Akbar Ali, Rodrigo César da Silva, Hao Jiang and Márcio W. Paixão*
DOI: 10.1039/x0xx00000x
www.rsc.org/
substrates association.[9]
A combined tris(trimethylsilyl)silane and visible-light-promoted
intramolecular reductive cyclization protocol for the synthesis of
indoles and oxindoles has been developed. This straightforward
and efficient method shows tolerance towards a broad spectrum
of functional-groups and enables rapid and practical synthesis of
functionalized nitrogen-based heterocycles in high yields under
additive and metal-free, mild photochemical conditions.
Nevertheless, in spite of the advances that have been achieved
in this field, photochemical strategies which allow the synthesis of
indoles and oxindoles skeleton have been scarcely explored.[10]
Therefore, the exploitation of new strategies and reagents to
develop straightforward and greener methodologies for the rapid
assembly of N-heterocycles is highly desirable.[11] An important
step in that direction was given by Jørgensen’s group, which
reported the radical-mediated reduction of aryl and alkyl iodides
using TTMSS under visible-light irradiation.[12]
Based in our experience in the synthesis of N-heterocyclic
compounds[13] and in the recent results disclosed by Jørgensen et
al., herein we are pleased to report the use of a mild and
operationally simple strategy for the synthesis of indoles and
Nitrogen-based heterocycles are a class of compounds of great
biological importance that also play a pivotal role in the Organic
Synthesis field.[1] In particular, indoles and oxindoles derivatives
have been attracting considerable interest from both synthetic and
medicinal chemistry due to the presence of these privileged
scaffolds in many important drugs and naturally occurring
compounds. [2]
oxindoles
frameworks
through
the
cyclization
radical-promoted
of 2-
dehalogenation/intramolecular
Due to the importance of these moieties, substantial efforts
have been devoted to the development of new synthetic
methodologies for the assembly of indole and oxindole ring
systems.[3] In addition to the classical synthetic methods, a handful
of strategies based on the use of both transition-metal catalysis and
oxidative conditions have been studied.[4]
halobenzenesulfonamides bearing terminal alkynes (Scheme 1).
Indole Synthesis
X
15 W household bulb
(compact fluorescent light)
R1
R1
N
R2
Recently, the use of visible-light as a source of energy for
promoting chemical transformation has emerged as a hot topic in
organic synthesis.[5] In this sense, several research groups have
demonstrated the versatility of Ru(II)- and Ir(III)-based complexes in
the presence of additives under visible-light irradiation,[6] as well
N
R2
TTMSS, Solvent,
Time, r.t.
Oxindole Synthesis
X
15 W household bulb
(compact fluorescent light)
O
R1
1
as photo-organocatalysts,[7]
to promote
a
variety of
R
O
N
R2
N
R2
TTMSS, Solvent,
Time, r.t.
transformations initiated by SETs.
The great interest in the development of environmentally
benign technologies and procedures in the last few years have
driven efforts in order to improve the processes of radical
generation. These improvements include avoiding the use of
expensive transition-metal catalysts, toxic reagents, as well as
explosive oxidants.[8] In this regard, Melchiorre’s group has explored
the photochemical reactivity of photon-absorbing electron donor-
acceptor complexes (EDA) generated in the ground state upon
Key Features
Mild Reaction Condition
Inexpensive Reagent
Metal and Additive-Free
Great Functional Group Tolerance
Scheme 1. General scheme for the synthesis of indole and
oxaindole derivatives.
We started our investigation on the dehalogenation followed by
5-exo-dig cyclization reaction of substrated 1a by evaluating the
effect of different parameters on the reaction conditions, such as
solvent, N-protecting group and halide substituent. The results
showed that both the nature of the solvent and amount of the
TTMSS were critical to obtain the indole product in high yields
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(under the optimized reaction condition product 2a was obtained in experiments. We started our mechanistic investigations through the
92% yield, see SI for complete details). reaction of aryl iodide 1a in the absence of TTMSS in optimized
We next turned our attention to the investigation of the reaction conditions, and, as expected, no conversion to the indole
scope and generality of the method by subjecting a wide range of 2a was observed. The reaction performed in the absence of light
substituted 2-halobenzenesulfonamides containing terminal also did not afford the desired product, with the starting material
alkynes to the optimized reaction conditions. Substrates bearing being fully recovered. Additionally, experiments with sequential
either electron-withdrawing or electron-donating groups furnished periods of irradiation and darkness support the hypothesis that a
the desired indole 2d-q in moderated to excellent yields (45-85%), radical chain process is not the predominant pathway under the
with electron-donating groups at different positions of the aromatic reaction conditions (see SI for complete details).
