Solvent- and ligand-induced switch of selectivity in gold(I)-catalyzed tandem reactions of 3-propargylindoles

Article abstract of DOI:10.3762/bjoc.7.89

The selectivity of our previously described gold-catalyzed tandem reaction, 1,2-indole migration followed by aura-iso-Nazarov cyclization, of 3-propargylindoles bearing (hetero)aromatic substituents at both the propargylic and terminal positions, was reversed by the proper choice of the catalyst and the reaction conditions. Thus, 3-(inden-2-yl)indoles, derived from an aura-Nazarov cyclization (instead of an aura-iso-Nazarov cyclization), were obtained in moderate to good yields from a variety of 3-propargylindoles.

Full text of DOI:10.3762/bjoc.7.89

Solvent- and ligand-induced switch of selectivity in
gold(I)-catalyzed tandem reactions
of 3-propargylindoles
Estela Álvarez1, Delia Miguel1, Patricia García-García1,
Manuel A. Fernández-Rodríguez1, Félix Rodríguez2 and Roberto Sanz*1
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2011, 7, 786–793.
1Área de Química Orgánica, Departamento de Química, Facultad de
Ciencias, Universidad de Burgos, Pza. Misael Bañuelos s/n, 09001
Burgos, Spain and 2Instituto Universitario de Química Organometálica
“Enrique Moles”, Universidad de Oviedo, C/Julián Clavería 8, 33006
Oviedo, Spain
doi:10.3762/bjoc.7.89
Received: 31 March 2011
Accepted: 16 May 2011
Published: 09 June 2011
Email:
This article is part of the Thematic Series "Gold catalysis for organic
synthesis".
Roberto Sanz* - rsd@ubu.es
* Corresponding author
Guest Editor: F. D. Toste
Keywords:
© 2011 Álvarez et al; licensee Beilstein-Institut.
License and terms: see end of document.
catalysis; gold; indoles; Nazarov cyclizations; selectivity
Abstract
The selectivity of our previously described gold-catalyzed tandem reaction, 1,2-indole migration followed by aura-iso-Nazarov
cyclization, of 3-propargylindoles bearing (hetero)aromatic substituents at both the propargylic and terminal positions, was reversed
by the proper choice of the catalyst and the reaction conditions. Thus, 3-(inden-2-yl)indoles, derived from an aura-Nazarov cycliza-
tion (instead of an aura-iso-Nazarov cyclization), were obtained in moderate to good yields from a variety of 3-propargylindoles.
Introduction
Catalysis with gold complexes as carbophilic π-acids has gold-catalyzed migration reactions in propargylic systems that
become a highly developed area in the last decade [1-7]. In par- involve a carbon-centered moiety, implying that carbon–carbon
ticular, 1,2-acyl migration reactions of propargylic esters have bonds are broken and formed instead of carbon–heteroatom
been extensively investigated. In these processes the gold- bonds [13,14]. Based on the nucleophilic nature of indoles [15],
carbenoid species generated are able to undergo a wide variety which are known to react with gold-activated alkynes or allenes
of further transformations [8-11]. In addition, propargylic [16,17], and by taking advantage of our reported methodology
sulfides have also been reported as useful substrates for this for the synthesis of 3-propargylindoles [18,19], we have shown
type of process, participating in related 1,2-sulfur migrations that the indole nucleus is able to participate in gold-catalyzed
[12]. Within this area we have reported the first examples of 1,2-migration reactions of propargylic systems. Thus, 3-prop-
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Beilstein J. Org. Chem. 2011, 7, 786–793.
argylindoles 1 give rise to α,β-unsaturated gold-carbenoid inter- and selectivity of reactions catalyzed by gold complexes can be
mediates 2 that evolve through different pathways depending on appropriately tuned [21-27], we thought that it should be
the substituents at the propargylic and terminal positions of the possible to reverse the selectivity of our tandem reaction in
alkyne moiety (Scheme 1). If (hetero)aromatic substituents are favor of the iso-Nazarov pathway, to obtain compounds 4 by a
present at either of these positions, they undergo further cycliza- proper setting of the reaction conditions (modulation of the
tions to afford 3-(inden-2-yl)indoles 3 or 4 (Scheme 1). An electronic properties of the ligands, counter ion, solvent, substi-
analysis of the aromaticity of the transition state structures for tution pattern of the substrates, etc.). Herein, we report our
these cyclizations by DFT calculations revealed that these elec- efforts to control the two competing pathways in the evolution
trocyclic ring closures could be considered as gold variants of of gold-carbenoid intermediates generated by an initial 1,2-
the Nazarov (cyclization from 2 to 4) or iso-Nazarov reactions indole migration in 3-propargylindoles.
