Versatile synthesis of polyfunctionalized carbazoles from (3-iodoindol-2-yl)butynols via a gold-catalyzed intramolecular iodine-transfer reaction

Article abstract of DOI:10.1021/acscatal.5b00471

The controlled gold-catalyzed preparation of 3-iodo 2,4,6-trisubstituted 9H-carbazoles has been developed by starting from (3-iodoindol-2-yl)butynols. These results could be explained through an initial 6-endo-dig alkyne carbocyclization by chemo- and regiospecific attack of the C3-indole position at the external alkyne carbon followed by a stepwise 1,3-iodine transfer and dehydration. This reaction outcome for the gold-catalyzed transformation of (3-iodoindol-2-yl)alkynols sharply contrasts with that observed for conventional metal-catalyzed processes of iodoarenes, because iodine transfer is feasible. This selective reaction has been studied experimentally; additionally, its mechanism has been investigated by means of density functional theory calculations.

Full text of DOI:10.1021/acscatal.5b00471

Research Article
pubs.acs.org/acscatalysis
Versatile Synthesis of Polyfunctionalized Carbazoles from (3-
Iodoindol-2-yl)butynols via a Gold-Catalyzed Intramolecular Iodine-
Transfer Reaction
,
†
,‡
†
†
dez,§
Benito Alcaide,* Pedro Almendros,* Jose
́
M. Alonso, Eduardo Busto, Israel Fernan
́
†
†
M. Pilar Ruiz, and Gulinigaer Xiaokaiti
†
Grupo de Lactamas y Heterociclos Bioactivos, Departamento de Quım
ica, Universidad Complutense de Madrid, 28040 Madrid, Spain
Instituto de Quımica Organica General, CSIC, Juan de la Cierva 3, 28006 Madrid, Spain
Departamento de Quımica Organica I, Facultad de Quımica, Universidad Complutense de Madrid, 28040 Madrid, Spain
́
́
ica Organica I, Unidad Asociada al CSIC, Facultad de
Quım
́
‡
́
́
§
́
́
́
*
S Supporting Information
ABSTRACT: The controlled gold-catalyzed preparation of 3-iodo
,4,6-trisubstituted 9H-carbazoles has been developed by starting
2
from (3-iodoindol-2-yl)butynols. These results could be explained
through an initial 6-endo-dig alkyne carbocyclization by chemo- and
regiospecific attack of the C3-indole position at the external alkyne
carbon followed by a stepwise 1,3-iodine transfer and dehydration.
This reaction outcome for the gold-catalyzed transformation of (3-
iodoindol-2-yl)alkynols sharply contrasts with that observed for
conventional metal-catalyzed processes of iodoarenes, because iodine
transfer is feasible. This selective reaction has been studied experimentally; additionally, its mechanism has been investigated by
means of density functional theory calculations.
KEYWORDS: alkynes, cyclization, density functional calculations, gold, iodine
INTRODUCTION
Scheme 1. Reactivity of 1-(Indol-2-yl)-3-alkyn-1-ols under
Gold Catalysis
■
The development of new chemical transformations based on
the catalytic functionalization of starting materials, where as
many of the atoms of the reactants as possible should end up in1
the final product, has great potential in organic synthesis.
Because of the low atom economy associated with the classica2l
cross-coupling reactions of saturated or unsaturated halides,
novel activation modes of C−halogen bonds provide a prime
opportunity to develop reactions paying attention to synthetic
efficiency. The most successful approaches in which the
halogen is reintegrated into the final product involve palladium
3
,4
̈
́
catalysis; only Furstner et al., Hashmi et al., and Gonzalez et
al. have reported gold-catalyzed iodine transfer in 1-
We were curious about the attractive possibility of divergent
reactivity if the reactive C3-indole position were substituted
with an iodine atom. We envisaged that, by blocking the more
nucleophilic carbon of the indole ring with an easily displaced
iodine moiety, the C2 side may become somewhat more
reactive. We wish to report herein that to our surprise the
reaction outcome for the gold-catalyzed transformation of (3-
iodoindol-2-yl)alkynols is in sharp contrast with that typically
observed for conventional metal-catalyzed processes involving
iodoarenes, because iodine transfer is feasible, therefore
5
iodoalkynes.