ring affording higher yields than those bearing electron-
Considering that a C-centered radical is likely the key
withdrawing groups (Scheme 2). It should be noted that besides to intermediate of the process, the presence of a radical scavenger
the isolated products, the unreacted starting materials are easily should inhibit the intramolecular reductive cyclization process. As
recovered.
expected, when a radical quencher such as 2,2,6,6-tetramethyl-1-
As seen in Scheme 2, substrates bearing additional halogens piperidinyloxy (TEMPO) was added to the system, a complete
such as Br and Cl, functionalities that allow subsequent inhibition of the reductive reaction was observed, once more
functionalizations by transition metal mediated cross-coupling strongly suggesting a radical pathway for this transformations.
reactions,[14] underwent the reaction smoothly, affording the
These experiments allowed us to reach the conclusion that
corresponding products in moderated yields (53% and 62% continuous visible-light irradiation and the silicon reagent are
respectively). Due to the interesting properties of fluorine atoms required to yield the indole scaffold under a radical reaction
and the trifluoromethyl (CF3) moiety in medicinal chemistry, mechanism. To further identify the hydrogen atom source in this
agrochemical and intelligent materials,[15] we envisioned the process, we performed labeling studies using MeOD (5 equiv) in the
synthesis of the indole scaffolds containing these important reaction (Scheme 3), and the desired product was obtained with
subunits employing our newly developed protocol. The reaction of 70% of deuterium incorporation.
the fluoro derivative led to isolation of product 2g in 50% yield,
D
while the one bearing the trifluoromethyl group similarly gave rise
to 51% yield. Furthermore, the feasibility of the cyclization process
showed no dependence on the substitution patterns (e.g., para-,
meta-,) of the aromatic ring of the substrates (compare: 2h vs 2i).
Notably, this process was also compatible with ester, ketone and
aldehyde functional groups attached to the aromatic ring (compare
2j vs 2k vs 2l), and interestingly, unprotected alcohols could also be
readily employed with no great effect in the yield (2m and 2n).
I
15 W household bulb
(compact fluorescent light)
N
N
TTMSS, MeCN/MeOD, 4h, r.t.
1a
Ts
Ts
Scheme 3. Deuterium exchange experiment.
Additionally, aiming to analyze which of these reagents are
involved in the rate-determining step, kinetic studies were
performed.[16] The kinetic studies were conducted for the reaction
between substrate 1a and the TTMSS having acetonitrile as solvent
(standard reaction under the optimized conditions). To our delight,
we could observe from these experiments that the kinetic profile
give us further evidence that the EDA-complex excitation is the
rate-limiting step of this photochemical protocol.[17]
X
15 W household bulb
R
(compact fluorescent light)
R
N
N
TTMSS, MeCN/EtOH, 4h, r.t.
Ts
Ts
2a and 2d-q
1a and 1d-q
Br
Cl
N
Based on aforementioned results and previous reports, a
plausible reaction mechanism was proposed (Scheme 4). Initially, a
silyl-centered radical (Me3Si)3Si*) is generated under visible-light
irradiation.[18] The formation of the silyl radical may occur through
a sequence of events that consists firstly on the formation of a
photon-absorbing electron donor-acceptor complex by the
association of the aryl substrate with TTMSS (A)[19] followed by the
visible-light-promoted excitation of (A), giving rise to (B). After an
energy-transfer step, the Si-based radical (C) is formed[20] and
subsequently promotes the single-electron reduction of the iodide
substrate, generating the aryl radical (D). Next, a kinetically favored
5-exo-dig cyclization occurs, leading to the formation of a vinyl
radical (E). Finally, an aromatization step via a 1,3-H shift produces
an allylic radical (F), which further undergoes the proton abstraction
of EtOH, yielding the corresponding heterocyclic product 2a.
N
Ts
N
Ts
N
Cl
Ts
2f, X=I, 70%
Ts
2e, X=I, 62%
2a, X=I, 92%
2d, X=I, 53%
O
F3C
F
O
N
Ts
N
Ts
N
F3C
N
Ts
Ts
2h, X=I, 51%
2i, X=Br, 52%
2j, X=I, 65%
OH
2g, X=Br, 50%
O
O
HO
H
N
N
N
N
Ts
2m, X=I, 82%
Ts
Ts
Ts
2k, X=I, 50%
2l, X=I, 45%
MeO
2n, X=I, 52%
Me
O
N
N
O
N
Ts
2o, X=I, 85%
Ts
Ts
2q, X=I, 82%
2p, X=I, 85%
Scheme 2. Substrate scope for the visible-light-mediated indole synthesis.