(cyclization from 2 to 3) [14]. These theoretical calculations
also showed that in those cases where both cyclization path- Results and Discussion
ways are possible (for example in 2a arising from 1a; It is an intriguing possibility that the aura-Nazarov reaction may
Scheme 1), the calculated energy barriers for the two cycliza- also take place with substrates bearing aromatic substituents at
tion modes favored the iso-Nazarov-product 3a (9.42 kcal/mol both the propargylic and terminal positions, and thus allow
vs 11.71 kcal/mol for the Nazarov cyclization assuming that the access to new functionalized indole derivatives (Scheme 2). It
initial gold coordination to the alkyne is anti to the indole). This should be remarked that, until now, only the aura-Nazarov
is in complete agreement with the experimental data, as we cyclization to give products 4 from substrates 1 (without an
always observed the selective formation of cyclization products aromatic substituent at the propargylic position, see R2, R3 in
3 in those cases where both 3 and 4 could be obtained. Scheme 1) has been observed.
However, for the model compound 1a, similar energy profiles
were obtained for the corresponding iso-Nazarov and Nazarov
pathways (ΔE = 2.29 kcal/mol) [20]. Since there are several
examples reported in the literature that show that the reactivity
Scheme 2: Tandem 1,2-indole migration/aura-Nazarov cyclization
from 3-propargylindoles bearing an aromatic substituent at the prop-
argylic position.
For the initial selectivity control experiments, 1-methyl-3-(1-
methyl-1,3-diphenylprop-2-ynyl)-1H-indole (1a) was selected
as the model compound and was treated with several gold cata-
lysts under different reaction conditions (Table 1). As expected,
under our standard reported conditions ((Ph3P)AuCl/AgSbF6 in
CH2Cl2 at room temperature), the 3-(inden-2-yl)indole 3a was
obtained as the major product. However, a minor isomer 4a was
also isolated along with 3a in a ca. 3.5/1 ratio (Table 1, entry 1).
The structure of the minor compound 4a was established by
X-ray diffraction (Figure 1), confirming that the gold-carbenoid
intermediate 2a could also undergo the aura-Nazarov cycliza-
tion [28,29].
The use of cationic gold complexes bearing different types of
phosphane ligands always provided the iso-Nazarov product 3a
as the major isomer, with a small increase in the competing
Scheme 1: Formation of 3-(inden-2-yl)indoles 3 and 4 from 3-prop-
argylindoles. Energy barriers (kcal/mol) for the cyclization reactions of
gold-carbenoid intermediate 2a.
Nazarov product 4a on switching the ligand to SPhos (Table 1,
entries 1–4). The use of complexes bearing N-heterocyclic
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Beilstein J. Org. Chem. 2011, 7, 786–793.