The carbazole nucleus is both widely distributed in nature
and constitutes a key molecular motif in materials science. For
these reasons, great interest has been focused on the
development of new and efficient methods for the synthesis
of this valuable heterocycle. On the other hand, gold salts have
been particularly successful at effecting activation of alkynes
6
7
because of their powerful, soft Lewis acidic nature. The gold-
catalyzed reactions using (indol-2-yl)alkynols as substrate8s
constitute an almost unexplored field of noble-metal catalysis.
The only report on the gold-catalyzed reaction of 3-alkyn-1-ol-
appended indoles is the benzannulation of 1-(indol-2-yl)-39-
alkyn-1-ols to provide substituted carbazoles (Scheme 1, eq 1).
Received: March 5, 2015
Revised: April 23, 2015
©
XXXX American Chemical Society
3417
DOI: 10.1021/acscatal.5b00471
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fulfilling the atom economy criterion for efficient reactions
Research Article
1
haloaldehydes 1 resulted in a loss of the C−X bond through
exposure to the reducing metal.
(
Scheme 1, eq 2). The known reports on iodine transfer
reactions catalyzed by gold complexes used 1-iodoalkynes as
starting materials and are based on vinylidine chemistry. Our
report is conceptually different (Scheme 1, eq 2).
Our next efforts focused on the application of gold catalysis
to the selective construction of functionalized carbazoles
starting from alkynols bearing a 3-iodoindole moiety. (3-
Iodoindol-2-yl)butynol 2a was chosen as a model substrate for
gold-catalyzed oxycyclization reactions. Attempts to generate a
tricyclic iodinated structure from 2a by using Au(III) catalysis
5
RESULTS AND DISCUSSION
■
To explore the effects of various substrates on gold-catalyzed
reactions, a number of new (3-haloindol-2-yl)butynols were
synthesized. The starting materials, alkynes 2a−j, were
prepared from the corresponding 3-haloindole-2-carbaldehydes
via zinc- or indium-mediated Barbier-type carbonyl propargy-
lation reactions in aqueous media, in a modification of
failed, because in the presence of AuCl
not only the
3
1
1
iodocarbazole 3a but also the carbazole 4a, the benzoanne-
lation adduct, was exclusively obtained, resulting in failure of
the carbocyclization−iodination sequence because the iodine
transfer step was missed (Scheme 3). Despite this failure,
alkynol 2a was exposed at room temperature to different gold
salts under Au(I) catalysis. However, complex reaction mixtures
were obtained. Apparently, (3-iodoindol-2-yl)butynols deriva-
tives 2 have inadequate reactivity for joint participation in the
benzoannelation−iodine transfer sequence. Fortunately, an
interesting and useful temperature effect emerged from the
observation that cooling the reaction mixture resulted in the
formation of the required 3-iodocarbazole 3a. Our catalyst
screening led to the identification of Gagosz’ catalyst
[(Ph P)AuNTf ] as the most suitable promoter. AuCl and
10
previously described methodologies. A model reaction was
carried out by the treatment of a THF/H O (1/1) solution of
2
aldehyde 1a with (3-bromobut-1-ynyl)benzene in the presence
of indium, to give regioselectively (3-iodoindol-2-yl)butynol 2b
in 31% yield. Similar results were obtained for the indium-
mediated propargylation reaction of 1a in the system solvent
THF/NH Cl (aqueous saturated), but the reaction times were
4
considerably lengthened. We were pleased to find that the
addition of NH Cl (aqueous saturated) in a 1/1.5 ratio over 1 h
4
to a solution of 2b and (3-bromobut-1-ynyl)benzene in THF/
3
2
H O (1/1) improved the efficiency (60% yield). Indium was
[(Ph P)Au(OTf)] were less effective for the carbocyclization−
2
3
the metal of choice for the preparation of nonterminal butynols
iodine transfer sequence. 1,2-Dichloroethane (DCE) was
selected as the solvent of choice. It was found that
2b,c,e,f,h, while a conversion enhancement for the carbonyl
propargylation with propargyl bromide itself was observed
using zinc (Scheme 2). The reaction progress was followed by
[
(Ph P)AuNTf ] is an effective reagent for the iodocarbocyc-
3 2
lization of indole-linked alkynol 2a at −30 °C to afford the
iodobenzene-fused indole 3a in 81% yield in a totally selective
fashion (Scheme 3).