Reaction conditions: 1 equiv of TTMSS, 5 equiv of EtOH, and 0.4 mL of
MeCN, irradiation with a 15 W household bulb. Yields of isolated products.
Aiming to gain further insight into the mechanism of the visible-
light-induced reaction, we performed additional control
2 | J. Name., 2012, 00, 1-3
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ring such as Cl and F were tolerated under the reaction conditions,
giving the desired products in 45-65% yield (4c, 4d and 4e).
Scheme 4. Proposed mechanism for the visible-light-mediated
indole synthesis.
With the aim of demonstrating the potential scalability of this
photochemical approach for the synthesis of indoles, the reaction
to produce indole 2a was performed on a 5 mmol scale, with the
desired product being isolated in 87% yield. Having in mind the
high degree of functionalization displayed by product 2a, further
manipulations were briefly explored in order to demonstrate its
synthetic utility. In order to accomplish that, initially 3-methylindole
Scheme 6. Substrate scope for the visible-light-mediated oxindole
synthesis. Reaction conditions: 2 equiv of TTMSS, and 0.4 mL of
MeCN, irradiation with a 15 W household bulb. Yields of isolated
products.
We then explored the scope of the reaction towards the
2a was treated with DDQ in aqueous THF to obtain the presence of a sensitive and strong electron-withdrawing group,
corresponding aldehyde in 75% yield.[21] Next, the N-Tosyl and, gratifyingly, product 4f bering an ester moiety afforded the
protecting group was further removed through a simple known desired product in 51% of yield. Substrates containing electron-
procedure, affording the structure bearing a free amino group in donating groups reacted smoothly under the visible-light
95% yield. [22] Thus, after two simple synthetic manipulations it was conditions, giving rise to the corresponding oxindoles 4g and 4h in
possible to isolate the 1H-indole-3-carboxaldehyde, which could be 70% and 72% yield, respectively. Furthermore, the reaction of
a valuable precursor for the synthesis of several FDA approved substrate 3i revealed that the insertion of a second donating group
drugs,[1f] in 71% overall yield (Scheme 5).
on the aromatic ring has a negative effect in terms of chemical
outcome (4i was obtained in 45% of yield).
In summary, we have developed a straightforward and highly
efficient photochemical protocol leading to the formation of
nitrogen-containing heterocycles. This metal- and additive-free
protocol is carried out by allying visible-light irradiation from simple
household fluorescent light bulbs and the use of
tris(trimethylsilyl)silane as hydride source. The high functional-
group tolerance, employment of readily available starting materials
and scalability of the reaction substantiate the versatility of this
method, making it attractive for both academia and industry.
Further studies focused on the application of this transformation
towards the synthesis of others heterocyclic compounds, as well as
a thorough mechanistic study of this protocol are underway and will
be reported in due course.
Scheme 5
In light of the encouraging results for the synthesis of indoles,
we envisioned that the visible-light-induced photochemical strategy
could be also employed for the synthesis of oxindoles, another
important class of heterocyclic compounds,[23] employing N-
protected 2-halophenylacrylamides as substrates. To our delight,
this procedure also worked for the synthesis of the oxindole
scaffold, and after optimization, the use of 2 equiv of TTMSS in
acetonitrile was found to be appropriate to obtain the product in
good yield.
Having identified the optimal conditions for the reaction, we
sought to evaluate its scope and limitations, starting with the effect
of different alkyl groups attached to the nitrogen atom on the
substrate (Scheme 6). On the basis of this study, we were able to
observe that the substrate containing N-benzyl moiety afforded the
5-exo-trig cyclization product in a higher yield when compared with
its N-methyl analogue (4a vs 4b, 80% and 54% yield, respectively).
Interestingly, substrates bearing halogen atoms on the aromatic
G. P. da Silva and R. C. da Siva are grateful to CNPq for their
fellowships. A. Ali acknowledges TWAS-CNPq for the fellowship. The
authors also kindly acknowledge financial support from CNPq (INCT-
Catálise and 475448/2013-8), CAPES and FAPESP.
Notes and references
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