Table 1: Effect of the catalyst and reaction conditions on the reactivity of 1a.a
Ratiob
3a/4a
Entry
Catalyst
Solvent
1
2
(Ph3P)AuCl/AgSbF6
(Ph3P)AuNTf2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
DME
3.5/1
3.3/1
2.5/1
3.3/1d
2.5/1
3/1
3
SPhosAuNTf2c
4
(Et3P)AuCl/AgSbF6
5
IMeAuCle/AgSbF6
6
IPrAuClf/AgSbF6
7
[(PhO)3P]AuCl/AgSbF6
2.2/1
1.5/1
1.5/1
2/1
8
[(2,4-(t-Bu)2C6H3O)3P]AuCl/AgSbF6
[(2,4-(t-Bu)2C6H3O)3P]AuCl/AgBF4
[(2,4-(t-Bu)2C6H3O)3P]AuCl/AgNTf2
[(2,4-(t-Bu)2C6H3O)3P]AuCl/AgOTs
[(2,4-(t-Bu)2C6H3O)3P]AuCl/AgOTf
[(2,4-(t-Bu)2C6H3O)3P]AuCl/AgSbF6
[(2,4-(t-Bu)2C6H3O)3P]AuCl/AgSbF6
[(2,4-(t-Bu)2C6H3O)3P]AuCl/AgSbF6
[(2,4-(t-Bu)2C6H3O)3P]AuCl/AgSbF6
[(2,4-(t-Bu)2C6H3O)3P]AuCl/AgSbF6
9
10
11
12
13
14
15
16
17
1.5/1
1.4/1
1.4/1
1.4/1
1/1.8
1/2.3
1/4i
THF
toluene
tolueneg
tolueneh
aReactions carried out until complete consumption of the starting material 1a, as judged by GC-MS and/or TLC analysis, unless otherwise stated.
bDetermined by 1H NMR analysis of the crude reaction mixture. cSPhos = 2-dicyclohexylphosphino-2´,6´-dimethoxybiphenyl. d66% of conversion after
24 h. eIMe = 1,3,4,5-tetramethylimidazol-2-ylidene. fIPr = 1,3-bis(2,6-di-isopropylphenyl)imidazol-2-ylidene. gConducted at 0 °C. A similar result was
obtained by using AgOTf as a silver salt. hCarried out at −20 °C. i50% conversion after 24 h.
carbene ligands [30] also produced 3a as the major compound
of the corresponding mixtures (Table 1, entries 5 and 6). It
was decided to increase the π-acceptor character of the
ligand [31], and, in this case, the employment of a triphenyl-
phosphite–gold(I) complex led to a slight increase in the ratio of
4a (Table 1, entry 7). Finally, the use of the bulky phosphite
ligand tris(2,4-di-t-butylphenyl)phosphite, gave rise to a 1.5/1
ratio of 3a/4a (Table 1, entry 8) [32].
Once tris(2,4-di-t-butylphenyl)phosphite was selected as the
best ligand to favor the desired tandem process, the influence of
the metal counter ion was then studied. Thus, several silver salts
were employed for the generation of the cationic catalytic active
gold(I) complex, and it was concluded that the effect on the
selectivity is almost negligible (Table 1, entries 9–12). Never-
Figure 1: ORTEP diagram for 4a. Ellipsoids are shown at 30% level
(hydrogen atoms are omitted for clarity).
theless, it should be noted that no reaction occurred when
AgOBz was employed whilst the reactions with AgBF4,
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Beilstein J. Org. Chem. 2011, 7, 786–793.
AgNTf2 and AgOTs were relatively slow. Therefore, AgOTf influence on the selectivity. To clear up this point the initial
and AgSbF6 were selected as silver salts, due to their availabil- catalyst [(Ph3P)AuCl/AgOTf] was revisited, and 1a was treated
ity and higher reactivity, and subsequently the effect of the with this catalytic system in toluene. Since the reaction was
solvent was studied. Ethereal solvents, such as DME and THF, very slow at 0 °C, the temperature was increased to rt. Under
led to similar results as CH2Cl2 (Table 1, entries 13 and 14), these conditions the observed 3a/4a ratio was 1/1.5. By
whereas acetonitrile proved to be unsuitable for the reaction. comparing this result with that in entry 1 of Table 1 led to the
Gratifyingly, it was found that the use of toluene as the solvent conclusion that the change of solvent is the main factor respon-
reverses the selectivity of the reaction, and, with this solvent, sible for the selectivity switch in favor of the Nazarov product.
the Nazarov product 4a became the major isomer in the mix- Nevertheless, the beneficial effect of the bulky phosphite ligand
ture (Table 1, entry 15). Finally, the effect of the temperature in is also significant factor with regards to both reactivity and
toluene was investigated: It was found that carrying out the selectivity.