Scheme 2. Regioselective Preparation of (3-Haloindol-2-
a
yl)butynols 2a−j
In order to demonstrate the synthetic utility of this selective
benzoannelation−iodine transfer process in the synthesis of
substituted 3-iodocarbazole derivatives, seven additional 3-
iodoindole-linked alkynols 2b−h were employed. Both aliphatic
and aromatic substitutions were well tolerated. The steric
properties of the substituents in the acetylenic moiety did not
significantly affect the reaction yield, with 4-aryl-functionalized
but-3-yn-1-ols 2b,e performing well in the 3-iodocarbazole
formation (Scheme 4). The placement of a chlorine atom or a
methoxy group at the C5 position of the indole ring was
tolerated in the presence of [(Ph P)AuNTf ] (Scheme 4),
3
2
providing a handle for subsequent orthogonal reactivity. 3-Iodo
2
,4,6-trisubstituted 9H-carbazole compounds 3b−g were
exclusively generated under these conditions (Scheme 4).
Complete conversion was observed by TLC and H NMR
1
analysis of the crude reaction mixtures of alkynols 2, and no
side products were usually detected. Unfortunately, some
decomposition was observed on sensitive 3-iodocarbazoles 3
during purification by flash chromatography, which may be
responsible for the moderate isolated yields in some cases (3d−
g). In particular, 3-iodocarbazoles 3d,h were very unstable
products. Adduct 3d was isolated in very low yield, while the
characterization of 3h was not possible. The overall trans-
formation of (3-iodoindol-2-yl)butynols into 3-iodocarbazoles
a
Reagents and conditions: (i) 200 mol % of metal (In or Zn), THF/
H O/NH Cl (aqueous saturated, 1/1/1.5), from 0 °C to room
temperature; 2a (Zn), 48 h; 2b (In), 48 h; 2c (In), 48 h; 2d (Zn), 2 h;
2
2
2
4
e (In), 72 h; 2f (In), 16 h; 2g (Zn), 2 h; 2h (In), 15 h; 2i (Zn), 2 h;
j (Zn), 2 h.
TLC but could also be noted by the disappearance of the green
solution, which turns yellow. Prolonged reaction times of
Scheme 3. Cyclization of (3-Iodoindol-2-yl)butynol 2a under Gold(III) or Gold(I) Catalysis
3
418
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Scheme 4. Gold-Catalyzed Preparation of 3-Iodocarbazoles
3
gold salt to the triple bond. A subsequent nucleophilic attack of
the C3-indole position onto the above complex delivers
auratetrahydrocarbazole 6 by 6-endo carbocyclization. This
mechanistic pathway becomes attractive due to the stability of
the resulting iminium cation. In the next step of the cascade,
cationic intermediate 6 liberates the gold catalyst through
formation of iododihydrocarbazoles 7 via 1,3-iodine shift.
Finally, dehydration yields 3-iodocarbazoles 3 (Scheme 5).
In order to test if the iodine transfer is an intramolecular or
an intermolecular process, a crossover experiment was planned.
Consequently, the reaction of a 1/1 mixture of 1-(3-iodo-5-
methoxy-1-methyl-1H-indol-2-yl)-2-methyl-4-phenylbut-3-yn-
a
1
2
-ol (iodoalkynol 2e) and 1-(5-chloro-1-methyl-1H-indol-2-yl)-
-methyl-4-phenylbut-3-yn-1-ol (alkynol without iodine sub-
a
Conditions: 3b, −30 °C, 18 h; 3c, −30 °C, 19 h; 3d, 0 °C, 168 h; 3e,
stituent) was carried out. In this event, the gold-catalyzed
treatment of both alkynylindoles generated 3-iodocarbazole 3e
and carbazole 4h (Scheme 6). As the above crossover
experiment did not show any appreciable formation of the
chloro-substituted iodocarbazole, it can be concluded that the
iodine shift occurs intramolecularly.