reaction at 0 °C afforded a 2.3/1 ratio of isomers in favor of 4a
(Table 1, entry 16). If the temperature is lowered to −20 °C the To examine further the scope of this switch of selectivity in
ratio in favor of 4a was even higher, although only a 50% favor of the Nazarov pathway in tandem gold-catalyzed reac-
conversion was observed after 24 h (Table 1, entry 17). Under tions of 3-propargylindoles initiated by 1,2-indole migrations, a
the optimized and synthetically useful reaction conditions, i.e., selection of substrates 1a–i, bearing a methyl group at one of
toluene at 0 °C, with [(2,4-(t-Bu)2C6H3O)3P]AuCl/AgOTf as the propargylic positions and different (hetero)aromatic groups
the catalytic system, 4a was obtained in 60% isolated yield.
at both the other propargylic and terminal positions, were
reacted under the established conditions (Table 2). From the
At this point it was unclear whether the observed change of results obtained, the selectivity in favor of the Nazarov prod-
selectivity in favor of the Nazarov product 4a was mainly a ucts 4 seems to be general for the selected indoles 1a–h
solvent effect, or if the nature of the ligand also exerted an (Table 2, entries 1–8). N-unsubstituted indole 1b also showed a
Table 2: Synthesis of 3-(inden-2-yl)indoles 4 by gold-catalyzed tandem 1,2-indole migration/Nazarov-type cyclization of 3-propargylindoles 1.
Entry
Substrate
Ratio Nazarov (4)/
iso-Nazarov (3)a
Product
Yield (%)b
1
2.3/1
60
2
1.8/1
41
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Beilstein J. Org. Chem. 2011, 7, 786–793.
Table 2: Synthesis of 3-(inden-2-yl)indoles 4 by gold-catalyzed tandem 1,2-indole migration/Nazarov-type cyclization of 3-propargylindoles 1.
(continued)
3
3/1
67
4
>10/1
86
5
3/1
62
6
4/1
71
7
3/1
60
8
1.2/1
47c
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Beilstein J. Org. Chem. 2011, 7, 786–793.
Table 2: Synthesis of 3-(inden-2-yl)indoles 4 by gold-catalyzed tandem 1,2-indole migration/Nazarov-type cyclization of 3-propargylindoles 1.
(continued)
9
<1/10
88d
aDetermined by 1H NMR analysis of the crude reaction mixture. bIsolated yield of compounds 4 after column chromatography, unless otherwise
stated. cDetermined by NMR from the mixture of 3h and 4h.dCombined yield for 3i and 3´i, in which the double bond has isomerized. Both com-
pounds have been isolated and characterized. See Supporting Information File 1 (Experimental and analytical data) and Supporting Information File 2
(NMR spectra).
preference for the corresponding Nazarov product 4b, although respectively, provided the corresponding indole derivatives 3j
in this case the selectivity was slightly lower compared to 1a and 3k with high selectivity when the reaction was conducted in
(Table 2, entry 2), and the reaction gave a poorer overall yield. CH2Cl2 with (Ph3P)AuCl/AgSbF6 as catalyst (Scheme 3). Inter-
When an electron-withdrawing substituent was present on the estingly, under the new conditions developed herein, i.e., treat-
aryl group at the propargylic position, selectivity in favor of ment with a cationic phosphite–gold complex in toluene, the
Nazarov products 4 appeared to be slightly increased (Table 2, reaction of 1j afforded a ca. 3.5/1 mixture of 3j/4j, whereas 1k
entry 3) [33]. Substrate 1d, with a bulky electron-withdrawing gave rise to a ca. 1.6/1 mixture of 3k/4k (Scheme 3). These
substituent at one of the ortho positions of the aromatic prop- results again show that the change of selectivity in the competi-
argylic group, afforded almost exclusively the Nazarov product tive iso-Nazarov/Nazarov pathways could be induced by a
4d in high yield (Table 2, entry 4). Similarly, the presence of a change of ligand and solvent, although complete reversal of
π-electron rich heteroaromatic group or an electron-rich selectivity was not achieved for these substrates.
aromatic group at the terminal position of the triple bond also
favors the Nazarov pathway (Table 2, entries 5–7). On the other
hand, the use of 3-propargylindole 1h as starting material,
bearing an electron-withdrawing substituent on the aromatic
ring at the terminal position, led to a slight decrease in the
selectivity (Table 2, entry 8). Moreover, the introduction of an
electron-donating group on the aromatic ring at the propargylic
position gave rise to the almost exclusive formation of the iso-
Nazarov product 3i (Table 2, entry 9). A comparison of these
selectivities with that obtained for the parent indole 1a, leads to
the conclusion that the electronic nature of the aryl groups at
both the propargylic and terminal positions also has a signifi-
cant influence on the preferred cyclization pathway. The
Nazarov products 4 seem to be more favored when electron-
withdrawing groups are present at the propargylic position and
electron-donating substituents are present at the terminal posi-
tion. Under these optimized conditions, new and interesting
3-(inden-2-yl)indoles 4a–h were isolated in good yields.
Scheme 3: Comparison of the reactivity of C-2 substituted indoles 1j
and 1k. Conditions: a) (Ph3P)AuCl/AgSbF6 (5 mol %), CH2Cl2, rt; b)
[(2,4-(t-Bu)2C6H3O)3P]AuCl/AgOTf (5 mol %), toluene, 0 °C.
By contrast, it was previously observed that substitution at C-2 It has also been observed that reactions of 3-propargylindoles
of the starting 3-propargylindole led almost exclusively to the bearing alkyl substituents bulkier than methyl at the prop-
formation of iso-Nazarov products 3 [13,14]. For instance, argylic position, such as 1l and 1m, almost exclusively
indoles 1j and 1k, bearing a methyl and a phenyl group at C-2, produced the corresponding iso-Nazarov products 3l and 3m
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Beilstein J. Org. Chem. 2011, 7, 786–793.
with (Ph3P)AuCl/AgSbF6 as catalyst in CH2Cl2 (Scheme 4)
[13,14]. Again, the use of the phosphite–gold complex as cata-
lyst and toluene as solvent slightly favored the Nazarov
pathway: Approximately 3/1 ratios of the corresponding indole
derivatives 3l, m/4l, m were obtained (Scheme 4) [32]. In add-
ition, we were able to isolate the new Nazarov compounds 4l
and 4m, albeit in low yields (Scheme 4). Finally, when the more
sterically demanding isopropyl group was present at the prop-
argylic position, the corresponding iso-Nazarov product was
produced exclusively irrespective of the conditions employed.
Although these results show that the change of the methyl group
at the propargylic position of the starting indole 1 to a bulkier
alkyl group strongly favors the iso-Nazarov pathway, they also
show that our new reported conditions make the Nazarov
pathway more accessible.
Supporting Information
Experimental procedures and spectroscopic data for all new
compounds. Copies of 1H NMR and 13C NMR spectra for
new compounds.
Supporting Information File 1
Experimental and analytical data.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-7-89-S1.pdf]
Supporting Information File 2
NMR spectra.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-7-89-S2.pdf]
Acknowledgements
We acknowledge MICINN (CTQ2010-15358 and CTQ2009-
09949) and Junta de Castilla y León (BU021A09 and GR-172)
for financial support. We are also grateful to MEC (FPU
predoctoral fellowship to D.M., “Ramón y Cajal” contract to M.
A. F. R. and “Juan de la Cierva” contract to P. G. G.). Many
thanks are due to Prof. Dr. A. R. de Lera (Universidade de
Vigo) for his helpful comments.
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J. Am. Chem. Soc. 2009, 131, 6348–6349. doi:10.1021/ja901649s
32.When [(2,4-(t-Bu)2C6H3O)3P]AuCl was used as catalyst the
iso-Nazarov products were obtained as a mixture of 3 and 3´, in which
the double bond is isomerized. So, the ratio of isomers 3/4 reflects the
ratio of both the iso-Nazarov compounds 3 and 3´ against the Nazarov
product 4.
33.We have also checked that 1b affords a ca. 2.2/1 mixture of 3b/4b
when (Ph3P)AuCl/AgSbF6 was used as catalytic system in CH2Cl2,
also proving the effect of the electron-withdrawing substituent on the
phenyl group at the propargylic position.
793