Density functional theory (DFT) calculations have been
carried out at the dispersion-corrected PCM(DCE)-B3LYP-
D3/def-TZVP//B3LYP-D3/def2-SVP level to gain more
insight into the reaction mechanism of the aforementioned
gold(I)-catalyzed carbocyclization/iodine transfer reactions of
−
20 °C, 17 h; 3f, −20 °C, 18 h; 3g, 0 °C, 168 h; 3h, −20 °C, 72 h.
can be roughly explained through a 6-endo-dig alkyne
carbocyclization by chemo- and regiospecific attack of the
C3-indole carbon to the external alkyne carbon followed by 1,3-
iodine transfer and dehydration.
The proposed reaction pathway summarized in Scheme 5
looks valid for the formation of 3-iodocarbazoles 3 from (3-
12
Scheme 5. Mechanistic Explanation for the Gold-Catalyzed
Iodocarbocyclization Reaction of (3-Iodoindol-2-yl)butynols
2
3
-iodoindole-linked alkynols 2. To this end, we have computed
the reaction profile involving alkynol 2M and the model gold(I)
catalyst [AuPMe3]+ (see Figure 1, which gathers the
corresponding relative free energies, ΔG298, in dichloroethane
solution).
As proposed above, the process begins with the exergonic
coordination of the catalyst to the triple bond of 2M, thus
forming the initial cationic complex INT1 (ΔG
= −14.7
R,298
kcal/mol). From this species two alternative nucleophilic
additions may occur, namely indole-C2 or indole-C3 addition
to the gold(I)-coordinated external alkyne carbon atom. Our
calculations suggest that, although the activation barriers of
both processes (via TS1-A and TS1-B, respectively) are quite
⧧
similar (ΔΔG = 0.3 kcal/mol), the formation of intermediate
INT2-B is clearly thermodynamically favored over the spiranic
species INT2-A. Moreover, whereas the formation of INT2-B
is exergonic (ΔG
= −3.4 kcal/mol), the process leading to
R,298
INT2-A is endergonic (ΔG
= 7.0 kcal/mol), which is
R,298
hardly compatible with a process occurring at low temperature.
Therefore, it can be concluded that the initial carbocyclization
reaction does not involve a 5-endo-dig cyclization from the
indole-C2 carbon atom but a 6-endo-dig process involving the
indole-C3 carbon atom. Unfortunately, we were not able to
locate on the potential energy surface a transition state
iodoindol-2-yl)butynols 2. The initial step consists of the
formation of the gold complex 5, through coordination of the
Scheme 6. Crossover Experiment To Assess the Intramolecular Nature of the Gold-Catalyzed Iodine Transfer
3
419
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Research Article
+
Figure 1. Computed reaction profile for the process involving alkynol 2M and the model gold(I) catalyst [Au(PMe )] (which is represented as
3
+
[
Au] ). Relative free energies (ΔG298, at 298 K) and bond distances are given in kcal/mol and Å, respectively. All data have been computed at the
PCM(DCE)-B3LYP-D3/def-TZVP//B3LYP-D3/def2-SVP level.
Scheme 7. Functionalization of 3-Iodocarbazoles through Suzuki Cross-Coupling
connecting INT2-B directly to intermediate 7 via a 1,3-iodine
shift (Scheme 5). Instead, we found that a dehydration reaction
occurs first, leading to INT3 in a highly exergonic process
observed in iodoalkynols did not occur in their chlorine or
bromine counterparts. The treatment of alkynols 2i,j under
otherwise identical conditions resulted in the recovery of the
starting material along with a complicated mixture of products.
13
(
ΔG
= −20.9 kcal/mol). The exergonicity of this
R,298
dehydration reaction is strongly related to the stabilization of
the positive charge in INT3 by conjugation (see some
resonance structures in Figure 1). Then, INT5 is formed
through two consecutive 1,2-iodine shifts via transition states
3
-Iodocarbazoles 3 can undergo a further derivatization
reaction through a Pd-catalyzed process to give the
corresponding Suzuki coupling products. The Pd-catalyzed
coupling between 3-iodocarbazole derivatives 3a,e and 4-
tolylboronic acid afforded products 8a,e; even Suzuki coupling
on the 3-iodocarbazole precursor where the iodine is flanked by
two substituents works efficiently (Scheme 7). The capture of
the iodine in a tandem sequence is more troublesome, because
the reaction of (3-iodoindol-2-yl)butynols 2a,e with 4-
tolylboronic acid under a bimetallic Au−Pd catalytic system
resulted in a complicated mixture.
⧧
⧧
TS2 (ΔG = 9.0 kcal/mol) and TS3 (ΔG = 1.8 kcal/mol) in a
strongly exergonic transformation (ΔG = −34.1 kcal/mol
R,298
from INT3) instead of occurring via a direct 1,3-iodine shift.
Finally, the transformation ends up with the decoordination of
the gold(I) catalyst in INT5 to produce the experimentally
observed 3-iodocarbazoles 3.
We also investigated the feasibility of the gold-catalyzed 1,3-
halogen shift in (3-chloroindol-2-yl)butynol 2i and (3-
bromoindol-2-yl)butynol 2j. However, the 1,3-iodine migration
3
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Research Article
Moghimi, S.; Rudolph, M.; Braun, I.; Rominger, F.; Hashmi, A. S. K.
CONCLUSIONS
■
Adv. Synth. Catal. 2015, 357, 500−506. (e) Moran
E.; Gonzalez, J. M. Angew. Chem., Int. Ed. 2015, 54, 3052−3055.
f) Mader, S.; Molinari, L.; Rudolph, M.; Rominger, F.; Hashmi, A. S.
́
-Poladura, P.; Rubio,
In conclusion, the controlled benzoannelation−iodine transfer
process of 3-iodoindole-tethered alkynols into 3-iodo 2,4,6-
trisubstituted 9H-carbazoles has been realized by using gold
catalysis. This reaction sequence has been studied experimen-
tally; additionally, its mechanism has been investigated by a
DFT study. The overall transformation can be rationalized
through a 6-endo-dig alkyne carbocyclization by chemo- and
regiospecific attack of the C3 indole carbon to the external
alkyne carbon followed by dehydration and two consecutive
́
(
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ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acscatal.5b00471.
■
4428.
(7) For selected reviews, see: (a) Jia, M.; Bandini, M. ACS Catal.
*
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Experimental procedures, characterization data of new
compounds, copies of NMR spectra, and computational
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Chem., Int. Ed. 2013, 52, 11714−11724. (h) Rudolph, M.; Hashmi, A.
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AUTHOR INFORMATION
Corresponding Authors
E-mail for B.A.: alcaideb@quim.ucm.es.
E-mail for P.A.: Palmendros@iqog.csic.es.
Notes
■
Per
́
ez, A.; Sabater, M. J. Chem. Rev. 2011, 111, 1657−1712.
(
6
2
j) Rudolph, M.; Hashmi, A. S. K. Chem. Commun. 2011, 47, 6536−
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*
1367. (m) Hashmi, A. S. K. Angew. Chem., Int. Ed. 2010, 49, 5232−
5
241.
8) (a) Zhang, Z.; Tang, X.; Xu, Q.; Shi, M. Chem. - Eur. J. 2013, 19,
The authors declare no competing financial interest.
(
1
0625−10631. (b) Cera, G.; Chiarucci, M.; Mazzanti, A.; Mancinelli,
ACKNOWLEDGMENTS
■
M.; Bandini, M. Org. Lett. 2012, 14, 1350−1353. (c) Noey, E. L.;
Wang, X.; Houk, K. N. J. Org. Chem. 2011, 76, 3477−3483.
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(10) (a) Alcaide, B.; Almendros, P.; Rodrı
Chem. 2005, 70, 3198−3204. (b) See ref 8.
(
Support for this work by the MINECO and FEDER (Projects
CTQ2012-33664-C02-01, CTQ2012-33664-C02-02, and
CTQ2013-44303-P) and the UCM-BANCO SANTANDER
Project GR3/14) is gratefully acknowledged. E.B. thanks the
MINECO for a postdoctoral contract. We thank Saeed
Khodabakhshi for optimization studies.
9) (a) Qiu, Y.; Kong, W.; Fu, C.; Ma, S. Org. Lett. 2012, 14, 6198−
(
guez-Acebes, R. J. Org.
́
11) The major proton source for the protodeauration step in the
formation of carbazole 4a would be water, generated in situ in the
aromatization step (in addition to moisture).
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